Smooth-muscle caldesmon phosphatase is SMP-I, a type 2A protein phosphatase

Protein phosphatases 1 and 2A and their naturally occurring inhibitors: current topics in smooth muscle physiology and chemical biology

Protein phosphatases 1 and 2A (PP1 and PP2A) are the most ubiquitous and abundant serine/threonine phosphatases in eukaryotic cells. They play fundamental roles in the regulation of various cellular functions. This review focuses on recent advances in the functional studies of these enzymes in the field of smooth muscle physiology. Many naturally occurring protein phosphatase inhibitors with different relative PP1/PP2A affinities have been discovered and are widely used as powerful research tools. Current topics in the chemical biology of PP1/PP2A inhibitors are introduced and discussed, highlighting the identification of the gene cluster responsible for the biosynthesis of calyculin A in a symbiont microorganism of a marine sponge.

Interconnected network motifs control podocyte morphology and kidney function

Podocytes are kidney cells with specialized morphology that is required for glomerular filtration. Diseases, such as diabetes, or drug exposure that causes disruption of the podocyte foot process morphology results in kidney pathophysiology. Proteomic analysis of glomeruli isolated from rats with puromycin-induced kidney disease and control rats indicated that protein kinase A (PKA), which is activated by adenosine 3',5'-monophosphate (cAMP), is a key regulator of podocyte morphology and function. In podocytes, cAMP signaling activates cAMP response element-binding protein (CREB) to enhance expression of the gene encoding a differentiation marker, synaptopodin, a protein that associates with actin and promotes its bundling. We constructed and experimentally verified a β-adrenergic receptor-driven network with multiple feedback and feedforward motifs that controls CREB activity. To determine how the motifs interacted to regulate gene expression, we mapped multicompartment dynamical models, including information about protein subcellular localization, onto the network topology using Petri net formalisms. These computational analyses indicated that the juxtaposition of multiple feedback and feedforward motifs enabled the prolonged CREB activation necessary for synaptopodin expression and actin bundling. Drug-induced modulation of these motifs in diseased rats led to recovery of normal morphology and physiological function in vivo. Thus, analysis of regulatory motifs using network dynamics can provide insights into pathophysiology that enable predictions for drug intervention strategies to treat kidney disease.

Role of serine-threonine phosphoprotein phosphatases in smooth muscle contractility

The degree of phosphorylation of myosin light chain 20 (MLC20) is a major determinant of force generation in smooth muscle. Myosin phosphatases (MPs) contain protein phosphatase (PP) 1 as catalytic subunits and are the major enzymes that dephosphorylate MLC20. MP regulatory targeting subunit 1 (MYPT1), the main regulatory subunit of MP in all smooth muscles, is a key convergence point of contractile and relaxatory pathways. Combinations of regulatory mechanisms, including isoform splicing, multiple phosphorylation sites, and scaffolding proteins, modulate MYPT1 activity with tissue and agonist specificities to affect contraction and relaxation. Other members of the PP1 family that do not target myosin, as well as PP2A and PP2B, dephosphorylate a range of proteins that affect smooth muscle contraction. This review discusses the role of phosphatases in smooth muscle contractility with a focus on MYPT1 in uterine smooth muscle. Myometrium shares characteristics of vascular and other visceral smooth muscles yet, during healthy pregnancy, undergoes hypertrophy, hyperplasia, quiescence, and labor as physiological processes. Myometrium presents an accessible model for the study of normal and pathological smooth muscle function, and a better understanding of myometrial physiology may allow the development of novel therapeutics for the many disorders of myometrial physiology from preterm labor to dysmenorrhea.

Regulation of vascular endothelial cell barrier function and cytoskeleton structure by protein phosphatases of the PPP family

Reversible phosphorylation of cytoskeletal and cytoskeleton-associated proteins is a significant element of endothelial barrier function regulation. Therefore, understanding the mechanisms of phosphorylation/dephosphorylation of endothelial cell cytoskeletal proteins is vital to the treatment of severe lung disorders such as high permeability pulmonary edema. In vivo, there is a controlled balance between the activities of protein kinases and phosphatases. Due to various external or internal signals, this balance may be shifted. The actual balances at a given time alter the phosphorylation level of certain proteins with appropriate physiological consequences. The latest information about the structure and regulation of different types of Ser/Thr protein phosphatases participating in the regulation of endothelial cytoskeletal organization and barrier function will be reviewed here.

