Caldesmon and heat shock protein 20 phosphorylation in nitroglycerin- and magnesium-induced relaxation of swine carotid artery

The Role of Small Heat Shock Proteins in Protein Misfolding Associated Motoneuron Diseases

Motoneuron diseases (MNDs) are neurodegenerative conditions associated with death of upper and/or lower motoneurons (MNs). Proteostasis alteration is a pathogenic mechanism involved in many MNDs and is due to the excessive presence of misfolded and aggregated proteins. Protein misfolding may be the product of gene mutations, or due to defects in the translation process, or to stress agents; all these conditions may alter the native conformation of proteins making them prone to aggregate. Alternatively, mutations in members of the protein quality control (PQC) system may determine a loss of function of the proteostasis network. This causes an impairment in the capability to handle and remove aberrant or damaged proteins. The PQC system consists of the degradative pathways, which are the autophagy and the proteasome, and a network of chaperones and co-chaperones. Among these components, Heat Shock Protein 70 represents the main factor in substrate triage to folding, refolding, or degradation, and it is assisted in this task by a subclass of the chaperone network, the small heat shock protein (sHSPs/HSPBs) family. HSPBs take part in proteostasis by bridging misfolded and aggregated proteins to the HSP70 machinery and to the degradative pathways, facilitating refolding or clearance of the potentially toxic proteins. Because of its activity against proteostasis alteration, the chaperone system plays a relevant role in the protection against proteotoxicity in MNDs. Here, we discuss the role of HSPBs in MNDs and which HSPBs may represent a valid target for therapeutic purposes.

Tissue length modulates "stimulated actin polymerization," force augmentation, and the rate of swine carotid arterial contraction

"Stimulated actin polymerization" has been proposed to be involved in force augmentation, in which prior submaximal activation of vascular smooth muscle increases the force of a subsequent maximal contraction by ∼15%. In this study, we altered stimulated actin polymerization by adjusting tissue length and then measured the effect on force augmentation. At optimal tissue length (1.0 L(o)), force augmentation was observed and was associated with increased prior stimulated actin polymerization, as evidenced by increased prior Y118 paxillin phosphorylation without changes in prior S3 cofilin or cross-bridge phosphorylation. Tissue length, per se, regulated Y118 paxillin, but not S3 cofilin, phosphorylation. At short tissue length (0.6 L(o)), force augmentation was observed and was associated with increased prior stimulated actin polymerization, as evidenced by reduced prior S3 cofilin phosphorylation without changes in Y118 paxillin or cross-bridge phosphorylation. At long tissue length (1.4 L(o)), force augmentation was not observed, and there were no prior changes in Y118 paxillin, S3 cofilin, or cross-bridge phosphorylation. There were no significant differences in the cross-bridge phosphorylation transients before and after the force augmentation protocol at all three lengths tested. Tissues contracted faster at longer tissue lengths; contractile rate correlated with prior Y118 paxillin phosphorylation. Total stress, per se, predicted Y118 paxillin phosphorylation. These data suggest that force augmentation is regulated by stimulated actin polymerization and that stimulated actin polymerization is regulated by total arterial stress. We suggest that K(+) depolarization first leads to cross-bridge phosphorylation and contraction, and the contraction-induced increase in mechanical strain increases Y118 paxillin phosphorylation, leading to stimulated actin polymerization, which further increases force, i.e., force augmentation and, possibly, latch.

Force augmentation and stimulated actin polymerization in swine carotid artery

The phenomenon of posttetanic potentiation, in which a single submaximal contraction or series of submaximal contractions strengthens a subsequent contraction, has been observed in both skeletal and cardiac muscle. In this study, we describe a similar phenomenon in swine carotid arterial smooth muscle. We find that a submaximal K(+) depolarization increases the force generation of a subsequent maximal K(+) depolarization; we term this "force augmentation." Force augmentation was not associated with a significant increase in crossbridge phosphorylation or shortening velocity during the maximal K(+) depolarization, suggesting that the augmented force was not caused by higher crossbridge phosphorylation or crossbridge cycling rates. We found that the characteristics of the tissue before the maximal K(+) depolarization predicted the degree of force augmentation. Specifically, measures of stimulated actin polymerization (higher prior Y118 paxillin phosphorylation, higher prior F-actin, and transition to a more solid rheology evidenced by lower noise temperature, hysteresivity, and phase angle) predicted the subsequent force augmentation. Increased prior contraction alone did not induce force augmentation since readdition of Ca(2+) to Ca(2+)-depleted tissues induced a partial contraction that was not associated with changes in noise temperature or with subsequent force augmentation. These data suggest that stimulated actin polymerization may produce a substrate for increased crossbridge mediated force, a process we observe as force augmentation.

