Many potential treatments for Alzheimer's disease target amyloid-beta peptides (Abeta), which are widely presumed to cause the disease. The microtubule-associated protein tau is also involved in the disease, but it is unclear whether treatments aimed at tau could block Abeta-induced cognitive impairments. Here, we found that reducing endogenous tau levels prevented behavioral deficits in transgenic mice expressing human amyloid precursor protein, without altering their high Abeta levels. Tau reduction also protected both transgenic and nontransgenic mice against excitotoxicity. Thus, tau reduction can block Abeta- and excitotoxin-induced neuronal dysfunction and may represent an effective strategy for treating Alzheimer's disease and related conditions.

Many proteins suspected of causing neurodegenerative diseases exist in diverse assembly states. For most, it is unclear whether shifts from one state to another would be helpful or harmful. We used mutagenesis to change the assembly state of Alzheimer disease (AD)-associated amyloid-␤ (A␤) peptides. In vitro, the "Arctic" mutation (A␤E22G) accelerated A␤ fibrillization but decreased the abundance of nonfibrillar A␤ assemblies, compared with wild-type A␤. In human amyloid precursor protein (hAPP) transgenic mice carrying mutations adjacent to A␤ that increase A␤ production, addition of the Arctic mutation markedly enhanced the formation of neuritic amyloid plaques but reduced the relative abundance of a specific nonfibrillar A␤ assembly (A␤*56). Mice overexpressing Arctic mutant or wildtype A␤ had similar behavioral and neuronal deficits when they were matched for A␤*56 levels but had vastly different plaque loads. Thus, A␤*56 is a likelier determinant of functional deficits in hAPP mice than fibrillar A␤ deposits. Therapeutic interventions that reduce A␤ fibrils at the cost of augmenting nonfibrillar A␤ assemblies could be harmful. Alzheimer disease (AD)3 and many other neurodegenerative disorders are associated with the accumulation of abnormal protein assemblies in the central nervous system (CNS). Much evidence suggests that this association reflects a causal relationship in which the abnormal proteins actually trigger the neuronal dysfunction and degeneration that characterize these conditions (1-3). The prevalence of AD and other neurodegenerative proteinopathies is increasing rapidly around the world, most likely because of their age dependence, the increasing longevity of many populations, and the lack of effective strategies for treatment and prevention (4 -6). This alarming trend underlines the need to better understand the relationship between the accumulation of abnormal proteins in the CNS and the decline of neurological function.This relationship has been difficult to analyze in depth because proteins associated with neurodegenerative disorders can exist in diverse assembly states, and distinct assemblies can differ markedly in pathogenic potential. For example, the amyloid-␤ (A␤) peptide, which seems to play a causal role in AD, can exist as monomers, low molecular weight oligomers (such as dimers and trimers), larger globular oligomers (such as A␤*56, A␤-derived diffusible ligands, amylospheroids, and globulomers), amyloid pores, protofibrils, fibrils, and amyloid plaques that contain densely packed A␤ fibrils and a large number of other molecules and cellular elements (7-15). Which of these structures contributes most critically to neurological decline in AD is a matter of active study and debate that has important implications for therapeutic interventions. Studies of transgenic mice with neuronal expression of human amyloid precursor proteins (hAPP), from which A␤ is released by proteolytic cleavage, suggest that nonfibrillar A␤ assemblies are more critical than amyloid plaques in the pathogene...

Human amyloid precursor protein (hAPP) transgenic mice with high levels of amyloid-␤ (A␤) develop behavioral deficits that correlate with the depletion of synaptic activity-related proteins in the dentate gyrus. The tyrosine kinase Fyn is altered in Alzheimer's disease brains and modulates premature mortality and synaptotoxicity in hAPP mice. To determine whether Fyn also modulates A␤-induced behavioral deficits and depletions of synaptic activity-dependent proteins, we overexpressed Fyn in neurons of hAPP mice with moderate levels of A␤ production. Compared with nontransgenic controls and singly transgenic mice expressing hAPP or FYN alone, doubly transgenic FYN/hAPP mice had striking depletions of calbindin, Fos, and phosphorylated ERK (extracellular signal-regulated kinase), impaired neuronal induction of Arc, and impaired spatial memory retention. These deficits were qualitatively and quantitatively similar to those otherwise seen only in hAPP mice with higher A␤ levels. Surprisingly, levels of active Fyn were lower in high expresser hAPP mice than in NTG controls and lower in FYN/hAPP mice than in FYN mice. Suppression of Fyn activity may result from dephosphorylation by striatal-enriched phosphatase, which was upregulated in FYN/hAPP mice and in hAPP mice with high levels of A␤. Thus, increased Fyn expression is sufficient to trigger prominent neuronal deficits in the context of even relatively moderate A␤ levels, and inhibition of Fyn activity may help counteract A␤-induced impairments.

The fibroblast growth factor family of secreted signaling molecules is essential for patterning in the central nervous system. Fibroblast growth factor 17 (Fgf17) has been shown to contribute to regionalization of the rodent frontal cortex. To determine how Fgf17 signaling modulates behavior, both during development and in adulthood, we studied mice lacking one or two copies of the Fgf17 gene. Fgf17-deficient mice showed no abnormalities in overall physical growth, activity level, exploration, anxiety-like behaviors, motor co-ordination, motor learning, acoustic startle, prepulse inhibition, feeding, fear conditioning, aggression and olfactory exploration. However, they displayed striking deficits in several behaviors involving specific social interactions. Fgf17-deficient pups vocalized less than wild-type controls when separated from their mother and siblings. Elimination of Fgf17 also decreased the interaction of adult males with a novel ovariectomized female in a social recognition test and reduced the amount of time opposite-sex pairs spent engaged in prolonged, affiliative interactions during exploration of a novel environment. After social exploration of a novel environment, Fgf17-deficient mice showed less activation of the immediateearly gene Fos in the frontal cortex than wild-type controls. Our findings show that Fgf17 is required for several complex social behaviors and suggest that disturbances in Fgf17 signaling may contribute to neuropsychiatric diseases that affect such behaviors.