Transplantation to the diseased and damaged retina

The Lrat-/- Rat: CRISPR/Cas9 Construction and Phenotyping of a New Animal Model for Retinitis Pigmentosa

PURPOSE

We developed and phenotyped a pigmented knockout rat model for lecithin retinol acyltransferase (LRAT) using CRISPR/Cas9. The introduced mutation (c.12delA) is based on a patient group harboring a homologous homozygous frameshift mutation in the LRAT gene (c.12delC), causing a dysfunctional visual (retinoid) cycle.

METHODS

The introduced mutation was confirmed by DNA and RNA sequencing. The expression of Lrat was determined on both the RNA and protein level in wildtype and knockout animals using RT-PCR and immunohistochemistry. The retinal structure and function, as well as the visual behavior of the Lrat^-/- and control rats, were characterized using scanning laser ophthalmoscopy (SLO), optical coherence tomography (OCT), electroretinography (ERG) and vision-based behavioral assays.

RESULTS

Wildtype animals had high Lrat mRNA expression in multiple tissues, including the eye and liver. In contrast, hardly any expression was detected in Lrat^-/- animals. LRAT protein was abundantly present in wildtype animals and absent in Lrat^-/- animals. Lrat^-/- animals showed progressively reduced ERG potentials compared to wildtype controls from two weeks of age onwards. Vison-based behavioral assays confirmed reduced vision. Structural abnormalities, such as overall retinal thinning, were observed in Lrat^-/- animals. The retinal thickness in knockout rats was decreased to roughly 80% by four months of age. No functional or structural differences were observed between wildtype and heterozygote animals.

CONCLUSIONS

Our Lrat^-/- rat is a new animal model for retinal dystrophy, especially for the LRAT-subtype of early-onset retinal dystrophies. This model has advantages over the existing mouse models and the RCS rat strain and can be used for translational studies of retinal dystrophies.

Comparison of trophic factors' expression between paralyzed and recovering muscles after facial nerve injury. A quantitative analysis in time course

After peripheral nerve injury, recovery of motor performance negatively correlates with the poly-innervation of neuromuscular junctions (NMJ) due to excessive sprouting of the terminal Schwann cells. Denervated muscles produce short-range diffusible sprouting stimuli, of which some are neurotrophic factors. Based on recent data that vibrissal whisking is restored perfectly during facial nerve regeneration in blind rats from the Sprague Dawley (SD)/RCS strain, we compared the expression of brain derived neurotrophic factor (BDNF), fibroblast growth factor-2 (FGF2), insulin growth factors 1 and 2 (IGF1, IGF2) and nerve growth factor (NGF) between SD/RCS and SD-rats with normal vision but poor recovery of whisking function after facial nerve injury. To establish which trophic factors might be responsible for proper NMJ-reinnervation, the transected facial nerve was surgically repaired (facial-facial anastomosis, FFA) for subsequent analysis of mRNA and proteins expressed in the levator labii superioris muscle. A complicated time course of expression included (1) a late rise in BDNF protein that followed earlier elevated gene expression, (2) an early increase in FGF2 and IGF2 protein after 2 days with sustained gene expression, (3) reduced IGF1 protein at 28 days coincident with decline of raised mRNA levels to baseline, and (4) reduced NGF protein between 2 and 14 days with maintained gene expression found in blind rats but not the rats with normal vision. These findings suggest that recovery of motor function after peripheral nerve injury is due, at least in part, to a complex regulation of lesion-associated neurotrophic factors and cytokines in denervated muscles. The increase of FGF-2 protein and concomittant decrease of NGF (with no significant changes in BDNF or IGF levels) during the first week following FFA in SD/RCS blind rats possibly prevents the distal branching of regenerating axons resulting in reduced poly-innervation of motor endplates.

