Calreticulin mediated glucocorticoid receptor export is involved in beta-catenin translocation and Wnt signalling inhibition in human osteoblastic cells

Protein kinase G2 activation restores Wnt signaling and bone mass in glucocorticoid-induced osteoporosis in mice

Osteoporotic fractures are a major complication of long-term glucocorticoid therapy. Glucocorticoids transiently increase bone resorption, but they predominantly inhibit bone formation and induce osteocyte apoptosis, leading to bone loss. Current treatments of glucocorticoid-induced osteoporosis aim mainly at reducing bone resorption and are, therefore, inadequate. We previously showed that signaling via the NO/cGMP/protein kinase G pathway plays a key role in skeletal homeostasis. Here, we show that pharmacological PKG activation with the guanylyl cyclase-1 activator cinaciguat or expression of a constitutively active, mutant PKG2R242Q restored proliferation, differentiation, and survival of primary mouse osteoblasts exposed to dexamethasone. Cinaciguat treatment of WT mice or osteoblast-specific expression of PKG2R242Q in transgenic mice prevented dexamethasone-induced loss of cortical bone mass and strength. These effects of cinaciguat and PKG2R242Q expression were due to preserved bone formation parameters and osteocyte survival. The basis for PKG2's effects appeared to be through recovery of Wnt/β-catenin signaling, which was suppressed by glucocorticoids but critical for proliferation, differentiation, and survival of osteoblast-lineage cells. Cinaciguat reduced dexamethasone activation of osteoclasts, but this did not occur in the PKG2R242Q transgenic mice, suggesting a minor role in osteoprotection. We propose that existing PKG-targeting drugs could represent a novel therapeutic approach to prevent glucocorticoid-induced osteoporosis.

Temporal gene expression changes and affected pathways in neurodevelopment of a mouse model of Smith-Lemli-Opitz syndrome

Smith-Lemli-Opitz syndrome is an autosomal recessive disorder that arises from mutations in the gene DHCR7, which encodes the terminal enzyme of cholesterol biosynthesis, leading to decreased production of cholesterol and accumulation of the cholesterol precursor, 7-dehydrocholesterol, and its oxysterol metabolites. The disorder displays a wide range of neurodevelopmental defects, intellectual disability, and behavioral problems. However, an in-depth study on the temporal changes of gene expression in the developing brains of SLOS mice has not been done before. In this work, we carried out the transcriptomic analysis of whole brains from WT and Dhcr7-KO mice at four-time points through postnatal day 0. First, we observed the expected downregulation of the Dhcr7 gene in the Dhcr7-KO mouse model, as well as gene expression changes of several other genes involved in cholesterol biosynthesis throughout all time points. Pathway and GO term enrichment analyses revealed affected signaling pathways and biological processes that were shared amongst time points and unique to individual time points. Specifically, the pathways important for embryonic development, including Hippo, Wnt, and TGF-β signaling pathways are the most significantly affected at the earliest time point, E12.5. Additionally, neurogenesis-related GO terms were enriched in earlier time points, consistent with the timing of development. Conversely, pathways related to synaptogenesis, which occurs later in development compared to neurogenesis, are significantly affected at the later time points, E16.5 and PND0, including the cholinergic, glutamatergic, and GABAergic synapses. The impact of these transcriptomic changes and enriched pathways is discussed in the context of known biological phenotypes of SLOS.

7-Dehydrocholesterol-derived oxysterols cause neurogenic defects in Smith-Lemli-Opitz syndrome

Defective 3β-hydroxysterol-Δ7 -reductase (DHCR7) in the developmental disorder, Smith-Lemli-Opitz syndrome (SLOS), results in a deficiency in cholesterol and accumulation of its precursor, 7-dehydrocholesterol (7-DHC). Here, we show that loss of DHCR7 causes accumulation of 7-DHC-derived oxysterol metabolites, premature neurogenesis from murine or human cortical neural precursors, and depletion of the cortical precursor pool, both in vitro and in vivo. We found that a major oxysterol, 3β,5α-dihydroxycholest-7-en-6-one (DHCEO), mediates these effects by initiating crosstalk between glucocorticoid receptor (GR) and neurotrophin receptor kinase TrkB. Either loss of DHCR7 or direct exposure to DHCEO causes hyperactivation of GR and TrkB and their downstream MEK-ERK-C/EBP signaling pathway in cortical neural precursors. Moreover, direct inhibition of GR activation with an antagonist or inhibition of DHCEO accumulation with antioxidants rescues the premature neurogenesis phenotype caused by the loss of DHCR7. These results suggest that GR could be a new therapeutic target against the neurological defects observed in SLOS.

