RhoA and its role in synaptic structural plasticity of isolated salamander photoreceptors.

Adult salamander photoreceptors retract existing axons and extend new neuritic processes in vitro. In mammalian retina, similar forms of structural plasticity occur in injury and disease. The authors asked whether RhoA is present in photoreceptor axon terminals and whether activity in the RhoA-ROCK pathway contributes to the structural plasticity observed in rod and cone cells. Antibodies against RhoA were used to immunolabel Western blots sections and isolated neurons from salamander retina. Isolated photoreceptors were treated with lysophosphatidic acid (LPA; a RhoA activator) or Y27632 (an inhibitor of RhoA effector ROCK) for the first 24 hours, the first 3 days, or the last 24 hours of culture. Growth and retraction were assessed with time-lapse and image analyses. RhoA protein was found throughout the retina, including in rod and cone synaptic terminals. When treated with LPA, photoreceptors significantly reduced the growth of new neuritic processes and presynaptic varicosities and retracted growth at the highest LPA concentrations. When treated with Y27632, rod cells significantly increased the number of varicosities, whereas cone cells increased process growth. Treatment with Y27632 also dramatically reduced retraction of the existing axon, which occurs spontaneously in rod cells during the first 24 hours of culture. Thus, RhoA-ROCK activity reduces and retracts neuritic growth, but inhibition of activity increases neuritic development and blocks retraction. The results suggest that RhoA activation contributes to axon retraction by rod cells after retinal detachment, whereas inhibition of RhoA contributes to the neuritic sprouting seen in reattached and degenerating retina.