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.

Migration, integration and maturation of photoreceptor precursors following transplantation in the mouse retina

Retinal degeneration leading to loss of photoreceptors is a major cause of untreatable blindness. Recent research has yielded definitive evidence for restoration of vision following the transplantation of rod photoreceptors in murine models of blindness, while advances in stem cell biology have enabled the generation of transplantable photoreceptors from embryonic stem cells. Importantly, the amount of visual function restored is dependent upon the number of photoreceptors that migrate correctly into the recipient retina. The developmental stage of the donor cells is important for their ability to migrate; they must be immature photoreceptor precursors. Little is known about how and when donor cell migration, integration, and maturation occurs. Here, we have performed a comprehensive histological analysis of the 6-week period following rod transplantation in mice. Donor cells migrate predominately as single entities during the first week undergoing a stereotyped sequence of morphological changes in their translocation from the site of transplantation, through the interphotoreceptor matrix and into the recipient retina. This includes initial polarization toward the outer nuclear layer (ONL), followed by formation of an apical attachment and rudimentary segment during migration into the ONL. Strikingly, acquisition of a nuclear architecture typical of mature rods was accelerated compared with normal development and a feature of migrating cells. Once within the ONL, precursors formed synaptic-like structures and outer segments in accordance with normal maturation. The restoration of visual function mediated by transplanted photoreceptors correlated with the later expression of rod α-transducin, achieving maximal function by 5 weeks.

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.

Outside-the-(cavity-prep)-box thinking

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Photoreceptor precursors derived from three-dimensional embryonic stem cell cultures integrate and mature within adult degenerate retina

Irreversible blindness caused by loss of photoreceptors may be amenable to cell therapy. We previously demonstrated retinal repair and restoration of vision through transplantation of photoreceptor precursors obtained from postnatal retinas into visually impaired adult mice. Considerable progress has been made in differentiating embryonic stem cells (ESCs) in vitro toward photoreceptor lineages. However, the capability of ESC-derived photoreceptors to integrate after transplantation has not been demonstrated unequivocally. Here, to isolate photoreceptor precursors fit for transplantation, we adapted a recently reported three-dimensional (3D) differentiation protocol that generates neuroretina from mouse ESCs. We show that rod precursors derived by this protocol and selected via a GFP reporter under the control of a Rhodopsin promoter integrate within degenerate retinas of adult mice and mature into outer segment-bearing photoreceptors. Notably, ESC-derived precursors at a developmental stage similar to postnatal days 4-8 integrate more efficiently compared with cells at other stages. This study shows conclusively that ESCs can provide a source of photoreceptors for retinal cell transplantation.

Photoreceptor precursors derived from three-dimensional embryonic stem cell cultures integrate and mature within adult degenerate retina

Irreversible blindness caused by loss of photoreceptors may be amenable to cell therapy. We previously demonstrated retinal repair and restoration of vision through transplantation of photoreceptor precursors obtained from postnatal retinas into visually impaired adult mice. Considerable progress has been made in differentiating embryonic stem cells (ESCs) in vitro toward photoreceptor lineages. However, the capability of ESC-derived photoreceptors to integrate after transplantation has not been demonstrated unequivocally. Here, to isolate photoreceptor precursors fit for transplantation, we adapted a recently reported three-dimensional (3D) differentiation protocol that generates neuroretina from mouse ESCs. We show that rod precursors derived by this protocol and selected via a GFP reporter under the control of a Rhodopsin promoter integrate within degenerate retinas of adult mice and mature into outer segment-bearing photoreceptors. Notably, ESC-derived precursors at a developmental stage similar to postnatal days 4-8 integrate more efficiently compared with cells at other stages. This study shows conclusively that ESCs can provide a source of photoreceptors for retinal cell transplantation.

