The human cerebral cortex is distinguished by its large size and abundant gyrification, or folding. However, the evolutionary mechanisms that drive cortical size and structure are unknown. Although genes that are essential for cortical developmental expansion have been identified from the genetics of human primary microcephaly (a disorder associated with reduced brain size and intellectual disability)1, studies of these genes in mice, which have a smooth cortex that is one thousand times smaller than the cortex of humans, have provided limited insight. Mutations in abnormal spindle-like microcephaly-associated (ASPM), the most common recessive microcephaly gene, reduce cortical volume by at least 50% in humans2,3,4, but have little effect on the brains of mice5,6,7,8,9; this probably reflects evolutionarily divergent functions of ASPM10,11. Here we used genome editing to create a germline knockout of Aspm in the ferret (Mustela putorius furo), a species with a larger, gyrified cortex and greater neural progenitor cell diversity12,13,14 than mice, and closer protein sequence homology to the human ASPM protein. Aspm knockout ferrets exhibit severe microcephaly (25–40% decreases in brain weight), reflecting reduced cortical surface area without significant change in cortical thickness, as has been found in human patients3,4, suggesting that loss of ‘cortical units’ has occurred. The cortex of fetal Aspm knockout ferrets displays a very large premature displacement of ventricular radial glial cells to the outer subventricular zone, where many resemble outer radial glia, a subtype of neural progenitor cells that are essentially absent in mice and have been implicated in cerebral cortical expansion in primates12,13,14,15,16. These data suggest an evolutionary mechanism by which ASPM regulates cortical expansion by controlling the affinity of ventricular radial glial cells for the ventricular surface, thus modulating the ratio of ventricular radial glial cells, the most undifferentiated cell type, to outer radial glia, a more differentiated progenitor.
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We thank the late R. W. Guillery, who first introduced ferrets as a model for developmental neuroscience; J. K. Joung for advice on genome editing; J. Bond for the ASPM antibody; L. Vasung, P. Herman, J. Neil and C. D. Kroenke for advice on ferret brain MRI; A. Lee, the G. M. Church laboratory (S. Biwas), the S. McCarroll laboratory (S. Burger), the P. Kharchenko laboratory (J. Fan), and the R. Satija laboratory (A. Butler) for advice on scRNA-seq; S. Wasiuk, E. Feiner, A. S. Kamumbu and M. Lee for technical assistance; Marshall BioResources for animal husbandry; and E. Pollack and the veterinary staff at Boston Children’s Hospital and Yale School of Medicine for surgical support. Animal silhouettes in Fig. 1 were designed by Freepik from https://www.flaticon.com/. This work was supported by P30NS052519 (F.H. and Yale’s QNMR Core Center), 2R01MH067528 (F.H.), 1R24MH114805 (X.P.), R21HD083956 (K.I.), R01EB017337 (P.E.G.), R24HL123482 (J.F.E.), 5R01NS032457 (C.A.W.), 5R21NS091865 (B.-I.B.) and the Allen Discovery Center program through The Paul G. Allen Frontiers Group. C.A.W. is an Investigator of the Howard Hughes Medical Institute.Reviewer information
Nature thanks S. Juliano, F. Tissir and the other anonymous reviewer(s) for their contribution to the peer review of this work.
Extended Data Figures and Tables
Extended Data Fig. 1 Cytoarchitecture and neuronal subtype lamination in the cortex of mature Aspm knockout ferrets.
a, Nissl stains of coronal sections from the brains of P41 littermates, as shown in Fig. 1l, with additional Aspm+/− and Aspm−/− littermates shown. b, c, Brain sections of P41 littermates immunostained for cortical layer-specific projection neurons including SATB2 (layer II–IV), CTIP2 (layer V), and FoxP2 (layer VI). The experiments were repeated independently three times with similar results. Scale bars, 2 mm (a, top), 200 μm (a, bottom) and 100 μm (b, c).
Extended Data Fig. 2 SOX2 and Ki-67 immunostaining in additional E35 and P0 littermates dorsal cortex.
Additional results to Fig. 2e, f. Each set of panels is from the brain of a different littermate, showing the high penetrance of the neural progenitor cell basal displacement phenotype. The experiments were repeated independently three times with similar results. Scale bar, 200 μm.
