Mitochondria are ancient organelles, with bacterial origins, that are found in virtually all of our body's cells. In addition to oxidative phosphorylation, mitochondria play key roles in cellular metabolism, signaling, and cell death. Defects in this organelle underlie some of the most devastating inborn errors of metabolism, and mitochondrial dysfunction accompanies virtually all age-associated degenerative diseases. Our laboratory is interested in defining the full spectrum of interconnected pathways resident within the mitochondria, understanding how the organelles' circuitry goes awry in disease, and developing therapeutics to target this dysfunction.
Deciphering the Molecular Circuitry of Mitochondria
Mitochondria contain their own genome (mtDNA), but over billions of years of evolution, nearly all of the coding capacity of the organelle has been transferred to the nucleus. Although the mtDNA encodes 13 essential components of oxidative phosphorylation, all the remaining 1,000-plus proteins are encoded in the nuclear genome and imported into the organelle. In recent years our lab has combined tandem mass spectrometry, computation, and microscopy to assemble a near-comprehensive protein parts list for mitochondria. We call this inventory MitoCarta, and it serves as a molecular framework for many of our lab's basic and clinical studies. Almost half of these proteins remain poorly studied. Ongoing work is aimed at rapidly predicting and validating their function and understanding how these 1,000 proteins are organized into pathways and complexes. To achieve this goal, we are combining large-scale computational and experimental genomics with classical biochemical physiology. Our computational approaches take advantage of the wealth of new genome sequences and activity profiles that are now available in the public domain. Our experimental approaches combine systematic perturbations (RNAi, small molecules) with mass spectrometry–based profiling. The long-term goal is to systematically map the complete molecular circuitry of mitochondria.
The Mitochondrial Calcium Uniporter
A major question in cell biology is how the metabolic state of mitochondria is coupled to rest of the cell. For example, mitochondrial ATP production must match cytosolic demand, and mitochondrial spatial positioning must support polarized activities within a cell. Multiple lines of evidence indicate that calcium may be the major signal, because mitochondrial calcium uptake stimulates ATP production, shapes cytosolic calcium oscillations, contributes to spatial positioning, and can trigger cell death. Mitochondrial calcium overload is observed in a wide range of pathology, including neurodegeneration and ischemic injury. Given these multifaceted roles, it has been hypothesized that mitochondrial calcium uptake may be central to physiology and disease. The major route of entry of calcium into mitochondria is the uniporter, a highly selective channel whose activity was first documented 50 years ago. Despite its importance and extensive characterization, its molecular identity remained elusive for decades. We used computational genomics, physiology, and biochemistry to solve this problem and identified the pore-forming and calcium-sensing regulatory subunits. Ongoing work is aimed at a full biochemical and mechanistic characterization of the channel complex and its role in disease.
Orphan Mitochondrial Disorders
The orphan mitochondrial disorders are a large class of individually rare genetic syndromes that can arise from mutations in the nuclear or mtDNA genomes. Because their diagnosis is incredibly challenging and we lack effective therapies, these disorders represent an important unmet clinical need. In recent years, we have pioneered the application of targeted next-generation sequencing to discover the molecular genetic basis for these disorders. We have worked with our collaborators throughout the world to define the basis of more than a dozen monogenic disorders. Ongoing work is aimed at applying metabolomics technologies to identify circulating biomarkers that can be used to monitor disease progression and response to therapeutic interventions. We are using chemical biology methods to search for mitochondria-active drugs that one day might effectively treat these orphan disorders. Such compounds may also be useful in targeting the mitochondrial basis of common age-associated diseases, including neurodegeneration, diabetes, and cancer.
Grants from the National Institutes of Health provided partial support for some of these projects.
As of March 4, 2016