Loss of IGF-IEa or IGF-IEb impairs myogenic differentiation

Differential activation of AKT isoforms by growth factors in human myotubes

AKT signaling plays a crucial role in muscle physiology, and is activated by stimuli, including insulin, growth factors, and exercise. Three AKT isoforms have been identified in mammals, and they possess both distinct and redundant functions. However, it is currently unknown what the predominant AKT isoform is in primary human skeletal myotubes, and very little is known regarding the effects of insulin and insulin-like growth factor-I (IGF-I) on AKT isoforms activation in human myotubes. Thus, we sought to determine the abundances of each AKT isoform in primary human skeletal myotubes and their responses to insulin or IGF-I. Analysis of protein lysates by liquid chromatography-parallel reaction monitoring/mass spectrometry revealed that AKT1 was the most abundant AKT isoform and AKT3 was the least-abundant isoform. Next, myotubes were treated with either 100 nM insulin or 10 nM IGF-I for 5, 20, 45, or 60 min. In response to insulin, there was a significant treatment effect on phosphorylation of AKT1 and AKT2, but not AKT3 (p < 0.01). In response to IGF-I, there was a significant treatment effect on phosphorylation of pan-AKT at all timepoints compared to control (p < 0.01). Next, we determined how much of the total AKT isoform pool was phosphorylated. For insulin stimulation, AKT1 was significantly higher than AKT2 at 5 min and 60 min posttreatment (p < 0.05 both) and significantly different than AKT3 at all timepoints (p < 0.05). For IGF-I stimulation, AKT1 was significantly higher than AKT2 at 45 and 60 min posttreatment (p < 0.05 both) and significantly higher than AKT3 at all timepoints (p < 0.05). Our findings reveal the differential phosphorylation patterns among the AKT isoforms and suggest a potential explanation for the regulatory role of AKT1 in skeletal muscle.

Molecular cloning, expression, and functional features of IGF1 splice variants in sheep

Insulin-like growth factor 1 (IGF1), also known as somatomedin C, is essential for the regulation of animal growth and development. In many species, the IGF1 gene can be alternatively spliced into multiple transcripts, encoding different pre-pro-IGF1 proteins. However, the exact alternative splicing patterns of IGF1 and the sequence information of different splice variants in sheep are still unclear. In this study, four splice variants (class 1-Ea, class 1-Eb, class 2-Ea, and class 2-Eb) were obtained, but no IGF1 Ec, similar to that found in other species, was discovered. Bioinformatics analysis showed that the four splice variants shared the same mature peptide (70 amino acids) and possessed distinct signal peptides and E peptides. Tissue expression analysis indicated that the four splice variants were broadly expressed in all tested tissues and were most abundantly expressed in the liver. In most tissues and stages, the expression of class 1-Ea was highest, and the expression of other splice variants was low. Overall, levels of the four IGF1 splice variants at the fetal and lamb stages were higher than those at the adult stage. Overexpression of the four splice variants significantly increased fibroblast proliferation and inhibited apoptosis (P < 0.05). In contrast, silencing IGF1 Ea or IGF1 Eb with siRNA significantly inhibited proliferation and promoted apoptosis (P < 0.05). Among the four splice variants, class 1-Ea had a more evident effect on cell proliferation and apoptosis. In summary, the four ovine IGF1 splice variants have different structures and expression patterns and might have different biological functions.

Implications of Insulin-Like Growth Factor-1 in Skeletal Muscle and Various Diseases

Skeletal muscle is an essential tissue that attaches to bones and facilitates body movements. Insulin-like growth factor-1 (IGF-1) is a hormone found in blood that plays an important role in skeletal myogenesis and is importantly associated with muscle mass entity, strength development, and degeneration and increases the proliferative capacity of muscle satellite cells (MSCs). IGF-1R is an IGF-1 receptor with a transmembrane location that activates PI3K/Akt signaling and possesses tyrosine kinase activity, and its expression is significant in terms of myoblast proliferation and normal muscle mass maintenance. IGF-1 synthesis is elevated in MSCs of injured muscles and stimulates MSCs proliferation and myogenic differentiation. Mechanical loading also affects skeletal muscle production by IGF-1, and low IGF-1 levels are associated with low handgrip strength and poor physical performance. IGF-1 is potentially useful in the management of Duchenne muscular dystrophy, muscle atrophy, and promotes neurite development. This review highlights the role of IGF-1 in skeletal muscle, its importance during myogenesis, and its involvement in different disease conditions.

