Patent Publication Number: US-2022220501-A1

Title: CELL-SPECIFIC EXPRESSION OF modRNA

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 16/354,814, filed Mar. 15, 2019, which issued as U.S. Pat. No. 11,299,749 on Apr. 12, 2022, which was a continuation of PCT/US2017/052035 filed Sep. 18, 2017 and published on Mar. 22, 2018 as WO 2018/053414, which claims priority to U.S. provisional application No. 62/395,701 filed Sep. 16, 2016, the contents of which are hereby incorporated by reference into the present application. 
    
    
     SEQUENCE LISTING 
     The instant application contains a Sequence Listing, created on Feb. 28, 2022; the file, in ASCII format, is designated H2277405.txt and is 17.9 kilobytes in size. The file is hereby incorporated by reference in its entirety into the instant application. 
     TECHNICAL FIELD 
     The present disclosure relates generally to a platform for the cell-specific expression of therapeutic proteins in vitro, ex vivo and in vivo, using a cell-specific transcriptional regulatory system based on cell-specific miR override of gene expression suppression. 
     BACKGROUND OF THE DISCLOSURE 
     Chemically modified messenger RNA (modRNA) is a therapeutic strategy that enables the cellular machinery to produce genes of interest without modifying the genome. Thus, modRNA avoids several of the problems that have arisen with conventional gene therapy, including lack of genomic integration, persistence of expression, immunogenicity, difficulty in scalability and production, need for life-long monitoring for tumorigenesis and other adverse clinical outcomes, and the potential for vector escape into the systemic circulation and long-term expression elsewhere in the body. 
     modRNA has considerable potential as a therapy for disease. Delivery of a cell cycle inducer via modRNA, for example, would trigger growth of beta cells in individuals with diabetes or restore proliferation of cardiomyocytes following myocardial infarction or heart failure. Diabetic neuropathy may be lessened by the ability to deliver genes encoding nerve growth factor. Additionally, with the advent of genome editing technology, CRISPR/Cas9 or transcription activator-like effector nuclease (TALEN) transfection will be safer if delivered in a transient and cell-specific manner. 
     However, none of the available transfection reagents for modRNA offers both a high level of gene expression and the ability to target any cell of interest. For example, a common in vivo transfection reagent is in vivo-jetPEI® (Polyplus-transfection® SA, Illkirch, France), which is a polymer based reagent that complexes with modRNA to form nanoparticles. However, in vivo-jetPEI primarily targets lung tissue in vivo and significantly lowers transfection efficacy compared to naked modRNA. 
     Therefore, what is needed is a modRNA-based gene delivery system that achieves a high level of gene expression exclusivity in a cell of interest. 
     SUMMARY OF THE DISCLOSURE 
     The present disclosure provides an expression regulatory platform for cell-specific transcription based on the exploitation of a repressor RNA-binding protein/k-motif interaction coupled with cell-specific miR override of the repressor function to control expression of a delivered modRNA in a cell-specific fashion. RNA-binding proteins such as the archaeal protein L7Ae and eukaryotic homologs thereof such as L30e recognize a distinctive RNA motif, the kink-turn (k-turn or k-motif as referred to herein). By incorporating the k-motif into a first construct that encodes a gene of interest (GOI) and including a recognition element for a cell-specific miR in a second construct that encodes the RNA-binding protein, suppression of expression of the GOI is overridden when the two constructs are co-transfected into the appropriate cell type. The platform incorporates modified mRNA 
     The present disclosure, therefore, relates to a method for achieving cell-specific expression of a modRNA of a gene of interest (GOI) the expression of which is desired only in the cell of interest. In one aspect, the disclosure describes an expression regulatory system for cell-specific transcription, the system comprising a first nucleic acid that encodes (1) a cell-specific microRNA (miR) recognition element, and (2) a translation suppressor protein; and a second nucleic acid that encodes (1) a suppressor protein interaction motif, for example a K-motif, downstream of its 5′UTR that binds the translation suppressor protein, and (2) a gene that encodes a protein of interest. The nucleic acids are modRNA. 
     By swapping out the miR recognition element, cell specificity can be modulated, making the system adaptable to other cell types. 
     In another aspect, the present disclosure relates to short-term expression of cardiomyocyte (CM)-specific modRNA of candidate genes, such as cell cycle inducer genes, the expression of which reactivates CM regeneration, which is important following post-myocardial infarction or in heart failure settings. The method is based on the observation that cell cycle inducer genes, for example, Lin28 and Pkm2, delivered as modRNA using the cell-specific delivery system of the disclosure following MI significantly induces CM and non-CM proliferation. Since increased non-CM proliferation can lead to enhanced cardiac scarring, it was necessary to develop a CM-specific modRNA that allows expression of genes only in cardiomyocytes. 
     The present disclosure describes CM-specific modRNA that allows modRNA translation exclusively in CMs. In one embodiment, CM-specific Lin28 or Pkm2 modRNA expression results in significant CM proliferation without significantly changing non-CM proliferation. In another embodiment, based on CM-specific modRNA, a novel lineage tracing adult mouse model that is based on co-expression destabilized Cre recombinase and candidate genes in Rosa26 tdTomato  using CM-specific modRNA was developed. 
     In one aspect, the disclosure relates to an expression regulatory system for cardiomyocyte-specific expression comprising a first nucleic acid that encodes a recognition element for microRNA (miR recognition elements serve as an anti-miR approach) that binds specifically to a target cardiomyocyte miR, and prevents the translation of a suppressor protein (L7Ae); and a second nucleic acid that comprises a gene of interest and a kink-turns motif (K-motif) that are bound by the suppressor protein (L7Ae). Binding of L7Ae to the K motif inhibits the expression of the genes that had the K motif. 
     In one embodiment of the translational regulatory system, the target cardiomyocyte miR is selected from the group consisting of miR1, miR29, miR126, miR133a, miR199, miR208a and miR378. In another embodiment, the target cardiomyocyte miR is selected from the group consisting of miR1, miR 208a and miR1 in combination with miR208a. 
     In one embodiment of the expression regulatory system, the suppressor protein is L7Ae and the protein interaction motif is K-motif. L7Ae is an RNA binding protein that represses translation of the targeted transcript. L7Ae targets a specific sequence called the k-motif or k-turn. Accordingly, the k-motif is built into the nucleic acid of the pair that encodes the GOI. Ordinarily, when the other nucleic acid of the pair that encodes L7Ae is expressed normally, L7Ae is able to bind to the k-motif, thereby repressing expression of the GOI encoded by that nucleic acid. 
     In an embodiment of the present system, the nucleic acid encoding L7Ae also contains a cell-specific miR recognition element. When expressed in the appropriate cell, cell-specific miR binds the miR recognition element to halt expression of L7Ae, eliminating suppression of the GOI on the other nucleic acid. 
     In one embodiment of the translational regulatory system, the protein of interest is a reporter protein or other gene of interest. In one embodiment of the translational regulatory system, the reporter protein or selection marker is a fluorescent protein, an antibiotic resistance marker or other gene of interest. In one embodiment of the translational regulatory system, the reporter protein or selection marker is selected from the group consisting of green fluorescence protein (GFP), inactive human CD25 (ihCD25). In one embodiment of the transcriptional/translational regulatory system of the disclosure, the protein of interest is a cell cycle inducer protein. In one embodiment of the translational regulatory system, the cell cycle inducer protein is selected from the group consisting of Lin28, Pkm2, and Cyclin D2. In one embodiment of the transcriptional regulatory system, said first nucleic acid comprises the nucleotide sequence of SEQ ID NO: 2, SEQ ID NO: 3, or SEQ ID NO: 4. In one embodiment of the transcriptional regulatory system, said second nucleic acid comprises the nucleotide sequence of SEQ ID NO: 5, SEQ ID NO: 6, SEQ ID NO: 7 or SEQ ID NO: 8. 
     In one aspect, the disclosure relates to a composition comprising first and second modified RNAs (modRNAs), wherein said first modRNA is an expression product of the first nucleic acid of claim  1 ,  2  or  3  and the second modRNA is an expression product of the second nucleic acid. 
     In one aspect, the disclosure relates to a method for expressing a protein in cardiomyocytes (CMs), the method comprising contacting said CMs with a modRNA encoding an miR recognition element specific for a cardiomyocyte miR target, wherein the modRNA comprises the nucleotide sequence of SEQ ID NO: 2, SEQ ID NO: 3, or SEQ ID NO: 3. 
     In one aspect, the disclosure relates to a vector comprising first and second nucleic acids as described herein. 
     In one aspect, the disclosure relates to a transcriptional/translational regulatory kit comprising the first and second nucleic acids as described herein or a vector comprising first and second nucleic acids as described herein. 
     In one aspect, the disclosure relates to a method for inducing/reactivating proliferation of cardiomyocytes following myocardial infarction (MI), the method comprising contacting said cardiomyocytes or a portion of said myocytes with a first modRNA that encodes a cardiomyocyte-specific miR and a second modRNA that encodes a cell cycle inducer gene. 
     In one aspect, the disclosure relates to the disclosed method, wherein the cell cycle inducer gene is selected from the group consisting of Lin28, Pkm2 and Cyclin D2. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a plasmid map of pTEMPLZ used in generating the modRNA of the disclosure. 
         FIG. 2  shows in graphic (top panel) and table form (bottom panel) the PCR settings for synthesizing DNA tailed template. Shown in box is the elongation step that must be set based on the size of the sequence insert. Elongation step requires 30 sec per KB of ORF insert. PCR setting is based of manufacturer instructions from 2×KAPA HiFi HotStart ReadyMix kit. 
         FIGS. 3A-3C  show the results of the quality control analysis for modRNA synthesis. A 1% agarose gel determining correct size of the plasmid pTEMPLZ with ORF insert and tailed DNA template for IVT. B Ideal Nanodrop result of final modRNA product. Ideal concentration is between 15-20 ug/ul. 260/280 values closer to 2 indicate purity. C Bioanalyzer result for quality control of synthesized modRNA. 
         FIGS. 4A-4C  A Whole heart view of mouse heart injected in vivo with modRNA encoded with LacZ gene. 24 hours after injection, mouse was sacrificed, the heart was fixed with 4% PFA, and stained with x-gal. B immunostaining of mouse heart injected in vivo with modRNA encoded with nuclear GFP. (left) Cardiomyocytes (TropT: Red), Endothelial cells (Pecam1: Red) and smooth muscle cells (smMHC: Red) positive for nuclear GFP (Green). (DAPI: Blue). C cross section of Rosa26 LacZ mouse heart injected with modRNA encoded with Cre Recombinase. Transfected cells with Cre Recombinase can be stained with x-gal resulting in dark blue color. 
         FIGS. 5A and 5B  shows adult mouse myocardial infarction and heart failure models. Adult mouse myocardial infarction model (MI model) is performed using a permanent ligation of left anterior descending coronary artery (LAD) following direct intramuscular injection of modRNA. One or more days post MI, hearts are collected and used for immunostaining. B Adult mouse heart after MI is highly transfected with Luc; LacZ and nGFP modRNAs. A Several cell types are transfected with modRNA, including cardiomyocytes (CM), cardiac fibroblasts (CF) and endothelial cells (EC). 
         FIGS. 6A-6H  show that Pkm2 expression in adult CMs induces proliferation after MI. A Relative expression of Pkm2 measured by qRT-PCR in mice&#39; hearts 1 or 10 days after birth. B Experimental plan for immunostaining of Pkm2 or α-Actinin (CMs marker) at different stages of mouse heart development. C Representative images of Pkm2 expression at different stages of mouse heart developmental. D pharmacokinetics of Pkm2 expression post modRNA delivery in vivo. E Experimental timeline for measuring the effect of Pkm2 on CMs proliferation F A Representative image of DNA synthesis (BrdU + ) in CMs (α-Actinin + ) and non-CMs cells (α-Actinin − ) 7 days post-MI. G-H Quantification of hallmark proliferation markers in CMs F or non-CMs G in adult mice 7 days post-MI. Results represent 2 independent experiments (n=4); white arrow heads point to CMs; yellow arrow heads point to non-CMs, ***, P&lt;0.001, **, P&lt;0.01, two-tailed student t-test, Scale bar 10 μm. 