VIP induces PKA-mediated rapid and sustained phosphorylation of HSP20

The small molecular weight heat shock protein HSP20 has been proposed to regulate smooth muscle relaxation in a manner dependent on its phosphorylated state. We present the first evidence of HSP20 phosphorylation in response to a naturally occurring neurotransmitter. HSP20 was rapidly phosphorylated in colonic circular smooth muscle cells exposed to the physiologically relevant relaxant neuropeptide, Vasoactive Intestinal Peptide (VIP). HSP20 phosphorylation was significantly and substantially increased by 30s following VIP treatment and remained elevated for 30 min. VIP-induced HSP20 phosphorylation was dose dependent. Both basal and VIP-induced HSP20 phosphorylations were solely mediated by Protein Kinase A. Maximal phosphorylation of HSP20 was induced by the same VIP concentration range which induces maximal relaxation. Increased phosphorylation of HSP20 occurred in both cytosolic and particulate cell fractions. Our findings represent evidence for neurogenic modulation of the cyclic molecular regulation of relaxation required for peristalsis via a VIP-PKA-HSP20 pathway.

Postnatal maturation modulates relationships among cytosolic Ca2+, myosin light chain phosphorylation, and contractile tone in ovine cerebral arteries

The present study tests the hypothesis that age-related changes in patterns of agonist-induced myofilament Ca(2+) sensitization involve corresponding differences in the relative contributions of thick- and thin-filament regulation to overall myofilament Ca(2+) sensitivity. Posterior communicating cerebral arteries from term fetal and nonpregnant adult sheep were used in measurements of cytosolic Ca(2+), myosin light chain (MLC) phosphorylation, and contractile tensions induced by varying concentrations of K(+) or serotonin [5-hydroxytryptamine (5-HT)]. The results were used to assess the relative contributions of the relationships between cytosolic Ca(2+) and MLC phosphorylation (thick-filament reactivity), along with the relationships between MLC phosphorylation and contractile tension (thin-filament reactivity), to overall myofilament Ca(2+) sensitivity. For K(+)-induced contractions, both fetal and adult arteries exhibited similar basal myofilament Ca(2+) sensitivity. Despite this similarity, thick-filament reactivity was greater in fetal arteries, whereas thin-filament reactivity was greater in adult arteries. In contrast, 5-HT-induced contractions exhibited increased myofilament Ca(2+) sensitivity compared with K(+)-induced contractions for both fetal and adult cerebral arteries, and the magnitude of this effect was greater in fetal compared with adult arteries. When interpreted together with our previous studies of 5-HT-induced myofilament Ca(2+) sensitization, we attributed the present effects to agonist enhancement of thick-filament reactivity in fetal arteries mediated by G protein receptor activation of a PKC-independent but RhoA-dependent pathway. In adult arteries, agonist stimulation enhanced thin-filament reactivity was also probably mediated through G protein-coupled activation of RhoA-dependent and PKC-independent mechanisms. Overall, the present data demonstrate that agonist-enhanced myofilament Ca(2+) sensitivity can be partitioned into separate thick- and thin-filament effects, the magnitudes of which are different between fetal and adult cerebral arteries.

Paxillin phosphorylation, actin polymerization, noise temperature, and the sustained phase of swine carotid artery contraction

