Elucidating The Molecular Mechanism Underlying Coronin 2B-Regulated Neuronal Migration In Cerebral Cortex

Neuronal migration is one of the most critical steps for facilitating the wellorganized lamination of the cerebral cortex and formation of neural circuitry.In the developmental cortex,excitatory pyramidal neurons,accounting for 80% of all neurons,are produced in the ventricular zone（VZ）of the basal cortex and migrate to their designated position in the cortex for further maturation and integration into neurocircuits.Neuronal migration defects are the major cause of neurodevelopmental disorders（NDDs）which result in symptoms such as mental retardation,epilepsy,and autism.Neuronal migration requires the regulation of cytoskeleton dynamics.The actin cytoskeleton is involved in regulating many critical processes for neuronal migration,such as maintaining growth cone structure and function,multipolar–bipolar transition,and leading process extension and stabilization.Especially at the multipolar–bipolar transition,actin cytoskeleton dynamics are precisely regulated by a series of actin binding proteins（ABPs）including coronins,a class of evolutionarily conserved actin regulatory protein family.Coronin 2B,a member of the coronin family,is abundantly expressed in the central nervous system,especially during the embryonic developmental stage of neurogenesis and neuronal migration,suggesting that Coronin2 B is essential for cortical development.However,the role of Coronin 2B in regulating neuronal migration and the underlying mechanisms has remained largely unknown;thus,our study examined these underlying mechanisms,targeting the cerebral cortex in mice,as follows:First,we elucidated the role of Coronin 2B in neuronal migration.Immunofluorescence staining revealed that Coronin 2B was mainly expressed in the intermediate zone（IZ）and cortical plate（CP）,where neuronal migration occurs.Cortical neurons were labeled with green fluorescent protein（GFP）through in utero electroporation（IUE）,and knockdown of Coronin 2B resulted in neuronal migration defects,which involved a significant increase of migrating neurons stalled in the IZ.In addition,these arrested neurons exhibited abnormal neuronal identity with Neu N positive,but not Cux1（marker for neurons in layers II–IV）positive nor Ctip2（marker for neurons in layers V–VI）positive,indicating that the retained neurons would remain immature.Notably,Coronin 2B knockdown resulted in increased susceptibility to pentylenetetrazol（PTZ）-induced seizures in young mice at P30.Next,we demonstrated that Coronin 2B regulates neuronal migration via controlling the neuronal multipolar–bipolar transition.Knockdown of Coronin 2B in vivo results in a failure of migrating neurons to polarize,while the proliferation or differentiation of neural progenitors is unaffected.During multipolar–bipolar transition,Coronin 2B maintains the stability of the actin cytoskeleton by regulating the activity of the actin severing protein cofilin,and promotes the establishment of neuronal polarity.Finally,we analyzed the molecular mechanism by which Coronin 2B regulates neuronal migration.We used PBD and Lifeact to label GTP-Rac1,the upstream regulator of cofilin,in primary cultured neurons and in the developmental cortex.Our results indicate that Coronin 2B regulates the distribution of activated Rac1,maintains the dynamic stability of the actin cytoskeleton,and promotes leading process formation and stability of migrating neurons.In summary,this study demonstrates the critical roles of Coronin 2B in regulating neuronal migration during cortical development.We also revealed that Coronin 2B mediates actin cytoskeleton dynamics through the Rac1–cofilin pathway to regulate neuronal migration.This study not only sheds light on the molecular mechanisms underlying neuronal migration,but also provides therapeutic clues for treating NDDs.