         FIGS. 7A-7K  show the design and function of  cms modRNA in vivo. A Construct design and experimental timeline used to identify  cms miRs. B Immunostaining images of ihCD25 modRNA expression (red) with or without recognition elements for different miRs post transfection. C Quantification of the experiment in c. D modRNAs constructs design used for  cms Cre or  cms nGFP modRNAs delivery in vivo. E-F nGFP-K modRNA (green) transfected alone or co-transfected miR1-208, 4 days post-MI. E Representative images of hearts 7 days post-MI. F Transfection efficiency with different ratios of nGFP-K and miR1-208. Rosa26 mTmG  mice co-transfected with Cre-K+miR1-208. G Co-transfection of Cre-K+miR1-208. Red: Troponin I. H Quantification of the experiment in g. I Experimental timeline for evaluation of  cms Pkm2 modRNA effect on proliferation. Quantification of hallmark proliferation markers in CMs J or non-CMs K 7 days post-MI. Results represent 2 independent experiments (n=3 mice); ****, P&lt;0.0001, ***, P&lt;0.001, **, P&lt;0.01, N.S, Not Significant, two-tailed student t-test (f) or One-way ANOVA, Bonferroni post-hoc test (j,c,k). Scale bar 10 μm or 50 μm in c and for h, respectively. 
         FIGS. 8A-8N  show that  cms Pkm2 modRNA improves cardiac function and outcome post MI. A Experimental timeline to evaluate cardiac function and outcome. B MRI assessments of left ventricular systolic function 1 month post-MI. Images depict left ventricular chamber (outlined in red) in diastole and systole. C Percentage of ejection fraction for the experiments in b. D Echo evaluation of delta in percentage of fractioning shorting differences between day 2 (baseline) and day 28 post-MI. E Representative pictures of masson trichrome staining to evaluate scar size 28 days post-MI. F-I Quantification of scar size F, heart weight to body weight ratio G, CMs size H, and capillary density I measured 28 days post-MI. J-M Number of CMs with different treatments 28 days post-MI. J Representative image of the number of CMs in each group. K Quantification of the experiment in j. L Representative images of nuclei of isolated CMs (mono, bi or multi). M Quantification of the experiment in I N Long-term post-MI survival curve for mice injected with Pkm2-K or luc-K modRNAs and co-transfected with miR1-208. Results represent 2 independent experiments (n=5 mice); ***, P&lt;0.001, **, P&lt;0.01, *, P&lt;0.05, One-way ANOVA, Bonferroni post hoc test. P-values for long term survival were calculated using the Mantel-Cox log-rank test. Scale bar 10 μm. 
         FIGS. 9A-9M  show a lineage tracing of CMs expressing cms Pkm2 post-MI and shows increased number of transfected CMs and induction of key downstream mediators of Pkm2&#39;s functions. A Experimental timeline used for cardiac lineage tracing in R26mTmG mice B Transfection efficiency (% GFP + ) of CMs or non-CMs 28 days post-MI. C Representative images of CMs and their progeny (GFP + ) 28 days post-MI. D Quantification of GFP +  CMs 3 or 28 days post-MI. Ratio of heart to body weight E, relative size of GFP +  CMs F, and number of nuclei in GFP +  CMs G in hearts, 28 days post-MI. H Representative image of GFP +  CMs, pH3 +  or Ki67 +  28 days post-MI. Quantification of GFP +  pH3 +  CMs I or GFP +  Ki67 +  CMs J 28 days post transfection with  cms Luc or  cms Pkm2 with  cms Cre modRNA in MI model. K-M 2 days post-MI and administration of  cms ihCD25 with  cms Luc or  cms Pkm2 modRNAs, adult CMs were isolated using magnetic beads. K qRT-PCR analysis to validate purity of isolated CMs. L Gene expression comparisons of key genes for both PPP (G6PD) and key downstream indirect transcriptional targets of Pkm2 in adult CMs. M Expression of cell-cycle promoting genes or cell-cycle inhibitors. Results represent 2 independent experiments (n=3 mice); ***, P&lt;0.001, **, P&lt;0.01, *, P&lt;0.05, N.S, Not Significant, two-tailed student t-test (b-j) or ANOVA with Bonferroni post hoc test (k-m). Scale bar 50 or 10 μm in c or h, respectively. 
         FIG. 10A-10B  shows that adult CMs are successfully transfected with modRNA in vitro. Isolated adult CMs were transfected with nuclear GFP (nGFP) and imaged 20 hours post transfection (bar=10 μm.) 
         FIG. 11  is a bar graph showing that activation of proliferation of adult CMs in vivo using cell cycle inducer modRNAs do not compromise CM integrity. CM size was measured using wheat germ agglutinin (WGA) staining for CMs cross-section area evaluation in hearts 7 days after MI with different modRNA treatments. Results indicate no significant differences in CM integrity and size when cell cycle inducer Lin28 or Pkm2 modRNAs were delivered in adult mouse MI model. Results represent two independent experiments with n=3 mice, N.S, not significant, two-tailed student t-test. 
         FIGS. 12A-12C  show that activation of proliferation of adult CMs in vivo using cell cycle inducer modRNAs reduces CM apoptosis and increases capillary density. CM proliferation (Ki67+) apoptosis (TUNEL+) and capillary density (Cd31+ luminal structures) were measured in the left ventricle of hearts 7 days post MI and different modRNA treatments. A representative data showing Lin28, cell cycle inducer modRNA treatment 7 days post MI induce CM proliferation, reduce apoptosis and increase capillary density. Quantifiable results for apoptosis B or capillary density C level with the different treatments. Results indicate significant reduction in CM apoptosis and elevation in capillary density using cell cycle inducer genes such as Lin28 (red), and PKM2. Results represent two independent experiments with n=3 mice, ***, P&lt;0.001, two-tailed student t-test. 
         FIGS. 13A-13C  show that miR-1 and miR-208 are expressed exclusively in rat neonatal CMs in vitro. A Inactivate human CD25 (ihCD25) modRNA with or without miR recognition element for miR-208, miR1, miR133a, miR126, miR199, miR378 and miR29a were transfected into neonatal CM in vitro. 20 hours post transfection cells were fixed and stained with anti CD25 (red) and Troponoin I (CM marker, green). B Images taken for different treatments showing that when ihCD25 modRNA had recognition elements of miR-1 or miR-208, CMs (Troponoin I+ cells) were unable to translate ihCD25 modRNA (Troponoin I+ and ihCD25+ CMs), other treatments resulted in ihCD25 translated in CMs. This indicate that only miR-1 or miR-208 are CMs specific. C quantification of the experiments. Results represent two independent experiments with n=3 wells, bar=10 mm. 
         FIGS. 14A-14C  show that miR-1 and miR-208 are expressed exclusively in rat neonatal CMs in vitro. A ihCD25 modRNA with or without miR recognition element for miR-208 or miR1 were co-transfected with nGFP into neonatal CM in vitro. 20 hours post transfection cells were fixed and stained with anti CD25 (red) and Troponin I (CMs marker, green nuclear). nGFP was used as transfection control. B images taken for different treatments showing that when ihCD25 modRNA had recognition elements of miR-1 or miR-208, CMs (Troponoin I+ cells) were unable to translate ihCD25 modRNA (Troponoin I+ and ihCD25+ cells), however ihCD25 modRNA without miR recognition elements was able to translate in CMs. all cells were transfected with nGFP indicating that modRNA was delivered successfully. C quantification of the experiments. Results represent two independent experiments with n=3 wells, bar=10 mm. 
         FIGS. 15A-15C  show that miR-1 and miR-208 express exclusively in adult mouse heart in vivo. A ihCD25 modRNA with or without miR recognition element for miR-208 or miR1 were co-transfected with nGFP into adult mouse heart in MI model. 20 hours post MI and delivery of modRNA hearts were collected, fixed and stained with anti CD25 (red), Troponoin I (CMs marker, white) and nGFP (green). nGFP was used as transfection control. B images taken for different treatments showing that when ihCD25 modRNA had recognition elements of miR-1 or miR-208, CMs (Troponoin I+ cells) were unable to translate ihCD25 modRNA (Troponoin I+ and ihCD25+ cells), transfecting ihCD25 without miR recognition site resulted in ihCD25 translated in CMs. all cells were transfected with nGFP indicating that modRNA was delivered successfully. C quantification of the experiments. Results represent two independent experiments with total n=5 mice, bar=10 mm. 
         FIGS. 16A-16C  show that nGFP CMs specific modRNA carrying recognition element for miR-208, miR-1, or both, in 1:1 ratio, show nGFP translation mostly in CMs in vitro. A CMs specific modRNA design. B nGFP CMs specific modRNA caring recognition element for miR-208, miR-1, or both, in 1:1 ratio, were transfected into neonatal rat CMs in vitro. 20 hours post transfection cells were fixed and stain for nGFP (nuclear green) and Troponin I (CM marker, red). C quantification of the experiment described in B. Results represent two independent experiments with n=3 wells, bar=10 mm. 
         FIGS. 17A-17C  show nGFP CMs-specific modRNA carrying recognition element for miR-208, miR-1, or both, in 1:2.5 ratio or higher, show nGFP translation exclusively in CMs in vitro. A CMs specific modRNA design. B nGFP CMs specific modRNA caring recognition element for miR-208, miR-1, or both, in different ratios, were transfected into neonatal rat CMs in vitro. 20 hours post transfection cells were fixed and stain for nGFP (green nuclear) and Troponin I (CM marker, red). C quantification of the experiment described in B. Results represent two independent experiments with n=3 wells, bar=10 mm. 
         FIG. 18  nGFP CM-specific modRNA carrying recognition element for both miR-1 and miR-208, in 1:0.5 ratio or higher, show nGFP translation exclusively in CMs in vivo. Results represent two independent experiments with n=3 mice, bar=10 mm. 
         FIGS. 19A-19C  Pkm2 CM-specific modRNA promotes proliferation exclusively in CMs. A Experimental time line. B modRNA design used in the experiments. C Lin28/PKM2 CMs-specific modRNA promotes proliferation exclusively in CMs. PKM2 modRNA carrying a k motif co-transfected with L7AE-miR1 and miR208 were tested for reactivation of adult mouse cardiomyocytes proliferation 7 days post-delivery in a mouse myocardial infarction model. Red dashed line represents control proliferation rate. Results represent 2 independent experiments with n=2 mice (total n=4 mice), ***P&lt;0.001, N.S., Not significant; two-tailed student t-test. 
         FIG. 20  L7AE modRNA did not elevate immune response in adult mouse myocardial infraction model. Quality images showing that no elevation in immune response (CD45+ cells, red) can be seen after L7AE is been delivered with or without recognition elements of different miRs. Scale bar=10 μm. 
         FIGS. 21A-21C  Evaluation of L7AE modRNA with or without miR recognition element for miR-208 or miR1 or both on CMs proliferation and size, in vivo. A Mouse adult heart after MI was injected with Luc, L7AE without miR recognition element or with recognition element for miR-1 or miR-208 or both. 7 days post MI hearts were collected, fixed and stain for different proliferation markers such as Ki67, BrdU, H3P and Aurora B B and WGA for measuring CMs size in the treated hearts C Results indicate that L7AE with miR recognition element for miR-208 or miR-1 or both induce CM proliferation without compromising CMs size. Results represent two independent experiments with n=3 mice, N.S, not significant, ***, P&lt;0.001, two-tailed student t-test. 
         FIGS. 22A-22C  show the transcriptional/translational regulatory system used to express nGFP. 
         FIGS. 23A-23D  show the results of isolation of transfected adult CMs from heart post MI using a CMs specific modRNA approach and magnetic bead sorting. A Isolation of adult CMs was performed 2 days post MI and modRNA administration. Anti-hCD25 magnetic beads were used to isolate CD25-positive cells. B CFW mice were transfected with ihCD25 and nGFP carrying the k motif. All positive cells isolated with this approach are GFP +  ihCD25+. C When transfected together with L7AE carrying recognition elements of miR1 and miR208 (CM specific modRNA) results in mixture of transfected CMs (nGFP+ and ihCD25+) and non-transfected CMs and no-cms. D Using hCD25 magnetic beads allows one to isolate only transfected CMs. Mice=3. 
         FIG. 24  shows that Lin28 or Pkm2 CMs specific modRNA improve cardiac function 28 days post MI and injection in mouse myocardial infraction model. Heart function was measured for different treated groups at day 2 and day 28 post MI using echocardiography. Results show improvement of cardiac function in Lin28 or Pkm2 treated groups with synergistic effect when Lin28 or Pkm2 were delivered in CMs specific manner (+L7Ae miR1+miR208). n=3 mice, ***, P&lt;0.001, two-tailed student t-test. 
     