Defining the integration capacity of embryonic stem cell-derived photoreceptor precursors

Retinal degeneration is a leading cause of irreversible blindness in the developed world. Differentiation of retinal cells, including photoreceptors, from both mouse and human embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs), potentially provide a renewable source of cells for retinal transplantation. Previously, we have shown both the functional integration of transplanted rod photoreceptor precursors, isolated from the postnatal retina, in the adult murine retina, and photoreceptor cell generation by stepwise treatment of ESCs with defined factors. In this study, we assessed the extent to which this protocol recapitulates retinal development and also evaluated differentiation and integration of ESC-derived retinal cells following transplantation using our established procedures. Optimized retinal differentiation via isolation of Rax.GFP retinal progenitors recreated a retinal niche and increased the yield of Crx(+) and Rhodopsin(+) photoreceptors. Rod birth peaked at day 20 of culture and expression of the early photoreceptor markers Crx and Nrl increased until day 28. Nrl levels were low in ESC-derived populations compared with developing retinae. Transplantation of early stage retinal cultures produced large tumors, which were avoided by prolonged retinal differentiation (up to day 28) prior to transplantation. Integrated mature photoreceptors were not observed in the adult retina, even when more than 60% of transplanted ESC-derived cells expressed Crx. We conclude that exclusion of proliferative cells from ESC-derived cultures is essential for effective transplantation. Despite showing expression profiles characteristic of immature photoreceptors, the ESC-derived precursors generated using this protocol did not display transplantation competence equivalent to precursors from the postnatal retina.

Restoration of vision after transplantation of photoreceptors

Cell transplantation is a potential strategy for treating blindness caused by the loss of photoreceptors. Although transplanted rod-precursor cells are able to migrate into the adult retina and differentiate to acquire the specialized morphological features of mature photoreceptor cells, the fundamental question remains whether transplantation of photoreceptor cells can actually improve vision. Here we provide evidence of functional rod-mediated vision after photoreceptor transplantation in adult Gnat1−/− mice, which lack rod function and are a model of congenital stationary night blindness. We show that transplanted rod precursors form classic triad synaptic connections with second-order bipolar and horizontal cells in the recipient retina. The newly integrated photoreceptor cells are light-responsive with dim-flash kinetics similar to adult wild-type photoreceptors. By using intrinsic imaging under scotopic conditions we demonstrate that visual signals generated by transplanted rods are projected to higher visual areas, including V1. Moreover, these cells are capable of driving optokinetic head tracking and visually guided behaviour in the Gnat1−/− mouse under scotopic conditions. Together, these results demonstrate the feasibility of photoreceptor transplantation as a therapeutic strategy for restoring vision after retinal degeneration.

Effective transplantation of photoreceptor precursor cells selected via cell surface antigen expression

Retinal degenerative diseases are a major cause of untreatable blindness. Stem cell therapy to replace lost photoreceptors represents a feasible future treatment. We previously demonstrated that postmitotic photoreceptor precursors expressing an NrlGFP transgene integrate into the diseased retina and restore some light sensitivity. As genetic modification of precursor cells derived from stem cell cultures is not desirable for therapy, we have tested cell selection strategies using fluorochrome-conjugated antibodies recognizing cell surface antigens to sort photoreceptor precursors. Microarray analysis of postnatal NrlGFP-expressing precursors identified four candidate genes encoding cell surface antigens (Nt5e, Prom1, Podxl, and Cd24a). To test the feasibility of using donor cells isolated using cell surface markers for retinal therapy, cells selected from developing retinae by fluorescence-activated cell sorting based on Cd24a expression (using CD24 antibody) and/or Nt5e expression (using CD73 antibody) were transplanted into the wild-type or Crb1(rd8/rd8) or Prph2(rd2/rd2) mouse eye. The CD73/CD24-sorted cells migrated into the outer nuclear layer, acquired the morphology of mature photoreceptors and expressed outer segment markers. They showed an 18-fold higher integration efficiency than that of unsorted cells and 2.3-fold higher than cells sorted based on a single genetic marker, NrlGFP, expression. These proof-of-principle studies show that transplantation competent photoreceptor precursor cells can be efficiently isolated from a heterogeneous mix of cells using cell surface antigens without loss of viability for the purpose of retinal stem cell therapy. Refinement of the selection of donorphotoreceptor precursor cells can increase the number of integrated photoreceptor cells,which is a prerequisite for the restoration of sight.

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.

