Alzheimer's disease constitutes a rising threat to public health. Despite extensive research in cellular and animal models, identifying the pathogenic agent present in the human brain and showing that it confers key features of Alzheimer's disease has not been achieved. We extracted soluble amyloid-beta protein (Abeta) oligomers directly from the cerebral cortex of subjects with Alzheimer's disease. The oligomers potently inhibited long-term potentiation (LTP), enhanced long-term depression (LTD) and reduced dendritic spine density in normal rodent hippocampus. Soluble Abeta from Alzheimer's disease brain also disrupted the memory of a learned behavior in normal rats. These various effects were specifically attributable to Abeta dimers. Mechanistically, metabotropic glutamate receptors were required for the LTD enhancement, and N-methyl D-aspartate receptors were required for the spine loss. Co-administering antibodies to the Abeta N-terminus prevented the LTP and LTD deficits, whereas antibodies to the midregion or C-terminus were less effective. Insoluble amyloid plaque cores from Alzheimer's disease cortex did not impair LTP unless they were first solubilized to release Abeta dimers, suggesting that plaque cores are largely inactive but sequester Abeta dimers that are synaptotoxic. We conclude that soluble Abeta oligomers extracted from Alzheimer's disease brains potently impair synapse structure and function and that dimers are the smallest synaptotoxic species.

Tar DNA Binding Protein-43 (TDP-43) is a principle component of inclusions in many cases of frontotemporal lobar degeneration (FTLD-U) and amyotrophic lateral sclerosis (ALS). TDP-43 resides predominantly in the nucleus, but in affected areas of ALS and FTLD-U central nervous system, TDP-43 is aberrantly processed and forms cytoplasmic inclusions. The mechanisms governing TDP-43 inclusion formation are poorly understood. Increasing evidence indicates that TDP-43 regulates mRNA metabolism by interacting with mRNA binding proteins that are known to associate with RNA granules. Here we show that TDP-43 can be induced to form inclusions in cell culture and that most TDP-43 inclusions co-localize with SGs. SGs are cytoplasmic RNA granules that consist of mixed protein - RNA complexes. Under stressful conditions SGs are generated by the reversible aggregation of prion-like proteins, such as TIA-1, to regulate mRNA metabolism and protein translation. We also show that disease-linked mutations in TDP-43 increased TDP-43 inclusion formation in response to stressful stimuli. Biochemical studies demonstrated that the increased TDP-43 inclusion formation is associated with accumulation of TDP-43 detergent insoluble complexes. TDP-43 associates with SG by interacting with SG proteins, such as TIA-1, via direct protein-protein interactions, as well as RNA-dependent interactions. The signaling pathway that regulates SGs formation also modulates TDP-43 inclusion formation. We observed that inclusion formation mediated by WT or mutant TDP-43 can be suppressed by treatment with translational inhibitors that suppress or reverse SG formation. Finally, using Sudan black to quench endogenous autofluorescence, we also demonstrate that TDP-43 positive-inclusions in pathological CNS tissue co-localize with multiple protein markers of stress granules, including TIA-1 and eIF3. These data provide support for accumulating evidence that TDP-43 participates in the SG pathway.

The accumulation of amyloid β-protein (Aβ) in brain regions serving memory and cognition is a central pathogenic feature of Alzheimer's disease (AD). We have shown that small soluble oligomers of human Aβ that are naturally secreted by cultured cells inhibit hippocampal long-term potentiation (LTP) in vitro and in vivo and transiently impair the recall of a complex learned behaviour in rats. These results support the hypothesis that diffusible oligomers of Aβ initiate a synaptic dysfunction that may be an early event in AD. We now report detailed electrophysiological analyses that define conditions under which acute application of soluble Aβ inhibits hippocampal synaptic plasticity in wild-type mice. To ascertain which Aβ assemblies contribute to the impairment of LTP, we fractionated oligomers by size-exclusion chromatography and found that Aβ trimers fully inhibit LTP, whereas dimers and tetramers have an intermediate potency. Natural Aβ oligomers are sensitive to heat denaturation, primarily inhibit the induction phase of LTP, and cause a sustained impairment of LTP even after extensive washout. We observed no effects of Aβ oligomers on presynaptic vesicle release. LTP in juvenile mice is resistant to the effects of Aβ oligomers, as is brain-derived-neurotrophic-factor-induced LTP in adult hippocampus. We conclude that specific assemblies, particularly timers, of naturally secreted Aβ oligomers are potent and selective inhibitors of certain forms of hippocampal LTP.

Numerous studies have now shown that the amyloid ␤-protein (A␤), the principal component of cerebral plaques in Alzheimer disease, rapidly and potently inhibits certain forms of synaptic plasticity. The amyloid (or A␤) hypothesis proposes that the continuous disruption of normal synaptic physiology by A␤ contributes to the development of Alzheimer disease. However, there is little consensus about how A␤ mediates this inhibition at the molecular level. Using mouse primary hippocampal neurons, we observed that a brief treatment with cell-derived, soluble, human A␤ disrupted the activation of three kinases (Erk/MAPK, CaMKII, and the phosphatidylinositol 3-kinaseactivated protein Akt/protein kinase B) that are required for long term potentiation, whereas two other kinases (protein kinase A and protein kinase C) were stimulated normally. An antagonist of the insulin receptor family of tyrosine kinases was found to mimic the pattern of A␤-mediated kinase inhibition. We then found that soluble A␤ binds to the insulin receptor and interferes with its insulin-induced autophosphorylation. Taken together, these data demonstrate that physiologically relevant levels of naturally secreted A␤ interfere with insulin receptor function in hippocampal neurons and prevent the rapid activation of specific kinases required for long term potentiation. Alzheimer disease (AD)2 is characterized by a progressive loss of memory and the accumulation of amyloid plaques and neurofibrillary tangles in discrete brain regions. The amyloid ␤-protein (A␤), the principal constituent of amyloid plaques observed in AD and to a lesser extent during normal brain aging, is generated by sequential proteolytic cleavages of the ␤-amyloid precursor protein (APP) (1). The recognition that several forms of early onset AD are caused by mutations that alter A␤ production and that transgenic mice expressing these mutant alleles develop several features of AD, including A␤ plaques and memory deficits, have provided strong evidence that A␤ plays an important role in the disease process (2). In particular, small oligomers of soluble A␤ have been shown to be potent inhibitors of memory in rodents (3, 4) and can be found in human cerebral spinal fluid and APP transgenic mice (3, 5, 6). However, how A␤ leads to a disruption in hippocampal synaptic function and deficits in memory remains unclear.There is considerable evidence that A␤ can bind to several different molecules on the neuronal surface. It has been reported that a brief treatment of cultured neurons with trypsin reduces synthetic A␤ binding (7). Moreover, Klein and co-workers (8) have used a far Western technique to demonstrate the binding of synthetic A␤ oligomers called A␤-derived diffusible ligands to discrete proteins in cortical brain homogenates. Various laboratories have identified candidate receptors that may play a role in A␤ neurotoxicity, including the ␣7-type nicotinic acetylcholine receptor (9 -11), the metabotropic glutamate receptor 5 (mGluR5) (12), the ␤-adrenergic receptor (13), and the IR (...

A small, orally available natural product penetrates into the brain in vivo to rescue the memory impairment produced by soluble Abeta oligomers through a mechanism that restores hippocampal synaptic plasticity.

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