CSRP3

Cysteine and glycine-rich protein 3 is_associated_with::gene codes for the Muscle LIM Protein (MLP) or CSRP3, a small 194 amino acid is_associated_with::protein, which is specifically expressed in is_associated_with::skeletal muscles and is_associated_with::cardiac muscle. Since the identification of MLP, 20 years ago, a multitude of studies have focused on delineating its functional significance.

Gene
The CSRP3 gene was discovered in rat in 1994. In humans it was mapped to is_associated_with::chromosome 11p15.1, where it spans a 20kb genomic region, organized in 6 is_associated_with::exons. The full length transcript is 0.8kb, while a splice variant, originating from the is_associated_with::alternative splicing of exons 3 and 4, was recently identified and designated MLP-b.

Structure
MLP contains two is_associated_with::LIM domains (LIM1 and LIM2), each being surrounded by glycine-rich regions, and the two separated by more than 50 residues. LIM domains offer a remarkable ability for is_associated_with::protein-protein interactions. Furthermore, MLP carries a is_associated_with::nuclear localization signal at is_associated_with::amino acid positions 64-69 MLP can be acetylated/deacetylated at the position 69 lysine residue (K69), by is_associated_with::acetyltransferase (PCAF) and histone deacetylase 4 (is_associated_with::HDAC4), respectively. In is_associated_with::myocytes, MLP has the ability to is_associated_with::oligomerize, forming dimers, trimers and tetramers, an attribute that impacts its interactions, localization and function.

Protein interactions and localization
MLP has been identified to bind to an increasing list of proteins, exhibiting variable subcellular localization and diverse functional properties. In particular, MLP interacts with proteins at the: is_associated_with::M-line as well as is_associated_with::plasma membrane localization of MLP has also been observed, however, the protein associations mediating this subcellular distribution are currently unknown. These diverse localization patterns and binding partners of MLP suggest a multitude of roles relating both to the striated myocyte is_associated_with::cytoskeleton and the nucleus. The role of MLP in each of these two cellular compartments appears to be dynamic, with studies demonstrating nucleocytoplasmic shuttling, driven by its nuclear localization signal, over time and under different conditions.
 * 1) is_associated_with::Z-line, including is_associated_with::telethonin (T-cap), alpha-is_associated_with::actinin (is_associated_with::ACTN), is_associated_with::cofilin-2 (CFL2), is_associated_with::calcineurin, is_associated_with::HDAC4, MLP-b as well as to MLP itself;
 * 2) is_associated_with::costameres, where it binds to is_associated_with::zyxin, integrin linked kinase (is_associated_with::ILK) and beta1-is_associated_with::spectrin;
 * 3) is_associated_with::intercalated discs, where it associates with the is_associated_with::nebulin-related anchoring protein (is_associated_with::NRAP);
 * 4) nucleus, where it binds to the transcription factors is_associated_with::MyoD, is_associated_with::myogenin and is_associated_with::MRF4.

Functions
In the nucleus, MLP acts as a positive regulator of is_associated_with::myogenesis and promotes myogenic differentiation. Overexpression of MLP enhances myotube differentiation, an effect attributed to the direct association of MLP with muscle specific transcription factors such as MyoD, myogenin and MRF4 and consequently the transcriptional control of fundamental muscle-specific genes. In the cytoplasm, MLP is an important is_associated_with::scaffold protein, implicated in various cytoskeletal macromolecular complexes, at the sarcomeric Z-line, the costameres, and the is_associated_with::microfilaments. At the Z-line, MLP interacts with different Z-line components        and acts as a scaffold protein promoting the assembly of macromolecular complexes along sarcomeres and is_associated_with::actin-based cytoskeleton    Moreover, since the Z-line acts as a stretch sensor,    MLP is believed to be involved in mechano-signaling processes. Indeed, is_associated_with::cardiomyocytes from MLP transgenic or is_associated_with::knock-out mouse exhibit defective intrinsic stretch responses, due to selective loss of passive stretch sensing. At the is_associated_with::costameres, another region implicated in force transmission, MLP is thought to be contributing in mechanosensing through its interactions with β1 is_associated_with::spectrin and is_associated_with::zyxin. However, the precise role of MLP at the costameres has not been extensively investigated yet.

At the microfilaments, MLP is implicated in is_associated_with::actin remodeling (or actin dynamics) through its interaction with is_associated_with::cofilin-2 (CFL2). Binding of MLP to CFL2 enhances the CFL2-dependent F-actin depolymerization, with a recent study demonstrating that MLP can act directly on actin cytoskeleton dynamics through direct binding that stabilizes and crosslinks is_associated_with::actin filaments into bundles.

