Transplantation of adult rat hippocampus-derived neural stem cells into retina injured by transient ischemia

Neuro-Visual and Vestibular Manifestations of Concussion and Mild TBI

PURPOSE OF REVIEW

Mild traumatic brain injury, or concussion, is a major cause of disability. Vestibular and visual dysfunction following concussion is common and can negatively affect patients' well-being and prolong recovery. Etiologies of visual and vestibular symptoms are numerous, including ocular, neuro-ophthalmic, otologic, and neuro-vestibular conditions. Some etiologies are benign and may be treatable, while others are potentially vision or life-threatening, making a focused history and examination essential. This review offers an approach to the evaluation and treatment of the most common neuro-visual and vestibular impairments that may result from concussion.

RECENT FINDINGS

Treatment of concussion including exercise, computerized programs, transcranial magnetic stimulation, gene therapy, stem cell therapy, and nanoparticles has shown promise. Many novel therapies are in the pipework for visual and vestibular recovery after concussion; however, the treatment mainstay remains therapy and evaluation for co-existing diseases.

Diversified Treatment Options of Adult Stem Cells for Optic Neuropathies

Optic neuropathies refer to a group of ocular disorders with abnormalities or dysfunction of the optic nerve, sharing a common pathophysiology of retinal ganglion cell (RGC) death and axonal loss. RGCs, as the retinal neurons in the central nervous system, show limited capacity in regeneration or recovery upon diseases or after injuries. Critically, there is still no effective clinical treatment to cure most types of optic neuropathies. Recently, stem cell therapy was proposed as a potential treatment strategy for optic neuropathies. Adult stem cells, including mesenchymal stem cells and hematopoietic stem cells, have been applied in clinical trials based on their neuroprotective properties. In this article, the applications of adult stem cells on different types of optic neuropathies and the related mechanisms will be reviewed. Research updates on the strategies to enhance the neuroprotective effects of human adult stem cells will be summarized. This review article aims to enlighten the research scientists on the diversified functions of adult stem cells and consideration of adult stem cells as a potential treatment for optic neuropathies in future clinical practices.

Novel Technique for Retinal Nerve Cell Regeneration with Electrophysiological Functions Using Human Iris-Derived iPS Cells

Regenerative medicine in ophthalmology that uses induced pluripotent stem cells (iPS) cells has been described, but those studies used iPS cells derived from fibroblasts. Here, we generated iPS cells derived from iris cells that develop from the same inner layer of the optic cup as the retina, to regenerate retinal nerves. We first identified cells positive for p75NTR, a marker of retinal tissue stem and progenitor cells, in human iris tissue. We then reprogrammed the cultured p75NTR-positive iris tissue stem/progenitor (H-iris stem/progenitor) cells to create iris-derived iPS (H-iris iPS) cells for the first time. These cells were positive for iPS cell markers and showed pluripotency to differentiate into three germ layers. When H-iris iPS cells were pre-differentiated into neural stem/progenitor cells, not all cells became positive for neural stem/progenitor and nerve cell markers. When these cells were pre-differentiated into neural stem/progenitor cells, sorted with p75NTR, and used as a medium for differentiating into retinal nerve cells, the cells differentiated into Recoverin-positive cells with electrophysiological functions. In a different medium, H-iris iPS cells differentiated into retinal ganglion cell marker-positive cells with electrophysiological functions. This is the first demonstration of H-iris iPS cells differentiating into retinal neurons that function physiologically as neurons.

The internal limiting membrane: Roles in retinal development and implications for emerging ocular therapies

Basement membranes help to establish, maintain, and separate their associated tissues. They also provide growth and signaling substrates for nearby resident cells. The internal limiting membrane (ILM) is the basement membrane at the ocular vitreoretinal interface. While the ILM is essential for normal retinal development, it is dispensable in adulthood. Moreover, the ILM may constitute a significant barrier to emerging ocular therapeutics, such as viral gene therapy or stem cell transplantation. Here we take a neurodevelopmental perspective in examining how retinal neurons, glia, and vasculature interact with individual extracellular matrix constituents at the ILM. In addition, we review evidence that the ILM may impede novel ocular therapies and discuss approaches for achieving retinal parenchymal targeting of gene vectors and cell transplants delivered into the vitreous cavity by manipulating interactions with the ILM.

Role of the Internal Limiting Membrane in Structural Engraftment and Topographic Spacing of Transplanted Human Stem Cell-Derived Retinal Ganglion Cells

Retinal ganglion cell (RGC) replacement holds potential for restoring vision lost to optic neuropathy. Transplanted RGCs must undergo neuroretinal integration to receive afferent visual signals for processing and efferent transmission. To date, retinal integration following RGC transplantation has been limited. We sought to overcome key barriers to transplanted human stem cell-derived RGC integration. Following co-culture ex vivo on organotypic mouse retinal explants, human RGCs cluster and extend bundled neurites that remain superficial to the neuroretina, hindering afferent synaptogenesis. To enhance integration, we increased the cellular permeability of the internal limiting membrane (ILM). Extracellular matrix digestion using proteolytic enzymes achieved ILM disruption while minimizing retinal toxicity and preserving glial reactivity. ILM disruption is associated with dispersion rather than clustering of co-cultured RGC bodies and neurites, and increased parenchymal neurite ingrowth. The ILM represents a significant obstacle to transplanted RGC connectivity and its circumvention may be necessary for functional RGC replacement.

Transplantation of Retinal Progenitor Cells from Optic Cup-Like Structures Differentiated from Human Embryonic Stem Cells In Vitro and In Vivo Generation of Retinal Ganglion-Like Cells

Human embryonic stem cells (hESCs) have the potential to differentiate along the retinal lineage. We have efficiently differentiated human pluripotent stem cells into optic cup-like structures by using a novel retinal differentiation medium (RDM). The purpose of this study was to determine whether the retinal progenitor cells (RPCs) derived from hESCs can integrate into the host retina and differentiate into retinal ganglion cells (RGCs) in vivo. In this study, hESCs (H9-GFP) were induced to differentiate into optic cup-like structures by using our novel differentiation system. The RPCs extracted from the optic cup-like structures were transplanted into the vitreous cavity of N-methyl-d-aspartic acid-treated mice. Sham-treated eyes received the same amount of RDM. The host retinas were analyzed by triple immunofluorescence on the fourth and fifth weeks after transplantation. The optic cup-like structures were efficiently differentiated from hESCs by using our novel differentiation system in vitro for 6-8 weeks. The RPCs extracted from the optic cup-like structures migrated and integrated into the ganglion cell layer (GCL) of the host retina. Furthermore, the remaining transplanted cells were spread over the GCL and had a complementary distribution with host residual RGCs in the GCL of the mouse retina. Surprisingly, some of the transplanted cells expressed the RGC-specific marker Brn3a. These findings demonstrated that the RPCs derived from hESCs could integrate into the host GCL and differentiate into retinal ganglion-like cells in vivo, suggesting that RPCs can be used as an ideal source in supplying countless RGC and embryonic stem cell-based replacement therapies may be a promising treatment to restore vision in patients with degenerative retinal diseases.

