Grp94 is Tyr-phosphorylated by Fyn in the lumen of the endoplasmic reticulum and translocates to Golgi in differentiating myoblasts

A proinsulin-dependent interaction between ENPL-1 and ASNA-1 in neurons is required to maintain insulin secretion in C. elegans

Neuropeptides, including insulin, are important regulators of physiological functions of the organisms. Trafficking through the Golgi is crucial for the regulation of secretion of insulin-like peptides. ASNA-1 (TRC40) and ENPL-1 (GRP94) are conserved insulin secretion regulators in Caenorhabditis elegans (and mammals), and mouse Grp94 mutants display type 2 diabetes. ENPL-1/GRP94 binds proinsulin and regulates proinsulin levels in C. elegans and mammalian cells. Here, we have found that ASNA-1 and ENPL-1 cooperate to regulate insulin secretion in worms via a physical interaction that is independent of the insulin-binding site of ENPL-1. The interaction occurs in DAF-28/insulin-expressing neurons and is sensitive to changes in DAF-28 pro-peptide levels. Consistently, ASNA-1 acted in neurons to promote DAF-28/insulin secretion. The chaperone form of ASNA-1 was likely the interaction partner of ENPL-1. Loss of asna-1 disrupted Golgi trafficking pathways. ASNA-1 localization to the Golgi was affected in enpl-1 mutants and ENPL-1 overexpression partially bypassed the ASNA-1 requirement. Taken together, we find a functional interaction between ENPL-1 and ASNA-1 that is necessary to maintain proper insulin secretion in C. elegans and provides insights into how their loss might cause diabetes in mammals.

CD28-CAR-T cell activation through FYN kinase signaling rather than LCK enhances therapeutic performance

Signal transduction induced by chimeric antigen receptors (CARs) is generally believed to rely on the activity of the SRC family kinase (SFK) LCK, as is the case with T cell receptor (TCR) signaling. Here, we show that CAR signaling occurs in the absence of LCK. This LCK-independent signaling requires the related SFK FYN and a CD28 intracellular domain within the CAR. LCK-deficient CAR-T cells are strongly signaled through CAR and have better in vivo efficacy with reduced exhaustion phenotype and enhanced induction of memory and proliferation. These distinctions can be attributed to the fact that FYN signaling tends to promote proliferation and survival, whereas LCK signaling promotes strong signaling that tends to lead to exhaustion. This non-canonical signaling of CAR-T cells provides insight into the initiation of both TCR and CAR signaling and has important clinical implications for improvement of CAR function.

Chronic Systemic Curcumin Administration Antagonizes Murine Sarcopenia and Presarcopenia

Curcumin administration attenuates muscle disuse atrophy, but its effectiveness against aging-induced, selective loss of mass or force (presarcopenia or asthenia/dynopenia), or combined loss (sarcopenia), remains controversial. A new systemic curcumin treatment was developed and tested in 18-month-old C57BL6J and C57BL10ScSn male mice. The effects on survival, liver toxicity, loss of muscle mass and force, and satellite cell responsivity and commitment were evaluated after 6-month treatment. Although only 24-month-old C57BL10ScSn mice displayed age-related muscle impairment, curcumin significantly increased survival of both strains (+20–35%), without signs of liver toxicity. Treatment prevented sarcopenia in soleus and presarcopenia in EDL of C57BL10ScSn mice, whereas it did not affect healthy-aged muscles of C57BL6J. Curcumin-treated old C57BL10ScSn soleus preserved type-1 myofiber size and increased type-2A one, whereas EDL maintained adult values of total myofiber number and fiber-type composition. Mechanistically, curcumin only partially prevented the age-related changes in protein level and subcellular distribution of major costamere components and regulators. Conversely, it affected satellite cells, by maintaining adult levels of myofiber maturation in old regenerating soleus and increasing percentage of isolated, MyoD-positive satellite cells from old hindlimb muscles. Therefore, curcumin treatment successfully prevents presarcopenia and sarcopenia development by improving satellite cell commitment and recruitment.

