The autophagy-inducing kinases, ULK1 and ULK2, regulate axon guidance in the developing mouse forebrain via a noncanonical pathway

Mammalian ULK1 (unc-51 like kinase 1) and ULK2, <i>Caenorhabditis elegans</i> UNC-51, and <i>Drosophila melanogaster</i> Atg1 are serine/threonine kinases that regulate flux through the autophagy pathway in response to various types of cellular stress. <i>C. elegans</i> UNC-51 and <i>D. melanogaster</i> Atg1 also promote axonal growth and defasciculation; disruption of these genes results in defective axon guidance in invertebrates. Although disrupting ULK1/2 function impairs normal neurite outgrowth in vitro, the role of ULK1 and ULK2 in the developing brain remains poorly characterized. Here, we show that ULK1 and ULK2 are required for proper projection of axons in the forebrain. Mice lacking <i>Ulk1</i> and <i>Ulk2</i> in their central nervous systems showed defects in axonal pathfinding and defasciculation affecting the corpus callosum, anterior commissure, corticothalamic axons and thalamocortical axons. These defects impaired the midline crossing of callosal axons and caused hypoplasia of the anterior commissure and disorganization of the somatosensory cortex. The axon guidance defects observed in <i>ulk1/2</i> double-knockout mice and central nervous system-specific (<i>Nes-Cre</i>) <i>Ulk1/2</i>-conditional double-knockout mice were not recapitulated in mice lacking other autophagy genes (i.e., <i>Atg7</i> or <i>Rb1cc1</i> [RB1-inducible coiled-coil 1]). The brains of <i>Ulk1/2</i>-deficient mice did not show stem cell defects previously attributed to defective autophagy in <i>ambra1</i> (autophagy/Beclin 1 regulator 1)- and <i>Rb1cc1</i>-deficient mice or accumulation of SQSTM1 (sequestosome 1)⁺ or ubiquitin⁺ deposits. Together, these data demonstrate that ULK1 and ULK2 regulate axon guidance during mammalian brain development via a noncanonical (i.e., autophagy-independent) pathway.

哺乳动物ULK1（unc-51 like kinase 1）与ULK2、秀丽隐杆线虫（Caenorhabditis elegans）UNC-51以及黑腹果蝇（Drosophila melanogaster）Atg1均为丝氨酸/苏氨酸激酶，可响应多种细胞应激调控自噬通路的通量。秀丽隐杆线虫UNC-51与黑腹果蝇Atg1还可促进轴突生长与脱束；上述基因的功能缺失会导致无脊椎动物出现轴突导向缺陷。尽管敲除ULK1/2的功能会在体外损害正常的神经突生长，但ULK1与ULK2在发育大脑中的功能仍未得到充分表征。
本研究证实，ULK1与ULK2对于前脑轴突的正常投射至关重要。中枢神经系统特异性缺失Ulk1和Ulk2的小鼠，其胼胝体、前连合、皮质丘脑轴突及丘脑皮质轴突均出现了轴突寻路与脱束缺陷。这些缺陷损害了胼胝体轴突的中线跨越，导致前连合发育不全以及体感皮层结构紊乱。
在ulk1/2双敲除小鼠与中枢神经系统特异性（Nes-Cre）Ulk1/2条件性双敲除小鼠中观察到的轴突导向缺陷，并未在其他自噬基因缺失（即Atg7或Rb1cc1[RB1-inducible coiled-coil 1]）的小鼠中重现。Ulk1/2缺陷小鼠的大脑并未出现此前在ambra1（autophagy/Beclin 1 regulator 1）与Rb1cc1缺陷小鼠中报道的干细胞缺陷，也未出现SQSTM1（sequestosome 1）⁺或泛素⁺沉积的积累。
综上，上述数据表明，ULK1与ULK2可通过非经典（即不依赖自噬）通路调控哺乳动物脑发育过程中的轴突导向。