Patent Publication Number: US-2020277570-A1

Title: Delivery method

Description:
FIELD OF THE INVENTION 
     This invention relates to a method of culturing cells. 
     BACKGROUND 
     The development and maintenance of the organisms requires many complex interactions between cells and extracellular matrix (ECM) components. Control of the cellular microenvironment is also important for assuring functionality of tissue engineered organ substitutes. The use of ECM mimics, which utilize collagen gel compaction, electromagnetic fields, electrospinning of nanofibers, mechanical stimulation and microstructured culture plates for artificial guidance of cells, have all been explored. 
     SUMMARY 
     The inventors have found that a polyhedron-based delivery system which releases growth factor (Polyhedra Delivery System, PODS) provides a stable physiologically relevant gradient of growth factor. The inventors&#39; work investigates the activity of a growth factor gradient generated by a PODS which is set up by sustained release over a period of time, and is able to direct changes in relevant growth factor sensitive cells. The inventors have surprisingly found that using a PODS in this way allows differentiated cells with desired properties, such as directional growth, to be produced in the absence of extracellular matrix (ECM). Part of their work concerns preparation of an unbranched chain of neurons connected by axons. 
     Accordingly a first aspect of the invention provides a method of altering cells comprising culturing them in a growth factor gradient, wherein said gradient is provided by a polyhedra delivery system (PODS) that releases the growth factor to set up the gradient. 
     A second aspect of the present invention provides PODS as a therapeutic agent for use in a method of therapy. 
     A third aspect of the invention provides cells altered by the methods of the invention, optionally with a delivery vehicle, for use in a method of therapy. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1 . Construction of NGF Polyhedra. Full length and mature NGF (full NGF and mature NGF, respectively) were fused with either a H1 or VP3 tag, and each recombinant NGF fusion protein was encapsulated into polyhedra. 
         FIG. 2 . PODS EGFP and PODS NGF were mixed and spotted. PC12 cells were seeded and incubated for 5 days (left). Green fluorescence was scanned within the white box (middle) and measured by counting the number of pixels that exhibited green fluorescence (right). The aligned PC12 cells were detected by the fluorescence from PODS field. The dotted line indicates the extent of the EGFP gradient 
         FIG. 3 . PC12 cells were incubated with PODS H1/full NGF. PC12 cells were incubated with PODS H1/full NGF. Expression of tau and neurofilament which was induced by PODS NGF was analyzed by immunofluorescence cytochemistry. Nuclei were stained with propidium iodide (PI). Connection of PC12 cells via the extended axon was observed. The extended axon which was induced by PODS NGF was detected by expression of neurofilament. The solid box shows the connection between the extended axon and the growth cone-like structure. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Terminology 
     The terms ‘PODS’, ‘microcrystal’ and ‘crystal’ are used interchangeably herein. However it should be understood that in embodiments which refer to ‘microcrystal’ or ‘crystal’ non-crystalline forms of PODS can also be used. 
     Characteristics of the Invention 
     When used in culture systems growth factors can be rapidly and homogenously diffused, but the imbalance and asymmetry of growth factors is important for the development of certain cells. The invention shows these complex phenomena can be reproduced in vitro by a PODS that releases a growth factor. The invention concerns altering cells by means of a gradient of growth factor generated by a PODS. The altering of the cells which occurs in the method of the invention may be due to one part of the cell being in contact with the growth factor at a different concentration from another part of the cell. It may be due to one cell being in communication or contact with another cell in contact with the growth factor at a different concentration. Through this mechanism the gradient is able to impart certain characteristic to the cell which would not be possible where the growth factor was present at a homogenous concentration, for example at the same concentration at each point of the cell surface. Further the fact that the gradient is unchanging is also an important in imparting the desired characteristic to the cell. 
     Culturing 
     The invention concerns culturing cells, which typically comprises placing them in conditions where they grow and/or differentiate. The conditions may be in vivo or in vitro, such as in a human or animal body. 
     Whilst the features of the invention will be described with reference to its in vitro embodiments these features may also apply to the in vivo embodiments as appropriate. 
     Culturing in vitro is typically in an aqueous medium. Optionally the culturing takes place in a medium comprising aqueous gel. The culturing may take place in a vessel, optionally a dish. The culturing may or may not take place on a coverslip, optionally in a culture medium in contact with a coverslip. 
     The culturing may be in any suitable system, for example a 2-dimensional or 3-dimensional system. The culturing may in static conditions, for example in which there is no flow of medium and/or there is no change in medium. In preferred embodiments medium is not changed for at least 10 hours, for example for at least 20, 30, 50, 100 or 500 hours. 
     The culturing will preferably take place in a medium in which is able to sustain the cells and/or allow them to grow. The medium typically comprises one or more nutrients. The medium may or may not be serum-free. The medium may comprise DMEM or neurobasal. 
     Cells 
     As used herein the term cells typically refers to eukaryotic cells, preferably human or animal cells, such as mammalian or avian cells. Optionally the cells described herein are neuronal cells. Preferably the cells are PC12 cells. Before culturing according to the methods of the invention the cells are typically undifferentiated cell such as stem cells or pluripotent precursor cells. The cells are typically non-specialized cells, or cells that are not mature, or cells that can undergo further stages in development. The shape of the cells before culturing is typically amorphous or spherical. The arrangement of the cells before culturing is typically incoherent, or without a well-defined order. The cells which are cultured may be continuous cell lines (e.g. with an immortal phenotype), primary cell cultures, transformed cell lines, finite cell lines (e.g. non-transformed cells), or any other cell population maintained in vitro. The cells may be isolated, purified or partially purified cells. 
     Neurons are preferred cells, and the method of the invention may result in formation of sympathetic neurons. 
     The cells are typically responsive to the growth factor. They may be of the same species as the growth factor. They may be cells associated with any condition mentioned herein and/or they may be cells which can be used to treat any condition mentioned herein. Whenever ‘treatment’ or ‘treating’ is mentioned it includes ‘preventative’ or ‘prophylactic’ uses. The cells may be associated with or responsible for any of the activities mentioned in herein or in Table 1. 
     Growth Factor 
     As used herein a growth factor is typically an extracellular protein capable of stimulating or inhibiting cellular growth, proliferation and cellular differentiation. Thus the growth factor may be a hormone or cytokine. The growth factor may be a natural or artificial one, and may for example be a homologue and/or fragment of a natural growth factor which retains growth factor activity. It may be a homologue and/or fragment of any specific growth factor mentioned herein, for example in Table 1. It may be a eukaryotic, preferably human or animal, such as mammalian or avian, growth factor. 
     The growth factor can be a neurotrophin. The expression ‘Neurotrophin’ is used interchangeably herein with the expression ‘Neurotrophic growth factor’. Neurotrophins are the family of biomolecules that support the growth, survival and differentiation of both developing and mature neurons. The growth factor can be one or more members of one of the three main families, such as neurotrophin family (for example, nerve growth factor NGF, neurotrophin-3 NT-3, brain-derived neurotrophic factor BDNF, neurotrophin 4 NT-4 (also known as NT-5), ciliary neurotrophic factor (CNTF) family (CNTF, Leukemia inhibitory factor LIF, Interleukin-6), and GDNF (Glial cell-line neurotrophic factor) family (for example GDNF, CDNF, Artemin, Neuturin, Persephin). The growth factor may be a semaphorin or slit molecule. 
     Neurons can be cultured using gradients of neurotrophic growth factors. Nerve growth factor (NGF) can promote myelination and/or differentiation of neurons and/or axon growth and/or dendrite formation and/or elongation of the neuron perpendicular to the direction of the gradient and/or intermediate filament growth and/or tau protein expression and/or microtubule growth. 
     The growth factor can be a bone growth factor. A bone growth factor is a growth factor that stimulates the growth of bone tissue. Bone growth factors include bone morphogenetic proteins (BMPs), insulin-like growth factor (IGF-1), insulin-like growth factor-2 (IGF-2), transforming growth factor beta (TGF-b), fibroblast growth factors (FGFs), platelet-derived growth factor (PDGF), parathyroid hormone-related peptide (PTHrP), bone morphogenetic proteins (BMPs), and certain members of the growth differentiation factor (GDF) group of proteins. Preferred GDF proteins include: 
     GDF1—Studies in rodents suggest that this protein is involved in the establishment of left-right asymmetry in early embryogenesis and in neural development in later embryogenesis. 
     GDF2—This protein regulates cartilage and bone development, angiogenesis and differentiation of cholinergic central nervous system neurons. 
     GDF5—This protein regulates the development of numerous tissue and cell types, including cartilage, joints, brown fat, teeth, and the growth of neuronal axons and dendrites 
     GDF7—This protein may play a role in the differentiation of tendon cells and spinal cord interneurons. 
     GDF10—This promotes neural repair after stroke. 
     GDF11—This protein plays a role in the development of the nervous and other organ systems, and may regulate aging. 
     The growth factor is preferably one which exerts an effect via microtubules, and or intermediate filaments/and or microfilaments, such as neuronal growth factors, or neurotrophins, for example NGF. 
     Table 1 describes preferred growth factors and exemplified constructs for expressing them, one or more of which may be used in the methods of the invention. 
     The growth factor in the gradient will be in purified or substantially purified form. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 GF PODS construct 
                 Tag 
                 Growth factor 
                 Roles include: 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 1 
                 Activin A-H1 
                 H1 
                 Activin 
                 Cell proliferation, differentiation, apoptosis, 
               
