The large and small SPEN family proteins stimulate axon outgrowth during neurosecretory cell remodeling in Drosophila

Split ends (SPEN) is the founding member of a well conserved family of nuclear proteins with critical functions in transcriptional regulation and the post-transcriptional processing and nuclear export of transcripts. In animals, the SPEN proteins fall into two size classes that perform either complementary or antagonistic functions in different cellular contexts. Here, we show that the two Drosophila representatives of this family, SPEN and Spenito (NITO), regulate metamorphic remodeling of the CCAP/bursicon neurosecretory cells. CCAP/bursicon cell-targeted overexpression of SPEN had no effect on the larval morphology or the pruning back of the CCAP/bursicon cell axons at the onset of metamorphosis. During the subsequent outgrowth phase of metamorphic remodeling, overexpression of either SPEN or NITO strongly inhibited axon extension, axon branching, peripheral neuropeptide accumulation, and soma growth. Cell-targeted loss-of-function alleles for both spen and nito caused similar reductions in axon outgrowth, indicating that the absolute levels of SPEN and NITO activity are critical to support the developmental plasticity of these neurons. Although nito RNAi did not affect SPEN protein levels, the phenotypes produced by SPEN overexpression were suppressed by nito RNAi. We propose that SPEN and NITO function additively or synergistically in the CCAP/bursicon neurons to regulate multiple aspects of neurite outgrowth during metamorphic remodeling.