Clustering of neurotransmitter receptors in postsynaptic densities involves proteins that aggregate the receptors and link them to the cytoskeleton. In the case of glycine and GABA(A) receptors, gephyrin has been shown to serve this function. However, it is unknown whether gephyrin is involved in the clustering of all glycine and GABA(A) receptors or whether it interacts only with specific isoforms. This was studied in the retinae of mice, whose gephyrin gene was disrupted, with immunocytochemistry and antibodies that recognize specific subunits of glycine and GABA(A) receptors. Because homozygous (geph -/-) mutants die around birth, an organotypic culture system of the mouse retina was established to study the clustering of gephyrin and the receptors in vitro. We found that all gephyrin and all glycine receptor clusters (hot spots) were abolished in the geph (-/-) mouse retina. In the case of GABA(A) receptors, there was a significant reduction of clusters incorporating the gamma2, alpha2, and alpha3 subunits; however, a substantial number of hot spots was still present in geph (-/-) mutant retinae. This shows that gephyrin interacts with all glycine receptor isoforms but with only certain forms of GABA(A) receptors. In heterozygous geph (+/-) mutants, no reduction of hot spots was observed in the retina in vivo, but a significant reduction was found in the organotypic cultures. This suggests that mechanisms may exist in vivo that allow for the compensation of a partial gephyrin deficit.

b-Site APP-cleaving enzyme (BACE) initiates the processing of the amyloid precursor protein (APP) leading to the generation of b-amyloid, the main component of Alzheimer's disease senile plaques. BACE (Asp2, memapsin 2) is a type I transmembrane aspartyl protease and is responsible for the b-secretase cleavage of APP producing different endoproteolytic fragments referred to as the carboxy-terminal C99, C89 and the soluble ectodomain sAPPb. Here we describe two transgenic mouse lines expressing human BACE in the brain. Overexpression of BACE augments the amyloidogenic processing of APP as demonstrated by decreased levels of full-length APP and increased levels of C99 and C89 in vivo. In mice expressing huBACE in addition to human APP wild-type or carrying the Swedish mutation, the induction of APP processing characterized by elevated C99, C89 and sAPPb, results in increased brain levels of b-amyloid peptides Ab40 and Ab42 at steady-state.

At least two splice variants of GLT-1 are expressed by rat brain astrocytes, albeit in different membrane domains. There is at present only limited data available as to the spatial relationship of such variants relative to the location of synapses and their functional properties. We have characterized the transport properties of GLT-1v in a heterologous expression system and conclude that its transport properties are similar to those of the originally described form of GLT-1, namely GLT-1alpha. We demonstrate that GLT-1alpha is localized to glial processes, some of which are interposed between multiple synapse types, including GABAergic synapses, whereas GLT-1v is expressed by astrocytic processes, at sites not interposed between synapses. Both splice variants can be expressed by a single astrocyte, but such expression is not uniform over the surface of the astrocytes. Neither splice variant of GLT-1 is evident in brain neurons, but both are abundantly expressed in some retinal neurons. We conclude that GLT-1v may not be involved in shaping the kinetics of synaptic signaling in the brain, but may be critical in preventing spillover of glutamate between adjacent synapses, thereby regulating intersynaptic glutamatergic and GABAergic transmission. Furthermore, GLT-1v may be crucial in ensuring that low levels of glutamate are maintained at extrasynaptic locations, especially in pathological conditions such as ischemia, motor neurone disease, and epilepsy.

␤-Secretase (BACE) initiates the amyloidogenic processing of the amyloid precursor protein leading to the generation of the ␤-amyloid, the main component of Alzheimer's disease senile plaques. BACE is a type I transmembrane aspartyl protease of 501 amino acids. Here we describe a novel BACE mRNA lacking 132 base pairs that is expressed in the pancreas but not in the brain. Sequence alignment indicates that the deleted fragment matches the terminal two-thirds of exon 3. The new BACE variant is short of a 44-amino acid region located between the two catalytic aspartyl residues. Accordingly, a 50-kDa form of BACE (BACE457) is detected in the human pancreas. When expressed in cells, BACE457 colocalizes with the marker for the endoplasmic reticulum BiP. Moreover, BACE457 remains in a proenzymatic and endoglycosidase H-sensitive state, suggesting that its transport along the secretory pathway is blocked at the level of the endoplasmic reticulum. Notably, this novel form of BACE does not contribute to the processing of the amyloid precursor protein. Our findings suggest that tissue-specific splicing of the BACE mRNA may explain the observation that in the human pancreas robust transcription of the BACE gene does not translate into recovered enzymatic activity.

b-Site APP-cleaving enzyme (BACE) initiates the processing of the amyloid precursor protein (APP) leading to the generation of b-amyloid, the main component of Alzheimer's disease senile plaques. BACE (Asp2, memapsin 2) is a type I transmembrane aspartic protease responsible for the b-secretase cleavage of APP producing a soluble form of the ectodomain (sAPPb) and the membrane-bound, carboxy-terminal intermediates C99 and C89. BACE maturation involves cysteine bridge formation, N-glycosylation and propeptide removal. We investigated variants of BACE in which the disulphide bonds of the catalytic domain spanning between Cys216/Cys420, Cys278/Cys443 and Cys330/Cys380 were removed by mutagenesis. When transfected in cultured cells, these mutants showed impaired maturation. Nevertheless, a fraction of mutated protein retained both the competence to mature as well as the activity to process APP. For the generation of a functional enzyme the conserved Cys330/Cys380 bond was the most critical, whereas the two bonds between Cys216/Cys420 and Cys278/Cys443, which are typical for the membrane-bound BACE, appeared to be less important.

We investigated the localization and possible function of EAAC1 in the rat retina. Immunocytochemical localization of EAAC1 at the light-microscopic level revealed a fine dust-like labelling pattern across the two synaptic layers. Horizontal cell and subpopulations of amacrine cell somata were labelled, as were some somata within the ganglion cell layer. Some immunoreactive puncta were observed within the cytoplasm of amacrine cells, in regions well away from synaptic sites. At the ultrastructural level, EAAC1 immunolabelled one postsynaptic element at synapses and also processes well away from the synaptic release site. Since EAAC1 was localized away from synaptic sites, we evaluated the role EAAC1 plays in GABA formation by measuring GABA concentrations via reversed-phase high-performance liquid chromatography following incubation of retinae in enzyme and glutamate uptake inhibitors. Incubation of retinae in D-threo-beta-hydroxyaspartate or D/ L-threo-beta-benzyloxyaspartate, which are known to inhibit the glutamate transporters GLAST1, GLT1, and EAAC1, caused a decrease in GABA synthesis by around 50%. Incubation in 6-diazo-5-oxo- L-norleucine, a phosphate-activated glutaminase inhibitor, decreased GABA formation by 40%. Taken together with the anatomical data, the results of this study suggest that EAAC1 plays very little role in GABA synthesis - indeed GABA formation occurs predominantly from glutamine. By virtue of its location both near and well away from synaptic release sites, EAAC1 may regulate glutamate uptake differentially.

The different decay kinetics of GABA(A)Rs expressed in amacrine cells of geph(+/+) and of geph(-/-) retinas suggests that these cells express at least two types of GABA(A)R subtypes. In amacrine cells of geph(-/-) mice, a specific GABA(A)R subtype that may contain the alpha2 subunit, is impaired by the absence of gephyrin, whereas other GABA(A)Rs appear to function normally.

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