# Patent

## Title
Transgenic mouse having a transgene that converts a prodrug into a cytotoxic compound in senescent cells

## Abstract

This invention provides a transgenic mouse for studying the role of senescent cells on an age-related disorder or an age-sensitive trait. The transgene contains a p16 promoter sequence that controls expression of an enzyme so as to cause the enzyme to be expressed in senescent cells in the mouse. The enzyme converts a prodrug to a cytotoxic agent, so that treating the mouse with the prodrug results in the prodrug selectively killing the senescent cells. As a result, progression of an age-related disorder or an age-sensitive trait is delayed. Included is the 3MR mouse model, which also expresses bioluminescent and fluorescent markers under control of the p16 promoter so that senescent cells in the mice can be visualized.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Application No. 61/692,613 filed Aug. 23, 2012, U.S. Provisional Application No. 61/837,096 filed Jun. 19, 2013, which applications are incorporated herein by reference in their entirety.


STATEMENT REGARDING SEQUENCE LISTING
The Sequence Listing associated with this application is provided in text format in lieu of a paper copy, and is hereby incorporated by reference into the specification. The name of the text file containing the Sequence Listing is 200201_410_SEQUENCE_LISTING.txt. The text file is 31 KB, was created on Aug. 21, 2013, and is being submitted electronically via EFS-Web.
BACKGROUND
Technical Field
This disclosure relates to non-human animal models for age-related disorders and age-sensitive traits associated with senescence-inducing stimuli, methods for identifying therapeutic agents for treating or preventing age-related disorders and age-sensitive traits associated with senescence-inducing stimuli, and methods for treating or preventing age-related disorders and age-sensitive traits. It also relates to therapeutic agents and pharmaceutical compositions for treating or preventing age-related disorders and age-sensitive traits associated with senescence-inducing stimuli.
Description of the Related Art
The American Cancer Society estimates that there will be more than 1.6 million new cases of cancer in 2012 in the United States alone. More than half of these patients will receive chemotherapy and/or treatment with radiation in addition to undergoing surgical resection of the tumor. Various chemotherapeutics and radiation are known to induce cellular senescence. Given that senescent cells have been causally implicated in certain aspects of age-related decline in health and may contribute to certain diseases, the induction of senescent cells resulting from necessary live-preserving chemotherapeutic and radiation treatments may have deleterious effects to millions of patients worldwide (e.g. fatigue, weakness, loss of physical agility). As such, treatments aimed at clearing therapy-induced senescent cells and improving age-sensitive traits associated with the same have the potential to markedly improve the health, lifespan and quality of life for patients exposed to senescence-inducing stimuli. The present disclosure addresses these needs and offers numerous related advantages.
BRIEF SUMMARY
Briefly, provided herein are animal models, methods for identifying therapeutic agents, and therapeutic agents useful for treating and/or preventing age-related disorders and age-sensitive traits, particularly those cause by senescence-inducing stimuli. For example, provided herein are the following embodiments.
The present disclosure provides, in one embodiment, a non-human animal model for aging comprising a non-human animal that (a) exhibits an age-related disorder or age-sensitive trait and (b) comprises a transgene selectively expressed by senescent cells, wherein the animal is exposed to or treated with a senescence-inducing stimulus. In another embodiment, the transgene comprises a senescent cell-specific promoter. In a more specific embodiment, the promoter is derived from p16Ink4a.
In another embodiment, the transgene expresses at least one detectable label, a cytotoxic agent, a cytotoxicity-activating molecule, an RNA, or a combination thereof. In still another embodiment, the detectable label is selected from the group consisting of (a) luciferase; (b) a red fluorescent protein; (c) a green fluorescent protein; and (d) a luciferase and a red fluorescent protein. In yet another embodiment, the cytotoxicity-activating molecule is a truncated herpes simplex virus thymidine kinase or a FK506-binding protein (FKBP)-caspase fusion polypeptide.
In a further embodiment, the detectable label is selected from the group consisting of (a) luciferase; (b) a red fluorescent protein; (c) a green fluorescent protein; and (d) a luciferase and a red fluorescent protein; and wherein the cytotoxicity-activating molecule is a truncated herpes simplex virus thymidine kinase or a FKBP-caspase fusion polypeptide.
In still another embodiment, the transgene comprises (a) a p16Ink4a promoter operatively linked to a polynucleotide sequence encoding a FKBP-caspase fusion polypeptide (p16-FKBP-caspase transgene), and to a polynucleotide sequence encoding a green fluorescence protein, or (b) a p16Ink4a promoter operatively linked to a polynucleotide sequence encoding a fusion polypeptide comprising a luciferase, a red fluorescent protein, and a truncated herpes simplex virus thymidine kinase (p16-3MR transgene).
In yet another embodiment, the transgene comprises a p16Ink4a promoter operatively linked to a polynucleotide sequence encoding a FKBP-caspase fusion polypeptide (p16-FKBP-caspase transgene), and to a polynucleotide sequence encoding a green fluorescence protein.
In a further embodiment, the transgene comprises a p16Ink4a promoter operatively linked to a polynucleotide sequence encoding a fusion polypeptide comprising a luciferase, a red fluorescent protein, and a truncated herpes simplex virus thymidine kinase (p16-3MR transgene).
In another embodiment, the senescence-inducing stimulus gives rise to an age-related disorder or age-sensitive trait. In a more specific embodiment, the senescence-inducing stimulus comprises exposure to irradiation or treatment with a chemotherapeutic agent. In another specific embodiment, the chemotherapeutic agent is doxorubicin, taxol, docetaxel, gemcitabine, or cisplatin. In yet another specific embodiment, the irradiation is whole body gamma radiation.
In still another embodiment, the age-related disorder or age-sensitive trait results at least in part from (1) a genetic modification; (2) a diet modification; (3) a chemical induction; (4) radiation induction; or (5) any combination thereof. For example, in a more specific embodiment, the age-related disorder or age-sensitive trait results at least in part from a genetic modification, wherein the genetic modification comprises (1) expression of a second transgene; (2) reduced or abrogated expression of an endogenous gene, or (3) a combination thereof. In another specific embodiment, the animal is a BubR1 mouse.
The present disclosure provides, in another embodiment, a non-human animal model comprising a transgene that comprises (1) a senescent cell-specific promoter operatively linked to a polynucleotide encoding (a) at least one detectable label, (b) a cytotoxic agent, (c) a cytotoxicity-activating molecule, (d) an RNA, or (e) any combination of (a), (b), (c), and (d); and wherein the animal exhibits an age-sensitive trait. In another embodiment, the transgene comprises a p16Ink4a promoter operatively linked to a polynucleotide sequence encoding a FKBP-caspase fusion polypeptide (p16-FKBP-caspase transgene) and a green fluorescent protein. In still another embodiment, the transgene comprises a p16Ink4a promoter operatively linked to a polynucleotide sequence encoding a fusion polypeptide comprising a luciferase, a red fluorescent protein, and a truncated herpes simplex virus thymidine kinase (p16-3MR transgene).
In yet another embodiment, the age-sensitive trait is measure of a specific T cell subset distribution, cataract formation, spontaneous activity, motor coordination, cognitive capacity, physical function, body composition, cardiac function, or any combination thereof. In a more specific embodiment, the body composition is a measure of sarcopenia, osteoporosis, bone mineral density, lean mass or fat mass. In a related embodiment, the lean mass, fat mass and bone mineral density are measured by QNMR, dual-energy X-ray absorptiometry, MRI, PET, or a combination thereof. In another related embodiment, the physical function is a measure of (1) running time, distance, and work using a motorized treadmill, (2) grip strength using a grip meter, or (3) any combination measure thereof. In another embodiment, the age-sensitive trait is a measure of a tissue or organ, wherein the measure is of fiber diameter on gastrocnemius muscle, DNA damage, renal and glomerulosclerosis, retinal atrophy, proteotoxic stress, oxidative stress, or hematopoietic system.
The present disclosure provides, in another embodiment, a method for identifying a therapeutic agent effective for treating or reducing the risk of developing an age-related disorder or age-sensitive trait, said method comprising:
(a) administering a candidate therapeutic agent to the animal of the aging animal model of the present disclosure, to provide a treated animal;
(b) (1) determining an age-related disorder or age-sensitive trait exhibited in the treated animal, and comparing to the age-related disorder or age-sensitive trait exhibited in an untreated control aging model animal, or
(2) determining the level of suppression of cellular senescence in the treated animal and comparing to the level of cellular senescence in the untreated control animal
wherein (1) suppression of an age-related disorder or age-sensitive trait or (2) suppression of cellular senescence in the treated animal compared with the untreated animal identifies an agent effective for treating or preventing an age-related disorder or age-sensitive trait.
In a more specific embodiment, step (b) comprises:
(1) determining an age-related disorder or age-sensitive trait exhibited in the treated animal, and comparing to the age-related disorder or age-sensitive trait exhibited in an untreated control aging model animal; and
(2) determining the level of suppression of cellular senescence in the treated animal and comparing to the level of cellular senescence in the untreated control animal; and
wherein (1) suppression of an age-related disorder or age-sensitive trait and (2) suppression of cellular senescence in the treated animal compared with the untreated animal identifies an agent effective for treating or preventing an age-related disorder or age-sensitive trait
In a related embodiment, suppression of cellular senescence comprises suppression of the expression or secretion of one or more senescent cell-associated molecules in the animal. In yet another related embodiment, suppression of cellular senescence comprises reducing the quantity of senescent cells in the animal.
The present disclosure provides, in another embodiment, a therapeutic agent for treating or reducing the likelihood of developing an age-related disorder or age-sensitive trait, wherein the therapeutic agent is identified according to a method of the present disclosure.
The present disclosure provides, in another embodiment, a method for treating or reducing the likelihood of developing an age-related disorder or age-sensitive trait in a subject who has an age-related disorder or age-sensitive trait, who is in remission for an age-related disorder or age-sensitive trait, or who is at risk of developing a recurrence of an age-related disorder or age-sensitive trait, comprising administering to the subject the therapeutic agent of the present disclosure.
The present disclosure also provides, in another embodiment, a method for treating or reducing the likelihood of an age-related disorder or age-sensitive trait in a subject who has an age-related disorder or age-sensitive trait, who is in remission for an age-related disorder or age-sensitive trait, or who is at risk of developing a recurrence of an age-related disorder or age-sensitive trait, said method comprising administering a therapeutic agent that selectively suppresses cellular senescence in the subject, thereby treating or reducing the likelihood of an age-related disorder or age-sensitive trait in the subject. In another embodiment, suppressing cellular senescence comprises suppressing the expression or secretion of one or more senescent cell-associated molecules in the subject. In a related embodiment, suppressing cellular senescence comprises reducing the quantity of senescent cells in the subject.
The present disclosure also provides, in another embodiment, an isolated cell or cell line derived from the animal model of the present disclosure.
The present disclosure further provides, in another embodiment, a method for producing the non-human animal of the model of the present disclosure, comprising (a) providing a non-human animal that comprises a transgene selectively expressed by senescent cells; and (b) aging the animal to produce an age-related disorder or age-sensitive trait.
In the following description, certain specific details are set forth in order to provide a thorough understanding of various embodiments. However, one skilled in the art will understand that the invention may be practiced without these details. In other instances, well-known structures have not been shown or described in detail to avoid unnecessarily obscuring descriptions of the embodiments. Unless the context requires otherwise, throughout the specification and claims which follow, the word “comprise” and variations thereof, such as, “comprises” and “comprising” are to be construed in an open, inclusive sense, that is, as “including, but not limited to.” In addition, the term “comprising” (and related terms such as “comprise” or “comprises” or “having” or “including”) is not intended to exclude that in other certain embodiments, for example, an embodiment of any composition of matter, composition, method, or process, or the like, described herein, may “consist of” or “consist essentially of” the described features. Headings provided herein are for convenience only and do not interpret the scope or meaning of the claimed embodiments.
Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.
Also, as used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the content clearly dictates otherwise. Thus, for example, reference to “a non-human animal” may refer to one or more non-human animals, or a plurality of such animals, and reference to “a cell” or “the cell” includes reference to one or more cells and equivalents thereof (e.g., plurality of cells) known to those skilled in the art, and so forth. When steps of a method are described or claimed, and the steps are described as occurring in a particular order, the description of a first step occurring (or being performed) “prior to” (i.e., before) a second step has the same meaning if rewritten to state that the second step occurs (or is performed) “subsequent” to the first step. The term “about” when referring to a number or a numerical range means that the number or numerical range referred to is an approximation within experimental variability (or within statistical experimental error), and thus the number or numerical range may vary between 1% and 15% of the stated number or numerical range. It should also be noted that the term “or” is generally employed in its sense including “and/or” unless the content clearly dictates otherwise. The term, “at least one,” for example, when referring to at least one compound or to at least one composition, has the same meaning and understanding as the term, “one or more.”



BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A-1D include a listing of an exemplary transgene selectively expressed in senescent cells, the nucleic acid sequence of a pBLUESCRIPT II KS vector containing a p16Ink4a promoter-FKBP-caspase 8-IRES-GFP nucleic acid construct.
FIGS. 2A-2F include a listing of the nucleic acid sequence of FIG. 1 with the various vector components and construct components labeled.
FIG. 3 represents an exemplary 3MR transgene sequence.
FIG. 4 illustrates lifespan difference between wild-type INK-ATTAC transgenic mice treated with AP20187 (TG TR) and untreated wild-type INK-ATTAC transgenic mice (TG UN).
FIG. 5 shows fat mass measurements for 18-month wild-type INK-ATTAC transgenic mice treated with AP20187 or control INK-ATTAC animals (treated with PBS). Treated female mice were compared with untreated female mice, and treated male mice were compared with untreated male mice.
FIG. 6 illustrates the difference in adipose cell (AC) diameter in AP20187-treated wild-type INK-ATTAC transgenic mice (TG3+/TG5+, indicating mice from two different INK-ATTAC strains) and control untreated (PBS) INK-ATTAC animals (TG3−).
FIG. 7 illustrates cardiac performance of AP20187-treated wild-type INK-ATTAC transgenic mice (treated) and untreated (PBS) INK-ATTAC mice. Animals were injected with isoproterenol, as an exogenous stress, prior to sacrifice of the animals.
FIG. 8 shows the extent of glomerulosclerosis in AP20187-treated wild-type INK-ATTAC transgenic mice (TG+) and untreated (PBS) wild-type INK-ATTAC mice.