Single-cell RNA sequencing of the retina in a model of retinitis pigmentosa reveals early responses to degeneration in rods and cones

Background

In inherited retinal disorders such as retinitis pigmentosa (RP), rod photoreceptor-specific mutations cause primary rod degeneration that is followed by secondary cone death and loss of high-acuity vision. Mechanistic studies of retinal degeneration are challenging because of retinal heterogeneity. Moreover, the detection of early cone responses to rod death is especially difficult due to the paucity of cones in the retina. To resolve heterogeneity in the degenerating retina and investigate events in both types of photoreceptors during primary rod degeneration, we utilized droplet-based single-cell RNA sequencing in an RP mouse model, rd10.

Results

Using trajectory analysis, we defined two consecutive phases of rod degeneration at P21, characterized by the early transient upregulation of Egr1 and the later induction of Cebpd. EGR1 was the transcription factor most significantly associated with the promoters of differentially regulated genes in Egr1-positive rods in silico. Silencing Egr1 affected the expression levels of two of these genes in vitro. Degenerating rods exhibited changes associated with metabolism, neuroprotection, and modifications to synapses and microtubules. Egr1 was also the most strongly upregulated transcript in cones. Its upregulation in cones accompanied potential early respiratory dysfunction and changes in signaling pathways. The expression pattern of EGR1 in the retina was dynamic during degeneration, with a transient increase of EGR1 immunoreactivity in both rods and cones during the early stages of their degenerative processes.

Conclusion

Our results identify early and late changes in degenerating rd10 rod photoreceptors and reveal early responses to rod degeneration in cones not expressing the disease-causing mutation, pointing to mechanisms relevant for secondary cone degeneration. In addition, our data implicate EGR1 as a potential key regulator of early degenerative events in rods and cones, providing a potential broad target for modulating photoreceptor degeneration.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01280-9.

Glucocorticoid regulates mesenchymal cell differentiation required for perinatal lung morphogenesis and function

While antenatal glucocorticoids are widely used to enhance lung function in preterm infants, cellular and molecular mechanisms by which glucocorticoid receptor (GR) signaling influences lung maturation remain poorly understood. Deletion of the glucocorticoid receptor gene (Nr3c1) from fetal pulmonary mesenchymal cells phenocopied defects caused by global Nr3c1 deletion, while lung epithelial- or endothelial-specific Nr3c1 deletion did not impair lung function at birth. We integrated genome-wide gene expression profiling, ATAC-seq, and single cell RNA-seq data in mice in which GR was deleted or activated to identify the cellular and molecular mechanisms by which glucocorticoids control prenatal lung maturation. GR enhanced differentiation of a newly defined proliferative mesenchymal progenitor cell (PMP) into matrix fibroblasts (MFBs), in part by directly activating extracellular matrix-associated target genes, including Fn1, Col16a4, and Eln and by modulating VEGF, JAK-STAT, and WNT signaling. Loss of mesenchymal GR signaling blocked fibroblast progenitor differentiation into mature MFBs, which in turn increased proliferation of SOX9+ alveolar epithelial progenitor cells and inhibited differentiation of mature alveolar type II (AT₂) and AT₁ cells. GR signaling controls genes required for differentiation of a subset of proliferative mesenchymal progenitors into matrix fibroblasts, in turn, regulating signals controlling AT₂/AT₁ progenitor cell proliferation and differentiation and identifying cells and processes by which glucocorticoid signaling regulates fetal lung maturation.

Effects of hypergravity on gene levels in anti-gravity muscle and bone through the vestibular system in mice

We recently reported that hypergravity with 3 g for 4 weeks affects muscle and bone through the vestibular system in mice. The purpose of this study was to investigate the effects of hypergravity with 2 g, which had no influence on circulating glucocorticoid level, on the gene levels in muscle and bone, as well as the roles of the vestibular system in those changes using vestibular lesioned (VL) mice. Hypergravity for 2 and 8 weeks or VL exerted little effects on the mRNA levels of muscle differentiation factors and myokines in the soleus muscle. Although hypergravity for 2 weeks significantly elevated alkaline phosphatase (ALP) and type I collagen mRNA levels in the tibia, VL significantly attenuated the levels of ALP mRNA enhanced by hypergravity. In conclusion, the present study suggests that a 2-g load for 2 weeks enhances osteoblast differentiation partly through the vestibular system in mice.

