CCK+ interneurons in CDKL5 deficiency disorder: characterization of the InSyn1-DGC complex in early brain maturation

CDKL5 deficiency disorder (CDD) is a rare genetic disorder caused by mutations in the X-linked cyclin-dependent kinase-like 5 (CDKL5) gene; it is characterized by early-onset drug-resistant epileptic encephalopathy, motor, cognitive, visual and autonomic deficiencies. The epileptic seizures and cognitive deficits that occur in CDD patients emphasize the importance of the protein in the correct regulation of neuronal activity, which is based on a complex balance between excitation and inhibition. While the part concerning excitation is largely investigated, the function of CDKL5 at the inhibitory compartment remains mostly unknown. In this regard, we have characterized the molecular interaction between CDKL5 and InSyn1, a TDark protein that, by interacting with the dystrophin-dystroglycan complex (DGC) controls the formation of cholecystokinin interneurons (CCK+ INs), essential for "turning off" the excitation that is physiologically generated by excitatory neurons. Our results demonstrated that this interaction seems to be prominent during the first stages of neuronal development. Of note, we further found a reduction in InSyn1 expression and an impairment in CCK+ innervation when Cdkl5 is missing, thus underling an strong relationship between the two proteins. Most of our analysis is focused on mouse models of CDD during adolescence and adulthood. However, we believe it is relevant to thoroughly investigate the relationship between Cdkl5-InSyn1 and CCK+ neurons also in the early stages of brain development, thus associated with the childhood period. Indeed, early identification of deregulated mechanisms involving the inhibitory circuit could provide important information to guide therapeutic treatments in CDD patients in a more precise time window.