An UNC-40 pathway directs postsynaptic membrane extension in Caenorhabditis elegans.

The postsynaptic membrane of the embryonic neuromuscular junction undergoes a dramatic expansion during later development to facilitate the depolarization of larger muscles. In C. elegans, the postsynaptic membrane resides at the termini of plasma membrane extensions called muscle arms. Membrane extension to the motor axons during larval development doubles the number of muscle arms, making them a tractable model to investigate both postsynaptic membrane expansion and guided membrane extension. To identify genes required for muscle arm extension, we performed a forward screen for mutants with fewer muscle arms. We isolated 23 mutations in 14 genes, including unc-40/Dcc, which encodes a transmembrane receptor that guides the migration of cells and extending axons in response to the secreted UNC-6/Netrin spatial cue. We discovered that UNC-40 is enriched at muscle arm termini and functions cell-autonomously to direct arm extension to the motor axons. Surprisingly, UNC-6 is dispensable for muscle arm extension, suggesting that UNC-40 relies on other spatial cues to direct arm extension. We provide the first evidence that the guanine-nucleotide exchange factor UNC-73/Trio, members of the WAVE actin-polymerization complex, and a homolog of the focal adhesion complex can function downstream of UNC-40 to direct membrane extension. Our work is the first to define a pathway for directed muscle membrane extension and illustrates that axon guidance components can play key roles in postsynaptic membrane expansion.

Differential sensitivities of CaV1.2 IIS5-S6 mutants to 1,4-dihydropyridine analogs.

1,4-Dihydropyridines (DHPs), L-type calcium channel (Ca(V)1) blockers, are known to interact with Ca(V)1.2 subunits through their binding site located at IIIS5-S6 and IVS6 regions. We recently identified two domain II residues (S666 and A752) critical for nifedipine blockade (Kwok et al., 2008). In this study, we examined the blockade effects of two DHP analogues, nemadipine and nicardipine, on wildtype, M1161A (in IIIS6), S666V (in IIS5) and A752T (in IIS6) mutants of the rat alpha(1C) subunit transiently expressed with beta(2a) and alpha(2)delta in cultured tsA201 cells. We found that the IC(50) ratio of the mutants to the wildtype channel was similar in S666V and M1161A mutants for both drugs, but in A752T it was lower for nemadipine than nicardipine (P<0.05). At saturating drug concentrations, not all the current was completely blocked in the mutants. The residual current recorded in 100 microM nemadipine was approximately 10% of the total current for the A752T channel, which was significantly higher than that in 100 microM nicardipine (approximately 2%). In wildtype, S666V and M1161A, there was no significant difference in residual current between nemadipine and nicardipine, although it was greater in S666V (approximately 15%) and M1161A approximately 30%) as compared to the wildtype channel (<5%). Taken together, our findings suggest that the domain II residues alter the DHP effect in a structure-specific manner and may be involved in a pathway downstream of DHP binding.

A genetic screen for dihydropyridine (DHP)-resistant worms reveals new residues required for DHP-blockage of mammalian calcium channels.

Dihydropyridines (DHPs) are L-type calcium channel (Ca(v)1) blockers prescribed to treat several diseases including hypertension. Ca(v)1 channels normally exist in three states: a resting closed state, an open state that is triggered by membrane depolarization, followed by a non-conducting inactivated state that is triggered by the influx of calcium ions, and a rapid change in voltage. DHP binding is thought to alter the conformation of the channel, possibly by engaging a mechanism similar to voltage dependent inactivation, and locking a calcium ion in the pore, thereby blocking channel conductance. As a Ca(v)1 channel crystal structure is lacking, the current model of DHP action has largely been achieved by investigating the role of candidate Ca(v)1 residues in mediating DHP-sensitivity. To better understand DHP-block and identify additional Ca(v)1 residues important for DHP-sensitivity, we screened 440,000 randomly mutated Caenorhabditis elegans genomes for worms resistant to DHP-induced growth defects. We identified 30 missense mutations in the worm Ca(v)1 pore-forming (alpha(1)) subunit, including eleven in conserved residues known to be necessary for DHP-binding. The remaining polymorphisms are in eight conserved residues not previously associated with DHP-sensitivity. Intriguingly, all of the worm mutants that we analyzed phenotypically exhibited increased channel activity. We also created orthologous mutations in the rat alpha(1C) subunit and examined the DHP-block of current through the mutant channels in culture. Six of the seven mutant channels examined either decreased the DHP-sensitivity of the channel and/or exhibited significant residual current at DHP concentrations sufficient to block wild-type channels. Our results further support the idea that DHP-block is intimately associated with voltage dependent inactivation and underscores the utility of C. elegans as a screening tool to identify residues important for DHP interaction with mammalian Ca(v)1 channels.

Insulin-like signaling negatively regulates muscle arm extension through DAF-12 in Caenorhabditis elegans.

The body wall muscles (BWMs) of nematodes are connected to motor axons by muscle membrane extensions called muscle arms. To better understand how muscle arm extension is regulated, we screened conserved receptor tyrosine kinases for muscle arm defects in Caenorhabditis elegans. We discovered that mutations in daf-2, which encodes the only insulin-like receptor tyrosine kinase, confer a supernumerary muscle arm (Sna) phenotype. The Sna phenotype of daf-2 mutants is suppressed by loss-of-function in the canonical downstream FOXO-family transcription factor DAF-16 in either the muscles or the intestine, demonstrating that insulin-like signaling can regulate muscle arm extension non-autonomously. Furthermore, supernumerary arm extension requires the B isoform of the down-stream DAF-12 nuclear hormone receptor, which lacks the DNA-binding domain, but retains the ligand-binding domain. daf-2 regulates many processes in C. elegans including entry into dauer, which is a diapause-like state that facilitates survival of harsh environmental conditions. We found that wild-type dauers are also Sna. Unlike other changes associated with dauer, however, the Sna phenotype of dauers persists in recovered adults. Finally, disruption of a TGF-beta pathway that regulates dauer formation in parallel to the insulin-like pathway also confers the Sna phenotype. We conclude that supernumerary muscle arms are a novel dauer-specific modification that may facilitate some aspect of dauer behavior.