GO Hessian for Expectation-Based Objectives

An unbiased low-variance gradient estimator, termed GO gradient, was proposed recently for expectation-based objectives $\mathbb{E}_{q_{\boldsymbol{\gamma}}(\boldsymbol{y})} [f(\boldsymbol{y})]$, where the random variable (RV) $\boldsymbol{y}$ may be drawn from a stochastic computation graph with continuous (non-reparameterizable) internal nodes and continuous/discrete leaves. Upgrading the GO gradient, we present for $\mathbb{E}_{q_{\boldsymbol{\boldsymbol{\gamma}}}(\boldsymbol{y})} [f(\boldsymbol{y})]$ an unbiased low-variance Hessian estimator, named GO Hessian. Considering practical implementation, we reveal that GO Hessian is easy-to-use with auto-differentiation and Hessian-vector products, enabling efficient cheap exploitation of curvature information over stochastic computation graphs. As representative examples, we present the GO Hessian for non-reparameterizable gamma and negative binomial RVs/nodes. Based on the GO Hessian, we design a new second-order method for $\mathbb{E}_{q_{\boldsymbol{\boldsymbol{\gamma}}}(\boldsymbol{y})} [f(\boldsymbol{y})]$, with rigorous experiments conducted to verify its effectiveness and efficiency

Cadmium and Secondary Structure-dependent Function of a Degron in the Pca1p Cadmium Exporter

Protein turnover is a critical cellular process regulating biochemical pathways and destroying terminally misfolded or damaged proteins. Pca1p, a cadmium exporter in the yeast Saccharomyces cerevisiae, is rapidly degraded by the endoplasmic reticulum-associated degradation (ERAD) system via a cis-acting degron that exists at the 250–350 amino acid region of Pca1p and is transferable to other proteins to serve as a degradation signal. Cadmium stabilizes Pca1p in a manner dependent on the degron. This suggested that cadmium-mediated masking of the degron impedes its interaction with the molecular factors involved in the ERAD. The characteristics and mechanisms of action of the degron in Pca1p and most of those in other proteins however remain to be determined. The results presented here indicate that specific cysteine residues in a degron of Pca1p sense cadmium.Anunbiased approach selecting non-functional degrons indicated a critical role of hydrophobic amino acids in the degron for its function.Asecondary structure modeling predicted the formation of an amphipathic helix. Site-directed mutagenesis confirmed the functional significance of the hydrophobic patch. Last, hydrophobic amino acids in the degron- and cadmium-binding region affected the interaction of Pca1p with the Ssa1p molecular chaperone, which is involved in ERAD. These results reveal the mechanism of action of the degron, which might be useful for the identification and characterization of other degrons

Glucagon regulates hepatic lipid metabolism via cAMP and Insig-2 signaling: implication for the pathogenesis of hypertriglyceridemia and hepatic steatosis

Insulin induced gene-2 (Insig-2) is an ER-resident protein that inhibits the activation of sterol regulatory element-binding proteins (SREBPs). However, cellular factors that regulate Insig-2 expression have not yet been identified. Here we reported that cyclic AMP-responsive element-binding protein H (CREBH) positively regulates mRNA and protein expression of a liver specific isoform of Insig-2, Insig-2a, which in turn hinders SREBP-1c activation and inhibits hepatic de novo lipogenesis. CREBH binds to the evolutionally conserved CRE-BP binding elements located in the enhancer region of Insig-2a and upregulates its mRNA and protein expression. Metabolic hormone glucagon and nutritional fasting activated CREBH, which upregulated expression of Insig-2a in hepatocytes and inhibited SREBP-1c activation. In contrast, genetic depletion of CREBH decreased Insig-2a expression, leading to the activation of SREBP-1c and its downstream lipogenic target enzymes. Compromising CREBH-Insig-2 signaling by siRNA interference against Insig-2 also disrupted the inhibitory effect of this signaling pathway on hepatic de novo triglyceride synthesis. These actions resulted in the accumulation of lipid droplets in hepatocytes and systemic hyperlipidemia. Our study identified CREBH as the first cellular protein that regulates Insig-2a expression. Glucagon activated the CREBH-Insig-2a signaling pathway to inhibit hepatic de novo lipogenesis and prevent the onset of hepatic steatosis and hypertriglyceridemia