Histamine stimulation of swine carotid artery induces both contraction and actin polymerization. The importance of stimulus-induced actin polymerization is not known. Tyrosine phosphorylation of the scaffolding protein paxillin is thought to be an important regulator of actin polymerization. Noise temperature, hysteresivity, and phase angle are rheological measures of the fluidity of a tissue, i.e., whether the muscle is more a "Hookean solid" or a "Newtonian liquid." Y118 paxillin phosphorylation, crossbridge phosphorylation, actin polymerization, noise temperature, hysteresivity, phase angle, real stiffness, and stress were measured in intact swine carotid arteries that were depolarized with high K(+) or stimulated with histamine. The initial rapid force development phase of high-K(+) or histamine-induced contraction was associated with increased crossbridge phosphorylation but no significant change in Y118 paxillin phosphorylation, actin polymerization, noise temperature, hysteresivity, or phase angle. This suggests that the initial contraction was caused by the increase in crossbridge phosphorylation and did not alter the tissue's rheology. Only after full force development was there a significant increase in Y118 paxillin phosphorylation and actin polymerization associated with a significant decrease in noise temperature and hysteresivity. These data suggest that some part of the sustained contraction may depend on stimulated actin polymerization and/or a transition to a more "solid" rheology. Supporting this contention was the finding that an inhibitor of actin polymerization, latrunculin-A, reduced force while increasing noise temperature/hysteresivity. Further research is needed to determine whether Y118 paxillin phosphorylation, actin polymerization, and changes in rheology could have a role in arterial smooth muscle contraction.

Force suppression and the crossbridge cycle in swine carotid artery

Cyclic nucleotides can relax arterial smooth muscle without reductions in crossbridge phosphorylation, a process termed force suppression. There are two potential mechanisms for force suppression: 1) phosphorylated crossbridges binding to thin filaments could be inhibited or 2) the attachment of thin filaments to anchoring structures could be disrupted. These mechanisms were evaluated by comparing histamine-stimulated swine arterial smooth muscle with and without forskolin-induced force suppression and with and without latrunculin-A-induced actin filament disruption. At matched force, force suppression was associated with higher crossbridge phosphorylation and shortening velocity at low loads when compared with tissues without force suppression. Shortening velocity at high loads, noise temperature, hysteresivity, and stiffness did not differ with and without force suppression. These data suggest that crossbridge phosphorylation regulates the crossbridge cycle during force suppression. Actin disruption with latrunculin-A was associated with higher crossbridge phosphorylation when compared with tissues without actin disruption. Shortening velocity, noise temperature, hysteresivity, and stiffness did not differ with and without actin disruption. These data suggest that actin disruption interferes with regulation of crossbridge cycling by crossbridge phosphorylation. Stiffness was linearly dependent on stress, suggesting that the force per attached crossbridge was not altered with force suppression or actin disruption. These data suggest a difference in the mechanical characteristics observed during force suppression and actin disruption, implying that force suppression does not mechanistically involve actin disruption. These data are most consistent with a model where force suppression involves the inhibition of phosphorylated crossbridge binding to thin filaments.

Longer muscle lengths recapitulate force suppression in swine carotid artery

Cyclic nucleotide can relax arterial smooth muscle without reductions in myosin regulatory light chain (MRLC) phosphorylation, a process termed force suppression. Smooth muscle contractile force also depends on tissue length. It is not known how tissue length affects force suppression. Swine carotid artery rings were equilibrated at various lengths (as a fraction of L(o), the optimal length for force development). They were then frozen during contractile activation with or without forskolin-induced relaxation. Frozen tissue homogenates were then analyzed for Ser(19)-MRLC phosphorylation and Ser(16)-heat shock protein 20 (HSP20) phosphorylation (HSP20 is the proposed mediator of force suppression). Higher values of MRLC phosphorylation were required to induce a histamine contraction at longer tissue lengths. At 1.4 L(o), the dependence of force on MRLC phosphorylation observed with histamine stimulation alone was shifted to the right, a response similar to that observed during force suppression at 1.0 L(o). The rightward shift in the dependence of force on MRLC phosphorylation seen with histamine stimulation alone at 1.4 L(o) was not associated with increased HSP20 phosphorylation. Addition of forskolin to histamine-stimulated tissues at 1.4 L(o) induced a relaxation associated with increased HSP20 phosphorylation and reduced MRLC phosphorylation, i.e., there was no additional force suppression. At shorter tissue lengths (0.6 L(o)), the dependence of force on MRLC phosphorylation with histamine stimulation alone was steep, a response similar to that observed during normal contractile activation at 1.0 L(o). Addition of forskolin induced force suppression at 0.6 L(o). The sensitivity of swine carotid to the concentration of histamine was greater at longer tissue lengths compared with shorter tissue lengths, suggesting a physiological mechanism to restore optimal tissue length. These data suggest that longer tissue lengths induced a force suppression-like state that was 1) not additive with forskolin and 2) not associated with HSP20 phosphorylation. Further research is required to determine this length-dependent mechanism.