Vision impairment after traumatic brain injury: present knowledge and future directions

Traumatic brain injury (TBI) is a major cause of mortality and morbidity in the USA as well as in the world. As a result of TBI, the visual system is also affected often causing complete or partial visual loss, which in turn affects the quality of life. It may also lead to ocular motor dysfunction, defective accommodation, and impaired visual perception. As a part of the therapeutic strategy, early rehabilitative optometric intervention is important. Orthoptic therapy, medication, stem cell therapy, motor and attention trainings are the available treatment options. Gene therapy is one of the most promising emerging strategies. Use of state-of-the-art nanomedicine approaches to deliver drug(s) and/or gene(s) might enhance the therapeutic efficacy of the present and future modalities. More research is needed in these fields to improve the outcome of this debilitating condition. This review focuses on different visual pathologies caused by TBI, advances in pre-clinical and clinical research, and available treatment options.

Potential therapeutic roles of stem cells in ischemia-reperfusion injury

Ischemia-reperfusion injury (I/RI), produced by an initial interruption of organ blood flow and its subsequent restoration, contributes significantly to the pathophysiologies of stroke, myocardial infarction, renal I/RI, intestinal I/RI and liver I/RI, which are major causes of disability (including transplant failure) and even mortality. While the restoration of blood flow is required to restore oxygen and nutrient requirements, reperfusion often triggers local and systemic inflammatory responses and subsequently elevate the ischemic insult where the duration of ischemia determines the magnitude of I/RI damage. I/RI increases vascular leakage, changes transcriptional and cell death programs, drives leukocyte entrapment and inflammation and oxidative stress in tissues. Therapeutic approaches which reduce complications associated with I/RI are desperately needed to address the clinical and economic burden created by I/RI. Stem cells (SC) represent ubiquitous and uncommitted cell populations with the ability to self-renew and differentiate into one or more developmental 'fates'. Like immune cells, stem cells can home to and penetrate I/R-injured tissues, where they can differentiate into target tissues and induce trophic paracrine signaling which suppress injury and maintain tissue functions perturbed by ischemia-reperfusion. This review article summarizes the present use and possible protective mechanisms underlying stem cell protection in diverse forms of ischemia-reperfusion.

Transplantation of Neuro2a Cells into the Developing Postnatal Mouse Eye

The present study aimed to investigate the influence of the host retinal microenvironment on cell migration and differentiation using Neuro2a (N2a) cells transduced with green fluorescent protein. N2a cells were transplanted into the vitreous cavities of developing mouse eyes (C57BL/6) on postnatal days 1, 5, and 10 (P1, 5, and 10). To analyze the effects of the host microenvironment on neural differentiation of N2a cells in vitro, cells were treated with a conditioned medium (CM) collected from retinal cells cultured at each developmental stage. We observed that numerous cells transplanted into P5 mice eyes migrated into all layers of the host retina, and the presence of processes indicated morphological differentiation. Some transplanted N2a cells expressed several neural markers. However, cells transplanted into the P1 and 10 mice eyes only proliferated within the vitreous cavity. Neurite length increased in N2a cells treated with CM collected from the cultured retinal cells from P5 and 10 mice, while western blotting revealed that the levels of proteins related to neural differentiation were not significantly altered in N2a cells treated with CM. We show that the migration and differentiation capacities of transplanted cells were differentially influenced by the microenvironment of the retinal postnatal ontogeny.

Development of gene and stem cell therapy for ocular neurodegeneration

Retinal degenerative diseases pose a serious threat to eye health, but there is currently no effective treatment available. Recent years have witnessed rapid development of several cutting-edge technologies, such as gene therapy, stem cell therapy, and tissue engineering. Due to the special features of ocular structure, some of these technologies have been translated into ophthalmological clinic practice with fruitful achievements, setting a good example for other fields. This paper reviews the development of the gene and stem cell therapies in ophthalmology.

βB2-Crystallin Promotes Axonal Regeneration in the Injured Optic Nerve in Adult Rats

The purpose of the study was to further scrutinize the potential of βB2-crystallin in supporting regeneration of injured retinal ganglion cell axons both in vitro and in vivo. Retinal explants obtained from animals after treatment either with lens injury (LI) alone or with combined LI 5 days or 3 days before or simultaneously with an optic nerve crush (ONC) were cultured for 96 h under regenerative conditions, and the regenerating axons were quantified and compared with untreated controls. These measurements were then repeated with LI replaced by intravitreal injections of γ-crystallin and β-crystallin at 5 days before ONC. Finally, βB2-crystallin-overexpressing transfected neural progenitor cells (βB2-crystallin-NPCs) in the eye were studied after crushing the optic nerve in vivo. Regeneration was monitored with the aid of immunoblotting of the retina and optic nerve both distal and proximal to the lesion site, and this was compared with controls that received injections of phosphate buffer only. LI performed 5 days or 3 days before ONC significantly promoted axonal outgrowth in vitro (p < 0.001), while LI performed alone before explantation did not. Intravitreal injections of β-crystallin and γ-crystallin mimicked the effects of LI and significantly increased axonal regeneration in culture at the same time intervals (p < 0.001). Western blot analysis revealed that crystallins were present in the proximal optic nerve stump at the lesion site in ONC, but were neither expressed in the undamaged distal optic nerve nor in uninjured tissue. βB2-crystallin-NPCs supported the regeneration of cut optic nerve axons within the distal optic nerve stump in vivo. The reported data suggest that βB2-crystallin-producing "cell factories" could be used to provide novel therapeutic drugs for central nervous system injuries.

Photoreceptor replacement therapy: Challenges presented by the diseased recipient retinal environment

Vision loss caused by the death of photoreceptors is the leading cause of irreversible blindness in the developed world. Rapid advances in stem cell biology and techniques in cell transplantation have made photoreceptor replacement by transplantation a very plausible therapeutic strategy. These advances include the demonstration of restoration of vision following photoreceptor transplantation and the generation of transplantable populations of donor cells from stem cells. In this review, we present a brief overview of the recent progress in photoreceptor transplantation. We then consider in more detail some of the challenges presented by the degenerating retinal environment that must play host to these transplanted cells, how these may influence transplanted photoreceptor cell integration and survival, and some of the progress in developing strategies to circumnavigate these issues.