Long-term preservation of cones and improvement in visual function following gene therapy in a mouse model of leber congenital amaurosis caused by guanylate cyclase-1 deficiency

Leber congenital amaurosis (LCA) is a severe retinal dystrophy manifesting from early infancy as poor vision or blindness. Loss-of-function mutations in GUCY2D cause LCA1 and are one of the most common causes of LCA, accounting for 20% of all cases. Human GUCY2D and mouse Gucy2e genes encode guanylate cyclase-1 (GC1), which is responsible for restoring the dark state in photoreceptors after light exposure. The Gucy2e(-/-) mouse shows partially diminished rod function, but an absence of cone function before degeneration. Although the cones appear morphologically normal, they exhibit mislocalization of proteins involved in phototransduction. In this study we tested the efficacy of an rAAV2/8 vector containing the human rhodopsin kinase promoter and the human GUCY2D gene. Following subretinal delivery of the vector in Gucy2e(-/-) mice, GC1 protein was detected in the rod and cone outer segments, and in transduced areas of retina cone transducin was appropriately localized to cone outer segments. Moreover, we observed a dose-dependent restoration of rod and cone function and an improvement in visual behavior of the treated mice. Most importantly, cone preservation was observed in transduced areas up to 6 months post injection. To date, this is the most effective rescue of the Gucy2e(-/-) mouse model of LCA and we propose that a vector, similar to the one used in this study, could be suitable for use in a clinical trial of gene therapy for LCA1.

Long-term and age-dependent restoration of visual function in a mouse model of CNGB3-associated achromatopsia following gene therapy

Mutations in the CNGB3 gene account for >50% of all known cases of achromatopsia. Although of early onset, its stationary character and the potential for rapid assessment of restoration of retinal function following therapy renders achromatopsia a very attractive candidate for gene therapy. Here we tested the efficacy of an rAAV2/8 vector containing a human cone arrestin promoter and a human CNGB3 cDNA in CNGB3 deficient mice. Following subretinal delivery of the vector, CNGB3 was detected in both M- and S-cones and resulted in increased levels of CNGA3, increased cone density and survival, improved cone outer segment structure and normal subcellular compartmentalization of cone opsins. Therapy also resulted in long-term improvement of retinal function, with restoration of cone ERG amplitudes of up to 90% of wild-type and a significant improvement in visual acuity. Remarkably, successful restoration of cone function was observed even when treatment was initiated at 6 months of age; however, restoration of normal visual acuity was only possible in younger animals (e.g. 2–4 weeks old). This study represents achievement of the most substantial restoration of visual function reported to date in an animal model of achromatopsia using a human gene construct, which has the potential to be utilized in clinical trials.

Dominant cone-rod dystrophy: a mouse model generated by gene targeting of the GCAP1/Guca1a gene

Cone dystrophy 3 (COD3) is a severe dominantly inherited retinal degeneration caused by missense mutations in GUCA1A, the gene encoding Guanylate Cyclase Activating Protein 1 (GCAP1). The role of GCAP1 in controlling cyclic nucleotide levels in photoreceptors has largely been elucidated using knock-out mice, but the disease pathology in these mice cannot be extrapolated directly to COD3 as this involves altered, rather than loss of, GCAP1 function. Therefore, in order to evaluate the pathology of this dominant disorder, we have introduced a point mutation into the murine Guca1a gene that causes an E155G amino acid substitution; this is one of the disease-causing mutations found in COD3 patients. Disease progression in this novel mouse model of cone dystrophy was determined by a variety of techniques including electroretinography (ERG), retinal histology, immunohistochemistry and measurement of cGMP levels. It was established that although retinal development was normal up to 3 months of age, there was a subsequent progressive decline in retinal function, with a far greater alteration in cone than rod responses, associated with a corresponding loss of photoreceptors. In addition, we have demonstrated that accumulation of cyclic GMP precedes the observed retinal degeneration and is likely to contribute to the disease mechanism. Importantly, this knock-in mutant mouse has many features in common with the human disease, thereby making it an excellent model to further probe disease pathogenesis and investigate therapeutic interventions.

