Rescue of retinal degeneration by intravitreally injected adult bone marrow–derived lineage-negative hematopoietic stem cells

Inherited retinal degenerations afflict 1 in 3,500 individuals and are a heterogeneous group of diseases that result in profound vision loss, usually the result of retinal neuronal apoptosis. Atrophic changes in the retinal vasculature are also observed in many of these degenerations. While it is thought that this atrophy is secondary to diminished metabolic demand in the face of retinal degeneration, the precise relationship between the retinal neuronal and vascular degeneration is not clear. In this study we demonstrate that whenever a fraction of mouse or human adult bone marrow–derived stem cells (lineage-negative hematopoietic stem cells [Lin^– HSCs]) containing endothelial precursors stabilizes and rescues retinal blood vessels that would ordinarily completely degenerate, a dramatic neurotrophic rescue effect is also observed. Retinal nuclear layers are preserved in 2 mouse models of retinal degeneration, rd1 and rd10, and detectable, albeit severely abnormal, electroretinogram recordings are observed in rescued mice at times when they are never observed in control-treated or untreated eyes. The normal mouse retina consists predominantly of rods, but the rescued cells after treatment with Lin^– HSCs are nearly all cones. Microarray analysis of rescued retinas demonstrates significant upregulation of many antiapoptotic genes, including small heat shock proteins and transcription factors. These results suggest a new paradigm for thinking about the relationship between vasculature and associated retinal neuronal tissue as well as a potential treatment for delaying the progression of vision loss associated with retinal degeneration regardless of the underlying genetic defect.

Inherited degenerations of the retina affect as many as 1 in 3,500 individuals and are characterized by progressive night blindness, visual field loss, optic nerve atrophy, arteriolar attenuation, altered vascular permeability, and central loss of vision often progressing to complete blindness (1). Molecular genetic analysis of these diseases has identified mutations in over 110 different genes accounting for only a relatively small percentage of the known affected individuals (2, 3); many of these mutations are associated with enzymatic and structural components of the phototransduction machinery including rhodopsin (4), cGMP phosphodiesterase (5), peripherin (6), and RPE65 (7). Despite these observations, there are still no effective treatments to slow or reverse the progression of these diseases. Recent advances in gene therapy have led to successful reversal of the retinal degeneration, slow (rds) (8) and retinal degeneration (rd) (9) phenotypes in mice and the RPE65 phenotype in dogs (10) when the WT transgene is delivered to photoreceptors (PRs) or the retinal pigmented epithelium in animals with a specific mutation. The potential use of calcium channel blockers (11), trophic factors (12), and dietary supplements (13) has also been explored in recent studies. Most inherited human retinal degenerations specifically affect rod PRs, but there is concomitant loss of cones, the principal cellular component of the macula, the region of the retina in humans that is responsible for central, fine visual acuity. Cone-specific survival factors have been described recently (14) and may facilitate cone survival in mouse models of retinal degeneration.

Adult bone marrow is known to contain a population of hematopoietic stem cells (HSCs) that can be divided into lineage positive (Lin⁺) and lineage negative (Lin^–) subpopulations with regard to their potential to differentiate into formed elements of the blood. The Lin^– population contains a variety of progenitor cells including those capable of becoming vascular endothelial cells (15). These endothelial progenitor cells (EPCs) mobilize from the bone marrow in response to a variety of signaling molecules (16, 17) and can target sites of angiogenesis in ischemic peripheral vasculature (15), myocardium (18), or induced ocular injury (19). We have reported that Lin^– HSCs injected directly into the eye can target activated astrocytes and participate in normal developmental angiogenesis in neonatal mice or injury-induced neovascularization in the adult (20). We now report that adult bone marrow–derived Lin^– HSCs, when injected intravitreally up to 2 weeks postnatally, can completely prevent retinal vascular degeneration ordinarily observed in the rd1 or rd10 mouse models of retinal degeneration, and this vascular rescue correlates with neuronal rescue. The inner nuclear layer (INL) remains nearly normal, and the outer nuclear layer (ONL) containing PRs is significantly preserved with the rescued cells being predominantly cones. Detectable, albeit severely abnormal, electroretinogram (ERG) recordings are observed in rescued mice at times when they are never observed in control-treated, or untreated, rd/rd eyes. This rescue effect is also observed when human bone marrow–derived (hBM-derived) Lin^– HSCs are used to treat SCID mice with retinal degeneration. Large-scale genomic analysis of rescued and nonrescued eyes revealed significant upregulation of antiapoptotic genes. These findings demonstrate that this newly described neurotrophic effect correlates with preservation of the vasculature and suggest that autologous bone marrow–derived EPCs may be useful in the treatment of currently untreatable blinding disease.

