Muscle mechano growth factor is preferentially induced by growth hormone in growth hormone-deficient lit/lit mice

Surface modification of electrospun fibers with mechano-growth factor for mitigating the foreign-body reaction

The implantation of synthetic polymeric scaffolds induced foreign-body reaction (FBR) seriously influence the wound healing and impair functionality recovery. A novel short peptide, mechano-growth factor (MGF), was introduced in this study to modify an electrospun polycaprolactone (PCL) fibrous scaffold to direct the macrophage phenotype transition and mitigate the FBR. In vitro studies discovered the cell signal transduction mechanism of MGF regulates the macrophage polarization via the expression of related genes and proteins. We found that macrophages response the MGF stimuli via endocytosis, then MGF promotes the histone acetylation and upregulates the STAT6 expression to direct an anti-inflammatory phenotype transition. Subsequently, an immunoregulatory electrospun PCL fibrous scaffold was modified by silk fibroin (SF) single-component layer-by-layer assembly, and the SF was decorated with MGF via click chemistry. Macrophages seeded on scaffold to identify the function of MGF modified scaffold in directing macrophage polarization in vitro. Parallelly, rat subcutaneous implantation model and rat tendon adhesion model were performed to detect the immunomodulatory ability of the MGF-modified scaffold in vivo. The results demonstrate that MGF-modified scaffold is beneficial to the transformation of macrophages to M2 phenotype in vitro. More importantly, MGF-functionalized scaffold can inhibit the FBR at the subcutaneous tissue and prevent tissue adhesion.

Chronic doxorubicin administration impacts satellite cell and capillary abundance in a muscle-specific manner

Anthracycline chemotherapies are effective at reducing disease recurrence and mortality in cancer patients. However, these drugs also contribute to skeletal muscle wasting and dysfunction. The purpose of this study was to assess the impact of chronic doxorubicin (DOX) administration on satellite cell and capillary densities in different skeletal muscles. We hypothesized that DOX would reduce satellite cell and capillary densities of the soleus (SOL) and extensor digitorum longus (EDL) muscles, along with muscle fiber size. Ovariectomized female Sprague-Dawley rats were randomized to receive three bi-weekly intraperitoneal injections of DOX (4 mg∙kg-1 ; cumulative dose 12 mg∙kg-1 ) or vehicle (VEH; saline). Animals were euthanized 5d following the last injection and the SOL and EDL were dissected and prepared for immunohistochemical and RT-qPCR analyses. Relative to VEH, CSA of the SOL and EDL fibers were 26% and 33% smaller, respectively, in DOX (P < 0.05). In the SOL, satellite cell and capillary densities were 39% and 35% lower, respectively, in DOX (P < 0.05), whereas in the EDL satellite cell and capillary densities were unaffected by DOX administration (P > 0.05). Proliferating satellite cells were unaffected by DOX in the SOL (P > 0.05). In the SOL, MYF5 mRNA expression was increased in DOX (P < 0.05), while in the EDL MGF mRNA expression was reduced in DOX (P < 0.05). Chronic DOX administration is associated with reduced fiber size in the SOL and EDL; however, DOX appeared to reduce satellite cell and capillary densities only in the SOL. These findings highlight that therapeutic targets to protect skeletal muscle from DOX may vary across muscles.

Putative roles of soluble trophic factors in facial nerve regeneration, target reinnervation, and recovery of vibrissal whisking

It is well-known that, after nerve transection and surgical repair, misdirected regrowth of regenerating motor axons may occur in three ways. The first way is that the axons enter into endoneurial tubes that they did not previously occupy, regenerate through incorrect fascicles and reinnervate muscles that they did not formerly supply. Consequently the activation of these muscles results in inappropriate movements. The second way is that, in contrast with the precise target-directed pathfinding by elongating motor nerves during embryonic development, several axons rather than a single axon grow out from each transected nerve fiber. The third way of misdirection occurs by the intramuscular terminal branching (sprouting) of each regenerating axon to culminate in some polyinnervation of neuromuscular junctions, i.e. reinnervation of junctions by more than a single axon. Presently, "fascicular" or "topographic specificity" cannot be achieved and hence target-directed nerve regeneration is, as yet, unattainable. Nonetheless, motor and sensory reinnervation of appropriate endoneurial tubes does occur and can be promoted by brief nerve electrical stimulation. This review considers the expression of neurotrophic factors in the neuromuscular system and how this expression can promote functional recovery, with emphasis on the whisking of vibrissae on the rat face in relationship to the expression of the factors. Evidence is reviewed for a role of neurotrophic factors as short-range diffusible sprouting stimuli in promoting complete functional recovery of vibrissal whisking in blind Sprague Dawley (SD)/RCS rats but not in SD rats with normal vision, after facial nerve transection and surgical repair. Briefly, a complicated time course of growth factor expression in the nerves and denervated muscles include (1) an early increase in FGF2 and IGF2, (2) reduced NGF between 2 and 14days after nerve transection and surgical repair, (3) a late rise in BDNF and (4) reduced IGF1 protein in the denervated muscles at 28days. These findings suggest that recovery of motor function after peripheral nerve injury is due, at least in part, to a complex regulation of nerve injury-associated neurotrophic factors and cytokines at the neuromuscular junctions of denervated muscles. In particular, the increase of FGF2 and concomittant decrease of NGF during the first week after facial nerve-nerve anastomosis in SD/RCS blind rats may prevent intramuscular axon sprouting and, in turn, reduce poly-innervation of the neuromuscular junction.