ENPL-1, the Caenorhabditis elegans homolog of GRP94, promotes insulin secretion via regulation of proinsulin processing and maturation

Insulin/IGF signaling in Caenorhabditis elegans is crucial for proper development of the dauer larva and growth control. Mutants disturbing insulin processing, secretion and downstream signaling perturb this process and have helped identify genes that affect progression of type 2 diabetes. Insulin maturation is required for its proper secretion by pancreatic β cells. The role of the endoplasmic reticulum (ER) chaperones in insulin processing and secretion needs further study. We show that the C. elegans ER chaperone ENPL-1/GRP94 (HSP90B1), acts in dauer development by promoting insulin secretion and signaling. Processing of a proinsulin likely involves binding between the two proteins via a specific domain. We show that, in enpl-1 mutants, an unprocessed insulin exits the ER lumen and is found in dense core vesicles, but is not secreted. The high ER stress in enpl-1 mutants does not cause the secretion defect. Importantly, increased ENPL-1 levels result in increased secretion. Taken together, our work indicates that ENPL-1 operates at the level of insulin availability and is an essential modulator of insulin processing and secretion.

Grp94 Regulates the Recruitment of Aneural AChR Clusters for the Assembly of Postsynaptic Specializations by Modulating ADF/Cofilin Activity and Turnover

Temperature is a physiological factor that affects neuronal growth and synaptic homeostasis at the invertebrate neuromuscular junctions (NMJs); however, whether temperature stress could also regulate the structure and function of the vertebrate NMJs remains unclear. In this study, we use Xenopus laevis primary cultures as a vertebrate model system for investigating the involvement of heat shock protein 90 (HSP90) family of stress proteins in NMJ development. First, cold temperature treatment or HSP90 inhibition attenuates the formation of aneural acetylcholine receptor (AChR) clusters, but increases their stability after they are formed, in cultured muscles. HSP90 inhibition specifically affects the stability of aneural AChR clusters and their associated intracellular scaffolding protein rapsyn, instead of causing a global change in cell metabolism and protein expression in Xenopus muscle cultures. Upon synaptogenic stimulation, a specific HSP90 family member, glucose-regulated protein 94 (Grp94), modulates the phosphorylation and dynamic turnover of actin depolymerizing factor (ADF)/cofilin at aneural AChR clusters, leading to the recruitment of AChR molecules from aneural clusters to the assembly of agrin-induced postsynaptic specializations. Finally, postsynaptic Grp94 knock-down significantly inhibits nerve-induced AChR clustering and postsynaptic activity in nerve-muscle co-cultures as demonstrated by live-cell imaging and electrophysiological recording, respectively. Collectively, this study suggests that temperature-dependent alteration in Grp94 expression and activity inhibits the assembly of postsynaptic specializations through modulating ADF/cofilin phosphorylation and activity at aneural AChR clusters, which prevents AChR molecules from being recruited to the postsynaptic sites via actin-dependent vesicular trafficking, at developing vertebrate NMJs.

Trafficking of Adhesion and Growth Factor Receptors and Their Effector Kinases

Cell adhesion to macromolecules in the microenvironment is essential for the development and maintenance of tissues, and its dysregulation can lead to a range of disease states, including inflammation, fibrosis, and cancer. The biomechanical and biochemical mechanisms that mediate cell adhesion rely on signaling by a range of effector proteins, including kinases and associated scaffolding proteins. The intracellular trafficking of these must be tightly controlled in space and time to enable effective cell adhesion and microenvironmental sensing and to integrate cell adhesion with, and compartmentalize it from, other cellular processes, such as gene transcription, protein degradation, and cell division. Delivery of adhesion receptors and signaling proteins from the plasma membrane to unanticipated subcellular locales is revealing novel biological functions. Here, we review the expected and unexpected trafficking, and sites of activity, of adhesion and growth factor receptors and intracellular kinase partners as we begin to appreciate the complexity and diversity of their spatial regulation.

Cell entry of a host-targeting protein of oomycetes requires gp96

The animal-pathogenic oomycete Saprolegnia parasitica causes serious losses in aquaculture by infecting and killing freshwater fish. Like plant-pathogenic oomycetes, S. parasitica employs similar infection structures and secretes effector proteins that translocate into host cells to manipulate the host. Here, we show that the host-targeting protein SpHtp3 enters fish cells in a pathogen-independent manner. This uptake process is guided by a gp96-like receptor and can be inhibited by supramolecular tweezers. The C-terminus of SpHtp3 (containing the amino acid sequence YKARK), and not the N-terminal RxLR motif, is responsible for the uptake into host cells. Following translocation, SpHtp3 is released from vesicles into the cytoplasm by another host-targeting protein where it degrades nucleic acids. The effector translocation mechanism described here, is potentially also relevant for other pathogen-host interactions as gp96 is found in both animals and plants.

