Molecular regulation of local translation at the synapse: a novel role for Elongator?

Genetic variants of Elongator subunits (Ep1-Elp6) have been associated with neurological disorders characterized by synaptic deficits. This complex is expressed in the developing cerebral cortex where it controls the early steps of neurogenesis preceding synapse formation. Importantly, Elongator colocalizes with synaptic vesicles and is enriched at the pre-synaptic side of neuromuscular junction buttons. At the molecular level, Elongator promotes the modification of selected tRNAs by catalysing the first step reaction of the methoxy-carbonyl-methyl (mcm5) group addition to wobble uridines. Through this activity Elongator controls the fidelity and efficiency of translation. Whether Elongator modulates cortical synaptogenesis by regulating tRNAs modification at synapses, thereby local protein synthesis, has not been investigated yet. Given the important contribution of local translation to synaptogenesis, we therefore hypothesize that impaired tRNA modifications could be a common pathophysiological mechanism in Elongator-related disorders characterized by connectivity defects. Here, we show that Elongator is expressed at axonal growth cone, dendrites, and excitatory/inhibitory synapses. In addition, western blotting on synaptosomes isolated from postnatal cortices detected the expression of Elongator subunits, Alkbh8 and Ctu1, the three enzymes that act in a multi-step reaction to promote mcm5s2 group addition to selected tRNAs. In cortical neuronal cultures, we observed a decreased density of excitatory and inhibitory synapses in the absence of Elongator. Moreover, Elp3-depleted neurons display reduced puromycin labelling in their soma, along their dendrites and axons, suggesting reduced total protein synthesis. Interestingly, the PLA-Puromycin assay revealed a decreased number of newly synthesized PSD95, the major scaffolding protein in the excitatory postsynaptic density. Taken together, these preliminary data support a possible role for Elongator in local protein synthesis at synapses.