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Randall L. Duncan, Ph.D.
- BISC 605 - Advanced Mammalian Physiology I
- BISC 606 - Advanced Mammalian Physiology II
The development of skeletal architecture and the maintenance of bone mass is dependent on the mechanical stresses and strains encountered throughout our lifetime. Removal of these physical forces during paralysis, microgravity encountered in spaceflight, and even extended bed rest, results in a rapid loss of bone with up to 2% of total body calcium lost per month. Conversely, increased activity and exercise has been shown to reduce the rate of bone loss in osteoporotic patients and application of exogenous mechanical loads can result in increased bone formation in a modeling skeleton. The focus of the research in my lab is to understand how mechanical signals are perceived by bone cells and converted into biochemical responses within the cell to promote bone formation.
One of the earliest responses of the osteoblast, or bone-forming cell, to a mechanical stimulus is an increase in intracellular calcium that occurs within seconds of stimulation. The primary focus of the research in my lab is to define the role of ion channels in this mechanically-induced increase in intracellular calcium and how the changes in channel activity influence the function of the cell. We also seek to determine how the cell can control the kinetics of these channels. Finally, we are using genetic manipulation to determine how mutation, overexpression and knockout of various components of the proposed mechanotransduction pathway alter the response of the skeleton to mechanical loading.
Adaptation of osteoblasts to mechanical stimulation. The skeleton rapidly responds to a novel mechanical stimulus with an increase in bone formation, in vivo. However, this skeletal mechanosensitivity is lost upon continued stimulation. In this project, we are examining changes in osteoblastic morphology and mechanosensitivity during prolonged application of fluid shear and whether allowing the cell to "rest" between bouts of shear alter the loss of sensitivity to stimulation. We hypothesize that the mechanosensitive, cation-selective channel (MSCC) activity will be reduced during prolonged shear and that this decrease is mediated through an increase in actin cytoskeletal organization. We are currently examining whether introduction of "rest periods" alters cytoskeletal rearrangement and channel activity and if disruption of the actin cytoskeleton enhances the mechanosensitivity of the cell. Finally, we are comparing the effects of different types of mechanical stimuli, such as pressure, strain and shear, on the loss of mechanosensitivity of osteoblasts.
PTH reduces the mechanical threshold of the osteoblasts. Well-defined strain thresholds for net bone resorption and formation have been observed, however the threshold for net bone formation exceeds the levels of strain that occur under physiologic conditions. We, and others, have postulated that parathyroid hormone (PTH) can interact with the signaling mechanisms of mechanotransduction to lower the mechanical threshold and prime the osteogenic cells of bone to respond to lesser magnitudes of mechanical stimulation to promote bone formation. We have previously made significant observations that indicate that PTH can alter both mechanosensitive and voltage sensitive channel kinetics to increase the Ca2+ signal in osteoblasts, resulting in increased osteogenic activity. In this project, we are studying the effects of PTH-induced second messenger signaling on channel kinetics and cytoskeletal structure on the L-type voltage sensitive calcium channel (LVSCC) and the MSCC. Using point mutation techniques, patch clamp analyses and intracellular Ca2+ imaging, we will identify sites of phosphorylation of the LVSCC and the MSCC to determine how PTH alters channel kinetics.
Calcium and purinergic signaling in response to mechanical stimulation in osteoblasts. We have observed that activation of phospholipase C (PLC) increases osteoblastic gene expression through IP3-mediated intracellular Ca2+ release. This has led us to ask how PLC is activated by fluid shear. Recent reports demonstrate the presence of purinergic receptors (P2) in osteoblasts and activation of these receptors have been shown to mediate intracellular Ca2+ signaling in a number of cell types including osteoblasts. We have recently shown that ATP is released from osteoblasts in response to mechanical stimulation, suggesting a possible mechanism for PLC activation and other Ca2+ dependent responses in osteoblasts and that shear-induced activation of the mechanosensitive channel (MSCC) and the L-VSCC is responsible for this release. Further we have shown that prostaglandin release results from ATP binding to the P2X7 receptor. In this project, we are developing knockout mice which are missing one of the isoforms of the LVSCC that we believe is important in mechanotransduction. We plan to mechanically load these mice in vivo and examine changes in the skeletal response to load. We will also isolate primary osteoblasts from these mice to discern changes in the mechanotransduction pathway when the LVSCC is absent. Secondly, we will examine the mechanisms of ATP release in osteoblasts and how changes in channel activity alter this release. Finally, we will define the P2 receptors responsible for the response of osteoblasts to mechanical stimulation.
