Effects of EMD57033, an activator of actomyosin ATPase activity, on the relaxation process of cell membrane-permeabilized carotid artery and taenia cecum from guinea pigs

Smooth muscle relaxation after contraction is thought to reflect "latch-like" slow cycling bridge formation and deformation. However, how actin-myosin interaction contributes to the transfer from fast-cycling cross bridges to slow-cycling bridges is still unclear. The thiadiazinone compound EMD57033 is known to bind to an allosteric pocket in the myosin motor domain and to increase basal and actin-activated myosin ATPase activity and contractile force in striated muscles. Therefore, we investigated whether EMD57033 affected the relaxation process after Ca2+ removal by affecting slow cycling bridge formation and/or deformation in β-escin skinned (cell membrane-permeabilized) carotid artery and taenia cecum from guinea pigs. EMD57033 at ≥30 µM decreased the force decay during relaxation in both the skinned carotid artery and taenia cecum, irrespective of the presence of ATP. A kinetic analysis in the present study indicated that EMD57033 significantly prolonged τslow-detach, a time constant of detachment of the slow cycling bridge, in both the skinned carotid artery and taenia cecum, irrespective of the presence of nucleoside triphosphates (ATP or ITP). Further studies are necessary to elucidate how EMD57033 modulates the smooth muscle myosin (SMM) structure, SMM activity, and thick filament organization, affecting slow cycling bridge formation and deformation, although EMD57033 might change slow cycling bridge formation, resulting in both cycling rate modulation and an increase in the affinity of SMM to actin.

Effects of cytochalasin D on relaxation process of skinned taenia cecum and carotid artery from guinea pig

Actin linked regulatory mechanisms are known to contribute contraction/relaxation in smooth muscle. In order to clarify whether modulation of polymerization/depolymerization of actin filaments affects relaxation process, we examined the effects of cytochalasin D on relaxation process by Ca2+ removal after Ca2+-induced contraction of β-escin skinned (cell membrane permeabilized) taenia cecum and carotid artery preparations from guinea pigs. Cytochalasin D, an inhibitor of actin polymerization, significantly suppressed the force during relaxation both in skinned taenia cecum and carotid artery. The data fitting analysis of the relaxation processes indicates that cytochalasin D accelerates slow (latch-like) bridge dissociation. Cytochalasin D seems to directly disrupts actin filament organization or its length, resulting in modulation of actin filament structure that prevents myosin binding.

Inhibitory effects of rubratoxin A, a potent inhibitor of protein phosphatase 2, on the Ca2+-dependent contraction of skinned carotid artery from guinea pig

Rubratoxin A, a potent inhibitor of PP2A, is known to suppress smooth muscle contraction. The inhibitory role of PP2A in smooth muscle contraction is still unclear. In order to clarify the regulatory mechanisms of PP2A on vascular smooth muscle contractility, we examined the effects of rubratoxin A on the Ca²⁺-induced contraction of β-escin skinned carotid artery preparations from guinea pigs. Rubratoxin A at 1 µM and 10 µM significantly inhibited skinned carotid artery contraction at any Ca²⁺ concentration. The data fitting to the Hill equation in [Ca²⁺]-contraction relationship indicated that rubratoxin A decreased F_max-Ca2+ and increased [Ca²⁺]₅₀, indices of Ca²⁺ sensitivity for the force and myosin-actin interaction, respectively. These results suggest that PP2A inhibition causes downregulation of the myosin light chain phosphorylation and direct interference with myosin-actin interaction.

Accelerating effects of blebbistatin on relaxation process of cell membrane permeabilized trachea and taenia cecum from guinea pig

Blebbistatin, a potent inhibitor of myosin II, is known to suppress smooth muscle contraction without affecting myosin light chain phosphorylation level. In order to clarify the regulatory mechanisms of blebbistatin on phasic and tonic smooth muscles in detail, we examined the effects of blebbistatin on relaxation process by Ca²⁺ removal after Ca²⁺-induced contraction of β-escin skinned (cell membrane permeabilized) trachea and taenia cecum preparations from guinea pigs. Blebbistatin significantly suppressed the force during relaxation both in skinned trachea and taenia cecum. The data fitting analysis of the relaxation processes indicates that blebbistatin accelerates slow (latch-like) bridge dissociation.