DETAILED DESCRIPTION
The present disclosure relates generally to non-human animal models of aging. More specifically, the present disclosure provides non-human animal models of aging, age-related disorders, age-sensitive traits, and the like, wherein the animals models comprise transgenes selectively expressed by senescent cells, and wherein the age-related disorders and age-sensitive traits result at least in part from senescence-inducing stimuli, such as treatment with chemotherapy or radiation. For example, in certain embodiments, the animal models comprise a transgene that allows for the selective and controllable ablation of senescent cells at a desired time or for a desired period relative to exposure of the animals to senescence-inducing stimuli. The animal models are advantageously used for determining the deleterious health effects of senescence-inducing stimuli and for identifying therapeutic agents effective for treating or preventing age-related disorders and age-sensitive traits associated with or caused by such stimuli. As described herein, the present disclosure also provides therapeutic agents identified using such methods, pharmaceutical compositions comprising the identified therapeutic agents, and methods of treating or preventing age-related disorders and age-specific traits associated with senescence-inducing stimuli.
Transgenic Animals with Senescent Cell-Specific Transgene Expression
Certain aspects of the present disclosure employ non-human animals, particularly genetically modified non-human animals, wherein the animals comprise a transgene expressed under the control of a senescent cell-specific promoter, and wherein the animals are exposed to or treated with at least one senescence-inducing stimulus, such as chemotherapy or radiation. In certain more specific embodiments, the animals comprise a transgene that allows for the selective and controllable ablation of senescent cells.
By operably linking a senescent cell-specific promoter of a transgene to a nucleic acid sequence encoding a polypeptide of interest (e.g., a detectable label or cytotoxicity-activating molecule), senescent cells within an animal can be monitored and/or deleted in a controlled and user-determined fashion. In certain embodiments, for example, the present disclosure employs transgenic non-human animals that can be induced to delete senescent cells in vivo at a predetermined and desired point in time, such as at a particular stage of development or disease.
In addition to the transgene selectively expressed by senescent cells, an animal model of the present disclosure, to which a senescence-inducing stimulus is administered, may further contain other genetic or non-genetic modifications, or may be exposed to or administered other agents or treatments that are known to accelerate or facilitate the aging process in the animal and/or that contribute to the development of an age-related disorder or age-sensitive trait in the animal. For example, as further described herein, the non-human animals comprising a senescent cell-specific transgene may be advantageously crossed with other animals, which, due to one or more genetic modifications, for example, are known to develop a desired age-related phenotype, age-related disorder and/or age-sensitive trait of interest.
In this way, an animal model may be generated according to the present disclosure which is genetically or otherwise modified to develop a desired age-related phenotype, age-related disorder and/or age-sensitive trait, wherein the role of senescence-inducing stimuli in relation to an animal's age-related phenotype, age-related disorder and/or age-sensitive trait may be evaluated. The animal model may be further employed in screening methods in order to identify therapeutic agent that suppress or otherwise advantageously effect senescent cell survival, viability and/or clearance in the context of the animal model.
A senescent cell-specific promoter sequence present within a transgene of the present disclosure can be essentially any sequence that selectively drives expression of a polypeptide encoded by the transgene or expression of a nucleic acid sequence (e.g., an RNA) in senescent cells, while driving less, little, or no expression of the encoded polypeptide or nucleic acid sequence in non-senescent cells. In certain exemplary embodiments, a senescent cell-specific promoter used in accordance with the present disclosure may include, without limitation, a p16Ink4a promoter sequence, a p21cip promoter sequence, or a Pail promoter sequence.
In certain embodiments, the present disclosure provides a non-human animal model comprising a transgene that comprises (1) a senescent cell-specific promoter operatively linked to a polynucleotide encoding (a) at least one detectable label, (b) a cytotoxic agent, (c) a cytotoxicity-activating molecule, (d) an RNA, or (e) any combination of (a), (b), (c) and (d); and that exhibits a phenotype of an age-related disease.
It will be understood that a senescent cell-specific promoter can be operably linked to a nucleic acid sequence encoding any polypeptide of interest. In certain embodiments, the polypeptide of interest is selected from a detectable label, a cytotoxic molecule (e.g., a polypeptide capable of killing a senescent cell in which it is expressed) and a cytotoxicity-activating molecule (e.g., a polypeptide capable of facilitating the killing of a cell in which it is expressed), or a combination thereof. In certain embodiments, the transgene can be operably linked to an RNA (e.g., siRNA, shRNA, microRNA, and the like, which reduces or abrogates the expression of one or more genes important or essential for senescent cell survival), or a combination thereof.
Depending on the polypeptide of interest that is encoded by the transgene, it will be understood that different promoter features may be advantageous or desired. For example, as will be understood, in embodiments wherein a transgene encodes a polypeptide that is directly cytotoxic to cells in which it is expressed, the senescent cell-specific promoter of the transgene will need to be an inducible promoter in order to control the timing of expression of the cytotoxic polypeptide, and thereby control the deletion of senescent cells in the animal. If, on the other hand, a transgene encodes a cytotoxicity-activating polypeptide, the transgene promoter need not be inducible. Rather, the inducibility of the system in this instance relies upon the timing of exposure of the cytotoxicity-activating molecule expressed by senescent cells to its activating agent, as further described below.
Any of a number of detectable labels may be operably linked to a senescent cell-specific promoter. Many such detectable labels, and the means by which they can be detected, have been described and are well known and established in the art. In some embodiments, the detectable label comprises one or more fluorescent or bioluminescent labels, many of which are well known and established in the art. For example, the fluorescent protein may be any protein that fluoresces and that may be visualized when expressed in senescent cells under the control of a senescent cell-specific promoter as described herein. Illustrative fluorescent proteins can include, for example, green fluorescent protein (GFP), modified or enhanced green fluorescent protein (EGFP), red fluorescent protein (RFP), and various other known fluorescent proteins such as EBFP, EBFP2, Azurite, mKalama1, ECFP, Cerulean, CyPet, YFP, Citrin, Venus, and Wet. Other illustrative fluorescent or bioluminescent proteins include, for example, infrared-fluorescent proteins (IFPs), mRFP1, mCherry, mOrange, DsRed, tdTomato, mKO, TagRFP, mOrange2, maple, TagRFP-T, Firefly Luciferase, Renilla Luciferase and Click Beetle Luciferase. Still other illustrative labels can include yellow fluorescent protein, cyan FP, blue FP, red FP and their enhanced versions. It will be understood that essentially any of a number of other luminescent or fluorescent proteins that can emit light can be used in this context.
In certain specific embodiments, a detectable label present within a transgene according to the disclosure is selected from the group consisting of (a) luciferase; (b) a red fluorescent protein; (c) a green fluorescent protein; and (d) a luciferase and a red fluorescent protein.
Any of a number of cytotoxicity-activating molecules may be operably linked to a senescent cell-specific promoter to produce a suitable transgene for use in the context of the present disclosure. Following its expression in a senescent cell-specific fashion, the cytotoxicity-activating molecule is one that is capable of inducing the controllable killing of the senescent cells in which it is expressed upon administration of an activating agent to the transgenic animal. Illustrative examples of cytotoxicity-activating molecules include, but are not limited to herpes simplex virus (HSV) thymidine kinase (TK) polypeptides and FK506 binding protein (FKBP)-caspase fusion polypeptide. FK506 binding protein includes variant thereof, such as a Phe36Val mutant.
For example, in a specific embodiment, the cytotoxicity-activating molecule encoded by the transgene is a herpes simplex virus (HSV) thymidine kinase (TK) polypeptide (including truncated TK polypeptides) and the activating agent is ganciclovir.
In other embodiments, the cytotoxicity-activating molecule encoded by the transgene comprises two or more polypeptide sequences fused together (e.g., a fusion polypeptide). An example of such a fusion polypeptide can be a FKBP-caspase-8 fusion polypeptide. See, e.g., Pajvani et al., Nat. Med., 11:797-803 (2005) and Baker et al., Nature 479:232-36 (2011). Such fusion polypeptides may comprise, for example, one or more catalytic domains of human caspase-8 fused to one or more FKBP domains. Following transgene expression, adjacent FKBP molecules in the encoded polypeptide can be activated via forced dimerization using a suitable activating agent, thereby allowing for the regulated ablation of cells in which the fusion polypeptide is expressed. In a specific example, the p20 and p10 domains of human caspase-8 are fused to serial FKBPv (Phe36Val mutant FKBP) domains. Other examples of such polypeptides include, without limitation, a FKBP-caspase-1 fusion polypeptide or FKBP-caspase-3 fusion polypeptide (see, e.g., Mallet et al., Nat. Biotechnol. 20:1234-39 (2002)). In these and related embodiments, an illustrative activating agent used to induce cytotoxicity of senescent cells expressing the fusion polypeptide include the compound FK1012 analog AP20187 (referred to herein as AP20187) and related analogs. (see, e.g., U.S. Patent Application Publication No. 2004/0006233, the disclosure of which is incorporated herein by reference). To increase local concentrations of a FKBP-caspase fusion polypeptide, a myristoylation sequence may be included in the transgene to provide membrane attachment for the FKBP-caspase fusion polypeptide.
In this way, administration of a suitable cytotoxicity activating molecule to an animal at a desired time provides an effective means for selectively killing (e.g., by apoptosis) the senescent cells which express the cytotoxicity-activating molecule in the animal. In certain specific embodiments, less than 1%, 5%, 10% or 20% of non-senescent cells of the transgenic mouse are killed when an activating compound is administered to a transgenic mouse comprising a transgene encoding a cytotoxicity activating molecule.
Any of a number of nucleotide sequences encoding small RNAs whose expression affects expression or secretion of senescence cell-associated molecules may be operatively linked to a senescent cell-specific promoter. Such small RNAs include siRNA, shRNA, microRNA and the like (see, Finnegan and Matzke, J. Cell Sci. 226:4689-93, 2003). In certain embodiments, the expression of such small RNAs is under the control of an inducible senescent cell-specific promoter. Upon induction, the expression of the small RNAs down-regulates the expression or secretion of senescent cell-associated molecules of interest.
In some cases, a polypeptide encoded by a transgene of the present disclosure will be engineered to include one or more other elements, such as an affinity tag (e.g., a Flag tag), cellular localization sequence (e.g., myristoylation sequence) or any other desired element of interest.
In light of the above general disclosure, it will be evident that various more specific transgenes and transgenic non-human animals are provided for use herein. For example, in a specific embodiment, the animal model will comprise a transgene wherein a senescent cell-specific promoter directs expression of a detectable label selected from the group consisting of (a) luciferase; (b) a red fluorescent protein; (c) a green fluorescent protein; and (d) a luciferase and a red fluorescent protein; and further directs expression of a cytotoxicity-activating molecule selected from the group consisting of a truncated herpes simplex virus thymidine kinase and a FKBP-caspase fusion polypeptide.
In another specific embodiment, an animal model includes a transgene comprising (a) a p16Ink4a promoter operatively linked to a polynucleotide sequence encoding a FKBP-caspase fusion polypeptide (p16-FKBP-caspase transgene) and to a polynucleotide sequence encoding a green fluorescent protein; or (b) a p16Ink4a promoter operatively linked to a polynucleotide sequence encoding a fusion polypeptide comprising a luciferase, a red fluorescent protein, and a truncated herpes simplex virus thymidine kinase (tTK) (p16-3MR transgene), which may be called herein a trimodal fusion protein (3MR). In more specific embodiments, luciferase is renilla luciferase and red fluorescent protein is a monomeric red fluorescent protein. A p16Ink4a promoter may comprise a full-length promoter sequence or may comprise a functional (i.e., operable) truncation (or fragment) thereof (see, e.g., Wang et al. J. Biol. Chem. 276:48655-61 (2001)).
The non-human animal models of the present disclosure can be implemented in essentially any type of animal. Most typically, the animal model will be a mammal. In more specific embodiments, illustrative animal models include, but are not limited to, models derived from farm animals such as pigs, goats, sheep, cows, horses, and rabbits, rodents such as rats, guinea pigs, and mice, and non-human primates such as baboons, monkeys, and chimpanzees.
Transgenic nonhuman animal of the present disclosure can include, without limitation, founder transgenic non-human animals as well as progeny of the founders, progeny of the progeny, and so forth, provided that the progeny retain the transgene. The nucleated cells of the transgenic nonhuman animals provided herein can contain a transgene that includes a senescent cell-specific promoter sequence (e.g., a p16Ink4a promoter sequence, or an operable truncation thereof) operably linked to a nucleic acid sequence encoding a polypeptide of interest, such as a polypeptide that comprises a detectable label, a polypeptide capable of killing a cell (e.g., a cytotoxic polypeptide) and/or a polypeptide capable of facilitating the killing of a cell (e.g., a cytotoxicity-activating polypeptide), or a combination thereof.
In the context of transgenic animal production, operably linking a promoter sequence of interest to a nucleic acid sequence encoding a polypeptide of interest is well known and established. This generally involves positioning a regulatory element (e.g., a promoter sequence, an inducible element and/or an enhancer sequence) relative to a nucleic acid sequence encoding a polypeptide in such a way as to permit or facilitate expression of the encoded polypeptide. In the transgenes disclosed herein, for example, a promoter sequence (e.g., a p16Ink4a promoter sequence, or an operable truncation thereof) can be positioned 5′ relative to a nucleic acid encoding a polypeptide of interest (e.g., an FKBP-caspase-8 fusion protein).
Various techniques known in the art can be used to introduce transgenes into non-human animals to produce founder lines, in which the transgene is integrated into the genome. Such techniques include, without limitation, pronuclear microinjection (See, e.g., U.S. Pat. No. 4,873,191), retrovirus mediated gene transfer into germ lines (Van der Putten et al., Proc. Natl. Acad. Sci. USA, 82:6148-1652 (1985)), gene targeting into embryonic stem cells (Thompson et al., Cell 56:313-321 (1989)), electroporation of embryos (Lo, Mol. Cell. Biol., 3:1803-1814 (1983)), and in vitro transformation of somatic cells, such as cumulus or mammary cells, followed by nuclear transplantation (Wilmut et al., Nature, 385:810-813 (1997); and Wakayama et al., Nature, 394:369-374 (1998)). For example, fetal fibroblasts can be genetically modified to contain a desired transgene construct, and then fused with enucleated oocytes. After activation of the oocytes, the eggs are cultured to the blastocyst stage. See, for example, Cibelli et al., Science, 280:1256-1258 (1998). Standard breeding techniques can be used to create animals that are homozygous for the transgene from the initial heterozygous founder animals. Homozygosity is not required, however, as the phenotype can be observed in hemizygotic animals.
Once transgenic non-human animals have been generated, expression of an encoded polypeptide can be assessed using standard techniques. Initial screening can be accomplished by Southern blot analysis to determine whether or not integration of the transgene has taken place. For a description of Southern analysis, see sections 9.37-9.52 of Sambrook et al., 1989, Molecular Cloning, A Laboratory Manual, second edition, Cold Spring Harbor Press, Plainview; NY. Polymerase chain reaction (PCR) techniques also can be used in the initial screening. PCR refers to a procedure or technique in which target nucleic acids are amplified. Generally, sequence information from the ends of the region of interest or beyond is employed to design oligonucleotide primers that are identical or similar in sequence to opposite strands of the template to be amplified. PCR can be used to amplify specific sequences from DNA as well as RNA, including sequences from total genomic DNA or total cellular RNA. Primers typically are 14 to 40 nucleotides in length, but can range from 10 nucleotides to hundreds of nucleotides in length. PCR is described in, for example PCR Primer: A Laboratory Manual, ed. Dieffenbach and Dveksler, Cold Spring Harbor Laboratory Press, 1995. Nucleic acids also can be amplified by ligase chain reaction, strand displacement amplification, self-sustained sequence replication, or nucleic acid sequence-based amplified. See, for example, Lewis, Genetic Engineering News, 12:1 (1992); Guatelli et al., Proc. Natl. Acad. Sci. USA, 87:1874-1878 (1990); and Weiss, Science, 254:1292-1293 (1991).
Expression of a nucleic acid sequence encoding a polypeptide of interest in senescent cells of transgenic non-human animals can be assessed using techniques that include, without limitation, Northern blot analysis of tissue samples obtained from the animal, in situ hybridization analysis, Western analysis (immunoblot analysis), immunoassays such as enzyme-linked immunosorbent assays, and reverse-transcriptase PCR (RT-PCR).
