ANKRD1

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: