Monitoring photoreceptor transplants with nuclear and cytoplasmic markers

Rebuilding the Missing Part-A Review on Photoreceptor Transplantation

Vision represents one of the main senses for humans to interact with their environment. Our sight relies on the presence of fully functional light sensitive cells – rod and cone photoreceptors — allowing us to see under dim (rods) and bright (cones) light conditions. Photoreceptor degeneration is one of the major causes for vision impairment in industrialized countries and it is highly predominant in the population above the age of 50. Thus, with the continuous increase in life expectancy it will make retinal degeneration reach an epidemic proportion. To date, there is no cure established for photoreceptor loss, but several therapeutic approaches, spanning from neuroprotection, pharmacological drugs, gene therapy, retinal prosthesis, and cell (RPE or photoreceptor) transplantation, have been developed over the last decade with some already introduced in clinical trials. In this review, we focus on current developments in photoreceptor transplantation strategies, its major breakthroughs, current limitations and the next challenges to translate such cell-based approaches toward clinical application.

Cell replacement and visual restoration by retinal sheet transplants

Retinal diseases such as age-related macular degeneration (ARMD) and retinitis pigmentosa (RP) affect millions of people. Replacing lost cells with new cells that connect with the still functional part of the host retina might repair a degenerating retina and restore eyesight to an unknown extent. A unique model, subretinal transplantation of freshly dissected sheets of fetal-derived retinal progenitor cells, combined with its retinal pigment epithelium (RPE), has demonstrated successful results in both animals and humans. Most other approaches are restricted to rescue endogenous retinal cells of the recipient in earlier disease stages by a 'nursing' role of the implanted cells and are not aimed at neural retinal cell replacement. Sheet transplants restore lost visual responses in several retinal degeneration models in the superior colliculus (SC) corresponding to the location of the transplant in the retina. They do not simply preserve visual performance - they increase visual responsiveness to light. Restoration of visual responses in the SC can be directly traced to neural cells in the transplant, demonstrating that synaptic connections between transplant and host contribute to the visual improvement. Transplant processes invade the inner plexiform layer of the host retina and form synapses with presumable host cells. In a Phase II trial of RP and ARMD patients, transplants of retina together with its RPE improved visual acuity. In summary, retinal progenitor sheet transplantation provides an excellent model to answer questions about how to repair and restore function of a degenerating retina. Supply of fetal donor tissue will always be limited but the model can set a standard and provide an informative base for optimal cell replacement therapies such as embryonic stem cell (ESC)-derived therapy.

Cell transplantation as a treatment for retinal disease

It has been shown that photoreceptor degeneration can be limited in experimental animals by transplantation of fresh RPE to the subretinal space. There is also evidence that retinal cell transplants can be used to reconstruct retinal circuitry in dystrophic animals. Here we describe and review recent developments that highlight the necessary steps that should be taken prior to embarking on clinical trials in humans.

The first decade of continuous progress in retinal transplantation

In recent months, neural fetal retina has been transplanted into blind human patients affected by Retinitis Pigmentosa. Initial success, as documented by improved visual activity, has been reported (del Cerro et al., Neuroscience Abstract, 1996). With the rapid progress in human patients, additional questions are arising concerning transplantation issues. Additional answers and further success in treating clinical disease will necessarily come from new laboratory research in animal models as well as in vitro systems. This increases the need for evaluation of the data already gathered over the first decade of retinal transplantation. The extensive experimental background work that preceded the current wave of human retinal transplants is reviewed in this paper, with particular emphasis given to the work dealing with the transplantation of neural retina.

Fibroblast growth factor receptor is required for in vivo cardiac myocyte proliferation at early embryonic stages of heart development

In birds and mammals, cardiac myocytes terminate mitotic activity in the neonatal period and regeneration of cardiac muscle does not occur after myocardial injury in adult hearts. Even embryonic myocytes, which actively proliferate in vivo, quickly lose mitotic activity when placed in cell culture. Several growth factors, including fibroblast growth factor (FGF), have been documented in embryonic hearts and some have been shown to influence myocyte terminal differentiation in culture. However, none of these growth factors have been shown to reactivate cell division in postmitotic myocytes nor have their in vivo functions been defined satisfactorily. To clarify the role of FGF signaling in heart growth, we prepared two retroviral vectors capable of suppressing (i) functions of FGF receptors (FGFRs) with a dominant-negative mutant of receptor type 1 (FGFR1) or (ii) the translation of endogenous FGFR1 by transcribing its antisense RNA. Both vectors inhibited myocyte proliferation and/or survival during the first week of chicken embryonic development but had much less effect after the second week. No apparent alteration of myocyte growth was observed after overexpression of full-length FGFR1. These results suggest that receptor-coupled FGF signaling regulates cardiac myocyte growth during tubular stages of cardiogenesis but that myocyte growth becomes FGF-independent after the second week of embryogenesis.

