Ankyrin repeat domain-containing protein 1, or Cardiac ankyrin repeat protein is a protein that in humans is encoded by the ANKRD1 gene also known as CARP. [4] [5] [6] CARP is highly expressed in cardiac and skeletal muscle, and is a transcription factor involved in development and under conditions of stress. CARP has been implicated in several diseases, including dilated cardiomyopathy, hypertrophic cardiomyopathy, and several skeletal muscle myopathies.
Human cardiac ankyrin repeat protein is a 36.2kDa protein composed of 319 amino acids., [7] though in cardiomyocytes, CARP can exist as multiple alternatively spliced forms. [8] CARP contains five tandem ankyrin repeats. Studies have shown that CARP can homodimerize. [9] Studies have also shown that CARP is N-terminally, post-translationally cleaved by calpain-3 in skeletal muscle, suggesting alternate bioactive forms of CARP exist. [10] CARP has been localized to nuclei and Z-discs in animal and human muscle cells, and at intercalated discs in human cardiac muscle cells. [11]
CARP was originally identified as a YB-1-associating, cardiac-restricted transcription co-repressor in the homeobox NKX2-5 pathway that is involved in cardiac ventricular chamber specification, maturation and morphogenesis, [12] [13] [14] and whose mRNA levels are exquisitely sensitive to Doxorubicin, mediated through a hydrogen peroxide/ERK/p38MAP kinase-dependent [15] [16] as well as M-CAT cis-element-dependent [17] mechanism. Subsequent studies showed that CARP expression in cardiomyocytes is regulated by alpha-adrenergic signaling, in part via the transcription factor GATA4. [18] [19] An additional study showed that beta-adrenergic signaling via protein kinase A and CaM kinase induces the expression of CARP, and that CARP may have a negative effect on contractile function. [20] CARP has also been identified as a transcriptional co-activator of tumor suppressor protein p53 for stimulating gene expression in muscle; p53 was found to be an upstream effector of CARP via upregulation of the proximal ANKRD1 promoter. [21] CARP has a relatively short half-life being longer in cardiomyocytes than endothelial cells; and CARP is degraded by the 26S proteasome via a PEST degron. [22] [23]
In animal models of disease and injury, CARP has been characterized to be a stress-inducible myofibrillar protein. CARP has been shown to play a role in skeletal muscle structure [24] remodeling, [25] and repair, being expressed in skeletal muscle near myotendinous junctions, [26] and in vascular smooth muscle cells, as a downstream target of TGF-beta/Smad sigmaling in response to balloon injury [27] and atherosclerotic plaques. [28] Further studies have identified a role for CARP in initiation and regulation of arteriogenesis. [29] [30] [31] Decreased expression of CARP in cardiac cells within the ischemic region was detected in a rat model of ischemic injury, and was thought to be linked to the induction of GADD153, an apoptosis-related gene. [32] In cardiomyocytes treated with doxorubicin, a model of anthracycline-induced cardiomyopathy, CARP mRNA and protein levels were depleted, myofilament gene transcription was attenuated and sarcomeres showed significant disarray. [33]
In a transgenic mouse model of cardiac-specific overexpression of CARP, mice exhibited normal physiology at baseline, but were protected against pathological cardiac hypertrophy induced via pressure-overload or isoproterenol, which could be attributed to the downregulation of the ERK1/2, MEK and TGFbeta-1 pathways. [34] Another study demonstrated that adenoviral overexpression of CARP in cardiomyocytes enhances cardiac hypertrophy induced by Angiotensin II or pressure-overload [35] and promotes cardiomyocyte apoptosis via p53 activation and mitochondrial dysfunction. [36] However, transgenic knockout models of either CARP alone or CARP in combination with the other muscle ankyrin repeat proteins (MARPs), ANKRD2 and ANKRD23 demonstrated a lack of cardiac phenotype; mice displayed normal cardiac function at baseline and in response to pressure overload-induced cardiac hypertrophy, suggesting that these proteins are not essential. [37]
Interactions between CARP and the sarcomeric proteins myopalladin and titin suggest that it may also be involved in the myofibrillar stretch-sensor system. Passive stretch in fetal cardiomyocytes induced differential CARP distribution at nuclei and I-band titin N2A regions. [38] In a mouse model of muscular dystrophy with myositis (mdm) caused by a small deletion in titin, CARP mRNA expression was shown to be 30-fold elevated in skeletal muscle tissue. [39]