Molecular mechanisms involved in development of cerebral vasospasm

Object

Although the agents responsible for production of vasospasm have not yet been clearly identified, the author reviews the molecular mechanisms involved in development of vasospasm mainly based on the experimental data in a canine two-hemorrhage model.

Methods

The blood products after subarachnoid hemorrhage most likely stimulate many cell membrane receptors, such as G protein–coupled receptors and receptor tyrosine kinases, to activate the tyrosine kinase pathway of the vascular smooth muscle cells. The activation of the tyrosine kinase pathway is associated with continuous elevation of intracellular Ca++levels and activation of μ-calpain; the former may result mainly not from Ca++release but from Ca++influx from outside the cells. The increased intracellular Ca++concentrations stimulate Ca++/calmodulin (CaM)–dependent myosin light chain kinase to phosphorylate myosin light chain continuously during vasospasm. A topical application of genistein, ethylene-glycol-bis(β-aminoethylether) N,N'-tetraacetic acid, or various L-type Ca++channel blockers likely induces reversal of vasospasm as a result of a decrease in intracellular Ca++levels. The blood products also activate the rho/rho-associated kinase pathway during vasospasm most likely via G protein–coupled receptors, and the activated rho-associated kinase inhibits myosin phosphatase through phosphorylation at its myosin-binding subunit to induce Ca++-independent development of vasospasm. The enhanced generation of arachidonic acid during vasospasm may also contribute to inhibition of myosin phosphatase, at least in part, through the rho/rho-associated kinase pathway. The activity of myosin phosphatase in vasospam can also be inhibited by activated protein kinase C independently of the rho/rho-associated kinase pathway, but the inhibition may play a minor and transient role in contractile regulation. The protein levels of thin filament–associated proteins, calponin and caldesmon, are progressively decreased in vasospasm, whereas their phosphorylation levels are increased. Both changes probably contribute to the enhancement of smooth muscle contractility. Contractile and cytoskeletal proteins appear to be degraded in vasospasm by proteolysis with activated μ-calpain, suggesting that the intracellular devices responsible for smooth-muscle contraction are severely degraded in vasospasm.

Conclusions

It remains to be determined the extent to which Ca++-dependent and -independent contractile regulations, proteolysis and phosphorylation of thin filament–associated proteins, and degradation of contractile and cytoskeletal proteins are involved in the development of vasospasm.

C2-ceramide attenuates phenylephrine-induced vasoconstriction and elevation in [Ca2+]i in rat aortic smooth muscle

In the present study, we examined the effects of cell-permeable C2-ceramide on contraction of aortic smooth muscle and intracellular free Ca2+ ([Ca2+]i). C2-ceramide (10(-7) to 10(-4) M) alone did not elicit any significant changes in either basal tension or resting levels of [Ca2+]i in rat aortic smooth muscle. However, C2-ceramide (10(-7) to 10(-4) M) attenuated phenylephrine-induced contractions in isolated rat aortic rings in a concentration-related manner, and inhibited elevations in [Ca2+]i in cultured rat aortic smooth muscle cells induced by phenylephrine. C2-ceramide-induced relaxation was found to be only slightly endothelium-dependent. However, nitric oxide inhibitors (L-NNA, L-NMMA), an inhibitor of prostanoid synthesis (indomethacin), an inhibitor of opiate actions, and several inhibitors of the pharmacologic actions of various vasoactive amines all failed to interfere with the vasorelaxant responses of C2-ceramide. Three different inhibitors of protein kinase C, when used in a wide concentration range, also failed to interfere with the ceramide-induced relaxations. Our results suggest that the sphingomyelin-signaling pathway may play an important regulatory role in arterial wall tone.

