LIM Kinase, a Newly Identified Regulator of Presynaptic Remodeling by Rod Photoreceptors After Injury.

Photoreceptors demonstrate presynaptic structural remodeling in response to injury or disease. After retinal detachment, rod cells retract their axons and terminals from the outer plexiform layer (OPL), whereas cone cells respond by rounding their synaptic terminal.1,2 In contrast, after retinal reattachment or during retinal disease, rod photoreceptors sprout neuritic processes.3–8 Both axonal/terminal retraction and neuritic sprouting presumably disrupt the normal connectivity between photoreceptors and second-order neurons, and thus are potentially disruptive for vision. Here we focus on rod photoreceptor axonal retraction in response to the injury of retinal detachment. Retinal detachment occurs with severe myopia, advancing age, complication from cataract surgery, or after trauma from high-impact or high-speed activities.9–14 To prevent potential visual loss after detachment due to disrupted circuitry, it is necessary to understand the mechanisms behind the reactive structural changes by photoreceptors. RhoA-Rho kinase (ROCK) activity15,16 and Ca2+ influx17,18 have been reported to play a role in regulating axonal retraction by rod photoreceptors in both isolated tiger salamander rod cells and detached porcine retinas. This study expands these findings by examining (1) the role of ROCK's downstream effector, LIM kinase (LIMK)19–21; (2) the role of p21-activated kinase (Pak), like ROCK, an upstream regulator of LIMK21–24; and (3) the relationship between Ca2+ influx and ROCK or LIMK in rod cell injury. LIM kinase regulates actin cytoskeletal dynamics, through cofilin, in various species and tissues.25–28 Since rod axonal and neuritic plasticity requires morphologic change, actin filament rearrangement is presumably necessary. Thus LIMK activity is very likely to be involved in the response of photoreceptors to injury. However, involvement of LIMK does not preclude a role for actomyosin contraction, which has been suggested to coordinate with increased actin dynamics in complex physiological activities, such as axon guidance29–31 and cell migration.32–34 ROCK and Ca2+ influx, previously demonstrated to be necessary for axonal retraction, promote actomyosin contraction through regulating myosin light chain (MLC) and myosin light chain phosphatase (MLCP), and myosin light chain kinase (MLCK), respectively.19,20,35–37 We postulate, therefore, that both actin filament turnover and actomyosin contraction contribute to rod cell axonal retraction (Fig. 1). Figure 1 Diagram of proposed pathways involved in rod cell axonal and neuritic plasticity. Two activities, actomyosin contraction and actin filament turnover, are suggested to contribute to plasticity in the axon and axon terminal after injury. Both RhoA-ROCK ... LIM kinase may also play a role in the photoreceptor response to injury in the form of neuritic process outgrowth.5,7,38 LIM kinase is well known to function in regulating actin polymerization in cooperation with the pathways that promote nucleation and extension of new actin filament branches.39–41 Our study therefore also investigates the effect of LIMK inhibition on process outgrowth and explores the possibility that using inhibition of a single kinase, LIMK, can reduce both forms of structural plasticity, axonal retraction and neuritic sprouting. Here we demonstrate the effectiveness of LIMK antagonism in stabilizing the structure of mature rod terminals of isolated salamander rod cells. We confirm the effect of LIMK inhibition on preventing axonal/synaptic retraction in the detached porcine retina. Pig eyes are a good model for retinal injury as porcine retina resembles human retina structurally and physiologically.42–46 Finally, we demonstrate that LIMK, in conjunction with Ca2+, also plays a role in neuritic sprouting. Thus, axonal retraction and neuritic sprouting appear to be linked by a common pathway.