Channelrhodopsins: visual regeneration and neural activation by a light switch

The advent of optogenetics provides a new direction for the field of neuroscience and biotechnology, serving both as a refined investigative tool and as potential cure for many medical conditions via genetic manipulation. Although still in its infancy, recent advances in optogenetics has made it possible to remotely manipulate in vivo cellular functions using light. Coined Nature Methods' 'Method of the Year' in 2010, the optogenetic toolbox has the potential to control cell, tissue and even animal behaviour. This optogenetic toolbox consists of light-sensitive proteins that are able to modulate membrane potential in response to light. Channelrhodopsins (ChR) are light-gated microbial ion channels, which were first described in green algae. ChR2 (a subset of ChR) is a seven transmembrane α helix protein, which evokes membrane depolarization and mediates an action potential upon photostimulation with blue (470 nm) light. By contrast to other seven-transmembrane proteins that require second messengers to open ion channels, ChR2 form ion channels themselves, allowing ultrafast depolarization (within 50 milliseconds of illumination). It has been shown that integration of ChR2 into various tissues of mice can activate neural circuits, control heart muscle contractions, and even restore breathing after spinal cord injury. More compellingly, a plethora of evidence has indicated that artificial expression of ChR2 in retinal ganglion cells can reinstate visual perception in mice with retinal degeneration.

Channelrhodopsins provide a breakthrough insight into strategies for curing blindness

Photoreceptor cells are the only retinal neurons that can absorb photons. Their degeneration due to some diseases or injuries leads to blindness. Retinal prostheses electrically stimulating surviving retinal cells and evoking a pseudo light sensation have been investigated over the past decade for restoring vision. Currently, a gene therapy approach is under development. Channelrhodopsin-2 derived from the green alga Chlamydomonas reinhardtii, is a microbial-type rhodopsin. Its specific characteristic is that it functions as a light-driven cation-selective channel. It has been reported that the channelrhodopsin-2 transforms inner light-insensitive retinal neurons to light-sensitive neurons. Herein, we introduce new strategies for restoring vision by using channelrhodopsins and discuss the properties of adeno-associated virus vectors widely used in gene therapy.

Recovery of whisking function promoted by manual stimulation of the vibrissal muscles after facial nerve injury requires insulin-like growth factor 1 (IGF-1)

Recently, we showed that manual stimulation (MS) of denervated vibrissal muscles enhanced functional recovery following facial nerve cut and suture (FFA) by reducing poly-innervation at the neuro-muscular junctions (NMJ). Although the cellular correlates of poly-innervation are established, with terminal Schwann cells (TSC) processes attracting axon sprouts to "bridge" adjacent NMJ, molecular correlates are poorly understood. Since quantitative RT-PCR revealed a rapid increase of IGF-1 mRNA in denervated muscles, we examined the effect of daily MS for 2 months after FFA in IGF-1(+/-) heterozygous mice; controls were wild-type (WT) littermates including intact animals. We quantified vibrissal motor performance and the percentage of NMJ bridged by S100-positive TSC. There were no differences between intact WT and IGF-1(+/-) mice for vibrissal whisking amplitude (48 degrees and 49 degrees ) or the percentage of bridged NMJ (0%). After FFA and handling alone (i.e. no MS) in WT animals, vibrissal whisking amplitude was reduced (60% lower than intact) and the percentage of bridged NMJ increased (42% more than intact). MS improved both the amplitude of vibrissal whisking (not significantly different from intact) and the percentage of bridged NMJ (12% more than intact). After FFA and handling in IGF-1(+/-) mice, the pattern was similar (whisking amplitude 57% lower than intact; proportion of bridged NMJ 42% more than intact). However, MS did not improve outcome (whisking amplitude 47% lower than intact; proportion of bridged NMJ 40% more than intact). We conclude that IGF-I is required to mediate the effects of MS on target muscle reinnervation and recovery of whisking function.

Preservation of photoreceptors in dystrophic RCS rats following allo- and xenotransplantation of IPE cells

PURPOSE

To examine whether iris pigment epithelial (IPE) cells transplanted into the subretinal space of Royal College of Surgeons (RCS) rats have the ability to rescue photoreceptors.

METHODS

Rat IPE (rIPE) or human IPE (hIPE) cells were transplanted subretinally in 23-day-old RCS rats. Sham injection and transplantation of ARPE-19 cells served as controls. After 12 weeks, eyes were evaluated for photoreceptor survival by morphometric analysis and electron microscopy.

