Thomas Simmen - Academia.edu (original) (raw)

Papers by Thomas Simmen

Molecular Immunology, 1996

Secreted glycoproteins generally contain oligosaccharides of the complex type. However, several m... more Secreted glycoproteins generally contain oligosaccharides of the complex type. However, several molecules have been described in which individual glycans are processed differently from one another. Folding, assembly and oligomerization could affect the maturation of certain glycans by hindering them to the Golgi processing machinery. We have tested this possibility by analysing a panel of engineered murine μ chains secreted as

Histology and histopathology

The endoplasmic reticulum (ER) is a multifunctional organelle that accommodates a large array of ... more The endoplasmic reticulum (ER) is a multifunctional organelle that accommodates a large array of functions. Recent publications have shown that many of these functions are influenced by the ongoing oxidative folding of secretory and membrane proteins. Conversely, successful ER protein folding critically depends on the cellular redox state, but also the availability of Ca²⁺. These findings suggest the existence of regulatory mechanisms that steer ER Ca²⁺ homeostasis according to the cellular redox state. Indeed, accumulating evidence demonstrates that ER Ca²⁺ uptake and release by sarco-endoplasmic reticulum Ca²⁺ transport ATPases (SERCAs), stromal interaction molecule 1 (STIM1), Orai1, inositol 1,4,5-trisphosphate (IP3) receptors (IP3Rs) and ryanodine receptors (RyR) depends on redox modifications of these channels and pumps. In addition, ER chaperones and oxidoreductases moonlight as regulators of ER Ca²⁺ channels and pumps. Discrete redox conditions of channels, pumps and oxidored...

Histology and histopathology, 2014

The EMBO journal, Jan 15, 2002

In human cells, Ero1-Lalpha and -Lbeta (hEROs) regulate oxidative protein folding by selectively ... more In human cells, Ero1-Lalpha and -Lbeta (hEROs) regulate oxidative protein folding by selectively oxidizing protein disulfide isomerase. Specific protein--protein interactions are probably crucial for regulating the formation, isomerization and reduction of disulfide bonds in the endoplasmic reticulum (ER). To identify molecules involved in ER redox control, we searched for proteins interacting with Ero1-Lalpha. Here, we characterize a novel ER resident protein (ERp44), which contains a thioredoxin domain with a CRFS motif and is induced during ER stress. ERp44 forms mixed disulfides with both hEROs and cargo folding intermediates. Whilst the interaction with transport-competent Ig-K chains is transient, ERp44 binds more stably with J chains, which are retained in the ER and eventually degraded by proteasomes. ERp44 does not bind a short-lived ribophorin mutant lacking cysteines. Its overexpression alters the equilibrium of the different Ero1-Lalpha redox isoforms, suggesting that ER...

Nature cell biology, 2002

Adaptors are heterotetrameric complexes that mediate the incorporation of cargo into transport ve... more Adaptors are heterotetrameric complexes that mediate the incorporation of cargo into transport vesicles by interacting with sorting signals present in the cytosolic domain of transmembrane proteins. Four adaptors, AP-1 (beta 1, gamma, mu 1A or mu 1B, sigma 1), AP-2 (beta 2, alpha, mu 2, sigma 2), AP-3 (beta 3 , delta, mu 3, sigma 3) or AP-4 (beta 4, epsilon, mu 4, sigma 4), have been characterized. AP-1 and AP-3 mediate sorting events at the level of the TGN and/or endosomes, whereas AP-2 functions in endocytic clathrin coated vesicle formation; no function is known so far for AP-4. Here, we show that AP-4 can bind different types of cytosolic signals known to mediate basolateral transport in epithelial cells. Furthermore, in MDCK cells with depleted mu 4 protein levels, several basolateral proteins are mis-sorted to the apical surface, showing that AP-4 participates in basolateral sorting in epithelial cells.

ACS synthetic biology, Jan 21, 2012

Dimerization-dependent fluorescent proteins (ddFP) are a recently introduced class of genetically... more Dimerization-dependent fluorescent proteins (ddFP) are a recently introduced class of genetically encoded reporters that can be used for the detection of protein interactions in live cells. The progenitor of this class of tools was a red fluorescent ddFP (ddRFP) derived from a homodimeric variant of Discosoma red fluorescent protein. Here, we describe the engineering and application of an expanded palette of ddFPs, which includes green (ddGFP) and yellow (ddYFP) variants. These optimized variants offer several advantages relative to ddRFP including increased in vitro contrast and brightness for ddGFP and increased brightness and a lowered pK a for ddYFP. We demonstrate that both variants are useful as biosensors for protease activity in live cells. Using the ddGFP tool, we generated a highly effective indicator of endomembrane proximity that can be used to image the mitochondria-associated membrane (MAM) interface of endoplasmic reticulum (ER) and mitochondria.

