Trophic control of cholinesterase activity in a testosterone-dependent muscle of the rat: effects of castration and denervation

Identification of potential target genes associated with the reversion of androgen-dependent skeletal muscle atrophy

Muscle wasting or atrophy is extensively associated with human systemic diseases including diabetes, cancer, and kidney failure. Accumulating evidence from transcriptional profiles has noted that a common set of genes, termed atrogenes, is modulated in atrophying muscles. However, the transcriptional changes that trigger the reversion or attenuation of muscle atrophy have not been characterized at the molecular level until now. Here, we applied cDNA microarrays to investigate the transcriptional response of androgen-sensitive Levator ani muscle (LA) during atrophy reversion. Most of the differentially expressed genes behaved as atrogenes and responded to castration-induced atrophy. However, seven genes (APLN, DUSP5, IGF1, PIK3IP1, KLHL38, PI15, and MKL1) did not respond to castration but instead responded exclusively to testosterone replacement. Considering that almost all proteins encoded by these genes are associated with the reversion of atrophy and may function as regulators of cell proliferation/growth, our results provide new perspectives on the existence of anti-atrogenes.

The kinin B1 receptor regulates muscle-specific E3 ligases expression and is involved in skeletal muscle mass control

Regulation of muscle mass depends on the balance between synthesis and degradation of proteins, which is under the control of different signalling pathways regulated by hormonal, neural and nutritional stimuli. Such stimuli are altered in several pathologies, including COPD (chronic obstructive pulmonary disease), diabetes, AIDS and cancer (cachexia), as well as in some conditions such as immobilization and aging (sarcopenia), leading to muscle atrophy, which represents a significant contribution to patient morbidity. The KKS (kallikrein-kinin system) is composed of the enzymes kallikreins, which generate active peptides called kinins that activate two G-protein-coupled receptors, namely B1 and B2, which are expressed in a variety of tissues. The local modulation of the KKS may account for its participation in different diseases, such as those of the cardiovascular, renal and central nervous systems, cancer and many inflammatory processes, including pain. Owing to such pleiotropic actions of the KKS by local modulatory events and the probable fine-tuning of associated signalling cascades involved in skeletal muscle catabolic disorders [for example, NF-κB (nuclear factor κB) and PI3K (phosphoinositide 3-kinase)/Akt pathways], we hypothesized that KKS might contribute to the modulation of intracellular responses in atrophying skeletal muscle. Our results show that kinin B1 receptor activation induced a decrease in the diameter of C2C12 myotubes, activation of NF-κB, a decrease in Akt phosphorylation levels, and an increase in the mRNA levels of the ubiquitin E3 ligases atrogin-1 and MuRF-1 (muscle RING-finger protein-1). In vivo, we observed an increase in kinin B1 receptor mRNA levels in an androgen-sensitive model of muscle atrophy. In the same model, inhibition of the kinin B1 receptor with a selective antagonist resulted in an impairment of atrogin-1 and MuRF-1 expression and IκB (inhibitor of NF-κB) phosphorylation. Moreover, knockout of the kinin B1 receptor in mice led to an impairment in MuRF-1 mRNA expression after induction of LA (levator ani) muscle atrophy. In conclusion, using pharmacological and gene-ablation tools, we have obtained evidence that the kinin B1 receptor plays a significant role in the regulation of skeletal muscle proteolysis in the LA muscle atrophy model.

Testosterone inhibits transforming growth factor-β signaling during myogenic differentiation and proliferation of mouse satellite cells: potential role of follistatin in mediating testosterone action

Testosterone (T) administration is associated with increased satellite cell number and skeletal muscle hypertrophy, although there is considerable heterogeneity in the response of different skeletal muscle groups to T in vivo. We investigated the effects of T on the growth and differentiation of satellite cells isolated from levator ani (LA) and gastrocnemius (gastroc) muscles. T up regulated follistatin (Fst) expression, but down regulated the mRNA and protein expression of a number of genes in the transforming growth factor-beta (TGF-β)-signaling pathway. Inhibition of Fst expression by small interfering RNA (siRNA) inhibited myogenic differentiation and blocked the pro-myogenic effects of T. Treatment of satellite cells with T or Fst up regulated the expression of Pax7 and PCNA, and increased their proliferation. T and Fst blocked TGF-β induced inhibition of growth and myogenic differentiation and down regulated TGF-β-dependent transcriptome in both LA and gastroc cells. We conclude that T stimulation of satellite cell proliferation and myogenic differentiation are associated with up regulation of Fst and inhibition of TGF-β-signaling.