Transcriptomic Profile of Primary Culture of Skeletal Muscle Cells Isolated from Semitendinosus Muscle of Beef and Dairy Bulls

The aim of the study was to identify differences in the transcriptomic profiles of primary muscle cell cultures derived from the semitendinosus muscle of bulls of beef breeds (Limousin (LIM) and Hereford (HER)) and a dairy breed (Holstein-Friesian (HF)) (n = 4 for each breed). Finding a common expression pattern for proliferating cells may point to such an early orientation of the cattle beef phenotype at the transcriptome level of unfused myogenic cells. To check this hypothesis, microarray analyses were performed. The analysis revealed 825 upregulated and 1300 downregulated transcripts similar in both beef breeds (LIM and HER) and significantly different when compared with the dairy breed (HF) used as a reference. Ontological analyses showed that the largest group of genes were involved in muscle organ development. Muscle cells of beef breeds showed higher expression of genes involved in myogenesis (including erbb-3, myf5, myog, des, igf-1, tgfb2) and those encoding proteins comprising the contractile apparatus (acta1, actc1, myh3, myh11, myl1, myl2, myl4, tpm1, tnnt2, tnnc1). The obtained results confirmed our hypothesis that the expression profile of several groups of genes is common in beef breeds at the level of proliferating satellite cells but differs from that observed in typical dairy breeds.

Association Analysis of Single-Cell RNA Sequencing and Proteomics Reveals a Vital Role of Ca2+ Signaling in the Determination of Skeletal Muscle Development Potential

This study is aimed at exploring the mechanism underlying the homeostasis between myogenesis and adipogenesis in skeletal muscle using a special porcine model with a distinct phenotype on muscle growth rate and intramuscular fat deposition. Differentiation potential of muscle-derived Myo-lineage cells of lean-type pigs was significantly enhanced relative to obese-type pigs, while that of their Adi-lineage cells was similar. Single-cell RNA sequencing revealed that lean-type pigs reserved a higher proportion of Myo-lineage cells in skeletal muscle relative to obese-type pigs. Besides, Myo-lineage cells of the lean-type pig settled closer to the original stage of muscle-derived progenitor cells. Proteomics analysis found that differentially expressed proteins between two sources of Myo-lineage cells are mainly involved in muscle development, cell proliferation and differentiation, ion homeostasis, apoptosis, and the MAPK signaling pathway. The regulation of intracellular ion homeostasis, Ca2+ in particular, significantly differed between two sources of Myo-lineage cells. Ca2+ concentration in both cytoplasm and endoplasmic reticulum was lower in Myo-lineage cells of lean-type pigs relative to obese-type pigs. In conclusion, a higher proportion and stronger differentiation capacity of Myo-lineage cells are the main causes for the higher capability of myogenic differentiation and lower intramuscular fat deposition. Relative low concentration of cellular Ca2+ is advantageous for Myo-lineage cells to keep a potent differentiation potential.

Stem Cells for Skeletal Muscle Tissue Engineering

Volumetric muscle loss (VML) is a debilitating condition wherein muscle loss overwhelms the body's normal physiological repair mechanism. VML is particularly common among military service members who have sustained war injuries. Because of the high social and medical cost associated with VML and suboptimal current surgical treatments, there is great interest in developing better VML therapies. Skeletal muscle tissue engineering (SMTE) is a promising alternative to traditional VML surgical treatments that use autogenic tissue grafts, and rather uses isolated stem cells with myogenic potential to generate de novo skeletal muscle tissues to treat VML. Satellite cells are the native precursors to skeletal muscle tissue, and are thus the most commonly studied starting source for SMTE. However, satellite cells are difficult to isolate and purify, and it is presently unknown whether they would be a practical source in clinical SMTE applications. Alternative myogenic stem cells, including adipose-derived stem cells, bone marrow-derived mesenchymal stem cells, perivascular stem cells, umbilical cord mesenchymal stem cells, induced pluripotent stem cells, and embryonic stem cells, each have myogenic potential and have been identified as possible starting sources for SMTE, although they have yet to be studied in detail for this purpose. These alternative stem cell varieties offer unique advantages and disadvantages that are worth exploring further to advance the SMTE field toward highly functional, safe, and practical VML treatments. The following review summarizes the current state of satellite cell-based SMTE, details the properties and practical advantages of alternative myogenic stem cells, and offers guidance to tissue engineers on how alternative myogenic stem cells can be incorporated into SMTE research.

The structure-function relationships of insulin-like growth factor 1 Ec in C2C12 cells

Insulin-like growth factor 1 (IGF1) is a crucial growth factor, that regulates skeletal muscles development during cell growth and repair. Recently, its alternative splicing variant, named IGF1Ec, also named mechano-growth factor (MGF), has gained attentions as a new damage repair factor. However, the structure-function relationships of IGF1Ec have not been fully clarified due to contradictory reports. In this study, we systematically investigated physiologic responses of C2C12 muscle cells to IGF1Ec, IGF1 and MGF E peptide. Our data indicate that while the N-terminal sequence of IGF1Ec, which is homolog in part with IGF1, promotes proliferation; the C-terminal sequence of IGF1Ec, which is identical to MGF E, promotes differentiation and migration of C2C12 cells. Our results suggest that MGF E cannot completely replace all the functions of IGF1Ec on muscle repair and regeneration, and elucidate the relationships between structure and function of IGF1Ec.

