A novel bipartite UNC-101/AP-1 μ1 binding signal mediates KVS-4/Kv2.1 somatodendritic distribution inCaenorhabditis elegans

WTS-1/LATS regulates endocytic recycling by restraining F-actin assembly in a synergistic manner

The disruption of endosomal actin architecture negatively affects endocytic recycling. However, the underlying homeostatic mechanisms that regulate actin organization during recycling remain unclear. In this study, we identified a synergistic endosomal actin assembly restricting mechanism in C. elegans involving WTS-1, the homolog of LATS kinases, which is a core component of the Hippo pathway. WTS-1 resides on the sorting endosomes and colocalizes with the actin polymerization regulator PTRN-1 [the homolog of the calmodulin-regulated spectrin-associated proteins (CAMSAPs)]. We observed an increase in PTRN-1-labeled structures in WTS-1-deficient cells, indicating that WTS-1 can limit the endosomal localization of PTRN-1. Accordingly, the actin overaccumulation phenotype in WTS-1-depleted cells was mitigated by the associated PTRN-1 loss. We further demonstrated that recycling defects and actin overaccumulation in WTS-1-deficient cells were reduced by the overexpression of constitutively active UNC-60A(S3A) (a cofilin protein homolog), which aligns with the role of LATS as a positive regulator of cofilin activity. Altogether, our data confirmed previous findings, and we propose an additional model, that WTS-1 acts alongside the UNC-60A-mediated actin disassembly to restrict the assembly of endosomal F-actin by curbing PTRN-1 dwelling on endosomes, preserving recycling transport.

AP-1 Recruits SMAP-1/SMAPs to the trans-Golgi Network to Promote Sorting in Polarized Epithelia

Coordinated AP-1 and clathrin coat assembly mediate secretory sorting on the trans-Golgi network (TGN) during conventional secretion. Here we found that SMAP-1/SMAPs deficiency caused the apical protein ERM-1 to accumulate on the basolateral side of the TGN. In contrast, the basolateral protein SLCF-1 appeared abnormally on the apical membrane. SMAP-1 colocalized with AP-1 on the TGN. The integrity of AP-1 is required for the subcellular presence of SMAP-1. Moreover, we found that the loss of SMAP-1 reduced clathrin-positive structures in the cytosol, suggesting that SMAP-1 has a regulatory role in clathrin assembly on the TGN. Functional experiments showed that overexpressing clathrin effectively alleviated exocytic defects due to the lack of SMAP-1, corroborating the role of SMAP-1 in promoting the assembly of clathrin on the TGN. Together, our results suggested that the AP-1 complex regulates the TGN localization of SMAP-1, promoting clathrin assembly to ensure polarized conventional secretion in C. elegans intestinal epithelia.

The HSPG syndecan is a core organizer of cholinergic synapses

The extracellular matrix has emerged as an active component of chemical synapses regulating synaptic formation, maintenance, and homeostasis. The heparan sulfate proteoglycan (HSPG) syndecans are known to regulate cellular and axonal migration in the brain. They are also enriched at synapses, but their synaptic functions remain more elusive. Here, we show that SDN-1, the sole orthologue of syndecan in C. elegans, is absolutely required for the synaptic clustering of homomeric α7-like acetylcholine receptors (AChRs) and regulates the synaptic content of heteromeric AChRs. SDN-1 is concentrated at neuromuscular junctions (NMJs) by the neurally secreted synaptic organizer Ce-Punctin/MADD-4, which also activates the transmembrane netrin receptor DCC. Those cooperatively recruit the FARP and CASK orthologues that localize α7-like-AChRs at cholinergic NMJs through physical interactions. Therefore, SDN-1 stands at the core of the cholinergic synapse organization by bridging the extracellular synaptic determinants to the intracellular synaptic scaffold that controls the postsynaptic receptor content.

LET-502/ROCK Regulates Endocytic Recycling by Promoting Activation of RAB-5 in a Distinct Subpopulation of Sorting Endosomes

To explore the mechanism of Rab5/RAB-5 activation during endocytic recycling, we perform a genome-wide RNAi screen and identify a recycling regulator, LET-502/ROCK. LET-502 preferentially interacts with RAB-5(GDP) and activates RABX-5 GEF activity toward RAB-5, presumably by disrupting the self-inhibiting conformation of RABX-5. Furthermore, we find that the concomitant loss of LET-502 and another CED-10 effector, TBC-2/RAB-5-GAP, results in an endosomal buildup of RAB-5, indicating that CED-10 directs TBC-2-mediated RAB-5 inactivation and re-activates RAB-5 via LET-502 afterward. Then, we compare the functional position of LET-502 with that of RME-6/RAB-5-GEF. Loss of LET-502-RABX-5 module or RME-6 leads to diminished RAB-5 presence in spatially distinct endosome groups. We conclude that in the intestine of C. elegans, RAB-5 resides in discrete endosome subpopulations. Under the oversight of CED-10, LET-502 synergizes with RABX-5 to revitalize RAB-5 on a subset of endosomes in the deep cytosol, ensuring the progress of basolateral recycling.

