Effects of mutations in the gamma-phosphate binding site of myosin on its motor function

The role of the highly conserved residues in the gamma-phosphate binding site of myosin upon myosin motor function was studied. Each of five residues (Ser181, Lys185, Asn235, Ser236, and Arg238) in smooth muscle myosin was mutated. K185Q has neither a steady state ATPase nor an initial Pi burst. Although ATP and actin bind to K185Q, it is not dissociated from actin by ATP. These results indicate that the hydrolysis of bound ATP by K185Q is inhibited. S236T has nearly normal basal Mg2+-ATPase activity, initial Pi burst, ATP-induced enhancement of intrinsic tryptophan fluorescence, and ATP-induced dissociation from actin. However, the actin activation of the Mg2+-ATPase activity and actin translocation of S236T were blocked. In contrast S236A has nearly normal enzymatic properties and actin-translocating activity. These results indicate that 1) the hydroxyl group of Ser236 is not critical as an intermediary of proton transfer during the ATP hydrolysis step, and 2) the bulk of the extra methyl group of the threonine residue in S236T blocks the acceleration of product release from the active site by actin. Arg238, which interacts with Glu459 at the Switch II region, was mutated to Lys and Ile, respectively. R238K has essentially normal enzymatic activity and motility. In contrast, R238I does not hydrolyze ATP or support motility, although it still binds ATP. These results indicate that the charge interaction between Glu459 and Arg238 is critical for ATP hydrolysis by myosin. Other mutants, S181A, S181T, and N235I, showed nearly normal enzymatic and motile activity.

A specific amino acid sequence at the head-rod junction is not critical for the phosphorylation-dependent regulation of smooth muscle myosin

It has been suggested that the structure at the head-rod junction of smooth muscle myosin is important for the phosphorylation-mediated regulation of myosin motor activity. To investigate whether a specific amino acid sequence at the head-rod junction is critical for the regulation, three smooth muscle myosin mutants in which the sequence at the N-terminal end of S2 is deleted to various extents were expressed in Sf9 cells; 28, 56, and 84 amino acid residues, respectively, at the position immediately C-terminal to the invariant proline (Pro849) were deleted, and the S1 domain was directly linked to the downstream sequence of the rod. The mutant myosins were expressed, purified, and biochemically characterized. All three myosin mutants showed a stable double-headed structure based upon electron microscopic observation. Both the actin-activated ATPase activity and the actin translocating activity of the mutants were completely regulated by the phosphorylation of the regulatory light chain. The actin sliding velocity of the three mutant myosins was the same as the wild-type recombinant myosin. These results indicate that a specific amino acid sequence at the head-rod junction is not required for the regulation of smooth muscle myosin. The results also suggest that there is no functionally important interaction between the regulatory light chain and the heavy chain at the head-rod junction.

ADP/vanadate mediated photocleavage of myosin light chain kinase at the autoinhibitory region

The vanadate (Vi)-mediated photocleavage reaction was used to study the interaction between the regulatory segment and the catalytic site of smooth muscle myosin light chain kinase (MLCK). When MLCK was irradiated with long-wave UV (366 nm) in the presence of ADP and Vi, kinase activity was substantially decreased, and the MLCK polypeptide of 130 kDa was cleaved into several smaller fragments with apparent molecular masses of 100, 70, 60, 32, and 28 kDa. Inhibition of kinase activity and photocleavage were both competitively antagonized by the addition of ATP. Inconsistency between the observed maximum levels of UV-induced inhibition of MLCK-mediated phosphorylation (80%) and photocleavage (15-20%) suggested that the photocleavage reaction proceeds as a two-step process. Monoclonal antibodies recognizing the C-terminus of MLCK labeled the 60- and 28-kDa fragments, indicating that MLCK was cleaved at two sites, at 28 and 60 kDa from the C-terminus, within what are believed to be the autoinhibitory region and the catalytic site, respectively. Moreover, Ca2+-calmodulin (Ca2+-CaM) protected against cleavage at the site at 28 kDa from the C-terminus. Analysis of the amino acid composition of the fragment revealed that the cleavage site at 28 kDa from C-terminus occurred at Lys 799 +/- 3 amino acid residues, which is in a region where the CaM-binding and pseudosubstrate regions overlap. These results suggest that the three-dimensional structure of MLCK brings the regulatory segment into direct contact with the ATP-binding site. Moreover, the binding of Ca2+-CaM displaces the regulatory segment away from the catalytic site.

