Citron-kinase loss leads to hypomyelination via cell autonomous and non-cell autonomous mechanisms

Citron kinase (Cit-K) is a serine-threonine kinase involved in DNA repair and cytoskeletal dynamics. Loss-of-function of Cit-K results in microcephaly and severe Central Nervous System (CNS) hypomyelination in both humans and mice (i.e. Cit-k KO). Former studies (Boda et al. 2022, doi: 10.1038/s41467-022-30010-6) have shown that Cit-k loss results in accumulation of DNA damage and cell-autonomously induces forebrain oligodendrocyte progenitor cells (OPCs) to undergo cell death or enter a senescent state. Yet, in Cit-k mutants, severe and diffuse hypomyelination persists even when forebrain OPC population is rescued, suggesting additional Cit-K loss -dependent mechanisms involved in myelin defect. To study these aspects, at first, we took a transplantation approach and compared the maturation potential of wild-type (WT) OPCs grafted in either Cit-k KO vs. WT postnatal mouse brain. Notably, when exposed to the Cit-k KO milieu, the fraction of grafted cells that reached a mature MBP+ phenotype was significantly lower than that in the WT brain, suggesting the presence of an inhibitory/non-supportive environment in the Cit-k KO CNS tissue. This was corroborated by in vitro assays, where Cit-k KO OPCs appeared able to progress along the lineage when isolated from the tissue and exposed to pro-myelinating factors. Yet, even in these conditions the maturation potential of Cit-k KO OPCs was lower than that of WT cells, indicating the co-existence of cell-autonomous brakes to maturation. Consistently, oligodendrocyte-specific Cit-k mutants (i.e. Sox10Cre;Cit-k^fl/fl mice) showed only a partial recovery of the forebrain myelination pattern compared to the germinal Cit-k KO mouse brain. In the adult Sox10Cre;Cit-k^fl/fl mice, hypomyelination persisted in the cerebral cortex –where OPC density was normal or only slightly reduced– and was associated with spatial memory defects and subtle motor deficits.
Overall, these data indicate that Cit-k loss leads to a functionally-relevant and regionalized hypomyelination via cell autonomous and non-cell autonomous mechanisms.