Daniel Sandoval

The ascorbic acid transporter SVCT2 is expressed in slow-twitch skeletal muscle fibres

Ascorbic acid, the reduced form of vitamin C, functions as a potent antioxidant as well as in cell differentiation. Ascorbate is taken up by mammalian cells through the specific sodium/ascorbate co-transporters SVCT1 and SVCT2. Although skeletal muscle contains about 50% of the whole-body vitamin C, the expression of SVCT transporters has not been clearly addressed in this tissue. In this work, we analysed the expression pattern of SVCT2 during embryonic myogenesis using the chick as model system. We cloned the chick orthologue of SVCT2 (cSVCT2) that shares 93% identity with the mouse transporter. cSVCT2 mRNA and protein are expressed during chick embryonic muscle development. Immunohistochemical analyses showed that SVCT2 is preferentially expressed by type I slow-twitch muscle fibres throughout chick myogenesis as well as in post-natal skeletal muscles of several species, including human. Our results suggest that SVCT2-mediated uptake of ascorbate is relevant to the oxidative nature of type I muscle fibres.

Up-regulation of the vitamin C transporter SVCT2 upon differentiation and depolarization of myotubes

In addition to its role as a strong antioxidant, vitamin C regulates the differentiation of several cell lineages. In vertebrate skeletal muscle, the vitamin C transporter SVCT2 is preferentially expressed in slow muscle fibers. To gain insights into the possible involvement of intracellular vitamin C on early myogenesis, we investigated the regulation of SVCT2 expression in cultures of chick fetal myoblasts. SVCT2 expression increases in cultures of both, slow and fast muscle‐derived myoblasts, as they fuse to form mainly fast myotubes. Interestingly, we found that SVCT2 could be positively modulated by potassium‐induced depolarization of myotubes. These findings suggest that SVCT2‐mediated uptake of vitamin C could play diverse roles on skeletal muscle development and physiology.

Frizzled-9 impairs acetylcholine receptor clustering in skeletal muscle cells

Cumulative evidence indicates that Wnt pathways play crucial and diverse roles to assemble the neuromuscular junction (NMJ), a peripheral synapse characterized by the clustering of acetylcholine receptors (AChR) on postsynaptic densities. The molecular determinants of Wnt effects at the NMJ are still to be fully elucidated. We report here that the Wnt receptor Frizzled-9 (Fzd9) is expressed in developing skeletal muscles during NMJ synaptogenesis. In cultured myotubes, gain- and loss-of-function experiments revealed that Fzd9-mediated signaling impairs the AChR-clustering activity of agrin, an organizer of postsynaptic differentiation. Overexpression of Fzd9 induced the cytosolic accumulation of β-catenin, a key regulator of Wnt signaling. Consistently, Fzd9 and β-catenin localize in the postsynaptic domain of embryonic NMJs in vivo. Our findings represent the first evidence pointing to a crucial role of a Fzd-mediated, β-catenin-dependent signaling on the assembly of the vertebrate NMJ.

The vitamin C transporter SVCT2 is down-regulated during postnatal development of slow skeletal muscles

Vitamin C plays key roles in cell homeostasis, acting as a potent antioxidant as well as a positive modulator of cell differentiation. In skeletal muscle, the vitamin C/sodium co-transporter SVCT2 is preferentially expressed in oxidative slow fibers. Besides, SVCT2 is up-regulated upon the early fusion of primary myoblasts. However, our knowledge of the postnatal expression profile of SVCT2 remains scarce. Here we have analyzed the expression of SVCT2 during postnatal development of the chicken slow anterior and fast posterior latissimus dorsi muscles, ranging from day 7 to adulthood. SVCT2 expression is consistently higher in the slow than in the fast muscle at all stages. After hatching, SVCT2 expression is significantly down-regulated in the anterior latissimus dorsi, which nevertheless maintains a robust slow phenotype. Taking advantage of the C2C12 cell line to recapitulate myogenesis, we confirmed that SVCT2 is expressed in a biphasic fashion, reaching maximal levels upon early myoblasts fusion and decreasing during myotube growth. Together, these findings suggest that the dynamic expression levels of SVCT2 could be relevant for different features of skeletal muscle physiology, such as muscle cell formation, growth and activity.

Enlarged adipocytes in subcutaneous adipose tissue associated to hyperandrogenism and visceral adipose tissue volume in women with polycystic ovary syndrome

Context: Polycystic ovary syndrome (PCOS) is an androgen excess disorder associated with obesity and adipose tissue disturbances. Our aim was to evaluate gene expression of adipocytokines and adipocyte characteristics in abdominal subcutaneous adipose tissue (SAT) of PCOS women. Design: Twelve PCOS (PCOSw) and 12 control (Cw) premenopausal women (BMI 20–35 kg/m2) were included, with measurements of whole‐body composition assessed by dual‐energy X‐ray absorptiometry, and abdominal subcutaneous and visceral adipose tissue (VAT) volume, by magnetic resonance imaging. An oral glucose tolerance test was performed with measurements of glucose and insulin, and sex steroids, lipid profile and serum adipocytokines were determined in the fasting sample. Adipocytokine gene expression, mean adipocyte area and macrophage infiltration were evaluated in SAT biopsies. Results: Both groups were comparable in age and BMI. Trunk fat mass amount (p = .043), serum and SAT leptin/adiponectin ratio (p = .034 and p = .028, respectively) and adipocyte area (p = .015) were higher in PCOSw compared to Cw. Interestingly, trunk fat mass was positively correlated with adipocyte area in PCOSw (r = 0.821, p = .023), while the inverse correlation was found in Cw (r = −0.786, p = .021). Only in PCOSw, adipocyte area was positively correlated with serum testosterone (r = 0.857, p = .014) and visceral adipose tissue volume (r = 0.857, p = .014). Conclusions: Our results indicate that PCOS women present adipose tissue dysfunction in the subcutaneous compartment, characterized by an alteration in adipocyte size and leptin/adiponectin expression and secretion, probably associated with higher androgen concentrations. HighlightsPCOS promote metabolic derangements which include adipose tissue dysfunction.PCOS women have larger subcutaneous adipocytes and augmented leptin/adiponectin ratio.Adipose tissue dysfunction is an inherent feature of PCOS caused by androgens.

Long-term testosterone treatment during pregnancy does not alter insulin or glucose profile in a sheep model of polycystic ovary syndrome

Abstract The administration of testosterone to pregnant sheep to resemble fetal programming of the polycystic ovary syndrome could alter other hormones/factors of maternal origin with known effects on fetal growth. Hence, we studied the weekly profile of insulin, progesterone and glucose during a treatment with testosterone propionate given biweekly from weeks 5 to 17 of pregnancy (term at 21 weeks) and checked the outcome of their fetuses at 17 weeks of gestation after C-section. Control dams were only exposed to the vehicle of the hormone. The testosterone administration did not cause any significant change in the maternal weekly profile of insulin, progesterone or glucose concentration, although the plasma levels of testosterone in the treated dams were inversely correlated to the levels of progesterone. Testosterone treatment also induced an inverse correlation between mean maternal insulin levels and fetal insulin levels; however, the fetal zoometric parameters, body weight, or insulin levels did not differ between exposed and not exposed fetuses. Therefore, treatment with testosterone during pregnancy does not cause significant impact on insulin levels in the mother, leading to less effect on the programming of fetal growth.