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. [5] [6] [7] [8] 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. [9] [10]
Two isoforms of ARPP have been documented; a 39.8 kDa protein isoform composed of 360 amino acids [11] and a 36.2 kDa protein isoform composed of 327 amino acids. [12] ANKRD2 has nine exons, four of which encode ankyrin repeats in the middle region of the protein, a PEST-like and Lysine-rich sequence in the N-terminal region, and a Proline-rich sequence containing consensus sequences for phosphorylation in the C-terminal region. [13] [14] It has been proposed that ARPP can homo- or hetero-dimerize with other MARPs in an antiparallel fashion. [15] ARPP is highly expressed in nuclei and I-bands in slow skeletal fibers [13] [16] and cardiac muscle, specifically in ventricular regions [14] at intercalated discs; [17] and expression in brain, pancreas and esophageal epithelium has also been documented. [16] [18] Though ARPP and CARP proteins show significant homology, their expression profiles in muscle cells are markedly different; CARP is expressed throughout atria and ventricles, in development and in adult myocytes, however ARPP is almost exclusively ventricular and only in adult myocytes. ARPP was also found to be expressed in rhabdomyosarcomas, exhibiting a pattern distinct from actin and desmin. [19]
ARPP localizes to both nuclei and sarcomeres in muscle cells. ARPP may play a role in the differentiation of myocytes, as ARPP expression was shown to be induced during the C2C12 differentiation in vitro. [19] A role for ARPP in regulating muscle gene expression and sensing stress signals was implicated in the finding that ARPP colocalizes with the transcriptional co-activator and co-repressor PML in myoblast nuclei, and binds p53 to enhance the p21(WAFI/CIPI) promoter. [20] It was further demonstrated that Nkx2.5 and p53 synergistically activate the ANKRD2 promoter to promote effects on myogenic differentiation. [21] At the sarcomere, ARPP binds titin at I-bands, which is potentiated by homo-dimerization and can alter the protein kinase A/protein kinase C phosphorylation status of itself or titin. [15] These studies demonstrate a stretch-responsive relationship between ARPP and Titin, which can be rapidly altered by post-translational mechanisms.
Functional insights into ARPP function have come from transgenic studies. In mice lacking all three muscle ankyrin repeat proteins (MARPs), ARPP, CARP, and DARP), skeletal muscles tended towards a more slower fiber type distribution, with longer resting sarcomere length, decreased fiber stiffness, expression of a longer titin isoform, greater degree of torque loss following eccentric contraction-related injury, and enhanced expression of MyoD and MLP. These findings suggest that ARPP and related MARP proteins may play a role in the passive stiffness and gene regulatory roles in skeletal muscle. [22] A study investigating ARPP function in cardiac muscle in which ARPP was knocked out alone or in combination with the other MARPs showed that mice displayed normal cardiac function at baseline and in response to pressure overload-induced cardiac hypertrophy, suggesting that these proteins are not essential for normal cardiac development or in response to a hypertrophic stimulus. [23]
ARPP has also shown to play a role in models of disease. ARPP has also exhibited elevated expression following skeletal muscle denervation, persisting for four weeks following the insult. [16] ARPP (ANKRD2) gene expression was also shown to be rapidly induced in a model of eccentric contraction-related injury, showing peak expression (6-11 times normal value) within 12–24 hours following injury, suggesting that ARPP may play a role in repair. [24] In a mouse model of muscular dystrophy with myositis (mdm) caused by a small deletion in titin, ANKRD2 mRNA expression was shown to be significantly elevated in skeletal muscle tissue along with that of CARP, suggesting a role for ARPP in titin-based signaling. [25] Levels of ARPP were also altered in a mouse model of diabetes. [26]
In patients with dilated cardiomyopathy, levels of ARPP were upregulated. [27]
ARPP expression patterns have been shown to be altered in patients with amyotrophic lateral sclerosis (ALS), with decreased expression in slow skeletal muscle fibers and increased expression in fast skeletal muscle fibers. [28]
ARPP has also been shown to be a potentially useful biomarker for the differential diagnosis between oncocytoma and chromophobe renal cell carcinomas. [29]
In non-pathologic physiology, ARPP mRNA expression in skeletal muscle of patients was shown to be elevated two days following fatiguing jumping exercises. Levels of CARP, MLP and calpain-2 mRNA levels were also enhanced, suggesting that these molecules may be part of a signaling network activated by physical exercise. [30]
Ankrd2 has been shown to be involved in the progression of some types of cancers, such as osteosarcoma [9] and head and neck squamous cell carcinoma. [10]
ANKRD2 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-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.
β-Tropomyosin, also known as tropomyosin beta chain is a protein that in humans is encoded by the TPM2 gene. β-tropomyosin is striated muscle-specific coiled coil dimer that functions to stabilize actin filaments and regulate muscle contraction.
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.
Troponin I, fast skeletal muscle is a protein that in humans is encoded by the TNNI2 gene.
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 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. 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.
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
Myomesin-1 is a protein that in humans is encoded by the MYOM1 gene. Myomesin-1 is expressed in muscle cells and functions to stabilize the three-dimensional conformation of the thick filament. Embryonic forms of Myomesin-1 have been detected in dilated cardiomyopathy.
Ankyrin repeat domain-containing protein 17 is a protein that in humans is encoded by the ANKRD17 gene.
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.
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.
Ankyrin repeat domain 11 is a protein that in humans is encoded by the ANKRD11 gene.