Identification of the IGF-1 processing product human Ec/rodent Eb peptide in various tissues: Evidence for its differential regulation after exercise-induced muscle damage in humans

OBJECTIVE

Insulin-like growth factor-1 (IGF-1) is a pleiotropic factor expressed in various tissues and plays a critical role in skeletal muscle physiology. Alternative splicing of the IGF-1 gene gives rise to different precursor polypeptides (isoforms) which could undergo post-translational cleavage, generating the common mature IGF-1 peptide and different carboxyl terminal extension (E-) peptides, with the fate of the latter being, so far, unknown. The objective if this study was to identify the IGF-1Ec forms or processing product(s), other than mature IGF-1, generated in different human and rodent tissues and particularly in human skeletal muscle after exercise-induced damage.

DESIGN

Protein lysates from a wide range of human and rodent tissues were immunoblotted with a rabbit anti-human Ec polyclonal antibody raised against the last 24 amino acids of the C-terminal of the Ec peptide. This antibody can recognize the Ec peptide, both as part of IGF-1Ec and alone, and also the corresponding rodent forms, due to the high homology that the human Ec shares with the rodent Eb.

RESULTS

We were able to confirm, for the first time, that the human Ec peptide and its rodent homologous Eb peptide are produced simultaneously with their precursor protein (pro-IGF-1Ec/Eb) in vivo, in a wide range of tissues (e.g. muscle, liver, heart). Proprotein convertase furin digestion of human muscle and liver protein lysates confirmed that the higher molecular form, pro-IGF-1Ec, can be cleaved to produce the free Ec peptide. Furthermore, initial evidence is provided that Ec peptide is differentially regulated during the process of muscle regeneration after exercise-induced damage in humans.

CONCLUSIONS

The findings of this study possibly imply that the post-translational modification of the IGF-1Ec pro-peptide may regulate the bioavailability and activity of the processing product(s).

RNA transcript expression of IGF-I/PI3K pathway components in regenerating skeletal muscle is sensitive to initial injury intensity

OBJECTIVE

Skeletal muscle regeneration is a complex process involving the coordinated input from multiple stimuli. Of these processes, actions of the insulin-like growth factor-I (IGF-I) and phosphoinositide 3-kinase (PI3K) pathways are vital; however, whether IGF-I or PI3K expression is modified during regeneration relative to initial damage intensity is unknown. The objective of this study was to determine whether mRNA expression of IGF-I/PI3K pathway components was differentially regulated during muscle regeneration in mice in response to traumatic injury induced by freezing of two different durations.

DESIGN

Traumatic injury was imposed by applying a 6-mm diameter cylindrical steel probe, cooled to the temperature of dry ice (-79°C), to the belly of the left tibialis anterior muscle of 12-week-old C57BL/6J mice for either 5s (5s) or 10s (10s). The right leg served as the uninjured control. RNA was obtained from injured and control muscles following 3, 7, and 21days recovery and examined by real-time PCR. Expression of transcripts within the IGF, PI3K, and Akt families, as well as for myogenic regulatory factors and micro-RNAs were studied.

RESULTS

Three days following injury, there was significantly increased expression of Igf1, Igf2, Igf1r, Igf2r, Pik3cb, Pik3cd, Pik3cg, Pik3r1, Pik3r5, Akt1, and Akt3 in response to either 5s or 10s injury compared to uninjured control muscle. There was a significantly greater expression of Pik3cb, Pik3cd, Pik3cg, Pik3r5, Akt1, and Akt3 in 10s injured muscle compared to 5s injured muscle. Seven days following injury, we observed significantly increased expression of Igf1, Igf2, Pik3cd, and Pik3cg in injured muscle compared to control muscle in response to 10s freeze injury. We also observed significantly reduced expression of Igf1r and miR-133a in response to 5s freeze injury compared to control muscle, and significantly reduced expression of Ckm, miR-1 and miR-133a in response to 10s freeze injury as compared to control. Twenty-one days following injury, 5s freeze-injured muscle exhibited significantly increased expression of Igf2, Igf2r, Pik3cg, Akt3, Myod1, Myog, Myf5, and miR-206 compared to control muscle, while 10s freeze-injured muscles showed significantly increased expression of Igf2, Igf2r, Pik3cb, Pik3cd, Pik3r5, Akt1, Akt3, and Myog compared to control. Expression of miR-1 was significantly reduced in 10s freeze-injured muscle compared to control muscle at this time. There were no significant differences in RNA expression between 5s and 10s injury at either 7d or 21d recovery in any transcript examined.

CONCLUSIONS

During early skeletal muscle regeneration in mice, transcript expressions for some components of the IGF-I/PI3K pathway are sensitive to initial injury intensity induced by freeze damage.

