Involvement of K(Ca) channels and stretch-activated channels in calcium influx, triggering membrane fusion of chick embryonic myoblasts

Gelatin-Polyaniline Composite Nanofibers Enhanced Excitation-Contraction Coupling System Maturation in Myotubes

In this study, composite gelatin-polyaniline (PANI) nanofibers doped with camphorsulfonic acid (CSA) were fabricated by electrospinning and used as substrates to culture C2C12 myoblast cells. We observed enhanced myotube formation on composite gelatin-PANI nanofibers compared to gelatin nanofibers, concomitantly with enhanced myotube maturation. Thus, in myotubes, intracellular organization, colocalization of the dihydropyridine receptor (DHPR) and ryanodine receptor (RyR), expression of genes correlated to the excitation-contraction (E-C) coupling apparatus, calcium transients, and myotube contractibility were increased. Such composite material scaffolds combining topographical and electrically conductive cues may be useful to direct skeletal muscle cell organization and to improve cellular maturation, functionality, and tissue formation.

Low Intensity Exercise Training Improves Skeletal Muscle Regeneration Potential

Purpose: The aim of this study was to determine whether 12 days of low-to-moderate exercise training at low altitude (598 m a.s.l.) improves skeletal muscle regeneration in sedentary adult women.

Methods: Satellite cells were obtained from the vastus lateralis skeletal muscle of seven women before and after this exercise training at low altitude. They were investigated for differentiation aspects, superoxide anion production, antioxidant enzymes, mitochondrial potential variation after a depolarizing insult, intracellular Ca²⁺ concentrations, and micro (mi)RNA expression (miR-1, miR-133, miR-206).

Results: In these myogenic populations of adult stem cells, those obtained after exercise training, showed increased Fusion Index and intracellular Ca²⁺ concentrations. This exercise training also generally reduced superoxide anion production in cells (by 12–67%), although not in two women, where there was an increase of ~15% along with a reduced superoxide dismutase activity. miRNA expression showed an exercise-induced epigenetic transcription profile that was specific according to the reduced or increased superoxide anion production of the cells.

Conclusions: The present study shows that low-to-moderate exercise training at low altitude improves the regenerative capacity of skeletal muscle in adult women. The differentiation of cells was favored by increased intracellular calcium concentration and increased the fusion index. This low-to-moderate training at low altitude also depicted the epigenetic signature of cells.

Transient Ca2+ depletion from the endoplasmic reticulum is critical for skeletal myoblast differentiation

Endoplasmic reticulum (ER) stress is a cellular condition in which unfolded proteins accumulate in the ER because of various but specific causes. Physiologic ER stress occurs transiently during myoblast differentiation, and although its cause remains unknown, it plays a critical role in myofiber formation. To examine the mechanism underlying ER stress, we monitored ER morphology during differentiation of murine myoblasts. Novel ER-derived structures transiently appeared prior to myoblast fusion both in vitro and in vivo. Electron microscopy studies revealed that these structures consisted of pseudoconcentric ER cisternae with narrow lumens. Similar structures specifically formed by pharmacologically induced ER Ca(2+) depletion, and inhibition of ER Ca(2+) efflux channels in differentiating myoblasts considerably suppressed ER-specific deformation and ER stress signaling. Thus, we named the novel structures stress-activated response to Ca(2+) depletion (SARC) bodies. Prior to SARC body formation, stromal interaction molecule 1 (STIM1), an ER Ca(2+) sensor protein, formed ER Ca(2+) depletion-specific clusters. Furthermore, myoblast differentiation manifested by myoblast fusion did not proceed under the same conditions as inhibition of ER Ca(2+) depletion. Altogether, these observations suggest that ER Ca(2+) depletion is a prerequisite for myoblast fusion, causing both physiologic ER stress signaling and SARC body formation.

