Regulated membrane trafficking of AMPA-type glutamate receptors (AMPARs) is a key mechanism underlying synaptic plasticity, yet the pathways used by AMPARs are not well understood. In this paper, we show that the AMPAR subunit GLR-1 in Caenorhabditis elegans utilizes the retrograde transport pathway to regulate AMPAR synaptic abundance. Mutants for rab-6.2, the retromer genes vps-35 and snx-1, and rme-8 failed to recycle GLR-1 receptors, resulting in GLR-1 turnover and behavioral defects indicative of diminished GLR-1 function. In contrast, expression of constitutively active RAB-6.2 drove the retrograde transport of GLR-1 from dendrites back to cell body Golgi. We also find that activated RAB-6.2 bound to and colocalized with the PDZ/phosphotyrosine binding domain protein LIN-10. RAB-6.2 recruited LIN-10. Moreover, the regulation of GLR-1 transport by RAB-6.2 required LIN-10 activity. Our results demonstrate a novel role for RAB-6.2, its effector LIN-10, and the retromer complex in maintaining synaptic strength by recycling AMPARs along the retrograde transport pathway.
Oxygen influences behaviour in many organisms, with low levels (hypoxia) having devastating consequences for neuron survival. How neurons respond physiologically to counter the effects of hypoxia is not fully understood. Here, we show that hypoxia regulates the trafficking of the glutamate receptor GLR-1 in C. elegans neurons. Either hypoxia or mutations in egl-9, a prolyl hydroxylase cellular oxygen sensor, result in the internalization of GLR-1, the reduction of glutamate-activated currents, and the depression of GLR-1-mediated behaviours. Surprisingly, hypoxia-inducible factor (HIF)-1, the canonical substrate of EGL-9, is not required for this effect. Instead, EGL-9 interacts with the Mint orthologue LIN-10, a mediator of GLR-1 membrane recycling, to promote LIN-10 subcellular localization in an oxygen-dependent manner. The observed effects of hypoxia and egl-9 mutations require the activity of the proline-directed CDK-5 kinase and the CDK-5 phosphorylation sites on LIN-10, suggesting that EGL-9 and CDK-5 compete in an oxygen-dependent manner to regulate LIN-10 activity and thus GLR-1 trafficking. Our findings demonstrate a novel mechanism by which neurons sense and respond to hypoxia.
Epidermal growth factor (EGF) signalling regulates growth and differentiation. Here, we examine the function of EGF signalling in Caenorhabditis elegans lifespan. We find that EGF signalling regulates lifespan via the Ras-MAPK pathway and the PLZF transcription factors EOR-1 and EOR-2. As animals enter adulthood, EGF signalling upregulates the expression of genes involved in the ubiquitin proteasome system (UPS), including the Skp1-like protein SKR-5, while downregulating the expression of HSP16-type chaperones. Using reporters for global UPS activity, protein aggregation, and oxidative stress, we find that EGF signalling alters protein homoeostasis in adults by increasing UPS activity and polyubiquitination, while decreasing protein aggregation. We show that SKR-5 and the E3/E4 ligases that comprise the ubiquitin fusion degradation (UFD) complex are required for the increase in UPS activity observed in adults, and that animals that lack SKR-5 or the UFD have reduced lifespans and indications of oxidative stress. We propose that as animals enter fertile adulthood, EGF signalling switches the mechanism for maintaining protein homoeostasis from a chaperone-based approach to an approach involving protein elimination via augmented UPS activity.