Reporter gene expression in cones in transgenic mice carrying bovine rhodopsin promoter/lacZ transgenes

Rhodopsin gene expression has been used as a model system to study the mechanisms regulating photoreceptor gene expression. Previous transgenic experiments using rhodopsin promoter/lacZ fusion constructs identified some of the cis-acting DNA elements responsible for photoreceptor cell-specific expression. However, the issue of rod specificity vs. photoreceptor (rod and cone) specificity of the elements was not resolved. To address this issue, the specificity of reporter gene expression in the retinas of transgenic mice carrying bovine rhodopsin promoter/lacZ (beta-galactosidase) fusion genes was assessed using X-gal staining and electron microscopy. Two independent transgenic lines, one carrying a rhodopsin promoter fragment extending from -2174 to +70 base pairs (bp) relative to the messenger RNA start site and another line carrying a fragment from -222 to +70 bp, both showed reporter gene expression in cones as well as rods, although the level of staining appeared to be less in the cones than in the rods. These results demonstrate that the -2174 to +70 bp and -222 to +70 bp bovine rhodopsin promoter fragments are not rod-specific in transgenic mice and indicate that the existence of rod promoter mediated-expression in cones must be considered when interpreting results from transgenic experiments utilizing the rhodopsin promoter.

Reversed ratio of color-specific cones in rabbit retinal cell transplants

Recently, we have reported on the emergence of various retinal cell types in embryonic rabbit retina transplanted to adult rabbits. When comparing the relative numbers of the spectrally different cone types in the transplants to those in the host or age-matched control retinas, a surprising shift was observed. While in the normal rabbit retina the middle-wavelength-sensitive (M) cones are considerably more abundant than the short-wave-sensitive (S) cones, the S/M cone ratio was found to be the opposite in the graft. The number of rosettes containing only S-cones in high density was found to be considerably higher than that of M-cone rich rosettes. The number of S-cones also exceeded that of the M-cones in each rosette that contained both cell types. Our results were obtained from the systematic immunocytochemical analysis of 15 different transplants derived from transplantations of embryonic rabbit retinas into adult hosts of the same species. The emergence and proportion of the two cone types were followed between 14 and 63 days after transplantation (between 29 and 78 postconceptional days of the donor tissue). Sections from various parts of the transplants were reacted with the monoclonal antibodies COS-1 and OS-2, specific for the middle- and short-wavelength-sensitive cones, respectively. The explanation for the reverse cone ratio in these transplants is not known yet, however, the observed phenomenon may indicate differences between the specification of the two basic cone types.

Clonal analysis of cardiac morphogenesis in the chicken embryo using a replication-defective retrovirus. III: Polyclonal origin of adjacent ventricular myocytes

Replication-incompetent variants of the avian spleen necrosis virus (SNV) encoding cytoplasmic or nuclear-directed beta-galactosidase (beta-gal) have been used to trace the clonal growth of myocytes during left ventricular free-wall formation. Tubular-stage hearts were infected with a mixed suspension of both retroviruses and, after hatching, the progeny of marked cells in the ventricular wall were examined by X-gal histochemistry. When a small number of virions was introduced individual blue patches contained myocytes with only one label type (cytoplasmic or nuclear). These results confirmed our previous conclusion that each cluster or patch represents a single clone (Mikawa et al., 1992, Dev. Dynamics, 193:11-23). Each of these clones formed a clone-shaped patch which often extended through the entire thickness of the ventricular myocardium, but typically each patch was heterogeneous, containing a mixture of labeled and unlabeled cells. We then asked whether the two populations of myocytes in each patch were clonally related or generated from more than one progenitor. When hearts were infected with high titer viral suspensions many patches were observed in which cytoplasmic-tagged myocytes were intermingled with nuclear-tagged myocytes. Thus, the cone-shaped myocyte patches in the ventricular wall are polyclones derived from separate progenitors in the precardiac mesoderm. This finding led us to examine the separation of clonally related ventricular myocytes in the developing hearts. Embryos were infected with retroviral suspensions at varying stages of development and the resulting colonies examined after hatching.(ABSTRACT TRUNCATED AT 250 WORDS)