Increased cutaneous miR-let-7d expression correlates with small nerve fiber pathology in patients with fibromyalgia syndrome

Fibromyalgia syndrome (FMS) is a chronic widespread pain condition probably comprising subgroups with different underlying pathomechanisms. There is increasing evidence for small nerve fiber impairment in subgroups of patients with FMS. MicroRNAs (miRNAs) regulate molecular factors determining nerve de- and re-generation. We investigated whether systemic and cutaneous miRNA expression in patients with FMS is related to small nerve fiber pathology. We confirmed previous findings of disturbed small fiber function and reduced intraepidermal nerve fiber density in subgroups of patients with FMS. We found 51 aberrantly expressed miRNAs in white blood cells of patients with FMS, of which miR-let-7d correlated with reduced small nerve fiber density in patients with FMS. Furthermore, we demonstrated miR-let-7d and its downstream target insulin-like growth factor-1 receptor as being aberrantly expressed in skin of patients with FMS with small nerve fiber impairment. Our study gives further evidence of small nerve fiber pathology in FMS subgroups and provides a missing link in the pathomechanism that may lead to small fiber loss in subgroups of patients with FMS.

Comparison of trophic factors' expression between paralyzed and recovering muscles after facial nerve injury. A quantitative analysis in time course

After peripheral nerve injury, recovery of motor performance negatively correlates with the poly-innervation of neuromuscular junctions (NMJ) due to excessive sprouting of the terminal Schwann cells. Denervated muscles produce short-range diffusible sprouting stimuli, of which some are neurotrophic factors. Based on recent data that vibrissal whisking is restored perfectly during facial nerve regeneration in blind rats from the Sprague Dawley (SD)/RCS strain, we compared the expression of brain derived neurotrophic factor (BDNF), fibroblast growth factor-2 (FGF2), insulin growth factors 1 and 2 (IGF1, IGF2) and nerve growth factor (NGF) between SD/RCS and SD-rats with normal vision but poor recovery of whisking function after facial nerve injury. To establish which trophic factors might be responsible for proper NMJ-reinnervation, the transected facial nerve was surgically repaired (facial-facial anastomosis, FFA) for subsequent analysis of mRNA and proteins expressed in the levator labii superioris muscle. A complicated time course of expression included (1) a late rise in BDNF protein that followed earlier elevated gene expression, (2) an early increase in FGF2 and IGF2 protein after 2 days with sustained gene expression, (3) reduced IGF1 protein at 28 days coincident with decline of raised mRNA levels to baseline, and (4) reduced NGF protein between 2 and 14 days with maintained gene expression found in blind rats but not the rats with normal vision. These findings suggest that recovery of motor function after peripheral nerve injury is due, at least in part, to a complex regulation of lesion-associated neurotrophic factors and cytokines in denervated muscles. The increase of FGF-2 protein and concomittant decrease of NGF (with no significant changes in BDNF or IGF levels) during the first week following FFA in SD/RCS blind rats possibly prevents the distal branching of regenerating axons resulting in reduced poly-innervation of motor endplates.

The effect of training volume and intensity on improvements in muscular strength and size in resistance-trained men

This investigation compared the effect of high-volume (VOL) versus high-intensity (INT) resistance training on stimulating changes in muscle size and strength in resistance-trained men. Following a 2-week preparatory phase, participants were randomly assigned to either a high-volume (VOL; n = 14, 4 × 10-12 repetitions with ~70% of one repetition maximum [1RM], 1-min rest intervals) or a high-intensity (INT; n = 15, 4 × 3-5 repetitions with ~90% of 1RM, 3-min rest intervals) training group for 8 weeks. Pre- and posttraining assessments included lean tissue mass via dual energy x-ray absorptiometry, muscle cross-sectional area and thickness of the vastus lateralis (VL), rectus femoris (RF), pectoralis major, and triceps brachii muscles via ultrasound images, and 1RM strength in the back squat and bench press (BP) exercises. Blood samples were collected at baseline, immediately post, 30 min post, and 60 min postexercise at week 3 (WK3) and week 10 (WK10) to assess the serum testosterone, growth hormone (GH), insulin-like growth factor-1 (IGF1), cortisol, and insulin concentrations. Compared to VOL, greater improvements (P < 0.05) in lean arm mass (5.2 ± 2.9% vs. 2.2 ± 5.6%) and 1RM BP (14.8 ± 9.7% vs. 6.9 ± 9.0%) were observed for INT. Compared to INT, area under the curve analysis revealed greater (P < 0.05) GH and cortisol responses for VOL at WK3 and cortisol only at WK10. Compared to WK3, the GH and cortisol responses were attenuated (P < 0.05) for VOL at WK10, while the IGF1 response was reduced (P < 0.05) for INT. It appears that high-intensity resistance training stimulates greater improvements in some measures of strength and hypertrophy in resistance-trained men during a short-term training period.

