Citron-Kinase, A Protein Essential to Cytokinesis in Neuronal Progenitors, Is Deleted in the FlatheadMutant Rat

Cytokinesis is an essential step in neurogenesis, yet the mechanisms that control cytokinesis in the developing CNS are not well understood. The flathead ( fh) mutation in rat results in cytokinesis failure in neural progenitors followed by apoptosis and a dramatic reduction in CNS growth. Here we present evidence that the fh mutation is caused by a single base deletion in exon 1 of the gene encoding Citron-Kinase (Citron-K). This base deletion causes a premature stop codon at the 27th codon in the N-terminal kinase domain of Citron-K, and Western blot and immunocytochemical analysis show that the Citron-K protein is absent in proliferative zones in fh/fh mutant embryos. We find that Citron-K protein is normally expressed along the ventricular zone (VZ) surface and localizes to cleavage furrows of both symmetrically and asymmetrically dividing progenitors. In addition, Citron-K colocalizes with RhoA at cleavage furrows in wild-type (wt) embryos, whereas RhoA expression is reduced at the VZ surface and is absent from many cytokinesis furrows in homozygous fh/fh mutants. These results, together with evidence from a recently described induced mutation in mice, indicate that the flathead mutation is in the Citron-K gene and that Citron-K may act with RhoA to ensure the progression of cytokinesis in neuronal progenitors.