Regulation of membrane assembly during development of sarcoplasmic reticulum: the possible role of calcium

ER stress signaling by regulated splicing: IRE1/HAC1/XBP1

The endoplasmic reticulum (ER) serves many specialized functions in the cell including calcium storage and gated release, biosynthesis of membrane and secretory proteins, and production of lipids and sterols. Therefore, the ER integrates many internal and external signals to coordinate downstream responses, although the mechanism(s) that maintain homeostasis are largely unknown. When misfolded or unfolded proteins accumulate in the ER, an intracellular signaling pathway termed the unfolded protein response (UPR) is activated. Identification of IRE1 in the yeast Saccharomyces cerevisiae as a proximal sensor in the UPR pathway was a milestone in understanding how the ER responds to the accumulation of unfolded protein and signals transcriptional activation through regulated nonconventional splicing of its substrate mRNA encoding the transcription factor Hac1p. Subsequent studies identified IRE1 and HAC1 homologues in mammalian cells. Here, we summarize various approaches to study the IRE1-Hac1 pathway in yeast and the homologous IRE1-XBP1 pathway in mammalian cells. We present microbiological growth assays for the UPR, reporter assays for UPR signaling, direct techniques to measure UPR activation in vivo, methods to study translation of HAC1 mRNA, and in vitro cleavage and ligation of HAC1 and XBP1 mRNA. Especially we think the newly developed quantitative and qualitative methods to detect IRE1 activity-dependent XBP1 mRNA splicing will be fast and accurate tools to show the activation of the UPR.

Cross-talk between transcriptional regulation by thyroid hormone and myogenin: new aspects of the Ca2+-dependent expression of the fast-type sarcoplasmic reticulum Ca2+-ATPase

We have previously demonstrated an interaction between the major determinants of skeletal muscle phenotype by showing that continuous contractile activity represses the thyroid hormone (3,3', 5-tri-iodothyronine; T3)-dependent transcriptional activity of fast-type sarcoplasmic/endoplasmic-reticulum Ca2+-ATPase (SERCA1), a characteristic of the fast phenotype. Both the free cytosolic Ca2+ concentration ([Ca2+]i) and the myogenic determination factors MyoD and myogenin have been implicated as mediators of the effect of contractile activity on skeletal muscle phenotype. Using L6 cells we have shown that an increase in the steady-state [Ca2+]i above the resting level of 120 nM indeed can mimic the effect of contractile activity on T3-dependent SERCA1 expression. We now show that the repressing effect of increased [Ca2+]i on T3-dependent SERCA1 expression in L6 cells is exerted at a pre-translational level and is accompanied by increased myogenin mRNA expression. Myogenin overexpression in these cells revealed that increased expression of myogenin alone strongly decreases the T3-dependent stimulation of SERCA1 promoter activity. These results suggest a pathway for the regulation of skeletal muscle phenotype in which [Ca2+]i mediates the effect of contractile activity by regulating the expression of myogenin, which in turn interferes with transcriptional regulation by T3.

Electrical stimulation of C2C12 myotubes induces contractions and represses thyroid-hormone-dependent transcription of the fast-type sarcoplasmic-reticulum Ca2+-ATPase gene

Chronic low-frequency contraction of skeletal muscle, either induced by a slow motor nerve or through direct electrical stimulation, generally induces expression of proteins associated with the slow phenotype, while repressing the corresponding fast isoforms. Contractions thereby counteract the primarily transcriptional effect of thyroid hormone (T3) which results in the selective induction and stimulation of expression of fast isoforms. We studied the regulation of expression of the fast-type sarcoplasmic-reticulum Ca2+-ATPase (SERCA1), a characteristic component of the fast phenotype. Previous work suggested that reduction of SERCA1 expression by contractile activity might result from interference with the T3-dependent transcriptional stimulation of the SERCA1 gene. The present study was set up to test this unexpected mode of action of contractile activity. We show that electrical stimulation of C2C12 mouse myotubes, which results in synchronous contractions at the imposed frequency, reduces basal but virtually abolishes T3-dependent SERCA1 expression. T3-dependent expression of a reporter gene driven by the SERCA1 promoter was similarly affected by electrical stimulation. This is the first demonstration that the counteracting effects on muscle gene expression of electrically induced contractions and T3 may interact at the transcriptional level.

