Inhibition of the Microglial Phagocytic Receptor MerTK Underlies ELA-induced Changes in Synapses and Behavior in Male Mice.

Background: Early-life adversity (ELA) is a significant risk factor for emotional disorders like depression, likely by provoking changes in stress-related circuit development. We have previously shown that ELA increases the number of excitatory synapses onto corticotropin-releasing hormone (CRH)-expressing neurons in the paraventricular nucleus (PVN) by decreasing microglial synapse engulfment. Here, we hypothesize that ELA induces microglial dysfunction via inhibition of the microglial phagocytic receptor, MerTK, thus resulting in the observed changes in synapses and stress-related behavior.

Methods: To determine whether deleting MerTK in microglia phenocopies the effects of ELA, microglia-specific (m)MerTK-KO (CX3CR1-Cre::MerTK) mice were crossed with 'wild-type' (CX3CR1-Cre::MerTK) mice and their litters were reared in either a control or ELA (induced by limited bedding and nesting paradigm) environment, from postnatal days (P)2-10. Excitatory synapses in the PVN were assessed at P10, and adult offspring were tested in a behavioral battery to measure threat-response (known to be dependent on PVN-CRH+neurons) and anxiety-like behavior, followed by acute restraint stress to measure the neuroendocrine stress response.

Results: Following ELA at P10, excitatory, but not inhibitory, synapses in the PVN were increased in males, which was mimicked by mMerTK-KO in control males, but caused no further increase in ELA males. However, females already had higher numbers of excitatory synapses at baseline, and showed no further increase with ELA or mMerTK-KO. Remarkably, the pattern of threat-response behavior in males closely matched the excitatory synapses, with mMerTK-KO control males escaping more from the simulated predator threat in the looming-shadow threat task, similar to ELA males. Again, females did not show any significant changes due to ELA or mMerTK-KO in the threat-response, although they did show ELA-induced changes in anxiety-like behavior. ELA provoked a greater corticosterone response to acute stress in males, but not females, although females were again higher at baseline.

Conclusions: Our results demonstrate that ELA provokes increased excitatory synapses in the PVN, leading to an increased active response to threat in the looming-shadow test in males only. Deleting MerTK specifically from microglia recapitulates both the synaptic and behavioral effects in control males, but does not have an effect in ELA males or control females, suggesting that the MerTK pathway is already inhibited by ELA in males and less active in females at baseline. Our work is the first to elucidate the mechanisms underlying the male-biased microglial dysfunction caused by ELA, with promise for the development of better preventative and therapeutic strategies for at-risk children.