Osteocytes burn out, give bone a break

Abstract

The pathogenic mechanisms of chronic kidney disease–induced osteoporosis are not well understood and might involve metabolic alterations and apoptosis of osteocytes. In this issue, Hsu et al. present experimental work in uremic mice and cultured mouse osteoblasts showing that impaired mitochondrial function and mitophagy in osteocytes contribute to chronic kidney disease–associated osteoporosis. They investigate new therapeutic approaches to improve mitochondrial function in the setting of increased uremic toxin levels.

Osteocytes are the most differentiated cells of the osteoblast lineage. They participate in tuning matrix remodeling and mineralization to exogenous signals, such as mechanical strain, inflammation, or metabolic alterations. In this issue, Hsu et al.¹ show that impaired mitochondrial function and mitophagy in osteocytes contribute to chronic kidney disease (CKD)-associated osteoporosis.

Loss of kidney function leads to major alterations of bone and mineral metabolism. As kidney function declines, osteocytes begin to secrete exponential amounts of the phosphate-regulating hormone FGF23, triggering a chain reaction of hormonal regulations resulting in reduced calcitriol production by the kidney and secondary hyperparathyroidism. In kidney failure, severe kidney damage prevents the phosphaturic actions of FGF23 and parathyroid hormone, which results in hyperphosphatemia.² Available therapies focus on correcting hyperparathyroidism and hyperphosphatemia; however, these have limited efficacy on bone in absence of overt osteitis fibrosa. In other skeletal pathologies such as postmenopausal osteoporosis, the uncoupling between the activities of bone formation and resorption is responsible for bone loss. Therefore, targeting bone remodeling has been a gold standard in the treatment of bone loss. However, this approach also has shown underwhelming benefits to treat bone disease in CKD. Currently, the pathogenic mechanisms of CKD-associated osteoporosis are not well understood and lead, in addition to bone loss, to impaired bone quality, which increases fracture and mortality risk.³ Emerging studies suggest that additional mechanisms including metabolic alterations and apoptosis of the osteocytes are at the center stage of this complex pathology.

In new supporting evidence, and consistent with recent study in patients and mice with CKD,⁴ Hsu et al.¹ used similar transcriptomics and show that genes associated with mitochondrial function and energy metabolism are downregulated in bone from mice with CKD induced by adenine feeding. Hsu et al.¹ further show that these genes are at the crossroads of central metabolic pathways like the tricarboxylic acid cycle, glycolysis, mitophagy, and mitochondrial electron transport chain complexes. Mitophagy consists in the selective triage of damaged mitochondria that leads to their degradation and recycling by autophagy. After mitochondrial damage or stress, the damaged mitochondrion is engulfed in a lysosome forming a mitolysosome, and the mitolysosome content is then degraded by specific enzymes. Using a reporter model, Hsu et al.¹ observed an accumulation of mitolysosomes in osteocytes from adenine-fed mice with CKD, which is highly suggestive of mitochondrial dysfunction and possibly stalled mitophagy (Figure 1). To rule out the potential indirect effects of metabolic stress induced by adenine feeding, and to demonstrate that mitochondrial dysfunction occurs in osteocytes in response specifically to kidney injury, Hsu et al.¹ also confirmed osseous mitolysosome accumulation in the 5/6 nephrectomy mouse model of CKD. In this second model, CKD is surgically induced and results in the traditional hallmarks of CKD-induced osteoporosis, including increased cortical bone porosity and cortical thinning.

Figure 1 |. Effects of increased mitochondrial stress on bone volume and mineralization in chronic kidney disease (CKD).

Mitochondria that are damaged by stress are selectively degraded by mitophagy. During this process, mitochondria are engulfed in lysosomes, forming mitolysosomes. In CKD, uremic toxins contribute to mitochondrial damage, increased mitophagy, and altered clearance of mitolysosomes, leading to impaired osteocyte function and bone loss. This is prevented by mitochondria antioxidant mitoquinone mesylate (MitoQ) and charcoal adsorbent AST-120. The proximal molecular mechanisms that regulate mitochondrial function and mitophagy in CKD require further investigation.

