Adipocytes are professional secretory cells. Many adipose tissue-secreted signaling mediators (adipokines) have been identified since the discovery of leptin and adiponectin. As such, much has been learned about how adipose tissue communicates with other organs of the body to maintain systemic homeostasis. Beyond proteins, additional factors such as lipids, metabolites, non-coding RNAs and extracellular vesicles released by adipose tissue participate in this process. However, major gaps still persist in our basic understanding of how leptin and adiponectin achieve their profound actions on target cells. In this renewal application, we focus on adiponectin, a protein we first described in 1995. The previous funding period has shed much light on how adiponectin and its receptors achieve anti-inflammatory, anti-apoptotic and insulin sensitizing effects through the ceramide axis, primarily through gain-of-function approaches. In the proposed studies in this application, we aim to elucidate why adiponectin knock out models generated by different groups give rise to seemingly different metabolic phenotypes. In preliminary studies, we attribute this phenomenon to different strategies for gene elimination. While our own knock out mouse completely lacks all relevant exons for the adiponectin gene, other models leave behind exon 3 that has the potential to encode a non-secreted form of globular adiponectin. We believe it is this remnant fragment that is responsible for the different phenotypes. Furthermore, we believe that endogenous adiponectin, even in the absence of any gene manipulation, may also have a non-secreted, cytoplasmic / nuclear form that exerts unique functions locally in adiponectin expressing cells. In fact, adiponectin is expressed in the proximal tubules of the kidney, specifically in the cortex. We show that adiponectin exerts a major role on renal gluconeogenesis. Adiponectin is also expressed in hepatic stellate cells, where it prevents the activation of stellate cells into fibrotic myofibroblasts. Inducible gain- and loss-of-function experiments will shed more light on the relevance of these phenomena. Finally, we are well positioned to eliminate both adiponectin receptors inducibly in target cells in the adult animal. This has not been possible so far due to embryonic lethality of double-knock out animals. We will focus our efforts initially on the hepatocyte and the adipocyte, in which we will inducibly eliminate both adiponectin receptors. Combined, these experiments will significantly broaden our knowledge of adiponectin function in ?non-conventional? areas of metabolism studies. ?Non-conventional? in the sense of adiponectin expression in cells other than adipocytes. Furthermore, ?non-conventional? in the sense of adiponectin action not mediated through its passing through the secretory pathway, but rather through its actions as a non-secreted fragment in the cytoplasm and/or nucleus. Particularly the latter efforts have been inspired by studies in the various adiponectin knock out models, which have given rise to a large amount of confusing and discrepant data that we aim to explain and clarify for the field.
Adiponectin has morphed into a widely-used biomarker reflecting the health of adipose tissue and systemic insulin sensitivity. Understanding the cellular and systemic physiology of adiponectin and its receptors has proven invaluable for a better understanding of the role of the adipocyte in systemic metabolic homeostasis. The experiments outlined here will not only focus on novel adiponectin fragments that act inside the cell, but will also focus on the role of adiponectin in non-adipocytes, particularly the cells of the proximal tubules in the kidney and in hepatic stellate cells.
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