Macrophage deficiency in osteopetrotic (op/op) mice inhibits activation of satellite cells and prevents hypertrophy in single soleus fibers

Abstract

Effects of macrophage on the responses of soleus fiber size to hind limb unloading and reloading were studied in osteopetrotic homozygous (op/op) mice with inactivated mutation of macrophage colony-stimulating factor (M-CSF) gene and in wild-type (ϩ/ϩ) and heterozygous (ϩ/op) mice. The basal levels of mitotically active and quiescent satellite cell (Ϫ46 and Ϫ39% vs. ϩ/ϩ, and Ϫ40 and Ϫ30% vs. ϩ/op) and myonuclear number (Ϫ29% vs. ϩ/ϩ and Ϫ28% vs. ϩ/op) in fibers of op/op mice were significantly less than controls. Fiber length and sarcomere number in op/op were also less than ϩ/ϩ (Ϫ22%) and ϩ/op (Ϫ21%) mice. Similar trend was noted in fiber cross-sectional area (CSA, Ϫ15% vs. ϩ/ϩ, P ϭ 0.06, and Ϫ14% vs. ϩ/op, P ϭ 0.07). The sizes of myonuclear domain, cytoplasmic volume per myonucleus, were identical in all types of mice. The CSA, length, and the whole number of sarcomeres, myonuclei, and mitotically active and quiescent satellite cells, as well as myonuclear domain, in single muscle fibers were decreased after 10 days of unloading in all types of mice, although all of these parameters in ϩ/ϩ and ϩ/op mice were increased toward the control values after 10 days of reloading. However, none of these levels in op/op mice were recovered. Data suggest that M-CSF and/or macrophages are important to activate satellite cells, which cause increase of myonuclear number during fiber hypertrophy. However, it is unclear why their responses to general growth and reloading after unloading are different. myonuclei; muscle fiber size; osteopetrotic mice; satellite cells; unloading and reloading SKELETAL MUSCLES ARE COMPOSED of multinucleated muscle fibers and have an extensive capacity for morphological and functional adaptation. For example, chronic unloading of muscles by bedrest in humans (45,46,64) and hind limb suspension and actual spaceflight in animals (6, 40 -42, 47, 59) result in muscle fiber atrophy and a shift toward a faster myosin heavy chain profile, particularly in antigravity muscles composed predominantly of slow-twitch fibers, such as the soleus (6,42,(45)(46)(47) and adductor longus (41, 59). Muscle fiber atrophy and the shift toward a faster myosin heavy chain profile are reversed in response to muscle reloading (41,45,46,59). The muscular response to reloading appears to be closely related to intramuscular stimuli that are activated by mechanical loading (20,21,41,47,59) and/or muscle activation with reloading (1,12,20,22,41).Macrophage colony-stimulating factor (M-CSF, CSF-1) is a cytokine that influences hematopoietic stem cells to differentiate into macrophages and other cell types (54). Deficiency of macrophages has been well-reported in osteopetrotic (op/op) mutant mice with inactivating mutation of M-CSF gene, which results in the absence of certain macrophage and in osteopetrosis, following a lack of osteoclasts (60, 63). Tidball and Wehling-Henricks (58) reported important roles of macrophages in the repair of muscle fiber membrane, regeneration, and regrowth after unloading-related atrophy in mouse s...