Hsu et al.¹ report that mitolysosome accumulation is not restricted to the bone in CKD and occurs in several other organs, including the kidney, where they observed mitolysosome accumulation in the renal tubules and glomeruli of adenine-fed mice. Therefore, in subsequent analyses, they tested the effects of mitochondrial antioxidant mitoquinone mesylate (MitoQ) on kidney function, uremic toxins levels, bone phenotype, and mitolysosome accumulation in osteocytes. Although Hsu et al.¹ do not report the effects of MitoQ treatment on mitolysosome accumulation in the kidney, they convincingly demonstrate that MitoQ prevents CKD-induced osteoporosis in adenine-fed mice independently of CKD progression. Indeed, MitoQ lowered mitolysosome accumulation in osteocytes and led to an improvement of cortical bone parameters (Figure 1), which occurred despite worsened kidney function as demonstrated by a further increase in serum creatinine levels post treatment. The beneficial effects of MitoQ on the bone, but not on the kidney, offer a potential new therapeutic strategy for a bone-targeted approach. Future studies should investigate the effects of MitoQ on mitochondrial dysfunction and on the bone phenotype in alternate models of CKD.

Hsu et al.¹ postulate that MitoQ ameliorates the bone phenotype by preventing uremic toxins from inducing mitochondrial damage. To test this hypothesis, they performed a series of additional in vivo and in vitro analyses. In vivo, they tested the effects of AST-120, a charcoal adsorbent used to slow CKD progression by reducing the levels of uremic toxins, in adenine-fed mice. AST-120 induced the expected reduction in uremic toxins indoxyl sulfate and p-cresyl sulfate levels and delayed the progression of kidney disease (Figure 1). It also led to a near complete rescue of the bone phenotype; however, this might have been an indirect consequence of delayed onset of CKD and associated parameters, as opposed to a response to the correction of uremic toxin levels specifically. They address this in vitro and demonstrate that indoxyl sulfate dose-dependently induces oxidative phosphorylation and reduces glycolysis when directly applied to cultured osteoblasts. The alterations in mitochondrial structure and expression of mitophagy markers caused by indoxyl sulfate were rescued by the mammalian target of rapamycin inhibitor, offering an additional potential therapeutic avenue of interest. While Hsu et al.¹ do not show mitolysosome accumulation or the effects of MitoQ in indoxyl sulfate–treated osteoblasts, in a final experiment they provide evidence for direct effects of MitoQ on mitolysosome accumulation using p-cresyl sulfate as a second uremic toxin. Consistent with promising results reported in MitoQ-treated mice with CKD, mitolysosome accumulation induced by p-cresyl sulfate was rescued by MitoQ in vitro. Whether MitoQ treatment also restores osteoblast differentiation, activity, and mineralization in vitro as a result of improved mitochondrial function remains to be shown.

In vitro, osteoblasts from patients and animals with CKD retain intrinsic defects of differentiation and mineralization weeks after isolation,^5,6 which suggests possible permanent epigenetic changes in the osteoblasts⁷ in response to kidney injury or inhibition of osteoprogenitor differentiation by locally produced factors.⁸ Future studies should address whether factors other than uremic toxins might also impact mitochondrial function in osteocytes. Stress in the mitochondria can lead to increased apoptosis, which was previously reported in osteocytes in patients and animals with CKD.⁵ Although Hsu et al.¹ did not determine the proximal upstream mechanisms leading to mitochondrial dysfunction in their models, several studies suggest that local factors control osteoblastic fate and activity. Indeed, the downregulation of transcription factor HNF4A in bone leads to bone loss, altered metabolism, and apoptosis of osteoblasts from patients and mice with CKD.⁴ In a separate study, CKD-associated osteocyte apoptosis was corrected by DMP1, an osteocyte-produced regulator of matrix mineralization and upstream inhibitor of FGF23.⁵ Finally, a recent study showed that FGF23-mediated activation of FGFR4 induced mitochondrial dysfunction in the heart, which contributes to the development of cardiac hypertrophy in CKD.⁹ Whether, and if so, how HNF4A, DMP1, and FGF23 might regulate mitochondrial function and mitophagy in osteocytes is currently unclear.

To conclude, while multiple studies showed a role for mitochondrial damage in altered bone remodeling, the study by Hsu et al.¹ is the first to implicate increased mitophagy in the development of bone disease in CKD. Additional data reported in their work raise the interesting concept that increased mitophagy in osteocytes from mice with CKD might be attributed to impaired clearance of damaged mitochondria. This ponders the question that in CKD, the loss of bone mass and quality might ultimately occur because osteocytes fail to keep up with CKD-induced stress and burn out. Together, this study opens interesting perspectives for bone research and therapeutics in CKD, including that MitoQ might be a promising new strategy for the treatment of mitochondrial dysfunction in CKD-induced osteoporosis.