The polarization of axon and dendrites underlies the ability of neurons to integrate and transmit information in the brain. In the past few years, studies have implicated several genes encoding kinases, phosphatases, small GTPases, microtubule-associated proteins, and scaffolding proteins in the establishment of neuronal polarity. Most of these studies used the classic model of dissociated hippocampal neuron culture and their relevance to brain development in vivo is uncertain. The most pressing challenges in the field are now (i) determining if these proteins play major roles in neuronal polarization in vivo and (ii) understanding how extracellular cues polarize intracellular responses leading to axon and dendrite specification. Using a candidate approach, we recently obtained preliminary results demonstrating that the serine/threonine kinase LKB1 (also known as Par4 or STK11) is required for polarization of cortical neurons in vivo (Barnes et al. Cell - in press). Using a conditional cortex-specific knockout, we found that LKB1 is specifically required for axon initiation during neuronal polarization in vivo but is not essential for neuronal migration or dendrite formation. Our preliminary results suggest that the polarizing activity of LKB1 requires both interaction with its necessary co-activator, the pseudo-kinase Strad1 and phosphorylation on Serine 431 (S431) by protein kinase A (PKA) or p90 Ribosomal S6 Kinase (p90RSK), two kinases known to mediate some of the effects of extracellular cues controlling axon morphogenesis. Once activated, LKB1 phosphorylates SAD-A/B kinases which are required for the ability of LKB1 to specify neuronal polarity in part through their ability to phosphorylate microtubule associated proteins (MAPs) such as Tau. Taken together, our preliminary results lead us to hypothesize that this novel multi-kinase pathway is required for axon specification in vivo by linking extracellular signals to the intracellular responses underlying neuronal polarization. We propose to test this hypothesis by (Aim 1) determining if LKB1 is required throughout the developing CNS and PNS for the specification of neuronal polarity, (Aim 2) determining if extracellular cues such as BDNF or Wnt7a previously shown to activate PKA and p90RSK can induce phosphorylation of Serine 431 and control axon specification, (Aim 3) testing if receptors for these extracellular cues, TrkB and Frz3 are required for cortical axon specification. Frz3 is of particular interest since Frz3 knockout embryos show a strong defect in axon formation, a phenotype almost indentical to our conditional LKB1 knockout. This proposal will provide novel insights into the molecular mechanisms specifying neuronal polarity in vivo.Project narrative One major question in modern neuroscience focuses on how neurons establish connections during development. We identified a gene (LKB1) that underlies the formation of axons in the cerebral cortex and we propose to study what regulate the function of this gene during development. Recent evidence suggested that LKB1 disruption could participate to tauopathies such as Alzheimer's disease. Therefore our work will provide important and novel clues regarding the molecular mechanisms underlying this devastating neurodegenerative pathology.
