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Fraser J. Moss, PhD
View Curriculum Vitae (pdf)

Mailing Address:
Robbins E518
Phone: 216-368-5405
Fax: 216-368-5586

Research Interests

Molecular mechanisms sensing and regulating physiological pH

My research studies the function of sodium-coupled bicarbonate cotransporters (NCBTs) in their roles regulating intracellular and whole-body pH and transepithelial transport. I employ two-electrode voltage clamp electrophysiology to record the currents and ion-sensitive microelectrodes to record the intracellular pH or sodium concentration in cells heterologously expressing NCBTs.  Delivery of out-of-equilibrium solutions allows exquisite control over the extracellular environment of cells heterologously expressing NCBTs in order to independently control extracellular [CO2] and [HCO3-] and pH. The molecular processes underlying how the proximal tubule senses Δ[CO2]BL and Δ[HCO3-]BL and transduces these changes to modulate the rate of H+ secretion or bicarbonate reabsorption during either respiratory or metabolic acidosis is another research focus. Receptor protein tyrosine phosphatase γ (RPTPγ) is a major candidate for the CO2/HCO3- sensor. Förster resonance energy transfer imaging monitors the oligomerization of RPTPγ when [HCO3-] or [CO2] change, or when RPTPγ interacts with downstream signaling targets in live cells.

Specific Projects
  1. Molecular mechanism for sensing [CO2] and [HCO3] by RPTPγ

    HCO3 reabsorption (JHCO3) from renal proximal tubules (PT) is acutely regulated by basolateral [CO2] and [HCO3], not by extracellular pH (pHo). More recently we reported that the knockout of receptor protein tyrosine phosphatase γ (RPTPγ), normally present in the PT basal membrane, eliminates the CO2 and HCO3 sensitivities of JHCO3, as well as the normal defense to whole-body metabolic acidosis (MAc). The RPTPγ intracellular region has both a D1 phosphatase domain and a D2 blocking domain. When RPTPγ dimerizes, the D2 domain of one monomer blocks the D1 domain of the other. The extracellular region possess a carbonic anhydrase (CA) like domain (CALD) that is strikingly similar to classic CAs. However, the CALD lacks the amino acid residues believed necessary for CA activity. If the CALD is no longer capable of interconverting CO2 and HCO3, we hypothesize that it can sense CO2 or HCO3 and that the identity of the ligand bound to the CALD favors either dimerization or monomerization of the intracellular RPTPγ phosphatase domains. To detect the interaction of two RPTPγ monomers, we fuse the protein with the pH- and halide-insensitive GFP variants Aquamarine (Aq) to serve as a Förster resonance energy transfer (FRET) donor, and Citrine (Cit) to serve as a FRET acceptor, and coexpress the fusion proteins in HEK cells. We subject the cells to solutions representing different acid-base disturbances and acquire data from coexpressing cells, normalizing FRET for donor and acceptor expression levels (NFRET). We are able to vizualize changes in RPTPγ dimerization as reported by changes in the NFRET in response to each acid-base disturbance.  In parallel experiments, we co-transfect cells with RPTPγ-Aq and ErbB1-Cit (a candidate downstream target for RPTPγ) and are able to measure changes in NFRET in response to acid-base disturbances that are reciprocal to those measured for RPTPγ dimerization. We hypothesize that HCO3 and CO2 compete at the CALD to control RPTPγ dimerization state (and presumably phosphatase activity). We also hyothesize that changes in the RPTPγ dimerization state in response to [CO2]o or [HCO3]o but not pH, influence its interaction with downstream effector molecules including ErbB1.

  2. Is NBCe1 an exchanger rather than a cotransporter?

    I am recording the membrane potential (Vm), slope conductance and pHi from oocytes expressing NBCe1 that are exposed to different OOE solutions designed to probe the dependence of the transport direction on different extracellular ions and HCO3.

  3. Structural determinants of NBCe1 electrogenicity

    To determine the structural elements of NBCe1 that are essential for electrogenicity, with emphasis on the contribution of extracellular loop 4.

  4. The directional dependence of DIDS block of NCBTs

    Identifying and examining the roles of extracellular DIDS binding sites and determining the directional dependence of DIDS block in NBCe1 and other Na+-coupled HCO3 transporters.

  5. The role of carbonic anhydrase II on HCO3- -initiated transport through the SLC4A4 transporter NBCe1

    Testing the metabolon hypothesis by investigating what influence, if any, of soluble carbonic anhydrase II (CAII) or CAII fused to the C-terminus of NBCe1A has on transporter slope conductance.

Honors and Awards
  • 2017                                    Key personnel & co-author NIH R01 DK113197
    2013                                    American Physiological Society Postdoctoral Travel Award IUPS 2013
    2005-2007                          American Heart Association (Western States Affiliate) Postdoctoral Fellowship.
    2005-2007                          Most Valuable Scientist –
    1998-2001                          GlaxoSmithKline/Medical Research Council CASE Ph. D. Scholarship
    US Patent
    1. Moss FJ, CD Son, R Srinivasan & HA Lester. Methods and systems for detection of stoichiometry by Förster resonance energy transfer. 2014.
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