               
                   
                 H29S ΔCC 
                   
                   
                 metabolism, homeostasis, immune response, 
               
               
                   
                   
                   
                   
                 wound repair and endocrine function 
               
               
                 2 
                 BDNF-H1 
                 H1 
                 Brain-derived 
                 Growth and differentiation of new neurons 
               
               
                   
                 H29S ΔCC 
                   
                 neurotrophic 
                 and synapses 
               
               
                   
                   
                   
                 factor 
               
               
                 3 
                 BMP2-Full-H1 
                 H1 
                 Bone 
                 Stimulates production of bone 
               
               
                   
                 H29S ΔCC 
                   
                 morphogenetic 
               
               
                   
                   
                   
                 protein 2 
               
               
                 4 
                 BMP-4-Full-H1 
                 H1 
                 Bone 
                 Differentiation in embryo, bone formation 
               
               
                   
                 H29S ΔCC 
                   
                 morphogenetic 
               
               
                   
                   
                   
                 protein 4 
               
               
                 5 
                 EGF-H1 
                 H1 
                 Epidermal growth 
                 Cellular proliferation, differentiation, and survival 
               
               
                   
                 H29S ΔCC 
                   
                 factor 
               
               
                 6 
                 Endostatin 
                 H1 
                 Endostatin 
                 Blocks the proliferation and organization of 
               
               
                   
                 B2-H1 
                   
                   
                 endothelial cells into new blood vessels. Inhibits 
               
               
                   
                 H29S ΔCC 
                   
                   
                 angiogenesis and growth of both primary 
               
               
                   
                   
                   
                   
                 tumours and secondary metastasis. 
               
               
                 7 
                 FGF2-H1 
                 H1 
                 Basic fibroblast 
                 FGF family members bind heparin and possess 
               
               
                   
                 H29S ΔCC 
                   
                 growth factor 
                 broad mitogenic and angiogenic activities. FGF-2 
               
               
                   
                   
                   
                   
                 causes limb and nervous system development, 
               
               
                   
                   
                   
                   
                 wound healing, and tumour growth. 
               
               
                 8 
                 FGF7-H1 
                 H1 
                 Keratinocyte 
                 A potent epithelial cell-specific growth factor, 
               
               
                   
                 H29S ΔCC 
                   
                 growth factor 
                 whose mitogenic activity is predominantly 
               
               
                   
                   
                   
                   
                 exhibited in keratinocytes but not in fibroblasts 
               
               
                   
                   
                   
                   
                 and endothelial cells. Role in morphogenesis of 
               
               
                   
                   
                   
                   
                 epithelium, reepithelialization of wounds, hair 
               
               
                   
                   
                   
                   
                 development and early lung organogenesis. 
               
               
                 9 
                 GDNF-H1 
                 H1 
                 Glial Cell Derived 
                 Promotes the survival and differentiation of 
               
               
                   
                 H29S ΔCC 
                   
                 Neurotrophic 
                 dopaminergic neurons in culture, and prevents 
               
               
                   
                   
                   
                 Factor 
                 apoptosis of motor neurons induced by axotomy. 
               
               
                 10 
                 IGF-1-H1 
                 H1 
                 Insulin-like 
                 Primarily made by the liver as an endocrine 
               
               
                   
                 H29S ΔCC 
                   
                 growth factor 1 
                 hormone as well as in target tissues in a 
               
               
                   
                   
                   
                   
                 paracrine/autocrine fashion; produced 
               
               
                   
                   
                   
                   
                 throughout life. The highest rates of IGF-1 
               
               
                   
                   
                   
                   
                 production occur during the pubertal growth 
               
               
                   
                   
                   
                   
                 spurt. The lowest levels occur in infancy and old 
               
               
                   
                   
                   
                   
                 age. 
               
               
                 11 
                 IL10-H1 
                 H1 
                 Interleukin 10 
                 This cytokine has pleiotropic effects in 
               
               
                   
                 H29S ΔCC 
                   
                   
                 immunoregulation and inflammation. It 
               
               
                   
                   
                   
                   
                 down-regulates the expression of Th1 cytokines, 
               
               
                   
                   
                   
                   
                 MHC class II Ags, and costimulatory molecules on 
               
               
                   
                   
                   
                   
                 macrophages. It also enhances B cell survival, 
               
               
                   
                   
                   
                   
                 proliferation, and antibody production. 
               
               
                 12 
                 IL6-H1 
                 H1 
                 Interleukin 6 
                 Functions in inflammation and the maturation of 
               
               
                   
                 H29S ΔCC 
                   
                   
                 B cells 
               
               
                 13 
                 LIF-H1 
                 H1 
                 Leukaemia 
                 Involved in the induction of hematopoietic 
               
               
                   
                 H29S ΔCC 
                   
                 Inhibitory Factor 
                 differentiation in normal and myeloid leukemia 
               
               
                   
                   
                   
                   
                 cells, induction of neuronal cell differentiation, 
               
               
                   
                   
                   
                   
                 regulator of mesenchymal to epithelial 
               
               
                   
                   
                   
                   
                 conversion during kidney development 
               
               
                 14 
                 mEphrin 
                 H1 
                 Ephrin B2 
                 Implicated in mediating developmental events, 
               
               
                   
                 B2-H1 
                   
                 (species: mouse) 
                 especially in the nervous system and in 
               
               
                   
                 H29S ΔCC 
                   
                   
                 erythropoiesis. 
               
               
                 15 
                 NGF-Full-H1 
                 H1 
                 Nerve Growth 
                 NGF is a neurotrophic factor and neuropeptide 
               
               
                   
                 H29S 
                   
                 Factor 
                 primarily involved in the regulation of growth, 
               
               
                   
                   
                   
                   
                 maintenance, proliferation, and survival of 
               
               
                   
                   
                   
                   
                 certain target neurons. In fact, NGF is critical for 
               
               
                   
                   
                   
                   
                 the survival and maintenance of sympathetic and 
               
               
                   
                   
                   
                   
                 sensory neurons, as they undergo apoptosis in its 
               
               
                   
                   
                   
                   
                 absence. 
               
               
                 16 
                 NGF-Mature-H1 
                 H1 
               
               
                   
                 H29S ΔCC 
               
               
                 17 
                 PDGF-B-H1 
                 H1 
                 Platelet Derived 
                 PDGF subunit B, which can homodimerize, bind 
               
               
                   
                 H29S ΔCC 
                   
                 Growth Factor 
                 and activate PDGF receptor tyrosine kinases, 
               
               
                   
                   
                   
                   
                 which play a role in a wide range of 
               
               
                   
                   
                   
                   
                 developmental processes. 
               
               
                 18 
                 Rank-L-H1 
                 H1 
                 Receptor 
                 RANK Ligand plays a critical role in bone 
               
               
                   
                 H29S ΔCC 
                   
                 activator of 
                 metabolism, particularly osteoclast 
               
               
                   
                   
                   
                 nuclear factor 
                 differentiation. In addition, RANK Ligand is 
               
               
                   
                   
                   
                 kappa-B ligand 
                 expressed by some T cells and promotes dendritic 
               
               
                   
                   
                   
                   
                 cell maturation. Recombinant soluble human 
               
               
                   
                   
                   
                   
                 RANK Ligand is a non-glycosylated protein. 
               
               
                 19 
                 Rank-L-VP3 
                 VP3 
               
               
                   
                 H29S ΔCC 
               
               
                 20 
                 SCF-H1 
                 H1 
                 Stem Cell Factor; 
                 Cytokine that binds to the c-KIT receptor. SCF can 
               
               
                   
                 H29S ΔCC 
                   
                 KIT Ligand 
                 exist both as a transmembrane protein and a 
               
               
                   
                   
                   
                   
                 soluble protein. This cytokine plays an important 
               
               
                   
                   
                   
                   
                 role in hematopoiesis (formation of blood cells), 
               
               
                   
                   
                   
                   
                 spermatogenesis, and melanogenesis. 
               
