Zellulärer Transport von Myelinproteinen

Während der Myelinbildung im zentralen Nervensystem (ZNS) umwinden Oligodendrozyten mit Ausläufern ihrer Plasmamembran mehrfach das Axon. Myelin ermöglicht die saltatorische Erregungsweiterleitung entlang der Axone und ist zudem für die Aufrechterhaltung der axonalen Integrität erforderlich (Edgar and Garbern, 2004). Ein Oligodendrozyt myelinisiert bis zu 40 Axonsegmente gleichzeitig, wodurch er in seiner aktivsten Myelinisierungsphase 5 bis 50 x 103 µm2 Membranfläche pro Tag produziert (Pfeiffer et al., 1993). Die vollständig ausgebildete Myelinscheide besteht aus Subdomänen mit charakteristischen Protein- und Lipidzusammensetzungen. Die Entwicklung und der Erhalt der komplexen Myelinmembran erfordert die kontinuierliche Kommunikation zwischen Neuronen und Glia-Zellen, die Koordination der Protein- und Lipidsynthese sowie angepasste intrazelluläre Sortier- und Transportwege der Myelinkomponenten. Über die molekularen Mechanismen, die zur Ausbildung des Myelins und seiner Domänen führen, ist bisher nicht sehr viel bekannt. Im Rahmen dieser Arbeit wurden Endo- und Exozytosemechanismen von Myelinproteinen analysiert. Dabei wurden drei Proteine untersucht, die in unterschiedlichen Subdomänen der Myelinmembran des ZNS lokalisiert sind. Das Hauptmyelinprotein Proteolipid Protein (PLP), das Myelin-assoziierte Glykoprotein (MAG) und das Myelin Oligodendrozyten Glykoprotein (MOG). Die Exozytose des Hauptmyelinproteins PLP erfolgt möglicherweise durch sekretorische Lysosomen (Trajkovic et al., 2006) und ist Ca2+-abhängig. Interessanterweise konnte gezeigt werden, dass PLP, MAG und MOG unterschiedlichen endosomalen Transportwegen und Sortierprozessen unterliegen. PLP wird über einen Clathrin-unabhängigen, MAG und MOG hingegen über einen Clathrin-abhängigen Mechanismus endozytiert. Zudem gelangen die Proteine zu unterschiedlichen endosomalen Zielkompartimenten und recyceln zu verschiedenen oligodendroglialen Membrandomänen. Diese Ergebnisse legen nahe, dass die endosomale Sortierung und das Recycling der Myelinproteine, die für die Bildung der Subdomänen erforderliche Umgestaltung der oligodendroglialen Plasmamembran unterstützen.The central nervous system myelin sheath is a multilayered specialized membrane, which is characterized by a highly organized subdomain structure. How these membrane domains evolve during myelin formation is unknown. Neuronal cells control the endocytic recycling of the major myelin protein proteolipid protein (PLP) (Trajkovic et al., 2006). We asked if endocytic trafficking is common to myelin proteins and analyzed the endocytic fates of proteins with distinct myelin subdomain localization. Interestingly, we found that PLP, myelin-associated glycoprotein (MAG), and myelin-oligodendrocyte glycoprotein (MOG), which localize to compact myelin, periaxonal loops, and abaxonal loops respectively, exhibit distinct endocytic fates. PLP was internalized via clathrin-independent endocytosis. MAG and MOG were both endocytosed by a clathrin-dependent pathway, however each reached a distinct endocytic compartment. Following endocytic sorting, PLP, MAG, and MOG recycled to distinct oligodendroglial membrane domains, which mimicked the biochemical characteristics of myelin subdomains. Thus, endocytic sorting and recycling is common to myelin proteins and promotes oligodendroglial plasma membrane remodeling necessary for myelin subdomain morphogenesis

Distinct endocytic recycling of myelin proteins promotes oligodendroglial membrane remodeling