Endoplasmic reticulum chaperones and oxidoreductases: critical regulators of tumor cell survival and immunorecognition

Endoplasmic reticulum (ER) chaperones and oxidoreductases are abundant enzymes that mediate the production of fully folded secretory and transmembrane proteins. Resisting the Golgi and plasma membrane-directed “bulk flow,” ER chaperones and oxidoreductases enter retrograde trafficking whenever they are pulled outside of the ER by their substrates. Solid tumors are characterized by the increased production of reactive oxygen species (ROS), combined with reduced blood flow that leads to low oxygen supply and ER stress. Under these conditions, hypoxia and the unfolded protein response upregulate their target genes. When this occurs, ER oxidoreductases and chaperones become important regulators of tumor growth. However, under these conditions, these proteins not only promote the folding of proteins, but also alter the properties of the plasma membrane and hence modulate tumor immune recognition. For instance, high levels of calreticulin serve as an “eat-me” signal on the surface of tumor cells. Conversely, both intracellular and surface BiP/GRP78 promotes tumor growth. Other ER folding assistants able to modulate the properties of tumor tissue include protein disulfide isomerase (PDI), Ero1α and GRP94. Understanding the roles and mechanisms of ER chaperones in regulating tumor cell functions and immunorecognition will lead to important insight for the development of novel cancer therapies.

The stress protein/chaperone Grp94 counteracts muscle disuse atrophy by stabilizing subsarcolemmal neuronal nitric oxide synthase

AIMS

Redox and growth-factor imbalance fosters muscle disuse atrophy. Since the endoplasmic-reticulum chaperone Grp94 is required for folding insulin-like growth factors (IGFs) and for antioxidant cytoprotection, we investigated its involvement in muscle mass loss due to inactivity.

RESULTS

Rat soleus muscles were transfected in vivo and analyzed after 7 days of hindlimb unloading, an experimental model of muscle disuse atrophy, or standard caging. Increased muscle protein carbonylation and decreased Grp94 protein levels (p<0.05) characterized atrophic unloaded solei. Recombinant Grp94 expression significantly reduced atrophy of transfected myofibers, compared with untransfected and empty-vector transfected ones (p<0.01), and decreased the percentage of carbonylated myofibers (p=0.001). Conversely, expression of two different N-terminal deleted Grp94 species did not attenuate myofiber atrophy. No change in myofiber trophism was detected in transfected ambulatory solei. The absence of effects on atrophic untransfected myofibers excluded a major role for IGFs folded by recombinant Grp94. Immunoprecipitation and confocal microscopy assays to investigate chaperone interaction with muscle atrophy regulators identified 160 kDa neuronal nitric oxide synthase (nNOS) as a new Grp94 partner. Unloading was demonstrated to untether nNOS from myofiber subsarcolemma; here, we show that such nNOS localization, revealed by means of NADPH-diaphorase histochemistry, appeared preserved in unloaded myofibers expressing recombinant Grp94, compared to those transfected with the empty vector or deleted Grp94 cDNA (p<0.02).

INNOVATION

Grp94 interacts with nNOS and prevents its untethering from sarcolemma in unloaded myofibers.

CONCLUSION

Maintenance of Grp94 expression is sufficient to counter unloading atrophy and oxidative stress by mechanistically stabilizing nNOS-multiprotein complex at the myofiber sarcolemma.

The small molecules AZD0530 and dasatinib inhibit dengue virus RNA replication via Fyn kinase

In this study, we characterized the antiviral mechanism of action of AZD0530 and dasatinib, two pharmacological inhibitors of host kinases, that also inhibit dengue virus (DV) infection. Using Northern blot and reporter replicon assays, we demonstrated that both small molecules inhibit the DV2 infectious cycle at the step of steady-state RNA replication. In order to identify the cellular target of AZD0530 and dasatinib mediating this anti-DV2 activity, we examined the effects of RNA interference (RNAi)-mediated depletion of the major kinases known to be inhibited by these small molecules. We determined that Fyn kinase, a target of both AZD0530 and dasatinib, is involved in DV2 RNA replication and is probably a major mediator of the anti-DV activity of these compounds. Furthermore, serial passaging of DV2 in the presence of dasatinib led to the identification of a mutation in the transmembrane domain 3 of the NS4B protein that overcomes the inhibition of RNA replication by AZD0530, dasatinib, and Fyn RNAi. Although we observed that dasatinib also inhibits DV2 particle assembly and/or secretion, this activity does not appear to be mediated by Src-family kinases. Together, our results suggest that AZD0530 and dasatinib inhibit DV at the step of viral RNA replication and demonstrate a critical role for Fyn kinase in this viral process. The antiviral activity of these compounds in vitro makes them useful pharmacological tools to validate Fyn or other host kinases as anti-DV targets in vivo.