Signaling mechanisms in mammalian myoblast fusion

Myoblast fusion is a critical process that contributes to the growth of muscle during development and to the regeneration of myofibers upon injury. Myoblasts fuse with each other as well as with multinucleated myotubes to enlarge the myofiber. Initial studies demonstrated that myoblast fusion requires extracellular calcium and changes in cell membrane topography and cytoskeletal organization. More recent studies have identified several cell-surface and intracellular proteins that mediate myoblast fusion. Furthermore, emerging evidence suggests that myoblast fusion is also regulated by the activation of specific cell-signaling pathways that lead to the expression of genes whose products are essential for the fusion process and for modulating the activity of molecules that are involved in cytoskeletal rearrangement. Here, we review the roles of the major signaling pathways in mammalian myoblast fusion.

Gadolinium ions block mechanosensitive channels by altering the packing and lateral pressure of anionic lipids

Effects of polyvalent ions on the lateral packing of phospholipids have been known for decades, but the physiological consequences have not been systematically studied. Gd(3+) is a relatively nonspecific agent that blocks mechano-gated channels with a variable affinity. In this study, we show that the large mechanosensitive channel MscL of Escherichia coli is effectively blocked by Gd(3+) only when reconstituted with negatively charged phospholipids (e.g., PS). Taking this lead, we studied effects of Gd(3+) on monolayers and unilamellar vesicles made of natural brain PS, DMPS, and its mixtures with DMPC. In monolayer experiments, we found that muM Gd(3+) present in the subphase leads to approximately 8% lateral compaction of brain PS (at 35 mN/m). Gd(3+) more strongly shrinks and rigidifies DMPS films causing a spontaneous liquid expanded-to-compact transition to the limiting 40 A(2)/mol. Pressure-area isotherms of uncharged DMPC were unaffected by Gd(3+), and neutralization of DMPS surface by low pH did not produce strong compaction. Upshifts of surface potential isotherms of DMPS monolayers reflected changes in the diffuse double layer due to neutralization of headgroup charges by Gd(3+), whereas the increased packing density produced up to a 200 mV change in the interfacial dipole potential. The slopes of surface potential versus reciprocal area predicted that Gd(3+) induced a modest ( approximately 18%) increase in the magnitude of the individual lipid dipoles in DMPS. Isothermal titration calorimetry indicated that binding of Gd(3+) to DMPS liposomes in the gel state is endothermic, whereas binding to liquid crystalline liposomes produces heat consistent with the isothermal liquid-to-gel phase transition induced by the ion. Both titration curves suggested a K(b) of approximately 10(6) M(-1). We conclude that anionic phospholipids serve as high-affinity receptors for Gd(3+) ions, and the ion-induced compaction generates a lateral pressure increase estimated as tens of mN/m. This pressure can "squeeze" the channel and shift the equilibrium toward the closed state.

Cytoskeleton/stretch-activated ion channel interaction regulates myogenic differentiation of skeletal myoblasts

In the present study, we investigated the functional interaction between stress fibers (SFs) and stretch-activated channels (SACs) and its possible role in the regulation of myoblast differentiation induced by switch to differentiation culture in the presence or absence of sphingosine 1-phosphate. It was found that there was a clear temporal correlation between SF formation and SAC activation in differentiating C2C12 myoblasts. Inhibition of actin polymerization with the specific Rho kinase inhibitor Y-27632, significantly decreased SAC sensitivity in these cells, suggesting a role for Rho-dependent actin remodeling in the regulation of the channel opening. The alteration of cytoskeletal/SAC functional correlation had also deleterious effects on myogenic differentiation of C2C12 cells as judged by combined confocal immunofluorescence, biochemical and electrophysiological analyses. Indeed, the treatment with Y-27632 or with DHCB, an actin disrupting agent, inhibited the expression of the myogenic markers (myogenin and sarcomeric proteins) and myoblast-myotube transition. The treatment with the channel blocker, GdCl(3), also affected myogenesis in these cells. It impaired, in fact, myoblast phenotypic maturation (i.e., reduced the expression of alpha-sarcomeric actin and skeletal myosin and the activity of creatine kinase) but did not modify promoter activity and protein expression levels of myogenin. The results of this study, together with being in agreement with the general idea that cytoskeletal remodeling is essential for muscle differentiation, describe a novel pathway whereby the formation of SFs and their contraction, generate a mechanical tension to the plasma membrane, activate SACs and trigger Ca(2+)-dependent signals, thus influencing the phenotypic maturation of myoblasts.

