Epilepsy in mice deficient in the 65-kDa isoform of glutamic acid decarboxylase

Epilepsy in mice deficient in the 65-kDa isoform of glutamic acid decarboxylase

^*Departments of Medicine and Microbiology/Immunology and Hormone Research Institute, ^†Department of Biochemistry and Biophysics and Hormone Research Institute, and ^§Department of Psychiatry and Center for Neurobiology and Psychiatry, School of Medicine, University of California at San Francisco, San Francisco, CA 94143; ^¶Department of Neurology, Baylor College of Medicine, Houston, TX 77030; and ^‖Department of Pharmacology, School of Medicine, University of Colorado Health Science Center, Denver, CO 80262

Edited by Stanley B. Prusiner, University of California at San Francisco, San Francisco, CA, and approved October 17, 1997 (received for review September 3, 1997)

Abstract

γ-Aminobutyric acid (GABA), the major inhibitory neurotransmitter in the mammalian brain, is synthesized by two glutamate decarboxylase isoforms, GAD65 and GAD67. The separate role of the two isoforms is unknown, but differences in saturation with cofactor and subcellular localization suggest that GAD65 may provide reserve pools of GABA for regulation of inhibitory neurotransmission. We have disrupted the gene encoding GAD65 and backcrossed the mutation into the C57BL/6 strain of mice. In contrast to GAD67−/− animals, which are born with developmental abnormalities and die shortly after birth, GAD65−/− mice appear normal at birth. Basal GABA levels and holo-GAD activity are normal, but the pyridoxal 5′ phosphate-inducible apo-enzyme reservoir is significantly decreased. GAD65−/− mice develop spontaneous seizures that result in increased mortality. Seizures can be precipitated by fear or mild stress. Seizure susceptibility is dramatically increased in GAD65−/− mice backcrossed into a second genetic background, the nonobese diabetic (NOD/LtJ) strain of mice enabling electroencephalogram analysis of the seizures. The generally higher basal brain GABA levels in this backcross are significantly decreased by the GAD65−/− mutation, suggesting that the relative contribution of GABA synthesized by GAD65 to total brain GABA levels is genetically determined. Seizure-associated c-fos-like immunoreactivity reveals the involvement of limbic regions of the brain. These data suggest that GABA synthesized by GAD65 is important in the dynamic regulation of neural network excitability, implicate at least one modifier locus in the NOD/LtJ strain, and present GAD65−/− animals as a model of epilepsy involving GABA-ergic pathways.

Footnotes

↵ ‡ Present address: Department of Biology, University of California at San Diego, La Jolla, CA 92093.
↵ ** To whom reprint requests should be addressed at: Hormone Research Institute, University of California at San Francisco, 513 Parnassus Avenue, RM HSW 1090, San Francisco, CA 94143-0534. e-mail: s_baekkeskov{at}biochem.ucsf.edu.
This paper was submitted directly (Track II) to the Proceedings Office.
Abbreviations: GABA, γ-aminobutyric acid; GAD, glutamic acid decarboxylase; PLP, pyridoxal 5′ phosphate; ES cell, embryonic stem cell; NOD, nonobese diabetic; EEG, electroencephalography.