A wide spectrum of clinical features have been associated with ANKRD1/CARP. Mutations in ANKRD1 have been associated with dilated cardiomyopathy, two of which result in altered binding with TLN1 and FHL2. [40] [41] Mutations in ANKRD1 have also been associated with hypertrophic cardiomyopathy, and have shown to increase binding of CARP to Titin and MYPN. [42] Examination of the functional effects of CARP hypertrophic cardiomyopathy mutations in engineered heart tissue demonstrated that Thr123Met was a gain-of-function mutation exhibiting augmented contractile properties; whereas Pro52Ala and Ile280Val were unstable and failed to incorporate into sarcomeres, an effect that was remedied upon proteasome inhibition via epoxomicin. [43]
A missense mutation in ANKRD1 was shown to be associated with the congenital heart defect, Anomalous pulmonary venous connection. [44] CARP has been found as a sensitive and specific biomarker for the differential diagnosis of rhabdomyosarcoma. [45] ANKRD1 mRNA levels correlate with patient platinum sensitivity, thus ANKRD1 associates with platinum-based chemotherapy treatment outcome in ovarian adenocarcinoma patients. [46]
CARP and mRNA expression has been shown to be upregulated in left ventricles of heart failure patients. [47] [48] [49] [50] Studies in patients with amyotrophic lateral sclerosis, [51] spinal muscular atrophy, and congenital myopathy, [52] also found altered expression of CARP in skeletal muscle fibers. Another study in congenital muscular dystrophy and Duchenne muscular dystrophy patients showed elevated expression of CARP. [53] CARP expression is also elevated in patients with lupus nephritis, and associates with proteinuria severity, suggesting that it may have biomarker potential. [54]
ANKRD1 has been shown to interact with:
Titin is a protein that in humans is encoded by the TTN gene. Titin is a giant protein, greater than 1 µm in length, that functions as a molecular spring that is responsible for the passive elasticity of muscle. It comprises 244 individually folded protein domains connected by unstructured peptide sequences. These domains unfold when the protein is stretched and refold when the tension is removed.
The ankyrin repeat is a 33-residue motif in proteins consisting of two alpha helices separated by loops, first discovered in signaling proteins in yeast Cdc10 and Drosophila Notch. Domains consisting of ankyrin tandem repeats mediate protein–protein interactions and are among the most common structural motifs in known proteins. They appear in bacterial, archaeal, and eukaryotic proteins, but are far more common in eukaryotes. Ankyrin repeat proteins, though absent in most viruses, are common among poxviruses. Most proteins that contain the motif have four to six repeats, although its namesake ankyrin contains 24, and the largest known number of repeats is 34, predicted in a protein expressed by Giardia lamblia.
Myomesin is a protein family found in the M-line of the sarcomere structure. Myomesin has various forms throughout the body in striated muscles with specialized functions. This includes both slow and fast muscle fibers. Myomesin are made of 13 domains including a unique N-terminal followed by two immunoglobulin-like (Ig) domains, five fibronectin type III (Fn) domains, five more Ig domains. These domains all promote binding which indicates that myomesin is regulated through binding.
Nebulette is a cardiac-specific isoform belonging to the nebulin family of proteins. It is encoded by the NEBL gene. This family is composed of 5 members: nebulette, nebulin, N-RAP, LASP-1 and LASP-2. Nebulette localizes to Z-discs of cardiac muscle and appears to regulate the length of actin thin filaments.
Alpha II-spectrin, also known as Spectrin alpha chain, brain is a protein that in humans is encoded by the SPTAN1 gene. Alpha II-spectrin is expressed in a variety of tissues, and is highly expressed in cardiac muscle at Z-disc structures, costameres and at the sarcolemma membrane. Mutations in alpha II-spectrin have been associated with early infantile epileptic encephalopathy-5, and alpha II-spectrin may be a valuable biomarker for Guillain–Barré syndrome and infantile congenital heart disease.
Alpha-actinin-2 is a protein which in humans is encoded by the ACTN2 gene. This gene encodes an alpha-actinin isoform that is expressed in both skeletal and cardiac muscles and functions to anchor myofibrillar actin thin filaments and titin to Z-discs.
The myosin-binding protein C, cardiac-type is a protein that in humans is encoded by the MYBPC3 gene. This isoform is expressed exclusively in heart muscle during human and mouse development, and is distinct from those expressed in slow skeletal muscle (MYBPC1) and fast skeletal muscle (MYBPC2).
