What we know and would like to know about CDKL5 and its involvement in epileptic encephalopathy

Abstract

In the last few years, the X-linked serine/threonine kinase cyclin-dependent kinase-like 5 (CDKL5) has been associated with early-onset epileptic encephalopathies characterized by the manifestation of intractable epilepsy within the first weeks of life, severe developmental delay, profound hypotonia, and often the presence of some Rett-syndrome-like features. The association of CDKL5 with neurodevelopmental disorders and its high expression levels in the maturing brain underscore the importance of this kinase for proper brain development. However, our present knowledge of CDKL5 functions is still rather limited. The picture that emerges from the molecular and cellular studies suggests that CDKL5 functions are important for regulating both neuronal morphology through cytoplasmic signaling pathways and activity-dependent gene expression in the nuclear compartment. This paper surveys the current state of CDKL5 research with emphasis on the clinical symptoms associated with mutations in CDKL5, the different mechanisms regulating its functions, and the connected molecular pathways. Finally, based on the available data we speculate that CDKL5 might play a role in neuronal plasticity and we adduce and discuss some possible arguments supporting this hypothesis.

Figure 1

The genomic structure of CDKL5 and its splice variants. (a) The human CDKL5 gene with nontranslated sequences in grey and exons encoding the catalytic domain in blue. Exons encoding the common C-terminal region appear in white, whereas isoform-specific sequences are shown in red, green, and as hatched. (b) hCDKL5 protein isoforms differing in the C-terminal region. CDKL5₁₁₅ [22] contains the primate-specific exons 19–21. In CDKL5₁₀₇ [3], intron 18 is retained. The inclusion of exon 16b would generate CDKL5_115+ex.16b and/or CDKL5_107+ex.16b [21, 23]. hCDKL5_−ex19 is a hypothetical splice variant in which exon 19 is excluded generating an alternative C-terminus (light blue; personal communication Limprasert.) (c) The murine CDKL5 isoforms. mCDKL5₁₀₅ harbors a distinct C-terminal region encoded by a mouse-specific exon 19 (orange). As in humans, the retention of intron 18 generates the common CDKL5₁₀₇ isoform [3].

Figure 2

Pathogenic CDKL5 mutations. (a) Schematic representation of CDKL5₁₁₅ with the functional domains and signatures indicated. NLS: nuclear localization signal; NES: nuclear export signal. (b) All mutations in CDKL5 reported to date are indicated corresponding to their location within the gene. Mutations shown above the CDKL5 gene are deletion and frame shift mutations as well as splice variants indicated with cDNA nomenclature. Missense and nonsense mutations (fuchsia and black, resp.) are represented with amino acid nomenclature below the CDKL5 gene. *: recurrent mutations; °: uncertain pathogenicity. The indicated mutations have been referred by: t(X;6) [2]; c.-162_99del261 [25]; c.39delT [10]; c.64+2delT [12]; c.99+1G>T [12, 26]; c.99+5G>A [8, 27]; c.100-2°>G [9]; c.145+2T>C [12, 25, 28]; c.229delGAAG [12]; c.183delT [5]; c.163_del166delGAAA [18, 29]; c.275_276insAA [8]; c.283-3_290del [8]; c.del458A>G [9]; c.463+1G>A [9]; IVS6-1G>T [30]; IVS7-2A>G [31]; c.del678_691ins683_673 [30]; c.800_801delAT [12, 26]; t(X;7) [2]; c 838_847del10 [22]; c.865insA [12]; c.903_904dupGA [25]; c.964dupA [18]; IVS11-2°>G [30]; c.1079Tdel [10]; c.1311dupC [12, 26]; c.1784dup [8]; c.1892_1893dupTA [12, 26]; c.2014-2015insC [10]; c.2016delC [12]; c.2045_2046delAGins18 [12, 26]; IVS13-1G>A [5]; c.2066delC [18]; c.2105_2106del [8]; c.2323_2324delGA [12, 26]; c2343delG [22]; c.2362-2366delAAGAAA [30]; IVS16+1G>C/A [30, 31]; c.2376+5G [25]; c.2635_2636delCT [12, 26, 29]; G20R [32]; A40V [9, 12, 26, 33]; R59X [30, 34]; N71D [7]; I72N [31]; I72T [25]; Q118X [12, 26]; H127R [25]; V132G [7]; L142X [12]; C152F [4]; R158 [35]; R175S [4]; R178P [9, 19]; R178W [7, 9]; R178Q [10]; P180L [30]; S196L [32]; E203D [7]; E203X [36]; L220P [12, 26, 33]; L227R [9]; T288I [19]; C291Y [19]; L302F [10]; Q347X [7, 10]; S413X [10]; R550X [25, 28]; R559X [10, 20]; E570X [36]; V718M [12]; Q834X [12, 26, 37]; R952X [6]; R970X [38].

Figure 3

Cdkl5 expression patterning in the adult male mouse brain (C57BL/6J, postnatal day 56). Images are collected from the Allen Brain Atlas website (http://www.brain-map.org/; probe name RP_051219_02_C01; experiment ID 74000486 for sagittal samples, experiment ID 75042239 for coronal samples). Each Nissl-stained section is coupled by a quantitative in situ hybridization screening at different sectioning levels. The color bar in (a) might be used to follow transcription level intensity. (b), (c), (d), (e), (f), and (g) are representative of different brain levels, therefore, different brain areas; in particular: cg: cingulated cortex, st: striatum, pc: piriform cortex, hyp: hypothalamus, ec: entorhinal cortex, dg: dentate gyrus, ca1,2,3: hippocampal CA fields, vta: ventral tegmental area, gn: geniculate nuclei, sn: substantia nigra, lc: locus ceruleus, pc: Purkinje cells.

Figure 4

A model depicting the different functions of CDKL5 in the cytoplasmic and nuclear compartments. As described in the text, CDKL5 functions are occurring both in the cytoplasm and in the nucleus. A. In the cytoplasm, CDKL5 is involved in the regulation of actin cytoskeleton and dendritic arborization. This function is mediated by the interaction of CDKL5 with Rac1. Importantly, a link between BDNF, CDKL5 phosphorylation, and Rac1 activation has been suggested by Chen et al. [24]. B. In the cytoplasmic compartment, the levels of CDKL5 are regulated by degradation. Furthermore, we assume that several phosphorylation targets of CDKL5 remain to be identified. C. In the nucleus, CDKL5 has been proposed capable of interacting with and phosphorylating MeCP2 and DNMT1, thereby influencing gene expression and DNA methylation. Furthermore, the protein has been shown to colocalize with RNA speckles involved in RNA splicing. As for the cytosol, we assume that several other targets of CDKL5 remain to be discovered. In this model, we have neglected the capability of the nuclear CDKL5 to influence dendritic arborization [24] and the transcriptional regulation of CDKL5.