Tsc2 Is a Molecular Checkpoint Controlling Osteoblast Development and Glucose Homeostasis

Abstract

g Insulin signaling in osteoblasts regulates global energy balance by stimulating the production of osteocalcin, a bone-derived protein that promotes insulin production and action. To identify the signaling pathways in osteoblasts that mediate insulin's effects on bone and energy metabolism, we examined the function of the tuberous sclerosis 2 (Tsc2) protein, a key target important in coordinating nutrient signaling. Here, we show that loss of Tsc2 in osteoblasts constitutively activates mTOR and destabilizes Irs1, causing osteoblasts to differentiate poorly and become resistant to insulin. Young Tsc2 mutant mice demonstrate hypoglycemia with increased levels of insulin and undercarboxylated osteocalcin. However, with age, Tsc2 mutants develop metabolic features similar to mice lacking the insulin receptor in the osteoblast, including peripheral adiposity, hyperglycemia, and decreased pancreatic ␤ cell mass. These metabolic abnormalities appear to result from chronic elevations in undercarboxylated osteocalcin that lead to downregulation of the osteocalcin receptor and desensitization of the ␤ cell to this hormone. Removal of a single mTOR allele from the Tsc2 mutant mice largely normalizes the bone and metabolic abnormalities. Together, these findings suggest that Tsc2 serves as a key checkpoint in the osteoblast that is required for proper insulin signaling and acts to ensure normal bone acquisition and energy homeostasis.T he formation, remodeling, and repair of the skeleton are performed by osteoblasts, osteoclasts, and osteocytes through a process that is increasingly recognized as energetically expensive (1). To coordinate bone formation and remodeling, bone cell activity is regulated by intricate networks of local factors and systemic hormones. In turn, bone cells secrete factors that communicate with other tissues to ensure homeostatic regulation of global energy utilization (2, 3). For example, the adipocyte-derived hormone leptin influences appetite and energy expenditure (4) and also regulates postnatal bone acquisition (5, 6). In addition, osteoblasts and osteocytes produce and secrete hormones, including osteocalcin, which enhances insulin secretion and peripheral insulin sensitivity (7).Recent studies (8, 9) have demonstrated that insulin signaling in osteoblasts is required for normal bone acquisition and the coordination of whole-body glucose metabolism. Osteoblasts lacking the insulin receptor fail to differentiate normally, which results in reduced postnatal bone acquisition. In addition, loss of insulin signaling in osteoblasts inhibits osteocalcin production via increased activity of FoxO1, a suppressor of osteocalcin gene transcription (10, 11). Thus, the metabolic abnormalities in mice lacking the insulin receptor in osteoblasts can be partially reversed by infusing mice with undercarboxylated osteocalcin (ucOCN) (8), strongly suggesting the existence of an osteoblast-pancreas endocrine loop in which bone production of ucOCN ensures appropriate insulin production in a feed-forward loop.Many...

mTORC1 Plays an Important Role in Skeletal Development by Controlling Preosteoblast Differentiation

“…Deletion of either Tsc1 in preosteoblasts using Osx:cre (ΔTsc1 mice [57]) or Tsc2 in mature osteoblasts using Ocn:cre (ΔTsc2 mice [58]), results in constitutive activation of mTORC1. In both models, hyperactivation of mTORC1 leads to an increase in postnatal bone mass that was evident in cortical and trabecular regions of the long bones and calvarial bones of the skull.…”

“…In contrast to gain-of-function mouse models, the evidence from loss-of-function studies suggests that mTORC1 may play a role in promoting osteoblast differentiation. Mice with the hom deletion of mTOR in mature osteoblasts (Ocn:cre ΔmTOR) demonstrate reduced tibial trabecular bone volume and trabecular number (58). This phenotype can be attributed to impaired mTORC1 function since mice with homozygous deletion of Rictor, an essential component of mTORC2, in mature osteoblasts have no tibial bone phenotype (60).…”

A Mouse Model of Craniofacial Bone Lesion of Tuberous Sclerosis Complex

“…Studies using rapamycin, an mTORC1 inhibitor, have showed both positive [9–11] and negative [12–15] effects on osteogenesis in vitro . Some recent reports demonstrated the important roles of mTOR signaling in skeletal development [16–18] . However, despite the high prevalence of bone lesions in TSC patient, the underlying bone lesion mechanism is largely unknown, which is mainly due to the lack of appropriate animal models of TSC bone lesions.…”

“…Interestingly, mutant bones had not only increased mass but also increased tissue mineral density (Xiaoxi Wei and Fei Liu, unpublished). Interestingly, TSC2 deletion in mature osteoblasts also leads to increased calvaria thickness [16] . However, the disorganized bone structure was reported in that study, which is clearly different from the highly mineralized bone in our model.…”

Searching for additional endocrine functions of the skeleton: genetic approaches and implications for therapeutics

“…This notion was supported by a study from the Clemens’ group showing that mice harboring osteoblast specific ablation of Tsc2 have improved glucose handling with increased levels of insulin and undercarboxylated osteocalcin at 1 week of age. However, with age, these mutant mice developed opposite metabolic phenotypes due to chronic elevations in undercarboxylated osteocalcin that down-regulating Gprc6a expression and therefore desensitizing osteocalcin signaling in pancreatic β-cells [133]. Taken together, these genetic and pharmacological studies reveal the pivotal role that osteocalcin plays in the development of metabolic diseases and suggests that harnessing osteocalcin signaling may be of therapeutic use for metabolic diseases.…”