Until recently, neurons in the healthy brain were considered immune-privileged because they did not appear to express MHC class I (MHCI). However, MHCI mRNA was found to be regulated by neural activity in the developing visual system and has been detected in other regions of the uninjured brain. Here we show that MHCI regulates aspects of synaptic function in response to activity. MHCI protein is colocalized postsynaptically with PSD-95 in dendrites of hippocampal neurons. In vitro, whole-cell recordings of hippocampal neurons from ␤2m/TAP1 knockout (KO) mice, which have reduced MHCI surface levels, indicate a 40% increase in mini-EPSC (mEPSC) frequency. mEPSC frequency is also increased 100% in layer 4 cortical neurons. Similarly, in KO hippocampal cultures, there is a modest increase in the size of presynaptic boutons relative to WT, whereas postsynaptic parameters (PSD-95 puncta size and mEPSC amplitude) are normal. In EM of intact hippocampus, KO synapses show a corresponding increase in vesicles number. Finally, KO neurons in vitro fail to respond normally to TTX treatment by scaling up synaptic parameters. Together, these results suggest that postsynaptically localized MHCl acts in homeostatic regulation of synaptic function and morphology during development and in response to activity blockade. The results also imply that MHCI acts retrogradely across the synapse to translate activity into lasting change in structure.homeostatic ͉ neuron ͉ plasticity ͉ synapsin ͉ PSD-95 E xperience transduced into neural activity is required for proper brain development (1). The process by which neural activity remodels synaptic connections during development is termed ''activity-dependent plasticity,'' in which electrical signals induce specific patterns of gene transcription to alter synaptic properties and structural connectivity. Genes including BDNF and CamKII are known to be critical for this plasticity (2-4); however, many other molecules are likely involved as well.MHC class I (MHCI) family members are well known for their roles in cellular immunity, but a neuronal function has not been generally appreciated. In the immune system, MHCI genes act in concert with T cell receptors to discriminate self-versus non-self-proteins. The CNS was considered ''immune privileged,'' in part, because it was thought that healthy neurons do not express MHCI protein (5, 6). Recently, MHCI gene family members have been found at low levels in CNS neurons (7-11). MHCI mRNA is expressed and regulated in cortical and thalamic neurons during development and is down-regulated by chronic activity blockade with Tetrodotoxin (TTX) in vivo (7). MHCI is also a downstream target of the transcription factor CREB, required for Hebbian synaptic plasticity (8,12,13). MHCI is thus implicated in several forms of activity-dependent synaptic plasticity.The MHCI gene family includes Ͼ70 members in rodents (14). The proteins encoded are heavy chains comprising the largest portion of the MHCI protein complex. Functional MHCI is usually a trimer consisting...

A series of bis(dithiolene)tungsten(IV,VI) complexes derived from benzene-1,2-dithiolate (bdt) has been prepared as an synthetic approach to pterin dithiolene-bound active sites of tungstoenzymes, one example of which, a archaeal oxidoreductase, has been established crystallographically (Chan et al. Science 1995, 267, 1463). With [WIVO(bdt)2]2- (2) as the starting compound, silylation with RR‘2SiCl afforded [WIV(bdt)2(OSiRR‘2)]1- (4). Oxidation of 4 with Me3NO gave [WVIO(bdt)2(OSiRR‘2)]1- (5), also accessible by silylation of [WVIO2(bdt)2]2- (3). Reaction of 3 or 5 with Me3SiCl resulted in [WVIO(bdt)2Cl]1- (6), from which the unstable species [WVIO(bdt)2L]1- (L = ButO-, PhS-) were generated in solution. Reductive oxo transfer of 6 with L‘ = P(OEt)3 or ButNC/P(OEt)3 gave [WIV(bdt)2L‘2] (8 and 9). Sulfido complex [WVIS(bdt)2(OSiRR‘2)]1- (12) was obtained in the reaction systems 4/(PhCH2S)2S and 5/(Me3Si)2S. Structures of [WO(SPh)4]1- and [W(bdt)3]2- and eight complexes of types 4−6, 8, 9, and 12 were determined by X-ray crystallography. Complexes 4 and 5 are tungsten analogues of the desoxo Mo(IV) and monooxo Mo(VI) states of Rhodobacter sphaeroides DMSO reductase. Six types of reactivity, including oxygen atom transfer, are recognized by the synthesis and interconversion of the set of complexes. The potential biological relevance of these complexes to the structure and function of active sites in two families of tungstoenzymes is considered (RR‘2 = Me3 (4); ButMe2 (4 and 5), ButPh2 (4, 5, and 12)).