RESULTS

Morphometric analysis showed photoreceptor rescue in all transplanted and sham-injected animals (number of photoreceptors/300 microm retina+/-sd: rIPE 41.67 +/- 28; hIPE 29.50 +/- 16; ARPE-19 36.12 +/- 21; sham 16.56 +/- 6) compared to age-matched, control rats (number of photoreceptors/300 microm retina+/-sd: 9.71 +/- 4). Photoreceptor rescue was prominent in IPE cell-transplanted rats and was significantly greater than sham-injected eyes (p = 0.02 for rIPE and p = 0.04 for hIPE).

CONCLUSION

Since IPE cells transplanted into the subretinal space have the ability to rescue photoreceptors from degeneration in the RCS rat without any harmful effects, IPE cells may represent an ideal cell to genetically modify and thus carry essential genetic information for the repair of defects in the subretinal space.

Fate and protective effect of marrow stromal cells after subretinal transplantation

Engraftment of marrow stromal cells (MSCs) has been proposed as a therapeutic approach for degenerative diseases. In this study we investigated the fate and dynamic progress of grafted MSCs in living retina with the aim of evaluating the use of transplanted MSCs to treat retinal degeneration. Approximately 1x10(5) gfp-MSCs in 2 microl phosphate-buffered saline were injected into the subretinal space of adult Sprague-Dawley rats. Two weeks later, approximately 0.174%+/-0.082% of the transplanted cells had survived and diffused into the subretinal space. Nine weeks after transplantation the surviving gfp-MSCs accounted for 0.049%+/-0.023% of the number of cells injected and were mainly located at the injection site. The same number of MSCs were transplanted into the left eye subretinal space of 3-week-old hereditary retinal degenerative Royal College of Surgeons rats, and phosphate-buffered saline was injected into their right eyes as a control. Five weeks after transplantation, the amount of rudimentary photoreceptors was more significantly increased in grafted eyes than in control eyes. The results indicated that grafted MSCs could survive and rescue retinal degeneration.

Factors limiting motor recovery after facial nerve transection in the rat: combined structural and functional analyses

It is believed that a major reason for the poor functional recovery after peripheral nerve lesion is collateral branching and regrowth of axons to incorrect muscles. Using a facial nerve injury protocol in rats, we previously identified a novel and clinically feasible approach to combat axonal misguidance--the application of neutralizing antibodies against neurotrophic factors to the injured nerve. Here, we investigated whether reduced collateral branching at the lesion site leads to better functional recovery. Treatment of rats with antibodies against nerve growth factor, brain-derived neurotrophic factor, fibroblast growth factor, insulin-like neurotrophic factor I, ciliary neurotrophic factor or glial cell line-derived neurotrophic factor increased the precision of reinnervation, as evaluated by multiple retrograde labelling of motoneurons, more than two-fold as compared with control animals. However, biometric analysis of vibrissae movements did not show positive effects on functional recovery, suggesting that polyneuronal reinnervation--rather than collateral branching --may be the critical limiting factor. In support of this hypothesis, we found that motor end-plates with morphological signs of multiple innervation were much more frequent in reinnervated muscles of rats that did not recover after injury (51% of all end-plates) than in animals with good functional performance (10%). Because polyneuronal innervation of muscle fibres is activity-dependent and can be manipulated, the present findings raise hopes that clinically feasible and effective therapies could be soon designed and tested.

Basal increase in c-Fos-like expression in superior colliculus of Royal College of Surgeons dystrophic rats can be abolished by intraocular injection of tetrodotoxin

In normal rats maintained in the dark, very few cells in the primary visual centers, including the superior colliculus, show Fos-like immunoreactivity. By contrast, in rats presented with flashing lights many Fos-like immunoreactivity cells are observed distributed throughout the visual centers. In the dystrophic Royal College of Surgeons rat, in which there is major loss of photoreceptors over the first 3 months of life, similar numbers of Fos-like immunoreactivity cells are seen on light presentation, but in marked contrast, cell densities in the rats maintained in the dark are many times higher than in non-dystrophic rats maintained under similar conditions. Here we show that this elevated dark response can be abolished by intravitreal injection of the sodium channel blocker tetrodotoxin, indicating that this effect results from changed retinal activity, rather than being centrally generated. We suggest that since Fos-like immunoreactivity is not usually elicited by steady state conditions, the elevated levels in the superior colliculus in these animals reflect the return of waves of activity, first seen in development coursing across the retina, but lost with photoreceptor maturation.