Segregation of lysosomal membrane proteins into the endosomal system occurs via clathrin coated v... more Segregation of lysosomal membrane proteins into the endosomal system occurs via clathrin coated vesicles, either from the trans-Golgi network or the cell surface. In both cases, cytosolic signals present in these proteins interact with clathrin adaptor proteins. In polarized kidney MDCK cells, transport of lysosomal membrane proteins via the plasma membrane occurs in a polarized fashion through the basolateral domain. Here, we discuss recent developments on lysosomal membrane protein trafficking, with a particular focus on the transport of these proteins in polarized MDCK cells.

Developmental Cell, 2015

Recent evidence suggests that endoplasmic reticulum (ER) tubules mark the sites where the GTPase ... more Recent evidence suggests that endoplasmic reticulum (ER) tubules mark the sites where the GTPase Drp1 promotes mitochondrial fission via a largely unknown mechanism. Here, we show that the SNARE protein syntaxin 17 (Syn17) is present on raft-like structures of ER-mitochondria contact sites and promotes mitochondrial fission by determining Drp1 localization and activity. The hairpin-like C-terminal hydrophobic domain, including Lys-254, but not the SNARE domain, is important for this regulation. Syn17 also regulates ER Ca(2+) homeostasis and interferes with Rab32-mediated regulation of mitochondrial dynamics. Starvation disrupts the Syn17-Drp1 interaction, thus favoring mitochondrial elongation during autophagy. Because we also demonstrate that Syn17 is an ancient SNARE, our findings suggest that Syn17 is one of the original key regulators for ER-mitochondria contact sites present in the last eukaryotic common ancestor. As such, Syn17 acts as a switch that responds to nutrient conditions and integrates functions for the ER and autophagosomes with mitochondrial dynamics.

Cellular Logistics, 2014

The mitochondria-associated membrane (MAM) is an endoplasmic reticulum (ER) domain that forms con... more The mitochondria-associated membrane (MAM) is an endoplasmic reticulum (ER) domain that forms contacts with mitochondria and accommodates Ca 2C transfer between the two organelles. The GTPase Rab32 regulates this function of the MAM via determining the localization of the Ca 2C regulatory transmembrane protein calnexin to the MAM. Another function of the MAM is the regulation of mitochondrial dynamics mediated by GTPases such as dynaminrelated protein 1 (Drp1). Consistent with the importance of the MAM for mitochondrial dynamics and the role of Rab32 in MAM enrichment, the inactivation of Rab32 leads to mitochondrial collapse around the nucleus. However, Rab32 and related Rabs also perform intracellular functions at locations other than the MAM including melanosomal trafficking, autophagosome formation and maturation, and retrograde trafficking to the trans-Golgi network (TGN). This plethora of functions raises questions concerning the original cellular role of Rab32 in the last common ancestor of animals and its possible role in the last eukaryotic common ancestor (LECA). Our results now shed light on this conundrum and identify a role in Drp1-mediated mitochondrial dynamics as one common denominator of this group of Rabs, which includes the paralogues Rab32A and Rab32B, as well as the more recently derived Rab29 and Rab38 proteins. Moreover, we provide evidence that this mitochondrial function is dictated by the extent of ER-association of Rab32 family proteins.

Frontiers in Oncology, 2014

The International Journal of Biochemistry & Cell Biology, 2014

Cellular organelles do not function as isolated or static units, they form dynamic contacts with ... more Cellular organelles do not function as isolated or static units, they form dynamic contacts with each other that can be modulated according to cellular needs. The physical interfaces between organelles are important for Ca 2+ and lipid homeostasis, and serve as a platform for many processes that include metabolic regulation, localized signaling mechanisms, organelle quality control and the apoptotic program. Emerging evidence also highlights the importance of organelle communication in disorders such as Alzheimer's disease, pulmonary arterial hypertension, cancer, skeletal and cardiac muscle dysfunction. Here, we overview the current literature on organelle communications and its link with human pathologies.

Transplantation Journal, 2010