Inhibition of mechanosensitive cation channels inhibits myogenic differentiation by suppressing the expression of myogenic regulatory factors and caspase-3 activity

Mechanosensitive cation channels (MSC) are ubiquitous in eukaryotic cell types. However, the physiological functions of MSC in several tissues remain in question. In this study we have investigated the role of MSC in skeletal myogenesis. Treatment of C2C12 myoblasts with gadolinium ions (MSC blocker) inhibited myotube formation and the myogenic index in differentiation medium (DM). The enzymatic activity of creatine kinase (CK) and the expression of myosin heavy chain-fast twitch (MyHCf) in C2C12 cultures were also blocked in response to gadolinium. Treatment of C2C12 myoblasts with gadolinium ions did not affect the expression of either cyclin A or cyclin D1 in DM. Other inhibitors of MSC such as streptomycin and GsTMx-4 also suppressed the expression of CK and MyHCf in C2C12 cultures. The inhibitory effect of gadolinium ions on myogenic differentiation was reversible and independent of myogenic cell type. Real-time-polymerase chain reaction analysis revealed that inhibition of MSC decreases the expression of myogenic transcription factors MyoD, myogenin, and Myf-5. Furthermore, the activity of skeletal alpha-actin promoter was suppressed on MSC blockade. Treatment of C2C12 myoblasts with gadolinium ions prevented differentiation-associated cell death and inhibited the cleavage of poly (ADP-ribose) polymerase and activation of caspase-3. On the other hand, delivery of active caspase-3 protein to C2C12 myoblasts reversed the inhibitory effect of gadolinium ions on myogenesis. Our data suggest that inhibition of MSC suppresses myogenic differentiation by inhibiting the caspase-3 activity and the expression of myogenic regulatory factors.

Calcium sources used by post-natal human myoblasts during initial differentiation

Increases in cytoplasmic Ca(2+) are crucial for inducing the initial steps of myoblast differentiation that ultimately lead to fusion; yet the mechanisms that produce this elevated Ca(2+) have not been fully resolved. For example, it is still unclear whether the increase comes exclusively from membrane Ca(2+) influx or also from Ca(2+) release from internal stores. To address this, we investigated early differentiation of myoblast clones each derived from single post-natal human satellite cells. Initial differentiation was assayed by immunostaining myonuclei for the transcription factor MEF2. When Ca(2+) influx was eliminated by using low external Ca(2+) media, we found that approximately half the clones could still differentiate. Of the clones that required influx of external Ca(2+), most clones used T-type Ca(2+) channels, but others used store-operated channels as influx-generating mechanisms. On the other hand, clones that differentiated in low external Ca(2+) relied on Ca(2+) release from internal stores through IP(3) receptors. Interestingly, by following clones over time, we observed that some switched their preferred Ca(2+) source: clones that initially used calcium release from internal stores to differentiate later required Ca(2+) influx and inversely. In conclusion, we show that human myoblasts can use three alternative mechanisms to increase cytoplasmic Ca(2+) at the onset of the differentiation process: influx through T-types Ca(2+) channels, influx through store operated channels and release from internal stores through IP(3) receptors. In addition, we suggest that, probably because Ca(2+) elevation is essential during initial differentiation, myoblasts may be able to select between these alternate Ca(2+) pathways.

