This study showed that tDC therapy in a preclinical model of MI was potentially translatable into an antiremodeling therapy for ischemic tissue repair.

Kang et al. show that the GCN2–ATF4 pathway induces 4E-BP transcription in response to amino acid deprivation and also during the development of certain Drosophila tissues. 4E-BP has selective effects on translation; therefore, this pathway helps to shift the mRNA expression profiles of cells.

Eukaryotic cells have evolved signaling pathways that help to restore cellular homeostasis in response to various physiological or pathological conditions. ATF4 is a transcription factor whose mRNA translation is stimulated in response to stress-activated eIF2alpha kinases. Established conditions that activate eIF2alpha phosphorylation and ATF4 translation include excessive stress in the endoplasmic reticulum (ER) and amino acid deprivation. ATF4 is activated through a unique translational activation mechanism that involves multiple upstream open reading frames (uORFs) in the 5’-untranslated region (UTR), which is conserved from yeast to mammals. Taking advantage of this, we developed a translational activation reporter of ATF4 in Drosophila, in which the dsRed reporter coding sequence was placed downstream of the Drosophila ATF4 5’ UTR. This reporter remained inactive in most tissues under normal conditions, but showed dsRed expression when starved, or when challenged with conditions that imposed ER stress. In normally developing flies, a small number of cell types showed reporter expression even without exogenous stress, which included the salivary gland, gut, the male reproductive organ, and the photoreceptor cells, suggestive of inherent stress during the normal development of these cell types. These results establish a new tool to study ATF4-mediated stress response in Drosophila development and disease.

Proper folding and assembly of major histocompatibility complex (MHC) class I complexes are essential for optimal peptide loading and subsequent antigen presentation. MHC class I folding involves the coordinated formation of multiple disulfide bonds within MHC class I molecules. However, the regulation of disulfide bond formation during the early process of MHC class I folding is uncharacterized. Here, we show that protein disulfide isomerase (PDI) catalyzes the disulfide bond formation of MHC class I molecules and thereby facilitates the assembly of MHC class I heavy chain with beta(2)-microglobulin (beta(2)m). Depletion of PDI but not ERp57 by RNAi interfered with the disulfide bond formation in the MHC class I molecules. In the absence of PDI, the association of free class I heavy chain with calnexin increased, whereas the assembly of MHC class I heavy chain-beta(2)m heterodimers was delayed. These observations suggest that PDI-catalyzed disulfide bond formation of MHC class I molecules is an event downstream of the interaction of class I molecules with calnexin and upstream of their interaction with beta(2)m. Thus, our data establish a critical function for PDI in the early assembly of MHC class I molecules.

Major histocompatibility complex (MHC) class I molecules present antigenic peptides to the cell surface for screening by CD8(+) T cells. A number of ER-resident chaperones assist the assembly of peptides onto MHC class I molecules, a process that can be divided into several steps. Early folding of the MHC class I heavy chain is followed by its association with beta(2)-microglobulin (beta(2)m). The MHC class I heavy chain-beta(2)m heterodimer is incorporated into the peptide-loading complex, leading to peptide loading, release of the peptide-filled MHC class I molecules from the peptide-loading complex, and exit of the complete MHC class I complex from the ER. Because proper antigen presentation is vital for normal immune responses, the assembly of MHC class I molecules requires tight regulation. Emerging evidence indicates that thiol-based redox regulation plays critical roles in MHC class I-restricted antigen processing and presentation, establishing an unexpected link between redox biology and antigen processing. We review the influences of redox regulation on antigen processing and presentation. Because redox signaling pathways are a rich source of validated drug targets, newly discovered redox biology-mediated mechanisms of antigen processing may facilitate the development of more selective and therapeutic drugs or vaccines against immune diseases.

Current stem cell-based therapy for cardiac repair and regeneration after myocardial infarction (MI) is not readily translatable into clinical scenarios due to the low retention and survival of the transplanted cells. Here, we evaluated a simple and feasible design of gelatin−hydroxyphenyl propionic acid (GH) hydrogel as an in situ crosslinkable and injectable cell delivery platform for cardiac tissue regeneration. The GH hydrogel exhibited improved cell retention and survival in vitro and in vivo when encapsulating mouse bone marrow-derived mesenchymal stem cells (MSCs) that were used as promising therapeutic candidates for stem cell therapy. Moreover, we demonstrated that MSC-encapsulating GH hydrogels led to a significant improvement in cardiac functional metrics, such as the fractional shortening (FS), ejection fraction (EF), and end-systolic volume (ESV). Similarly, MSC-encapsulating GH hydrogels induced favorable effects in the cardiac structures of the infarcted heart, producing less fibrosis and thicker infarcted walls. These results suggest that GH hydrogels can be used as an instructive cell delivery platform to provide a suitable microenvironment for transplanted cells; therefore, their in vivo applications combined with MSCs may provide great potential for repair and regeneration of injured cardiac tissues after MI.

