Mitochondrial respiration in microglia is essential for response to demyelinating injury but not proliferation.

Microglia are necessary for central nervous system (CNS) function during development and play roles in ageing, Alzheimer's disease and the response to demyelinating injury1-5. The mitochondrial respiratory chain (RC) is necessary for conventional T cell proliferation6 and macrophage-dependent immune responses7-10. However, whether mitochondrial RC is essential for microglia proliferation or function is not known. We conditionally deleted the mitochondrial complex III subunit Uqcrfs1 (Rieske iron-sulfur polypeptide 1) in the microglia of adult mice to assess the requirement of microglial RC for survival, proliferation and adult CNS function in vivo. Notably, mitochondrial RC function was not required for survival or proliferation of microglia in vivo. RNA sequencing analysis showed that loss of RC function in microglia caused changes in gene expression distinct from aged or disease-associated microglia. Microglia-specific loss of mitochondrial RC function is not sufficient to induce cognitive decline. Amyloid-ß plaque coverage decreased and microglial interaction with amyloid-ß plaques increased in the hippocampus of 5xFAD mice with mitochondrial RC-deficient microglia. Microglia-specific loss of mitochondrial RC function did impair remyelination following an acute, reversible demyelinating event. Thus, mitochondrial respiration in microglia is dispensable for proliferation but is essential to maintain a proper response to CNS demyelinating injury.

Reduced expression of mitochondrial complex I subunit Ndufs2 does not impact healthspan in mice.

Aging in mammals leads to reduction in genes encoding the 45-subunit mitochondrial electron transport chain complex I. It has been hypothesized that normal aging and age-related diseases such as Parkinson's disease are in part due to modest decrease in expression of mitochondrial complex I subunits. By contrast, diminishing expression of mitochondrial complex I genes in lower organisms increases lifespan. Furthermore, metformin, a putative complex I inhibitor, increases healthspan in mice and humans. In the present study, we investigated whether loss of one allele of Ndufs2, the catalytic subunit of mitochondrial complex I, impacts healthspan and lifespan in mice. Our results indicate that Ndufs2 hemizygous mice (Ndufs2+/-) show no overt impairment in aging-related motor function, learning, tissue histology, organismal metabolism, or sensitivity to metformin in a C57BL6/J background. Despite a significant reduction of Ndufs2 mRNA, the mice do not demonstrate a significant decrease in complex I function. However, there are detectable transcriptomic changes in individual cell types and tissues due to loss of one allele of Ndufs2. Our data indicate that a 50% decline in mRNA of the core mitochondrial complex I subunit Ndufs2 is neither beneficial nor detrimental to healthspan.

Ciliary gene RPGRIP1L is required for hypothalamic arcuate neuron development.

Intronic polymorphisms in the α-ketoglutarate-dependent dioxygenase gene (FTO) that are highly associated with increased body weight have been implicated in the transcriptional control of a nearby ciliary gene, retinitis pigmentosa GTPase regulator-interacting protein-1 like (RPGRIP1L). Previous studies have shown that congenital Rpgrip1l hypomorphism in murine proopiomelanocortin (Pomc) neurons causes obesity by increasing food intake. Here, we show by congenital and adult-onset Rpgrip1l deletion in Pomc-expressing neurons that the hyperphagia and obesity are likely due to neurodevelopmental effects that are characterized by a reduction in the Pomc/Neuropeptide Y (Npy) neuronal number ratio and marked increases in arcuate hypothalamic-paraventricular hypothalamic (ARH-PVH) axonal projections. Biallelic RPGRIP1L mutations result in fewer cilia-positive human induced pluripotent stem cell-derived (iPSC-derived) neurons and blunted responses to Sonic Hedgehog (SHH). Isogenic human ARH-like embryonic stem cell-derived (ESc-derived) neurons homozygous for the obesity-risk alleles at rs8050136 or rs1421085 have decreased RPGRIP1L expression and have lower numbers of POMC neurons. RPGRIP1L overexpression increases POMC cell number. These findings suggest that apparently functional intronic polymorphisms affect hypothalamic RPGRIP1L expression and impact development of POMC neurons and their derivatives, leading to hyperphagia and increased adiposity.