Cell-type-specific differences in KDEL receptor clustering in mammalian cells

Exploring the potential biological significance of KDELR family genes in lung adenocarcinoma

The Lys-Asp-Glu-Leu receptor (KDELR) family genes play critical roles in a variety of biological processes in different tumors. Our study aimed to provide a comprehensive analysis of the potential roles of KDELRs in lung adenocarcinoma (LUAD). Utilizing data from The Cancer Genome Atlas (TCGA) and Gene Expression Omnibus (GEO) database, as well as clinical samples, we conducted a series of analyses and validations using R software tools and various online resources. The results showed that KDELR family genes and proteins were highly expressed and associated with a poor prognosis of LUAD. Promoter hypomethylation and the competing endogenous RNA (ceRNA) network of PCAT6/hsa-miR-326/KDELR1 might be potential causes of aberrant KDELR1 overexpression in LUAD. Three key Transcription factors (TFs) (SPI1, EP300, and MAZ) and a TFs-miRNAs-KDELRs network (involving 11 TFs) might be involved in modulating KDELRs expression abnormalities. Gene Set Enrichment Analysis (GSEA) indicated enrichment of genes highly expressing KDELR1, KDELR2, and KDELR3 in MTORC1_SIGNALING, P53_PATHWAY, and ANGIOGENESIS. Negative correlations between KDELRs expression and CD8 + T cell infiltration, as well as CTLA-4 expression. Our multiple analyses suggested that the KDELRs are important signaling molecules in LUAD. These results provided novel insights for developing prognostic markers and novel therapies of LUAD.

Crosstalk between KDEL receptor and EGF receptor mediates cell proliferation and migration via STAT3 signaling

Hostile microenvironment of cancer cells provoke a stressful condition for endoplasmic reticulum (ER) and stimulate the expression and secretion of ER chaperones, leading to tumorigenic effects. However, the molecular mechanism underlying these effects is largely unknown. In this study, we reveal that the last four residues of ER chaperones, which are recognized by KDEL receptor (KDELR), is required for cell proliferation and migration induced by secreted chaperones. By combining proximity-based mass spectrometry analysis, split venus imaging and membrane yeast two hybrid assay, we present that EGF receptor (EGFR) may be a co-receptor for KDELR on the surface. Prior to ligand addition, KDELR spontaneously oligomerizes and constantly undergoes recycling near the plasma membrane. Upon KDEL ligand binding, the interactions of KDELR with itself and with EGFR increase rapidly, leading to augmented internalization of KDELR and tyrosine phosphorylation in the C-terminus of EGFR. STAT3, which binds the phosphorylated tyrosine motif on EGFR, is subsequently activated by EGFR and mediates cell growth and migration. Taken together, our results suggest that KDELR serves as a bona fide cell surface receptor for secreted ER chaperones and transactivates EGFR-STAT3 signaling pathway.

KDEL Receptor Trafficking to the Plasma Membrane Is Regulated by ACBD3 and Rab4A-GTP

KDEL receptor-1 maintains homeostasis in the early secretory pathway by capturing and retrieving ER chaperones to the ER during heavy secretory activity. Unexpectedly, a fraction of the receptor is also known to reside in the plasma membrane (PM), although it is largely unknown exactly how the KDEL receptor gets exported from the Golgi and travels to the PM. We have previously shown that a Golgi scaffolding protein (ACBD3) facilitates KDEL receptor localization at the Golgi via the regulating cargo wave-induced cAMP/PKA-dependent signaling pathway. Upon endocytosis, surface-expressed KDEL receptor undergoes highly complex itineraries through the Golgi and the endo-lysosomal compartments, where the endocytosed receptor utilizes Rab14A- and Rab11A-positive recycling endosomes and clathrin-decorated tubulovesicular carriers. In this study, we sought to investigate the mechanism through which the KDEL receptor gets exported from the Golgi en route to the PM. We report here that ACBD3 depletion results in greatly increased trafficking of KDEL receptor to the PM via Rab4A-positive tubular carriers emanating from the Golgi. Expression of constitutively activated Rab4A mutant (Q72L) increases the surface expression of KDEL receptor up to 2~3-fold, whereas Rab4A knockdown or the expression of GDP-locked Rab4A mutant (S27N) inhibits KDEL receptor targeting of the PM. Importantly, KDELR trafficking from the Golgi to the PM is independent of PKA- and Src kinase-mediated mechanisms. Taken together, these results reveal that ACBD3 and Rab4A play a key role in regulating KDEL receptor trafficking to the cell surface.

Cell surface biotinylation to identify the receptors involved in nanoparticle uptake into endothelial cells

