Aspm knockout ferret reveals an evolutionary mechanism governing cerebral cortical size

Abstract

The human cerebral cortex is distinguished by its large size and abundant gyrification, or folding, yet the evolutionary mechanisms driving cortical size and structure are unknown. While genes essential for cortical developmental expansion have been identified from the genetics of human primary microcephaly (“small head”, associated with reduced brain size and intellectual disability)1, studies of these genes in mice, whose smooth cortex is one thousand times smaller than that of humans, have provided limited insight. Mutations of abnormal spindle-like microcephaly-associated (ASPM), the most common recessive microcephaly gene, reduce cortical volume by ≥50% in humans2–4, but have little effect in mice5–9, likely reflecting evolutionarily divergent functions of ASPM10,11. We used genome editing to create a germline knockout (KO) of Aspm in the ferret (Mustela putorius furo), a species with a larger, gyrified cortex and greater neural progenitor cell (NPC) diversity12–14 than mice, and closer Aspm protein sequence homology to human. Aspm KO ferrets exhibit severe microcephaly (25–40% decreases in brain weight), reflecting reduced cortical surface area without significant change in cortical thickness, as in human patients3,4, suggesting loss of “cortical units”. The mutant ferret fetal cortex displays a massive premature displacement of ventricular radial glial cells (VRG) to the outer subventricular zone (OSVZ), where many resemble outer radial glia (ORG), an NPC subtype essentially absent in mice and implicated in cerebral cortical expansion in primates12–16. These data suggest an evolutionary mechanism whereby Aspm regulates cortical expansion by controlling the affinity of VRG for the ventricular surface, thus modulating the ratio of VRG, the most undifferentiated cell type, to ORG, a more differentiated progenitor.

Ferret brain possesses young interneuron collections equivalent to human postnatal migratory streams

“…Their brain size is significantly increased compared to rodents (Barnette et al, 2009;Raju et al, 2018). Like humans, and unlike rodents, they possess a gyrencephalic cortex (Barnette et al, 2009;Duque & McCormick, 2010;Johnson et al, 2018). Furthermore, they undergo extensive postnatal brain development (McConnell, 1985(McConnell, , 1988Morton et al, 2017).…”

Evolution of ASPM coding variation in apes and associations with brain structure in chimpanzees

“…56,57 Thus, variation in ventricular volume may affect the circulation of growth factors that could potentially influence the regulation of cerebral cortical development. Alternatively, because ASPM has a significant effect on neural progenitor cycling along the ventricles in fetal life, 26,59 the association shown in our study may be a result of how brain size is patterned by ASPM during neurogenesis in early development. It has been shown that ASPM plays a role in regulating the affinity of ventricular radial glial cells (VRGs) for the ventricular surface.…”

“…Alternatively, because ASPM has a significant effect on neural progenitor cycling along the ventricles in fetal life,26,59 the association shown in our study may be a result of how brain size is patterned by ASPM during neurogenesis in early development. Alternatively, because ASPM has a significant effect on neural progenitor cycling along the ventricles in fetal life,26,59 the association shown in our study may be a result of how brain size is patterned by ASPM during neurogenesis in early development.…”

“…56 Previous studies have shown that certain ASPM variants can have a significant impact on volume and surface area of cerebral cortical regions, 14,18,25,59 while other research suggests that common variants of ASPM do not contribute to human brain size variability. Ventricles are a critical site of neuronal proliferation in early development.…”

“…volume may affect the circulation of growth factors that could potentially influence the regulation of cerebral cortical development. It has been shown that ASPM plays a role in regulating the affinity of ventricular radial glial cells (VRGs) for the ventricular surface 59. It has been shown that ASPM plays a role in regulating the affinity of ventricular radial glial cells (VRGs) for the ventricular surface 59.…”