Chronic alterations in growth hormone/insulin-like growth factor-I signaling lead to changes in mouse tendon structure

The growth hormone/insulin-like growth factor-I (GH/IGF-I) axis is an important stimulator of collagen synthesis in connective tissue, but the effect of chronically altered GH/IGF-I levels on connective tissue of the muscle-tendon unit is not known. We studied three groups of mice; 1) giant transgenic mice that expressed bovine GH (bGH) and had high circulating levels of GH and IGF-I, 2) dwarf mice with a disrupted GH receptor gene (GHR-/-) leading to GH resistance and low circulating IGF-I, and 3) a wild-type control group (CTRL). We measured the ultra-structure, collagen content and mRNA expression (targets: GAPDH, RPLP0, IGF-IEa, IGF-IR, COL1A1, COL3A1, TGF-β1, TGF-β2, TGF-β3, versican, scleraxis, tenascin C, fibronectin, fibromodulin, decorin) in the Achilles tendon, and the mRNA expression was also measured in calf muscle (same targets as tendon plus IGF-IEb, IGF-IEc). We found that GHR-/- mice had significantly lower collagen fibril volume fraction in Achilles tendon, as well as decreased mRNA expression of IGF-I isoforms and collagen types I and III in muscle compared to CTRL. In contrast, the mRNA expression of IGF-I isoforms and collagens in bGH mice was generally high in both tendon and muscle compared to CTRL. Mean collagen fibril diameter was significantly decreased with both high and low GH/IGF-I signaling, but the GHR-/- mouse tendons were most severely affected with a total loss of the normal bimodal diameter distribution. In conclusion, chronic manipulation of the GH/IGF-I axis influenced both morphology and mRNA levels of selected genes in the muscle-tendon unit of mice. Whereas only moderate structural changes were observed with up-regulation of GH/IGF-I axis, disruption of the GH receptor had pronounced effects upon tendon ultra-structure.

Calcium-dependent deceleration of the cell cycle in muscle cells by simulated microgravity

Of all our mechanosensitive tissues, skeletal muscle is the most developmentally responsive to physical activity. Conversely, restricted mobility due to injury or disease results in muscle atrophy. Gravitational force is another form of mechanical input with profound developmental consequences. The mechanical unloading resulting from the reduced gravitational force experienced during spaceflight results in oxidative muscle loss. We examined the early stages of myogenesis under conditions of simulated microgravity (SM). C2C12 mouse myoblasts in SM proliferated more slowly (2.23× less) as a result of their being retained longer within the G2/M phase of the cell cycle (2.10× more) relative to control myoblasts at terrestrial gravity. Blocking calcium entry via TRP channels with SKF-96365 (10-20 μM) accumulated myoblasts within the G2/M phase of the cell cycle and retarded their proliferation. On the genetic level, SM resulted in the reduced expression of TRPC1 and IGF-1 isoforms, transcriptional events regulated by calcium downstream of mechanical input. A decrease in TRPC1-mediated calcium entry thus appears to be a pivotal event in the muscle atrophy brought on by gravitational mechanical unloading. Hence, relieving the constant force of gravity on cells might prove one valid experimental approach to expose the underlying mechanisms modulating mechanically regulated developmental programs.

Hormone replacement therapy and physical function in healthy older men. Time to talk hormones?

Improving physical function and mobility in a continuously expanding elderly population emerges as a high priority of medicine today. Muscle mass, strength/power, and maximal exercise capacity are major determinants of physical function, and all decline with aging. This contributes to the incidence of frailty and disability observed in older men. Furthermore, it facilitates the accumulation of body fat and development of insulin resistance. Muscle adaptation to exercise is strongly influenced by anabolic endocrine hormones and local load-sensitive autocrine/paracrine growth factors. GH, IGF-I, and testosterone (T) are directly involved in muscle adaptation to exercise because they promote muscle protein synthesis, whereas T and locally expressed IGF-I have been reported to activate muscle stem cells. Although exercise programs improve physical function, in the long-term most older men fail to comply. The GH/IGF-I axis and T levels decline markedly with aging, whereas accumulating evidence supports their indispensable role in maintaining physical function integrity. Several studies have reported that the administration of T improves lean body mass and maximal voluntary strength in healthy older men. On the other hand, most studies have shown that administration of GH alone failed to improve muscle strength despite amelioration of the detrimental somatic changes of aging. Both GH and T are anabolic agents that promote muscle protein synthesis and hypertrophy but work through separate mechanisms, and the combined administration of GH and T, albeit in only a few studies, has resulted in greater efficacy than either hormone alone. Although it is clear that this combined approach is effective, this review concludes that further studies are needed to assess the long-term efficacy and safety of combined hormone replacement therapy in older men before the medical rationale of prescribing hormone replacement therapy for combating the sarcopenia of aging can be established.