    
    
     DETAILED DESCRIPTION OF THE DISCLOSURE 
     All patents, published applications and other references cited herein are hereby incorporated by reference into the present application. Methodologies used in developing the present invention are well known to those of skill in the art unless otherwise indicated. 
     In the description that follows, certain conventions will be followed as regards the usage of terminology. In general, terms used herein are intended to be interpreted consistently with the meaning of those terms as they are known to those of skill in the art. Some definitions are provide purely for the convenience of the reader. 
     The term “recognition element for miRNA” or “miRNA recognition element refers to single-stranded RNA-based oligonucleotides that are designed to bind endogenous miRNA and inhibit the expression of a construct containing the recognition element when it is introduced into cells. 
     The term “miRNA” refers to sequences that are complementary to mRNA that are involved in the cleavage of RNA or the suppression of the translation. Endogenous mature miRNAs function as part of the RNA-induced complex, which has the capacity to post-transcriptionally regulate mRNAs that have sequences with partial complementarity to the bound miRNA. Through the hybridization of the anti-miRNA sequence to the miRNA sequence, the function of the miRNA sequence is neutralized by preventing its selective binding to the target. 
     The term “modRNA” refers to a synthetic modified RNA that can be used for expression of a gene of interest. Chemical modifications made in the modRNA, for example substitution of pseudouridine for uridine, stabilize the molecule and enhance transcription. Additionally, unlike delivery of protein agents directly to a cell, which can activate the immune system, the delivery of modRNA can be achieved without immune impact. The use of modRNA for in vivo and in vitro expression is described in more detail in for example, WO 2012/138453. 
     The term “inactive human CD25” (ihCD25) refers to a truncated interleukin-2 receptor that has only the extracellular domain and is unable to signal into the cell. Other species, for example, inactive mouse CD25 may also be used in the disclosed method. 
     The present disclosure relates to methodology for achieving cell-specific expression of a modRNA encoding a gene of interest (GOI) the expression of which is desired in a cell of interest. In one aspect, the disclosure describes an expression regulatory system for cell-specific transcription, the system comprising a first nucleic acid having a 5′ untranslated region (UTR) and a 3′ UTR, where the nucleic acid encodes (1) a cell-specific microRNA (miR) recognition element upstream of its 3′UTR, and (2) a translation suppressor protein; and a second nucleic acid having a 5′ UTR and a 3′ UTR that encodes (1) a suppressor protein interaction motif, for example a K-motif, downstream of its 5′UTR that binds the translation suppressor protein, and (2) a gene that encodes a protein of interest. 
     Current treatments for MI address the consequences of myocyte loss, but are not effective in enhancing myocardial repair of lost heart muscle (3, 5). Recently, it was demonstrated that one day adult mammalian heart cells (mice) can regenerate heart themselves via CMs proliferation (7). Examining the genetic differences between the regenerative and the non-regenerative stages it was found that the most differentially expressed gene between these stages belong to mitosis and cell cycle categories (7). 
     Modified mRNA (modRNA) has emerged as an effective and safe tool for somatic gene transfer, and has been successfully used by us and others for gene delivery to the heart. 10,12-15  Here we show that Pyruvate Kinase Muscle Isozyme M2 (Pkm2), a pro-proliferative factor, frequently dysregulated in cancer, 16,17  is highly expressed in regenerative fetal and early neonatal CMs, but not in adult CMs. Restoration of Pkm2 levels using the modRNA delivery of the disclosure exclusively into adult CMs ( cms Pkm2) post-MI significantly and exclusively induced CMs proliferation, and was associated with improved cardiac function, reduced scar size, increased heart to body weight ratio, reduced CMs size, reduced apoptosis and increased capillary density. Those regenerative processes translated into increased long-term survival post-MI. Using lineage tracing and isolation of Pkm2-transfected CMs followed by gene expression analysis post-MI we show an increase in number of Pkm2-transfected CMs colonies and the potential involvement of key downstream effectors of the pro-proliferative cytoplasmic (via the pentose phosphate pathway (PPP) 18,19 ) and nuclear (via trans-activation of β-catenin and Hif1α 20,21 ) functions of Pkm2. Our results show that a short pulse of a pro-proliferative gene, using a highly translatable, clinically adaptable platform is sufficient to induce CM proliferation and cardiac regeneration. Those findings underline the therapeutic potential of  cms Pkm2 modRNA in cardiac disease. 
     Reactivation of CMs proliferation has been a key element in cardiac regeneration strategies. Zebrafish and newt cardiac regeneration is mostly mediated by CMs proliferation 3,5,7,8 . In mammals fetal development, CMs proliferation is a distinct pathway for heart growth and regeneration 9,22.  It has been shown that after injury adult CMs upregulate a subset of fetal genes suggesting that adult CM are not terminally differentiated and possess some degree of cell plasticity 4,9 . Adult mammalian CMs can divide in vitro and in vivo and this ability can be stimulated by upregulating pro-proliferative genes 9,22-33 . Over the years, several publications have shown that reactivation of adult CMs cell cycle re-entry is possible via proteins 23,24,26,30,34  viruses  26,39,31,35  or transgenic mouse models of pro-proliferation genes 25,28,33  Protein administration for the purpose of cell cycle induction is challenging due to the very short half-life, the difficulty of local administration, lack of CMs specificity and the inability to deliver intracellular genes, such as transcription factors. The cardiac specific adeno-associated virus ( cms AAV) vector is not immunogenic and used in many heart studies but has a very long and sustained expression time that may lead to increased uncontrolled CMs size and cardiac hypertrophy and arrhythmia. Although transgenic mice can be used in CM-specific and transient way, they are not clinically-relevant for gene delivery. Challenges with current approaches highlight the need for an efficient gene delivery approach that can safely, and locally deliver cell cycle inducer genes to the CMs, with a transient, efficient, and controlled manner. Pyruvate Kinase Muscle Isozyme M2 (Pkm2) is a cell cycle inducer. During development, Pkm2 is expressed in many adult tissues including the spleen and lung, however during adulthood Pkm2 is strictly expressed in proliferating cells with high anabolic activity 16,17.  Pkm2 was found to increase adult cell and cancer cells proliferation, angiogenesis and prevent apoptosis caused by oxidative stress 18,20,36-42 . Pkm2 exerts its functions by its two distinct functions: In the cytoplasm, Pkm2 shifts the metabolic fate from glycolysis to pentose phosphate pathway (PPP) by reducing the conversion of phosphoenolpyruvate to pyruvate 18,19 . This leads to the accumulation of galactose, a glycolysis intermediate, and activation of PPP via Glucose-6-phosphate dehydrogenase (G6pd) 43-45 . The PPP pathway activation leads to the synthesis of nucleotides, amino acids, and lipids and the production of reduced NADPH, increase nitric oxide synthase and DNA repair 38,39,41,46-48 . In addition, Pkm2 has a role also in the nucleus. Pkm2 directly interacting with the transcription factors β-catenin and Hif1α. This interaction promotes the expression of genes such as in Ccdn1, c-Myc and Vegfa, and Bcl2 20,21  See summary of Pkm2 role in proliferative or cancer cells in  FIG. 5  (S1). 
     Several studies indicate that cell cycle inducer genes can induce CMs to proliferate (8-22). However, activation of these genes for long periods in CMs may lead to CMs hypertrophy and in some cases to hypertrophic cardiomyopathy and HF(14). In addition, systemic delivery of cell cycle inducer genes can lead to uncontrolled cell growth of non-CM cells in the heart and throughout the body, and can raise safety issues. 
     The differential expression of different cell cycle inducer genes in the heart changes during heart development. Others and we focused on two different time points after birth (Day 1 and Day 10) as they represent developmental stages that the heart has regenerative ability via CMs proliferation (day 1) and lacking this ability (day 10). As can be seen in  FIG. 1 a    several cell cycle inducer changes significantly between the two stages of development. However Pkm2 levels in mice hearts are high during fetal development 49 and are very significantly decreased by day 10 after birth. As Pkm2 most highly significant is upstream to several cell cycle inducer genes and his changes is the to Co-immunostaining of Pkm2 and the CMs marker α-Actinin revealed that Pkm2 was highly expressed in CMs during development and at one-day post birth, however, its expression was undetectable 8 weeks after birth ( FIG. 1 b   &amp; c ). Pkm2 expression in the heart post-MI was restricted to immune cells (CD45+) and non-CMs but not upregulated in CMs ( FIG. 11 ). We have restored Pkm2 levels by direct injection of Pkm2 modRNA into the myocardium ( FIG. 1 c   ). Pharmacokinetics study of Pkm2 levels after myocardial injection indicated that Pkm2 protein expression occurred a few hours post injection, and lasted for at least 8 days, but no longer than 12 days ( FIG. 1 d   ). To test the effect Pkm2 expression on CMs proliferation we isolated 4-day old neonatal rat CMs and transfected them with Luc control or Pkm2 modRNAs ( FIGS. 12A-12C ). Pkm2 modRNA was translated 12 hours post transfection and levels remained up to 10 days post transfection ( FIGS. 12A-12C ). Three days post transfection with Pkm2 or Luc modRNAs there was a significant increase in proliferation of Pkm2-transfected CMs ( FIGS. 12A-12C ). To test Pkm2 effects in MI setting, we directly injected Pkm2 or Luc modRNAs into the myocardium immediately after LAD ligation.13-15 One week post-MI and injection, Pkm2 significantly induced proliferation of CMs and non-CMs ( FIG. 1 e - h   ). We hypothesized that the observed improvement in proliferative capacity may translate into better regeneration, and result in improved outcome post-MI. However, inducing non-CMs proliferation in the heart frequently results in undesired effects, mainly by promoting fibrosis and immune response. Hence, we developed a unique CM-specific modRNA (cmsmodRNA) system that is based on two distinct modRNAs ( FIG. 2 ,  FIGS. 13A-13C  and  FIGS. 14A-14C ). The first construct contains L7AE, an archaeal ribosomal protein that regulates the translation of genes containing a kink-turn motif (K-motif), a specific binding site for L7AE.50,51 Translation of L7AE modRNA suppresses the translation of the designed gene of interest modRNA when the two are co-transfected into the cell. By adding a CM-specific microRNA (cmsmiR) recognition element to the L7AE gene 3′UTR, we were able to prevent L7AE translation in CMs that abundantly and mostly exclusively express those miRs, allowing the translation of the gene of interest modRNA strictly in CMs ( FIGS. 13A-13C ). miR1-1 (miR-1), miR-208a (miR-208) and miR-199a (miR-199) are reported to be expressed mostly in CMs.52-54 We tested the expression of those miRs in our model by generating an inactive human CD25 (ihCD25)—a truncated gene containing only the extracellular domain of hCD25− as a reporter gene that can be immunostained when expressed on the surface of cells/tissues. We have designed two versions of the ihCD25 construct, with or without recognition elements for miR-1, miR-208 or miR-199. modRNAs were transfected into neonatal CMs ( FIGS. 14A-14C ), or injected using the MI model ( FIG. 2   ga - c  and  FIGS. 14A-14C ). miR-1 and miR-208 were found to be CM-specific, as indicated by positive ihCD25 staining in non-CM but not in CMs. We designed a L7AE modRNA that contains both miR-1 and miR-208 recognition elements (miR-1-208) ( FIG. 2 d - k   ), and used a nuclear GFP modRNA (nGFP-K) and a Cre recombinase (Cre-K) modRNAs that contains a K-motif. In our MI model, transfection of nGFP-K resulted in the translation of nGFP in both CMs and non-CMs. However, when nGFP-K was co-transfected with miR-1-208, nGFP was exclusively translated in CMs ( FIG. 2 e   &amp; f ). Co-transfection Cre-K with miR-1-208 in our MI model using Rosa26 reporter mice (Rosa26mTmG) resulted in GFP expression strictly in CMs ( FIG. 2 g   ). Injection of Cre-K alone resulted in transfection efficiency of ˜24.8% of heart section/˜2600 cells in left ventricle (both CMs and non-CMs). However, Cre-K+miR1-208 combination resulted in transfection efficiency of 7.7%/˜800 cells of exclusively CMs ( FIG. 2 h   ). We also show that the non-mammalian protein L7AE does not exacerbate immune response post MI ( FIGS. 15A-15C ). We hypothesize that this is due to the already active immune response in the heart immediately post MI. We concluded that the use of L7AE in mice model is immunologically safe. To test the functionality of our cmsmodRNA delivery platform in our MI model, we directly injected Luc-K, miR1-208, Luc K+miR1-208, Pkm2-K, Pkm2+miR1-208 or Pkm2-K+miR1-208 (cmsPkm2). Seven days post transfection we measured the proliferation rate in the heart ( FIG. 2 i   ). Pkm2-K modRNA alone or Pkm2+miR1-208 significantly increased proliferation of both CMs and non-CMs (P&lt;0.001) compared to Luc modRNA ( FIG. 2   j &amp; k ). However, cmsPkm2 modRNA significantly reactivated the proliferation of only CMs (P&lt;0.001), with no significant influence on the proliferation of non-CMs. Using live imaging of neonatal rat CMs for 24 hours, we found that co-transfection of cmsPkm2 modRNA with cmsnGFP modRNA increased CMs proliferation in comparison to transfection with cmsnGFP modRNA alone (Supplemental Movie 1). Additionally, cmsPkm2 modRNA significantly reduced apoptosis and increased capillary density in the myocardium 2 or 7 days post-MI ( FIGS. 16A-16C ). MRI or echo showed that cmsPkm2 significantly increased the percentage of ejection fraction ( FIG. 3 a - d    and Supplemental Movies 2&amp;3) and delta of percentage fractioning shortening from day 2 (baseline) to day 28 post-MI ( FIG. 3 d   ). Left ventricular internal diameter end systole was increased, while left ventricular internal diameter end diastole was significantly increased in cmsPkm2 mice compared to control 28 days post-MI ( FIGS. 16A-6C ). 28 days post-MI, Pkm2 or cmsPkm2 expression significantly reduced cardiac scar formation Additionally, no abnormality in the cardiac tissue (e.g. angioma, edema) was observed post injection of cmsPkm2 ( FIG. 3 e   &amp; f ), heart weight to body weight ratio was significantly increased ( FIG. 3 g   ) while CMs size was significantly decreased, indicating the CMs proliferation ( FIG. 3 h    and  FIGS. 16A-16C ), and capillary density was significantly increased ( FIG. 3 i   ) in Pkm2 or cmsPkm2 modRNA transfections compared to controls. Lastly, cmsPkm2 significantly increased CMs number in the heart without elevating the number of nuclei per CM, while increasing the mononuclear fraction compared to control ( FIG. 3 j - m   ). Importantly, long-term survival curve for mice treated immediately after MI with cmsLuc or cmsPkm2 modRNAs showed significant improvement in mice survival post-MI and cmsPkm2 transfection ( FIG. 3 n   ). To understand the mechanism by which cmsPkm2 improves cardiac function post-MI, we used a lineage-tracing model that combines cmsmodRNAs and R26mTmG ( FIG. 4 a - j   ) to exclusively express Pkm2/Luc in CMs (by mixing cmsPkm2/cmsLuc+cmsCre modRNA; GFP-labeled CMs,  FIG. 4 a   &amp; b ), and trace the fate and properties of transfected CMs over time, after the cmsmodRNA was no longer expressed. The number of CMs transfected with cmsPkm2+Cre modRNAs was higher 3 days post-MI and significantly higher 28 days post-MI compared to control ( FIG. 4 c   &amp; d ). Heart weight to body weight ratio was significantly increased ( FIG. 4 e   ), while GFP +  CMs size ( FIG. 4 f   ) and nuclei number/cell ( FIG. 4 g   ) was significantly decreased in mice treated with cmsPkm2+Cre modRNA compared to control. Importantly, 28 days post treatment with cmsPkm2+Cre modRNA, GFP +  CMs showed elevated expression of proliferative markers such as pH3 and Ki67 ( FIG. 4 h - j   ), long after Pkm2 was not expressed. Changes in gene expression post cmsPkm2 or cmsLuc together with cmsihCD25 modRNAs delivery in MI setting were measured 2 days post injection. Isolated cells were enriched for CMs markers with significantly lower expression of Troponin T ( FIG. 4 k   ). Pkm2 expressing cells significantly upregulated effectors downstream of its cytoplasmic (G6pd) and nuclear (c-Myc, Cyclin D1, Bcl2, VEGF-A and Pdk1) functions ( FIG. 4 l   ). In accordance with the increased proliferation, we observed an upregulation of cell cycle-promoting genes (Cdc20, Cdk1 and Ccnd2, Ccnb1), and downregulation of the cell cycle inhibitors (p21 and p27) in Pkm2+ ihCD25+ CMs compared Luc+ ihCD25+ CMs ( FIG. 4M ). 
