Adult-Onset Leukoencephalopathy with Axonal Spheroids and Pigmented Glia (ALSP) is a rare autosomal dominant neurodegenerative white matter disease. Pathologically, this disease is characterized by the presence of cerebral white matter lesions, which induce early thinning of the corpus collosum followed by cortical atrophy in affected regions. Genome-wide linkage analyses (GWLAs) have discovered a genetic origin of disease with the identification of multiple mutations that affect CSF1R in families with ALSP. In the adult brain, CSF1R is primarily expressed by microglia. However, the cellular biology underlying detrimental ALSP phenotypes induced by mutations in Csf1r (e.g. cellular source(s) of pathology) remains elusive. Moreover, the expression of CSF1R in neurons during development has been noted in mice, and thus it is critical to delineate microglial versus non-microglial, as well as the developmental versus non-developmental contributions to disease onset. As recent studies into myeloid biology have clarified that microglia are not merely sentinel cells, but rather active participants in the maintenance of the central nervous system at large, it is important to understand the extent to which dysfunctional microglia may be able to affect central nervous system health. Moreover, microgliosis is common to many neurogenerative diseases and central nervous system injuries; however, it remains unclear whether microglia play a causative role in neurodegeneration or a reactive role by responding to the presence of pathology/injury.Given these outstanding gaps in our knowledge of microglia cell biology, the primary goal of my thesis was to identify the role of microglial CSF1R haploinsufficiency in mediating pathogenesis. Using an inducible Cx3cr1CreERT2/+-Csf1r+/fl system, we found that postdevelopmental, microglia-specific Csf1r haploinsufficiency resulted in reduced expression of the homeostatic microglial markers P2RY12 and TMEM119. This was associated with loss of presynaptic surrogates and perineuronal nets. Similar phenotypes were observed in constitutive global Csf1r haploinsufficient mice and could be reversed/prevented by elimination of microglia in adulthood. As microglial elimination is unlikely to be clinically feasible for extended durations, we treated adult CSF1R+/- mice with a microglia-modulating dose of the CSF1R inhibitor PLX5622, which prevented microglial dyshomeostasis along with synaptic- and PNN-related deficits. Intriguingly, treatment of wild-type microglia with the microglia-modulating dose of PLX5622 similarly reduced expression of canonical microglia homeostatic marker expression. However, aside from reductions in presynaptic markers, behavioral and ECM outputs were unchanged. These data thus highlight microglial dyshomeostasis as a driver of pathogenesis and show that CSF1R inhibition can mitigate these phenotypes.
As proper diagnosis for ALSP tends to occur once pathology has set in, we further aimed to verify whether CSF1R inhibitor treatment could be used during late stage disease timepoints as a viable treatment paradigm. To do so we extended the time of observation to 14 months at which point mice were given CSF1R inhibitor for the next two months. Here we observed losses to cognitive functions as assessed by Novel Object and Novel Place tasks in CSF1R+/- mice, however, treatment with CSF1R inhibitor had marginal affects on the recovery of performance. However, deviations from microglial homeostasis, with respect to lowered P2RY12, and heightened CD68 and Lamp1 expression, were still attenuated with CSF1R inhibitor treatment. Importantly, corpus callosum damage was attenuated with CSF1R inhibitor treatment. This was seen with increased MBP and SMI-31 immunostaining as well as decreased levels of Nf-L, a marker for axonal damage. Unexpectedly, wild-type mice that were treated with CSF1R inhibitor treatment showed striking parallels to CSF1R+/- mice when comparing levels of corpus callosum damage, a feature that was absent when comparing younger cohorts, suggesting an environmental influence of aging that compounds the effect of CSF1R inhibitor treatment. Given that many CSF1R inhibitors are currently going towards phase 3 clinical trials, it is highly relevant and crucial to determine the consequences of altered CSF1R signaling on CNS homeostasis. Collectively the results from the studies throughout this thesis highlight the extent to which microglia dyshomeostasis can affect central nervous system health and provide crucial insight into the impact of globally altering CSF1R signaling on disrupting central nervous system homeostasis.
