We have synthesized a new class of photoaffinity analogs, 2',3'-O-(2,4,6-trinitrophenyl)-8-azido-ATP, -ADP and -AMP (TNP- 8N₃ATP, -ADP and -AMP), and their radiolabeled derivatives, and characterized their interaction with the sarcoplasmic reticulum Ca²⁺-ATPase. The TNP-8N₃-nucleotides were synthesized from ATP in three steps involving bromination in the 8-position of the adenine ring followed by displacement with an azido group and then trinitrophenylation of the resulting 8N₃-nucleotide with TNBS. Inclusion of the oxidizing agent, DTNB, in the final reaction was found to be necessary to prevent reduction of the azido group by the released sulfite anion and also elevated the yield of trinitrophenylation to about 80%. Purity was determined spectrophotometrically, as well as by anion exchange TLC and reversed phase HPLC. In the dark, the compounds were found to display most of the features of the parent TNP-nucleotides and interacted with the Ca²⁺-ATPase in a similar way. When activated by illumination, the probes were specifically incorporated into SR vesicles with high efficiency at alkaline pH. The site of labeling was identified as being on the A₁ tryptic fragment.

Properties of the nucleotide binding site of the Ca²⁺-ATPase of skeletal muscle sarcoplasmic reticulum have been investigated. The study centred around interaction of the high affinity ATP analog, 2'-3'-0-(2,4,6-trinitrophenyl)adenosine 5'-triphosphate, (TNP-ATP), with the Ca²⁺-ATPase. Defined fractions of the sarcoplasmic reticulum (SR), corresponding to the terminal cisternae (TC) and light SR (LSR), were isolated. The TC were shown to have distinctive morphological characteristics that differ from the LSR. The TC vesicles contained electron dense intravesicular material representative of Ca²⁺ binding proteins, and visible membranous "feet" structures, which are reported to interconnect with the transverse tubule. Functional characterisation of the isolated fractions provided evidence for the predominant localisation of Ca²⁺ release channels in TC, and concentration of Ca²⁺-ATPase molecules in LSR. These conclusions were based on the following observations: (a) decreased Ca²⁺ transport of TC versus LSR; ruthenium red, a Ca²⁺ channel blocker, enhanced Ca²⁺ transport and pumping efficiency in TC, (b) higher Ca²⁺-ATPase activity for LSR in the presence and absence of ionophore, (c) rapid Ca²⁺ efflux from TC which is inhibited by ruthenium red. Of special interest was the characterisation of the TC and LSR with respect to turnover-dependent TNP-ATP fluorescence. Fluorescence observed for TC was approximately 65% of that for LSR. This phenomenon may be attributable to either the decreased Ca²⁺ ATPase content of the TC vesicles or open Ca²⁺ release channels. Hence the TNP-ATP fluorescence characteristics appear to reflect the morphological and functional subspecialisation of the defined SR fractions.

Although cytoplasmic lipid droplets (LDs) are the major reserves for energy-dense neutral lipids in plants, the cellular mechanisms for packaging neutral lipids into LDs remain poorly understood. To gain insights into the cellular processes of neutral lipid accumulation and compartmentalization, a necessary step forward would be to characterize functional roles of lipogenic proteins that participate in the compartmentalization of neutral lipids in plant cells. In this study, the lipogenic proteins, Arabidopsis thaliana SEIPIN homologues and mouse (Mus Musculus) fat storage-inducing transmembrane protein 2 (FIT2), were characterized for their functional roles in the biogenesis of cytoplasmic LDs in various plant tissues. Both Arabidopsis SEIPINs and mouse FIT2 supported the accumulation of neutral lipids and cytoplasmic LDs in plants. The three Arabidopsis SEIPIN isoforms play distinct roles in compartmentalizing neutral lipids by enhancing the numbers and sizes of LDs in various plant tissues and developmental stages. Further, the potential applications of Arabidopsis SEIPINs and mouse FIT2 in engineering neutral lipids and terpenes in plant vegetative tissues were evaluated by co-expressing these and other lipogenic proteins in Nicotiana benthamiana leaves. Arabidopsis SEIPINs and mouse FIT2 represent effective tools that may complement ongoing strategies to enhance the accumulation of desired neutral lipids and terpenes in plant vegetative tissues. Collectively, our findings in this study expand our knowledge of the broader cellular mechanisms of LD biogenesis that are partially conserved in eukaryotes and distinct in plants and suggest novel targets that can be introduced into plants to collaborate with other factors in lipid metabolism and elevate oil content in plant tissues.