Structure, properties, and probable physiological role of small heat shock protein with molecular mass 20 kD (Hsp20, HspB6)

This review is devoted to critical analysis of data concerning the structure and functions of small heat shock proteins with apparent molecular mass 20 kD (Hsp20). We describe the structure of Hsp20, its phosphorylation by different protein kinases, interaction of Hsp20 with other small heat shock proteins, and chaperone activity of Hsp20. The distribution of Hsp20 in different animal tissues and the factors affecting expression of Hsp20 are also described. Data on the possible involvement of Hsp20 in regulation of platelet aggregation and glucose transport are presented and analyzed. Special attention is paid to literature data describing probable regulatory effect of Hsp20 on contraction of smooth muscle. Two hypotheses postulating direct effect of Hsp20 on actomyosin interaction or its effect on cytoskeleton are compared and analyzed. The most recent data on the effect of Hsp20 on apoptosis and contractile activity of cardiomyocytes are also presented.

HSP20 phosphorylation and interstitial metabolites in hypoxia-induced dilation of swine coronary arteries

OBJECTIVE

Hypoxia induces coronary artery dilation, but the responsible mechanism is largely unknown. Many stimuli induce arterial smooth muscle relaxation by reducing ser19-myosin regulatory light chain (MLC) phosphorylation. Other stimuli can induce smooth muscle relaxation without reductions in ser19-MLC phosphorylation. This form of relaxation has been termed force suppression and appears to be associated with heat shock protein 20 (HSP20) phosphorylation on ser16. We investigated whether hypoxia-induced sustained dilation in swine coronary arteries was promoted without ser19-MLC dephosphorylation and associated with ser16-HSP20 phosphorylation. Nitroglycerin vasodilation served as control.

METHODS

In a pressure myograph, the tunica media of intact pre-contracted (PGF(2alpha); 10(-5) m) porcine coronary artery segments were cannulated using a microdialysis catheter. Diameter responses and interstitial lactate/pyruvate ratios were studied during 90 min hypoxia, hypoxia + reoxygenation (60 min), nitroglycerin (100 microm, 90 min), and nitroglycerin + wash-out (60 min). The arterial segments were snap-frozen and analysed for ser16-HSP20 phosphorylation and ser19-MLC phosphorylation.

RESULTS

The normalized diameter responses to hypoxia (6.1 +/- 4.3%) and nitroglycerin (12.6 +/- 1.6%) were both significantly greater than normoxic control arteries (-10.5 +/- 1.8%, anova, P < 0.05). Ser16-HSP20 phosphorylation was increased with hypoxia and nitroglycerin treatment and ser16-HSP20 phosphorylation correlated with changes in diameters (n = 29, r2 = 0.64, P < 0.001). Ser19-MLC phosphorylation was not significantly altered by hypoxia. The lactate/pyruvate ratio was significantly increased in hypoxic arteries but did not correlate with diameters or ser16-HSP20 phosphorylation.

CONCLUSION

Ser16-HSP20 phosphorylation is a potential regulator of hypoxia-induced dilation in coronary arteries.

Heat shock protein modulation of KATPand KCachannel cerebrovasodilation after brain injury

Fluid percussion brain injury (FPI) impairs pial artery dilation to activators of the ATP-sensitive (KATP) and calcium-activated (KCa) K+channels. This study investigated the role of heat shock protein (HSP) in the modulation of K+channel-induced pial artery dilation after FPI in newborn pigs equipped with a closed cranial window. Under nonbrain injury conditions, topical coadministration of exogenous HSP-27 (1 μg/ml) blunted dilation to cromakalim, CGRP, and NS-1619 (10−8and 10−6M; cromakalim and CGRP are KATPagonists and NS-1619 is a KCaagonist). In contrast, coadministration of exogenous HSP-70 (1 μg/ml) potentiated dilation to cromakalim, CGRP, and NS-1619. FPI increased the cerebrospinal fluid (CSF) concentration of HSP-27 from 0.051 ± 0.012 to 0.113 ± 0.035 ng/ml but decreased the CSF concentration of HSP-70 from 50.42 ± 8.96 to 30.9 ± 9.9 ng/ml at 1 h postinsult. Pretreatment with topical exogenous HSP-70 (1 μg/ml) before FPI fully blocked injury-induced impairment of cromakalim and CGRP dilation and partially blocked injury-induced impairment of dilation to NS-1619. These data indicate that HSP-27 and HSP-70 contribute to modulation of K+channel-induced pial artery dilation. These data suggest that HSP-70 is an endogenous protectant of which its actions may be unmasked and/or potentiated with exogenous administration before brain injury.