It will be understood that the present disclosure also provides tissues (e.g., skin, eye, fat, muscle, lung, heart, bone, liver, intestine, kidney, spleen, brain, cartilage, marrow, adrenal glands, ovaries, and testes) and cells or cell lines (e.g., fat cells, preadipocytes, skin or lung fibroblasts, muscle satellite cells, osteoblasts, bone marrow progenitor cells, neuronal progenitor cells, hepatocytes, endothelial cells, chondroblasts, and splenocytes cells) obtained from a transgenic nonhuman animal provided herein.
Polypeptide sequences and the encoding polynucleotide sequences for proteins, protein domains and fragments thereof, for proteins described herein such as HSV thymidine kinase (TK) polypeptides, FK506 binding protein (FKBP) and domains thereof, caspase(s) and domains thereof, the detectable fluorescent, bioluminescent polypeptides that are described herein include natural and recombinantly engineered variants. These variants retain the function and biological activity (including enzymatic activities if applicable) associated with the parent (or wildtype) protein. Conservative substitutions of amino acids are well known and may occur naturally in the polypeptide (e.g., naturally occurring genetic variants) or may be introduced when the polypeptide is recombinantly produced. Amino acid substitutions, deletions, and additions may be introduced into a polypeptide using well-known and routinely practiced mutagenesis methods (see, e.g., Sambrook et al. Molecular Cloning: A Laboratory Manual, 3d ed., Cold Spring Harbor Laboratory Press, N Y 2001)). Oligonucleotide-directed site-specific (or segment specific) mutagenesis procedures may be employed to provide an altered polynucleotide that has particular codons altered according to the substitution, deletion, or insertion desired. Deletion or truncation variants of proteins may also be constructed by using convenient restriction endonuclease sites adjacent to the desired deletion. Alternatively, random mutagenesis techniques, such as alanine scanning mutagenesis, error prone polymerase chain reaction mutagenesis, and oligonucleotide-directed mutagenesis may be used to prepare polypeptide variants (see, e.g., Sambrook et al., supra).
Differences between a wild type (or parent) polynucleotide or polypeptide and the variant thereof, may be determined by methods routinely practiced in the art to determine identity, which are designed to give the greatest match between the sequences tested. Methods to determine sequence identity can be applied from publicly available computer programs. Computer program methods to determine identity between two sequences include, for example, BLASTP, BLASTN (Altschul, S. F. et al., J. Mol. Biol. 215: 403-410 (1990), and FASTA (Pearson and Lipman Proc. Natl. Acad. Sci. USA 85; 2444-2448 (1988). The BLAST family of programs is publicly available from NCBI and other sources (BLAST Manual, Altschul, S., et al., NCBI NLM NIH Bethesda, Md.
Assays for determining whether a polypeptide variant folds into a conformation comparable to the non-variant polypeptide or fragment include, for example, the ability of the protein to react with mono- or polyclonal antibodies that are specific for native or unfolded epitopes, the retention of ligand-binding functions, the retention of enzymatic activity (if applicable), and the sensitivity or resistance of the mutant protein to digestion with proteases (see Sambrook et al., supra). Polypeptides, variants and fragments thereof, can be prepared without altering a biological activity of the resulting protein molecule (i.e., without altering one or more functional activities in a statistically significant or biologically significant manner). For example, such substitutions are generally made by interchanging an amino acid with another amino acid that is included within the same group, such as the group of polar residues, charged residues, hydrophobic residues, and/or small residues, and the like. The effect of any amino acid substitution may be determined empirically merely by testing the resulting modified protein for the ability to function in a biological assay, or to bind to a cognate ligand or target molecule.
Non-Human Animals Models for Aging, Age-Related Disorders and Age-Sensitive Traits Caused by Senescence-Inducing Stimuli
The present disclosure provides transgenic non-human animal models of aging, which exhibit, for example, age-related phenotypes, age-related disorders, age-sensitive traits, and the like, wherein the animals comprise a transgene that allows for the selective and controllable ablation of senescent cells. The animal models are particularly advantageous for determining the contribution of senescence-inducing stimuli to age-related health parameters and for treating the deleterious effects that result from such stimuli.
A non-human animal model of the disclosure exhibits features of an age-related phenotype, age-related disorder or age-sensitive trait at least in part as a result of contact with a senescence-inducing stimulus. Exemplary features of an age-related phenotype, age-related disorder or age-sensitive trait will be apparent in light of the present disclosure.
In certain specific embodiments, for example, an age-sensitive trait evaluated in the context of the animal models is selected from one or more of T cell subset distribution, cataract formation, spontaneous activity, motor coordination and cognitive capacity, physical function, body composition (e.g., sarcopenia, osteoporosis, loss of fat mass, adipose tissue loss, adipocyte cell size) and cardiac function. Still other illustrative age-sensitive traits can be measured using tissues and organs of test and control mice, including fiber diameter analysis on muscle (e.g., gastrocnemius muscle, abdominal muscle), DNA damage analysis, analysis of renal and glomerulosclerosis, analysis for retinal atrophy, proteotoxic stress analysis, oxidative stress analysis, analysis of the hematopoietic system, and the like.
These exemplary age-sensitive traits can be assessed using standard techniques known and available in the art. Spontaneous activity of individual mice can be measured, for example, over a 48-hour period using comprehensive laboratory animal monitoring systems equipped with photocells (e.g., Columbus Instruments) as previously described (e.g., Handschin et al., J. Biol. Chem., Vol. 282, 41, 30014, Oct. 12, 2007; Pack et al., Physiol. Genomics (Sep. 19, 2006)). Motor coordination can be analyzed, for example, by performing an accelerating rotarod test. For measuring cognitive capacity, a modified Stone T-maze, which is sensitive to age-related changes in learning and memory, can be used. Physical function can be assessed, for example, by measuring running time, distance and work using a motorized treadmill, and grip strength using a grip meter, according to previously described protocols (e.g., Zhang et al., Animal Models of Inflammatory Pain Neuromethods, Volume 49, Oct. 20, 2010, 23-40; Balkaya et al., Behavioral Testing in Mouse Models of Stroke, Neuromethods, Volume 47, 2010, 179-197). Lean mass, fat mass and bone mineral density can be assessed, for example, by QNMR and/or dual-energy X-ray absorptiometry measurements as previously described (e.g., Reed et al., Physiology & Behavior, Vol. 91, 2007, 593-600; Halldorsdottir et al., Int. J. Body Compos. Res., 2009; 7(4), 147-154; Brommage et al., AJP-Endo, Sep. 1, 2003, Vol. 285, No. 3 E454-E459).
These and other methods for detecting, monitoring or quantifying age-related phenotypes associated with senescence-inducing stimuli will be apparent in light of the present disclosure, such as histological studies, molecular studies, biochemical studies, cognitive studies, behavioral assessment, and others. Exemplary methods are also provided in the examples of the present disclosure.
A senescence-inducing stimulus may include, for example, one or more of a chemical stimulus, an environmental stimulus, a genetic modification, a diet modification, or a combination thereof. In certain specific embodiments, the senescence-inducing stimulus comprises irradiation treatment or treatment with one or more chemotherapeutic agents. In certain more specific embodiments, the chemotherapeutic agents are illustratively selected from doxorubicin, taxol, docetaxel, gemcitabine, or cisplatin. In other embodiments, the senescence-inducing stimulus is cigarette smoking or exposure to cigarette smoke or other environmental insults.
An animal comprising a transgene selectively expressed in senescent cells may be exposed to one or more senescence-inducing stimuli (e.g., radiation or chemotherapy) at any desired points in time.
The aging process in an animal model, or the induction of an age-related disorder or age-sensitive trait, can be manipulated by any desired means. For example, a non-human animal model as described herein may be a multi-transgenic model comprising, in addition to a first transgene selectively expressed in senescent cells, a second transgene which expressed a gene product that contributes to a desired age-related phenotype (i.e., a transgenic aging model) or having reduced or abrogated expression of an endogenous gene (i.e., gene knockout aging model), or a combination thereof, which results in or contributes to an age-related phenotype, including an age-related disorder or age-sensitive trait, in the animal. Alternatively, a non-human animal model as described herein may contain one or more naturally occurring mutations that result in or contribute to an age-related phenotype of interest. As such, the effects of senescence-inducing stimuli on the age-related phenotype of the animals can be advantageously determined.
In one embodiment, to create an animal model as described herein, a transgenic parent animal comprising a transgene selectively expressed by senescent cell (for convenience, referred to as a “first transgenic animal”) and a genetically modified model parent animal, e.g., which develops a desired age-related phenotype (for convenience, referred to as a “second genetically modified parent animal”) may be crossed (or bred) with each other. A transgenic animal comprising a transgene selectively expressed by a senescent cell may be any transgenic animal that comprises a senescent cell-specific promoter within a transgene that selectively drives expression of a polypeptide encoded by the transgene in senescent cells. In certain embodiments, a first transgenic parent animal comprises a transgene comprising p16Ink4a promoter operatively linked to a polynucleotide sequence encoding a FKBP-caspase fusion polypeptide (p16-FKBP-caspase transgene), and to a polynucleotide sequence encoding a green fluorescent protein; or a p16Ink4a promoter operatively linked to a polynucleotide sequence encoding a fusion polypeptide comprising a luciferase, a red fluorescent protein, and a truncated herpes simplex virus thymidine kinase (p16-3MR transgene). In other embodiments, a parent animal comprises a transgene comprising a p16Ink4a promoter in frame to express an at least one FKBP domain and at least one caspase domain (e.g., FKBP-caspase fusion polypeptide described herein) and comprises the p16-3MR transgene described herein. In still another embodiment, a parent animal comprises a transgene comprising a p16Ink4a promoter in frame to express a at least one FKBP domain and at least one caspase domain (e.g., FKBP-caspase fusion polypeptide described herein) or the transgene comprises the p16-3MR transgene described herein, and which parent animal has a BubR1 hypomorphic (BubR1H/H) genetic background (see, e.g., Baker et al., Nature 479:232-236 (2011); International Application Publication No. WO 2012/177927). The second genetically modified parent animal may be an animal that develops an age-related phenotype (e.g., transgenic or knockout models that develop an age-related phenotype). F1 progeny from a cross between these parental animals are multi-transgenic animals which express the transgenes from each parent. Additional crosses for selection of progeny with heterozygous or homozygous knockout genes may be necessary.
In an alternative embodiment, a multi-transgenic model may be derived by directly introducing a transgene into the germline of another genetically modified animal. For example, a transgene selectively expressed by senescent cells (e.g., p16-3MR transgene) may be injected into single cell embryos harvested from a genetically modified transgenic mouse model. A person having skill in the art will understand that various injection/recipient embryo combinations may be employed to create a multi-transgenic animal model. Co-injection of different transgenes for generating multi-transgenic mice can be accomplished by these and other methods routinely practiced in the art (see, e.g., Oddo et al., 2003, Neuron 39:409-421; U.S. Pat. No. 7,479,579).
Non-human animal models with senescent cell specific transgene expression created by cross-breeding, transgene injection, or other methods may be confirmed for genotype or transgene expression. Resulting offspring may be genotyped by Southern blot analysis or PCR techniques on DNA extracted from tissue samples (e.g., tail tips or ear punches) using transgene specific probes or primers, respectively. The level of mRNA expression of the transgenes in tissues of transgenic animals may also be assessed using techniques including Northern blot analysis, in situ hybridization, RT-PCR, or real-time PCR. Transgenic proteins may also be detected in tissue samples from transgenic animals using antibodies specific for a polypeptide expressed by the transgene and/or antibodies that are specific for a detectable label that is co-expressed by the transgene, for example.
In another embodiment, for producing the animal model described herein, a knock out or knock-down animal (e.g., which develops an age-related phenotype of interest) may be crossed with a transgenic animal that expresses a senescent cell specific transgene. Gene knock-outs allow assessment of in vivo function of a gene which has been altered and used to replace a normal copy. Knock-out modifications include insertion of mutant stop codons, deletion of DNA sequences, or inclusion of recombination elements (lox p sites) recognized by enzymes such as Cre recombinase. Cre-lox system allows for the ablation of a given gene or the ablation of a certain portion of gene sequence. To create a transgenic animal, an altered version of a gene of interest (e.g., a human age-related gene or a gene encoding a senescent cell associated polypeptide) can be inserted into an animal germ line using standard techniques of oocyte microinjection or transfection, or microinjection into stem cells. For oocyte injection, one or more copies of the altered/mutated gene of interest (e.g., human age-related gene) can be inserted into the pronucleus of a just-fertilized oocyte. The oocyte is then re-implanted into a pseudo-pregnant foster mother. The liveborn progeny can be screened for transgene integrants by analyzing the DNA from tissue samples. Retroviral infection of early embryos may also be performed to insert the altered gene. Embryos are infected during early stages of development to generate a chimera, some of which will lead to germline transmission. Alternatively, if it is desired to inactivate or replace the endogenous gene, mutant alleles may be introduced by homologous recombination into embryonic stem cells. Embryonic stem cells containing a knock out mutation in one allele of the gene being studied are introduced into early embryos. The resultant progeny are chimeras containing tissues derived from both the transplanted ES cells and host cells. Chimeric animals may be mated to assess whether the mutation is incorporated into the germ line. Chimeric animals that are each heterozygous for the knock-out mutation are mated to produce homozygous knock out animals. Mutations in the mouse germline may also be created by injecting oligonucleotides containing the mutation of interest. Gene knock down uses RNAi technology to repress endogenous gene expression in vivo or in vitro. Lentiviral vectors expressing siRNAs or shRNAs may be used to transduce preimplantation mouse embryos for silencing of their specific target genes (see e.g., Tiscornia et al., 2003, Proc. Natl. Acad. Sci. USA 100:1844-1848; Singer et al., Nature Protocols 1:286-292; Szulc et al., 2006, Nature Methods 3:109-116).
Transgenic non-human animal models may comprise at least a second transgene associated with an age-related phenotype or disorder, in addition to the first transgene selectively expressed by senescent cells. The expression of the second transgene will generally alter one or more age-related phenotype or age-sensitive trait in the animal.
Gene knock out models for aging are known in the art and may comprise heterozygous or homozygous mutation of a gene associated with aging to reduce or abrogate expression of the gene. The reduction or abrogation of the gene associated with aging will generally result in or contributes to one or more age-related phenotype or disorder, or will modulate one or more age-sensitive traits.
Animal models of aging (including transgenic models, knockout models, and models having naturally occurring gene modifications) have been reviewed (e.g., Exp Mol Pathol. 2002 February; 72(1):49-55; ILAR J. 2011 Feb. 8; 52(1):4-15; and elsewhere, each of the review articles as well as the references cited therein is incorporated herein by reference in its entirety).
Additionally, many transgenic/knockout mouse lines and information about mouse models of aging are available from the Jackson Laboratory (Bar Harbor, Me.).
In certain embodiments, the present disclosure provides for an animal model comprising a p16Ink4a promoter, or an operable truncation thereof, operatively linked to a polynucleotide sequence encoding a FKBP-caspase fusion polypeptide (p16-FKBP-caspase transgene), and to a polynucleotide sequence encoding a green fluorescent protein; or a p16Ink4a promoter operatively linked to a polynucleotide sequence encoding a fusion polypeptide comprising a luciferase, a red fluorescent protein, and a truncated herpes simplex virus thymidine kinase (p16-3MR transgene); wherein the model further comprises a second transgene associated with aging (e.g., one that gives rise to or contributes to an age-related phenotype, age-related disorder and/or age-sensitive trait).
In yet another embodiment, the present disclosure provides for a method of producing a non-human animal model for aging comprising a non-human animal that exhibits an age-related phenotype and comprises a transgene selectively expressed by senescent cells, wherein the method comprises (a) providing an animal comprising the transgene selectively expressed by senescent cells; and (b) breeding the animal of step (a) with an animal comprising a genetic modification associated with an age-related phenotype to produce a multi-transgenic animal. An animal comprising a genetic modification associated with an age-related phenotype may be a knockout, knockdown, or transgenic animal. In a specific embodiment, the method may further comprise administering to, or exposing the animal to, for example, a chemical stimulus, whole body irradiation, an environmental stimulus, a diet modification, or another stimulus, to produce or facilitate an age-related phenotype.
