Jonathan Soboloff, PhD

Store-operated Ca2+ entry

Ca²⁺ signals mediate a vast range of both short-term (contraction, secretion) and long-term (mitogenesis, differentiation and survival) cellular events. Receptor-dependent changes in Ca²⁺ concentration involve two closely coupled components – rapid, inositol 1,4,5-trisphosphate-mediated Ca²⁺ release from ER stores, followed by Ca²⁺ entry. A major mechanism of receptor-mediated Ca²⁺ entry is store-operated Ca²⁺ entry (SOCe), whereby endoplasmic reticulum (ER) Ca²⁺ depletion results in the activation of plasma membrane (PM) Ca²⁺ channels. This process has been extensively studied since its initial discovery over 20 years ago, ultimately leading to the discovery of members of the Stromal-Interacting Molecule (STIM) and Orai families as the mediators of this process. Thus, changes in ER Ca²⁺ content are “sensed” by the ER Ca²⁺ sensors STIM1 and STIM2 which transduce this signal to the channel (Orai1) via ER-PM interactions (see Figure). These increases in cytosolic Ca²⁺ concentration are short-term, as excess Ca²⁺ is rapidly removed from the cytosol via pumps located on the plasma membrane (PMCA) and endoplasmic reticulum (SERCA)

Integration of SOCe in physiological responses

The short-term nature of Ca²⁺ signals has created a longstanding conceptual challenge; how do event that occur in seconds to minutes regulate physiological events that can last for hours to days? Recent investigations in my lab have revealed that the expression of STIM1 is controlled by members of the Early Growth Response (EGR) of zinc finger transcription factors along with the closely related Wilms Tumor suppressor 1 (WT1). EGR proteins are immediate early genes whose expression can be rapidly upregulated by the same receptors that initiate Ca²⁺ transients. Hence, our lab is greatly interested in understanding how changes in EGR protein expression patterns modulate receptor-mediated Ca²⁺ signals and assessing how this modulation relates to receptor-mediated control of physiological responses. To achieve our goals, we use a wide variety of molecular biological, biochemical and biophysical techniques to study transcriptional control of protein expression and Ca²⁺ signals. Model systems examined range from human cell lines to genetically manipulated mouse models.

Identification and characterization of new roles and targets for STIM1

Whereas STIM and Orai were originally envisioned as partners designed specifically for the control of store-operated Ca²⁺ entry, recent investigations by us and others have revealed a much wider role for STIM1. Hence, whereas the primary activator of STIM1 is loss of ER Ca²⁺, its ability to bind Ca²⁺ is dependent on both redox state and temperature. Further, in addition to Orai1, CaV1.2 and TRPC family members have both been identified as additional Ca²⁺ channels within the PM under the control of STIM1. In the process of investigating EGR-mediated changes in Ca²⁺ homeostasis during T cell activation, we have recently identified PMCA as a target for STIM1. Hence, during T cell activation, the cell dramatically reorganizes itself, such that STIM1 and PMCA come into close physically apposition. This leads to inhibition of PMCA function and local elevation of cytosolic Ca²⁺ concentration. While the critical role of Ca²⁺ in T cell activation has been well described, precisely how STIM1-mediated PMCA inhibition contributes to T cell activation is under current investigation in my lab.

Physiological and pathophysiological roles for STIM/Orai in control of bone density

The density of bone is under the control of osteoblasts (build bone) and osteoclasts (degrade bone). In collaboration with investigators at the University of Pittsburgh (Harry Blair) and the University of West Virginia (John Barnett), we have recently identified a critical role for Orai1 in osteoclast differentiation. Hence, in the absence of Orai1 expression and/or function, terminal osteoclast differentiation cannot complete. Hence, cells exhibit multiple markers and characteristics of mature osteoclasts, but fail to fuse into multi-nucleated cells. Ongoing collaborative efforts with the Blair and Barnett groups to determine the implications of these findings have revealed a novel and unexpected bone phenotype. We are also investigating the therapeutic potential of these findings towards the treatment of bone erosion associate with arthritis using novel inhibitors of Orai1 function.

Control of Ca²⁺ homeostasis and cell survival

Although SOCe serves an important signaling role, SOCe also cooperates with SERCA to maintain ER Ca²⁺ concentration by creating a source for Ca2+ pumping. We find that EGR/WT1 proteins also regulate SERCA expression. Further, since EGR/WT1 proteins function as tumor suppressors in cancer types of multiple lineages, dysregulation of both SERCA and STIM expression is a common feature of multiple tumor types. An important objective of this laboratory is to characterize these novel features of tumor cells, both to better understand tumor cell biology and to assess the potential of targeting Ca²⁺ entry pathways as a therapeutic strategy. Tumor types under investigation in the lab include Wilms Tumor, Acute Myeloid Leukemia, glioblastoma and rhabdomyosarcoma.