SOX7 Is Required for Muscle Satellite Cell Development and Maintenance.

Satellite cells are skeletal-muscle-specific stem cells that are activated by injury to proliferate, differentiate, and fuse to enable repair. SOX7, a member of the SRY-related HMG-box family of transcription factors is expressed in quiescent satellite cells. To elucidate SOX7 function in skeletal muscle, we knocked down Sox7 expression in embryonic stem cells and primary myoblasts and generated a conditional knockout mouse in which Sox7 is excised in PAX3+ cells. Loss of Sox7 in embryonic stem cells reduced Pax3 and Pax7 expression. In vivo, conditional knockdown of Sox7 reduced the satellite cell population from birth, reduced myofiber caliber, and impaired regeneration after acute injury. Although Sox7-deficient primary myoblasts differentiated normally, impaired myoblast fusion and increased sensitivity to apoptosis in culture and in vivo were observed. Taken together, these results indicate that SOX7 is dispensable for myogenesis but is necessary to promote satellite cell development and survival.

Acute exposure to progesterone attenuates cardiac contraction by modifying myofilament calcium sensitivity in the female mouse heart.

Acute application of progesterone attenuates cardiac contraction, although the underlying mechanisms are unclear. We investigated whether progesterone modified contraction in isolated ventricular myocytes and identified the Ca2+ handling mechanisms involved in female C57BL/6 mice (6-9 mo; sodium pentobarbital anesthesia). Cells were field-stimulated (4 Hz; 37°C) and exposed to progesterone (0.001-10.0 µM) or vehicle (35 min). Ca2+ transients (fura-2) and cell shortening were recorded simultaneously. Maximal concentrations of progesterone inhibited peak contraction by 71.4% (IC50 = 160 ± 50 nM; n = 12) and slowed relaxation by 75.4%. By contrast, progesterone had no effect on amplitudes or time courses of underlying Ca2+ transients. Progesterone (1 µM) also abbreviated action potential duration. When the duration of depolarization was controlled by voltage-clamp, progesterone attenuated contraction and slowed relaxation but did not affect Ca2+ currents, Ca2+ transients, sarcoplasmic reticulum (SR) content, or fractional release of SR Ca2+ Actomyosin MgATPase activity was assayed in myofilaments from hearts perfused with progesterone (1 µM) or vehicle (35 min). While maximal responses to Ca2+ were not affected by progesterone, myofilament Ca2+ sensitivity was reduced (EC50 = 0.94 ± 0.01 µM for control, n = 7 vs. 1.13 ± 0.05 µM for progesterone, n = 6; P < 0.05) and progesterone increased phosphorylation of myosin binding protein C. The effects on contraction were inhibited by lonaprisan (progesterone receptor antagonist) and levosimendan (Ca2+ sensitizer). Unlike results in females, progesterone had no effect on contraction or myofilament Ca2+ sensitivity in age-matched male mice. These data indicate that progesterone reduces myofilament Ca2+ sensitivity in female hearts, which may exacerbate manifestations of cardiovascular disease late in pregnancy when progesterone levels are high. NEW & NOTEWORTHY: We investigated myocardial effects of acute application of progesterone. In females, but not males, progesterone attenuates and slows cardiomyocyte contraction with no effect on calcium transients. Progesterone also reduces myofilament calcium sensitivity in female hearts. This may adversely affect heart function, especially when serum progesterone levels are high in pregnancy.Listen to this article's corresponding podcast at https://ajpheart.podbean.com/e/acute-progesterone-modifies-cardiac-contraction/.

Cardiac contraction, calcium transients, and myofilament calcium sensitivity fluctuate with the estrous cycle in young adult female mice.

