RNA-binding protein Hermes/RBPMS inversely affects synapse density and axon arbor formation in retinal ganglion cells in vivo.

The RNA-binding protein Hermes [RNA-binding protein with multiple splicing (RBPMS)] is expressed exclusively in retinal ganglion cells (RGCs) in the CNS, but its function in these cells is not known. Here we show that Hermes protein translocates in granules from RGC bodies down the growing axons. Hermes loss of function in both Xenopus laevis and zebrafish embryos leads to a significant reduction in retinal axon arbor complexity in the optic tectum, and expression of a dominant acting mutant Hermes protein, defective in RNA-granule localization, causes similar defects in arborization. Time-lapse analysis of branch dynamics reveals that the decrease in arbor complexity is caused by a reduction in new branches rather than a decrease in branch stability. Surprisingly, Hermes depletion also leads to enhanced early visual behavior and an increase in the density of presynaptic puncta, suggesting that reduced arborization is accompanied by increased synaptogenesis to maintain synapse number.

Rac controls PIP5K localisation and PtdIns(4,5)P2 synthesis, which modulates vinculin localisation and neurite dynamics.

In N1E-115 cells, neurite retraction induced by neurite remodelling factors such as lysophosphatidic acid, sphingosine 1-phosphate and semaphorin 3A require the activity of phosphatidylinositol 4-phosphate 5-kinases (PIP5Ks). PIP5Ks synthesise the phosphoinositide lipid second messenger phosphatidylinositol(4,5)bisphosphate [PtdIns(4,5)P2], and overexpression of active PIP5K is sufficient to induce neurite retraction in both N1E-115 cells and cerebellar granule neurones. However, how PIP5Ks are regulated or how they induce neurite retraction is not well defined. Here, we show that neurite retraction induced by PIP5Kß is dependent on its interaction with the low molecular weight G protein Rac. We identified the interaction site between PIP5Kß and Rac1 and generated a point mutant of PIP5Kß that no longer interacts with endogenous Rac. Using this mutant, we show that Rac controls the plasma membrane localisation of PIP5Kß and thereby the localised synthesis of PtdIns(4,5)P2 required to induce neurite retraction. Mutation of this residue in other PIP5K isoforms also attenuates their ability to induce neurite retraction and to localise at the membrane. To clarify how increased levels of PtdIns(4,5)P2 induce neurite retraction, we show that mutants of vinculin that are unable to interact with PtdIns(4,5)P2, attenuate PIP5K- and LPA-induced neurite retraction. Our findings support a role for PtdIns(4,5)P2 synthesis in the regulation of vinculin localisation at focal complexes and ultimately in the regulation of neurite dynamics.

The role of cyclic nucleotides in axon guidance.

During the formation of the nervous system, axonal growth cones navigate through the complex environment of the developing embryo to innervate their targets. Growth cones achieve this formidable feat by responding to attractive or repulsive guidance cues expressed at specific points along the trajectory of their growth, which impart the directional information required for accurate pathfinding. While much is known about guidance molecules and their receptors, many questions remain unanswered. Which signal transduction pathways are activated within the growth cone after encountering a guidance cue? How is this related to rearrangement of the growth cone cytoskeleton? Do different cues use different signal transduction pathways? This chapter will review some of the work that has addressed these fundamental questions, with a specific focus on the role of the cyclic nucleotides, cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP), in axon guidance.

Signaling mechanisms underlying Slit2-induced collapse of Xenopus retinal growth cones.

Slits mediate multiple axon guidance decisions, but the mechanisms underlying the responses of growth cones to these cues remain poorly defined. We show here that collapse induced by Slit2-conditioned medium (Slit2-CM) in Xenopus retinal growth cones requires local protein synthesis (PS) and endocytosis. Slit2-CM elicits rapid activation of translation regulators and MAP kinases in growth cones, and inhibition of MAPKs or disruption of heparan sulfate blocks Slit2-CM-induced PS and repulsion. Interestingly, Slit2-CM causes a fast PS-dependent decrease in cytoskeletal F-actin concomitant with a PS-dependent increase in the actin-depolymerizing protein cofilin. Our findings reveal an unexpected link between Slit2 and cofilin in growth cones and suggest that local translation of actin regulatory proteins contributes to repulsion.

Inhibition of RhoA-mediated SRF activation by p116Rip.

p116Rip, originally identified as a binding partner of activated RhoA, is an actin-binding protein that interacts with the regulatory myosin-binding subunit (MBS) of myosin-II phosphatase and is essential for Rho-regulated cytoskeletal contractility. Here, we have examined the role of p116Rip in RhoA-mediated activation of the transcription factor SRF. We show that p116Rip oligomerizes via its C-terminal coiled-coil domain and, when overexpressed, inhibits RhoA-induced SRF activation without affecting RhoA-GTP levels. Mutant forms of p116Rip that fail to oligomerize or bind to MBS are still capable of inhibiting SRF activity. Our results suggest that p116Rip interferes with RhoA-mediated transcription through its ability to disassemble the actomyosin cytoskeleton downstream of RhoA.

Retinal axon guidance: novel mechanisms for steering.

Axons from the retina traverse different molecular territories as they navigate to the tectum. A single territory might span only a few cell diameters and harbour multiple guidance cues, many of which are beginning to be characterized. Also present in the pathway are 'modulators' that influence a growth cone's response to a coincident signal but do not guide growth directly. An emerging principle is that the growth cone, itself, changes molecularly as it journeys through the visual pathway. Growing retinal axons contain mRNAs, ubiquitinating and apoptotic enzymes, translation and degradation machinery. Guidance cues can trigger rapid and local synthesis, degradation and endocytosis of proteins, providing a fast and flexible way for growth cones to respond to cues in their microenvironment and to alter their responsiveness. The data raise the idea that the localized synthesis and downregulation of proteins might help to steer retinal axon growth and, further, might contribute to the changing character of a growth cone as it ages.

Essential role of type I(alpha) phosphatidylinositol 4-phosphate 5-kinase in neurite remodeling.

Rapid neurite remodeling is fundamental to nervous system development and plasticity and is regulated by Rho family GTPases that signal f-actin reorganization in response to various receptor ligands. Neuronal N1E-115 cells show dramatic neurite retraction and cell rounding in response to serum factors such as lysophosphatidic acid (LPA), sphingosine-1 phosphate (S1P), and thrombin, due to activation of the RhoA-Rho kinase pathway. Type I phosphatidylinositol 4-phosphate 5-kinases (PIPkinase), which regulate cellular levels of PtdIns(4,5)P(2), have been suggested as targets of the RhoA-Rho kinase pathway able to modulate cytoskeletal dynamics. Here, we show that the introduction of Type Ialpha PIPkinase into N1E-115 cells leads to cell rounding and complete inhibition of neurite outgrowth, perhaps through the dissociation of vinculin and the destabilization of focal adhesions. This occurs independently of RhoA, Rho kinase, and the activation of actomyosin contraction. Strikingly, expression of kinase-dead PIPkinase promotes the outgrowth of neurites, which fail to retract in response to LPA, S1P, thrombin, or active RhoA. Moreover, neurite retraction in response to an endogenous neuronal guidance cue, Semaphorin3A, was also dependent on Type Ialpha PIPkinase. Our results suggest an essential role for a Type I PIPkinase during neurite retraction in response to a number of diverse stimuli.