Cristae, the organized invaginations of the mitochondrial inner membrane, respond structurally to the energetic demands of the cell. The mechanism by which these dynamic changes are regulated and the consequences thereof are largely unknown. Optic atrophy 1 (OPA1) is the mitochondrial GTPase responsible for inner membrane fusion and maintenance of cristae structure. Here, we report that OPA1 responds dynamically to changes in energetic conditions to regulate cristae structure. This cristae regulation is independent of OPA1's role in mitochondrial fusion, since an OPA1 mutant that can still oligomerize but has no fusion activity was able to maintain cristae structure. Importantly, OPA1 was required for resistance to starvation-induced cell death, for mitochondrial respiration, for growth in galactose media and for maintenance of ATP synthase assembly, independently of its fusion activity. We identified mitochondrial solute carriers (SLC25A) as OPA1 interactors and show that their pharmacological and genetic blockade inhibited OPA1 oligomerization and function. Thus, we propose a novel way in which OPA1 senses energy substrate availability, which modulates its function in the regulation of mitochondrial architecture in a SLC25A protein-dependent manner.

Overexpression of the EGF receptor (EGFR) is a recurrent theme in human cancer and is thought to cause aggressive phenotypes and resistance to standard therapy. There has, thus, been a concerted effort in identifying EGFR gene mutations to explain misregulation of EGFR expression as well as differential sensitivity to anti-EGFR drugs. However, such genetic alterations have proven to be rare occurrences in most types of cancer, suggesting the existence of a more general physiological trigger for aberrant EGFR expression. Here, we provide evidence that overexpression of wild-type EGFR can be induced by the hypoxic microenvironment and activation of hypoxia-inducible factor 2-␣ (HIF2␣) in the core of solid tumors. Our data suggest that hypoxia/HIF2␣ activation represents a common mechanism for EGFR overexpression by increasing EGFR mRNA translation, thereby diminishing the necessity for gene mutations. This allows for the accumulation of elevated EGFR levels, increasing its availability for the autocrine signaling required for tumor cell growth autonomy. Taken together, our findings provide a nonmutational explanation for EGFR overexpression in human tumors and highlight a role for HIF2␣ activation in the regulation of EGFR protein synthesis.hypoxia inducible factor ͉ receptor tyrosine kinase signaling ͉ tumor microenvironment ͉ VHL

Abstract. Neuronal cell loss associated with neurodegeneration has recently been linked to mitochondrial dysfunction. Electron transport chain defects and reactive oxygen species (ROS) production are emerging as important players in the etiology of neurodegenerative diseases. Proper management of ROS and disposal of damaged cellular components are vital to the survival and function of neurons. Proteins involved in these pathways are often mutated in neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and Huntington's disease. In this review, we will discuss the roles of ROS in normal physiology, how changes in ROS production affect neuronal survival in neurodegenerative diseases, and the recent advances in mitochondrial antioxidants as potential therapeutics.

Objective: High levels of reactive oxygen species (ROS) are intricately linked to obesity and associated pathologies, notably insulin resistance and type 2 diabetes. However, ROS are also thought to be important in intracellular signaling, which may paradoxically be required for insulin sensitivity. Many theories have been developed to explain this apparent paradox, which have broadened our understanding of these important small molecules. While many sites for intracellular ROS production have been described, mitochondrial generated ROS remain a major contributor in most cell types. Mitochondrial ROS generation is controlled by a number of factors described in this review. Moreover, these studies have established both a demand for novel sensitive approaches to measure ROS, as well as a need to standardize and review their suitability for different applications. Methods: To properly assess levels of ROS and mitochondrial ROS in the development of obesity and its complications, a growing number of tools have been developed. This paper reviews many of the common methods for the investigation of ROS in mitochondria, cell, animal, and human models. Results: Available approaches can be generally divided into those that measure ROS-induced damage (e.g., DNA, lipid, and protein damage); those that measure antioxidant levels and redox ratios; and those that use novel biosensors and probes for a more direct measure of different forms of ROS (e.g., 2 0 ,7 0 -dichlorofluorescein (DCF), dihydroethidium (DHE) and its mitochondrial targeted form (MitoSOX), Amplex Red, roGFP, HyPer, mt-cpYFP, ratiometric H 2 O 2 probes, and their derivatives). Moreover, this review provides caveats and strengths for the use of these techniques in different models. Conclusions: Advances in these techniques will undoubtedly advance the understanding of ROS in obesity and may help resolve unanswered questions in the field.

Sustained cellular function and viability of high-energy demanding post-mitotic cells rely on the continuous supply of ATP. The utilization of mitochondrial oxidative phosphorylation for efficient ATP generation is a function of oxygen levels. As such, oxygen deprivation, in physiological or pathological settings, has profound effects on cell metabolism and survival. Here we show that mild extracellular acidosis, a physiological consequence of anaerobic metabolism, can reprogramme the mitochondrial metabolic pathway to preserve efficient ATP production regardless of oxygen levels. Acidosis initiates a rapid and reversible homeostatic programme that restructures mitochondria, by regulating mitochondrial dynamics and cristae architecture, to reconfigure mitochondrial efficiency, maintain mitochondrial function and cell survival. Preventing mitochondrial remodelling results in mitochondrial dysfunction, fragmentation and cell death. Our findings challenge the notion that oxygen availability is a key limiting factor in oxidative metabolism and brings forth the concept that mitochondrial morphology can dictate the bioenergetic status of post-mitotic cells.