Bone equilibria and disruptions

Abstract

Osteoporosis is considered a disease of adulthood, but there is increasing recognition that poor bone density during childhood can have effects decades later. To understand the pathogenesis of osteoporosis, it is important to understand normal bone maintenance and remodeling, since disruptions of these processes lead to pathologic bone. Bone maintenance is a complex and highly regulated system, consisting of several homeostatic equilibria. This article highlights three homeostatic systems. The first, the interplay between the differentiation of osteoblasts from mesenchymal stem cells and osteoclasts from hematopoietic stem cells, is the most important. Estrogen has a direct effect on the system, and its absence is pivotal. The second is a lesser-known homeostasis that functions between bone and bone marrow adipose tissue, which can insidiously drive osteoporosis. Bone marrow adipose tissue acts as a regulator of bone metabolism, negatively affecting bone formation. The third homeostatic system covered is the microbiota-gut-bone axis, where the make-up of the gut microbiome can influence a balance between osteoblastic and osteoclastic T-cells. Understanding these systems has provided avenues of study for existing and future treatments.

Key concepts: (1)The balance between bone formation and bone resorption is driven by factors that initiate the differentiation of mesenchymal stem cells to osteoblasts and hematopoietic stem cells to osteoclasts.(2)Bone marrow adipose tissue is formed by adipocytes that are the result of diversion of mesenchymal stem cells from the osteoblastic differentiation pathway.(3)The health of the gut microbiome has direct effects on the bone homeostatic processes.

Keywords: Bone formation; Bone metabolism; Osteoblast; Osteoclast; Osteocyte; Osteoporosis.

Figure 1

Stages of the osteoblast differentiation pathway. The uncommitted mesenchymal stem cell is stimulated by exogenous BMPs and the Wnt signaling pathway to transcribe RUNX2 and OSX, directing terminal differentiation in the osteoblastic direction. RUNX2 functions upstream of OSX, both of which are required for osteoblastogenesis. Red arrows are osteoblastic and osteoblastogenic, green arrows indicate stimulation by exogenous factors, purple arrows indicate stimulation by endogenous/autocrine factors. BMP, bone morphogenic proteins; OSX, Osterix; RUNX2, runt-related transcript factor 2.

Figure 2

Stages of the osteoclast differentiation pathway. Osteoblasts release the factors M-CSF and RANKL which stimulate hematopoietic stem (myeloid precursor) cells to transcribe several factors, most importantly PU-1, MITF, NF-κB, and NFATc1. These factors are all required for development of a mature osteoclast. Black arrows are osteoclastic and osteoclastogenic, green arrows indicate stimulation by exogenous factors, purple arrows indicate stimulation by endogenous/autocrine factors. M-CSF, macrophage colony-stimulating factor; MITF, microphthalmia-associated transcription factors; NFATc1, nuclear factor-activated T-cells cytoplasmic 1; RANKL, receptor activators of nuclear factor-κB ligand.

Figure 3

The RANKL/RANK/OPG regulating system. Osteoclast differentiation requires RANKL to bind the cell surface receptor RANK. RANK is present on the cell surface of osteoclast progenitors as well as mature osteoclasts. When RANK binds RANKL, an intracellular cascade leads to the transcription of factors such as NF-κB, NFATc1, and MITF. Osteocytes directly induce osteoblasts to express RANKL. Osteoblasts also release osteoprotegerin (OPG) in response to cytokines, hormones, growth factors and Wnt signaling. OPG then competitively binds RANKL, preventing formation of the RANKL/RANK complex. Red arrows and factors are osteoblastic and osteoblastogenic, black arrows and blue factors are osteoclastic or osteoclastogenic. MITF, microphthalmia-associated transcription factors; NFATc1, nuclear factor-activated T-cells cytoplasmic 1; OPG, Osteoprotegerin; RANKL, receptor activators of nuclear factor-κB ligand.