The rd1/rd1 mouse retinas exhibit profound vascular and neuronal degeneration. Normal postnatal retinal vascular and neuronal development in mice has been well described and is analogous to changes observed in the third-trimester human fetus (21). Mice homozygous for the rd1 gene share many characteristics of human retinal degeneration (11) and exhibit rapid PR loss accompanied by severe vascular atrophy as the result of a mutation in the gene encoding PR cGMP phosphodiesterase (5). To examine the vasculature during retinal development and its subsequent degeneration, we used Ab’s against collagen IV (CIV), an ECM protein of mature vasculature, and CD31 (PECAM-1), a marker for endothelial cells (Figure 1). Retinas of rd1/rd1 (C3H/HeJ) developed normally until approximately postnatal day 8 (P8), when degeneration of the PR containing the ONL begins. The ONL rapidly degenerates, and cells die by apoptosis (22), such that only a single layer of nuclei remains by P20 (23). Double staining of the whole-mounted retinas with Ab’s to both CIV and CD31 revealed details of the vascular degeneration in rd1/rd1 mice similar to that described by others (24). The primary and deep retinal vascular layers appear to develop normally through P12, after which there is a rapid loss of endothelial cells as evidenced by the absence of CD31 staining; CD31-positive endothelial cells are present in a normal distribution through P12 but rapidly disappear after that. Interestingly, CIV-positive staining remains present throughout the time points examined, suggesting that the vessels and associated ECM form normally, but that only the matrix remains after P13, by which time no CD31-positive cells are observed. (Figure 1, middle panels). The intermediate vascular plexus also degenerates after P21, but the progression is slower than that observed in the deep plexus (Figure 1, upper panel). Retinal vascular and neural cell layers of a normal mouse are shown for comparison with the rd1/rd1 mouse (right panels, Figure 1).

Figure 1

Retinal vasculature and neural cell changes in rd1/rd1 (C3H/HeJ, left panels) or WT mice (C57BL/6, right panels). Retinal vasculature of intermediate (top row) or deep (middle row) vascular plexuses in whole-mounted retinas (red: CIV; green: CD31) and sections (bottom row; red: DAPI; green: CD31) of the same retinas are shown. GCL, ganglion cell layer; Int., intermediate plexus; deep, deep plexus.

Neuroprotective effect of bone marrow–derived Lin^– HSCs in rd1/rd1 mice. We have reported that intravitreally injected Lin^– HSCs incorporate into endogenous retinal vasculature in all three vascular plexuses and prevent the vessels from degenerating (20). Interestingly, the injected cells are never observed in the ONL; these cells either incorporate into the forming retinal vessels or are observed in close proximity to these vessels. To investigate whether Lin^– HSCs could also slow or prevent the degeneration of neuronal cells in mouse eyes with retinal degeneration we injected Lin^– HSCs (from C3H/HeJ) intravitreally into C3H/HeJ (rd1/rd1) mouse eyes at P6, just prior to the onset of degeneration. By P30, control cell–injected (CD31^–) eyes exhibited the typical rd1/rd1 phenotype; nearly complete degeneration of the deep vascular plexus, and ONL was observed in every retina examined. Eyes injected with Lin^– HSCs maintained normal-appearing intermediate and deep vascular plexuses and, surprisingly, significantly more cells in the INL and ONL than control cell–injected eyes (Figure 2A). This rescue effect of Lin^– HSCs can be observed at 2 months (Figure 2B) and for as long as 6 months after injection (Figure 2C). The effect is observed by injecting Lin^– HSCs up to 16 days postnatally; injection after this time did not produce significant rescue (data not shown). Differences in the vasculature of the intermediate and deep plexuses, as well as the neuronal cell–containing INL and ONL, were significant at all time points measured when rescued and nonrescued eyes were compared (Figure 2, A–C). This effect was quantified by measuring the total length of the vasculature (Figure 2D) and counting the number of DAPI-positive cell nuclei observed in the ONL (Figure 2E).

