Actin-microtubule cytoskeletal interplay mediated by MRTF-A/SRF signaling promotes dilated cardiomyopathy caused by LMNA mutations.

Mutations in the lamin A/C gene (LMNA) cause dilated cardiomyopathy associated with increased activity of ERK1/2 in the heart. We recently showed that ERK1/2 phosphorylates cofilin-1 on threonine 25 (phospho(T25)-cofilin-1) that in turn disassembles the actin cytoskeleton. Here, we show that in muscle cells carrying a cardiomyopathy-causing LMNA mutation, phospho(T25)-cofilin-1 binds to myocardin-related transcription factor A (MRTF-A) in the cytoplasm, thus preventing the stimulation of serum response factor (SRF) in the nucleus. Inhibiting the MRTF-A/SRF axis leads to decreased α-tubulin acetylation by reducing the expression of ATAT1 gene encoding α-tubulin acetyltransferase 1. Hence, tubulin acetylation is decreased in cardiomyocytes derived from male patients with LMNA mutations and in heart and isolated cardiomyocytes from Lmnap.H222P/H222P male mice. In Atat1 knockout mice, deficient for acetylated α-tubulin, we observe left ventricular dilation and mislocalization of Connexin 43 (Cx43) in heart. Increasing α-tubulin acetylation levels in Lmnap.H222P/H222P mice with tubastatin A treatment restores the proper localization of Cx43 and improves cardiac function. In summary, we show for the first time an actin-microtubule cytoskeletal interplay mediated by cofilin-1 and MRTF-A/SRF, promoting the dilated cardiomyopathy caused by LMNA mutations. Our findings suggest that modulating α-tubulin acetylation levels is a feasible strategy for improving cardiac function.

The non-muscle ADF/cofilin-1 controls sarcomeric actin filament integrity and force production in striated muscle laminopathies.

Cofilins are important for the regulation of the actin cytoskeleton, sarcomere organization, and force production. The role of cofilin-1, the non-muscle-specific isoform, in muscle function remains unclear. Mutations in LMNA encoding A-type lamins, intermediate filament proteins of the nuclear envelope, cause autosomal Emery-Dreifuss muscular dystrophy (EDMD). Here, we report increased cofilin-1 expression in LMNA mutant muscle cells caused by the inability of proteasome degradation, suggesting a protective role by ERK1/2. It is known that phosphorylated ERK1/2 directly binds to and catalyzes phosphorylation of the actin-depolymerizing factor cofilin-1 on Thr25. In vivo ectopic expression of cofilin-1, as well as its phosphorylated form on Thr25, impairs sarcomere structure and force generation. These findings present a mechanism that provides insight into the molecular pathogenesis of muscular dystrophies caused by LMNA mutations.

Microtubule cytoskeleton regulates Connexin 43 localization and cardiac conduction in cardiomyopathy caused by mutation in A-type lamins gene.

Mutations in the lamin A/C gene (LMNA) cause an autosomal dominant inherited form of dilated cardiomyopathy associated with cardiac conduction disease (hereafter referred to as LMNA cardiomyopathy). Compared with other forms of dilated cardiomyopathy, mutations in LMNA are responsible for a more aggressive clinical course owing to a high rate of malignant ventricular arrhythmias. Gap junctions are intercellular channels that allow direct communication between neighboring cells, which are involved in electrical impulse propagation and coordinated contraction of the heart. For gap junctions to properly control electrical synchronization in the heart, connexin-based hemichannels must be correctly targeted to intercalated discs, Cx43 being the major connexin in the working myocytes. We here showed an altered distribution of Cx43 in a mouse model of LMNA cardiomyopathy. However, little is known on the molecular mechanisms of Cx43 remodeling in pathological context. We now show that microtubule cytoskeleton alteration and decreased acetylation of α-tubulin lead to remodeling of Cx43 in LMNA cardiomyopathy, which alters the correct communication between cardiomyocytes, ultimately leading to electrical conduction disturbances. Preventing or reversing this process could offer a strategy to repair damaged heart. Stabilization of microtubule cytoskeleton using Paclitaxel improved intraventricular conduction defects. These results indicate that microtubule cytoskeleton contributes to the pathogenesis of LMNA cardiomyopathy and that drugs stabilizing the microtubule may be beneficial for patients.

Cofilin-1 phosphorylation catalyzed by ERK1/2 alters cardiac actin dynamics in dilated cardiomyopathy caused by lamin A/C gene mutation.

Hyper-activation of extracellular signal-regulated kinase (ERK) 1/2 contributes to heart dysfunction in cardiomyopathy caused by mutations in the lamin A/C gene (LMNA cardiomyopathy). The mechanism of how this affects cardiac function is unknown. We show that active phosphorylated ERK1/2 directly binds to and catalyzes the phosphorylation of the actin depolymerizing factor cofilin-1 on Thr25. Cofilin-1 becomes active and disassembles actin filaments in a large array of cellular and animal models of LMNA cardiomyopathy. In vivo expression of cofilin-1, phosphorylated on Thr25 by endogenous ERK1/2 signaling, leads to alterations in left ventricular function and cardiac actin. These results demonstrate a novel role for cofilin-1 on actin dynamics in cardiac muscle and provide a rationale on how increased ERK1/2 signaling leads to LMNA cardiomyopathy.

Decreased WNT/ß-catenin signalling contributes to the pathogenesis of dilated cardiomyopathy caused by mutations in the lamin a/C gene.

Cardiomyopathy caused by lamin A/C gene (LMNA) mutations (hereafter referred as LMNA cardiomyopathy) is characterized by cardiac conduction abnormalities and left ventricular systolic dysfunction predisposing to heart failure. Previous cardiac transcriptional profiling of LmnaH222P/H222P mouse, a small animal model of LMNA cardiomyopathy, suggested decreased WNT/ß-catenin signalling. We confirmed decreased WNT/ß-catenin signalling in the hearts of these mice by demonstrating decreased ß-catenin and WNT proteins. This was correlated with increased expression of soluble Frizzled-related proteins that modulate the WNT/ß-catenin signalling pathway. Hearts of LmnaH222P/H222P mice also demonstrated lowered expression of the gap junction connexin 43. Activation of WNT/ß-catenin activity with 6-bromoindirubin-3'-oxime improved cardiac contractility and ameliorated intraventricular conduction defects in LmnaH222P/H222P mice, which was associated with increased expression of myocardial connexin 43. These results indicate that decreased WNT/ß-catenin contributes to the pathophysiology of LMNA cardiomyopathy and that drugs activating ß-catenin may be beneficial in affected individuals.