Non-human animal models for aging with senescent cell specific transgene expression, as described herein, are useful in tracking or monitoring senescence cells. For example, animal models comprising transgenes expressing detectable labels may be used in imaging senescent cells, determining ratio of senescent cells in a tissue, and/or monitoring the elimination of senescent cells. In addition, such models may also be used in characterizing drug candidates for treating or preventing development of an age-related disorder or age-sensitive trait, such as the tissue specificity of such candidates. For example, these models expressing detectable labels under the control of senescent cell-specific promoters may be used to determine the tissue type in which a drug candidate suppresses cellular senescence using the detectable labels. Animal models comprising transgenes expressing cytotoxicity-activating molecules allow for titrating the elimination of senescent cells by modulating concentrations of activating agents. In yet another embodiment, the present disclosure provides for a method of producing a non-human animal model for aging comprising a non-human animal that exhibits an age-related phenotype and comprises a transgene selectively expressed by senescent cells, wherein the method comprises (a) providing an animal comprising the transgene selectively expressed by senescent cells; and (b) administering a senescence-inducing stimulus.
Non-human animal models for aging with senescent cell specific transgene expression, as described herein, may be used to evaluate effects of senescent cell ablation on an age-related phenotype. By eliminating senescent cells or effects of senescent cell associated molecules from the animal at various times in an animal model for aging, the role of senescent cells in vivo on an age-related phenotype, particularly an age-related phenotype associated at least in part with senescence-inducing stimuli, may be tested. In certain embodiments, senescent cell ablation may be accomplished by administration of FK1012 analog AP20187, which induces dimerization of membrane-bound myristolylated FKBP-caspase fusion protein expressed in senescent cells via the p16Ink4a promoter, resulting in activation of apoptosis. Senescent cell ablation may also be performed by candidate therapeutic agents. In certain other embodiments, senescent cell ablation may be accomplished by administration of the pro-drug ganciclovir (GCV), which is converted to a cytotoxic moiety by a truncated herpes simplex virus thymidine kinase expressed in senescent cells via the p16Ink4a promoter.
In certain embodiments, also provided herein are isolated cells and/or cell lines derived from the non-human animal models with senescent cell-specific transgene expression, as described herein. Primary cell cultures derived from the non-human animal models as described herein may be used. In certain embodiments, continuous cell lines are generated from the non-human animal models. Methods for deriving a continuous cell line from transgenic animals are known in the art (see, e.g., Small et al., 1985, Mol. Cell. Biol. 5:642-648; Morgan et al., 1994, Dev. Biol. 162:486-98; U.S. Pat. No. 5,814,716; U.S. Pat. No. 6,583,333; U.S. Pat. No. 6,825,394). Isolated cells or cell lines may be derived from any organ, tissue, or bodily fluid from the animal model, including, but not limited to the brain, cerebrospinal fluid, or spinal cord. The cells and cell lines may be cultured under conditions and in media appropriate to maintain the health and propagation of the cells, as desired, using techniques and procedures routinely practiced in the cell culture art. These isolated cells or cell lines may be used to identify and characterize therapeutic agents that suppress cellular senescence and that are useful for treating or preventing age-related phenotypes associated with senescence-inducing stimuli.
Methods for Identifying Therapeutic Agents
The non-human animal models and cell lines derived therefrom as described herein may be used to identify therapeutic agents effective for treating or preventing age-related disorders, and, more particularly, for treating or preventing the deleterious age-related health effects caused by various senescence-inducing stimuli, such as radiation or chemotherapy treatment. Such animal models and cell lines are particularly useful for identifying therapeutic agents effective for treating or preventing age-related disorders via suppressing cellular senescence. Therapeutic agents include small molecules, antibodies, antigen-binding fragments, polypeptides, peptides, peptibodies, hormones, and nucleic acids.
In one embodiment, the present disclosure provides a method for identifying a therapeutic agent effective for treating or preventing the development of an age-related disorder or age-sensitive trait, comprising: (a) administering a candidate therapeutic agent to the animal of the animal model provided herein to provide a treated animal; and (b) determining the level of suppression of cellular senescence in the treated animal and comparing to the level of cellular senescence in the untreated control animal; wherein suppression of cellular senescence in the treated animal compared with the untreated animal identifies an agent effective for treating or preventing the age-related disorder or age-sensitive trait.
In a related embodiment, the present disclosure provides a method for identifying a therapeutic agent effective for treating or preventing the development of an age-related disorder or age-sensitive trait, comprising: (a) administering a candidate therapeutic agent to the animal of the animal model provided herein to provide a treated animal; and (b) (1) determining the phenotype of the development of an age-related disorder or age-sensitive trait exhibited in the treated animal, and comparing to the phenotype of the development of an age-related disorder or age-sensitive trait exhibited in an untreated control model animal, and (2) determining the level of suppression of cellular senescence in the treated animal and comparing to the level of cellular senescence in the untreated control animal; wherein (1) suppression of one or more age-related disorder or age-sensitive traits and (2) suppression of cellular senescence in the treated animal compared with the untreated animal identifies an agent effective for treating or preventing the development of an age-related disorder or age-sensitive trait.
Candidate therapeutic agents include any agents that are potentially capable of treating or preventing one or more age-related disorder or age-sensitive trait, such as small molecules, antibodies, polypeptides, peptides, hormones, and nucleic acids. Potential therapeutic agents may be identified from “libraries” or collections of compounds, compositions, or molecules. A source of small molecules, peptides, and oligonucleotides includes combinatorial libraries that may be screened to identify a therapeutic agent useful for treating of preventing an age-related disorder or age-sensitive trait. Other exemplary libraries comprise peptides or polypeptides that represent a complementarity determining region (CDR) of an antibody.
Candidate therapeutic agents may be administered to the animal of the animal model provided herein before, concurrently or after onset of an age-related disorder or age-sensitive trait. The onset of an age-related disorder or age-sensitive trait may be triggered by inducing the expression of an inducible transgene that is associated with an age-related disorder or age-sensitive trait, such as by chemical or mechanical induction, by radiation, or by diet manipulation.
A person skilled in the art will also readily appreciate that when performing analyses in animal models, appropriate controls are included. By way of non-limiting example, a group of animals will receive a candidate therapeutic agent (or a composition or a vehicle comprising the agent), which animals may be referred to as treated animals. Another group of animals will receive vehicle only or another appropriate control composition that does not include an agent effective in treating or preventing an age-related disorder or age-sensitive trait. In certain embodiments, a group of animals is treated with a agent known as effective in treating or preventing an age-related disorder or age-sensitive trait as a positive control. The phenotype (or one or more phenotypic markers or characteristics) of the treated animal is then compared with the phenotype of the control animals that do not receive the candidate agent.
For these embodiments, a group of animals may receive a therapeutic agent known to be effective in treating or preventing an age-related disorder or age-sensitive trait by suppressing cellular senescence to be used as a positive control group. Additional positive controls include the animal models provided herein that comprise a transgene that includes a senescent cell specific promoter that is operatively linked to a polypeptide (which may be a fusion polypeptide) comprising a cytotoxicity-activating molecule (e.g., HSV truncated TK, FKBP-caspase polypeptide). Accordingly, when cells are induced to senescence by normal aging or by any one of the molecules, methods, or genetic modifications described herein, the fusion polypeptide expresses the cytotoxicity-activating molecule. By way of example, a positive control animal model group that expresses the FKBP-caspase polypeptide (e.g., an INK-ATTAC animal (see Baker et al., Nature 479:232-36 (2011)) may be treated with AP20187 and related analogs, for example, which results in senescent cell destruction. By way of additional example, a transgenic animal (e.g., a P16-3MR animal) that expresses a fusion polypeptide comprising HSV truncated TK can be administered a prodrug, such as ganciclovir, that is activated when the truncated thymidine kinase is expressed in senescent cells, resulting in destruction of the senescent cell.
Suppressing cellular senescence may comprise one or both of (1) selectively destroying or facilitating selective destruction of a senescent cell; and (2) inhibiting expression or secretion of one or more senescence cell-associated molecules including senescence-cell associated polypeptides (e.g., cytokines, chemokines, growth factors) by the senescent cell.
Determining the effectiveness of a therapeutic agent or a candidate therapeutic agent to inhibit induction or progression of an age-related disorder or age-sensitive trait or to suppress senescence as described herein in an animal model is typically performed using one or more statistical analyses with which a skilled person will be familiar. By way of example, statistical analyses such as two-way analysis of variance (ANOVA) may be used for determining the statistical significance of differences between animal groups treated with an agent and those that are not treated with the agent (i.e., negative control group). Statistical packages such as SPSS, MINITAB, SAS, Statistika, Graphpad, GLIM, Genstat, and BMDP are readily available and routinely used by a person skilled in the animal art.
Cellular senescence is a stable and essentially permanent arrest of cell proliferation, which is accompanied by extensive changes in gene expression. Many types of cells, both normal cells and tumor cells, undergo senescence in response to stress. As described in the art, the phenotype of a senescence cell, such as the phenotype referred to as senescence associated secretory phenotype (SASP), is typified by secretion of numerous cytokines (e.g., inflammatory cytokines), growth factors, extracellular matrix components (ECM) and ECM-degrading enzymes, and proteases, for example. While proliferative arrest poses a formidable barrier to tumor progression (see, e.g., Campisi, Curr. Opin. Genet. Dev. 21:107-12 (2011); Campisi, Trends Cell Biol. 11:S27-31 (2001); Prieur et al., Curr. Opin. Cell Biol. 20:150-55 (2008)), and molecules secreted by senescent cells can stimulate tissue repair (see, e.g., Adams, Molec. Cell 36:2-14 (2009); Rodier et al., J. Cell Biol. 192:547-56 (2011)), senescent cells also secrete molecules that can cause inflammation (see, e.g., Freund et al., Trends Mol. Med. 16:238-46 (2010); Davalos et al., Cancer Metastasis Rev. 29:273-83 (2010)). Low-level, chronic inflammation is a hallmark of aging tissues, and inflammation is a major cause of, or contributor to, virtually every major age-related pathology, including cancer (Ferrucci et al., 2004, Aging Clin. Exp. Res. 16:240-243; Franceschi et al., 2007, Mech. Ageing Dev. 128:192-105; Chung et al., 2009, Ageing Res. Rev. 8:18-30; Davalos et al., 2010, Cancer Metastasis Rev. 29:273-283; Freund et al., 2010, Trends Molec. Med. 16:238-248). Thus, senescent cells, which increase with age and at sites of age-related pathology, might stimulate local chronic inflammation and tissue remodeling, thereby fueling both the degenerative diseases of aging as well as age-related cancer.
A senescent cell may exhibit any one or more of the following characteristics. (1) Senescence growth arrest is essentially permanent and cannot be reversed by known physiological stimuli. (2) Senescent cells increase in size, sometimes enlarging more than twofold relative to the size of non-senescent counterparts. (3) Senescent cells express a senescence-associated β-galactosidase (SA-β-gal), which partly reflects the increase in lysosomal mass. (4) As described herein, most senescent cells express p16INK4a, which is not commonly expressed by quiescent or terminally differentiated cells. (5) Cells that senesce with persistent DNA damage response signaling harbor persistent nuclear foci, termed DNA segments with chromatin alterations reinforcing senescence (DNA-SCARS). These foci contain activated DDR proteins and are distinguishable from transient damage foci. DNA-SCARS include dysfunctional telomeres or telomere dysfunction-induced foci (TIF). (6) Senescent cells express and may secrete molecules called herein senescent cell-associated molecules, which in certain instances may be dependent on persistent DDR signaling for their expression. (7) The nuclei of senescent cells lose structural proteins such as Lamin B1 or chromatin-associated proteins such as histones and HMGB1. See, e.g., Freund et al., Mol. Biol. Cell 23:2066-75 (2012); Davalos et al., J. Cell Biol. 201:613-29 (2013); Ivanov et al., J. Cell Biol. DOI: 10.1083/jcb.201212110, page 1-15; published online Jul. 1, 2013; Funayama et al., J. Cell Biol. 175:869-80 (2006)).
Senescent cell-associated molecules include growth factors, proteases, cytokines (e.g., inflammatory cytokines), chemokines, cell-related metabolites, reactive oxygen species (e.g., H2O2), and other molecules that stimulate inflammation and/or other biological effects or reactions that may promote or exacerbate the underlying disease of the subject. Senescent cell-associated molecules include those that are described in the art as comprising the senescence-associated secretory phenotype (SASP), senescent-messaging secretome, and DNA damage secretory program (DDSP). These groupings of senescent cell associated molecules, as described in the art, contain molecules in common and are not intended to describe three separate distinct groupings of molecules. Senescent cell-associated molecules include certain expressed and secreted growth factors, proteases, cytokines, and other factors that may have potent autocrine and paracrine activities. Without wishing to be bound by theory, the negative effects of senescent cells are believed to be the result of, at least in part, the secretion of pro-inflammatory cytokines, chemokines, growth factors, and proteases that comprise the SASP of a senescent cell (see, e.g., Coppe et al., PLoS Biol. 6:2853-68 (2008)). Senescent cell-associated molecules that comprise the SASP can disrupt normal tissue structure and function and stimulate malignant phenotypes in pre-malignant or non-aggressive cancer cells (see, e.g., Coppe et al., supra; Coppe et al. J. Biol. Chem. 281:29568-74 (2006); Coppe et al. PLoS One 5:39188 (2010); Krtolica et al. Proc. Natl. Acad. Sci. U.S.A. 98:12072-77 (2001); Parrinello et al., J. Cell Sci. 118:485-96 (2005). ECM associated factors include inflammatory proteins and mediators of ECM remodeling and which are strongly induced in senescent cells (see, e.g., Kuilman et al., Nature Reviews 9:81-94 (2009)). Other senescent cell-associated molecules include extracellular polypeptides (proteins) described collectively as the DNA damage secretory program (DDSP) (see, e.g., Sun et al., Nature Medicine published online 5 Aug. 2012; doi:10.1038/nm.2890). Senescent cell-associated proteins also include cell surface proteins (or receptors) that are expressed on senescent cells, which include proteins that are present at a detectably lower amount or are not present on the cell surface of a non-senescent cell.
Senescence cell-associated molecules include secreted factors which may make up the pro-inflammatory phenotype of a senescent cell (e.g., SASP). These factors include, without limitation, GM-CSF, GROα, GROα,β,γ, IGFBP-7, IL-1α, IL-6, IL-7, IL-8, MCP-1, MCP-2, MIP-1α, MMP-1, MMP-10, MMP-3, Amphiregulin, ENA-78, Eotaxin-3, GCP-2, GITR, HGF, ICAM-1, IGFBP-2, IGFBP-4, IGFBP-5, IGFBP-6, IL-13, IL-1β, MCP-4, MIF, MIP-3α, MMP-12, MMP-13, MMP-14, NAP2, Oncostatin M, osteoprotegerin, PIGF, RANTES, sgp130, TIMP-2, TRAIL-R3, Acrp30, angiogenin, Ax1, bFGF, BLC, BTC, CTACK, EGF-R, Fas, FGF-7, G-CSF, GDNF, HCC-4, I-309, IFN-γ, IGFBP-1, IGFBP-3, IL-1 R1, IL-11, IL-15, IL-2R-α, IL-6 R, I-TAC, Leptin, LIF, MMP-2, MSP-a, PAI-1, PAI-2, PDGF-BB, SCF, SDF-1, sTNF RI, sTNF RII, Thrombopoietin, TIMP-1, tPA, uPA, uPAR, VEGF, MCP-3, IGF-1, TGF-β3, MIP-1-delta, IL-4, FGF-7, PDGF-BB, IL-16, BMP-4, MDC, MCP-4, IL-10, TIMP-1, Fit-3 Ligand, ICAM-1, Ax1, CNTF, INF-γ, EGF, BMP-6. Cell-associated molecules also include those sometimes referred to in the art as senescence messaging secretome (SMS) factors, some of which are included in the listing of SASP polypeptides, include without limitation, IGF1, IGF2, and IGF2R, IGFBP3, IDFBP5, IGFBP7, PA11, TGF-β, WNT2, IL-1α, IL-6, IL-8, and CXCR2-binding chemokines. Factors, including those referred to in the art, include without limitation the factors described in Sun et al., Nature Medicine, supra, including, for example, products of the genes, MMP1, WNT16B, SFRP2, MMP12, SPINK1, MMP10, ENPP5, EREG, BMP6, ANGPTL4, CSGALNACT, CCL26, AREG, ANGPT1, CCK, THBD, CXCL14, NOV, GAL, NPPC, FAM150B, CST1, GDNF, MUCL1, NPTX2, TMEM155, EDN1, PSG9, ADAMTS3, CD24, PPBP, CXCL3, MMP3, CST2, PSG8, PCOLCE2, PSG7, TNFSF15, C17orf67, CALCA, FGF18, IL8, BMP2, MATN3, TFP1, SERPINI 1, TNFRSF25, and IL23A. Senescent cell-associated proteins also include cell surface proteins (or receptors) that are expressed on senescent cells, which include proteins that are present at a detectably lower amount or are not present on the cell surface of a non-senescent cell.