Figure 2

Lin^– HSC injection rescues the degeneration of vascular and neural cells in rd1/rd1 mice. (A_C) Retinal vasculature of intermediate or deep plexus and cross sections of Lin^– HSC_injected eye (Lin^–, right panels in each part) and contralateral control cell_injected (CD31^–) eye (left panels in each part) at P30 (A), P60 (B), and P180 (C). (D) The average total length of vasculature (± SEM) in Lin^– HSC_injected or control cell_injected (CD31^–) retinas at P30 (left, n = 10), P60 (middle, n = 10), and P180 (right, n = 6). Data of intermediate and deep vascular plexus are shown separately (y axis: relative length of vasculature). (E) The average numbers of cell nuclei in the ONL at P30 (left, n = 10), P60 (middle, n = 10), or P180 (right, n = 6) of control cell_injected (CD31^–) or Lin^– HSC_injected retinas (y axis: relative number of cell nuclei in the ONL). (F) Simple linear correlations between the length of vasculature (x axis) and the number of cell nuclei in the ONL (y axis) at P30 (left), P60 (middle), and P180 (right) of Lin^– HSC_injected or control cell_injected retinas.

To determine whether there is a relationship between the length of the vasculature and the number of ONL cells, simple linear-regression analysis was applied to the data at all time points. We consistently observed a statistically significant correlation between vascular rescue and neuronal (e.g., ONL thickness) rescue at P30 (P < 0.024) and P60 (P < 0.034) in the Lin^– HSC–injected eyes (Figure 2F). The correlation remains high, although not statistically significant (P < 0.14), at P180 when comparing Lin^– HSC–injected retinas to control cell–injected retinas (Figure 2F). In contrast, control cell–injected retinas showed no significant correlation between the preservation of vasculature and ONL at any time point (Figure 2F). These data demonstrate that intravitreal injection of Lin^– HSCs results in concomitant retinal vascular and neuronal rescue in retinas of rd1/rd1 mice. Injected cells were never observed in the ONL or any place other than within, or in close proximity to, retinal blood vessels.

Functional rescue of Lin^– HSC–injected rd/rd retinas. To test whether Lin^– HSC treatment could also induce functional rescue of vision in rd1/rd1 mice, we performed ERGs on mice 2 months after injection of control or Lin^– HSCs. Immunohistochemical and microscopic analyses (Figure 3, A and B) were done with each eye following ERG recordings to confirm that vascular and neuronal rescue had, in fact, occurred. Representative ERG recordings from treated rescued and control nonrescued eyes are shown in Figure 3C. Clearly, the signal from the rescued eye is severely abnormal. Consistent and detectable ERGs with amplitudes on the order of 8–10 μV were recordable from the Lin^– HSC–treated eyes, however; in all cases the ERG from the control eye was nondetectable. While the amplitudes of the signals in rescued eyes were considerably lower than normal and their pattern atypical, they were consistently observed whenever there was histological rescue and are on the order of magnitude of those reported by other, gene-based, rescue studies. Overall, these results are suggestive of some degree of functional rescue in treated eyes.

Figure 3

Retinal function is rescued by Lin^– HSC injection. ERG recordings were used to measure the function of Lin^– HSC_injected or control cell_injected (CD31^–) retinas. (A and B) Representative cases of rescued and nonrescued retinas 2 months after injection. Retinal section of Lin^– HSC_injected right eye (A) and CD31^– control cell_injected left eye (B) of the same animal are shown (green: CD31-stained vasculature; red: DAPI-stained nuclei). (C) ERG responses from the same eyes shown in A and B.

Rescued rd/rd retinal cell types are predominantly cones. To determine the cellular basis of the functional rescue, we analyzed rescued and nonrescued retinas immunohistochemically with Ab’s specific for rod or cone opsin. The same eyes used for the ERG recordings presented in Figure 3 were analyzed for rod or cone opsin. In WT mouse retinas, less than 5% of PRs present are cones (25), and the immunohistochemical staining patterns we observed with red/green cone opsin (Figure 4A) or rod rhodopsin (Figure 4B) are consistent with this. When WT retinas are stained with preimmune IgG, no staining is observed anywhere in the neurosensory retinas other than autofluorescence of the blood vessels (Figure 4C). Two months after birth, retinas of noninjected rd/rd mice have an essentially atrophic ONL that does not exhibit any staining with Ab’s to red/green cone opsin (Figure 4D) or rhodopsin (Figure 4G). Eyes injected with control CD31^– HSCs also do not stain positively for the presence of either cone (Figure 4E) or rod (Figure 4H) opsin. In contrast, contralateral eyes injected with Lin^– HSC have three to six rows of nuclei in a preserved ONL; most of these cells are positive for cone opsin (Figure 4F), with approximately 1–3% positive for rod opsin (Figure 4I). Remarkably, this is nearly the reverse of what is ordinarily observed in the normal mouse retina, which is rod dominated (26). These data suggest that the injection of Lin^– HSCs preserves cones for extended periods of time during which they would ordinarily degenerate.

