<strong>Calcium signaling mechanisms in health and disease</strong>
One of the most ubiquitous intracellular signaling messengers found in nature is calcium. In cells ranging from simple bacteria to complex neurons, calcium ions (Ca2+) mediate a variety of essential functions as diverse as chemotaxis, muscle contraction, and neurotransmitter release. We are interested in elucidating the generation, regulation, and functional consequences of calcium events with the long-term goal of understanding the physiological functions and dysregulation in disease states of this widespread signaling pathway. A primary focus of our laboratory is on calcium signaling mechanisms involving store-operated channels (SOCs). SOCs are a family of Ca2+ permeable ion channels expressed by most cells. The signal for the activation of these ion channels is a decrease in the calcium concentration ([Ca2+]) in the endoplasmic reticulum (ER), a vast membranous network within the cell that serves as a reservoir for stored calcium. SOCs are activated by a variety of extracellular signals such as hormones, neurotransmitters, and growth factors whose binding to receptors mobilizes calcium from the ER. Although the phenomenon of store-operated Ca2+ entry was first described decades ago, the molecular composition of SOCs and the mechanisms by which a decrease in ER [Ca2+] is linked to activation of these elusive channels remain two of the most enduring mysteries in biology.
Activation of SOCs elicits cytosolic Ca2+ signals that initiate a variety of critical cellular processes such as NFAT (nuclear factor of activated T cells)-driven gene expression in T cells, the regulation of blood vessel tone in endothelial cells, and the modulation of neurotransmitter release in neurons. Aberrant calcium signals arising from improper functioning of the store-operated calcium signaling machinery lead to devastating diseases. We have shown that T lymphocytes from patients with a hereditary severe combined immune deficiency (SCID) syndrome are defective in store-operated Ca2+ entry. In these patients, the loss of SOC function in T cells results in devastating propensity for fungal and viral infections. Malfunctions in SOCs have additionally been linked to the etiology of Alzheimer’s disease, underscoring their critical importance for human physiology. Yet SOCs have remained highly enigmatic, perhaps the last bastion of ion channels whose molecular mechanisms still await elucidation.
Two recent breakthroughs promise to lift this veil of ignorance and mystery. The first is the identification of the Ca2+ sensor in the ER that couples Ca2+ store depletion to SOC activation, a molecule termed STIM (stromal interaction molecule). The second is our identification of a novel transmembrane molecule, termed Orai1, as a pore subunit of the store-operated channel in T cells. We hypothesize that Orai1 and its related genes (Orai2 and Orai3) comprise the extended family of store-operated channels found in most cells. Using site-directed mutagenesis and functional expression, electrophysiology, and video imaging, we are currently examining the molecular mechanisms of SOC function: their activation mechanisms, localization, regulation by physiological pathways, and their cellular functions. In particular, we are investigating three issues.
What are the molecular mechanisms by which Orai and STIM recapitulate SOC entry? Does Orai1 alone constitute the SOC channel or does it need other obligate subunits? What are the roles of Orai2 and Orai3? Does STIM interact directly with the Orai to trigger the activation of SOCs?
We have discovered that SOCs are widely expressed in a variety of cell types including the central nervous system. What are the properties of these SOCs and how do they compare with the well-studied SOCs of the immune system? Do Orai1/2/3 encode these SOCs? What are the cellular functions of this Ca2+ entry pathway in the nervous system?
Dysregulation of Ca2+ homeostasis involving the ER and SOCs is manifested not only in a variety of neurodegenerative disorders such as Alzheimer’s disease but also more generally during aging. What are the molecular mechanisms of aberrant ER and SOC Ca2+ signaling in these pathological states? What cellular end-points are affected by the changes in ER and SOC Ca2+ signaling?