LAUSR

It has long been known that adipose tissue has functions beyond the storage of lipids. More particularly, adipocytes secrete hormones and other molecular cues, including adipokines that actively influence metabolism at distant tissue sites. However, until now researchers have not understood how these molecules are targeted to distant organs and, more importantly, how they regulate metabolic homeostasis. Scientists at Joslin Diabetes Center and Harvard Medical School in Boston have revealed a mechanism whereby small pieces of genetic material called microRNAs (miRNAs) act as messenger molecules that regulate gene expression in other organs such as the liver. miRNAs are snips of noncoding RNA produced intracellularly and secreted into the circulation either as free entities or packaged into small vesicles called exosomes. They mediate their effects either by mRNA cleavage, translational repression, or mRNA destabilization following binding to target transcript sequences. Using genetically engineered mice deficient for the miRNA-processing enzyme (Dicer) in their adipose tissue (ADicerKO), the researchers demonstrated that the levels of exosomal miRNA in the circulation were significantly lower compared to control animals. These animals also had less adipose tissue and showed some degree of insulin resistance. Transplanting thermogenic brown adipose tissue and, to a lesser extent, energystoring white adipose tissue into ADicerKO animals restored exosomal miRNA levels and their capacity to process glucose. These experiments elegantly indicated that adipose tissue is an important source of circulating exosomal miRNAs and that distinct adipose depots contribute differently to the type of exosomal miRNAs as well as the capacity to regulate whole-body metabolism. Thomou et al. then studied whether these mi-RNAs effectively direct gene regulation in distant tissues such as the liver. They focused specifically on fibroblast growth factor 21 (FGF21) and observed that this parameter was elevated within the liver of ADicerKO mice. They identified miR-99b as the predicted regulator of FGF21 expression. Further analysis showed that miR-99b was reduced in circulating exosomes from ADicerKO mice and that its levels were restored by brown adipose tissue transplantation. They then developed tools to measure the exosomal miRNA-dependent hepatic expression of FGF21 by transfecting ADicerKO mice with an Fgf21-luciferase reporter (FGF21-30-untranslated region [UTR] reporter). In a control condition, these animals featured a bright luminescent signal from the liver when subjected to in vivo imaging, correlating with the absence of miRNAs and their inability to repress hepatic Fgf21. When the mice were transferred with exosomes from wild-type mice or exosomes from knockout mice electroporated with miR-99b, hepatic Fgf21 expression was successfully repressed as visualized by the dampened intensity of the luminescence signal (originating from the FGF21-30UTR reporter). It seems, therefore, that in healthy conditions miRNA-99b is needed to keep Fgf21 repressed and that in its absence the repressive effect is lost, resulting in elevated hepatic Fgf21. Another important observation that the group of Kahn made using a similar in vitro system was that the regulation of Fgf21 was dependent on exosomal delivery and could not be recapitulated with naked miR-99. It is therefore not only the regulatory molecule and its actions that are important for the observed effects but also the barcode information provided by its packaging that is crucial in directing these vesicles to the desired tissue site. Additionally, packaging of the miRNA within exosomal vesicles may increase miRNA stability and in this way aid their delivery to distant sites in an intact state. Although FGF21 has emerged as an important regulator of metabolism and its association with lipodystrophy has been established, it is plausible that many more targets and tissues may be involved because