Figure 4

Rescued PRs in rd1 ONL following intravitreal injection of Lin^– HSCs are predominantly cones. A small percentage of PRs in the WT mouse retina (upper images) are cones as evidenced by expression of red/green cone opsin (A), while most cells of the ONL are positive for rod-specific rhodopsin (B). Retinal vasculature autofluoresces with preimmune serum (C), but nuclear layers are completely negative for staining with rod- or cone-specific opsins. The rd/rd mouse retinas (lower images) have a diminished INL and a nearly completely atrophic ONL, both negative for cone (D) or rod (G) opsin. Control, CD31^– HSC_treated eyes are identical to noninjected rd/rd retinas, without any staining for cone (E) or rod (H) opsin. Lin^– HSC_treated contralateral eyes have a markedly reduced, but clearly present, ONL that is predominantly made up of cones as evidenced by positive immunoreactivity for red/green cone opsin (F). A small number of rods are also observed (I). rd, red; gr, green.

The hBM–derived Lin^– HSCs also rescue degenerating retinas. We have also examined whether there is a population of cells in hBM that behave similarly to mouse Lin^– HSCs. Bone marrow was collected from human donors and the Lin⁺ HSCs were depleted, producing a population of human Lin^– HSCs (hLin^– HSCs). These cells were labeled ex vivo with fluorescent dye and injected into C3SnSmn.CB17-Prkdc^SCID mouse eyes. The injected hLin^– HSCs migrated to and targeted sites of retinal angiogenesis in a fashion identical to that observed when mouse Lin^– HSCs were injected (Figure 5A) (20). In addition to the vascular targeting, the human stem cells also provided a robust rescue effect on both the vascular and neuronal cell layers of the rd1/rd1 mice (Figure 5, B and C). This observation confirms the presence of cells in hBM that target retinal vasculature and can prevent its degeneration in mice with inherited retinal degeneration.

Figure 5

A population of hBM cells can rescue degenerating retinas in the rd1 mouse (A_C), and the rescue is also observed using mouse bone marrow_derived Lin^– HSCs in another model of retinal degeneration, rd10 (D_K). (A) hLin^– HSCs labeled with green dye targeted to developing vasculature after intravitreal injection into C3SnSmn.CB17-Prkdc^SCID mice. (B and C) Retinal vasculature (upper-left panels: intermediate plexus; lower-left panels, deep plexus) and neural cells (right panels) in hLin^– HSC_injected eye (B) or contralateral control eye (C) 1.5 months after injection. (D_K) Rescue of rd10 mice by Lin^– HSCs (injected at P6). Representative retinas at P21 (D: Lin^– HSCs; H: control cells), P30 (E: Lin^– HSCs; I: control cells); P60 (F: Lin^– HSCs; J: control cells), and P105 (G: Lin^– HSCs; K: control cells) are shown (treated and control eyes are from the same animal at each time point). Retinal vasculature (upper image in each part is the intermediate plexus; the middle image in each part is the deep plexus) was stained with CD31 (green) and CIV (red). The lower image in each part shows a cross section made from the same retina (red: DAPI; green: CD31).

