The lipid droplet–associated protein Fsp27 mediates lipid droplet growth by promoting directional lipid transfer from smaller to larger lipid droplets.

Fsp27, a member of the Cide family proteins, was shown to localize to lipid droplet and promote lipid storage in adipocytes. We aimed to understand the biological role of Fsp27 in regulating adipose tissue differentiation, insulin sensitivity and energy balance. Fsp27 −/− mice and Fsp27/lep double deficient mice were generated and we examined the adiposity, whole body metabolism, BAT and WAT morphology, insulin sensitivity, mitochondrial activity, and gene expression changes in these mouse strains. Furthermore, we isolated mouse embryonic fibroblasts (MEFs) from wildtype and Fsp27 −/− mice, followed by their differentiation into adipocytes in vitro. We found that Fsp27 is expressed in both brown adipose tissue (BAT) and white adipose tissue (WAT) and its levels were significantly elevated in the WAT and liver of leptin-deficient ob/ob mice. Fsp27 −/− mice had increased energy expenditure, lower levels of plasma triglycerides and free fatty acids. Furthermore, Fsp27 −/− and Fsp27/lep double-deficient mice are resistant to diet-induced obesity and display increased insulin sensitivity. Moreover, white adipocytes in Fsp27 −/− mice have reduced triglycerides accumulation and smaller lipid droplets, while levels of mitochondrial proteins, mitochondrial size and activity are dramatically increased. We further demonstrated that BAT-specific genes and key metabolic controlling factors such as FoxC2, PPAR and PGC1α were all markedly upregulated. In contrast, factors inhibiting BAT differentiation such as Rb, p107 and RIP140 were down-regulated in the WAT of Fsp27 −/− mice. Remarkably, Fsp27 −/− MEFs differentiated in vitro show many brown adipocyte characteristics in the presence of the thyroid hormone triiodothyronine (T3). Our data thus suggest that Fsp27 acts as a novel regulator in vivo to control WAT identity, mitochondrial activity and insulin sensitivity.

Background: Immune checkpoint inhibitors (ICI) treat an expanding range of cancers. Consistent basic data suggest that these same checkpoints are critical negative regulators of atherosclerosis. Therefore, our objectives were to test whether ICIs were associated with accelerated atherosclerosis and a higher risk of atherosclerosis-related cardiovascular events. Methods: The study was situated in a single academic medical center. The primary analysis evaluated whether exposure to an ICI was associated with atherosclerotic cardiovascular events in 2842 patients and 2842 controls, matched by age, a history of cardiovascular events and cancer type. In a second design, a case-crossover analysis was performed with an "at-risk period" defined as the two-year period after and the "control period" as the two-year prior to treatment. The primary outcome was a composite of atherosclerotic cardiovascular events (myocardial infarction, coronary revascularization and ischemic stroke). Secondary outcomes included the individual components of the primary outcome. Additionally, in an imaging sub-study (n=40), the rate of atherosclerotic plaque progression was compared from before and after starting an ICI. All study measures and outcomes were blindly adjudicated. Results: In the matched cohort study, there was a 3-fold higher risk for cardiovascular events after starting an ICI (HR, 3.3 [95% CI, 2.0-5.5]; P <0.001). There was a similar increase in each of the individual components of the primary outcome. In the case-crossover, there was also an increase in cardiovascular events from 1.37 to 6.55 per 100 person-years at two years (adjusted HR, 4.8 [95% CI, 3.5-6.5]; P <0.001). In the imaging study, the rate of progression of total aortic plaque volume was >3-fold higher with ICIs (from 2.1%/year pre-to 6.7%/year post). This association between ICI use and increased atherosclerotic plaque progression was attenuated with concomitant use of statins or corticosteroids. Conclusions: Cardiovascular events were higher after initiation of ICIs, potentially mediated by accelerated progression of atherosclerosis. Optimization of cardiovascular risk factors and increased awareness of cardiovascular risk, prior to, during and after treatment, should be considered among patients on an ICI.

