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Calcium Regulation Group

Ca2+ Pools Involved in Cellular Signaling

James W. Putney, Jr., Ph.D.
James Putney, Jr., Ph.D.
Principal Investigator

Tel (919) 541-1420
Fax (919) 541-1898
P.O. Box 12233
Mail Drop F2-02
Research Triangle Park, North Carolina 27709
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Research Summary

Calcium signaling in non-excitable cells is most generally initiated by mobilization of Ca2+ from intracellular stores by the second messenger, inositol 1,4,5-trisphosphate (IP3). Although it is clear that the locus of this intracellular store of Ca2+ is either a part of or is derived from endoplasmic reticulum, the precise locus of these calcium stores and their relationship to other cellular structures is not clear. In addition, the interaction of IP3 with its receptor on these Ca2+ stores often results in complex kinetics of Ca2+ release and reaccumulation, resulting in the appearance of regenerative cytoplasmic intracellular Ca2+ ([Ca2+]i) oscillations. One aim of research in this laboratory is to better identify and localize the intracellular Ca2+ pools involved in cellular signaling, and to understand better the basis for intracellular [Ca2+]i oscillations.

Agonist (Ag) activation of a plasma membrane receptor (R) results in formation of IP3, which activates the IP3 receptor (IP3R) causing discharge of stored Ca2+ from a subcompartment of the endoplasmic reticulum. Within this subcompartment, Ca2+ binds reversibly to an EF hand motif in Stim1; depletion of Ca2+ results in Ca2+ dissociation from Stim1, which causes Stim1 to redistribute within the endoplasmic reticulum to areas near Orai channels that reside in the plasma membrane. Stim1 then activates Ca2+-selective Orai channels; the mechanism by which this activation is accomplished is unknown at present. Stim1 is also present in the plasma membrane, although its function there is unclear. TRPC channels can also be activated by phospholipase C (PLC) -dependent mechanisms. There is evidence that in some instances, perhaps when combined with other subunits, they can function as store-operated channels, perhaps also involving Stim1 as an activator.
Mechanisms of regulated calcium entry.

When hormones, neurotransmitters or growth factors activate calcium signaling through the formation of inositol 1,4,5-trisphosphate (IP3), the rise in intracellular calcium ([Ca2+]i) is typically biphasic. The initial rise results from a direct effect of IP3 on the ligand-activated calcium channels in intracellular Ca2+-storing organelles. This release of intracellular Ca2+ is transient in nature and is usually followed by a more sustained elevation of the intracellular Ca2+ concentration ([Ca2+]i) due to Ca2+ entry across the plasma membrane. The predominant mechanism by which phospholipase C-coupled receptors activate Ca2+ entry is an indirect one. Ca2+ entry across the plasma membrane is somehow coupled to the depletion of intracellular Ca2+ stores by IP3, a process termed capacitative calcium entry. The most compelling evidence for capacitative entry is the finding that inhibitors of the Ca2+-ATPase of the intracellular Ca2+ storage compartment, such as thapsigargin, cause depletion of intracellular Ca2+ pools without IP3 production, and as a result mimic the ability of surface membrane, IP3-linked agonists to activate Ca2+ entry. Since the inception of the concept of capacitative calcium entry, the two key issues have been the nature of the calcium channel, and the signal generated by depletion of intracellular Ca2+ stores. Recent work has indicated that two proteins, Stim and Orai, play essential roles in this pathway. A major focus of research in this project is to gain insights into the cellular and molecular aspects of this key signaling mechanism.

The group’s approach to both of these research problems utilizes a combination of molecular, electrophysiological, and morphological approaches.

Major areas of research:

  • Identification and localization of the intracellular Ca2+ pools involved in cellular signaling.
  • Improving understanding of the basis of intracellular [Ca2+]i oscillations.
  • The nature of the calcium channel, and the signal generated by depletion of intracellular Ca2+ stores.

Current projects:

  • Understanding the biophysical properties of Orai calcium channels
  • Understanding the cellular mechanisms underlying movements of the calcium sensor, Stim1, within the endoplasmic reticulum.
  • Determining the roles of store-operated and non-store-operated calcium channel mechanism in significant physiological contexts using primary cells.
  • Investigating the signaling mechanisms for non-store-operated TRP channels.

James W. Putney, Jr., Ph.D., heads the Calcium Regulation Group within the Laboratory of Signal Transduction.  He received his Ph.D. in 1972 from The Medical College of Virginia in Richmond, Virginia. He has published 165 peer-reviewed articles in leading biomedical journals, as well as several book chapters.  He served as Professor of Pharmacology at The Medical College of Virginia, Virginia Commonwealth University, before joining NIEHS in 1986.

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Last Reviewed: May 22, 2007