Lin^– HSCs have vasculotrophic and neurotrophic effects in the rd10/rd10 mouse. While the rd1/rd1 mouse is the most widely used and best-characterized model for retinal degeneration (26), the degeneration is very rapid and in this regard differs from the usual, slower time course observed in the human disease. In this strain, PR cell degeneration begins around P8, a time when the retinal vasculature is still rapidly expanding (Figure 1). Subsequent degeneration of the deep retinal vasculature occurs even while the intermediate plexus is still forming, and thus the retinas of rd1/rd1 mice never completely develop, unlike that observed in most humans with this disease. To investigate whether HSC-mediated vascular rescue could prevent retinal neuronal degeneration that occurs after the neurosensory retina and its corresponding vasculature is largely developed, we decided to use another model for retinal degeneration that has a slower time course of degeneration and, thus, may more closely resemble the human condition. In the rd10 mouse, PR cell degeneration begins around P21 (26), and shortly after this, vascular degeneration begins. Since normal neurosensory retinal development is largely complete by P21, the degeneration is observed to start after the retina has completed differentiation and in this way is more analogous to human retinal degenerations. We injected Lin^– HSCs or control cells from rd10 mice into P6 eyes and evaluated the retinas at varying time points. At P21 the retinas from both Lin^– HSC– and control cell–injected eyes appeared normal, with complete development of all vascular layers and normal development of the INL and ONL (Figure 5, D and H). At approximately P21, the retinal degeneration begins and progresses with age. By P30, the control cell–injected retinas exhibited severe vascular and neuronal degeneration (Figure 5I), while the Lin^– HSC–injected retinas maintained nearly normal vascular layers and PR cells (Figure 5E). The difference between rescued and nonrescued eyes was most pronounced at P30 (compare Figure 5E with Figure 5I), but is also observed at later time points (compare Figure 5, F and G with J and K). In the control-treated eyes, the progression of vascular degeneration was very clearly observed by immunohistochemical staining for CD31 and CIV (Figure 5, I–K); these eyes were nearly completely negative for CD31 whereas CIV-positive vascular “tracks” remained evident, indicating that vascular regression, rather than incomplete vascular formation, had occurred. In contrast, Lin^– HSC-injected eyes had both CD31 and CIV-positive vessels that appeared very similar to normal, WT eyes (compare Figure 5E with Figure 5I and with WT vasculature in Figure 1, rightmost panel).

Gene expression analysis of rd/rd mouse retinas after Lin^– HSC treatment. To begin to identify putative mediators of the neurotrophic rescue effect in mice with retinal degeneration, we used large-scale genomics (microarray analysis) to analyze rescued and nonrescued retinas. Gene expression in rd1/rd1 mouse retinas treated with Lin^– HSCs was compared with uninjected retinas as well as retinas injected with control cells (CD31^–). These experiments were each done in triplicate. To be considered present, genes were required to have expression levels at least twofold higher than background levels in all three triplicates. Genes that were upregulated threefold in Lin^– HSC–protected retinas compared with control cell–injected and noninjected rd/rd mouse retinas are shown in Table 1. Coefficient of variance levels were calculated for the expressed genes by dividing the SD by the mean expression level of each copy RNA (cRNA) replicate. In addition, the correlation between expression levels and noise variance was calculated by correlating the mean [ln(expression)] and SD [ln(expression)]. This demonstrated a correlation between gene-expression levels and SDs for each gene, allowing us to determine background levels and reliable expression-level thresholds. As a whole, our data fell well within acceptable limits (27), and individually discussed genes fell above these critical expression levels. Paired t test values for the discussed genes are also presented. In each case, P values are reasonable (near or below 0.05), demonstrating similarities between replicates and probable significant differences between the different test groups. Many of the significantly upregulated genes, including MAD and Ying Yang-1 (28), encode proteins with functions involving the protection of cells from apoptosis. A number of crystallin genes, which have sequence homology and functions similar to known heat shock proteins involving protection of cells from stress, were also upregulated by Lin^– HSC treatment. Expression of α-crystallin was localized to the ONL by immunohistochemical analysis (Figure 6).

Figure 6

Crystallin αA is upregulated in rescued ONL cells after treatment with Lin^– HSCs but not in contralateral eyes treated with control cells. Left: IgG control in rescued retina; middle: crystallin αA in rescued retina; right: crystallin αA in nonrescued retina.

Table 1

Gene expression analysis

To identify factors directly expressed by the HSCs themselves, we hybridized mRNA from rd1/rd1 mouse retinas rescued with hBM-derived HSCs to human-specific microarray chips. After stringent analysis, a number of genes were found whose mRNA expression was human specific, above background, and significantly higher in the human HSC-rescued retinas compared with the mouse HSC-rescued and human control cell–injected nonrescued retinas (Table 2). CD6, a cell adhesion molecule expressed at the surface of primitive and newly differentiated CD34⁺ hematopoietic stem cells (29), and IFNα-13, another gene expressed by hematopoietic stem cells (30), were both found by our microarray bioinformatics, helping to validate the experimental protocol. In addition, several growth factors and neurotrophic factors were expressed above background by human HSCs in rescued mouse retina samples (Table 2).