C2-ceramide attenuates prostaglandin F2alpha-induced vasoconstriction and elevation of [Ca2+]i in canine cerebral vascular smooth muscle

Sphingolipids have emerged as important components of signal transduction pathways involved in a variety of cellular processes. In the present study, we examined the effects of C2-ceramide, a cell-permeable sphingolipid, on contraction of canine cerebral vascular smooth muscle and intracellular free Ca2+ ([Ca2+]i). C2-ceramide (10(-8)-10(-4) M) alone did not elicit any significant changes in either basal tension or resting levels of [Ca2+]i in canine cerebrovascular muscle. However, C2-ceramide (10(-7)-10(-4) M) attenuated prostaglandin F2alpha (PGF2alpha)-induced contractions in isolated canine cerebrovascular smooth muscle rings. C2-ceramide (10(-5) M) inhibited the secondary phasic rise of [Ca2+]i evoked by PGF2alpha in cultured canine cerebral vascular smooth muscle cells, resulting in decreases in the elevation in [Ca2+]i. NO inhibitors (L-NNA, L-NMMA), an inhibitor of prostanoid synthesis (indomethacin), an inhibitor of opiate actions and several inhibitors of the pharmacologic actions of various vasoactive amines all failed to interfere with the vasorelaxant response of C2-ceramide. Our results suggest that the sphingomyelin signaling pathway may play an important regulatory role in cerebral arterial wall tone.

Activity of smooth muscle phosphatases 1 and 2A in rabbit basilar artery in vasospasm

BACKGROUND AND PURPOSE

Subarachnoid hemorrhage frequently leads to a long-term cerebral artery narrowing called vasospasm. Recently, the involvement of myosin light chain kinase has been found in experimental vasospasm in our laboratory. We therefore measured the activity of serine/threonine protein phosphatases 1 and 2A in the rabbit basilar artery in vasospasm and in vasocontraction to study their role, particularly in regard to vasospasm compared with vasocontraction.

METHODS

Vasospasm was produced in the rabbit basilar artery by a two-hemorrhage method. Vasocontraction was induced by local application of KCl or serotonin to the rabbit basilar artery after a transclival exposure. The control animals were treated with saline instead of fresh blood. Serine/threonine protein phosphatase activity in the basilar artery was assayed with the use of [32P]phosphorylase-a as a substrate; protein phosphatase 1 activity was evaluated as protein phosphatase activity in the presence of 1 nmol/L okadaic acid, whereas protein phosphatase 2A activity was assessed as protein phosphatase activity inhibited by 1 nmol/L okadaic acid.

RESULTS

Values of mean activity of protein phosphatase 1 in myofibrillar extract were 3.58 +/- 0.26 nmol/min per milligram in the control group, 3.22 +/- 0.12 nmol/min per milligram in the spastic group on day 2, and 3.01 +/- 0.16 nmol/min per milligram in the spastic group on day 4 (a significant decrease in protein phosphatase 1 activity in the spastic group on days 2 and 4). In contrast, these values did not show any significant changes in the KCl and serotonin groups. Values of mean activity of protein phosphatase 2A in cytosolic extract were 0.90 +/- 0.07 nmol/min per milligram in the control group, 0.75 +/- 0.10 nmol/min per milligram in the spastic group on day 2, and 0.62 +/- 0.17 nmol/min per milligram in the spastic group on day 4 (a significant reduction in protein phosphatase 2A in the spastic group on days 2 and 4). There was no evidence of significant changes of protein phosphatase 2A in cytosolic extract in the KCl and serotonin groups.

CONCLUSIONS

Protein phosphatase 1 in myofibrillar extract is reported to catalyze the dephosphorylation of myosin light chain and calponin, whereas protein phosphatase 2A in cytosolic extract catalyzes the dephosphorylation of calponin and caldesmon. In addition, the phosphorylation of calponin and caldesmon results in the loss of their ability to inhibit smooth muscle contraction. Therefore, the significant decrease in activity of protein phosphatases 1 and 2A in vasospasm may result in uninterrupted vascular smooth muscle contraction by the preservation of phosphorylation of not only myosin light chain but also calponin and caldesmon.