Identification and localization of caldesmon in cardiac muscle

Caldesmon has been detected in smooth muscle and in a number of non-muscle cells. It binds both actin and myosin and may act as a regulator of contraction or a structural element in smooth muscle. The presence of caldesmon in striated muscle has not been well established. To address this issue, polyclonal antibodies and a panel of monoclonal antibodies were raised against chicken gizzard smooth muscle caldesmon and used to demonstrate that caldesmon is present in adult cardiac muscle of a variety of mammalian species. Western-blot analysis revealed the presence of caldesmon in ventricular myocytes isolated from rat heart. The epitopes for the individual monoclonal antibodies were mapped to the caldesmon primary structure using chymotryptic and 2-nitro-5-thiocyanatobenzoic acid fragments. Bovine and rat cardiac caldesmons were recognized only by a subset of these monoclonal antibodies, indicating primary sequence differences from the chicken smooth muscle protein. Immunofluorescence labelling of isolated myocytes from rat, rabbit and guinea pig cardiac muscle revealed a striated pattern of fluorescence labelling. Dual labelling of caldesmon and myosin or caldesmon and alpha-actinin demonstrated that caldesmon was present at the centre of the I-band rather than in the A-band, as might have been expected from the myosin binding properties of the smooth muscle protein. These results suggest a structural role for caldesmon in cardiac muscle cells.

Characterization of the interaction of myosin with ATP analogues having the syn conformation with respect to the adenine-ribose bond

Numerous analytical experiments have shown that, in solution, ATP analogues with bulky substitutions at the eighth position of the adenine ring predominantly assume the syn conformation with respect to the adenine-ribose bond. Two such analogues, 3'-O-(N-methylanthraniloyl)-8-azido-ATP (Mant-8-N3-ATP) and 8-Br-ATP, were synthesized and used to probe the conformation of the ATP-binding site of myosin. In the presence of these analogues, actomyosin was rapidly dissociated; Mg2+-dependent ATP hydrolysis was significantly activated by actin; and Pi bursting was observed. For skeletal myosin, however, these analogues failed to support actin translocation, and they did not significantly enhance the intrinsic tryptophan fluorescence of skeletal muscle myosin subfragment-1 (SKE S-1). These results suggest that although myosin**/ADP/Pi intermediates can be formed with these analogues, the crucial conformational changes required for cross-bridge cycling do not occur in skeletal muscle myosin. The conformations of the ATP-binding sites of skeletal and smooth-muscle myosin were compared using the ternary complexes, myosin-ADP-beryllium fluoride (BeFn) or myosin-ADP-aluminium fluoride (AIF4-). In AlF4- complexes, Mant-8-N3-ADP affinity labeled the N-terminal 29-kDa domain of smooth-muscle myosin subfragment-1 (SM S-1), as did ATP analogues having the anti conformation, whereas it labeled the C-terminal 20-kDa domain of skeletal S-1. In smooth muscle BeFn complexes, Mant-8-N3-ADP was equally likely to cross-link to the 29-kDa N-terminal and the 25-kDa C-terminal domains. These analogues induced smooth muscle actomyosin super-precipitation and increased intrinsic tryptophan fluorescence to the same degree as ATP itself. As was expected from above results, the analogues supported smooth-muscle-myosin-induced actin translocation. These results suggest that smooth-muscle myosin adopts the eight-substituted ATP analogue in the normal conformation, but skeletal muscle myosin does not. This reflects the likely differences in the structures of their respective ATPase sites.

High affinity Ca2+ binding sites of calmodulin are critical for the regulation of myosin Ibeta motor function