     The rapid downregulation of Pkm2 after birth, which coincides with the loss of cardiac regeneration ability, 55  points to its involvement in fetal and neonatal cardiac regeneration. Additionally, its previously-described pro-proliferative and pro-survival roles in cancer, make it an ideal candidate to promote cardiac function/regeneration. Our finding that  cms Pkm2 improves outcome after MI, most likely by improving cardiac function, has physiological and clinical implications, as they underline the potential therapeutic value of  cms Pkm2 expression immediately post-MI. Our results are in agreement with a recent publication showing that a short expression of synthetic miRs is sufficient for the induction of CMs proliferation and cardiac regeneration 56 . In addition, the cardiac specificity of our modRNA along with its short expression time make it a safe and translatable strategy for cardiac regeneration. Our data point to the high potency of Pkm2 and its ability to induce metabolic reprogramming that better supports CMs homeostasis with long-term beneficial effects, lasting weeks after the protein was no longer expressed. Our experimental approach and tools will allow us to further investigate other relevant pro-proliferative and metabolic reprogramming genes and their therapeutic potential in different disease models, and to efficiently and precisely study CMs cell fate. Notably, our isolation approach using  cms ihCD25 ( FIGS. 17A-17C ) overcomes the challenge of FACS sorting of adult transfected CMs. 31  This study pioneers the use of  cms modRNAs to manipulate cellular behavior and holds a great therapeutic potential for cardiac disease, as modRNA is a safe, transient, local, and non-immunogenic platform for gene transfer. 
     The field of cardiac gene therapy is expanding, yet its use in the clinical setting is limited. Currently the most widely used method for targeting gene expression to the heart is through viral vectors, particularly the adeno-associated virus (AAV) vector (1-3). During the past few decades several attempts were made to insert genes of interest into CMs using adenovirus, associate adeno virus (AAV), lentivirus and DNA plasmid. While both AAV and adenovirus possess high CM transfection levels, lentivirus and DNA plasmid CMs transfection efficiency is low. Adenoviruses can elicit a robust immune response, leaving only AAV as a suitable option for gene delivery system to the heart. Using CMs-specific promoters in AAV may allow for cell-cycle inducers gene expression strictly in CMs, however its pharmacokinetics in the heart (expression starts at day 4 and remains for at least 11 months) may lead to uncontrolled growth and hypertrophic cardiomyopathy and HF (3, 5). Additionally, over 60% of healthy human individuals possess neutralizing antibodies directed against the AAV capsid that can efficiently neutralize gene expression delivered by this method (21). Viral gene therapy shows promise yet its applications are limited due to its length of expression and inability to regulate gene expression in a quantifiable dose manner (1-3). 
     While the use of unmodified exogenous RNA as a gene delivery method is appealing because it may be safer than plasmid DNA owing to a reduced risk of genomic integration, it is ineffective due to its instability outside the cell and the strong innate immune response it elicits when transfected into cells (10,11). 
     Kariko et al. discovered that the substitution of Uridine and Cytidine with Pseudouridine and 5-methylcytidine, respectively, drastically reduced the immune response elicited from exogenous RNA (11,12). In order to increase stability and translational efficiency, a 3″-O-Me-m7G(5′)ppp(5′)G Anti Reverse Cap Analog (ARCA) cap is substituted at the 5′ end of the RNA molecule (4,5,10). Modified mRNA (modRNA) therefore provides a novel and effective gene delivery method that provides short-term (1-2 weeks), titratable gene expression for use both in vitro or in vivo (4-9). 
     Modified mRNA (modRNA) has emerged as an effective and safe tool for somatic gene transfer, and has been successfully used by us and others for gene delivery to the heart. 10,12-15  Here we show that Pyruvate Kinase Muscle Isozyme M2 (Pkm2), a pro-proliferative factor, frequently dysregulated in cancer, 16,17  is highly expressed in regenerative fetal and early neonatal CMs, but not in adult CMs. Restoration of Pkm2 levels using modRNA delivery exclusively into adult CMs ( cms Pkm2) post-MI significantly and exclusively induced CMs proliferation, and was associated with improved cardiac function, reduced scar size, increased heart to body weight ratio, reduced CMs size, reduced apoptosis and increased capillary density. Those regenerative processes translated into increased long-term survival post-MI. Using lineage tracing and isolation of Pkm2-transfected CMs followed by gene expression analysis post-MI we show an increase in number of Pkm2-transfected CMs colonies and the potential involvement of key downstream effectors of the pro-proliferative cytoplasmic (via the pentose phosphate pathway (PPP) 18,19) and nuclear (via trans-activation of β-catenin and Hif1α 20,21 ) functions of Pkm2. Our results show that a short pulse of a pro-proliferative gene, using a highly translatable, clinically adaptable platform is sufficient to induce CM proliferation and cardiac regeneration. Those findings underline the therapeutic potential of  cms Pkm2 modRNA in cardiac disease. 
     It has recently been shown (1) that by using modified mRNA (modRNA) technology, modRNA can drive a transient, safe gene expression in the heart with high transfection levels without eliciting immune response or compromising the genome(5, 22). Exogenous unmodified mRNA that enters the cell via the cell membrane is recognized by endosomal Toll-like receptors 7/8 and 3(23, 24). This process inhibits protein translation and activates the innate immune response, ultimately leading to apoptosis of the hosting cell. ModRNA is synthesized by substituting ribonucleotides with naturally modified ribonucleotides. The use of these modified ribonucleotides results in changing the secondary structure of the synthesized mRNA, which prevents the Toll-like receptors from recognizing the modRNA and therefore permitting its translation to a functional protein by the ribosomal machinery without eliciting immune response or compromising the genome (5, 22). 
     Applicants previously showed that modRNA transfects different cell types in the heart including CMs with high efficiency, leading to immediate and high levels of protein expression in a transient, pulse like kinetic (duration of 3-5 days in vitro and 7-10 days in vivo). Co-transfection of two individual modRNAs resulted in co-translation of both. Using the MI model (5) and Luc, LacZ and nGFP modRNAs delivery in myocardium, Applicants show that the cardiac tissue after MI is well transfected with modRNA and several cell types such as CMs and non-CMs are highly transfected in the left ventricle. Applicants then selected several candidate cell cycle inducer genes that had previously been shown to have the ability to induce neonatal CMs during cardiac development (CDK2, β catenin) (16) or reactivation of adult CMs proliferation in transgenic mouse models (CyclinD2, cMYC)(12, 14) and others that had shown robust proliferative potential in different organs and cell types but had never been tested in cardiomyocytes and heart (Lin28, PKM2)(24, 25). 
     Generally, a platform for making cell specific modified mRNA (modRNA) is as follows. 
     First, choose a cell type of interest for making cell specific modRNA. Identify candidate microRNA (miR) that have been reported to express in the cell of interest and preferably only in the cell of interest (e.g., in the case of cardiomyocytes, miR1, miR29, miR126, miR133, miR199, miR208, miR378). Identify reverse complement sequences for each miR sequence that allows recognition of the specific miR to this sequence. Add to 3′UTR each of the previous calculated miR reverse complement sequence to ihCD25 k motif, a truncated receptor for hCD25 carrying a k motif. This allows ihCD25 to express only in those cells that are lacking the specific miR that the reverse complement sequence is targeting. 
     Co-transfect a mixture of cells that contains the cell of interest and other cell type (e.g fibroblasts) as well with nGFP modRNA and with different miR-ihCD25 modRNAs. After about 18 hours, fix the cells and stain the cells for GFP (show transfected cells with modRNA) and for reporter gene (with anti hCD25, show cells that are lacking the miR that was target) and cell specific markers (e.g., for cardiomyocytes Troponin I, for endothelial cells, Pecam1, etc.). 
     Identify GFP-positive cells that are also positive for cell specific marker (e.g Troponin I for cardiomyocytes) but negative for reporter gene (hCD25). This means that this specific miR-ihCD25 was not translated although the modRNA was delivered to this cell type. This will indicate that this miR is specifically expressed in the cell type of interest and can be used to create cell specific modRNA. Create cell specific modRNA by adding to the 3′UTR of L7AE the sequence that inhibits ihCD25 in the cell of interest. Co-transfect with mir-L7AE and gene of interest that carrying in his 5′ UTR k-motif. These two modRNAs will allow you to specifically deliver a gene of interest to a specific cell type. 
     In one embodiment, Applicant designed and generated modRNAs for each of the above genes. Using rat neonatal CMs, Applicant tested the translation of each modRNA. In addition, the functionality of the protein was tested by measuring the proliferation rate of rat neonatal CMs with control and the candidate cell inducer modRNAs. All candidate cell cycle inducer modRNAs increase the proliferation of neonatal rat CMs and adult CMs proliferation after MI to various extents. Both Lin28 and PKM2 significantly increased CMs proliferative capacity. Therefore, those genes were selected for further investigation. 
     Lin28 is a known suppressor of Let7 that tightly controls cell cycle regulators (25-29). To test whether Lin28 induces cell cycle regulators, nGFP (control modRNA) or Lin28 modRNA was injected immediately after LAD ligation and found a significant increase in the expression of Ccnb1, Ccnb2, Cdc20, Cdk1 and Aurka cell cycle genes after 3 days using RT-PCR. The use of cell cycle inducer modRNAs such as Lin28 modRNA in a non-specific manner increases proliferation not only in CMs, but also non-CMs representing an experiential challenge since the model and hypothesis were aimed to test aimed to test CMs proliferation as a mean to achieve increased cardiac regeneration. 
     To address this challenge Applicants designed a CM-specific modRNA system that is based on two distinct modRNAs ( FIG. 5 ). The first construct is a suppressor modRNA the carries L7AE, an archaeal ribosomal protein that regulates the translation of a designed gene of interest modRNA with kink-turn motif—a specific binding site for L7AE(30, 31). Translation of L7AE modRNA will suppress the translation of the designed gene of interest modRNA when the two are co-transfected into the cell. By adding a CMs-specific microRNA (miR) recognition element to the L7AE gene, we are able to prevent L7AE translation in CMs that abundantly and mostly exclusively express the miR (“suppress the suppressor” approach) allowing the translation of the gene of interest modRNA strictly in CMs. It was shown previously, using miR recognition element, results in a reduction of the number of copies of the targeted miR (32,33). Reduction in number of miRs in the heart can be detrimental or beneficial to the heart (33-42). In our approach we need to be sure we don&#39;t reduce miR expression that is beneficial to cardiac regeneration but rather reducing miR expression that is detrimental to cardiac regeneration. miR1-2 (miR1), miR208a (miR208) and miR199a (miR199) are expressed mostly in CMs (33, 39, 41). miR1 and miR208 were found to be upregulate after MI in adult animal study and humans (33, 38, 41, 43). miR1 and miR208 up regulation has detrimental effects, while its down regulation has beneficial effects after MI and heart diseases (32-42) 
     To test the expression of these miRs in CMs, we have made an inactive human CD25 (ihCD25) gene, a truncated gene containing only the extracellular domain (ECD) of hCD25− as a reporter gene. We have designed two versions of the ihCD25 construct, with or without the miR recognition elements for miR-1, miR-208 or miR-199. We then transfected the modRNAs into neonatal CMs in vitro and in vivo using the MI model ( FIG. 2 ). As can be seen in  FIG. 6  both miR-1 and miR-208 were found to be CM-specific, as translation of ihCD25 was observed in non-CM but not in CMs. In contrast, modRNAs with or without miR-199 recognition element was found not to be CMs specific, in vitro and in vivo. Next, we designed a L7AE modRNA that carries both miR-1 and miR-208 recognition elements (L7AE miR-1+miR-208). We have also generated a nuclear GFP modRNA (nGFP-k-motif) and a destabilized Cre recombinase (DD-Cre-k motif) modRNAs that includes the k-motif (L7AE recognition site). Using our adult mouse MI model, we show that transfection of nGFP-k motif resulted in the translation of nGFP in both CMs and non-CMs ( FIG. 7 ). However, when nGFP-k motif was co-transfected with L7AE miR-1+miR-208 only CMs translated the nGFP. In addition, co-transfecting L7AE miR-1+miR-208 with a DD-Cre-k motif in a MI model using Rosa26 Tdtomato  resulted in gene activation (Tomato fluorescence) strictly in CMs. The combination of these two methods allows us to elegantly express our gene/genes of interest exclusively in CMs, and to allow for linage tracing over longer time periods after the gene of interest modRNA is no longer expressed. 
     To test the functionality of our CMs-specific modRNA, we directly inject Luc control modRNA or Lin28-K and PKM2-k motif modRNA alone or together with L7AE miR-1+miR-208 (Lin28/PKM2 CMs specific modRNA) using our MI model. Seven days post transfection we measured the proliferation (using hallmark proliferation markers, BrdU, Ki67, H3P and Aurora B) of both CMs and non-CMs. As depicted in  FIG. 8  Lin28-k or PKM2-k motif modRNA alone significantly increased proliferation of both CMs and non-CMs (P&lt;0.001) in comparison to Luc modRNA. However, Lin28 and PKM2 CMs-specific modRNA significantly reactivated the proliferation of only CMs (P&lt;0.001), with no significant influence on the proliferation of non-CMs. Importantly, since L7AE is not a mammalian protein, to test the immunogenicity of L7AE after MI we have injected Luc control modRNA or L7AE modRNA with or without miR-1, miR-208 or both in adult mouse MI model. As can be seen in  FIG. 8  we did not witness significant elevations in immune response and increased apoptosis with all L7AE modRNAs after 7 day post MI. We concluded that the use of L7AE in mice is immunologically safe. 
     Plasmids 
     pTEMPLZ is a cloning vector into which an ORF of interest can be inserted between the UTRs. In one embodiment, plasmids for use in the disclosed method include those shown in Table 1. 
                         TABLE 1                  1   No miR-L7AE                                     2   miR 1-L7AE                                     3   miR 208a-L7AE                                     4   miR 1-miR 208a-L7AE                                     5   Lin28-K motif                                     6   Pkm2-K motif                                     7   nucGFP-K motif                                     8   ihCD25 K motif-non modified                                                                            
ihCD25 modRNA-Based CM-Specific Cell Sorting
 
