Yanai Duran

Effective gene therapy with nonintegrating lentiviral vectors

Retroviral and lentiviral vector integration into host-cell chromosomes carries with it a finite chance of causing insertional mutagenesis. This risk has been highlighted by the induction of malignancy in mouse models, and development of lymphoproliferative disease in three individuals with severe combined immunodeficiency–X1 (refs. 2,3). Therefore, a key challenge for clinical therapies based on retroviral vectors is to achieve stable transgene expression while minimizing insertional mutagenesis. Recent in vitro studies have shown that integration-deficient lentiviral vectors can mediate stable transduction. With similar vectors, we now show efficient and sustained transgene expression in vivo in rodent ocular and brain tissues. We also show substantial rescue of clinically relevant rodent models of retinal degeneration. Therefore, the high efficiency of gene transfer and expression mediated by lentiviruses can be harnessed in vivo without a requirement for vector integration. For therapeutic application to postmitotic tissues, this system substantially reduces the risk of insertional mutagenesis.

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.

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 repair1 and restoration of vision through transplantation of photoreceptor precursors obtained from post-natal retinas into visually impaired adult mice2,3. Considerable progress has been made in differentiating embryonic stem cells (ESCs) in vitro toward photoreceptor lineages4-6. 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 ESCs6. We show that Rhop.GFP-selected rod precursors derived by this protocol integrate within degenerate retinae 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 than cells at other stages. This study shows conclusively that ESCs can provide a source of photoreceptors for retinal cell transplantation.

The Drusenlike Phenotype in Aging Ccl2-Knockout Mice Is Caused by an Accelerated Accumulation of Swollen Autofluorescent Subretinal Macrophages

PURPOSE Drusen, which are defined clinically as yellowish white spots in the outer retina, are cardinal features of age-related macular degeneration (AMD). Ccl2-knockout (Ccl2(-/-)) mice have been reported to develop drusen and phenotypic features similar to AMD, including an increased susceptibility to choroidal neovascularization (CNV). This study was conducted to investigate the nature of the drusenlike lesions in vivo and further evaluate the Ccl2(-/-) mouse as a model of AMD. METHODS The eyes of 2- to 25-month-old Ccl2(-/-) and C57Bl/6 mice were examined in vivo by autofluorescence scanning laser ophthalmoscopy (AF-SLO) and electroretinography, and the extent of laser-induced CNV was measured by fluorescein fundus angiography. The retinal morphology was also assessed by immunohistochemistry and quantitative histologic and ultrastructural morphometry. RESULTS The drusenlike lesions of Ccl2(-/-) mice comprised accelerated accumulation of swollen CD68(+), F4/80(+) macrophages in the subretinal space that were apparent as autofluorescent foci on AF-SLO. These macrophages contained pigment granules and phagosomes with outer segment and lipofuscin inclusions that may account for their autofluorescence. Only age-related retinal pigment epithelium (RPE) damage, photoreceptor loss, and sub-RPE deposits were observed but, despite the accelerated accumulation of macrophages, we identified no spontaneous development of CNV in the senescent mice and found a reduced susceptibility to laser-induced CNV in the Ccl2(-/-) mice. CONCLUSIONS These findings suggest that the lack of Ccl2 leads to a monocyte/macrophage-trafficking defect during aging and to an impaired recruitment of these cells to sites of laser injury. Other, previously described features of Ccl2(-/-) mice that are similar to AMD may be the result of aging alone.

Repair of the degenerate retina by photoreceptor transplantation

Despite different aetiologies, age-related macular degeneration and most inherited retinal disorders culminate in the same final common pathway, the loss of photoreceptors. There are few treatments and none reverse the loss of vision. Photoreceptor replacement by transplantation is proposed as a broad treatment strategy applicable to all degenerations. Recently, we demonstrated restoration of vision following rod-photoreceptor transplantation into a mouse model of stationary night-blindness, raising the critical question of whether photoreceptor replacement is equally effective in different types and stages of degeneration. We present a comprehensive assessment of rod-photoreceptor transplantation across six murine models of inherited photoreceptor degeneration. Transplantation is feasible in all models examined but disease type has a major impact on outcome, as assessed both by the morphology and number of integrated rod-photoreceptors. Integration can increase (Prph2+/Δ307), decrease (Crb1rd8/rd8, Gnat1−/−, Rho−/−), or remain constant (PDE6βrd1/rd1, Prph2rd2/rd2) with disease progression, depending upon the gene defect, with no correlation with severity. Robust integration is possible even in late-stage disease. Glial scarring and outer limiting membrane integrity, features that change with degeneration, significantly affect transplanted photoreceptor integration. Combined breakdown of these barriers markedly increases integration in a model with an intact outer limiting membrane, strong gliotic response, and otherwise poor transplantation outcome (Rho−/−), leading to an eightfold increase in integration and restoration of visual function. Thus, it is possible to achieve robust integration across a broad range of inherited retinopathies. Moreover, transplantation outcome can be improved by administering appropriate, tailored manipulations of the recipient environment.