Dual Role of an mps-2/KCNE-Dependent Pathway in Long-Term Memory and Age-Dependent Memory Decline

Activity-dependent persistent changes in neuronal intrinsic excitability and synaptic strength are underlying learning and memory. Voltage-gated potassium (K_v) channels are potential regulators of memory and may be linked to age-dependent neuronal disfunction. MinK-related peptides (MiRPs) are conserved transmembrane proteins modulating K_v channels; however, their possible role in the regulation of memory and age-dependent memory decline are unknown. Here, we show that, in C. elegans, mps-2 is the sole member of the MiRP family that controls exclusively long-term associative memory (LTAM) in AVA neuron. In addition, we demonstrate that mps-2 also plays a critical role in age-dependent memory decline. In young adult worms, mps-2 is transcriptionally upregulated by CRH-1/cyclic AMP (cAMP)-response-binding protein (CREB) during LTAM, although the mps-2 baseline expression is CREB independent and instead, during aging, relies on nhr-66, which acts as an age-dependent repressor. Deletion of nhr-66 or its binding element in the mps-2 promoter prevents age-dependent transcriptional repression of mps-2 and memory decline. Finally, MPS-2 acts through the modulation of the K_v2.1/KVS-3 and K_v2.2/KVS-4 heteromeric potassium channels. Altogether, we describe a conserved MPS-2/KVS-3/KVS-4 pathway essential for LTAM and also for a programmed control of physiological age-dependent memory decline.

Spatial control of membrane traffic in neuronal dendrites

Neuronal dendrites are highly branched and specialized compartments with distinct structures and secretory organelles (e.g., spines, Golgi outposts), and a unique cytoskeletal organization that includes microtubules of mixed polarity. Dendritic membranes are enriched with proteins, which specialize in the formation and function of the post-synaptic membrane of the neuronal synapse. How these proteins partition preferentially in dendrites, and how they traffic in a manner that is spatiotemporally accurate and regulated by synaptic activity are long-standing questions of neuronal cell biology. Recent studies have shed new insights into the spatial control of dendritic membrane traffic, revealing new classes of proteins (e.g., septins) and cytoskeleton-based mechanisms with dendrite-specific functions. Here, we review these advances by revisiting the fundamental mechanisms that control membrane traffic at the levels of protein sorting and motor-driven transport on microtubules and actin filaments. Overall, dendrites possess unique mechanisms for the spatial control of membrane traffic, which might have specialized and co-evolved with their highly arborized morphology.

The netrin receptor UNC-40/DCC assembles a postsynaptic scaffold and sets the synaptic content of GABAA receptors

Increasing evidence indicates that guidance molecules used during development for cellular and axonal navigation also play roles in synapse maturation and homeostasis. In C. elegans the netrin receptor UNC-40/DCC controls the growth of dendritic-like muscle cell extensions towards motoneurons and is required to recruit type A GABA receptors (GABA_ARs) at inhibitory neuromuscular junctions. Here we show that activation of UNC-40 assembles an intracellular synaptic scaffold by physically interacting with FRM-3, a FERM protein orthologous to FARP1/2. FRM-3 then recruits LIN-2, the ortholog of CASK, that binds the synaptic adhesion molecule NLG-1/Neuroligin and physically connects GABA_ARs to prepositioned NLG-1 clusters. These processes are orchestrated by the synaptic organizer CePunctin/MADD-4, which controls the localization of GABA_ARs by positioning NLG-1/neuroligin at synapses and regulates the synaptic content of GABA_ARs through the UNC-40-dependent intracellular scaffold. Since DCC is detected at GABA synapses in mammals, DCC might also tune inhibitory neurotransmission in the mammalian brain.

The netrin receptor UNC-40/DCC is required to recruit GABA_AR at neuromuscular junctions in C. elegans. Here, the authors show that UNC-40/DCC assembles an intracellular synaptic scaffold, regulating the content of GABA_AR and inhibitory neurotransmission.

