RNA sequencing of all transcripts and how islet β-cells fail.

In this issue, Cnop et al. (1) report on using RNA-sequencing methodology and the response of the whole transcriptome of human islets to 48-h exposure to saturated free fatty acid (FFA) palmitate. They demonstrated that palmitate altered the expression of 1,325 genes and shifted alternate splicing of 3,525 transcripts. This follows on from a similar study by the same group on the effects on human islets of 48-h exposure to the proinflammatory cytokines interleukin-1b and interferon-g in which 3,065 (16%) of transcripts were modified and, again, alternate splicing of transcripts was commonly seen (2). However, islet b-cell failure causing diabetes, particularly type 2 diabetes, develops over years and not 48 h. So how do these technically remarkable in vitro experiments on human islets help us to understand islet b-cell failure? Islet b-cell failure causing diabetes is due to impaired b-cell function and/or loss of b-cell mass. The pathways to islet b-cell failure, however, are very heterogeneous, requiring interaction between extrinsic stressors, such as cytokines or nutrient oversupply, and genetic or acquired islet susceptibility factors (3,4) (Fig. 1). In some circumstances, islet susceptibility may predominant such that extrinsic stressors are less important (e.g., some forms of monogenic diabetes [5]); while at the other end of the spectrum, severe extrinsic insults may be enough to cause failure of quite robust b-cells (e.g., aggressive autoimmune attack of type 1 diabetes or as a consequence of morbid obesity) (Fig. 1). Very often though, it will be the interaction of extrinsic and intrinsic factors that will result in the b-cell failure. Of note, the separation of type 1 from type 2 diabetes is becoming more blurred with overweight and insulin resistance possibly underlying some of the increasing prevalence of type 1 diabetes (4,6). Complex interactions among various external islet stressor factors and islet susceptibility factors are likely to underlie much of the heterogeneity in the diabetes phenotypes that we see (Fig. 1). The above-mentioned in vitro studies are really looking at the effects of severe insults from the outside, that of toxic saturated FFA or cytokines, on normal human islets (Fig. 1). They are not designed to investigate how various islet susceptibility factors interact with these extrinsic stresses. The experimental conditions are very different from those that occur in vivo. For example, palmitate is not the only FFA present in the circulation in vivo, but occurs within a mix of other FFAs. Importantly, monounsaturates, such as oleate, are well known to attenuate palmitate toxicity (7). The experimental conditions used, however, will result in islet damage, allowing analysis of the mechanisms involved, many of which will be important in islet b-cell failure in diabetes. The RNA-sequencing method of analysis used here confirms potential mechanisms already suspected, but also is a powerful tool of discovery for previously unsuspected processes by which islets may fail, with the new hypotheses generated requiring confirmation in other systems. Of note, this RNA-sequencing methodology identified the expression of 15,200 and 18,463 genes in human islets in the first and second of these studies, respectively (1,2). This has increased, by more than twofold, the number of genes known to be expressed in human b-cells. Importantly, 25/41 of type 1 candidate genes and 59/69 type 2 candidate genes were found expressed in human islets, consistent with genetic factors being involved in islet susceptibility. It also has become apparent how prone the human islet transcriptome is to alternative splicing in response to extrinsic stresses. Both cytokines and palmitate altered the expression of splicing factors, but the pattern of alternate splice variants in response to the two stresses was very different. These studies show, however, that altered transcript splicing is likely to be an important mechanism by which extrinsic stresses alter b-cell function and survival.