Isoaspartyl protein damage and repair in mouse retina.

PURPOSE: To determine the propensity of retinal proteins for spontaneous damage via formation of isoaspartyl sites, a common type of protein damage that could contribute to retinal disease. METHODS: Tissue extracts were obtained from retinas and brains of control mice and from mice in which the gene for protein L-isoaspartate O-methyltransferase (PIMT; an enzyme that repairs isoaspartyl protein damage) was knocked out. PIMT expression in these extracts was measured by Western blot, and its specific activity was assayed by monitoring the rate of [(3)H]methyl transfer from S-adenosyl-[methyl-(3)H]L-methionine to γ-globulin. Isoaspartate levels in extracts were measured by their capacity to accept [(3)H]methyl groups via the PIMT-catalyzed methylation reaction. To compare molecular weight distributions of isoaspartyl-rich proteins in retina versus brain, proteins from PIMT knockout (KO) and control mice were separated by SDS-PAGE and transferred to polyvinylidene difluoride (PVDF). Isoaspartyl proteins were (3)H-labeled on-blot using a PIMT overlay and imaged by autoradiography. RESULTS: When normalized to the ß-actin content of each tissue, retina was found to be nearly identical to brain with regard to expression and activity of PIMT and its propensity to accumulate isoaspartyl sites when PIMT is absent. The two tissues show distinct differences in the molecular weight distribution of isoaspartyl proteins. CONCLUSIONS: The retina is rich in PIMT activity and contains a wide range of proteins that are highly susceptible to this type of protein damage. Recoverin may be one such protein. Isoaspartate formation, along with oxidation, should be considered as a potential source of protein dysfunction and autoimmunity in retinal disease.

Feline neural progenitor cells II: use of novel plasmid vector and hybrid promoter to drive expression of glial cell line-derived neurotrophic factor transgene.

Sustained transgene expression is required for the success of cell transplant-based gene therapy. Most widely used are lentiviral-based vectors which integrate into the host genome and thereby maintain sustained transgene expression. This requires integration into the nuclear genome, and potential risks include activation of oncogenes and inactivation of tumor suppressor genes. Plasmids have been used; however lack of sustained expression presents an additional challenge. Here we used the pCAG-PyF101-eGFP plasmid to deliver the human GDNF gene to cat neural progenitor cells (cNPCs). This vector consists of a CAGG composite promoter linked to the polyoma virus mutant enhancer PyF101. Expression of an episomal eGFP reporter and GDNF transgene were stably maintained by the cells, even following induction of differentiation. These genetically modified cells appear suitable for use in allogeneic models of cell-based delivery of GDNF in the cat and may find veterinary applications should such strategies prove clinically beneficial.

Transplantation of adult mouse iPS cell-derived photoreceptor precursors restores retinal structure and function in degenerative mice.

This study was designed to determine whether adult mouse induced pluripotent stem cells (iPSCs), could be used to produce retinal precursors and subsequently photoreceptor cells for retinal transplantation to restore retinal function in degenerative hosts. iPSCs were generated using adult dsRed mouse dermal fibroblasts via retroviral induction of the transcription factors Oct4, Sox2, KLF4 and c-Myc. As with normal mouse ES cells, adult dsRed iPSCs expressed the pluripotency genes SSEA1, Oct4, Sox2, KLF4, c-Myc and Nanog. Following transplantation into the eye of immune-compromised retinal degenerative mice these cells proceeded to form teratomas containing tissue comprising all three germ layers. At 33 days post-differentiation a large proportion of the cells expressed the retinal progenitor cell marker Pax6 and went on to express the photoreceptor markers, CRX, recoverin, and rhodopsin. When tested using calcium imaging these cells were shown to exhibit characteristics of normal retinal physiology, responding to delivery of neurotransmitters. Following subretinal transplantation into degenerative hosts differentiated iPSCs took up residence in the retinal outer nuclear layer and gave rise to increased electro retinal function as determined by ERG and functional anatomy. As such, adult fibroblast-derived iPSCs provide a viable source for the production of retinal precursors to be used for transplantation and treatment of retinal degenerative disease.

