The role of leiomodin in actin dynamics: a new road or a secret gate

Lmod2 is necessary for effective skeletal muscle contraction

Muscle contraction is a regulated process driven by the sliding of actin-thin filaments over myosin-thick filaments. Lmod2 is an actin filament length regulator and essential for life since human mutations and complete loss of Lmod2 in mice lead to dilated cardiomyopathy and death. To study the little-known role of Lmod2 in skeletal muscle, we created a mouse model with Lmod2 expressed exclusively in the heart but absent in skeletal muscle. Loss of Lmod2 in skeletal muscle results in decreased force production in fast- and slow-twitch muscles. Soleus muscle from rescued Lmod2 knockout mice have shorter thin filaments, increased Lmod3 levels, and present with a myosin fiber type switch from fast myosin heavy chain (MHC) IIA to the slower MHC I isoform. Since Lmod2 regulates thin-filament length in slow-twitch but not fast-twitch skeletal muscle and force deficits were observed in both muscle types, this work demonstrates that Lmod2 regulates skeletal muscle contraction, independent of its role in thin-filament length regulation.

A nemaline myopathy-linked mutation inhibits the actin-regulatory functions of tropomodulin and leiomodin

Actin is a highly expressed protein in eukaryotic cells and is essential for numerous cellular processes. In particular, efficient striated muscle contraction is dependent upon the precise regulation of actin-based thin filament structure and function. Alterations in the lengths of actin-thin filaments can lead to the development of myopathies. Leiomodins and tropomodulins are members of an actin-binding protein family that fine-tune thin filament lengths, and their dysfunction is implicated in muscle diseases. An Lmod3 mutation [G326R] was previously identified in patients with nemaline myopathy (NM), a severe skeletal muscle disorder; this residue is conserved among Lmod and Tmod isoforms and resides within their homologous leucine-rich repeat (LRR) domain. We mutated this glycine to arginine in Lmod and Tmod to determine the physiological function of this residue and domain. This G-to-R substitution disrupts Lmod and Tmod's LRR domain structure, altering their binding interface with actin and destroying their abilities to regulate thin filament lengths. Additionally, this mutation renders Lmod3 nonfunctional in vivo. We found that one single amino acid is essential for folding of Lmod and Tmod LRR domains, and thus is essential for the opposing actin-regulatory functions of Lmod (filament elongation) and Tmod (filament shortening), revealing a mechanism underlying the development of NM.

Postnatal skeletal muscle myogenesis governed by signal transduction networks: MAPKs and PI3K-Akt control multiple steps

Skeletal muscle myogenesis represents one of the most intensively and extensively examined systems of cell differentiation, tissue formation, and regeneration. Muscle regeneration provides an in vivo model system of postnatal myogenesis. It comprises multiple steps including muscle stem cell (or satellite cell) quiescence, activation, migration, myogenic determination, myoblast proliferation, myocyte differentiation, myofiber maturation, and hypertrophy. A variety of extracellular signaling and subsequent intracellular signal transduction pathways or networks govern the individual steps of postnatal myogenesis. Among them, MAPK pathways (the ERK, JNK, p38 MAPK, and ERK5 pathways) and PI3K-Akt signaling regulate multiple steps of myogenesis. Ca2+, cytokine, and Wnt signaling also participate in several myogenesis steps. These signaling pathways often control cell cycle regulatory proteins or the muscle-specific MyoD family and the MEF2 family of transcription factors. This article comprehensively reviews molecular mechanisms of the individual steps of postnatal skeletal muscle myogenesis by focusing on signal transduction pathways or networks. Nevertheless, no or only a partial signaling molecules or pathways have been identified in some responses during myogenesis. The elucidation of these unidentified signaling molecules and pathways leads to an extensive understanding of the molecular mechanisms of myogenesis.

Nuclear Magnetic Resonance-Guided Structural Analysis of Moderate-Affinity Protein Complexes with Intrinsically Disordered Polypeptides

Binding affinity of an individual binding site of an intrinsically disordered protein for its folded partner may be moderate. In such cases, a straightforward determination of the structure of the binding interface is difficult. We offer a hybrid protocol combining NMR chemical shift information, NMR spectral data on amino acid residue sequence substitution effects, residual dipolar coupling, and molecular dynamics simulation that allowed us to determine the structure of a complex between the intrinsically disordered tropomyosin-binding site of leiomodin and a coiled-coil peptide modeling the N-terminal fragment of tropomyosin. The protocol can be used for other moderate-affinity complexes composed of an intrinsically disordered peptide bound to a structured protein partner.