Expression of muscle anabolic and metabolic factors in mechanically loaded MLO-Y4 osteocytes

Lack of physical activity results in muscle atrophy and bone loss, which can be counteracted by mechanical loading. Similar molecular signaling pathways are involved in the adaptation of muscle and bone mass to mechanical loading. Whether anabolic and metabolic factors regulating muscle mass, i.e., insulin-like growth factor-I isoforms (IGF-I Ea), mechano growth factor (MGF), myostatin, vascular endothelial growth factor (VEGF), or hepatocyte growth factor (HGF), are also produced by osteocytes in bone in response to mechanical loading is largely unknown. Therefore, we investigated whether mechanical loading by pulsating fluid flow (PFF) modulates the mRNA and/or protein levels of muscle anabolic and metabolic factors in MLO-Y4 osteocytes. Unloaded MLO-Y4 osteocytes expressed mRNA of VEGF, HGF, IGF-I Ea, and MGF, but not myostatin. PFF increased mRNA levels of IGF-I Ea (2.1-fold) and MGF (2.0-fold) at a peak shear stress rate of 44Pa/s, but not at 22Pa/s. PFF at 22 Pa/s increased VEGF mRNA levels (1.8- to 2.5-fold) and VEGF protein release (2.0- to 2.9-fold). Inhibition of nitric oxide production decreased (2.0-fold) PFF-induced VEGF protein release. PFF at 22 Pa/s decreased HGF mRNA levels (1.5-fold) but increased HGF protein release (2.3-fold). PFF-induced HGF protein release was nitric oxide dependent. Our data show that mechanically loaded MLO-Y4 osteocytes differentially express anabolic and metabolic factors involved in the adaptive response of muscle to mechanical loading (i.e., IGF-I Ea, MGF, VEGF, and HGF). Similarly to muscle fibers, mechanical loading enhanced expression levels of these growth factors in MLO-Y4 osteocytes. Although in MLO-Y4 osteocytes expression levels of IGF-I Ea and MGF of myostatin were very low or absent, it is known that the activity of osteoblasts and osteoclasts is strongly affected by them. The abundant expression levels of these factors in muscle cells, in combination with low expression in MLO-Y4 osteocytes, provide a possibility that growth factors expressed in muscle could affect signaling in bone cells.

Expression of insulin-like growth factor 1 isoforms in the rabbit oculomotor system

OBJECTIVE

The insulin-like growth factor-1 (IGF-1) gene encodes two isoforms, IGF-1Ea and IGF-1Eb. Both isoforms can regulate skeletal muscle growth and strength. It has been suggested that IGF-Eb may be more potent in promoting skeletal muscle hypertrophy. Precise contractile force regulation is particularly important in the oculomotor system. However, expression of these isoforms in mammalian extraocular muscles (EOMs) is unknown. Here, we examined their expression in rabbit EOMs and the innervating nerve, two potential sources for myogenic growth factors, and compared isoform expression between EOMs and limb skeletal muscles.

DESIGN

Expression of IGF-1 isoforms was quantified by real-time RT-PCR in adult rabbit EOMs, trochlear and ophthalmic nerves, and compared with expression in rabbit limb skeletal muscles. The presence of mature IGF-1 peptide in the muscles was further examined by Western blot.

RESULTS

Both IGF-1Ea and IGF-1Eb were expressed in the EOM and the trochlear nerve. Both isoforms were expressed at significantly higher levels (9-fold) in EOM than in limb skeletal muscle. Transcripts of IGF-1 isoforms, of IGF-1 receptor and of IGF binding proteins showed a gradient distribution along the EOM from proximal to distal. The mature IGF-1 protein showed the same gradient distribution in the EOM.

CONCLUSIONS

Expression of relatively abundant amounts of both IGF-1 splicing isoforms in EOMs, and at a significantly higher level than in limb skeletal muscle, underscores the potential relevance of these myogenic growth factors in EOM plasticity and force regulation.

