Do microglia promote the development of Alzheimer disease (AD) or hinder it? These immune cells are known to massively infiltrate amyloid plaques, lured by the presence of Aβ. However, in the process, microglia become activated, and there is continuing debate about whether activated microglia serve a neuroprotective function in the diseased brain. Yesterday in Neuron, the laboratories of Jean-Pierre Julien and Serge Rivest at Laval University in Quebec, Canada, entered this debate with a study that attempts to distinguish between resident and migratory, blood-derived microglia. First author Alain Simard and his colleagues report that bone marrow-derived microglia can reduce the number and size of amyloid deposits in an APPSwe/PS1 transgenic mouse model of AD. They show that this set of microglia actually infiltrate the core of amyloid plaques and phagocytose β-amyloid, and suggest these cells may be key in curbing the progression of AD.
The researchers transplanted green fluorescent protein (GFP)-expressing bone marrow cells into the bloodstream of irradiated APPSwe/PS1 transgenic mice. Then they immunohistochemically visualized an age-dependent increase in the number of plaque-associated GFP-positive microglia that peaked at 6 months of age, then slightly decreased around 9 months. Meanwhile, immunohistochemical staining of brain sections from treated animals revealed a consistent age-associated increase in the number and size of plaques. Based on these data, the authors argue that blood-derived microglia infiltrate the brain and migrate toward amyloid plaques after they have already formed and reached a particular size.
But how do these microglial cells get recruited and activated? To address this question, Simard and colleagues transplanted GFP-expressing bone marrow cells into irradiated wild-type mice and injected different isoforms of Aβ into their hippocampi. Upon examination of brain sections, they found that both Aβ40 and Aβ42 were able to stimulate the infiltration of GFP+ cells, suggesting that both proteins are behaving as chemoattractants for the bone marrow-derived microglia. Neither the control peptides Aβ31 and Aβ57, nor saline injections elicited this effect. Further investigation of the activation of microglia in response to Aβ42 in both wild-type and transgenic mice demonstrated the induction of an innate immune response without the inflammatory molecule TNF-α. The authors take this to mean that both exogenous and endogenous Aβ42 are capable of triggering similar, yet highly specific and atypical immune responses.
The dramatic infiltration of these blood-derived microglia around the age of 6 months and slight decrease at 9 months, when the number and size of plaques are still increasing, begs the question of whether these cells hinder or exacerbate senile plaque formation. To take a closer look, the researchers crossed the APPSwe/PS1 transgenic mice with ones that express mutant thymidine kinase (TK) and treated these animals with the antiviral drug ganciclovir. This drug is known to suppress bone marrow production of white blood cells, and thus, the treatment inhibited the recruitment of newly differentiated bone marrow-derived microglia, but not activation of resident microglia. Tissue analysis showed that ganciclovir treatment resulted in an increase in the number and size of plaques, as compared to matched saline-injected animals, indicating that the bone marrow-derived microglia restricted plaque size and number. Furthermore, ganciclovir treatment appeared to attenuate the immune response associated with the presence of amyloid plaques, a result the authors attribute to the prevention of infiltration of blood-derived microglia.
Finally, in analyzing stained tissues used in these studies, Simard and colleagues observed Aβ42 staining inside subcellular compartments within the GFP-positive, bone marrow-derived microglia, which colocalized with the lysosomal marker LAMP-2. From these data, the researchers assume that the blood-derived microglia were attempting to clear the amyloid deposits via phagocytosis in vivo. To confirm this observation, they treated cultured BV2 microglial cells with fluorescent red (Cy3)- conjugated Aβ42, and saw that the Aβ42 localized within the microglia, in line with their in vivo data.
In the end, the authors emphasize that previous studies have not distinguished between blood-derived and resident microglia, but have tended to fault microglia generally for contributing to plaque formation. Here the authors instead propose that resident microglia are present early on and perhaps play a role in plaque formation, while blood-derived ones appear later in an attempt to restrict the number and growth of plaques and/or clear them via phagocytosis. “The fact that newly recruited microglia are more efficient immune cells compared to their resident counterparts is clearly a beneficial mechanism in restricting disease progression,” they write. It is worth noting that some researchers interested in this topic raise a technical caveat about studies using irradiation, because that procedure is thought to temporarily weaken the blood-brain barrier. This would allow blood-borne cells to enter the brain in numbers that may not reflect the situation in human AD brain.—Erene Mina