Zebrafish models of human-duplicatedSRGAP2reveal novel functions in microglia and visual system development
Authors: Uribe-Salazar José M. , Kaya Gulhan , Weyenberg KaeChandra , Radke Brittany , Hino Keiko , Soto Daniela C. , Shiu Jia-Lin , Zhang Wenzhu , Ingamells Cole , Haghani Nicholas K. , Xu Emily , Rosas Joseph , Simó Sergi , Miesfeld Joel , Glaser Tom , Baraban Scott C. , Jao Li-En , Dennis Megan Y. 
Published: September 2024

The expansion of the humanSRGAP2family, resulting in a human-specific paralogSRGAP2C,likely contributed to altered evolutionary brain features. The introduction ofSRGAP2Cin mouse models is associated with changes in cortical neuronal migration, axon guidance, synaptogenesis, and sensory-task performance. Truncated SRGAP2C heterodimerizes with the full-length ancestral gene product SRGAP2A and antagonizes its functions. However, the significance ofSRGAP2duplication beyond neocortex development has not been elucidated due to the embryonic lethality of completeSrgap2knockout in mice. Using zebrafish, we show thatsrgap2knockout results in viable offspring and that these larvae phenocopy “humanized”SRGAP2Clarvae, including altered morphometric features (i.e., reduced body length and inter-eye distance) and differential expression of synapse-, axonogenesis-, and vision-related genes. Through single-cell transcriptome analysis, we demonstrate a skewed balance of excitatory and inhibitory neurons that likely contribute to increased susceptibility to seizures displayed bySrgap2mutant larvae, a phenotype resemblingSRGAP2loss-of-function in a child with early infantile epileptic encephalopathy. Single-cell data also shows strong endogenous expression ofsrgap2in microglia with mutants exhibiting altered membrane dynamics and likely delayed maturation of microglial cells. Microglia cells expressingsrgap2were also detected in the developing eye together with altered expression of genes related to axonogenesis in mutant retinal cells. Consistent with the perturbed gene expression in the retina, we found thatSRGAP2mutant larvae exhibited increased sensitivity to broad and fine visual cues. Finally, comparing the transcriptomes of relevant cell types between human (+SRGAP2C) and non-human primates (–SRGAP2C) revealed significant overlaps of gene alterations with mutant cells in our zebrafish models; this suggests thatSRGAP2Cplays a similar role altering microglia and the visual system in modern humans. Together, our functional characterization of conserved ortholog Srgap2 and human SRGAP2C in zebrafish uncovered novel gene functions and highlights the strength of cross-species analysis in understanding the development of human-specific features.

Abstract (short)

SRGAP2Chas been implicated in contributing to altered brain features in the evolution of humans. However, the significance ofSRGAP2duplication beyond neocortex development has not been elucidated due to the embryonic lethality of completeSrgap2knockout in mice. Using zebrafish, we show thatsrgap2knockout results in viable offspring that phenocopy “humanized”SRGAP2Clarvae. Morphometric, behavioral, and transcriptome analyses collectively suggestsrgap2impacts axonal guidance, synaptogenesis, and seizure susceptibility. Beyond neurons,Srgap2functions in controlling membrane dynamics and maturation of microglial cells, possibly leading to altered axonogenesis in the developing retina and increased sensitivity to broad and fine visual cues. Comparing relevant transcriptomes between human and nonhuman primates suggests thatSRGAP2Csimilarly impacts microglia and vision in modern humans. Our functional characterization of conserved ortholog Srgap2 and human SRGAP2C in zebrafish uncovered novel gene functions and highlights the strength of cross-species analysis in understanding the development of human-specific features.