Structure-function relationships in the Ca(2+)-ATPase of sarcoplasmic reticulum: facts, speculations and questions for the future

Structural data on the Ca(2+)-ATPase of sarcoplasmic reticulum are integrated with kinetic data on Ca2+ transport. The emphasis is upon ATPase-ATPase interactions, the requirement for phospholipids, and the mechanism of Ca2+ translocation. The possible role of cytoplasmic [Ca2+] in the regulation of the synthesis of Ca(2+)-ATPase is discussed.

Giant sarcoplasmic reticulum vesicles: a study of membrane morphogenesis

Rabbit sarcoplasmic reticulum vesicles were fused into giant proteoliposomes in a medium of 0.1 M KCl, 10 mM Tris-maleate, pH 7.0, 10 micrograms ml-1 antipain, 10 micrograms ml-1 leupeptin, 25 IU per ml Trasylol, 3 mM NaN3, 3.75% PEG 1500 and 3% DMSO by brief exposure to 37 degrees C, followed by incubation for 4 h at 25 degrees C. Approximately 5-10% of the sarcoplasmic reticulum elements underwent fusion, forming single-walled spherical vesicles of 1-25 microns diameter, in which the polarity of the native membrane was preserved. The Ca(2+)-stimulated ATPase activity remained essentially unchanged after fusion. On exposure to decavanadate in a Ca(2+)-free medium the spherical vesicles assumed a corrugated appearance with the formation of long ridges separated by deep furrows that eventually pinched off longitudinally and separated into numerous long crystalline tubules of uniform (approximately 0.1 microns) diameter. The vanadate-induced transformation of giant vesicles into tubules implies that the geometry of the sarcoplasmic reticulum membrane is determined by the conformation of the Ca(2+)-ATPase.

Ca2+ homeostasis and fast-type sarcoplasmic reticulum Ca(2+)-ATPase expression in L6 muscle cells. Role of thyroid hormone

The effect of thyroid hormone (L-tri-iodothyronine; T3) on the cytosolic free Ca2+ concentration ([Ca2+]i) in L6 myotubes was studied at rest and during activation to explore the possible mediating role of [Ca2+]i in the T3-induced net synthesis of fast-type sarcoplasmic reticulum (SR) Ca(2+)-ATPase. The mean [Ca2+]i at rest was approx. 115 nM in myoblasts, control myotubes and T3-treated myotubes. Therefore it is unlikely that the T3-induced elevation of Ca(2+)-ATPase levels is mediated by [Ca2+]i changes. To investigate the influence of the 4-fold higher Ca(2+)-ATPase levels in T3-treated myotubes (compared with controls) on [Ca2+]i, interventions with caffeine (10 mM) and a high extracellular K+ concentration ([K+]o) (30 mM) were applied which initially mobilize Ca2+ predominantly from the SR. The results showed a lower (caffeine) or not significantly different (high [K+]o) increase in [Ca2+]i in T3-treated myotubes compared with controls. No rise in [Ca2+]i was found in myoblasts with caffeine or high [K+]o. The role of [Ca2+]i in the regulation of Ca(2+)-ATPase levels was investigated by varying [Ca2+]i through exposure of cells to different concentrations of extracellular Ca2+ (0.2-1.8 mM) and ionomycin (0.1-0.25 microM). At subnormal [Ca2+]i (55 nM) the T3-induced net synthesis of Ca(2+)-ATPase was virtually abolished, and at supranormal [Ca2+]i (195 nM) it was greatly depressed. Intermediate stimulation of net Ca(2+)-ATPase synthesis was found at [Ca2+]i of 95 and 165 nM, with an optimum at approx. 125 nM. Similar but less pronounced effects were found for the basal Ca(2+)-ATPase levels. In contracting primary rat myotubes, Ca(2+)-ATPase levels were significantly lower than in tetrodotoxin-arrested myotubes. The same results were obtained in the presence of T3. Since the mean [Ca2+]i in contracting cells is higher than in resting cells, these data agree with those obtained in the L6 cells with ionomycin. A major conclusion of this study is the existence of a [Ca2+]i optimum, near resting levels, for the expression of the fast-type Ca(2+)-ATPase in the L6 muscle cell line.

