Shi Yu Yang

Different roles of the IGF‐I Ec peptide (MGF) and mature IGF‐I in myoblast proliferation and differentiation

The physiological function of a recently cloned splice variant of insulin‐like growth factor‐I (IGF‐I; mechano growth factor (MGF)) was studied using an in vitro cell model. Unlike mature IGF‐I, the distinct E domain of MGF inhibits terminal differentiation whilst increasing myoblast proliferation. Blocking the IGF‐I receptor with a specific antibody indicated that the function of MGF E domain is mediated via a different receptor. The results provide a basis for localized tissue adaptation and helps explain why loss of muscle mass occurs in the elderly and in dystrophic muscle in which MGF production is markedly affected.

Cloning and characterization of an IGF-1 isoform expressed in skeletal muscle subjected to stretch

SummaryTo ascertain if IGF-1 is a regulator of local muscle growth, total RNA was extracted from rabbit muscle induced to undergo rapid hypertrophy using active stretch and from control muscles. This was analysed by Northern hybridization with a 280 base pair probe containing sequences derived from exons 3 and 4 of the insulin-like growth factor 1 gene. Two types of insulin-like growth factor 1 mRNA were shown to be strong expressed in the stretched muscles. In situ hybridization using the same probe (280 base pair) showed that IGF-1 is strongly expressed in muscle that is induced to grow rapidly and is expressed in the muscle fibres themselves. Using RT-PCR a single insulin-like growth factor 1 isoform cDNA (IGF-1Ea) could be cloned from the normal resting muscles. However, an additional isoform of insulin-like growth factor 1 (insulin-like growth factor 1Eb) was found to be expressed in stretched muscle undergoing hypertrophy. The E domain sequence of the additional isoform differs from the liver insulin-like growth factor 1Ea by the presence a 52 base pair insert. This changes the reading frame of the derived carboxyl-terminal resulting in a different precursor insulin-like growth factor 1 isoform. This insulin-like growth factor 1 mRNA probably encodes the precursor insulin-like growth factor 1 isoform that is responsible for local muscle growth regulation in response to mechanical stimulation. To confirm that alternative splicing of the insulin-like growth factor 1 gene occurs in muscle in response to physical activity, oligonucleotide primers were made which specifically amplify the cDNAs of two isoforms (insulin-like growth factors 1Ea and Eb) in the human as well as the rabbit. Following altered physical activity for 2 h to 6 days, appreciable levels of insulin-like growth factor 1Eb (in human the Ec) isoform were detected in skeletal muscle by using RT-PCR. In contrast very little if any of this splice variant could be detected in control muscle not subjected to stretch or extra physical activity.

Expression of insulin growth factor-1 splice variants and structural genes in rabbit skeletal muscle induced by stretch and stimulation

1 Skeletal muscle is a major source of circulating insulin growth factor‐1 (IGF‐1), particularly during exercise. It expresses two main isoforms. One of the muscle IGF‐1 isoforms (muscle L.IGF‐1) is similar to the main liver IGF‐1 and presumably has an endocrine action. The other muscle isoform as a result of alternative splicing has a different 3′ exon sequence and is apparently designed for an autocrine/paracrine action (mechano‐growth factor, MGF). Using RNase protection assays with a probe that distinguishes these differently spliced forms of IGF‐1, their expression and also the expression of two structural genes was measured in rabbit extensor digitorum longus muscles subjected to different mechanical signals. 2 Within 4 days, stretch using plaster cast immobilization with the limb in the plantar flexed position resulted in marked upregulation of both forms of IGF‐1 mRNA. Electrical stimulation at 10 Hz combined with stretch (overload) resulted in an even greater increase of both types of IGF‐1 transcript, whereas electrical stimulation alone, i.e. without stretch, resulted in no significant increase over muscle from sham‐operated controls. Previously, it was shown that stretch combined with electrical stimulation of the dorsiflexor muscles in the adult rabbit results in a marked increase in muscle mass involving increases in both length and girth, within a few days. The expression of both systemic and autocrine IGF‐1 growth factors provides a link between the mechanical signal and the marked increase in the structural gene expression involved in tissue remodelling and repair. 3 The expression of the β actin gene was seen to be markedly upregulated in the stretched and stretched/stimulated muscles. It was concluded that the increased expression of this cytoskeletal protein gene is an indication that the production of IGF‐1 may initially be a response to local damage. 4 Switches in muscle fibre phenotype were studied using a specific gene probe for the 2X myosin heavy chain gene. Type 2X expression was found to decrease markedly with stimulation alone and when electrical stimulation was combined with stretch. Unlike the induction of IGF‐1 and β actin, the decreased expression of the 2X myosin mRNA was less marked in the ‘stretch only’ muscles. This indicates that the interconversion of fibre type 2X to 2A may in some situations be commensurate with, but not under the control of IGF‐1.

Age-related loss of skeletal muscle function and the inability to express the autocrine form of insulin-like growth factor-1 (MGF) in response to mechanical overload

The response of insulin‐like growth factor‐1 (IGF‐1) signalling and the capacity of skeletal muscle to adapt to mechanical overload was studied using synergistic muscle ablation. Overload of the plantaris and soleus resulted in marked hypertrophy and activation of satellite cells (as indicated by MyoD expression), particularly in young rats. Two muscle IGF‐1 splice variants were measured and found to be differentially regulated at the RNA level. The significant changes associated with the inability of the older muscles to respond to mechanical overload included the considerably lower expression of the local splice variant mechano growth factor, and the failure to up‐regulate IGF‐1 receptor and MyoD mRNA.