Trafficking mechanisms of synaptogenic cell adhesion molecules

Nearly every aspect of neuronal function, from wiring to information processing, critically depends on the highly polarized architecture of neurons. Establishing and maintaining the distinct molecular composition of axonal and dendritic compartments requires precise control over the trafficking of the proteins that make up these cellular domains. Synaptic cell adhesion molecules (CAMs), membrane proteins with a critical role in the formation, differentiation and plasticity of synapses, require targeting to the correct pre- or postsynaptic compartment for proper functioning of neural circuits. However, the mechanisms that control the polarized trafficking, synaptic targeting, and synaptic abundance of CAMs are poorly understood. Here, we summarize current knowledge about the sequential trafficking events along the secretory pathway that control the polarized surface distribution of synaptic CAMs, and discuss how their synaptic targeting and abundance is additionally influenced by post-secretory determinants. The identification of trafficking-impairing mutations in CAMs associated with various neurodevelopmental disorders underscores the importance of correct protein trafficking for normal brain function.

SAC-1 ensures epithelial endocytic recycling by restricting ARF-6 activity

Arf6/ARF-6 is a crucial regulator of the endosomal phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) pool in endocytic recycling. To further characterize ARF-6 regulation, we performed an ARF-6 interactor screen in Caenorhabditis elegans and identified SAC-1, the homologue of the phosphoinositide phosphatase Sac1p in yeast, as a novel ARF-6 partner. In the absence of ARF-6, basolateral endosomes show a loss of SAC-1 staining in epithelial cells. Steady-state cargo distribution assays revealed that loss of SAC-1 specifically affected apical secretory delivery and basolateral recycling. PI(4,5)P2 levels and the endosomal labeling of the ARF-6 effector UNC-16 were significantly elevated in sac-1 mutants, suggesting that SAC-1 functions as a negative regulator of ARF-6. Further analyses revealed an interaction between SAC-1 and the ARF-6-GEF BRIS-1. This interaction outcompeted ARF-6(guanosine diphosphate [GDP]) for binding to BRIS-1 in a concentration-dependent manner. Consequently, loss of SAC-1 promotes the intracellular overlap between ARF-6 and BRIS-1. BRIS-1 knockdown resulted in a significant reduction in PI(4,5)P2 levels in SAC-1-depleted cells. Interestingly, the action of SAC-1 in sequestering BRIS-1 is independent of SAC-1's catalytic activity. Our results suggest that the interaction of SAC-1 with ARF-6 curbs ARF-6 activity by limiting the access of ARF-6(GDP) to its guanine nucleotide exchange factor, BRIS-1.

LET-413/Erbin acts as a RAB-5 effector to promote RAB-10 activation during endocytic recycling

RAB-10 is a master regulator of endocytic recycling in polarized epithelial cells. Liu et al. identify LET-413, the Caenorhabditis elegans homolog of Scrib/Erbin, as a RAB-5 effector that is required for the DENN-4–mediated activation of RAB-10 and the control of membrane expansion in the C. elegans intestine.

RAB-10/Rab10 is a master regulator of endocytic recycling in epithelial cells. To better understand the regulation of RAB-10 activity, we sought to identify RAB-10(GDP)–interacting proteins. One novel RAB-10(GDP)–binding partner that we identified, LET-413, is the Caenorhabditis elegans homologue of Scrib/Erbin. Here, we focus on the mechanistic role of LET-413 in the regulation of RAB-10 within the C. elegans intestine. We show that LET-413 is a RAB-5 effector and colocalizes with RAB-10 on endosomes, and the overlap of LET-413 with RAB-10 is RAB-5 dependent. Notably, LET-413 enhances the interaction of DENN-4 with RAB-10(GDP) and promotes DENN-4 guanine nucleotide exchange factor activity toward RAB-10. Loss of LET-413 leads to cytosolic dispersion of the RAB-10 effectors TBC-2 and CNT-1. Finally, we demonstrate that the loss of RAB-10 or LET-413 results in abnormal overextensions of lateral membrane. Hence, our studies indicate that LET-413 is required for DENN-4–mediated RAB-10 activation, and the LET-413–assisted RAB-5 to RAB-10 cascade contributes to the integrity of C. elegans intestinal epithelia.

Two Clathrin Adaptor Protein Complexes Instruct Axon-Dendrite Polarity

The cardinal feature of neuronal polarization is the establishment and maintenance of axons and dendrites. How axonal and dendritic proteins are sorted and targeted to different compartments is poorly understood. Here, we identified distinct dileucine motifs that are necessary and sufficient to target transmembrane proteins to either the axon or the dendrite through direct interactions with the clathrin-associated adaptor protein complexes (APs) in C. elegans. Axonal targeting requires AP-3, while dendritic targeting is mediated by AP-1. The axonal dileucine motif binds to AP-3 with higher efficiency than to AP-1. Both AP-3 and AP-1 are localized to the Golgi but occupy adjacent domains. We propose that AP-3 and AP-1 directly select transmembrane proteins and target them to axon and dendrite, respectively, by sorting them into distinct vesicle pools.