Preface-Autophagy: An old mechanism with new challenges ahead

MicroRNAs in Age-Related Proteostasis and Stress Responses

Aging is associated with the accumulation of damaged and misfolded proteins through a decline in the protein homeostasis (proteostasis) machinery, leading to various age-associated protein misfolding diseases such as Huntington's or Parkinson's. The efficiency of cellular stress response pathways also weakens with age, further contributing to the failure to maintain proteostasis. MicroRNAs (miRNAs or miRs) are a class of small, non-coding RNAs (ncRNAs) that bind target messenger RNAs at their 3'UTR, resulting in the post-transcriptional repression of gene expression. From the discovery of aging roles for lin-4 in C. elegans, the role of numerous miRNAs in controlling the aging process has been uncovered in different organisms. Recent studies have also shown that miRNAs regulate different components of proteostasis machinery as well as cellular response pathways to proteotoxic stress, some of which are very important during aging or in age-related pathologies. Here, we present a review of these findings, highlighting the role of individual miRNAs in age-associated protein folding and degradation across different organisms. We also broadly summarize the relationships between miRNAs and organelle-specific stress response pathways during aging and in various age-associated diseases.

Regulation of autophagy by miRNAs in human diseases

Autophagy is a homeostatic process designed to eliminate dysfunctional and aging organelles and misfolded proteins through a well-concerted pathway, starting with forming a double-membrane vesicle and culminating in the lysosomal degradation of the cargo enclosed inside the mature vesicle. As a vital sentry of cellular health, autophagy is regulated in every human disease condition and is an essential target for non-coding RNAs like microRNAs (miRNAs). miRNAs are short oligonucleotides that specifically bind to the 3'-untranslated region (UTR) of target mRNAs, thus leading to mRNA silencing, degradation, or translation blockage. This review summarizes the recent findings regarding the regulation of autophagy and autophagy-related genes by different miRNAs in various pathological conditions, including cancer, kidney and liver disorders, neurodegeneration, cardiovascular disorders, infectious diseases, aging-related conditions, and inflammation-related diseases. As miRNAs are being identified as prime regulators of autophagy in human disease, pharmacological molecules and traditional medicines targeting these miRNAs are also being tested in disease models, thus initiating a new series of therapeutic interventions targeting autophagy.