The formation of skeletal muscle myotubes requires functional membrane receptors activated by extracellular ATP

Skeletal muscle differentiation follows an organized sequence of events including commitment, cell cycle withdrawal, and cell fusion to form multinucleated myotubes. The role of adenosine 5'-triphosphate (ATP)-mediated signaling in differentiation of skeletal muscle myoblasts was evaluated in C(2)C(12) cells, a myoblast cell line. Cell differentiation was inhibited by P2X receptor blockers or by degradation of endogenous ATP with apyrase. However, pertussis toxin, known to block only a group of P2Y receptors, did not alter the differentiation process. Cells were heterogeneous in their expression of functional P2X receptors, evaluated by the uptake of fluorescent permeability tracers (Lucifer yellow and ethidium bromide), and by immunofluorescence of P2X(7) receptors. Moreover, xestospongin C, a selective and membrane-permeable inhibitor of IP(3) receptors, inhibited both myotube formation and myogenin expression. Based on these results, we suggest that the known increase in intracellular Ca(2+) concentration required for differentiation is due at least in part to Ca(2+) influx through P2X receptors and Ca(2+) release from intracellular stores. The possible involvement of P2X receptors and other pathways that might set the intracellular Ca(2+) at the level required for myoblast differentiation as well as the possible involvement of gap junction channels in the intercellular transfer of second messengers involved in coordinating myogenesis is proposed.

NMDA receptor-mediated calcium influx plays an essential role in myoblast fusion

Ca2+ influx is known to be prerequisite for myoblast fusion during skeletal muscle differentiation. Here, we show that the N-methyl-D-aspartate (NMDA) receptor is involved in the Ca2+ influx of C2C12 myoblasts. NMDA receptor (NR) 1 and NR2D were expressed in the myoblasts during muscle differentiation. Using Ca2+ imaging analysis, Ca2+ influx through NRs was directly measured at a single-cell level. l-Glutamate increased myoblast fusion as well as intracellular Ca2+ levels, and both effects were completely blocked by MK801, a selective antagonist of NRs. Furthermore, treatment with the Ca2+ ionophore A23187 recovered MK801-mediated inhibition of myoblast fusion. These results suggest that the NRs may play an important role in myoblast fusion by mediating Ca2+ influx.

Actin filaments regulate the stretch sensitivity of large-conductance, Ca2+-activated K+ channels in coronary artery smooth muscle cells

Using the inside-out patch-clamp technique, large-conductance Ca2+ -activated K+ channel (BK(Ca)) currents were recorded from coronary artery smooth muscle cells. Cytochalasin D, an actin filament disrupter, increased channel activity ( NP(o), where N is the number of channels and P(o) the open probability), and this increase was reversed by phalloidin, an actin filament stabilizer. NP(o) was also increased by colchicine, a microtubule disrupter, and decreased by taxol, a microtubule stabilizer. With the stepwise increase of negative pressure in the patch pipettes, the activity of BK(Ca) gradually increased: the maximum effect (527% increase in NP(o)) was achieved at -40 cmH(2)O and the half-maximum effect at -25 cmH(2)O. The increase in NP(o) in response to negative pressure was abolished by phalloidin but not by taxol. These results imply that both actin filaments and microtubules inhibit the opening of BK(Ca) in coronary artery smooth muscle cells, but that only actin filaments are involved in the stretch sensitivity of BK(Ca).

Increase in cytosolic Ca2+ induced by elevation of extracellular Ca2+ in skeletal myogenic cells

Cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) variation is a key event in myoblast differentiation, but the mechanism by which it occurs is still debated. Here we show that increases of extracellular Ca(2+) concentration ([Ca(2+)](o)) produced membrane hyperpolarization and a concentration-dependent increase of [Ca(2+)](i) due to Ca(2+) influx across the plasma membrane. Responses were not related to inositol phosphate turnover and Ca(2+)-sensing receptor. [Ca(2+)](o)-induced [Ca(2+)](i) increase was inhibited by Ca(2+) channel inhibitors and appeared to be modulated by several kinase activities. [Ca(2+)](i) increase was potentiated by depletion of intracellular Ca(2+) stores and depressed by inactivation of the Na(+)/Ca(2+) exchanger. The response to arginine vasopressin (AVP), which induces inositol 1,4,5-trisphosphate-dependent [Ca(2+)](i) increase in L6-C5 cells, was not modified by high [Ca(2+)](o). On the contrary, AVP potentiated the [Ca(2+)](i) increase in the presence of elevated [Ca(2+)](o). Other clones of the L6 line as well as the rhabdomyosarcoma RD cell line and the satellite cell-derived C2-C12 line expressed similar responses to high [Ca(2+)](o), and the amplitude of the responses was correlated with the myogenic potential of the cells.