Endocrine parameters and phenotypes of the growth hormone receptor gene disrupted (GHR-/-) mouse

Disruption of the GH receptor (GHR) gene eliminates GH-induced intracellular signaling and, thus, its biological actions. Therefore, the GHR gene disrupted mouse (GHR-/-) has been and is a valuable tool for helping to define various parameters of GH physiology. Since its creation in 1995, this mouse strain has been used by our laboratory and others for numerous studies ranging from growth to aging. Some of the most notable discoveries are their extreme insulin sensitivity in the presence of obesity. Also, the animals have an extended lifespan, which has generated a large number of investigations into the roles of GH and IGF-I in the aging process. This review summarizes the many results derived from the GHR-/- mice. We have attempted to present the findings in the context of current knowledge regarding GH action and, where applicable, to discuss how these mice compare to GH insensitivity syndrome in humans.

Reactive oxygen species play an essential role in IGF-I signaling and IGF-I-induced myocyte hypertrophy in C2C12 myocytes

IGF-I induces skeletal muscle hypertrophy by stimulating protein synthesis and suppressing the protein degradation pathway; the downstream signaling pathways Akt-mammalian target of rapamycin (mTOR)-p70-kDA-S6-kinase (p70S6K), and Forkhead box O1 (FoxO1) play essential roles in this regulation. Reactive oxygen species (ROS) modulate the signaling of various growth factors via redox regulation. However, the role of ROS in IGF-I signaling is not fully understood. In this study, we investigated whether ROS regulate the signaling and biological action of IGF-I in C2C12 myocytes. We found that IGF-I induces ROS in C2C12 myocytes. While treatment with H(2)O(2) significantly enhanced IGF-I-induced phosphorylation of the IGF-I receptor (IGF-IR), IGF-IR phosphorylation was markedly attenuated when cells were treated with antioxidants. The downstream signaling pathway, Akt-mTOR-p70S6K was subsequently down-regulated. Furthermore, the phosphorylation of FoxO1 by IGF-I decreased concomitantly with the restoration of the expression of its target genes, Atrogin-1 and muscle RING finger 1, which are related to muscle atrophy. Nox4 knockdown, which is reportedly to produce ROS in insulin signaling, attenuated IGF-I-induced IGF-IR phosphorylation, indicating that Nox4 is involved in the regulation of IGF-I signaling. Importantly, antioxidant treatments inhibited IGF-I-induced myocyte hypertrophy, demonstrating that ROS are necessary for IGF-I-induced myocyte hypertrophy in vitro. These results indicate that ROS play an essential role in the signaling and biological action of IGF-I in C2C12 myocytes.

Insulin-like growth factor-I E peptides: implications for aging skeletal muscle

In skeletal muscle there is good evidence to suggest that locally produced insulin-like growth factor-1 (IGF-I), rather than circulating IGF-I, is important in regard to muscle mass maintenance, repair and hypertrophy. This "mature" IGF-I comprises exons 3 and 4 of the IGF-I gene, but during processing the full length gene (which contains six exons) is subject to a process of alternative splicing. As a result smaller peptides (E peptides) are believed to be cleaved from the mature IGF-I peptide during processing of the prohormone and the likelihood is that they have different biological roles. In human skeletal muscle three transcripts encoding for these splice variants (IGF-IEa, IGF-IEb and IGF-IEc, also known as MGF) can be identified. When studied at the mRNA level these three transcripts are known to be upregulated in the muscles of elderly people following high resistance exercise, albeit with different time courses. However, compared with mature IGF-I relatively little is known about the mechanism of action of the different E peptides.

IGF-IEc expression, regulation and biological function in different tissues

Insulin-like growth factor I (IGF-I) is an important growth factor for embryonic development, postnatal growth, tissue repair and maintenance of homeostasis. IGF-I functions and regulations are complex and tissue-specific. IGF-I mediates growth hormone signaling to target tissues during growth, but many IGF-I variants have been discovered, resulting in complex models to describe IGF-I function and regulation. Mechano-growth factor (MGF) is an alternative splicing variant of IGF-I and serves as a local tissue repair factor that responds to changes in physiological conditions or environmental stimuli. MGF expression is significantly increased in muscle, bone and tendon following damage resulting from mechanical stimuli and in the brain and heart following ischemia. MGF has been shown to activate satellite cells in muscle resulting in hypertrophy or regeneration, and functions as a neuroprotectant in brain ischemia. Both expression and processing of this IGF-I variant are tissue specific, but the functional mechanism is poorly understood. MGF and its short derivative have been examined as a potential therapy for muscular dystrophy and cerebral hypoxia-ischemia using experimental animals. Although the unique mode of action of MGF has been identified, the details remain elusive. Here we review the expression and regulation of MGF and the function of this IGF-I isoform in tissue protection.

