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.

Mutations in the X-linked cyclin-dependent kinase-like 5 (CDKL5) gene have been identified in patients with Rett syndrome, West syndrome, and X-linked infantile spasms sharing the common features of generally intractable early seizures and mental retardation. Disease-causing mutations are distributed in both the catalytic domain and in the large COOH terminus. In this report, we examine the functional consequences of some Rett mutations of CDKL5 together with some synthetically designed derivatives useful to underline the functional domains of the protein. The mutated CDKL5 derivatives have been subjected to in vitro kinase assays and analyzed for phosphorylation of the TEY (Thr-Glu-Tyr) motif within the activation loop, their subcellular localization, and the capacity of CDKL5 to interact with itself. Whereas wild-type CDKL5 autophosphorylates and mediates the phosphorylation of the methyl-CpG-binding protein 2 (MeCP2) in vitro, Rett-mutated proteins show both impaired and increased catalytic activity suggesting that a tight regulation of CDKL5 is required for correct brain functions. Furthermore, we show that CDKL5 can self-associate and mediate the phosphorylation of its own TEY (Thr-Glu-Tyr) motif. Eventually, we show that the COOH terminus regulates CDKL5 properties; in particular, it negatively influences the catalytic activity and is required for its proper sub-nuclear localization. We propose a model in which CDKL5 phosphorylation is required for its entrance into the nucleus whereas a portion of the COOH-terminal domain is responsible for a stable residency in this cellular compartment probably through protein-protein interactions.X-linked cyclin-dependent kinase-like 5 (CDKL5, 3 previously named STK9) was originally identified in a transcriptional mapping project focused on the human chromosome Xp22.3-p21.3, spanning a region critical for several diseases. Expression studies demonstrated that CDKL5 was transcribed in several tissues, including brain (1). However, the possible link between CDKL5 and human diseases was drawn only few years later when balanced translocating events disrupting the gene were identified in two female patients affected by severe infantile spasms and mental retardation (2). Retrospectively, a previous publication had identified a large deletion involving CDKL5 in a male patient with X-linked retinoschisis and seizure (3); it has recently been hypothesized that retinoschisis is due to deletion of the XLRS1 gene, whereas epilepsy is caused by truncation of at least the last exon of CDKL5 (2). The importance of CDKL5 in early onset seizures and severe mental retardation in females has been further reinforced by five recent reports linking mutations in CDKL5 to patients with Rett syndrome but only in those affected by a variant form characterized by seizure onset before 6 months of age (4 -8). Very recently the frequency of CDKL5 mutations in patients affected by infantile spasms or early onset epilepsy of unknown cause has been investigated. The identification of several novel lik...

Alzheimer disease (AD) is characterized by cerebral deposits of β‐amyloid (Aβ) peptides, which are surrounded by neuroinflammatory cells. Epidemiological studies have shown that prolonged use of non‐steroidal anti‐inflammatory drugs (NSAIDs) reduces the risk of developing AD. In addition, biological data indicate that certain NSAIDs specifically lower Aβ42 levels in cultures of peripheral cells independently of cyclooxygenase (COX) activity and reduce cerebral Aβ levels in AD transgenic mice. Whether other NSAIDs, including COX‐selective compounds, modulate Aβ levels in neuronal cells remains unexploited. Here, we investigated the effects of compounds from every chemical class of NSAIDs on Aβ40 and Aβ42 secretion using both Neuro‐2a cells and rat primary cortical neurons. Among non‐selective NSAIDs, flurbiprofen and sulindac sulfide concentration‐dependently reduced the secretion not only of Aβ42 but also of Aβ40. Surprisingly, both COX‐2 (celecoxib; sc‐125) or COX‐1 (sc‐560) selective compounds significantly increased Aβ42 secretion, and either did not alter (sc‐560; sc‐125) or reduced (celecoxib) Aβ40 levels. The levels of βAPP C‐terminal fragments and Notch cleavage were not altered by any of the NSAIDs, indicating that γ‐secretase activity was not overall changed by these drugs. The present findings show that only a few non‐selective NSAIDs possess Aβ‐lowering properties and therefore have a profile potentially relevant to their clinical use in AD.

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.

Although Rett syndrome (RTT) represents one of the most frequent forms of severe intellectual disability in females worldwide, we still have an inadequate knowledge of the many roles played by MeCP2 (whose mutations are responsible for most cases of RTT) and their relevance for RTT pathobiology. Several studies support a role of MeCP2 in the regulation of synaptic plasticity and homeostasis. At the molecular level, MeCP2 is described as a repressor capable of inhibiting gene transcription through chromatin compaction. Indeed, it interacts with several chromatin remodeling factors, such as HDAC-containing complexes and ATRX. Other studies have inferred that MeCP2 functions also as an activator; a role in regulating mRNA splicing and in modulating protein synthesis has also been proposed. Further, MeCP2 avidly binds both 5-methyl- and 5-hydroxymethyl-cytosine. Recent evidence suggests that it is the highly disorganized structure of MeCP2, together with its post-translational modifications (PTMs) that generate and regulate this functional versatility. Indeed, several reports have demonstrated that differential phosphorylation of MeCP2 is a key mechanism by which the methyl binding protein modulates its affinity for its partners, gene expression and cellular adaptations to stimuli and neuronal plasticity. As logic consequence, generation of phospho-defective Mecp2 knock-in mice has permitted associating alterations in neuronal morphology, circuit formation, and mouse behavioral phenotypes with specific phosphorylation events. MeCP2 undergoes various other PTMs, including acetylation, ubiquitination and sumoylation, whose functional roles remain largely unexplored. These results, together with the genome-wide distribution of MeCP2 and its capability to substitute histone H1, recall the complex regulation of histones and suggest the relevance of quickly gaining a deeper comprehension of MeCP2 PTMs, the respective writers and readers and the consequent functional outcomes.