a, Wild type human SVBP (hSVBP), but not its double mutant I39A Y40A, rescues axonal specification in mouse hippocampal neurons knocked down for mouse SVBP (mSVBP). Representative immunofluorescence images of neurons cotransfected with plasmids expressing a shRNA (CTRL or mSVBP) and hSVBP-myc-Flag (wild type or I39A Y40A). Tyrosinated and detyrosinated α-tubulin contents were assayed at 1DIV and staining images were merged. Detyrosinated tubulin levels and number of stage III neuron (bearing an axon) are significantly reduced in knocked down neurons (top right: sh mSVBP) compared to control neurons (top left: sh control), and rescued with native hSVBP (bottom left: sh mSVBP + hSVBP) but not with the hSVBP I39A Y40A mutant (bottom right: sh mSVBP + hSVBP I39A Y40A). Scale bar is indicated. Arrowheads and arrows highlight single and double axons emanating from a cell body, respectively.