Stem cell therapy for glaucoma: possibilities and practicalities

Glaucoma is a progressive, neurodegenerative, optic neuropathy in which currently available therapies cannot always prevent, and do not reverse, vision loss. Stem cell transplantation may provide a promising new avenue for treating many presently incurable degenerative conditions, including glaucoma. This article will explore the various ways in which transplantation of stem or progenitor cells may be applied for the treatment of glaucoma. We will critically discuss the translational prospects of two cell transplantation-based treatment modalities: neuroprotection and retinal ganglion cell replacement. In addition, we will identify specific questions that need to be addressed and obstacles to overcome on the path to clinical translation, and offer insight into potential strategies for approaching this goal.

Neural Stem Cells Derived by Small Molecules Preserve Vision

PURPOSE

The advances in stem cell biology hold a great potential to treat retinal degeneration. Importantly, specific cell types can be generated efficiently with small molecules and maintained stably over numerous passages. Here, we investigated whether neural stem cell (NSC) derived from human embryonic stem cells (hESC) by small molecules can preserve vision following grafting into the Royal College Surgeon (RCS) rats; a model for retinal degeneration.

METHODS

A cell suspension containing 3 × 10⁴ NSCs or NSCs labeled with green fluorescent protein (GFP) was injected into the subretinal space or the vitreous cavity of RCS rats at postnatal day (P) 22; animals injected with cell-carry medium and those left untreated were used as controls. The efficacy of treatment was evaluated by testing optokinetic response, recording luminance threshold, and examining retinal histology.

RESULTS

NSCs offered significant preservation of both photoreceptors and visual function. The grafted NSCs survived for long term without evidence of tumor formation. Functionally, NSC treated eyes had significantly better visual acuity and lower luminance threshold than controls. Morphologically, photoreceptors and retinal connections were well preserved. There was an increase in expression of cillary neurotrophic factor (CNTF) in Müller cells in the graft-protected retina.

CONCLUSIONS

This study reveals that NSCs derived from hESC by small molecules can survive and preserve vision for long term following subretinal transplantation in the RCS rats. These cells migrate extensively in the subretinal space and inner retina; there is no evidence of tumor formation or unwanted changes after grafting into the eyes.

TRANSLATIONAL RELEVANCE

The NSCs derived from hESC by small molecules can be generated efficiently and provide an unlimited supply of cells for the treatment of some forms of human outer retinal degenerative diseases. The capacity of NSCs migrating into inner retina offers a potential as a vehicle to delivery drugs/factors to treat inner retinal disorders.

Recent Advances towards the Clinical Application of Stem Cells for Retinal Regeneration

Retinal degenerative diseases constitute a major cause of irreversible blindness in the world. Stem cell-based therapies offer hope for these patients at risk of or suffering from blindness due to the deterioration of the neural retina. Various sources of stem cells are currently being investigated, ranging from human embryonic stem cells to adult-derived induced pluripotent stem cells as well as human Müller stem cells, with the first clinical trials to investigate the safety and tolerability of human embryonic stem cell-derived retinal pigment epithelium cells having recently commenced. This review aims to summarize the latest advances in the development of stem cell strategies for the replacement of retinal neurons and their supportive cells, the retinal pigment epithelium (RPE) affected by retinal degenerative conditions. Particular emphasis will be given to the advances in stem cell transplantation and the challenges associated with their translation into clinical practice.

Transplantation of bone marrow-derived mesenchymal stem cells into the developing mouse eye

Mesenchymal stem cells (MSCs) have been studied widely for their potential to differentiate into various lineage cells including neural cells in vitro and in vivo. To investigate the influence of the developing host environment on the integration and morphological and molecular differentiation of MSCs, human bone marrow-derived mesenchymal stem cells (BM-MSCs) were transplanted into the developing mouse retina. Enhanced green fluorescent protein (GFP)-expressing BM-MSCs were transplanted by intraocular injections into mice, ranging in ages from 1 day postnatal (PN) to 10 days PN. The survival dates ranged from 7 days post-transplantation (DPT) to 28DPT, at which time an immunohistochemical analysis was performed on the eyes. The transplanted BM-MSCs survived and showed morphological differentiation into neural cells and some processes within the host retina. Some transplanted cells expressed microtubule associated protein 2 (MAP2ab, marker for mature neural cells) or glial fibrillary acid protein (GFAP, marker for glial cells) at 5PN 7DPT. In addition, some transplanted cells integrated into the developing retina. The morphological and molecular differentiation and integration within the 5PN 7DPT eye was greater than those of other-aged host eye. The present findings suggest that the age of the host environment can strongly influence the differentiation and integration of BM-MSCs.

Stem cells for retinal replacement therapy

Retinal degenerative disease has limited therapeutic options and the possibility of stem cell-mediated regenerative treatments is being actively explored for these blinding retinal conditions. The relative accessibility of this central nervous system tissue and the ability to visually monitor changes after transplantation make the retina and adjacent retinal pigment epithelium prime targets for pioneering stem cell therapeutics. Prior work conducted for several decades indicated the promise of cell transplantation for retinal disease, and new strategies that combine these established surgical approaches with stem cell-derived donor cells is ongoing. A variety of tissue-specific and pluripotent-derived donor cells are being advanced to replace lost or damaged retinal cells and/or to slow the disease processes by providing neuroprotective factors, with the ultimate aim of long-term improvement in visual function. Clinical trials are in the early stages, and data on safety and efficacy are widely anticipated. Positive outcomes from these stem cell-based clinical studies would radically change the way that blinding disorders are approached in the clinic.

Current approaches and future prospects for stem cell rescue and regeneration of the retina and optic nerve

The 3 most common causes of visual impairment and legal blindness in developed countries (age-related macular degeneration, glaucoma, and diabetic retinopathy) share 1 end point: the loss of neural cells of the eye. Although recent treatment advances can slow down the progression of these conditions, many individuals still suffer irreversible loss of vision. Research is aimed at developing new treatment strategies to rescue damaged photoreceptors and retinal ganglion cells (RGC) and to replace lost cells by transplant. The neuroprotective and regenerative potential of stem and progenitor cells from a variety of sources has been explored in models of retinal disease and ganglion cell loss. Continuous intraocular delivery of neurotrophic factors via stem cells (SC) slows down photoreceptor cells and RGC loss in experimental models. Following intraocular transplantation, SC are capable of expressing proteins and of developing a morphology characteristic of photoreceptors or RGC. Recently, recovery of vision has been achieved for the first time in a rodent model of retinal dystrophy, using embryonic SC differentiated into photoreceptors prior to transplant. This indicates that clinically significant synapse formation and acquisition of the functional properties of retinal neurons, and restoration of vision, are distinct future possibilities.