Skeletal muscle growth defect in human growth hormone transgenic rat is accompanied by phenotypic changes in progenitor cells

Growth hormone (GH) is known to have a pivotal role in the maintenance of skeletal muscle mass. Sarcopenia, the loss of skeletal muscle mass, is a common phenomenon in aging, and it is widely accepted that sarcopenia is largely attributed to age-related decline in GH secretion. In the present study, we tested if human growth hormone transgenic rats (GH-TG rats) whose plasma GH levels are maintained relatively low could be an appropriate model for sarcopenia. Analyses of GH-TG rats revealed that they exhibit skeletal muscle growth defect as well as atrophy of myofibers. The number of myofibers in tibialis anterior muscle was comparable to that of WT rats, while the proportion of type I slow myofibers in tibialis anterior muscle was increased in GH-TG rats after 5 months. Neither increased expression of ubiquitin ligases, MuRF1 and MAFbx, nor indication of apoptotic cell death was observed. Notably, myogenic differentiation potential of skeletal muscle progenitor cells in GH-TG rats was lower than WT rats, and this was accompanied by increased adipogenic potential. These results indicate that GH-TG rats could be a useful model to elucidate the mechanism of sarcopenia induced by reduced GH action and raised the possibility that decreased GH action may cause an alteration of differentiation potential of skeletal muscle progenitor cells.

Recovery of whisking function promoted by manual stimulation of the vibrissal muscles after facial nerve injury requires insulin-like growth factor 1 (IGF-1)

Recently, we showed that manual stimulation (MS) of denervated vibrissal muscles enhanced functional recovery following facial nerve cut and suture (FFA) by reducing poly-innervation at the neuro-muscular junctions (NMJ). Although the cellular correlates of poly-innervation are established, with terminal Schwann cells (TSC) processes attracting axon sprouts to "bridge" adjacent NMJ, molecular correlates are poorly understood. Since quantitative RT-PCR revealed a rapid increase of IGF-1 mRNA in denervated muscles, we examined the effect of daily MS for 2 months after FFA in IGF-1(+/-) heterozygous mice; controls were wild-type (WT) littermates including intact animals. We quantified vibrissal motor performance and the percentage of NMJ bridged by S100-positive TSC. There were no differences between intact WT and IGF-1(+/-) mice for vibrissal whisking amplitude (48 degrees and 49 degrees ) or the percentage of bridged NMJ (0%). After FFA and handling alone (i.e. no MS) in WT animals, vibrissal whisking amplitude was reduced (60% lower than intact) and the percentage of bridged NMJ increased (42% more than intact). MS improved both the amplitude of vibrissal whisking (not significantly different from intact) and the percentage of bridged NMJ (12% more than intact). After FFA and handling in IGF-1(+/-) mice, the pattern was similar (whisking amplitude 57% lower than intact; proportion of bridged NMJ 42% more than intact). However, MS did not improve outcome (whisking amplitude 47% lower than intact; proportion of bridged NMJ 40% more than intact). We conclude that IGF-I is required to mediate the effects of MS on target muscle reinnervation and recovery of whisking function.

Effect of circulating growth hormone on muscle IGF-I protein concentration in female mice with growth hormone receptor gene disruption

Growth hormone (GH) is a potent secretague for circulating insulin-like growth factor-I (IGF-I). The purpose of this study was to examine the effect of circulating GH on muscle IGF-I protein expression using GH transgenic animal models. Three different models were used: mice that overexpress bovine GH (bGH; n=10), mice without a functional GH receptor (GHR-/-; n=10), and wildtype mice (n=10). All mice were 16-week old females and each group differed in their basic phenotypic characteristics. Immediately after euthanization the triceps surae muscle group (soleus, plantaris, and gastrocnemius muscles) was removed. IGF-I was extracted from the muscle with an acid-ethanol solution (12.5% 2N hydrochloric acid and 87.5% ethanol, pH 1.5) followed by neutralization with Tris-base and subsequently quantified using a radioimmunoassay. Analysis revealed that bGH mice had significantly greater muscle IGF-I protein expression compared to GHR-/- and wildtype mice. No difference in IGF-I protein concentration was found between GHR-/- and wildtype animals. This study found that overexpression of GH leading to high circulating GH concentrations increase muscle IGF-I protein expression. However, the absence of a functional GHR did not affect muscle IGF-I protein expression compared to wildtype despite high circulating levels of GH and low circulating levels of IGF-I. In conclusion, it appears that at rest high circulating levels of GH augment muscle IGF-I protein expression only in the presence of an intact GHR but that the absence of a functional GH receptor does not affect basal levels of muscle IGF-I protein in female mice.

