Dysfunctional Microglia Identified in Friedreich Ataxia, Providing Potential Target to Halt Neurodegeneration

Nadia D’Ambrosi, MD

A recently published preclinical study assessing a murine model of Friedreich ataxia (FA) described the involvement of microglia, the brain’s immune cells, and their role in the pathogenesis of the disease. All told, the research was the first to provide multilater evidence—using phenomics, transcriptomics, and metabolic analysis—that FA microglia are dysfunctional, suggesting a contribution of non-cell autonomous mechanisms in FA etiology.¹

Senior author Nadia D’Ambrosi, MD, researcher at the Department of Biology, University of Rome Tor Vergata, and colleagues characterized microglia derived from a knock-in knock-out (KIKO) mouse model of FA and analyzed whether they contribute neuron demise in the disease. Primary microglia were isolated specifically from the cerebellum, the most affected central nervous system (CNS) area in FA.

When compared with wild-type (WT) mice, investigators observed that KIKO microglia displayed changes in cell shape and complexity, with decreased circularity and increased perimeter and Feret’s diameter. Between these 2 models, KIKO microglia displayed significantly lower spontaneous migration than WT cells, as well as increased number of phagocytic cells and phagocytic activity per cell. Prior to these comparisons, it was confirmed that the frataxin mRNA was down-regulated in KIKO cells, and that frataxin loss did not affect the overall number of microglia cells obtained from the cerebella of KIKO mice.

"Migration and phagocytosis are partially rescued by the iron chelator deferiprone in KIKO cells, suggesting that disturbed iron regulation, resulting from frataxin deficiency, could contribute to the observed changes in microglial phenotypes,” the study authors wrote. “Overall, these results demonstrate that primary microglia derived from the cerebellum of WT and KIKO mice are morphologically and functionally different, with the KIKO cells less motile but more phagocyting than the WT counterpart."

A transcriptomics analysis found 184 genes that were significantly (P <.001) differentially expressed between WT and KIKO microglia. Of these, 87 were down-regulated and 97 were up-regulated. All told, a volcano polo revealed that the top 10 differentially expressed genes by P-value encompass genes were involved in mitochondria oxidative phosphorylation (Ndufb6, ND2, ND4, and Atp5b), inflammation (Socs, Csf2ra), and proteostasis (Rps2, Hsp90aa1, and Fbxw7).

KIKO microglia exhibited altered morphology, proinflammatory features, and dysfunctional microchondria, as indicated by reactome pathway and gene ontology enrichment analyses. Here, differentially expressed genes between the 2 models were related to categories as “cellular response to cytokine stimulus," "signaling by 53 interleukins," ":cytokine signaling,” “immune system,” “mitochondrial membrane,” “mitochondria 54 respiratory chain complex I,” “aerobic electron transport chain,” “NADH oxidoreductase activity,” and "positive regulation of cellular component organization."

Between WT and KIKO microglia, oxygen consumption rates revealed defects in mitochrondrial respiration. Although basal respiration and ATP production ere not significantly affected in KIKO microglia, maximal respiration and spare respiratory capacity were decreased. Notably, findings revealed that KIKO microglia up-regulated overall glycolytic function, by increasing glycolysis and glycolytic capacity.

READ MORE: Vatiquinone Treatment Shows Benefit Across End Points for Friedreich Ataxia in Phase 3 MOVE-FA Trial

The study authors noted that, "Overall, these results suggest that the lack of frataxin impairs mitochondria functionality, with decreased oxygen consumption, enhanced glycolysis, and oxidative stress, together with increased phagocytic activity. These alterations are typical of reactive microglia and are generally associated with a cytotoxic function."

Immunoflurescence analysis demonstrated that the microglial conditioned medium derived from KIKO microglia decreased neurite length, neurite number per cell, and overall neuron viability, supporting the notion that microglia can participate in neuron degeneration in FA. When observing the cerebellum of WT and KIKO mice at postnatal day 15, frataxin levels were significantly reduced in cerebellar homogenates and sections of KIKO mice compared with age-matched controls. Furthermore, mRNA levels of P2ry12, as well as of triggering receptor expressed on myeloid cells 2 (Trem2) and chemokine (C-X3-C motif) receptor 1 (Cx3cr1), key microglial molecules mediating the process of synapse refinement during neurodevelopment, were decreased in the cerebellum of KIKO mice.

In both molecule and cortical cerebellar regions of KIKO mice, investigators observed decreased P2Y12 protein, a characterization of cells in a homeostatic context. Changes in morphology of both models of mice were evident, as the number of branches per cell, in the branch length, and in the triple end points in KIKO microglia, were reduced, and were less ramified compared with WT cells. This indicated that their ability to generate complex branches is impaired in KIKO cerebellum.

"Considering the established role of microglia in neurodegeneration, the comprehension of the mechanisms of underlying microglial-related pathological mechanisms in FA could be instrumental in designing time- and molecule-targeted therapeutic interventions to halt cerebellar degeneration in the disease," D’Ambrosi et al concluded.

REFERENCE
1. Della Valle I, Rossi S, Turchi R, et al. Loss of homeostatic functions in microglia from a murine model of Friedreich’s ataxia. Genes & Diseases. Published October 29, 2023. doi:10.1016/j.gendis.2023.101178.