Calcium and ionophore A23187 stimulates deposition of extracellular matrix and acetylcholinesterase release in cultured myotubes

Calcium (Ca2+) and calcium-transporting ionophores stimulate protein secretion in many cellular systems. We demonstrate here than increases in intracellular calcium concentration induce a time- and concentration-dependent deposition of extracellular matrix and an increase in acetylcholinesterase secretion. Scanning and transmission electron-microscopy revealed that treatment with the calcium ionophore A23187, or high extracellular Ca2+ levels (5 mM to 15 mM) produce significant deposits of extracellular matrix around the myotubes, as well as a marked increase in the acetylcholinesterase reaction-product. Blocking muscle contraction was not necessary for the induction of AChE secretory activity. Sucrose density-gradients of media conditioned by muscle cells revealed 3 separate acetylcholinesterase molecular forms. However, incubation with A23187 increased only the 4.5 S and the 7.2 S molecular forms, whereas the 12.0 S form showed no significant differences from controls. Polyacrylamide gel electrophoresis, and autoradiography using [3H]diisopropyl fluorophosphate revealed a broad band at 65,000 daltons. This band was broader than for controls when medium was obtained from A23187-treated cells. Our results show that increasing intracellular Ca2+ concentration induces marked deposition of extracellular matrix and increased acetylcholinesterase secretion, with an apparent selectivity for the monomeric and dimeric acetylcholinesterase molecular forms.

The elevation of sarcoplasmic reticulum Ca2(+)-ATPase levels by thyroid hormone in the L6 muscle cell line is potentiated by insulin-like growth factor-I

Net synthesis of the fast-type sarcoplasmic reticulum (SR) Ca2(+)-ATPase was studied in the muscle cell line L6AM using an immunochemical assay (e.l.i.s.a.). In addition, Ca2+ uptake by SR was monitored in muscle cell homogenates by a method employing the fluorescent Ca2+ indicator fura-2. Measurements were done both in differentiating myoblasts and in myotubes. Ca2(+)-ATPase levels were low (1 pmol/mg of protein) in undifferentiated myoblasts (controls) and only doubled over a period of 8 days in the absence of thyroid hormone (L-triiodothyronine; T3). This corresponded to a similar increase in Ca2+ uptake activity. Only half of the myoblasts fused under these conditions. Fusion was not increased in the presence of T3 (5 nM), but Ca2(+)-ATPase levels increased 4-fold and the Ca2+ uptake activity doubled compared with controls. In contrast, insulin-like growth factor-I (IGF-I) induced almost complete myotube formation (greater than 90% fusion), but only slightly stimulated (50%) net Ca2(+)-ATPase synthesis above control levels. However, the doubling of the Ca2+ uptake stimulation by IGF-I was comparable with that caused by T3. The effects of T3 plus IGF-I on Ca2(+)-ATPase levels and Ca2+ uptake activity were more than additive. Furthermore, the temporal relationship between the induction of Ca2(+)-ATPase net synthesis and Ca2+ uptake activity was identical with the two hormones. Qualitatively similar results were obtained when T3 and IGF-I were added to maximally fused cell cultures. The enhanced effect of T3 on Ca2(+)-ATPase net synthesis and Ca2+ uptake activity in the presence of IGF-I cannot therefore be explained by an increased myotube formation stimulated by the latter. In both differentiating myoblasts and myotubes the effect of T3 was more prominent on Ca2(+)-ATPase net synthesis than on Ca2+ uptake activity, whereas in myotubes the opposite was observed for IGF-I. This could imply complementary actions of the two agents in the development of a functional SR.

Regulation of the number of alpha-bungarotoxin binding sites in cultured chick myotubes by a 1,4 dihydropyridine calcium channel antagonist

Calcium has been suggested as the second messenger link between skeletal muscle activity and AChR gene expression and synthesis. We have compared the concentrations of the Ca2+ channel antagonists D600 and nisoldipine needed both to block Ca2+ uptake into cultured myotubes and to increase AChR expression. The good correspondence between these two measurements and the use of the highly specific Ca2+ channel antagonist nisoldipine strengthens the hypothesis that AChR expression is regulated by levels of intracellular Ca2+.

