Characterization of adipocytes derived from fibro/adipogenic progenitors resident in human skeletal muscle

A population of fibro/adipogenic but non-myogenic progenitors located between skeletal muscle fibers was recently discovered. The aim of this study was to determine the extent to which these progenitors differentiate into fully functional adipocytes. The characterization of muscle progenitor-derived adipocytes is a central issue in understanding muscle homeostasis. They are considered as being the cellular origin of intermuscular adipose tissue that develops in several pathophysiological situations. Here fibro/adipogenic progenitors were isolated from a panel of 15 human muscle biopsies on the basis of the specific cell-surface immunophenotype CD15⁺/PDGFRα⁺CD56⁻. This allowed investigations of their differentiation into adipocytes and the cellular functions of terminally differentiated adipocytes. Adipogenic differentiation was found to be regulated by the same effectors as those regulating differentiation of progenitors derived from white subcutaneous adipose tissue. Similarly, basic adipocyte functions, such as triglyceride synthesis and lipolysis occurred at levels similar to those observed with subcutaneous adipose tissue progenitor-derived adipocytes. However, muscle progenitor-derived adipocytes were found to be insensitive to insulin-induced glucose uptake, in association with the impairment of phosphorylation of key insulin-signaling effectors. Our findings indicate that muscle adipogenic progenitors give rise to bona fide white adipocytes that have the unexpected feature of being insulin-resistant.Adipose tissue consists of several distinct anatomical compartments. It is not completely clear how all of them are formed despite the current high interest in understanding adipose tissue specificities. One intriguing compartment is the so-called intermuscular adipose tissue (IMAT), which can be found between muscle fibers.^{1, 2} Adipocytes accumulate and replace a large proportion of muscle fibers in muscular dystrophies,³ and muscle adiposity was even shown to be an accurate measurement of the severity of Duchenne muscular dystrophy.⁴ IMAT accumulation has also been reported in type II diabetes,^{5, 6} aged muscles,^{7, 8, 9} denervation,¹⁰ and in chronic disuse-induced muscle atrophy.^{11, 12} IMAT accumulation also occurs in muscles of healthy younger individuals after only 4 weeks of immobilization.¹¹ In pathological and nonpathological models, IMAT accumulation is linked to insulin resistance.^{5, 13, 14} The fat infiltration of muscle has not been markedly investigated for many years, whereas this process likely has deep impacts on muscle function because of the profound alterations induced in muscle structure and the important interplay between muscle and adipose tissues – which are both known to be very active factor-secreting tissues.¹⁵Muscle regeneration is supported by the extensively characterized satellite cells, which are myogenic progenitors laying along muscle fibers.¹⁶ In addition, a few groups have recently identified adipogenic progenitors resident in skeletal muscle. Among them are progenitors identified on the basis of specific cell-surface marker expressions, which can thus be physically separated by cell sorting. In humans, muscle adipogenic progenitors have been separated by flow cytometry as a CD15⁺CD56⁻ subpopulation by us and others.^{17, 18, 19, 20} CD56, the neural cell adhesion molecule 1, is known to be expressed by muscle satellite cells (which have the CD15⁻CD56⁺ immunophenotype). CD15 is an antigenic carbohydrate molecule found in several glycoproteins. Before its implication in the muscle adipogenic lineage, it was essentially known to be present in hematopoietic and neural cells. The CD15⁺CD56⁻ adipogenic progenitors express the mesenchymal stem or progenitor cell markers CD13, CD34, CD44, CD49, CD90, and CD105. They are negative for the lineage markers CD31, CD45, CD106, CD117, CD133, and STRO-1.^{18, 19, 20}In parallel in mice, muscle fibro/adipogenic progenitors (FAPs) have been identified as lin⁻(α7 integrin)⁻Sca-1⁺CD34⁺ cells²¹ and muscle mesenchymal progenitors with the immunophenotype CD31⁻CD45⁻SM/C-2.6⁻PDGFRα⁺ have been shown to contribute to fat cell formation in skeletal muscle.²² Further studies indicated that the two mouse immunophenotypes in fact specifically label the same progenitors that should be recognized as skeletal muscle-resident mesenchymal progenitors.²³ Finally, PDGFRα has also been used very recently in human to isolate muscle mesenchymal progenitors, which are equivalent to the mouse FAPs.^{24, 25}Despite the physiological importance of adipocytes derived from human or mouse skeletal muscle, characterization of these terminally differentiated cells is essentially limited to the expression of adipogenic markers. No comprehensive analyses have been reported, and the extent to which muscle adipogenic progenitors differentiate into fully functional adipocytes is unknown.Here we benefited from the recent identification of these progenitors to investigate their differentiation, as well as the functional characteristics and specificities of the derived adipocytes. The whole study has been performed in humans considering the functional importance of human IMAT. Muscle biopsies were taken from a panel of 15 donors. Canonical adipose stroma cells (ASCs) prepared from subcutaneous adipose tissue depots, and their derived adipocytes were used as references. In this study, we established first that the PDGFRα⁺CD56⁻ muscle progenitors are identical to the CD15⁺CD56⁻ progenitors, which therefore, can be also considered as the human counterparts of the FAPs isolated in mice. Then, our cellular, molecular, and biochemical data showed that bona fide white adipocytes are derived from human muscle-resident progenitors. However, these adipocytes have an unexpected impairment in insulin signaling associated with insulin resistance with reduced glucose uptake.