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

Cellular strategies for retinal repair by photoreceptor replacement

Loss of photoreceptors due to retinal degeneration is a major cause of blindness in the developed world. While no effective treatment is currently available, cell replacement therapy, using pluripotent stem cell-derived photoreceptor precursor cells, may be a feasible future treatment. Recent reports have demonstrated rescue of visual function following the transplantation of immature photoreceptors and we have seen major advances in our ability to generate transplantation-competent donor cells from stem cell sources. Moreover, we are beginning to realise the possibilities of using endogenous populations of cells from within the retina itself to mediate retinal repair. Here, we present a review of our current understanding of endogenous repair mechanisms together with recent progress in the use of both ocular and pluripotent stem cells for the treatment of photoreceptor loss. We consider how our understanding of retinal development has underpinned many of the recent major advances in translation and moved us closer to the goal of restoring vision by cellular means.

Multipotent adult hippocampal progenitor cells maintained as neurospheres favor differentiation toward glial lineages

Adult hippocampal progenitor cells (AHPCs) are generally maintained as a dispersed monolayer population of multipotent neural progenitors. To better understand cell-cell interactions among neural progenitors and their influences on cellular characteristics, we generated free-floating cellular aggregates, or neurospheres, from the adherent monolayer population of AHPCs. Results from in vitro analyses demonstrated that both populations of AHPCs were highly proliferative under maintenance conditions, but AHPCs formed in neurospheres favored differentiation along a glial lineage and displayed greater migrational activity than the traditionally cultured AHPCs. To study the plasticity of AHPCs from both populations in vivo, we transplanted green fluorescent protein (GFP)-expressing AHPCs via intraocular injection into the developing rat eyes. Both AHPC populations were capable of surviving and integrating into developing host central nervous system, but considerably more GFP-positive cells were observed in the retinas transplanted with neurosphere AHPCs, compared to adherent AHPCs. These results suggest that the culture configuration during maintenance for neural progenitor cells (NPCs) influences cell fate and motility in vitro as well as in vivo. Our findings have implication for understanding different cellular characteristics of NPCs according to distinct intercellular architectures and for developing cell-based therapeutic strategies using lineage-committed NPCs.

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.

Advances in repairing the degenerate retina by rod photoreceptor transplantation

Despite very different aetiologies, age-related macular degeneration (AMD) and most inherited retinal disorders culminate in the same final common pathway, loss of the light-sensitive photoreceptors. There are few clinical treatments and none can reverse the loss of vision. Photoreceptor replacement by transplantation is proposed as a broad treatment strategy applicable to all degenerations. The past decade has seen a number of landmark achievements in this field, which together provide strong justification for continuing investigation into photoreceptor replacement strategies. These include proof of principle for restoring vision by rod-photoreceptor transplantation in mice with congenital stationary night blindness and advances in stem cell biology, which have led to the generation of complete optic structures in vitro from embryonic stem cells. The latter represents enormous potential for generating suitable and renewable donor cells with which to achieve the former. However, there are still challenges presented by the degenerating recipient retinal environment that must be addressed as we move to translating these technologies towards clinical application.

Mouse retinal progenitor cell dynamics on electrospun poly (ϵ-caprolactone)

Age-related macular degeneration, retinitis pigmentosa and glaucoma are among the many retinal degenerative diseases where retinal cell death leads to irreversible vision loss and blindness. Working toward a cell-replacement-based therapy for such diseases, a number of research groups have recently evaluated the feasibility of using retinal progenitor cells (RPCs) cultured and transplanted on biodegradable polymer substrates to replace damaged retinal tissue. Appropriate polymer substrate design is essential to providing a three-dimensional environment that can facilitate cell adhesion, proliferation and post-transplantation migration into the host environment. In this study, we have designed and fabricated a novel, ultra-thin electrospun poly(ϵ-caprolactone) (PCL) scaffold with microscale fiber diameters, appropriate porosity for infiltration by RPCs, and biologically compatible mechanical characteristics. We have verified that our electrospun PCL scaffold supports robust mouse RPC proliferation, adhesion, and differentiation in vitro, as well as migration into mouse retinal explants. These promising results make PCL a strong candidate for further development as a cell transplantation substrate in retinal regenerative research.

Manipulation of the recipient retinal environment by ectopic expression of neurotrophic growth factors can improve transplanted photoreceptor integration and survival

Degeneration of the neural retina is the leading cause of untreatable blindness in the developed world. Stem cell replacement therapy offers a novel strategy for retinal repair. Postmitotic photoreceptor precursors derived from the early postnatal (P) retina are able to migrate and integrate into the adult mouse retina following transplantation into the subretinal space, but it is likely that a large number of these cells would be required to restore vision. The adult recipient retina presents a very different environment to that from which photoreceptor precursor donor cells isolated from the developing postnatal retina are derived. Here we considered the possibility that modulation of the recipient environment by ectopic expression of developmentally regulated growth factors, normally present during photoreceptor development, might enhance the migration and integration of transplanted cells into the adult neural retina. Adeno-associated viral (AAV) vectors were used to introduce three growth factors previously reported to play a role in photoreceptor development, IGF1, FGF2, and CNTF, into the adult retina, prior to transplantation of P4 cells derived from the Nrl.GFP(+ve) neural retina. At 3 weeks posttransplantation the number of integrated, differentiated photoreceptor cells present in AAV-mediated neurotrophic factor-treated eyes was assessed and compared to control treated contralateral eyes. We show, firstly, that it is possible to manipulate the recipient retinal microenvironment via rAAV-mediated gene transfer with respect to these developmentally relevant growth factors. Moreover, when combined with cell transplantation, AAV-mediated expression of IGF1 led to significantly increased levels of cell integration, while overexpression of FGF2 had no significant effect on integrated cell number. Conversely, expression of CNTF led to a significant decrease in cell integration and an exacerbated glial response that led to glial scarring. Together, these findings demonstrate the importance of the extrinsic environment of the recipient retina for photoreceptor cell transplantation and show for the first time that it is possible to manipulate this environment using viral vectors to influence photoreceptor transplantation efficiency.