Endgültig fehlgefaltete oder anderweitig beschädigte Proteine des Endoplasmatischen Retikulums (ER) werden durch das Proteasom in einem Prozess abgebaut, der als Endoplasmatischer Retikulum Assoziierter Abbau (ERAD) bezeichnet wird. Der Hrd1-Komplex ist ein aus mehreren Komponenten bestehender Transmembran-Proteinkomplex, der die Ubiquitinierung und den Export von Proteinen aus dem ER vermittelt, welche dann im Zytosol abgebaut werden. Trotz erheblicher Anstrengungen in den letzten zwei Jahrzehnten führte die biochemische Charakterisierung seiner Architektur und seines Mechanismus zu inkonsistenten und sogar widersprüchlichen Ergebnissen, sodass kein Konsens darüber besteht, wie Hrd1 den Proteintransport realisiert. In diesem Projekt habe ich Fluoreszenz-Mehrfarben-Einzelmolekül-Mikroskopie verwendet, um eine neue Perspektive auf die Architektur, Bildung und Dynamik des Hrd1-Komplexes zu eröffnen. Im Projektverlauf habe ich zellbiologische, experimentelle und analytische Werkzeuge entwickelt, um die Hrd1-Oligomerisierung in vivo robust zu quantifizieren und zu charakterisieren. Durch die Kombination von Mehrfarben-Einzelmolekül-Mikroskopie mit chemischer Inhibierung, der Herunterregulierung anderer Komplexkomponenten und einem neuartigen, auf Bindungswettbewerb basierenden Assay konnte ich nachweisen, dass Hrd1 ein stabiles Homo-Tetramer bildet, das über seine zytosolische Domäne Hrd1480-529 geformt wird. Durch Strukturmodellierung über AlphaFold konnte ich nachweisen, dass sich diese Domäne unabhängig von anderen Komplexkomponenten oder der Aktivität von Hrd1 zu einer kanonischen „coiled-coil“ Domäne zusammensetzt. Während diese Arbeit neue spezifische biologische Einblicke in die Hrd1-Komplexbildung liefert, dient sie auch als allgemeine Blaupause dafür, wie Einzelpartikel-Tracking verwendet werden kann, um Fragen zu beantworten, die mit klassischer Biochemie in der Regel nur begrenzt untersucht werden können. / Terminally misfolded or otherwise damaged proteins of the Endoplasmic Reticulum (ER) are degraded by the proteasome in a process termed Endoplasmic Reticulum Associated Degradation (ERAD). The Hrd1 complex is a multicomponent transmembrane protein complex that mediates ubiquitination and export of proteins from the ER to be degraded in the cytosol. Despite substantial effort in the past two decades, the biochemical characterization of its architecture and mechanism produced inconsistent and even contradictory results, yielding no consensus on how it mediates protein transport. Its elusive nature is representative of the limitations of classical biochemical approaches, whose often harsh experimental conditions directly interfere with the objects they study. In this project I used fluorescence multi-color single molecule microscopy to offer a new perspective on the architecture, formation and dynamics of the Hrd1 complex. In this process I developed cell biological, experimental and analytical tools to robustly quantify and characterize Hrd1 oligomerization in vivo. Combining live-cell dual-color single-particle tracking with chemical inhibition, downregulation of complex components and a novel, binding-competition based tracking assay, I demonstrated that Hrd1 forms a stable homo-tetramer via its cytosolic domain Hrd1480-529. By structural modeling via AlphaFold, results of which were validated with both single-particle tracking and recombinant protein expression, I showed that this domain assembles into a canonical coiled-coil domain independently of other complex components or Hrd1's activity. While yielding specific novel biological insight into Hrd1 complex formation, it also serves as a general blueprint on how dual-color single particle tracking can be used to address questions that bring classical biochemistry to its limits.