Serum IGF-I-Deficiency Does Not Prevent Compensatory Skeletal Muscle Hypertrophy in Resistance Exercise

The involvement of circulating insulin-like growth factor-I (IGF-I) in the skeletal muscle response to resistance exercise is currently unclear. To address this, we utilized the liver IGF-I-deficient (LID) mouse model, in which the igf1 gene has been disrupted in the hepatocytes, resulting in ~80% reduction in serum IGF-I. Twelve- to 13-month-old male LID and control (L/L) mice were subjected to 16 weeks of resistance training. Resistance exercise resulted in equal strength gains in both L/L and LID mice. Basal IGF-I mRNA levels were greater in LID muscles than in L/L, and exercise increased IGF-I mRNA in quadriceps, gastrocnemius, and plantaris muscles. LID mice had elevated tyrosine phosphorylation of IGF-IR and Stat5b, the latter possibly reflective of increased serum GH. Tyrosine phosphorylation of IGF-IR was increased, while phospho-Stat5b was reduced after resistance training of both wild-type and LID mice. These data suggest that: 1) performance and recovery in response to resistance training is normal even when there is severe deficiency of circulating IGF-I; and 2) upregulation of local IGF-I may be involved in the compensatory growth of muscle that occurs in response to resistance training. Decreased levels of p-Stat5b in exercised mice suggests that the upregulation of local IGF-I gene expression in response to exercise may be GH-independent.

Serum IGF-I levels and IGF-I gene splicing in muscle of healthy young males receiving rhGH

OBJECTIVE

Elevated growth hormone (GH) levels lead to increased circulating insulin-like growth factor-I (IGF-I), but the effects on localised muscle IGF-I splice variant expression is not known. The effects of rhGH administration, with or without an acute bout of high resistance exercise, were measured on serum IGF-I and on the mRNA levels of IGF-I splice variants in the vastus lateralis muscle of healthy young men.

DESIGN

The study was a randomised double blind trial with a crossover design. Seven subjects were randomly assigned to a group receiving daily injections of rhGH (0.075IU kg(-1)day(-1)) or placebo for a two week period. Following a one month washout, the groups were reversed.

RESULTS

Administration of rhGH increased circulating IGF-I from 31.8+/-3.2 to 109+/-5.4 nmol/L (p<0.05). There was no effect of the exercise bout. RNA was extracted from muscle biopsies obtained from exercised and non-exercised legs 2.5h after the cessation of the exercise. Transcript expression was measured using Real-time QPCR. There was no effect of either exercise or rhGH administration on IGF-I 5' (Class 1 or Class 2) or 3' (IGF-IEa, or MGF) transcripts.

CONCLUSION

Although rhGH administration has an effect on liver IGF-I expression, as shown by increase in circulating IGF-I, muscle IGF-I expression is unaffected in young healthy subjects with normal GH profile. The findings contrast with those of a previous study in which GH deficient elderly men showed higher muscle IGF-I 3' splice variant levels following rhGH administration with and without resistance training. Unlike in the liver, muscle Class1 and 2 IGF-I expression do not change significantly following administration of rhGH.

Running on empty: how p53 controls INS/IGF signaling and affects life span

In higher organisms dependent on the regenerative ability of tissue stem cells to maintain tissue integrity throughout adulthood, the failure of stem cells to replace worn out, dead, or damaged cells is seen as one mechanism that limits life span. In these organisms, tumor suppressors such as p53 are central participants in the control of longevity because they regulate stem cell proliferation. Several recent reports have identified p53 as a longevity gene in organisms such as Caenorhabditis elegans and Drosophila melanogaster, which lack proliferative stem cells in all but the germline and have relatively short life spans. This has forced us to reevaluate the role of p53 in the control of life span. We discuss how p53 might regulate longevity in both long- and short-lived species by controlling the activity of insulin-like molecules that operate in proliferating and non-proliferating compartments of adult somatic tissues. We also discuss the hierarchical structure of life span regulation where loss of p53 has life span extending effects. Finally, we suggest a molecular mechanism by which p53 might facilitate the response to severe nutrient deprivation that allows metabolically active cells to survive periods of starvation. Paradoxically, loss of p53 function in these cells would compromise life span.

Cellular and molecular regulation of muscle growth and development in meat animals1,2

Although in vivo and in vitro studies have established that anabolic steroids, transforming growth factor-beta (TGF-beta), and myostatin affect muscle growth in meat-producing animals, their mechanisms of action are not completely understood. Anabolic steroids have been widely used as growth promoters in feedlot cattle for over 50 yr. A growing body of evidence suggests that increased muscle levels of IGF-I and increased muscle satellite cell numbers play a role in anabolic steroid enhanced muscle growth. In contrast to anabolic steroids, the members of the TGF-beta-myostatin family suppress muscle growth in vivo and suppress both proliferation and differentiation of cultured myogenic cells. Recent evidence suggests that IGFBP-3 and IGFBP-5 play a role in mediating the proliferation-suppressing actions of both TGF-beta and myostatin on cultured myogenic cells. Consequently, this review will focus on the roles of IGF-I and IGFBP in the cellular and molecular mechanisms of action of anabolic steroids and TGF-beta and myostatin, respectively.

Therapeutic Ultrasound Decreases Mechano-Growth Factor Messenger Ribonucleic Acid Expression After Muscle Contusion Injury

OBJECTIVE

To examine the influence of nonthermal ultrasound on mechano-growth factor (MGF) messenger ribonucleic acid (mRNA) expression after blunt trauma.