A therapeutic agent of interest includes an agent that selectively destroys or facilitates selective destruction of a senescent cell and/or in some manner is effective for inhibiting expression or secretion of a senescence cell-associated molecule, including a senescence cell-associated protein, or a protein that is present on the cell surface of a senescent cell. Therapeutic agents of interest also include agents that inhibit transcription or translation of a senescence cell-associated polypeptide (protein), or a protein that is present on the cell surface of a senescent cell. Such agents are useful for treating or preventing an age-related disorder or age-sensitive trait.
A therapeutic agent that “selectively” destroys or facilitates “selective” destruction of a senescent cell is an agent that preferentially (or to a greater degree) destroys or facilitates destruction or facilitates clearance of a senescent cell. In other words, the therapeutic agent destroys or facilitates destruction of a senescent cell in a biologically, clinically, and/or statistically significant manner compared with its capability to destroy or facilitate destruction of a non-senescent cell. By way of non-limiting example, the therapeutic agent may directly or indirectly kill a senescent cell by disrupting the integrity of the cell membrane; inhibiting one or more metabolic processes in the cell; enhancing or stimulating a signaling pathway that leads to apoptosis or necrosis of the senescent cell; disrupt transcription or translation of genes or proteins, respectively, necessary for cell survival; and/or binding to the senescent cell to facilitate clearance or removal of the cell, for example, clearance by immune cells. As described herein, the presence of senescent cells in the transgenic animals comprising a senescent cell specific promoter can be monitored and determined by expression or presence (or lack of expression or presence) of one or more detectable labels (e.g., a luciferase or fluorescent polypeptide) that is operatively linked to the promoter.
In particular embodiments, the level of transcription, expression, or secretion can be determined for one or more senescence cell-associated polypeptides. An effective therapeutic agent that suppresses cellular senescence reduces or inhibits expression, secretion, or production of a senescence cell-associated polypeptide in a statistically significant or biologically significant manner compared to the appropriate controls. Proteins that comprise senescence cell-associated molecules and methods for evaluating expression and secretion of SASP proteins are described in the art (see, e.g., Freund et al., Trends Mol. Med. 16:283-46 (2010) and references cited therein; Sun et al., Nature Med., published online 5 Aug. 2012; doi: 10.1038/nm.2890). Senescent cells may also be detected by determining the presence and level of senescence-associated-β-galactosidase (SA-β-Gal). A decrease or reduction in the level of expression or secretion of one or more senescence cell-associated molecules (including senescence cell-associated polypeptides), SA-β-Gal, or reduction in the quantity of senescent cells and a suppression of one or more phenotypic markers of an age-related disorder or age-sensitive trait in the test animal compared with the control animal identifies a therapeutic agent.
Senescent cells and senescent cell associated molecules can be detected by techniques and procedures described in the art. For example, the presence of senescent cells in tissues can be analyzed by histochemistry or immunohistochemistry techniques that detect the senescence marker, SA-beta gal (SA-Bgal) (see, e.g., Dimri et al., Proc. Natl. Acad. Sci. USA 92: 9363-9367 (1995). The presence of the senescent cell-associated polypeptide p16 can be determined by any one of numerous immunochemistry methods practiced in the art, such as immunoblotting analysis. Expression of p16 mRNA in a cell can be measured by a variety of techniques practiced in the art including quantitative PCR. The presence and level of senescence cell associated polypeptides (e.g., polypeptides of the SASP) can be determined by using automated and high throughput assays, such as an automated Luminex array assay described in the art (see, e.g., Coppe et al., PLoS Biol 6: 2853-68 (2008)). For monitoring a DNA damage response, the various DNA damage response indicators can be detected according to the method of Rodier et al., Nature Cell Biol 11: 973-979 (2009)).
Therapeutic agents of interest include those that are activated or that are pro-drugs which are converted to the active form by enzymes that are expressed at a higher level in senescent cells than in non-senescent cells. Other therapeutic agents of interest include those that bind to proteins on the cell surface of a cell that are present exclusively or at a greater level on senescent cells compared with non-senescent cells (see, e.g., International Patent Application Publication No. WO 2009/085216). In certain embodiments, a therapeutic agent that specifically binds to a senescent cell has at least 2, 4, 8, 10, 50, 100, or 1000 fold greater affinity for a senescent cell than for a non-senescent cell, or in certain embodiments, the therapeutic agent does not detectably bind to a non-senescent cell. A protein present at a greater level on a senescent cell than on a non-senescent cell may be a protein that is typically an intracellular protein and not detectable on the cell surface of a non-senescent cell. Other therapeutic agents of interest that suppress cellular senescence include those that are activated by a metabolic process that occurs more frequently or at a higher rate in senescent cells than in a non-senescent cell.
In one embodiment, therapeutic agents useful for treating or preventing age-related disorder or an age-sensitive trait are small organic molecules that suppress cellular senescence. A small molecule compound of interest may be further derivatized, either randomly or by SAR, to obtain compounds with improved anti-cellular senescence activity and more effective agents for treating an age-related disorder or age-sensitive trait. Small organic molecules typically have molecular weights less than 105 daltons, less than 104 daltons, or less than 103 daltons.
A therapeutic agent includes an antibody, or antigen-binding fragment thereof, that specifically binds to a cognate antigen that is overly expressed, selectively expressed, or only expressed by senescent cell compared with a non-senescent, normal cell. The antibody may be an antibody that is internalized by the senescent cell via interaction with its cognate antigen. These specific antibodies may be polyclonal or monoclonal, prepared by immunization of animals and subsequent isolation of the antibody, or cloned from specific B cells according to methods and techniques routinely practiced in the art and described herein. A variable region or one or more complementarity determining regions (CDRs) may be identified and isolated from antigen-binding fragment or peptide libraries. An antibody, or antigen-binding fragment thereof, may be recombinantly engineered and/or recombinantly produced.
An antibody may belong to any immunoglobulin class, for example IgG, IgE, IgM, IgD, or IgA and may be obtained from or derived from an animal, for example, fowl (e.g., chicken) and mammals, which include but are not limited to a mouse, rat, hamster, rabbit, or other rodent, a cow, horse, sheep, goat, camel, human, or other primate. The antibody may be an internalising antibody. For use in human subjects, antibodies and antigen-binding fragments are typically human, humanized, or chimeric to reduce an immunogenic response by the subject to non-human peptides and polypeptide sequences.
Binding properties of an antibody to its cognate antigen may generally be determined and assessed using immunodetection methods including, for example, an enzyme-linked immunosorbent assay (ELISA), immunoprecipitation, immunoblotting, countercurrent immunoelectrophoresis, radioimmunoassays, dot blot assays, inhibition or competition assays, and the like, which may be readily performed by those having ordinary skill in the art (see, e.g., Harlow et al., Antibodies: A Laboratory Manual, Cold Spring Harbor Laboratory (1988)). As used herein, an antibody is said to be “immunospecific,” “specific for” or to “specifically bind” to a cognate antigen if it reacts at a detectable level with the antigen or immunogen. Affinities of antibodies and antigen binding fragments thereof can be readily determined using conventional techniques, for example, those described by Scatchard et al. (Ann. N.Y. Acad. Sci. USA 51:660 (1949)) and by surface plasmon resonance (SPR; BIAcoreTM, Biosensor, Piscataway, N.J.).
The antibody may be a monoclonal antibody that is a human antibody, humanized antibody, chimeric antibody, bispecific antibody, or an antigen-binding fragment (e.g., F(ab′)2, Fab, Fab′, Fv, and Fd) prepared or derived therefrom. An antigen-binding fragment may also be any synthetic or genetically engineered protein that acts like an antibody in that it binds to a specific antigen to form a complex. For example, antibody fragments include isolated fragments consisting of the light chain variable region, Fv fragments consisting of the variable regions of the heavy and light chains, recombinant single chain polypeptide molecules (scFv proteins), and minimal recognition units consisting of the amino acid residues that mimic the hypervariable region. In certain other embodiments, antibodies are multimeric antibody fragments such as miniantibodies, bispecific and bifunctional antibodies comprising a first Fv specific for an antigen associated with a second Fv having a different antigen specificity, and diabodies and the like. Useful methodologies are described generally, for example in Hayden et al., Curr Opin. Immunol. 9:201-12 (1997) and Coloma et al., Nat. Biotechnol. 15:159-63 (1997); U.S. Pat. No. 5,910,573); (Holliger et al., Cancer Immunol. Immunother. 45:128-30 (1997); Drakeman et al., Expert Opin. Investig. Drugs 6:1169-78 (1997); Koelemij et al., J. Immunother. 22:514-24 (1999); Marvin et al., Acta Pharmacol. Sin. 26:649-58 (2005); Das et al., Methods Mol. Med. 109:329-46 (2005)).
A minimal recognition unit or other antigen binding fragment may be identified from a peptide library. Such peptides may be identified and isolated from combinatorial libraries (see, e.g., International Patent Application Nos. PCT/US91/08694 and PCT/US91/04666) and from phage display peptide libraries (see, e.g., Scott et al., Science 249:386 (1990); Devlin et al., Science 249:404 (1990); Cwirla et al., Science 276: 1696-99 (1997); U.S. Pat. Nos. 5,223,409; 5,733,731; 5,498,530; 5,432,018; 5,338,665; 1994; 5,922,545; International Application Publication Nos. WO 96/40987 and WO 98/15833). A peptide that is a minimal recognition unit or a CDR (i.e., any one or more of three CDRs present in a heavy chain variable region and/or one or more of three CDRs present in a light chain variable region) may be identified by computer modeling techniques, which can be used for comparing and predicting a peptide sequence that will specifically bind to a polypeptide of interest as described herein (see, e.g., Bradley et al., Science 309:1868 (2005); Schueler-Furman et al., Science 310:638 (2005)).
Antibodies may generally be prepared by any of a variety of techniques known to persons having ordinary skill in the art. Immunogens used to immunize animals and/or to screen for antibodies of desired specificity include proteins isolated from senescent cells that, for example, are present on the cell surface of a senescent cell in greater quantity or having a different conformation than on a non-senescent cell; and senescent cell extracts, including outer membrane preparations, organelles isolated from senescent cells, and the like. Antibodies may also be identified and isolated from human immunoglobulin phage libraries, from rabbit immunoglobulin phage libraries, from mouse immunoglobulin phage libraries, and/or from chicken immunoglobulin phage libraries (see, e.g., Winter et al., Annu. Rev. Immunol. 12:433-55 (1994); Burton et al., Adv. Immunol. 57:191-280 (1994); U.S. Pat. No. 5,223,409; Huse et al., Science 246:1275-81 (1989); Schlebusch et al., Hybridoma 16:47-52 (1997) and references cited therein; Rader et al., J. Biol. Chem. 275:13668-76 (2000); Popkov et al., J. Mol. Biol. 325:325-35 (2003); Andris-Widhopf et al., J. Immunol. Methods 242:159-31(2000)). Antibodies isolated from non-human species or non-human immunoglobulin libraries may be genetically engineered according to methods described herein and known in the art to “humanize” the antibody or fragment thereof.
Useful strategies for designing humanized antibodies may include, for example by way of illustration and not limitation, identification of human variable framework regions that are most homologous to the non-human framework regions of a chimeric antibody (see, e.g., Jones et al., Nature 321:522-25 (1986); Riechmann et al., Nature 332:323-27 (1988)). A humanized antibody may be designed to include CDR loop conformations and structural determinants of non-human variable regions, for example, by computer modeling, and then comparing the CDR loops and determinants to known human CDR loop structures and determinants (see, e.g., Padlan et al., FASEB 9:133-39 (1995); Chothia et al., Nature, 342:377-83 (1989)). Computer modeling may also be used to compare human structural templates selected by sequence homology with the non-human variable regions.
A therapeutic agent also includes a peptide-immunoglobulin (Ig) constant region fusion polypeptide, which includes a peptide-IgFc fusion polypeptide. The peptide may be any naturally occurring or recombinantly prepared molecule. A peptide-Ig constant region fusion polypeptide, such as a peptide-IgFc fusion polypeptide (also referred to in the art as a peptibody (see, e.g., U.S. Pat. No. 6,660,843)).
Therapeutic agents such as polypeptides, peptides, peptibodies, antibodies, and antigen binding fragments (i.e., peptides or polypeptides comprising at least one antibody V region) or other agents that specifically to a senescent cell can be linked to (i.e., conjugated to, fused to, or in some manner joined to or attached to) a second agent that selectively destroys or facilitates selective destruction of senescent cells. When delivered to the senescent cell by binding of the agent to the senescent cell, the cytotoxic moiety selectively destroys the senescent cell. If the agent is recombinantly produced, a nucleotide sequence encoding the cytotoxic moiety may be linked in frame to the agent and to one or more regulatory expression sequences to produce a fusion protein comprising the agent and cytotoxic moiety. Such second agents include cytotoxic molecules, including toxins derived from plants and microorganisms, as well as small molecules that do not selectively bind to senescent cells in the absence of being linked to the aforementioned antibody, polypeptide, or peptide.
In certain embodiments, a therapeutic agent is a polynucleotide or oligonucleotide that specifically hybridize to a portion of the genome or mRNA of a cell that is a senescent cell or that is in a disease microenvironment and may be induced to senescence by a cell damaging (i.e., biologically damaging) medical therapy. Polynucleotides and oligonucleotides are provided that are complementary to at least a portion of a nucleotide sequence encoding a senescent cellular polypeptide of interest (e.g., a short interfering nucleic acid, an antisense polynucleotide, a ribozyme, or a peptide nucleic acid) and that may be used to alter gene and/or protein expression. As described herein, these polynucleotides that specifically bind to or hybridize to nucleic acid molecules that encode a cellular polypeptide may be prepared using the nucleotide sequences available in the art. In another embodiment, nucleic acid molecules such as aptamers that are not sequence-specific may also be used to alter gene and/or protein expression.
Antisense polynucleotides bind in a sequence-specific manner to nucleic acids such as mRNA or DNA. Identification of oligonucleotides and ribozymes for use as antisense agents and identification of DNA encoding the genes for targeted delivery involve methods well known in the art. For example, the desirable properties, lengths, and other characteristics of such oligonucleotides are well known. Antisense technology can be used to control gene expression through interference with binding of polymerases, transcription factors, or other regulatory molecules (see Gee et al., In Huber and Carr, Molecular and Immunologic Approaches, Futura Publishing Co. (Mt. Kisco, N.Y.; 1994)).
Short interfering RNAs may be used for modulating (decreasing or inhibiting) the expression of a gene encoding a cellular polypeptide of interest. For example, small nucleic acid molecules, such as short interfering RNA (siRNA), micro-RNA (miRNA), and short hairpin RNA (shRNA) molecules may be used according to the methods described herein to modulate the expression of a cellular polypeptide of interest. A siRNA polynucleotide preferably comprises a double-stranded RNA (dsRNA) but may comprise a single-stranded RNA (see, e.g., Martinez et al. Cell 110:563-74 (2002)). A siRNA polynucleotide may comprise other naturally occurring, recombinant, or synthetic single-stranded or double-stranded polymers of nucleotides (ribonucleotides or deoxyribonucleotides or a combination of both) and/or nucleotide analogues as provided herein and known and used by persons skilled in the art.
In certain other embodiments, methods are provided herein for identifying a therapeutic agent that suppresses cellular senescence by using the animal models described herein. Candidate therapeutic agents may be administered to an animal of the animal model to provide a treated animal, followed by determining suppression of cellular senescence as described herein (i.e., determining the level or extent to which the candidate agent kills or facilitates killing of a senescent cell or determining the level of one or more senescence cell-associated molecules including senescence cell-associated polypeptides expressed or secreted by a senescent cell). The capability of the agent to suppress cellular senescence is determined by comparing the level or extent to which the candidate agent kills or facilitates killing of a senescent cell and/or comparing the level of one or more senescence cell-associated molecules such as senescence cell-associated proteins expressed or secreted by a senescent cell in the treated animal with an untreated (i.e., vehicle only or placebo) control animal. A statistically or biologically significant decrease or reduction of cellular senescence in the treated animal compared with untreated, control animal thereby identifies an agent that suppresses cellular senescence. As described herein, positive control animal groups (i.e., those that include an agent capable of destroying or facilitating destruction of a senescent cell or that inhibit or reduce expression or secretion of one or more senescence cell-associated including senescence cell-associated polypeptides) may also be included in such methods.
