Khris G. Griffis

Mammalian UNC119 is a ciliary trafficking chaperone highly expressed in the inner segment of retinal photoreceptors. Previous research has shown that UNC119 can bind to transducin, the synaptic ribbon protein RIBEYE, and the calcium-binding protein CaBP4, suggesting that UNC119 may have a role in synaptic transmission. We made patch-clamp recordings from retinal slices in mice with the UNC119 gene deleted and showed that removal of even one gene of UNC119 has no effect on the rod outer segment photocurrent, but acted on bipolar cells much like background light: it depolarized membrane potential, decreased sensitivity, accelerated response decay, and decreased the Hill coefficient of the response–intensity relationship. Similar effects were seen on rod bipolar-cell current and voltage responses, and after exposure to bright light to translocate transducin into the rod inner segment. These findings indicate that UNC119 deletion reduces the steady-state glutamate release rate at rod synapses, though no change in the voltage dependence of the synaptic Ca current was detected. We conclude that UNC119, either by itself or together with transducin, can facilitate the release of glutamate at rod synapses, probably by some interaction with RIBEYE or other synaptic proteins rather than by binding to CaBP4 or calcium channels.

Key points: Most vertebrate eyes have rods for dim‐light vision and cones for brighter light and higher temporal sensitivity. Rods evolved from cone‐like precursors through expression of different transduction genes or the same genes at different expression levels, but we do not know which molecular differences were most important. We approached this problem by analysing rod and cone responses with the same model but with different values for model parameters. We showed that, in addition to outer‐segment volume, the most important differences between rods and cones are: (1) decreased transduction gain, reflecting smaller amplification in the G‐protein cascade; (2) a faster rate of turnover of the second messenger cGMP in darkness; and (3) an accelerated rate of decay of the effector enzyme phosphodiesterase and perhaps also of activated visual pigment. We believe our analysis has identified the principal alterations during evolution responsible for the duplex retina. Most vertebrates have rod and cone photoreceptors, which differ in their sensitivity and response kinetics. We know that rods evolved from cone‐like precursors through the expression of different transduction genes or the same genes at different levels, but we do not know which molecular differences were most important. We have approached this problem in mouse retina by analysing the kinetic differences between rod flash responses and recent voltage‐clamp recordings of cone flash responses, using a model incorporating the principal features of photoreceptor transduction. We apply a novel method of analysis using the log‐transform of the current, and we ask which of the model's dynamic parameters need be changed to transform the flash response of a rod into that of a cone. The most important changes are a decrease in the gain of the response, reflecting a reduction in amplification of the transduction cascade; an increase in the rate of turnover of cGMP in darkness; and an increase in the rate of decay of activated phosphodiesterase, with perhaps also an increase in the rate of decay of light‐activated visual pigment. Although we cannot exclude other differences, and in particular alterations in the Ca²⁺ economy of the photoreceptors, we believe that we have identified the kinetic parameters principally responsible for the differences in the flash responses of the two kinds of photoreceptors, which were likely during evolution to have resulted in the duplex retina.

Multiple neural and peripheral cell types rapidly respond to tissue damage after spinal cord injury to form a structurally and chemically inhibitory scar that limits axon regeneration. Astrocytes form an astroglial scar and produce chondroitin sulfate proteoglycans (CSPGs), activate microglia, and recruit blood-derived immune cells to the lesion for debris removal. One beneficial therapy, olfactory ensheathing cell (OEC) transplantation, results in functional improvements and promotes axon regeneration after spinal cord injury. The lack of an OEC-specific marker, however, has limited the investigation of mechanisms underlying their proregenerative effects. We compared the effects of enhanced green fluorescent protein-labeled fibroblast (FB) and OEC transplants acutely after a complete low-thoracic spinal cord transection in adult rats. We assessed the preservation of neurons and serotonergic axons, the levels of inhibitory CSPGs and myelin debris, and the extent of immune cell activation between 1 and 8 weeks postinjury. Our findings indicate that OECs survive longer than FBs post-transplantation, preserve axons and neurons, and reduce inhibitory molecules in the lesion core. Additionally, we show that OECs limit immune-cell activation and infiltration, whereas FBs alter astroglial scar formation and increase immune-cell infiltration and concomitant secondary tissue damage. Administration of cyclosporine-A to enhance graft survival demonstrated that immune suppression can augment OEC contact-mediated protection of axons and neurons during the first 2 weeks postinjury. Collectively, these data suggest that OECs have neuroprotective and immunomodulatory mechanisms that create a supportive environment for neuronal survival and axon regeneration after spinal cord injury. SIGNIFICANCE STATEMENT Spinal cord injury creates physical and chemical barriers to axon regeneration. We used a complete spinal cord transection model and olfactory ensheathing cell (OEC) or fibroblast (FB; control) transplantation as a repair strategy. OECs, but not FBs, intermingled with astrocytes, facilitated astroglial scar border formation and sequestered invading peripheral cells. OECs attenuated immune cell infiltration, reduced secondary tissue damage, protected neurons and axons in the lesion core, and helped clear myelin debris. Immunosuppression enhanced survival of OECs and FBs, but only OEC transplantation promoted scaffold formation in the lesion site that facilitated axon regeneration and neuron preservation.