The plasma membrane calcium ATPase MCA-3 is required for clathrin-mediated endocytosis in scavenger cells of Caenorhabditis elegans

Suppression of Selective Voltage-Gated Calcium Channels Alleviates Neuronal Degeneration and Dysfunction through Glutathione S-Transferase-Mediated Oxidative Stress Resistance in a Caenorhabditis elegans Model of Alzheimer's Disease

Calcium homeostasis plays a vital role in protecting against Alzheimer's disease (AD). In this study, amyloid-β (Aβ)-induced C. elegans models of AD were used to elucidate the mechanisms underlying calcium homeostasis in AD. Calcium acetate increased the intracellular calcium content, exacerbated Aβ _1-42 aggregation, which is closely associated with oxidative stress, aggravated neuronal degeneration and dysfunction, and shortened the lifespan of the C. elegans models. Ethylene glycol tetraacetic acid (EGTA) and nimodipine were used to decrease the intracellular calcium content. Both EGTA and nimodipine showed remarkable inhibitory effects on Aβ _1-42 aggregations by increasing oxidative stress resistance. Moreover, both compounds significantly delayed the onset of Aβ-induced paralysis, rescued memory deficits, ameliorated behavioral dysfunction, decreased the vulnerability of two major (GABAergic and dopaminergic) neurons and synapses, and extended the lifespan of the C. elegans AD models. Furthermore, RNA sequencing of nimodipine-treated worms revealed numerous downstream differentially expressed genes related to calcium signaling. Nimodipine-induced inhibition of selective voltage-gated calcium channels was shown to activate other calcium channels of the plasma membrane (clhm-1) and endoplasmic reticulum (unc-68), in addition to sodium-calcium exchanger channels (ncx-1). These channels collaborated to activate downstream events to resist oxidative stress through glutathione S-transferase activity mediated by HPGD and skn-1, as verified by RNA interference. These results may be applied for the treatment of Alzheimer's disease.

A C. elegans genome-wide RNAi screen for altered levamisole sensitivity identifies genes required for muscle function

At the neuromuscular junction (NMJ), postsynaptic ionotropic acetylcholine receptors (AChRs) transduce a chemical signal released from a cholinergic motor neuron into an electrical signal to induce muscle contraction. To identify regulators of postsynaptic function, we conducted a genome-wide RNAi screen for genes required for proper response to levamisole, a pharmacological agonist of ionotropic L-AChRs at the Caenorhabditis elegans NMJ. A total of 117 gene knockdowns were found to cause levamisole hypersensitivity, while 18 resulted in levamisole resistance. Our screen identified conserved genes important for muscle function including some that are mutated in congenital myasthenic syndrome, congenital muscular dystrophy, congenital myopathy, myotonic dystrophy, and mitochondrial myopathy. Of the genes found in the screen, we further investigated those predicted to play a role in endocytosis of cell surface receptors. Loss of the Epsin homolog epn-1 caused levamisole hypersensitivity and had opposing effects on the levels of postsynaptic L-AChRs and GABA_A receptors, resulting in increased and decreased abundance, respectively. We also examined other genes that resulted in a levamisole-hypersensitive phenotype when knocked down including gas-1, which functions in Complex I of the mitochondrial electron transport chain. Consistent with altered ATP synthesis impacting levamisole response, treatment of wild-type animals with levamisole resulted in L-AChR–dependent depletion of ATP levels. These results suggest that the paralytic effects of levamisole ultimately lead to metabolic exhaustion.