In contrast to the fairly well-characterized mechanism of assembly of MHC class I-peptide complexes, the disassembly mechanism by which peptide-loaded MHC class I molecules are released from the peptide-loading complex and exit the endoplasmic reticulum (ER) is poorly understood. Optimal peptide binding by MHC class I molecules is assumed to be sufficient for triggering exit of peptide-filled MHC class I molecules from the ER. We now show that protein disulfide isomerase (PDI) controls MHC class I disassembly by regulating dissociation of the tapasin-ERp57 disulfide conjugate. PDI acts as a peptide-dependent molecular switch; in the peptide-bound state, it binds to tapasin and ERp57 and induces dissociation of the tapasin-ERp57 conjugate. In the peptide-free state, PDI is incompetent to bind to tapasin or ERp57 and fails to dissociate the tapasin-ERp57 conjugates, resulting in ER retention of MHC class I molecules. Thus, our results indicate that even after optimal peptide loading, MHC class I disassembly does not occur by default but, rather, is a regulated process involving PDI-mediated interactions within the peptide-loading complex. INTRODUCTIONAssembly of major histocompatibility complex (MHC) class I molecules within the endoplasmic reticulum (ER) is essential for bringing antigenic peptides to CD8 ϩ T-cells, which scan and destroy the infected and transformed cells. MHC class I quality control ensures both the ER retention of suboptimally loaded MHC class I molecules and the release of those with a higher affinity to the cell surface. Therefore, the peptide-editing process for optimal peptide loading is critical for increasing the recognition by CD8 ϩ T-cells (Van Kaer, 2002;Cresswell, 2005).MHC class I assembly is a highly regulated process mediated by several ER-resident chaperones and accessory molecules (Cresswell, 2000;Williams et al., 2002a). After initial interaction with the lectin-like chaperone calnexin, MHC class I heavy chain-␤ 2 -microglobulin (␤ 2 m) heterodimers are incorporated into the peptide-loading complex (PLC), a multimolecular unit of ER proteins that promotes optimal peptide loading into MHC class I molecules (Hammerling et al., 1998;Pamer and Cresswell, 1998;Elliott and Williams, 2005). The PLC contains calreticulin, tapasin, transporters associated with antigen processing (TAP) and two thioldependent oxidoreductases, ERp57 and protein disulfide isomerase (PDI) (Garbi et al., 2005;Park et al., 2006;Santos et al., 2007).Several functions have been proposed for tapasin in facilitating optimal peptide loading and optimization of the peptide cargo (Grandea and Van Kaer, 2001;Purcell et al., 2001;Momburg and Tan, 2002). Tapasin bridges heavy chain-␤ 2 m heterodimers to TAP (Sadasivan et al., 1996) and thus provides physical proximity between MHC class I molecules and TAP. Tapasin also retains MHC class I molecules in the ER (Schoenhals et al., 1999;Barnden et al., 2000;Grandea et al., 2000), which might enhance peptide loading. In addition, tapasin optimizes the repertoire of bound pep...

Atrazine (ATR) is one of the most commonly applied broad-spectrum herbicides. Although ATR is well known to be a biologically hazardous molecule with potential toxicity in the immune system, the molecular mechanisms responsible for ATR-induced immunotoxicity remain unclear. In this study, we found that the immunotoxic properties of ATR were mediated through the induction of apoptotic changes in T lymphocytes. Mice exposed to ATR for 4 weeks exhibited a significant decrease in the number of spleen CD3(+) T lymphocytes, while CD19(+) B lymphocytes and nonlymphoid cells were unaffected. ATR exposure also led to inhibition of cell growth and induction of apoptosis in human Jurkat T-cells. Importantly, ATR triggered the activation of caspase-3 and the cleavage of caspase-8 and PARP, whereas it did not affect the release of cytochrome c from the mitochondria in Jurkat T-cells. In addition, ATR activated the unfolded protein response signaling pathway, as indicated by eIF2α phosphorylation and CHOP induction. Our results demonstrate that ATR elicited an immunotoxic effect by inducing ER stress-induced apoptosis in T-cells, therefore providing evidence for the molecular mechanism by which ATR induces dysregulation of the immune system. © 2015 Wiley Periodicals, Inc. Environ Toxicol 31: 998-1008, 2016.