Rod-cone interactions: developmental and clinical significance

During the last decade, numerous research reports have considerably improved our knowledge about the physiopathology of retinal degenerations. Three non-mutually exclusive general areas dealing with therapeutic approaches have been proposed; gene therapy, pharmacology and retinal transplantations. The first approach involving correction of the initial mutation, will need a great deal of time and further development before becoming a therapeutic tool in human clinical practice. The observation that cone photoreceptors, even those seemingly unaffected by any described anomaly, die secondarily to rod disappearance related to mutations expressed specifically in the latter, led us to study the interactions between these two photoreceptor populations to search for possible causal links between rod degeneration and cone death. These in vivo and in vitro studies suggest that paracrine interactions between both cell types exist and that rods are necessary for continued cone survival. Since the role of cones in visual perception is essential, pending the identification of the factors mediating these interactions underway, rod replacement by transplantation and/or neuroprotection by trophic factors or alternative pharmacological means appear as promising approaches for limiting secondary cone loss in currently untreatable blinding conditions.

Development and cellular functions of the iris pigment epithelium

A number of studies have shown that transplantation of retinal pigment epithelial (RPE) cells to the subretinal space offers a promising treatment modality for retinal degenerative diseases. However, it is necessary to transplant autologous cells to avoid rejection; unfortunately, obtaining autologous RPE cells necessitates such traumatic surgical intervention as to make this approach irrelevant. It has been hypothesized that iris pigment epithelial (IPE) cells may be a possible substitute for RPE cells for transplantation into the subretinal space. The iris pigment epithelium, which has the same embryonic origin as retinal pigment epithelium, has not received much attention from visual scientists. Even though it forms a highly specialized tissue, it is not clear whether the iris pigment epithelium contributes critical functions to the health of the visual system. In vivo the IPE does not appear to have any of the functions characteristic of RPE; however, in vitro cultured IPE cells do acquire functions, such as specific phagocytosis of rod outer segments, that are characteristic of RPE cells, and have been shown to have the potential to carry out many functions characteristic of RPE cells, e.g., retinol metabolism. This review outlines the development and cellular functions of the IPE with special emphasis on the modulation of those functions that can allow the IPE cells to be transplanted to the subretinal space where they appear to acquire differentiated properties of retinal pigment epithelium (RPE).

Subretinal injection of rod outer segments leads to an increase in the number of early-stage melanosomes in retinal pigment epithelial cells

Our study was performed to test the hypothesis that subretinally injected protein can induce melanogenesis in the retinal pigment epithelium (RPE). Rod outer segments (ROS) were isolated from cattle eyes and injected into the subretinal space of Long Evans rats. Five days after surgery, the injected eyes were investigated by electron microscopy. The number of early-stage melanosomes and small melanin granules was compared in injected and noninjected eyes. It was found that the injected ROS were phagocytized by the RPE cells, and the number of early-stage melanosomes in the RPE was significantly increased in injected eyes compared to eyes without injection. The ROS-containing endosomes fused with melanolysosomes in which melanogenesis took place. The increased number of early-stage melanosomes indicates new formation of melanin.

Can subretinal microphotodiodes successfully replace degenerated photoreceptors?

The idea of implanting microphotodiode arrays as visual prostheses has aroused controversy on its feasibility from the moment it appeared in print. We now present results which basically support the concept of replacing damaged photoreceptors with subretinally implanted stimulation devices. Network activity in degenerated rat retinae could be modulated through local electrical stimulation in vitro. We also investigated the long term stability and biocompatibility of the subretinal implants and their impact on retinal physiology in rats. Ganzfeld electroretinograms and histology showed no significant side effect of subretinal implants on retinal function or the architecture of the inner retina.