Visualization of Ca2+ entry through single stretch-activated cation channels

Stretch-activated channels (SACs) have been found in smooth muscle and are thought to be involved in myogenic responses. Although SACs have been shown to be Ca(2+) permeable when Ca(2+) is the only charge carrier, it has not been clearly demonstrated that significant Ca(2+) passes through SACs in physiological solutions. By imaging at high temporal and spatial resolution the single-channel Ca(2+) fluorescence transient (SCCaFT) arising from Ca(2+) entry through a single SAC opening, we provide direct evidence that significant Ca(2+) can indeed pass through SACs and increase the local [Ca(2+)]. Results were obtained under conditions where the only source of Ca(2+) was the physiological salt solution in the patch pipette containing 2 mM Ca(2+). Single smooth muscle cells were loaded with fluo-3 acetoxymethyl ester, and the fluorescence was recorded by using a wide-field digital imaging microscope while SAC currents were simultaneously recorded from cell-attached patches. Fluorescence increases at the cell-attached patch were clearly visualized before the simultaneous global Ca(2+) increase that occurred because of Ca(2+) influx through voltage-gated Ca(2+) channels when the membrane was depolarized by inward SAC current. From measurements of total fluorescence ("signal mass") we determined that about 18% of the SAC current is carried by Ca(2+) at membrane potentials more negative than the resting level. This would translate into at least a 0.35-pA unitary Ca(2+) current at the resting potential. Such Ca(2+) currents passing through SACs are sufficient to activate large-conductance Ca(2+)-activated K(+) channels and, as shown previously, to trigger Ca(2+) release from intracellular stores.

Hyperpolarization, but not depolarization, increases intracellular Ca(2+) level in cultured chick myoblasts

Ca(2+) influx appears to be important for triggering myoblast fusion. It remains, however, unclear how Ca(2+) influx rises prior to myoblast fusion. The present study examines a possible involvement of the voltage-dependent Ca(2+) influx pathways. Treatment with the L-type Ca(2+) channel blockers, diltiazem, and nifedipine did not alter cytosolic Ca(2+) levels. Depolarization with high K(+) solution and activation of Ca(2+) channel with Bay K 8644, and agonist of voltage dependent Ca(2+) channels, failed to elicit increases intracellular Ca(2+) level, indicating the absence of depolarization-operated mechanisms. In contrast, phloretin, an agonist of Ca(2+)-activated potassium (K(Ca)) channels, was able to hyperpolarize membrane potential and promoted Ca(2+) influx. These effects were completely abolished by treatment of charybdotoxin, a specific inhibitor of K(Ca) channels. In addition, gadolinium, a potent stretch-activated channel (SAC) blocker, prevented the phloretin-mediated Ca(2+) increase, indicating the involvement of SACs in Ca(2+) influx. Furthermore, phloretin stimulated precocious myoblast fusion and this effect was blocked with gadolinium or charybdotoxin. Taken together, these results suggest that induced hyperpolarization, but not depolarization increases Ca(2+) influx through stretch-activated channels, and in turn triggers myoblast fusion.

Human Myoblast Differentiation: Ca2+ Channels are Activated by K+ Channels

In a paradigm of cellular differentiation, human myoblast fusion, we investigated how a Ca2+ influx, indispensable for fusion, is triggered. We show how newly expressed Kir2.1 K+ channels, via their hyperpolarizing effect on the membrane potential, generate a window Ca2+ current (mediated by a1H T-type Ca2+ channels), which causes intracellular Ca2+ to rise.

