Conference Agenda

Chloroplasts and mitochondria are subcellular bioenergetic organelles that contain their own genomes and genetic systems. DNA replication and its transmission to daughter organelles produces cytoplasmic inheritance of characters associated with primary events in photosynthesis and respiration. The prokaryotic ancestors of chloroplasts and mitochondria were endosymbionts whose genes became copied to the genomes of their cellular hosts. Those copies that survive are now nuclear, chromosomal genes that encode either cytosolic proteins or precursor proteins imported into the organelle into which the endosymbiont evolved. What accounts for the retention of genes for the complete synthesis within chloroplasts and mitochondria of only a tiny minority of their protein subunits? One hypothesis is that expression of genes for components of electron transport chains must respond to physical environmental change by means of a direct and unconditional regulatory control—control exerted by change in the redox state of the corresponding gene product (1). This hypothesis proposes that, to preserve function, an entire redox regulatory system must be retained within its original membrane-bound compartment together with the DNA upon which it acts. Co-location of gene and gene product for Redox Regulation of gene expression (CoRR) (2) is an hypothesis in agreement with the results of a variety of experiments designed to test it and that seem to have no other satisfactory explanation (3). I present evidence relating to the CoRR hypothesis, and describe a two-component mechanism by which transcription of genes for reaction centre apoproteins is coupled to photosynthetic electron transport in chloroplasts.

1. Allen JF (1993) Control of Gene Expression by Redox Potential and the Requirement for Chloroplast and Mitochondrial Genomes. J. Theor. Biol. 165: 609-631.

2. Allen JF (2003) The function of genomes in bioenergetic organelles. Phil. Trans. R. Soc. B 358: 19-37.

3. Allen JF (2015) Why chloroplasts and mitochondria retain their own genomes and genetic systems: colocation for redox regulation of gene expression. Proc. Natl. Acad. Sci (USA) 112: 10231–10238.

Life is an exergonic chemical reaction. Many individual reactions in metabolism entail slightly endergonic processes that are coupled to free energy release, typically as ATP hydrolysis, in order to go forward. ATP is almost always supplied by the rotor-stator ATP synthetase (the ATPase), which harnesses chemiosmotic ion gradients. Because the ATPase is a protein, it arose after the ribosome did. Here we address two questions using comparative physiology: What was the energy currency of metabolism before the origin of the ATPase? How (and why) did ATP come to be the universal energy currency? About 27% of a cell’s energy budget is consumed as GTP during translation. The universality of GTP-dependence in ribosome function indicates that GTP was the ancestral energy currency of protein synthesis [1]. The use of GTP in translation and ATP in small molecule synthesis are conserved across all prokaryotic lineages, representing energetic compartments that arose in the last universal common ancestor, LUCA.

[1] N. Mrnjavac, W.F. Martin, GTP before ATP: The energy currency at the origin of genes. arXiv (2024) https://doi.org/10.48550/arXiv.2403.08744

Non-vesicular lipid transfer mediated by lipid transfer protein (LTPs) at membrane Contact Sites (CS) is essential to maintain the membrane lipid composition of cellular organelles, especially those that are excluded from the classical vesicular routes, such as mitochondria and lipid droplets (LDs). In order to maintain their integrities and functions, mitochondria and LDs require a constant and regulated exchange of lipids with the Endoplasmic Reticulum (ER), major site of lipid synthesis, at CS. However how lipids are shuttled from the ER to the mitochondria and to LDs is largely unknown. Also, where and how LDs originate from the ER is still unclear. We have recently shown that ER Membrane subdomains Associated with Mitochondria (MAMs) are hotspots for the formation of LDs, uncovering the existence of a three-way CS connection between mitochondria, ER and LD. We have also found that ORP5 and ORP8, two ER-anchored LTP of the Oxysterol-binding protein (OSBP)-related protein family, mainly localize and interact at MAMs where they facilitate LD biogenesis and growth. Currently, by using a combination of biochemical and imaging approaches, we are investigating the mechanisms by which ORP5-8 mediate LD biogenesis at MAM-LD contact sites and the functional relevance of these three-way contacts for lipid transfer and storage in the cell. Also, by using 3D Focused Ion Beam-Scanning Electron Microscopy (FIB-SEM) we have uncovered the unique morphology of the MAM subdomains connected to LD, that we propose is instrumental to promote LD growth. Overall, our studies provide new insights on the metabolic and physical crosstalk between ER, mitochondria and LD held at three-way contacts between these organelles.

The regulation of adult stem cell quiescence is essential for stem cell maintenance, longevity and sustained tissue regeneration, however, the upstream regulatory mechanisms are not fully understood. Our studies have uncovered that changes in stem cell mitochondrial shape serve as an upstream signaling mechanism that communicates retrogradely with the nucleus to regulate the quiescent state of stem cells [1]. We show that mitochondria in stem cells rapidly fragment in response to an environmental activation stimulus to drive the exit from deep quiescence. Manipulation of mitochondrial dynamics, by changing OPA1 levels, or redox states is sufficient to alter stem cells function and muscle regeneration [1]. This occurs through activation of an intracellular reactive oxygen species (ROS) and glutathione (GSH) mediated signaling pathway that initiates a nuclear transcriptional program to suppress quiescence and self-renewal genes and promote myogenic gene expression. we provide evidence that impaired mitochondrial dynamics and GSH homeostasis may be an early contributor to age-related muscle stem cell dysfunction. Together, our data demonstrate that mitochondrial plasticity and redox balance are essential upstream regulators of stem cell function and longevity.

1. Baker N, Wade S, Triolo M, Girgis J, Chwastek D, Larrigan S, Feige P, Fujita R, Crist C, Rudnicki MA, Burelle Y, Khacho M (2022) The mitochondrial protein OPA1 regulates the quiescent state of adult muscle stem cells. Cell Stem Cell 29 (9):1315-1332.