Blebbistatin, a myosin II inhibitor, suppresses Ca(2+)-induced and "sensitized"-contraction of skinned tracheal muscles from guinea pig

Blebbistatin, a potent inhibitor of myosin II, has inhibiting effects on Ca(2+)-induced contraction and contractile filament organization without affecting the Ca(2+)-sensitivity to the force and phosphorylation level of myosin regulatory light chain (MLC20) in skinned (cell membrane permeabilized) taenia cecum from the guinea pig (Watanabe et al., Am J Physiol Cell Physiol. 2010; 298: C1118-26). In the present study, we investigated blebbistatin effects on the contractile force of skinned tracheal muscle, in which myosin filaments organization is more labile than that in the taenia cecum. Blebbistatin at 10 μM or higher suppressed Ca(2+)-induced tension development at any given Ca(2+) concentration, but had little effects on the Ca(2+)- induced myosin light chain phosphorylation. Also blebbistatin at 10 μM and higher significantly suppressed GTP-γS-induced "sensitized" force development. Since the force inhibiting effects of blebbistatin on the skinned trachea were much stronger than those in skinned taenia cecum, blebbistatin might directly affect myosin filaments organization.

Large potentials of small heat shock proteins

Modern classification of the family of human small heat shock proteins (the so-called HSPB) is presented, and the structure and properties of three members of this family are analyzed in detail. Ubiquitously expressed HSPB1 (HSP27) is involved in the control of protein folding and, when mutated, plays a significant role in the development of certain neurodegenerative disorders. HSPB1 directly or indirectly participates in the regulation of apoptosis, protects the cell against oxidative stress, and is involved in the regulation of the cytoskeleton. HSPB6 (HSP20) also possesses chaperone-like activity, is involved in regulation of smooth muscle contraction, has pronounced cardioprotective activity, and seems to participate in insulin-dependent regulation of muscle metabolism. HSPB8 (HSP22) prevents accumulation of aggregated proteins in the cell and participates in the regulation of proteolysis of unfolded proteins. HSPB8 also seems to be directly or indirectly involved in regulation of apoptosis and carcinogenesis, contributes to cardiac cell hypertrophy and survival and, when mutated, might be involved in development of neurodegenerative diseases. All small heat shock proteins play important "housekeeping" roles and regulate many vital processes; therefore, they are considered as attractive therapeutic targets.

Versatility of the small heat shock protein HSPB6 (Hsp20)

The recently published review by Dreiza et al. (Cell Stress and Chaperones DOI 10.1007/s12192-0090127-8 ) dealing with the functional role of HSPB6 in muscle regulation is critically analyzed. Published data indicate that the chaperone-like activity of HSPB6 is comparable with that of HSPB5 and that phosphorylation of HSPB6 does not affect its oligomeric structure. Different hypotheses concerning the molecular mechanisms of HSPB6 action on smooth muscle contraction and on the reorganization of the cytoskeleton are compared, and it is concluded that although HSPB6 is not a genuine actin-binding protein, it can affect the actin cytoskeleton indirectly. Phosphorylated HSPB6 interacts with 14-3-3 and thereby displaces other binding partners of 14-3-3; among them, certain phosphatases, protein kinases, and various actin-binding proteins, which can participate in the reorganization of the actin cytoskeleton. In addition, HSPB6 seems to regulate the activity of certain protein kinases. All of these processes are dependent on HSPB6 phosphorylation which in turn might be regulated by the formation of heterooligomeric complexes of HSPB6 with other small heat shock proteins.

Blebbistatin, a myosin II inhibitor, suppresses contraction and disrupts contractile filaments organization of skinned taenia cecum from guinea pig

To explore the precise mechanisms of the inhibitory effects of blebbistatin, a potent inhibitor of myosin II, on smooth muscle contraction, we studied the blebbistatin effects on the mechanical properties and the structure of contractile filaments of skinned (cell membrane permeabilized) preparations from guinea pig taenia cecum. Blebbistatin at 10 microM or higher suppressed Ca(2+)-induced tension development at any given Ca(2+) concentration but had little effects on the Ca(2+)-induced myosin light chain phosphorylation. Blebbistatin also suppressed the 10 and 2.75 mM Mg(2+)-induced, "myosin light chain phosphorylation-independent" tension development at more than 10 microM. Furthermore, blebbistatin induced conformational change of smooth muscle myosin (SMM) and disrupted arrangement of SMM and thin filaments, resulting in inhibition of actin-SMM interaction irrespective of activation with Ca(2+). In addition, blebbistatin partially inhibited Mg(2+)-ATPase activity of native actomyosin from guinea pig taenia cecum at around 10 microM. These results suggested that blebbistatin suppressed skinned smooth muscle contraction through disruption of structure of SMM by the agent.