Gender-related differences in circadian rhythm of rat plasma acetyl- and butyrylcholinesterase: effects of sex hormone withdrawal

The role of acetylcholinesterase (AChE) in the termination of the cholinergic response through acetylcholine (ACh) hydrolysis and the involvement of plasma butyrylcholinesterase (BuChE), mainly of hepatic origin, in the metabolism of xenobiotics with ester bonds is well known. Besides, BuChE has a crucial role in ACh hydrolysis, especially when selective anticholinesterases inhibit AChE. Herein, we analyzed the gender-related differences and the circadian changes of rat plasma cholinesterases. Plasma and liver cholinesterase activities were evaluated in control or 2-30-day castrated adult male and female rats. Plasma and liver AChE activities did not differ between genders and were not influenced by sex hormone deprivation. BuChE plasma activity was 7 times greater in female, reflecting gender differences in liver enzyme expression. Castration increased liver and plasma BuChE activity in male, while reduced it in female, abolishing gender differences in enzyme activity. Interestingly, female AChE and BuChE plasma activities varied throughout the day, reaching values 27% and 42% lower, respectively, between 2 p.m. and 6 p.m. when compared to the morning peaks at 8 a.m. Castration attenuated daily female BuChE oscillation. On the other hand, male plasma enzymes remained constant throughout the day. In summary, our results show that liver and plasma BuChE, but not AChE, expression is influenced by sex hormones, leading to high levels of blood BuChE in females. The fluctuation of female plasma BuChE during the day should be taken into account to adjust the bioavailability and the therapeutic effects of cholinesterase inhibitors used in cholinergic-based conditions such Alzheimer's disease.

Testosterone represses ubiquitin ligases atrogin-1 and Murf-1 expression in an androgen-sensitive rat skeletal muscle in vivo

Skeletal muscle atrophy induced by denervation and metabolic diseases has been associated with increased ubiquitin ligase expression. In the present study, we evaluate the influence of androgens on muscle ubiquitin ligases atrogin-1/MAFbx/FBXO32 and Murf-1/Trim63 expression and its correlation with maintenance of muscle mass by using the testosterone-dependent fast-twitch levator ani muscle (LA) from normal or castrated adult male Wistar rats. Gene expression was determined by qRT-PCR and/or immunoblotting. Castration induced progressive loss of LA mass (30% of control, 90 days) and an exponential decrease of LA cytoplasm-to-nucleus ratio (nuclear domain; 22% of control after 60 days). Testosterone deprivation induced a 31-fold increase in LA atrogin-1 mRNA and an 18-fold increase in Murf-1 mRNA detected after 2 and 7 days of castration, respectively. Acute (24 h) testosterone administration fully repressed atrogin-1 and Murf-1 mRNA expression to control levels. Atrogin-1 protein was also increased by castration up to 170% after 30 days. Testosterone administration for 7 days restored atrogin-1 protein to control levels. In addition to the well known stimulus of protein synthesis, our results show that testosterone maintains muscle mass by repressing ubiquitin ligases, indicating that inhibition of ubiquitin-proteasome catabolic system is critical for trophic action of androgens in skeletal muscle. Besides, since neither castration nor androgen treatment had any effect on weight or ubiquitin ligases mRNA levels of extensor digitorum longus muscle, a fast-twitch muscle with low androgen sensitivity, our study shows that perineal muscle LA is a suitable in vivo model to evaluate regulation of muscle proteolysis, closely resembling human muscle responsiveness to androgens.