Long-term survival of photoreceptors transplanted into the adult murine neural retina requires immune modulation

Stem cell therapy presents an opportunity to replace photoreceptors that are lost as a result of inherited and age-related degenerative disease. We have previously shown that murine postmitotic rod photoreceptor precursor cells, identified by expression of the rod-specific transcription factor Nrl, are able to migrate into and integrate within the adult murine neural retina. However, their long-term survival has yet to be determined. Here, we found that integrated Nrl.gfp(+ve) photoreceptors were present up to 12 months post-transplantation, albeit in significantly reduced numbers. Surviving cells had rod-like morphology, including inner/outer segments and spherule synapses. In a minority of eyes, we observed an early, marked reduction in integrated photoreceptors within 1 month post-transplantation, which correlated with increased numbers of amoeboid macrophages, indicating acute loss of transplanted cells due to an inflammatory response. In the majority of transplants, similar numbers of integrated cells were observed between 1 and 2 months post-transplantation. By 4 months, however, we observed a significant decrease in integrated cell survival. Macrophages and T cells were present around the transplantation site, indicating a chronic immune response. Immune suppression of recipients significantly increased transplanted photoreceptor survival, indicating that the loss observed in unsuppressed recipients resulted from T cell-mediated host immune responses. Thus, if immune responses are modulated, correctly integrated transplanted photoreceptors can survive for extended periods of time in hosts with partially mismatched H-2 haplotypes. These findings suggest that autologous donor cells are optimal for therapeutic approaches to repair the neural retina, though with immune suppression nonautologous donors may be effective.

Comparative analysis of the retinal potential of embryonic stem cells and amniotic fluid-derived stem cells

Photoreceptors have recently been generated from mouse and human embryonic stem cells (ESCs), although ethics concerns impede their utilization for cell replacement therapy for retinal disease. Extra-embryonic tissues have received attention as alternative therapeutic sources of stem cells. Human and mouse amniotic fluid-derived stem cells (AFCs) have been reported to be multipotent and express embryonic and adult stem cell markers. Here, in vitro conditions that generate retinal cells from ESCs were used to analyze and compare the retinal potential of murine AFCs and ESCs. We show that AFCs express pluripotency markers (Nanog, Sox2, and Oct3/4) as well as retinal transcription factor genes (Et, Lhx2, Tll1, Six6, Otx2, Pax6, and Fgf15). AFCs from amniotic fluid of Fgf15.gfp, Nrl.gfp, and Crx.gfp embryos cultured in retinal proliferation and differentiation conditions failed to switch on these retinal transgenes. AFCs cultured in retinal-promoting conditions, effective on ESCs, showed reduced expression of retinal markers. Retinal co-cultures activated retinal genes in ESCs but not in AFCs, and migration assays in retinal explants showed limited migration of AFCs compared with ESCs. Unlike ESCs, AFCs do not express the early embryonic ectodermal gene Utf1 and Western analysis of AFCs identified only the B isoform of Oct3/4, rather than the isoform A present in ESCs. We conclude that AFCs have restricted potential and differ considerably from ESCs and retinal progenitor cells. Reprogramming to induce pluripotency or new differentiation protocols will be required to confer retinal potential to AFCs as expression of a subset of pluripotency and retinal markers is not sufficient.

Cone and rod photoreceptor transplantation in models of the childhood retinopathy Leber congenital amaurosis using flow-sorted Crx-positive donor cells

Retinal degenerative disease causing loss of photoreceptor cells is the leading cause of untreatable blindness in the developed world, with inherited degeneration affecting 1 in 3000 people. Visual acuity deteriorates rapidly once the cone photoreceptors die, as these cells provide daylight and colour vision. Here, in proof-of-principle experiments, we demonstrate the feasibility of cone photoreceptor transplantation into the wild-type and degenerating retina of two genetic models of Leber congenital amaurosis, the Crb1^rd8/rd8 and Gucy2e^−/− mouse. Crx-expressing cells were flow-sorted from the developing retina of CrxGFP transgenic mice and transplanted into adult recipient retinae; CrxGFP is a marker of cone and rod photoreceptor commitment. Only the embryonic-stage Crx-positive donor cells integrated within the outer nuclear layer of the recipient and differentiated into new cones, whereas postnatal cells generated a 10-fold higher number of rods compared with embryonic-stage donors. New cone photoreceptors displayed unambiguous morphological cone features and expressed mature cone markers. Importantly, we found that the adult environment influences the number of integrating cones and favours rod integration. New cones and rods were observed in ratios similar to that of the host retina (1:35) even when the transplanted population consisted primarily of cone precursors. Cone integration efficiency was highest in the cone-deficient Gucy2e^−/− retina suggesting that cone depletion creates a more optimal environment for cone transplantation. This is the first comprehensive study demonstrating the feasibility of cone transplantation into the adult retina. We conclude that flow-sorted embryonic-stage Crx-positive donor cells have the potential to replace lost cones, as well as rods, an important requirement for retinal disease therapy.