Hsp90: structure and function

Hsp90 is a highly abundant and ubiquitous molecular chaperone which plays an essential role in many cellular processes including cell cycle control, cell survival, hormone and other signalling pathways. It is important for the cell's response to stress and is a key player in maintaining cellular homeostasis. In the last ten years, it has become a major therapeutic target for cancer, and there has also been increasing interest in it as a therapeutic target in neurodegenerative disorders, and in the development of anti-virals and anti-protozoan infections. The focus of this review is the structural and mechanistic studies which have been performed in order to understand how this important chaperone acts on a wide variety of different proteins (its client proteins) and cellular processes. As with many of the other classes of molecular chaperone, Hsp90 has a critical ATPase activity, and ATP binding and hydrolysis known to modulate the conformational dynamics of the protein. It also uses a host of cochaperones which not only regulate the ATPase activity and conformational dynamics but which also mediate interactions with Hsp90 client proteins. The system is also regulated by post-translational modifications including phosphorylation and acetylation. This review discusses all these aspects of Hsp90 structure and function.

Interaction between the SH3 domain of Src family kinases and the proline-rich motif of HTLV-1 p13: a novel mechanism underlying delivery of Src family kinases to mitochondria

The association of the SH3 (Src homology 3) domain of SFKs (Src family kinases) with protein partners bearing proline-rich motifs has been implicated in the regulation of SFK activity, and has been described as a possible mechanism of relocalization of SFKs to subcellular compartments. We demonstrate in the present study for the first time that p13, an accessory protein encoded by the HTLV-1 (human T-cell leukaemia virus type 1), binds the SH3 domain of SFKs via its C-terminal proline-rich motif, forming a stable heterodimer that translocates to mitochondria by virtue of its N-terminal mitochondrial localization signal. As a result, the activity of SFKs is dramatically enhanced, with a subsequent increase in mitochondrial tyrosine phosphorylation, and the recognized ability of p13 to insert itself into the inner mitochondrial membrane and to perturb the mitochondrial membrane potential is abolished. Overall, the present study, in addition to confirming that the catalytic activity of SFKs is modulated by interactors of their SH3 domain, leads us to hypothesize a general mechanism by which proteins bearing a proline-rich motif and a mitochondrial localization signal at the same time may act as carriers of SFKs into mitochondria, thus contributing to the regulation of mitochondrial functions under various pathophysiological conditions.

GRP94: An HSP90-like protein specialized for protein folding and quality control in the endoplasmic reticulum

Glucose-regulated protein 94 is the HSP90-like protein in the lumen of the endoplasmic reticulum and therefore it chaperones secreted and membrane proteins. It has essential functions in development and physiology of multicellular organisms, at least in part because of this unique clientele. GRP94 shares many biochemical features with other HSP90 proteins, in particular its domain structure and ATPase activity, but also displays distinct activities, such as calcium binding, necessitated by the conditions in the endoplasmic reticulum. GRP94's mode of action varies from the general HSP90 theme in the conformational changes induced by nucleotide binding, and in its interactions with co-chaperones, which are very different from known cytosolic co-chaperones. GRP94 is more selective than many of the ER chaperones and the basis for this selectivity remains obscure. Recent development of molecular tools and functional assays has expanded the spectrum of clients that rely on GRP94 activity, but it is still not clear how the chaperone binds them, or what aspect of folding it impacts. These mechanistic questions and the regulation of GRP94 activity by other proteins and by post-translational modification differences pose new questions and present future research avenues. This article is part of a Special Issue entitled: Heat Shock Protein 90 (HSP90).

