Illuminating Mitochondrial Function and Dysfunction using Multiphoton Technology

Abstract

Studying differential cell function or dysfunction within the kidney is made difficult by cellular heterogeneity, alterations in regional blood flow, oxygen tension, and interstitial tonicity, resulting in extremely complex anatomic and physiologic arrangements. Creative investigators have developed techniques to reduce this heterogeneity but often with loss or alteration of three-dimensional cellular associations, anatomic and physiologic cell-to-cell interactions, and harsh isolation and experimental conditions. These limitations have made "absolutes" difficult to determine, especially in relation to pathologic processes such as cell type involvement in renal ischemia. In this issue of JASN, Hall et al. 1 use multiphoton microscopy of kidney slices to compare mitochondrial parameters and differential cellular responses to stress. This approach allows them to quantify several key parameters of mitochondrial function and dysfunction in proximal tubular (PT) cells and directly compare different PT segments with each other and with distal tubular (DT) cells of the thick ascending limb (TAL). Because mitochondrial alterations play a central role in normal cell function and in response to injury, the article adds to our previous knowledge about an important area.To understand the significance and limitations of their contribution, one must first understand what is known about differences between PT and TAL cells. Abundant mitochondria in cortical and outer medullary renal epithelial cells are necessary to meet the ATP demands of sodium transport by high-capacity aerobic metabolism. They compose 33% of the volume of proximal convoluted tubular S1 cells, 39% of the cells in the S2 segment, and 22% of the volume of cells in the proximal straight S3 segment; in the medullary and cortical TAL cells, mitochondria account for 30 to 44% of cell volume. 2 Differential tubular cell metabolism is also notable for the absence of aerobic and anaerobic glycolysis in the proximal convoluted tubule (S1), 3,4 which removes an important mechanism for preserving cell ATP and viability during injury. 5 In contrast, DT segments, including the TAL, have well-developed glycolytic pathways. 3,4,6,7 Glycolysis occurs in the S3 segment of the proximal tubule, albeit to a lesser extent than in distal tubules, and contributes to maintaining ATP there when mitochondrial function is impaired. 3,8 Mitochondrial ATP production depends on substrate-supported, electron transport-mediated proton extrusion from the mitochondrial matrix that generates a proton gradient across the inner mitochondrial membrane. In turn, this is used to drive phosphorylation of ADP to ATP by proton movement down the gradient back into the matrix through the inner membrane F 1 F O -ATPase. Electron transport-driven proton extrusion is also responsible for the net potential across the inner membrane (⌬⌿ m ), and ⌬⌿ m in combination with the pH gradient accounts for the net proton electrochemical gradient across the membrane, also called the proton motive force. ⌬⌿ m is the ...

Hyperpolarized 13C magnetic resonance evaluation of renal ischemia reperfusion injury in a murine model

“…We found a mean decrease of 36% in the HP 13 C Bic/Pyr ratio from baseline to day 7 in the kidneys subjected to IRI. In the kidneys, multiple tubular transport processes require a high level of ATP generation, which is derived almost exclusively from mitochondrial oxidative phosphorylation . In renal tubular cells, Pyr is taken up by the mitochondria for oxidative phosphorylation and ATP production.…”

“…In the kidneys, multiple tubular transport processes require a high level of ATP generation, which is derived almost exclusively from mitochondrial oxidative phosphorylation. 35 In renal tubular cells, Pyr is taken up by the mitochondria for oxidative phosphorylation and ATP production. In the first step of oxidative phosphorylation, the mitochondrial enzyme PDH irreversibly converts Pyr into acetyl-CoA and CO 2 , which is in equilibrium with Bic.…”

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“…These obligately aerobic and highly metabolically active cells are especially prone to hypoxic injury both in vivo and in vitro because glycolytic pathways for preservation of ATP content in the absence of mitochondrial oxidative phosphorylation are either absent or only minimally expressed in healthy proximal tubule cells [16] so that, in the absence of modifying interventions, oxygen deprivation of 15–30 minutes results in necrotic cell death to most cells as manifested by LDH release [17–19], and classical oncotic structural changes [18] that disrupt cellular structure and decrease numbers of intact cells that can be recovered by centrifugation [5]. As a result of these events the cells lose their ability to maintain integrated metabolic, energetic and transport functions during reoxygenation as manifested by failed recovery of mitochondrial respiration and intracellular ATP and K + levels.…”

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