Les protéines SNARE (Soluble N-ethylmaleimide-sensitive factor Attachment protein REceptor) sont essentielles pour la fusion membranaire. J'ai étudié chez Arabidopsis thaliana la SNARE Memb11 de l’appareil de Golgi qui intervient au début de la voie sécrétoire à l'interface Réticulum endoplasmique (RE)-appareil de Golgi. Dans les cellules de mammifères, l'orthologue de Memb11 (Membrine) est un « récepteur » potentiel de la GTPase Arf1 à la membrane golgienne. Cette dernière est impliquée dans le recrutement de la machinerie COPI nécessaire au transport rétrograde de l'appareil de Golgi vers le RE. Le but de ce travail était de déterminer si Memb11 pouvait interagir avec Arf1 dans les cellules végétales. Des anticorps dirigés contre la partie cytosolique de Memb11 ont été obtenus et ont été utilisés sur tissus végétaux pour réaliser des immunomarquages en microscopie électronique à transmission et des immunoprécipitations sur extraits de plantes. Il a été démontré que Memb11 est située au niveau de la membrane cis-golgienne et qu'elle co-immunoprécipite avec Arf1, suggérant ainsi que Arf1 peut interagir avec Memb11. J'ai confirmé l'interaction de Memb11 et Arf1 au niveau de l'appareil de Golgi par des expériences de BiFC (Bimolecular Fluorescence Complementation) in vivo. Cette interaction est spécifique puisque ni Memb12 (90% d'identité avec Memb11) ni Sec22 interagissent avec Arf1. Grâce à une approche de bioinformatique structurale, j'ai déterminé les régions de Memb11 (différentes de Memb12) qui pourraient être critiques pour l'interaction et j’ai commencé à tester in vivo les mutants correspondants par BiFC. En outre, des expériences d’immunoprécipitations avec des protéines recombinantes produites in vitro suggèrent que la forme d’Arf1 liée au GTP interagit avec Memb11. / The SNARE proteins (Soluble N-ethylmaleimide-sensitive factor Attachment protein REceptor) are critical for membrane fusion in the secretory pathway. I have studied the Golgi SNARE Memb11 in Arabidopsis thaliana cells. Memb11 is involved at the ER-Golgi interface. In mammalian cells, the ortholog of Memb11 (Membrin) is the potential “receptor” of the GTPase Arf1 in the Golgi membrane. This protein is involved for the recruitment of the COPI machinery, required for retrograde transport from the Golgi to the ER. The aim of this work was to determine whether Memb11 can interact with Arf1 in plant cells. Antibodies against the cytosolic part of Memb11 were obtained and were applied on plant tissues to perform immunolabeling by transmission electron microscopy and immunoprecipitation (IP) studies. It has been shown that Memb11 is located at the cis-Golgi and that it co-immunoprecipated with Arf1, suggesting that Arf1 may interact with Memb11. I confirmed the interaction of Memb11 and Arf1 at the Golgi by in vivo BiFC (Bimolecular Fluorescence Complementation) experiments. This interaction was specific since neither Memb12 (90% identity with Memb11) nor Sec22 interacted with ARF1. Thanks to a structural bioinformatic approach, I determined the regions in Memb11 (different from Memb12) that could be critical for the interaction and started to test corresponding mutants in vivo by BiFC. In addition, IP experiments with recombinant proteins produced in vitro suggest that the GTP-bound form of ARF1 interacts with Memb11.

L’importance des activités protéolytiques associées à la membrane plasmique dans divers processus biologiques fondamentaux est de mieux en mieux définie. Les protéases de la famille ADAM (A Disintegrin and Metalloprotease), et ADAM10 en particulier, ont suscité un intérêt tout particulier du fait de l’importance de leurs substrats (récepteur de l’EGF, TNFα, Notch, APP…). Néanmoins, peu d’études se sont intéressées aux mécanismes régulant le trafic d’ADAM10.Les tétraspanines sont une super-famille de protéines de surface impliquées dans de nombreux processus biologiques fondamentaux parmi lesquels la migration, les interactions intercellulaires, la réponse immunitaire, la fusion des gamètes… L’une des caractéristiques majeure des tétraspanines est leur capacité à organiser un réseau d’interactions moléculaires appelé le « tetraspanin web ». De précédentes études menées dans le laboratoire ont montré qu’ADAM10 est associé au « tetraspanin web ». Néanmoins, la tétraspanine en interaction directe avec ADAM10 permettant son association au réseau n’est pas encore connue. Dans cette étude nous nous sommes intéressés à la régulation d’ADAM10 par les tétraspanines. Nous avons ainsi pu identifier une sous-famille de tétraspanines à 8 cystéines, les TspanC8 (Tspan5, Tspan10, Tspan14, Tspan15, Tspan17 et Tspan33), comme étant capables d’interagir directement avec ADAM10 et de réguler sa sortie du réticulum endoplasmique. Nous avons montré que Tspan5, Tspan14, Tspan15 et Tspan33 sont capables de réguler l’expression de surface d’ADAM10 et que Tspan10 et Tspan17 entrainent l’accumulation d’ADAM10 dans un compartiment endosomal tardif. Les TspanC8 pourraient également contribuer à la régulation de la spécificité de substrat d’ADAM10 puisque nous avons montré que l’expression des TspanC8 humaines Tspan5 et Tspan14 augmente l’activation de la voie Notch alors que Tspan15 n’a pas d’effet. Par ailleurs, les TspanC8 de Drosophile sont capables d’interagir directement avec Kuzbanian (l’orthologue d’ADAM10), permettent son accumulation à la surface cellulaire et régulent l’activation de la voie Notch dans différents contextes développementaux. Nous proposons que les TspanC8 soient une nouvelle famille de protéines ayant une fonction très conservée dans la régulation de l’activité et du trafic d’ADAM10, capables de réguler l’activation de la voie Notch. / Increasing evidence suggests a critical implication of membrane-associated protease activities in numerous biological processes. ADAM (A Disintegrin and