Professor, Department of Cell Biology
John S. Gammill Endowed Chair in Polycystic Kidney Disease, Warren Medical Research Institute
B.S., Pharmacy, Aristotle University, Greece
Ph.D., Pharmacology, University of Medicine and Dentistry, New Jersey
Project 1: Molecular and cellular basis of Autosomal Dominant Polycystic Kidney Disease
Autosomal dominant polycystic kidney disease (ADPKD) is one of the most common genetic diseases affecting more than half a million Americans and 12.5 million people, worldwide. Mutations in two genes, PKD1 and PKD2 are responsible for ADPKD. Their protein products, originally called PKD1 and PKD2 or polycystin-1 and 2 (currently named, TRPP2), respectively, are plasma membrane-spanning proteins present also in the primary cilium. It has been speculated that PKD1 and TRPP2 assemble into a receptor-channel complex linking extracellular stimuli to Ca2+ signaling. However, there are four major, yet unresolved questions that form the focus of our research interests: 1) What is the nature of the extracellular stimuli that activate the PKD1/TRPP2 receptor/channel complex? 2) How activation of PKD1 leads to the activation of TRPP2? 3) What are the downstream effectors of the PKD1/TRPP2 complex? 4) What is the role of the primary cilium in PKD2/TRPP2-based signaling? These questions are being investigated using complementary approaches in cell culture and the mouse.
Project 2: Role of primary cilia in cell division and development
While primary cilia are present in most quiescent mammalian cell types, they are never seen in mitosis. This observation has prompted investigators to propose that ciliogenesis and cell cycle progression are mutually exclusive processes. However, the mechanisms coordinating cilium biogenesis with the cell cycle have been a mystery for more than 30 years. We have identified NDE1 (nuclear distribution element 1 or nuclear distribution E homolog), as part of a network of proteins that mediates this coordination. NDE1 functions as a negative regulator of ciliary length. Consistent with such a role, NDE1 levels oscillate between high levels in mitosis and low levels in quiescence. Knockdown of NDE1 in cell culture and in zebrafish embryos leads to abnormally long cilia and a delay in the initiation of cell division. Embryonic defects induced by the loss of NDE1 include the random positioning of internal organs (left-right patterning defects) and the formation of abnormally small heads, which appears to be somewhat reminiscent to a human condition called microcephaly. Naturally occurring mutations in the NDE1 gene in humans or deletion of the Nde1 gene in mice leads to microcephaly. Using Nde1 as a model protein, we have been investigating how ciliogenesis is integrated with the cell cycle aiming at obtaining mechanistic insights into microcephaly and cancer.
Project 3: Ca2+ signaling in osteoclasts
Osteoclasts are constantly made throughout life from hematopoietic stem cells residing in the bone marrow through a series of complex events involving cytokine signaling and the microenvironment. Ca2+ signaling has an essential role in the regulation of osteoclastogenesis. Ca2+ channels activated in response to the depletion of intracellular Ca2+ stores have been suggested to mediate Ca2+ signaling in early stages of osteoclast formation. However, the exact molecules and mechanism(s) by which these channels control Ca2+ signaling are largely unknown. Using a combination of molecular, cell biological and whole animal studies, we investigate the role of the Transient Receptor Potential channel, TRPC1, in this process. Our data show that TRPC1 promotes osteoclastogenesis at an early stage, whereas its inhibitor, the small cytosolic protein, I-mfa has the opposite effect. Enhanced osteoclastogenesis in I-mfa-null mice is corrected in mice lacking both genes indicating that TRPC1-mediated Ca2+ signaling has a dominant effect over I-mfa in osteoclast formation. Our model is that TRPC1 and I-mfa have an essential role in osteoclastogenesis by regulating Ca2+ signaling. This hypothesis has been tested by asking whether and how TRPC1 and I-mfa affect “priming” of early osteoclast progenitors, specifically at the myeloid precursor stage, whether TRPC1 and I-mfa affect osteoclastostogenesis in a cell-autonomous fashion and whether they affect osteoclast recruitment in experimentally induced animal models of osteoclastogenesis. We hope that our studies will lead to better understanding of critical pathways in the regulation of osteoclast development and function, which is needed to identify and develop new therapeutic interventions to control osteoclastogenesis and prevent bone loss.
2. Ong EC, Nesin V, Long CL, Bai CX, Guz JL, Ivanov IP, Abramowitz J, Birnbaumer L, Humphrey MB, and Tsiokas L. A TRPC1-dependent pathway regulates osteoclast formation and function. Journal of Biological Chemistry, 288(31):22219-32, 2013.
3. Kim S., Zaghloul NA., Bubenshchikova E., Oh EC., Rankin S., Katsanis N., Obara T., and Tsiokas L. Nde-1 mediated suppression of ciliogenesis affects cell cycle re-entry. Nature Cell Biology, 13(4): 351-60, 2011. “New and Views” 
4. Bai CX., Kim S., Li WP., Streets AJ., Ong AC., and Tsiokas L. Activation of TRPP2 through mDia1-dependent voltage gating. EMBO Journal, 27: 1345-1356, 2008.
5. Bai CX., Giamarchi A., Rodat-Despoix L., Padilla F., Downs T., Tsiokas L.,* and Delmas P.* Formation of a novel receptor-operated channel by heteromeric assembly of TRPP2 and TRPC1 subunits. EMBO Reports, 9:472-9, 2008. *Corresponding authors
6. Tsiokas, L., Kim, S., and Ong, EC. Cell biology of Polycystin-2 (invited review). Cellular Signalling, 19: 444-453, 2007.