Actin caldesmon

Moran and colleagues have reported the existence of primary mediastinal leiomyosarcomas, which may be observed in either the anterior or posterior compartments. These spindle-cell malignancies are uniformly immunoreactive for desmin (Fig. 11.35), muscle-specific actin, alpha-isoform actin, caldesmon, and calponin vimentin also may be present. Conversely, EMA, keratin, Myo-Dl, myoglobin, myogenin, and... 

Finally, a 150 kDa calmodulin binding protein, caldesmon is found bound to actin filaments at low calcium concentrations. The ratio of caldesmon to actin molecules seems to vary from 1 20 in the aorta to as much as 1 200 in chicken gizzard. It has been reported and disputed that caldesmon crosslinks actin filaments into large bundles. 

In in vitro experiments, caldesmon can inhibit the activation of myosin by actin and this inhibition can be reversed by calmodulin. Thus, there is a potentiality for... 

It has also been proposed that, since Ca-calmodulin combines with caldesmon and the complex has little affinity for actin, this may also contribute to the activation process. The notion here is that, in parallel with the activation of MLCK disinhibi-tion, regulation via caldesmon may occur. Unfortunately, such a mechanism would work only at cytosolic levels too high in respect of typical contraction. 

The calcium mediated contraction of smooth muscle, which unlike striated muscle does not contain troponin, is quite different and requires a particular calcium-binding protein called calmodulin. Calmodulin (CM) is a widely distributed regulatory protein able to bind, with high affinity, four Ca2+ per protein molecule. The calcium—calmodulin (CaCM) complex associates with, and activates, regulatory proteins, usually enzymes, in many different cell types in smooth muscle the target regulatory proteins are caldesmon (CDM) and the enzyme myosin light chain kinase (MLCK). As described below, CaCM impacts on both actin and myosin filaments. 

In smooth muscle, caldesmon plays an analogous role to that of troponin in striated muscle in that it blocks the myosin binding sites. The CaCM complex removes caldesmon from its binding on the thin actin filaments allowing tropomyosin to reposition in the helical grooves of F-actin leading to myosin ATP ase activation. 

In addition to the displacement of caldesmon, smooth muscle cell contraction requires kinase-induced phosphorylation of myosin. Smooth muscle has a unique type of myosin filament called p-light chains which are the target (substrate) for MLCK, but MLCK is only active when complexed with CaCM. Myosin light chain phosphatase reverses the PKA-mediated process and when cytosolic calcium ion concentration falls, CDM is released from CaCM and re-associates with the actin. The central role of calcium-calmodulin in smooth muscle contraction is shown in Figure 7.4. 

A cytoskeletal regulatory protein that attaches laterally to actin filament without resulting in filament cross-linking and/or bundling. These include troponin, tropomyosin, calponin, tropomodulin, adducin, caldesmon, and hisactophilin. See also ABM-1 ABM-2 Sequences in Actin-Based Motors Actin-Based Bacterial Motility Actin Assembly Kinetics... 

Additional information <1> (<1> isozyme of calmodulin-dependent multifunctional protein kinase II in smooth-muscle [5] <1> caldesmon is not a substrate of smooth-muscle myosin light-chain kinase [3] <1> no substrates are bovine cardiac C-protein, bovine brain fodrin, rabbit skeletal muscle glycogen synthase, phosphorylase B, troponon (I -I- T -I- C), actin, tropomyosin, smooth-muscle actin, filamin, vinculin, cr-actinin, protamine or phosvitin [1]) [1-3]... 

Ngai, P.K. Walsh, M.P. Inhibition of smooth muscle actin-activated myosin Mg -ATPase activity by caldesmon. J. Biol. Chem., 259, 13656-13659 (1984)... 

Apart from the phosphorylation theory, other regulatory mechanisms have also been suggested for smooth muscle contraction. A thin-filament protein that has been proposed as a regulatory component is caldesmon [102], Purified caldesmon is a potent inhibitor of actin-tropomyosin interaction with myosin. The mechanisms by which calcium removes this inhibition are controversial. Furthermore, phosphorylation of caldesmon by a caldesmon kinase in vitro has also been implicated in this... 

There is an additional controlling pathway which is thin filament regulated this is dependent upon another protein, caldesmon, which is closely associated with the actin/tropomyosin-myosin domain. Caldesmon derives its name fi-om its intrinsic ability to bind to calmodulin with a much greater affinity than to the thin... 

Sobue, K., Morimoto, K., Inui, M., Kanda, K. and Kakiuchi, S. (1982). Control of actin-myosin interaction of gjzzard smooth muscle by calmodulin- and caldesmon-linked flip-flop mechanism. Biomed. Res. 3, 188-196. 

In skeletal muscle, disinhibition of actin is necessary for contraction to occur, and control of contraction is said to be actin-linked. In smooth muscle, phosphorylation of myosin light chains (MLCs) is required for contraction. Several mechanisms alter MLC phosphorylation, and so in smooth muscle, control of contraction is primarily myosin-linked. Three control proteins have been identified in smooth muscle myosin light chain kinase (MLCK) caldesmon (CaD) and calponin (CaP). Figure 21-14... 