Additional results for Fig. 2h, i. Immunostaining of SOX2, Ki-67 and VIM shows that displaced neural progenitors have basal radial fibres. The experiments were repeated independently three times with similar results. Scale bars, 100 μm.
Extended Data Fig. 4 Aspm knockout mice do not demonstrate displaced progenitors in the intermediate zone.
a, b, Unlike Aspm−/− ferrets, Aspm−/− mice do not have displaced NPCs in the intermediate zone. However, they show a variable increase in the number of intermediate progenitors (PAX6−Ki-67+ cells in a and TBR2+ cells in b), which is enhanced by heterozygous, compound mutation in Wdr62, a microcephaly gene causing more severe microcephaly9. The experiments were repeated independently three times with similar results. Scale bars, 100 μm.
Extended Data Fig. 5 Modest increase in apoptosis throughout the germinal zones of the Aspm knockout telencephalon.
Apoptotic cells (yellow) are indicated by enzymatic fluorescence detection of double-stranded DNA damage with DAPI nuclear counterstaining (blue). The experiments were repeated independently three times with similar results. a, Whole section. b, c, Cortical wall columns. Scale bars, 500 μm (a) and 100 μm (b, c).
Extended Data Fig. 6 Additional immunohistochemical analyses of displaced progenitors in the Aspm knockout cortex.
a, E35 knockout cortex stained for VRG and ORG markers SOX2 and HOPX reveals extensive co-labelling in both the ventricular zone (VZ) and SVZ, including in displaced OSVZ progenitors. b, In the E35 knockout OSVZ, clusters of supernumerary displaced neural progenitor cells include numerous TBR2+ intermediate progenitors and are surrounded by DCX+ newborn neurons, indicating preserved neurogenesis within the precocious OSVZ niche of the Aspm knockout cortex. The experiments were repeated independently three times with similar results. Scale bars, 50 μm.
a, t-SNE plot from Fig. 3a with cells coloured by biological replicate (that is, animal). Most clusters include cells from all samples, except for a cluster expressing blood genes and a cluster expressing choroid plexus epithelial cells that are mostly from animal WT5E. These two cell clusters were not included in downstream analyses. HET, heterozygote; KO, knockout; WT, wild type. Numbers and letters indicate litter and animal identification number, respectively. b, t-SNE plot from Fig. 3a with cells coloured by the batch they were processed in. Clusters are composed of cells from all batches. c, Per-cell gene count and UMI count per sample. Each violin plot is one biological replicate and each dot is one cell. Sample WT5D was not included in the analysis due to the lower gene and UMI count compared to other samples as well as the inconsistent clustering compared to other wild-type samples (data not shown). d, Per-cell gene count and UMI count for identified clusters. Each violin plot is one cell cluster and each dot is one cell. The three clusters in grey (EN4, BL, CPE) were not included in downstream analyses. See Methods for details. This scRNA-seq experiment was performed once with n = 22,211 cells (8,037 cells from two Aspm+/+ and one Aspm+/− ferrets and 14,174 cells from four Aspm−/− ferrets). RG1, cycling radial glial progenitors; RG2, interphase radial glial progenitors; IP, intermediate progenitors; EN1, upper-layer excitatory neurons; EN2, deep-layer excitatory neurons; EN3, Cajal–Retzius cells; IN1, immature inhibitory neurons; IN2, SST+ inhibitory neurons; IN3, ventral/inhibitory progenitors; ENDO1, endothelial cells 1; ENDO2, endothelial cells 2; OPC, oligodendrocyte precursors; MG, microglia; EN4, mixed excitatory neuron identity; BL, blood cells; CPE, choroid plexus epithelial cells.
Mitotic Aspm knockout FEFs, identified by staining for pH3 and co-stained for the centriolar marker centrin, display a significant loss of centrioles. The percentage of cells with an abnormal number (less than 4) of centrioles is increased eightfold in Aspm−/− FEFs compared to Aspm+/+ FEFs (n = 100 cells per genotype for three independent experiments; P = 0.003). The experiments were repeated independently three times with similar results. Statistical analysis was performed using a two-tailed t-test; data are mean ± s.e.m.