Randall L. Duncan, Ph.D.

Teaching

• BISC 605 - Advanced Mammalian Physiology I - syllabus
• BISC 606 - Advanced Mammalian Physiology II - syllabus

These courses focus on cell, organ and systemic physiology of three selected systems each semester,with particular emphasis on human physiology.  BISC 605 covers renal, cardiovascular and blood/pulmonary physiology while BISC 606 concentrates on endocrinology, musculoskeletal system physiology and neurophysiology.   Classrooom discussions center on the primary literature in each of the respective physiologic fields and medical case studies of pathophysiologies associated with each system. There are three goals for the student in these courses: 1) to be able to integrate the principles and concepts learned from each system into an understanding of the physiology of the organism, 2) to learn to research the literature and other available resources to answer a problem and 3) to logically and succinctly defend an answer to a problem.

Research Interests

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.

Current Projects

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.

Research Group

• 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.

Selected Publications

Mechanotransduction:

• Gardinier J, Yang W, Madden GR, Malik M, Kronbergs A, Adams E, Czymmek K, Duncan RL. (2014) P2Y2 receptors regulate MC3T3 osteoblast mechano-sensitivity during fluid flow. Am. J. Physiol: Cell Physiol. 306(11): C1058-C1067. [PDF]
• Gardinier JD, Gangadharan V, Wang L, Duncan RL. (2014) Hydraulic pressure during fluid flow regulates purinergic signaling in the cytoskeleton organization of MC3T3 osteoblasts. Cell Mol. Bioeng. 7(2): 266-277. [PDF]
• Katiyar A, Duncan RL and Sarkar, K. (2014) Ultrasound stimulation increases proliferation of MC3T3-E1 preosteoblast-like cells. J. Therap. Ultrasound 2(1) 1-10.  [PDF]
• Gangadharan V, Nohe A, Caplan J, Czymmek K and Duncan RL. (2015) Caveolin-1 regulates P2X7 receptor signaling in osteoblasts. Am. J. Physiol.: Cell Physiol. 308: C41-C50, 2015. [PDF]
• Heo S-J, Thorpe SD, Driscoll TP, Duncan RL, Lee DA, and Mauck RL. (2015) Biophysical regulation of chromatin architecture instills a mechanical memory in mesenchymal stem cells. Scientific Reports 5:16895 doi:10.1038/srep16895. [PDF]

Chondrocyte Biology:

• Srinivasan PP, Parjuli A, Price C, Wang L, Duncan RL, Kirn-Safran, CB. (2015) Inhibition of T-type voltage sensitive calcium channel reduces load-induced OA in mice and suppresses the catabolic effect of bone mechanical stress in chondrocytes.  PlosOne. 10: e0127290. [PDF]
• Zhou Y, David MA, Chen X, Wan LQ, Duncan RL, Wang L, Lu XL. (2015) Effects of osmolarity on spontaneous calcium signaling of in situ juvenile and adult articular chondrocytes.  Ann. Biomed. Eng. doi: 10.1007/s10439-015-1406-4. [PDF]
• Hurd LM, Kirwin SM, Boggs ME, Mackenzie WG, Bober MB, Funanage VL and Duncan RL. (2015)  A mutation in TRPV4 results in altered chondrocyte calcium signaling in severe metatropic dysplasia. Am. J. Med. Genetics, Part A. 167:2286-2293. [PDF]

Tissue Engineering:

• Pradhan-Bhatt S, Harrington DA, Duncan, RL, Jia X, Witt RL, Farach-Carson MC. (2013) Implantable three-dimensional salivary spheroid assemblies demonstrate fluid and protein secretory responses to neurotransmitters. Tissue Eng. Part A. 19(13-14); 1610-1620 (Cover art for Tissue Eng. Part A, July issue; Tissue Eng. Part B, August issue and Tissue Eng. Part C, August issue). [PDF]
• Tong Z, Zerdourm AB, Duncan RL, Jia X. (2014) Dynamic vibration cooperates with connective tissue growth factor to modulate stem cell behaviors. Tissue Eng. Part A. 20(13-14): 1922-1934. [PDF]
• Han W, Heo S-J, Driscoll TP, Delucca J, McLeod C, Smith L, Duncan RL, Mauck RL, and Elliott D.  (in press) Emergent microstructural heterogeneity in native and engineered fibrocartilage directs micromechanics and mechanobiology. Nature Materials (accepted: 11/20/2015) [PDF]

For full list of publications, go to: http://www.ncbi.nlm.nih.gov/sites/myncbi/1PST30ueRAoQ2/bibliography/46869260/public/?sort=date&direction=ascending