Polarizing the Neuron through Sustained Co-expression of Alternatively Spliced Isoforms

Alternative splicing (AS) is an important source of proteome diversity in eukaryotes. However, how this affects protein repertoires at a single-cell level remains an open question. Here, we show that many 3′-terminal exons are persistently co-expressed with their alternatives in mammalian neurons. In an important example of this scenario, cell polarity gene Cdc42, a combination of polypyrimidine tract-binding, protein-dependent, and constitutive splicing mechanisms ensures a halfway switch from the general (E7) to the neuron-specific (E6) alternative 3′-terminal exon during neuronal differentiation. Perturbing the nearly equimolar E6/E7 ratio in neurons results in defects in both axonal and dendritic compartments and suggests that Cdc42E7 is involved in axonogenesis, whereas Cdc42E6 is required for normal development of dendritic spines. Thus, co-expression of a precise blend of functionally distinct splice isoforms rather than a complete switch from one isoform to another underlies proper structural and functional polarization of neurons. [Display omitted] •Alternative 3′-terminal isoforms often become co-expressed in developing neurons•Ptbp1/2-dependent and constitutive mechanisms ensure co-expression of Cdc42 isoforms•Controlled utilization of Cdc42 exon 7 in neurons is required for proper axonogenesis•Exon-6-containing isoform of Cdc42 promotes formation of dendritic spines in vivo Alternative splicing diversifies eukaryotic proteomes, but how this affects protein functions in individual cells remains poorly understood. Yap et al. show that polypyrimidine tract-binding, protein-dependent, and constitutive mechanisms ensure co-expression of two alternative isoforms of Cdc42 in neurons. Importantly, this is required for proper development of axons and dendritic spines.