The use of progenitor cell/biodegradable MMP2-PLGA polymer constructs to enhance cellular integration and retinal repopulation.

The inability of the adult mammalian retina to regenerate can be partly attributed to the expression of injury-induced inhibitory extracellular matrix (ECM) and cell adhesion molecules. In particular, photoreceptor degeneration stimulates deposition of the inhibitory ECM proteins neurocan and CD44 at the outer limits of the dystrophic retina, where they act as a barrier against cellular migration and axonal extension. We have previously shown that degradation of these molecules, via induction of MMP2, promotes host-donor integration and retinal repopulation following transplantation. Here we present a biodegradable/biocompatible polymer scaffold that has the ability to deliver MMP2, in conjunction with retinal progenitor cells, directly to the site of retinal injury in an attempt to enhance cellular integration and promote retinal repopulation. Pre-activated MMP2, loaded into a PLGA polymer, maintained its activity throughout polymer fabrication and hydrolysis. Following delivery, significant degradation of CD44 and neurocan from the outer limits of the dystrophic retina, without further disruption of retinal architecture, was observed. As a result, the number of retinal progenitor cells that migrated beyond the glial barrier into the degenerating host increased significantly. These cells took up residence in the retinal outer nuclear layer, adopted appropriate photoreceptor morphology and expressed the mature photoreceptor markers recoverin and rhodopsin. Thus, we have created a cell delivery platform that upon transplantation provides controlled release of active-MMP2 directly to the site of retinal injury, stimulating inhibitory ECM barrier removal and enhancement of stem cell integration and retinal repopulation.

CNS progenitor cells promote a permissive environment for neurite outgrowth via a matrix metalloproteinase-2-dependent mechanism.

Transplantation of progenitor cells to the CNS has shown promise in neuronal and glial replacement and as a means of rescuing host neurons from apoptosis. Here we examined the effect of progenitor grafts on neurite extension in the degenerating retina of rd1 (retinal degeneration 1) mice. Transplantation of retinal progenitor cells induced increased matrix metalloproteinase-2 (MMP2) secretion, partly from activated glial cells, which was then activated by neuronally expressed MMP14. Active MMP2 resulted in proteolysis of the neurite outgrowth inhibitors CD44 and neurocan in the degenerative retina, allowing significantly increased neurite outgrowth across the border between abutting nondystrophic and rd1 retinas. Progenitor-induced enhancement of outgrowth was abrogated by an MMP inhibitor or by coculture with retinal explants from MMP2-/- mice. This study provides the first identification of an MMP2-dependent mechanism by which exogenous progenitor cells alter the host environment to promote neural regeneration. This suggests a novel therapeutic role for progenitor cells in the treatment of CNS degenerative diseases.

Retinal transplantation.

Degenerative diseases of the retina afflict millions of Americans, and very few effective treatments are available at present. Transplantation of solid tissue or stem cell grafts represents a promising, albeit challenging, approach to replace photoreceptor cells lost due to injury or disease. However, there remain a number of formidable obstacles to be overcome before these techniques can be applied in a clinical setting. Foremost of these challenges is immunological acceptance and survival of the graft. We will refer to studies performed in collaboration with J. Wayne Streilein over the past decade that address this issue. The immune-privileged status of the subretinal space, as well as the inherent immune privilege of retinal pigment epithelium, neuronal retina and neural stem cells will be described. The goal of these studies is to gain a better understanding of the immunological properties of both the donor tissues and recipient graft site in retinal transplantation. This information will allow for the development of strategies to improve graft outcome and lead to successful repair of the diseased eye.

Retinal progenitor cell xenografts to the pig retina: morphologic integration and cytochemical differentiation.