We coexpressed myosin Ibeta heavy chain with three different calmodulin mutants in which the two Ca2+-binding sites of the two N-terminal domain (E12Q), C-terminal domain (E34Q), or all four sites (E1234Q) are mutated in order to define the importance of these Ca2+ binding sites to the regulation of myosin Ibeta. The calmodulin mutated at the two Ca2+ binding sites in N-terminal domain and C-terminal domain lost its lower affinity Ca2+ binding site and higher affinity Ca2+ binding site, respectively. We found that, based upon the change in the actin-activated ATPase activities and actin translocating activities, myosin Ibeta with E12Q calmodulin has the regulatory characteristics similar to myosin Ibeta containing wild-type calmodulin, while myosin Ibeta with E34Q or E1234Q calmodulin lose all Ca2+ regulation. While the increase in myosin Ibeta ATPase activity paralleled the dissociation of 1 mol of calmodulin from myosin Ibeta heavy chain for both wild type (above pCa 5) and E12Q calmodulin (above pCa 6), the Ca2+ level required for the inhibition of actin-translocating activity of myosin Ibeta was lower than that required for dissociation of calmodulin, suggesting that the conformational change induced by the binding of Ca2+ at the high affinity site but not the dissociation of calmodulin is critical for the inhibition of the motor activity. Our results suggest that the regulation of unconventional myosins by Ca2+ is directly mediated by the Ca2+ binding to calmodulin, and that the C-terminal pair of Ca2+-binding sites are critical for this regulation.

Biologically active peptides caged on tyrosine

We have demonstrated the feasibility of preparing caged peptides by derivatizing a single amino acid side chain in peptides up to 20 amino acids long. Two peptides are illustrated whose activities are reduced by nearly 2 orders of magnitude using this caging approach. The specific strategy described here of derivatizing tyrosine side chains with a charged caging moiety should be generally applicable in the preparation of caged peptides that have a critical tyrosine residue (e.g., LSM1) or that have critical hydrophobic patches (e.g., RS-20). Other amino acid side chains are also accessible via this caging strategy. Derivatives of threonine, serine, lysine, cysteine, glutamate, aspartate, glutamine, and asparagine can be prepared and site specifically inserted into peptides in an analogous manner. The caged peptides synthesized and purified by the methods described here are compatible with biological samples, including living cells, and have been used to demonstrate the central importance of calmodulin, MLCK, and, by inference, myosin II in ameboid locomotion in polarized eosinophil cells. Photoactivation of peptides within cells should provide a wealth of new information in future investigations by allowing specific protein activities to be knocked out in an acute and spatially defined way.

A hinge at the central helix of the regulatory light chain of myosin is critical for phosphorylation-dependent regulation of smooth muscle myosin motor activity

The motor function of smooth muscle myosin is activated by phosphorylation of the regulatory light chain (RLC) at Ser19. However, the molecular mechanism by which the phosphorylation activates the motor function is not yet understood. In the present study, we focused our attention on the role of the central helix of RLC for regulation. The flexible region at the middle of the central helix (Gly95-Pro98) was substituted or deleted to various extents, and the effects of the deletion or substitution on the regulation of the motor activity of myosin were examined. Deletion of Gly95-Asp97, Gly95-Thr96, or Thr96-Asp97 decreased the actin-translocating activity of myosin a little, but the phosphorylation-dependent regulation of the motor activity was not disrupted. In contrast, the deletion of Gly95-Pro98 of RLC completely abolished the actin translocating activity of phosphorylated myosin. However, the unregulated myosin long subfragment 1 containing this RLC mutant showed motor activity the same as that containing the wild type RLC. Since long subfragment 1 motor activity is unregulated by phosphorylation, i.e. constitutively active, these results suggest that the deletion of these residues at the central helix of RLC disrupts the phosphorylation-mediated activation mechanism but not the motor function of myosin itself. On the other hand, the elimination of Pro98 or substitution of Gly95-Pro98 by Ala resulted in the activation of actin translocating activity of dephosphorylated myosin, whereas it did not affect the motor activity of phosphorylated myosin. Together, these results clearly indicate the importance of the hinge at the central helix of RLC on the phosphorylation-mediated regulation of smooth muscle myosin.

Regulation of the renal Na/K/2Cl cotransporter gene. Physiological modulation in health and abnormal function in disease

The recently cloned bumetanide-sensitive Na/K/2Cl cotransporters (ENCC2-3) and thiazide-sensitive Na/Cl cotransporter (ENCC1) have important roles in sodium chloride absorption and secretion, cell volume regulation, ammonium accumulation in medullary interstitium, and ammonium excretion. Genetic mutations of bumetanide-sensitive Na/K/2Cl cotransporters and thiazide-sensitive Na/Cl cotransporter cause the hereditary hypokalemic alkalosis of some Bartter's syndrome and Gitelman's syndrome, respectively. The transporters are sensitive to diuretics and several hormones. Dehydration and acidosis influence the gene expression of ENCC3 but not of ENCC1-2. In contrast, sodium loading increases ENCC2 protein expression in the outer medulla.

