As presented at the SfN meeting, graduate student Robert Bell tested this hypothesis using cultured, normal human vascular smooth muscle cells (VSMC) obtained postmortem from non-demented seniors. He infected some of those cells with adenovirus to boost MYOCD and SRF protein expression to levels seen in AD. He found that while non-infected cells cleared Aβ in an in vitro assay after five days, MYOCD/SRF-overexpressing cells did not. In vitro, cultured VSMC from age-matched AD patients also did not efficiently clear Aβ in this assay. Using real-time PCR and Western blotting, Bell and colleagues found that MYOCD/SRF overexpression bumps up mRNA and protein levels of sterol regulatory element binding protein-2 (SREBP2), which downregulates expression of low-density lipoprotein receptor-related protein 1 (LRP1), a key Aβ clearance receptor (see ARF related news story). Based on data from luciferase reporter assays, Bell found that myocardin/SRF-mediated activation of SREBP2 requires the two CArG motifs in its first intron.

Turning to postmortem and in vivo experiments, Bell and colleagues found that levels of SREBP2 are elevated in VSMCs of AD patients with CAA compared to age-matched controls, and that the elevated SREBP2 correlates with reduces LRP levels. Local adenovirus-mediated silencing of SRF expression in mice carrying the Dutch/Iowa human APP mutation led to enhanced clearance of Aβ from vessels in the immediate vicinity, as well as a focal reduction of Aβ in underlying brain tissue. A similar clearance effect was seen when SRF was silenced in APPsw+/- mice, which develop CAA after about 12 months. In both cases, the improvement in Aβ clearance was accompanied by more than 2.5-fold reduction in SREBP2. In contrast, adenovirus-mediated MYOCD/SRF overexpression sped up Aβ deposition in both mouse models, and increased SREBP2. Low-density lipoprotein receptor (LDLR), an SREBP2 target gene, was also upregulated.

“In summary, we suggest that elevated SRF and MYOCD activity enhances Aβ accumulation in the vessel wall and this may initiate the development of CAA, focal Aβ brain accumulations, cerebral arterial hypoperfusion and neurovascular uncoupling, commonly seen in AD,” write the authors. What leads to increased SRF and MYOCD to begin with is not clear, but the authors demonstrate that hypoxia and hypoxia-inducing factor 1α (HIF-1α) drive MYOCD expression in vitro in human VSMC cells and also in pial blood vessels in APPsw+/- mice. “Thus, hypoxia can set in motion the SRF-MYOCD pathogenic pathway in AD specifically in VSMCs by upregulating MYOCD, whose expression in brain is restricted to VSMCs,” write the authors. Given that hypoxia can increase SRF/MYOCD, which in turn constricts blood vessels—leading to more hypoxia—and that hypoxia/HIF-1α may increase BACE expression (see Zhang et al., 2007) and contribute to γ-secretase maturation and processing of APP and Notch (see Wang et al., 2006), the study of hypoxia in CAA and AD might be worthy of further scrutiny.—Esther Landhuis, Tom Fagan.

Reference:
Bell RD, Deane R, Chow N, Long X, Sagare A, Singh I, Streb JW, Guo H, Rubio A, Van Nostrand W, Miano JM, Zlokovic BV. SRF and myocardin regulate LRP-mediated amyloid-beta clearance in brain vascular cells. Nature Cell Biol. 2008 December 21 advanced online publication. Abstract