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Plasticity of skeletal muscle metabolism via insulin and IGF receptor-mediated AKT activation

Reference	BB/C516279/1
Principal Investigator / Supervisor	Dr Jennifer Pell
Co-Investigators / Co-Supervisors
Institution	Babraham Institute
Department	Molecular Signalling
Funding type	Research
Value (£)	246,842
Status	Completed
Type	Research Grant
Start date	28/11/2005
End date	27/01/2009
Duration	38 months

Abstract

Administration of insulin or insulin-like growth factor (IGF) ligands has determined that the insulin receptor (IR) regulates cell metabolism, whereas the IGF type 1 receptor (IGFR) controls cell growth and survival. However, novel studies in which the IR or IGFR are genetically ablated have challenged this clear-cut delineation of receptor function. For example, the IR entirely supports growth in IGF type 1 and type 2 receptor double null mice; conversely, the IGFR stimulates glucose uptake in skeletal muscle lacking the IR. Remarkable in these models is the ability of different tissues eg. muscle and adipose, to communicate and adapt. In more physiological models, the diurnal models, the diurnal variation in IGF binding protein-1 controls a glucoregulatory role of IGF-1, and the incidence of type 2 diabetes is greater in short than tall subjects, who usually have higher circulating IGF-1 concentrations. In skeletal muscle, IR and IGFR signalling converges on Akt, which has multiple downstream targets and regulates basic cell processes such as glucose metabolism, protein synthesis, apoptosis, cell cycle, differentiation and transcription. The first hypothesis to be examined in this proposal is therefore that Akt provides a mechanism for communication and cooperation between IR and IGFR. Akt exists as three isoforms; Akt2 is highly expressed in skeletal muscle, with somewhat less Akt1 and negligible amounts of Akt3. Data from our lab and others suggest that Akt2 rather Akt1 activates key downstream targets in myocytes. Therefore our second hypothesis is that Akt2 is responsible for IR and IGFR compensation in differentiated muscle. We will examine these hypotheses in vivo and ex vivo by use of novel and existing genetically targeted mice models. Mice will be generated that overexpress different forms of Akt in skeletal muscle: i, constitutively active (ca) Akt1 or Akt2 which will be active independent of upstream signals, ii, wild-type (wt) Akt2 that could be activated if appropriate upstream signals are generated; and iii, dominant negative (dn) Akt2 that in vitro studies suggest will inhibit all forms of Akt. These mice will be generated from two strains: i, Akt1 or Akt2 will be inserted downstream of a CMV enhancer beta-actin promoter that is silenced by IoxP-flanked CAT, ii, these mice will be crossed with mice that express Cre-recombinase under the regulation of the muscle creatine kinase promoter, activating Akt in differentiated skeletal muscle. This approach will give experimental flexibility, eg. by crossing mice with myogenin-Cre mice we could examine the role of Akt earlier in muscle development, but will not involve the generation of more novel strains than would a direct transgenic. The phenotype of the different Akt overexpressing mice models will be determined, in vivo in terms of muscle growth, morphology and glucose utilisation. As these mice will overexpress Akt on a normal Akt background, we will cross them with mice that are null for Akt1 or Akt2, thus addressing Akt function isolated to skeletal muscle. The next objective is to determine whether Akt overexpression can rescue the phenotypes of i, muscle-specific IR null mice, that develop compensatory hypertriglyceridaemia adipose tissue hyperplasia and ii, mice expressing a dn IGFR that inhibits both IR and IGFR function. The key comparison will be between responses of ca and wt Akt2 overexpressing mice as these will inform whether Akt can rescue and if upstream signals can compensate. We will next examine whether muscle Akt can rescue IR and IGFR resistance syndromes: i, insulin-resistance in the type 2 diabetic db db mouse and ii, age-related decrease in IGF insensitivity in muscle atrophy. Finally, mechanisms of Akt action and isoform specificity will be examined in myocyte cultures derived from the Akt mouse models: i, Akt localisation, which may specify its function ii, Akt downstream phospho-targets and iii, Akt binding partners scaffold proteins.

Summary

unavailable

Committee	Closed Committee - Animal Sciences (AS)
Research Topics	Ageing
Research Priority	X – Research Priority information not available
Research Initiative	X - not in an Initiative
Funding Scheme	X – not Funded via a specific Funding Scheme

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