Jeffrey C Freedman, PhD
- Associate Professor of Neuroscience and Physiology
Research Programs and Affiliations
- Research Pillars
Education & Fellowships
- PhD: University of Pennsylvania, 1973
- Membrane physiology in normal and sickle human red blood cells; Optical indicators of membrane potential and intracellular calcium; Membrane biophysics
Electrophysiology of Human Red Blood Cells
Biophysical studies of chloride and proton transport, and of calcium-dependent potassium conductance in human red blood cells are designed to evaluate the relationship between transmembrane voltage and ionic fluxes. These studies are intended to constrain models for the mechanism of specific ion transport pathways, to elucidate the number of different red cell ion conductance pathways, to increase understanding of voltage-dependent permeabilities between normal and abnormal red cells. A principal technique being developed is the use of fluorescent dyes for optical measurement of membrane potential in suspensions of cells and isolated membranes and organelles. Detailed consideration is being given to the calibration of optical signals and to their mechanism of response to evaluate their use as a new quantitative technique.
Other research interests in red cell physiology include membrane pathologies associated with congenital hemolytic anemias and systemic diseases, the cytotoxic effects of calcium, determinants of red cell life-time and survival, red cell shape, membrane lipid fluidity, the development of membrane transport properties, neonatal red cells, cell volume regulation, and reconstitution of red cell transport systems into planar lipid bilayers.
Electrostatic Charging of Mitochondrial Membranes.
Knowledge of the Mitochondrial transmembrane voltage-its magnitude, spatial profile, and time-dependence-is of fundamental importance for understanding the mechanism of chemiosmosis, and for distinguishing among various kinetic models of proton pumping. Recently, in a study of the Na/K-ATPase, L"auger and colleagues discovered that certain fluorescent dyes that had been thought to measure transmembrane voltage actually measure changes in the intra-membrane electrostatic potential due to alterations in the charge state of the transport protein during the pumping cycle. In the course of our studies of the Mitochondrial FoF1-ATPase, we have acquired evidence that the dye Oxonal V likewise monitors the electrostatic charging, as well as the transmembrane voltage, of the membranes of submitochondrial particles. In this project we are studying the mechanism by which the fluorescence of Oxonal V changes when the membranes are energized by the addition of respiratory substrates of ATP. We are attempting to relate the new information about the electrostatic charging of Mitochondrial membranes to the mechanism of chemiosmosis. The major hypothesis to be tested in this project is that Oxonol V senses and the accumulation of positive charge in proton wells when Mitochondrial membranes are energized either with respiratory substrates or with ATP. The experiments also have relevance to the question of the location of the pumped protons, whether these be localized or delocalized.
Freedman, J.C., T.S. Novak, J.D. Bisognano, and P.R. Pratap. Voltage-Dependence of DIDS-Insensitive Chloride Conductance in Human Red Blood Cells Treated with Vilinomycin or Gramicidin. J. Gen. Physiol. 104:1-23, 1994.
Bisognano, J.D., J.A. Dix, P.R. Pratap, T.S. Novak, and J.C. Freedman. Proton (or Hydroxide) Fluxes and the Biphasic Osmotic Response of Human Red Blood Cells. J. Gen. Physiol. 102:99-103, 1993.