FATP4 contributes as an enzyme to the basal and insulin-mediated fatty acid uptake of C₂C₁₂ muscle cells

The function of membrane proteins in long-chain fatty acid transport is controversial. The acyl-CoA synthetase fatty acid transport protein-4 (FATP4) has been suggested to facilitate fatty acid uptake indirectly by its enzymatic activity, or directly by transport across the plasma membrane. Here, we investigated the function of FATP4 in basal and insulin mediated fatty acid uptake in C(2)C(12) muscle cells, a model system relevant for fatty acid metabolism. Stable expression of exogenous FATP4 resulted in a twofold higher fatty acyl-CoA synthetase activity, and cellular uptake of oleate was enhanced similarly. Kinetic analysis demonstrated that FATP4 allowed the cells to reach apparent saturation of fatty acid uptake at a twofold higher level compared with control. Short-term treatment with insulin increased fatty acid uptake in line with previous reports. Surprisingly, insulin increased the acyl-CoA synthetase activity of C(2)C(12) cells within minutes. This effect was sensitive to inhibition of insulin signaling by wortmannin. Affinity purified FATP4 prepared from insulin-treated cells showed an enhanced enzyme activity, suggesting it constitutes a novel target of short-term metabolic regulation by insulin. This offers a new mechanistic explanation for the concomitantly observed enhanced fatty acid uptake. FATP4 was colocalized to the endoplasmic reticulum by double immunofluorescence and subcellular fractionation, clearly distinct from the plasma membrane. Importantly, neither differentiation into myotubes nor insulin treatment changed the localization of FATP4. We conclude that FATP4 functions by its intrinsic enzymatic activity. This is in line with the concept that intracellular metabolism plays a significant role in cellular fatty acid uptake.

Src family kinases and paclitaxel sensitivity

Src-family Kinases (SFKs) participate in the regulation of proliferation, differentiation, apoptosis, autophagy, adhesion, migration, invasion and angiogenesis in normal and cancer cells. Abnormal expression of SFKs has been documented in cancers that arise in breast, colon, ovary, melanocyte, gastric mucosa, head and neck, pancreas, lung, and brain. Targeting SFKs in cancer cells has been shown to be a promising therapeutic strategy in solid tumors, particularly in ovarian, colon and breast cancers. Paclitaxel is one of most widely used chemotherapeutic agents for the management of ovarian, breast, lung and head/neck cancers. As a microtubule-stabilizing agent, paclitaxel possesses both mitosis-dependent and mitosis-independent activities against cancer cells. A variety of mechanisms such as deregulation of P-glycoprotein, alteration of tubulin isotypes, alteration of microtubule-regulatory proteins, deregulation of apoptotic signaling pathways, mutation of tubulins and overexpression of copper transporters have been implicated in the development of primary or secondary resistance to paclitaxel. By affecting cancer cell survival, proliferation, autophagy, microtubule stability, motility, and/or angiogenesis, SFKs interact with mechanisms that regulate paclitaxel sensitivity. Inhibition of SFKs can potentiate the anti-tumor activity of paclitaxel by enhancing apoptosis, autophagy and microtubule stability. Based on pre-clinical observations, administration of SFK inhibitors in combination with paclitaxel could improve treatment for ovarian, breast, lung and head/neck cancers. Identification and validation of predictive biomarkers could also permit personalization of the therapy.

Grp94 acts as a mediator of curcumin-induced antioxidant defence in myogenic cells

Curcumin is a non-toxic polyphenol with pleiotropic activities and limited bioavailability. We investigated whether a brief exposure to low doses of curcumin would induce in the myogenic C2C12 cell line an endoplasmic reticulum (ER) stress response and protect against oxidative stress. A 3-hr curcumin administration (5-10 microM) increased protein levels of the ER chaperone Grp94, without affecting those of Grp78, calreticulin and haeme-oxygenase-1 (HO-1). Exposure of cells to hydrogen peroxide 24 hrs after the curcumin treatment decreased caspase-12 activation, total protein oxidation and translocation of NF-kappaB to the nucleus, compared with untreated cells. Grp94 overexpression, achieved by means of either stable or transient trasfection, induced comparable cytoprotective effects to hydrogen peroxide. The delayed cytoprotection induced by curcumin acted through Grp94, because the curcumin-induced increase in Grp94 expression was hampered by either stable or transient transfection with antisense cDNA; in these latter cells, the extent of total protein oxidation, as well as the translocation of NF-kappaB to the nucleus, and the percentage of apoptotic cells were comparable to those observed in both curcumin-untreated wild-type and empty vector transfected cells. Defining the mechanism(s) by which Grp94 exerts its antioxidant defence, the determination of cytosolic calcium levels in C2C12 cells by fura-2 showed a significantly reduced amount of releasable calcium from intracellular stores, both in conditions of Grp94 overexpression and after curcumin pre-treatment. Therefore, a brief exposure to curcumin induces a delayed cytoprotection against oxidative stress in myogenic cells by increasing Grp94 protein level, which acts as a regulator of calcium homeostasis.

