Calnexin is required for zebrafish posterior lateral line development

Endoplasmic reticulum calnexins participate in the primary root growth response to phosphate deficiency

Accumulation of incompletely folded proteins in the endoplasmic reticulum (ER) leads to ER stress, activates ER protein degradation pathways, and upregulates genes involved in protein folding. This process is known as the unfolded protein response (UPR). The role of ER protein folding in plant responses to nutrient deficiencies is unclear. We analyzed Arabidopsis (Arabidopsis thaliana) mutants affected in ER protein quality control and established that both CALNEXIN (CNX) genes function in the primary root response to phosphate (Pi) deficiency. CNX1 and CNX2 are homologous ER lectins promoting protein folding of N-glycosylated proteins via the recognition of the GlcMan9GlcNAc2 glycan. Growth of cnx1-1 and cnx2-2 single mutants was similar to that of the wild type under high and low Pi conditions, but the cnx1-1 cnx2-2 double mutant showed decreased primary root growth under low Pi conditions due to reduced meristematic cell division. This phenotype was specific to Pi deficiency; the double mutant responded normally to osmotic and salt stress. Expression of CNX2 mutated in amino acids involved in binding the GlcMan9GlcNAc2 glycan failed to complement the cnx1-1 cnx2-2 mutant. The root growth phenotype was Fe-dependent and was associated with root apoplastic Fe accumulation. Two genes involved in Fe-dependent inhibition of primary root growth under Pi deficiency, the ferroxidase LOW PHOSPHATE 1 (LPR1) and P5-type ATPase PLEIOTROPIC DRUG RESISTANCE 2 (PDR2) were epistatic to CNX1/CNX2. Overexpressing PDR2 failed to complement the cnx1-1 cnx2-2 root phenotype. The cnx1-1 cnx2-2 mutant showed no evidence of UPR activation, indicating a limited effect on ER protein folding. CNX might process a set of N-glycosylated proteins specifically involved in the response to Pi deficiency.

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 new calnexin modulates antibacterial immune response in obscure puffer Takifugu obscurus

Calnexin (Cnx) is a membrane-bound lectin chaperone of the endoplasmic reticulum. In this study, a novel Cnx homologue from the obscure puffer Takifugu obscurus was characterized, tentatively named ToCnx. The cDNA of ToCnx was 1803 bp, and it contained an open reading frame encoding a polypeptide of 600 amino acid residues with a calculated molecular weight of 67.5 kDa. Multiple alignment of the deduced amino acid sequences of ToCnx and other related fish Cnxs revealed that ToCnx had typical characteristics of fish Cnxs. Sequence comparison and phylogenetic tree analysis showed that ToCnx had the closest relationship with Cnxs from Takifugu flavidus and Takifugu rubripes. ToCnx transcripts were detected in all the tissues examined, and they were mainly expressed in the liver, kidney, and intestine. Upon Vibrio harveyi, Edwardsiella tarda, and Aeromonas hydrophila infection, ToCnx transcripts were all significantly upregulated in the kidneys. The recombinant calreticulin domain of ToCnx (rToCnx) was prepared by prokaryotic expression. In the absence of calcium, rToCnx was able to bind three Gram-negative bacteria (V. harveyi, E. tarda, and A. hydrophila) and two bacterial saccharides, such as lipopolysaccharide and peptidoglycan. In the presence of calcium, rToCnx could agglutinate all the detected microorganisms. In addition, rToCnx possessed the effect of inhibiting the growth of three microbe strains. These observations suggested that ToCnx is an important participant in host immune defense against bacteria.

A transgenic zebrafish line for in vivo visualisation of neutrophil myeloperoxidase

The neutrophil enzyme myeloperoxidase (MPO) is a major enzyme made by neutrophils to generate antimicrobial and immunomodulatory compounds, notably hypochlorous acid (HOCl), amplifying their capacity for destroying pathogens and regulating inflammation. Despite its roles in innate immunity, the importance of MPO in preventing infection is unclear, as individuals with MPO deficiency are asymptomatic with the exception of an increased risk of candidiasis. Dysregulation of MPO activity is also linked with inflammatory conditions such as atherosclerosis, emphasising a need to understand the roles of the enzyme in greater detail. Consequently, new tools for investigating granular dynamics in vivo can provide useful insights into how MPO localises within neutrophils, aiding understanding of its role in preventing and exacerbating disease. The zebrafish is a powerful model for investigating the immune system in vivo, as it is genetically tractable, and optically transparent. To visualise MPO activity within zebrafish neutrophils, we created a genetic construct that expresses human MPO as a fusion protein with a C-terminal fluorescent tag, driven by the neutrophil-specific promoter lyz. After introducing the construct into the zebrafish genome by Tol2 transgenesis, we established the Tg(lyz:Hsa.MPO-mEmerald,cmlc2:EGFP)sh496 line, and confirmed transgene expression in zebrafish neutrophils. We observed localisation of MPO-mEmerald within a subcellular location resembling neutrophil granules, mirroring MPO in human neutrophils. In Spotless (mpxNL144) larvae-which express a non-functional zebrafish myeloperoxidase-the MPO-mEmerald transgene does not disrupt neutrophil migration to sites of infection or inflammation, suggesting that it is a suitable line for the study of neutrophil granule function. We present a new transgenic line that can be used to investigate neutrophil granule dynamics in vivo without disrupting neutrophil behaviour, with potential applications in studying processing and maturation of MPO during development.