Glucocorticoids suppress Wnt16 expression in osteoblasts in vitro and in vivo

Glucocorticoid-induced osteoporosis is a frequent complication of systemic glucocorticoid (GC) therapy and mainly characterized by suppressed osteoblast activity. Wnt16 derived from osteogenic cells is a key determinant of bone mass. Here, we assessed whether GC suppress bone formation via inhibiting Wnt16 expression. GC treatment with dexamethasone (DEX) decreased Wnt16 mRNA levels in murine bone marrow stromal cells (mBMSCs) time- and dose-dependently. Similarly, Wnt16 expression was also suppressed after DEX treatment in calvarial organ cultures. Consistently, mice receiving GC-containing slow-release prednisolone pellets showed lower skeletal Wnt16 mRNA levels and bone mineral density than placebo-treated mice. The suppression of Wnt16 by GCs was GC-receptor-dependent as co-treatment of mBMSCs with DEX and the GR antagonist RU-486 abrogated the GC-mediated suppression of Wnt16. Likewise, DEX failed to suppress Wnt16 expression in GR knockout-mBMSCs. In addition, Wnt16 mRNA levels were unaltered in bone tissue of GC-treated GR dimerization-defective GR^dim mice, suggesting that GCs suppress Wnt16 via direct DNA-binding mechanisms. Consistently, DEX treatment reduced Wnt16 promoter activity in MC3T3-E1 cells. Finally, recombinant Wnt16 restored DEX-induced suppression of bone formation in mouse calvaria. Thus, this study identifies Wnt16 as a novel target of GC action in GC-induced suppression of bone formation.

Possible roles for glucocorticoid signalling in breast cancer

Our understanding of breast cancer biology, and our ability to manipulate breast cancers have grown exponentially in the last 20 years. Much of that expansion has focused on the roles of steroids in driving these neoplasms. Initially this research focused on estrogens and progesterone receptors, and more recently on androgen actions in breast cancers. This review aims to make the case for glucocorticoids as the next essential steroid subclass that contributes significantly to our understanding of steroidogenic regulation of these neoplasms. Glucocorticoids have the potential to play multiple roles in the regulation of breast cancers including their control of cellular differentiation, apoptosis and proliferation. Beyond this they also act as a master integrator of organ homeostats in relation to such as circadian rhythms and stress responses. Therefore a better understanding of glucocorticoids and breast cancer could help to explain some of the epidemiological links between circadian disruption and/or stress and breast cancer development. Finally glucocorticoids are currently used during chemotherapeutic treatment in breast cancer therapy and yet results of various studies suggest that this may have an adverse impact on treatment success. This review aims to summarise the current evidence for glucocorticoids as actors in breast cancer and then suggest future essential approaches in order to determine the roles of glucocorticoids in this disease.

Molecular pathways involved in the transport of nuclear receptors from the nucleus to cytoplasm

Nuclear receptors (NRs) are transcription regulators that direct the expression of many genes linked to cellular processes, such as proliferation, differentiation, and apoptosis. Additionally, some cellular events are also modulated by signaling pathways induced by NRs outside of the nucleus. Hence, the subcellular transport of NRs is dynamic and is modulated by several signals, protein-protein interactions, and posttranslational modifications. Particularly, the exit of NRs from the nucleus to cytoplasm and/or other compartments is transcendental, as it is this export event, which determines their abundance in the cells' compartments, the activation or attenuation of nuclear or extranuclear pathways, and the magnitude and duration of their effects inside or outside of the nucleus. Consequently, an adequate control of the distribution of NRs is critical for homeostasis, because a deregulation in the nucleo-cytoplasmic transport of NRs could be involved in diseases including cancer as well as metabolic and vascular alterations. In this review, we investigated the pathways and molecular and biological aspects that have been described for the nuclear export of NRs so far and their functional relevance in some diseases. This information suggests that the transport of NRs out of the nucleus is a key mechanism for the identification of new therapeutic targets for alterations associated with the deregulation of the function of NRs.

