In Science Journals

Highlights from the Science family of journals.

In Science Journals

Highlights from the Science family of journals.

From mitochondria to cells to humans: Targeting bioenergetics in aging and disease

In vivo control over metabolism is at the cutting edge of biomedical research. The particulars of mitochondrial function are especially important to understand in vivo to progress metabolic therapies that will be relevant for diseases of aging. Understanding the differences between how mitochondria function in vitro versus in vivo will be a necessary challenge to overcome to achieve mitochondrial medicine. In this article we outline how discoveries in invertebrate models will be informative for understanding the basic biology of mitochondria to streamline translation to mammals and eventually to humans. Further, we highlight examples of how what is known about mitochondria in vitro is translatable to in vivo models and, in some cases, to human diseases.

Optogenetic rejuvenation of mitochondrial membrane potential extends C. elegans lifespan

Mitochondrial dysfunction plays a central role in aging but the exact biological causes are still being determined. Here, we show that optogenetically increasing mitochondrial membrane potential during adulthood using a light-activated proton pump improves age-associated phenotypes and extends lifespan in C. elegans. Our findings provide direct causal evidence that rescuing the age-related decline in mitochondrial membrane potential is sufficient to slow the rate of aging and extend healthspan and lifespan.

Decreased Mitochondrial Membrane Potential Activates the Mitochondrial Unfolded Protein Response

Mitochondria are ATP-producing organelles that also signal throughout the cell. Mitochondrial protein homeostasis is regulated through membrane potential-dependent protein import and quality control signaling. The mitochondrial unfolded protein response (UPR^mt) is a specific program that responds to imbalances in nuclear and mitochondrial gene expression. Mounting evidence suggests that the electrochemical gradient that powers mitochondrial function, the mitochondrial membrane potential (Δψ_m), is a core regulator of the UPR^mt. Here we tested this notion directly by pharmacologically dissipating Δψ_m and monitoring UPR^mt activation. We found that chemical dissipation of Δψ_m using FCCP indeed activated UPR^mt dose-dependently in C. elegans assayed by the HSP-60::GFP reporter strain.

Optical Control of CD8+ T Cell Metabolism and Effector Functions

Although cancer immunotherapy is effective against hematological malignancies, it is less effective against solid tumors due in part to significant metabolic challenges present in the tumor microenvironment (TME), where infiltrated CD8+ T cells face fierce competition with cancer cells for limited nutrients. Strong metabolic suppression in the TME is often associated with impaired T cell recruitment to the tumor site and hyporesponsive effector function via T cell exhaustion. Increasing evidence suggests that mitochondria play a key role in CD8+ T cell activation, effector function, and persistence in tumors. In this study, we showed that there was an increase in overall mitochondrial function, including mitochondrial mass and membrane potential, during both mouse and human CD8+ T cell activation. CD8+ T cell mitochondrial membrane potential was closely correlated with granzyme B and IFN-γ production, demonstrating the significance of mitochondria in effector T cell function. Additionally, activated CD8+ T cells that migrate on ICAM-1 and CXCL12 consumed significantly more oxygen than stationary CD8+ T cells. Inhibition of mitochondrial respiration decreased the velocity of CD8+ T cell migration, indicating the importance of mitochondrial metabolism in CD8+ T cell migration. Remote optical stimulation of CD8+ T cells that express our newly developed "OptoMito-On" successfully enhanced mitochondrial ATP production and improved overall CD8+ T cell migration and effector function. Our study provides new insight into the effect of the mitochondrial membrane potential on CD8+ T cell effector function and demonstrates the development of a novel optogenetic technique to remotely control T cell metabolism and effector function at the target tumor site with outstanding specificity and temporospatial resolution.

