S-34-4 - Opposing effects of stress andantidepressants of neurotrophic factors

Abstract

Development and maintenance of the mammalian nervous system require neurotrophic factors. The most well-characterized neurotrophic factor nerve growth factor (NGF) has recently been shown to be a member of a family of structurally related proteins that also includes brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3) and neurotrophin-4 (NT-4). An evolutionary study of these factors, collectively known as neurotrophins, in representative groups of vertebrates showed that the four factors are highly conserved from fishes to mammals. Chimeric molecules were constructed where different combinations of sequences from BDNF replaced the corresponding sequences in NGF. Assays for biological activity of such molecules showed that the biological specificities of the two proteins are obtained by specific combinations of a set of sequences that differ between the two molecules. In the rat embryo, expression of both the neurotrophins and the essential components of their high-affinity receptors (trk, trkB and trkC) are developmentally regulated. From their sites of synthesis, both target-derived and local modes of neurotrophic actions can be inferred and the development pattern of mRNA expression suggests the existence of serval neurotrophin dependent neuronal systems where these factors have not previously been implicated. In the adult rat brain, the highest levels of NGF, BDNF and NT-3 mRNAs are found within the hippocampus where they are expressed in a set of partially non-overlapping neurons. Expression of NGF and BDNF mRNAs in the brain appears to be regulated by neuronal activity and the levels of these mRNAs show a marked and transient increase following epileptic seizures, cerebral ischemia and hypoglycemic coma. The increases in NGF and BDNF mRNAs appear to be mediated via a release of glutamate activating AMPA receptors within the hippocampus as well as by a rise in the intracellular calcium levels. As a first step towards an understanding of the molecular mechanisms governing BDNF mRNA expression the rat BDNF gene was characterized. Multiple transcripts with different 5′-untranslated sequences are transcribed from this gene. These mRNAs are synthesized by a differential, tissue specific, usage of four different promoters within the BDNF gene. Two of these promoters appear to confirm a marked seizure-induced increase of BDNF mRNA in the brain. The product of the trkB gene encoding an essential component of the high-affinity BDNF receptor, is also expressed in the hippocampus where it is spatially and temporally regulated in a manner similar to BDNF. These data suggest that BDNF and its receptor could, in a paracrine/autocrine fashion, play a role in kindling-associated neural plasticity and in neuronal protection following ischemic and hypoglycemic insults. Alanine-scanning mutagenesis combined with receptor binding and biological assays were used to assess the functional importance of amino acid residues exposed on the surface of the NGF dimer. The results show that Lys-32, Lys-34 and Lys-95 form a positively charged interface involved in binding to the low-affinity NGF receptor (p75ngfr). Mutated molecules that do not bind to p75ngfr but still retain binding to p 140 trk and biological activity demonstrate for the first time a functional dissociation between the two NGF receptors.