Table 2

Gene expression analysis

We have demonstrated that adult bone marrow–derived Lin^– HSCs exert profound vasculotrophic and neurotrophic effects when injected intravitreally into mice with retinal degenerative disease. This rescue effect persists for up to 6 months after treatment and is most efficacious when the Lin^– HSCs are injected prior to complete retinal degeneration (up to 16 days after birth in mice that ordinarily exhibit practically complete ONL degeneration by 30 days postnatally). This rescue is observed in two mouse models of retinal degeneration and, remarkably, can be accomplished with adult hBM-derived HSCs when the recipient is an immunodeficient rodent with retinal degeneration (e.g., the SCID mouse) or when the donor is itself a mouse with retinal degeneration. PRs in the normal mouse retina are predominantly rods, but the ONL observed after rescue with Lin^– HSCs contains predominantly cones. While several recent reports have described a partial phenotypic rescue in mice or dogs with retinal degeneration after viral-based gene therapy with the WT gene (8–10), this is the first report of a generic cell-based therapeutic effect correlating with vascular rescue. When attempting to treat a group of diseases (e.g., retinitis pigmentosa, or RP) that can be caused by mutations in over 100 known genes, such an approach is potentially more practical than creating individual gene therapies to treat each mutation. Most inherited human retinal degenerations occur as a result of primary rod-specific defects, and loss of the cones is believed to be secondary to rod dysfunction, perhaps related to the loss of some trophic factor expressed by rods (14). Our observation that cone survival in the face of rod/retinal degeneration is facilitated by Lin^– HSCs suggests that it may be possible to better preserve the cone-dominated human macula in diseases such as RP.

The precise molecular basis of the neurotrophic rescue effect remains unknown but is observed only when there is concomitant vascular stabilization/rescue. The presence of injected stem cells, per se, is not sufficient to generate a neurotrophic rescue, and the clear absence of stem cell–derived neurons in the ONL rules out the possibility that the injected cells are transforming into PRs. Data obtained by microarray gene expression analysis demonstrated a significant upregulation of genes known to have antiapoptotic effects. Since most neuronal death observed in retinal degenerations is by apoptosis, such protection may be of great therapeutic benefit in prolonging the life of PRs and other neurons critical to visual function in these diseases. C-myc is a transcription factor that participates in apoptosis (31) by upregulating various downstream apoptosis-inducing factors. C-myc expression was increased 4.5-fold in rd/rd mice over WT (data not shown), indicating potential involvement in the PR degeneration observed in the rd1/rd1 mouse. Mad1 and YY-1, two genes dramatically upregulated in Lin^–-protected retinas (Table 1), are known to suppress the activity of c-myc, thus inhibiting c-myc–induced apoptosis (28, 32). Overexpression of Mad1 has also been shown to suppress Fas-induced activation of caspase-8 (32), another critical component of the apoptotic pathway. Upregulation of these two molecules may play a role in protection of the retina from vascular and neural degeneration by preventing the initiation of apoptosis that normally leads to degeneration in rd/rd mice.

Another set of genes that were greatly upregulated in Lin^– HSC–protected retinas includes members of the crystallin family (Table 1B). Similar to heat shock and other stress-induced proteins, crystallins may be activated by retinal stress and provide a protective effect against apoptosis (33). Abnormally low expression of αA-crystallins is correlated with PR loss in a rat model of retinal dystrophy (34), and a recent proteomic analysis of the retina in the rd/rd mouse demonstrated induction of crystallin upregulation in response to retinal degeneration (35). Based on our microarray data of Lin^– HSC rescued rd/rd mouse retinas, upregulation of crystallins may also play a key role in EPC-mediated retinal neuroprotection.

Genes such as c-myc, MadI, Yy-1, and the crystallins are likely to be downstream mediators of neuronal rescue. Presumably, neurotrophic agents serve to regulate antiapoptotic gene expression, although our microarray analysis of retinas rescued with mouse stem cells did not demonstrate induction of increased levels of known neurotrophic factors. Analysis of hBM-derived stem cell–mediated rescue with human-specific chips did, on the other hand, demonstrate low but significant increases in the expression of multiple growth factor genes. While these data are preliminary, they include several members of the fibroblast growth factor family and otoferlin. Mutations in the otoferlin gene are associated with genetic disorders leading to deafness due to auditory neuropathy (36, 37). It is possible that otoferlin production by injected Lin^– HSCs is critical to the prevention of retinal neuropathy as well.