Basic study of retinal stem/progenitor cell separation from mouse iris tissue

We described the possibility of retinal regeneration using a novel and efficient technique for culturing and separating retinal stem/progenitor cells from iris tissue. Immunohistochemical staining of adult agouti mouse iris tissue revealed the presence of nestin/low-affinity neurotrophin receptor p75 (p75(NTR))-positive cells on the endothelium camerae anterioris side. Cultured mouse iris-derived cells contained little or no melanin and were found to be positive for nestin. Most nestin-positive cells were analyzed for the coexpression of p75(NTR) as a cell membrane protein. When the p75(NTR) was used as a marker to sort the cells, we obtained a dense population of nestin-positive cells. Furthermore, the nestin/p75(NTR)-positive cells were able to differentiate into neural retina cells. Thus, this culture and separation technique is useful for obtaining retinal stem/progenitor cells from adult mouse iris tissue and for the efficient production of neural retina cells.

Using stem cells to mend the retina in ocular disease

Retinal degenerative diseases are the leading cause of incurable blindness worldwide. Furthermore, existing pharmacological and surgical interventions are only partially effective in halting disease progression, thus adjunctive neuroprotective strategies are desperately needed to preserve vision. Stem cells appear to possess inherent neuroprotective abilities, at least in part by providing neurotrophic support to injured neurons. Advances in stem cell biology offer the hope of new therapies for a broad range of neurodegenerative conditions, including those of the retina. Experimental cell-mediated therapies also hint at the tantalizing possibility of achieving retinal neuronal replacement and regeneration, once cells are lost to the disease process. This article summarizes the latest advances in cell therapies for neuroprotection and regeneration in neurodegenerative pathologies of both the inner and outer retina.

Targeted disruption of outer limiting membrane junctional proteins (Crb1 and ZO-1) increases integration of transplanted photoreceptor precursors into the adult wild-type and degenerating retina

Diseases culminating in photoreceptor loss are a major cause of untreatable blindness. Transplantation of rod photoreceptors is feasible, provided donor cells are at an appropriate stage of development when transplanted. Nevertheless, the proportion of cells that integrate into the recipient outer nuclear layer (ONL) is low. The outer limiting membrane (OLM), formed by adherens junctions between Müller glia and photoreceptors, may impede transplanted cells from migrating into the recipient ONL. Adaptor proteins such as Crumbs homologue 1 (Crb1) and zona occludins (ZO-1) are essential for localization of the OLM adherens junctions. We investigated whether targeted disruption of these proteins enhances donor cell integration. Transplantation of rod precursors in wild-type mice achieved 949 +/- 141 integrated cells. By contrast, integration is significantly higher when rod precursors are transplanted into Crb1(rd8/rd8) mice, a model of retinitis pigmentosa and Lebers congenital amaurosis that lacks functional CRB1 protein and displays disruption of the OLM (7,819 +/- 1,297; maximum 15,721 cells). We next used small interfering (si)RNA to transiently reduce the expression of ZO-1 and generate a reversible disruption of the OLM. ZO-1 knockdown resulted in similar, significantly improved, integration of transplanted cells in wild-type mice (7,037 +/- 1,293; maximum 11,965 cells). Finally, as the OLM remains largely intact in many retinal disorders, we tested whether transient ZO-1 knockdown increased integration in a model of retinitis pigmentosa, the rho(-/-) mouse; donor cell integration was significantly increased from 313 +/- 58 cells without treatment to 919 +/- 198 cells after ZO-1 knockdown. This study shows that targeted disruption of OLM junctional proteins enhances integration in the wild-type and degenerating retina and may be a useful approach for developing photoreceptor transplantation strategies.

Stem Cell Therapy for Retinal Degeneration: Retinal Neurons from Heterologous Sources

Over the past few years a great deal of interest has been generated in using stem cells/progenitors to treat degenerative diseases that afflict different tissues, including retina. This interest is due to the defining properties of stem cells/progenitors, the ability of these cells to self-renew and generate all the basic cell types of the particular tissue to which they belong. In addition, the recent reports of plasticity of the adult tissue-specific stem cells/progenitors and directed differentiation of the embryonic cells (ES) has fueled the hope for cell and gene therapy using stem cells from heterologous sources. Will this approach work for treating retinal degeneration? Here, we review the current state of knowledge about obtaining retinal cells from heterologous sources, including ES cells.

A library of tunable poly(ethylene glycol)/poly(L-lysine) hydrogels to investigate the material cues that influence neural stem cell differentiation

Neural stem cells (NSCs) have the potential to replace the major cell types of the central nervous system (CNS) and may be important in therapies for injuries to and diseases of the CNS. However, for such treatments to be safe and successful, NSCs must survive and differentiate appropriately following transplantation. A number of polymer scaffolds have shown promise in improving the survival and promoting the differentiation of NSCs. To capitalize on the interaction between scaffolds and NSCs, we need to determine the fundamental material properties that influence NSC behavior. To investigate the role of material properties on NSCs, we synthesized a library of 52 hydrogels composed of poly(ethylene glycol) and poly(L-lysine) (PLL). This library of hydrogels allows independent variation of chemical and mechanical properties across a wide range of values. By culturing NSCs on this library, we have identified a subset of gels that promotes NSC migration and a further subset that promotes NSC differentiation. By combining the material properties of these subsets with the cell behavior, we determined that mechanical properties play a critical role in NSC behavior with elastic moduli promoting NSC migration and neuronal differentiation. Amine concentration is less critical, but PLL molecular weight also plays a role in NSC differentiation.

Effects of bone-marrow mesenchymal stem cells transplanted into vitreous cavity of rat injured by ischemia/reperfusion

OBJECTIVE

To examine the survival, migration, integration, differentiation and the expression of various neurotrophic factors of bone-marrow mesenchymal stem cells (BMSCs) transplanted into the vitreous cavity of rats injured by ischemia/reperfusion(I/R).

METHODS

The BMSCs were separated from rat marrow using the wall-sticking method, and cultured in vitro to expand. Flow cytometry detected the surface antigens of BMSCs. Ninety-six rats were randomly divided into four groups: normal control injected PBS(C+P), normal control injected BMSCs (C+B), ischemic/reperfusion injected PBS(I/R+P)and ischemic/reperfusion injected BMSCs(I/R+B). After retinal I/R injury was induced in each group by increasing intraocular pressure, 10 microl PBS and BMSC suspensions labeled by red fluorescence CM-Dil were immediately injected into the vitreous cavity. We observed the survival, migration and integration of BMSCs using confocal microscopy. The differentiation and expression of basic fibroblast growth factor (bFGF), brain-derived neurotrophic factor (BDNF) and ciliary neurotrophic factor (CNTF) of CM-Dil-labeled BMSCs were detected by immunofluorescent labeling and reserved by confocal microscopy. The expression of mRNA and proteins of bFGF, BDNF and CNTF were assayed by RT-PCR and Western Blot respectively.

