Lipolysis of bone marrow adipocytes is required to fuel bone and the marrow niche during energy deficits

Abstract

To investigate roles for bone marrow adipocyte (BMAd) lipolysis in bone homeostasis, we created a BMAd-specific Cre mouse model in which we knocked out adipose triglyceride lipase (ATGL, Pnpla2 gene). BMAd-Pnpla2-/- mice have impaired BMAd lipolysis, and increased size and number of BMAds at baseline. Although energy from BMAd lipid stores is largely dispensable when mice are fed ad libitum, BMAd lipolysis is necessary to maintain myelopoiesis and bone mass under caloric restriction. BMAd-specific Pnpla2 deficiency compounds the effects of caloric restriction on loss of trabecular bone in male mice, likely due to impaired osteoblast expression of collagen genes and reduced osteoid synthesis. RNA sequencing analysis of bone marrow adipose tissue reveals that caloric restriction induces dramatic elevations in extracellular matrix organization and skeletal development genes, and energy from BMAd is required for these adaptations. BMAd-derived energy supply is also required for bone regeneration upon injury, and maintenance of bone mass with cold exposure.

Constitutive bone marrow adipocytes suppress local bone formation

“…Cell-specific DTA expression depletes BMAds. We recently created a novel BMAd-specific Cre mouse model (6), that uses an intersectional strategy with dual recombinases: Osterix-FLPo and FLPo-dependent Adipoq-Cre (FAC). Briefly, endogenous Osterix-driven FLPo recognizes Frt sites and flips the reverse orientation of the Cre cassette located in the 3'untranslated region of endogenous Adipoq into the sense orientation (Supplemental Figure 1A-1C).…”

“…Osterix-FLPo, FLPo-activated Adipoq-Cre (FAC), and BMAd-Cre mice were generated in a C57BL/6J and SJL mixed background by University of Michigan Transgenic Animal Model Core and MDRC Molecular Genetics Core. Mouse generation and BMAd-Cre recombinase cell specificity and efficiency were validated in our recent publication (6). mT/mG reporter (Stock No.…”

“…004584) were purchased from The Jackson Laboratory. Since BMAd-Cre mice were shown to have hypoadiponectinemia (35), all mice used in the studies included within this manuscript were positive for both Osterix-FLPo and FAC. Recombination events leading to DTA expression or Pparg knockout was determined and confirmed by genotyping across floxed alleles.…”

Raman microspectroscopy reveals unsaturation heterogeneity at the lipid droplet level and validates anin vitromodel of bone marrow adipocyte subtypes

“…The mechanisms for differential hematopoietic support in vivo are still poorly understood, but extrapolation from the collective findings of the field suggests that cBMAd-dense regions of the distal skeleton maintain HSCs, while rBMAd-containing red marrow would contribute to expansion of their progenitors, possibly through increased FA transfer (83). Most HSCs have low mitochondrial potential and rely mainly on anaerobic glycolysis, while downstream progenitors rely on high energy production through mitochondrial oxidative phosphorylation (93)(94)(95).…”

“…In particular BMAds were increasingly present in the mid-diaphysis with age (81, 82). Furthermore, specific depletion of BMAds through a double conditional Osterix and Adipoq-dependent strategy caused severe reduction in rBMAds with a milder reduction in the cBMAd mass, while ATGL ( Pnpla2 gene) loss in BMAd through the same strategy caused massive rBMAd expansion in the mid diaphysis (83, 84). To what extent these discrepancies are due to divergent transcriptional programs or part of a stabilization of the adipocytic differentiation program remains to be determined.…”