Development of Light-activated Pupilloconstriction in Rats as Mediated by Normal and Transplanted Retinae

Repair of Retinal Degeneration following Ex Vivo Minicircle DNA Gene Therapy and Transplantation of Corrected Photoreceptor Progenitors

The authors describe retinal reconstruction and restoration of visual function in heritably blind mice missing the rhodopsin gene using a novel method of ex vivo gene therapy and cell transplantation. Photoreceptor precursors with the same chromosomal genetic mutation were treated ex vivo using minicircle DNA, a non-viral technique that does not present the packaging limitations of adeno-associated virus (AAV) vectors. Following transplantation, genetically modified cells reconstructed a functional retina and supported vision in blind mice harboring the same founder gene mutation. Gene delivery by minicircles showed comparable long-term efficiency to AAV in delivering the missing gene, representing the first non-viral system for robust treatment of photoreceptors. This important proof-of-concept finding provides an innovative convergence of cell and gene therapies for the treatment of hereditary neurodegenerative disease and may be applied in future studies toward ex vivo correction of patient-specific cells to provide an autologous source of tissue to replace lost photoreceptors in inherited retinal blindness. This is the first report using minicircles in photoreceptor progenitors and the first to transplant corrected photoreceptor precursors to restore vision in blind animals.

Development of melanopsin-based irradiance detecting circuitry

BACKGROUND

Most retinal ganglion cells (RGCs) convey contrast and motion information to visual brain centers. Approximately 2% of RGCs are intrinsically photosensitive (ipRGCs), express melanopsin and are necessary for light to modulate specific physiological processes in mice. The ipRGCs directly target the suprachiasmatic nucleus (SCN) to photoentrain circadian rhythms, and the olivary pretectal nucleus (OPN) to mediate the pupillary light response. How and when this ipRGC circuitry develops is unknown.

RESULTS

Here, we show that some ipRGCs follow a delayed developmental time course relative to other image-forming RGCs. Specifically, ipRGC neurogenesis extends beyond that of other RGCs, and ipRGCs begin innervating the SCN at postnatal ages, unlike most RGCs, which innervate their image-forming targets embryonically. Moreover, the appearance of ipRGC axons in the OPN coincides precisely with the onset of the pupillary light response.

CONCLUSIONS

Some ipRGCs differ not only functionally but also developmentally from RGCs that mediate pattern-forming vision.

Retinal transplantation--advantages of intact fetal sheets

Retinal transplantation aims to prevent blindness and to restore eyesight, i.e., to rescue photoreceptors or to replace damaged photoreceptors with the hope of reestablishing neural circuitry. Retinal donor tissue has been transplanted as dissociated cells or intact sheets. A promising experimental paradigm is the subretinal transplantation of sheets of fetal retina with or without its attached retinal pigment epithelium (RPE) into recipient rats with retinal degeneration. As long as healthy RPE either from the host or from the graft is present, such transplants can develop lamination resembling a normal retina. Different methods have been used to demonstrate transplant/host connectivity. In two different rat retinal degeneration models, visually evoked responses can be demonstrated in an area of the superior colliculus corresponding to the placement of the transplant in the retina. In summary, sheets of fetal retina can morphologically repair an area of a degenerated retina, and there is evidence to suggest that transplants form synaptic connections with the host and restore visual responses in blind rats.

Progressive visual sensitivity loss in the Royal College of Surgeons rat: perimetric study in the superior colliculus

The Royal College of Surgeons rat has a retinal pigment epithelial cell defect which causes a progressive loss of rods occurring primarily over the first few months of life. We have studied the consequences of this degenerative process on visual sensitivity across the visual field. Sensitivities were determined in the superior colliculus for unit responses recorded from 22 days up to one year of age from sites encompassing the whole visual field representation. Following visual sensitivity assessment, retinae were examined anatomically at the light and electron microscopic level. At 22 days of age, sensitivities in dystrophic rats were comparable to those of non-dystrophics at any age (40+/-1 and 41+/-1dB, respectively), despite the fact that signs of degenerative events were clear at the electron microscopic level, including presence of pyknotic photoreceptor nuclei, disorganised outer segments and accumulation of debris. However, loss in sensitivity was first detected only at 28-36 days of age (27+/-4dB). From then on, sensitivities progressively decreased to reach a plateau by 180-240 days (4+/-2dB). Starting around 90 days and onward, there was a positive gradient of sensitivities from temporal to nasal field. Drops in visual sensitivity were parallelled by several changes in visual response properties, including prolonged latency, inconsistent responsiveness, appearance of bursting spontaneous activity and activation of units by stimuli presented outside their classical receptive fields. The measure of visual sensitivities by recording visual responses at specific sites in the superior colliculus provides a reliable point-to-point assessment of retinal function comparable to visual perimetry testing in humans. This experimental approach provides the background for answering questions arising during the development of potential experimental therapies for retinal degeneration using animal models like the Royal College of Surgeons rat.