Neuronal AMPK coordinates mitochondrial energy sensing and hypoxia resistance in C. elegans

Organisms adapt to their environment through coordinated changes in mitochondrial function and metabolism. The mitochondrial protonmotive force (PMF) is an electrochemical gradient that powers ATP synthesis and adjusts metabolism to energetic demands via cellular signaling. It is unknown how or where transient PMF changes are sensed and signaled due to the lack of precise spatiotemporal control in vivo. We addressed this by expressing a light-activated proton pump in mitochondria to spatiotemporally "turn off" mitochondrial function through PMF dissipation in tissues with light. We applied our construct-mitochondria-OFF (mtOFF)-to understand how metabolic status impacts hypoxia resistance, a response that relies on mitochondrial function. Activation of mtOFF induced starvation-like behavior mediated by AMP-activated protein kinase (AMPK). We found prophylactic mtOFF activation increased survival following hypoxia, and that protection relied on neuronal AMPK. Our study links spatiotemporal control of mitochondrial PMF to cellular metabolic changes that mediate behavior and stress resistance.

Mitochondrial light switches: optogenetic approaches to control metabolism

Developing new technologies to study metabolism is increasingly important as metabolic disease prevalence increases. Mitochondria control cellular metabolism and dynamic changes in mitochondrial function are associated with metabolic abnormalities in cardiovascular disease, cancer, and obesity. However, a lack of precise and reversible methods to control mitochondrial function has prevented moving from association to causation. Recent advances in optogenetics have addressed this challenge, and mitochondrial function can now be precisely controlled in vivo using light. A class of genetically encoded, light-activated membrane channels and pumps has addressed mechanistic questions that promise to provide new insights into how cellular metabolism downstream of mitochondrial function contributes to disease. Here, we highlight emerging reagents-mitochondria-targeted light-activated cation channels or proton pumps-to decrease or increase mitochondrial activity upon light exposure, a technique we refer to as mitochondrial light switches, or mt^SWITCH . The mt^SWITCH technique is broadly applicable, as energy availability and metabolic signaling are conserved aspects of cellular function and health. Here, we outline the use of these tools in diverse cellular models of disease. We review the molecular details of each optogenetic tool, summarize the results obtained with each, and outline best practices for using optogenetic approaches to control mitochondrial function and downstream metabolism.

Optogenetic control of mitochondrial protonmotive force to impact cellular stress resistance

Mitochondrial respiration generates an electrochemical proton gradient across the mitochondrial inner membrane called protonmotive force (PMF) to drive diverse functions and synthesize ATP. Current techniques to manipulate the PMF are limited to its dissipation; yet, there is no precise and reversible method to increase the PMF. To address this issue, we aimed to use an optogenetic approach and engineered a mitochondria-targeted light-activated proton pump that we name mitochondria-ON (mtON) to selectively increase the PMF in Caenorhabditis elegans. Here we show that mtON photoactivation increases the PMF in a dose-dependent manner, supports ATP synthesis, increases resistance to mitochondrial toxins, and modulates energy-sensing behavior. Moreover, transient mtON activation during hypoxic preconditioning prevents the well-characterized adaptive response of hypoxia resistance. Our results show that optogenetic manipulation of the PMF is a powerful tool to modulate metabolism and cell signaling.

Mitochondrial Reactive Oxygen Species Generated at the Complex-II Matrix or Intermembrane Space Microdomain Have Distinct Effects on Redox Signaling and Stress Sensitivity in Caenorhabditis elegans

Aims: How mitochondrial reactive oxygen species (ROS) impact physiological function may depend on the quantity of ROS generated or removed, and the subcellular microdomain in which this occurs. However, pharmacological tools currently available to alter ROS production in vivo lack precise spatial and temporal control. Results: We used CRISPR/Cas9 to fuse the light-sensitive ROS-generating protein, SuperNova to the C-terminus of mitochondrial complex II succinate dehydrogenase subunits B (SDHB-1::SuperNova) and C (SDHC-1::SuperNova) in Caenorhabditis elegans to localize SuperNova to the matrix-side of the inner mitochondrial membrane, and to the intermembrane space (IMS), respectively. The presence of the SuperNova protein did not impact complex II activity, mitochondrial respiration, or C. elegans development rate under dark conditions. ROS production by SuperNova protein in vitro in the form of superoxide (O₂˙^-) was both specific and proportional to total light irradiance in the 540-590 nm spectra, and was unaffected by varying the buffer pH to resemble the mitochondrial matrix or IMS environments. We then determined using SuperNova whether stoichiometric ROS generation in the mitochondrial matrix or IMS had distinct effects on redox signaling in vivo. Phosphorylation of PMK-1 (a p38 MAPK homolog) and transcriptional activity of SKN-1 (an Nrf2 homolog) were each dependent on both the site and duration of ROS production, with matrix-generated ROS having more prominent effects. Furthermore, matrix- but not IMS-generated ROS attenuated susceptibility to simulated ischemia reperfusion injury in C. elegans. Innovation and Conclusion: Overall, these data demonstrate that the physiological output of ROS depends on the microdomain in which it is produced. Antioxid. Redox Signal. 31, 594-607.