Figure 4

Osteocytes’ central role in bone maintenance. Many important functions of bone maintenance homeostasis are regulated by osteocytes, primarily by controlling the rate of osteoblastogenesis and osteoclastogenesis. On the left are listed the stimulating and inhibiting factors exogenous to osteoblasts. On the right are the exogenous factors stimulatory or inhibitory to the differentiation of osteoclasts. Some of the factors are hormonal (PTH, estrogen), others have more of a paracrine function. The osteocyte is responsible for the release, either directly or indirectly, of many of the listed factors, but not all of them. Factors in red are osteoblastogenic, those in black are osteoclastogenic. FGF, fibroblast growth factor; PTH, parathyroid hormone.

Figure 5

Estrogen’s role in bone homeostasis. The RANKL-RANK signaling pathway within the osteoclast requires the factor Traf6 as an intermediary. On the osteoclast’s surface or in its cytoplasm, estrogen binds its receptor, ERα and the complex interrupts the pathway by sequestering Traf6, preventing formation of NF-κB and NFATc1, and thereby inhibiting both osteoclastogenesis and osteoclast functioning. The estrogen-ERα complex also transits into the cell nucleus to inhibit NF-κB from binding promoters within the osteoclast’s nucleus, and separately causes overexpression of the calcium-transport proteins TRPV5 and TRPV6 within the osteoclast. The latter leads to apoptosis of the osteoclast. Estrogen stimulates osteoblastogenesis by enhancing both Wnt and BMP signaling cascades in the mesenchymal stem cells and the osteoblast progenitor cells. Estrogen-ERα can potentiate the Wnt and BMP signaling pathways in the extranuclear space directly and also function as an adjunct to transcription. In the osteoblasts, estrogen suppresses RANKL formation while promoting OPG expression. Note that the estrogen can directly traverse the cell membranes to bind its receptor in the cytoplasm, or can act via specific cell surface receptors. Red factors and arrows are osteoblastic and osteoblastogenic, blue factors and black arrows are osteoclastic or osteoclastogenic. Dashed arrows indicate multistep signaling pathways. Right angled arrows indicate transcription, with relevant transcription factors positioned above. BMP, bone morphogenic proteins; NFATc1, nuclear factor-activated T-cells cytoplasmic 1; OPG, Osteoprotegerin; RANKL, receptor activators of nuclear factor-κB ligand.

Figure 6

Bone marrow adipocyte-osteoclast co-stimulation. Hematopoietic stem cells release PPARγ and C/EBPs, activating transcription of osteoclastogenic factors such as NFATc1, and also driving commitment of the mesenchymal stem cell towards the adipocytic pathway. The adipocytes release RANKL and inflammatory cytokines such as TNF-α, IL-1β, IL-6, which significantly promote osteoclast differentiation and function. TNF-α, IL-1β, and IL-6 also stimulate MSCs to differentiate along adipocytic lines. The adipocytes release a BMP receptor antagonist, inhibiting BMP’s influence on the mesenchymal stem cell to differentiate along osteoblastic lines. PPARγ, C/EBP, and TNF-α stimulate osteoblasts to also release RANKL, with further osteoclastic effects. Red factors and arrows are osteoblastic and osteoblastogenic, blue factors and black arrows are osteoclastic or osteoclastogenic, yellow factors and arrows are adipocytogenic, green arrows indicate stimulation by exogenous factors, purple arrows indicate stimulation by endogenous/autocrine factors. Dashed arrows indicate a multi-step process. BMP, bone morphogenic proteins; MSCs, mesenchymal stem cells; NFATc1, nuclear factor-activated T-cells cytoplasmic 1; PPARγ, peroxisome proliferator activated receptor gamma; RANKL, receptor activators of nuclear factor-κB ligand.

Figure 7

The probiotic bacteria Lactobacillus rhamnosus, as a dietary supplement, augments butyrate levels in intestinal tissue and serum. Butyrate augments the differentiation of naïve helper CD4+ cells into Treg cells in the intestine, spleen, and bone marrow. Tregs cells activate a signaling pathway that increases the production of TGF-β by Treg cells, stimulating CD8+ cells to transcribe the osteogenic Wnt ligand. Wnt signaling is activated in bone marrow stromal cells, causing their proliferation and differentiation of those stomal cells into osteoblasts. The expansion of the osteoblastic population results in increased bone formation and improved bone structure. Red factors and arrows are osteoblastic and osteoblastogenic, green arrows indicate stimulation by exogenous factors, dashed arrows indicate a multi-step process. TGF-β, transforming growth factor β.