Adipose CaMKI: a pivotal controller of triglyceride metabolism

Obesity is a significant, and growing, public health issue in the United Kingdom. Though overweight itself is often not unhealthy, obesity can predispose people to develop other, more serious, health problems. One of the most common diseases caused by obesity is type-2 diabetes, in which the body no longer responds properly to insulin. Many aspects of obesity itself, and the diseases it can cause, are not well understood: we don't fully understand how other parts of the body communicate with adipose (fat) cells, and we don't fully understand how adipose cells communicate with one another, to regulate the amount of fat they contain; we don't fully understand how the communications between these cells actually causes changes in fat accumulation. The answers to these cellular questions would help us understand why some people become obese and others don't, and why some obese people develop diabetes and others don't.

We will study these issues in fruit flies. Many of the mechanisms of metabolic regulation are similar between flies and people, but flies are relatively easy and inexpensive to work with. We have unexpectedly found that a gene called CaMKI is required for normal fat metabolism in flies. Flies that don't have CaMKI are extremely obese--they have about four times as much fat as normal flies--but they are also healthy: they live as long as normal flies and apparently are not diabetic. This suggests that CaMKI is important in controlling fat accumulation and also in the development of diabetes.

CaMKI is a kind of protein called a kinase. Kinases function by modifying other proteins; kinase activity is often responsive to signals from outside the cell. Our experiments will test how CaMKI is involved in the adipose cells' ability to receive signals from other tissues, whether CaMKI is involved in the development of diabetes, and how CaMKI actually works to change the accumulation of fat--what proteins are modified by CaMKI and what the effects are of that modification.

Our preliminary results, combined with the experiments we propose, represent a dramatic step in our knowledge of how adipose cells control their fat level and how fat accumulation can lead to diabetes. Our work will have significant implications regarding metabolic regulation and metabolic disease and may help us understand the nature of "healthy" obesity.

Signalling mechanisms that control adiposity are only partially understood. It is clear that adipose calcium signalling is important in the regulation of triglyceride balance, but the important effectors of calcium in this context are not known. We show that Drosophila carrying a mutation in calcium/calmodulin dependent kinase I (CaMKI) carry dramatically more triglyceride than wild-type flies. These animals are apparently healthy: they have normal lifespans and normal or low glycemia. Anatomically, this effect maps (at least in part) to a requirement for CaMKI in fat cells. We have, with collaborators, undertaken phosphoproteomic analysis on these animals; this has identified a large number of phosphorylations that differ between CaMKI mutants and wild-type animals, but we have not yet determined which of these phosphorylation sites are direct CaMKI targets or which of these sites are important drivers of the observed phenotype.

This project has three aims. First, we will follow the results of our phosphoproteomic survey to identify putative direct targets of CaMKI and define their relevance in driving the remarkable adiposity of CaMKI mutants. Second, we will explore the role of CaMKI in the response to endocrine regulators of metabolism; we observe that CaMKI is required for acute responses to the catabolic hormone AKH, but we don't know the physiological relevance of this interaction, and we don't know how CaMKI interacts with anabolic hormones such as insulin. Finally, recent work in mammals has indicated that calcium signalling is important in insulin resistance driven by proinflammatory cytokines; we will define the role played by CaMKI in this process.

This work will shed significant new light on the signalling mechanisms of adiposity control and the related issue of development of obesity-related disease.