Direct observation of cortactin protecting Arp2/3-actin filament branch junctions from GMF-mediated destabilization.

How cells tightly control the formation and turnover of branched actin filament arrays to drive cell motility, endocytosis, and other cellular processes is still not well understood. Here, we investigated the mechanistic relationship between two binding partners of the Arp2/3 complex, glia maturation factor (GMF) and cortactin. Individually, GMF and cortactin have opposite effects on the stability of actin filament branches, but it is unknown how they work in concert with each other to govern branch turnover. Using TIRF microscopy, we observe that GMF's branch destabilizing activities are potently blocked by cortactin (IC50 = 1.3 nM) and that this inhibition requires direct interactions of cortactin with Arp2/3 complex. The simplest model that would explain these results is competition for binding Arp2/3 complex. However, we find that cortactin and GMF do not compete for free Arp2/3 complex in solution. Further, we use single molecule analysis to show that cortactin's on-rate (3 ×107 s-1 M-1) and off-rate (0.03 s-1) at branch junctions are minimally affected by excess GMF. Together, these results show that cortactin binds with high affinity to branch junctions, where it blocks the destabilizing effects of GMF, possibly by a mechanism that is allosteric in nature. In addition, the affinities we measure for cortactin at actin filament branch junctions (Kd = 0.9 nM) and filament sides (Kd = 206 nM) are approximately 20-fold stronger than previously reported. These observations contribute to an emerging view of molecular complexity in how Arp2/3 complex is regulated through the integration of multiple inputs.

Role of microRNAs in regulation of insulin secretion and insulin signaling involved in type 2 diabetes mellitus.

Type 2 diabetes mellitus (T2DM) is a chronic metabolic disorder due to insulin resistance that can be caused by both genetic and environmental factors. In 2018, the American Diabetes Association (ADA) estimated more than 500 million T2DM cases globally. In recent years, studies conducted on humans and animals have suggested that non-coding RNAs, namely, microRNAs (miRNAs), post-transcriptionally regulate gene expression that can bring changes in normal physiology, resulting in the development of metabolic diseases. miRNAs also regulate different cellular processes including insulin synthesis and its secretion from pancreatic ß-islet cells, its development and function, insulin signaling and glucose homeostasis. Dysregulation of miRNA can affect the functioning of different tissues during the progression of T2DM. This review focuses on various miRNAs that influence the development of ß-cells and insulin secretion, various protein cascades that play an important role in insulin signaling and glucose uptake, and their role in insulin resistance. Similarly, the long noncoding RNAs also known as lncRNAs and their ß-cell characteristics involved in T2DM have been discussed. Finally, the significance of miRNAs and their mRNA targets as effective biomarkers and therapeutics in studying the early onset and progression of T2DM have been highlighted.