Focal Rings and Filopodial Emergence in Neuronal Growth Cones

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0212326TosneyAxon Guidance & Neural Plasticity Oral preferred, Poster is OK THREE FUNCTIONALLY DISTINCT ADHESIONS IN FILOPODIA Tosney, K.W., K. Balazovich and M. Steketee, MCDB Department and Neuroscience Program, University of Michigan, Ann Arbor, MI 48109 Filopodia are sensory and motile structures, vital for the motility and guidance of growth cones. Growth cones can advance but cannot respond to guidance cues if bereft of filopodia, the tips of their filopodia are specialized for signal reception, and adhesion of just a single filopodial tip to a cue can alter discrete elements of motility as well as axonal trajectory. Individual filopodia were analyzed with correlated optical recording, electron microscopy and immunolabeling, to elucidate the role of filopodial adhesions in motility and guidance. We report that, contrary to common belief, all substrate adhesions are not created equal. Individual filopodia develop adhesions at three sites, and these adhesions differ distinctively in function. Tip adhesions suffice to signal. Basal adhesions lie at filopodial bases and along internal actin bundles, associate with a novel organelle, the "focal ring," and function in filopodial emergence and dynamics. Shaft adhesions lie along filopodial shafts, lack a focal ring, and control the extent of lamellar ("veil") advance. When present, shaft adhesions inhibit veil advance. Veils are unaffected by basal adhesions, but stop advancing as they encountered shaft adhesions. Veils readily advance along filopodia lacking shaft adhesions, but not along filopodia displaying shaft adhesions. This relationship is not due to veils removing adhesions as they advance. Reducing adhesion with antibodies decreases the proportion of filopodia with shaft adhesions and coordinately increases veil advance. Of particular interest, guidance cues can act by targeting shaft adhesions. When a filopodial tip contacts an inhibitory cue, the signal induces shaft adhesions and coordinately abolishes veil advance, whereas contact with a permissive cue prohibits shaft adhesions and coordinately promotes veil advance. Cues that target shaft adhesions produce systematic changes in veil advance and thereby modulate axonal trajectory. Individual growth cone filopodia thus develop three functionally distinct adhesions that can help regulate both motility and navigation. These adhesions also have structural and compositional distinctions. For instance, basal and shaft adhesions differentially associate with Rho GTPases, basal adhesions with Rac1 and shaft adhesions with Cdc42. We are currently testing the hypothesis that one way in which Rac1 and Cdc42 control motility is by selectively inducing adhesions that differ in function. Supported by NIH-21308 and NSF-0212326.