Morphologic and neurochemical target selectivity of regenerating adult photoreceptors in vitro

Regenerating adult central nervous system (CNS) neurons must re-establish synaptic circuits in an environment very different from that present during development. However, the complexity of CNS circuitry has made it extremely difficult to assess the selectivity and mechanisms of synaptic regeneration at the cellular level in vivo. The synaptic preferences of adult photoreceptors were examined by using a defined cell culture system known to support regenerative process growth, presynaptic varicosity formation, and establishment of functional synapses. Immunolabeling for synaptic vesicle protein 2 and ultrastructural analysis demonstrated that cell-cell contacts made by photoreceptors were synaptic in nature. Target selectivity was determined by quantitative analysis of contacts onto normal and novel target cell types in cultures in which opportunities to contact all retinal cell types were present. Target cells were identified by morphology and immunolabeling for the amino acid neurotransmitters glutamate, aspartate, gamma-aminobutyric acid (GABA), and glycine. Regenerating photoreceptors showed a strong preference for novel multipolar cell targets (amacrine and ganglion cells) over normal photoreceptor, horizontal, and bipolar cell targets. Additionally, photoreceptors were selective for targets containing the transmitter GABA. These results indicate first, that the normal synaptic partners for photoreceptors are not intrinsically the optimal targets for regenerative synapse formation, and second, that GABA may modulate synaptic targeting by adult photoreceptors.

Genesis of the retinal pigment epithelium in the macaque monkey

The development of the retinal pigment epithelium (RPE) was studied in rhesus monkey (Macaca mulatta) fetuses, neonates, and juveniles exposed to a pulse of 3H-thymidine (3H-TdR) between embryonic day (E) 25 and postnatal day (P) 204 and examined at short and long intervals after the injection of the isotope. The RPE develops from the outer layer of the optic cup which by E45 consists of a multistratified epithelium. The outer layer appears immature near the retina's edge and gradually becomes monostratified and more mature centrally. Even at this early stage, all cells contain pigmented melanosomes, although peripherally the pigment is limited to the apical portion of the cells. Examination of autoradiograms from animals allowed to survive for several postnatal months shows that monkey RPE cell genesis begins just after E27, increasing to a peak frequency of 0.38 cells/mm at E43. Between E30 and E85 the density of radiolabelled cells varies within a restricted range of from 0.2 to 0.4 cells/mm (mean = 0.25 +/- 0.09). From the density of radiolabelled cells, and data on the overall density of RPE cells in the juvenile retina, we determined the labelling index. During the first half of gestation, between 0.38% and 0.99% (mean = 0.65 +/- 0.22) of RPE cells are generated during the short interval of isotope availability after pulse injection. Approximately 5% of RPE cells were generated by E33, and 50% by E71. After E85, RPE cytogenesis begins gradually to decrease, and 95% of the cells have been generated by the time of birth. Continued, very low density (0.01 cells/mm) cytogenesis in the RPE is seen at P17, and persists at least until seven months postnatally. RPE cell genesis begins near the fovea, and proceeds towards the periphery. Cell division largely ceases in both foveal and perifoveal regions by E56, at which time labelled cells first begin to appear peripheral to the equator. Besides the timing differences, RPE genesis in the central retina differs from that in the peripheral retina in that it proceeds at a higher rate, and lasts for a shorter time period. A prolonged postnatal period of low density RPE cell genesis persists in both central and peripheral retina. Comparison of the pattern of expansion of the area containing radiolabelled cells in the RPE and neuroretina demonstrates a remarkable spatial and temporal correspondence. Close analysis suggests that at any point on the retina, the last cells are generated in the neuroretina slightly before the last cells in the RPE.

