Study reveals sex-specific differences in microglial response to CSF1R inhibition

By Dr. Priyom Bose, Ph.D.Reviewed by Benedette Cuffari, M.Sc.Jan 24 2025

Research shows male microglia are more sensitive to disrupted CSF1R signaling, uncovering potential links to sex-based disparities in neurological disorders.

Study: The microglial response to inhibition of Colonystimulating-factor-1 receptor by PLX3397 differs by sex in adult mice. Image Credit: ART-ur / Shutterstock.com

A recent Cell Reports study investigates the sex-dependent microglial response to pexidartinib (PLX3397), a colony-stimulating factor 1 receptor (CSF1R) inhibitor.

The role of CSF1R in microglia formation

Microglia, the immune cells of the central nervous system (CNS) that respond to infection, injury, and other stimuli, are crucial for CNS development and plasticity due to their dynamic interactions with other cells. During development, microglia arise from uncommitted embryonic myeloid progenitors and subsequently migrate to colonize in the CNS, even before the formation of the blood-brain barrier (BBB).

CSF1R signaling is essential for the migration of microglia to the CNS during early development, as it ensures a stable microglial population throughout the animal’s lifespan. Upon migration to the CNS, microglia expand and mature sequentially until adulthood with minimal exchange with circulating monocytes.

Postnatal maturation of microglia is dependent on transforming growth factor β (TGF-β) signaling. Comparatively, adult homeostatic microglia are dependent on characteristic transcriptomic signatures including microglia-enriched genes like P2ry12, Tmem119, and Sall1.

To investigate the role of CSF1R in the adult mouse CNS, several studies have used small-molecule CSF1R inhibitors PLX3397 and PLX5622 to deplete microglia. Although the depletion rate of microglia significantly varies with dose and duration of administration, both inhibitors fail to completely remove microglia.

In addition to their limited efficacy, partial resistance to CSF1R inhibitor treatment has also been observed, thus necessitating additional research to clarify how certain individual characteristics, such as sex, may impact the response to CSF1R inhibitors.

Is differential microglial response to CSF1R inhibitor sex-specific?

The current study explored sex-dependent differences in the microglial response to CSF1R inhibition with PLX3397 (PLX). Microglia density and soma size in male and female mice in the absence of PLX treatment were recorded at baseline.

The cortex, followed by the hippocampus, exhibited higher microglia density as compared to the midbrain and thalamus. Although comparable microglia density was observed between sexes, the size of soma in the hippocampus was much smaller in females as compared to males. However, upregulation of the microglial mammalian target of rapamycin (mTOR) pathway was observed in males only at baseline.

Microglial responsiveness to CSF1R inhibition was determined by measuring the number of remaining immunolabeled microglia in different brain regions of male and female mice after PLX treatment. For this experiment, adult mice were fed a diet containing the CSF1R inhibitor PLX for two weeks, following which the number of remaining microglia was compared to males and females fed a control diet. At this time point, all brain regions in male mice exhibited greater microglial depletion than females.

In both male and female mice, the remaining microglia after PLX treatment exhibited reactive morphologies with decreased ramification. A higher PLX dose of 400mg/kg led to greater microglia depletion in both male and female mice, with almost complete ablation of microglia observed in male brains, as compared to the standard dose of 290 mg/kg.

Fluorescence-activated cell sorting (FACS) was used to characterize surviving microglia after PLX treatment. PLX treatment significantly reduced the expression of microglial homeostatic proteins P2RY12 and transmembrane protein 119 (TMEM119), whereas the fluorescence intensity of CD68 increased. This finding suggests differential expression in homeostatic and phagocytotic markers as compared to control levels.

Microglia in both male and female mice were found to downregulate homeostatic and upregulate immunoreactivity signatures under CSF1R inhibition. Gene ontology analysis indicated that the upregulation of immune activity was associated with cytokine-mediated signaling.

Galectin-3+ upregulation was observed in a proportion of surviving microglia, thus demonstrating that the galectin-3-expressing subpopulation of microglia can survive CSF1R signaling inhibition. These experiments also revealed that PLX induces mitogenesis to a greater degree in males than females.

Galectin-3+ microglia in both sexes exhibited high mitochondrial content, even after PLX treatment. The loss of CX3CR1, and not TREM2, was associated with higher susceptibility to PLX-induced microglia depletion in female mice.

Conclusions

Microglia in male mice are significantly more sensitive to disrupted CSF1R signaling than microglia in female mice. After PLX treatment, the remaining microglia upregulated autophagy and proteostasis pathways in females, whereas this treatment led to increased expression of electron transport chain complexes and mitobiogenesis in male mice.

Taken together, the current study provides evidence for potential sex-specific differences in microglial response to PLX. In the future, additional studies are needed to elucidate whether differential survival mechanisms between male and female microglia contribute to sex-related disparities in the prevalence of many neurological disorders.