Department of Molecular Biosciences, The University of Kansas, Lawrence, KS 66045, USA.
Genetics. 2022 May 5;221(1). doi: 10.1093/genetics/iyac047.
UNC-6/Netrin is a secreted conserved guidance cue that regulates dorsal-ventral axon guidance of Caenorhabditis elegans and in the vertebral spinal cord. In the polarity/protrusion model of VD growth cone guidance away from ventrally expressed UNC-6 (repulsion), UNC-6 first polarizes the growth cone via the UNC-5 receptor such that filopodial protrusions are biased dorsally. UNC-6 then regulates a balance of protrusion in the growth cone based upon this polarity. UNC-5 inhibits protrusion ventrally, and the UNC-6 receptor UNC-40/DCC stimulates protrusion dorsally, resulting in net dorsal growth cone outgrowth. UNC-5 inhibits protrusion through the flavin monooxygenases FMO-1, 4, and 5 and possible actin destabilization, and inhibits pro-protrusive microtubule entry into the growth cone utilizing UNC-33/CRMP. The PH/MyTH4/FERM myosin-like protein was previously shown to act with UNC-5 in VD axon guidance utilizing axon guidance endpoint analysis. Here, we analyzed the effects of MAX-1 on VD growth cone morphology during outgrowth. We found that max-1 mutant growth cones were smaller and less protrusive than wild type, the opposite of the unc-5 mutant phenotype. Furthermore, genetic interactions suggest that MAX-1 might normally inhibit UNC-5 activity, such that in a max-1 mutant growth cone, UNC-5 is overactive. Our results, combined with previous studies suggesting that MAX-1 might regulate UNC-5 levels in the cell or plasma membrane localization, suggest that MAX-1 attenuates UNC-5 signaling by regulating UNC-5 stability or trafficking. Alternately, MAX-1 might inhibit UNC-5 independent of this known mechanism. We also show that the effects of MAX-1 in growth cone protrusion are independent of UNC-40/DCC, UNC-33/CRMP, and UNC-34/Enabled. In summary, in the context of growth cone protrusion, MAX-1 inhibits UNC-5, demonstrating the mechanistic insight that can be gained by analyzing growth cones during outgrowth in addition to axon guidance endpoint analysis.
UNC-6/Netrin 是一种保守的分泌导向因子，可调节秀丽隐杆线虫的背腹轴导向，并在脊椎脊髓中起作用。在 VD 生长锥远离腹侧表达的 UNC-6（排斥）的极性/突起模型中，UNC-6 通过 UNC-5 受体首先使生长锥极化，使得丝状伪足突起偏向背侧。然后，UNC-6 根据这种极性调节生长锥中的突起平衡。UNC-5 抑制腹侧突起，而 UNC-6 受体 UNC-40/DCC 则刺激背侧突起，从而导致净背侧生长锥外突。UNC-5 通过黄素单加氧酶 FMO-1、4 和 5 以及可能的肌动蛋白去稳定化来抑制突起，并且通过 UNC-33/CRMP 抑制促进突起的微管进入生长锥。先前的研究表明，PH/MyTH4/FERM 肌球蛋白样蛋白与 UNC-5 一起在 VD 轴突导向中起作用，利用轴突导向终点分析。在这里，我们分析了 MAX-1 在生长锥突起过程中对 VD 生长锥形态的影响。我们发现，max-1 突变体生长锥比野生型小且突起少，这与 unc-5 突变体的表型相反。此外，遗传相互作用表明，MAX-1 可能正常抑制 UNC-5 的活性，使得在 max-1 突变体生长锥中，UNC-5 过度活跃。我们的结果结合先前的研究表明，MAX-1 可能通过调节 UNC-5 在细胞中的水平或质膜定位来调节 UNC-5 活性，这表明 MAX-1 通过调节 UNC-5 的稳定性或运输来减弱 UNC-5 信号。或者，MAX-1 可能独立于这种已知机制抑制 UNC-5。我们还表明，MAX-1 在生长锥突起中的作用独立于 UNC-40/DCC、UNC-33/CRMP 和 UNC-34/Enabled。总之，在生长锥突起的背景下，MAX-1 抑制 UNC-5，证明了通过在生长锥突起过程中进行分析而不是在轴突导向终点分析中可以获得的机制见解。