Metalloprotease) proteases, and especially ADAM10, are of particular interest because of the importance of their substrates (EGF receptor, TNF α, Notch, APP…). However, few studies focus on the mechanisms of ADAM10 trafficking. Tetraspanins are a super-family of proteins implicated in numerous biological processes including migration, intercellular interactions, immune response, gamete fusion… One of the most striking features of tetraspanins is their ability to organise multi-molecular complexes called « Tetraspanin Web ». Previous studies in the laboratory have shown that ADAM10 is associated to the « Tetraspanin Web ». Nevertheless, the tetraspanin in direct interaction with ADAM10 that drives its association to the web is not known. In this study, we focused on ADAM10 regulation by tetraspanins. We identified a subfamily of tetraspanins with 8 cysteines in their large extracellular domain that we called TspanC8 (Tspan5, Tspan10, Tspan14, Tspan15, Tspan17 and Tspan33) that can directly interact with ADAM10 and regulate its egress from the endoplasmic reticulum. We have shown that Tspan5, Tspan14, Tspan15 and Tspan33 regulate the surface expression of ADAM10 and that Tspan10 and Tspan17 accumulate ADAM10 in a late endosomal compartment. TspanC8 could also contribute to substrate specificity since Tspan5 and Tspan14 can increase Notch activation when Tspan15 cannot. Drosophila TspanC8 directly interact with the Drosophila ADAM10 ortholog Kuzbanian, increase its accumulation at the cell surface and modulate Notch activation in several developmental contexts. We propose that TspanC8 constitute a new family of Notch regulators with conserved functions in the regulation of ADAM10 trafficking and activity.

The central role played by the Endoplasmic Reticulum (ER) in the correct folding and assembly of secretary and membrane proteins cannot be overstated. As the first compartment in the secretary pathway, it is responsible for the synthesis, modification and targeting of proteins to their proper destinations within the secretary pathway and the extracellular space. Protein folding in this specialized compartment is dynamic and involves a host of molecular chaperones and folding catalysts. Once inside the ER lumen, proteins fold into their native conformation and undergo a multitude of post-translational modifications, including N-linked glycosylation and disulfide oxidation. The proper conformational maturation of nascent proteins that traverse the secretary pathway is both aided and monitored by a complex process termed ER quality control. A variety of quality control mechanisms that rely on the chaperone systems operate in the ER. These act in close concert with the molecular machinery involved in degradation of non-native proteins to maintain homeostasis. The common goal of these mechanisms is to prevent expression and secretion of misfolded proteins. As a general rule, only those proteins that have successfully completed their folding and passed a stringent selection process are allowed to exit the ER on their way to their final destinations. The importance of the normal functioning of the ER is underlined by the fact that disruption in protein folding, resulting in ER stress, has now been identified as the biochemical basis of many ER storage diseases including Diabetes mellitus, Endocrinopathies and Hemophilia A. Processing events occurring inside the ER lumen are known to influence the efficiency of protein secretion. Vastly different rates of exocytose observed among secretary proteins have been found to correlate with the rate of exit from the ER. One such example is the interesting secretion property exhibited by Retinol Binding Protein (RBP) The principal carrier of retinol (Vitamin A) in plasma. RBP is a single domain protein consisting of three intramolecular disulfide bonds and helps transport retinol from the liver stores to the various target tissues in the body. Availability of its ligand, retinol, while not affecting its synthesis, is known to be the major factor in regulating RBP secretion from the liver. In the absence of retinol, apo-RBP has been shown to be retained in the ER by a hitherto unclear mechanism. Like most other secretary proteins, RBP is co-translationally targeted to the ER lumen, where it undergoes disulfide oxidation as the only modification. It has been shown to form a complex with another secretary protein, Transthyretin (TTR) in the ER and this complex formation is thought to prevent premature glomerular filtration of the otherwise small RBP with its bound retinol. Despite attaining a mature conformation, apo-RBP is not secreted and awaits conversion to its ligand-bound, holo form in order to exit the ER. It is widely believed that ligand binding may relieve this retention of RBP from the ER quality control machinery. However the precise mechanisms that mediate and regulate RBP folding, ligand binding, TTR assembly and secretion are not clearly understood. Though the folding and secretion properties of RBP have been described in HepG2 cells, its interactions with the ER resident chaperones have not been addressed. Apart from being an important cell biological question, the study of RBP assumes a lot of significance with its recent emergence as a key player in the pathogenesis of type 2 diabetes mellitus. It has been proposed that lowering of serum RBP levels could be a new strategy for treating type 2 diabetes mellitus. The present study was undertaken with the intention of analyzing the oxidative folding of RBP in the ER more closely. A systematic approach aimed at understanding the early events associated with folding and maturation of RBP, with particular emphasis on the role of ER-resident chaperones and the quality control machinery, is likely to provide interesting insights into the mechanisms involved in its ligand dependent secretion. Reconstitution of RBP biogenesis in a cell free system. The folding of RBP in cells is extremely quick with rapid oxidation kinetics. This makes it difficult to systematically analyze the early folding events in cultured cells. It was necessary to make use of a simplified system that would faithfully recapitulate the folding process in the ER. Therefore, a cell free translation system consisting of rabbit reticulocyte lysate and canine pancreatic microcosms as a source of ER-derived membranes was developed. This system affords the advantage of easy manipulation while still preserving the overall environment that prevails in the ER of intact cells. Extensive biochemical and functional characterization of the isolated microcosms was carried out and in vitro translation and microsomal translocation of RBP was established. Though initially confined to studies on membrane insertion and core glycosylate, the cell free system supplemented with microcosms has subsequently been used to analyze folding and assembly of a number of secretary and membrane proteins. A similar strategy has been adopted in the present study of RBP folding and maturation. Oxidative folding of RBP in isolated microcosms: Delineation of its disulfide oxidation pathway Using glutathione (GSSG) as the oxidant, co- and posttranslational disulfide oxidation of RBP was carried out in isolated microcosms. The ability to manipulate the redox status of this cell free system has helped to considerably slow down the oxidative folding of RBP so that a more careful analysis of the folding process could be performed. RBP was found to undergo oxidative folding with a t1/2 of 30 minutes and folding proceeded through at least one disulfide-bonded intermediate. Non-reducing SDS PAGE was used to resolve the folding intermediates. The pattern of oxidation was in good agreement with that reported earlier in HepG2 cells. No significant effect of retinol was observed on either the folding kinetics or the pattern of disulfide oxidation of RBP in isolated microsomes.A DTT sensitivity assay, used to probe the conformational maturity of folding RBP, revealed that RBP was capable of maturing into a DTT-resistant conformation in isolated microsomes. With the aid of disulfide mutants, the probable disulfide oxidation pathway of RBP in the ER has been determined. Single and double disulfide mutants of RBP were generated by site-directed mutagenesis and their posttranslational oxidation patterns were analyzed and compared with that of the wild type protein. Based on the results obtained, it was clear that the folding intermediate was made up of one of the two big disulfide loops and that the presence of both these loops was essential for RBP to fold into a fully oxidized, compact form. It has not been possible to determine the contribution of the third, smallest disulfide loop to the oxidative folding of RBP. Molecular events associated with the early oxidative folding of RBP To gain insights into the possible role of ER chaperones in the oxidative folding of RBP, the oligomeric state of folding RBP was analyzed by velocity sedimentation and chemical crosslinking assays. Velocity sedimentation analysis revealed that the reduced form of RBP was present in a large complex of size >100 S20,W. Upon disulfide oxidation, it readily dissociated from the complex and assumed a monomeric state. This was evident even during co-translational oxidation which suggested that RBP transiently associated with the large complex during its oxidative folding. Dynamic nature of this complex indicated that this could be a folding complex containing the chaperone machinery of the ER. These results were also supported by crosslinking analysis performed in unbroken microsomes using the homo-bifunctional crosslinker, DSP. The early folding forms of RBP could be crosslinked to a large complex while upon disulfide oxidation, RBP matured to its monomeric form and was no longer crosslinkable. Sedimentation and crosslinking analyses of the RBP disulfide mutants revealed that while the double disulfide mutant remained irreversibly associated with the large complex, the single mutants were released upon acquiring one of the two big disulfide loops. This suggested that despite the lack of one of the two major disulfides, these mutants were considered ‘folded’ by the quality control machinery in the ER while the double mutant probably resembled a molten globule state and was therefore considered ‘unfolded’ and irreversibly retained. Results from crosslinking analysis in microsomes not engaged in active translation suggested that chaperones of the ER were organized in a complex constitutively thereby lending support to the concept of ER-matrix, a large network of luminal proteins consisting of ER chaperones and accessory factors. Given this scenario, it is not unlikely that newly synthesized protein substrates transiently associate with this large pre-existing complex of chaperones and dissociate during late stages of their maturation. Conclusion In all, this study provides significant insights into some of the early events associated with the oxidative folding of RBP in the ER. The delineation of the disulfide oxidation pathway of RBP has been possible. The results obtained from this study suggest that RBP probably dissociates from the quality control quite early during its folding process and this step in its maturation might not be influenced by retinol. The stimulus for its ligant dependent secretion is likely to operate at a later stage of its sojourn in the ER, possibly consequent to positive cues from accessory binding factors such as TTR. Lastly, Perservation of the ER microenvironment in isolated microsomes, as evidenced from this study, augurs well for the use of this system to analyze mechanisms underlying folding, maturation, secretion and/or retention of secretory proteins.

