Increasing evidence suggests that amyloid peptides associated with a variety of degenerative diseases induce neurotoxicity in their intermediate oligomeric state, rather than as monomers or fibrils. To test this hypothesis and investigate the possible involvement of Ca 2؉ signaling disruptions in amyloid-induced cytotoxicity, we made homogeneous preparations of diseaserelated amyloids (A␤, prion, islet amyloid polypeptide, polyglutamine, and lysozyme) in various aggregation states and tested their actions on fluo-3-loaded SH-SY5Y cells. Application of oligomeric forms of all amyloids tested (0.6 -6 g ml ؊1 ) rapidly (ϳ5 s) elevated intracellular Ca 2؉ , whereas equivalent amounts of monomers and fibrils did not. Ca 2؉ signals evoked by A␤42 oligomers persisted after depletion of intracellular Ca 2؉ stores, and small signals remained in Ca 2؉ -free medium, indicating contributions from both extracellular and intracellular Ca 2؉ sources. The increased membrane permeability to Ca 2؉ cannot be attributed to activation of endogenous Ca 2؉ channels, because responses were unaffected by the potent Ca 2؉ -channel blocker cobalt (20 m). Instead, observations that A␤42 and other oligomers caused rapid cellular leakage of anionic fluorescent dyes point to a generalized increase in membrane permeability. The resulting unregulated flux of ions and molecules may provide a common mechanism for oligomer-mediated toxicity in many amyloidogenic diseases, with dysregulation of Ca 2؉ ions playing a crucial role because of their strong trans-membrane concentration gradient and involvement in cell dysfunction and death.Alzheimer disease (AD) 1 is characterized by the appearance in the brain of plaques, containing extracellular deposits of amyloid ␤-peptide (A␤) that result from altered proteolytic processing of amyloid precursor protein, together with intracellular neurofibrillary tangles containing misfolded tau (1).Brain regions with plaques and tangles exhibit reduced numbers of synapses, and neurites associated with plaques and tangles are often damaged, suggesting a pivotal role for A␤ in the neuropathology of AD (2-5). Moreover, numerous other neurodegenerative disorders (including Huntington, Parkinson, and prion diseases) are also associated with the formation and accumulation of amyloid fibrils in specific brain areas (6, 7). These commonalities suggest a general mechanism of action for the more than 100 human amyloid-related diseases, whereby normally soluble peptides and proteins undergo aberrant folding (8).Aggregation of A␤ proceeds through several conformational states, including dimers, spherical oligomers composed of 10 -24 monomers, and strings of oligomers (protofibrils), before finally assuming an insoluble fibrillar conformation (9). The initial formulation of the "amyloid hypothesis" of AD specifically implicated fibrillar amyloid deposits (10). However, more recent evidence suggests that soluble oligomers may be the principal neurotoxic agent (11-15). Soluble A␤ oligomers are found in the cerebrospinal fluid of ...

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Seizures induced by fever (febrile seizures) are the most frequent seizures affecting infants and children; however, their impact on the developing hippocampal formation is not completely understood. Such understanding is highly important because of the potential relationship of prolonged febrile seizures to temporal lobe epilepsy. Using an immature rat model, we have previously demonstrated that prolonged experimental febrile seizures render the hippocampus hyperexcitable throughout life. Here we examined whether (1) neuronal loss, (2) altered neurogenesis, or (3) mossy fiber sprouting, all implicated in epileptogenesis in both animal models and humans, were involved in the generation of a pro-epileptic, hyperexcitable hippocampus by these seizures. The results demonstrated that prolonged experimental febrile seizures did not result in appreciable loss of any vulnerable hippocampal cell population, though causing strikingly enhanced sensitivity to hippocampal excitants later in life. In addition, experimental febrile seizures on postnatal day 10 did not enhance proliferation of granule cells, whereas seizures generated by kainic acid during the same developmental age increased neurogenesis in the immature hippocampus. However, prolonged febrile seizures resulted in long-term axonal reorganization in the immature hippocampal formation: Mossy fiber densities in granule cell-and molecular layers were significantly increased by 3 months (but not 10 days) after the seizures. Thus, the data indicate that prolonged febrile seizures influence connectivity of the immature hippocampus long-term, and this process requires neither significant neuronal loss nor altered neurogenesis. In addition, the temporal course of the augmented mossy fiber invasion of the granule cell and molecular layers suggests that it is a consequence, rather than the cause, of the hyperexcitable hippocampal network resulting from these seizures.

Different strategies proposed as therapy for Alzheimer disease (AD) have aimed to reduce the level of toxic forms of A␤ peptide in the brain. Here, we directly analyze the therapeutic utility of the polyclonal anti-A␤ 1-11 antibody induced in 3xTg-AD mice vaccinated with the second generation prototype epitope vaccine. Substoichiometric concentrations of purified anti-A␤ 1-11 antibody prevented aggregation of A␤ 42 and induced disaggregation of preformed A␤ 42 fibrils down to nonfilamentous and nontoxic species. Anti-A␤ 1-11 antibody delayed A␤ 42 oligomer formation but ultimately appeared to stabilize nonfibrillar conformations, including oligomer-like assemblies. The reduced oligomer-mediated cytotoxicity observed upon preincubation of A␤ oligomers with the anti-A␤ 1-11 antibody in the absence of oligomer disaggregation suggests a possible oligomer rearrangement in the presence of the antibody. These in vitro observations suggest that preventive vaccination may protect from AD or may delay the onset of the disease, whereas therapeutic vaccination cannot disrupt the toxic oligomers and may only minimally alleviate preexisting AD pathology. AD4 is characterized by deposition of fibrillar forms of A␤ peptide in senile plaques, appearance of A␤ congophilic deposits and neurofibrillary tangles in the cerebrovasculature, and neuronal loss (1-4). A␤ peptide is cleaved from the amyloid precursor protein (APP) by ␤-and ␥-secretases (5-7) and is thought to play a central role in the onset and progression of AD (8 -10). In AD, the normally soluble A␤ molecule (39 -43 aa) undergoes conformational changes and is deposited as insoluble fibrils, oligomers, and protofibrills. Previously, it was demonstrated that A␤ neurotoxicity requires insoluble fibril formation (11) and that these fibrils serve as inducers of neuronal apoptosis (12). Recently, emphasis has shifted to smaller soluble oligomers of A␤ 42 , such as the 12-mers known as A␤-derived diffusible ligands, increased about 70-fold in AD patients' brains over controls (13). More recently, it was shown that extracellular accumulation of 56-kDa soluble A␤ assembly impairs memory in middle-aged APP/Tg 2576 mice in the absence of neuritic plaques (14). A␤ 42 dimers and trimers naturally secreted from a 7PA2 cell line were also suggested to be responsible for the disruption of cognitive functions (15). Importantly, intraventricular injection of such A␤ 42 small oligomers inhibited long term potentiation in rat hippocampus, and an injection of anti-A␤ monoclonal antibody 6E10 prevented this inhibition (16). It has also been demonstrated that passive immunization with monoclonal antibodies (NAB61) that specifically recognize a pathologic conformation present in A␤ oligomers resulted in a rapid improvement in spatial learning and memory (17).The therapeutic potency of polyclonal and monoclonal anti-A␤ antibodies was documented in different mouse models of AD (18 -25). Collectively, these data suggest that antibodies specific to the N-terminal region of A␤ are capable of reduci...

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