Research Interests
My research concerns functional aspects of two components in the vertebrate peripheral motor system: Myelinated axons and Motor nerve terminals.
Functional role of the internodal
axolemma in myelinated axons: For many years, the ionic conductances responsible for maintaining action potential conduction were thought to be activated exclusively at the nodes of Ranvier, while the
internodal axolemma (the axonal membrane covered by myelin) was thought to be a passive component. We have shown that K+ channels in the internodal
axolemma open during axonal activity and play an important role in limiting axonal depolarization during trains of action potentials. By comparing electrical and morphological properties of
myelinated axons in different species we learned that the effect of the internodal
axolemma on membrane
repolarization is influenced by both the thickness of the myelin sheath and by activation of axolemmal K+ channels. Current and future projects in this field include studies of the ionic composition of the peri-axonal space and the mechanisms that control it, with an emphasis on activity related ionic changes (such as K+ accumulation)
Response of
myelinated axons to mechanical injury:
Transection of motor nerve fibers results in immediate loss of motor function in the target muscle and initiates a sequence of events which might lead to regeneration and functional recovery. These processes require the re-establishment of the axonal internal environment, which has been disrupted by the injury. We investigated the early changes in ionic composition of transected axons and found that entry of Na+ through the injured site leads to a large elevation of intra-axonal [Na+] which spreads
diffusionally hundreds of microns proximal to the site of injury. This is followed by resealing of the axonal membrane near the injury site and by restoration of intra-axonal [Na+] to its normal low levels. We showed that this recovery is achieved by a combination of
diffusional redistribution of Na+ in the axon and by the action of an electrogenic Na+/K+ pump, located in part in the internodal axolemma. Current and future projects in this field include studies of the
spatio-temporal profile of [Ca++] in transected vertebrate axons and identification of conditions that improve resealing.
Motor nerve terminals - Calcium handling and transmitter release: The invasion of action potentials to motor nerve terminals causes the opening of voltage activated Ca++ channels followed by an increase in intra-terminal [Ca++] and rapid exocytosis of transmitter vesicles. We are studying the time course and spatial distribution of intra-terminal [Ca++] during action potential activity and how these parameters relate to transmitter release. We are also interested in mechanisms by which the terminal clears large Ca++ loads induced by short periods of intense activity (occurring during physiological neuromuscular transmission) Mechanisms addressed in present and future projects include mitochondrial Ca++ uptake, plasma membrane Ca++/
ATPase and plasma membrane Na+/Ca++ exchanger.
Techniques used in the lab include: intracellular and
extracellular recordings from
myelinated axons, motor nerve terminals and muscle cells, measurements of activity-related cytoplasmic concentrations of Ca++ , Na+ , K+ and H+ using ion-sensitive fluorescent dyes (some of which are targeted to mitochondria) imaged with laser scanning
confocal microscopy and CCD cameras.
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Ca2+ transient in a mouse motor nerve terminal stimulated with a train of action potentials. A fluorescent Calcium-sensitive dye was injected into the axon and imaged with a confocal microscope. |
Ca2+ transient in the mitochondria of a mouse motor nerve terminal stimulated with a train of action potentials. A fluorescent Calcium-sensitive dye was loaded into the mitochondrial matrix and imaged with a confocal microscope. |
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Stimulation-evoked increases in cytoplasmic calcium in a lizard motor nerve terminal imaged with the fluorescent calcium indicator Calcium Green-5N. Blue to red color-scale represents low to high calcium concentrations.
Source: David, Barrett & Barrett (1997), J. Physiol, 504.1, pp 83-96. |
Lizard motor nerve terminal labeled with a calcium sensitive dye in the cytoplasm (green) and in the mitochondria (red) Source: David et al, (1998) J. Physiol. 509.1, 59-65. |
Selected Publications
- Talbot JD, Barrett JN, Barrett EF, David G (2008) Rapid, stimulation-induced reduction of C12-resorufin in motor nerve terminals:linkage to mitochondrial metabolism. J Neurochem. 105(3):807-19.
- David G, Nguyen K, Barrett EF (2007) Early vulnerability to ischemia/reperfusion injury in motor terminals innervating fast muscles of SOD1-G93A mice. Exp Neurol. 204(1):411-20.
- Talbot J, Barrett JN, Barrett EF, David G (2007) Stimulation-induced changes in NADH fluorescence and mitochondrial membrane potential in lizard motor nerve terminals. J Physiol. 579(Pt 3):783-98.
- Garcia-Chacon LE, Nguyen KT, David G, Barrett EF (2006)
Extrusion of Ca2+ from Mouse Motor Terminal Mitochondria via a Na+/Ca2+ Exchanger Increases Post-tetanic Evoked Release.
J Physiol. 574(Pt 3):663-75.
- David G and Barrett EF (2003) Mitochondrial Ca2+ uptake prevents desynchronization of quantal release and minimizes depletion during repetitive stimulation of mouse motor nerve terminals. J Physiol. 548(Pt 2):425-38.
- David G, Talbot J, Barrett EF (2003) Quantitative estimate of mitochondrial [Ca2+]in stimulated motor nerve terminals. Cell Calcium. 33(3):197-206.
- Talbot JD, David G, Barrett EF (2003) Inhibition of mitochondrial Ca2+ uptake affects phasic release from motor terminals differently depending on external [Ca2+]. J Neurophysiol. 90(1):491-502.
- David G and Barrett EF (2000) Stimulation-evoked increases in cytosolic [Ca2+] in mouse motor nerve terminals are limited by mitochondrial uptake and are temperature-dependent. J Neurosci. 20(19):7290-6.
- David G (1999) Mitochondrial clearance of cytosolic Ca(2+) in stimulated lizard motor nerve terminals proceeds without progressive elevation of mitochondrial matrix [Ca2+]. J Neurosci 19(17):7495-506.
Curriculum Vitae
- 1982
B.Sc. in Basic Medical Sciences, Hebrew University-Hadassah
Medical School, Jerusalem, Israel
- 1986
Ph.D. in Neurobiology, Hebrew University-Hadassah Medical
School, Jerusalem, Israel
- 1988
Intern, Hadassah Medical Center, Jerusalem, Israel
- 1989
M.D, Hebrew University-Hadassah Medical School, Jerusalem,
Israel
- 1989
Post-Doctoral Fellow, Dept. of Physiology and Biophysics,
University of Miami School of Medicine
- 1991
Research Associate, Dept. of Physiology and Biophysics,
University of Miami School of Medicine.
- 1993-2000
Research Assistant Professor, Dept. of Physiology and
Biophysics, University of Miami School of Medicine.
- 2000 - present
Research Associate Professor, Dept. of Physiology and
Biophysics, University of Miami School of Medicine.
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