How the immune system helps build your brain

Have you gotten your flu shot yet? Staying healthy this flu season may do more than just save you a few days of work - it may help protect the development of babies’ brains.

Okay, well, that claim may be a bit of an exaggeration, but it’s not as far-fetched as you might think. Several studies have found that pregnant women who are hospitalized for infections, including the flu, have a slightly higher risk for giving birth to a child with a neurodevelopmental disorder like autism (Atladóttir 2010, Lee 2015). Of course, flus sometimes happen to even the most vigilant germophobes - so in order to help babies exposed to illness in the womb, scientists need to ask why immune systems impact the developing brain at all.

A microglia cell, in the mouse hippocampus. Credit: Simon Beggs, Wellcome Images

The answer will likely involve microglia, the brain’s resident immune cells, whose role in shaping the developing brain is just now being illuminated. A 2014 study by Arnó et. al in developing mice (Arnò 2014) suggests that building a brain may require interaction between microglia and neural progenitor cells (NPC) that will divide and become most of the cells in the brain. First, they observed that microglia migrate across a long distance in the embryo to nestle alongside NPCs in the developing brain. A certain subtype of NPC, called a basal progenitor, is known to secrete a protein called Cxcl12 that acts as a “come-hither” signal for other types of migrating cells in the developing cortex. Microglia express receptors for Cxcl12, so it is entirely feasible that they respond to that particular signal. The authors then asked whether NPCs - and, specifically, basal progenitors - were responsible for recruiting microglia to the cortex with Cxcl12. Last, they asked what microglia did when they reached the NPCs. Their answer suggests that NPCs choreograph microglia in the cortex so that microglia can help protect the precious NPCs and make sure they grow at the right rate to form a functional brain.

To confirm that Cxcl12 is important in recruiting microglia into the brain, the researchers attempted to dial the number of microglia in the embryonic brain up or down by changing NPC Cxcl12 expression. To increase the amount of Cxcl12 expressed by NPCs, researchers used a technique called in utero electroporation. For this technique, the researchers inject a small circle of Cxcl12-encoding DNA into the ventricle of fetal brains. Applying a small electrical current across the fetal brain stuffs the negatively-charged DNA into NPCs lining the ventricle, where they begin to express surplus Cxcl12. As predicted, forcing NPCs to produce more Cxcl12 with this technique led to a greater number of microglia two days later.

From this experiment alone, it was impossible to tell whether there were more microglia migrating in, or whether the microglia that would normally exist in the brain were just dividing more. Therefore, two days after the electroporation experiment, fetuses were injected with a special, manufactured nucleotide that integrates into DNA during the DNA replication step of the cell cycle. Thus, by imaging fluorescently-labeled antibodies that target this protein and the microglial marker Iba1+, they were able to determine how many microglia cells were actively dividing in brains with surplus Cxcl12. They failed to find a difference between the Cxcl12 surplus and unaffected conditions, suggesting that the observed increase in microglia was due to an increase in microglial recruitment into the brain, rather than mere proliferation.

Conversely, the authors looked at what would happen if they deafened microglia to the call of Cxcl12. To do this, they injected a drug that blocked receptors for Cxcl12, called AMD3100. Sure enough, there were significantly fewer microglia twenty hours after injection.

A further series of experiments verified that basal progenitors secreting Cxcl12 were important in recruiting microglia. To upregulate basal progenitor-derived Cxcl12, they specifically increased the number of basal progenitors in the brain.  First, they used in utero electroporation to introduce a construct that increased the formation of basal progenitors. Two different genetic strategies were applied for the converse experiment. In one experiment, they created a genetic knockout with far fewer basal progenitors than normal. In a separate experiment, they electroporated healthy mice with a construct that boosts the formation of non-basal progenitor NPCs. In all of these experiments, the effects of microglia were as they had predicted: more basal progenitors meant more microglia, and fewer basal progenitors meant fewer microglia.

The final issue this paper addresses is what microglia are doing in the brain - why do they need to be so close to the zone where the NPCs birth new neurons? This study gives two hints. First, they observe that microglia proliferate when NPCs die. To do this, they electroporated NPCs with the so-called “suicide gene”, Thymidine Kinase, that kills NPCs in the fetus once the mother has been fed the drug Ganciclovir. Surprisingly, this seemed to stimulate microglia in the brain to proliferate. The authors suggest that this is one potential mechanism by which the microglia protect the developing brain - if cells are dying, the bodyguard microglia know they need to bulk up and find the source of the disturbance to protect the delicate neural precursors. The next hint they give is to observe that there are fewer progenitor cells in mice that never develop microglia.  This finding is corroborated by other papers (Cunningham 2013).

Taken together, microglia and basal progenitors seem to work in a feedback loop: basal progenitors recruit microglia to the developing brain, where microglia can proliferate, remove any dying cells, and promote the presence of basal progenitors. The authors suggest that this is one potential mechanism for keeping the brain’s development in check in case of injury. Understanding the steps in this dance may prove important in figuring out how to help ensure the best outcomes for children of mothers who fall ill during pregnancy. Until those answers arrive, don’t forget to take advantage of the best-vetted advice science has to offer: wash your hands with soap, sneeze into your elbow, and don’t forget that flu shot!

Sources:

Arnò, Benedetta, et al. "Neural progenitor cells orchestrate microglia migration and positioning into the developing cortex." Nature communications 5 (2014).

Atladóttir, Hjördis O., et al. "Maternal infection requiring hospitalization during pregnancy and autism spectrum disorders." Journal of autism and developmental disorders 40.12 (2010): 1423-1430.

Cunningham, Christopher L., Verónica Martínez-Cerdeño, and Stephen C. Noctor. "Microglia regulate the number of neural precursor cells in the developing cerebral cortex." The Journal of Neuroscience 33.10 (2013): 4216-4233.

Lee, Brian K., et al. "Maternal hospitalization with infection during pregnancy and risk of autism spectrum disorders." Brain, behavior, and immunity 44 (2015): 100-105.