               
                 21 
                 SHH-H1 
                 H1 
                 Sonic Hedge Hog 
                 Member of a small group of secreted proteins 
               
               
                   
                 H29S ΔCC 
                   
                   
                 that are essential for development in both 
               
               
                   
                   
                   
                   
                 vertebrates and invertebrates. 
               
               
                 22 
                 TGFb1-H1 
                 H1 
                 Transforming 
                 The protein regulates cell proliferation, 
               
               
                   
                 H29S ΔCC 
                   
                 Growth Factor 
                 differentiation and growth, and can modulate 
               
               
                   
                   
                   
                 Beta 1 
                 expression and activation of other growth factors 
               
               
                   
                   
                   
                   
                 including interferon gamma and tumor necrosis 
               
               
                   
                   
                   
                   
                 factor alpha. This gene is frequently upregulated 
               
               
                   
                   
                   
                   
                 in tumor cells. 
               
               
                 23 
                 TGFb3-H1 
                 H1 
                 Transforming 
                 The protein is involved in embryogenesis and cell 
               
               
                   
                 H29S ΔCC 
                   
                 Growth Factor 
                 differentiation, and plays a role in wound healing. 
               
               
                   
                   
                   
                 Beta 3 
               
               
                 24 
                 mVEGF-164-H1 
                 H1 
                 Vascular 
                 The protein induces proliferation and migration 
               
               
                   
                 H29S ΔCC 
                   
                 Endothelial 
                 of vascular endothelial cells, and is essential for 
               
               
                   
                   
                   
                 Growth Factor 
                 both physiological and pathological angiogenesis. 
               
               
                   
                   
                   
                 (e.g. in the 
               
               
                   
                   
                   
                 construct 164 
               
               
                   
                   
                   
                 amino acid 
               
               
                   
                   
                   
                 variant, species 
               
               
                   
                   
                   
                 mouse) 
               
               
                 25 
                 Wnt3a-H1 
                 H1 
                 Wingless-related 
                 A role in oncogenesis and in several 
               
               
                   
                 H29S ΔCC 
                   
                 integration site 
                 developmental processes, including regulation of 
               
               
                   
                   
                   
                 family, member3a 
                 cell fate and patterning during embryogenesis. 
               
               
                   
               
            
           
         
       
     
