A 3D Look at Microglia in the Developing Brain
Neurobiology
Dr. Rosa C. Paolicelli and colleagues, European Molecular Biology Laboratory, University of Rome, and the University of Turin
When the brain is injured, specialized phagocytic microglial
cells engulf and remove cellular debris. However, the role of
these cells in uninjured brains is not clear. Researchers led by
Dr. Cornelius Gross from the European Molecular Biology
Laboratory (EMBL) in Italy studied microglia in uninjured
developing mouse brains to find out if these cells help monitor
and maintain synapses.
“Studies on microglial function so far were mainly performed in
pathological conditions. The idea that microglia can also exert
an active role in the intact developing brain is a relatively new
concept,” said Dr. Rosa Paolicelli, who was part of the
research team. A better understanding of microglia-mediated
synaptic pruning might reveal more about how the brain and
immune system interact and how neuronal connections form in
the brain.
With confocal, simulated emission depletion (STED), and
electron microscopy the researchers observed microglia in
developing mouse brains. They used GFP to label microglial
processes and immunohistochemistry was used to mark the
presence of PSD95, a protein found at the post-synaptic
density of dendritic spines. The confocal images showed that
some PSD95 puncta colocalized with GFP. This observation
Figure 1: An Imaris surface-rendered image of GFP-labeled microglia
(green) surrounded by nuclei of other cells in the mouse hippocampus
(blue).
was confirmed with STED microscopy, which resolved PSD95 puncta as small as ~ 80 nm.
The researchers acquired serial optical sections (0.4 m Z-step
size) of hippocampal stratum radiatum with confocal
microscopy and then used Imaris software to create 3-D
reconstructions. The reconstructions showed that, in some
cases, the PSD95 puncta that colocalized with GFP were
surrounded by GFP-labeled microglial cytoplasm. Dr. Paolicelli
said that the 3-D reconstruction of microglial cell processes
containing synaptic material helped support their idea that the
microglia engulfed dendritic spines and provided information
that was complementary to the 2-D colocalization analysis.
Figure 2: Surface rendering of GFP presence located exclusively in the
microglia in the hippocampal CA1 section
Collectively, the microscopy images taken with the three
techniques showed that microglia engulf and eliminate
synapses during development. The researchers also found that
the developing brains of mice without CX3CR1 gene encoding
a chemokine receptor expressed by microglia in the brain –
had less microglia and delayed synaptic pruning, which
resulted in extra dendritic spines and immature synapses. In
other words, microglia are critical for setting up the right
connectivity in the brain. “These findings shed light on a new
important role for microglia in synaptic pruning, refining neural
connections and circuit maturation,” Dr. Paolicelli said.
The researchers say that genetic variation in expression of
CX3CR1 together with environmental pathogens that impact
microglia function might lead to an increased susceptibility to
developmental disorders such as autism that are associated
with alterations in the number of synapses in the brain. The
researchers plan to investigate microglia in the healthy adult
brain, where their role is essentially unknown.
Research Paper: Synaptic Pruning by Microglia Is Necessary
for Normal Brain Development, Science, DOI:
10.1126/science.1202529.
Figure 3: The terminal processes of microglia (green) contain synaptic
material (red), showing in this surface-rendered image that the
microglia engulfed the synapses. The red particles are positive for
PSD95, a protein located at the post-synaptic density of dendritic
spines.