Changes in Communication between Muscle Stem Cells and their Environment with Aging

Aging is associated with both muscle weakness and a loss of muscle mass, contributing towards overall frailty in the elderly. Aging skeletal muscle is also characterised by a decreasing efficiency in repair and regeneration, together with a decline in the number of adult stem cells. Commensurate with this are general changes in whole body endocrine signalling, in local muscle secretory environment, as well as in intrinsic properties of the stem cells themselves. The present review discusses the various mechanisms that may be implicated in these age-associated changes, focusing on aspects of cell-cell communication and long-distance signalling factors, such as levels of circulating growth hormone, IL-6, IGF1, sex hormones, and inflammatory cytokines. Changes in the local environment are also discussed, implicating IL-6, IL-4, FGF-2, as well as other myokines, and processes that lead to thickening of the extra-cellular matrix. These factors, involved primarily in communication, can also modulate the intrinsic properties of muscle stem cells, including reduced DNA accessibility and repression of specific genes by methylation. Finally we discuss the decrease in the stem cell pool, particularly the failure of elderly myoblasts to re-quiesce after activation, and the consequences of all these changes on general muscle homeostasis.

Skeletal muscle tissue engineering: methods to form skeletal myotubes and their applications

Skeletal muscle tissue engineering (SMTE) aims to repair or regenerate defective skeletal muscle tissue lost by traumatic injury, tumor ablation, or muscular disease. However, two decades after the introduction of SMTE, the engineering of functional skeletal muscle in the laboratory still remains a great challenge, and numerous techniques for growing functional muscle tissues are constantly being developed. This article reviews the recent findings regarding the methodology and various technical aspects of SMTE, including cell alignment and differentiation. We describe the structure and organization of muscle and discuss the methods for myoblast alignment cultured in vitro. To better understand muscle formation and to enhance the engineering of skeletal muscle, we also address the molecular basics of myogenesis and discuss different methods to induce myoblast differentiation into myotubes. We then provide an overview of different coculture systems involving skeletal muscle cells, and highlight major applications of engineered skeletal muscle tissues. Finally, potential challenges and future research directions for SMTE are outlined.

Early intervention with intranasal NPY prevents single prolonged stress-triggered impairments in hypothalamus and ventral hippocampus in male rats

Intranasal administration of neuropeptide Y (NPY) is a promising treatment strategy to reduce traumatic stress-induced neuropsychiatric symptoms of posttraumatic stress disorder (PTSD). We evaluated the potential of intranasal NPY to prevent dysfunction of the hypothalamic-pituitary-adrenal (HPA) axis, a core neuroendocrine feature of PTSD. Rats were exposed to single prolonged stress (SPS), a PTSD animal model, and infused intranasally with vehicle or NPY immediately after SPS stressors. After 7 days undisturbed, hypothalamus and hippocampus, 2 structures regulating the HPA axis activity, were examined for changes in glucocorticoid receptor (GR) and CRH expression. Plasma ACTH and corticosterone, and hypothalamic CRH mRNA, were significantly higher in the vehicle but not NPY-treated group, compared with unstressed controls. Although total GR levels were not altered in hypothalamus, a significant decrease of GR phosphorylated on Ser232 and increased FK506-binding protein 5 mRNA were observed with the vehicle but not in animals infused with intranasal NPY. In contrast, in the ventral hippocampus, only vehicle-treated animals demonstrated elevated GR protein expression and increased GR phosphorylation on Ser232, specifically in the nuclear fraction. Additionally, SPS-induced increase of CRH mRNA in the ventral hippocampus was accompanied by apparent decrease of CRH peptide particularly in the CA3 subfield, both prevented by NPY. The results show that early intervention with intranasal NPY can prevent traumatic stress-triggered dysregulation of the HPA axis likely by restoring HPA axis proper negative feedback inhibition via GR. Thus, intranasal NPY has a potential as a noninvasive therapy to prevent negative effects of traumatic stress.

IGF expression in HPV-related and HPV-unrelated human cancer cells

The human Igf-1 gene not only produces insulin‑like growth factor-I (IGF-I), but also different carboxy‑terminal extensions, known as E peptides, through alternative splicing. We and others have shown that human Eb peptide (hEb) derived from Igf-1 has intrinsic biological activity and is localized to nuclei of transfected cells. Since hEb actions can complement the activity of IGF-I itself, the aim of the present study was to compare IGF-I isoforms at the endogenous protein and transcript level in cancer cell lines, including HeLa, U2OS, HepG2 and K562 cells. Quantitative real-time PCR (qRT‑PCR) using Igf-1 isoform specific primers was performed to determine expression patterns, using β-actin as a reference gene. The overall relative Igf-1 transcript level was different across the cell lines, with ~80-fold higher expression in K562 (130.2±31.2) than in U2OS cells (1.7±1.1). The relative copy number of Igf-1b was the highest in HepG2 (69.9±28.6) and K562 cells (28.3±6.7), whereas the relative copy numbers of Igf-1a and Igf-1c were significantly higher in K562 cells compared to all other cell lines. Immunoblotting using total cell lysates, cytoplasmic and nuclear fractions were carried out to determine the level and distribution of IGF-I proteins. K562 cells exhibited the highest level of hEb in total cell lysates and nuclear fractions and no cell lines displayed hEb in the cytoplasmic fractions. In contrast, IGF-IA was the highest in HeLa cells and was enriched only in the cytoplasmic fraction. Since relatively low IGF-1A transcript level but relatively high pro‑IGF-1A protein level is plausible, we hypothesized that these transcripts could be processed with higher efficiency and/or the protein product may be stabilized by viral HPV oncogenes in HeLa cells. We assert that while it is important to analyze Igf-1 transcript level, it may be more relevant to determine the IGF isoforms at the protein level.

