ABSTRACT
The host response to neural injury, which can include axonal sprouting and synaptic reorganization is likely to be under tight genetic regulatory control at the level of the genome and may be implicated in epileptogenesis. Despite its importance, however, the molecular basis of synaptic reorganization is unclear. We have studied the development of synaptic reorganization, semaphorin gene expression, and epileptogenesis in hippocampus of epileptogenic sensitive (FVB/NJ) and epileptogenic resistant (C57BL/6J) mice (i.e. distinct genetic backgrounds) after kainic acid-induced status epilepticus. Our results support the hypothesis that disruption of transcriptional regulation of axon guidance genes leads to a differential loss of tonic neuropilin-2 dependent activation of semaphorin 3F receptors on hippocampal neurons on distinct genetic backgrounds. This results in rearranged synaptic circuitry and thus promotes epileptogenesis. These findings may define biologic principles underlying the role of semaphorin signaling which may broadly apply to other systems undergoing neural regeneration.
Subject(s)
Brain Chemistry/genetics , Excitatory Amino Acid Agonists , Gene Expression Regulation/physiology , Kainic Acid , Semaphorins/biosynthesis , Status Epilepticus/metabolism , Animals , Axons/metabolism , Blotting, Northern , Cell Count , Cloning, Molecular , Dentate Gyrus/metabolism , Deoxyglucose/metabolism , GAP-43 Protein/metabolism , Immunoblotting , Immunohistochemistry , In Situ Hybridization , Male , Mice , Mice, Inbred C57BL , Mice, Inbred Strains , Pyramidal Tracts/metabolism , RNA Probes , Semaphorins/genetics , Species Specificity , Status Epilepticus/chemically induced , Status Epilepticus/genetics , Synapses/physiologyABSTRACT
Glutamate receptor signaling is essential to normal synaptic function in the central nervous system. The major ionotropic glutamate receptors (AMPA, Kainic, and NMDA) have different synaptic functions depending upon cellular and subcellular localization, subunit composition, and second messenger systems linked to the receptors. In this review, we examine major advances in glutamate receptor biology whose physiology plays a central role in neurologic disease such as epilepsy and stroke. A key feature of glutamate receptor activation in neurologic disease is the downstream effects on cell survival, genetic expression of axon guidance cues, synaptic connectivity/formation of networks, and neuronal excitability. Identification of therapeutic pharmacologic targets and development of antagonists specific to the disease process remain central themes in epilepsy and stroke research.
Subject(s)
Nervous System Diseases/etiology , Receptors, Glutamate/physiology , Synapses/physiology , Excitatory Amino Acids/history , Excitatory Amino Acids/physiology , Hippocampus/anatomy & histology , Hippocampus/cytology , Hippocampus/metabolism , History, 20th Century , History, 21st Century , Humans , Receptors, AMPA/metabolism , Receptors, AMPA/physiology , Receptors, Glutamate/metabolism , Receptors, Kainic Acid/metabolism , Receptors, Kainic Acid/physiology , Synapses/metabolismABSTRACT
Protein phosphatase 2A (PP2A) isolated from whole rat brain homogenate supernatants has been compared with that extracted from rat synaptosomal membranes. Both purified enzymes are comprised of the three known PP2A polypeptide chains of 65 (A subunit), 55 (B/B' subunit), and 38 (C subunit) kDa and have okadaic acid inhibition curves (Ki = 0.05 nM) nearly identical to that reported for skeletal muscle PP2A. The isolated 38-kDa subunit of rat brain PP2A appears to contain phosphotyrosine based on cross-reactivity with a specific monoclonal antibody (PY-20). Amino acid compositions and sequences of peptides isolated from the 65- and 38-kDa species correspond to regions of the cDNA-deduced sequences of the regulatory and catalytic subunits of protein phosphatase 2A from several sources. Studies reported here also demonstrate that autophosphorylated protein kinases, particularly Ca2+/calmodulin-dependent protein kinase II (CaM kinase II), are excellent substrates for brain PP2A. Furthermore, Ca(2+)-dependent K(+)-depolarization of hippocampal synaptosomes was accompanied by a sequential increase, then decrease, in CaM kinase II phosphorylation level over a 45-s time course. The decrease was blocked by 1 nM okadaic acid. These data demonstrate that the type 2A protein phosphatase is present at the synapses of CNS neurons where its localization could alter the functions of phosphoproteins involved in synaptic plasticity.