Calponin is another polypeptide monomer (M.W. 32,000) that can inhibit actin-activated myosin ATPase activity. In contrast to CaD, CaP exerts its effect in the absence of tropomyosin and completely inhibits motility in a 2/3 ratio with actin. CaP inhibits myosin binding to actin, but does so by reducing the affinity of actin for myosin rather than competing for the same site. CaP can be phosphorylated by PKC and CaMKII, both of which reverse CaP s inhibitory activity. As with caldesmon, many questions remain. The ratio of CaP to actin actually observed in smooth muscle is in the range 1 10 to 1 16, far from the 2/3 ratio found to produce near-complete inhibition of motility. Therefore, the importance of CaP and its regulation by phosphorylation is still debatable. 

KER, keratin, detected by a mixture of GAMS.2, MAK-6, and AEl /AE3 EMA, epithelial membrane antigen VIM, vimentin DES, desmin MSA, muscle-specific actin SMA, smooth muscle (alpha isoform) actin GALD, h-caldesmon S-IOOP, S-100 protein OCN, osteocalcin LM, laminin UL, Ulex europaeus I lectin binding FS, fibrosarcoma SGRMS, spindle cell rhabdomyosarcoma LMS, leiomyosarcoma MPNST, malignant peripheral nerve sheath tumor MSS, monophasic spindle cell synovial sarcoma SCAS, spindle cell angiosarcoma KS, Kaposi s sarcoma FOS, fibroblastic osteosarcoma. 

Caldesmon is a cytoplasmic protein with two isoform classes, one of which is found predominantly in smooth muscle cells and other cell types with partial myogenic differentiation. High-molecular-weight isoforms with molecular weights between 89 and 93 kD are capable of binding to actin, tropomyosin, calmodulin, myosin, and phospholipids, and they function to counteract actin-tropomyosin-activated myosin adenosine triphosphatase (ATPase). As such, they are mediators for the inhibition of calcium-dependent smooth muscle contraction." ... 

Commercial antibodies to caldesmon are now applied to diagnostic problems in surgical pathology. They appear to be relatively specific for smooth muscle differentiation and, as such, are useful adjuncts to des-min and actin immunostains." " Caldesmon is also expressed in the majority of gastrointestinal stromal tumors (GISTs), glomus tumors, and myopericytomas, and is therefore not entirely specific." >"9... 

Alpha) smooth muscle actin is less specific for myogenous differentiation than h-caldesmon or HHF-35, muscle-specific actin. 

Certain markers with specificity for muscle differentiation may be useful in recognizing leiomyomas, beyond desmin, caldesmon, and muscle-associated actins, but their application in diagnostic immunohistopathology is not generally considered routine. Smooth muscle myosin and Z-band protein have been advocated by some, particularly when either the histologic pattern is unusual (such as myxoid or hyalinized lesions) or the interpretation of a myogenous lesion is not corroborated by other stains. 

Fuji T, Machino K, Andoh H, et al. Galcium-dependent control of caldesmon-actin interaction by SlOO protein. J Biochem. 1990 107 133-137. 

Because muscle-specific actin (recognized by monoclonal antibody HHF-35), alpha-isoform ( smooth muscle ) actin (recognized by antibody 1A4), and caldesmon are also preferentially seen in nonepithelial, nonmelanocytic, nonglial tissues, those determinants would be unexpected in melanocytic neoplasms. Indeed,... 

Perez-Montiel MD, Plaza JA, Dominguez-Malagon H, Suster S. Differential expression of smooth muscle myosin, smooth muscle actin, h-caldesmon, and calponin in the diagnosis of myofi-broblastic and smooth muscle lesions of skin and soft tissue. Am J Dermatopathol. 2006 28 105-111. 

The ovarian-like stroma cells also label with antibodies to smooth muscle actin, muscle-specific actin, desmin, h-caldesmon, and also variably for alpha-inhibin and calretinin. 

The muscle actins (MSA and SMSA), desmin, and h-caldesmon are useful in identifying smooth muscle cells. Normal endometrial stromal cells express vimen-tin and muscle actins, but they generally lack expression... 

Endometrial stromal neoplasms, both nodules and sarcomas, almost always express CDIO, ER, PR, and WTl (Fig. 18.17).138>141-143,213,214 Many also express P-catenin.2i 2i They frequently express smooth muscle actin,i°°40i,i38,2i4 cytokeratin, 4i4 qj- androgen receptors patchily. They only unusually express des-j inioo,i35,2i9,220 CD34 (unpublished observation) in varieties where the constituent cells resemble non-neoplastic proliferative endometrial stroma. Diffuse desmin and h-caldesmon expression supports smooth muscle differentiation and disqualifies categorization as an endometrial stromal tumor except in rare cases. 

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