Grp94 acts as a mediator of curcumin-induced antioxidant defence in myogenic cells

Curcumin is a non-toxic polyphenol with pleiotropic activities and limited bioavailability. We investigated whether a brief exposure to low doses of curcumin would induce in the myogenic C2C12 cell line an endoplasmic reticulum (ER) stress response and protect against oxidative stress. A 3-hr curcumin administration (5-10 microM) increased protein levels of the ER chaperone Grp94, without affecting those of Grp78, calreticulin and haeme-oxygenase-1 (HO-1). Exposure of cells to hydrogen peroxide 24 hrs after the curcumin treatment decreased caspase-12 activation, total protein oxidation and translocation of NF-kappaB to the nucleus, compared with untreated cells. Grp94 overexpression, achieved by means of either stable or transient trasfection, induced comparable cytoprotective effects to hydrogen peroxide. The delayed cytoprotection induced by curcumin acted through Grp94, because the curcumin-induced increase in Grp94 expression was hampered by either stable or transient transfection with antisense cDNA; in these latter cells, the extent of total protein oxidation, as well as the translocation of NF-kappaB to the nucleus, and the percentage of apoptotic cells were comparable to those observed in both curcumin-untreated wild-type and empty vector transfected cells. Defining the mechanism(s) by which Grp94 exerts its antioxidant defence, the determination of cytosolic calcium levels in C2C12 cells by fura-2 showed a significantly reduced amount of releasable calcium from intracellular stores, both in conditions of Grp94 overexpression and after curcumin pre-treatment. Therefore, a brief exposure to curcumin induces a delayed cytoprotection against oxidative stress in myogenic cells by increasing Grp94 protein level, which acts as a regulator of calcium homeostasis.

Differential proteome and phosphoproteome signatures in human T-lymphoblast cells induced by sirolimus

OBJECTIVES

The present study was designed to investigate early proteome and phosphoproteome changes during inhibition of lymphocyte proliferation induced by sirolimus (SRL).

MATERIALS AND METHODS

Proliferation assays were conducted using human CCRF-CEM T lymphoblasts under different SRL concentrations. Total protein lysates after SRL treatment were used to identify significantly regulated proteins and phosphorylated proteins by 2-DE and Q-TOF Ultima Global mass spectrometer.

RESULTS AND CONCLUSIONS

Incubation with 2.5 micromol/l SRL resulted in a approximately 70% inhibition of cell proliferation. Cells incubated with 2.5 micromol/l for 30 min showed a differential phosphorylation pattern with one higher (TCPQ) and six lower phosphorylation signals (TBA1B, VIME, HNRPD, ENPL, SEPT9, PLSL). On investigating the differential protein expression, five proteins were found to be up-regulated (ECHB, PSB3, MTDC, LDHB and NDKA) and four were down-regulated (EHD1, AATC, LMNB1 and MDHC). Nine of these differentially regulated proteins/phosphoproteins (TCPQ, TBA1B, VIME, HNRPD, ENPL, ECHB, PSB3, LDHB and LMNB1) showed significant interaction potential, through binding protein YWHAZ using MINT software.

CONCLUSIONS

We report for the first time the simultaneous early influence of SRL on phosphorylation status and on protein expression in the total proteome of CCRF-CEM T lymphoblasts and predict that 56% of the proteins interact with each other, highlighting significance of these results.

Myofiber stress-response in myositis: parallel investigations on patients and experimental animal models of muscle regeneration and systemic inflammation

Introduction

The endoplasmic reticulum (ER) stress-response, evoked in mice by the overexpression of class I major histocompatibility complex antigen (MHC-I), was proposed as a major mechanism responsible for skeletal muscle damage and dysfunction in autoimmune myositis. The present study was undertaken to characterize in more detail the ER stress-response occurring in myofibers of patients with inflammatory myopathies, focusing on the expression and distribution of Grp94, calreticulin and Grp75, three ER chaperones involved in immunomodulation.