The complexity of the IGF1 gene splicing, posttranslational modification and bioactivity

The insulinlike growth factor-I (IGF-I) is an important factor which regulates a variety of cellular responses in multiple biological systems. The IGF1 gene comprises a highly conserved sequence and contains six exons, which give rise to heterogeneous mRNA transcripts by a combination of multiple transcription initiation sites and alternative splicing. These multiple transcripts code for different precursor IGF-I polypeptides, namely the IGF-IEa, IGF-IEb and IGF-IEc isoforms in humans, which also undergo posttranslational modifications, such as proteolytic processing and glycosylation. IGF-I actions are mediated through its binding to several cell-membrane receptors and the IGF-I domain responsible for the receptor binding is the bioactive mature IGF-I peptide, which is derived after the posttranslational cleavage of the pro-IGF-I isoforms and the removal of their carboxy-terminal E-peptides (that is, the Ea, Eb and Ec). Interestingly, differential biological activities have been reported for the different IGF-I isoforms, or for their E-peptides, implying that IGF-I peptides other than the IGF-I ligand also possess bioactivity and, thus, both common and unique or complementary pathways exist for the IGF-I isoforms to promote biological effects. The multiple peptides derived from IGF-I and the differential expression of its various transcripts in different conditions and pathologies appear to be compatible with the distinct cellular responses observed to the different IGF-I peptides and with the concept of a complex and possibly isoform-specific IGF-I bioactivity. This concept is discussed in the present review, in the context of the broad range of modifications that this growth factor undergoes which might regulate its mechanism(s) of action.

Biological activity of the e domain of the IGF-1Ec as addressed by synthetic peptides

Insulin-like growth factor-1 (IGF-1) is a multipotent growth factor involved in the growth, development and regulation of homeostasis in a tissue-specific manner. Alternative splicing, multiple transcription initiation sites and different polyadelynation signals give rise to diverse mRNA isoforms, such as IGF-1Ea, IGF-1Eb and IGF-1Ec transcripts. There is increasing interest in the expression of the IGF-1 isoforms and their potential distinct biological role. IGF-1Ec results from alternative splicing of exons 4-5-6 and its expression is upregulated in various conditions and pathologies. Recent studies have shown that IGF-1Ec is preferentially increased after injury in skeletal muscle during post-infarctal myocardium remodelling and in cancer tissues and cell lines. A synthetic analogue corresponding to the last 24 aa of the E domain of the IGF-1Ec isoform has been used to elucidate its potential biological role. The aim of the present review is to describe and discuss the putative bioactivity of the E domain of the IGF-1Ec isoform.

Mechano-growth factor peptide, the COOH terminus of unprocessed insulin-like growth factor 1, has no apparent effect on myoblasts or primary muscle stem cells

A splice form of IGF-1, IGF-1Eb, is upregulated after exercise or injury. Physiological responses have been ascribed to the 24-amino acid COOH-terminal peptide that is cleaved from the NH3-terminal 70-amino acid mature IGF-1 protein. This COOH-terminal peptide was termed "mechano-growth factor" (MGF). Activities claimed for the MGF peptide included enhancing muscle satellite cell proliferation and delaying myoblast fusion. As such, MGF could represent a promising strategy to improve muscle regeneration. Thus, at our two pharmaceutical companies, we attempted to reproduce the claimed effect of MGF peptides on human and mouse muscle myoblast proliferation and differentiation in vitro. Concentrations of peptide up to 500 ng/ml failed to increase the proliferation of C2C12 cells or primary human skeletal muscle myoblasts. In contrast, all cell types exhibited a proliferative response to mature IGF-1 or full-length IGF-1Eb. MGF also failed to inhibit the differentiation of myoblasts into myotubes. To address whether the response to MGF was lost in these tissue culture lines, we measured proliferation and differentiation of primary mouse skeletal muscle stem cells exposed to MGF. This, too, failed to demonstrate a significant effect. Finally, we tested whether MGF could alter a separate documented in vitro effect of the peptide, activation of p-ERK, but not p-Akt, in cardiac myocytes. Although a robust response to IGF-1 was observed, there were no demonstrated activating responses from the native or a stabilized MGF peptide. These results call in to question whether there is a physiological role for MGF.