Historically, it has long been assumed that vascular changes observed in patients and animals with retinal degeneration were secondary to decreased metabolic demand as the PRs die. Our data suggest that, at least for mice with inherited retinal degeneration due to mutations in the cGMP phosphodiesterase gene, preserving normal vasculature correlates with preservation of cones in the ONL as well. Recent reports in the literature would support the concept that tissue-specific vasculature has trophic effects that go beyond that expected from simply providing vascular “nourishment.” For example, liver endothelial cells can be induced to produce, after VEGFR1 activation, growth factors critical to hepatocyte regeneration and maintenance in the face of hepatic injury (38). Similar interactions between vascular endothelial cells and adjacent hepatic parenchymal cells are necessary for liver organogenesis, as well as for the formation of functional blood vessels (39). Endogenous retinal vasculature in individuals with retinal degeneration may not facilitate so dramatic a rescue, but perhaps the presence of endothelial progenitors derived from bone marrow hematopoietic stem cell populations makes the vasculature more resistant to degeneration and at the same time facilitates retinal neuronal survival. In humans with this disease, slowing the rate of degeneration, even to a limited extent, may provide years of additional sight. While the animals treated in this study had significant, but not dramatic, preservation of ERG signals, even such minimal signs of electrical function in the retina may be sufficient to support vision.

Clinically, it is widely appreciated that there may be substantial loss of PRs and other neurons while still preserving functional vision. At some point, the critical threshold is crossed and vision is lost. Since nearly all of the human inherited retinal degenerations are of early, but slow, onset, it may be possible to identify an individual with retinal degeneration, treat them intravitreally with an autologous bone marrow stem cell graft, and delay retinal degeneration with concomitant loss of vision. This would be particularly effective if, similar to our observations in the mouse model presented in this study, rescue of the cones would occur. To enhance targeting and incorporation of these stem cells, the presence of activated astrocytes would be desirable (20); this could be accomplished by early treatment when there is an associated gliosis or by using laser to stimulate local proliferation of activated astrocytes. Since the actual mechanism of stem cell–mediated rescue remains unknown, ex vivo transfection of the cells with one or several neurotrophic substances prior to intraocular injection may enhance the rescue effect. If the observed neuronal rescue effect is achieved by Lin^– HSC–derived vascular endothelial cell induction of antiapoptotic factor synthesis in a paracrine fashion, it may be possible to apply this approach to the treatment of other visual neuronal degenerative disorders such as glaucoma, in which there is retinal ganglion cell degeneration. Such investigations are currently in progress.

We thank Edith Aguilar and other members of the Friedlander laboratory for assistance and helpful discussions during the course of this study, Matthew Ritter for critical comments on the manuscript, and Maria Concepcion Lillo Delgado of the Scripps Research Institute/University of California, San Diego Eye Core for her expertise in the EM analysis of tissue specimens. We are grateful to Robert Molday of the University of British Columbia for generously providing us with the rod opsin–specific Ab. This work was supported by grants from the National Eye Institute (EY11254 to M. Friedlander), the Robert Mealey Program for the Study of Macular Degenerations, and the Kovner Family Fund.

See the related Commentary beginning on page 755.

John Heckenlively’s present address is: Kellogg Eye Center, University of Michigan, Ann Arbor, Michigan, USA.

Atsushi Otani’s present address is: Department of Ophthalmology, Kyoto University, Kyoto, Japan.

Nonstandard abbreviations used: CIV, collagen IV; EPC, endothelial progenitor cell; ERG, electroretinogram; hBM, human bone marrow; hLin^– HSC, human lineage-negative hematopoietic stem cell; HSC, hematopoietic stem cell; INL, inner nuclear layer; Lin^–, lineage negative; Lin⁺, lineage positive; ONL, outer nuclear layer; P, postnatal day; PR, photoreceptor; rd, retinal degeneration; rd1, retinal degeneration 1 mutation; rds, retinal degeneration, slow; RP, retinitis pigmentosa.

Conflict of interest: The authors have declared that no conflict of interest exists.

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