Methods

Muscle biopsies were obtained from seven healthy subjects and 29 myositis patients, who were subdivided into groups based on the morphological evidence of inflammation and/or sarcolemmal immunoreactivity for MHC-I. Biopsies were analyzed by means of immunohistochemistry and western blot using anti-Grp94, anti-calreticulin and anti-Grp75 specific antibodies. Parallel analyses on these ER chaperones were conducted in rabbit and/or murine skeletal muscle after experimental induction of regeneration or systemic inflammation.

Results

Upregulation of Grp94 characterized regenerating myofibers of myositis patients (P = 0.03, compared with values detected in biopsies without signs of muscle regeneration) and developing and regenerating myofibers of mouse muscles. Conversely, levels of calreticulin and Grp75 increased about fourfold and twofold, respectively, in patient biopsies positive for sarcolemmal MHC-I immunoreactivity, compared with healthy subjects and patients negative for both inflammation and MHC-I labeling (P < 0.005). Differently from calreticulin, the Grp75 level increased significantly also in patient biopsies that displayed occasional sarcolemmal MHC-I immunoreactivity (P = 0.002), suggesting the interference of other mechanisms. Experimental systemic inflammation achieved in mice and rabbits by a single injection of bacterial lipopolysaccharide significantly increased Grp75 and calreticulin but not MHC-I expression in muscles.

Conclusions

These results indicate that, in myositis patients, muscle regeneration and inflammation, in addition to MHC-I upregulation, do evoke an ER stress-response characterized by the increased expression of Grp94 and Grp75, respectively. The increase in the muscle Grp75 level in patients showing occasional immunoreactivity for sarcolemmal MHC-I might be considered further as a broader indicator of idiopathic inflammatory myopathy.

Characterization of an endoplasmic reticulum-associated silaffin kinase from the diatom Thalassiosira pseudonana

The formation of SiO(2)-based cell walls by diatoms (a large group of unicellular microalgae) is a well established model system for the study of molecular mechanisms of biological mineral morphogenesis (biomineralization). Diatom biomineralization involves highly phosphorylated proteins (silaffins and silacidins), analogous to other biomineralization systems, which also depend on diverse sets of phosphoproteins (e.g. mammalian teeth and bone, mollusk shells, and sponge silica). The phosphate moieties on biomineralization proteins play an essential role in mineral formation, yet the kinases catalyzing the phosphorylation of these proteins have remained poorly characterized. Recent functional genomics studies on the diatom Thalassiosira pseudonana have revealed >100 proteins potentially involved in diatom silica formation. Here we have characterized the biochemical properties and biological function of one of these proteins, tpSTK1. Multiple tpSTK1-like proteins are encoded in diatom genomes, all of which exhibit low but significant sequence similarity to kinases from other organisms. We show that tpSTK1 has serine/threonine kinase activity capable of phosphorylating silaffins but not silacidins. Cell biological and biochemical analysis demonstrated that tpSTK1 is an abundant component of the lumen of the endoplasmic reticulum. The present study provides the first molecular structure of a kinase that appears to catalyze phosphorylation of biomineral forming proteins in vivo.

Glucose regulated protein 94 is required for muscle differentiation through its control of the autocrine production of insulin-like growth factors

The endoplasmic reticulum chaperone GRP94 is essential for early embryonic development and in particular affects differentiation of muscle lineages. To determine why an ubiquitously expressed protein has such a specific effect, we investigated the function of GRP94 in the differentiation of established myogenic cell lines in culture. Using both genetic suppression of expression, via RNA interference, and inhibition of function, via specific chemical inhibitors, we show that GRP94 expression and activity are needed for the in vitro fusion of myoblasts precursors into myotubes and the expression of contractile proteins that mark terminal differentiation. The inhibition can be complemented by addition of insulin-like growth factors to the cultures. GRP94 is not needed for the initial steps of myogenesis, only for the steps downstream of MyoD up-regulation, coinciding with the known need for synergistic input from growth factor signaling. Indeed, GRP94 is needed for the production of insulin-like growth factors I and II (IGF-I and IGF-II) by the differentiating cells. Moreover, the depletion of the chaperone does not increase the rate of apoptosis that always accompanies myogenic differentiation. Thus, the major effect of GRP94 on muscle differentiation is mediated by its regulation of IGF production.