RESULTS

After transplantation to normal eyes, BMSCs labeled by CM-Dil were mostly present in the vitreous cavity, and did not migrate. After transplantation to I/R eyes, BMSCs labeled by CM-Dil were mostly present along with the inner limiting membrane. Only a few cells were integrated into the ganglion cell layer. Two or 4 weeks after transplantation, a few BMSCs labeled by CM-Dil were observed to express markers of neuron- neurone specific enolase (NSE), neurofilament (NF) and various neurotrophic factors. The BMSC-injected I/R model eyes showed less reduction in the number of RGCs than that of the I/R eyes with PBS injection.

CONCLUSIONS

BMSC transplantation is a valuable neuroprotection tool for the treatment of retina and optic nerve diseases.

Transplantation of cells from eye-like structures differentiated from embryonic stem cells in vitro and in vivo regeneration of retinal ganglion-like cells

BACKGROUND

An embryonic stem (ES) cell-derived eye-like structure, made up of neural retinal lineage cells, retinal pigment epithelial (RPE) cells, and lens cells was constructed in our laboratory. We have shown that cells from these eye-like structures can be integrated into the developing optic vesicle of chicks. The purpose of this study was to determine whether the cells from these eye-like structures can differentiate into retinal ganglion cells (RGCs) when transplanted into the vitreous of an injured adult mouse retina.

METHODS

ES cells were induced to differentiate into eye-like structures in vitro for 6 or 11 days. Recipient mouse eyes were injected with NMDA to injure the RGCs prior to the transplantation. Sham-treated eyes received the same amount of carrier vehicle. Cells were extracted from the eye-like structures and transplanted into the vitreous of damaged and control eyes. The host eyes were analyzed both qualitatively and quantitatively by immunohistochemistry 10 days or 8 weeks after transplantation.

RESULTS

Cells from the ES cell-derived eye-like structures were integrated into the RGC layer, and differentiated into neurons when transplanted into control (non-NMDA-treated) adult eyes. However, they rarely expressed RGC markers. When they were transplanted into NMDA-treated eyes, the cells spread on the surface of the retina and covered a relatively large area of the host RGC layer that had been injured by the NMDA. The cells from the ES cell-derived eye cells frequently differentiated into cells expressing RGC-specific markers, and formed a new RGC layer. In addition, a small number of these ES cell-derived cells were observed to extend axon-like processes toward the optic disc of the host. However, visually evoked responses could not be recorded from the visual cortex.

DISCUSSION

These findings suggest that ES cell-derived eye-like structures contain cells that can differentiate into RG-like cells and regenerate a new RGC layer. These cells also appeared to be integrated into the retina and extend axon-like processes toward the optic nerve head.

The effect of dendritic cells on the retinal cell transplantation

The potential of bone marrow cell-derived immature dendritic cells (myeloid iDCs) in modulating the efficacy of retinal cell transplantation therapy was investigated. (1) In vitro, myeloid iDCs but not BMCs enhanced the survival and proliferation of embryonic retinal cells, and the expression of various neurotrophic factors by myeloid iDCs was confirmed with RT-PCR. (2) In subretinal transplantation, neonatal retinal cells co-transplanted with myeloid iDCs showed higher survival rate compared to those transplanted without myeloid iDCs. (3) CD8 T-cells reactive against donor retinal cells were significantly increased in the mice with transplantation of retinal cells alone. These results suggested the beneficial effects of the use of myeloid iDCs in retinal cell transplantation therapy.

An in vitro mouse model for retinal ganglion cell replacement therapy using eye-like structures differentiated from ES cells

The aim of this study was to investigate the developmental potential of embryonic stem (ES) cell-derived eye-like structures as a replacement cell therapy model for retinas with N-methyl-d-aspartate (NMDA)-induced damage. For this purpose mouse ES cells were induced to differentiate into eye-like structures in vitro for 10 days and co-cultured with adult mouse retina treated with or without NMDA treatment. NMDA induces excitotoxic neuronal cell death in the inner neural retina, specifically within the ganglion cell layer. After 10 days of co-culture, the specimens were fixed, embedded in paraffin wax and analyzed by immunohistochemistry. Transplanted eye-like structures differentiate into Tuj1-positive neurons when co-cultured with an adult mouse retina and the cells then migrate into the ganglion cell layer. When co-cultured with an NMDA-treated retina, most of the cells migrating into the ganglion cell layer express the ganglion cell-specific markers Hu and Brn3b. Murine ES cell-derived eye-like structures contain cell populations that can differentiate into ganglion-like cells in the host ganglion cell layer in vitro. Moreover, their contribution to the ganglion cell layer was more prominent when ganglion cell specific damage was induced by NMDA administration. These findings suggest that cells prepared from the eye-like structures generated from ES cells may be useful for cell replacement therapy and may also serve as a model system for such therapies.

Protection of visual functions by human neural progenitors in a rat model of retinal disease

Background

A promising clinical application for stem and progenitor cell transplantation is in rescue therapy for degenerative diseases. This strategy seeks to preserve rather than restore host tissue function by taking advantage of unique properties often displayed by these versatile cells. In studies using different neurodegenerative disease models, transplanted human neural progenitor cells (hNPC) protected dying host neurons within both the brain and spinal cord. Based on these reports, we explored the potential of hNPC transplantation to rescue visual function in an animal model of retinal degeneration, the Royal College of Surgeons rat.

Methodology/Principal Findings

Animals received unilateral subretinal injections of hNPC or medium alone at an age preceding major photoreceptor loss. Principal outcomes were quantified using electroretinography, visual acuity measurements and luminance threshold recordings from the superior colliculus. At 90–100 days postnatal, a time point when untreated rats exhibit little or no retinal or visual function, hNPC-treated eyes retained substantial retinal electrical activity and visual field with near-normal visual acuity. Functional efficacy was further enhanced when hNPC were genetically engineered to secrete glial cell line-derived neurotrophic factor. Histological examination at 150 days postnatal showed hNPC had formed a nearly continuous pigmented layer between the neural retina and retinal pigment epithelium, as well as distributed within the inner retina. A concomitant preservation of host cone photoreceptors was also observed.

Conclusions/Significance

Wild type and genetically modified human neural progenitor cells survive for prolonged periods, migrate extensively, secrete growth factors and rescue visual functions following subretinal transplantation in the Royal College of Surgeons rat. These results underscore the potential therapeutic utility of hNPC in the treatment of retinal degenerative diseases and suggest potential mechanisms underlying their effect in vivo.