Protein S100-A1, also known as S100 calcium-binding protein A1 is a protein which in humans is encoded by the S100A1 gene. S100A1 is highly expressed in cardiac and skeletal muscle, and localizes to Z-discs and sarcoplasmic reticulum. S100A1 has shown promise as an effective candidate for gene therapy to treat post-myocardially infarcted cardiac tissue.
Telethonin, also known as Tcap, is a protein that in humans is encoded by the TCAP gene. Telethonin is expressed in cardiac and skeletal muscle at Z-discs and functions to regulate sarcomere assembly, T-tubule function and apoptosis. Telethonin has been implicated in several diseases, including limb-girdle muscular dystrophy, hypertrophic cardiomyopathy, dilated cardiomyopathy and idiopathic cardiomyopathy.
Plakophilin-2 is a protein that in humans is encoded by the PKP2 gene. Plakophilin 2 is expressed in skin and cardiac muscle, where it functions to link cadherins to intermediate filaments in the cytoskeleton. In cardiac muscle, plakophilin-2 is found in desmosome structures located within intercalated discs. Mutations in PKP2 have been shown to be causal in arrhythmogenic right ventricular cardiomyopathy.
Obscurin is a protein that in humans is encoded by the OBSCN gene. Obscurin belongs to the family of giant sarcomeric signaling proteins that includes titin and nebulin. Obscurin is expressed in cardiac and skeletal muscle, and plays a role in the organization of myofibrils during sarcomere assembly. A mutation in the OBSCN gene has been associated with hypertrophic cardiomyopathy and altered obscurin protein properties have been associated with other muscle diseases.
Ankyrin Repeat, PEST sequence and Proline-rich region (ARPP), also known as Ankyrin repeat domain-containing protein 2 is a protein that in humans is encoded by the ANKRD2 gene. ARPP is a member of the muscle ankyrin repeat proteins (MARP), which also includes CARP and DARP, and is highly expressed in cardiac and skeletal muscle and in other tissues. Expression of ARPP has been shown to be altered in patients with dilated cardiomyopathy and amyotrophic lateral sclerosis. A role for Ankrd2 in tumor progression and metastases spreading has also been described.
Nebulin-related-anchoring protein(N-RAP) is a protein that in humans is encoded by the NRAP gene. N-RAP is a muscle-specific isoform belonging to the nebulin family of proteins. This family is composed of 5 members: N-RAP, nebulin, nebulette, LASP-1 and LASP-2. N-RAP is involved in both myofibrillar myogenesis during development and cell-cell connections in mature muscle.
E3 ubiquitin-protein ligase TRIM63, also known as "MuRF1", is an enzyme that in humans is encoded by the TRIM63 gene.
Myopalladin is a protein that in humans is encoded by the MYPN gene. Myopalladin is a muscle protein responsible for tethering proteins at the Z-disc and for communicating between the sarcomere and the nucleus in cardiac and skeletal muscle
Ankyrin 1, also known as ANK-1, and erythrocyte ankyrin, is a protein that in humans is encoded by the ANK1 gene.
Ankyrin repeat domain-containing protein 23 is a protein that in humans is encoded by the ANKRD23 gene.
Atrial Light Chain-2 (ALC-2) also known as Myosin regulatory light chain 2, atrial isoform (MLC2a) is a protein that in humans is encoded by the MYL7 gene. ALC-2 expression is restricted to cardiac muscle atria in healthy individuals, where it functions to modulate cardiac development and contractility. In human diseases, including hypertrophic cardiomyopathy, dilated cardiomyopathy, ischemic cardiomyopathy and others, ALC-2 expression is altered.
Myomesin-2, also known as M-protein is a protein that in humans is encoded by the MYOM2 gene. M-protein is expressed in adult cardiac muscle and fast skeletal muscle, and functions to stabilize the three-dimensional arrangement of proteins comprising M-band structures in a sarcomere.
Ankyrin-2, also known as Ankyrin-B, and Brain ankyrin, is a protein which in humans is encoded by the ANK2 gene. Ankyrin-2 is ubiquitously expressed, but shows high expression in cardiac muscle. Ankyrin-2 plays an essential role in the localization and membrane stabilization of ion transporters and ion channels in cardiomyocytes, as well as in costamere structures. Mutations in ANK2 cause a dominantly-inherited, cardiac arrhythmia syndrome known as long QT syndrome 4 as well as sick sinus syndrome; mutations have also been associated to a lesser degree with hypertrophic cardiomyopathy. Alterations in ankyrin-2 expression levels are observed in human heart failure.
