Autophagy unleashes noncanonical microRNA functions

Abstract

MicroRNAs (miRNAs) are post-transcriptional regulators of gene expression which act by guiding AGO (argonaute) proteins to target RNA transcripts in the RNA-induced silencing complex (RISC). This macromolecular complex includes multiple additional components (e.g., TNRC6A) that allow for interaction with enzymes mediating inhibition of translation or RNA decay. However, miRNAs also reside in low-molecular weight complexes without being engaged in target repression, and their function in this context is largely unknown. Our recent findings show that endothelial cells exposed to protective high-shear stress or MTORC inhibition activate the macroautophagy/autophagy machinery to sustain viability by promoting differential trafficking of MIR126 strands and by enabling unconventional features of MIR126-5p. Whereas MIR126-3p is degraded upon autophagy activation, MIR126-5p interacts with the RNA-binding protein MEX3A to form a ternary complex with AGO2. This complex forms on the autophagosomal surface and facilitates its nuclear localization. Once in the nucleus, MIR126-5p dissociates from AGO2 and establishes aptamer-like interactions with the effector CASP3 (caspase 3). The binding to MIR126-5p prevents dimerization and proper active site formation of CASP3, thus inhibiting proteolytic activity and limiting apoptosis. Disrupting this pathway in vivo by genetic deletion of Mex3a or by specific deficiency of endothelial autophagy aggravates endothelial apoptosis and exacerbates the progression of atherosclerosis. The direct inhibition of CASP3 by MIR126-5p reveals a non-canonical mechanism by which miRNAs can modulate protein function and mediate the autophagy-apoptosis crosstalk.

Keywords: Atherosclerosis; Autophagy; Endothelial cells; MEX3A; Noncanonical miRNA functions; miR-126-5p; microRNA.

Figure 1.

Summary of the MEX3A-guided nuclear MIR126-5p pathway. Atheroprotective high shear stress (HSS), activation of KLF2 (Krüppel like factor 2), and MTOR-inhibition by rapamycin favors the assembly of a ternary complex of MIR126-5p with MEX3A and AGO2 (A). This complex localizes on the extralumenal surface of autophagosome, which preserves it from degradation and leads to its nuclear transfer (B). In the nucleus, MIR126-5p dissociates from AGO2 and MEX3A and becomes available for binding the effector caspase, CASP3, which can be transferred to the nucleus when activated (C). The direct interaction inhibits CASP3 catalytic activity, thus reducing its function as the terminal effector of apoptotic cell death (D). The reduced apoptotic rate preserves endothelial cell viability to protect against atherosclerosis. The direct interference with protein activity expands the range of post-transcriptional regulation by miRNAs and highlights an anti-apoptotic mechanism for protecting integrity of the endothelium in health and disease