ABSTRACT
Pharmacological activation of nicotinic acetylcholine receptors (nAChRs) exerts neuroprotective effects in cultured neurons and the intact animal. Much less is known about a physiological protective role of nAChRs. To understand whether endogenous activation of beta2* nAChRs contributes to the maintenance of the functional and morphological integrity of neural tissue, adult beta2-/- mice were subjected to in vivo challenges that cause neurodegeneration and cognitive impairment (intrahippocampal injection of the excitotoxin quinolinic acid), or neuroprotection and cognitive potentiation (2-month exposure to an enriched environment). The excitotoxic insult caused an increased deficit in the Morris water maze learning curve and increased loss of hippocampal pyramidal cells in beta2-/- mice. Exposure to an enriched environment improved performance in contextual and cued fear conditioning and object recognition tests in beta2+/+, whereas the improvement was absent in beta2-/- mice. In addition, beta2+/+, but not beta2-/-, mice exposed to an enriched environment showed a significant hypertrophy of the CA1/3 regions. Thus, lack of beta2* nAChRs increased susceptibility to an excitotoxic insult and diminished the positive effects of an enriched environment. These results may be relevant to understanding the pathophysiological consequences of the marked decrease in nAChRs that occurs in neurodegenerative diseases such as Alzheimer's disease and Parkinson's disease.
Subject(s)
Behavior, Animal/drug effects , Environment Design , Hippocampus/drug effects , Neurotoxins/administration & dosage , Quinolinic Acid/administration & dosage , Receptors, Nicotinic/deficiency , Receptors, Nicotinic/genetics , Animals , Behavior, Animal/physiology , Exploratory Behavior/drug effects , Exploratory Behavior/physiology , Female , Hippocampus/metabolism , Hippocampus/pathology , Male , Maze Learning/drug effects , Maze Learning/physiology , Mice , Mice, Inbred C57BL , Mice, Knockout , Neurotoxins/toxicity , Protein Binding/genetics , Quinolinic Acid/toxicity , Receptors, Nicotinic/physiologyABSTRACT
The Ca2+-sensor protein S100A1 was recently shown to bind in vitro to synapsins, a family of synaptic vesicle phosphoproteins involved in the regulation of neurotransmitter release. In this paper, we analyzed the distribution of S100A1 and synapsin I in the CNS and investigated the effects of the S100A1/synapsin binding on the synapsin functional properties. Subcellular fractionation of rat brain homogenate revealed that S100A1 is present in the soluble fraction of isolated nerve endings. Confocal laser scanning microscopy and immunogold immunocytochemistry demonstrated that S100A1 and synapsin codistribute in a subpopulation (5-20%) of nerve terminals in the mouse cerebral and cerebellar cortices. By forming heterocomplexes with either dephosphorylated or phosphorylated synapsin I, S100A1 caused a dose- and Ca2+-dependent inhibition of synapsin-induced F-actin bundling and abolished synapsin dimerization, without affecting the binding of synapsin to F-actin, G-actin or synaptic vesicles. These data indicate that: (i) synapsins and S100A1 can interact in the nerve terminals where they are coexpresssed; (ii) S100A1 is unable to bind to SV-associated synapsin I and may function as a cytoplasmic store of monomeric synapsin I; and (iii) synapsin dimerization and interaction with S100A1 are mutually exclusive, suggesting an involvement of S100A1 in the Ca2+-dependent regulation of synaptic vesicle trafficking.
Subject(s)
Actins/metabolism , Brain/metabolism , Calcium-Binding Proteins/metabolism , Synapsins/metabolism , Actins/chemistry , Animals , Brain/cytology , Brain Chemistry , Calcium-Binding Proteins/pharmacology , Dimerization , Humans , Immunohistochemistry , Mice , Presynaptic Terminals/chemistry , Presynaptic Terminals/metabolism , Protein Binding/drug effects , Protein Binding/physiology , Rats , S100 Proteins , Subcellular Fractions/chemistry , Subcellular Fractions/metabolism , Synapsins/chemistry , Synaptic Vesicles/chemistry , Synaptic Vesicles/metabolismABSTRACT
Focal adhesion kinase (FAK) and the related proline-rich tyrosine kinase 2 (PYK2) are non-receptor protein tyrosine kinases that transduce extracellular signals through the activation of Src family kinases and are highly enriched in neurones. To further elucidate the regulation of FAK and PYK2 in nervous tissue, we investigated their distribution in brain subcellular fractions and analysed their translocation between membrane and cytosolic compartments. We have found that FAK and PYK2 are present in a small membrane-associated pool and a larger cytosolic pool in various neuronal compartments including nerve terminals. In intact nerve terminals, inhibition of Src kinases inhibited the membrane association of FAK, but not of PYK2, whereas tyrosine phosphatase inhibition sharply increased the membrane association of both FAK and PYK2. Disruption of the actin cytoskeleton was followed by a decrease in the membrane-associated pool of FAK, but not of PYK2. For both kinases, a significant correlation was found between autophosphorylation and membrane association. The data indicate that FAK and PYK2 are present in nerve terminals and that the membrane association of FAK is regulated by both phosphorylation and actin assembly, whereas that of PKY2 is primarily dependent on its phosphorylation state.