Triclocarban at environmentally relevant concentrations induces the endoplasmic reticulum stress in zebrafish

Triclocarban (TCC) is an antibacterial agent commonly found in environmental, wildlife, and human samples. However, with in-depth study of TCC, its negative effects are increasingly presented. Toxicological studies of TCC at environmentally relevant concentrations have been conducted in zebrafish embryos and indicated that TCC leads to deformity of development causes developmental deformities. However, the molecular mechanisms underlying the toxicity of TCC in zebrafish embryos have not been entirely elucidated. We investigated whether exposure to TCC at environmentally relevant concentrations induces endoplasmic reticulum (ER) stress and unfolded protein response (UPR) in zebrafish. Zebrafish embryos were grown to 32 hours post fertilization and exposed to 2.5, 5, and 10 μg/L TCC and used in whole-mount in situ hybridization to visualize the expression of ER chaperone hspa5 and ER stress-related apoptosis factor chop. Zebrafish livers were exposed to different concentrations of TCC to elaborate the relationships between fatty degeneration and ER stress. Then, a human hepatic cell line (HL-7702) was used to test whether TCC induced ER stress in human livers similar to those of zebrafish. In zebrafish embryos, TCC induced high hspa5 expression, which could defend against external stimulations. Furthermore, hapa5, hsp90b1, and chop exhibited ectopic expressions in the neuromast, intestinal tract, and tail tip of zebrafish embryos. On the one hand, significant differences were observed in the mRNA and protein expressions of the ER stress molecular chaperone pPERK-pEIF2a-ATF4 and ATF6 pathways in HL-7702 cells exposed to TCC. On the other hand, lipid droplet accumulation slightly increased in zebrafish livers exposed to 10 μg/L TCC in vitro. These results demonstrate that TCC not only damages the development of zebrafish embryos and structure of zebrafish liver but also influences human hepatic cells by activating ER stress and the UPR signaling pathway.

Lack of evidence for a causal role of CALR3 in monogenic cardiomyopathy

The pathogenicity of previously published disease-associated genes and variants is sometimes questionable. Large-scale, population-based sequencing studies have uncovered numerous false assignments of pathogenicity. Misinterpretation of sequence variants may have serious implications for the patients and families involved, as genetic test results are increasingly being used in medical decision making. In this study, we assessed the role of the calreticulin-3 gene (CALR3) in cardiomyopathy. CALR3 has been included in several cardiomyopathy gene panels worldwide. Its inclusion is based on a single publication describing two missense variants in patients with hypertrophic cardiomyopathy. In our national cardiomyopathy cohort (n = 6154), we identified 17 unique, rare heterozygous CALR3 variants in 48 probands. Overall, our patient cohort contained a significantly higher number of rare CALR3 variants compared to the ExAC population (p = 0.0036). However, after removing a potential Dutch founder variant, no statistically significant difference was found (p = 0.89). In nine probands, the CALR3 variant was accompanied by a disease-causing variant in another, well-known cardiomyopathy gene. In three families, the CALR3 variant did not segregate with the disease. Furthermore, we could not demonstrate calreticulin-3 protein expression in myocardial tissues at various ages. On the basis of these findings, it seems highly questionable that variants in CALR3 are a monogenic cause of cardiomyopathy.

Brachyury-YAP Regulatory Axis Drives Stemness and Growth in Cancer

Molecular factors that define stem cell identity have recently emerged as oncogenic drivers. For instance, brachyury, a key developmental transcriptional factor, is also implicated in carcinogenesis, most notably of chordoma, through mechanisms that remain elusive. Here, we show that brachyury is a crucial regulator of stemness in chordoma and in more common aggressive cancers. Furthermore, this effect of brachyury is mediated by control of synthesis and stability of Yes-associated protein (YAP), a key regulator of tissue growth and homeostasis, providing an unexpected mechanism of control of YAP expression. We further demonstrate that the brachyury-YAP regulatory pathway is associated with tumor aggressiveness. These results elucidate a mechanism of controlling both tumor stemness and aggressiveness through regulatory coupling of two developmental factors.

Rainbow trout (Oncorhynchus mykiss) contain two calnexin genes which encode distinct proteins

Calnexin (IP90/P88) is an integral membrane protein of the endoplasmic reticulum that binds newly synthesized N-linked glycoproteins during their folding in the ER including MHC class I molecule. This manuscript reports the identification of two unique cDNA clones of calnexin in rainbow trout. Both encode putative mature proteins of 579 and 592 aa respectively in addition to a 24 aa signal peptide. Sequence analysis revealed that only one of the two cDNA clones encodes a putative ER retention signal, K/QEDDL, followed by a serine phosphorylation site conserved with mammalian homologs. Amino acid sequence alignment illustrated conservation of the calnexin luminal domain, which consists of a globular and a P domain, in both copies. Southern blotting revealed that there are at least two copies of the calnexin gene in the trout genome and northern blotting showed a wide tissue distribution of an estimated 3 kbp calnexin transcript with an additional minor transcript of 2.3 kbp expressed only in head kidney, spleen PBLs and strongly in RTS11. Importantly, the smaller transcript was predominantly upregulated in RTS11 after a 24h treatment with the calcium ionophore A23187. In western blots, calnexin was detected primarily as a 120 kDa protein and upon A23187 treatment; a 100 kDa band was most prominently expressed. These results suggest that in salmonids there are two differentiated versions of the calnexin gene which encode proteins that may have diverged to perform unique biological functions.