Molecular mechanisms of cortical axon guidance

The establishment of correct connectivity in the mammalian neocortex is one of the most elaborate and least understood developmental processes. In an initial step, multiple guidance molecules cooperatively direct axons in their trajectories. The contribution of individual guidance cues and the molecular mechanisms they employ to effect refined and precise axonal targeting are little known. In my thesis work I took the particular case of semaphorin 3A, a cue expressed in the developing neocortex, and I addressed two key questions regarding its function in guiding cortical efferents.;First, I asked how diverse cellular responses downstream of this cue coordinate and result in a unitary growth cone behavior. Together with my collaborators in the lab, I showed that ezrin radixin and moesin (ERM) proteins, by virtue of their ability to link membranes and the cytoskeleton, take an active part to this process. ERMs regulate endocytosis of semaphorin 3A receptors, growth cone collapse and axon guidance in cortical neurons. Moreover, their active state is responsive to semaphorin 3A stimulation.;In the second part of my thesis I investigated the molecular bases of the differential responsiveness to a cue. I found that two molecularly defined populations of cortical efferents respond differently to semaphorin 3A in several functional assays. Neurons that lack the transcription factor Satb2 are more responsive and internalize more ligand via a pathway mediated by flotillin-1, a resident of lipid rafts known to induce membrane curvature and budding. Knocking down flotillin-1 in these neurons prevents internalization and full axonal responsiveness to semaphorin 3A. Collectively, my data indicate novel roles for ERMs and flotillin-1 proteins in establishing the proper connectivity of the neocortex, a conclusion that is currently tested in vivo.