Adult ciliary epithelial cells, previously identified as retinal stem cells with potential for retinal repair, fail to differentiate into new rod photoreceptors

The ciliary margin in lower vertebrates is a site of continual retinal neurogenesis and a stem cell niche. By contrast, the human eye ceases retinal neuron production before birth and loss of photoreceptors during life is permanent and a major cause of blindness. The discovery of a proliferative cell population in the ciliary epithelium (CE) of the adult mammalian eye, designated retinal stem cells, raised the possibility that these cells could help to restore sight by replacing lost photoreceptors. We previously demonstrated the feasibility of photoreceptor transplantation using cells from the developing retina. CE cells could provide a renewable source of photoreceptors for transplantation. Several laboratories reported that these cells generate new photoreceptors, whereas a recent report questioned the existence of retinal stem cells. We used Nrl.gfp transgenic mice that express green fluorescent protein in rod photoreceptors to assess definitively the ability of CE cells to generate new photoreceptors. We report that CE cells expanded in monolayer cultures, lose pigmentation, and express a subset of eye field and retinal progenitor cell markers. Simultaneously, they continue to express some markers characteristic of differentiated CE and typically lack a neuronal morphology. Previously reported photoreceptor differentiation conditions used for CE cells, as well as conditions used to differentiate embryonic retinal progenitor cells (RPCs) and embryonic stem cell-derived RPCs, do not effectively activate the Nrl-regulated photoreceptor differentiation program. Therefore, we conclude that CE cells lack potential for photoreceptor differentiation and would require reprogramming to be useful as a source of new photoreceptors.

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.

Neuron network activity scales exponentially with synapse density

Neuronal network output in the cortex as a function of synapse density during development has not been explicitly determined. Synaptic scaling in cortical brain networks seems to alter excitatory and inhibitory synaptic inputs to produce a representative rate of synaptic output. Here, we cultured rat hippocampal neurons over a three-week period to correlate synapse density with the increase in spontaneous spiking activity. We followed the network development as synapse formation and spike rate in two serum-free media optimized for either (a) neuron survival (Neurobasal/B27) or (b) spike rate (NbActiv4). We found that while synaptophysin synapse density increased linearly with development, spike rates increased exponentially in developing neuronal networks. Synaptic receptor components NR1, GluR1 and GABA-A also increase linearly but with more excitatory receptors than inhibitory. These results suggest that the brain's information processing capability gains more from increasing connectivity of the processing units than increasing processing units, much as Internet information flow increases much faster than the linear number of nodes and connections.

Relative economic benefits of tactical anthelmintic treatment and urea-molasses block supplementation of Boer goats raised under extensive grazing conditions at Onderstepoort, Pretoria, South Africa

The potential economic benefits of combining tactical anthelmintic treatment for gastrointestinal nematodes and nutritional supplementation with urea-molasses blocks were examined in Boer goats raised under extensive grazing conditions in the summer rainfall area of South Africa. Eight groups of nine goats were monitored over a 12-month period from 1 October 2002 to 9 October 2003. Ad libitum nutritional supplementation with urea-molasses blocks was provided when the goats were housed at night, during the summer (wet season--December 2002 to February 2003), and/or the winter (dry season--June 2003 to August 2003). All the goats were treated symptomatically for Haemonchus contortus infection when deemed necessary by clinical examination of the conjunctiva for anaemia using the FAMACHA system. Half the groups were tactically treated for gastrointestinal nematodes in mid-summer (28 January 2003). Under the symptomatic treatment, climatic and extensive grazing conditions encountered during the trial, feed supplementation in the winter dry season had the greatest economic benefit and is therefore recommended. Tactical anthelmintic treatment afforded no additional advantage, but the nematode challenge was low.