     To test whether our novel CMs-specific modRNA-based design that allows a transient gene expression of our genes of interest exclusively in CMs can be used to isolate only transfected CMs with innovative inactive (only extracellular domain) human CD25 (ihCD25)-based sorting (magnetic beads) system we injected nGFP k-motif with ihCD25 k-motif modRNA in heart after MI, cells were Isolated and sorted out with CD25-specific magnetic beads. Both nGFP-positive CM&#39;s and non-CM&#39;s were observed. When nGFP k-motif, ihCD25 k-motif with L7AE miR1-miR208 were co-transfected and without magnetic separation, CM specific nGFP expression was seen. The culture also contains nGFP-negative CMs and non-CMs. When magnetic separation was applied, only pure nGFP-positive CMs were observed ( FIG. 9 ). 
     In one embodiment, production of exogenous genes is driven by expression of anti-miRs from a first replicon that also encodes a repressor protein. Expressed anti-miRs bind miRs that occur naturally in human and primate cardiomyocytes and transcription of the repressor protein is prevented. In the absence of repressor protein, expression of a gene of interest from a second replicon encoding the gene and containing the repressor protein recognition site can proceed. 
     Cell Cycle Inducer Genes 
     Expression of a gene of interest, for example, a proliferation-inducing gene can be made cardiomyocyte-specific by placing transcription/translation of the gene under the control of a transcription/translational regulatory system in which one of a pair of nucleic acids encodes an anti-microRNA (anti-miR) that binds specifically to a target cardiomyocyte-specific miR. A second nucleic acid translation suppressor protein and a second nucleic acid that comprises a suppressor protein interaction motif that binds the translation suppressor protein and a gene that encodes a protein of interest. 
     Using the method described herein, the expression is transient, avoiding the problems associated with unlimited expression, such as hypertrophy. 
     Repressor/suppressor protein that binds a specific RNA motif inserted in the 5′-untranslated region of an mRNA modulates the translation of that message in mammalian cells. The expression specificity to human and primate cardiomyocytes is achieved by the inclusion in the repressor/suppressor oligonucleotide of a sequence that encodes a recognition element specific to endogenous miRNAs found in mouse, pig, human and non-human primate cardiomyocytes. 
     The synthesis of modRNA for in vivo use involves four stages: DNA template creation containing the desired transcript, in vitro transcription (IVT), 5′ phosphate removal with Antarctic phosphatase, and precipitation with 5M ammonium acetate salt. Investigation into the use of modRNA for experimental and clinical purposes is growing rapidly. Daily transfection with modRNA encoding reprograming factors OCT4, SOX2, MYC, and KLF4 were successful at reprogramming human fibroblasts back to pluripotency (5,8). Additionally, modRNA has been shown to be capable of directing cell fate in vitro by using MyoD modRNA that resulted in the conversion of fibroblasts to skeletal muscle cells (2). ModRNA has also shown promise in directing cell fate in vivo. The expanding use of modRNA technology in vivo and its potential use in the field of cardiac gene therapy motivated us to generate a step-wise, streamlined protocol for the effective synthesis of modRNA for in vivo use. 
     Cardiomyocytes 
     In one embodiment, the present disclosure relates to a method of treating a subject following myocardial infarction (MI) or heart failure (HF) in a subject comprising administering an effective amount of a composition comprising at least two synthetic modRNAs to a subject in need thereof. 
     Protein expression lasts for from 5 to 20 days, in some embodiments from 7 to 14 days, and results in a low immunological response as compared to non-modified RNA. 
     Inter alia, the present disclosure describes a new set of candidate cell cycle inducer genes: Lin28, Pkm2, and Cyclin D2, which when delivered as modRNA, can reactivate mammalian cardiomyocyte (CM) proliferation in vivo (without increasing CM size or nuclei number), reduce CM apoptosis and increase overall left ventricle vascularization post myocardial infarction (MI). When expression of the cell cycle inducer genes is placed under the control of a transcriptional/translational regulatory (an expression regulatory) system for cardiomyocyte-specific transcription (expression), the result is a tool for cardiomyocyte specific expression of the cell cycle inducer gene-driven proliferation following injury, for example, as the result of myocardial infarction (MI) or heart failure (HF). 
     Modified mRNA (modRNA) is a safe, efficient, transient, and non-immunogenic gene delivery system that allows one to investigate the effect of cell cycle inducer genes on CMs following MI or HF. Kariko et al. discovered that the substitution of uridine and cytidine with pseudouridine and 5-methylcytidine, respectively, drastically reduced the immune response elicited from exogenous RNA (11,12). Investigation into the mechanism revealed that the nucleoside substitutions resulted in a conformational change in the RNA that caused reduced response by toll-like receptors 3, 7, and 8 (TLR3, TLR7, TLR 8), and retinoic acid-inducible gene 1 (RIG-1) (13). A further decrease in RIG-1 response from modRNA was seen upon removal of the 5′ triphosphates (4,10). In order to increase stability and translational efficiency, a 3″-O-Me-m7G(5′)ppp(5′)G Anti Reverse Cap Analog (ARCA) cap is substituted at the 5′ end of the RNA molecule (4,5,10). 
     Cell Selection by Anti-hCD25 Affinity 
     Cell selection of cardiomyocytes by traditional FACS cell sorting can be problematic due to the size of the cells. An alternative approach to cell selection was devised. In one embodiment, using the transcription regulatory system of the disclosure, a nucleic acid that encodes the hCD25 extracellular domain (ECD) is included in the construct that contains the nucleic acid that encodes the gene of interest. To isolate CMs that transiently express either control or candidate gene modRNA, cells that co-express the hCD25 ECD plus the gene of interest are selected using an anti-CD25 ECD antibody in an affinity chromatography column or using a panning method. These cells are used to generate gene expression profiles using RNA-seq technique, and identify differentially expressed genes 
     EXAMPLES 
     Example 1: Materials 
     The following materials are used in conjunction with the disclosed method. 
     All solutions should be made in Nuclease Free water unless otherwise specified. All materials used in this protocol should be nuclease free. 
     Equipment used includes the following:
         1. PCR thermocycler   2. Microfuge   3. Vortex mixer   4. Thermomixer (EPPENDORF™)   5. Nano-Drop   6. Nuclease-free water   7. 15 ml Nuclease Free conical tubes   8. Nuclease free strip PCR tubes   9. Ethanol (100% and 70%)   10. 2 ml Ambion Elution Tubes       

