Calpain-mediated regulation of AMPA receptors in adult rat brain

Changes in AMPA receptors have been proposed to underlie changes in synaptic efficacy in hippocampus and other brain structures. Activation of calpain has also been discussed as a potential mechanism to produce lasting modifications of synaptic structure and function. We report here that preincubation of thin (10 microns) frozen rat brain sections with calcium at physiological temperature changes the immunological properties of AMPA receptors, an effect totally blocked by calpain inhibitors. Immunocytochemistry indicates that in situ calpain activation produces a decreased immunoreactivity for GluR1, and to a lesser extent for GluR2/3, in the neuropil throughout the brain and an increased immunoreactivity in cell bodies, particularly in hippocampus. Western blots of calcium-treated sections suggest that the decreased immunoreactivity for GluR subunits is due to partial proteolysis. These results strongly suggest the involvement of calpain in the regulation of glutamatergic synapses.

Hypernegative supercoiling of the DNA template during transcription elongation in vitro

Supercoiled plasmid DNAs with negative superhelicity several times higher than normal have been isolated from Escherichia coli topA mutants. The formation of these hypernegatively supercoiled plasmid DNAs is apparently induced by transcription. We show that hypernegatively supercoiled plasmid DNAs isolated from topA mutants contain R-loop(s). To study the mechanism of formation of hypernegatively supercoiled plasmid DNA, we have been able to reproduce hypernegatively supercoiled DNA in vitro using purified RNA polymerase and DNA gyrase. The formation of hypernegatively supercoiled plasmid DNA template in vitro is shown to require transcription elongation and is tightly linked to R-loop formation. We propose that one of the roles of topoisomerase I is to suppress R-loop formation during transcription elongation.

recA-independent and recA-dependent intramolecular plasmid recombination. Differential homology requirement and distance effect

We show that recombination between directly repeated sequences in plasmids occurs via both recA-independent and recA-dependent mechanisms in Escherichia coli. They are differentially affected by two factors, the distance separating the homologies and the size of the homology. Recombination between tandem duplications up to 300 base-pairs shows virtually no recA dependence. Increasing the size of the duplications beyond 300 base-pairs gradually increases the recA dependence. Furthermore, insertion of a sizable DNA sequence in between the duplications, substantially increases the recA dependence. We conclude that increasing the distance separating the homologous regions preferentially inhibits the recA-independent recombination. On the other hand, shortening of the homology preferentially inhibits recA-dependent recombination. Consequently, recombination between short tandem duplications is totally recA-independent.