Hippocampal slices often have more synapses than perfusion-fixed hippocampus, but the cause of this synaptogenesis is unclear. Ultrastructural evidence for synaptogenic triggers during slice preparation was investigated in 21-day-old rats. Slices chopped under warm or chilled conditions and fixed after 0, 5, 25, 60, or 180 minutes of incubation in an interface chamber were compared with hippocampi fixed by perfusion or by immersion of the whole hippocampus. There was no significant synaptogenesis in these slices compared with perfusion-fixed hippocampus, but there were other structural changes during slice preparation and recovery in vitro. Whole hippocampus and slices prepared under warm conditions exhibited an increase in axonal coated vesicles, suggesting widespread neurotransmitter release. Glycogen granules were depleted from astrocytes and neurons in 0-min slices, began to reappear by 1 hour, and had fully recovered by 3 hours. Dendritic microtubules were initially disassembled in slices, but reassembled into normal axial arrays after 5 minutes. Microtubules were short at 5 minutes (12.3 +/- 1.1 microm) but had recovered normal lengths by 3 hours (84.6 +/- 20.0 microm) compared with perfusion-fixed hippocampus (91 +/- 22 microm). Microtubules appeared transiently in 15 +/- 3% and 9 +/- 4% of dendritic spines 5 and 25 minutes after incubation, respectively. Spine microtubules were absent from perfusion-fixed hippocampus and 3-hour slices. Ice-cold dissection and vibratomy in media that blocked activity initially produced less glycogen loss, coated vesicles, and microtubule disassembly. Submersing these slices in normal oxygenated media at 34 degrees C led to glycogen depletion, as well as increased coated vesicles and microtubule disassembly within 1 minute.

An improved synthetic route to the previously reported dicarbonyl complexes [W(CO)2(S2C2R2)2] (R = Ph (1), Me (2)) has been developed via the thermal reaction of [W(CO)3(MeCN)3] and [Ni(S2C2R2)2] in dichloromethane (60−70%). Complexes 1 and 2 are shown to be useful synthetic precursors by means of carbonyl ligand displacement. Reactions of 1 with Et4NOH, Na2S, and Li2Se afford the previously unknown bis(dithiolene) series [WIVQ(S2C2Ph2)2]2- (Q = O (3), S (6), Se (7), 65−76%). Complex 2 and Et4NOH give [WIVO(S2C2Me2)2]2- (5, 68%). Members of the series manifest absorption spectra that are strongly dependent on Q and redox potentials for WIVQ/WVQ couples that are independent of Q. Reaction of 3 and 5 with MeI or EtI results in mono-S-alkylation as shown by the 1H NMR spectra of [WO(EtS2C2R2)(S2C2R2)]1-, which indicate a single stereoisomer with a diastereotopic methylene group. S-alkylation of 3 was further confirmed by the structure of the reaction product with MeI, [WO(MeS2C2Ph2)(S2C2Ph2)]1- (8, 65%), which reveals the exo stereoisomer with a pyramidal sulfur atom whose methyl carbon atom is displaced 1.27 Å from the chelate ring plane. Treatment of 3 with hard alkylating agents caused oxidation to [WVO(S2C2Ph2)2]-, independently prepared by reaction of 3 with iodine (78%). Sulfido complex 6 with soft alkylating agents such as MeI gave mixtures. Reaction of 6 with C7H7 + resulted in electron transfer rather than alkylation and the formation of binuclear [WV 2(μ-S)2(S2C2Ph2)4]2- (11, 53%). No alkylated species were isolated from selenido complex 7; [W(S2C2Ph2)3] (13) was identified as a reaction product. Electrochemical data and X-ray structural results for 1, 2, 13 (trigonal prismatic), Et4N+ salts of 3, 5, 6 (square pyramidal), 8 (distorted square pyramidal), and 11 (distorted octahedral), and (PhCH2NEt3)[W(S2C2Ph2)3] (distorted trigonal prismatic) are presented.