Comparative proteomic and metabolomic analysis reveal the antiosteoporotic molecular mechanism of icariin from Epimedium brevicornu maxim

ETHNOPHARMACOLOGICAL RELEVANCE

Icariin, a principal flavonoid glycoside of Epimedium brevicornu Maxim, has been widely proved to possess antiosteoporotic activity with promoting bone formation and decreasing bone resorption. However, the involving mechanisms remain unclear.

AIM OF THE STUDY

To clear a global insight of signal pathways involved in anti-osteoporotic mechanism of icariin at proteins and metabolites level by integrating the proteomics and NMR metabonomics, in a systems biology approach.

MATERIAL AND METHODS

Mice were divided into sham, OVX model and icariin-treated OVX group, after 90 days treatment, difference gel electrophoresis combined with MALDI-TOF/TOF proteomics analysis on bone femur and serum metabolomics were carried out for monitor intracellular processes and elucidate anti-osteoporotic mechanism of icariin. Osteoblast and osteoclast were applied to evaluate the potential signal pathways.

RESULTS

Twenty three proteins in bone femur, and 8 metabolites in serum, were significantly altered and identified, involving in bone remodeling, energy metabolism, cytoskeleton, lipid metabolism, MAPK signaling, Ca²⁺ signaling et, al. Furthermore, animal experiment show icariin could enhance the BMD and BMC, decrease CTX-I level in ovariectomized mice. The mitochondrial membrane potential and the intracellular ATP levels were increased significantly, and the cytoskeleton were improved in icariin-treatment osteoblast and osteoclast. Icariin also increased mRNA expression of Runx2 and osterix of OB, decreased CTR and CAII mRNA expression and protein expression of P38 and JNK. However, icariin did not reveal any inhibition of the collagenolytic activity of cathepsin K, mRNA expression of MMP-9 and protein expression of ERK in osteoclast.

CONCLUSION

we consider icariin as multi-targeting compounds for treating with osteoporosis, involve initiating osteoblastogenesis, inhibiting adipogenesis, and preventing osteoclast differentiation.

Glucocorticoid-Induced Osteoporosis

Osteoporosis is among the most devastating side effects of glucocorticoid (GC) therapy for the management of inflammatory and auto-immune diseases. Evidence from both humans and mice indicate deleterious skeletal effects within weeks of pharmacological GC administration, both related and unrelated to a decrease in bone mineral density (BMD). Osteoclast numbers and bone resorption are also rapidly increased, and together with osteoblast inactivation and decreased bone formation, these changes lead the fastest loss in BMD during the initial disease phase. Bone resorption then decreases to sub-physiological levels, but persistent and severe inhibition of bone formation leads to further bone loss and progressively increased fracture risk, up to an order of magnitude higher than that observed in untreated individuals. Bone forming osteoblasts are thus considered the main culprits in GC-induced osteoporosis (GIO). Accordingly, we focus this review primarily on deleterious effects on osteoblasts: inhibition of cell replication and function and acceleration of apoptosis. Mediating these adverse effects, GCs target pivotal regulatory mechanisms that govern osteoblast growth, differentiation and survival. Specifically, GCs inhibit growth factor pathways, including Insulin Growth Factors, Growth Hormone, Hepatocyte Growth/Scatter Factor and IL6-type cytokines. They also inhibit downstream kinases, including PI3-kinase and the MAP kinase ERK, the latter attributable in part to direct transcriptional stimulation of MAP kinase phosphatase 1. Most importantly, however, GCs inhibit the Wnt signaling pathway, which plays a pivotal role in osteoblast replication, function and survival. They transcriptionally stimulate expression of Wnt inhibitors of both the Dkk and Sfrp families, and they induce reactive oxygen species (ROS), which result in loss of ß-catenin to ROS-activated FoxO transcription factors. Identification of dissociated GCs, which would suppress the immune system without causing osteoporosis, is proving more challenging than initially thought, and GIO is currently managed by co-treatment with bisphosphonates or PTH. These drugs, however, are not ideally suited for GIO. Future therapeutic approaches may aim at GC targets such as those mentioned above, or newly identified targets including the Notch pathway, the AP-1/Il11 axis and the osteoblast master regulator RUNX2.