Heat shock protein 20-mediated force suppression in forskolin-relaxed swine carotid artery

Increases in cyclic nucleotide levels induce smooth muscle relaxation by deactivation [reductions in myosin regulatory light chain (MRLC) phosphorylation (e.g., by reduced [Ca(2+)])] or force suppression (reduction in force without reduction in MRLC phosphorylation). Ser(16)-heat shock protein 20 (HSP20) phosphorylation is the proposed mediator of force suppression. We evaluated three potential hypotheses whereby Ser(16)-HSP20 phosphorylation could regulate smooth muscle force: 1) a threshold level of HSP20 phosphorylation could inactivate a thin filament as a whole, 2) phosphorylation of a single HSP20 could fully inactivate a small region of a thin filament, or 3) HSP20 phosphorylation could weakly inhibit myosin binding at either the thin- or thick-filament level. We tested these hypotheses by analyzing the dependence of force on Ser(16)-HSP20 phosphorylation in swine carotid media. First, we determined that swine HSP20 has a second phosphorylation site at Ser(157). Ser(157)-HSP20 phosphorylation values were high and did not change during contractile activation or forskolin-induced relaxation. Forskolin significantly increased Ser(16)-HSP20 phosphorylation. The relationship between Ser(16)-HSP20 phosphorylation and force remained linear and was shifted downward in partially activated muscles relaxed with forskolin. Neither forskolin nor nitroglycerin induced actin depolymerization as detected using the F/G-actin ratio method in smooth muscle homogenates. These results suggest that force suppression does not occur in accordance with the first hypothesis (inactivation of a thin filament as a whole). Our data are more consistent with the second and third hypotheses that force suppression is mediated by full or partial inhibition of local myosin binding at the thin- or thick-filament level.

Heat induced HSP20 phosphorylation without increased cyclic nucleotide levels in swine carotid media

BACKGROUND

Heat pretreatment of swine carotid artery has been shown to increase ser16-heat shock protein 20 (HSP20) phosphorylation and suppress force, i.e., reduce force with only minimal reduction in ser19-myosin regulatory light chain (MRLC) phosphorylation.

RESULTS

We further investigated this response in intact histamine stimulated swine carotid artery rings. There was a heat threshold such that increased ser16-HSP20 phosphorylation and force suppression were observed between 43 degrees C and 46 degrees C. The increased ser16-HSP20 phosphorylation persisted up to 16 hours after 44.5 degrees C heat treatment. Pretreatment of swine carotid media at 44.5 degrees C increased ser16-HSP20 phosphorylation without increases in [cAMP] or [cGMP], suggesting an alternate mechanism, perhaps phosphatase inhibition, for the increase in ser16-HSP20 phosphorylation. Heat pretreatment at 47.5 degrees C reduced force by decreasing MRLC phosphorylation rather than by large increases in ser16-HSP20 phosphorylation. HSP20 phosphorylation at the putative PKC site did not change with any treatment.

CONCLUSION

These results demonstrate that multiple mechanisms can induce force suppression that is correlated with ser16-HSP20 phosphorylation: 1) nitrovasodilators via cGMP, 2) forskolin via cAMP, and 2) thermal stress in a cyclic nucleotide independent manner.

