Increased Plin2 Expression in Human Skeletal Muscle Is Associated with Sarcopenia and Muscle Weakness

et al. (2013) Increased Plin2 Expression in Human Skeletal Muscle Is Associated with Sarcopenia and Muscle Weakness. PLoS ONE 8(8): e73709. doi:10.1371/journal.pone.0073709 Increased Plin2 Expression in Human Skeletal Muscle Is Associated with Sarcopenia and Muscle Weakness Maria Conte 0 Francesco Vasuri 0 Giovanni Trisolino 0 Elena Bellavista 0 Aurelia Santoro 0 Alessio 0 Degiovanni 0 Ermanno Martucci 0 Antonia D'Errico-Grigioni 0 Daniela Caporossi 0 Miriam Capri 0 Andrea 0 B. Maier 0 Olivier Seynnes 0 Laura Barberi 0 Antonio Musar 0 Marco V. Narici 0 Claudio Franceschi 0 Stefano Salvioli 0 Maurilio Sampaolesi, Stem Cell Research Institute, Belgium 0 1 Department of Experimental, Diagnostic and Specialty Medicine and Interdepartmental Centre L. Galvani (CIG), University of Bologna , Bologna, Italy, 2 F. Addarii Institute of Oncology and Transplant Pathology, S. Orsola-Malpighi Hospital, University of Bologna , Bologna , Italy , 3 Reconstructive Hip and Knee Joint Surgery, Istituto Ortopedico Rizzoli , Bologna , Italy , 4 Department of Health Science, University of Rome Foro Italico , Rome , Italy , 5 Department of Gerontology and Geriatrics, Leiden University Medical Center , Leiden , The Netherlands , 6 Norwegian School of Sport Sciences , Oslo , Norway , 7 Institute Pasteur Cenci- Bolognetti, DAHFMO-unit of Histology and Medical Embryology, IIM, Sapienza University of Rome , Rome , Italy , 8 School of Graduate Entry Medicine and Health, Division of Clinical Physiology, Derby Royal Hospital, University of Nottingham , Derby , United Kingdom Human aging is associated with a progressive loss of muscle mass and strength and a concomitant fat accumulation in form of inter-muscular adipose tissue, causing skeletal muscle function decline and immobilization. Fat accumulation can also occur as intra-muscular triglycerides (IMTG) deposition in lipid droplets, which are associated with perilipin proteins, such as Perilipin2 (Plin2). It is not known whether Plin2 expression changes with age and if this has consequences on muscle mass and strength. We studied the expression of Plin2 in the vastus lateralis (VL) muscle of both healthy subjects and patients affected by lower limb mobility limitation of different age. We found that Plin2 expression increases with age, this phenomenon being particularly evident in patients. Moreover, Plin2 expression is inversely correlated with quadriceps strength and VL thickness. To investigate the molecular mechanisms underpinning this phenomenon, we focused on IGF-1/p53 network/signalling pathway, involved in muscle physiology. We found that Plin2 expression strongly correlates with increased p53 activation and reduced IGF-1 expression. To confirm these observations made on humans, we studied mice overexpressing muscle-specific IGF-1, which are protected from sarcopenia. These mice resulted almost negative for the expression of Plin2 and p53 at two years of age. We conclude that fat deposition within skeletal muscle in form of Plin2-coated lipid droplets increases with age and is associated with decreased muscle strength and thickness, likely through an IGF-1- and p53-dependent mechanism. The data also suggest that excessive intramuscular fat accumulation could be the initial trigger for p53 activation and consequent loss of muscle mass and strength. - Funding: The research leading to these results has received funding from the European Union's Seventh Framework Programme (FP7/2007-2011) under grant agreement n 223576 (MYOAGE) to CF, MN and AM. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing interests: The authors have declared that no competing interests exist. Human aging is characterized by increased levels of physical disability due at least in part to loss of muscle strength. This loss depends on both decrease in muscle mass and accumulation of inter-muscular adipose tissue (IMAT) [1,2]. In particular, emerging evidence suggests that high level of IMAT contributes to the decline of muscle quality, predicting sarcopenia and increasing risk of mobility impairment [3,4]. Nevertheless, the precise mechanisms leading to age-related loss of muscle quality and strength are far from being elucidated. Many studies have focused on the role of IMAT, as it is known to be a source of inflammatory mediators [5], while much less is known about the possible role of intracellular lipid deposition, which occurs in form of lipid droplets (LDs). LDs are vesicles formed by a phospholipid monolayer whose dynamics appears to be determined by a family of proteins named Perilipins (Plins), previously referred to as PAT proteins, which play a critical role in regulating intracellular lipid storage and mobilization [6,7]. The PAT family consists of five specific members: Perilipin (Plin1), Adipocyte differentiation-related protein (ADRP or also referred to as Plin2), Tail-interacting protein of 47 kDa (TIP47 or Plin3), S3-12 (Plin4) and OXPAT (Plin5). The expression patterns of the five Perilipins vary in different tissues. For instance, Plin1 expression is nearly exclusive of adipocytes, whereas Plin2 has been reported to be a marker for LDs in human skeletal muscle [8,9], and its content is closely related to intramuscular triglycerides [1012]. Plin2 is mostly expressed in the type I fibres of skeletal muscle, which contain more fat than the type II fibres and where Plin2 promotes the uptake of fatty acids and their storage as triacylglycerols [13,14]. The exact function of Plin2 remains to be clearly defined, nevertheless, recent findings suggest that Plin2 is essential for lipid storage in skeletal muscle by enhancing the partitioning of excess fatty acids toward triglycerol storage in lipid droplets, thereby blunting lipotoxicity-associated insulin resistance [13,14]. Since insulin is a potent anti-proteolytic agent, the development of insulin resistance likely contributes to the loss of muscle mass [15]. Recent data indicate that Plin2 is strictly associated to LDs [16] and thus can be an indicator of intramuscular triglyceride (IMTG) deposition; moreover, increased levels of Plin2 are related to mechanisms promoting IMTG utilization during exercise and to improvements in insulin sensitivity [17]. It is not known whether Plin2 expression is modified with age in human skeletal muscle and whether this can be associated with alterations of muscle mass and strength. Therefore the objective of this study was to investigate the different expression levels of Plin2 in skeletal muscle from subjects of different age (from 20 to over 80 years), either healthy people or patients affected by mobilitylimiting pathologies (osteoarthritis) leading to sedentary life style. In the framework of the EU 7th Program Project MYOAGE (Understanding and combating human age-related muscle weakness), we analysed Plin2 expression and distribution in Vastus lateralis (VL) muscle and associa (...truncated)