Abstract Skeletal muscle cells (myofibers) are huge multinucleated cells responsible for muscle contraction and hence for the everyday movements of the joints. The structure of these voluminous cells differs greatly from that of the mononucleated cells – the characteristic features of the myofibers include dozens of peripherally located nuclei, tightly packed contractile apparatus and a sophisticatedly organized endomembrane system. The basic physiology involving myofibers is quite well known, but scarce data exist on the membrane biology of the myofibers. The purpose of this study was to examine the localization of mRNA and the site of protein synthesis in the myofibers. The characterization of the membrane dynamics in muscle cells was also performed. In this study we utilized a primary cell culture model obtained from the rat flexor digitorum brevis (FDB) muscle. Also frozen sections from the rat extensor digitorum longus muscle were used. The precursor cells of the myofibers – myoblasts and myotubes – were also utilized in some experiments. Furthermore, methods of immunohistochemistry and molecular biology were applied extensively in this study. We found that in FDB myofibers the mRNA lies just under the plasma membrane. Protein synthesis seemed to be concentrated in the vicinity of nuclei locating beneath the plasma membrane but also in interfibrillar dot-like structures. Protein products moved hundreds of micrometers away from the nuclei of origin. Moreover, there were no barriers for protein movement into the core regions of the myofibers. Movement of proteins was found to be rapid in the cytosol and in the endomembrane system, too. Interestingly, when examining exocytic trafficking we observed that ER-to-Golgi trafficking significantly differed from that of mononucleated cells. Finally, myofibers were found to be able to generate lipid bodies under stress conditions. The dynamics of lipid bodies seemed to deviate from the dynamics found in other cells types. Nowadays not much muscle research with primary myofibers is done worldwide, and therefore dilemmas involving myofibers such as insulin resistance and myotoxicity of statins are mostly unresolved. The knowledge gained from this study may be used in the future to solve clinical problems related to the cell biology of the myofibers. / Tiivistelmä Luurankolihassolut eli myofiiberit ovat jättimäisiä monitumaisia soluja, jotka vastaavat lihassupistuksen aikaansaamisesta ja siten mahdollistavat jokapäiväisen liikkumisemme. Näiden suurten solujen rakenne poikkeaa selkeästi yksitumaisten solujen rakenteesta: myofiiberien tunnusomaisia piirteitä ovat kymmenet solun reunoille sijoittuneet tumat, tiiviisti pakkautunut supistumiskoneisto ja monimutkaisesti järjestynyt solukalvostojärjestelmä. Vaikka myofiiberien perusfysiologia tunnetaankin hyvin, niin tiedetään itse myofiiberien kalvostobiologiasta sangen vähän. Kokonaisuutena tämän tutkimuksen tarkoituksena oli tarkastella mRNA:n ja proteiinisynteesin sijaintia myofiibereissä. Lisäksi selvitimme lihassolujen kalvostodynamiikkaa. Tässä tutkimuksessa käytimme rotan flexor digitorum brevis (FDB) -lihaksesta saatua primääristä soluviljelymallia. Lisäksi hyödynsimme rotan extensor digitorum longus -lihaksesta hankittuja jääleikkeitä. Joissakin kokeissa käytimme myös myofiiberien esiastesoluja (myoblasteja ja myotuubeja). Immunohistokemian ja molekyylibiologian menetelmiä sovellettiin tutkimuksessa laajasti. Havaitsimme, että FDB –myofiibereissä mRNA sijaitsee aivan solukalvon alla. Proteiinisynteesi vaikutti olevan keskittynyt solukalvon alla sijaitsevien tumien ympärille, mutta myös solusisäisiin pistemäisiin rakenteisiin. Proteiinituotteet ylsivät satojen mikrometrien päähän alkuperäisestä tumastaan. Lisäksi proteiineille ei ilmennyt leviämisestettä myofiiberin sisäosiin. Leviämisen havaittiin olevan nopeaa sekä solulimassa että solulimakalvostoissa. Tutkiessamme solun eritystoimintaa huomasimme, että kuljetus ER:stä Golgin laitteeseen eroaa huomattavasti yksitumaisten solujen vastaavasta kuljetuksesta. Lopuksi havaitsimme myofiiberien pystyvän muodostamaan rasvapisaroita rasitusolosuhteissa. Rasvapisaroiden käyttäytyminen näytti myös poikkeavan siitä, mitä muissa soluissa on havaittu. Nykyään lihastutkimusta primäärisoluilla ei juuri tehdä maailmalla, minkä vuoksi myofiibereihin liittyvät lääketieteelliset pulmat kuten insuliiniresistenssi ja statiinien lihashaitat ovat suurelta osin ratkaisematta. Tästä tutkimuksesta saatuja tuloksia voitaneen jatkossa käyttää myofiiberien solubiologiaan liittyvien kliinisten ongelmien selvittämiseen.