Formation of the transition zone by Mks5/Rpgrip1L establishes a ciliary zone of exclusion (CIZE) that compartmentalises ciliary signalling proteins and controls PIP2 ciliary abundance

Cilia are thought to harbour a membrane diffusion barrier within their transition zone (TZ) that compartmentalises signalling proteins. How this "ciliary gate" assembles and functions remains largely unknown. Contrary to current models, we present evidence that Caenorhabditis elegans MKS-5 (orthologue of mammalian Mks5/Rpgrip1L/Nphp8 and Rpgrip1) may not be a simple structural scaffold for anchoring > 10 different proteins at the TZ, but instead, functions as an assembly factor. This activity is needed to form TZ ultrastructure, which comprises Y-shaped axoneme-to-membrane connectors. Coiled-coil and C2 domains within MKS-5 enable TZ localisation and functional interactions with two TZ modules, consisting of Meckel syndrome (MKS) and nephronophthisis (NPHP) proteins. Discrete roles for these modules at basal body-associated transition fibres and TZ explain their redundant functions in making essential membrane connections and thus sealing the ciliary compartment. Furthermore, MKS-5 establishes a ciliary zone of exclusion (CIZE) at the TZ that confines signalling proteins, including GPCRs and NPHP-2/inversin, to distal ciliary subdomains. The TZ/CIZE, potentially acting as a lipid gate, limits the abundance of the phosphoinositide PIP2 within cilia and is required for cell signalling. Together, our findings suggest a new model for Mks5/Rpgrip1L in TZ assembly and function that is essential for establishing the ciliary signalling compartment.

Identification of nonviable genes affecting touch sensitivity in Caenorhabditis elegans using neuronally enhanced feeding RNA interference

Caenorhabditis elegans senses gentle touch along the body via six touch receptor neurons. Although genetic screens and microarray analyses have identified several genes needed for touch sensitivity, these methods miss pleiotropic genes that are essential for the viability, movement, or fertility of the animals. We used neuronally enhanced feeding RNA interference to screen genes that cause lethality or paralysis when mutated, and we identified 61 such genes affecting touch sensitivity, including five positive controls. We confirmed 18 genes by using available alleles, and further studied one of them, tag-170, now renamed txdc-9. txdc-9 preferentially affects anterior touch response but is needed for tubulin acetylation and microtubule formation in both the anterior and posterior touch receptor neurons. Our results indicate that neuronally enhanced feeding RNA interference screens complement traditional mutageneses by identifying additional nonviable genes needed for specific neuronal functions.

C. elegans as a model for membrane traffic

The counterbalancing action of the endocytosis and secretory pathways maintains a dynamic equilibrium that regulates the composition of the plasma membrane, allowing it to maintain homeostasis and to change rapidly in response to alterations in the extracellular environment and/or intracellular metabolism. These pathways are intimately integrated with intercellular signaling systems and play critical roles in all cells. Studies in Caenorhabditis elegans have revealed diverse roles of membrane trafficking in physiology and development and have also provided molecular insight into the fundamental mechanisms that direct cargo sorting, vesicle budding, and membrane fisson and fusion. In this review, we summarize progress in understanding membrane trafficking mechanisms derived from work in C. elegans, focusing mainly on work done in non-neuronal cell-types, especially the germline, early embryo, coelomocytes, and intestine.

Sequential breakdown of 3-phosphorylated phosphoinositides is essential for the completion of macropinocytosis

Macropinocytosis is a highly conserved endocytic process by which extracellular fluid and solutes are internalized into cells. Macropinocytosis starts with the formation of membrane ruffles at the plasma membrane and ends with their closure. The transient and sequential emergence of phosphoinositides PI(3,4,5)P3 and PI(3,4)P2 in the membrane ruffles is essential for macropinocytosis. By making use of information in the Caenorhabditis elegans mutants defective in fluid-phase endocytosis, we found that mammalian phosphoinositide phosphatase MTMR6 that dephosphorylates PI(3)P to PI, and its binding partner MTMR9, are required for macropinocytosis. INPP4B, which dephosphorylates PI(3,4)P2 to PI(3)P, was also found to be essential for macropinocytosis. These phosphatases operate after the formation of membrane ruffles to complete macropinocytosis. Finally, we showed that KCa3.1, a Ca(2+)-activated K(+) channel that is activated by PI(3)P, is required for macropinocytosis. We propose that the sequential breakdown of PI(3,4,5)P3 → PI(3,4)P2 → PI(3)P → PI controls macropinocytosis through specific effectors of the intermediate phosphoinositides.