Liver ischemia/reperfusion injury: Processes in inflammatory networks—A review

Liver ischemia/reperfusion (IR) injury is typified by an inflammatory response. Understanding the cellular and molecular events underpinning this inflammation is fundamental to developing therapeutic strategies. Great strides have been made in this respect recently. Liver IR involves a complex web of interactions between the various cellular and humoral contributors to the inflammatory response. Kupffer cells, CD4+ lymphocytes, neutrophils, and hepatocytes are central cellular players. Various cytokines, chemokines, and complement proteins form the communication system between the cellular components. The contribution of the danger‐associated molecular patterns and pattern recognition receptors to the pathophysiology of liver IR injury are slowly being elucidated. Our knowledge on the role of mitochondria in generating reactive oxygen and nitrogen species, in contributing to ionic disturbances, and in initiating the mitochondrial permeability transition with subsequent cellular death in liver IR injury is continuously being expanded. Here, we discuss recent findings pertaining to the aforementioned factors of liver IR, and we highlight areas with gaps in our knowledge, necessitating further research. Liver Transpl 16:1016–1032, 2010.

Changes in muscle fibre type, muscle mass and IGF-I gene expression in rabbit skeletal muscle subjected to stretch

The relationship between IGF‐I and changes in muscle fibre phenotype in response to 6 d of stretch or disuse of the lower limb muscles of the rabbit was studied by combining in situ hybridisation and immunohistochemistry procedures. Passive stretch by plaster cast immobilisation of the muscle in its lengthened position not only induced an increase in IGF‐I mRNA expression within the individual muscle fibres but also an increase in the percentage of fibres expressing neonatal and slow myosin. This change in phenotype was also found to be accompanied by a rapid and marked increase of muscle mass, total RNA content as well as IGF‐I gene expression. In contrast, IGF‐I appears not to be involved in muscle atrophy induced by immobilisation in the shortened position and the inactivity which results from this procedure. The level of increase in expression of IGF‐I mRNA varied from fibre to fibre. By using adjacent serial sections, the fibres which expressed IGF‐I mRNA at the highest levels were identified as expressing neonatal and the slow type 1 myosin. These data suggest that the expression of IGF‐I within individual muscle fibres is correlated not only with hypertrophy but also with the muscle phenotypic adaptation that results from stretch and overload.

Mechanical signals and IGF-I gene splicing in vitro in relation to development of skeletal muscle

It has been shown that the insulin‐like growth factor (IGF‐I) gene is spliced in response to mechanical signals producing forms of IGF‐I which have different actions. In order to study how mechanical signals influence this gene splicing in developing muscle, C2C12 cells were grown in three‐dimensional (3D) culture and subjected to different regimens of mechanical strain. IGF‐IEa which initiates the fusion of myoblasts to form myotubes was found to be constitutively expressed in myoblasts and myotubes (held under endogenous tension) and its expression upregulated by a single ramp stretch of 1‐h duration but reduced by repeated cyclical stretch. In contrast, mechano growth factor (MGF), which is involved in the proliferation of mononucleated myoblasts that are required for secondary myotube formation and to establish the muscle satellite (stem) cell pool, showed no significant constitutive expression in static cultures, but was upregulated by a single ramp stretch and by cycling loading. The latter types of force simulate those generated in myoblasts by the first contractions of myotubes. These data indicate the importance of seeking to understand the physiological signals that determine the ratios of splice variants of some growth factor/tissue factor genes in the early stages of development of skeletal muscle.

The IGF-I splice variant MGF increases progenitor cells in ALS, dystrophic, and normal muscle

The effects of muscle splice variants of insulin‐like growth factor I (IGF‐I) on proliferation and differentiation were studied in human primary muscle cell cultures from healthy subjects as well as from muscular dystrophy and ALS patients. Although the initial numbers of mononucleated progenitor cells expressing desmin were lower in diseased muscle, the E domain peptide of IGF‐IEc (MGF) significantly increased the numbers of progenitor cells in healthy and diseased muscle. IGF‐I significantly enhances myogenic differentiation whereas MGF E peptide blocks this pathway, resulting in an increased progenitor (stem) cell pool and thus potentially facilitating repair and maintenance of this postmitotic tissue.

Modern surgical management of peripheral nerve gap

The management of peripheral nerve injury requires a thorough understanding of the complex physiology of nerve regeneration. The ability to perform surgery under magnification has improved our understanding of the anatomy of the peripheral nerves. However, the level of functional improvement that can be expected following peripheral nerve injury has plateaued. Advancements in the field of tissue engineering have led to an exciting complement of commercially available products that can be used to bridge peripheral nerve gaps. However, the quest for enhanced options is ongoing. This article provides a review of the current treatment options available following peripheral nerve injury, a summary of the published studies using commercially available nerve conduits and nerve allografts in humans and the emerging hopes for the next generation of nerve conduits with the advancement of nanotechnology.

Apoptosis and colorectal cancer: implications for therapy

Colorectal cancer (CRC) is characterized by the partial suppression of apoptosis, which in turn gives tumours a selective advantage for survival and can cause current chemotherapy approaches to be ineffective. Recent progress in understanding the mechanisms of apoptosis in colorectal carcinogenesis has provided potential new targets for therapy. Here, we review recent studies of the regulation of apoptosis and its role in CRC initiation and progression, and we discuss the relationship between chemoresistance and the suppression of apoptosis. Recent progress in targeting apoptotic pathways and their regulators provide strategies for the exploration of novel therapies for CRC.