     Primers used for tail PCR are as follows: 
     
       
         
           
               
            
               
                 Forward Primer:  
               
               
                 (SEQ ID NO: 9) 
               
               
                 5&#39;-TTG GAC CCT CGT ACA GAA GCT AAT ACG-3&#39;  
               
               
                   
               
               
                 Reverse Primer:  
               
               
                 (SEQ ID NO: 10) 
               
               
                 5&#39;-TTT TTT TTT TTT TTT TTT TTT TTT TTT TTT TTT  
               
               
                   
               
               
                 TTT TTT TTT TTT TTT TTT TTT TTT TTT TTT TTT TTT   
               
               
                   
               
               
                 TTT TTT TTT TTT TTT TTT TTT TTT TTT TTT TTT TTT   
               
               
                   
               
               
                 TTT TTT TTT TTT TTT TCT TCC TAC TCA GGC TTT ATT 
               
               
                   
               
               
                 CAA AGA CCA-3&#39; 
               
            
           
         
       
     
     Construction of DNA template for in vitro transcription using pTEMPLZ plasmid is as follows:
         1. T4 Polynucleotide kinase enzyme   2. 100 mM ATP   3. 2×KAPA HiFi HotStart ReadyMix PCR master mix   4. Alel enzyme   5. Afel enzyme   6. Antarctic phosphatase enzyme   7. T4 DNA ligase enzyme   8. One Shot® ccdB Survival™ 2 T1 Phage-Resistant (T1R) cells   9. QIAquick® gel extraction kit   10. QIAquick® PCR purification kit   11. One shot chemically competent  E. coli      12. QIAprep® spin Miniprep kit   13. 10× Phosphorylation Buffer       

     Synthesis of Linear DNA Template with a Poly T Tail for IVT Reaction
         1. 2×KAPA HiFi HotStart ReadyMix PCR master mix   2. Primer Solution: 1 μM each of forward and reverse primer   3. Dpnl enzyme   4. QIAquick® PCR purification kit       

     In Vitro Transcription Reaction
         1. Ambion T7 Megascript Kit (life technologies Cat #: am1334-5)   2. GTP 75 mM solution (provided in Megascript® kit)   3. ATP 75 mM solution (provided in Megascript® kit)   4. CTP 75 mM solution (provided in Megascript® kit)   5. 5-Methylpseudouridine-5′-Triphosphate (Trilink)   6. Trilink Biotechnologies Anti Reverse Cap Analog, 3′-O-Me-m 7 G(5′)ppp(5′)G 10 μmoles (Cat #: N-7003)   7. T7 TURBO DNase enzyme (provided in Megascript kit)   8. Ambion MEGAclear™ Transcription Clean-Up kit. (life technologies; cat #: AM1908).       

     RNA Phosphatase Treatment
         1. Antarctic phosphatase enzyme       

     RNA Precipitation
         1. 5M Ammonium Acetate Salt solution (Provided in AmbionMEGAclear™ Kit)   2. Elution Buffer (Provided in AmbionMEGAclear™ Kit)       

     Preparation for modRNA injection
         1. Lipofectamine® RNAiMAXtransfection reagent (Thermofisher Cat #: 13778150)   2. OptiMEM Reduced Serum Medium, no phenol red   3. Ultra-Fine insulin syringe needle 31 g 8 mm       

     Methods 
     The following methods are used in conjunction with the disclosed method. All procedures were carried out at room temperature, in a non-sterile environment unless otherwise specified. All materials used should be nuclease free. 
     Synthesis of modRNA 
     ModRNAs were transcribed in vitro from a plasmid templates using a custom ribonucleotide blend of anti-reverse cap analog, 3″-O-Me-m7G(5′)ppp(5′)G (6 mM, TriLink Biotechnologies), guanosine triphosphate (1.5 mM, Life Technology), adenosine triphosphate (7.5 mM, Life Technology), cytidine triphosphate (7.5 mM, Life Technology) and N1-Methylpseudouridine-5′-Triphosphate (7.5 mM, TriLink Biotechnologies) as described previously. 48-50  mRNA was purified using megaclear kit (Life Technology) and was treated with antarctic phosphatase (New England Biolabs), followed by re-purification using Megaclear kit. mRNA was quantitated by Nanodrop (Thermo Scientific), precipitated with ethanol and ammonium acetate, and resuspended in 10 mM TrisHCl, 1 mM EDTA. For a detailed protocol please see our recent publication. 48    
     modRNA transfection. In vivo transfection of modRNA was done using sucrose citrate buffer containing 20 μl of sucrose in nuclease-free water (0.3 g/ml), 20 μl of citrate (0.1M pH=7; Sigma) mixed with 20 μl of different concentrations of modRNA in saline to a total volume of 60p1. The transfection mixture was directly injected (3 individual injections, 20 μl each) into the myocardium. For in vitro transfection, we used RNAiMAX transfection reagent (Life Technologies) that was used according to manufacturer&#39;s recommendation. 
     Construction of DNA Template for In Vitro Transcription Using pTEMPLZ Plasmid Carrying k Motif. 
     pTEMPLZ is a cloning vector into which an ORF of interest can be inserted between the UTR&#39;s ( FIG. 1 ). The 5′- and 3′-UTRs are synthesized de novo by synthetic oligos. The synthesized UTR&#39;s are annealed together and amplified using forward and reverse primers. To provide an entry site for the ORF, Alel and Afel restriction sites are introduced in between 5′ and 3′ UTRs. (The adenine nucleotide (A) of the 1st codon (ATG) can be omitted from forward primer sequence as it is provided by the Alel site.) The PCR-amplified fragment and pZErO-2 vector with ampicillin resistance are digested with HindIII and NotI, and ligated together to create pTEMPLZ. (pTEMPLZ plasmid and derivatives should be propagated in bacterial strain resistant to the ccdB gene product such as One Shot ccdB Survival™ 2 T1 Phage-Resistant (T1R) cells.) 
     Before insertion into pTEMPLZ, the ORF is amplified by using phosphorylated forward and reverse primer pair for the gene of interest. Phosphorylation of the primers is done using the T4 polynucleotide kinase enzyme according to reaction below: 
                                                    10 x Phosphorylation Buffer   5    μl           Forward primer (100 μM)   3    μl           Reverse Primer (100 μM)   3    μl           100 mM ATP   0.5    μl           T4 polynucleotide kinase   10    U           Nuclease free Water   50    μl                        
Incubate reaction at 37° C. for 1 hour.
 
To inactivate enzyme, the reaction is incubated at 65° C. for 20 min. The reaction is diluted to 300 μl by adding 250 μl of water giving final 1 μM primer mixture.
 
     Amplification of the ORF of interest is done using the PCR reaction below: 
                                                    Primer mix (1 μM) from above   10    μl           Template DNA   1-100   ng           Water   50    μl           HiFi HotStart ready mix (2 x)   25    μl                        
The mixture is run in Thermocycler with settings according to  FIG. 2 . The amplified target is isolated using QIAquick gel extraction kit. Before insertion of ORF, pTEMPIz is linearized and dephosphorylated. To linearize plasmid, pTEMPIz is digested with Alel and Afel according to the reaction below:
 
                                                    pTEMPLZ Plasmid DNA   2    μg           Nuclease Free Water   30   μl           10 x Buffer 4   3   μl           Alel   5   U           Afel   5   U                        
Incubate in thermomixer for 1 hour at 37° C. Digest was purified using QIAquick® PCR purification kit and eluted in 30 μl of elution buffer.
 
     Linearized pTEMPLZ is dephosphorylated according to reaction below: 
                                                    Linearized μlasmid from step 1   30   μl           10 x antarctic phosphatase buffer   5    μl           Antarctic phosphatase   5    U           Nuclease Free Water   50   μl                        
The reaction is incubated at 37° C. for 1 hour. Enzyme is inactivated by incubating at 65° C. for 15 min.
 
     Linearized and dephosphorylated plasmid is isolated using QIAquick gel extraction kit and the quantity of pTEMPLZ product is determined using nanodrop. Plasmid can be stored in −20° C. for future use. 
     Blunt end ligation of ORF of interest is performed into pTEMPLZ according to the reaction below: 
                                    Linearized dephosphorylated TEMPlz μlasm id   50 ng       Amplified ORF    3-fold molar excess       10 x T4 DNA ligase buffer   2 μl       T4 DNA ligase   4 U       Nuclease Free Water   20 μl                    
Mix reagents and incubate overnight on melting ice at room temp or at 16° C. Negative control ligation reaction might be necessary to monitor self-ligation of plasmid. Transformation of plasmid is performed with competent cells and grow on an ampicillin agar plate.
 
     To isolate positive clones with the correct orientation colony PCR is performed. Between 8-10 colonies are extracted from ampicillin agar plates with a pipette tip. Individual tips are stabbed in 200 μl of Luria Broth (LB) and rinsed several times in 75 μl of TE buffer under pH 8.0, tips are incubated in 37° C. in a shaker. Tubes are then boiled for 5 min to lyse bacteria and spun to pellet debris. Colony PCR is performed with 2 μl of supernatant using forward primer and gene specific reverse primer. PCR sample is run on 1% agarose gel to identify clones with positive orientation. 200 μl of LB is cultured with correct orientation clones in larger volume of LB overnight in a 37° C. shaker and extracted using QIAprep® spin Miniprep kit. The quantity of plasmid product is determined using NANODROP™ (Thermo Fisher Scientific) and diluted to a concentration between 1-5 ng/ul. 
     Synthesis of Tailed DNA Template 
     A 1600 μl PCR master solution was prepared according to the reaction below: 
                                                    Plasmid solution (1-5 ng/μl) (see Note 3)   400    μl           Primer solution (1 μM primers)   400    μl           2X KAPA HiFi HotStart ReadyMix.   800    μl                        
50 μl of PCR master solution was aliquoted into 32 separate PCR tubes. PCR is run using the thermo cycler (setting listed in  FIG. 2 ). The length of elongation step may vary depending on DNA polymerase used and ORF length (If using 2×KAPA HiFi HotStart ReadyMix, for example, elongation step of Thermocycler should be set at a ratio of 30 sec per Kb of ORF length.).
 
     To digest methylated plasmid DNA, product is combined into one EPPENDORF™ tube and digested with 30 μl of Dpnl. The PCR product is purified using QIAquick PCR Purification Kit (Qiagen cat #: 28106) and the final product eluted in nuclease free water. The concentration of tailed product is measured using nanodrop machine and concentration is adjusted using nuclease free water to 100-200 ng/μl. 
     For quality control analysis, purity of Tailed DNA template product is checked on a 1% agarose gel together with the original DNA plasmid ( FIG. 3 a   ). 
     In Vitro Transcription (IVT) Reaction (1 ml Reaction Volume) 
     A custom NTP mixture is prepared in one EPPENDORF™ tube according to Table 2. Reagents for IVT reaction are mixed in the following order into one EPPENDORF™ tube:
         a. 400 μl of custom NTP&#39;s from table 2.   b. 400 μl of the DNA tailed template (200 ng/μl).   c. Vortex 10× Reaction Buffer from the T7 megascript kit to dissolve any precipitate and add 100 μl.   d. Add 100 μl of T7 Enzyme. This will give you a 1 ml IVT reaction   e. Mix thoroughly and Incubate in thermomixer at 37° C. for 4-6 hours. 30 μl of T7 Turbo DNase was added and mixed gently and then incubated at 37° C. in Thermomixer for 15-20 min to halt the reaction (see Note 5).
 
Purify reaction using Ambion MEGAclear™ Transcription Clean-Up kit and elute each tube three times with 50 μl of 95° C. elution buffer to obtain 150 μl of RNA product in each tube. Combine the RNA mixture from each tube into one EPPENDORF™ tube.
       

