Fatty acid binding protein (Fabp) 5 interacts with the calnexin cytoplasmic domain at the endoplasmic reticulum

Endothelial Cell-Derived Soluble CD200 Determines the Ability of Immune Cells to Cross the Blood-Brain Barrier

The infiltration of immune cells into the central nervous system mediates the development of autoimmune neuroinflammatory diseases. We previously showed that the loss of either Fabp5 or calnexin causes resistance to the induction of experimental autoimmune encephalomyelitis (EAE) in mice, an animal model of multiple sclerosis (MS). Here we show that brain endothelial cells lacking either Fabp5 or calnexin have an increased abundance of cell surface CD200 and soluble CD200 (sCD200) as well as decreased T-cell adhesion. In a tissue culture model of the blood-brain barrier, antagonizing the interaction of CD200 and sCD200 with T-cell CD200 receptor (CD200R1) via anti-CD200 blocking antibodies or the RNAi-mediated inhibition of CD200 production by endothelial cells increased T-cell adhesion and transmigration across monolayers of endothelial cells. Our findings demonstrate that sCD200 produced by brain endothelial cells regulates immune cell trafficking through the blood-brain barrier and is primarily responsible for preventing activated T-cells from entering the brain.

Importance of fatty acid binding proteins in cellular function and organismal metabolism

Fatty acid binding proteins (Fabps) are small soluble proteins that are abundant in the cytosol. These proteins are known to bind a myriad of small hydrophobic molecules and have been postulated to serve a variety of roles, yet their precise functions have remained an enigma over half a century of study. Here, we consider recent findings, along with the cumulative findings contributed by many laboratories working on Fabps over the last half century, to synthesize a new outlook for what functions Fabps serve in cells and organisms. Collectively, the findings illustrate that Fabps function as versatile multi-purpose devices serving as sensors, conveyors and modulators to enable cells to detect and handle a specific class of metabolites, and to adjust their metabolic capacity and efficiency.

Calnexin, More Than Just a Molecular Chaperone

Calnexin is a type I integral endoplasmic reticulum (ER) membrane protein with an N-terminal domain that resides in the lumen of the ER and a C-terminal domain that extends into the cytosol. Calnexin is commonly referred to as a molecular chaperone involved in the folding and quality control of membrane-associated and secreted proteins, a function that is attributed to its ER- localized domain with a structure that bears a strong resemblance to another luminal ER chaperone and Ca²⁺-binding protein known as calreticulin. Studies have discovered that the cytosolic C-terminal domain of calnexin undergoes distinct post-translational modifications and interacts with a variety of proteins. Here, we discuss recent findings and hypothesize that the post-translational modifications of the calnexin C-terminal domain and its interaction with specific cytosolic proteins play a role in coordinating ER functions with events taking place in the cytosol and other cellular compartments.

A View of the Endoplasmic Reticulum Through the Calreticulin Lens

Calreticulin is well known as an ER-resident protein that serves as the major endoplasmic reticulum (ER) Ca²⁺ binding protein. This protein has been the major topic of discussion in an international workshop that has been meeting for a quarter of a century. In sharing information about this protein, the field also witnessed remarkable insights into the importance of the ER as an organelle and the role of ER Ca²⁺ in coordinating ER and cellular functions. Recent technological advances have helped to uncover the contributions of calreticulin in maintaining Ca²⁺ homeostasis in the ER and to unravel its involvement in a multitude of cellular processes as highlighted in this collection of articles. The continuing revelations of unexpected involvement of calreticulin and Ca²⁺ in many critical aspects of cellular function promises to further improve insights into the significance of this protein in the promotion of physiology as well as prevention of pathology.

FABP5 Deficiency Impairs Mitochondrial Function and Aggravates Pathological Cardiac Remodeling and Dysfunction

Fatty acid-binding protein 5 (FABP5) is an important member of the FABP family and plays a vital role in the metabolism of fatty acids. However, few studies have examined the role of FABP5 in pathological cardiac remodeling and heart failure. The aim of this study was to explore the role of FABP5 in transverse aortic constriction (TAC)-induced pathological cardiac remodeling and dysfunction in mice. Quantitative RT-PCR (qRT-PCR) and western blotting (WB) analysis showed that the levels of FABP5 mRNA and protein, respectively, were upregulated in hearts of the TAC model. Ten weeks after TAC in FABP5 knockout and wild type control mice, echocardiography, histopathology, qRT-PCR, and WB demonstrated that FABP5 deficiency aggravated cardiac injury (both cardiac hypertrophy and fibrosis) and dysfunction. In addition, transmission electron microscopy, ATP detection, and WB revealed that TAC caused severe impairment to mitochondria in the hearts of FABP5-deficient mice compared with that in control mice. When FABP5 was downregulated by siRNA in primary mouse cardiac fibroblasts, FABP5 silencing increased oxidative stress, reduced mitochondrial respiration, and increased the expression of myofibroblast activation marker genes in response to treatment with transforming growth factor-β. Our findings demonstrate that FABP5 deficiency aggravates cardiac pathological remodeling and dysfunction by damaging cardiac mitochondrial function.