Analysis of stress in the active site of myosin accompanied by conformational changes in transient state intermediate complexes using photoaffinity labeling and 19F-NMR spectroscopy

Myosin forms stable ternary complexes with ADP and the phosphate analogues, fluoroaluminate (Al F4-), fluoroberyllate (BeFn) or orthovanadate (Vi); these ternary complexes mimic transient intermediates in the myosin ATPase cycle. Moreover, we previously demonstrated that these complexes may mimic different myosin ATPase reaction intermediates corresponding to separate steps in the cross-bridge cycle [Maruta, S., Henry, G. D., Sykes, B. D. & Ikebe, M. (1993) J. Biol. Chem. 268, 7093-7100]. Park et al. suggested that the changing conformation of ATP during hydrolysis stresses the active site of myosin subfragment-1 (S-1) through protein-nucleotide contacts at the gamma-phosphate and nucleotide base, and the stress-induced strain in the cross-bridge may be the mechanism by which energy in ATP is transferred to the myosin structure [Park, S., Ajtai, K. & Burghardt, T. P. (1997) Biochemistry 36, 3368-3372]. In the present study, the photoactive ADP analogue, 3'-O-(N-methylanthraniloyl)-2-azido-ADP (Mant-2-N3-ADP), and the 19F-labeled ADP analogue, 2-[(trifluoromethylnitrophenyl)aminoethyl]diphosphate, were employed to examine conformational differences in protein-nucleotide contact in the ATP-binding site that may correlate with energy transduction. Mant-2-N3-ADP was trapped within the active site of skeletal and smooth muscle myosin in the presence of AlF4-, BeFn or Vi. For both skeletal and smooth muscle myosins, trapped Mant-2-N3-ADP was covalently linked to the 25-kDa N-terminal fragment of S-1 of both myosin/Mant-2-N3-ADP/AlF4- and BeFn complexes, presumably at Trp130. However, the efficiency of the incorporation was much higher for skeletal than for smooth muscle myosin suggesting that the conformations of the adenine-binding pockets of the two myosins are somewhat different. Although the amount of Mant-2-N3-ADP trapped in the presence of AlF4- and BeFn was the same for both myosins, the efficiency of photolabeling skeletal muscle myosin was approximately two times higher for BeFn complex than for AlF4- complex. The 19F-NMR spectra of the bound 2-[(trifluoromethylnitrophenyl)aminoethyl]diphosphate in the ternary complexes formed in the presence of AlF4-, BeFn or Vi showed small but distinguishable differences. Taken together, these results indicate that there is some variation in the protein-nucleotide contacts at the nucleotide base among the ternary complexes studied, and these differences mimic separate steps occurring transiently during the contractile cycle.

Myosin II activation promotes neurite retraction during the action of Rho and Rho-kinase

BACKGROUND

The Rho small GTPase regulates myosin II activity through the phosphorylation of the myosin light chain (MLC) by activating Rho-kinase, which is a target of Rho. Several lines of evidence point to an important role of Rho in the action of lysophosphatidic acid (LPA) and thrombin in provoking neurite retraction in N1E-115 neuroblastoma cells.

RESULTS

Here we examined whether Rho-kinase and myosin II are involved in neurite retraction in N1E-115 cells. We showed that the expression of constitutively active forms of Rho-kinase induced neurite retraction in NIE-115 cells and MLC phosphorylation in NIH 3T3 cells, whereas the expression of dominant negative forms of Rho-kinase inhibited the LPA-induced neurite retraction in N1E-115 cells and the serum-induced MLC phosphorylation in NIH 3T3 cells. The expression of mutant MLCT18D,S19D (substitution of Thr and Ser by Asp), which is known to lead to the activation of myosin ATPase and a conformational change of myosin II when reconstituted with myosin heavy chains in vitro, also promoted neurite retraction.

CONCLUSION

These results indicate that Rho-kinase is involved in the LPA-induced neurite retraction downstream of Rho, and that myosin II activation promotes neurite retraction downstream of Rho and Rho-kinase.

