Molecular profiling of microglia in steady state and under inflammatory conditions. (Doctoral thesis)

Microglia are specialized parenchymal-resident phagocytes of the central nervous system (CNS) that actively support, defend and modulate the neural environment. As the first sentinels protecting against invading pathogens and tissue damage, microglia act as central communicators between the nervous and the immune systems. Although much is known about the response of microglia towards specific pro- or anti-inflammatory cues, their phenotypic and functional behavior under threatening signals in the CNS, such as inflammation and neurodegeneration, are only starting to emerge. Many CNS diseases are believed to be strengthened by dysfunctional microglia responses, therefore discriminating microglia from other myeloid cells is fundamental for understanding their specificity in brain development, function and disease. Difficulties in addressing microglia functions were so far mostly associated to limitations in isolation protocols and lack of techniques or specific markers to specifically target these cells. Genome-wide profiling of acutely isolated cell populations have opened up new opportunities to overcome such limitations. Here, using a combination of multicolor flow cytometry, single-cell mRNA sequencing and immunohistochemical analyses, we comprehensively profiled microglia under physiological and inflammatory conditions.
Our results showed that the microglia homeostatic signature is lost under inflammation. Importantly, we provide evidences that this is an intrinsic feature of microglia under inflammatory conditions. Notably, we identified unforeseen heterogeneity in microglia states of activation. In addition, we showed that microglial inflammatory signatures are distinct from neurodegenerative disease-associated profiles.
In parallel, we systematically analyzed the effects of DJ-1 deficiency on microglia transcriptional signature under steady state and upon systemic inflammatory conditions. DJ-1 acts as an intracellular redox sensor and several evidences suggest functional roles for DJ-1 in cell homeostasis. Here, and for the first time, we report an unforeseen link between DJ-1, cytoskeleton dynamics, and microglia polarization.
We believe that the identification and characterization of specific microglia signatures may pave the way to design new therapeutic approaches to restore detrimental microglia phenotypes found in several brain disorders, such as neurodegenerative diseases and tumors.
Lastly, in a side project, we investigated the link between itaconate production and succinate accumulation in macrophages under inflammatory conditions. We showed that Irg1 expression and itaconate levels markedly correlate with succinate amounts and that itaconate acts as an endogenous succinate dehydrogenase inhibitor resulting in succinate accumulation. The emerging role of itaconate as a regulatory molecule to reprogram immune cell metabolism provides an intriguing link between innate immunity, metabolism, and disease pathogenesis.