Voltage-sensitive calcium channels in bone: the role of electrically excitable calcium channels in a non-excitable tissue
Date
2020
Authors
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Publisher
University of Delaware
Abstract
Bone is a dynamic, living tissue that serves many functions throughout the body, including locomotion, protection of vital organs, and mineral homeostasis. Central to these roles, is the ability of bone to adapt its mass, architecture, and strength according to its environment and the body’s needs (R. L. Duncan & Turner, 1995). These changes take place throughout one’s life and occur via bone remodeling, a process by which specialized cells called “osteoclasts” resorb old or damaged bone and allow for “osteoblasts” to form new bone in its place. Here, I seek to define the roles that specific calcium channels play within the osteoblast to help build new bone and explore novel means of therapeutically targeting these channels to address conditions of bone loss. ☐ Voltage-sensitive calcium channels (VSCC’s) are activated by membrane depolarization, allowing calcium influx into the cell to regulate a number of cellular processes. VSCC’s are found in both excitable and non-excitable tissues throughout the body, yet how these channels are activated in non-excitable tissue and how they alter cell function are unknown (Catterall, 2011). In bone, L-type and T-type VSCCs are found in all cells of the osteogenic lineage except for osteocytes, where the L-VSCC is lost (Shao & et al., 2005). The loss of the L-VSCC in the osteocyte parallels the loss of a proliferative phenotype, which leads me to postulate that the L-VSCC is essential for proliferation of osteoprogenitor cells and osteoblasts. The presence of the T-VSCC throughout the entire osteogenic lineage suggests that it may play a consistent and critical role in all bone cells. This is further supported by the unique electrophysiological properties of the T-VSCC which allow it to become activated easily and remain activated at a steady state by low levels of stimulation. Together, these activation properties and expression patterns suggest that the T-VSCC serves a fundamental process that needs to be steadily maintained; this leads me to hypothesize that the T-VSCC is necessary for the survival of osteogenic cells. ☐ The role of VSCC’s is well-characterized in excitable tissues, and VSCC inhibitors have been successfully used in excitable tissues to address conditions such as angina, high blood pressure, and premature labor (Pontremoli, Leoncini, & Parodi, 2014; Songthamwat, 2018; Terry, 1982). However, VSCC’s inhibitors are deleterious to bone, and thus are not viable candidates for treating issues of bone loss, but rather are more akin to analogs for osteoporosis and other conditions of bone loss. VSCC inhibitors have been effective as research tools to help researchers better understand the role of these channels in bone, but there is still an urgent need for agents which can activate bone in a manner that will offset bone loss. The ability of VSCC’s to be activated by mechanical load poses limited therapeutic potential for those who are unable to exert the necessary mechanical strains to counteract bone loss in vivo. Pulsed electromagnetic field (PEMF) stimulation shows promising therapeutic potential as a non-invasive means of combating issues of bone loss, since evidence suggests that PEMF’s can be adjusted to selectively target specific VSCC’s within certain tissues (Buckner, Buckner, Koren, Persinger, & Lafrenie, 2015). Here, I explore pulsed electromagnetic fields as an alternative stimulus to mechanical stimuli in bone and hypothesize that pulsed electromagnetic fields can be used to activate VSCC-mediated processes within bone cells. ☐ Our lab has previously shown that both the L-VSCC and the T-VSCC are essential to the increase in bone formation in response to mechanical loading (J. Li, Liu, Ke, Duncan, & Turner, 2005; Owan, Ibaraki, Duncan, Turner, & Burr, 1999; Ryder & Duncan, 2001). I now show that these channels are necessary for proliferation and survival of osteoblasts even in the absence of mechanical load. Here, I demonstrate that T-VSCC inhibition, but not L-VSCC inhibition, results in reduced cell survival of pre-osteoblast-like MC3T3-E1 cells. Rather, inhibition of the L-VSCC reduces proliferation by slowing cell cycle progression. These studies, in combination with unpublished work from our lab, suggest that the L-VSCC works in conjunction with purinergic signaling to mediate cell cycle progression of MC3T3-E1 cells, and that this occurs through two distinct pathways that eventually converge within the nucleus (Jones, 2011). I further demonstrate that pulsed electromagnetic field (PEMF) stimulation of MC3T3-E1 cells has a proliferative effect in vitro that does not occur when the L-VSCC and the T-VSCC are blocked. Preliminary evidence indicates that this effect may be partially dependent on purinergic signaling, in a manner that is similar to that of mechanical stimulation (Jones, 2011). This provides valuable insight into the mechanisms by which PEMF technology has been used to treat bone fractures and supports the therapeutic potential of this approach. Furthermore, it suggests that we can expand upon and improve the way this approach is currently being used within the clinical setting by selectively targeting specific VSCC’s. In studying VSCC activation and function, we can better understand how we can treat abnormal bone loss and perhaps shed new light on their roles within other tissues.
Description
Keywords
Calcium channels, Bone, Electricity, Osteoblast