TOM-1/Tomosyn acts with the UNC-6/Netrin receptor UNC-5 to inhibit growth cone protrusion in Caenorhabditis elegans

In the polarity/protrusion model of growth cone repulsion from UNC-6/Netrin, UNC-6 first polarizes the VD growth cone via the UNC-5 receptor, and then regulates protrusion asymmetrically across the growth cone based on this polarity. Through the UNC-40/DCC receptor, UNC-6 stimulates protrusion dorsally, and through UNC-5 inhibits protrusion ventrally and laterally, resulting in net dorsal growth. Previous studies showed that UNC-5 inhibits growth cone protrusion via the flavin monooxygenases and potential destabilization of F-actin, and via UNC-33/CRMP and restriction of microtubule + end entry into the growth cone. To explore the role of vesicle fusion in growth cone protrusion, we analyzed tom-1/tomosyn mutants. Tomosyn normally occludes formation of the SNARE complex by interacting with and inhibiting Syntaxin and thus preventing vesicle fusion. VD growth cones of tom-1 null mutants were similar to wild-type. However, tom-1 null mutants suppressed the effects of constitutively-activated MYR::UNC-5, which alone causes small growth cones with little protrusion. This suggests that TOM-1 is normally required for the inhibitory effects of MYR::UNC-5 on growth cone protrusion. tom-1 encodes long and short isoforms, and results here indicate that tom-1S is required downstream of UNC-5 to inhibit protrusion, whereas the tom-1L has a pro-protrusive role. unc-64/Syntaxin mutants displayed reduced growth cone protrusion, suggesting that TOM-1 inhibits growth cone protrusion by inhibiting UNC-64/Syntaxin, similar to its role in neurotransmission. TOM-1L, TOM-1S, and UNC-64/Syntaxin were all required for VD growth cone polarity of protrusion, indicating that regulated vesicle fusion is required for the establishment and/or maintenance of VD growth cone polarity. These studies show that, in addition to effects on actin and microtubules, UNC-5 might inhibit VD growth cone protrusion by inhibiting growth cone vesicle fusion and thus the ability of growth cones to add plasma membrane necessary for protrusive growth.