     Differentiation and/or Proliferation 
     Differentiation is typically the process whereby cells become specialized in order to perform a specific function. Through culturing according to the method of the invention cells can alter by acquiring specialized structural and/or functional features. 
     Proliferation is typically a process that results in an increase in the number of cells. Proliferation can be regulated by the slope (change in concentration per unit distance) of a gradient and/or by the direction of gradient. 
     Changes that Typically Occur in the Method of the Invention 
     The cells may alter in one or more structural features including: 
     Change of type: Cells can change to a different type in response to a gradient of growth factor. Cells can become a first type at one part of the gradient. Cells can become a second type at a second part of the gradient. For example for Wnt in mammals at the highest concentration establishes the posterior region whilst in areas of lowest concentration establishes the anterior region. 
     Change in arrangement or long range order: The cells can become more or less prevalent at one part of the gradient in comparison to uncultured cells or cultured cells grown in an equivalent uniform concentration of growth factor. The cells can become more or less prevalent at one part of the gradient in comparison to cells grown at a second part of the gradient. In one embodiment the cells are completely absent from certain points of the gradient. The cells or prominent cellular features, for example projections, such as neurites, optionally in axons or dendrites, can align in a direction which is well defined with respect to the direction of the gradient, for example along the gradient, or perpendicular to the gradient. 
     In one embodiment the cells align and/or attach to other (for example as described in the Examples). Such cells may align at a distance of 50 to 150 microns from the PODS, such as at a distance of 80 to 120 microns. In one embodiment there may be only a single set of aligned and/or attached cells (for example forming a circle), or there may be no aligned and/or attached cells outside the above distance ranges. The alignment is preferably autonomous, in that the gradient is the only cause of the alignment. 
     Change of shape: Cells can change shape optionally becoming elongated in shape, or can alter so that there is an increase in the cell surface to volume ratio. In an elongated cell the ratio of longest to shortest dimension is typically at least 2 to 1, at least 5 to 1, at least 10 to 1, at least 20 to 1, at least 100 to 1, at least 500 to 1, and optionally less than 100,000 to 1 or less than 10,000 to 1. 
     Change in expression profile: Cells can upregulate expression of proteins not expressed or minimally expressed compared to uncultured cells or cultured cells grown in equivalent uniform concentration of growth factor. Cells can downregulate expression of proteins not expressed or minimally expressed compared to uncultured cells or cultured cells grown in equivalent uniform concentration of growth factor. Cells can upregulate expression of one or more proteins and downregulate expression of one or more different proteins not expressed or minimally expressed compared to uncultured cells or cultured cells grown in equivalent uniform concentration of growth factor. Expression of proteins relevant to the structure of the cells may be upregulated, optionally proteins in filaments such as microfilaments, neurofilaments, microtubules, associated with microtubules such as tau. Expression of receptors can be upregulated. In one embodiment specific markers are expressed when the cells are altered. 
     Change in physical features: A change in the microtubules and/or microtubule associated proteins and/or intermediate filaments, for example tau and neurofilaments can occur, such as alignment of the microtubules or intermediate filaments, optionally neurofilaments, with respect to the gradient of growth factor, for example alignment of the longest dimension of the cell along the gradient of the growth factor or alignment of the shortest dimension of the cell along the gradient of the growth factor. 
     Cells cultured according to the methods of the invention may form lines (e.g. straight, curved or circular lines) of connecting cells, optionally where the line of cells is orientated perpendicular to the direction of the growth factor gradient. 
     Nerve cells, or neurons, cultured according to the method of the invention are preferably elongated in shape, and/or unbranched and/or have neurite projections at polar positions. Nerve cells may have axons and/or a growth cone/and or dendrites. 
     The Gradient 
     In the method of the invention the growth factor is not present at a homogenous concentration. Instead it is in the form of a gradient. As used herein, the gradient is typically the change in the value of growth factor concentration per unit distance in a particular direction, normally determined by sustained release by the PODS and diffusion through the medium. The concentration is preferably highest at the PODS and decreases with distance away from the PODS. The direction of the gradient is from the/a region containing PODS towards the/a region containing no PODS. For a circular arrangement of PODS in 2D or 3D the direction of the gradient is radially outwards from the PODS. For a linear arrangement of PODS the direction of the gradient is perpendicular to the axis along which the PODS are arranged. The gradient is provided by the PODS which, for example, break down and release growth factor in situ. 
     Growth factor is typically slowly released from the microcrystals field (which is the location where the PODS are), preferably resulting in a steady physiologically relevant gradient in growth factor at the periphery of the field. This microenvironment can result in the alteration of the cells in one or more ways as described in the paragraphs above. Typically for neurons, culturing in a growth factor gradient results in alteration of the cells in all of the ways described in the paragraphs above. The altering for neurons cultured in a growth factor gradient includes induction of differentiation and can induce alignment of cells, for example nerve cells, preferably PC12 cells. 
     Gradients will normally occur when the PODS is confined to a small part of the culture space. An important property of a PODS is that it typically generates short gradients. Short gradients are important for reducing off-target effects caused by high concentrations of growth factor diffusing to the surrounding area, for example to neighbouring tissues in therapeutic use. The concentration of growth factor in the gradients described herein is typically at a maximum in the microcrystals field. The concentration reduces as the distance from the microcrystals field increases, typically dropping to half of the maximum value at up to 5 microns, up to 10 microns, or up to 30 microns, or up to 60 microns, or up to 100 microns, or up to 150 microns from the microcrystals field. The PODS usually provide a ‘short range’ gradient, and in certain situations problems can arise with long range gradients. In one embodiment the gradient has a maximum size of up to 300 microns, such as up to 200 microns. 
     The gradients of the invention will be established in up to a day. The gradient may persist/last for at least 12 hours, 1 day, 5 days, 14 days, 30 days or 90 days. In a preferred embodiment the gradient is unchanging, or substantially unchanging. For example the concentration of the growth factor does not decrease by more than 10% over 12 hours, over 1 day or over 2 days at 10 microns from the PODS. 
     Microcrystals 
     Viruses such as cypoviruses and baculoviruses produce microcrystals which are crystals of the protein polyhedrin, also known as polyhedra. The microcrystals of the invention, or polyhedra, are typically regular arrays of the polyhedrin protein assembled in a cubic crystal lattice. The microcrystals are typically cubic in shape, or optionally form as irregular crystals. The microcrystals of the invention are typically isolated from cells, such as insect cells, which have been infected with virus, preferably baculovirus. The microcrystals typically range in size from 0.1 to 10 micron, for example 2 to 5 micron (measured as the maximum distance inside the microcrystal between different surfaces or the maximum length of an edge). The microcrystals typically comprise, for example encapsulate, one or more types of growth factor. 
     PODS and Polyhedra 
     The PODS used in the invention is typically in the form of, or comprises, microcrystals which comprise a structural protein and a growth factor. Each microcrystal (PODS) comprises more than one copy of the structural protein and growth factor, and typically comprises 10 7  to 10 9 , for example 10 8  polyhedron proteins and/or 10 7  to 10 9 , for example 10 8  growth factor molecules. Typically each PODS comprises 5×10 7  to 5×10 8  polyhedron proteins and/or 5×10 7  to 5×10 8  growth factor molecules. 
     The structural protein is typically capable of forming polyhedra, and is preferably a natural or artificial polyhedrin protein. The polyhedrin protein is typically of viral origin, for example a virus of the genus Cypovirus (CPV) or Baculovirus, such as a  Bombyx mori  cypovirus polyhedrin. Polyhedra are typically in the form of microcrystals which are preferably cubic crystals. Optionally PODS used in the methods of the invention have a maximum dimension (defined by distance between surfaces or length of an edge) of 1 micron, 2 micron, 1-4 micron, up to 10 micron, up to 15 micron or up to 20 micron. The polyhedrin protein typically has homology to all or a part of any such protein described herein. 
     Characteristics of the Crystal 
     Typically a crystal unit cell has a dimension of 100 angstroms, i.e. 1×10 −8  m. In a 1 micron crystal, a cube with each edge 10 −6  m long, there are 100 unit cells along each edge, and so a 1 micron crystal has 100×100×100 unit cells, i.e. 10 6  unit cells. Each unit cell typically contains 24 copies of polyhedrin, so a 1 μm crystal has 24×10 6 , or 2.4×10 7  copies of polyhedron, a density of 2.4×10 7  per μm 3 . That means a 2 μm crystal has 2×2×2×2.4×10 7 =19.2×10 7 =approx. 2×10 8  copies of polyhedrin. 
     H1 tag: this replaces H1 in the polyhedrin. Assuming a 1 in 4 incorporation to minimise disrupting the crystal (H1 forms bunches of four), this would give 6 copies of H1-Growth factor per micron crystal, 6×10 6  per μm 3 , and so typically so a 1 μm 3  sized PODS contains 6×10 6  per μm 3  copies of growth factor. 
     VP3 tag: potentially 1 VP3 binding site per polyhedrin molecule, so up to 2.4×10 7  per μm 3 , and 2.4×10 7  copies in a 1 μm crystal. 
     Typically a crystal comprises 10 7  to 10 9  molecules of growth factor, such as about 10 8  molecules. 
     Attaching, Encapsulating or Packaging Growth Factor 
     The PODS of the invention comprises growth factor. This is typically immobilised and/or attached to the PODS in manner that allows release to provide the gradient. In a preferred embodiment the growth factor comprises a tag that is used to attach it to the PODS. 
     The growth factor can be targeted for packaging in a polyhedra by attachment of the tag to the growth factor. The growth factor polypeptide can typically comprise the growth factor amino acid sequence and the amino acid sequence of part of the polyhedrin protein, preferably helix 1 (H1), even more preferably a sequence with at least 80% homology to an H1 sequence mentioned herein. The growth factor polypeptide can comprise the growth factor amino acid sequence and the amino acid sequence of part of the baculovirus coat protein, preferably VP3, even more preferably a sequence with at least 80% homology to a VP3 sequence disclosed herein. A short VP3 sequence is preferred to minimise any disruptive effect on the PODS. 
     PODS incorporating growth factor can be prepared by coexpressing polyhedrin protein and polypeptide comprising a targeting portion and a growth factor portion. Cells can be inoculated with recombinant baculovirus expressing polyhedrin to generate empty polyhedra. Cells can be coinfected with recombinant baculovirus expressing polyhedrin and recombinant baculovirus expressing a polypeptide comprising the growth factor and a tag which targets the growth factor for packaging, for example VP3 or H1. PODS comprising polyhedrin and the polypeptide can be prepared. Cells can be coinfected with recombinant baculovirus expressing polyhedrin and one or more different recombinant baculoviruses each expressing a different polypeptide comprising the growth factor and a tag which targets the growth factor for packaging. 
     PODS comprising polyhedrin and one or more different polypeptides, encoding one or more different growth factors, can be prepared. The different polypeptides can have the same tag or different tags. 
     The polyhedrin protein can be  Bombyx mori  cypovirus polyhedrin protein. The recombinant baculovirus can be AcCP-H29 expressing  Bombyx mori  cypovirus polyhedrin. The cells used to produce the PODS, or growth factor-encapsulated polyhedral, can be  Spodoptera frugiperda  IPLB-SF21-AE cells (Sf cells). The PODS can be isolated from the cells, optionally insect cells, by centrifugation. 
     The PODS can be prepared in an aqueous suspension. The concentration of the PODS suspension can be more than 10 4  PODS per microliter volume, typically from 10 4  to 10 7  PODS per microliter volume, preferably from 5×10 4  to 5×10 6  or 10 5  to 10 6  PODS per microliter volume. PODS can be applied to (contacted with) the culture system/medium at these concentrations. These can be used in therapy at concentrations of 10 5  to 10 7  crystals per microliter, for example at 3×10 6  crystals per microliter. 
     The Microcrystals (PODS) Field 
     The microcrystals field is the location of the microcrystals (that set up the gradient). The term ‘microcrystals field’ includes the feature of a ‘PODS field’ where the PODS are not in crystalline form. 
     The geometry and size of the microcrystals field may depend on the growth factors. The microcrystals field can be a circular, linear or rectangular field. The shape of the field can be customized for a particular use, for example it can be a curved, including S-shaped, or it can be a shape consisting of straight lines, such as a square, hexagon or an octagon. The microcrystals field can comprise 10 4 -10 8 , preferably 10 5 -10 7  or 5×10 5 -10 6  microcrystals in total, or this range of microcrystals in 20 mm 2 . The field can comprise up to 10 5  microcrystals, or up to 10 6  microcrystals, for example in 20 mm 2 . The microcrystals field can comprise one or more shape elements, for example one or more lines. Each shape element can comprise these stated ranges of microcrystals, for example 10 4 -10 5  microcrystals, or up to 10 5  microcrystals, or up to 10 6  microcrystals. 
     A microcrystals field can comprise one or more lines or shapes. The lines can be straight lines. The lines and/or shapes may be spaced so that the concentration of growth factor reaches substantially zero between the lines and/or shapes. Adjacent lines or shapes may each comprise the same growth factor. Adjacent lines or shapes may each comprise a different growth factor. The lines and/or shapes may intersect. For example, the lines may form a regular grid pattern. One or more lines or shapes may comprise one or more different growth factors. In a preferred embodiment the field is a 0.5 to 10 mm diameter, for example 1 to 8 or 2 to 4 mm diameter circular field. 
     A grid structure is envisaged in which individual lines of the grid can contain different growth factor combinations. Adjacent and intersecting grid lines can provide areas where complex gradients will develop, allowing the creation of a wide range of defined and addressable culture conditions in a small area. The lines of the grid can be placed up to 0.1 mm, up to 1 mm, up to 5 mm, from 0.1 mm to 1 mm, from 1 mm to 5 mm, apart. 
     In the methods herein, the microcrystals field may be applied to a surface, for example the surface of a coverslip, the surface of a cell culture vessel, or the surface of a delivery vehicle such as hydrogel. The microcrystals field may be applied by hand, for example using a micropipette, or may be applied using a robot or other methods of surface deposition including lithography. The microcrystals field can be dried after application to a surface, for example for more than 3 hours. The microcrystals field may be extruded into hydrogel for example using a syringe. The use of a hydrogel or other carrier can increase the duration of the gradient. Synthetic hydrogels or natural materials, preferably collagen, can be used as a delivery vehicle. A field of PODS as described herein can be provided in a delivery vehicle such as a layer of hydrogel which can be manipulated, for example to provide a gradient of growth factor in a defined direction along a pre-determined path. For example, in one embodiment the methods provide an insulated line of PODS spanning an injury to encourage axons to navigate across the injury. 
     For neuronal growth factors a linear pattern is envisaged. There are any number of different geometries that may be effective. One possibility is creating 3D sandwiches of cells and PODS. A microcrystal field comprising parallel lines of PODS (0.1 to 1 mm apart) can be created. A layer of soft hydrogel or other delivery system vehicle could be layered on this and optionally seeded with neuronal cells or neuronal precursor cells. A stiffer layer of hydrogel can be layered above. After the gel has set, the gel can be manipulated to change the 3D disposition of the lines of PODS. For example, the structure could be rolled to create a “Swiss roll” of cells and PODS. The nerve cells would form linear, unbranched connections perpendicular to the gradient, and therefore along the lines of PODS. This entire structure comprising PODS and cells could be used as an implant, for example to restore nerves that had been lost due to neurodegeneration or injury. 
     PODS are typically denser than water, and can sink to the bottom of aqueous cell culture medium in a few seconds to within 10 minutes. PODS may be held in place in a culture system by gravity. The microcrystals may be added to a drop of cell culture medium suspended from a surface, optionally example a plastic surface, for example a coverslip. This would establish a gradient across the drop with cells at the bottom experiencing the highest concentrations. 
     Any suitable may be used to place or attach the PODS. In one embodiment the PODS is attached via a micropatterning system, for example using a photoactivatable reagent and a UV illumination system. 
     Embodiments 
     In one embodiment the method of the invention does not comprise a component that mimics an extracellular matrix, for example it might not comprise collagen gel compaction, an electromagnetic field, electrospinning of cells, mechanical stimulation and microstructured culture plates. 
     For the avoidance of doubt any feature described herein, such as specific growth factors or specific properties of cells may be excluded in certain embodiments of the invention. 
     Medical Use 
     The invention includes setting up a gradient for use in therapy. Therefore the invention includes administering the PODS for preventing or treating a condition, such as any condition mentioned herein. 
     Preferred conditions include growth factor deficiency, degeneration, injury or nerve damage. The PODS may be used in the manufacture of a medicament for preventing or treating a condition. Thus the PODS may be used in therapy, optionally together with a delivery vehicle, for example collagen or a gel (such as a hydrogel). The PODS may be delivered using a scaffold. Examples of diseases that can be treated by the PODS are mentioned herein, including in Table 1. 
     In preferred embodiments:
         PODS for therapy of ALS can comprise IGF-1.   PODS for therapy of Parkinson&#39;s can comprise GDNF, GDF-5, and/or CDNF.   PODS for cartilage repair can comprise the TGFb family.   PODS for therapy of multiple sclerosis can comprise LIF.   PODS for therapy of bone fracture can comprise bone morphogenetic protein 2 or bone morphogenetic protein 4   PODS for therapy of ALS or cardiac conditions can comprise VEGF.       

