BBX protein restrains p53-dependent p21 activation in neural stem cells of the developing mouse cortex

Researchers reported in EMBO Reports that bobby sox homolog (BBX), a nuclear protein with an HMG box domain, regulates the development of the cerebral cortex. The work was done in mouse embryos and in cultured cortical precursors. BBX is enriched in the germinal zones of the embryonic mouse cortex, and its transcripts were found in the ventricular and subventricular zones at embryonic day 14.5 nature.comEMBO Reportsnature.comOpen source ↗.

In the embryonic cortex, depletion of BBX reduced the number of SOX2-expressing neural stem cells and promoted premature cell-cycle exit. It also disrupted the positioning of cortical cells, caused abnormal clustering of newborn neurons and impaired the shape of migrating neurons.

BBX and neural stem cell maintenance

In neurosphere assays, overexpression of BBX increased the number of primary neurospheres and sustained their growth through serial replating, with higher SOX2 levels. Silencing of Bbx lowered neurosphere yield and reduced SOX2 and Cyclin D1.

The BBX protein has 888 amino acids, and its HMG box spans residues 80 to 148. A construct lacking this region bound chromatin less well, yet it still stimulated neurosphere formation and sustained SOX2 to a similar degree as the full-length protein. According to the authors, the classical DNA-binding motif is not required for this function.

After in utero electroporation, cells with reduced BBX stalled in the intermediate zone and formed dense clusters. Migrating neurons had shortened leading processes, and neurons in the lower cortical plate were multipolar rather than bipolar.

The p53-p21-RB axis

RNA sequencing of BBX-depleted neural stem cells showed 661 upregulated and 695 downregulated genes. Pathways of DNA damage signalling and cellular senescence were upregulated. Among cell-cycle inhibitors, p21 was the only one strongly induced.

Biochemical assays showed that BBX interacts with p53 and suppresses p53-dependent p21 expression. BBX knockdown increased γH2AX accumulation and senescence-associated gene expression. Co-depletion of p21 rescued the defects in stem cell maintenance, neuronal distribution and morphology, and depletion of retinoblastoma protein (RB) restored the abnormal cortical cell distribution. The authors conclude that BBX maintains normal cortical development by restraining the p53-p21-RB axis nature.comEMBO Reportsnature.comOpen source ↗.