Signaling pathways underlying eosinophil cell motility revealed by using caged peptides

Insights into structure-function relations of many proteins opens the possibility of engineering peptides to selectively interfere with a protein's activity. To facilitate the use of peptides as probes of cellular processes, we have developed caged peptides whose influence on specific proteins can be suddenly and uniformly changed by near-UV light. Two peptides are described which, on photolysis of a caging moiety, block the action of calcium-calmodulin or myosin light chain kinase (MLCK). The efficacy of theses peptides is demonstrated in vitro and in vivo by determining their effect before and after photolysis on activities of isolated enzymes and cellular functions known to depend on calcium-calmodulin and MLCK. These caged peptides each were injected into motile, polarized eosinophils, and when exposed to light promptly blocked cell locomotion in a similar manner. The results indicate that the action of calcium-calmodulin and MLCK, and by inference myosin II, are required for the ameboid locomotion of these cells. This methodology provides a powerful means for assessing the role of these and other proteins in a wide range of spatio-temporally complex functions in intact living cells.

Fas ligand gene transfer to the vessel wall inhibits neointima formation and overrides the adenovirus-mediated T cell response

Proliferation of vascular smooth muscle cells (VSMCs) in response to injury plays a key role in the pathogenesis of vascular disorders. Fas ligand (FasL) induces apoptosis in Fas-bearing cells, and its expression on activated T cells contributes to the regulation of the immune response and physiological cell turnover. Here, we show that a replication-defective adenovirus encoding FasL (Ad-FasL) induced apoptosis in Fas-bearing VSMCs. When introduced locally to balloon-injured rat carotid arteries, a well characterized model of a VSMC-derived lesion, Ad-FasL functioned as a potent inhibitor of neointima formation. In rats immunized with an empty adenoviral vector, robust T cell infiltration of the vessel wall was detected after local delivery of a beta-galactosidase-expressing virus (Ad-betagal), whereas T cell infiltrates were not detected after local delivery of Ad-FasL. Prior immunization prevented beta-galactosidase expression from Ad-betagal, whereas the expression of the FasL transgene was unaffected. When Ad-betagal and Ad-FasL were delivered together to preimmunized animals, T cell infiltration was reduced and beta-galactosidase expression was restored. These data demonstrate that Fas ligand gene transfer can effectively inhibit injury-induced vessel lesion formation and can allow adenovirus-harboring cells to evade immune destruction.

Transforming growth factor-beta 1 overexpression produces drug resistance in vivo: reversal by decorin

The gene for transforming growth factor-beta 1 (TGF-beta 1) was transfected into the murine EMT-6/Parent mammary carcinoma tumor line to form the EMT6/PRK5 beta 1E tumor line. In monolayer culture the EMT-6/PRK5 beta 1E tumor line secretes about 15-times as much TGF-beta 1 into the medium as the EMT-6/Parent line. There was no difference in the response of these two cell lines to 4-hydroperoxycyclophosphamide, cisplatin, melphalan or thiotepa in monolayer culture. When the EMT-6/PRK5 beta 1E cells were grown as a solid tumor in Balb/C mice, plasma levels of TGF-beta 1 were about 5-fold higher than in animals bearing the EMT-6/Parent tumor. The EMT-6/PRK5 beta 1E tumor was markedly resistant to a dosage range of cyclophosphamide, cisplatin, melphalan and thiotepa compared with the EMT-6/Parent tumor. The bone marrow CFU-GM from the animals bearing the EMT-6/PRK5 beta 1E tumor were spared from the cytotoxicity of the drugs compared with the bone marrow CFU-GM from animals bearing the EMT-6/Parent tumor. Administration of decorin, a naturally occurring inhibitor of TGF-beta 1, to animals bearing the EMT-6/PRK5 beta 1E tumor prior to treatment of the animals with the antitumor alkylating agents restored drug sensitivity to the tumor and to the bone marrow CFU-GM. Administration of decorin prior to the antitumor alkylating agents produced very little or no increase in the response of the EMT6/Parent tumor or the bone marrow CFU-GM from those animals. The EMT6/PRK5 beta 1E tumor model allows the effect of secretion of TGF-beta 1 on therapeutic resistance to be assessed directly compared with the EMT-6/Parent tumor. In vivo resistance occurred in the presence of high levels of TGF-beta 1 and was reversed by the TGF-beta 1 inhibitor decorin.