Redox Signaling Through Compartmentalization of Reactive Oxygen Species: Implications for Health and Disease

The cell maintains a balance between the production and removal of reactive oxygen species (ROS). Changes in ROS levels can impact many cellular functions, and dysregulation contributes to pathologies. How a specific cellular environment or microdomain influences the ROS-generating systems and biological impact of ROS remains an active area of research. This Forum highlights the complexity of ROS microdomains and their contributions to health and disease. Novel technologies to measure or generate ROS in defined regions are important developments in the spatial control of ROS. Using these advances, the articles herein demonstrate how site-specific redox environments influence cellular function and pathology. Antioxid. Redox Signal. 31, 591-593.

Light-induced oxidant production by fluorescent proteins

Oxidants play an important role in the cell and are involved in many redox processes. Oxidant concentrations are maintained through coordinated production and removal systems. The dysregulation of oxidant homeostasis is a hallmark of many disease pathologies. The local oxidant microdomain is crucial for the initiation of many redox signaling events; however, methods to control oxidant product are limited. Some fluorescent proteins, including GFP, TagRFP, KillerRed, miniSOG, and their derivatives, generate oxidants in response to light. These genetically-encoded photosensitizers produce singlet oxygen and superoxide upon illumination and offer spatial and temporal control over oxidant production. In this review, we will examine the photosensitization properties of fluorescent proteins and their application to redox biology. Emerging concepts of selective oxidant species production via photosensitization and the impact of light on biological systems are discussed.

Use the Protonmotive Force: Mitochondrial Uncoupling and Reactive Oxygen Species

Mitochondrial respiration results in an electrochemical proton gradient, or protonmotive force (pmf), across the mitochondrial inner membrane. The pmf is a form of potential energy consisting of charge (∆ψ_m) and chemical (∆pH) components, that together drive ATP production. In a process called uncoupling, proton leak into the mitochondrial matrix independent of ATP production dissipates the pmf and energy is lost as heat. Other events can directly dissipate the pmf independent of ATP production as well, such as chemical exposure or mechanisms involving regulated mitochondrial membrane electrolyte transport. Uncoupling has defined roles in metabolic plasticity and can be linked through signal transduction to physiologic events. In the latter case, the pmf impacts mitochondrial reactive oxygen species (ROS) production. Although capable of molecular damage, ROS also have signaling properties that depend on the timing, location, and quantity of their production. In this review, we provide a general overview of mitochondrial ROS production, mechanisms of uncoupling, and how these work in tandem to affect physiology and pathologies, including obesity, cardiovascular disease, and immunity. Overall, we highlight that isolated bioenergetic models-mitochondria and cells-only partially recapitulate the complex link between the pmf and ROS signaling that occurs in vivo.

Migration reversals in perspective: the long-wave evidence

"Although the pattern of polarization reversal reported by Vining and Pallone in the 1970s and the re-emergence of core-ward net migration that they now report for the 1980s are problematic if viewed from the perspective of a short-term time horizon, they are easily understandable in a long-wave context. Evidence is provided for 55-year waves of urbanward migration [in the United States], each of which reached its nadir during the nation's major stagflation crises. The periodic repetitions of the phenomena described by Vining and Pallone suggest the relevance of the interpretations provided by long-wave theory."