19 August 2005. The vascular hypothesis of Alzheimer disease suggests that pathology starts with hypoperfusion, or decreased blood flow to the brain. This, in turn, leads to a crisis among glia and neurons, eventually culminating in neurodegeneration and cognitive impairment (see de la Torre, 2004 and ARF Live Discussion). The theory just got a little shot in the arm. In a Nature Medicine paper published online last Sunday, Berislav Zlokovic and colleagues show that expression of MEOX2, a homeobox gene that is upregulated during vascular differentiation, is significantly reduced in late-stage AD and is required for optimal clearance of amyloid-β (Aβ) from the brains of mice.
Zlokovic, from the University of Rochester, New York, together with colleagues there and at other labs in New York and California, found the link between AD and MEOX2 when they carried out microarray analysis of messenger RNA levels in brain tissue samples. Joint first authors Zhenhua Wu, Huang Guo, Nienwen Chow and Jan Sallstrom—the last three from Socratech Research Laboratories in Rochester—measured levels of almost 13,000 genes in brain endothelial cells (BECs) isolated from frontal cortex autopsies. Wu and colleagues studied six patients with late-stage AD, six age-matched controls (mean age 70 years) having no dementia, and five younger (average age 24) subjects. The authors found about 34 genes that were differentially expressed in the AD samples compared to samples from nondemented and younger individuals.
Homing in on these 34 genes, Wu and colleagues found that MEOX2, encoding the homeobox protein GAX, and TGM2, encoding a transglutaminase, were the most dramatically affected. Quantitative PCR analysis on BECs captured by laser microdissection revealed that MEOX2 expression was down almost 20-fold, whereas TGM2 expression was elevated by about the same degree. Other genes that are involved in angiogenesis were also downregulated (NR4A2 and PLEC1), while the proapoptotic, forkhead transcription factor gene MLLT7 was upregulated.
The loss of MEOX2 expression could have important consequences for vascularization in the CNS and for clearance of Aβ. Wu and colleagues found that if they silenced MEOX2 to reduce GAX to about 40 percent of normal in endothelial cells, then capillaries formed in culture were shorter by about 60 percent while forkhead expression increased about twofold. This suggests that loss of MEOX2 in AD patients might be driving endothelial cell apoptosis and loss of capillaries. Furthermore, the authors found that MEOX2+/- heterozygote mice had about a 50 percent reduction in cerebral blood flow at 2-3 months of age, and they were unable to mount an angiogenesis response to hypoxia; normal mice increased total cortical capillary length by about 40 percent after 3 weeks of hypoxia, whereas in MEOX2+/- animals there was no change in the capillary network. Furthermore, these heterozygotes retained about twice as much Aβ40 in the brain as did normal animals and clearance across the blood-brain barrier of radioactive Aβ40 injected into the brain was reduced by 80 percent.
The failure to clear Aβ may not only be related to poor vascularization. Wu and colleagues also found that when MEOX2 is silenced, lipoprotein receptor-related protein 1 (LRP), a major carrier for Aβ, is reduced by about 40 percent in human BECs (though see ARF related news story which questions the link between LRP and Aβ clearance).
All told, the data seem to support the idea that vascular losses could contribute to AD pathology. While six patients is a small sample set, the authors were able to confirm their transcriptional profiling data using samples from a further ten patients in late-stage AD and nine age-matched controls having no AD-like pathology. Also of interest is that the authors found no changes in GAX levels in Tg2576 mice which, although rife with Aβ deposits, show very little neurodegeneration.
The authors write that low expression of MEOX2 in Alzheimer disease could have two major effects: “(i) impaired angiogenesis associated with apoptosis, vessel malformation and regression, ultimately resulting in reductions in brain capillary density and CBF [cerebral blood flow], as seen in Alzheimer disease and (ii) a pathological BBB [blood brain barrier] phenotype with little or no Aβ-clearing capability resulting from low levels of LRP, which may lead to Aβ accumulation as seen in Alzheimer disease models and the disease itself.”—Tom Fagan.
Wu Z, Guo H, Chow N, Sallstrom J, Bell RD, Deane R, Brooks AI, Kanagala S, Rubio A, Sagare A, Liu D, Li F, Armstrong D, Gasiewicz T, Zidovetski R, Song X, Hofman F, Zlokovic BV. Role of the MEOX2 homeobox gene in neurovascular dysfunction in Alzheimer disease. Nature Medicine. 2005. August 14, advanced online publication. Abstract