The Blind Lab

 

 

The exogenous plant phospholipid dilauryl-phosphatidylcholine (DLPC) has dramatic anti-diabetic effects in mice, dramatically improving glucose homeostasis and diet-induced fatty liver, completely dependent on the nuclear receptor Liver Receptor Homolog-1 (LRH-1, NR5A2). Despite early promise, DLPC therapies have not translated into human patients – but what has remained clear is that understanding how DLPC works would have tremendous impact on the development of therapeutics for diabetic liver pathophysiology.

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At the molecular level, DLPC is thought to activate NR5A2 the same way all nuclear receptors are thought to be activated – by allosterically changing NR5A2 shape. However, we established that NR5A2 is activated by the endogenous phospholipid PI(3,4,5)P3 (PIP3) and inhibited by PI(4,5)P2 (PIP2) through an atypical mechanism independent of allostery: NR5A2 acts as a scaffold for phosphoinositide (PIP) headgroups to form interaction surfaces, akin to PIP function in membranes. Importantly, we also discovered that a PIP2-kinase called Inositol Polyphosphate Multikinase (IPMK) directly phosphorylates PIP2 bound to NR5A2, which activates NR5A2.

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Elucidating these unusual ways NR5A2 is regulated suggests two even more novel mechanisms DLPC may use to activate NR5A2, both of which we test here. The first mechanism we test proposes that since NR5A2 functions as a scaffold for PIP headgroups, that DLPC activates NR5A2 by displacing an inhibitory phospholipid endogenous to liver cells, such as PIP2 . This would explain why NR5A2 has been so resistant to new drug discovery, as compound screens have only looked for allosteric modulators. The second mechanism we test proposes that since NR5A2 is by definition a phospholipid transfer protein, that DLPC activates NR5A2 by increasing “PIP2 shuffling” with nuclear membranes, enhancing IPMK PIP2-kinase activity. The act of phospholipid shuffling between transfer proteins and membranes is known to stimulate the activity of PI-kinases: the Sec14 transfer protein shuffles phosphatidylcholine (PC) and PI in membranes, exposing the PI-headgroup to the Pik1 PI-kinase, increasing kinase activity. Translated to our model, NR5A2 would stimulate IPMK PIP2-kinase by shuffling DLPC with PIP2 in membranes, helping to expose the PIP2 headgroup to IPMK, enhancing catalysis and generation of the activated NR5A2/PIP3 complex.

 

Details aside, our overall hypothesis is that DLPC activates NR5A2 through non-traditional mechanisms, which our group is ideally suited to address. Understanding the molecular details of how DLPC activates NR5A2 will drive translation of the anti-diabetic effects of DLPC seen in mice into the diabetes clinic most rapidly, by providing proof of principle for drug companies to invest in new approaches to activate NR5A2.

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