[Paleopsych] Science: Brain Under Surveillance: The Microglia Patrol

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Brain Under Surveillance: The Microglia Patrol
http://www.sciencemag.org/cgi/content/full/309/5733/392
Science, Vol 309, Issue 5733, 392-393 , 15 July 2005
[DOI: 10.1126/science.1114852]
Luc Fetler and Sebastian Amigorena*

Biophysicists and biologists have long worked together to develop tools to 
analyze the limits of the living world--molecules and cells at one end and 
whole organisms at the other. In contrast, important intermediate levels of 
organization, namely the organs and tissues, had received little attention 
until very recently. For instance, we understand very little about what 
controls the density, shape, and size of organs, or how cells direct their 
movements or communicate with each other within tissues. This has been due in 
part to technical limitations. The environment that cells encounter within 
tissues can include multiple three-dimensional chemotactic gradients and 
numerous physical constraints imposed by interactions with the extracellular 
matrix and with other cells. Such a complex milieu is virtually impossible to 
reconstitute in vitro.

The advent of two-photon microscopy (1) and its use on tissues of living 
animals is rapidly advancing our understanding of cell behavior and fate within 
tissues (2, 3). Based on the simultaneous absorption of two photons, the 
technique allows greater imaging depth and minimal phototoxicity compared to 
conventional fluorescence microscopy. Whereas other imaging techniques require 
surgical dissection that can damage tissue, two-photon microscopy allows direct 
imaging of cells in the undisturbed physiological environment of an intact 
organ. Two recent studies by Nimmerjahn et al. (4) and Davalos et al. (5) used 
this powerful imaging approach to examine the activity of microglia, the most 
abundant immune cell in the brain, in live mice. Microglia comprise ~10% of the 
cells in the central nervous system. Under pathological conditions such as 
neurodegenerative disease, stroke, and tumor invasion, these cells become 
activated, surround damaged and dead cells, and clear cellular debris from the 
area, much like phagocytic macrophages of the immune system. In healthy 
mammalian brain tissue, microglia display characteristically elongated cell 
bodies with spine-like processes that often branch perpendicularly. Until 
Nimmerjahn and Davalos applied two-photon microscropy to a live and healthy 
mammalian brain, it was generally thought that microglia are essentially 
quiescent cells--dormant and nonmotile. But a static state is hardly what was 
observed.

The technique allowed the Nimmerjahn and Davalos groups to transcranially 
visualize microglia in live animals. Information was recorded from up to 200 m 
below the brain's surface through a surgically thinned section of skull. Both 
groups generated transgenic mice whose microglia were fluorescently labeled. 
The easily detectable cells were observed for several hours in the brains of 
anesthesized mice. Whereas microglial cell bodies and main branches were stable 
for hours, their evenly distributed and highly ramified processes were 
remarkably motile, continuously and randomly undergoing cycles of formation, 
extension, and withdrawal on time scales of minutes (1.5 m/min). These 
processes also displayed motile (4 m/min), filopodia-like protrusions that 
typically formed bulbous tips with an average lifetime of 4 min. Although the 
function of these tips remains unclear, it is possible that they constitute 
specialized phagocytosis domains that clear accumulated metabolic products and 
deteriorated tissue. This high "resting" motility may serve a housekeeping 
function, enabling microglia to effectively sample and assess the status of the 
local surroundings and control their microenvironment. The restructuring 
activity of microglial processes is in sharp contrast to the apparent stability 
of dendritic processes of surrounding neurons. Microglial processes and 
protrusions were also observed to directly contact astrocytes, neuronal cell 
bodies, and blood vessels, suggesting that in healthy brain tissue, microglia 
communicate with other cortical cells to coordinately monitor the general 
health of the brain.

Both groups also performed laser-induced injury of individual capillaries in 
the brains of the transgenic mice. Within a few minutes, time-lapse imaging 
revealed rapid, targeted movements of nearby microglial processes toward the 
injured site (see the figure). The average velocity of microglial extensions 
radially impinging on the target site was similar to extension rates during the 
resting state. Within 30 min after laser ablation, processes of nearby 
microglia reached the damaged site and appeared to accumulate and fuse 
together, forming a spherical containment around the damaged area and 
establishing a potential barrier between healthy and injured tissue (5). 
Microglia responded to mechanical injury in a similar way. The shielding of 
injured sites suggests a neuroprotective role for microglia. Furthermore, the 
early formation of spherical inclusions within the microglial processes 
suggests immediate phagocytic engulfment and removal of damaged tissue or 
leaked blood components. Together, these findings confirm the idea that 
microglia represent the first line of defense against invading pathogens or 
other types of brain tissue injury.

