Splice form-dependent regulation of axonal arbor complexity by FMRP

The autism-related protein Fragile X mental retardation protein (FMRP) is an RNA binding protein that plays important roles during both nervous system development and experience dependent plasticity. Alternative splicing of the Fmr1 locus gives rise to 12 different FMRP splice forms that differ in the functional and regulatory domains they contain as well as in their expression profile among brain regions and across development. Complete loss of FMRP leads to morphological and functional changes in neurons, including an increase in the size and complexity of the axonal arbor. To investigate the relative contribution of the FMRP splice forms to the regulation of axon morphology, we overexpressed individual splice forms in cultured wild type rat cortical neurons. FMRP overexpression led to a decrease in axonal arbor complexity that suggests that FMRP regulates axon branching. This reduction in complexity was specific to three splice forms-the full-length splice form 1, the most highly expressed splice form 7, and splice form 9. A focused analysis of splice form 7 revealed that this regulation is independent of RNA binding. Instead this regulation is disrupted by mutations affecting phosphorylation of a conserved serine as well as by mutating the nuclear export sequence. Surprisingly, this mutation in the nuclear export sequence also led to increased localization to the distal axonal arbor. Together, these findings reveal domain-specific functions of FMRP in the regulation of axonal complexity that may be controlled by differential expression of FMRP splice forms. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 738-752, 2017.