Calreticulin translocation aggravates endoplasmic reticulum stress-associated apoptosis during cardiomyocyte hypoxia/reoxygenation

Background:

Calreticulin (CRT) is major Ca²⁺-binding chaperone mainly resident in the endoplasmic reticulum (ER) lumen. Recently, it has been shown that non-ER CRT regulates a wide array of cellular responses. We previously found that CRT was up-regulated during hypoxia/reoxygenation (H/R) and this study was aimed to investigate whether CRT nuclear translocation aggravates ER stress (ERS)-associated apoptosis during H/R injury in neonatal rat cardiomyocytes.

Methods:

Apoptosis rate and lactate dehydrogenase (LDH) leakage in culture medium were measured as indices of cell injury. Immunofluorescence staining showed the morphological changes of ER and intracellular translocation of CRT. Western blotting or reverse transcription polymerase chain reaction was used to detect the expression of target molecules.

Results:

Compared with control, H/R increased apoptosis rate and LDH activity. The ER became condensed and bubbled, and CRT translocated to the nucleus. Western blotting showed up-regulation of CRT, Nrf2, activating transcription factor 4 (ATF4), CHOP and caspase-12 expression after H/R. Exogenous CRT overexpression induced by plasmid transfection before H/R increased cell apoptosis, LDH leakage, ER disorder, CRT nuclear translocation and the expression of ERS-associated molecules. However, administration of the ERS inhibitor, taurine, or CRT siRNA alleviated cell injury, ER disorder, and inhibited ERS-associated apoptosis.

Conclusions:

Our results indicated that during H/R stress, CRT translocation increases cell apoptosis and LDH leakage, aggravates ER disorder, up-regulates expression of nuclear transcription factors, Nrf2 and ATF4, and activates ERS-associated apoptosis.

Glucocorticoid Receptor β Acts as a Co-activator of T-Cell Factor 4 and Enhances Glioma Cell Proliferation

We previously reported that glucocorticoid receptor β (GRβ) regulates injury-mediated astrocyte activation and contributes to glioma pathogenesis via modulation of β-catenin/T-cell factor/lymphoid enhancer factor (TCF/LEF) transcriptional activity. The aim of this study was to characterize the mechanism behind cross-talk between GRβ and β-catenin/TCF in the progression of glioma. Here, we reported that GRβ knockdown reduced U118 and Shg44 glioma cell proliferation in vitro and in vivo. Mechanistically, we found that GRβ knockdown decreased TCF/LEF transcriptional activity without affecting β-catenin/TCF complex. Both GRα and GRβ directly interact with TCF-4, while only GRβ is required for sustaining TCF/LEF activity under hormone-free condition. GRβ bound to the N-terminus domain of TCF-4 its influence on Wnt signaling required both ligand- and DNA-binding domains (LBD and DBD, respectively). GRβ and TCF-4 interaction is enough to maintain the TCF/LEF activity at a high level in the absence of β-catenin stabilization. Taken together, these results suggest a novel cross-talk between GRβ and TCF-4 which regulates Wnt signaling and the proliferation in gliomas.

Purification and characterization of calreticulin: a Ca²⁺-binding chaperone from sheep kidney

Calreticulin (CRT) is a molecular chaperone with a molecular mass of 46 kDa present in the endoplasmic reticulum (ER). This protein is primarily involved in the regulation of intracellular Ca(2+) homeostasis and Ca(2+) storage in the ER. CRT also plays a significant role in autoimmunity and cancer. This protein contains three distinct structural domains with specialized functions. Here, we are reporting a simple procedure for the purification of CRT from mammalian kidney. To isolate CRT, sheep kidney was crushed and kept for 12 h in the extraction buffer. The lysate was centrifuged, and supernatant was precipitated by ammonium sulphate. The precipitate of 90 % ammonium sulphate was extensively dialyzed and loaded on DEAE-Hi-Trap FF and Mono Q chromatography columns. The purity of CRT was confirmed by SDS-PAGE. Finally, the protein was identified by matrix-assisted laser desorption/ionization time of flight. The purified protein was further characterized for secondary structural elements using the far-UV circular dichroism measurements. Our purification procedure is fast and simple with high yield.

