Intervertebral disc degeneration is highly prevalent within the elderly population and is a leading cause of chronic back pain and disability. Due to the link between disc degeneration and senescence, we explored the ability of the Dasatinib and Quercetin drug combination (D + Q) to prevent an age-dependent progression of disc degeneration in mice. We treated C57BL/6 mice beginning at 6, 14, and 18 months of age, and analyzed them at 23 months of age. Interestingly, 6- and 14-month D + Q cohorts show lower incidences of degeneration, and the treatment results in a significant decrease in senescence markers p16INK4a, p19ARF, and SASP molecules IL-6 and MMP13. Treatment also preserves cell viability, phenotype, and matrix content. Although transcriptomic analysis shows disc compartment-specific effects of the treatment, cell death and cytokine response pathways are commonly modulated across tissue types. Results suggest that senolytics may provide an attractive strategy to mitigating age-dependent disc degeneration.

Hypoxia‐inducible factors (HIFs) are critical to the development and homeostasis of hypoxic tissues. Although HIF‐2α, one of the main HIF‐α isoforms, is expressed in nucleus pulposus (NP) cells, its functions remain unknown. We deleted HIF‐2α in the NP tissue using a notochord‐specific FoxA2Cre allele to study HIF‐2α function in the adult intervertebral disc. Unlike observations in HIF‐1αcKO mice, fate mapping studies using Rosa26‐mTmG reporter showed that HIF‐2α loss in NP did not negatively impact cell survival or affect compartment development. Rather, loss of HIF‐2α resulted in slightly better attributes of NP morphology in 14‐month‐old HIF‐2αcKO mice as evident from lower scores of degeneration. These 14‐month‐old HIF‐2αcKO mice also exhibited significant reduction in NP tissue fibrosis and lower collagen turnover in the annulus fibrosis (AF) compartment. Imaging‐Fourier transform‐infrared (FTIR) analyses showed decreased collagen and protein content in the NP and maintained chondroitin sulfate levels in 14‐month‐old HIF‐2αcKO. Mechanistically, global transcriptomic analysis showed enrichment of differentially expressed genes with Gene Ontology (GO) terms related to metabolic processes and cell development, molecular functions concerned with histone and protein binding, and associated pathways, including oxidative stress. Noteworthy, these morphological differences were not apparent in 24‐month‐old HIF‐2αcKO, indicating that aging is the dominant factor in governing disc health. Together these data suggest that loss of HIF‐2α in the NP compartment is not detrimental to the intervertebral disc development but rather mitigates NP tissue fibrosis and offers mild but transient protection from age‐dependent early degenerative changes. © 2022 American Society for Bone and Mineral Research (ASBMR).

Glycolysis is central to homeostasis of nucleus pulposus (NP) cells in the avascular intervertebral disc. Since the glucose importer, GLUT1, is a well-established phenotypic marker of NP cells, we hypothesized that it is vital for development and post-natal maintenance of the disc. Surprisingly, primary NP cells treated with two well-characterized GLUT1 inhibitors maintained normal rates of glycolysis and ATP production, indicating intrinsic compensatory mechanisms. We show that NP cells mitigate the loss of GLUT1 function by rewiring glucose import through alternative transporters, particularly GLUT3 and possibly GLUT9/SGLT1, all of which are expressed in the disc. Noteworthy, we demonstrate that alternative substrates, such as glutamine and palmitate, do not compensate glucose restriction resulting from dual inhibition of GLUT1/3 by Glutor and inhibition compromises long-term cell viability. To investigate redundancy of GLUT1 function in NP in vivo, we generated two NP-specific knockout mice: Krt19CreERT; Glut1f/f and Foxa2Cre; Glut1f/f. In contrast to the strong phenotype of tissue-specific Glut1 knockout mice in other connective tissues and supporting our in vitro findings, there were no apparent defects in post-natal disc maintenance (K19CreERT; Glut1f/f) or development and maturation (Foxa2Cre; Glut1f/f) in mutant mice. Microarray analysis confirmed that GLUT1 loss in the NP compartment did not cause transcriptomic alterations, supporting that NP cells are refractory to GLUT1 loss. These findings provide the first evidence of functional redundancy in GLUT transporters in the physiologically hypoxic intervertebral disc and underscore the importance of glucose as the indispensable metabolic substrate for NP cells.

Glycolysis is central to homeostasis of nucleus pulposus (NP) cells in the avascular intervertebral disc. Since the glucose importer, GLUT1, is a highly enriched phenotypic marker of NP cells, we hypothesized that it is vital for the development and post-natal maintenance of the disc. Surprisingly, primary NP cells treated with two well-characterized GLUT1 inhibitors maintained normal rates of glycolysis and ATP production, indicating intrinsic compensatory mechanisms.We show in vitro that NP cells mitigate GLUT1 loss by rewiring glucose import through GLUT3.Noteworthy, we demonstrate that substrates, such as glutamine and palmitate, do not compensate for glucose restriction resulting from dual inhibition of GLUT1/3 and inhibition compromises long-term cell viability. To investigate the redundancy of GLUT1 function in NP, we generated two NP-specific knockout mice: Krt19 CreERT ; Glut1 f/f and Foxa2 Cre ; Glut1 f/f . Noteworthy, there were no apparent defects in post-natal disc health or development and maturation in mutant mice. Microarray analysis confirmed that GLUT1 loss did not cause transcriptomic alterations in the NP, supporting that cells are refractory to GLUT1 loss. These observations provide the first evidence of functional redundancy in GLUT transporters in the physiologically hypoxic intervertebral disc and underscore the importance of glucose as the indispensable substrate for NP cells.

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