The central nervous system myelin sheath is a multilayered specialized membrane with compacted and non-compacted domains of defined protein composition. How oligodendrocytes regulate myelin membrane trafficking and establish membrane domains during myelination is largely unknown. Oligodendroglial cells respond to neuronal signals by adjusting the relative levels of endocytosis and exocytosis of the major myelin protein, proteolipid protein (PLP). We investigated whether endocytic trafficking is common to myelin proteins and analyzed the endocytic fates of proteins with distinct myelin subdomain localization. Interestingly, we found that PLP, myelin-associated glycoprotein (MAG) and myelin-oligodendrocyte glycoprotein (MOG), which localize to compact myelin, periaxonal loops and abaxonal loops, respectively, exhibit distinct endocytic fates. PLP was internalized via clathrin-independent endocytosis, whereas MAG was endocytosed by a clathrin-dependent pathway, although both proteins were targeted to the late-endosomal/lysosomal compartment. MOG was also endocytosed by a clathrin-dependent pathway, but in contrast to MAG, trafficked to the recycling endosome. Endocytic recycling resulted in the association of PLP, MAG and MOG with oligodendroglial membrane domains mimicking the biochemical characteristics of myelin domains. Our results suggest that endocytic sorting and recycling of myelin proteins may assist plasma membrane remodeling, which is necessary for the morphogenesis of myelin subdomains

An epidemiological study on the course of disease and therapeutic considerations in relapsing–remitting multiple sclerosis patients receiving injectable first-line disease-modifying therapies in Germany (EPIDEM)

Background: In relapsing–remitting multiple sclerosis (RRMS), ‘no evidence of disease activity’ (NEDA) is regarded as a key treatment goal. The increasing number of treatments allows for individualized treatment optimization in patients with suboptimal response to first-line disease-modifying therapies (DMTs). Therefore, monitoring of clinical and subclinical disease activity on DMTs has been recognized as an important component of long-term patient management. Methods: EPIDEM was a multicenter non-interventional retrospective study in a large cohort of RRMS patients receiving injectable DMTs for at least 2 years in outpatient centers throughout Germany. It documented measures and ratings of disease activity on DMTs to characterize the factors that made the treating neurologists consider to switch therapy towards potentially more effective or better-tolerated drugs. Results: The cohort included predominantly female patients with a mean age of 45 years and a mean disease duration of 9.6 years, who had been continuously treated with an injectable DMT for a median duration of 54 months. Overall, 34.0% of the patients had experienced ⩾1 relapse on any DMT in the previous 2 years; 21.0% exhibited magnetic resonance imaging (MRI) activity, and the Kurtzke Expanded Disability Status Scale (EDSS) score increased by at least 0.5 points in 20.1%. Overall, 50.3% of the patients with EDSS progression and 70.6% of the patients with relapses were assessed as clinically stable by the neurologists. A change of treatment was considered in a fraction of patients with disease activity: in 22.8% of those with relapse activity, in 37.8% of those with MRI activity and in 20.1% of those with EDSS progression. Conclusion: The results of EPIDEM underline the importance of standardized evaluation and documentation of ongoing disease activity and disability deterioration. Judged from the present data, the current paradigm of low tolerance for disease activity and recommendations for early treatment optimization have not been turned fully into action as yet. More widespread implementation of current guideline recommendations may allow patients to more benefit from the growing panel of effective treatment options

Soluble syntaxin 3 functions as a transcriptional regulator

Syntaxins are a conserved family of SNARE proteins and contain C-terminal transmembrane anchors required for their membrane fusion activity. Here we show that Stx3 (syntaxin 3) unexpectedly also functions as a nuclear regulator of gene expression. We found that alternative splicing creates a soluble isoform that we termed Stx3S, lacking the transmembrane anchor. Soluble Stx3S binds to the nuclear import factor RanBP5 (RAN-binding protein 5), targets to the nucleus, and interacts physically and functionally with several transcription factors, including ETV4 (ETS variant 4) and ATF2 (activating transcription factor 2). Stx3S is differentially expressed in normal human tissues, during epithelial cell polarization, and in breast cancer versus normal breast tissue. Inhibition of endogenous Stx3S expression alters the expression of cancer-associated genes and promotes cell proliferation. Similar nuclear-targeted, soluble forms of other syntaxins were identified, suggesting that nuclear signaling is a conserved, novel function common among these membrane-trafficking proteins

Transport of the major myelin proteolipid protein is directed by VAMP3 and VAMP7.

