Disruption of the Cellular Regulation of CDKL5 Might be Relevant for Rett Syndrome

Mutations in the human X-linked cyclin dependent kinase like 5 (CDKL5) gene have recently been identified in some Rett patients with the Hanefeld variant as well as in girls with mental retardation associated with early seizures. We have previously shown that CDKL5 works in a pathway common with that of MeCP2, the main cause of classic Rett Syndrome (RTT). In fact, the two proteins associate and the kinase is able to mediate the phosphorylation of MeCP2 in vitro. This suggests that mutations in CDKL5 cause RTT in part because important MeCP2 functions are impaired. Furthermore, CDKL5 might play a secondary role in RTT by acting as a modifier gene thereby influencing disease severity in patients with mutations in MECP2. Even though our results do suggest a common molecular pathway belonging to CDKL5 and MeCP2, we still have to reveal in which brain areas and when the two factors are communicating. Furthermore, we have to get insight into the functional relevance of the identified interaction. Our immunohistochemistry and western blot experiments show that in adult brain the two proteins have overlapping expression patterns. However, whereas MeCP2 levels appear rather uniform through late embryogenesis and postnatal stages as well as in the different brain areas, CDKL5 levels appear to be subject to a dynamic modulation. In particular, the CDKL5 protein is virtually absent in the mouse embryo brain and is strongly induced in early post-natal stages. Furthermore, whereas MeCP2 is confined to the nuclear compartment, CDKL5 is found in both the nuclear and cytoplasmic compartments indicating a possible role in transmitting signals between these two compartments. Interestingly, the subcellular distribution of CDKL5 varies in the different areas of the adult mouse brain as well as during development. Experiments in cell cultures have allowed us to show that CDKL5 shuttles between the nucleus and the cytoplasm and that an active nuclear export mechanism depending on the C-terminal tail of the protein is responsible for the cytoplasmic localization. The relevance of this regulation seems to be demonstrated by the fact that all the late RTT truncating mutations identified so far lead to an abnormal accumulation of the kinase into the nucleus.