Genetic modulation of tau phosphorylation in the mouse

Abstract

The axonal microtubule stabilizing protein tau is hyperphosphorylated in several neurodegenerative conditions, including Alzheimer's disease, yet the genes that regulate tau phosphorylation are largely unknown. Disabled-1 (Dab1) is a cytoplasmic adapter protein that interacts with apolipoprotein E (ApoE) receptors and controls neuronal positioning during embryonic brain development. We have investigated the role of Dab1 in tau phosphorylation. We found that wild-type Dab1, but not a mutant lacking tyrosine phosphorylation sites, protects mice from the hyperphosphorylation of tau. However, the absence of Dab1 is not sufficient to cause tau hyperphosphorylation, because hyperphosphorylation is manifested only when Dab1 is mutated in specific mouse strain backgrounds. Tau hyperphosphorylation correlates with early death in susceptible mouse strains, and it occurs in the neurons of the hippocampus and dentate gyrus. By quantitative trait locus (QTL) analysis of Dab1-deficient mice on a hybrid strain background, we uncovered one significant and three suggestive chromosomal loci that modulate tau phosphorylation. Two of these QTL regions contain genes that are defective in early onset Alzheimer's disease. Our findings suggest that Dab1 gene disruption sensitizes mice to tau hyperphosphorylation contingent on specific haplotypes that are linked to Alzheimer's disease loci. Dab1 mutant mice provide an animal model for studying the relationships between ApoE receptors, tau hyperphosphorylation, and Alzheimer's disease.

Fig. 1.

Tau phosphorylation at several sites is regulated by genes of the Reln–Dab1 pathway. Brain extracts were prepared at P18–P20, and equal quantities of heat-stable protein were analyzed by SDS PAGE and Western blotting. A, Samples from various mutant mice of different strain backgrounds were analyzed simultaneously with antibodies recognizing phosphorylated (top) and dephosphorylated (bottom) tau.B, Selected samples were reanalyzed with antibodies recognizing additional epitopes on tau. Antibody specificity was determined previously using human tau as the antigen:+P, Phosphorylation dependent; −P, dephosphorylation dependent; Pi, phosphorylation independent (see Materials and Methods for references). Note the altered migration of tau from mutant brains (asterisks), which is indicative of increased phosphorylation.

Fig. 2.

Presence of phosphorylated tau in the hippocampus. All sections were stained for phosphorylated tau, andbrown areas indicate immunoreactivity.A–D, Sections from aDab1^−/−SB strain mouse showing the hippocampus (A), CA2 region (B), and dentate gyrus (C, D). E, F, Sections from the hippocampus ofDab1^+/+ SB strain mouse (E) andDab1^−/− CCstrain mouse (F). Scale bars: A, E, F, 400 μm; B, C, 200 μm; D, 100 μm.

Fig. 3.

Identification of genetic modifiers of tau phosphorylation. A,Dab1^−/+ BALB/cByJ (CC) mice were bred to C57BL/6J (BB) mice, andDab1^−/+ F₁(BC) offspring were intercrossed. F₂offspring that were Dab1^−/− were identified, and the brain samples were analyzed by Western blotting. The figure shows data on the first 12 F₂ brains analyzed. Phenotypes were scored as 0 (lowest quartile of tau phosphorylation), 1 (intermediate), or 2 (highest). Mice with phenotype 0 or 2 were genotyped. +P, Phosphorylation dependent;−P, dephosphorylation dependent; Pi, phosphorylation independent (see Materials and Methods for references).B, Table of genotypes. Rows correspond to individual mice, grouped according to phenotype; columnscorrespond to markers analyzed, grouped by chromosome (see Materials and Methods for a list of markers). Genotypes are color-coded asCC, BC, or BB. Black indicates no data. Note predominance ofCC (red) on chromosome 4, whereDab1 is located. Markers at the right end of chromosome 1 are predominantly BB (blue) in phenotype 2 mice (high tau phosphorylation), whereas markers on chromosome 16 are predominantly BB in phenotype 0 mice (low tau phosphorylation). C, Genome scan for linkage between genotype and phenotype. Results are plotted as LOD scores compared with the LOD scores required for significant linkage (top dotted line) and suggestive linkage (bottom dotted line). A locus on chromosome 16 (60 cm) is significantly linked to phenotype. D, Direction of effects. Tau phosphorylation is increased by a dominant Ballele on chromosome 16, by a recessive B allele on 1, and by a dominant C allele on 12.