The dynamics of mitochondrial-linked gene expression among tissues and life stages in two contrasting strains of laying hens

The cellular energy metabolism is one of the most conserved processes, as it is present in all living organisms. Mitochondria are providing the eukaryotic cell with energy and thus their genome and gene expression has been of broad interest for a long time. Mitochondrial gene expression changes under different conditions and is regulated by genes encoded in the nucleus of the cell. In this context, little is known about non-model organisms and we provide the first large-scaled gene expression analysis of mitochondrial-linked genes in laying hens. We analysed 28 mitochondrial and nuclear genes in 100 individuals in the context of five life-stages and strain differences among five tissues. Our study showed that mitochondrial gene expression increases during the productive life span, and reacts tissue and strain specific. In addition, the strains react different to potential increased oxidative stress, resulting from the increase in mitochondrial gene expression. The results suggest that the cellular energy metabolism as part of a complex regulatory system is strongly affected by the productive life span in laying hens and thus partly comparable to model organisms. This study provides a starting point for further analyses in this field on non-model organisms, especially in laying-hens.

Sirtuin-1 isoforms differentially regulate mitochondrial function

Alternative splicing generates multiple distinct isoforms that increase transcriptome and proteome diversity. Alternatively spliced isoforms may lose part of the protein domain and have different intracellular localization as well as distinct functions. The main form of the SIRT1 (SIRT1v1) protein contains 11 exons. We have identified two new isoforms, SIRT1v2 (lost 2 exons), and SIRT1v3 (lost 3 exons), but their effect on mitochondrial gene expression has not been reported. To study the effect of the three SIRT1 isoforms on mitochondrial gene expression and function, neuronal cells were transfected with SIRT1 isoforms v1, v2 or v3 plasmids, respectively. Gene expression was measured by quantitative reverse transcription PCR (RT-qPCR). Our data showed SIRT1 isoforms v1, v2 and v3 differentially regulated PCG-1alpha and PCG-1beta, which are the upstream regulators of mitochondrial structure and function. SIRT1v1 upregulated mitofusin-1 (MFN1), the mitochondrial dynamin-like GTPase (OPA1) gene, and the transcription factor A mitochondrial (TFAM) gene. In contrast, the SIRT1-v2 isoform repressed the MFN1, MFN2, and TFAM genes, while the SIRT1-v3 isoform repressed the MFN1 gene. In addition, the three SIRT1 isoforms differentially affected the mitochondrial respiratory complex I genes, including NDUFAB1, NDUFS1, NDUFV1, NDUFV2. The data indicates that SIRT1 regulates mitochondrial biogenesis and function through a signaling pathway involving PGC-1alpha, PCG-1beta, mitofusin 1 and 2, OPA1, and TFAM genes. Taken together, alternative splicing generated three SIRT1 isoform proteins with diverse functions. Age-related changes in the alternative splicing events are likely to impact sirtuin-regulated cellular functions and signaling pathways in aging and senescence.

Skeletal muscle bioenergetic health and function in people living with HIV: association with glucose tolerance and alcohol use

At-risk alcohol use is prevalent and increases dysglycemia among people living with human immunodeficiency virus (PLWH). Skeletal muscle (SKM) bioenergetic dysregulation is implicated in dysglycemia and type 2 diabetes. The objective of this study was to determine the relationship between at-risk alcohol, glucose tolerance, and SKM bioenergetic function in PLWH. Thirty-five PLWH (11 females, 24 males, age: 53±9 yrs, body mass index: 29.0±6.6 kg/m2) with elevated fasting glucose enrolled in the ALIVE-Ex study provided medical history and alcohol use information (Alcohol Use Disorders Identification Test, AUDIT), then underwent an oral glucose tolerance test (OGTT) and SKM biopsy. Bioenergetic health and function and mitochondrial volume were measured in isolated myoblasts. Mitochondrial gene expression was measured in SKM. Linear regression adjusting for age, sex, and smoking was performed to examine the relationship between glucose tolerance (2-h glucose post-OGTT), AUDIT, and their interaction with each outcome measure. Negative indicators of bioenergetic health were significantly (p<0.05) greater with higher 2-h glucose (proton leak) and AUDIT (proton leak, non-mitochondrial oxygen consumption, and bioenergetic health index). Mitochondrial volume was increased with the interaction of higher 2-h glucose and AUDIT. Mitochondrial gene expression decreased with higher 2-h glucose (TFAM, PGC1B, PPARG, MFN1), AUDIT (MFN1, DRP1, MFF), and their interaction (PPARG, PPARD, MFF). Decreased expression of mitochondrial genes were coupled with increased mitochondrial volume and decreased bioenergetic health in SKM of PLWH with higher AUDIT and 2-h glucose. We hypothesize these mechanisms reflect poorer mitochondrial health and may precede overt SKM bioenergetic dysregulation observed in type 2 diabetes.

