Insights into Glycolytic Metabolism and pH Homeostasis in the Hypoxic Intervertebral Disc
Low back pain and associated intervertebral disc degeneration are the leading causes of disability in the United States and largely contribute to the recent opioid epidemic. While mechanisms of disc degeneration have been studied in many contexts, the relationship between disc degeneration and low back pain remains elusive. Very recently, researchers validated intradiscal acidity as a biomarker for identifying pain-causing discs and severity of disc degeneration. The goal of this work was to elucidate how and why changes in acid/base metabolism and glucose consumption synergize with age-dependent disc degeneration. We therefore investigated the role and regulation of a network of pH regulatory enzymes, Carbonic Anhydrases (CAs), and plasma-membrane transporters, Monocarboxylate Transporters (MCTs) and Glucose Transporter 1 (GLUT1), in the physiologically hypoxic and acidic intervertebral disc niche. We hypothesized that loss of function of these key regulatory proteins may contribute to the pathogenesis of disc degeneration. Our studies confirm that the hypoxia-inducible and HIF-1-dependent CA9/12 isozymes and MCT4 isoform are critical for NP cell survival and maintenance of the aging spine, respectively. Mechanistically, CA9/12 are required for intracellular pH regulation through HCO3– recycling, and MCT4 is critical for lactic acid efflux from the cell, as lactate accumulation was shown to instigate metabolic and transcriptional reprogramming in NP cells. Furthermore, investigations into the role of CA3 revealed that it is, in fact, not involved in intracellular pH regulation, but rather acts as a robustly-expressed antioxidant that protcts NP cells from oxidative-stress induced apoptosis. Finally, we tested the hypothesis that glucose import into NP cells, via the NP phenotypic marker, GLUT1, was required for disc development and maintenance. However, using an NP-specific GLUT1 knock-out mouse model, we discovered that GLUT1 was not required for NP cell viability or disc development. Taken together, our work contributes to a new model of the pH regulatory network that enables NP cell survival in their harsh microenvironmental niche. In addition, we add nuanced mechanistic insights into the role and regulation of NP phenotypic markers, CA3 and GLUT1 which, in fact, are not required for pH regulation or maintenance of NP cell metabolism.
Silagi, Elizabeth S, "Insights into Glycolytic Metabolism and pH Homeostasis in the Hypoxic Intervertebral Disc" (2019). ETD Collection for Thomas Jefferson University. AAI13878960.