Actin Dynamics, Regulated by RhoA-LIMK-Cofilin Signaling, Mediates Rod Photoreceptor Axonal Retraction After Retinal Injury

Abstract

Purpose Retraction of the axon terminals of rod photoreceptors after retinal detachment breaks the first synapse in the visual pathway, resulting in visual impairment. Previous work showed that the mechanism of axonal retraction involves RhoA signaling and its downstream effector LIM Kinase (LIMK) activation. We examined the response of the downstream component cofilin, a direct binding protein of actin filaments, as well as the regulation by RhoA-LIMK-Cofilin signaling of actin assembly/disassembly, in the presynaptic ribbon terminal of injured rod cells. Methods Injury was produced by retinal detachment or rod cell isolation. Detached porcine retina was probed for levels and localization of phosphorylated cofilin with Western blots and confocal microscopy, whereas rod cell cultures of dissociated salamander retina were examined for filamentous actin assembly/disassembly with a barbed end assay and phalloidin staining. Results A detachment increased phosphorylation of cofilin in retinal explants; phosphorylation occurred in rod terminals in sections of detached retina. Isolation of rod cells resulted in axon retraction accompanied by an increase in actin barbed ends and a decrease in net filament labeling. All changes were significantly reduced by either Rho kinase (ROCK) or LIMK inhibition, using Y27632 or BMS-5, respectively. Cytochalasin D also reduced retraction and stabilized filaments in isolated rod cells. Conclusions These results indicate that actin depolymerization via activation of RhoA downstream kinases and cofilin contributes to axon retraction. Preventing depolymerization, in addition to actomyosin contraction, may stabilize ribbon synapses after trauma.

Critical Decrease in the Level of Axon Guidance Receptor ROBO1 in Rod Synaptic Terminals Is Followed by Axon Retraction

“…2,3,17,20 Such remodeling events are not unique for RP, because they have also been reported in other genetic 3 and environmental diseases such as AMD 22 and the aging retina, 39 as well as retinal detachment. 21,23 However, there is a considerable heterogeneity in the progression of neuronal morphologic changes and network rewiring in retinal dystrophies, owing to the diverse molecular pathways underlying the retinal cell dysfunction. In this study, we examined the disease-associated remodeling of axonal and dendritic processes in the outer retina of affected dogs carrying the XLPRA1 mutation in the RPGR gene.…”

“…Prior studies showed that activation of RHOA GTPase, a key regulator of the cytoskeleton, and its downstream signaling effectors mediate rod axonal retraction after retinal detachment and injury. 21,23 We hypothesized that disease-associated perturbations in axon guidance signaling pathways, known to regulate the activity of RHO GTPases, underlie or contribute to rod axon retraction and degeneration. Two evolutionary conserved axon guidance receptors ROBO1 and ROBO2 that mediate repulsion in response to binding of SLIT ligands during neuronal development 25,27,28,41 were tested in this study for possible association between their expression pattern and synaptic remodeling in XLPRA1.…”

“…In this study, we examined remodeling of PR synaptic terminals and secondorder retinal neurons in XLPRA1 retinas. Disruption of these synaptic contacts contributes to impairment of visual circuits, and is the underlying basis of disease-associated neuronal remodeling, including rod neurite sprouting 17,20 and retraction of the axon terminals of rod PRs, 3,[21][22][23][24] as well as the aberrant outgrowth of rod bipolar cell (RBC) dendrites and HC processes into the outer nuclear layer (ONL). 3,22 A limited number of molecular players modulating rod PR axonal retraction have been identified.…”

“…Prior studies showed that activation of the Ras homolog family member A (RHOA, a member of the RHO family of small GTPases) and downstream RHO kinase activity contributes to retraction of the axon terminals of rod PRs. 21,23 However, the upstream molecular events responsible for triggering RHOA activity remain uncertain. During the development of the nervous system, a number of axon guidance cues and receptors guide axon movement by regulating the activity of RHO GTPases.…”

LPAR2 receptor activation attenuates radiation‐induced disruption of apical junctional complexes and mucosal barrier dysfunction in mouse colon

“…Rho-GTPase is known to be involved in the activation of cofilin and the promotion of actin cytoskeletal organization. 32,33 The data showed that toxin-B, an inhibitor of Rho-GTPase, blocked LPA-mediated prevention of radiation effects on cofilin and F-actin organization ( Figure 1D). To confirm the effects of radiation on the tight junction, we examined the effects of radiation and LPA in I-mC C12 cell monolayers, a non-transformed mouse intestinal epithelial cell line with rich in LPA2 receptors.…”