Posttranscriptional regulation of insulin family ligands and receptors

Insulin system including ligands (insulin and IGFs) and their shared receptors (IR and IGFR) are critical regulators of insulin signaling and glucose homeostasis. Altered insulin system is associated with major pathological conditions like diabetes and cancer. The mRNAs encoding for these ligands and their receptors are posttranscriptionally controlled by three major groups of regulators; (i) alternative splicing regulatory factors; (ii) turnover and translation regulator RNA-binding proteins (TTR-RBPs); and (iii) non-coding RNAs including miRNAs and long non-coding RNAs (lncRNAs). In this review, we discuss the influence of these regulators on alternative splicing, mRNA stability and translation. Due to the pathological impacts of insulin system, we also discussed the possibilities of discovering new potential regulators which will improve understanding of insulin system and associated diseases.

Insulin-like growth factors effects on the expression of myogenic regulatory factors in gilthead sea bream muscle cells

Gilthead sea bream (Sparus aurata) is a widely cultured fish; however, muscle development regulation is poorly known. Myogenesis can be activated by the myogenic regulatory factors (MRFs: MyoD, Myf5, myogenin and MRF4) and by endocrine signals from the growth hormone (GH)/insulin-like growth factors (IGFs) axis. We cultured gilthead sea bream myocytes to better understand the role of IGFs in muscle growth and differentiation through the regulation of MRFs expression. First, we studied the expression pattern during culture development of IGFs and IGF-I splice variants. The expression of igf-II was highest at the beginning of the culture and decreased when the cells started to differentiate, similarly to that observed for total igf-I. Igf-Ib showed a paralleled expression pattern as that of total igf-I, whereas igf-Ic was more stable during culture progression. Next, we analyzed the expression of IGFs and MRFs after incubation of cells at day 4 with GH, IGF-I, IGF-II and combinations of them at 3, 6 and 18 h. IGF-II increased myod2 and myf5 expression, genes involved in early muscle cell proliferation. Moreover, IGF-I caused an increase on mrf4 and myogenin expression, both involved in the later stages of development corresponding to differentiation. Regarding the regulation of IGFs expression, igf-I was stimulated by GH and IGF-II alone and combined, whereas igf-II expression was increased in response to IGF-I, suggesting a nice model of crossed regulation. Overall, the present model could be very useful to understand the different regulatory roles of these endocrine and transcription factors on fish myogenesis.

The therapeutic potential of IGF-I in skeletal muscle repair

Skeletal muscle loss due to aging, motor-neuron degeneration, cancer, heart failure, and ischemia is a serious condition for which currently there is no effective treatment. Insulin-like growth factor 1 (IGF-I) plays an important role in muscle maintenance and repair. Preclinical studies have shown that IGF-I is involved in increasing muscle mass and strength, reducing degeneration, inhibiting the prolonged and excessive inflammatory process due to toxin injury, and increasing the proliferation potential of satellite cells. However, clinical trials have not been successful due to ineffective delivery methods. Choosing the appropriate isoforms or peptides and developing targeted delivery techniques can resolve this issue. Here we discuss the latest development in the field with special emphasis on novel therapeutic approaches.

IGF-I 3' untranslated region: strain-specific polymorphisms and motifs regulating IGF-I in osteoblasts

Reduced IGF-I is associated with low bone mass in humans and mice. C3H/He/J (C3H) mice have higher skeletal IGF-I and greater bone mass than C57BL/6J (B6). We hypothesized that strain-related genotypic differences in Igf1 affected skeletal function. The Igf1 coding region is nonpolymorphic, but its 3' untranslated region (UTR) is polymorphic between C3H and B6. Luciferase-Igf1 3' UTR reporter constructs showed that these polymorphic regions did not affect UTR function. IGF-I splice variants give rise to a common mature IGF-I peptide, but different E peptides. We identified two splice products, exon 4+6 (Ea) and exon 4+5+6 (Eb, mechano-growth factor) and found that their abundance was unchanged during osteoblastic differentiation. The Igf1 3' UTR encoded by exon 6 contains alternative polyadenylation sites. Proximal site use produces a short 3' UTR of approximately 195 bases, whereas distal site usage results in an approximately 6300-base UTR. Although Igf1 mRNA levels did not change during osteoblastic differentiation, distal polyadenylation site usage was increased in B6 cells but not in C3H. The resulting long Igf1 RNA isoform is less stable and has decreased translation efficiency, which may be one mechanism contributing to decreased IGF-I in B6 vs. C3H mice. Although the long UTR contains a conserved [GU](18) repeat, which is a positive regulator of UTR activity, it is also targeted by negative regulators, miR-29 and miR-365. These microRNAs are increased in B6 and C3H cells during osteoblastic differentiation. Differential expression of the long Igf1 3' UTR isoform may be a possible mechanism for enhanced IGF-I regulation in B6 vs. C3H mice.