The Fabp5/calnexin complex is a prerequisite for sensitization of mice to experimental autoimmune encephalomyelitis

We previously showed that calnexin (Canx)-deficient mice are desensitized to experimental autoimmune encephalomyelitis (EAE) induction, a model that is frequently used to study inflammatory demyelinating diseases, due to increased resistance of the blood-brain barrier to immune cell transmigration. We also discovered that Fabp5, an abundant cytoplasmic lipid-binding protein found in brain endothelial cells, makes protein-protein contact with the cytoplasmic C-tail domain of Canx. Remarkably, both Canx-deficient and Fabp5-deficient mice commonly manifest resistance to EAE induction. Here, we evaluated the importance of Fabp5/Canx interactions on EAE pathogenesis and on the patency of a model blood-brain barrier to T-cell transcellular migration. The results demonstrate that formation of a complex comprised of Fabp5 and the C-tail domain of Canx dictates the permeability of the model blood-brain barrier to immune cells and is also a prerequisite for EAE pathogenesis.

Identification of calnexin as a diacylglycerol acyltransferase-2 interacting protein

Triacylglycerol synthesis is catalyzed by acyl CoA:diacylglycerol acyltransferase-2 (DGAT2). DGAT2 is an integral membrane protein that is localized to the endoplasmic reticulum and interacts with lipid droplets. Using BioId, a method to detect proximal and interacting proteins, we identified calnexin as a DGAT2-interacting protein. Co-immunoprecipitation and proximity ligation assays confirmed this finding. We found that calnexin-deficient mouse embryonic fibroblasts had reduced intracellular triacylglycerol levels and fewer large lipid droplets (>1.0 μm² area). Despite the alterations in triacylglycerol metabolism, in vitro DGAT2 activity, localization and protein stability were not affected by the absence of calnexin.

Fatty-acid binding protein 5 modulates the SAR1 GTPase cycle and enhances budding of large COPII cargoes

COPII-coated vesicles are the primary mediators of ER-to-Golgi trafficking. Sar1, one of the five core COPII components, is a highly conserved small GTPase, which, upon GTP binding, recruits the other COPII proteins to the ER membrane. It has been hypothesized that the changes in the kinetics of SAR1 GTPase may allow for the secretion of large cargoes. Here we developed a cell-free assay to recapitulate COPII-dependent budding of large lipoprotein cargoes from the ER. We identified fatty-acid binding protein 5 (FABP5) as an enhancer of this budding process. We found that FABP5 promotes the budding of particles ∼150 nm in diameter and modulates the kinetics of the SAR1 GTPase cycle. We further found that FABP5 enhances the trafficking of lipoproteins and of other cargoes, including collagen. These data identify a novel regulator of SAR1 GTPase activity and highlight the importance of this activity for trafficking of large cargoes.

Proteomic profiling of liver tissue from the mdx-4cv mouse model of Duchenne muscular dystrophy

Background

Duchenne muscular dystrophy is a highly complex multi-system disease caused by primary abnormalities in the membrane cytoskeletal protein dystrophin. Besides progressive skeletal muscle degeneration, this neuromuscular disorder is also associated with pathophysiological perturbations in many other organs including the liver. To determine potential proteome-wide alterations in liver tissue, we have used a comparative and mass spectrometry-based approach to study the dystrophic mdx-4cv mouse model of dystrophinopathy.

Methods

The comparative proteomic profiling of mdx-4cv versus wild type liver extracts was carried out with an Orbitrap Fusion Tribrid mass spectrometer. The distribution of identified liver proteins within protein families and potential protein interaction patterns were analysed by systems bioinformatics. Key findings on fatty acid binding proteins were confirmed by immunoblot analysis and immunofluorescence microscopy.

Results

The proteomic analysis revealed changes in a variety of protein families, affecting especially fatty acid, carbohydrate and amino acid metabolism, biotransformation, the cellular stress response and ion handling in the mdx-4cv liver. Drastically increased protein species were identified as fatty acid binding protein FABP5, ferritin and calumenin. Decreased liver proteins included phosphoglycerate kinase, apolipoprotein and perilipin. The drastic change in FABP5 was independently verified by immunoblotting and immunofluorescence microscopy.

Conclusions

The proteomic results presented here indicate that the intricate and multifaceted pathogenesis of the mdx-4cv model of dystrophinopathy is associated with secondary alterations in the liver affecting especially fatty acid transportation. Since FABP5 levels were also shown to be elevated in serum from dystrophic mice, this protein might be a useful indicator for monitoring liver changes in X-linked muscular dystrophy.

Calnexin is necessary for T cell transmigration into the central nervous system

In multiple sclerosis (MS), a demyelinating inflammatory disease of the CNS, and its animal model (experimental autoimmune encephalomyelitis; EAE), circulating immune cells gain access to the CNS across the blood-brain barrier to cause inflammation, myelin destruction, and neuronal damage. Here, we discovered that calnexin, an ER chaperone, is highly abundant in human brain endothelial cells of MS patients. Conversely, mice lacking calnexin exhibited resistance to EAE induction, no evidence of immune cell infiltration into the CNS, and no induction of inflammation markers within the CNS. Furthermore, calnexin deficiency in mice did not alter the development or function of the immune system. Instead, the loss of calnexin led to a defect in brain endothelial cell function that resulted in reduced T cell trafficking across the blood-brain barrier. These findings identify calnexin in brain endothelial cells as a potentially novel target for developing strategies aimed at managing or preventing the pathogenic cascade that drives neuroinflammation and destruction of the myelin sheath in MS.