     RNA Phosphatase Treatment 
     Nuclease-free water is added to the RNA to obtain a 1.5 ml solution. 150 μl of Antarctic Phosphatase Buffer (10×) and 150 μl of Antarctic Phosphatase enzyme is added, mixed thoroughly and incubated in thermomixer at 37° C. for 1 hour. 
     RNA Precipitation Using Ammonium Acetate 
     The 1800 μl RNA solution is transferred to a 15 ml conical tube. 180 μl of 5M ammonium acetate is added and mixed thoroughly. 5200 μl of cold (−20° C.) 100% ethanol is added to solution and aliquoted into 3-4 2 ml EPPENDORF™ tubes. Let tubes stand in −20° C. overnight. The tubes are centrifuged at 10,000 rpm for 30 min at 4° C. The supernatant is then carefully discarded. Each pellet is dissolved in 500 μl of 70% ethanol. modRNA ethanol solutions from each tube are consolidated into 1 EPPENDORF™ tube. The tube is centrifuged at 10,000 rpm for 30 min at 4° C. The supernatant is gently poured out and discarded, and using a kimwipe, the inside of the tube is gently cleaned. Care is taken not to disturb the pellet. The tube is inverted and let stand for no more than 2 min to air-dry pellet. Using a pipette, any small drops of ethanol left around the pellet are gently removed. The pellet is resuspended using 45-50 μl of elution buffer. modRNA is left in elution buffer for 5 min then gently pipetted until the pellet is dissolved. RNA solution can now be used in vivo, stored in −20° C. for up to 6 months, or −80° C. for 5 years. 
     ModRNA Yield 
     Concentration is measured using nanodrop machine ( FIG. 3 b   ). The ratio of A260/A280 should be greater than 1.8 with values closer to 2.0 indicating higher purity. Depending on yield, concentration should be close to 20 μg/ul. For better quality control analysis a 1 μl sample from the final modRNA solution is diluted in 100 μl of nuclease free water. The sample is analyzed using a bioanalyzer machine ( FIG. 3 c   ). 
     Preparation of modRNA for Myocardial Injection in Mice 
     40 μl of RNAiMax is combined with 5 μl of OptiMEM in an EPPENDORF™ tube and vortexed. The mixture is allowed to sit for 10 min at room temperature. In another EPPENDORF™ tube 150-200 ug of modRNA is combined with 5 μl of OptiMEM. The tube is spun down to eliminate liquid on the sides of the tube. After letting the RNAiMAX and OptiMEM mixture sit for 10 min at room temperature, the liquid from the tube with the modRNA mixture is added to the tube with the RNAiMAX mixture. (In some embodiments, it is important to add the modRNA mixture to the RNAiMAX mixture and not the other way around.) The combined mixture is allowed to stand for 15 min at room temperature. The mixture is extracted into a 31 gauge insulin syringe and injected into mouse myocardium. (Example of result shown in  FIG. 4 ). 
     Mice 
     All animal procedures were performed under protocols approved by the Icahn School of Medicine at Mount Sinai Institutional Care and Use Committee. CFW (Swiss Webster) mice or Rosa26 mTmG  mice, male and female, were used. ModRNAs are synthesized by in vitro transcription as described above. Modified nucleotides (Trident) are pseudouridine, 5-methyl-cytidine, and cap analog. A total of 100-200 μg modified RNA complexed with RNAiMax transfection reagent is injected into the peri-infarct region of the myocardium in an open chest surgery post induction of MI. MRI is performed under light anesthesia (titrated to heart rate and sedation level). LAD ligation and histological analysis is performed as described previously (46.). Three to eight animals used for each experiment. For long-term survival, CFW (8-10-week-old) treated with CM-specific Luc or Pkm2 modRNAs (n=10) post induction of MI, and were left to recover for 6 months in the animal facility. Deaths were monitored and documented over time. 
     Isolation of Cells from Adult Mice Heart 
     Hearts are excised and perfused using the Langendorff technique, the cells are processed further by using CD25 specific magnetic beads (dynabeads CD25, Thermo Fisher Scientific) and RNA is isolated from cells using a RNeasy mini kit (Qiagen). The RNA is further used for RNA-seq and RT-PCR analysis. 
     Adult Mouse Myocardial Infarction and Heart Failure Models 
     The MI model described in  FIG. 5  was used to test the therapeutic effect after treatments with CMs-specific Lin28, Pkm2 in CFW or Rosa26 CD25  mice. The experimental design includes 4 control groups treated with a) vehicle only, b) 100 μg/heart Luc carrying a K-motif modRNA, c) 100 μg/heart L7AE modRNA that carry both miR-1 and miR-208 recognition elements (L7AE miR1+miR208) and d) Luc CMs specific modRNA contain a mixture of Luc carrying a K-motif modRNA and L7AE miR1+miR208, 100 μg/heart from each modRNA (total 200 μg/heart). Controls groups will serve to assess any unspecific effect of reduction of miR-1 and miR-208 in the heart that is not directly related to cell cycle inducer CMs-specific modRNAs. Applicants compared the control groups with 4 experimental groups using 100 μg/heart of a) Lin28 and b) Pkm2, carrying a K-motif modRNA and mixture of d) Lin28, and e) Pkm2, carrying a K-motif modRNA with L7AE miR1+miR208 (cell cycle inducer CMs specific modRNA, 100 μg/heart from each modRNA with (total modRNA 200 μg/heart). We will analyze improved cardiac function after 28 days post MI using MRI and reduced scar formation, and increased capillary density into higher rates of long term survival in comparison to control modRNA. We will also use the Rosa26Tdtomato mice for lineage tracing model of the transfected CMs in MI model. Our experimental design includes 1 control group treated with CMs-specific Luc (50 μg/heart) and DD-Cre (50 μg/heart) mixed together with 100 μg/heart L7AE miR1+miR208. We will compare the control groups with 3 experimental groups treated with CMs-specific cell cycle inducer modRNAs. We will use a mixture containing DD-Cre (50 μg/heart) and cell cycle inducer gene (Lin28 and Pkm2 50 pg/heart) carrying a K-motif modRNA mixed together with 100 μg/heart of L7AE miR-1+miR-208 (total modRNA 200 μg/heart). The heart of Rosa26 Tdtomato  mice, will be directed intramuscular injected with total of 100 or 200 μg modRNA. Using CMs lineage tracing model we will test 28 days post injection transfect CMs size using CMs cross-section area evaluation with anti wheat germ agglutinin (WGA) antibody in immunofluorescence analysis. We will count the number of transfected CMs per left ventricle and evaluate the number of nuclei per CMs in each of the treatments. These testing using Rosa26 Tdtomato  mice will allow us to evaluate the changes in CMs function after different treatments with CMs specific modRNAs over time. 
     The MI model described in  FIG. 5  was also used for testing gene expression changes in transfected CMs and non-transfected cells of the left ventricle. Our experimental design includes 3 control groups treated with different CMs-specific Luc (50 μg/heart) and inactivate (only extracellular domain) human CD25 (ihCD25, see  FIGS. 1 and 8 , 50 μg/heart) mixed together with 100 μg/heart L7AE miR1+miR208 (total modRNA 200 μg/heart). Control groups are compared with 3 experimental groups treated with CMs-specific cell cycle inducer modRNAs. Applicants used a mixture containing ihCD25, (50 μg/heart) and cell cycle inducer gene (Lin28 and Pkm2, 50 μg/heart) carrying a K-motif modRNA mixed together with 100 μg/heart of L7AE miR-1+miR-208 (total modRNA 200 μg/heart). Three days&#39; post treatment with 200 μg CMs-specific of different Luc controls or cell cycle inducer, Lin28 or Pkm2 in CFW mice (n=10), mice will be sacrificed and hearts will be dissociated with collagenase. Transfected CMs will be isolated from cardiac cell suspension using our CMs-specific modRNAsorting approach. This sorting approach is based on the use of CMs-specific modRNA ihCD25 and commercially available anti hCD25 magnetic beads (Thermo Fisher). Magnetic beads isolation is been used for variety of application, including cell sorting, for over 30 years. The magnetic beads that been used for cell sorting are pre-coupled with antibody that can recognize cell surface gene. As transfected CMs usually don&#39;t carry a specific cell surface genes, we use the truncated hCD25 to mark the transfected CMs and allow anti hCD25 magnetic beads to recognize and to attach exclusively to transfected CMs. Using a magnet and elution of residual beads and un-transfected cells will result in pure isolated transfected CMs cell population. Immediately after isolation RNA is extracted from the sorted transfected CMs and from the eluted fraction of cells (non-transfected cells) and sent for RNA-seq using HIseq2500 system in the Mount Sinai Genomics Core Facility. Some RNA is used for validation of RNA-seq measurements using Quantitative reverse transcription polymerase chain reaction (qRT-PCR). Downstream targets selection is based on: a) differentially expressed genes in control Luc vs. modRNA specifically in CMs; b) overlapping candidates between the different cell cycle inducer modRNAs treatments; c) Differentially expressed genes in CMs that may influence the gene expression observed in non-transfected cells. For example, up regulation of a relevant receptor in the non-transfected population indicates the secretion of its ligand from CMs. d) data mining of the literature. 3-5 targets are selected for validation using above approach. To test the hypothesis, we injected with CMs-specific nGFP k motif, inactivate (only extracellular domain) human CD25 (ihCD25) mixed together with L7AE miR1-miR208 after MI and we successfully sorted out nGFP and ihCD25 expressing CMs after 24 hrs post injection  FIG. 19 . 
     Magnetic Resonance Imaging (MRI) and Echocardiography (Echo). 
     CFW mice (8-weeks old) treated with Luc k motif, Luc k motif+miR1-208, miR1-208, Pkm2 k motif and Pkm2 k motif+miR1-208 modRNA were subjected to MRI assessment on day 28 post LAD ligation. 11  We obtained delayed-enhancement CINE images on a 7-T Bruker Pharmascan with cardiac and respiratory gating (SA Instruments, Inc, Stony Brook, N.Y.). Mice were anesthetized with 1-2% isoflurane/air mixture. ECG, respiratory, and temperature probes were placed on the mouse, which was kept warm during scans. Imaging was performed 10 to 20 min after IV injection of 0.3 mmol/kg gadolinium-diethylene triamine pentaacetic acid. A stack of eight to ten short-axis slices of the heart spanning the apex to the base were acquired with an ECG-triggered and respiratory-gated FLASH sequence with the following parameters: echo time (TE) 2.7 msec with resolution of 200 μm×200 μm; slice thickness of 1 mm; 16 frames per R-R interval; 4 excitations with flip angle at 60°. Ejection fraction was calculated as the difference in end-diastolic and end-systolic volumes, divided by the end-diastolic volume. MRI acquisition and analyses were performed blinded to treatment groups. For Echo evaluation of left ventricular systolic function a GE cares in site (V7R5049) equipped with a 40 MHz mouse ultrasound probe were used. Fractional shortening was calculated based on end diastolic and end systolic dimensions obtained from M-mode ultrasound. Echocardiograms were performed on 6-8 hearts/treatment groups. 
     RNA Isolation and Gene Expression Profiling Using Real-Time PCR 
     Total RNA was isolated using the RNeasy mini kit (Qiagen) and reverse transcribed using Superscript III reverse transcriptase (Invitrogen), according to the manufacturer&#39;s instructions. Real-time qPCR analyses were performed on a Mastercycler realplex 4 Sequence Detector (Eppendorf) using SYBR Green (Quantitect™ SYBR Green PCR Kit, Qiagen). Data were normalized to 18s expression, where appropriate (endogenous controls). Fold-changes in gene expression were determined by the ∂∂CT method and were presented relative to an internal control. PCR primer sequences are shown in Supplemental Table 3. 
     