Also provided herein are methods for identifying a therapeutic agent for treating and/or preventing age-related disorders and age-specific traits associated with senescence-inducing stimuli, which employ primary cells from an animal model (e.g., an animal model mouse) or a cell line prepared from cells isolated from the animal model. In one embodiment, primary cells or a cell line derived from the animal model may be used in screening (including high throughput methods) for therapeutic agents that suppress cellular senescence. The cells may be exposed to, contacted, mixed with, or in some manner permitted to interact with an agent (e.g., a medical therapy) that induces cellular senescence prior to, concurrent with, or subsequent to contact with a candidate therapeutic agent. Suppression of cellular senescence may be determined by any of the methods described herein or in the art.
High throughput screening, typically automated screening, of a large number of candidate therapeutic agents from synthetic or natural product libraries may be used to identify therapeutic agents. The candidate therapeutic agents to be screened may be organized in a high throughput screening format such as using microfluidics-based devices, or a 96-well plate format, or other regular two dimensional array, such as a 384-well, 48-well or 24-well plate format, or an array of test tubes. The format is therefore amenable to automation. An automated apparatus that is under the control of a computer or other programmable controller may be used for one or more steps of the methods described herein. A controller may monitor the results of each step of the method and may automatically alter the testing paradigm in response to those results.
Pharmaceutical Compositions and Methods of Treatment
The present disclosure further provides for pharmaceutical compositions comprising any of the agents that suppress cellular senescence identified according to the methods described herein and a pharmaceutically acceptable excipient. The therapeutic agents described herein may be formulated in a pharmaceutical composition for use in treatment or preventive (or prophylactic) treatment (e.g., reducing the likelihood of occurrence, of exacerbation of disease, or occurrence or recurrence of one or more symptoms of the disease). The methods and excipients described herein are exemplary and are in no way limiting. Pharmaceutical acceptable excipients are well known in the pharmaceutical art and described, for example, in Rowe et al., Handbook of Pharmaceutical Excipients: A Comprehensive Guide to Uses, Properties, and Safety, 5th Ed., 2006, and in Remington: The Science and Practice of Pharmacy (Gennaro, 21st Ed. Mack Pub. Co., Easton, Pa. (2005)). Exemplary pharmaceutically acceptable excipients include sterile saline and phosphate buffered saline at physiological pH. Preservatives, stabilizers, dyes, buffers, and the like may be provided in the pharmaceutical composition. In addition, antioxidants and suspending agents may also be used.
The pharmaceutical compositions may be in the form of a solution. Alternatively, they may be in the form of a solid, such as powder, tablets, or the like.
The present disclosure also provides a method for treating or preventing age-related disorders and age-sensitive traits associated with senescence-inducing stimuli in a subject who has or who is at risk of developing an age-related disorder or age-sensitive trait associated with a senescence-inducing stimulus comprising administering a therapeutic agent that selectively suppresses cellular senescence in the subject, thereby treating or preventing an age-related disorder or age-sensitive trait associated with a senescence-inducing stimulus in the subject.
As understood by a person skilled in the medical art, the terms, “treat” and “treatment,” refer to medical management of a disease, disorder, or condition of a subject (i.e., patient) (see, e.g., Stedman's Medical Dictionary). “Treating an age-related disorder or age-sensitive trait associated with a senescence-inducing stimulus” refers to reducing the number of symptoms of an age-related disorder or age-sensitive trait associated with a senescence-inducing stimulus, decreasing the severity of one or more symptoms, or delaying the progression of an age-related disorder or age-sensitive trait associated with a senescence-inducing stimulus.
“Preventing an age-related disorder or age-sensitive trait associated with a senescence-inducing stimulus” refers to preventing or delaying onset of an age-related disorder or age-sensitive trait associated with a senescence-inducing stimulus, or reoccurrence of one or more age-related disorder or age-sensitive trait associated with a senescence-inducing stimulus.
Subjects at risk of developing an age-related disorder or age-sensitive trait associated with a senescence-inducing stimulus include subjects exposed to conditions which induce senescence. For example, in specific embodiments, subjects at risk include those exposed to a senescence-inducing stimulus, such as a chemical stimulus, an environmental stimulus, a genetic modification, a diet modification, or a combination thereof. In certain more specific embodiments, the senescence-inducing stimulus comprises irradiation treatment (e.g., whole body treatment) or treatment with one or more chemotherapeutic agents (e.g., doxorubicin, taxol, docetaxel, gemcitabine, cisplatin, and the like). In other embodiments, the senescence-inducing stimulus comprises cigarette smoking, or exposure to cigarette smoke or other environmental insults.
The effectiveness of a therapeutic agent can readily be determined by a person skilled in the medical and clinical arts. One or any combination of diagnostic methods, including physical examination, assessment and monitoring of clinical symptoms, and performance of analytical tests and methods described herein, may be used for monitoring the health status of the subject. The effects of the treatment of a therapeutic agent or pharmaceutical composition can be analyzed using techniques known in the art, such as comparing symptoms of patients suffering from or at risk of an age-related disorder or age-sensitive trait that have received the treatment with those of patients without such a treatment or with placebo treatment.
In certain embodiments of the method for treating or preventing an age-related disorder or age-sensitive trait, the therapeutic agents are identified according to the screening methods provided herein. In certain other embodiments, the therapeutic agents may be other agents known in the art that selectively suppresses cellular senescence and that treat and/or prevent an age-related disorder or age-sensitive trait.
The therapeutic agents or pharmaceutical compositions that selectively suppress cellular senescence and that are useful for treating or preventing an age-related disorder or age-sensitive trait may be administered orally, topically, transdermally, parenterally, by inhalation spray, vaginally, rectally, or by intracranial injection, or any combination thereof. In one embodiment, the therapeutic agents or compositions comprising the agents are administered parenterally, such as via subcutaneous, intravenous, intramuscular, or intracisternal injection, or via infusion techniques.
The therapeutic agents or pharmaceutical compositions that selectively suppress cellular senescence provided herein are administered to a subject who has or is at risk of developing an age-related disorder or age-sensitive trait associated with a senescence-inducing stimulus at a therapeutically effective dose. A “therapeutically effective dose” of a specific therapeutic agent refers to that amount of the agent sufficient to result in reducing the severity of, eliminating, or delaying the onset or reoccurrence of one or more symptoms of an age-related disorder or age-sensitive trait in a statistically significant manner. Such a dose may be determined or adjusted depending on various factors including the specific therapeutic agents or pharmaceutical compositions, the routes of administration, the subject's condition, that is, stage of the disease, severity of symptoms caused by the disease, general health status, as well as age, gender, and weight, and other factors apparent to a person skilled in the medical art. Similarly, the dose of the therapeutic for treating a disease or disorder may be determined according to parameters understood by a person skilled in the medical art. Optimal doses may generally be determined using experimental models and/or clinical trials. Design and execution of pre-clinical and clinical studies for a therapeutic agent (including when administered for prophylactic benefit) described herein are well within the skill of a person skilled in the relevant art. The optimal dose of a therapeutic may depend upon the body mass, weight, or blood volume of the subject. For example, an amount between 0.01 mg/kg and 1000 mg/kg (e.g., about 0.1 to 1 mg/kg, about 1 to 10 mg/kg, about 10-50 mg/kg, about 50-100 mg/kg, about 100-500 mg/kg, or about 500-1000 mg/kg) body weight (which can be administered as a single dose, daily, weekly, monthly, or at any appropriate interval) of a therapeutic agent may be administered.
Also contemplated is the administration of a therapeutic agent or a pharmaceutical composition that selectively suppresses cellular senescence in combination with a second agent useful in treating or preventing an age-related disorder or age-sensitive trait.
In certain embodiments, a therapeutic agent that selectively suppresses cellular senescence and a second agent useful in treating or preventing an age-related disorder or age-sensitive trait act synergistically. In other words, these two agents interact such that the combined effect of the agents is greater than the sum of the individual effects of each agent when administered alone.
In certain other embodiments, a therapeutic agent that selectively suppresses cellular senescence and a second agent useful in treating or preventing an age-related disorder or age-sensitive trait act additively. In other words, these two agents interact such that the combined effect of the agents is the same as the sum of the individual effects of each agent when administered alone.
It is contemplated the therapeutic agent and the second agent may be given simultaneously in the same formulation. Alternatively, the second agents may be administered in a separate formulation but concurrently (i.e., given within less than one hour of each other).
In certain embodiments, the second agent useful in treating or preventing an age-related disorder or age-sensitive trait may be administered prior to administration of a therapeutic agent that selectively suppresses cellular senescence. Prior administration refers to administration of the second agent at least one hour prior to treatment with the therapeutic agent. It is also contemplated that the second agent may be administered subsequent to administration of the therapeutic agent. Subsequent administration is meant to describe administration of the second agent at least one hour after the administration of the therapeutic agent.
This disclosure contemplates a dosage unit comprising a pharmaceutical composition provided herein. Such dosage units include, for example, a single-dose or a multi-dose vial or syringe, including a two-compartment vial or syringe, one comprising the pharmaceutical composition of this disclosure in lyophilized form and the other a diluent for reconstitution. A multi-dose dosage unit can also be, e.g., a bag or tube for connection to an intravenous infusion device.
The following examples are for illustration and are not limiting.
EXAMPLES
Example 1
Preparation of P16-3MR Transgenic Mice
This Example describes preparation of a transgenic mouse comprising a p16ink4a promoter operatively linked to a trimodal fusion protein.
The promoter, p16Ink4a, which is transcriptionally active in senescent cells but not in non-senescent cells (see, e.g., Wang et al., J. Biol. Chem. 276:48655-61 (2001); Baker et al., Nature, supra) was engineered into a nucleic acid construct. The p16Ink4a gene promoter (approximately 100 kilobase pairs) was introduced upstream of a nucleotide sequence encoding a trimodal reporter fusion protein. Alternatively, a truncated p16Ink4a promoter may be used (see FIGS. 1 and 2 providing an exemplary vector and exemplary promoter sequence) (see, e.g., Baker et al., Nature, supra; International Application Publication No. WO/2012/177927; Wang et al., J. Biol. Chem. 276:48655-61 (2001)). The trimodal reporter protein, the expression of which is driven by the p16ink4a promoter in senescent cells, is termed 3MR and consists of renilla luciferase (rLUC), monomeric red fluorescent protein (mRFP) and a truncated herpes simplex virus thymidine kinase (tTK) (see, e.g., Ray et al., Cancer Res. 64:1323-30 (2004)). The polypeptide sequences and the encoding polynucleotides for each of the three proteins are known in the art and are available in public databases, such as GenBank. An exemplary sequence (SEQ ID NO:25) for the 3MR transgene is provided in FIG. 3. The 3MR transgene was inserted into a BAC vector using techniques routinely practiced by person skilled in the molecular biology art. The detectable markers, rLUC and mRFP permitted detection of senescent cells by bioluminescence and fluorescence, respectively. The expression of tTK permitted selective killing of senescent cells by exposure to the pro-drug ganciclovir (GCV), which is converted to a cytotoxic moiety by tTK. Transgenic founder animals, which have a C57B16 background, were established and bred according to routinely procedures for introducing transgenes into animals (see, e.g., Baker et al., Nature, supra). The transgenic mice are called p16-3MR herein.
Example 2
Effects of Therapy-Induced Senescence on Aging
This example identifies the role of senescence-inducing stimuli, particularly radiation and chemotherapy treatment, on declining health parameters and age-sensitive traits using a p16-FKBP-caspase-8 (INK-ATTAC) transgenic animal model. In particular, the example determines the extent to which chemotherapy and gamma radiation induce cellular senescence, identifies that cellular senescence induced by various treatment regimens is deleterious for tissue structure and function, and identifies that clearance of therapy-induced senescent cells improves age-sensitive traits and other health parameters following chemotherapy and radiation.
Briefly, INK-ATTAC mice are aged 10-12 weeks. Cohorts of mice are treated with 4 Gy IR (4 cycles), doxorubicin (3-4 mg/kg i.p. daily for five consecutive days: 3 cycles), docetaxel (25 mg/kg i.p. every seven days for 3 cycles), or cisplatin (6 mg/kg i.p. every twenty days for 3 cycles) to induce cellular senescence. The mice are also treated with AP20187 or PBS on a weekly basis from the start of treatment.
The rate of aging is assessed by various measures of health and lifespan. Specifically, mice in each of the above cohorts are analyzed for the following age-sensitive traits, using methods known in the art, will not harm the animals, and do not require anesthesia: (1) T cell subset distribution, (2) cataract formation, (3) spontaneous activity, (4) motor coordination, and (5) cognitive capacity. These traits provide assessments of systems known to change with age and which have important implications for human health. Spontaneous activity of individual mice is measured over a 48-hour period using comprehensive laboratory animal monitoring systems equipped with photocells (e.g., Columbus Instruments). Motor coordination is analyzed by performing the accelerating rotarod test. For measuring cognitive capacity, a modified Stone T-maze, which is sensitive to age-related changes in learning and memory, is used.
Three additional age-sensitive traits known to change with age and highly relevant to human health are also measured using known techniques: (1) physical function, (2) body composition (sarcopenia, osteoporosis, fat atrophy), and (3) cardiac function. Physical function is assessed by measuring running time, distance, and work using a motorized treadmill, and grip strength using a grip meter. Lean mass, fat mass and bone mineral density is assessed by QNMR and/or dual-energy X-ray absorptiometry measurements.
The above analyses are complemented with assessments of age-sensitive traits on tissues and organs of test and control mice. These analyses include the following assays which are performed using known techniques: (1) fiber diameter analysis on gastrocnemius muscle, (2) DNA damage analysis, (3) analysis of renal and glomerulosclerosis, (4) analysis for retinal atrophy, (5) proteotoxic stress analysis, (6) oxidative stress analysis, and (7) analysis of the hematopoietic system.
Example 3
Effects of Removing Senescent Cells from Aging Animal
Clearance of therapy-induced senescent cells improves age-sensitive traits and other health parameters in aged animals. Wild type INK-ATTAC transgenic animals from each of lineage 3 (TG3) and lineage 5 (TG5) were used in the experiments (see Baker et al., Nature, supra). INK-ATTAC transgenic mice were aged 10-12 weeks, and then cohorts of animals were treated with AP20187 or PBS (control) on a weekly basis from the start of treatment. As shown in FIG. 4, animals treated with AP20187 had an average lifespan of approximately 26 months compared with 21 months for control.
When the animals reached 18 months of age, several measures of health and lifespan were taken. Weight, fat mass, activity measurements (treadmill exercise tests to measure distance traveled and duration of exercise) were compared between untreated female mice (n=16) and AP20187-treated female mice (n=21), and compared between untreated male mice (n=19) and AP20187-treated male mice (n=21). Fat mass was greater in both treated females and males compared with the untreated groups (see FIG. 5). No statistically significant differences were observed with respect to weight, or activity measurements between the treated and untreated animals. Treated and untreated mice (5-6 per group) were sacrificed, and adipose cell (AC) diameter, gastrocnemius muscle fiber diameter (μm), abdominal muscle fiber diameter (μm), dermal thickness (μm), subdermal adipose thickness (μm), cardiac performance (introduction of isoproterenol as an exogenous stress in animals prior to sacrifice), and extent of glomerulosclerosis were determined. Treated animals had greater adipose cell diameter (see FIG. 6); better cardiac performance (FIG. 7); and had less sclerotic glomeruli (see FIG. 8) than untreated animals. No significant differences between treated animals were observed with respect to gastrocnemius muscle fiber diameter (μm), abdominal muscle fiber diameter (μm), dermal thickness (μm), and subdermal adipose thickness (μm). Removal of senescent cells in AP20187-treated animals prevented loss of adipose tissue, prevented loss of cardiac performance, and reduced the extent of glomerulosclerosis.
The various embodiments described above can be combined to provide further embodiments. All U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet are incorporated herein by reference, in their entirety. Aspects of the embodiments can be modified, if necessary to employ concepts of the various patents, applications and publications to provide yet further embodiments.
These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.