Glucocorticoid receptor β regulates injury-mediated astrocyte activation and contributes to glioma pathogenesis via modulation of β-catenin/TCF transcriptional activity

Astrocytes react to central nervous system (CNS) injury and participate in gliotic responses, imparting negative, as well as positive effects on axonal regeneration. Despite the considerable biochemical and morphological changes astrocytes undergo following insult, and the known influence of steroids on glial activation, details surrounding glucocorticoid receptor expression and activity are lacking. Such mechanistic information is essential for advancing and enhancing therapies in the treatment of CNS injuries. Using an in vitro wound-healing assay, we found glucocorticoid receptor β (GRβ), not GRα, is upregulated and acts as a regulator of gliosis after injury. In addition, our results suggest that GRβ interacts with β-catenin and is a necessary component for proliferation and migration in both injured astrocytes and glioma cells. Further analysis indicated GRβ/β-catenin interaction as a key modulator of astrocyte reactivity through sustained Wnt/β-catenin/TCF signaling in its dominant-negative effect on GRα mediated trans-repression by a GSK-3β-independent manner. These findings expand our knowledge of the mechanism of GRβ action in promoting astrocyte proliferation and migration following injury and in glioma. This information furthers our understanding the function of glucocorticoid receptor in CNS injury and disease, as well as in the basic biochemical responses astrocytes undergo in response to injury and glioma pathogenesis.

Novel evidence of the involvement of calreticulin in major psychiatric disorders

Calreticulin (CALR) is a multi-functional protein that is strictly conserved across species. Two mRNA transcripts have been recognized for the CALR gene in humans, which use a common promoter sequence. We have recently reported mutations in the CALR promoter that co-occur with psychosis. One of those mutations at -220A increases gene expression in human BE(2)-C and HEK-293 cell lines. This mutation is the first instance of a functional cognition-deficit mutation reversing a human gene promoter to the primitive type. In the current study, we analyzed the effect of the most widely-used mood-stabilizing drug, valproic acid (VPA), on nucleotide -220 in two neuronal cell lines, LAN-5 and N2A. Remarkably, VPA increased gene expression in the cells with the wild-type -220C construct, whereas a dramatic decrease in gene expression was observed in the cell lines with the mutant construct (p<0.000004 and p<0.016, respectively). We also sequenced the 600-bp CALR promoter, and the highly conserved intron 1 sequence in an independent sample of patients afflicted with major psychiatric disorders and controls. A new case of major depressive disorder with psychotic features with the -220A mutation was identified. A novel 1-bp insertion was also detected in intron 1 at IVSI-310, in a case of amphetamine-induced psychosis. As for the psychosis-linked CALR promoter mutations identified to-date, the IVSI mutation was not detected in the control pool. This mutation creates a RREB-1 transcription factor binding site within the first intron. Our present findings identify the site of action of VPA in the CALR promoter, and introduce a novel mutation in a case of substance-induced psychosis in the first intron of CALR.

Glucocorticoid receptor signaling in bone cells

Glucocorticoids are used for treating a wide range of diseases including inflammation and autoimmune disorders. However, there are drawbacks, primarily due to adverse effects on bone cells resulting in osteoporosis. Evidence indicates that the ratio of benefits to adverse effects depends greatly on glucocorticoid receptor (GR)-mediated mechanisms. Delineating GR-mediated signaling in bone cells will allow development of selective GR ligands/agonists (SEGRAs), which would dissociate the positive therapeutic (anti-inflammatory) effects from the negative effects on the skeleton. The present review provides an in-depth account of the current knowledge of GR-mediated transcriptional regulation of specific genes and proteins engaged in the proliferation, differentiation, and apoptosis of bone cells (osteoblasts, osteocytes, osteoclasts). We hope this knowledge will advance research in the development of SEGRAs with improved benefit/risk ratios.