Aqueous Humor Enhances the Proliferation of Rat Retinal Precursor Cells in Culture, and This Effect Is Partially Reproduced by Ascorbic Acid

Aqueous humor has been shown to influence the proliferation of various ocular cell types, but the effect on immature retinal cells is not known. Here, the effect of pig aqueous humor on the proliferation of rat retinal precursor cells (RPCs) was investigated. RPCs were prepared from embryonic day 19 Sprague-Dawley rats and cultured in the presence or absence of aqueous humor from healthy pigs along with a medium consisting of Dulbecco's modified Eagle's medium:Ham's F-12 medium, N2 supplement, and epidermal growth factor. Proliferation was quantified by [(3)H]thymidine incorporation under different treatment conditions, and any associated morphological changes were noted. Potential active components of porcine aqueous humor were partially characterized by gel filtration chromatography, and the effect on RPC proliferation was determined. Results showed that adding 20% aqueous humor increased [(3)H]thymidine incorporation by as much as 317%, as compared with controls. Aqueous supplementation also increased both the number and size of RPC spherical aggregates ("spheres") over the first 4 days, consistent with increased proliferative activity. Using gel filtration and the in vitro proliferation assay, the growth-promoting activity of aqueous humor was localized to two different molecular mass ranges, namely, around 30 kDa and less than 1 kDa. Ascorbic acid was present in the lower molecular mass fraction, and use of this molecule reproduced some, but not all, of the proliferative activity present in aqueous humor. These results highlight the potential role of soluble factors present in the cellular microenvironment with respect to modulation of endogenous progenitor cell activity.

Morphological integration and functional assessment of transplanted neural progenitor cells in healthy and acute ischemic rat eyes

We have functionally and morphologically characterized the retina and optic nerve after neural progenitor cell transplants to healthy rat eyes and eyes damaged by acute elevation of intraocular pressure (IOP). Green fluorescent protein-expressing adult rat hippocampal progenitor cells (AHPCs) were transplanted by intravitreal injection into healthy eyes and eyes damaged with acute ocular hypertension. Pupil light reflexes (PLR) and electroretinograms (ERGs) were recorded preoperatively and postoperatively. Eyes were subsequently prepared for immunohistochemical analysis and confocal imaging. Transplanted AHPCs were found in 8 of 15 (53%) acute ischemic eyes 62 days after surgery and 5 of 10 (50%) healthy eyes 32 days after grafting. Analysis of PLR and ERG function in acute ischemic eyes revealed no statistically significant difference compared to controls after transplantation for all observed functional parameters. Transplant into healthy rat eyes revealed no PLR or ERG amplitude deficits between transplanted and non-transplanted (control) eyes. Morphological and immunohistochemical analysis revealed that transplanted AHPCs survived and differentiated in both normal and injured retinal environments. Morphological integration occurred primarily within the inner retinal layers of the acute ischemic eyes. AHPCs were found to express neuronal and glial markers following transplantation. Transplanted AHPCs have the ability to integrate and differentiate in ischemia damaged retinas. PLR and ERG analysis revealed no significant difference in functional outcome in transplant recipient eyes.

Cellules souches rétiniennes : mécanisme de différenciation et potentiel thérapeutique

Retinal dystrophies are rarely curable diseases and several avenues of research are being pursued, such replacement therapies and pharmacological treatment. Among them, the transplantation of functional retinal cells has been envisaged in order to restore vision in patients who have these diseases by repopulating the damaged retina and/or by rescuing retinal neurons from further degeneration. Over the past few years, identification and characterization of stem cells has opened new avenues in cell-replacement therapy. Since retinal stem cells are already present during embryonic development, they persist in the adult mammalian eye only in the ciliary marginal zone, even a stem cell potential has been described for the Müller glia in the retina. This result opened possibilities of regeneration by mobilizing endogenous stem cells to respond to injury. Regarding the transplantation studies, in all experiments using different types of stem cells (retinal progenitors, neural stem cells, bone marrow-derived stem cells and ES cells), despite their incorporation within the host's retina, the transplanted cells failed to express retina-specific markers and to establish synaptic connections. Therefore, the true potential of the different stem cells in retina repair can only be realized with more information about mechanisms that regulate their proliferation and differentiation; and by development of techniques that allow their prospective identification and enrichment.

Embryonic stem cells that differentiate into RPE cell precursors in vitro develop into RPE cell monolayers in vivo

A culture system to generate eye-like structures consisting of lens, neural retina, and retinal pigmented epithelium (RPE) cells from undifferentiated embryonic stem cells has been established. Precursors of RPE cells that differentiated in the cultures were responsive to Wnt2b signaling and identified retrospectively to form secondary colonies consisting of only RPE-like cells in eye-like structures. These transplanted eye-like structures were capable of populating the developing chick eye as neuronal retina and RPE cells. The outgrowth of a single cell layer of mature RPE cells from the grafted eye-like structures confirmed the existence of precursors for RPE cells. These results suggest that the eye-like structures resulted from the normal developmental pathway responsible for generating eyes in vivo. If a functional effect of these cells can be established, such eye-like structures may be potentially used to establish therapy models for various eye diseases.

Neural Progenitor Cell Transplants into the Developing and Mature Central Nervous System

When developing cell transplant strategies to repair the diseased or injured central nervous system (CNS), it is essential to consider host-graft interactions and how they may influence the outcome of the transplants. Recent studies have demonstrated that transplanted neural progenitor cells (NPCs) can differentiate and integrate morphologically into developing mammalian retinas. Is the ability to differentiate and to undergo structural integration into the CNS unique to specific progenitor cells, or is this plasticity a function of host environment, or both? To address these issues we have used the developing retina of the Brazilian opossum and have compared the structural integration of brain and retinal progenitor cells transplanted into the eyes at different developmental stages. The Brazilian opossum, Monodelphis domestica, is a small pouchless marsupial native to South America. This animal's lack of a pouch and fetal-like nature at birth circumvents the need for in utero surgical procedures, and thus provides an ideal environment in which to study the interactions between developing host tissues and transplanted NPCs. To test whether NPCs affect visual function we transplanted adult hippocampal progenitor cells (AHPCs) into normal, healthy adult rat eyes and performed noninvasive functional recordings. Monitoring of the retina and optic nerve over time by electroretinography and pupillometry revealed no severe perturbation in visual function in the transplant recipient eyes. Taken together, our findings suggest that the age of the host environment can strongly influence NPC differentiation and that transplantation of neural progenitor cells may be a useful strategy aimed at treating neurodegeneration and pathology of the CNS.

Retinal neurospheres prepared as tissue for transplantation

The present work was conducted to study the cellular composition and developmental capacity of retinal neurospheres. Furthermore, the ability of grafted neurospheres to integrate into adult retinal tissue was studied in an in vitro model. Retinal progenitor cells isolated from rat embryos were expanded into neurospheres in vitro in the presence of basic fibroblast growth factor (bFGF), epidermal growth factor (EGF) and leukemia inhibitory factor (LIF). Neurospheres labeled with a lipophilic dye were placed onto explants, and tissue interactions were analyzed after 2-6 days of culture. Immunocytochemical analysis of neurospheres revealed the presence of neuronal and glial cells. Proliferating neuronal and glial cells were observed after 2 weeks, whereas the neuronal cell proliferation declined considerably after 4 weeks. Few apoptotic cells were observed in the neurospheres. Neurospheres cultured on explanted adult retina engrafted with the surrounding tissue, but progenitor cell migration into the explants was low. However, the grafted neurospheres appeared to limit the experimentally induced photoreceptor apoptosis in the surrounding explant tissue.