Targeted drug delivery requires -among others- specific interaction of nanocarriers with cell surface receptors enabling efficient internalization into the targeted cells. Thus, identification of receptors allowing efficient nanocarrier uptake is essential to improve the design of targeted nanomedicines. Here we used methods based on cell surface biotinylation to identify cell surface receptors mediating nanoparticle uptake by cells. We used human brain and liver endothelial cells as representative examples of cells typically showing very low and very high nanoparticle uptake, respectively. Amino-modified and carboxylated silica were used as model nanoparticles usually associated with high and low uptake into cells, respectively, and carrying different coronas after exposure in full human plasma. Using cell surface biotinylation of live cells and receptor pull-down assays, we compared the receptors internalized in control untreated cells and those internalized upon exposure to nanoparticles. In this way, we identified receptors associated with (high) nanoparticle uptake. The candidate receptors were further validated by decorating the nanoparticles with an artificial corona consisting of the respective receptor ligands. We found that a vitronectin corona can be used to target integrin receptors and strongly enhances nanoparticle uptake in brain and liver endothelial cells. The increased uptake was maintained in the presence of serum, suggesting that the vitronectin-corona could resist interaction and competition with serum. Furthermore, plasminogen-coated nanoparticles promoted uptake in endothelial cells of the liver, but not of the brain. The presented approach using reversible biotinylation of cell surface receptors in live cells allows for receptor-based targeting of nanocarriers that are instrumental in nanoparticle uptake, which can be exploited for targeted drug delivery. STATEMENT OF SIGNIFICANCE: In order to deliver drugs to their site of action, drug-loaded nanocarriers can be targeted to cell receptors enabling efficient uptake into target cells. Thus, methods to identify nanocarrier receptors are invaluable. Here we used reversible biotinylation of live cells and receptor pull-down approaches for receptor identification. By comparative analysis of the individual receptors internalized in untreated cells and cells exposed to nanoparticles, we identified receptors enabling high nanoparticle uptake into liver and brain endothelial cells. Their role was confirmed by decorating nanoparticles with an artificial corona composed of the receptor ligands. In conclusion, live cell reversible biotinylation of cell surface proteins is a powerful tool for the identification of potential receptors for receptor-based targeting of nanocarriers.

KDEL Receptors: Pathophysiological Functions, Therapeutic Options, and Biotechnological Opportunities

KDEL receptors (KDELRs) are ubiquitous seven-transmembrane domain proteins encoded by three mammalian genes. They bind to and retro-transport endoplasmic reticulum (ER)-resident proteins with a C-terminal Lys-Asp-Glu-Leu (KDEL) sequence or variants thereof. In doing this, KDELR participates in the ER quality control of newly synthesized proteins and the unfolded protein response. The binding of KDEL proteins to KDELR initiates signaling cascades involving three alpha subunits of heterotrimeric G proteins, Src family kinases, protein kinases A (PKAs), and mitogen-activated protein kinases (MAPKs). These signaling pathways coordinate membrane trafficking flows between secretory compartments and control the degradation of the extracellular matrix (ECM), an important step in cancer progression. Considering the basic cellular functions performed by KDELRs, their association with various diseases is not surprising. KDELR mutants unable to bind the collagen-specific chaperon heat-shock protein 47 (HSP47) cause the osteogenesis imperfecta. Moreover, the overexpression of KDELRs appears to be linked to neurodegenerative diseases that share pathological ER-stress and activation of the unfolded protein response (UPR). Even immune function requires a functional KDELR1, as its mutants reduce the number of T lymphocytes and impair antiviral immunity. Several studies have also brought to light the exploitation of the shuttle activity of KDELR during the intoxication and maturation/exit of viral particles. Based on the above, KDELRs can be considered potential targets for the development of novel therapeutic strategies for a variety of diseases involving proteostasis disruption, cancer progression, and infectious disease. However, no drugs targeting KDELR functions are available to date; rather, KDELR has been leveraged to deliver drugs efficiently into cells or improve antigen presentation.

Engineering a Rapid Insulin Release System Controlled By Oral Drug Administration

Rapid insulin release plays an essential role in maintaining blood-glucose homeostasis in mammalians. Patients diagnosed with type-I diabetes mellitus experience chronic and remarkably high blood-sugar levels, and require lifelong insulin injection therapy, so there is a need for more convenient and less invasive insulin delivery systems to increase patients' compliance and also to enhance their quality of life. Here, an endoplasmic-reticulum-localized split sec-tobacco etch virus protease (TEVp)-based rapamycin-actuated protein-induction device (RAPID) is engineered, which is composed of the rapamycin-inducible dimerization domains FK506 binding protein (FKBP) and FKBP-rapamycin binding protein fused with modified split sec-TEVp components. Insulin accumulation inside the endoplasmic reticulum (ER) is achieved through tagging its C-terminus with KDEL, an ER-retention signal, spaced by a TEVp cleavage site. In the presence of rapamycin, the split sec-TEVp-based RAPID components dimerize, regain their proteolytic activity, and remove the KDEL retention signal from insulin. This leads to rapid secretion of accumulated insulin from cells within few minutes. Using liver hydrodynamic transfection methodology, it is shown that RAPID quickly restores glucose homeostasis in type-1-diabetic (T1DM) mice treated with an oral dose of clinically licensed rapamycin. This rapid-release technology may become the foundation for other cell-based therapies requiring instantaneous biopharmaceutical availability.

The Function of KDEL Receptors as UPR Genes in Disease

The KDEL receptor retrieval pathway is essential for maintaining resident proteins in the endoplasmic reticulum (ER) lumen. ER resident proteins serve a variety of functions, including protein folding and maturation. Perturbations to the lumenal ER microenvironment, such as calcium depletion, can cause protein misfolding and activation of the unfolded protein response (UPR). Additionally, ER resident proteins are secreted from the cell by overwhelming the KDEL receptor retrieval pathway. Recent data show that KDEL receptors are also activated during the UPR through the IRE1/XBP1 signaling pathway as an adaptive response to cellular stress set forth to reduce the loss of ER resident proteins. This review will discuss the emerging connection between UPR activation and KDEL receptors as it pertains to ER proteostasis and disease states.