To assess the relationship between dementia, neuronal loss, and neuropathological findings in Alzheimer's disease (AD), we counted the number of neurons, senile plaques, and neurofibrillary tangles in a high-order association cortex. We studied the superior temporal sulcus of 34 individuals with AD and 17 nondemented control subjects, using statistically unbiased, stereological counting techniques. The number of superior temporal sulcus neurons in nondemented control subjects was stable across the sixth to ninth decades. In AD, more than 50% of the neurons were lost. Both neuronal loss and neurofibrillary tangles increased in parallel with the duration and severity of illness, but the amount of neuronal loss exceeded by manyfold the amount of neurofibrillary tangles accumulated. In contrast to the correlation between neurofibrillary tangles and neuronal loss, the number of senile plaques and the percentage of the superior temporal sulcus that was covered by Abeta (amyloid burden) were not related to neuronal loss, number of neurofibrillary tangles, or duration of disease. Neither the amount nor the rate of neuronal loss in the superior temporal sulcus in AD correlated with apolipoprotein E genotype. These data suggest that neuronal loss in association areas such as the superior temporal sulcus contributes directly to cognitive impairment in AD.
The vertebrate retina and optic nerve are strikingly different in terms of their size, organization, and cellular diversity, yet these two structures develop from the same embryonic neuroepithelium. Precursor cells in the most ventral domain of this epithelium give rise only to the astrocytes of the optic nerve, whereas immediately adjacent, more dorsal precursors give rise to the myriad cell types of the retina. We provide genetic evidence that two closely related, ventrally expressed homeodomain proteins-Vax1 and Vax2-control this neuroepithelial segregation. In the absence of both proteins, we find that the optic nerve is transformed in its entirety into fully differentiated retina. We demonstrate that this transformation results from the loss of ventralizing actvity in the developing eye field, and that ventralization is mediated, at least in part, via Vax repression of the Pax6 gene, a potent inducer of retinal development.
The homeodomain protein Vax1 is expressed in a highly circumscribed set of cells at the ventral anterior midline of the embryonic CNS. These cells populate the choroid fissure of the optic disk, the body of the optic stalk and nerve, the optic chiasm and ventral diencephalon, and the anterior midline zones that abut developing commissural tracts. We have generated mutant mice that lack Vax1. In these mice (1) the optic disks fail to close, leading to coloboma and loss of the eye-nerve boundary; (2) optic nerve glia fail to associate with and appear to repulse ingrowing retinal axons, resulting in a fascicle of axons that are completely segregated from optic nerve astrocytes; (3) retinal axons fail to penetrate the brain in significant numbers and fail to form an optic chiasm; and (4) axons in multiple commissural tracts of the anterior CNS, including the corpus callosum and the hippocampal and anterior commissures, fail to cross the midline. These axon guidance defects do not result from the death of normally Vax1 + midline cells but, instead, correlate with markedly diminished expression of attractive guidance cues in these cells. Vax1 therefore regulates the guidance properties of a set of anterior midline cells that orchestrate axon trajectories in the developing mammalian forebrain.
Apolipoprotein E allele 4 (apoE epsilon 4) is a major risk factor for late-onset AD. Inheritance of this allele is associated with an earlier age of onset of dementia in individuals with AD. It is unknown whether other polymorphisms in the apoE gene may influence the effect of apoE epsilon 4 on AD. We screened portions of the promoter enhancer element and of the apoE receptor binding domain for other polymorphisms that could affect risk of AD. In particular, a C/G polymorphism at position +113 of the apoE mRNA in the apoE intron 1 enhancer element (IE1) has been recently identified. We found no other polymorphisms. We studied the relationship of the two alleles of the IE1 polymorphism with AD and found an apparent association between IE1 G and AD (n = 94; p = 0.0515). However, the IE1 G allele is also closely associated with apoE epsilon 4 (p < 0.0001). When the presence of apoE epsilon 4 is covaried, the association between the IE1 G allele and AD is no longer statistically significant (odds ratio = 1.29, 95% confidence interval: 0.44, 3.78). In contrast, epsilon 4 is still highly associated with AD when IE1 G is controlled for (odds ratio = 5.91, 95% confidence interval: 3.29, 10.63). Furthermore, there is no significant association between the age of onset of dementia and the inheritance of the G allele. We believe that the apparent association between IE1 G and AD is a consequence of the association between the epsilon 4 and IE1 G alleles.
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