Preferential differentiation of neural progenitor cells into the glial lineage through gp130 signaling in N-methyl-d-aspartate-treated retinas

The purpose of this study was to investigate the differentiation of neural progenitor cells (NPCs) following retinal transplantation in N-methyl-D-aspartate (NMDA)-treated eyes. NMDA was injected into the vitreous cavity of adult rat eyes. NPCs were prepared from telencephalic neuroepithelium of enhanced green fluorescence protein (EGFP) transgenic mice on embryonic day 14.5. A cell suspension was injected into the vitreous cavity in experimental eyes. Immunohistochemistry was conducted at 1, 2 or 4 weeks after transplantation of NPCs in an effort to determine the survival and differentiation of transplanted NPCs. Similar experiments were conducted using glycoprotein (gp)130-null (-/-) mice. Examination of retinal sections revealed that transplanted NPCs could survive for at least 4 weeks in NMDA-treated retinas. Immunohistochemical studies for specific cell-type markers revealed that, among the transplanted NPCs at 2 weeks after transplantation, the mean percentage (+/-standard deviation) of glial fibrillary acidic protein (GFAP)-positive (glial) cells was 63.5 +/- 7.4%, demonstrating the differentiation of transplanted NPCs with a preference for the glial lineage. Furthermore, the mean percentage of betaIII-tubulin-positive (mature neuronal) cells was 18.8 +/- 4.5%. Following transplantation of NPCs isolated from gp130-/- mice into NMDA-treated retinas, the mean percentage of GFAP-positive cells (17.6 +/- 7.0%), was significantly lower than that in NPCs isolated from wild-type mice (59.1 +/- 6.0%, P = 0.04, Mann-Whitney U test). Preferential differentiation of NPCs into the glial lineage is induced through gp130 signaling in NMDA-treated eyes.

[Stem cell-based therapies for retinal disorders]

In the context of cell-based therapies for hereditary retinal dystrophies and other retinal disorders, interest has focussed on the therapeutic potential of embryonic and tissue-specific stem cells. Stem cells are characterised by their capacity for self-renewal and by their multipotentiality. Because of these properties, they can be expanded in vitro and eventually differentiated into "desired" specialized cell types. Stem cells are not only candidate cells for the development of cell replacement strategies, but are also interesting cells for the establishment of ex vivo gene therapies. Here, we discuss recent experimental work performed to evaluate the therapeutic potential of embryonic, mesenchymal, hematopoietic, neural and retinal stem cells for the treatment of inherited retinal dystrophies and other retinal diseases.

Transplantation of Neural Progenitor Cells into the Developing Retina of the Brazilian Opossum: An in vivo System for Studying Stem/Progenitor Cell Plasticity

In developing cell transplant strategies to repair the diseased or injured retina is essential to consider host-graft interactions and how they may influence the outcome of the transplants. In the present study we evaluated the influence of the host microenvironment upon neural progenitor cells (NPCs) transplanted into the developing and mature retina of the Brazilian opossum, Monodelphis domestica. Monodelphis pups are born in an extremely immature state and the neonatal pups provide a fetal-like environment in which to study the interactions between host tissues and transplanted NPCs. Three different populations of GFP-expressing NPCs were transplanted by intraocular injection in hosts ranging in age from 5 days postnatal to adult. Extensive survival, differentiation and morphological integration of NPCs were observed within the developing retina. These results suggest that the age of the host environment can strongly influence NPC differentiation and integration.

Photoreceptor differentiation and integration of retinal progenitor cells transplanted into transgenic rats

Previous studies evaluating neural stem cells transplanted into the mature retina have demonstrated limited levels of graft-host integration and photoreceptor differentiation. The purpose of this investigation is to enhance photoreceptor cell differentiation and integration of retinal progenitor cells (RPC) following subretinal transplantation into retinal degenerate rats by optimization of isolation, expansion, and transplantation procedures. RPCs were isolated from human placental alkaline phosphatase (hPAP)-positive embryonic day 17 (E17) rat retina and expanded in serum-free defined media. RPCs at passage 2 underwent in vitro induction with all trans retinoic acid or were transplanted into the subretinal space of post-natal day (P) 17 S334ter-3 and S334ter-5 transgenic rats. Animals were examined post-operatively by ophthalmoscopy and optical coherence tomography (OCT) at weeks 1 and 4. Differentiation profiles of RPCs, both in vitro and in vivo were analysed microscopically by immunohistochemistry for various retinal cell specific markers. Our results demonstrated that the majority of passage 2 RPCs differentiated into retina-specific neurons expressing rhodopsin after in vitro induction. Following subretinal transplantation, grafted cells formed a multi-layer cellular sheet in the subretinal space in both S334ter-3 and S334ter-5 rats. Prominent retina-specific neuronal differentiation was observed in both rat lines as evidenced by recoverin or rhodopsin staining in 80% of grafted cells. Less than 5% of the grafted cells expressed glial fibrillary acidic protein. Synapsin-1 (label for nerve terminals) positive neural processes were present at the graft-host interface. Expression profiles of the grafted RPCs were similar to those of RPCs induced to differentiate in vitro using all-trans retinoic acid. In contrast to our previous study, grafted RPCs can demonstrate extensive rhodopsin expression, organize into layers, and show some features of apparent integration with the host retina following subretinal transplantation in slow and fast retinal degenerate rats. The similarity of the in vitro and in vivo RPC differentiation profiles suggests that intrinsic signals may have a significant contribution to RPC cell fate determination.

Transplantation of cultured human neural stem cells in rabbits with experimental laser-induced damage to the retina

Cultured neural stem/progenitor cells from human fetal brain were transplanted into the retrobulbar and suprachoroid space in rabbits with laser-induced damage to the retina. Transplanted cells survived, retained multipotent activity, migrated into the zone of injury, and stimulated reparation and regeneration in the traumatized retina.

Genetic and Cellular Therapies for Cerebral Infarction

Neurosurgeons, working as surgical scientists, can have a prominent role in developing and implementing genetic and cellular therapies for cerebral ischemia. The rapid emergence of both genetic and cellular therapies for neural regeneration warrants a careful analysis before implementation of human studies to understand the pitfalls and promises of this strategy. In this article, we review the topic of genetic and cellular therapy for stroke to provide a foundation for practicing neurosurgeons and clinical scientists who may become involved in this type of work. In Part 1, we review preclinical approaches with gene transfer, such as 1) improved energy delivery, 2) reduction of intracellular calcium availability, 3) abrogation of effects of reactive oxygen species, 4) reduction of proinflammatory cytokine signaling, 5) inhibition of apoptosis mediators, and 6) restorative gene therapy, that are paving the way to develop new strategies to treat cerebral infarction. In Part 2, we discuss the results of studies that address the possibility of using cellular therapies for stroke in animal models and in human trials by reviewing 1) the basics of stem cell biology, 2) exogenous and 3) and endogenous cell sources for therapy, and 4) clinical considerations in cell therapy applications. These emerging technologies based on the advancements made in recent years in the fields of genetics, therapeutic cloning, neuroscience, stem cell biology, and gene therapy provide significant potential for new therapies for stroke.