Physical shock wave mediates membrane hyperpolarization and Ras activation for osteogenesis in human bone marrow stromal cells

Physical shock wave (SW) has shown effectiveness on promotion of bone growth. We have recently demonstrated that SW could promote bone marrow stromal cell differentiation toward osteoprogenitor associated with induction of TGF-beta1. We have further demonstrated that SW-induced membrane hyperpolarization and Ras activation acted an early signal for the osteogenesis in human bone marrow stromal cells. An optimal dose of SW treatment at 0.16 mJ/mm(2) for 500 impulses induced a rapid membrane hyperpolarization in 5 min, activation of Ras in 30 min, and cell proliferation in 2 days. The SW-promoted cell growth was related to osteogenesis as demonstrated by increase of bone alkaline phosphatase activity in 6 days and osteocalcin mRNA expression in 12 days. In support that SW-induced Ras activation mediated osteogenesis of human bone marrow stromal cells, we further demonstrated that transfection of bone marrow stromal cells with a dominant negative Ras mutant (Asn-17 ras(H)) abrogated the SW enhancement of osteogenic transcription factor (CBFA1) activation, osteocalcin mRNA expression, and bone nodule formations. These results suggest that physical SW promotes bone marrow stromal cell differentiation toward osteogenic lineage via membrane hyperpolarization, followed by Ras activation and specific osteogenic transcription factor CBFA1 expression. A link between physical SW and biomembrane perturbation-mediated Ras activation may highlight how noninvasive physical agents could be used to promote fracture healing and to rescue patients with osteoporosis and osteopenic disorders in the future.

Phloretin differentially inhibits volume-sensitive and cyclic AMP-activated, but not Ca-activated, Cl(-) channels

Some phenol derivatives are known to block volume-sensitive Cl(-) channels. However, effects on the channel of the bisphenol phloretin, which is a known blocker of glucose uniport and anion antiport, have not been examined. In the present study, we investigated the effects of phloretin on volume-sensitive Cl(-) channels in comparison with cyclic AMP-activated CFTR Cl(-) channels and Ca(2+)-activated Cl(-) channels using the whole-cell patch-clamp technique. Extracellular application of phloretin (over 10 microM) voltage-independently, and in a concentration-dependent manner (IC(50) approximately 30 microM), inhibited the Cl(-) current activated by a hypotonic challenge in human epithelial T84, Intestine 407 cells and mouse mammary C127/CFTR cells. In contrast, at 30 microM phloretin failed to inhibit cyclic AMP-activated Cl(-) currents in T84 and C127/CFTR cells. Higher concentrations (over 100 microM) of phloretin, however, partially inhibited the CFTR Cl(-) currents in a voltage-dependent manner. At 30 and 300 microM, phloretin showed no inhibitory effect on Ca(2+)-dependent Cl(-) currents induced by ionomycin in T84 cells. It is concluded that phloretin preferentially blocks volume-sensitive Cl(-) channels at low concentrations (below 100 microM) and also inhibits cyclic AMP-activated Cl(-) channels at higher concentrations, whereas phloretin does not inhibit Ca(2+)-activated Cl(-) channels in epithelial cells.

Involvement of gap junctional communication in myogenesis

Cell-to-cell communication plays important roles in development and in tissue morphogenesis. Gap junctional intercellular communication (GJIC) has been implicated in embryonic development of various tissues and provides a pathway to exchange ions, secondary messengers, and metabolites through the intercellular gap junction channels. Although GJIC is absent in adult skeletal muscles, the formation of skeletal muscles involves a sequence of complex events including cell-cell interaction processes where myogenic cells closely adhere to each other. Much experimental evidence has shown that myogenic precursors and developing muscle fibers can directly communicate through junctional channels. This review summarizes current knowledge on the GJIC and developmental events involved in the formation of skeletal muscle fibers and describes recent progress in the investigation of the role of GJIC in myogenesis: evidence of gap junctions in somitic and myotomal tissue as well as in developing muscle fibers in situ, GJIC between perfusion myoblasts in culture, and involvement of GJIC in cytodifferentiation of skeletal muscle cells and in myoblast fusion. A model of intercellular signaling is proposed where GJIC participates to coordinate a multicellular population of interacting myogenic precursors to allow commitment to the skeletal muscle fate.

