NMDA-type glutamate receptors play a critical role in the activity-dependent development and structural remodeling of dendritic arbors and spines. However, the molecular mechanisms that link NMDA receptor activation to changes in dendritic morphology remain unclear. We report that the Rac1-GEF Tiam1 is present in dendrites and spines and is required for their development. Tiam1 interacts with the NMDA receptor and is phosphorylated in a calcium-dependent manner in response to NMDA receptor stimulation. Blockade of Tiam1 function with RNAi and dominant interfering mutants of Tiam1 suggests that Tiam1 mediates effects of the NMDA receptor on dendritic development by inducing Rac1-dependent actin remodeling and protein synthesis. Taken together, these findings define a molecular mechanism by which NMDA receptor signaling controls the growth and morphology of dendritic arbors and spines.

The development of dendritic spines is thought to be crucial for synaptic plasticity.Dendritic spines are retracted upon EphA4 activation, but the mechanisms that control this process are not well understood. Here we report an important role of Cdk5 in EphA4-dependent spine retraction. We find that blockade of Cdk5 activity inhibits ephrin-A1-triggered spine retraction and reduction of mEPSC frequency at hippocampal synapses.Activation of EphA4 results in the recruitment of Cdk5 to EphA4, leading to the tyrosine phosphorylation and activation of Cdk5. EphA4 and Cdk5 then enhance the activation of ephexin1, a GEF that regulates RhoA activation. We show that the association between EphA4 and ephexin1 is significantly reduced in Cdk5 -/-brains and that Cdk5-dependent phosphorylation of ephexin1 is required for ephrin-A1-mediated regulation of spine density.These findings suggest that ephrin-A1 promotes EphA4-dependent spine retraction through the activation of Cdk5 and ephexin1 which in turn modulates actin cytoskeletal dynamics.3

SUMMARY Animals generate movement by engaging spinal circuits that direct precise sequences of muscle contraction, but the identity and organizational logic of local interneurons that lie at the core of these circuits remain unresolved. Here we show that V1 interneurons, a major inhibitory population that controls motor output, fractionate into highly diverse subsets on the basis of the expression of nineteen transcription factors. Transcriptionally defined V1 subsets exhibit distinct physiological signatures and highly structured spatial distributions with mediolateral and dorsoventral positional biases. These positional distinctions constrain patterns of input from sensory and motor neurons, arguing that interneuron position is a determinant of microcircuit organization. Moreover, V1 diversity indicates that different inhibitory microcircuits exist for motor pools controlling hip, ankle, and foot muscles, revealing a variable circuit architecture for interneurons that control limb movement.

Wnts make up a large family of extracellular signaling molecules that play crucial roles in development and disease. A subset of noncanonical Wnts signal independently of the transcription factor β-catenin by a mechanism that regulates key morphogenetic movements during embryogenesis. The best characterized noncanonical Wnt, Wnt5a, has been suggested to signal via a variety of different receptors, including the Ror family of receptor tyrosine kinases, the Ryk receptor tyrosine kinase, and the Frizzled seven-transmembrane receptors. Whether one or several of these receptors mediates the effects of Wnt5a in vivo is not known. Through loss-of-function experiments in mice, we provide conclusive evidence that Ror receptors mediate Wnt5a-dependent processes in vivo and identify Dishevelled phosphorylation as a physiological target of Wnt5a–Ror signaling. The absence of Ror signaling leads to defects that mirror phenotypes observed in Wnt5a null mutant mice, including decreased branching of sympathetic neuron axons and major defects in aspects of embryonic development that are dependent upon morphogenetic movements, such as severe truncation of the caudal axis, the limbs, and facial structures. These findings suggest that Wnt5a–Ror–Dishevelled signaling constitutes a core noncanonical Wnt pathway that is conserved through evolution and is crucial during embryonic development.

The mechanisms that promote excitatory synapse formation and maturation have been extensively studied. However, the molecular events that limit excitatory synapse development so that synapses form at the right time and place and in the correct numbers are less well understood. We have identified a RhoA guanine nucleotide exchange factor, Ephexin5, which negatively regulates excitatory synapse development until EphrinB binding to the EphB receptor tyrosine kinase triggers Ephexin5 phosphorylation, ubiquitination, and degradation. The degradation of Ephexin5 promotes EphB-dependent excitatory synapse development and is mediated by Ube3A, a ubiquitin ligase that is mutated in the human cognitive disorder Angelman syndrome and duplicated in some forms of Autism Spectrum Disorders (ASDs). These findings suggest that aberrant EphB/Ephexin5 signaling during the development of synapses may contribute to the abnormal cognitive function that occurs in Angelman syndrome and, possibly, ASDs.

Dendritic spines are small, actin-rich protrusions on the surface of dendrites that receive the majority of excitatory synaptic inputs in the brain. The formation and remodeling of spines, processes that underlie synaptic development and plasticity, are regulated in part by Eph receptor tyrosine kinases. However, the mechanism by which Ephs regulate actin cytoskeletal remodeling necessary for spine development is not fully understood. Here, we report that the Rac1 guanine nucleotide exchange factor Tiam1 interacts with the EphB2 receptor in a kinase-dependent manner. Activation of EphBs by their ephrinB ligands induces the tyrosine phosphorylation and recruitment of Tiam1 to EphB complexes containing NMDA-type glutamate receptors. Either knockdown of Tiam1 protein by RNAi or inhibition of Tiam1 function with a dominant-negative Tiam1 mutant blocks dendritic spine formation induced by ephrinB1 stimulation. Taken together, these findings suggest that EphBs regulate spine development in part by recruiting, phosphorylating, and activating Tiam1. Tiam1 can then promote Rac1-dependent actin cytoskeletal remodeling required for dendritic spine morphogenesis.synapses ͉ Rho GTPase ͉ actin cytoskeleton ͉ NMDA receptor

During development, neural precursors migrate in response to positional cues such as growth factor gradients. However, the mechanisms that enable precursors to sense and respond to such gradients are poorly understood. Here we show that cerebellar granule cell precursors (GCPs) migrate along a gradient of brain-derived neurotrophic factor (BDNF), and we demonstrate that vesicle trafficking is critical for this chemotactic process. Activation of TrkB, the BDNF receptor, stimulates GCPs to secrete BDNF, thereby amplifying the ambient gradient. The BDNF gradient stimulates endocytosis of TrkB and associated signaling molecules, causing asymmetric accumulation of signaling endosomes at the subcellular location where BDNF concentration is maximal. Thus, regulated BDNF exocytosis and TrkB endocytosis enable precursors to polarize and migrate in a directed fashion along a shallow BDNF gradient.

Motor output varies along the rostro-caudal axis of the tetrapod spinal cord. At limb levels, ∼60 motor pools control the alternation of flexor and extensor muscles about each joint, whereas at thoracic levels as few as 10 motor pools supply muscle groups that support posture, inspiration, and expiration. Whether such differences in motor neuron identity and muscle number are associated with segmental distinctions in interneuron diversity has not been resolved. We show that select combinations of nineteen transcription factors that specify lumbar V1 inhibitory interneurons generate subpopulations enriched at limb and thoracic levels. Specification of limb and thoracic V1 interneurons involves the Hox gene Hoxc9 independently of motor neurons. Thus, early Hox patterning of the spinal cord determines the identity of V1 interneurons and motor neurons. These studies reveal a developmental program of V1 interneuron diversity, providing insight into the organization of inhibitory interneurons associated with differential motor output.