LIMK2-1, a new isoform of human LIMK2, regulates actin cytoskeleton remodeling via a different signaling pathway than that of its two homologs, LIMK2a and LIMK2b.

LIMK1 and LIMK2 (LIMKs, LIM kinases) are kinases that play a crucial role in cytoskeleton dynamics by independently regulating both actin filament and microtubule remodeling. LIMK1 and, more recently, LIMK2 have been shown to be involved in cancer development and metastasis, resistance of cancer cells to microtubule-targeted treatments, neurological diseases, and viral infection. LIMKs have thus recently emerged as new therapeutic targets. Databanks describe three isoforms of human LIMK2: LIMK2a, LIMK2b, and LIMK2-1. Evidence suggests that they may not have completely overlapping functions. We biochemically characterized the three isoforms to better delineate their potential roles, focusing on LIMK2-1, which has only been described at the mRNA level in a single study. LIMK2-1 has a protein phosphatase 1 (PP1) inhibitory domain at its C-terminus which its two counterparts do not. We showed that the LIMK2-1 protein is indeed synthesized. LIMK2-1 does not phosphorylate cofilin, the canonical substrate of LIMKs, although it has kinase activity and promotes actin stress fiber formation. Instead, it interacts with PP1 and partially inhibits its activity towards cofilin. Our data suggest that LIMK2-1 regulates actin cytoskeleton dynamics by preventing PP1-mediated cofilin dephosphorylation, rather than by directly phosphorylating cofilin as its two counterparts, LIMK2a and LIMK2b. This specificity may allow for tight regulation of the phospho-cofilin pool, determining the fate of the cell.

Epileptic and nonepileptic features in patients with early onset epileptic encephalopathy and STXBP1 mutations.

PURPOSE: STXBP1 (MUNC18-1) mutations have been associated with various types of epilepsies, mostly beginning early in life. To refine the phenotype associated with STXBP1 aberrations in early onset epileptic syndromes, we studied this gene in a cohort of patients with early onset epileptic encephalopathy. METHODS: STXBP1 was screened in a multicenter cohort of 52 patients with early onset epilepsy (first seizure observed before the age of 3 months), no cortical malformation on brain magnetic resonance imaging (MRI), and negative metabolic screening. Three groups of patients could be distinguished in this cohort: (1) Ohtahara syndromes (n = 38); (2) early myoclonic encephalopathies (n = 7); and (3) early onset epileptic encephalopathies that did not match any familiar syndrome (n = 7). None of the patients displayed any cortical malformation on brain MRI and all were screened through multiple video-electroencephalography (EEG) recordings for a time period spanning from birth to their sixth postnatal month. Subsequently, patients had standard EEG or video-EEG recordings. KEY FINDINGS: We found five novel STXBP1 mutations in patients for whom video-EEG recordings could be sampled from the beginning of the disease. All patients with a mutation displayed Ohtahara syndrome, since most early seizures could be classified as epileptic spasms and since the silent EEG periods were on average shorter than bursts. However, each patient in addition displayed a particular clinical and EEG feature: In two patients, early seizures were clonic, with very early EEG studies exhibiting relatively low amplitude bursts of activity before progressing into a typical suppression-burst pattern, whereas the three other patients displayed epileptic spasms associated with typical suppression-burst patterns starting from the early recordings. Epilepsy dramatically improved after 6 months and finally disappeared before the end of the first year of life for four patients; the remaining one patient had few seizures until 18 months of age. In parallel, EEG paroxysmal abnormalities disappeared in three patients and decreased in two, giving place to continuous activity with fast rhythms. Each patient displayed frequent nonepileptic movement disorders that could easily be mistaken for epileptic seizures. These movements could be observed as early as the neonatal period and, unlike seizures, persisted during all the follow-up period. SIGNIFICANCE: We confirm that STXBP1 is a major gene to screen in cases of Ohtahara syndrome, since it is mutated in >10% of the Ohtahara patients within our cohort. This gene should particularly be tested in the case of a surprising evolution of the patient condition if epileptic seizures and EEG paroxysmal activity disappear and are replaced by fast rhythms after the end of the first postnatal year.

LIMK2-1 is a Hominidae-Specific Isoform of LIMK2 Expressed in Central Nervous System and Associated with Intellectual Disability.

LIMK2 is involved in neuronal functions by regulating actin dynamics. Different isoforms of LIMK2 are described in databanks. LIMK2a and LIMK2b are the most characterized. A few pieces of evidence suggest that LIMK2 isoforms might not have overlapping functions. In this study, we focused our attention on a less studied human LIMK2 isoform, LIMK2-1. Compared to the other LIMK2 isoforms, LIMK2-1 contains a supplementary C-terminal phosphatase 1 inhibitory domain (PP1i). We found out that this isoform was hominidae-specific and showed that it was expressed in human fetal brain and faintly in adult brain. Its coding sequence was sequenced in 173 patients with sporadic non-syndromic intellectual disability (ID), and we observed an association of a rare missense variant in the PP1i domain (rs151191437, p.S668P) with ID. Our results also suggest an implication of LIMK2-1 in neurite outgrowth and neurons arborization which appears to be affected by the p.S668P variation. Therefore our results suggest that LIMK2-1 plays a role in the developing brain, and that a rare variation of this isoform is a susceptibility factor in ID.