- Victor P. Fomin, Ph.D. - Research Assistant Scientist (Ph.D., A.V. Palladin Institute of Biochemistry, Ukraine). Role of PKC in mechanotransduction and myometrial contraction.
- Dawei Liu, Ph.D., D.D.S. - Research Associate (Ph.D., Beijing Medical University, China). Cellular response of osteoblasts to mechanical loading and how these responses effect orthodontic tooth movement.
- Jinsong Zhang, M.D. - Research Associate (M.D., Capital University of Medical Sciences, China). Ion channel and intracellular Ca2+ signaling in osteoblasts in response to loading and PTH.
- Liu D, Genetos DC, Shao Y, et al. Activation of extracellular-signal regulated kinase (ERK1/2) by fluid shear is Ca(2+)- and ATP-dependent in MC3T3-E1 osteoblasts. Bone. 2008;42(4):644–652.
- Bergh JJ, Shao Y, Puente E, Duncan RL, Farach-Carson MC. Osteoblast Ca(2+) permeability and voltage-sensitive Ca(2+) channel expression is temporally regulated by 1,25-dihydroxyvitamin D(3). Am J Physiol Cell Physiol. 2006;290(3):C822–31.
- Fiske JL, Fomin VP, Brown ML, Duncan RL, Sikes RA. Voltage-sensitive ion channels and cancer. Cancer Metastasis Rev. 2006;25(3):493–500.
- Sawakami K, Robling AG, Ai M, et al. The Wnt co-receptor LRP5 is essential for skeletal mechanotransduction but not for the anabolic bone response to parathyroid hormone treatment. J Biol Chem. 2006;281(33):23698–23711.
- Zhang J, Ryder KD, Bethel JA, Ramirez R, Duncan RL. PTH-induced actin depolymerization increases mechanosensitive channel activity to enhance mechanically stimulated Ca2+ signaling in osteoblasts. J Bone Miner Res. 2006;21(11):1729–1737.
- Duncan RL, Genetos DG. Calcium signaling in osteoblasts in response to mechanical stimulation. In: Aaron R, Bolander M, eds. Physical Regulation of Skeletal Repair. Rosemont, IL: American Association of Orthopaedic Surgeons Press; 2005:247–258.
- Genetos DC, Geist DJ, Liu D, Donahue HJ, Duncan RL. Fluid shear-induced ATP secretion mediates prostaglandin release in MC3T3-E1 osteoblasts. J Bone Miner Res. 2005;20(1):41–49.
- Li J, Liu D, Ke HZ, Duncan RL, Turner CH. The P2X7 nucleotide receptor mediates skeletal mechanotransduction. J Biol Chem. 2005;280(52):42952–42959.
- Chen NX, Geist DJ, Genetos DC, Pavalko FM, Duncan RL. Fluid shear-induced NFkappaB translocation in osteoblasts is mediated by intracellular calcium release. Bone. 2003;33(3):399–410.
- Li J, Duncan RL, Burr DB, Gattone VH, Turner CH. Parathyroid hormone enhances mechanically induced bone formation, possibly involving L-type voltage-sensitive calcium channels. Endocrinology. 2003;144(4):1226–1233.
- Pavalko FM, Norvell SM, Burr DB, Turner CH, Duncan RL, Bidwell JP. A model for mechanotransduction in bone cells: the load-bearing mechanosomes. J Cell Biochem. 2003;88(1):104–112.
- Ryder KD, Duncan RL. Parathyroid hormone enhances fluid shear-induced [Ca2+]i signaling in osteoblastic cells through activation of mechanosensitive and voltage-sensitive Ca2+ channels. J Bone Miner Res. 2001;16(2):240–248.
Phone: (302) 831-4296
Fax: (302) 831-2281
Office: 118C Wolf Hall
Lab: 351 Wolf Hall
Department of Biological Sciences
University of Delaware
Newark, DE 19716
- B.S. - Southwestern College
- Ph.D. - Oklahoma State University
- Postdoctoral - Medical College of Virginia