The pupillary light response: functional and anatomical interaction among inputs to the pretectum from transplanted retinae and host eyes

Pupilloconstriction to light can be mediated in rats through direct illumination of retinae previously transplanted to intracranial locations. Transplant-driven and normal pupillary light responses are stable under optimal testing conditions, and parameters describing the response can be quantified precisely. The present study demonstrates the interaction between transplant-driven and normal pupillary response patterns. When stimuli are presented concurrently to a transplanted retina and to the remaining eye in host rats, a greater degree of pupilloconstriction occurs than when either the transplanted or the host eye is illuminated independently. This suggests that transplant and host retinal inputs act in concert to determine pupil diameter. The second portion of this study investigates the pattern of retinal input to the pretectum to determine if a structural basis for such functional interactions may exist. Crossed and uncrossed retinal projections to the olivary pretectal nucleus occupy non-overlapping regions of this bilaterally represented nucleus in normal rats, with a greater number of optic axons directed to the contralateral olivary pretectal nucleus. Retinae transplanted to the midbrain of neonatal rats, from whom the contralateral eye had been removed, also project to the olivary pretectal nucleus at maturity. By contrast with the normal pattern of segregated retinal inputs, however, the terminal fields of transplant axons were found to overlap extensively with the retinal projection from the remaining host eye. In addition, the relative proportion of transplant axons directed to the ipsilateral and contralateral olivary pretectal nucleus varied among animals. The lack of spatial segregation between inputs from transplant and host sources and the relative proportion of ipsilateral and contralateral transplant axons together may represent a structural basis for the observed functional interactoin of these inputs to the neural circuit subserving pupilloconstriction to light. These features may also relate to the marked improvements in transplant-mediated responses that frequently occur when optic input from the remaining host eye is eliminated. The results presented here, together with our previous transplant studies, show that this preparation can be used to provide insight into more general questions as to the dynamic interactions that occur between converging sensory inputs in the generation of integrated output responses.

The pupillary light response: assessment of function mediated by intracranial retinal transplants

We have adapted a pupillometry measurement system to test the functional efficacy of retinae previously transplanted over the midbrain of neonatal rats in mediating a pupillary light reflex in the host eye. This has permitted us to examine factors influencing various parameters of the response, and to study transplant-mediated responses in comparison with responses mediated by way of the normal consensual pathway. Despite the unusual location of these transplanted retinae and the absence of supportive tissues normally associated with retinae in situ, it is clear that pupilloconstriction in the host eye can be elicited by transplant illumination. Under the optimal conditions here defined, response parameters for individual animals were stable with repeated testing over extended periods. When considered as individual cases, response amplitude, constriction rate and response latency were intensity dependent, although responses elicited by transplant illumination were less sensitive than normal, typically by 2-3 log units. Large-amplitude transplant-mediated pupillary responses could, however, be elicited repeatedly throughout long trains of stimuli, unlike normal responses, which rapidly failed to recover to baseline under similar test conditions. Finally, even though some cellular elements of the visual cycle are absent in transplanted retinae, pupilloconstriction in the host eye could be elicited repeatedly by transplant illumination as long as two years after transplantation took place. These observations indicate the applicability of this preparation as an assay for the effects of experimental manipulations on information processing and response plasticity in the visual system, and as a tool for examining, in general, the necessary conditions for optimal function of grafts that work by synthesizing and relaying neural signals.