     Cells prepared using the method of the invention may be used in therapy, for example to treat any condition mentioned herein, such as degeneration, injury or nerve damage. In a preferred embodiment nerve cells prepared in a gradient of neurotrophic growth factors, can be used in a method of treatment for nerve injury or for neurodegeneration. 
     Homologues and Fragments 
     Homologues of polypeptide sequences are referred to herein (for example growth factors and tags). Such homologues typically have at least 70% homology, preferably at least 80%, at least 85%, at least 90%, at least 95%, at least 97%, at least 98% or at least 99% homology, for example over a region of at least 10, 15, 20, 30, 100 or more contiguous amino acids. The homology may be calculated on the basis of amino acid identity (sometimes referred to as “hard homology”). Preferred homologues retain activity, for example growth factor or tag activity. 
     The UWGCG Package provides the BESTFIT program which can be used to calculate homology and/or % sequence identity (for example used on its default settings) (Devereux et al (1984) Nucleic Acids Research 12, p 387-395). The PILEUP and BLAST algorithms can be used to calculate homology and/or % sequence identity and/or line up sequences, such as identifying equivalent or corresponding sequences (typically on their default settings), for example as described in Altschul S. F. (1993) J Mol Evol 36:290-300; Altschul, S, F et al (1990) J Mol Biol 215:403-10. 
     Software for performing BLAST analyses is publicly available through the National Center for Biotechnology Information. This algorithm involves first identifying high scoring sequence pair (HSPs) by identifying short words of length W in the query sequence that either match or satisfy some positive-valued threshold score T when aligned with a word of the same length in a database sequence. T is referred to as the neighbourhood word score threshold (Altschul et al, supra). These initial neighbourhood word hits act as seeds for initiating searches to find HSPs containing them. The word hits are extended in both directions along each sequence for as far as the cumulative alignment score can be increased. Extensions for the word hits in each direction are halted when: the cumulative alignment score falls off by the quantity X from its maximum achieved value; the cumulative score goes to zero or below, due to the accumulation of one or more negative-scoring residue alignments; or the end of either sequence is reached. The BLAST algorithm parameters W5 T and X determine the sensitivity and speed of the alignment. The BLAST program uses as defaults a word length (W) of 11, the BLOSUM62 scoring matrix (see Henikoff and Henikoff (1992) Proc. Natl. Acad. Sci. USA 89: 10915-10919) alignments (B) of 50, expectation (E) of 10, M=5, N=4, and a comparison of both sequences. 
     The BLAST algorithm performs a statistical analysis of the similarity between two sequences; see e.g., Karlin and Altschul (1993) Proc. Natl. Acad. Sci. USA 90: 5873-5787. One measure of similarity provided by the BLAST algorithm is the smallest sum probability (P(N)), which provides an indication of the probability by which a match between two polynucleotide sequences would occur by chance. For example, a sequence is considered similar to another sequence if the smallest sum probability in comparison of the first sequence to the second sequence is less than about 1, preferably less than about 0.1, more preferably less than about 0.01, and most preferably less than about 0.001. 
     The homologous sequence typically differs by 1, 2, 3, 4 or more amino acids, such as less than 10, 15 or 20 amino acids (which may be substitutions, deletions or insertions of amino acids). These changes may be measured across any of the regions mentioned above in relation to calculating homology and/or % sequence identity. 
     Fragments of any of the polypeptides mentioned herein can be used. Typically such fragments retain the original activity, for example growth factor or tag activity. The fragments typically comprise at least 60%, for example at least 70%, 80%, 90% or 95% of the original sequence. 
     Therapeutic Agents 
     Therapeutic agents (PODS and cells) and uses are mentioned herein. The invention provides such agents for use in preventing or treating the relevant condition. This may comprise administering to an individual in need a therapeutically effective amount of the agent. The invention provides use of the agent in the manufacture of a medicament to prevent or treat the disease. 
     The formulation of the agent will depend upon the nature of the agent. The agent will be provided in the form of a pharmaceutical composition containing the agent and a pharmaceutically acceptable carrier or diluent. Suitable carriers and diluents include isotonic saline solutions, for example phosphate-buffered saline. The agent may be formulated for parenteral, intravenous, intramuscular, subcutaneous, transdermal or oral administration. Preferably administration can be by injection, nasal (a spray or dry powder), by inhaler or as eye drops. 
     The dose of an agent may be determined according to various parameters, especially according to the substance used; the age, weight and condition of the individual to be treated; the route of administration; and the required regimen. A physician will be able to determine the required route of administration and dosage for any particular agent. A suitable dose may however be from 0.1 to 100 mg/kg body weight such as 1 to 40 mg/kg body weight, for example, to be taken from 1 to 3 times daily. 
     EXAMPLES 
     Example 1. Generic Method for Construction, Expression and Purification of Growth Factor Polyhedra of Table 1 
     The requisite cDNA for selected growth factors (see Table 1) was purchased from commercial suppliers. Using standard cloning techniques, growth factors sequences were cloned and subsequently subcloned into transfection the plasmids pDEST/VP3 and/or pDEST/H1), resulting in the production of transfer vectors encoding a growth factor sequence fused to VP3 or H1 tags, C-terminally or N-terminally, respectively. Then,  Spodoptera frugiperda  cells were co-transfected with growth factor transfection vectors and a linear version of non-recombinant parent baculovirus DNA. The successful homologues recombination within insect cells inserted growth factor sequences into the baculovirus genome and so created a recombinant baculovirus variant that allowed the co-expression of a tagged growth factor protein and the polyhedrin protein, both under the control of the polyhedrin promoter for maximum efficiency. 
     Recombinant baculovirus was amplified as required and growth factor polyhedra were subsequently expressed in suspension insect cell cultures using a shaking incubator for 7-10 days at 27° C. The rapid progress of the cubic growth factor polyhedra expression was monitored by microscope. After culturing for 7-10 days, polyhedra containing cells were harvested by centrifugation and cell pellets stored at −20° C. 
     Growth factor polyhedra were isolated from insect cells by cell lysis and next purified by at least 3 rounds of PBS washes (from commercial suppliers, pH 7.6-7.9), followed by centrifugation at 3000×g, 4° C., or until a pure product has been achieved, where pure polyhedrin is characterised by a milky-white appearance in aqueous liquid, depending on the concentration of polyhedra, and was readily assessed under a standard bright field microscope with a 20× and 40× magnification. Lastly, growth factor polyhedra were counted and stored at 4° C. in PBS (pH 7.6-7-9). 
     Example 2. Construction of NGF Polyhedra 
     The cDNA encoding the NGF ORF was purchased from Toyobo in an entry vector. The full-length (241 amino acids) and mature (120 amino acids) form NGF were cloned and subcloned into each destination vector (pDEST/VP3, and pDEST/H1), resulting in production of the transfer vectors encoding the full-length or mature form of NGF fused with VP3 or H1 tags (pTransH1/full NGF, pTransH1/mature NGF, pTransVP3/full NGF, and pTransVP3/mature NGF). These transfer vectors were co-transfected into Sf21 and/or Sf9 insect cells with BaculoGold™ baculovirus linearized DNA. After incubation for 5 days at 27° C., recombinant baculoviruses expressing the full-length and mature forms of NGF with VP3 or H1 tags were harvested and stored at 4° C. 
     Full-length or mature NGF was fused with polyhedron-targeting tags (H1 or VP3) to obtain NGF-encapsulated polyhedra (PODS NGF) ( FIG. 1 ). 
     Example 3. Purification of NGF-Encapsulated Polyhedra 
     To generate empty polyhedra,  Spodoptera frugiperda  IPLB-5F21-AE cells (Sf cells) were inoculated with recombinant baculovirus AcCP-H29S expressing  Bombyx mori  cypovirus polyhedrin under the control of the baculovirus polyhedrin promoter. For production of NGF polyhedra (PODS NGF), Sf cells were co-infected with AcCP-H29S and another recombinant baculovirus expressing recombinant NGF fused with the VP3 or H1 tags. The infected cells were cultured for 10 days at 27° C. and then the cells were harvested in a conical tube by centrifugation. The cell pellet was resuspended in phosphate-buffered saline (PBS; pH 7.2) and treated with an ultrasonic homogenizer at 6% power for 30 sec. The cell homogenate was centrifuged at 1500×g at 4° C. and the supernatant was removed. These treatments were repeated and the purification was complete. The polyhedron suspension was adjusted to 5×10 4  or 1×10 5  numbers per microliter volume and stored at 4° C. in distilled water containing 100 units/ml penicillin and 100 μg/ml streptomycin. 
     Example 4. Methods for Preparing a Gradient 
     