International audienceCNS myelination by oligodendrocytes requires directed transport of myelin membrane components and a timely and spatially controlled membrane expansion. In this study, we show the functional involvement of the R-soluble N-ethylmaleimide-sensitive factor attachment protein receptor (R-SNARE) proteins VAMP3/cellubrevin and VAMP7/TI-VAMP in myelin membrane trafficking. VAMP3 and VAMP7 colocalize with the major myelin proteolipid protein (PLP) in recycling endosomes and late endosomes/lysosomes, respectively. Interference with VAMP3 or VAMP7 function using small interfering RNA-mediated silencing and exogenous expression of dominant-negative proteins diminished transport of PLP to the oligodendroglial cell surface. In addition, the association of PLP with myelin-like membranes produced by oligodendrocytes cocultured with cortical neurons was reduced. We furthermore identified Syntaxin-4 and Syntaxin-3 as prime acceptor Q-SNAREs of VAMP3 and VAMP7, respectively. Analysis of VAMP3-deficient mice revealed no myelination defects. Interestingly, AP-3δ-deficient mocha mice, which suffer from impaired secretion of lysosome-related organelles and missorting of VAMP7, exhibit a mild dysmyelination characterized by reduced levels of select myelin proteins, including PLP. We conclude that PLP reaches the cell surface via at least two trafficking pathways with distinct regulations: (1) VAMP3 mediates fusion of recycling endosome-derived vesicles with the oligodendroglial plasma membrane in the course of the secretory pathway; (2) VAMP7 controls exocytosis of PLP from late endosomal/lysosomal organelles as part of a transcytosis pathway. Our in vivo data suggest that exocytosis of lysosome-related organelles controlled by VAMP7 contributes to myelin biogenesis by delivering cargo to the myelin membrane

Mobile Cytochrome c(2) and Membrane-Anchored Cytochrome c(y) Are Both Efficient Electron Donors to the cbb(3)- and aa(3)-Type Cytochrome c Oxidases during Respiratory Growth of Rhodobacter sphaeroides

We have recently established that the facultative phototrophic bacterium Rhodobacter sphaeroides, like the closely related Rhodobacter capsulatus species, contains both the previously characterized mobile electron carrier cytochrome c(2) (cyt c(2)) and the more recently discovered membrane-anchored cyt c(y). However, R. sphaeroides cyt c(y), unlike that of R. capsulatus, is unable to function as an efficient electron carrier between the photochemical reaction center and the cyt bc(1) complex during photosynthetic growth. Nonetheless, R. sphaeroides cyt c(y) can act at least in R. capsulatus as an electron carrier between the cyt bc(1) complex and the cbb(3)-type cyt c oxidase (cbb(3)-C(ox)) to support respiratory growth. Since R. sphaeroides harbors both a cbb(3)-C(ox) and an aa(3)-type cyt c oxidase (aa(3)-C(ox)), we examined whether R. sphaeroides cyt c(y) can act as an electron carrier to either or both of these respiratory terminal oxidases. R. sphaeroides mutants which lacked either cyt c(2) or cyt c(y) and either the aa(3)-C(ox) or the cbb(3)-C(ox) were obtained. These double mutants contained linear respiratory electron transport pathways between the cyt bc(1) complex and the cyt c oxidases. They were characterized with respect to growth phenotypes, contents of a-, b-, and c-type cytochromes, cyt c oxidase activities, and kinetics of electron transfer mediated by cyt c(2) or cyt c(y). The findings demonstrated that both cyt c(2) and cyt c(y) are able to carry electrons efficiently from the cyt bc(1) complex to either the cbb(3)-C(ox) or the aa(3)-C(ox). Thus, no dedicated electron carrier for either of the cyt c oxidases is present in R. sphaeroides. However, under semiaerobic growth conditions, a larger portion of the electron flow out of the cyt bc(1) complex appears to be mediated via the cyt c(2)-to-cbb(3)-C(ox) and cyt c(y)-to-cbb(3)-C(ox) subbranches. The presence of multiple electron carriers and cyt c oxidases with different properties that can operate concurrently reveals that the respiratory electron transport pathways of R. sphaeroides are more complex than those of R. capsulatus