Cytoskeletal rearrangements and transcriptional activation of c-fos serum response element by Rho-kinase

The small GTPase Rho is implicated in cytoskeletal rearrangements including stress fiber and focal adhesion formation and in the transcriptional activation of c-fos serum response element. In vitro, Rho-kinase, which is activated by Rho, phosphorylates not only myosin light chain (MLC) (thereby activating myosin ATPase) but also myosin phosphatase, thus inactivating myosin phosphatase. Rho-kinase is involved in the formation of stress fibers and focal adhesions in fibroblasts. Here we show that the expression of constitutively active Rho-kinase increased the level of MLC phosphorylation. The activity of Rho-kinase was necessary for maintaining the vinculin-containing focal adhesions, whereas organized actin stress fibers were not necessary for this. The microinjection of constitutively active Rho-kinase into fibroblasts induced the formation of focal adhesions to some extent under the conditions where organized actin stress fibers were disrupted. The expression of constitutively active Rho-kinase also stimulated the transcriptional activity of c-fos serum response element. These results suggest that Rho-kinase has distinct roles in divergent pathways downstream of Rho, which include MLC phosphorylation leading to stress fiber formation, focal adhesion formation, and gene expression.

Differential activation of PKC-delta and -zeta by alpha 1-adrenergic stimulation in human airway epithelial cells

Phorbol ester and alpha 1-adrenergic stimulation of Na-Cl-K cotransport in human tracheal epithelial cells was investigated by determining the expression of protein kinase C (PKC) isotypes and their activation by phorbol 12-myristate 13-acetate (PMA) and methoxamine, an alpha 1-adrenergic agonist. PKC-alpha, -beta II, -delta, -epsilon, and -zeta were expressed in confluent cell cultures. PKC-beta I, -gamma, and -eta were not detected. PKC-alpha and -zeta were localized to the cytosol, and PKC-beta II and -delta were distributed approximately evenly between cytosolic and particulate fractions. Treatment with PMA for 30 min increased PKC activity in subcellular fractions and induced a redistribution of PKC-beta II and -delta to a particulate fraction. PMA treatment for 18 h reduced PKC activity to levels found in untreated cells and reduced, but did not deplete, PKC isotype mass. Methoxamine transiently increased PKC activity, with maximal levels at 40 s, and caused a shift in PKC-delta and -zeta mass to a particulate fraction. Methoxamine selectively induced a sustained increase in PKC-zeta activity but only a transient increase in PKC-delta. These results suggest that PKC-delta and -zeta mediate hormonal activation of Na-Cl-K cotransport.

Lymphohaematopoietic abnormalities and systemic lymphoproliferative disorder in interleukin-2 receptor gamma chain-deficient mice

Interleukin-2 (IL-2) receptor gamma chain-deficient mice with a truncated mutation showed the absence or severe reduction of natural killer cells, decreased numbers of T- and B-cells, marked hypoplasia of the thymus and peripheral lymphoid tissues, defective formation of lymphoid follicles and germinal centre in the peripheral lymphoid tissues, and the absence of Peyer's patches in the intestinal mucosa. In addition, marked splenomegaly with extramedullary haematopoiesis, increased level of IgM and decreased levels of IgG and IgE in serum, severe reduction of conventional B cells (B-2) in the peripheral lymphoid tissues, the presence of IgM-producing CD5+ B cells (B-1) and their differentiation into plasma cells and Motto cells in the spleen, and increased production and differentiation of macrophages in various tissues were found in the mutant mice. However, the development of both marginal metallophilic macrophage populations in the spleen and of their related macrophages in the other tissues of the mutant mice was severely impaired. All these abnormalities seem to be induced by the loss-of-function of the IL-2 receptor gamma chain. From 8 weeks of age on, inflammatory changes occurred in the intestines, mesenteric lymph nodes, lungs, liver, and kidneys of the mutant mice. Besides the absence of Hassall's corpuscles, thymic cysts were frequently observed in the mutant mice. These pathological abnormalities suggest that the gamma chain is implicated not only in lymphoid and haematopoietic development but also in thymic epithelial cell ontogeny.

