Regular articleInitial brain aging: heterogeneity of mitochondrial size is associated with decline in complex I-linked respiration in cortex and hippocampus
Introduction
Physiological brain aging is associated with decreased complex I-dependent respiration early in the aging process (Braidy et al., 2014, Lores-Arnaiz and Bustamante, 2011, Navarro et al., 2011, Petrosillo et al., 2008), supporting a central role for complex I-linked oxidative phosphorylation (OXPHOS) in aging. Indeed, complex I is the site at which OXPHOS efficiency is regulated in the brain (Cocco et al., 2009). With further aging, that is, at senescence, the OXPHOS deficit extends to also include complexes II–IV (Braidy et al., 2014). While increased gene expression of mitochondrial encoded subunits of the respiratory complexes occurs in the hippocampus and cortex and may counteract the reduction in mitochondrial respiration at middle age, this compensatory mechanism does not extend into senescence (Manczak et al., 2005). Especially the hippocampus is metabolically vulnerable as its respiratory reserve capacity, that is, the difference between maximal and basal respiration, becomes obliterated with age, such that mitochondrial OXPHOS activity cannot meet tissue needs for adenosine triphosphate (ATP) (Braidy et al., 2014).
Mitochondrial dynamics, including fusion and fission events, mitochondrial-specific autophagy (mitophagy), and biogenesis remodel mitochondrial morphology during physiological conditions, optimizing energy production in the face of changing cellular environments (McBride et al., 2006). A bidirectional relationship between mitochondrial morphology and bioenergetics exists (Benard et al., 2007, Galloway et al., 2012). Induction of mitochondrial fragmentation by inhibiting mitochondrial fusion results in reduced cellular respiration (Benard et al., 2007, Chen et al., 2005). Conversely, inhibition of respiratory complexes results in mitochondrial fragmentation (Benard et al., 2007). Notably, impaired enzyme activity of complex I leads to mitochondrial shortening or fragmentation (Benard et al., 2007, Koopman et al., 2005, Valsecchi et al., 2013). Another factor affecting both bioenergetics and mitochondrial morphology is the cellular content of mitochondrial DNA (mtDNA). Reduced mtDNA content decreases complex I activity and OXPHOS-derived ATP generation (Machado et al., 2013), while cells entirely devoid of mtDNA have no OXPHOS and glycolysis increases to compensate for the lack of OXPHOS-derived ATP (Qian and Van Houten, 2010). Concomitantly, mitochondria become extremely fragmented (Gilkerson et al., 2000, Qian and Van Houten, 2010).
Focusing on the initial events of brain aging, we used a murine model of the segmental progeria Cockayne syndrome (CS) and age-matched WT C57Bl/6 mice. CS results from mutations in either the ERCC6 (encoding Cockayne syndrome B [CSB] protein) or ERCC8 (encoding CSA protein) genes (Henning et al., 1995, Troelstra et al., 1992). CSB-deficient cells are extremely sensitive to UV radiation and oxidative stress due to defective transcription and DNA repair, including transcription-coupled nucleotide excision repair and base excision repair (Aamann et al., 2013, Brooks, 2013, Pascucci et al., 2012). The CS phenotype is mild in mice lacking functional CSB protein (CSBm/m mice) (Jaarsma et al., 2013). Nonetheless, CSBm/m mice display some of the same neurological manifestations as human CS patients, including neuronal degeneration and inflammation (Laposa et al., 2007, Nagtegaal et al., 2015, Scheibye-Knudsen et al., 2014).
Mitochondria in old brain have been reported to be enlarged and fragile with reduced respiratory and enzymatic activities of complexes I and III (Navarro and Boveris, 2004). We hypothesized that the reduction in mitochondrial respiration with age is a function of mitochondrial morphology and examined this in relation to mitochondrial dynamics and mtDNA in the frontal cortex and hippocampus of young and middle-aged CSBm/m and WT mice.
Section snippets
Material and methods
All procedures involving animals followed ARRIVA guidelines and were approved by the Danish National Ethics Committee according to the guidelines set forth in the European Council's Convention for the Protection of Vertebrate Animals used for Experimental and Other Scientific Purposes. Male CSBm/m mice and WT C57Bl/6j controls (Janvier Labs, France) at two ages were used: young, 2–3 months (respirometry experiments) or 5–6 months (transmission electron microscopy and biochemical analyses) and
Decreased complex I-linked state 3 respiration in CSBm/m hippocampus with age
In WT mice, mitochondrial respiration did not decrease with age in either the cortex or the hippocampus (cortex: p = 0.239, hippo: p = 0.074; MEM; Fig. 1A). In contrast, in the progeriod CSBm/m mice, an interaction between age and respiration was found in both the cortex and the hippocampus (cortex: p = 0.026, hippocampus: p = 1.36 × 10−9; MEM). The reduction in mitochondrial respiration in CSBm/m cortex could not be pinpointed to any particular respiratory state. In contrast, post hoc t-tests
Discussion
In the present study, we examined brain mitochondrial function, morphology, dynamics, and DNA content during initial aging with special focus on the cortex and hippocampus, as these areas of the brain are metabolically vulnerable to aging and neurodegeneration (Liu et al., 2013, Navarro et al., 2011). We compared brains from WT and CSBm/m mice, as CSB in humans is a known segmental progeria. We found an overall age-related reduction in mitochondrial respiration in CSBm/m cortex and a specific
Conclusion
To conclude, the significant association of complex I-linked respiration and mitochondrial size heterogeneity in both WT and CSBm/m mice indicates that age-related alterations in mitochondrial morphology occurred in middle-aged brains of both genotypes. The reduction in mitochondrial respiration and the increased heterogeneity of mitochondrial size are two early events in brain aging, occurring in the WT brain and more so in the progeroid brain. Relative expression levels of mitochondrial
Disclosure statement
The authors have no actual or potential conflicts of interest.
Acknowledgements
CSBm/m mice were a gracious gift from prof. Vilhelm Bohr (National Institutes of Health, Bethesda, MD, USA). This study was supported by grants from the Manpei Suzuki Diabetes Foundation (T.Y.); the Norwegian Health Association and the Research Council of Norway, project 214458 (L.H.B., C.E.R., and M.M.H-.O.); the Nordea Foundation (L.J.R., C.D., N.B.F., K.T., N.S., and M.L.); the Marie-Curie Ageing Network–MARRIAGE, funded by the European Union 7th Framework Programme (FP7/2007-2013) under
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- 1
These authors have contributed equally.
- 2
Present address: Department of Cardiovascular Medicine, Hokkaido University Graduate School of Medicine, Sapporo, Japan.
- 3
Present address: Department of Neurosurgery, Oslo University Hospital, Oslo, Norway.
- 4
Present address: National Institute on Aging, National Institutes of Health, Bethesda, Maryland, USA.