Two Distinct Modes of Hypoosmotic Medium-Induced Release of Excitatory Amino Acids and Taurine in the Rat Brain In Vivo

A variety of physiological and pathological factors induce cellular swelling in the brain. Changes in cell volume activate several types of ion channels, which mediate the release of inorganic and organic osmolytes and allow for compensatory cell volume decrease. Volume-regulated anion channels (VRAC) are thought to be responsible for the release of some of organic osmolytes, including the excitatory neurotransmitters glutamate and aspartate. In the present study, we compared the in vivo properties of the swelling-activated release of glutamate, aspartate, and another major brain osmolyte taurine. Cell swelling was induced by perfusion of hypoosmotic (low [NaCl]) medium via a microdialysis probe placed in the rat cortex. The hypoosmotic medium produced several-fold increases in the extracellular levels of glutamate, aspartate and taurine. However, the release of the excitatory amino acids differed from the release of taurine in several respects including: (i) kinetic properties, (ii) sensitivity to isoosmotic changes in [NaCl], and (iii) sensitivity to hydrogen peroxide, which is known to modulate VRAC. Consistent with the involvement of VRAC, hypoosmotic medium-induced release of the excitatory amino acids was inhibited by the anion channel blocker DNDS, but not by the glutamate transporter inhibitor TBOA or Cd2+, which inhibits exocytosis. In order to elucidate the mechanisms contributing to taurine release, we studied its release properties in cultured astrocytes and cortical synaptosomes. Similarities between the results obtained in vivo and in synaptosomes suggest that the swelling-activated release of taurine in vivo may be of neuronal origin. Taken together, our findings indicate that different transport mechanisms and/or distinct cellular sources mediate hypoosmotic medium-induced release of the excitatory amino acids and taurine in vivo

Toxin-Based Therapeutic Approaches

Protein toxins confer a defense against predation/grazing or a superior pathogenic competence upon the producing organism. Such toxins have been perfected through evolution in poisonous animals/plants and pathogenic bacteria. Over the past five decades, a lot of effort has been invested in studying their mechanism of action, the way they contribute to pathogenicity and in the development of antidotes that neutralize their action. In parallel, many research groups turned to explore the pharmaceutical potential of such toxins when they are used to efficiently impair essential cellular processes and/or damage the integrity of their target cells. The following review summarizes major advances in the field of toxin based therapeutics and offers a comprehensive description of the mode of action of each applied toxin

Glutamate neurotoxicity, transport and alternate splicing of transporters

Glutamate is the major excitatory neurotransmitter in the central nervous system and its levels in the synaptic cleft are tightly controlled by high affinity glutamate transporters (also known as Excitatory Amino Acid Transporters or EAATs). The EAAT family is comprised of five members (EAAT1-5), and these transporters are subject to alternative splicing. Alternative splicing of the EAAT genes is a fundamental mechanism that can give rise to multiple distinct mRNA transcripts, producing protein isoforms with potentially altered functions. Numerous splice variants of EAATs have been identified in humans, rodents, and other mammalian species. All splice variants of EAATs cloned to date are either exon-skipping and/or intron-retaining types. These modifications may impact upon protein structure, posttranslational modification, function, cellular localization, and trafficking. Message and protein for these splice variants are detectable in the normal brain and, in many instances, have been shown to be induced by pathophysiological insults such as hypoxia. In addition, aberrant expression of EAAT splice variants has been reported in neurodegenerative conditions such as amyotrophic lateral sclerosis, Alzheimer's disease, ischemic stroke, and age- related macular degeneration. These EAAT variants may represent therapeutic targets and thus require an improved understanding of their regulation. This chapter describes recent developments in investigating the molecular heterogeneity, localization, function, structure, and regulation of the EAATs and their splice variants