Increased expression of acetylcholine receptors in the diaphragm muscle of MDX mice

The absence of dystrophin in Duchenne muscular dystrophy (DMD) and in the mutant mdx mouse causes muscle degeneration and disruption of the neuromuscular junction. Based on evidence from the denervation-like properties of these muscles, we assessed the ligand-binding constants of nicotinic acetylcholine receptors (nAChRs) and the mRNA expression of individual subunits in membrane preparations of diaphragm muscles from adult (4-month-old) and aged (20-month-old) control and mdx mice. The concentration of nAChRs as determined by the maximal specific [(125)I]-alpha-bungarotoxin binding (Bmax) in the muscle membranes did not change with aging in both animal strains. When compared to age-matched control groups, the Bmax in mdx muscles was increased by 65% in adults, and by 103% in aged mice with no alteration of toxin affinity for nAChRs. Reverse-transcription polymerase chain reaction assays showed that mRNA transcripts for the nAChR alpha1, gamma, alpha7, and beta2, but not the epsilon subunits, were more abundant in mdx than in control muscles. The results indicate increased expression of extrajunctional nAChRs in the mdx diaphragm and reflect impairment of nAChR regulation in dystrophin-deficient muscles. These observations may be related to the resistance to nondepolarizing muscle relaxants and the high sensitivity to depolarizing agents reported in DMD patients.

Evidence of apoptosis in the castration-induced atrophy of the rat levator ani muscle

Apoptosis or programmed cell death has been previously reported in androgen-responsive tissue such as the prostate. We tested the hypothesis that apoptosis may also represent a component of the castration-induced atrophy of the sexually dimorphic levator ani (LA) muscle of the male rat. Gonadectomy (GDX) induced a severe decrease in the LA muscle wet weight accompanied by important modifications in cytoarchitecture including an increase in myofibrillar interspace and condensed mitochondria. These alterations were almost completely reversed after 7 days of testosterone propionate replacement therapy (GDX + TP). In GDX rats, internucleosomal DNA fragmentation was confirmed by both agarose gel electrophoresis and electron microscopy. DNA fragmentation was no longer detected in GDX + TP rats. In GDX rats, overexpression of the trpm-2/clusterin gene, used as an early marker of apoptosis, further confirmed that an apoptotic response, typical of androgen-responsive tissues, is taking place following androgen withdrawal.

Gender does not influence neuromuscular properties in dimorphic skeletal muscles of the toad

The aim of this work was to study gender differences on the physiology of the dimorphic brachial musculature involved in the clasp reflex of the toad (Bufo marinus L.). The neuromuscular transmission, the sensitivity to acetylcholine (ACh) and the cholinesterase activity were compared on the forelimb sternoradialis muscles (SR) from male and female toads. The interosseous muscles of the first finger were used to compare the properties of the nicotinic receptor/ionic channel complex (AChR). All the muscles studied were dimorphic, i.e. significantly smaller in the female than in the male frog in otherwise similar size animals. The SR of either sex contracted to bath application of ACh with similar EC50. In physiological solution the frequency of the miniature end-plate potentials (mepps) was very low (0.1 s-1) and no gender difference was detected. The mepp amplitudes were 0.62 +/- 0.03 and 0.58 +/- 0.03 mV in SR from male and female toads, respectively. To increase exocytosis the muscles were incubated in hypertonic solution (158 mM NaCl). Under this condition mepp frequency was increased by five and seven times and mepp amplitude increased by 1.3 and 1.6 times in SR from male and female toads, respectively. The cholinesterase activity measured by the colorimetric method, did not differ in SR from male and female toads. In muscle fibers dissociated from the dimorphic interosseous muscles of male and female toads, the ionic channel conductance was 43 +/- 5.3 and 44 +/- 4.5 pS, respectively. The mean channel open time was voltage-dependent and not significantly different in preparations from both genders. These observations indicate that neither the ACh-nicotinic receptor interaction, nor the AChR complex kinetics and the nicotinic excitation-contraction coupling or the cholinesterase activity differ in dimorphic muscles from Bufo genders. No gender difference was detected in neuromuscular transmission of the studied muscle. Only a slight increase in mepp frequency and amplitude could be detected when the muscles were incubated in hypertonic solution.