       
         
           
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                 Gene 
                 Forward (SEQ ID NO:) 
                 Reverse (SEQ ID NO:) 
               
               
                   
               
             
            
               
                 Pkm2 
                 gtctggagaaacagccaagg (11) 
                 cggagttcctcgaatagctg (12) 
               
               
                   
               
               
                 Tnnt2 
                 ctgagacagaggaggccaac (13) 
                 ttccgctctgtcttctggat (14) 
               
               
                   
               
               
                 Mhy6 
                 cagaacaccagcctcatcaa (15) 
                 cccagtacctccgaaagtca (16) 
               
               
                   
               
               
                 Pecam1 
                 ctgccagtccgaaaatggaac (17) 
                 cttcatccaccggggctatc (18) 
               
               
                   
               
               
                 Cdh5 
                 attgagacagaccccaaacg (19) 
                 ttctggttttctggcagctt (20) 
               
               
                   
               
               
                 αSMA 
                 aagctgcggctagaggtca (21) 
                 ccctccctttgatggctgag (22) 
               
               
                   
               
               
                 WT1 
                 agacacacaggtgtgaaacca (23) 
                 atgagtcctggtgtgggtct (24) 
               
               
                   
               
               
                 Myc 
                 aggcagctctggagtgagag (25) 
                 cctggctcgcagattgtaag (26) 
               
               
                   
               
               
                 Hif1a 
                 gggtacaagaaaccacccat (27) 
                 gaggctgtgtcgactgagaa (28) 
               
               
                   
               
               
                 Pdk1 
                 accaggacagccaatacaag (29) 
                 cctcggtcactcatcttcac (30) 
               
               
                   
               
               
                 Cdc20 
                 ttcgtgttcgagagcgatttg (31) 
                 accttggaactagatttgccag (32) 
               
               
                   
               
               
                 Cdk1 
                 tttcggccttgccagagcgtt (33) 
                 gtggagtagcgagccgagcc (34) 
               
               
                   
               
               
                 Ccnd2 
                 gtcacccctcacgacttcat (35) 
                 ttccagttgcaatcatcgac (36) 
               
               
                   
               
               
                 Ccnb1 
                 aaggtgcctgtgtgtgaacc (37) 
                 gtcagccccatcatctgcg (38) 
               
               
                   
               
               
                 18s 
                 agtccctgccctttgtacaca (39) 
                 cgatccgagggcctcacta (40) 
               
               
                   
               
               
                 HDac4 
                 aaccttagtggggtgctgtg (41) 
                 aaggcacaaactcgcatctt (42) 
               
               
                   
               
               
                 Hand2 
                 ccagctacatcgcctacctc (43) 
                 tggttttcttgtcgttgctg (44) 
               
               
                   
               
               
                 Meox2 
                 cacagtgcctgaaatcacca (45) 
                 ctggctgtgtttgtcaatgg (46) 
               
               
                   
               
               
                 Gata4 
                 tccagcctgaacatctaccc (47) 
                 ccatagtcaccaaggctgct (48) 
               
               
                   
               
               
                 Mstn 
                 tggctcctactggacctctc (49) 
                 tgccttttaagatgcagcag (50) 
               
               
                   
               
               
                 MYHC 
                 cagaacaccagcctcatcaa (51) 
                 gctccttcttcagctcctca (52) 
               
               
                   
               
            
           
         
       
     
     Lineage Tracing in R26 mTmG  Mice 
     Rosa26 mTmG  mice were obtained from the Jackson Laboratory. All experiments were performed on age- and sex-matched mice with equal ratio of male and female mice. Healthy mice were chosen randomly from the expansion colony for each experiment. In this mice line, membrane-targeted tdTomato is expressed under the control of ubiquitous promoter on Rosa26 locus, whereas membrane-targeted eGFP becomes active after Cre-mediated excision of floxed tdTomato. CM-specific Cre modRNA (Cre K-motif+miR1-miR208) was used to exclusively express Cre in transfected CMs. This allowed for lineage tracing of the transfected CMs and their progeny long after the modRNA expression was diminished (&gt;10 days). Rosa26mTmG mice were genotyped by PCR with tail DNA as described in the Jackson Laboratory Genotyping Protocols. Primer sequences are as follows: Rosa26mT/mG, wild type forward, 5′ CTCTGCTGCCTCCTGGCTTCT-3′ (SEQ ID NO: 53), wild type reverse, 5′-CGAGGCGGATCACAAGCAATA-3′ (SEQ ID NO:54), and mutant reverse, 5′-TCAATGGGCGGGGGTCGTT-3′ (SEQ ID NO: 55). In this model, we measured the transfection level of CM-specific Cre modRNA, CMs size and number, and the number of nuclei in CMs post transfection with CM-specific Luc or Pkm2 modRNAs. 
     Neonatal Rat and Adult Mouse CMs Isolation 
     CMs from 3-4 day old neonatal rat&#39;s heart were isolated as previously described. 1  Neonatal rats&#39; ventricular CMs were isolated from 4 day-old Sprague Dawley rats (Jackson). We used multiple rounds of digestion with 0.14-mg/mL collagenase II (Invitrogen). After each digestion, the supernatant was collected in Horse serum (Invitrogen). Total cell suspension was centrifuged at 1500 rpm for 5 min. Supernatants were discarded and cells were resuspended in DMEM (GIBCO) medium with 0.1 mM ascorbic acid (Sigma), 0.5% Insulin-Transferrin-Selenium (100×), penicillin (100 U/mL) and streptomycin (100 μg/mL). Cells were plated in plastic culture dishes for 90 min until most of the non-myocytes attached to the dish and myocytes remained in suspension. Myocytes were then seeded at 1×10 5  cells/well in a 24 well plate. Neonatal rat CMs were incubated for 48 hours in DMEM medium containing 5% horse serum plus Ara c. After incubation, cells were transfected with different doses of different modRNAs as described in the text. Adult CMs were isolated from CFW mice after 28 days post MI and modRNA injection using Langendorff&#39;s method as previously described. For CMs count, we averaged 3 different counts/sample and 3 hearts/group using a hemocytometer. The total number of CMs counted was approximately 150-200 CMs/aliquot (10 ul aliquots samples using a wide-bore pipette from the total volume of CMs obtained following digestion). The cultured CMs were stained with α-actinin (CMs, Red) antibody (abcam) and Hoechst 33342 for nuclei counts. For nuclei count, approximately 1×10 3 CMs were counted per sample, using 3-4 independent samples per group. nuclei count was plotted as percentage of counted CMs. For isolation of transfected adult CMs and RNA isolation please see  FIGS. 17A-17C . 
     Mouse MI Model and Histology 
     All surgical and experimental procedures with mice were performed in accordance with protocols approved by Institutional Animal Care and Use Committees at Icahn School of Medicine at Mount Sinai Institutional Animal Care and Use Committee (IACUC) and the MSSM Center for Comparative Medicine and Surgery (CCMS). CFW, R26 mTmG  mice (6-8 weeks old) were anesthetized with isoflurane. MI was induced by permanent ligation of the LAD, as previously described 3 . Briefly, the left thoracic region was shaved and sterilized. After intubation, the heart was exposed through a left thoracotomy. A suture was placed to ligate the LAD. The thoracotomy and skin were sutured closed in layers. Excess air was removed from the thoracic cavity, and the mouse was removed from ventilation when normal breathing was established. In order to determine the effect of modRNA on cardiovascular outcome after MI, modRNAs (100-150 μg/heart) were injected into the infarct zone immediately after LAD ligation. The peri-infarct zone near the apex was either snap-frozen for RNA isolation and subsequent real-time qPCR studies, or fixed in 4% PFA for cryo-sectioning and immunostaining. In all experiments, the surgeon was blinded to the treatment group. For assessment of heart histology, hearts were collected at the end of each study. The hearts were excised, briefly washed in PBS, perfused with perfusion buffer, weighted and fixed in 4% PFA at 4° C. overnight. On the next day hearts were washed with PBS and incubated overnight in 30% sucrose. Next, hearts were put in OCT, were frozen and stored at −80° C. The heart blocks were transverse sectioned at 8-9 μm using cryostat. The slides were further processed for evaluation using immunostaining (see below) or histological scar staining using Masson&#39;s trichrome staining kit (Sigma) and were performed according to standard procedures. Measuring ratio of heart-weight to body-weight was done using a scale. The ratio was measured at the end point of each experiment. This ratio was calculated as the heart tissue weight relative to the mouse total body-weight in grams (g). 
     Immunostaining of Heart Sections Following modRNA Treatment 
     The mouse hearts were harvested and perfused using perfusion buffer and 4% paraformaldehyde (PFA). Hearts were fixed in 4% PFA/PBS overnight on shaker and then washed with PBS for 1 hr and incubated in 30% sucrose/PBS at 4° C. overnight. The next day, hearts were fixed in OCT and frozen at −80° C. Transverse heart sections (8-10 μM) were made by cryostat. Frozen sections were rehydrated in PBS for 5 min followed by permeabilization with PBS with 0.1% triton X100 (PBST) for 7 min. Slides were then treated with 3% H 2 O 2  for 5 min. After 3 washes with PBST for 5 minutes each, the samples were blocked with PBS+5% Donkey normal serum+0.1% Triton X100 (PBSST) for 2 hours at room temperature and primary antibodies diluted in PBSST were added. Slides were then incubated overnight at 4° C. Slides were washed with PBST (5 times for 4 minutes each) followed by incubation with a secondary antibody (Invitrogen, 1:200) diluted in PBST for 2 hours at room temperature. The samples were further washed with PBST (3 times for 5 min each) and stained with Hoechst 33342 (1 μg/m I) diluted in PBST for 7 min. After 5 washes with PBST for 4 minutes each, and one time with tap water (for 4 minutes), slides were mounted with mounting medium (VECTASHIELD) for imaging. Stained slides were stored at 4° C. All staining were performed on 3-8 hearts/group, with 2-3 sections/heart. In the case of immunostaining with wheat germ agglutinin (WGA) for CMs size quantification, images at 20× magnification were captured and ImageJ was used to determine the area of each cell. Quantitative analyses involved counting of multiple fields from 3-6 independent hearts per group, and 3 sections/heart (˜50 cells per field assessed, to a total ˜250 cells per sample). For BrdU immunostaining, BrdU (1 mg/ml, Sigma) was added to the drinking water of adult mice (2-3-month-old) for 7-10 days before harvesting the hearts. Quantitative analyses involved counting BrdU positive CMs in multiple fields from three independent samples per group, and 3 sections/heart. The total number of CMs counted was ˜1-2×10 3  CMs per section. TUNEL immunostaining, was performed according to manufacturer&#39;s recommendations (In-Situ Cell Death Detection Kit, Fluorescein, Cat #11684795910, Roche). For Immunostaining of neonatal CMs following modRNA treatment, modRNA-transfected neonatal CMs were fixed on coverslips with 3.7% PFA for 15 min at room temperature. Following permeabilization with 0.5% Triton X in PBS for 10 min at room temperature, cells were blocked with 5% normal goat/Donkey serum+0.5% Tween 20 for 30 minutes. Coverslips were incubated with primary antibodies (see supplemental Table 1) in humid chamber for 1 hour at room temperature followed by incubation with corresponding secondary antibodies conjugated to Alexa Fluor 488, Alexa Fluor 647 and Alexa Fluor 555, and Hoechst 33342 staining for nuclei visualization (all from Invitrogene). The fluorescent images were taken on Zeiss fluorescent microscopy at 10×, 20× and 40× magnification. 
     Live Cell Imaging of Isolated Rat Neonatal Cardiomyocytes 
     The time-lapse images of isolated rat neonatal cardiomyocytes post transfection with nGFP CMs specific modRNA or co-transfected with nGFP and CM-specific Pkm2 modRNAs were acquired with a 10× objective lens every 10 sec with a confocal spinning disk microscope (Zeiss) following 24 hours of time-lapse acquisition. 
     Statistical Analysis 
     Statistical significance was determined by paired t-test for the MRI results, Log-rank (Mantel-Cox) test for survival curves or Student&#39;s t-test or One-way ANOVA, Bonferroni post hoc test for other experiments as detailed in the respective figure legends, with *p&lt;0.05 or lower considered significant. All graphs represent average values, and values were reported as mean±standard error of the mean. Two-sided Student&#39;s t-test was based on assumed normal distributions. For the quantification of the number of CD31 luminal structure, WGA, CD45, CD3, TUNEL, BrdU + , ki67 + , pH3 +  or Aurora B +  CMs, the results acquired from at least 3 heart sections. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                 Stock concentration 
                   
                 Final  
               
               
                   
                 (mM)/stock amount 
                 How much to add 
                 concentration 
               
               
                 Nucleotide 
                 (μmoles) 
                 (μl) 
                 (mM) 
               
               
                   
               
             
            
               
                 ARCA 
                 10 μmoles 
                 Use entire Trilink 
                 6 
               
               
                   
                   
                 vial 
                   
               
               
                 GTP 
                 75 mM 
                 36 μl from Ambion 
                 1.5 
               
               
                   
                   
                 kit 
                   
               
               
                 ATP 
                 75 mM 
                 183 μl from Ambion 
                 7.5 
               
               
                   
                   
                 kit 
                   
               
               
                 CTP 
                 75 mM 
                 183 μl from Ambion 
                 7.5 
               
               
                   
                   
                 kit 
                   
               
               
                 1.1-methyl 
                 100 mM 
                 138 μl from Trilink 
                 7.5 
               
               
                 pseudouridine 
                   
                 vial 
                   
               
               
                 Nuclease Free 
                 N/A 
                 205 μl from 
                 — 
               
               
                 Water 
                   
                 Ambion kit 
               
               
                   
               
            
           
         
       
     
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