Glioblastoma multiforme (GBM) is a deadly grade IV brain tumor characterized by a heterogeneous population of cells that are drug resistant, aggressive, and infiltrative. The current standard of care, which has not changed in over a decade, only provides GBM patients with 12-14 months survival post diagnosis. We asked if the addition of a novel endoplasmic reticulum (ER) stress inducing agent, JLK1486, to the standard chemotherapy, temozolomide (TMZ), which induces DNA double strand breaks (DSBs), would enhance TMZ’s efficacy. Because GBMs rely on the ER to mitigate their hypoxic environment and DNA repair to fix TMZ induced DSBs, we reasoned that DSBs occurring during heightened ER stress would be deleterious. Treatment of GBM cells with TMZ+JLK1486 decreased cell viability and increased cell death due to apoptosis. We found that TMZ+JLK1486 prolonged ER stress induction, as indicated by elevated ER stress marker BiP, ATF4, and CHOP, while sustaining activation of the DNA damage response pathway. This combination produced unresolved DNA DSBs due to RAD51 reduction, a key DNA repair factor. The combination of TMZ+JLK1486 is a potential novel therapeutic combination and suggests an inverse relationship between ER stress and DNA repair pathways.

The endoplasmic reticulum (ER) is the organelle responsible for synthesis and folding of secreted and membranous protein and lipid biosynthesis. It also functions as one of the main cellular calcium stores. Pancreatic beta-cells evolved to produce and secrete insulin upon demand in order to regulate blood glucose homeostasis. In response to increases in serum glucose, insulin synthesis represents nearly 50% of the total protein biosynthesis by beta-cells. This poses an enormous burden on the ER, rendering beta-cells vulnerable to agents that perturb ER function. Alterations of ER homeostasis lead to accumulation of misfolded proteins and activation of an adaptive response named the unfolded protein response (UPR). The UPR is transduced via 3 ER transmembrane proteins, namely PERK, IRE-1 and ATF6. The signaling cascades activated downstream of these proteins: a) induce expression of ER resident chaperones and protein foldases. Increasing the protein folding capacity of the ER; b) attenuate general protein translations which avoids overloading the stressed ER with new proteins; c) upregulate ER-associated degradation (ERAD) genes, which decreases the unfolded protein load of the ER. In severe cases, failure by the UPR to solve the ER stress leads to apoptosis. The mechanisms linking ER stress to apoptosis are still poorly understood, but potential mediators include the transcription factors Chop and ATF3, pro-apoptotic members of the Bcl-2 familly, the caspase 12 and the kinase JNK. Accumulating evidence suggest that ER stress contributes to beta-cell apoptosis in both type 1 and type 2 diabetes. Type 1 diabetes is characterized by a severe insulin deficiency resulting from chronic and progressive destruction of pancreatic beta-cells by the immune system. During this autoimmune assault, beta-cells are exposed to cytokines secreted by the immune cells infiltrating the pancreatic islets. Our group has previously shown that the pro-inflamatory cytokines interleukin-1beta (IL1-beta and interferon-gamma (IFN-gamma), via nitric oxide (NO) formation, downregulate expression and function of the ER Ca2+ pump SERCA2. This depletes beta-cell ER Ca2+ stores, leading to ER stress and apoptosis. Of note, IL1-beta alone triggers ER stress but does not induce beta-cell death, while IFN-gamma neither causes ER stress nor induces beta-cell death. Together, these cytokines cause beta-cell apoptosis but the mechanisms behind this synergistic effect were unknown. Type 2 diabetes is characterized by both peripheral resistance to insulin, usually as a result of obesity, and deficient insulin secretion secondary to beta cell failure. Obese patients have high levels of circulating free fatty acids (FFA) and several studies have shown that the FFA palmitate induces ER stress and beta-cell apoptosis. In the present work we initially established an experimental model to specifically activate the ER stress response in pancreatic beta-cells. For this purpose, insulinoma cells (INS-1E) or primary rat beta-cells were exposed to the reversible chemical SERCA pump blocker cyclopiazonic acid (CPA). Dose-response and time course experiments determined the best conditions to induce a marked ER stress without excessive cell death (<25%). The first goal of the work was to understand the synergistic effects of