     Figure 1 Microglia patrol the brain and shield it from injury. Microglia 
continually extend (green) and retract (yellow) processes, surveying their 
immediate environment within the brain. The processes move rapidly toward a 
site of injury, such as a damaged blood vessel in the brain, in response to the 
localized release of a chemoattractant (gradient of orange) from the injured 
sited. Once at the target site, the processes form a barrier to protect healthy 
tissue.

     CREDIT: PRESTON HUEY/SCIENCE

To identify the molecular signals that mediate this targeted microglial 
response, Davalos et al. (5) made use of the observation that microglial 
migration can be induced in cell culture with nucleotides that signal through 
P2Y receptors expressed at the cell surface. They demonstrate that localized 
application of ATP to the mouse brain (through either a craniotomy or a small 
electrode; neither invasive technique itself elicited a response from 
microglia), which mimics nucleotide release from injured tissue, attracted 
microglial processes, similar to the microglial response to injury. Apyrase, an 
ATPase (adenosine triphosphatase) that hydrolyzes ATP and ADP, substantially 
reduced both the baseline motility of microglial processes as well as their 
response to laser-induced tissue injury. Furthermore, they showed that 
activation of P2Y receptors on microglia in the surrounding tissue is necessary 
for the rapid microglial response toward the injured site. Previous studies 
showed that extracellular ATP can induce ATP release from astrocytes. ATP also 
mediates communication between astrocytes and between astrocytes and microglia. 
This ATP-induced ATP release was essential for attracting microglial processes. 
Indeed, when the authors applied apyrase and then a nonhydrolyzable ATP analog 
from a microelectrode, they observed no such rapid microglial response. 
Applying connexin channel inhibitors, which inhibit ATP release from 
astrocytes, also blocked the microglial response toward the laser ablation 
site. Resting motility of microglial processes in the intact brain also seems 
to be modulated by the same ATP signaling mechanisms that mediate 
injury-induced responses, because apyrase and connexin channel inhibitors 
nearly abolished microglial baseline dynamics.

These two elegant studies provide direct evidence for the highly dynamic nature 
of microglia, indicating that the brain is under constant immune surveillance 
by these cells. In the adult mammalian brain, there is generally little 
movement of cellular processes, except perhaps for those associated with 
synaptic plasticity that underlie learning and memory. Microglia are apparently 
never at physical rest either.

Although the development of two-photon microscopy opens new perspectives for 
the analysis of intact organs, some major technical issues remain. Improvements 
in the resolution, depth penetration, image acquisition speed, and photon 
detector sensitivity of the microscopes will enhance our ability to follow 
intracellular signaling events and cellular traffic in living tissues. 
Likewise, the generation of mice expressing fluorescent proteins under the 
control of different cell type-specific promoters or inducible promoters should 
allow the study of multiple cell functions within intact organs using 
two-photon microscopy. Accurate methods to quantify image information are also 
needed. Despite these obstacles, the development of intact organ imaging should 
continue to have a major impact in biology over the coming years.

References

    1. W. R. Zipfel, R. M. Williams, W. W. Webb, Nat. Biotechnol. 21, 1369 
(2003). [Medline]
    2. J. W. Wang, A. M. Wong, J. Flores, L. B. Vosshall, R. Axel, Cell 112, 271 
(2003). [Medline]
    3. B. J. Bacskai et al., Nat.Med. 7, 369 (2001). [Medline]
    4. A. Nimmerjahn, F. Kirchhoff, F. Helmchen, Science 308, [1314] (2005).
    5. D. Davalos et al., Nat. Neurosci. 8, 752 (2005). [Medline]

10.1126/science.1114852
The authors are at the Curie Institute, 26 rue d'Ulm, 75245 Paris Cedex 05, 
France. L. Fetler is in the Laboratoire Physico-Chimie Curie, CNRS UMR 168, 
Institut Curie, Paris, France. E-mail: luc.fetler at curie.fr S. Amigorena is in 
the Immunité et Cancer, INSERM U365, Institut Curie, Paris, France. E-mail: 
sebastian.amigorena at curie.fr

10.1126/science.1114852
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