An important characteristic of neurons is their intrinsic ability to grow and extend neurites, even in the absence of external cues. Neurite initiation is critical to neuronal morphogenesis and early neural circuit development. However, As compared to our understanding of the last stages of neuronal morphogenesis, including neuronal polarization, dendrite morphogenesis and synapse formation, relatively little is known regarding the mechanism regulating neuritogenesis？ – the initial extension of neurites from the spherical somata.

　　Using cultured rat hippocampal neurons and a combination of live imaging and pharmacological treatments, researchers from the Institute of Neuroscience,CAS, showed that one or two large actin aggregates formed underneath the membrane of newborn neurons and served as the start sites of neurite initiation. They further showed that actin dynamics was required for neurite initiation, while microtubule dynamics and protein synthesis were required for the later stage of neurite stabilization.

　　In addressing the molecular cue regulating actin aggregation during neurite initiation, the authors demonstrate that local clustering of phosphatidylinositol 3, 4-bisphosphate [PI(3,4)P2], a phospholipid with relatively few known signaling functions, is necessary and sufficient for aggregating actin and promoting neuritogenesis. In contrast, the related and more extensively studied phosphatidylinositol 4, 5-bisphosphate [PI(4,5)P2] or phosphatidylinositol (3,4,5)-trisphosphate (PIP3) molecules did not have such functions. Specifically, they showed that beads coated with PI(3,4)P2 promoted actin aggregation and neurite initiation, while interference with PI(3,4)P2 synthesis through pharmacological treatment inhibited both processes. PI(3,4)P2 clusters occurred even when actin aggregation was pharmacologically blocked, demonstrating that PI(3,4)P2 functioned as the upstream signaling molecule. Two enzymes critical for PI(3,4)P2 generation, namely SH2 domain containing inositol 5-phosphatase (SHIP2) and class II phosphoinositide 3-kinase alpha (PI3K C2α) were non-redundantly and complementarily required for actin aggregation and neuritogenesis, as well as for subsequent dendritogenesis. Finally, the authors demonstrate that neural Wiskott-Aldrich syndrome protein (N-WASP) and the Arp 2/3 complex functioned downstream of PI(3,4)P2 to mediate neuritogenesis and dendritogenesis. Together, these resultsidentify PI(3,4)P2 as an important signaling molecule during early development and demonstrate its critical role in regulating actin aggregation and neuritogenesis.

　　This work was carried out by graduate student ZHANG Shu-Xin and DUAN Li-Hui, under the supervision of Dr. YU Xiang, at the Institute of Neuroscience and State Key Laboratory of Neuroscience, CAS Center for Excellence in Brain Science and Intelligence Technology, Chinese Academy of Sciences. This work entitled "Phosphatidylinositol 3,4-bisphosphate regulates neurite initiation and dendrite morphogenesis via actin aggregation" was published online in the journal Cell Research on Jan 20th. It was supported by grants from the National Natural Science Foundation of China,the Strategic Priority Research Program of the Chinese Academyof Sciences and the Science and Technology Commission of Shanghai Municipality.

　　(A)Image of a cultured rat hippocampus neuron fluorescently labelled for PI(3,4)P2 (green channel), F-actin (phalloidin, red channel), DAPI (blue channel) and MAP 2 (magenta channel).(B)A working model of the mechanism underlying PI(3,4)P2-dependent actin aggregation and neurite initiation:SHIP2 and PI3KC2αcoordinate to promote PI(3,4)P2 clustering, initiating a cascade of events involving N-WASP and Arp3 and resulting in local actin clustering and neurite initiation.