OBJECTIVE: To investigate the survival, integration, and differentiation of mouse retinal progenitor cells after transplantation to the subretinal space of adult pigs. METHODS: Green fluorescent protein-positive (GFP+) murine retinal progenitor cells were transplanted subretinally as single cells, spheres, or biodegradable polymer-progenitor composites into 24 nonimmunosuppressed adult pigs. Of these, 14 pigs received laser lesions (n = 11) or outer retinal scraping injury (n = 3). Recipients were killed at 30 minutes to 5 weeks after grafting. RESULTS: The GFP+ murine retinal progenitor cells survived well for up to 14 days after transplantation to the pig retina. After 5 weeks, fewer GFP+ cells were found. In the pigs that received laser treatment before grafting of cell suspension, GFP+ cells integrated into the retinal pigment epithelium and all layers of the retina. The GFP+ cells exhibited morphologic evidence of differentiation into mature retinal neurons, although evaluation of marker expression found only nestin and glial fibrillary acidic protein colocalization. In noninjured pigs, cells mainly integrated into the retinal pigment epithelium. In pigs that received composites, cells appeared to mature and extended processes through pores in the polymer matrix. CONCLUSIONS: Retinal progenitor cell xenografts survive for a sufficiently long period to integrate into areas of injury and exhibit morphologic differentiation. By 5 weeks, survival diminishes. Biodegradable polymers may be useful for transplanting retinal progenitor cells in a structurally organized manner. Clinical Relevance Central nervous system (CNS) diseases may cause long-term disabilities. Substantial tissue destruction can be sustained by the complex structures of the brain, spinal cord, or retina without loss of life, yet the lack of effective CNS regeneration frequently results in disruption of activities of daily living and marked degradation in quality of life. It has become clear that an enormous potential for repair is present within the mammalian CNS. The challenge is to harness this potential to treat disease. Transplantation of neuronal tissue to the CNS represents a promising, albeit challenging, approach to the replacement of neurons lost owing to injury or disease.

Multipotent retinal progenitors express developmental markers, differentiate into retinal neurons, and preserve light-mediated behavior.

PURPOSE: To use progenitor cells isolated from the neural retina for transplantation studies in mice with retinal degeneration. METHODS: Retinal progenitor cells from postnatal day 1 green fluorescent protein-transgenic mice were isolated and characterized. These cells can be expanded greatly in culture and express markers characteristic of neural progenitor cells and/or retinal development. RESULTS: After they were grafted to the degenerating retina of mature mice, a subset of the retinal progenitor cells developed into mature neurons, including presumptive photoreceptors expressing recoverin, rhodopsin, or cone opsin. In rho-/- hosts, there was rescue of cells in the outer nuclear layer (ONL), along with widespread integration of donor cells into the inner retina, and recipient mice showed improved light-mediated behavior compared with control animals. CONCLUSIONS: These findings have implications for the treatment of retinal degeneration, in which neuronal replacement and photoreceptor rescue are major therapeutic goals.

Expression of cytokines by multipotent neural progenitor cells.

Recent work with mammalian neural stem cells has highlighted the role of cytokine signaling in the proliferation and differentiation of these multipotent cells. While the responsiveness of neural progenitors to exogenously applied growth factors has been demonstrated in vivo as well as in vitro, little attention has been given to the production of cytokines by these cells. Here we use immunocytochemistry, RT-PCR, and ELISA to show that under standard growth conditions multipotent neural progenitor cells from humans express multiple cytokines including IL-1alpha, IL-1beta, IL-6, TGF-beta1, TGF-beta2, TNF-alpha, but not IL-2, IL-4, or IFN-gamma. Neural progenitor cells from rat and mouse express some, but not all, of these cytokines under similar conditions. While the function of cytokine expression by neural progenitor cells remains to be elucidated, these signaling molecules are known to be involved in neural development and may play a role in the activation of quiescent stem cells by a variety of pathological processes.