Mitochondria are critical components of cells, and they are the hub of energy (ATP) production.Recent reports indicate that mitochondria also play a role in cell survival/death, calcium homeostasis, and reactive oxygen species (ROS) production/oxidative stress. A unique feature of mitochondria as organelles is their organization into a highly dynamic network within the cell characterized by the interrelated processes of fusion, fission and removal via mitophagy. Although still not completely understood, this dynamic regulation of the mitochondrial network seems to be important for the maintenance of healthy, properly functioning mitochondria. To support their roles in building and remodeling bone, osteoblasts and osteoclasts have high energy demands, and their activities are dependent on metabolic status. Therefore, we arecurrently working to understand the role of mitochondrial dynamics in both osteoclasts and osteoblasts utilizing conditional deletion of Mfn2, a critical protein for mitochondrial fusion and mitophagy. Since these bone cells each have a distinct and highly specialized cell biology to support their function, we expect to gain insight into specific mitochondrial functions in the context of bone mass regulation.
Veis (Novack) Lab
Molecular and Metabolic Regulation of Bone Mass in Health and Disease
Bone is a dynamic and complex organ whose integrity is controlled by the interaction of many cell types, including osteoclasts which remove bone and osteoblasts which build it. Normally, activity of bone cells is coordinated, and many bone cells use similar signaling pathways for different purposes. My laboratory studies NF-κB signaling, particularly alternative/non-canonical NF-κB, in bone cells in the context of pathological bone loss, such as in osteoporosis, inflammatory arthritis, osteomyelitis, and cancer metastasis to bone. We recently showed that new anti-cancer drugs (IAP antagonists) activate alternative NF-κB in osteoclasts, causing an increase in tumor growth specifically in bone. We also found that alternative NF-κB regulates mitochondrial biogenesis in osteoclasts, and that work has led us to directly examine aspects of mitochondrial biology in bone cells independent of NF-κB. We are using many transgenic mouse models as well as pharmacological approaches, combining in vivo disease modeling with in vitro cell cultures, to understand the molecular and metabolic regulation of bone mass.
Bone cell mitochondria: what do they do?
Alternative NF-κB in the osteoclast: sex matters
In osteoclasts, the alternative NF-κB pathway provides signals that promote osteoclast differentiation downstream of RANKL, and also enhance resorptive activity. We have found that disruption of the alternative NF-κB pathway causes sexually dimorphic effects on bone mass in mice. Recently there has been increased attention paid to sex differences in biological phenomena that might impact disease progression or response to therapy. A class of drugs known as IAP antagonists or SMAC-mimetics is in development for anti-cancer effects, and we have shown that these activate the alternative NF-κB pathway and cause high turnover bone loss, driven by increased OC activity. We have found that IAP antagonist BV6 causes trabecular bone loss in males but not females, and promotes bone metastasis of breast cancer in males. High bone turnover, and in particular high osteoclast activity, is seen with estrogen-deficiency in both sexes and has been linked to increased bone metastasis in patients and animal models. We are working to determine whether estrogen is the primary factor in the differential effects of BV6 on bone mass, and whether the male and female bone microenvironments are substantially with regard to bone metastasis, especially in the presence of this drug. Little attention has been paid to potential sex differences in the bone microenvironment for metastasis in either mouse or man, nor has the possibility for sexually dimorphic effects for drugs outside of those directly impacting sex hormones been explored. Therefore, it is important to begin to address these questions.
Alternative NF-κB and bone formation: not a simple story
There are some reports indicating that alternative NF-κB activation inhibits bone formation, but the data is all derived from globally mutant animals in which the osteoclasts are also dysfunctional. In order to determine whether alternative NF-κB controls bone formation in a cell autonomous manner, we are taking advantage of conditional alleles that allow us to activate or ablate activation of the pathway specifically in osteoblasts. Under basal and mechanical loading conditions, loss of IKKα in osteoblasts has little effect on cortical or trabecular bone mass. However, unexpectedly, constitutive activation of the alternative NF-κB pathway increases bone mass basally, and improves the anabolic response to mechanical loading. Stimuli for alternative NF-κB activation in osteoblasts, and pathways affected downstream are currently under investigation. Understanding this pathway in bone forming cells may lead to novel ways to improve bone mass.
Osteomyelitis: an active role for bone cells
Just as the host tumor microenvironment has gained attention in the area of cancer development and spread, the field of infectious disease research has recognized that host-pathogen interactions play a significant role in the progression and therapeutic response during infection. We are now applying some of the lesions we have learned from the study of bone metastasis to the context of bacterial osteomyelitis, to elucidate the active role of bone cells in infection.
- Anna Ballard, PhD Student
- Linda Cox, Research Assistant
- Jennifer Davis, PhD Student
- Emily Goering, Undergraduate Student
- Christine Shao, Undergraduate Student
- Ali Zarei, PhD, Post Doctoral Research Associate