Additionally, MLP is indirectly related to calcium homeostasis and energy metabolism. Specifically, acetylation of MLP increases the calcium sensitivity of myofilaments and regulates is_associated_with::contractility, while the absence of MLP causes alterations in is_associated_with::calcium signaling (intracellular calcium handling) with defects in is_associated_with::excitation-contraction coupling. Furthermore, lack of MLP leads to local loss of is_associated_with::mitochondria and energy deficiency.

Clinical significance
MLP is directly associated with striated muscle diseases. is_associated_with::Mutations in the CSRP3 gene have been detected in patients with is_associated_with::dilated cardiomyopathy (DCM) [e.g. G72R and K69R], and is_associated_with::hypertrophic cardiomyopathy (HCM) [e.g. L44P, S46R, S54R/E55G, C58G, R64C, Y66C, Q91L, K42/fs165], while the most frequent MLP mutation, W4R, has been found in both of these patient populations. Biochemical and functional studies have been performed for some of these mutant proteins, and reveal aberrant localization and interaction patterns, leading to impaired molecular and cellular functions. For example, the W4R mutation abolishes the MLP/T-cap interaction, leading to mislocalization of T-cap, Z-line instability and severe sarcomeric structural defects. The C58G mutation causes reduced protein stability due to enhanced is_associated_with::ubiquitin-dependent is_associated_with::proteasome degradation while the K69L mutation, which is within the predicted nuclear localization signal of MLP, abolishes the MLP/α-actinin interaction and causes altered subcellular distribution of the mutant protein, showing predominant perinuclear localization. Alterations in the protein expression levels of MLP, its is_associated_with::oligomerization or splicing have also been described in human cardiac and skeletal muscle diseases: both MLP protein levels and oligomerization are down-regulated in human is_associated_with::heart failure, while MLP levels are significantly changed in different skeletal is_associated_with::myopathies, including is_associated_with::facioscapulohumeral muscular dystrophy, is_associated_with::nemaline myopathy and limb girdle muscular dystrophy type 2B. Moreover, significant changes in MLP-b protein expression levels, as well as deregulation of the MLP:MLP-b ratio have been detected in is_associated_with::limb girdle muscular dystrophy type 2A, is_associated_with::Duchenne muscular dystrophy and is_associated_with::dermatomyositis patients.

Animal models
Animal models are providing valuable insight on MLP’s functional significance in striated muscle pathophysiology. Ablation of Mlp (MLP-/-) in mice affects all striated muscles, although the cardiac phenotype is more severe, leading to alterations in cardiac pressure and volume, aberrant is_associated_with::contractility, development of dilated cardiomyopathy with hypertrophy and progressive heart failure. At the histological level there is dramatic disruption of the cardiomyocyte is_associated_with::cytoarchitecture at multiple levels, and pronounced is_associated_with::fibrosis. Other cellular changes included alterations in intracellular calcium handling, local loss of mitochondria and energy deficiency. Crossing MLP-/- mice with is_associated_with::phospholamban (PLN) -/-, or is_associated_with::β2-adrenergic receptor (β2-AR) -/-, or is_associated_with::angiotensin II type 1a receptor (AT1a) -/-, or β-adrenergic receptor kinase 1 inhibitor (is_associated_with::bARK1) -/- mice, as well as overexpressing calcineurin rescued their cardiac function, through a series of only partly understood molecular mechanisms. Conversely crossing MLP-/- mice with is_associated_with::β1-adrenergic receptor (β1-AR) -/- mice was lethal, while crossing MLP-/- mice with calcineurin -/- mice, enhanced fibrosis and is_associated_with::cardiomyopathy. A is_associated_with::gene knockin mouse model harboring the human MLP-W4R mutation developed HCM and heart failure, while ultrastructural analysis of its cardiac tissue revealed is_associated_with::myocardial disarray and significant fibrosis, increased nuclear localization of MLP concomitantly with reduced sarcomeric Z-line distribution. Alterations in MLP nucleocytoplasmic shuttling, which are possibly modulated by changes in its oligomerization status, have also been implicated in hypertrophy and heart failure, independently of mutations. Studies in is_associated_with::Drosophila revealed that genetic ablation of Mlp84B, the Drosophila homolog of MLP, was associated with pupal lethality and impaired muscle function. Mechanical studies of Mlp84B-null flight muscles indicate that loss of Mlp84B results in decreased muscle stiffness and power generation. Cardiac-specific ablation of Mlp84B caused decreased lifespan, impaired diastolic function and disturbances in cardiac rhythm. Overall, these animal models have provided critical evidence on the functional significance of MLP in striated muscle physiology and pathophysiology.