Role of hyperpolarization attained by linoleic acid in chick myoblast fusion

Our previous report has suggested that hyperpolarization generated by reciprocal activation of calcium-activated potassium (K(Ca)) channels and stretch-activated channels induces calcium influx that triggers myoblast fusion. Here we show that linoleic acid is involved in the process of generating hyperpolarization in cultured chick myoblasts and hence in promotion of the cell fusion. Linoleic acid dramatically hyperpolarized the membrane potential from -14 +/- 3 to -58 +/- 5 mV within 10 min. This effect was partially blocked by 1 mM tetraethylammonium (TEA) or 30 nM charybdotoxin, a selective K(Ca) channel inhibitor, and completely abolished by 10 mM TEA. Single-channel recordings revealed that linoleic acid activates TEA-resistant potassium channels as well as K(Ca) channels. Furthermore, linoleic acid induced calcium influx from extracellular solution, and this effect was partially blocked by 1 mM TEA and completely prevented at 10 mM, similar to the effect of TEA on linoleic acid-mediated hyperpolarization. Since the valinomycin-mediated hyperpolarization promoted calcium influx, hyperpolarization itself appears capable of inducing calcium influx. In addition, gadolinium prevented the valinomycin-mediated increase in intracellular calcium level under hypotonic conditions, revealing the involvement of stretch-activated channels in calcium influx. Furthermore, linoleic acid stimulated myoblast fusion, and this stimulatory effect could completely be prevented by 10 mM TEA. These results suggest that linoleic acid induces hyperpolarization of membrane potential by activation of potassium channels, which induces calcium influx through stretch-activated channels, and thereby triggers myoblast fusion.

Signaling mechanisms underlying the vascular myogenic response

The vascular myogenic response refers to the acute reaction of a blood vessel to a change in transmural pressure. This response is critically important for the development of resting vascular tone, upon which other control mechanisms exert vasodilator and vasoconstrictor influences. The purpose of this review is to summarize and synthesize information regarding the cellular mechanism(s) underlying the myogenic response in blood vessels, with particular emphasis on arterioles. When necessary, experiments performed on larger blood vessels, visceral smooth muscle, and even striated muscle are cited. Mechanical aspects of myogenic behavior are discussed first, followed by electromechanical coupling mechanisms. Next, mechanotransduction by membrane-bound enzymes and involvement of second messengers, including calcium, are discussed. After this, the roles of the extracellular matrix, integrins, and the smooth muscle cytoskeleton are reviewed, with emphasis on short-term signaling mechanisms. Finally, suggestions are offered for possible future studies.

A possible role for the pentose phosphate pathway of spermatozoa in gamete fusion in the mouse

Glucose metabolism is essential for successful gamete fusion in the mouse. Although the metabolic activity of the oocyte does not appear to play a significant role in the fusion step, the metabolic role of the spermatozoon is not known. The aim of this study was therefore to characterize the role of glucose metabolism in mouse spermatozoa. Initially, the high-affinity glucose transporter GLUT3 was identified in mouse sperm. In characterizing the glucose metabolism of mouse sperm, we have shown 1) that mouse epididymal spermatozoa have a functional pentose phosphate pathway (PPP), implying that they produce NADPH, which is required for reducing reactions, and ribose 5-phosphate, which is required for nucleic acid synthesis; and 2) that sperm are able to fuse with the oocyte when NADPH is substituted for glucose, suggesting that sperm need to produce NADPH via the PPP in order to be able to achieve fertilization. The existence of an NADPH-regulated event that influences the ability of the sperm to fuse with the oocyte is envisaged.