Structural genomic variations represent a major driving force of evolution and a burst of large segmental gene duplications occurred in the human lineage during its separation from non-human primates. SRGAP2, a gene recently implicated in neocortical development, has undergone two human-specific duplications. Here we find that both duplications (SRGAP2B and SRGAP2C) are partial and encode a truncated F-BAR domain. SRGAP2C is expressed in the developing and adult human brain and dimerizes with ancestral SRGAP2 to inhibit its function. In the mouse neocortex, SRGAP2 promotes spine maturation and limits spine density. Expression of SRGAP2C phenocopies SRGAP2 deficiency. It underlies sustained radial migration and leads to the emergence of human-specific features, including neoteny during spine maturation and increased density of longer spines. These results suggest that inhibition of SRGAP2 function by its human-specific paralogs has contributed to the evolution of the human neocortex and plays an important role during human brain development.

Spinal motor neurons (MNs) and V2 interneurons (V2-INs) are specified by two related LIM-complexes, MN-hexamer and V2-tetramer, respectively. Here we show how multiple parallel and complementary feedback loops are integrated to assign these two cell fates accurately. While MN-hexamer response elements (REs) are specific to MN-hexamer, V2-tetramer-REs can bind both LIM-complexes. In embryonic MNs, however, two factors cooperatively suppress the aberrant activation of V2-tetramer-REs. First, LMO4 blocks V2-tetramer assembly. Second, MN-hexamer induces a repressor, Hb9, which binds V2-tetramer-REs and suppresses their activation. V2-INs use a similar approach; V2-tetramer induces a repressor, Chx10, which binds MN-hexamer-REs and blocks their activation. Thus, our study uncovers a regulatory network to segregate related cell fates, which involves reciprocal feedforward gene regulatory loops.

Summary Multiple excitatory and inhibitory interneurons form the motor circuit with motor neurons in the ventral spinal cord. Notch signaling initiates the diversification of immature V2-interneurons into excitatory V2a-interneurons and inhibitory V2b-interneurons. Here we provide a transcriptional regulatory mechanism underlying their balanced production. LIM-only protein LMO4 controls this binary cell fate choice by regulating the activity of V2a- and V2b-specific LIM-complexes inversely. In the spinal cord, LMO4 induces GABAergic V2b-interneurons in collaboration with SCL and inhibits Lhx3 from generating glutamatergic V2a-interneuons. In LMO4;SCL compound mutant embryos, V2a-interneurons increase markedly at the expense of V2b-interneurons. We further demonstrate that LMO4 nucleates the assembly of a novel LIM-complex containing SCL, Gata2 and NLI. This complex activates specific enhancers in V2b-genes consisting of binding sites for SCL and Gata2, thereby promoting V2b-interneuron fate. Thus, LOM4 plays essential roles in directing a balanced generation of inhibitory and excitatory neurons in the ventral spinal cord.

No abstract

mRNA localization and local translation enable exquisite spatial and temporal control of gene expression, particularly in highly polarized and elongated cells. These features are especially prominent in radial glial cells (RGCs), which serve as neural and glial precursors of the developing cerebral cortex, and scaffolds for migrating neurons. Yet the mechanisms by which distinct sub-cellular compartments of RGCs accomplish their diverse functions are poorly understood. Here, we demonstrate that subcellular RNA localization and translation of the RhoGAP Arhgap11a controls RGC morphology and mediates cortical cytoarchitecture. Arhgap11a mRNA and protein exhibit conserved localization to RGC basal structures in mice and humans, conferred by a 5′UTR cis-element. Proper RGC morphology relies upon active Arhgap11a mRNA transport and localization to basal structures, where ARHGAP11A is locally synthesized. Thus, RhoA activity is spatially and acutely activated via local translation in RGCs to promote neuron positioning and cortical cytoarchitecture. Altogether, our study demonstrates that mRNA localization and local translation mediate compartmentalization of neural progenitor functions to control brain development.