Biological events and molecular signaling following MLKL activation during necroptosis

<p>Necroptosis is a form of programmed necrotic cell death mediated by the kinase RIPK3 and its substrate MLKL. MLKL, which displays plasma membrane (PM) pore-forming activity upon phosphorylation, functions as the executioner during necroptosis. Thus, it was previously assumed that MLKL phosphorylation is the endpoint of the necroptotic signaling pathway. Here, we summarize several events that characterize the dying necroptotic cells after MLKL phosphorylation, including Ca²⁺ influx, phosphatidylserine (PS) externalization, PM repair by ESCRT-III activation, and the final compromise of PM integrity. These processes add several unexpected regulatory events downstream of MLKL signaling. We have also observed that CoCl₂, which may mimic hypoxia, can induce necroptosis, which suggests that <i>in vivo</i> triggers of necroptosis might include a transient lack of O₂.</p

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

<p>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.</p