Efferent projections of the olivary pretectal nucleus in the albino rat subserving the pupillary light reflex and related reflexes. A light microscopic tracing study

The olivary pretectal nucleus is a primary visual centre sensitive to luminance changes. It is involved in the pupillary light reflex, the consensual pupillary light reflex and related reflexes, such as the lid closure reflex whereby pupillary constriction takes place. Since the olivary pretectal nucleus is a small nucleus, previous studies using degeneration, horseradish peroxidase and radioactive amino acid tracing were limited regarding to the exclusiveness of the projections from the olivary pretectal nucleus. In the present study the position of the olivary pretectal nucleus in the rat was first localized by physiological recording of the neurons upon luminance stimulation. Subsequently, an anterograde tracer Phaseolus vulgaris leucoagglutinin was injected iontophoretically. This allows a much more precise localization of the olivary pretectal nucleus projections. Ascending and descending pathways originating from the olivary pretectal nucleus were observed. Ascending fibres project bilaterally to the intergeniculate leaflet, the ventral part of the lateral geniculate nucleus and ipsilaterally to the anterior pretectal nucleus. In addition, contralateral projections were observed to the zona incerta and the fields of Forel. Descending fibres project bilaterally to the periaqueductal gray, the nucleus of Darkschewitsch, the interstitial nucleus of Cajal, the Edinger-Westphal nucleus and the intermediate gray layer of the superior colliculus. Also a contralateral projection to the oculomotor nucleus and an ipsilateral projection to the pontine nucleus and the nucleus of the optic tract were found. Furthermore, the contralateral olivary pretectal nucleus received a small projection. Retrograde tracing experiments using two fluorescent dyes revealed that the fibres projecting to the contralateral olivary pretectal nucleus and to the contralateral interstitial nucleus of Cajal are collaterals. The projection from the olivary pretectal nucleus to the facial nucleus which has been described to receive an input in cats could not be confirmed for the rat. The fact that the Edinger-Westphal nucleus, the interstitial nucleus of Cajal and the superior colliculus receive an input from the olivary pretectal nucleus suggests that this primary visual centre is not only involved in the pupillary light reflex, but also in controlling eye and head position and saccadic eye movements. Although visual acuity largely depends on receptive field sizes of retinal ganglion cells and their central connections, the stronger sympathetic influence during the pupillary light reflex in animals with frontally placed eyes compared to animals with laterally placed eyes may also contribute to the higher visual acuity in animals with frontally placed eyes.

Visual information processing by intracerebral retinal transplants in rats

We have developed a simple system involving the implantation of retinae over the midbrain of rodents to examine whether, in a clearly defined system such as the primary optic pathway, it is possible to re-create circuits lost as a result of injury or developmental disorder. For much of the work, immature rat hosts have been used, in part to maximise optimal conditions and to provide a baseline for similar transplants in adults. In this review we summarise the sequence of studies that has led us to the conclusion that transplanted retinae are capable not only of differentiating and responding to light but also of relaying luminance information to visual centres of the host brain where appropriate behavioural responses are elaborated.

PDGF and its receptors in the developing rodent retina and optic nerve

We have used in situ hybridization to visualize cells in the developing rat retina and optic nerve that express mRNAs encoding the A and B chains of platelet-derived growth factor (PDGF-A and PDGF-B), and the alpha and beta subunits of the PDGF receptor (PDGF-alpha R and PDGF-beta R). We have also visualized PDGF-A protein in these tissues by immunohistochemistry. In the retina, PDGF-A mRNA is present in pigment epithelial cells, ganglion neurons and a subset of amacrine neurons. PDGF-A transcripts accumulate in ganglion neurons during target innervation and in amacrine neurons around the time of eye opening, suggesting that PDGF-A expression in these cells may be regulated by target-derived signals or by electrical activity. In the mouse retina, PDGF-A immunoreactivity is present in the cell bodies, dendrites and proximal axons of ganglion neurons, and throughout the inner nuclear layer. PDGF-alpha R mRNA is expressed in the retina by astrocytes in the optic fibre layer and by a subset of cells in the inner nuclear layer that might be Muller glia or bipolar neurons. Taken together, our data suggest short-range paracrine interactions between PDGF-A and PDGF-alpha R, the ligand and its receptor being expressed in neighbouring layers of cells in the retina. In the optic nerve, PDGF-A immunoreactivity is present in astrocytes but apparently not in the retinal ganglion cell axons. PDGF-alpha R+ cells in the optic nerve first appear near the optic chiasm and subsequently spread to the retinal end of the nerve; these PDGF-alpha R+ cells are probably oligodendrocyte precursors (Pringle et al., 1992). RNA transcripts encoding PDGF-B and PDGF-beta R are expressed by cells of the hyaloid and mature vascular systems in the eye and optic nerve.