Sabine Levi

Gamma Protocadherins Are Required for Survival of Spinal Interneurons

The murine genome contains approximately 70 protocadherin (Pcdh) genes. Many are expressed in the nervous system, suggesting that Pcdhs may specify neuronal connectivity. Here, we analyze the 22 contiguous genes of the Pcdh-gamma cluster. Individual neurons express subsets of Pcdh-gamma genes. Pcdh-gamma proteins are present in most neurons and associated with, but not confined to, synapses. Early steps in neuronal migration, axon outgrowth, and synapse formation proceed in mutant mice lacking all 22 Pcdh-gamma genes. At late embryonic stages, however, dramatic neurodegeneration leads to neonatal death. In mutant spinal cord, many interneurons are lost, but sensory and motor neurons are relatively spared. In cultures from mutant spinal cord, neurons differentiate and form synapses but then die. Thus, Pcdh-gamma genes are dispensable for at least some aspects of connectivity but required for survival of specific neuronal types.

Gephyrin Is Critical for Glycine Receptor Clustering But Not for the Formation of Functional GABAergic Synapses in Hippocampal Neurons

The role of the scaffolding protein gephyrin at hippocampal inhibitory synapses is not well understood. A previous study (Kneussel et al., 1999) reported a complete loss of synaptic clusters of the major GABAAR subunits α2 and γ2 in hippocampal neurons lacking gephyrin. In contrast, we show here that GABAAR α2 and γ2 subunits do cluster at pyramidal synapses in hippocampal cultures from gephyrin-/- mice, albeit at reduced levels compared with control neurons. Synaptic aggregation of GABAAR α1 on interneurons was identical between the culture types. Furthermore, we recorded miniature IPSCs (mIPSCs) from gephyrin-/- neurons. Although the mean mIPSC amplitude was reduced (by 23%) compared with control, the frequency of these events was unchanged. Cell surface labeling experiments indicated that gephyrin contributes, in part, to aggregation but not to insertion or stabilization of GABAAR α2 and γ2 in the plasma membrane. Thus, a major gephyrin-independent component of hippocampal inhibitory synapse development must exist. We also report that glycine receptors cluster at GABAergic synapses in a subset of hippocampal interneurons and pyramidal neurons. Unlike GABAARs, synaptic clustering of glycine receptors was completely abolished in gephyrin-/- neurons. Finally, artificial extrasynaptic aggregation of GABAAR was able to redistribute and cocluster gephyrin by a mechanism requiring a neuron-specific modification or intermediary protein. We propose a model of hippocampal inhibitory synapse development in which some GABAARs cluster at synapses by a gephyrin-independent mechanism and recruit gephyrin. This clustered gephyrin may then recruit glycine receptors, additional GABAARs, and other signal-transducing components.

Birth of a Synapse: Not Such Long Labor

Clearly, the brain has yet to yield up many if not most of its secrets regarding synaptic plasticity, but the ability of a timing-based mechanism to explain system level plasticity with physiologically plausible firing patterns strongly suggests that this is the right track. Time will tell! cule PSD-95 can form clusters. These preassembled aggregates may contribute to subsequent rapid synaptogenesis. and AMPA receptor cluster at the postsynaptic site after another 30–45 min. (e) Further maturation may be necessary to form a fully differentiated glutamatergic synapse. tergic central synapses may be surprisingly short. The authors combined time lapse imaging of newly formed presynaptic specializations in living neurons with retrospective immunolabeling to follow the differen-Synapses are specialized junctions between cells where tiation of single synaptic contacts at defined times after the presynaptic cell accumulates synaptic vesicles and formation. Functional presynaptic specializations were release machinery and a postsynaptic cell concentrates identified by stimulation-driven uptake into and release neurotransmitter receptors and signaling machinery. from synaptic vesicles of the amphipathic fluorescent Previous studies of synaptogenesis using population dye FM 4-64. Newly formed FM 4-64 puncta could be analyses of fixed tissue have suggested that it may take assigned a time of birth within a time frame of minutes days to weeks to form this junction. In this issue of by repeated loading and unloading of the dye at various Neuron, Ziv and colleagues (Friedman et al., 2000) pro-time intervals throughout which the cell was imaged. vide evidence from dynamic analyses of cultured hippo-campal neurons that the time required to concentrate Using this approach, Ziv and colleagues determined