Phosphorylation of caldesmon by mitogen-activated protein kinase with no effect on Ca2+ sensitivity in rabbit smooth muscle

1. Recombinant, activated mitogen-activated protein kinase (3.3 microM; p42mapk) phosphorylated caldesmon in phasic (rabbit portal vein) and tonic (rabbit femoral artery) smooth muscle strips permeabilized with Triton X-100. 2. Phosphorylation of caldesmon by p42mapk neither induced contraction of relaxed smooth muscle nor affected the Ca2+ sensitivity of submaximally contracted permeabilized phasic or tonic smooth muscle.

A retinoic acid-induced clonal cell line derived from multipotential P19 embryonal carcinoma cells expresses smooth muscle characteristics

Despite intense interest in understanding the differentiation of vascular smooth muscle, very little is known about the cellular and molecular mechanisms that control differentiation of this cell type. Progress in this field has been hampered by the lack of an inducible in vitro system for study of the early steps of smooth muscle differentiation. In this study, we describe a model system in which multipotential mouse P19 embryonal carcinoma cells (P19s) can be induced to express multiple characteristics of differentiated smooth muscle. Treatment of P19s with retinoic acid was associated with profound changes in cell morphology and with the appearance at high frequency of smooth muscle alpha-actin-positive cells that were absent or present at extremely low frequency in parental P19s. A clonal line derived from retinoic acid-treated P19s (9E11G) stably expressed multiple characteristics of differentiated smooth muscle, including smooth muscle-specific isoforms of alpha-actin and myosin heavy chain, as well as functional responses to the contractile agonists phenylephrine, angiotensin II, ATP, bradykinin, histamine, platelet-derived growth factor (PDGF)-AA, and PDGF-BB. Additionally, 9E11G cells expressed transcripts for MHox, a muscle homeobox gene expressed in smooth, cardiac, and skeletal muscles, but not the skeletal muscle-specific regulatory factors, MyoD and myogenin. Results demonstrate that retinoic acid treatment of multipotential P19 cells is associated with formation of cell lines that stably express multiple properties of differentiated smooth muscle.(ABSTRACT TRUNCATED AT 250 WORDS)

Phosphorylation of caldesmon by smooth-muscle casein kinase II

A caldesmon kinase activity was partially purified from an extract of chicken gizzard smooth muscle by sequential chromatography on columns of DEAE-Sephacel, MonoQ and Superose 12. This kinase was identified as casein kinase II by Western blotting using peptide-directed antibodies raised against the alpha, alpha' and beta subunits of human casein kinase II; the smooth muscle enzyme consisted of similar subunits of M(r) 43,000 (alpha), 39,000 (alpha'), and 27,000 (beta). Phosphorylation of caldesmon and casein by smooth muscle casein kinase II was optimal at approximately 0.1 M NaCl, did not require second messengers, and was inhibited by heparin. The kinase utilized either GTP or ATP as a substrate. Caldesmon was phosphorylated to approximately 1 mol Pi mol-1 caldesmon by smooth muscle casein kinase II with a Km for caldesmon of 4.9 microM. Two-dimensional thin-layer electrophoresis indicated phosphate incorporation into both serine and threonine. All the incorporated phosphate was recovered in the N-terminal peptide (residues 1-152) generated by cleavage at cysteine 153 with 2-nitro-5-thiocyanobenzoic acid. Purification of tryptic phosphopeptides and N-terminal sequencing revealed two principal sites of phosphorylation: serine 73 and threonine 83. The following four synthetic peptides corresponding to this domain of caldesmon were examined as substrates of casein kinase II: A = RRREVNAQNSVAEEE; B = AQNSVAEEE; C = RSTDDEAA; D = SVAEEETKRSTDDE. Interestingly, only peptides C and D were phosphorylated and both only at threonine. Phosphorylation of intact caldesmon did not affect the pattern of chymotryptic digestion suggesting that it does not induce a significant conformational change in the protein substrate. Phosphorylation also had no effect on the binding of caldesmon to actin or on the caldesmon-mediated inhibition of actomyosin MgATPase activity. However, phosphorylation completely abolished the interaction of caldesmon with immobilized smooth muscle myosin. These results are consistent with the localization of the myosin-binding domain near the N-terminus of caldesmon and of the actin-binding domain near the opposite end of the elongated molecule. Casein kinase II may therefore play a role in regulating caldesmon-myosin interaction and the ability of caldesmon to cross-link actin and myosin filaments in smooth muscle.