         
         
           
             a. Circular patch: Cover slip coating. Cover slips were sterilized with 70% ethanol and subsequently washed with sterilized water. Cellmatrix Type IV was dissolved in 0.15 M acetic acid (30 μg/mL). Sterilized cover slips were put and 0.5 mL of Cellmatrix Type IV solution was then added to culture wells. Following overnight incubation, the Cellmatrix Type IV solution was discarded and the cover slips were allowed to dry for 1 h. Collagen-coated cover slips were washed twice with serum-free DMEM. One microliter of polyhedron suspension (5×10 4  or 1×10 5  numbers/μl) was spotted on the cover slips and allowed to dry for more than 3 h. 
           
         
       
    
     Example 5. Culturing NGF 
     One microliter of PODS NGF suspension (5×10 4  or 1×10 5  crystals/μl) was manually spotted onto a gelatin-coated cover slip using a micropipette to create a circular field of approximately 2-4 mm in diameter of either PODS full-length NGF or PODS mature NGF ( FIG. 1 ). PC12 cells were seeded and cultured with serum-free medium under static conditions for 96 hours without media change. At the end of this period, alignment of PC12 cells along the periphery of the PODS fields was observed. PODS full-length NGF prominently induced axon extension from each cell. This axon extension was parallel with the edge of the PODS field resulting in the formation of a connected chain of migrated cells. 
     Example 6. Observation of Cells by SEM Imaging 
     Alignment and axon extension of PC12 cells surrounding PODS NGF field was also observed by scanning electron microscopy (SEM). 
     PC12 Cell Culture. 
     The PC12 DMEM cell culture medium was discarded and 0.02% EDTA solution was added. After the EDTA solution was discarded, 1 ml of 0.25% trypsin-EDTA was added and incubated for 1 min. One millilitre of DMEM culture medium was added and the cell suspension was centrifuged at 1,200 rpm for 2 min. After the supernatant was discarded, cells were suspended in 1 ml of serum-free DMEM and the number of cells was counted. Cells (7×10 4  cells/well) were then seeded into a well. Half of the medium (serum-free DMEM) was gently exchanged after 3 days. Images of cell alignment were obtained via scanning electron microscopy (SEM) on the 5th day following cell seeding. 
     Preparation of Cells for SEM Imaging. 
     After the cell medium was gently discarded, cells were fixed by 4% paraformaldehyde phosphate buffer solution, 1% osmium tetroxide and 1% tannic. Dehydration was carried out by immersing the cover slips in a series of ethanol solutions of increasing concentrations until 100% dehydration was achieved. Cover slips were covered with hexamethyldisilazane and allowed to dry overnight. Images of cell alignment were obtained via SEM after gold-sputtering (200 Å). 
     Immunocytochemistry. 
     For immunofluorescence, cells on the cover slips were fixed for 30 min at room temp with 4% paraformaldehyde. After three washes with PBS for 5 min each, the cells were permeabilized with 0.3% Triton X-100 in PBS for 15 min at room temperature. After three washes with PBS for 5 min each, the cells were incubated in blocking buffer (3% FBS in PBS) for 1 h at room temperature and then incubated with primary antibodies (anti-tau antibody (Merck Millipore) or anti-neurofilament heavy polypeptide antibody (SIGMA)) for overnight at 4° C. After three washes with PBS for 10 min each, the cells were incubated with FITC-conjugated secondary antibody (goat anti-mouse IgG antibody (Invitrogen)) for 1 h at room temperature. Cover slips were then washed three times with PBS and finally mounted on microscope slides in mounting medium with propidium Iodide (Invitrogen) for nuclei staining. Stained cells were observed using an Olympus Fluoview FV1000-IX81 confocal microscope. 
     These results indicate that PODS NGF field regulates the direction of alignment and axon extension of PC12 cells. We confirmed the differentiation of PC12 cells using tau and neurofilament antibodies. Tau and neurofilament proteins were observed in the aligned PC12 cells ( FIG. 2 a   ). Neurofilament was notably detected extending from the extended axon, indicating that the aligned cells are differentiated to nerve cells. A helical structure of neurofilament was also observed in the neighborhood of the tip of the extended axons. In contrast, expression of tau and neurofilament was not observed in PC12 cells incubated with empty polyhedra. In some cases the connections of PC12 cells were seen to be formed between the extended axon and the growth cone-like structure which were induced by PODS H1/full NGF (a solid box in  FIG. 2 b   ), but in other cases the connection was not observed (a dotted box in  FIG. 2 b   ). 
     Example 7 Direct Visualisation of a Gradient 
     To determine the extent of the gradient, PODS NGF were mixed with polyhedra encapsulating enhanced green fluorescent protein (PODS EGFP), and subsequently spotted on a cover slip. PC12 cells were then seeded and incubated with PODS NGF and PODS EGFP, and the fluorescence emission on the periphery of the aligned PC12 cells was measured ( FIG. 3 ). A gradient of green fluorescence was observed from the polyhedra field. 
     
       
         
           
               
            
               
                 DNA and Protein Sequences 
               
               
                 NGF (mature) sequence: 
               
               
                 TCATCATCCCATCCCATCTTCCACAGGGGCGAATTCTCGGTGTGTGACAG 
               
               
                   
               
               
                 TGTCAGCGTGTGGGTTGGGGATAAGACCACCGCCACAGACATCAAGGGCA 
               
               
                   
               
               
                 AGGAGGTGATGGTGTTGGGAGAGGTGAACATTAACAACAGTGTATTCAAA 
               
               
                   
               
               
                 CAGTACTTTTTTGAGACCAAGTGCCGGGACCCAAATCCCGTTGACAGCGG 
               
               
                   
               
               
                 GTGCCGGGGCATTGACTCAAAGCACTGGAACTCATATTGTACCACGACTC 
               
               
                   
               
               
                 ACACCTTTGTCAAGGCGCTGACCATGGATGGCAAGCAGGCTGCCTGGCGG 
               
               
                   
               
               
                 TTTATCCGGATAGATACGGCCTGTGTGTGTGTGCTCAGCAGGAAGGCTGT 
               
               
                   
               
               
                 GAGAAGAGCCTGA 
               
               
                   
               
               
                 SSSHPIFHRGEFSVCDSVSVWVGDKTTATDIKGKEVMVLGEVNINNSVFK 
               
               
                   
               
               
                 QYFFETKCRDPNPVDSGCRGIDSKHWNSYCTTTHTFVKALTMDGKQAAWR 
               
               
                   
               
               
                 FIRIDTACVCVLSRKAVRRA* 
               
               
                   
               
               
                 H1-NGF fusion protein sequence 
               
               
                 atggcagacgtagcaggaacaagtaaccgagactttcgcggacgcgaaca 
               
               
                   
               
               
                 aagactattcaatagcgaacaatacaactataacaAcagcAAGAATTCTA 
               
               
                   
               
               
                 GACCATCAACAAGTTTGTACAAAAAAGCAGGCTCCTCATCATCCCATCCC 
               
               
                   
               
               
                 ATCTTCCACAGGGGCGAATTCTCGGTGTGTGACAGTGTCAGCGTGTGGGT 
               
               
                   
               
               
                 TGGGGATAAGACCACCGCCACAGACATCAAGGGCAAGGAGGTGATGGTGT 
               
               
                   
               
               
                 TGGGAGAGGTGAACATTAACAACAGTGTATTCAAACAGTACTTTTTTGAG 
               
               
                   
               
               
                 ACCAAGTGCCGGGACCCAAATCCCGTTGACAGCGGGTGCCGGGGCATTGA 
               
               
                   
               
               
                 CTCAAAGCACTGGAACTCATATTGTACCACGACTCACACCTTTGTCAAGG 
               
               
                   
               
               
                 CGCTGACCATGGATGGCAAGCAGGCTGCCTGGCGGTTTATCCGGATAGAT 
               
               
                   
               
               
                 ACGGCCTGTGTGTGTGTGCTCAGCAGGAAGGCTGTGAGAAGAGCCTGA 
               
               
                   
               
               
                 MADVAGTSNRDFRGREQRLFNSEQYNYNNSKNSRPSTSLYKKAGSSSSHP 
               
               
                   
               
               
                 IFHRGEFSVCDSVSVWVGDKTTATDIKGKEVMVLGEVNINNSVFKQYFFE 
               
               
                   
               
               
                 TKCRDPNPVDSGCRGIDSKHWNSYCTTTHTFVKALTMDGKQAAWRFIRID 
               
               
                   
               
               
                 TACVCVLSRKAVRRA* 
               
               
                   
               
               
                 NGF-VP3 fusion protein sequence 
               
               
                 ATGTCATCATCCCATCCCATCTTCCACAGGGGCGAATTCTCGGTGTGTGA 
               
               
                   
               
               
                 CAGTGTCAGCGTGTGGGTTGGGGATAAGACCACCGCCACAGACATCAAGG 
               
               
                   
               
               
                 GCAAGGAGGTGATGGTGTTGGGAGAGGTGAACATTAACAACAGTGTATTC 
               
               
                   
               
               
                 AAACAGTACTTTTTTGAGACCAAGTGCCGGGACCCAAATCCCGTTGACAG 
               
               
                   
               
               
                 CGGGTGCCGGGGCATTGACTCAAAGCACTGGAACTCATATTGTACCACGA 
               
               
                   
               
               
                 CTCACACCTTTGTCAAGGCGCTGACCATGGATGGCAAGCAGGCTGCCTGG 
               
               
                   
               
               
                 CGGTTTATCCGGATAGATACGGCCTGTGTGTGTGTGCTCAGCAGGAAGGC 
               
               
                   
               
               
                 TGTGAGAAGAGCCATGGGTCGAAAGAACATGTTTCACCATGATGGGTACC 
               
               
                   
               
               
                 TTCTAGCTTTCAACTCACAACGACGATCACACACGTTACGACTACTAGGG 
               
               
                   
               
               
                 CCTTTTCAGTACTTCAACTTCTCCGAGACAGATAGAGGACATCCATTATT 
               
               
                   
               
               
                 TCGCCTACCTCTTAAGTATCCATCAAAAGCAATACCAGCAGATGAGTTAA 
               
               
                   
               
               
                 TTGACAATTTACACTAGTAACGGCGGAATAA 
               
               
                   
               
               
                 MSSSHPIFHRGEFSVCDSVSVWVGDKTTATDIKGKEVMVLGEVNINNSVF 
               
               
                   
               
               
                 KQYFFETKCRDPNPVDSGCRGIDSKHWNSYCTTTHTFVKALTMDGKQAAW 
               
               
                   
               
               
                 RFIRIDTACVCVLSRKAVRRAMGRKNMFHHDGYLLAFNSQRRSHTLRLLG 
               
               
                   
               
               
                 PFQYFNFSETDRGHPLFRLPLKYPSKAIPADELIDNLH* 
               
               
                   
               
               
                 NGF (full length) sequence: 
               
               
                 GAACCACACTCAGAGAGCAATGTCCCTGCAGGACACACCATCCCCCAAGT 
               
               
                   
               
               
                 CCACTGGACTAAACTTCAGCATTCCCTTGACACTGCCCTTCGCAGAGCCC 
               
               
                   
               
               
                 GCAGCGCCCCGGCAGCGGCGATAGCTGCACGCGTGGCGGGGCAGACCCGC 
               
               
                   
               
               
                 AACATTACTGTGGACCCCAGGCTGTTTAAAAAGCGGCGACTCCGTTCACC 
               
               
                   
               
               
                 CCGTGTGCTGTTTAGCACCCAGCCTCCCCGTGAAGCTGCAGACACTCAGG 
               
               
                   
               
               
                 ATCTGGACTTCGAGGTCGGTGGTGCTGCCCCCTTCAACAGGACTCACAGG 
               
               
                   
               
               
                 AGCAAGCGGTCATCATCCCATCCCATCTTCCACAGGGGCGAATTCTCGGT 
               
               
                   
               
               
                 GTGTGACAGTGTCAGCGTGTGGGTTGGGGATAAGACCACCGCCACAGACA 
               
               
                   
               
               
                 TCAAGGGCAAGGAGGTGATGGTGTTGGGAGAGGTGAACATTAACAACAGT 
               
               
                   
               
               
                 GTATTCAAACAGTACTTTTTTGAGACCAAGTGCCGGGACCCAAATCCCGT 
               
               
                   
               
               
                 TGACAGCGGGTGCCGGGGCATTGACTCAAAGCACTGGAACTCATATTGTA 
               
               
                   
               
               
                 CCACGACTCACACCTTTGTCAAGGCGCTGACCATGGATGGCAAGCAGGCT 
               
               
                   
               
               
                 GCCTGGCGGTTTATCCGGATAGATACGGCCTGTGTGTGTGTGCTCAGCAG 
               
               
                   
               
               
                 GAAGGCTGTGAGAAGAGCCTGA 
               
               
                   
               
               
                 EPHSESNVPAGHTIPQVHWTKLQHSLDTALRRARSAPAAAIAARVAGQTR 
               
               
                   
               
               
                 NITVDPRLFKKRRLRSPRVLFSTQPPREAADTQDLDFEVGGAAPFNRTHR 
               
               
                   
               
               
                 SKRSSSHPIFHRGEFSVCDSVSVWVGDKTTATDIKGKEVMVLGEVNINNS 
               
               
                   
               
               
                 VFKQYFFETKCRDPNPVDSGCRGIDSKHWNSYCTTTHTFVKALTMDGKQA 
               
               
                   
               
               
                 AWRFIRIDTACVCVLSRKAVRRA* 
               
               
                   
               
               
                 H1-NGF (full length) fusion protein sequence: 
               
               
                 atggcagacgtagcaggaacaagtaaccgagactttcgcggacgcgaaca 
               
               
                   
               
               
                 aagactattcaatagcgaacaatacaactataacaAcagcAAGAATTCTA 
               
               
                   
               
               
                 GACCATCAACAAGTTTGTACAAAAAAGCAGGCTCCGAACCACACTCAGAG 
               
               
                   
               
               
                 AGCAATGTCCCTGCAGGACACACCATCCCCCAAGTCCACTGGACTAAACT 
               
               
                   
               
               
                 TCAGCATTCCCTTGACACTGCCCTTCGCAGAGCCCGCAGCGCCCCGGCAG 
               
               
                   
               
               
                 CGGCGATAGCTGCACGCGTGGCGGGGCAGACCCGCAACATTACTGTGGAC 
               
               
                   
               
               
                 CCCAGGCTGTTTAAAAAGCGGCGACTCCGTTCACCCCGTGTGCTGTTTAG 
               
               
                   
               
               
                 CACCCAGCCTCCCCGTGAAGCTGCAGACACTCAGGATCTGGACTTCGAGG 
               
               
                   
               
               
                 TCGGTGGTGCTGCCCCCTTCAACAGGACTCACAGGAGCAAGCGGTCATCA 
               
               
                   
