EML2 and EML4 splice variants regulate microtubule remodeling during neuronal cell differentiation

Neurons depend on microtubule organization for axon and dendrite formation during differentiation. Yet, our understanding of how microtubules are remodeled during this process is far from complete. Echinoderm microtubule-associated protein-like (EML) is a family of microtubule-associated proteins (MAPs) highly expressed in neuronal cells. Database analysis revealed that EMLs are subject to alternative splicing in their N-terminal regions, leading to the production of one long (L) variant containing a complete N-terminus and TAPE domain, and possibly several short (S) variants with a truncated N-terminal region. We investigated EML4 and EML2 splice variants during SH-SY5Y neuronal cell differentiation, examining their expression, localization, and effects on neurite outgrowth and branching. We found that differentiation led to decreased expression of the EML4-L but not the -S variant. Moreover, expression of the EML2-L variant increased, while EML2-S expression decreased. Overexpression of EML2-L and EML2-S led to increased and decreased neurite lengths, respectively. Interestingly, neurite branching increased in EML2-S-transfected cells. Using immunofluorescence microscopy, we found that EML2-L and EML4-L but not EML2-S and EML4-S localized strongly to interphase microtubules regardless of whether cells had differentiated or not. Depletion of both EML4 and EML2 led to neurite outgrowth. We propose that the presence of the N-terminal microtubule-binding region in the long variants of EML2 and EML4 promotes stabilization of microtubules and neurite extension, while short variants favor branching. Together, these data suggest that the regulated expression and localization of different splice variants of EML proteins are crucial to microtubule remodeling during neuronal differentiation.