Endothelin-1 alters the contractile phenotype of cultured embryonic smooth muscle cells

Smooth muscle tissues may be classified into phasic (fast) or tonic (slow) contractile phenotypes. This study was initiated to examine the specification of these phenotypes during development and the role of growth factors in this process. We used myosin light chain 17 (MLC17) and myosin heavy chain transcript splice variants as markers of the tonic (aortic) and phasic (intestinal) smooth muscle phenotypes in chick embryos. By reverse transcription-polymerase chain reaction, we determined embryonic days 6 to 16 to be a critical period for the establishment of these phenotypes. During this period, endothelin-1 is present at 40-fold-higher levels in aortic compared with intestinal tissues. To test the hypothesis that endothelin-1 may be involved in establishing the aortic (tonic) phenotype, we developed a system in which embryonic smooth muscle cells exhibit phasic and tonic contractile properties in vitro. Single-cell force measurements showed that cultured embryonic gizzard (phasic) cells developed force more rapidly (8 +/- 2 seconds) and achieved greater force (3.0 +/- 0.7 microN) than did cultured embryonic aortic (tonic) cells (20 +/- 0.7 seconds, 0.76 +/- 0.01 microN; P < .05) in response to depolarization. Chronic exposure of the phasic (gizzard) cells to endothelin-1 prolonged the time to peak force (24 +/- 3 seconds) and reduced the peak force (1.0 +/- 0.1 microN), so that the contraction resembled the tonic type. This effect, mediated by the endothelin-A receptor, was associated with a shift in MLC17 splicing to the tonic pattern. These results demonstrate that endothelin-1 is highly enriched in developing aortic compared with intestinal tissues and can convert phasic smooth muscle cells to the tonic type in vitro, suggesting a role for this growth factor during development in determining the contractile phenotype of smooth muscle cells.

Dynamics of tumor oxygenation, CD31 staining and transforming growth factor-beta levels after treatment with radiation or cyclophosphamide in the rat 13762 mammary carcinoma

PURPOSE

Tumors are dynamic tissues that undergo marked molecular, biochemical, and physiologic changes in response to cytotoxic anticancer therapies. Understanding the changes in tumor oxygenation and transforming growth factor-beta expression may allow improved treatment regimens to be developed.

METHODS AND MATERIALS

The effects of a single dose of radiation therapy (20 Gy) or a single dose of chemotherapy (cyclophosphamide, 250 mg/kg) on several molecular and physiologic parameters of the rat 13762 mammary carcinoma growing subcutaneously in female Fischer 344 rats were explored.

RESULTS

Treatment of the tumor-bearing animals with 20 Gy of radiation killed about two logs (99%) of the 13762 tumor cells, and treatment with cyclophosphamide (250 mg/kg) killed about 1.5 logs (95%) of the 13762 tumor cells. Hypoxia, as determined by a pO2 electrode, initially decreased in the tumors of treated animals until 6 h. posttreatment and then increased, so that 24 h. after administration of the radiation therapy or the chemotherapy the number of intratumoral vessels as determined by CD31 staining increased until about 24 h after cytotoxic therapy. Transforming growth factor-beta1, measured by radioimmunoassay, peaked in the serum between 6 h and 18 h and again between 72 h and 96 h after radiation therapy and peaked in the tumor at 24 h and again at 72 h after radiation therapy. The first serum peak after cyclophosphamide was 3 h after drug injection, with second peaks at 36 h and 48 h after drug administration. In the tumor, transforming growth factor-beta1 peaked between 6 h and 8 h after drug administration and again 36 h and 72 h after drug. Apoptosis was maximal 6 h after 20 Gy and 24 h after cyclophosphamide. Vascular endothelial growth factor was also increased in tumors after cytotoxic therapy.

CONCLUSIONS

These changes in the tumor physiologic status are sufficient to protect the tumor from a second cytotoxic insult administered days afterwards and to result in a restructuring of the tissue.