Expression of fast and slow isoforms of the Ca2+-ATPase in developing chick skeletal muscle

The expression of fast and slow isoforms of the sarcoplasmic reticulum Ca2+-ATPase was studied in the developing chick embryo and in tissue-cultured myotubes. Monoclonal antibodies specific for each isoform were used as probes of protein expression. Analysis of expression of Ca2+-ATPase isoforms in chick thigh muscles by immunofluorescence microscopy revealed that all muscle fibers expressed both isoforms during their development. Primary generation muscle fibers expressed predominantly the slow isoform. Secondary generation fibers expressed both isoforms at comparable levels. Loss of the "inappropriate" isoforms occurred late in embryonic development. Immunoblot analysis of embryonic thigh muscle proteins indicated that the expression of the slow isoform varied little from embryonic Day 6 (ED6) to ED19, while expression of the fast isoform increased dramatically just prior to ED19. Tissue-cultured myotubes derived from ED12 chick thigh muscle myoblasts, plated at high density, expressed both isoforms of the Ca2+-ATPase at very similar levels. Clonal analysis of myoblasts taken from early (ED6) and late (ED12) chick thigh muscles showed that all muscle colonies expressed both forms, consistent with in vivo results. Fiber-type specific isoforms of the Ca2+-ATPase and myosin heavy chain are not coordinately expressed in developing chick skeletal muscle.

Calcium ionophore A23187 as a regulator of gene expression in mammalian cells

The calcium ionophore A23187 can reversibly induce the expression of two glucose-regulated genes, p3C5 and p4A3. This induction requires a continuous presence of the ionophore for over 2 h. Although extracellular Ca2+ is important for the optimal effect of A23187, it is not necessary for the induction, since a similar response with a lower magnitude can be triggered in cells cultured in low Ca2+ medium buffered with EGTA. Both the basal and induced levels of p3C5 and p4A3 transcripts can be modulated by the calmodulin antagonist W-7, indicating the involvement of Ca2+/calmodulin-associated pathways. In addition, the sensitivity of the A23187 induction to cycloheximide suggests that the induction process is dependent on de novo protein synthesis.

Measurement of cytoplasmic free Ca2+ concentration in cultured muscle cells by aequorin and quin 2

Ca2+ has been proposed to regulate expression of the gene for the Ca2+ pump of the sarcoplasmic reticulum in developing chicken myoblasts (A. N. Martonosi, L. Dux, R. L. Terjung, and D. Roufa. Ann. NY Acad. Sci. 402: 485-514, 1982. In the present study, intracellular free Ca2+ ([Ca2+]i) was measured with the photoprotein, aequorin, incorporated by a reversible hyperpermeabilization technique, and with the fluorescent probe, quin 2. Because aequorin reacts irreversibly with Ca2+ and is inactivated by heat, at 37 degrees C the active aequorin content declined markedly. The resting glow (1/10(6) of the maximum light) disappeared after a few hours, whereas the light from the total active aequorin remaining was still detectable when cells were lysed 3 days later. A new approach, which compared the rate of disappearance of aequorin in treated and control cells, was developed on the basis that aequorin would be inactivated more quickly in cells with higher [Ca2+]i. A23187, ionomycin, and trifluoperazine, all of which accelerated gene expression (A. N. Martonosi et al.), increased the rate of decay of active aequorin and therefore increased [Ca2+]i. "Ca shock", and ethyleneglycol-bis(beta-aminoethylether)-N,N'-tetraacetic acid, which also increased the gene expression (A. N. Martonosi et al.), caused small increases of [Ca2+]i not detectable by aequorin but detectable by quin 2. X537A and ouabain had no effect on the gene expression (A. N. Martonosi et al.) and did not raise [Ca2+]i. The results support the proposal that Ca2+ may regulate expression of this gene.

Membrane crystals of Ca2+-ATPase in sarcoplasmic reticulum of developing muscle

The vanadate-induced crystallization of Ca2+-ATPase was analyzed on sarcoplasmic reticulum vesicles isolated between 10 and 28 days of development from pectoralis muscles of chicken. After exposure to Na3VO4 in a Ca2+-free medium, Ca2+-ATPase crystals begin to appear on portions of the surface of a few vesicles, isolated at 18 days of development. Thereafter, the number of vesicles containing Ca2+-ATPase crystals rapidly increases and after 1 week of postnatal development (28 days), it reaches the adult level of about 30% of the vesicle population. These observations are discussed with reference to the mechanism of Ca2+-ATPase crystallization and the regulation of sarcoplasmic reticulum biosynthesis.