CDKL5 Expression Is Modulated during Neuronal Development and Its Subcellular Distribution Is Tightly Regulated by the C-terminal Tail

Abstract

Mutations in the human X-linked cyclin-dependent kinase-like 5 (CDKL5) gene have been identified in patients with Rett syndrome (RTT), West syndrome, and X-linked infantile spasms, sharing the common feature of mental retardation and early seizures. CDKL5 is a rather uncharacterized kinase, but its involvement in RTT seems to be explained by the fact that it works upstream of MeCP2, the main cause of Rett syndrome. To understand the role of this kinase for nervous system functions and to address if molecular mechanisms are involved in regulating its distribution and activity, we studied the ontogeny of CDKL5 expression in developing mouse brains by immunostaining and Western blotting. The expression profile of CDKL5 was compared with that of MeCP2. The two proteins share a general expression profile in the adult mouse brain, but CDKL5 levels appear to be highly modulated at the regional level. Its expression is strongly induced in early postnatal stages, and in the adult brain CDKL5 is present in mature neurons, but not in astroglia. Interestingly, the presence of CDKL5 in the cell nucleus varies at the regional level of the adult brain and is developmentally regulated. CDKL5 shuttles between the cytoplasm and the nucleus and the C-terminal tail is involved in localizing the protein to the cytoplasm in a mechanism depending on active nuclear export. Accordingly, Rett derivatives containing disease-causing truncations of the C terminus are constitutively nuclear, suggesting that they might act as gain of function mutations in this cellular compartment.

Neurodevelopmental and neurobehavioral characteristics in males and females with CDKL5 duplications

“…20 CDKL5 levels were found to be low in the embryonic cortex and strongly induced at perinatal and postnatal stages in maturing neurons residing in both cerebral cortex and hippocampus until day P14, where after it declines. 12 A detailed analysis of Cdkl5 expression in adult mouse brain (Allen Brain Atlas) showed that its mRNA levels are particularly high in the adult forebrain, 13 consistent with its role in neuronal maturation. 12 Conversely, isoform II, transcribed from exons 1a and 1b (but not exon 1), is only expressed at very low levels in human fetal brain and testes.…”

“…12 A detailed analysis of Cdkl5 expression in adult mouse brain (Allen Brain Atlas) showed that its mRNA levels are particularly high in the adult forebrain, 13 consistent with its role in neuronal maturation. 12 Conversely, isoform II, transcribed from exons 1a and 1b (but not exon 1), is only expressed at very low levels in human fetal brain and testes. 46,47 It remains to be determined in what brain compartment(s) and in which cells this isoform is predominantly expressed.…”

“…CDKL5-signaling cascades have been proposed to be involved in synaptic plasticity and learning, affecting spines, dendritic branching, and actin cytoskeleton in the cytoplasm and neuronal activity-dependent gene expression in the nucleus. 12,13 To date, over 200 sequence variants or genomic deletions in CDKL5 have been reported in both female and male patients with X-linked dominant early-onset seizures manifesting before 5 months of age, severe intellectual disability with absent speech, and Rett-like features such as hand stereotypies and deceleration of head growth classified as Epileptic encephalopathy, early infantile, 2 (OMIM 300672) or Angelman syndrome-like (OMIM 105830). 7,[14][15][16][17][18] The phenotypic resemblance to Rett syndrome has been proposed to result from similar functions or interactions between CDKL5 and MECP2 in their molecular pathways during neurodevelopment (OMIM 30005); 4,13,[19][20][21][22][23] however, more studies are needed to verify this hypothesis.…”

Mutations in the C-terminus of CDKL5: proceed with caution

“…As truncations of the C-terminus seem to enhance autophosphorylation and kinase activity of CDKL5 and cause its nuclear accumulation and activity, it has been suggested that these sequence variations may be pathogenic. 11 On the basis of this line of evidence, a nonsense mutation p.Arg970X (c.2908C4T) was reported in 2009 in a girl, suggestive of 'atypical' Rett syndrome (severe intellectual disability, microcephaly, poor eye fixation, stereotypies, not able to walk, breathing irregularities, mild seizures at 17 months of age) but without regression, and was suggested to be pathogenic. 2 Although the p.Arg970X was not found in the mother of the proband, DNA analysis in her father and a further functional study were not performed.…”

An isoform of the severe encephalopathy-related CDKL5 gene, including a novel exon with extremely high sequence conservation, is specifically expressed in brain

“…At the protein level, the N-terminal domain is involved in the catalytic activity of the protein, whereas the C-terminal region may regulate its subcellular localization. 6,23 CDKL5 phosphorylation targets are currently largely unknown. So far, it has been shown that the protein, in vitro, may autophosphorylate, and it phosphorylates the product of the methyl-CpG-binding protein 2 (MECP2, OMIM 300005) gene, which is mutated in 490% of classic RTT patients, 24 suggesting a common signaling pathway between these two proteins; 6,9,25 and the N-terminal region of the DNA methyltransferase 1 in nuclei.…”