               
               
                 TCCCATCCCATCTTCCACAGGGGCGAATTCTCGGTGTGTGACAGTGTCAG 
               
               
                   
               
               
                 CGTGTGGGTTGGGGATAAGACCACCGCCACAGACATCAAGGGCAAGGAGG 
               
               
                   
               
               
                 TGATGGTGTTGGGAGAGGTGAACATTAACAACAGTGTATTCAAACAGTAC 
               
               
                   
               
               
                 TTTTTTGAGACCAAGTGCCGGGACCCAAATCCCGTTGACAGCGGGTGCCG 
               
               
                   
               
               
                 GGGCATTGACTCAAAGCACTGGAACTCATATTGTACCACGACTCACACCT 
               
               
                   
               
               
                 TTGTCAAGGCGCTGACCATGGATGGCAAGCAGGCTGCCTGGCGGTTTATC 
               
               
                   
               
               
                 CGGATAGATACGGCCTGTGTGTGTGTGCTCAGCAGGAAGGCTGTGAGAAG 
               
               
                   
               
               
                 AGCCTGA 
               
               
                   
               
               
                 MADVAGTSNRDFRGREQRLFNSEQYNYNNSKNSRPSTSLYKKAGSEPHSE 
               
               
                   
               
               
                 SNVPAGHTIPQVHWTKLQHSLDTALRRARSAPAAAIAARVAGQTRNITVD 
               
               
                   
               
               
                 PRLFKKRRLRSPRVLFSTQPPREAADTQDLDFEVGGAAPFNRTHRSKRSS 
               
               
                   
               
               
                 SHPIFHRGEFSVCDSVSVWVGDKTTATDIKGKEVMVLGEVNINNSVFKQY 
               
               
                   
               
               
                 FFETKCRDPNPVDSGCRGIDSKHWNSYCTTTHTFVKALTMDGKQAAWRFI 
               
               
                   
               
               
                 RIDTACVCVLSRKAVRRA* 
               
               
                   
               
               
                 NGF-VP3 (full length) fusion protein sequence: 
               
               
                 ATGGAACCACACTCAGAGAGCAATGTCCCTGCAGGACACACCATCCCCCA 
               
               
                   
               
               
                 AGTCCACTGGACTAAACTTCAGCATTCCCTTGACACTGCCCTTCGCAGAG 
               
               
                   
               
               
                 CCCGCAGCGCCCCGGCAGCGGCGATAGCTGCACGCGTGGCGGGGCAGACC 
               
               
                   
               
               
                 CGCAACATTACTGTGGACCCCAGGCTGTTTAAAAAGCGGCGACTCCGTTC 
               
               
                   
               
               
                 ACCCCGTGTGCTGTTTAGCACCCAGCCTCCCCGTGAAGCTGCAGACACTC 
               
               
                   
               
               
                 AGGATCTGGACTTCGAGGTCGGTGGTGCTGCCCCCTTCAACAGGACTCAC 
               
               
                   
               
               
                 AGGAGCAAGCGGTCATCATCCCATCCCATCTTCCACAGGGGCGAATTCTC 
               
               
                   
               
               
                 GGTGTGTGACAGTGTCAGCGTGTGGGTTGGGGATAAGACCACCGCCACAG 
               
               
                   
               
               
                 ACATCAAGGGCAAGGAGGTGATGGTGTTGGGAGAGGTGAACATTAACAAC 
               
               
                   
               
               
                 AGTGTATTCAAACAGTACTTTTTTGAGACCAAGTGCCGGGACCCAAATCC 
               
               
                   
               
               
                 CGTTGACAGCGGGTGCCGGGGCATTGACTCAAAGCACTGGAACTCATATT 
               
               
                   
               
               
                 GTACCACGACTCACACCTTTGTCAAGGCGCTGACCATGGATGGCAAGCAG 
               
               
                   
               
               
                 GCTGCCTGGCGGTTTATCCGGATAGATACGGCCTGTGTGTGTGTGCTCAG 
               
               
                   
               
               
                 CAGGAAGGCTGTGAGAAGAGCCATGGGTCGAAAGAACATGTTTCACCATG 
               
               
                   
               
               
                 ATGGGTACCTTCTAGCTTTCAACTCACAACGACGATCACACACGTTACGA 
               
               
                   
               
               
                 CTACTAGGGCCTTTTCAGTACTTCAACTTCTCCGAGACAGATAGAGGACA 
               
               
                   
               
               
                 TCCATTATTTCGCCTACCTCTTAAGTATCCATCAAAAGCAATACCAGCAG 
               
               
                   
               
               
                 ATGAGTTAATTGACAATTTACACTAGTAACGGCGGAATAA 
               
               
                   
               
               
                 MEPHSESNVPAGHTIPQVHWTKLQHSLDTALRRARSAPAAAIAARVAGQT 
               
               
                   
               
               
                 RNITVDPRLFKKRRLRSPRVLFSTQPPREAADTQDLDFEVGGAAPFNRTH 
               
               
                   
               
               
                 RSKRSSSHPIFHRGEFSVCDSVSVWVGDKTTATDIKGKEVMVLGEVNINN 
               
               
                   
               
               
                 SVFKQYFFETKCRDPNPVDSGCRGIDSKHWNSYCTTTHTFVKALTMDGKQ 
               
               
                   
               
               
                 AAWRFIRIDTACVCVLSRKAVRRAMGRKNMFHHDGYLLAFNSQRRSHTLR 
               
               
                   
               
               
                 LLGPFQYFNFSETDRGHPLFRLPLKYPSKAIPADELIDNLH* 
               
               
                   
               
               
                 Bombyx mori polyhedrin  
               
               
                 sequence (GenBank acc: D37771.1): 
               
               
                 atggcagacgtagcaggaacaagtaaccgagactttcgcggacgcgaaca 
               
               
                   
               
               
                 aagactattcaatagcgaacaatacaactataacaacagcttgaacggag 
               
               
                   
               
               
                 aagtgagcgtgtgggtatacgcatattactcagacgggtctgtactcgta 
               
               
                   
               
               
                 atcaacaagaactcgcaatacaaggttggcatttcagagacattcaaggc 
               
               
                   
               
               
                 acttaaggaatatcgcgagggacaacacaacgactcttacgatgagtatg 
               
               
                   
               
               
                 aagtgaatcagagcatctactatcctaacggcggtgacgctcgcaaattc 
               
               
                   
               
               
                 cactcaaatgctaaaccacgcgcgatccagatcatcttcagtcctagtgt 
               
               
                   
               
               
                 gaatgtgcgtactatcaagatggccaaaggtaacgcggtatccgtgcccg 
               
               
                   
               
               
                 atgagtacctacagcgatctcacccatgggaagcgaccggaatcaagtac 
               
               
                   
               
               
                 cgcaagattaagagagacggggaaatcgttggttacagccattacttcga 
               
               
                   
               
               
                 actaccccacgaatacaactccatctccctagcggtaagtggtgtacata 
               
               
                   
               
               
                 agaacccatcatcatacaatgtcggatcagcacataacgtaatggacgtc 
               
               
                   
               
               
                 ttccaatcatgcgactcggctctcagattctgcaaccgctactgggccga 
               
               
                   
               
               
                 actcgaattggtgaaccactacatttcgccgaacgcctacccatacctct 
               
               
                   
               
               
                 atatcaacaatcatagctatggagtagctctgagtaaccgtcagcgattg 
               
               
                   
               
               
                 ctcgtgtaa 
               
               
                   
               
               
                 MADVAGTSNRDFRGREQRLFNSEQYNYNNSLNGEVSVWVYAYYSDGSVLV 
               
               
                   
               
               
                 INKNSQYKVGISETFKALKEYREGQHNDSYDEYEVNQSIYYPNGGDARKF 
               
               
                   
               
               
                 HSNAKPRAIQIIFSPSVNVRTIKMAKGNAVSVPDEYLQRSHPWEATGIKY 
               
               
                   
               
               
                 RKIKRDGEIVGYSHYFELPHEYNSISLAVSGVHKNPSSYNVGSAHNVMDV 
               
               
                   
               
               
                 FQSCDSALRFCNRYWAELELVNHYISPNAYPYLYINNHSYGVALSNRQRL 
               
               
                   
               
               
                 LV* 
               
               
                   
               
               
                 H1-tag sequence 
               
               
                 MADVAGTSNRDFRGREQRLFNSEQYNYNNS 
               
               
                   
               
               
                 VP3-tag sequence 
               
               
                 AFNSQRRSHTLRLLGPFQYFNFSETDRGHPLFRLPLKYPSKAIPADELID 
               
               
                   
               
               
                 NLH