IL1-beta and IFN-gamma leading to pancreatic beta-cell apoptosis. Our group previously observed, by microarray analysis of primary beta-cells, that IFN-gamma down-regulates mRNAs encoding for some ER chaperones. Against this background, our hypothesis was that IFN-gamma aggravates beta-cell ER stress by decreasing the ability of these cells to mount an adequate UPR. To test this hypothesis, we investigated whether IFN-gamma pre-treatment augments CPA-induced ER stress and beta cell death. The results obtained indicated that IFN-gamma pre-treatment potentiates CPA-induced apoptosis in INS-1E and primary beta-cells. This effect was specific for IFN-gamma since neither IL1-beta nor a low dose CPA pre-treatment potentiated CPA-induced apoptosis in INS-1E cells. These effects of IFN-gamma were mediated via the down regulation of genes involved in beta cell defense against ER stress, including the ER chaperones BiP, Orp150 and Grp94 as well as Sec61, a component of the ERAD pathway. This had functional consequences as evidenced by a decreased basal and CPA-induced activity of a reporter construct for the unfolded protein response element (UPRE) and augmented expression of the pro-apoptotic transcription factor Chop. We next investigated the molecular regulation of the Chop gene in INS-1E cells in response to several pro-apoptotic and ER stress inducing agents, namely cytokines (IL1-beta and IFN-gamma), palmitate, or CPA. Detailed mutagenesis studies of the Chop promoter showed differential regulation of Chop transcription by these compounds. While cytokines (via NO production)- and palmitate-induced Chop expression was mediated via a C/EBP-ATF composite and AP-1 binding sites, CPA induction required the C/EBP-ATF site and the ER stress response element (ERSE). Cytokines, palmitate and CPA induced ATF4 protein expression and further binding to the C/EBP-ATF composite site, as shown by Western blot and EMSA experiments. There was also formation of distinct AP-1 dimers and binding to the AP-1 site after exposure to cytokines or palmitate. The third objective of this work was to obtain a broad picture of the pancreatic beta-cell molecular responses during and after (recovery period) a severe ER stress. For this purpose, we utilized an “in home” spotted microarray, the APOCHIP, containing nearly 600 probes selected for the study of beta-cell apoptosis. Time-dependent gene expression profiles were measured in INS-1E cells exposed to CPA. CPA-induced ER-stress modified expression of 183 genes in at least one of the time points studied. Most of theses genes returned to control levels 3h after CPA removal from the culture medium. We observed full beta-cell recovery and survival, indicating that these cells trigger efficient defenses against ER stress. Beta-cell recovery is associated with a sustained increase in the expression of ER chaperones and a rapid decrease of pro-apoptotic mRNAs following CPA removal. Two groups of genes were particularly affected by CPA, namely those related to the cellular responses to ER stress, which were mostly up-regulated, and those related to differentiated beta-cell functions, which were down-regulated. Among this last group, we observed a 40-90% decrease of the mRNAs for insulin-1 and -2. These findings were confirmed in INS-1E cells exposed to cytokines or thapsigargin (another SERCA blocker), and in primary beta-cells exposed to the same treatments. This decrease in insulin mRNA expression is due to transcript degradation, most probably caused by IRE-1 activation and triggering of its endoribonuclease activity, as recently described in Drosophila cells. In conclusion, our work enabled a better understanding of the pancreatic beta-cell responses to ER stress: 1.)We identified a sensitizing effect of IFN-gamma to ER stress in beta-cells via downregulation of key ER chaperones. 2.)We observed a differential regulation of Chop transcription by different treatments suggesting distinct responses of pancreatic beta-cells to diverse ER stress inducers. 3.)We provided the first global analysis of gene expression modifications in pancreatic beta-cells following ER stress. 4.)We demonstrated a high capacity of beta-cells to cope and recover from a severe ER stress. 5.)We identified a new protective mechanism against ER stress, namely the degradation of insulin mRNA which limits the load posed on the ER by insulin synthesis. This, coupled to a marked increase in ER chaperones and a fast degradation of pro-apoptotic mRNAs, enables beta cells to recover from ER stress after the causes of this stress are removed.