28 June 2013. Growing evidence suggests that innate immune cells prevent or slow Alzheimer's disease by chewing up Aβ and promoting its clearance. In a June 25 Nature Communications paper, researchers led by Joseph El Khoury at Massachusetts General Hospital, Charlestown, report that macrophages require the scavenger receptor Scara1 to clear Aβ. Whereas many prior studies identified receptors for fibrillar amyloid, the current research shows Scara1 is essential for clearing soluble forms of the peptide, including oligomers, which are now seen as the most neurotoxic in AD. The findings add to a flurry of recent advances in understanding the role of microglia in AD at the molecular level and point to Scara1 as a promising therapeutic target.
Previously, El Khoury and others identified Scara1 (aka SRA for “scavenger receptor A” or MSR1 in humans) as a microglial receptor that mediates uptake of fibrillar amyloid in vitro (El Khoury et al., 1996; Chung et al., 2001), and levels of this receptor fall with age in AD mice (Hickman et al., 2008). Other research suggests that microglia and peripheral macrophages curb plaque deposition in mouse models of AD (see ARF related news story on El Khoury et al., 2007; ARF related news story; and ARF related news story on Yamanaka et al., 2012) and cerebral amyloid angiopathy (CAA) (see ARF related news story). As soluble oligomers emerged as the most menacing Aβ species in AD (see Shankar et al., 2008; McDonald et al., 2010), El Khoury and colleagues turned their attention to receptors that mediate their clearance.
To identify such proteins, co-lead author Kim Wilkinson and colleagues used a short hairpin RNA (shRNA) library to silence innate immune receptors in a mouse macrophage cell line, then measured uptake of fluorescently labeled synthetic Aβ1-42 by flow cytometry. Downregulation of Scara1 and CD36 caused a drop in Aβ uptake, whereas silencing of other scavenger receptors (Scarb1, Scarf, CD68, and CXCL16) had no effect, suggesting Scara1 and CD36 are the prime receptors involved in phagocytic clearance of soluble amyloid.
When Wilkinson and colleagues repeated the Aβ uptake assays using primary mouse monocytes and microglia from scavenger receptor knockout lines, only Scara1-deficient cells took up less Aβ. Phagocytes lacking other receptors—Lox1, RAGE, even CD36—internalized Aβ about as well as wild-type cells. This suggests CD36 may be redundant for clearing soluble amyloid. “There’s no doubt that CD36 binds Aβ,” El Khoury said. “However, in the absence of CD36, other receptors kick in, suggesting CD36 is important but not necessary for Aβ clearance.” Scara1, on the other hand, seems required for this process. Primary monocytes lacking the receptor took up only half as much Aβ as wild-type cells.
To confirm this in vivo, the researchers crossed AD transgenic mice (PS1/APP) with Scara1 knockouts and analyzed PS1/APP Scara1-/- and PS1/APP Scara1+/- progeny for Aβ levels and pathology. “The first thing we noticed was the Scara1 heterozygotes and Scara1 knockout AD mice started dying earlier,” El Khoury said. By 160 days of age, 39 percent of the PS1/APP Scara1+/- and 53 percent of the PS1/APP Scara1-/- mice had died, compared with just 16 percent of PS1/APP mice. In addition, eight-month-old Scara1-deficient AD mice had more brain amyloid, as measured by immunohistochemistry and ELISA—even the Scara1 heterozygotes.
“That caught my attention,” said Terrence Town of the University of Southern California, Los Angeles, who was not involved in the study. “If you only need a 50 percent genetic reduction to see a profound effect on plaque load, that suggests Scara1 may only need to be pharmacologically upregulated 25-50 percent to get a therapeutically beneficial effect. This is important from a translational standpoint.” Typically, though, it is easier to identify small molecules that inhibit, rather than promote the activity of, an enzyme.
As a therapeutic proof of principle, co-lead author Dan Frenkel of Tel Aviv University, Israel, exposed N9 mouse microglial cells to an adjuvant (Protollin) that tripled Scara1 transcript levels but left CD36 unchanged. When incubated with PS1/APP brain slices, the adjuvant-treated cells reduced Aβ deposition compared with sections exposed to none or to unstimulated microglia. When the researchers incubated PS1/APP brain slices with Scara1-deficient microglia, amyloid clearance remained poor, even with Protollin, suggesting the adjuvant promotes clearance primarily through Scara1. Previously, Frenkel and coauthor Howard Weiner of Brigham and Women’s Hospital, Boston, reported that Protollin enhances microglial clearance of Aβ in another AD transgenic strain (J20) (Frenkel et al., 2005; Frenkel et al., 2008), and others have safely used the adjuvant in human vaccines against bacterial pathogens (Fries et al., 2001).
The current findings follow a wave of recent work that implicates sluggish Aβ clearance in sporadic AD (Mawuenyega et al., 2010) and highlights the importance of innate immune molecules, said Michael Heneka of the German Center for Neurodegenerative Diseases in Bonn. Last year, two groups found rare mutations in the microglial receptor TREM2 that triple a person’s risk for AD (ARF related news story), and just months ago, researchers reported that AD risk variants that reduce expression of CD33—a transmembrane protein on myeloid cells—curb Aβ uptake and pathology in AD mice and patients (Griciuc et al., 2013; Bradshaw et al., 2013). Furthermore, a recent systems analysis of AD brain showed strong disturbances in molecular networks related to immunity and microglia. The human form of Scara1 (MSR1) showed up in the network that includes the microglial protein TYROBP, which binds TREM2 and appears to regulate other innate immune proteins including CD33 (Zhang et al., 2013; see also ARF Webinar). Heneka and others have detected neuroinflammation well in advance of plaque deposition in several AD mouse strains (Heneka et al., 2005;
Wright et al., 2013), suggesting that activated microglia play an important role early in the disease.
El Khoury and colleagues are checking Scara1 expression on macrophages and microglia from people with probable AD, and are working on molecular approaches to achieve similar effects as Protollin.—Esther Landhuis.
Frenkel D, Wilkinson K, Zhao L, Hickman SE, Means TK, Puckett L, Farfara D, Kingery ND, Weiner HL, El Khoury J. Scara1 deficiency impairs clearance of soluble amyloid-β by mononuclear phagocytes and accelerates Alzheimer’s-like disease progression. Nat Comm. 25 June 2013. Abstract