Androgens regulate gene expression in avian skeletal muscles

Circulating androgens in adult reproductively active male vertebrates influence a diversity of organ systems and thus are considered costly. Recently, we obtained evidence that androgen receptors (AR) are expressed in several skeletal muscles of three passeriform birds, the golden-collared manakin (Manacus vitellinus), zebra finch (Taenopygia guttata), and ochre-bellied flycatcher (Mionectes oleagieus). Because skeletal muscles that control wing movement make up the bulk of a bird’s body mass, evidence for widespread effects of androgen action on these muscles would greatly expand the functional impact of androgens beyond their well-characterized effects on relatively discrete targets throughout the avian body. To investigate this issue, we use quantitative PCR (qPCR) to determine if androgens alter gene mRNA expression patterns in wing musculature of wild golden-collared manakins and captive zebra finches. In manakins, the androgen testosterone (T) up-regulated expression of parvalbumin (PV) and insulin-like growth factor I (IGF-I), two genes whose products enhance cellular Ca²⁺ cycling and hypertrophy of skeletal muscle fibers. In T-treated zebra finches, the anti-androgen flutamide blunted PV and IGF-I expression. These results suggest that certain transcriptional effects of androgen action via AR are conserved in passerine skeletal muscle tissue. When we examined wing muscles of manakins, zebra finches and ochre-bellied flycatchers, we found that expression of PV and IGF-I varied across species and in a manner consistent with a function for AR-dependent gene regulation. Together, these findings imply that androgens have the potential to act on avian muscle in a way that may enhance the physicality required for successful reproduction.

Enhanced Akt phosphorylation and myogenic differentiation in PI3K p110β-deficient myoblasts is mediated by PI3K p110α and mTORC2

Phosphoinositide 3-kinase (PI3K) is a principal regulator of Akt activation and myogenesis; however, the function of PI3K p110β in these processes is not well defined. To address this, we investigated the role of p110β in Akt activation and skeletal muscle cell differentiation. We found that Akt phosphorylation was enhanced in p110β-deficient myoblasts in response to Insulin-like Growth Factor-I (IGF-I), epidermal growth factor, or p110α overexpression, as compared to p110β-sufficient cells. This effect was associated with increased mammalian target of rapamycin complex 2 activation, even in myoblasts deficient in mSin1 and rictor. Conversely, in response to the G-protein-coupled receptor agonist lysophosphatidic acid, Akt phosphorylation was attenuated in p110β-deficient myoblasts. Loss of p110β also enhanced the expression of myogenic markers at the myoblast stage and during the first 48 h of differentiation. These data demonstrate that reductions in p110β are associated with agonist-specific Akt hyperactivation and accelerated myogenesis, thus revealing a negative role for p110β in Akt activation and during myoblast differentiation.

Insulin-like growth factor-I E-peptide activity is dependent on the IGF-I receptor

Insulin-like growth factor-I (IGF-I) is an essential growth factor that regulates the processes necessary for cell proliferation, differentiation, and survival. The Igf1 gene encodes mature IGF-I and a carboxy-terminal extension called the E-peptide. In rodents, alternative splicing and post-translational processing produce two E-peptides (EA and EB). EB has been studied extensively and has been reported to promote cell proliferation and migration independently of IGF-I and its receptor (IGF-IR), but the mechanism by which EB causes these actions has not been identified. Further, the properties of EA have not been evaluated. Therefore, the goals of this study were to determine if EA and EB possessed similar activity and if these actions were IGF-IR independent. We utilized synthetic peptides for EA, EB, and a scrambled control to examine cellular responses. Both E-peptides increased MAPK signaling, which was blocked by pharmacologic IGF-IR inhibition. Although the E-peptides did not directly induce IGF-IR phosphorylation, the presence of either E-peptide increased IGF-IR activation by IGF-I, and this was achieved through enhanced cell surface bioavailability of the receptor. To determine if E-peptide biological actions required the IGF-IR, we took advantage of the murine C2C12 cell line as a platform to examine the key steps of skeletal muscle proliferation, migration and differentiation. EB increased myoblast proliferation and migration while EA delayed differentiation. The proliferation and migration effects were inhibited by MAPK or IGF-IR signaling blockade. Thus, in contrast to previous studies, we find that E-peptide signaling, mitogenic, and motogenic effects are dependent upon IGF-IR. We propose that the E-peptides have little independent activity, but instead affect growth via modulating IGF-I signaling, thereby increasing the complexity of IGF-I biological activity.

