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 inCDKL5, 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.

In the last years, the X-linked cyclin-dependent kinase-like 5 (CDKL5) gene has been associated with epileptic encephalopathies characterized by the early onset of intractable epilepsy, severe developmental delay, autistic features, and often the development of Rett syndrome-like features. Still, the role of CDKL5 in neuronal functions is not fully understood. By way of a yeast two hybrid screening we identified the interaction of CDKL5 with shootin1, a brain specific protein acting as a determinant of axon formation during neuronal polarization. We found evidence that CDKL5 is involved, at least in part, in regulating neuronal polarization through its interaction with shootin1. Indeed, the two proteins interact in vivo and both are localized in the distal tip of outgrowing axons. By using primary hippocampal neurons as model system we find that adequate CDKL5 levels are required for axon specification. In fact, a significant number of neurons overexpressing CDKL5 is characterized by supernumerary axons, while the silencing of CDKL5 disrupts neuronal polarization. Interestingly, shootin1 phosphorylation is reduced in neurons silenced for CDKL5 suggesting that the kinase affects, directly or indirectly, the post-translational modification of shootin1. Finally, we find that the capacity of CDKL5 to generate surplus axons is attenuated in neurons with reduced shootin1 levels, in agreement with the notion that two proteins act in a common pathway. Altogether, these results point to a role of CDKL5 in the early steps of neuronal differentiation that can be explained, at least in part, by its association with shootin1.

Mutations in the X-linked cyclin-dependent kinase-like 5 (CDKL5) gene have been associated with several forms of neurodevelopmental disorders, including atypical Rett syndrome, autism spectrum disorders, and early infantile epileptic encephalopathy. Accordingly, loss of CDKL5 in mice results in autistic-like features and impaired neuronal communication. Although the biological functions of CDKL5 remain largely unknown, recent pieces of evidence suggest that CDKL5 is involved in neuronal plasticity. Herein, we show that, at all stages of development, neuronal depolarization induces a rapid increase in CDKL5 levels, mostly mediated by extrasomatic synthesis. In young neurons, this induction is prolonged whereas, in more mature neurons, NMDA receptor stimulation induces a PP1-dependent dephosphorylation of CDKL5 that is mandatory for its proteasome dependent degradation. As a corollary, neuronal activity leads to a prolonged induction of CDKL5 levels in immature neurons but to a short-lasting increase of the kinase in mature neurons. Recent results demonstrate that many genes associated with autism spectrum disorders are crucial components of the activity-dependent signaling networks regulating the composition, shape and strength of the synapse. Thus, we speculate that CDKL5 deficiency disrupts activity-dependent signaling and the consequent synapse development, maturation and refinement.

Cyclin-dependent–like kinase 5 (CDKL5) gene mutations lead to an X-linked disorder that is characterized by infantile epileptic encephalopathy, developmental delay, and hypotonia. However, we found that a substantial percentage of these patients also report a previously unrecognized anamnestic deficiency in pain perception. Consistent with a role in nociception, we found that CDKL5 is expressed selectively in nociceptive dorsal root ganglia (DRG) neurons in mice and in induced pluripotent stem cell (iPS)–derived human nociceptors. CDKL5-deficient mice display defective epidermal innervation, and conditional deletion of CDKL5 in DRG sensory neurons impairs nociception, phenocopying CDKL5 deficiency disorder in patients. Mechanistically, CDKL5 interacts with calcium/calmodulin-dependent protein kinase II α (CaMKIIα) to control outgrowth and transient receptor potential cation channel subfamily V member 1 (TRPV1)–dependent signaling, which are disrupted in both CDKL5 mutant murine DRG and human iPS–derived nociceptors. Together, these findings unveil a previously unrecognized role for CDKL5 in nociception, proposing an original regulatory mechanism for pain perception with implications for future therapeutics in CDKL5 deficiency disorder.

Introduction: The nuclear mitogen- and stress-activated kinases (MSKs) play a critical role in the development and persistence of pain after tissue injury. Objectives: Here, we ascertained the MSK isoform, the cells and mechanisms, which mediate MSKs' pronociceptive function. Methods: Nocifensive behaviour evoked by subcutaneous formalin injection into the paw was quantified in wild type (WT), MSK1 and MSK2 global knock out (MSK1^−/− and MSK2^−/−) mice, and a month after injecting adeno-associated viral vector carrying short-hairpin (sh) RNA directed towards the MSK1-encoding gene Rps6ka5 mRNA or scrambled shRNA into the sciatic nerve of WT mice. Rps6ka5 expression in nociceptors was ascertained by analysing publicly available single cell and single nucleus RNA sequencing datasets on primary sensory neurons and reverse transcription polymerase chain reaction on dorsal root ganglia (DRG). Mitogen- and stress-activated kinase 1 expression was verified by immunofluorescent staining on DRG sections. Results: MSK1^−/− but not MSK2^−/− mice exhibited significantly attenuated evoked nocifensive behaviour specifically in the second but not the first phase of the formalin test. Downregulating Rps6ka5 in nociceptors by the viral vector tool attenuated formalin-induced pain behaviour to the same extent as observed in MSK1^−/− animals. Rps6ka5 expression was found in DRG and various transcriptionally defined groups of nociceptive primary sensory neurons (nociceptors). Immunofluorescence confirmed the presence of MSK1 predominantly in peptidergic nociceptors. Conclusion: MSK1 constitutes the principal MSK isoform, which is critically important for regulating cellular components that enable the transient activation of a specific subpopulation of nociceptors by formalin.