This study established conditions to induce regular estrous cycles in female C57BL/6J mice and investigated the impact of the estrous cycle on contractions, Ca2+ transients, and underlying cardiac excitation-contraction (EC)-coupling mechanisms. Daily vaginal smears from group-housed virgin female mice were stained to distinguish estrous stage (proestrus, estrus, metestrus, diestrus). Ventricular myocytes were isolated from anesthetized mice. Contractions and Ca2+ transients were measured simultaneously (4 Hz, 37 °C). Interestingly, mice did not exhibit regular cycles unless they were exposed to male pheromones in bedding added to their cages. Field-stimulated myocytes from mice in estrus had larger contractions (∼2-fold increase), larger Ca2+ transients (∼1.11-fold increase), and longer action potentials (>2-fold increase) compared with other stages. Larger contractions and Ca2+ transients were not observed in estrus myocytes voltage-clamped with shorter action potentials. Voltage-clamp experiments also demonstrated that estrous stage had no effect on Ca2+ current, EC-coupling gain, diastolic Ca2+, sarcoplasmic reticulum (SR) Ca2+ content, or fractional release. Although contractions were largest in estrus, myofilament Ca2+ sensitivity was lowest (EC50 values ∼1.15-fold higher) in conjunction with increased phosphorylation of myosin binding protein C in estrus. Contractions were enhanced in ventricular myocytes from mice in estrus because action potential prolongation increased SR Ca2+ release. These findings demonstrate that cyclical changes in reproductive hormones associated with the estrous cycle can influence myocardial electrical and contractile function and modify Ca2+ homeostasis. However, such changes are unlikely to occur in female mice housed in groups under conventional conditions, since these mice do not exhibit regular estrous cycles.

Ovariectomy enhances SR Ca²âº release and increases Ca²âº spark amplitudes in isolated ventricular myocytes.

This study compared Ca(2+) homeostasis in ventricular myocytes from 8-month-old female C57BL/6J mice that had either a bilateral ovariectomy (OVX) or a sham surgery at 3 weeks of age. Cells were loaded with fura-2 and field-stimulated or voltage-clamped with steps to membrane potentials between -40 and +80 mV (37°C). Peak Ca(2+) transients increased by two-fold in OVX myocytes when compared to sham, and Ca(2+) transient rates of rise and decay were faster in OVX cells. In contrast, Ca(2+) current densities were similar in sham and OVX cells. Sarcoplasmic reticulum (SR) Ca(2+) content, assessed by caffeine, also was higher in OVX compared to sham cells (111.7 ± 11.9 vs. 61.2 ± 10.4 nM; p<0.05). Furthermore, the gain of Ca(2+) release (Ca(2+) release/Ca(2+) current) was significantly greater in OVX than in sham cells (16.3 ± 2.5 vs. 7.7 ± 2.0 nM/pApF(-1) at 0 mV; p<0.05). As changes in unitary Ca(2+) release might account for the increased gain in OVX cells, spontaneous Ca(2+) sparks were compared in fluo-4-loaded myocytes (37°C). Spark frequency was higher in OVX cells than in sham cells. In addition, spark amplitudes were greater in OVX than in sham myocytes (ΔF/F(0)=0.379 ± 0.006 vs. 0.342 ± 0.006; p<0.05). However, spark widths and time courses were similar in the two groups. These data suggest that the size of individual SR Ca(2+) release units is larger and the SR Ca(2+) content is greater in myocytes of OVX mice, producing augmented gain and SR Ca(2+) release. These observations show that OVX disrupts intracellular Ca(2+) homeostasis and suggest that sex steroid hormones modulate unitary Ca(2+) release in individual cardiac myocytes.

A procedure for creating a frailty index based on deficit accumulation in aging mice.

This study developed an approach to quantify frailty with a frailty index (FI) and investigated whether age-related changes in contractions, calcium transients, and ventricular myocyte length were more prominent in mice with a high FI. The FI combined 31 variables that reflect different aspects of health in middle-aged (∼12 months) and aged (∼30 months) mice of both sexes. Aged animals had a higher FI than younger animals (FI = 0.43 ± 0.03 vs 0.08 ± 0.02, p < .001, n = 12). Myocyte hypertrophy increased by 30%-50% as the FI increased in aged animals. Peak contractions decreased more than threefold from lowest to highest FI values in aged mice (p < .037), but calcium transients were unaffected. Similar results were seen with an FI based on eight noninvasive variables identified as underlying factors. These results show that an FI can be developed for murine models and suggest that age-associated changes in myocytes are more prominent in animals with a high FI.