20 April 2008. Microglia are rather cozy with amyloid plaques, but how they snuggle up to these protein deposits and what ensues thereafter is not well understood. In the April 16 Journal of Neuroscience, researchers led by Michael Calhoun at the University of Tuebingen, Germany, give a vivid account of the microglia/plaque relationship. Their time-delay images show that dynamic microglia play an active role in seeking out plaques and engulfing Aβ. The cells’ prowess is not without limitation, however. “One surprising thing we found was there is really a recruitment issue, in that regardless of plaque size the number of microglia that surround individual plaques reach an upper limit,” said Calhoun in an interview with ARF. Understanding and overcoming that limitation is one idea worth thinking about in the context of therapy, he added.
The glia, in fact, may already be overcoming that limitation themselves—by beefing up. First author Tristan Bolmont and colleagues found that individual microglia can increase in size when they contact plaques, and that larger plaques consorted with larger microglia. The findings are based on multiphoton microscopy of mouse brain taken at intervals through a cranial window. The researchers used three different fluorescent markers to visualize microglia (expressing green fluorescent protein driven by the Iba-1 microglial promoter), amyloid (labeled with methoxy –X04, a Congo red derivative), and blood vessels (labeled with Texas red dextran). By using software to keep track of major landmarks in the field of view, including the blood vessels, the researchers were able to come back time and again to the same plaques to record microglial activity (see figure below).
With the ability to re-image the same location in the brain, the researchers have been able to begin to quantify the glia/plaque relationship. They found that the volume of small- and medium-sized plaques increases over a month in three-month-old transgenic animals (APPPS+ mice), while the volume of large plaques actually decreases. The number of microglia surrounding the plaques increases over that same time interval, but as Calhoun noted, there is a limit to the density of the glia surrounding plaques.
The researchers found that the microglia are incredibly dynamic, morphing in both position and size (see ARF related news story). In APPPS+ mice, for example, about half of the microglia that lie close to plaques migrate so that their cell bodies are in immediate contact with the deposits. This migration typically took one to two days, as seen in the figure below.
Microglia on the Move
A microglia (green) making contact with an amyloid plaque (yellow) via long cellular processes moves in to cover the protein deposit over the course of two days. Blood vessels are seen as red fluorescence. Image credit: Michael Calhoun
The data indicate that resting microglia can respond to their environment. The glia seem to first become polarized, extending processes in the direction of the plaque, then, on finally reaching it, they become enlarged. That process plays out in the movie below.
Polarized processes aid movement of resident microglia around amyloid
This movie shows the dynamic nature of microglia, which extend and retract processes to the amyloid plaque (center) as if sampling the plaque environment. Movie credit: Michael Calhoun
The findings help address one of the important and oft-debated questions about plaque microglia, namely, where do they come from? There are indications that some of them might come from the blood (see ARF related news story), and some through division of existing cells. “We were able to show that some of them actually transform from normal resting microglia into a more macrophagic type of microglia that surrounds plaque,” said Calhoun. He added that figuring out percentages of where all the cells come from needs more quantitative study. “But I think it is interesting to know that you can transform these resting microglia,” he said.
What does this all mean for plaque removal? The findings show a dynamic response to plaques. Microglia both actively migrated to plaques—presumably to help control plaque growth since the researchers found that the glia actively engulfed Aβ—and grew in size. Microglia surrounding medium-sized plaques were some 35 percent bigger than those associated with small plaques, for example, while microglia on large plaques were more than twice the size of those on medium plaques. Interestingly, the number of adjacent glia is also indicative of whether plaques will increase or decrease in size; more glia means either a smaller increase or a decrease. “This is both predictive and correlative, so from microglia coverage we can determine the likelihood of what the plaque will do,” said Calhoun.
However, despite the microglia’s rallying around plaques, the researchers never saw complete plaque removal. That is consistent with the observation that when Aβ production is halted in transgenic mice, plaques do not disappear (see ARF related news story). “That makes me wonder if there is homeostasis, or if the microglia at some point need an additional stimulus to get at the plaques,” said Calhoun. Brad Hyman, Massachusetts General Hospital, Boston, agrees. “I think this [study] is really intriguing and agrees quite well with our hypothesis that microglia respond to plaques but are not able to ingest them under ordinary circumstances,” he e-mailed ARF. Hyman recently used similar imaging techniques to show that amyloid plaques pop up quite rapidly in the brain (see ARF related news story), a phenomenon that Bolmont and colleagues also recorded.
Calhoun said it is not clear how one might coax microglia to completely remove plaques and take their contents away into the blood, but he thinks that some microglia may be capable or that some factors might persuade them. “We need to look at the heterogeneity in microglia and what molecular factors we can use to drive that process,” he said.—Tom Fagan.
Bolmont T, Haiss F, Eicke D, Radde R, Mathis CA, Klunk WE, Kohsaka S, Jucker M, Calhoun ME. Dynamics of the microglial/amyloid interaction indicate a role in plaque maintenance. J. Neurosci. 2008 April 16;28:4283-1292. Abstract