The key role of insulin-like growth factor I in limbal stem cell differentiation and the corneal wound-healing process

Limbal stem cells (LSC), which reside in the basal layer of the limbus, are thought to be responsible for corneal epithelial healing after injury. When the cornea is damaged, LSC start to proliferate, differentiate, and migrate to the site of injury. To characterize the signaling molecules ensuring communication between the cornea and LSC, we established a mouse model of mechanical corneal damage. The central cornea or limbal tissue was excised at different time intervals after injury, and the expression of genes in the explants was determined. It was observed that a number of genes for growth and differentiation factors were significantly upregulated in the cornea rapidly after injury. The ability of these factors to regulate the differentiation and proliferation of limbal cells was tested. It was found that the insulin-like growth factor-I (IGF-I), which is rapidly overexpressed after injury, enhances the expression of IGF receptor in limbal cells and induces the differentiation of LSC into cells expressing the corneal cell marker, cytokeratin K12, without any effect on limbal cell proliferation. In contrast, the epidermal growth factor (EGF) and fibroblast growth factor-β (FGF-β), which are also produced by the damaged corneal epithelium, supported limbal cell proliferation without any effect on their differentiation. Other factors did not affect limbal cell differentiation or proliferation. Thus, IGF-I was identified as the main factor stimulating the expression of IGF receptors in limbal cells and inducing the differentiation of LSC into cells expressing corneal epithelial cell markers. The proliferation of these cells was supported by EGF and FGF.

IGF-I measurement across blood, interstitial fluid, and muscle biocompartments following explosive, high-power exercise

Insulin-like growth factor-I (IGF-I) resides across different biocompartments [blood, interstitial fluid (ISF), and muscle]. Whether circulating IGF-I responses to exercise reflect local events remains uncertain. We measured the IGF-I response to plyometric exercise across blood, ISF, and muscle biopsy from the vastus lateralis. Twenty volunteers (8 men, 12 women, 22 ± 1 yr) performed 10 sets of 10 plyometric jump repetitions at a 40% 1-repetition maximum. Blood, ISF, and muscle samples were taken pre- and postexercise. Circulating IGF-I increased postexercise: total IGF-I (preexercise = 546 ± 42, midexercise = 585 ± 43, postexercise = 597 ± 45, +30 = 557 ± 42, +60 = 536 ± 40, +120 = 567 ± 42 ng/ml; midexercise, postexercise, and +120 greater than preexercise, P < 0.05); Free IGF-I (preexercise = 0.83 ± 0.09, midexercise = 0.78 ± 0.10, postexercise = 0.79 ± 0.11, +30 = 0.93 ± 0.10, +60 = 0.88 ± 0.10, + 120 = 0.91 ± 0.11 ng/ml; +30 greater than all other preceding time points, P < 0.05). No exercise-induced changes were observed for ISF IGF-I (preexercise = 2.35 ± 0.29, postexercise = 2.46 ± 0.35 ng/ml). No changes were observed for skeletal muscle IGF-I protein, although IGF-I mRNA content increased ∼40% postexercise. The increase in circulating total and free IGF-I was not correlated with increases in ISF IGF-I or muscle IGF-I protein content. Our data indicate that exercise-induced increases in circulating IGF-I are not reflective of local IGF-I signaling.

Rates of molecular evolution vary in vertebrates for insulin-like growth factor-1 (IGF-1), a pleiotropic locus that regulates life history traits

Insulin-like growth factor-1 (IGF-1) is a member of the vertebrate insulin/insulin-like growth factor/relaxin gene family necessary for growth, reproduction, and survival at both the cellular and organismal level. Its sequence, protein structure, and function have been characterized in mammals, birds, and fish; however, a notable gap in our current knowledge of the function of IGF-1 and its molecular evolution is information in ectothermic reptiles. To address this disparity, we sequenced the coding region of IGF-1 in 11 reptile species-one crocodilian, three turtles, three lizards, and four snakes. Complete sequencing of the full mRNA transcript of a snake revealed the Ea-isoform, the predominant isoform of IGF-1 also reported in other vertebrate groups. A gene tree of the IGF-1 protein-coding region that incorporated sequences from diverse vertebrate groups showed similarity to the species phylogeny, with the exception of the placement of Testudines as sister group to Aves, due to their high nucleotide sequence similarity. In contrast, long-branch lengths indicate more rapid divergence in IGF-1 among lizards and snakes. Additionally, lepidosaurs (i.e., lizards and snakes) had higher rates of non-synonymous:synonymous substitutions (dN/dS) relative to archosaurs (i.e., birds and crocodilians) and turtles. Tests for positive selection on specific codons within branches and evaluation of the changes in the amino acid properties, suggested positive selection in lepidosaurs on the C domain of IGF-1, which is involved in binding affinity to the IGF-1 receptor. Predicted structural changes suggest that major alterations in protein structure and function may have occurred in reptiles. These data propose new insights into the molecular co-evolution of IGF-1 and its receptors, and ultimately the evolution of IGF-1's role in regulating life-history traits across vertebrates.