Caenorhabditis elegans nicotinic acetylcholine receptors are required for nociception

Polymodal nociceptors sense and integrate information on injurious mechanical, thermal, and chemical stimuli. Chemical signals either activate nociceptors or modulate their responses to other stimuli. One chemical known to activate or modulate responses of nociceptors is acetylcholine (ACh). Across evolution nociceptors express subunits of the nicotinic acetylcholine receptor (nAChR) family, a family of ACh-gated ion channels. The roles of ACh and nAChRs in nociceptor function are, however, poorly understood. Caenorhabditis elegans polymodal nociceptors, PVD, express nAChR subunits on their sensory arbor. Here we show that mutations reducing ACh synthesis and mutations in nAChR subunits lead to defects in PVD function and morphology. A likely cause for these defects is a reduction in cytosolic calcium measured in ACh and nAChR mutants. Indeed, overexpression of a calcium pump in PVD mimics defects in PVD function and morphology found in nAChR mutants. Our results demonstrate, for the first time, a central role for nAChRs and ACh in nociceptor function and suggest that calcium permeating via nAChRs facilitates activity of several signaling pathways within this neuron.

Different endocytic functions of AGEF-1 in C. elegans coelomocytes

BACKGROUND

ADP-ribosylation factors (ARFs) are a family of small GTP-binding proteins that play roles in membrane dynamics and vesicle trafficking. AGEF-1, which is thought to act as a guanine nucleotide exchange factor of class I ARFs, is required for caveolin-1 body formation and receptor-mediated endocytosis in oocytes of Caenorhabditis elegans. This study explores additional roles of AGEF-1 in endocytic transport.

METHODS

agef-1 expression was knocked down by using RNAi in C. elegans. Markers that allow analysis of endocytic transport in scavenger cells were investigated for studying the effect of AGEF-1 on different steps of membrane transport.

RESULTS

Knockdown of AGEF-1 levels results in two apparent trafficking defects in coelomocytes of C. elegans. First, there is a delay in the uptake of solutes from the extracellular medium. Second, there is a dramatic enlargement of the sizes of lysosomes, even though lysosomal acidification is normal and degradation still occurs.

CONCLUSION

Our results suggest that AGEF-1 regulates endosome/lysosome fusion or fission events, in addition to earlier steps in endocytic transport.

GENERAL SIGNIFICANCE

AGEF-1 is the first identified GTPase regulator that functions at the lysosome fusion or fission stage of the endocytic pathway. Our study provides insight into lysosome dynamics in C. elegans.

Derlin-dependent accumulation of integral membrane proteins at cell surfaces

Quality-control mechanisms of protein folding of transmembrane and secreted proteins is mediated by endoplasmic-reticulum-associated degradation (ERAD), which is used to detect and to degrade misfolded proteins in the ER. The ERAD machinery consists of chaperones, transmembrane proteins and ubiquitin-associated enzymes that detect, modify, and retro-translocate the misfolded proteins to the cytoplasm for degradation by the proteasome. In contrast to ERAD, little is known about the fates of integral membrane and secreted proteins that become misfolded at the plasma membrane or in the extracellular space. Derlin proteins are a family of proteins that are conserved in all eukaryotes, where they function in ERAD. Here, we show that loss of Derlin function in Caenorhabditis elegans and in mouse macrophages results in the accumulation of integral membrane proteins at the plasma membrane. Induction of LDL receptor misfolding at the plasma membrane results in a sharp decrease in its half-life, which can be rescued by proteasomal inhibitors or by reduction of Derlin-1 levels. We also show that Derlin proteins localize to endosomes as well as to the ER. Our data are consistent with a model where Derlin proteins function in a spatially segregated quality control pathway that is used for the recognition and degradation of transmembrane proteins that become misfolded at the plasma membrane and/or in endosomes.