Implantation of neural stem cells via cerebrospinal fluid into the injured root

In avulsion injury of the dorsal root, regenerating axons cannot extend through the entry zone, i.e. the transition zone between peripheral and central nervous systems, due to the discontinuity between Schwann cells and astrocytes. We infused neural stem cells through the 4th ventricle in an attempt to enhance axonal growth in injured dorsal roots. Infused stem cells were attached to, and integrated into, the lesion of the root and became associated with axons in the same manner as Schwann cells or perineurial sheath cells in the peripheral nerve, and as astrocytes in the central nerve area. These findings suggest that neural stem cells integrated by infusion through CSF might have a beneficial effect on nerve regeneration by inducing a continuity of Schwann cells and astrocytes at the transition zone.

The neurogenic competence of progenitors from the postnatal rat retina in vitro

The mammalian retina develops from stem or progenitor cells that are of neuroectodermal origin and derive from bilateral invaginations of the neuroepithelium, the optic vesicles. Shortly after birth, around 12 days postnatal in rats, the retina is fully developed in its cellular parts. Even though different cell types in the adult might be potential sources for retinal stem cells or progenitor cells, the retina is a non-neurogenic region and the diseased retina is devoid of any spontaneous regeneration. In an attempt to link late developmental processes to the adult situation, we analyzed the presence and the neurogenic potential of retinal progenitors during the postnatal period and compared it to adult ciliary body (CB) derived retinal progenitors and subventricular zone (SVZ) derived neural stem cells. Retinal progenitor properties were identified by the capacity to proliferate and by the expression of the progenitor markers Nestin, Flk-1, Chx10, Pax6 and the radial glia marker BLBP. The neurogenic potential was assayed by the expression of the neuronal markers doublecortin, betaIII Tubulin, Map2 and NSE, the glial makers A2B5, NG2, GalC and GFAP, and by incorporation of BrdU. The number of Flk-1 positive cells and concomitantly the number of newly born betaIII Tubulin-positive cells decreased within the first postnatal week in retinal progenitor cultures and no newly generated betaIII Tubulin, but GFAP positive cells were detected thereafter. In contrast to neural stem cells derived from the adult SVZ, postnatal and adult CB derived progenitors had a lower and a restricted proliferation potential and did not generate oligodendrocytes. The work demonstrates, however, that the existence of retinal progenitor cells is not restricted to embryonic development. In the sensory retina the differentiation potential of late retinal progenitors becomes restricted to the glial lineage, whereas neurogenic progenitor cells are still present in the CB. In addition, major differences in growth and differentiation potential of adult neural stem cells and postnatal and adult retinal progenitors are presented.

Fate of multipotent neural precursor cells transplanted into mouse retina selectively depleted of retinal ganglion cells

In some parts of the CNS, depletion of a particular class of neuron might induce changes in the microenvironment that influence the differentiation of newly grafted neural precursor cells. This hypothesis was tested in the retina by inducing apoptotic retinal ganglion cell (RGC) death in neonatal and adult female mice and examining whether intravitreally grafted male neural precursor cells (C17.2), a neural stem cell (NSC)-like clonal line, become incorporated into these selectively depleted retinae. In neonates, rapid RGC death was induced by removal of the contralateral superior colliculus (SC), in adults, delayed RGC death was induced by unilateral optic nerve (ON) transection. Cells were injected intravitreally 6-48 h after SC ablation (neonates) or 0-7 days after ON injury (adults). Cells were also injected into non-RGC depleted neonatal and adult retinae. At 4 or 8 weeks, transplanted cells were identified using a Y-chromosome marker and in situ hybridisation or by their expression of the lacZ reporter gene product Escherichia coli beta-galactosidase (beta-gal). No C17.2 cells were identified in axotomised adult-injected eyes undergoing delayed RGC apoptosis (n = 16). Donor cells were however stably integrated within the retina in 29% (15/55) of mice that received C17.2 cell injections 24 h after neonatal SC ablation; 6-31% of surviving cells were found in the RGC layer (GCL). These NSC-like cells were also present in intact retinae, but on average, there were fewer cells in GCL. In SC-ablated mice, most grafted cells did not express retinal-specific markers, although occasional donor cells in the GCL were immunopositive for beta-III tubulin, a protein highly expressed by, but not specific to, developing RGCs. Targeted rapid RGC depletion thus increased cell incorporation into the GCL, but grafted C17.2 cells did not appear to differentiate into an RGC phenotype.

Embryonic stem cells are capable of generating a neuronal network in the adult mouse retina

The integration of embryonic stem (ES) cells as well-differentiated neuronal cells into the retinas of adult mice was investigated. ES cells were transplanted in adult mouse retinas by intravitreal injections. Neuronal differentiation of the ES cells was investigated by morphological and immunohistochemical examinations on post-operative days 5 and 30. ES cell apoptosis was examined by in situ terminal dUTP-biotin nick end labeling of DNA fragments. Differentiated ES cells growing along the retinal surface developed fine neuronal cell processes around cell nuclei and generated neuronal networks into the retinal inner plexiform layer (IPL) 30 days after transplantation. The differentiated ES cells expressed retinal and neuronal markers. Many apoptotic cells were recognized in transplanted ES cells at day 5 but not at day 30 after transplantation. ES cells may be useful for neural tissue regeneration in the adult mammalian retina.

Transplantation of neural stem cells into explants of rat inner ear

Damage and loss of hair cells in the inner ear is the most frequent cause of hearing loss and balance disorders. Mammalian hair cells do not regenerate in the conventional ways. To regenerate the hair cell in the mammalian inner ear we transplanted neural stem cells into explants of rat inner ear. The stem cells integrated successfully into the sensory epithelium of the vestibular organs, but not into the organ of Corti. This method is useful to investigate efficient ways to transplant stem cells into the inner ear.

Lens and retina regeneration: transdifferentiation, stem cells and clinical applications

In this review we present a synthesis on the potential of vertebrate eye tissue regeneration, such as lens and retina. Particular emphasis is given to two different strategies used for regeneration, transdifferentiation and stem cells. Similarities and differences between these two strategies are outlined and it is proposed that both strategies might follow common pathways. Furthermore, we elaborate on specific clinical applications as the outcome of regeneration-based research.