DESIGN

A 2x4 factorial multivariate analysis of variance design.

SETTING

University research laboratory.

ANIMALS

Thirty-six 3- to 4-month-old male Wistar rats (mean weight, 280.8+/-21.5g). Thirty-two received a bilateral contusion injury to the gastrocnemius via a drop mass technique. Four were control animals.

INTERVENTION

Ultrasound treatment (frequency, 3MHz; intensity, 0.3W/cm(2); continuous duty cycle) was started 24 hours postinjury and delivered for 5 minutes daily on 4 consecutive days. Treatment was on the left hindlimb and the contralateral right hindlimb was the nonultrasound control.

MAIN OUTCOME MEASURES

Muscle mass (in grams) and MGF mRNA expression as measured via real-time reverse transcriptase polymerase chain reaction.

RESULTS

Ultrasound had no effect on muscle mass (F(1,28)=2.723, P=.110, 1-beta=.357, eta(2)=.089). Ultrasound treatments decreased MGF mRNA expression in the treated limb compared with the nontreated hindlimb (F(1,28)=6.605, P=.016, 1-beta=.699, eta(2)=.191).

CONCLUSIONS

The nonthermal ultrasound treatments resulted in decreased MGF mRNA expression after blunt trauma to the gastrocnemius muscles.

Muscle IGF-I Ea, MGF, and myostatin mRNA expressions after compensatory overload in hypophysectomized rats

To determine whether IGF-I Ea, MGF, and myostatin mRNAs are related to GH-independent overload-induced muscle growth, we examined the expressions of IGF-I Ea and MGF mRNAs in the plantaris muscle after compensatory overload in hypophysectomized rats. The muscles were divided into four groups: normal-control, normal-overloaded, hypophysectomized-control, and hypophysectomized-overloaded. The weights of the plantaris muscle in the normal-overloaded were significantly higher than those of the normal-control. The weights of the hypophysectomized-overloaded were also significantly higher than those of the hypophysectomized-control. IGF-I Ea and MGF mRNAs in normal-overloaded and hypophysectomized-overloaded 3 days after overload were significantly higher than those of normal-control and hypophysectomized-control, respectively. Myostatin mRNAs in normal-overloaded and hypophysectomized-overloaded 3 days after the overload were significantly lower than those of normal-control and hypophysectomized-control, respectively. Thus, it was shown that IGF-I Ea, MGF, and myostatin mRNAs were expressed in association with muscle enlargement after compensatory overload independently of pituitary state. These observations suggest that the expression of IGF-I Ea, MGF, and myostatin mRNAs due to compensatory overload would be associated in a growth-hormone-independent manner.

The expression of IGF-I and myostatin mRNAs in skeletal muscle of hypophysectomized and underfed rats during postnatal growth

AIM

To determine the roles of myostatin and insulin-like growth factor-I (IGF-I) during postnatal growth, we examined IGF-I and myostatin mRNA expression in the skeletal muscles of hypophysectomized and underfed rats during postnatal growth.

METHODS

Five-week-old rats were divided into four groups: freely fed control, moderately underfed, severely underfed and hypophysectomized. Four weeks later, blood and muscle samples were gathered to determine serum IGF-I, myosin heavy chain (MHC) isoforms, IGF-I Ea, IGF-I Eb and myostatin mRNA.

RESULTS

The weights of soleus, plantaris and masseter muscles were decreased in underfed and hypophysectomized rats. Hypophysectomy resulted in significant increases of type I MHC at the expense of type IIx in plantaris muscle and of neonatal MHC at the expense of types IIx and IIb in masseter muscle. Serum IGF-I was decreased by underfeeding and hypophysectomy. Plantaris muscle IGF-I Ea mRNA in underfed and hypophysectomized rats was significantly lower than in normal controls. Plantaris muscle IGF-I Eb mRNA in underfed rats was significantly lower than in normal controls. Masseter muscle IGF-I Eb mRNA in severely underfed rats was significantly lower than in normal control and hypophysectomized rats. Soleus muscle myostatin mRNA in hypophysectomized rats was significantly higher than in normal and significantly underfed rats. No significant differences in plantaris and masseter muscle myostatin mRNA were observed between groups.

CONCLUSION

Suppressed muscle growth caused by hypophysectomy and underfeeding may be attributed mainly to reduced circulating IGF-I and partially to reduced IGF-I mRNA, rather than to a change in myostatin.

Mechanical signals, IGF-I gene splicing, and muscle adaptation

Combining physiological and molecular biology methods made it possible to identify and characterize a local muscle growth/repair factor (MGF). Following resistance exercise, MGF "kick starts" muscle hypertrophy and is important in local tissue repair. Loss of muscle mass in old age and certain diseases is associated with an impaired ability to express MGF.