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.

Cell transplantation strategies for retinal repair

Cell transplantation is a novel therapeutic strategy to restore visual responses to the degenerate adult neural retina and represents an exciting area of regenerative neurotherapy. So far, it has been shown that transplanted postmitotic photoreceptor precursors are able to functionally integrate into the adult mouse neural retina. In this review, we discuss the differentiation of photoreceptor cells from both adult and embryonic-derived stem cells and their potential for retinal cell transplantation. We also discuss the strategies used to overcome barriers present in the degenerate neural retina and improve retinal cell integration. Finally, we consider the future translation of retinal cell therapy as a therapeutic strategy to treat retinal degeneration.

The opossum genome: insights and opportunities from an alternative mammal

The strategic importance of the genome sequence of the gray, short-tailed opossum, Monodelphis domestica, accrues from both the unique phylogenetic position of metatherian (marsupial) mammals and the fundamental biologic characteristics of metatherians that distinguish them from other mammalian species. Metatherian and eutherian (placental) mammals are more closely related to one another than to other vertebrate groups, and owing to this close relationship they share fundamentally similar genetic structures and molecular processes. However, during their long evolutionary separation these alternative mammals have developed distinctive anatomical, physiologic, and genetic features that hold tremendous potential for examining relationships between the molecular structures of mammalian genomes and the functional attributes of their components. Comparative analyses using the opossum genome have already provided a wealth of new evidence regarding the importance of noncoding elements in the evolution of mammalian genomes, the role of transposable elements in driving genomic innovation, and the relationships between recombination rate, nucleotide composition, and the genomic distributions of repetitive elements. The genome sequence is also beginning to enlarge our understanding of the evolution and function of the vertebrate immune system, and it provides an alternative model for investigating mechanisms of genomic imprinting. Equally important, availability of the genome sequence is fostering the development of new research tools for physical and functional genomic analyses of M. domestica that are expanding its versatility as an experimental system for a broad range of research applications in basic biology and biomedically oriented research.

Optic Nerve Restoration

Neural regeneration and repair in the central nervous system are currently hot topics in neuroscience. For many years there has been a hope that neurodegenerative diseases which are resistant to current therapies may be treated by the selective replacement of cells. Yet it is only recently that we have started to acquire the knowledge, tools, and techniques that may translate such optimism into new therapies. In this article, we will consider the potential to restore function to the damaged optic nerve. We will consider the technical issues involved and suggest a strategy for research progress.

Neural stem cells for the treatment of disorders of the enteric nervous system: strategies and challenges

The main goal of this review is to summarize the status of the research in the field of stem cells transplantation, as it is applicable to the treatment of gastrointestinal motility. This field of research has advanced tremendously in the past 10 years, and recent data produced in our laboratories as well as others is contributing to the excitement on the use of neural stem cells (NSC) as a valuable therapeutic approach for disorders of the enteric nervous system characterized by a loss of critical neuronal subpopulations. There are several sources of NSC, and here we describe therapeutic strategies for NSC transplantation in the gut. These include using NSC as a relatively nonspecific cellular replacement strategy in conditions where large populations of neurons or their subsets are missing or destroyed. As with many other recent "breakthroughs" stem cell therapy may eventually prove to be overrated. However, at the present time, it does appear to provide the hope for a true cure for many currently intractable diseases of both the central and the peripheral nervous system. Certainly more extensive research is needed in this field. We hope that our review will encourage new investigators in entering this field of research ad contribute to our knowledge of the potentials of NSC and other cells for the treatment of gastrointestinal dysmotility.

Status and applications of genomic resources for the gray, short-tailed opossum, Monodelphis domestica, an American marsupial model for comparative biology

Owing to its small size, favourable reproductive characteristics, and simple husbandry, the gray, short-tailed opossum, Monodelphis domestica, has become the most widely distributed and intensively utilised laboratory-bred research marsupial in the world today. This article provides an overview of the current state and future projections of genomic resources for this species and discusses the potential impact of this growing resource base on active research areas that use M. domestica as a model system. The resources discussed include: fully arrayed, bacterial artificial chromosome (BAC) libraries; an expanding linkage map; developing full-genome BAC-contig and chromosomal fluorescence in situ hybridisation maps; public websites providing access to the M. domestica whole-genome-shotgun sequence trace database and the whole-genome sequence assembly; and a new project underway to create an expressed-sequence database and microchip expression arrays for functional genomics applications. Major research areas discussed span a variety of genetic, evolutionary, physiologic, reproductive, developmental, and behavioural topics, including: comparative immunogenetics; genomic imprinting; reproductive biology; neurobiology; photobiology and carcinogenesis; genetics of lipoprotein metabolism; developmental and behavioural endocrinology; sexual differentiation and development; embryonic and fetal development; meiotic recombination; genome evolution; molecular evolution and phylogenetics; and more.