Human neural transplantation

Great advances in neurobiology have resulted from 100 years of neural transplantation research. In the last 20 years, there has been a focus on using neural transplantation to repair the damaged central nervous system (CNS) utilising experimental animal models of various human neurodegenerative disease and CNS injury. Since 1985, there has been a rapid proliferation of adrenal medullary autograft transplantation to the caudate nucleus of humans with Parkinson's disease. However, this operation proved to be unsuccessful and was associated with unacceptable morbidity. Implantation of human fetal mesencephalon into patients with severe parkinsonism has supplanted the adrenal operation and has produced promising results, with some patients reported to improve markedly and some evidence of graft survival noted on positron emission tomography (PET). Host tissue recovery appears to be an important mechanism for this clinical improvement. The optimal technique is to use three to four fetuses from induced abortions of 6.5 to 8 weeks gestation, with multiple stereotactic implants into the putamen and caudate nucleus. Many biological questions still remain and the community remains troubled by the ethical problems of using fetal tissue obtained from abortions. This procedure is still experimental and should be restricted to a few centres with excellence in cell and molecular biology. A multicentre study is needed to more carefully evaluate CNS transplantation. Cloned neural precursor cells or immortalized embryonic cell lines genetically modified to manufacture selected growth factors or neurotransmitters may offer an alternative to the use of human fetal tissue. Much more experimental animal research is necessary before transplantation can be used to treat other CNS maladies.

Human fetal tissue research: practice, prospects, and policy

Tissue from human fetal cadavers has long been used for medical research, experimental therapies, and various other purposes. Research within the last two decades has led to substantial progress in many of these areas, particularly in the application of fetal tissue transplantation to the treatment of human disease. As a result, clinical trials have now been initiated at centers around the world to evaluate the use of human fetal tissue transplantation for the therapy of Parkinson's disease, insulin-dependent diabetes mellitus, and a number of blood, immunological and, metabolic disorders. Laboratory studies suggest a much wider range of disorders may in the future be treatable by transplantation of various types of human fetal tissue. A combination of characteristics renders fetal tissue uniquely valuable for such transplantation, as well as for basic research, the development of vaccines, and a range of other applications. Although substitutes for human fetal tissue are being actively sought, for many of these applications there are at present no satisfactory alternatives. Important issues remain unresolved concerning the procurement, distribution, and use of human fetal cadaver tissue as well as the effects of such use on abortion procedures and incidence. These issues can be addressed by the introduction of appropriate guidelines or legislation, and need not be an impediment to legitimate research and therapeutic use of fetal tissue.

Synaptic and photoreceptor components in retinal pigment epithelial cell transplanted retinas of Royal College of Surgeons dystrophic rats

Plexiform layer synaptic and photoreceptor cell components were investigated in retinas of Royal College of Surgeons (RCS) dystrophic rats transplanted with normal retinal pigment epithelial (RPE) cells by immunocytochemistry using previously characterized monoclonal antibodies. In retinas of normal adult rats and RPE-cell transplanted retinas of 4 month-old RCS rats, HNK-1, a marker for a carbohydrate of the neural cell adhesion molecule (N-CAM), was detected immunocytochemically in the inner and outer plexiform layers and ganglion cell bodies and their axons. HNK-1 was also detected in the inner plexiform layer of nontreated retinas of 4 month-old RCS rats, but was reduced to scattered patches in the outer plexiform layer. In addition, immunoreactivity for the SVP-38 antibody recognizing synaptophysin was found in both plexiform layers of normal adult rat retinas and RPE-transplanted retinas of 4 month-old RCS rats. Furthermore, photoreceptor cell bodies and their inner and outer segments were immunostained for the opsin monoclonal antibody RET-P1 in retinas of normal adult rats and RPE-cell transplanted retinas of 4 month-old RCS rats. However, in nontreated retinas of 4-month-old RCS rats, only immunostained debris material was detected. These results strongly suggest that normal RPE transplants not only rescue photoreceptor cells in RCS rats, but also maintain an essential functional capacity, in this case, synaptic components in the plexiform layers.

Control of photoreceptor development

Recent studies of cell type determination in the vertebrate retina suggest that rod photoreceptor development involves interactions among cells that are mediated, at least in part, by temporally regulated diffusible signals. In this review the strategies used to generate rods in the vertebrate retina are compared with those described for photoreceptor development in the Drosophila retina.