## Claims

1. A transgenic mouse for studying the role of senescent cells on an age-related disorder or an age-sensitive trait, wherein the genome of the mouse contains a recombinant polynucleotide,
wherein the polynucleotide contains a p16INK4a promoter sequence that controls expression of an enzyme so as to cause the enzyme to be expressed in senescent cells in the mouse;
wherein the enzyme converts a prodrug to a cytotoxic agent;
wherein treating the mouse with the prodrug results in the prodrug being converted to the cytotoxic agent by the enzyme expressed from the recombinant polynucleotide in the senescent cells, thereby killing the senescent cells but killing less than 10% of non-senescent cells;
whereby progression of an age-related disorder or an age-sensitive trait is delayed in the mouse treated with the prodrug compared with another such transgenic mouse that is not treated with the prodrug.

2. The transgenic mouse of claim 1, wherein the p16INK4a promoter also controls expression of a luminescent protein so as to cause it to be expressed in senescent cells in the mouse.

3. The transgenic mouse of claim 1, wherein the p16INK4a promoter also controls expression of a fluorescent protein so as to cause it to be expressed in senescent cells in the mouse.

4. The transgenic mouse of claim 1, wherein the p16INK4a promoter also controls expression of a luminescent protein and a fluorescent protein so as to cause the enzyme, the luminescent protein, and the fluorescent protein all to be expressed in senescent cells in the mouse.

5. The transgenic mouse of claim 1, wherein the enzyme is a thymidine kinase.

6. The transgenic mouse of claim 1, wherein the prodrug is ganciclovir.

7. The transgenic mouse of claim 2, wherein the luminescent protein is luciferase.

8. The transgenic mouse of claim 3, wherein the fluorescent protein is monomeric red fluorescent protein (mRFP).

9. A mouse from a strain of mice having a transgene inserted in their genome, the transgene containing:
a p16INK4a promoter sequence;
a nucleotide sequence encoding an enzyme that converts a prodrug to a cytotoxic agent, wherein the enzyme is expressed under control of the p16INK4a promoter so as to be produced by senescent cells in the mouse; and
a nucleotide sequence containing a luminescent and/or a fluorescent marker protein, expressed under control of the p16INK4a promoter so as to be produced by senescent cells in the mouse;
wherein treating the mouse with the prodrug results in the enzyme converting the prodrug to the cytotoxic agent in senescent cells, thereby killing the senescent cells but killing less than 10% of non-senescent cells;
whereby progression of an age-related disorder or an age-sensitive trait is delayed in the mouse treated with the prodrug compared with an untreated mouse of the same strain.

10. The mouse of claim 9, wherein the enzyme and the marker protein are expressed as a fusion protein.

11. The mouse of claim 9, wherein the enzyme is a thymidine kinase and the prodrug is ganciclovir.

12. The mouse of claim 9, wherein the marker protein is luciferase or monomeric red fluorescent protein (mRFP).

13. A method for determining the effect of a test compound on an age-related disorder or an age-sensitive trait mediated by senescent cells; the method comprising:
obtaining a plurality of mice according to claim 1,
dividing the plurality into several treatment groups,
treating mice in a first treatment group with the prodrug,
treating mice in a second treatment group with the test compound, and
imaging or measuring an effect of the test compound on the age-related disorder or the age-sensitive trait in the mice in the second treatment group.

14. The method of claim 13, further comprising aging the mice or treating the mice with chemotherapy or radiation to promote the age-related disorder or the age-sensitive condition.

15. A method of visualizing senescent cells that mediate an age-related disorder or an age-sensitive trait, the method comprising:
obtaining a mouse according to claim 9;
promoting the age-related disorder or the age-sensitive trait in the mouse; and then
imaging or measuring cells expressing the marker protein in the mouse.

16. A method for determining the effect of a test compound on an age-related disorder or an age-sensitive trait mediated by senescent cells; the method comprising:
obtaining a plurality of mice according to claim 9,
dividing the plurality into several treatment groups,
treating mice in a first treatment group with the prodrug,
treating mice in a second treatment group with the test compound, and
imaging or measuring an effect of the test compound on the age-related disorder or the age-sensitive trait in the mice in the second treatment group.

17. A method for removing senescent cells from a tissue, comprising contacting the senescent cells with a prodrug,
wherein the genome of cells in the tissue contains a recombinant polynucleotide in which a p16INK4a promoter sequence controls expression of an enzyme so as to cause the enzyme to be expressed in senescent cells;
wherein the enzyme converts the prodrug to a cytotoxic agent;
wherein contacting the senescent cells with the prodrug results in the prodrug being converted to the cytotoxic agent by the enzyme expressed from the recombinant polynucleotide in the senescent cells, thereby killing the senescent cells but killing less than 10% of non-senescent cells in the tissue;
whereby progression of an age-related disorder or an age-sensitive trait is delayed in the tissue as a result of contacting the senescent cells in the tissue with the prodrug.

18. The method of claim 17, wherein the p16INK4a promoter also controls expression of a luminescent protein so as to cause it to be expressed in senescent cells in the tissue.

19. The method of claim 17, wherein the p16INK4a promoter also controls expression of a fluorescent protein so as to cause it to be expressed in senescent cells in the method.

20. The method of claim 17, wherein the enzyme is a thymidine kinase and the prodrug is ganciclovir.