Arginylated calreticulin at plasma membrane increases susceptibility of cells to apoptosis

Post-translational modifications of proteins are important for the regulation of cell fate and functions; one of these post-translational modifications is arginylation. We have previously established that calreticulin (CRT), an endoplasmic reticulum resident, is also one of the arginylated substrates found in the cytoplasm. In the present study, we describe that arginylated CRT (R-CRT) binds to the cell membrane and identified its role as a preapoptotic signal. We also show that cells lacking arginyl-tRNA protein transferase are less susceptible to apoptosis than wild type cells. Under these conditions R-CRT is present on the cell membrane but at early stages is differently localized in stress granules. Moreover, cells induced to undergo apoptosis by arsenite show increased R-CRT on their cell surface. Exogenously applied R-CRT binds to the cell membrane and is able to both increase the number of cells undergoing apoptosis in wild type cells and overcome apoptosis resistance in cells lacking arginyl-tRNA protein transferase that express R-CRT on the cell surface. Thus, these results demonstrate the importance of surface R-CRT in the apoptotic response of cells, implying that post-translational arginylation of CRT can regulate its intracellular localization, cell function, and survival.

On the trail of the glucocorticoid receptor: into the nucleus and back

The glucocorticoid receptor (GR) belongs to the superfamily of steroid receptors and is an important regulator of physiological and metabolic processes. In its inactive state, GR is unbound by ligand and resides in the cytoplasm in a chaperone complex. When it binds glucocorticoids, it is activated and translocates to the nucleus, where it functions as a transcription factor. However, the subcellular localization of GR is determined by the balance between its rates of nuclear import and export. The mechanism of GR nuclear transport has been extensively studied. Originally, it was believed that nuclear import of GR is initiated by dissociation of the chaperone complex in the cytoplasm. However, several studies show that the chaperone machinery is required for nuclear transport of GR. In this review, we summarize the contribution of various chaperone components involved in the nuclear transport of GR and propose an updated model of its nuclear import and export. Moreover, we review the importance of ligand-independent nuclear transport and compare the nuclear transport of GR with that of other steroid receptors.

Interaction of ERp57 with calreticulin: Analysis of complex formation and effects of vancomycin

The protein ERp57 (also known as PDIA3) is a widely distributed protein, mainly localized in the endoplasmic reticulum, where it acts as disulfide isomerase, oxidoreductase and chaperone, in concert with the lectins calreticulin (CRT) and calnexin. The ERp57/CRT complex has been detected on the cell surface and previous studies have suggested its involvement in programmed cell death. Although the ERp57-CRT complex has been characterized, little is known about its role in different cellular compartments as well as inhibitors of this interaction. We focused on the kinetic, extent and stability of the ERp57-CRT complex, using the surface plasmon resonance spectroscopy, investigating the possible role as inhibitor of the antibiotic vancomycin. Equilibrium thermodynamic data suggested that vancomycin may hinder the interaction between the two proteins and could interfere with the ERp57 conformational changes that stabilize the complex. Furthermore, by means of confocal microscopy, we evaluated the effect of the in vivo administration of vancomycin on the ERp57/CRT complex on the surface of HeLa cells. The model presented here could be used for the search of other specific inhibitors/interactors of ERp57, which can be extremely helpful to understand the biological pathways where the protein is involved and to modulate its activity.

Support for down-tuning of the calreticulin gene in the process of human evolution

Tissue-specific expression of the CALR gene in the brain gray matter in late-adolescence and early adulthood coincides with the expression of the psychoses phenotypes. Indeed, increased expression of the chaperone genes in the prefrontal cortex has been reported in patients affected by schizophrenia. We have previously reported cases of psychosis-associated mutations in the CALR gene promoter. One of those mutations at -48 was found to increase the expression of the gene in comparison with the wild type sequence. A recently identified mutation at -220 reverts the conserved block harboring nucleotide -220 to the ancestral type, and has an approximate prevalence of 0.7% in psychoses. In this study, we analyzed the functional implication of this mutation in the human neuroblastoma cell line BE(2)-C, and non-neural Human Embryonic Kidney 293 (HEK-293), and show that the -220A mutation results in a constitutive increase in the expression of the CALR gene (p<0.0003). We checked homology of the first 1000-bp CALR promoter sequence across species, and found that nucleotide -220C is the only human-unique nucleotide in that stretch. The -220A mutation, on the other hand, co-occurs with severe cognition deficit in humans, and is the rule across the species except humans. To our knowledge, the -220A mutation is the first reported instance of a cognition-deficit-associated mutation which reverses a human gene promoter to the primitive type. It may be speculated that, at least the basal transcription of the CALR gene, relating to the proximal promoter region, has been decreased during the process of evolution to humans.

