The Bioenergetic Health Index: a new concept in mitochondrial translational research

Abstract

Bioenergetics has become central to our understanding of pathological mechanisms, the development of new therapeutic strategies and as a biomarker for disease progression in neurodegeneration, diabetes, cancer and cardiovascular disease. A key concept is that the mitochondrion can act as the 'canary in the coal mine' by serving as an early warning of bioenergetic crisis in patient populations. We propose that new clinical tests to monitor changes in bioenergetics in patient populations are needed to take advantage of the early and sensitive ability of bioenergetics to determine severity and progression in complex and multifactorial diseases. With the recent development of high-throughput assays to measure cellular energetic function in the small number of cells that can be isolated from human blood these clinical tests are now feasible. We have shown that the sequential addition of well-characterized inhibitors of oxidative phosphorylation allows a bioenergetic profile to be measured in cells isolated from normal or pathological samples. From these data we propose that a single value-the Bioenergetic Health Index (BHI)-can be calculated to represent the patient's composite mitochondrial profile for a selected cell type. In the present Hypothesis paper, we discuss how BHI could serve as a dynamic index of bioenergetic health and how it can be measured in platelets and leucocytes. We propose that, ultimately, BHI has the potential to be a new biomarker for assessing patient health with both prognostic and diagnostic value.

Figure 1. BHI as a dynamic measure of the response of the body to stress

In this scheme, healthy subjects have a high BHI with a high bioenergetic reserve capacity, high ATP-linked respiration (AL) and low proton leak (PL). The population of mitochondria is maintained by regenerative biogenesis. During normal metabolism, a sub-healthy mitochondrial population, still capable of meeting the energetic demand of the cell, accumulates functional defects, which can be repaired or turned over by mitophagy. Chronic metabolic stress induces damage in the mitochondrial respiratory machinery by progressively decreasing mitochondrial function and this manifests as low ATP-linked respiration, low reserve capacity and high non-mitochondrial (e.g. ROS generation) respiration. These bioenergetically inefficient damaged mitochondria exhibit increased proton leak and require higher levels of ATP for maintaining organelle integrity, which increases the basal oxygen consumption. In addition, chronic metabolic stress also promotes mitochondrial superoxide generation leading to increased oxidative stress, which can amplify mitochondrial damage, the population of unhealthy mitochondria and basal cellular energy requirements. The persistence of unhealthy mitochondria damages the mtDNA, which impairs the integrity of the biogenesis programme, leading to a progressive deterioration in bioenergetic function, which we propose can be identified by changes in different parameters of the bioenergetics profile and decreasing BHI.

Figure 2. Cellular mitochondrial profile in human monocytes

This assay defines cellular mitochondrial function using the well-defined inhibitors, oligomycin (Oligo), FCCP and antimycin A (AntiA) [12]. The interpretation of the different parameters defined by the assay is described in the accompanying text. Data is typically normalized to total protein or cell number in each well. Values are means±S.E.M., n=3–5.

Figure 3. Change in the BHI of monocytes subjected to oxidative stress

(A) The bioenergetic profiles of freshly isolated CD14⁺ monocytes from healthy volunteers were exposed to 4-HNE (20 μM for 1 h at 37°C) before the assay. AntiA, antimycin A; Oligo, oligomycin. (B) The BHI calculated using the mathematical relationship described in the text from the profile in (A) is demonstrated. Mean data (n=3–5 replicates) were plotted with ±S.E.M. (A) and +S.D. (B). #P ≤ 0.0001. All study protocols for collection and handling of human samples were reviewed and approved by the Institutional Review Board, University of Alabama at Birmingham.