Heat-induced force suppression and HSP20 phosphorylation in swine carotid media

In vascular smooth muscle, cyclic nucleotide-dependent phosphorylation of heat shock protein 20 (HSP20) on serine-16 (Ser16) has been suggested to cause force suppression, i.e., reduced force with only minimal myosin regulatory light chain (MRLC) dephosphorylation. We hypothesized that heat pretreatment also suppresses force by increasing HSP20 phosphorylation. After heat pretreatment of swine carotid artery at 44.5 degrees C for 4 h and reduction to 37 degrees C for 1 h, Ser16-HSP20 phosphorylation was increased and histamine-induced increases in contractile force were suppressed. Subsequent addition of nitroglycerin induced additive force suppression. Heat and nitroglycerin induced a similar relation between Ser16-HSP20 phosphorylation and force. Heat pretreatment induced a small, but significant, increase in total HSP20 immunostaining. These results demonstrate that vascular smooth muscle responds to thermal stress by increasing Ser16-HSP20 phosphorylation in addition to a possible small increase in total HSP20 concentration. The resulting heat-induced reduction in force should be considered "force suppression" because histamine-induced increases in MRLC phosphorylation were not significantly altered by heat pretreatment. These processes may bring about a resistance to contractile agonists, which could have clinical significance in conditions such as hyperthermia and/or sepsis with vasodilatory shock.

Activation of p38 mitogen-activated protein kinases by endothelin and noradrenaline in small arteries, regulation by calcium influx and tyrosine kinases, and their role in contraction

Small-artery responses to vasoconstrictor agonists are important for vascular function. To investigate the signaling pathways involved in contraction, we studied the activation and regulation of p38 mitogen-activated protein kinases (p38MAPKs) and heat shock protein (HSP) kinase by endothelin and noradrenaline in rat mesenteric arteries. Both vasoconstrictors activated p38alpha and/or p38beta but not p38gamma or p38delta, leading to increased HSP kinase activity. p38MAPK activation by noradrenaline was maximum between 2 and 10 minutes and was wholly dependent on calcium influx but insensitive to the tyrosine kinase inhibitor herbimycin A. In contrast, endothelin induced a biphasic response, with activation at 2 and 10 minutes. The early activity was wholly dependent on calcium influx and inhibited by herbimycin A. The later activity was only 50% calcium dependent, was insensitive to herbimycin A, but was 50% inhibited by genistein, a nonselective tyrosine kinase inhibitor. With both agonists, p38MAPK activity returned to basal by 30 minutes. SB203580, a p38MAPK inhibitor, blocked agonist-induced HSP kinase activity, and herbimycin A inhibited activation by endothelin but not by noradrenaline. In addition, SB203580 inhibited noradrenaline-induced contraction but had little effect on contraction to endothelin. These data show that vasoconstrictors use different upstream activators of p38MAPK in vascular tissue and that the p38MAPK pathway is selectively implicated in the contractile response to noradrenaline in small arteries.

Selected contribution: HSP20 phosphorylation in nitroglycerin- and forskolin-induced sustained reductions in swine carotid media tone

Cyclic nucleotide-induced relaxation of maximally activated arterial smooth muscle has two phases. 1) The initial relaxation transient is typically characterized by a rapid reduction in force associated with brief reductions in myoplasmic Ca(2+) concentration ([Ca(2+)](i)) and myosin regulatory light chain (MRLC) phosphorylation on serine (Ser)-19 (Ser(19)). 2) The sustained inhibitory response is typically associated with Ser(16) phosphorylation of heat shock protein 20 (HSP20) without sustained reductions in [Ca(2+)](i) or MRLC phosphorylation. We investigated whether the extent of Ser(16)-HSP20 phosphorylation quantitatively correlated with the sustained inhibitory response. With addition of nitroglycerin to histamine-stimulated swine carotid media, the initial relaxation transient was associated with a decrease in MRLC phosphorylation without an increase in Ser(16)-HSP20 phosphorylation. During the sustained phase of nitroglycerin-induced relaxation and during force redevelopment induced by washout of nitroglycerin in the continued presence of histamine, the level of Ser(16)-HSP20 phosphorylation, but not MRLC phosphorylation, correlated with inhibition of force. Forskolin, which increases cAMP concentration, also induced a sustained inhibitory response that was associated with increases in Ser(16)-HSP20 phosphorylation without reductions in MRLC phosphorylation levels. Forskolin increased Ser(16)-HSP20 phosphorylation to a greater extent and inhibited force more completely than that observed with nitroglycerin. Increases in Ser(16)-HSP20 phosphorylation correlated with the degree of force inhibition regardless of whether the relaxation was induced by nitroglycerin or forskolin. These data are consistent with the hypothesis that Ser(16)-HSP20 phosphorylation may be a cyclic nucleotide-dependent, yet MRLC phosphorylation-independent, inhibitor of smooth muscle contractile force.