Endoplasmic Reticulum-associated Degradation of Pca1p, a Polytopic Protein, via Interaction with the Proteasome at the Membrane

Endoplasmic reticulum-associated degradation (ERAD) plays a critical role in the destruction of terminally misfolded proteins at the secretory pathway. The system also regulates expression levels of several proteins such as Pca1p, a cadmium exporter in yeast. To gain better insight into the mechanisms underlying ERAD of Pca1p and other polytopic proteins by the proteasome in the cytosol, our study determined the roles for the molecular factors of ERAD in dislodging Pca1p from the endoplasmic reticulum (ER). Inactivation of the 20S proteasome leads to accumulation of ubiquitinated Pca1p in the ER membrane, suggesting a role for the proteasome in extraction of Pca1p from the ER. Pca1p formed a complex with the proteasome at the membrane in a Doa10p E3 ligase-dependent manner. Cdc48p is required for recruiting the proteasome to Pca1p. Although the Ufd2p E4 ubiquitin chain extension enzyme is involved in efficient degradation of Pca1p, Ufd2p-deficient cells did not affect the formation of a complex between Pca1p and the proteasome. Two other polytopic membrane proteins undergoing ERAD, Ste6*p and Hmg2p, also displayed the same outcomes observed for Pca1p. However, poly-ubiquitinated Cpy1*p, a luminal ERAD substrate, was detected in the cytosol independent of proteolytic activities of the proteasome. These results indicate that extraction and degradation of polytopic membrane proteins at the ER is a coupled event. This mechanism would relieve the cost of exposed hydrophobic domains in the cytosol during ERAD

Influenza C and D Viruses Demonstrated a Differential Respiratory Tissue Tropism in a Comparative Pathogenesis Study in Guinea Pigs

Influenza C virus (ICV) is increasingly associated with community-acquired pneumonia (CAP) in children and its disease severity is worse than the influenza B virus, but similar to influenza A virus associated CAP. Despite the ubiquitous infection landscape of ICV in humans, little is known about its replication and pathobiology in animals. The goal of this study was to understand the replication kinetics, tissue tropism, and pathogenesis of human ICV (huICV) in comparison to the swine influenza D virus (swIDV) in guinea pigs. Intranasal inoculation of both viruses did not cause clinical signs, however, the infected animals shed virus in nasal washes. The huICV replicated in the nasal turbinates, soft palate, and trachea but not in the lungs while swIDV replicated in all four tissues. A comparative analysis of tropism and pathogenesis of these two related seven-segmented influenza viruses revealed that swIDV-infected animals exhibited broad tissue tropism with an increased rate of shedding on 3, 5, and 7 dpi and high viral loads in the lungs compared to huICV. Seroconversion occurred late in the huICV group at 14 dpi, while swIDV-infected animals seroconverted at 7 dpi. Guinea pigs infected with huICV exhibited mild to moderate inflammatory changes in the epithelium of the soft palate and trachea, along with mucosal damage and multifocal alveolitis in the lungs. In summary, the replication kinetics and pathobiological characteristics of ICV in guinea pigs agree with the clinical manifestation of ICV infection in humans, and hence guinea pigs could be used to study these distantly related influenza viruses

The CREBH in hepatic lipid and lipoprotein metabolism: Implication for the pathogenesis of hyperlipidemia and metainflammation