SUMMARY Neuronal arborization is regulated by cell autonomous and non-autonomous mechanisms including endosomal signaling via BDNF/TrkB. The endosomal Na+/H+ exchanger 6 (NHE6) is mutated in a new autism-related disorder. NHE6 functions to permit proton leak from endosomes yet the mechanisms causing disease are unknown. We demonstrate that loss of NHE6 results in over-acidification of the endosomal compartment and attenuated TrkB signaling. Mouse brains with disrupted NHE6 display reduced axonal and dendritic branching, reduced synapse number and circuit strength. Site-directed mutagenesis shows that the proton leak function of NHE6 is required for neuronal arborization. We find that TrkB receptor co-localizes to NHE6-associated endosomes. TrkB protein and phosphorylation are reduced in NHE6 mutant neurons in response to BDNF signaling. Finally, exogenous BDNF rescues defects in neuronal arborization. We propose that NHE6 mutation leads to circuit defects that are in part due to impoverished neuronal arborization that may be treatable by enhanced TrkB signaling.

Mature white adipocytes contain a characteristic unilocular lipid droplet. However, the molecular mechanisms underlying unilocular lipid droplet formation are poorly understood. We previously showed that Fsp27, an adipocyte-specific lipid droplet-associated protein, promotes lipid droplet growth by initiating lipid exchange and transfer. Here, we identify Perilipin1 (Plin1), another adipocyte-specific lipid droplet-associated protein, as an Fsp27 activator. Plin1 interacts with the CIDE-N domain of Fsp27 and markedly increases Fsp27-mediated lipid exchange, lipid transfer and lipid droplet growth. Functional cooperation between Plin1 and Fsp27 is required for efficient lipid droplet growth in adipocytes, as depletion of either protein impairs lipid droplet growth. The CIDE-N domain of Fsp27 forms homodimers and disruption of CIDE-N homodimerization abolishes Fsp27-mediated lipid exchange and transfer. Interestingly, Plin1 can restore the activity of CIDE-N homodimerization-defective mutants of Fsp27. We thus uncover a novel mechanism underlying lipid droplet growth and unilocular lipid droplet formation that involves the cooperative action of Fsp27 and Plin1 in adipocytes.

Targeted and inducible regulation of mammalian gene expression is a broadly important capability. We engineered drug-inducible catalytically inactive Cpf1 fused to transcriptional activation domains to tune the expression of endogenous genes in human cells. Leveraging the multiplex capability of the Cpf1 platform, we demonstrate both synergistic and combinatorial gene expression in human cells. Our work should enable the development of multiplex gene perturbation library screens for understanding complex cellular phenotypes.

CIDE proteins are important regulators of energy homeostasis and are closely linked to the development of metabolic disorders including obesity, diabetes, and liver steatosis. They may serve as potential molecular targets for the screening of therapeutic drugs for these diseases.

We previously showed that Cidea À/À mice are resistant to diet-induced obesity through the upregulation of energy expenditure. The AMP-activated protein kinase (AMPK), consisting of catalytic a subunit and regulatory subunits b and c, has a pivotal function in energy homoeostasis. We show here that AMPK protein levels and enzymatic activity were significantly increased in the brown adipose tissue of Cidea À/À mice. We also found that Cidea is colocalized with AMPK in the endoplasmic reticulum and forms a complex with AMPK in vivo through specific interaction with the b subunit of AMPK, but not with the a or c subunit. When co-expressed with Cidea, the stability of AMPK-b subunit was dramatically reduced due to increased ubiquitinationmediated degradation, which depends on a physical interaction between Cidea and AMPK. Furthermore, AMPK stability and enzymatic activity were increased in Cidea À/À adipocytes differentiated from mouse embryonic fibroblasts or preadipocytes. Our data strongly suggest that AMPK can be regulated by Cidea-mediated ubiquitindependent proteosome degradation, and provide a molecular explanation for the increased energy expenditure and lean phenotype in Cidea-null mice.