Reversion of the human calreticulin gene promoter to the ancestral type as a result of a novel psychosis-associated mutation

Development-dependent, tissue-specific expression of the calreticulin (CALR) gene in the gray matter coincides with the expression of psychoses phenotypes. We have recently reported instances of mutations within the core promoter sequence of the gene in schizoaffective disorder. In view of the mounting evidence on the genetic overlap in the psychiatric spectrum, we investigated this gene in a spectrum of patients afflicted with schizophrenia, schizoaffective disorder and major affective disorder. We found that a unique mutation at nucleotide -220 from the transcription start site, located at a conserved genomic block in the promoter region of the gene, co-occurs with the spectrum of psychoses (p<0.005). This mutation reverts the human promoter sequence to the ancestral type observed in chimpanzee, mouse, and several other species, implying that the genomic block harboring nucleotide -220 may be involved in the evolution of human-specific higher-order functions of the brain (e.g. language, conceptual thinking, and judgment), that are ubiquitously impaired in psychoses. We propose that CALR is not only a promising candidate in the spectrum of psychoses, but also, a gene that may be important in the human-unique brain processes.

The arginylation-dependent association of calreticulin with stress granules is regulated by calcium

Post-translational modifications of proteins are important for the regulation of cell functions; one of these modifications is post-translational arginylation. In the present study, we show that cytoplasmic CRT (calreticulin) is arginylated by ATE1 (arginyl-tRNA protein transferase). We also show that a pool of CRT undergoes retrotranslocation from the ER (endoplasmic reticulum) to the cytosol, because in CRT-knockout cells transfected with full-length CRT (that has the signal peptide), cytoplasmic CRT appears as a consequence of its expression and processing in the ER. After the cleavage of the signal peptide, an N-terminal arginylatable residue is revealed prior to retrotranslocation to the cytoplasm where arginylation takes place. SGs (stress granules) from ATE1-knockout cells do not contain CRT, indicating that CRT arginylation is required for its association to SGs. Furthermore, R-CRT (arginylated CRT) in the cytoplasm associates with SGs in cells treated with several stressors that lead to a reduction of intracellular Ca2+ levels. However, in the presence of stressors that do not affect Ca2+ levels, R-CRT is not recruited to these loci despite the fact that SGs are formed, demonstrating Ca2+-dependent R-CRT association to SGs. We conclude that post-translational arginylation of retrotranslocated CRT, together with the decrease in intracellular Ca2+, promotes the association of CRT to SGs.

Genome-wide identification of new Wnt/beta-catenin target genes in the human genome using CART method

Background

The importance of in silico predictions for understanding cellular processes is now widely accepted, and a variety of algorithms useful for studying different biological features have been designed. In particular, the prediction of cis regulatory modules in non-coding human genome regions represents a major challenge for understanding gene regulation in several diseases. Recently, studies of the Wnt signaling pathway revealed a connection with neurodegenerative diseases such as Alzheimer's. In this article, we construct a classification tool that uses the transcription factor binding site motifs composition of some gene promoters to identify new Wnt/β-catenin pathway target genes potentially involved in brain diseases.

Results

In this study, we propose 89 new Wnt/β-catenin pathway target genes predicted in silico by using a method based on multiple Classification and Regression Tree (CART) analysis. We used as decision variables the presence of transcription factor binding site motifs in the upstream region of each gene. This prediction was validated by RT-qPCR in a sample of 9 genes. As expected, LEF1, a member of the T-cell factor/lymphoid enhancer-binding factor family (TCF/LEF1), was relevant for the classification algorithm and, remarkably, other factors related directly or indirectly to the inflammatory response and amyloidogenic processes also appeared to be relevant for the classification. Among the 89 new Wnt/β-catenin pathway targets, we found a group expressed in brain tissue that could be involved in diverse responses to neurodegenerative diseases, like Alzheimer's disease (AD). These genes represent new candidates to protect cells against amyloid β toxicity, in agreement with the proposed neuroprotective role of the Wnt signaling pathway.

Conclusions

Our multiple CART strategy proved to be an effective tool to identify new Wnt/β-catenin pathway targets based on the study of their regulatory regions in the human genome. In particular, several of these genes represent a new group of transcriptional dependent targets of the canonical Wnt pathway. The functions of these genes indicate that they are involved in pathophysiology related to Alzheimer's disease or other brain disorders.