Hyperlipidemia is reemerging as an important cardiovascular disease risk factor and other metabolic disorders, such as obesity, insulin resistance and type-2 diabetes. Hepatic de novo lipogenesis is controlled by a family of the b-zip transcription factors, the sterol regulatory element-binding proteins (SREBPs), which targets downstream genes involved in free fatty acids, triglycerides and cholesterol synthesis. Regulation of SREBPs signaling is controlled by a cluster of the ER membrane-bound proteins, the insulin induced gene-1(Insig-1) and gene-2 (Insig-2) and the SCAP protein. The cAMP responsive element binding protein H (CREBH) is a recently identified member of this family. CREBH is selectively and highly expressed in the liver and small intestine which is actively involved in lipid and glucose metabolism. However, its underlying molecular mechanisms are not fully understood. Here we demonstrated CREBH inhibits hepatic lipid de novo synthesis through modulating the expression of insulin induced gene-2 (Insig-2) isoform-a, a liver specific isoform of Insig-2, which is involved in the activation of sterol regulatory element-binding proteins (SREBPs). Metabolic cues, such as fasting, glucagon and cAMP agonist, activated CREBH which in turn inhibited SREBP-1c and SREBP-2 activation via upregulate the abundance of Insig-2a in hepatocytes. Depletion or suppression of CREBH expression by refeeding inhibited Insig-2a expression, which in turn hyperactivated SREBP-1c and -2, leading to the activation of hepatic de novo lipid synthesis, accumulation of lipids in hepatocytes and systemic hyperlipidemia. We further demonstrated that depletion of CREBH reduced both mRNA and protein expression of apolipoprotein B (apoB). In vitro, transient expression of CREBH cDNAs in McA cells induced significant increase of apoB mRNA and protein expression, which indicated the positive regulatory impact of CREBH on apoB biosynthesis. This study establishes the CREBH-Insig-2a as a novel metabolic pathway that regulates hepatic de novo lipogenesis. This novel finding provides new mechanistic insight into the pathogenesis of hyperlipidemia in metabolic diseases. Advisor: Qiaozhu S

Copper in Adipocyte Functions and Energy Homeostasis

Adipocytes play critical roles in metabolism by storing excess energy and utilizing fat through lipolysis and thermogenesis. Copper is an essential micronutrient for mammals in energy generation, thermogenic signaling, and lipid metabolism; nevertheless, uptake, distribution, and utilization of copper to support the adipocyte functions and implication of copper dysregulation in adipocyte-dependent metabolic disorders remain elusive. The long-term goal of this research project is to discover copper homeostasis in adipocytes with their metabolic activities and reveal the impacts of copper dysregulation in adipocytes on energy metabolism and metabolic diseases. To determine the regulation of copper as a function of adipocyte activities, we assessed copper homeostasis markers in the mice housed at a cold temperature demanding thermogenic energy expenditure or fed a high-fat diet (HFD) supplying excess energy. Our data demonstrated that adipocyte-dependent non-shivering thermogenesis increased the demand for copper in white adipose tissues (WAT), reflecting the recruitment of beige adipocytes possessing higher amounts of mitochondria that sequester copper. The reduced hepatic copper content suggests a redistribution of copper from the liver to adipose tissues. Conversely, the intake of a high-fat diet increased copper availability in the cytosol of WAT. We next generated and characterized a mouse line where the copper transporter 1 (Ctr1) gene encoding a high-affinity copper importer was deleted in adipocytes. The tissue copper levels and biomarkers revealed that CTR1 imports a significant portion of copper in the fat tissues. Ctr1 gene knockout mice tolerated copper limitation in the recommended/optimal growing conditions. Interestingly, Ctr1 gene knockout increased beige adipocyte recruitment and uncoupling protein 1 (UCP1) expression in the inguinal WAT, suggesting activation of thermogenic signaling pathways. When Ctr1-deficient mice were on HFD, the mice manifested reduced adipose tissue weight, increased energy expenditure, improved glucose homeostasis, and less hepatic fat accumulation. Indirect calorimetry analysis revealed a higher basal metabolism in the mice. Collectively, our study revealed a previously unrecognized relationship between copper and adipocyte-dependent macronutrient metabolism. These results are anticipated to advance our ability to combat obesity and metabolic diseases