To examine the in vivo function of presenilin-1 (PS1), we selectively deleted the PS1 gene in excitatory neurons of the adult mouse forebrain. These conditional knockout mice were viable and grew normally, but they exhibited a pronounced deficiency in enrichment-induced neurogenesis in the dentate gyrus. This reduction in neurogenesis did not result in appreciable learning deficits, indicating that addition of new neurons is not required for memory formation. However, our postlearning enrichment experiments lead us to postulate that adult dentate neurogenesis may play a role in the periodic clearance of outdated hippocampal memory traces after cortical memory consolidation, thereby ensuring that the hippocampus is continuously available to process new memories. A chronic, abnormal clearance process in the hippocampus may conceivably lead to memory disorders in the mammalian brain.

In total, 335 serum samples were collected from rabbits from two farms in Gansu province, China, and tested for anti-hepatitis E virus (HEV) antibody using EIA and for HEV RNA using nested RT- PCR with ORF2 primers. The overall prevalence of anti-HEV antibody and HEV RNA was 57.0% (191/335) and 7.5% (25/335), respectively. The positivity rate of HEV RNA in the anti-HEV antibody negative group (7.6% (11/144)) did not differ significantly from that in the positive group (7.3% (14/191)). The concordance between HEV RNA and anti-HEV antibody was 43.3% with no significant correlation (P < 0.05). All 25 amplicons from the ORF2 region were cloned and sequenced. On the basis of nucleotide sequence comparison, they had 84-99% identity to each other and 73-77%, 70-76%, 75-82%, 71-77%, and 53-65% with the corresponding regions of genotypes 1, 2, 3, 4, and avian HEV, respectively. Samples that were positive with the ORF2 primers were amplified using ORF1 region primers; 17 were positive and shared 71-78%, 73-76%, 74-82%, 72-78%, and 39-58% identity with the corresponding regions of genotypes 1, 2, 3, 4, and avian HEV, respectively, at the nucleotide level. Two representative full-length sequences were determined. These two sequences shared 85% identity with each other and had 74%, 73%, 78-79%, 74-75%, and 46-47% identity to full-length genotypes 1, 2, 3, 4, and avian HEV, respectively. Thus, the sequences isolated from the rabbits represent a novel genotype of HEV. This study provides novel information about HEV genotypes infecting rabbits as well as evidence of a new mammalian genotype of HEV.

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By integrating convergent protein engineering and rational inhibitor design, we have developed an in vivo conditional protein knockout and͞or manipulation technology. This method is based on the creation of a specific interaction interface between a modified protein domain and sensitized inhibitors. By introducing this system into genetically modified mice, we can readily manipulate the activity of a targeted protein, such as ␣-Ca 2؉ ͞calmodulin-dependent protein kinase II (␣CAMKII), on the time scale of minutes in specific brain subregions of freely behaving mice. With this inducible and regionspecific protein knockout technique, we analyzed the temporal stages of memory consolidation process and revealed the first postlearning week as the critical time window during which a precise level of CaMKII reactivation is essential for the consolidation of long-term memories in the brain. C urrent-inducible and region-specific gene knockout techniques are powerful for molecular and temporal analysis of biological processes (1, 2). However, because the inactivation event occurs at the DNA level, manifestation of any phenotype depends on the turnover rate of the existing protein, which takes days or weeks. This inherently slow process has excluded precise investigation of many in vivo biological processes that occur within minutes and hours. Therefore, it is highly desirable to develop new types of techniques that can direct the knockout event at the protein level, rather than at the DNA level, for achieving almost instantaneous effects. Furthermore, the molecular specificity of such a knockout should surpass the conventional pharmacological inhibitors. We decided to explore methods to integrate the molecular and regional specificity of genetics with the high temporal resolution of chemical inhibition for the development of an inducible, reversible, and regionspecific protein knockout technique.Such a technique would be valuable for elucidation of molecular mechanisms underlying various temporal stages of brain function such as memory processes. The N-methyl-D-aspartate (NMDA) receptor has been established as a crucial molecular switch for synaptic plasticity (3, 4) and for memory formation (1,(5)(6)(7)(8). At the molecular level, long-term memory was widely assumed to be stored in the form of synaptic structural changes resulting from a single molecular cascade triggered by learning. However, this ''single cascade hypothesis'' has its conceptual difficulties in accounting for long-term memory formation in the brain. For example, the time scale of a single molecular cascade (typically between hours to days) is too short for describing the hippocampus-mediated consolidation process that is known to occur over a timescale of week(s) in rodents (9-12) and years in humans (13-15). Moreover, synaptic structures in the adult brain are dynamic, and synaptic proteins such as the NMDA receptor are known to be degraded within 5 days in the brain of freely behaving animals (1). Thus, it raises fundamental concerns whether any struc...