Cholinergic-androgenic interaction in the regulation of male sexual behavior in rats

It is well known that testosterone (T) plays a major role in the adequate expression of male sexual behavior. On the other hand, the stimulation of central muscarinic receptors with oxotremorine (OXO), a specific agonist, facilitates the expression of masculine sexual behavior in rats. In this study, we analyzed the relationship between T and the cholinergic system, recording the effect of OXO administration on masculine sexual behavior in gonadectomized rats, before and after receiving T treatment. Sexually experienced and unexperienced male rats were gonadectomized and periodically tested for sexual behavior. Once this behavior was absent or drastically reduced, males were treated with OXO or saline and sexual behavior was assessed. 112 or 167 days later, animals were treated daily with T during 21 days, recording the response to OXO in days 12 and 21. Sexual behavior decreased more rapidly after gonadectomy in inexperienced males. OXO administration did not improve sexual performance in either group. The administration of T rapidly restored sexual behavior of experienced males: recovery of inexperienced males was slower. In the presence of T, OXO administration exerted a facilitative effect on sexual performance of both groups. These results suggest that the facilitative effect of muscarinic stimulation on sexual behavior requires the presence of testosterone.

Prejunctional regulatory actions of androgens on a hormone sensitive muscle

The influence of androgens and time course of effects induced by hormone deprivation were examined on the spontaneous transmitter release in the levator ani (LA) muscle of 30-180-day-old male rats. The resting membrane potential (RMP) and miniature endplate potentials (mepps) were recorded intracellularly from LA muscle fibers of intact animals or gonadectomized at different ages. In intact animals, the frequency of mepps increased proportionately to the muscle fiber growth up to 60 days, stabilizing thereafter. Gonadectomy at any age did not affect the RMP, but increased the frequency of mepps by 65% to 140%. The effect was detected after 15 days and was unrelated to the degree of muscle atrophy. Independently of the age of gonadectomy control values of mepp frequency were restored after 90 days, while the accompanying postjunctional changes persisted. These results indicate that androgens exert a prejunctional inhibitory influence on the spontaneous transmitter release in the rat LA muscle. The transient nature of the prejunctional effect induced by hormone deprivation indicates an adjustment of nerve terminals to persistent postjunctional alterations.

Testosterone control of endplate and non-endplate acetylcholinesterase in the rat levator ani muscle

The time course of effects of castration (5-60 days) and testosterone treatment (15-60 days) of adult male rats were examined on the endplate (+EP) and non-endplate (-EP) acetylcholinesterase (AChE) of the androgen-dependent levator ani (LA) muscle. The thiocholine method was used to determine the enzyme activity. Castration caused LA muscle atrophy within 5 days but reduced the -EP and +EP AChE after 10 and 20 days, respectively. Following 30 days castration -EP and +EP AChE reached respectively 41% and 35% of control activity. Testosterone retrieval restored the control values of both muscle weight and total AChE after 15 and 60 days, respectively. Recovery of the +EP AChE preceded that of -EP AChE by 30 days. The results showed that in the rat LA muscle, +EP and -EP AChE depend on a continuous testosterone regulation that predominates at +EP region spreading thereafter to -EP region. Those data suggest a hormone regulation of AChE exerted indirectly through the synthesis and release of neurotrophic substances.

The roles of sex, innervation, and androgen in laryngeal muscle of Xenopus laevis

The relative contributions of innervation and androgen to three muscle fiber properties--twitch type, size, and number--were examined in the sexually dimorphic, androgen-sensitive laryngeal muscle of Xenopus laevis. In adults, the muscle contains all fast-twitch fibers in males and fast- and slow-twitch fibers in females; laryngeal muscle fibers are larger and more numerous in males than in females. Juvenile larynges are female-like in both sexes; male laryngeal muscle is subsequently masculinized by androgen secretion during postmetamorphic development. Because both laryngeal motor neurons and muscle fibers are androgen sensitive during masculinization, we examined the role of the nerve in androgen-regulated muscle fiber development. Laryngeal muscle of male and female juvenile frogs was unilaterally denervated, and effects on muscle fiber type, size, and number were examined 4 weeks later. Half of the frogs received a dihydrotestosterone pellet at the time of denervation. Androgen treatment converts laryngeal muscle from mixed slow and fast to all fast twitch in both innervated and denervated muscle. Thus, the nerve is not required for androgen-regulated fiber type expression in either sex. Denervation produces muscle fiber atrophy and androgen treatment induces muscle fiber hypertrophy in male and female larynx. Nerve and hormone effects are independent and additive; fiber size in androgen-treated denervated muscle is greater than in untreated innervated muscle, and fiber size in androgen-treated denervated muscle is smaller than in androgen-treated innervated muscle. There is no sex difference in the effects of innervation or androgen on fiber size. Denervation causes laryngeal muscle fiber loss in males but not in females. Androgen treatment protects male laryngeal muscle from denervation-induced fiber loss and causes fiber addition in innervated female laryngeal muscle. We conclude that there is a sexually dimorphic interaction between innervation and androgen in control of laryngeal muscle fiber number.

