Identification and characterization of an endoplasmic reticulum localization motif

Injection of YiQiFuMai powder protects against heart failure via inhibiting p38 and ERK1/2 MAPKs activation

CONTEXT

Injection of YiQiFuMai (YQFM) powder, a modern Chinese plant-derived medical preparation, has a therapeutic effect in heart failure (HF). However, its therapeutic mechanism remains largely unknown.

OBJECTIVE

To investigate the molecular mechanisms of YQFM in HF.

MATERIALS AND METHODS

Kinase inhibition profiling assays with 2 mg/mL YQFM were performed against a series of 408 kinases. In addition, the effects of kinase inhibition were validated in cardiomyocyte cell line H9c2. In vivo, HF with reduced ejection fraction (HFrEF) was induced by permanent left anterior descending (LAD) coronary artery ligation for 6 weeks in male Sprague-Dawley rats. Then, HFrEF mice were treated with 0.46 g/kg YQFM or placebo once a day for 2 weeks. Echocardiography, immunohistochemistry, histological staining and Western blotting analysis were performed to assess the myocardial damage and molecular mechanisms.

RESULTS

Kinase inhibition profiling analysis demonstrated that mitogen-activated protein kinases (MAPKs) mediated the signalling cascades of YQFM during HF therapy. Meanwhile, p38 and extracellular signal-regulated kinases (ERK1/2) were inhibited after YQFM treatment in H9c2 cells. In rats, the control group had lower left ventricular ejection fraction (LVEF) at 37 ± 1.7% compared with the YQFM group at 54 ± 1.1% (p < 0.0001). Cardiac fibrosis levels in control group rats were significantly higher than YQFM group (30.5 ± 3.0 vs. 14.1 ± 1.0, p < 0.0001).

CONCLUSIONS

Our collective in vitro and in vivo experiments demonstrated that YQFM improves left ventricular (LV) function and inhibits fibrosis in HFrEF rats by inhibiting MAPK signalling pathways.

Cleavage of FNDC5 and insights into its maturation process

FNDC5 corresponds to an irisin precursor that increases with exercise. Studies suggest that irisin mediates beneficial effects in adipose tissues, skeletal muscle, bone, and brain. However, the cleavage and maturation processes of FNDC5 have not been clearly identified. This study aimed to show that the signal peptide and transmembrane domain of FNDC5 were associated with the secretion of its ectodomain. Localization studies identified the signal peptide that was responsible for endoplasmic reticulum targeting activity of nascent FNDC5 and showed that the FNDC5 ectodomain corresponding to irisin could be transported across the membrane by a transmembrane domain. Analysis of cleavage constructs revealed that the ectodomain of FNDC5 could be cleaved from its signal peptide and transmembrane attachment. Genetic ablation of the signal peptide cleavage site blocked N-glycosylation of FNDC5. Identification of the FNDC5 maturation process should facilitate our understanding of irisin secretion.

N-Glycosylation is required for FDNC5 stabilization and irisin secretion

Irisin, a myokine derived from the extracellular domain of FNDC5, has been shown to mediate thermogenesis of white adipose tissue. Biochemical data have shown that N-glycosylation of FNDC5 is unlikely to affect ligand or receptor activation of irisin. The N-glycosylation of FNDC5 remains poorly understood. In the present study, we analysed N-glycosylation sites of FNDC5 and found that two potential N-glycosylation sites (Asn³⁶ and Asn⁸¹) could indeed be occupied by N-glycan. Furthermore we showed that the lack of N-glycosylation decreases the secretion of irisin, which is relevant to the instability of FNDC5 and the deficiency of cleavage of the signal peptide. We also found that the expression level of N-glycosylated FNDC5 was elevated after myoblast differentiation. These findings show that the secretion of irisin is modulated by N-glycosylation, which in turn enhances our understanding of the secretion of glycosylated irisin.