G?2 Regulates The Multipolar-bipolar Transition Of Newborn Neurons In The Developing Neocortex

The development of mammalian cerebral cortex is composed of several sequential steps,including neural progenitor cell proliferation,cell differentiation,neuronal migration and projection.Postmitotic neurons originating from the ventricular zone(VZ)and the subventricular zone(SVZ)migrate radially through the intermediate zone(IZ)to the appropriate positions in the cortical plate via several migration modes,including multipolar migration,locomotion and translocation.Then the neurons will develop their neurites to form the functional neural circuits once they reach their positions.Series researches suggest that aberrant neuronal migration is associated with severe developmental defects,such as epilepsy,mental retardation and schizophrenia.Therefore,it is still urgent to further reveal the cellular mechanism of neuronal migration in the developmental stage,which will be also helpful to investigate the pathology of neurological disorders.Numerous intracellular and extracellular molecules have been reported to participate in neuronal migration.Heterotrimeric G protein,composed of ?,?,and ? subunits,is a central mediator of intracellular signal transduction.The classical function of G protein is to transduce signals from G protein coupled receptors to their downstream effectors,which in turn regulate kinase activity,ion channel activity and cytoskeleton stability and so on.Originally,G?? dimer was only considered as an inactivator of G?,but it now has been shown to regulate the activity of a variety of downstream effectors to participate in cytoskeleton dynamics,vesicle trafficking and regulation of transcriptional activity.Previous studies have suggested that G?? dimer plays a key role in cell chemotaxis and migration.In the central nervous system,G?? dimer has been verified to regulate the outgrowth of neurites,the release of neurotransmitter and the proliferation and differentiation of neuronal progenitor cells.There are five G? isoforms in mammals to serve their broad roles in cellular signaling.However,little is known about the function of each subunit in G?? dimer for neuronal development.Firstly,we found that the expression of G protein ? subunit 2(G?2)is highest among five G?s,and G?2 is highly expressed from embryonic day(E)13 to postnatal day(P)7.The results from the immunohischemistry staining illustrated that G?2 is broadly expressed in the developing neocortex but accumulates in the IZ.In utero suppression of G?2 at E14.5 or 15.5 impaired neuron migration temporarily at E18.5 or P0.Moreover,the morphological analysis revealed that impairment of G?2 affects the morphology establishment of bipolar neurons,and the dendritic arborization of the neurons in P7.The abnormal location of Gogli in G?2 knockdown neurons suggested that G?2 controls the polarization of migrating neurons.Extracellular regulated protein kinases 1/2(ERK1/2)are classical effectors of G?? dimer.Further mechanism analysis revealed that G?2 interacts with MEK1/2(MAPKK),ERK1/2 and the scalfold protein MP1.Reducing the expression of G?2 in NLT cells and neurons would inhibit the activation of ERK1/2 pathway.We also noticed that expressing dominant negative mutants of ERK1/2(ERK1 K71 R,ERK2 K52R)in migrating neuron disturbed the multipolar to bipolar transition,which phenocopied with G?2 knockdown.Importantly,restoring ERK1/2 activity by expression of a constitutively active MEK1(CAMEK1)rescues neuronal migration and the multipolar-bipolar transition defects of G?2-silenced neurons in a dose-dependent manner.In summary,this study revealed a crucial role of G?2 in neuronal migration.Loss of G?2 or impairment of ERK1/2 signaling would delay neuronal migration due to multipolar to bipolar transition defect.Collectively,our findings demonstrate that G?2-mediated modulation of ERK1/2 activity during neocortex development is required for neuronal migration.