Differential regulation of alpha-calcitonin gene-related peptide and preprocholecystokinin messenger RNA expression in alpha-motoneurons: effects of testosterone and inactivity induced factors

alpha-Calcitonin gene-related peptide expression in alpha-motoneurons is regulated by spinal cord transection, axotomy and testosterone, but to date there are no studies which examine the regulation of cholecystokinin expression in motoneurons. In the present study, we compared the regulation of preprocholecystokinin and alpha-calcitonin gene-related peptide messenger RNA levels in motoneurons of the spinal nucleus of the bulbocavernosus. Previously, we demonstrated that manipulations which decrease activity in target muscles of the spinal nucleus of the bulbocavernosus motoneurons increase alpha-calcitonin gene-related peptide message and peptide levels in spinal nucleus of the bulbocavernosus motoneurons. This muscle-nerve interaction is mediated by a soluble factor which is increased by castration. We now report that decreasing plasma testosterone levels decreased preprocholecystokinin messenger RNA levels. Testosterone replacement at the time of castration restored preprocholecystokinin messenger RNA levels to intact values. Injections of crude extracts prepared from denervated bulbocavernosus/levator ani into the homologous muscles of gonadally intact rats increased the levels of alpha-calcitonin gene-related peptide messenger RNA in spinal nucleus of the bulbocavernosus motoneurons. The levels of preprocholecystokinin messenger RNA did not differ in rats injected with denervated bulbocavernosus/levator ani extract or buffer, both of which were significantly higher than in intact, untreated rats. The results of the present experiments imply that levels of preprocholecystokinin and alpha-calcitonin gene-related peptide messenger ribonucleic acid are differentially regulated in spinal nucleus of the bulbocavernosus motoneurons.

Effect of denervation on the androgen-induced expression of actin and CPK mRNAs in the levator ani muscle of the rat

In the adult male rat, the castration-induced atrophy of the levator ani (LA) muscle was found to be associated with a decrease in the relative levels of both actin and creatine phosphokinase (CPK) mRNAs. The typical recovery of these two sequences following 5 days of testosterone propionate (TP) replacement therapy was not impaired by the bilateral denervation of the LA. This indicated that TP was the sole trophic factor regulating the plasticity of these two mRNAs and challenged the hypothesis that androgen action might be neuronally mediated. The observation that denervation led to a severe repression of both actin and CPK messages only in the absence of TP replacement therapy suggested that the nerve impulse could play an accessory role in the control of their expression.

Trophic control of cholinesterase activity in a testosterone-dependent muscle of the rat. II. Effects of testosterone administration

The effects of testosterone on the weight, protein content, and acetylcholinesterase (AChE) activity were investigated in the hormone-dependent levator ani and nondependent extensor digitorum longus and soleus muscles from normal or castrated male rats. In either group some muscles were also chronically denervated. Testosterone propionate treatment (3 mg/week for 2 weeks, s.c.) of normal rats increased the weight and protein content of the levator ani, respectively, by 19% and 63%; the muscle AChE was not affected. Protein content, but not the weight of the normal extensor digitorum longus and soleus was also increased after testosterone; AChE was reduced by 20% in the extensor digitorum longus and unaltered in the soleus. In castrated rats, testosterone reversed the levator ani atrophy and slowed down the decay of AChE, but it did not restore the normal enzyme activity. Testosterone did not prevent the atrophy and AChE decrease induced by denervation of either muscle. The weight and protein content of the denervated levator ani from castrated rats were increased by testosterone to the values found in denervated muscles from normal rats; AChE in the same muscles was not increased. The results confirm that separate mechanisms regulate protein synthesis and AChE in the rat levator ani. AChE is mainly regulated by neural factors which in turn appear to be influenced by circulating androgens. Similar hormonal influence on the muscle AChE was not detected in the extensor digitorum longus and soleus muscles.