Obesity induces resistance to central action of BMP8B through a mechanism involving the BBSome

Objective

Bone morphogenetic protein 8B (BMP8B) plays a major role in the regulation of energy homeostasis by modulating brown adipose tissue (BAT) thermogenesis and white adipose tissue (WAT) browning. Here, we investigated whether BMP8B's role in metabolism is affected by obesity and the possible molecular mechanisms underlying that action.

Methods

Central treatments with BMP8B were performed in rats fed a standard (SD) and high-fat diet (HFD), as well as in genetically modified mice. Energy balance studies, infrared thermographic analysis of BAT and molecular analysis of the hypothalamus, BAT and WAT were carried out.

Results

We show for the first time that HFD-induced obesity elicits resistance to the central actions of BMP8B on energy balance. This obesity-induced BMP8B resistance is explained by i) lack of effects on AMP-activated protein kinase (AMPK) signaling, ii) decreased BMP receptors signaling and iii) reduced expression of Bardet-Biedl Syndrome 1 (BBS1) protein, a key component of the protein complex BBSome in the ventromedial nucleus of the hypothalamus (VMH). The possible mechanistic involvement of BBS1 in this process is demonstrated by lack of a central response to BMP8B in mice carrying a single missense disease-causing mutation in the Bbs1 gene.

Conclusions

Overall, our data uncover a new mechanism of central resistance to hormonal action that may be of relevance in the pathophysiology of obesity.

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Central BMP8B induces BAT activation and browning through hypothalamic AMPK.

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Obesity elicits resistance to the central effects of BMP8B on energy balance.

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Obesity impairs the effect of BMP8B on AMPK, BMP Type I receptors signaling and BBS1 in the hypothalamus.

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Lack of BBS1 function recapitulates the thermogenic-induced resistance to central BMP8B.

ADAR1-Dependent RNA Editing Promotes MET and iPSC Reprogramming by Alleviating ER Stress

RNA editing of adenosine to inosine (A to I) is catalyzed by ADAR1 and dramatically alters the cellular transcriptome, although its functional roles in somatic cell reprogramming are largely unexplored. Here, we show that loss of ADAR1-mediated A-to-I editing disrupts mesenchymal-to-epithelial transition (MET) during induced pluripotent stem cell (iPSC) reprogramming and impedes acquisition of induced pluripotency. Using chemical and genetic approaches, we show that absence of ADAR1-dependent RNA editing induces aberrant innate immune responses through the double-stranded RNA (dsRNA) sensor MDA5, unleashing endoplasmic reticulum (ER) stress and hindering epithelial fate acquisition. We found that A-to-I editing impedes MDA5 sensing and sequestration of dsRNAs encoding membrane proteins, which promote ER homeostasis by activating the PERK-dependent unfolded protein response pathway to consequently facilitate MET. This study therefore establishes a critical role for ADAR1 and its A-to-I editing activity during cell fate transitions and delineates a key regulatory layer underlying MET to control efficient reprogramming.