New Insights into the Lactate Shuttle: Lesson Learned from the MCT4 Knock Out Mouse
For a long time, lactate has been considered just a waste by-product of anaerobic glycolysis and cause of muscle fatigue after intense exercise. Extensive research, however, showed that lactate is a physiological product of both aerobic and anaerobic glycolysis and its metabolism is much more complex, as it can serve both as energy substrate and signaling molecule. In 1986 George Brooks proposed that during exercise, lactate is shuttled from fast-twitch fibers to slow-twitch ones, as well as other tissue like heart and brain, where it is metabolized through oxidative phosphorylation. The flux of lactate is facilitated by the Monocarboxylate Transporters (MCT), H+-coupled transporters found at the plasma membrane of several tissues. MCT1 and MCT4 mediate the influx and efflux of lactate in slow twitch and fast twitch muscle respectively. Besides their role in maintaining the physiological concentrations of lactate and other metabolites, MCT1 and MCT4 have a relevant role in several pathologies including cancer and inflammation, as well as neurodegenerative diseases. In the first of the presented studies, we investigated the importance of the lactate shuttle during exercise by using whole-body (MCT4-/-) and muscle-specific BSG knockout mice. Using a combination of in vivo and ex vivo functional assays, we found that MCT4-/- mice are exercise intolerant. However, the disruption of the lactate flux does not affect the morphological and intrinsic contractile properties of muscle. To understand the causes of the limited exercise capacity in the MCT4-/- mice, we investigated the role of MCT4 in the neuronal components of the motor unit. Although we detected MCT4 expression in the lumbar spinal cord, the deletion in the MCT4-/- mice did not cause any morphological changes to the motoneurons or the sciatic nerve. Morphological analysis of the neuromuscular junctions (NMJs) showed a significant age-dependent, pre- and post-synaptic degeneration in the fast EDL muscles (but not in the slow, soleus) from MCT4-/- mice. Consistently, in vivo analysis of compound muscle action potentials (CMAPs) in control and MCT4-/- mice showed a significant reduction of the electromyographic (EMG) signal amplitude indicative of functional impairment of the propagation of action potentials. In conclusion, this first study showed that, while well-tolerated in muscle, the ablation of MCT4 causes significant age-dependent alterations of stability and function of the motor unit and in particular of the NMJs. As mentioned, the metabolic and signaling effects of MCT4 are relevant for a variety of metabolically active tissues such as exercising muscle, and also cancer cells. Indeed, MCT4 expression is significantly high in several type of cancers,and is associated with poor prognosis. In the second study presented here, we investigated the consequences of MCT4 ablation in a model carcinogen-induced head and neck squamous cell cancer (HNSCC). We found that MCT4-/- mice developed fewer and less severe tumor lesions compared to the control animals. We showed that MCT4 is expressed in small foci of early lesions. In more severe lesions, like in situ carcinomas, it was found in the hypoxic core of the tumors. Widespread expression of the transporter in both tumor cells and stroma was found in invasive lesions. Moreover, we detected MCT4 positive macrophages infiltrating the tumors and in the adjacent stroma, suggesting that the lactate transporter plays a role in immune activation and tumor progression. This was confirmed also in samples of human HNSCC. The result of this study showed that MCT4 is a potential new marker for early HNSCC lesions and the inhibition of lactate efflux could have a clinical benefit for the prevention of invasive tumors. Overall, the study of the MCT4-/- mice allowed us to better understand the physiological role of the lactate transporter in metabolically active tissues, providing new evidence supporting the potential applicability of MCT4 inhibitors for the treatment of pathologies such cancer.
Bisetto, Sara, "New Insights into the Lactate Shuttle: Lesson Learned from the MCT4 Knock Out Mouse" (2019). ETD Collection for Thomas Jefferson University. AAI27543024.