Metabolic design in a mammalian model of extreme metabolism, the North American least shrew (Cryptotis parva).

KEY POINTS Least shrews were studied to explore the relationship between metabolic function, mitochondrial morphology and protein content in different tissues. Liver and kidney mitochondrial content and enzymatic activity approaches the heart indicating similar metabolic demand among tissues that contribute to basal and maximum metabolism. This allows examination of mitochondrial structure and composition in tissues with similar maximum metabolic demands. Mitochondrial networks only occur in striated muscle. In contrast, the liver and kidney maintain individual mitochondria with limited reticulation. Muscle mitochondrial reticulation is the result of dense ATPase activity and cell-spanning myofibrils which require networking for adequate metabolic support. In contrast, liver and kidney ATPase activity is localized to the endoplasmic reticulum and basolateral membrane respectively, generating a locally balanced energy conversion and utilization Mitochondrial morphology is not driven by maximum metabolic demand, but by the cytosolic distribution of energy utilizing systems set by the functions of the tissue. ABSTRACT Mitochondrial adaptations are fundamental to differentiated function and energetic homeostasis in mammalian cells. But the mechanisms that underlie these relationships remain poorly understood. Here, we investigated organ-specific mitochondrial morphology, connectivity and protein composition in a model of extreme mammalian metabolism, the Least shrew (Cryptotis parva). This was achieved through a combination of high-resolution 3D focused-ion-beam EM imaging and tandem-mass-tag MS proteomics. We demonstrate that liver and kidney mitochondrial content are equivalent to the heart permitting assessment of mitochondrial adaptations in different organs with similar metabolic demand. Muscle mitochondrial networks (cardiac and skeletal) are extensive, with a high incidence of nanotunnels - which collectively support the metabolism of large muscle cells. Mitochondrial networks were not detected in the liver and kidney as individual mitochondria are localized with sites of ATP consumption. This configuration is not observed in striated muscle, likely due to a homogenous ATPase distribution and the structural requirements of contraction. These results demonstrate distinct, fundamental mitochondrial structural adaptations for similar metabolic demand that are dependent on the topology of energy utilization process in a mammalian model of extreme metabolism. Abstract figure. This study investigates the role of mitochondrial morphology and protein composition in setting the extreme metabolic rates of one of the smallest extant mammals - the North American least shrew (Cryptotis parva). To do this, mitochondrial characteristics from liver, kidney, skeletal muscle and heart tissues were compared as these tissues are major contributors to basal and maximum metabolic states. Liver and kidney mitochondrial volume density and protein content approach levels observed in the heart - indicating that these former tissues are major contributors to the high basal metabolic rates of small mammals. Despite this high mitochondrial content, the liver and kidney do not exhibit mitochondrial networking - structures that are proposed to conduct mitochondrial proton motive force at the scale of the cell. Shrew skeletal muscle and cardiac mitochondrial network organization is consistent with networks observed in larger mammals while also exhibiting increased connectivity at the nm-scale. Instead of forming networks, kidney and liver mitochondria are directly associated with sites of ATP utilization. These results identify conditions that dictate the formation of mitochondrial networks and processes that drive mammalian allometric scaling of metabolic rates. This article is protected by copyright. All rights reserved.