Protein tyrosine phosphatases as novel therapeutic targets to overcome inflammation-induced insulin resistance and skeletal muscle atrophy

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Critically ill patients frequently develop muscle wasting and atrophy leading to a pronounced loss of muscle mass and strength. This condition is known as intensive care unit (ICU)-acquired weakness (ICUAW) and is associated with high morbidity and mortality. A specific therapy is not available. Risk factors include systemic inflammation, sepsis and uncontrolled hyperglycemia. How these factors are interconnected and lead to ICUAW is unclear.In ICUAW patients, peripheral insulin resistance often occurs, which reduces muscular glucose uptake. Since energy production in muscle is mainly driven by glucose, the result is a reduced synthesis of adenosine triphosphate (ATP) and an increased protein breakdown, which leads to muscle atrophy and weakness. In addition, systemic inflammation occurs in critically ill patients, which aggravates insulin resistance. Clinical studies have shown that correcting high blood glucose levels by intensive insulin therapy reduces but does not prevent ICUAW. Insulin resistance develops at the level of the insulin receptor or its post-receptor signaling. Protein tyrosine phosphatases (PTPs) inhibit insulin receptor activity. Since our preliminary work implicates the phosphatases PTP1B and TC-PTP in inflammatory muscle weakness, we aim to test the hypothesis that these PTPs are therapeutic targets of inflammatory insulin resistance and muscle weakness.Using myocytes in culture and a mouse model of polymicrobial sepsis, we will evaluate PTP1B and TC-PTP as key molecules for inflammatory insulin resistance and muscle atrophy. For this approach, PTP activity will be measured by tyrosine dephosphorylation assays. Protein content and gene expression of components of insulin signaling, including PTPs, will be analyzed under basal and inflammatory conditions in myocytes and skeletal muscle. In addition, we investigate the insulin receptor phosphorylation, the activity of insulin signaling and the effect of pharmacological PTP1B and TC-PTP inhibition or knockdown / knockout by RNAi and CRISPR / Cas9 on inflammatory myocyte atrophy in vitro. Further, we will investigate whether specific PTP1B-inhibition prevents inflammatory insulin resistance and muscle atrophy in vivo as well. For this purpose, insulin sensitivity will be quantified by hyperinsulinemic/euglycemic clamp in vivo as well as glucose uptake assays in myocytes and muscles of myocyte-specific PTP1B- and TC-PTP knockout mice. The impact of PTPs on cell- and fiber-specific insulin receptor phosphorylation will be investigated by proximity ligation assays. The effect of a specific PTP deletion or PTP inhibition on inflammatory muscle weakness will be evaluated by muscle strength measurements.We aim to establish PTP1B and TC-PTP as therapeutic targets in ICUAW and to pave the way for a novel causally-oriented approach to overcome insulin resistance and to prevent inflammatory muscle weakness in critically ill patients.