Assessment of ocular transduction using single-stranded and self-complementary recombinant adeno-associated virus serotype 2/8

To date adeno-associated viral (AAV) vectors are the only gene therapy vectors that have been shown to efficiently transduce photoreceptor cells and have thus become the most commonly used vector for ocular transduction. Various AAV serotypes have been evaluated in the eye, the first of which was AAV2, which is able to transduce photoreceptors, retinal pigment epithelium (RPE) and retinal ganglion cells. AAV serotypes 1 and 4, as well as AAV2 pseudotyped with these capsids, only transduce the RPE. AAV serotype 5 and AAV2/5 transduce the photoreceptors as well as RPE, but not retinal ganglion cells. Here, we assessed the capacity of the novel serotype AAV2/8 to transduce various ocular tissues of the adult murine retina by administering AAV2/8 green fluorescent protein intravitreally, subretinally and intracamerally. We also determined the kinetics and efficiency of self-complementary AAV (scAAV) vectors of serotypes 2/2, 2/5 and 2/8 and compared them with single-stranded AAV (ssAAV). We found that ssAAV2/8 transduces photoreceptors and RPE more efficiently than ssAAV2/2 and ssAAV2/5, and that scAAV2/8 had faster onset and higher transgene expression than ssAAV2/8. This improved transduction efficiency might facilitate the development of improved gene therapy protocols for inherited retinal degenerations, particularly those caused by defects in photoreceptor-specific genes.

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 Crb1rd8/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.

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. STEM CELLS 2010;28:1997–2007

HIF-1alpha and HIF-2alpha are differentially activated in distinct cell populations in retinal ischaemia.

Background Hypoxia plays a key role in ischaemic and neovascular disorders of the retina. Cellular responses to oxygen are mediated by hypoxia-inducible transcription factors (HIFs) that are stabilised in hypoxia and induce the expression of a diverse range of genes. The purpose of this study was to define the cellular specificities of HIF-1alpha and HIF-2alpha in retinal ischaemia, and to determine their correlation with the pattern of retinal hypoxia and the expression profiles of induced molecular mediators. Methodology/Principal Findings We investigated the tissue distribution of retinal hypoxia during oxygen-induced retinopathy (OIR) in mice using the bio-reductive drug pimonidazole. We measured the levels of HIF-1alpha and HIF-2alpha proteins by Western blotting and determined their cellular distribution by immunohistochemistry during the development of OIR. We measured the temporal expression profiles of two downstream mediators, vascular endothelial growth factor (VEGF) and erythropoietin (Epo) by ELISA. Pimonidazole labelling was evident specifically in the inner retina. Labelling peaked at 2 hours after the onset of hypoxia and gradually declined thereafter. Marked binding to Müller glia was evident during the early hypoxic stages of OIR. Both HIF-1alpha and HIF-2alpha protein levels were significantly increased during retinal hypoxia but were evident in distinct cellular distributions; HIF-1alpha stabilisation was evident in neuronal cells throughout the inner retinal layers whereas HIF-2alpha was restricted to Müller glia and astrocytes. Hypoxia and HIF-alpha stabilisation in the retina were closely followed by upregulated expression of the downstream mediators VEGF and EPO. Conclusions/Significance Both HIF-1alpha and HIF-2alpha are activated in close correlation with retinal hypoxia but have contrasting cell specificities, consistent with differential roles in retinal ischaemia. Our findings suggest that HIF-2alpha activation plays a key role in regulating the response of Müller glia to hypoxia.

Donor and host photoreceptors engage in material transfer following transplantation of post-mitotic photoreceptor precursors

Photoreceptor replacement by transplantation is proposed as a treatment for blindness. Transplantation of healthy photoreceptor precursor cells into diseased murine eyes leads to the presence of functional photoreceptors within host retinae that express an array of donor-specific proteins. The resulting improvement in visual function was understood to be due to donor cells integrating within host retinae. Here, however, we show that while integration occurs the majority of donor-reporter-labelled cells in the host arises as a result of material transfer between donor and host photoreceptors. Material transfer does not involve permanent donor–host nuclear or cell–cell fusion, or the uptake of free protein or nucleic acid from the extracellular environment. Instead, RNA and/or protein are exchanged between donor and host cells in vivo. These data require a re-evaluation of the mechanisms underlying rescue by photoreceptor transplantation and raise the possibility of material transfer as a strategy for the treatment of retinal disorders.