Cell type-specific genetic and optogenetic tools reveal hippocampal CA2 circuits.

The formation and recall of episodic memory requires precise information processing by the entorhinal-hippocampal network. For several decades, the trisynaptic circuit entorhinal cortex layer II (ECII)→dentate gyrus→CA3→CA1 and the monosynaptic circuit ECIII→CA1 have been considered the primary substrates of the network responsible for learning and memory. Circuits linked to another hippocampal region, CA2, have only recently come to light. Using highly cell type-specific transgenic mouse lines, optogenetics and patch-clamp recordings, we found that dentate gyrus cells, long believed to not project to CA2, send functional monosynaptic inputs to CA2 pyramidal cells through abundant longitudinal projections. CA2 innervated CA1 to complete an alternate trisynaptic circuit, but, unlike CA3, projected preferentially to the deep, rather than to the superficial, sublayer of CA1. Furthermore, contrary to existing knowledge, ECIII did not project to CA2. Our results allow a deeper understanding of the biology of learning and memory.

Entorhinal cortex layer III input to the hippocampus is crucial for temporal association memory.

Associating temporally discontinuous elements is crucial for the formation of episodic and working memories that depend on the hippocampal-entorhinal network. However, the neural circuits subserving these associations have remained unknown. The layer III inputs of the entorhinal cortex to the hippocampus may contribute to this process. To test this hypothesis, we generated a transgenic mouse in which these inputs are specifically inhibited. The mutant mice displayed significant impairments in spatial working-memory tasks and in the encoding phase of trace fear-conditioning. These results indicate a critical role of the entorhinal cortex layer III inputs to the hippocampus in temporal association memory.

Characterization of beta amyloid assemblies in drusen: the deposits associated with aging and age-related macular degeneration.

PURPOSE: Recent studies strongly suggest that drusen, the extracellular deposits associated with age-related macular degeneration (AMD), are a manifestation of local inflammatory events. New evidence indicates that substructural elements within drusen contain activated complement components as well as amyloid beta (Abeta), a major pro-inflammatory component of Alzheimer's disease plaques. We characterized the ultrastructural organization and histochemical staining properties of these Abeta-containing elements in order to further assess their significance in drusen formation and AMD pathogenesis. METHODS: We used differential interference contrast optics, laser scanning confocal immunofluorescence, and immunogold electron microscopy to characterize the structural properties and molecular composition of Abeta-containing elements in drusen. We obtained estimates of their frequency from montages of electron micrographs gathered from 152 human donor eyes ranging from 9 to 91 years of age. RESULTS: Spherical Abeta-containing elements, which are typically organized as concentric ring-like structures, are common substructural components of drusen. They stain with thioflavin T, but are not stained by Congo red; nor do they bind cationic, lipophilic, or nucleic acid-binding fluorescent dyes. Ultrastructurally, they are composed of a central core, one or more concentric inner rings with intervening electron lucent layers, and an electron dense outer shell. Immunogold labeling indicates that most Abeta immunoreactivity is associated with the outer layers that consist of densely-packed spherical subunits. No longitudinally-oriented fibril arrays, characteristic of aggregated amyloid fibrils in the brain, are evident. Other prominent drusen-associated proteins including the terminal complement complex C5b-9, vitronectin, apolipoprotein E, serum amyloid P component, and ubiquitin are excluded from the spheres.Conclusions. These structures embedded in drusen appear to represent a new type of macromolecular assembly that contains Abeta as well as activated complement components. The presence of Abeta in these extracellular deposits is an additional indication that some of the pathogenic pathways that give rise to drusen and AMD may be shared with other neurodegenerative diseases characterized by misfolded protein deposition and aggregation.

The Alzheimer's A beta -peptide is deposited at sites of complement activation in pathologic deposits associated with aging and age-related macular degeneration.

Age-related macular degeneration (AMD) is a leading cause of irreversible vision loss in older individuals worldwide. The disease is characterized by abnormal extracellular deposits, known as drusen, that accumulate along the basal surface of the retinal pigmented epithelium. Although drusen deposition is common in older individuals, large numbers of drusen and/or extensive areas of confluent drusen represent a significant risk factor for AMD. Widespread drusen deposition is associated with retinal pigmented epithelial cell dysfunction and degeneration of the photoreceptor cells of the neural retina. Recent studies have shown that drusen contain a variety of immunomodulatory molecules, suggesting that the process of drusen formation involves local inflammatory events, including activation of the complement cascade. Similar observations in Alzheimer's disease (AD) have lead to the hypothesis that chronic localized inflammation is an important element of AD pathogenesis, with significant neurodegenerative consequences. Accordingly, the amyloid beta (A beta) peptide, a major constituent of neuritic plaques in AD, has been implicated as a primary activator of complement in AD. Here we show that A beta is associated with a substructural vesicular component within drusen. A beta colocalizes with activated complement components in these "amyloid vesicles," thereby identifying them as potential primary sites of complement activation. Thus, A beta deposition could be an important component of the local inflammatory events that contribute to atrophy of the retinal pigmented epithelium, drusen biogenesis, and the pathogenesis of AMD.