Cellular distribution of metastasis suppressor 1 and the shape of cell bodies are temporarily altered in Engrailed-2 overexpressing cerebellar Purkinje cells

Repulsive Sema3E-Plexin-D1 signaling coordinates both axonal extension and steering via activating an autoregulatory factor, Mtss1

Axon guidance molecules are critical for neuronal pathfinding because they regulate directionality and growth pace during nervous system development. However, the molecular mechanisms coordinating proper axonal extension and turning are poorly understood. Here, metastasis suppressor 1 (Mtss1), a membrane protrusion protein, ensured axonal extension while sensitizing axons to the Semaphorin 3E (Sema3E)-Plexin-D1 repulsive cue. Sema3E-Plexin-D1 signaling enhanced Mtss1 expression in projecting striatonigral neurons. Mtss1 localized to the neurite axonal side and regulated neurite outgrowth in cultured neurons. Mtss1 also aided Plexin-D1 trafficking to the growth cone, where it signaled a repulsive cue to Sema3E. Mtss1 ablation reduced neurite extension and growth cone collapse in cultured neurons. Mtss1-knockout mice exhibited fewer striatonigral projections and irregular axonal routes, and these defects were recapitulated in Plxnd1- or Sema3e-knockout mice. These findings demonstrate that repulsive axon guidance activates an exquisite autoregulatory program coordinating both axonal extension and steering during neuronal pathfinding.

MIM-Induced Membrane Bending Promotes Dendritic Spine Initiation

Proper morphogenesis of neuronal dendritic spines is essential for the formation of functional synaptic networks. However, it is not known how spines are initiated. Here, we identify the inverse-BAR (I-BAR) protein MIM/MTSS1 as a nucleator of dendritic spines. MIM accumulated to future spine initiation sites in a PIP2-dependent manner and deformed the plasma membrane outward into a proto-protrusion via its I-BAR domain. Unexpectedly, the initial protrusion formation did not involve actin polymerization. However, PIP2-dependent activation of Arp2/3-mediated actin assembly was required for protrusion elongation. Overexpression of MIM increased the density of dendritic protrusions and suppressed spine maturation. In contrast, MIM deficiency led to decreased density of dendritic protrusions and larger spine heads. Moreover, MIM-deficient mice displayed altered glutamatergic synaptic transmission and compatible behavioral defects. Collectively, our data identify an important morphogenetic pathway, which initiates spine protrusions by coupling phosphoinositide signaling, direct membrane bending, and actin assembly to ensure proper synaptogenesis.