Controlling the topology of mammalian mitochondrial DNA

The genome of mitochondria, called mtDNA, is a small circular DNA molecule present at thousands of copies per human cell. MtDNA is packaged into nucleoprotein complexes called nucleoids, and the density of mtDNA packaging affects mitochondrial gene expression. Genetic processes such as transcription, DNA replication and DNA packaging alter DNA topology, and these topological problems are solved by a family of enzymes called topoisomerases. Within mitochondria, topoisomerases are involved firstly in the regulation of mtDNA supercoiling and secondly in disentangling interlinked mtDNA molecules following mtDNA replication. The loss of mitochondrial topoisomerase activity leads to defects in mitochondrial function, and variants in the dual-localized type IA topoisomerase TOP3A have also been reported to cause human mitochondrial disease. We review the current knowledge on processes that alter mtDNA topology, how mtDNA topology is modulated by the action of topoisomerases, and the consequences of altered mtDNA topology for mitochondrial function and human health.

Cerebroventricular Microinjections of MPTP on Adult Zebrafish Induces Dopaminergic Neuronal Death, Mitochondrial Fragmentation, and Sensorimotor Impairments

Mitochondria are dynamic organelles that mediate the energetic supply to cells and mitigate oxidative stress through the intricate balance of fission and fusion. Mitochondrial dysfunction is a prominent feature within Parkinson disease (PD) etiologies. To date, there have been conflicting studies of neurotoxin impact on dopaminergic cell death, mitochondrial function and behavioral impairment using adult zebrafish. Here, we performed cerebroventricular microinjections (CVMIs) of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) on adult transgenic zebrafish that resulted in significant reductions in dopaminergic neurons within the telencephalon and olfactory bulbs (OB) of Tg(dat:eGFP) fish. Visualization of mCherry and mitochondrial gene expression analysis in Tg(dat:tom20 MLS:mCherry) fish reveal that MPTP induces mitochondrial fragmentation in dopaminergic neurons and the activation of the pink1/parkin pathway involved mitophagy. Moreover, the loss of dopaminergic neurons translated into a transient locomotor and olfactory phenotype. Taken together, these data can contribute to a better understanding of the mitochondrial impact on dopaminergic survivability.

Mitochondrial Glucocorticoid Receptors and Their Actions

Mitochondria are membrane organelles present in almost all eukaryotic cells. In addition to their well-known role in energy production, mitochondria regulate central cellular processes, including calcium homeostasis, Reactive Oxygen Species (ROS) generation, cell death, thermogenesis, and biosynthesis of lipids, nucleic acids, and steroid hormones. Glucocorticoids (GCs) regulate the mitochondrially encoded oxidative phosphorylation gene expression and mitochondrial energy metabolism. The identification of Glucocorticoid Response Elements (GREs) in mitochondrial sequences and the detection of Glucocorticoid Receptor (GR) in mitochondria of different cell types gave support to hypothesis that mitochondrial GR directly regulates mitochondrial gene expression. Numerous studies have revealed changes in mitochondrial gene expression alongside with GR import/export in mitochondria, confirming the direct effects of GCs on mitochondrial genome. Further evidence has made clear that mitochondrial GR is involved in mitochondrial function and apoptosis-mediated processes, through interacting or altering the distribution of Bcl2 family members. Even though its exact translocation mechanisms remain unknown, data have shown that GR chaperones (Hsp70/90, Bag-1, FKBP51), the anti-apoptotic protein Bcl-2, the HDAC6- mediated deacetylation and the outer mitochondrial translocation complexes (Tom complexes) co-ordinate GR mitochondrial trafficking. A role of mitochondrial GR in stress and depression as well as in lung and hepatic inflammation has also been demonstrated.

Contribution of PGC-1α to Obesity- and Caloric Restriction-Related Physiological Changes in White Adipose Tissue

Peroxisome proliferator-activated receptor γ coactivator-1 α (PGC-1α) regulates mitochondrial DNA replication and mitochondrial gene expression by interacting with several transcription factors. White adipose tissue (WAT) mainly comprises adipocytes that store triglycerides as an energy resource and secrete adipokines. The characteristics of WAT vary in response to systemic and chronic metabolic alterations, including obesity or caloric restriction. Despite a small amount of mitochondria in white adipocytes, accumulated evidence suggests that mitochondria are strongly related to adipocyte-specific functions, such as adipogenesis and lipogenesis, as well as oxidative metabolism for energy supply. Therefore, PGC-1α is expected to play an important role in WAT. In this review, we provide an overview of the involvement of mitochondria and PGC-1α with obesity- and caloric restriction-related physiological changes in adipocytes and WAT.