Cx3cr1-deficiency shifts behavior and microglia characteristics in the motor cortex towards stress vulnerability in a sex-dependent manner

Abstract

Prolonged exposure to stress has been demonstrated to contribute to the onset of various diseases, notably major depressive disorder. This mental health condition is among the most prevalent, yet current therapeutic strategies have proven ineffective in preventing relapses (Vinogradov et al., 2026). Although women have a higher prevalence of major depressive disorder, preclinical research predominantly focuses on males. Motor symptoms, which are often associated with mood disorders, remain largely unexplored in rodent studies. The CX3CL1-CX3CR1 signaling pathway has been associated with neuroplasticity, microglial reactivity, and neuroinflammatory regulation from development into adulthood. The loss of Cx3cr1 has been linked to cognitive, learning, and motor impairments, particularly in relation to stress exposure. The aim of my research is to investigate how partial and complete Cx3cr1 deficiency shapes stress-like phenotypes at the behavioral, cellular, and molecular levels under non-challenging conditions, after chronic restraint stress, and following prior exercise training as a non-pharmacological preventive approach, focusing on the motor cortex. Under physiological conditions, females lacking Cx3cr1 displayed increased vulnerability to stress-like behaviors, including reduced self-care, heightened anxiety-like responses, and decreased exploratory activity. Meanwhile, males exhibited results comparable to those of wild-type controls. The microglial responses of Cx3cr1-deficient mice appeared to be maladaptive, limiting their ability to appropriately respond to aversive or positive environmental cues, such as stress or exercise. In this paradigm, exercise performed before stress exposure did not mitigate the effects of stress, though long-term running in the absence of stress has been reported to be beneficial. This suggests that the stressor may override the effects of training after a period of cessation. Molecular analyses identified potential compensatory mechanisms involving TGF-β signaling that may support microglial homeostasis and that may be altered in females. The role of CX3CL1-CX3CR1 signaling was dependent on sex and region-specific. Motor cortex outcomes differed from those described in hippocampal studies. Genotype effects persisted over time, whereas stress effects were transient. Although training did not significantly alter behavior, the dynamics of microglia in previously stressed mice remained closer to baseline. Overall, these findings underscore the importance of considering the physiological baseline of Cx3cr1 deficiency when interpreting neuroprotective or neurotoxic outcomes. The apparent resilience of Cx3cr1-deficient mice may reflect an altered baseline state and maladaptive responses rather than true protection. This could potentially mask deficits in other domains, such as motor function.