miR‐422a suppresses SMAD4 protein expression and promotes resistance to muscle loss

Abstract

BackgroundLoss of muscle mass and strength are important sequelae of chronic disease, but the response of individuals is remarkably variable, suggesting important genetic and epigenetic modulators of muscle homeostasis. Such factors are likely to modify the activity of pathways that regulate wasting, but to date, few such factors have been identified.MethodsThe effect of miR‐422a on SMAD4 expression and transforming growth factor (TGF)‐β signalling were determined by western blotting and luciferase assay. miRNA expression was determined by qPCR in plasma and muscle biopsy samples from a cross‐sectional study of patients with chronic obstructive pulmonary disease (COPD) and a longitudinal study of patients undergoing aortic surgery, who were subsequently admitted to the intensive care unit (ICU).ResultsmiR‐422a was identified, by a screen, as a microRNA that was present in the plasma of patients with COPD and negatively associated with muscle strength as well as being readily detectable in the muscle of patients. In vitro, miR‐422a suppressed SMAD4 expression and inhibited TGF‐beta and bone morphogenetic protein‐dependent luciferase activity in muscle cells. In male patients with COPD and those undergoing aortic surgery and on the ICU, a model of ICU‐associated muscle weakness, quadriceps expression of miR‐422a was positively associated with muscle strength (maximal voluntary contraction r = 0.59, P < 0.001 and r = 0.51, P = 0.004, for COPD and aortic surgery, respectively). Furthermore, pre‐surgery levels of miR‐422a were inversely associated with the amount of muscle that would be lost in the first post‐operative week (r = −0.57, P < 0.001).ConclusionsThese data suggest that differences in miR‐422a expression contribute to the susceptibility to muscle wasting associated with chronic and acute disease and that at least part of this activity may be mediated by reduced TGF‐beta signalling in skeletal muscle.

Muscle wasting in the presence of disease, why is it so variable?

“…miR‐422a is only present in primates and is expressed from an intragenic region. Circulating levels of miR‐422a were originally inversely associated with muscle strength and size in patients with COPD (Paul et al ., ). In the muscle, expression of miR‐422a was higher in patients than in controls but surprisingly miR‐422a expression was directly proportional to muscle size in COPD patients but not in normal healthy elderly individuals.…”

“…Consistent with this suggestion, SMAD4 was identified as a target of miR‐422a and transfection of human skeletal myoblasts with miR‐422a inhibited TGF‐β signalling. As canonical TGF‐β signalling is dependent on SMAD4, these data suggest that at least one mechanism by which miR‐422 provides resistance to muscle wasting is through the inhibition of SMAD‐dependent wasting (Paul et al ., ). These data suggest a microRNA‐dependent mechanism that regulates the TGF‐β signalling pathway in skeletal muscle as shown in Fig.…”

“…Consequently, increased miR‐422a is likely to reduce p53 activity. In addition to targeting these mRNAs, miR‐422a was able to bind to the mRNAs for several transcription regulators that contribute to skeletal muscle phenotype, including myogenic factor 6 (myf6), inhibitor of DNA binding 2 (ID2) and myocyte enhancer factor 2D (MEF2D) (Paul et al ., ). Whether and in what direction it regulates the expression of these proteins remains to be determined.…”

“…The TGF‐β signalling system provides a good example of the regulation of signalling by miRNAs. Increased miR‐422a reduces SMAD4 expression, thereby reducing the ability of the muscle to respond to myostatin and contributing to increased strength (Paul et al ., ). The second area of variability is myonuclear recruitment, which, in muscle, will depend on the balance of satellite cell/myoblast proliferation and differentiation.…”

Nucleic Acid-Based Therapeutics for Pulmonary Diseases

“…Elevated miR-424-5p expression in patients with muscle wasting might contribute to the inhibition of protein synthesis and loss of muscle mass (88). It was demonstrated that miR-422a, as a suppressor of TGF-β signaling by reducing the expression of SMAD4 protein, might attribute to the maintenance of muscle mass (89).…”