Stem cell therapy and the retina

Retinal degeneration culminating in photoreceptor loss is the leading cause of untreatable blindness in the developed world. In this review, we consider how photoreceptors might be replaced by transplantation and how stem cells might be optimised for use as donor cells in future clinical strategies for retinal repair. We discuss the current advances in human and animal models of retinal cell transplantation, focussing on stem cell and reproductive cloning biology, in relation to the practical issues of retinal transplantation surgery. Stem and progenitor cells can be isolated from a number of sources including embryonic tissue, adult brain and even the retina, prompting many researchers to investigate the potential for using these cells to generate photoreceptors for transplantation. Nevertheless, several obstacles need to be overcome before these techniques can be applied in a clinical setting. Embryonic or stem cells have so far shown little ability to differentiate into retinal phenotypes when transplanted into the adult retina. We have recently noted, however, that donor cells harvested much later, at the photoreceptor precursor developmental stage, can be transplanted successfully and restore visual function. The current challenge is to understand the developmental processes that guide embryonic or adult stem cells towards photoreceptor differentiation, so that large numbers of these cells might be transplanted at the optimal stage. Future advances in reproductive cloning technology could lead to the successful generation of stem cells from adult somatic cells, thereby facilitating auto-transplantation of genetically identical cells in patients requiring photoreceptor replacement.

Pharmacological disruption of the outer limiting membrane leads to increased retinal integration of transplanted photoreceptor precursors

Retinal degeneration is the leading cause of untreatable blindness in the developed world. Cell transplantation strategies provide a novel therapeutic approach to repair the retina and restore sight. Previously, we have shown that photoreceptor precursor cells can integrate and form functional photoreceptors after transplantation into the subretinal space of the adult mouse. In a clinical setting, however, it is likely that far greater numbers of integrated photoreceptors would be required to restore visual function. We therefore sought to assess whether the outer limiting membrane (OLM), a natural barrier between the subretinal space and the outer nuclear layer (ONL), could be reversibly disrupted and if disruption of this barrier could lead to enhanced numbers of transplanted photoreceptors integrating into the ONL. Transient chemical disruption of the OLM was induced in adult mice using the glial toxin, dl-alpha-aminoadipic acid (AAA). Dissociated early post-natal neural retinal cells were transplanted via subretinal injection at various time-points after AAA administration. At 3 weeks post-injection, the number of integrated, differentiated photoreceptor cells was assessed and compared with those found in the PBS-treated contralateral eye. We demonstrate for the first time that the OLM can be reversibly disrupted in adult mice, using a specific dose of AAA administered by intravitreal injection. In this model, OLM disruption is maximal at 72 h, and recovers by 2 weeks. When combined with cell transplantation, disruption of the OLM leads to a significant increase in the number of photoreceptors integrated within the ONL compared with PBS-treated controls. This effect was only seen in animals in which AAA had been administered 72 h prior to transplantation, i.e. when precursor cells were delivered into the subretinal space at a time coincident with maximal OLM disruption. These findings suggest that the OLM presents a physical barrier to photoreceptor integration following transplantation into the subretinal space in the adult mouse. Reversible disruption of the OLM may provide a strategy for increasing cell integration in future therapeutic applications.

Benefits of urea-molasses block supplementation and symptomatic and tactical anthelmintic treatments of communally grazed indigenous goats in the Bulwer area, Kwazulu-Natal Province, South Africa

This study was carried out with the cooperation of farmers owning communally grazed indigenous goats in southwestern KwaZulu-Natal Province, South Africa, where farmers had identified poor reproductive performance in their herds as one of their major problems. The aim was to quantify the effects of 3 interventions and the interaction between these interventions on goat productivity and gastrointestinal nematode infection. The interventions were: urea-molasses block supplementation during the dry winter seasons of 2004 and 2005, tactical anthelmintic treatment with ivermectin (400 microg/kg) during the wet summer period (on 3 January 2005) and symptomatic treatment with ivermectin (400 microg/kg) of all goats judged anaemic throughout the entire study period. The FAMACHA system was used as a gauge of anaemia. It was noted that goats considered anaemic tended to remain so throughout the study period. The tactical anthelmintic treatment was effective as it markedly reduced (P = 0.066) the summer peak in faecal egg counts and is therefore recommended. By contrast, while the urea-molasses block supplementation appeared to reduce the faecal egg counts immediately following the 2004 supplementation (P < 0.05), this did not hold true in 2005. Interestingly, in the tactically treated anaemic goats, the improvement in the number of kids suckled per doe year-on-year tended to be greater than in the non-anaemic goats. It is considered that the routine symptomatic treatment of anaemic goats may have been a key factor. More detailed investigations into the routine symptomatic treatment of anaemic goats are therefore recommended.