Local translation of cell adhesion molecules in axons

Expression of Genes Involved in Axon Guidance: How Much Have We Learned?

Neuronal axons are guided to their target during the development of the brain. Axon guidance allows the formation of intricate neural circuits that control the function of the brain, and thus the behavior. As the axons travel in the brain to find their target, they encounter various axon guidance cues, which interact with the receptors on the tip of the growth cone to permit growth along different signaling pathways. Although many scientists have performed numerous studies on axon guidance signaling pathways, we still have an incomplete understanding of the axon guidance system. Lately, studies on axon guidance have shifted from studying the signal transduction pathways to studying other molecular features of axon guidance, such as the gene expression. These new studies present evidence for different molecular features that broaden our understanding of axon guidance. Hence, in this review we will introduce recent studies that illustrate different molecular features of axon guidance. In particular, we will review literature that demonstrates how axon guidance cues and receptors regulate local translation of axonal genes and how the expression of guidance cues and receptors are regulated both transcriptionally and post-transcriptionally. Moreover, we will highlight the pathological relevance of axon guidance molecules to specific diseases.

Global and local mechanisms sustain axonal proteostasis of transmembrane proteins

The control of neuronal protein homeostasis or proteostasis is tightly regulated both spatially and temporally, assuring accurate and integrated responses to external or intrinsic stimuli. Local or autonomous responses in dendritic and axonal compartments are crucial to sustain function during development, physiology and in response to damage or disease. Axons are responsible for generating and propagating electrical impulses in neurons, and the establishment and maintenance of their molecular composition are subject to extreme constraints exerted by length and size. Proteins that require the secretory pathway, such as receptors, transporters, ion channels or cell adhesion molecules, are fundamental for axonal function, but whether axons regulate their abundance autonomously and how they achieve this is not clear. Evidence supports the role of three complementary mechanisms to maintain proteostasis of these axonal proteins, namely vesicular transport, local translation and trafficking and transfer from supporting cells. Here, we review these mechanisms, their molecular machineries and contribution to neuronal function. We also examine the signaling pathways involved in local translation and their role during development and nerve injury. We discuss the relative contributions of a transport-controlled proteome directed by the soma (global regulation) versus a local-controlled proteome based on local translation or cell transfer (local regulation).

Hermes Regulates Axon Sorting in the Optic Tract by Post-Trancriptional Regulation of Neuropilin 1

The establishment of precise topographic maps during neural development is facilitated by the presorting of axons in the pathway before they reach their targets. In the vertebrate visual system, such topography is seen clearly in the optic tract (OT) and in the optic radiations. However, the molecular mechanisms involved in pretarget axon sorting are poorly understood. Here, we show in zebrafish that the RNA-binding protein Hermes, which is expressed exclusively in retinal ganglion cells (RGCs), is involved in this process. Using a RiboTag approach, we show that Hermes acts as a negative translational regulator of specific mRNAs in RGCs. One of these targets is the guidance cue receptor Neuropilin 1 (Nrp1), which is sensitive to the repellent cue Semaphorin 3A (Sema3A). Hermes knock-down leads to topographic missorting in the OT through the upregulation of Nrp1. Restoring Nrp1 to appropriate levels in Hermes-depleted embryos rescues this effect and corrects the axon-sorting defect in the OT. Our data indicate that axon sorting relies on Hermes-regulated translation of Nrp1.

SIGNIFICANCE STATEMENT An important mechanism governing the formation of the mature neural map is pretarget axon sorting within the sensory tract; however, the molecular mechanisms involved in this process remain largely unknown. The work presented here reveals a novel function for the RNA-binding protein Hermes in regulating the topographic sorting of retinal ganglion cell (RGC) axons in the optic tract and tectum. We find that Hermes negatively controls the translation of the guidance cue receptor Neuropilin-1 in RGCs, with Hermes knock-down resulting in aberrant growth cone cue sensitivity and axonal topographic misprojections. We characterize a novel RNA-based mechanism by which axons restrict their translatome developmentally to achieve proper targeting.