The motor domain and the regulatory domain of myosin solely dictate enzymatic activity and phosphorylation-dependent regulation, respectively

While the structures of skeletal and smooth muscle myosins are homologous, they differ functionally from each other in several respects, i.e., motor activities and regulation. To investigate the molecular basis for these differences, we have produced a skeletal/smooth chimeric myosin molecule and analyzed the motor activities and regulation of this myosin. The produced chimeric myosin is composed of the globular motor domain of skeletal muscle myosin (Met1-Gly773) and the C-terminal long alpha-helix domain of myosin subfragment 1 as well as myosin subfragment 2 (Gly773-Ser1104) and light chains of smooth muscle myosin. Both the actin-activated ATPase activity and the actin-translocating activity of the chimeric myosin were completely regulated by light chain phosphorylation. On the other hand, the maximum actin-activated ATPase activity of the chimeric myosin was the same as skeletal myosin and thus much higher than smooth myosin. These results show that the C-terminal light chain-associated domain of myosin head solely confers regulation by light chain phosphorylation, whereas the motor domain determines the rate of ATP hydrolysis. This is the first report, to our knowledge, that directly determines the function of the two structurally separated domains in myosin head.

Strain increases airway smooth muscle contractile and cytoskeletal proteins in vitro

Mechanical stress contributes to lung development and the progression of some lung diseases, although its effects on individual lung cells are unknown. Because increased airway smooth muscle (ASM) is found in lung diseases where abnormal stress is present, we determined if strain (change in resting length) causes ASM hypertrophy independently of other in vivo influences. Cultured canine ASM cells were subjected to two levels of cyclic deformational strain for 14 days and compared with nonstrained cells by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blotting. Cells subjected to 16-30% strain demonstrated increases in total cellular protein, myosin, myosin light chain kinase (MLCK), and desmin, whereas the cellular contents of actin, vimentin, and tubulin were similar. Changes in myosin appeared mostly due to the smooth muscle isoform, whereas nonmuscle myosin was unchanged. The increases in myosin and MLCK were disproportionate to increases in total protein, suggesting selective changes in contractile proteins. These relative increases in content of proteins were not as pronounced with 0-16% strain, suggesting a graded response. These data suggest that strain per se can increase the contractile proteins of ASM cells independently of other in vivo factors and modulates cultured cell phenotype to a more differentiated state, since it increases smooth muscle-specific proteins, such as smooth muscle myosin isoforms and desmin.

Leiomyoma of the esophagus with special reference to the characteristics of this tumor in teenagers

BACKGROUND/AIMS

Leiomyoma, the most common benign esophageal tumor, occurs rarely in youths. We herein reported our experiences with this tumor, paying particular attention to two cases in teenagers.

MATERIALS AND METHODS

We have surgically treated 12 patients with leiomyoma of the esophagus from 1978 to 1995. Ten of them occurred in adult and two occurred in teenagers aged 17 and 18. We evaluated the characteristics, diagnosis and treatment of these cases, particularly in young patients.

RESULTS

The age range was from 17 to 55, and the mean was 40.5 years. The mean longitudinal size of 12 tumors in 10 adult cases was 4.9 cm, while both tumors in the teenagers were larger in size, over 10 cm. Endoscopic ultrasonography was useful in the diagnosis of the tumors. All tumors were enucleated without any complication. In young cases, in spite of the large size of the tumors, an enucleation was also possible and the long-term result was satisfactory.

CONCLUSION

Regarding the young cases, in spite of the large size of the tumor, an enucleation of the tumor should be tried whenever possible, in consideration of the patient's postoperative quality of life.

Functional analysis of the mutations in the human cardiac beta-myosin that are responsible for familial hypertrophic cardiomyopathy. Implication for the clinical outcome

More than 30 missense mutations in the beta-cardiac myosin heavy chain gene have been shown to be responsible for familial hypertrophic cardiomyopathy. To clarify the effects of these point mutations on myosin motor function, we expressed wild-type and mutant human beta-cardiac myosin heavy chains in insect cells with human cardiac light chains. The wild-type myosin was well purified with similar enzymatic and motor activities to those of the naturally isolated V3 cardiac myosin. Arg249-->Gln and Arg453-->Cys mutations resulted in decreased actin translocating activity (61 and 23% of the wild-type, respectively) with decreased intrinsic ATPase activity. Arg403-->Gln mutation greatly decreased actin translocating activity (27% of wild type) with a 3.3-fold increased dissociation constant for actin, while intrinsic ATPase activity was unchanged. Val606-->Met mutation only mildly affected the actin translocating activity as well as ATPase activity of myosin. The degree of deterioration by each mutation was closely correlated with the prognosis of the affected kindreds, indicating that myosin dysfunction caused by the point mutations is responsible for the pathogenesis of the disease. Structure/function relationship of myosin is discussed.