The Mechanisms of Thin Filament Assembly and Length Regulation in Muscles

The actin containing tropomyosin and troponin decorated thin filaments form one of the crucial components of the contractile apparatus in muscles. The thin filaments are organized into densely packed lattices interdigitated with myosin-based thick filaments. The crossbridge interactions between these myofilaments drive muscle contraction, and the degree of myofilament overlap is a key factor of contractile force determination. As such, the optimal length of the thin filaments is critical for efficient activity, therefore, this parameter is precisely controlled according to the workload of a given muscle. Thin filament length is thought to be regulated by two major, but only partially understood mechanisms: it is set by (i) factors that mediate the assembly of filaments from monomers and catalyze their elongation, and (ii) by factors that specify their length and uniformity. Mutations affecting these factors can alter the length of thin filaments, and in human cases, many of them are linked to debilitating diseases such as nemaline myopathy and dilated cardiomyopathy.

Actin-Associated Proteins and Small Molecules Targeting the Actin Cytoskeleton

Actin-associated proteins (AAPs) act on monomeric globular actin (G-actin) and polymerized filamentous actin (F-actin) to regulate their dynamics and architectures which ultimately control cell movement, shape change, division; organelle localization and trafficking. Actin-binding proteins (ABPs) are a subset of AAPs. Since actin was discovered as a myosin-activating protein (hence named actin) in 1942, the protein has also been found to be expressed in non-muscle cells, and numerous AAPs continue to be discovered. This review article lists all of the AAPs discovered so far while also allowing readers to sort the list based on the names, sizes, functions, related human diseases, and the dates of discovery. The list also contains links to the UniProt and Protein Atlas databases for accessing further, related details such as protein structures, associated proteins, subcellular localization, the expression levels in cells and tissues, mutations, and pathology. Because the actin cytoskeleton is involved in many pathological processes such as tumorigenesis, invasion, and developmental diseases, small molecules that target actin and AAPs which hold potential to treat these diseases are also listed.

PECAM1, COL4A2, PHACTR1, and LMOD1 Gene Polymorphisms in Patients with Unstable Angina

Coronary artery disease (CAD) is a syndrome resulting from myocardial ischaemia of heterogeneous pathomechanism. Environmental and genetic factors contribute to its development. Atherosclerotic plaques that significantly narrow the lumen of coronary arteries cause symptoms of myocardial ischaemia. Acute coronary incidents are most often associated with plaque rupture or erosion accompanied by local activation of the coagulation system with thrombus formation. Plaque formation and stability are influenced by endothelial function and vascular smooth muscle cell function. In this study, we investigated the association between polymorphisms in genes affecting endothelial and vascular smooth muscle cell (VSMC) function and the occurrence of unstable angina pectoris. The aim of this study was to evaluate the association between the PECAM1 (rs1867624), COL4A2 (rs4773144), PHACTR1 (rs9349379) and LMOD1 (rs2820315) gene polymorphisms and the risk of unstable angina. The study included 232 patients with unstable angina diagnosed on the basis of clinical symptoms and coronary angiography and 144 healthy subjects with no significant coronary lumen stenosis at coronary angiography. There were no statistically significant differences in the distribution of COL4A2 rs4773144 and PECAM1 rs1867624 gene polymorphisms between patients with unstable angina and control subjects. In patients with unstable angina, there was an increased frequency of PHACTR1 rs9349379 G allele carriers (GG and AG genotypes) (GG+AG vs. AA, OR 1.71; 95% CI 1.10-2.66, p = 0.017) and carriers of the LMOD1 rs2820315 T allele (TT and CT genotypes) (TT+CT vs. CC, OR 1.65; 95% CI 1.09-2.51, p = 0.019) compared to the control group. The association between these alleles and unstable angina was confirmed by multivariate logistic regression analysis, in which the number of G (PHACTR1 rs9349379) and T (LMOD1 rs2820315) alleles was an independent risk factor for unstable angina. The results suggest an association between PHACTR1 rs9349379 and LMOD1 rs2820315 polymorphisms and the risk of unstable angina.