CNS myelination requires VAMP2/3-mediated membrane expansion in oligodendrocytes.

Myelin is required for rapid nerve signaling and is emerging as a key driver of CNS plasticity and disease. How myelin is built and remodeled remains a fundamental question of neurobiology. Central to myelination is the ability of oligodendrocytes to add vast amounts of new cell membrane, expanding their surface areas by many thousand-fold. However, how oligodendrocytes add new membrane to build or remodel myelin is not fully understood. Here, we show that CNS myelin membrane addition requires exocytosis mediated by the vesicular SNARE proteins VAMP2/3. Genetic inactivation of VAMP2/3 in myelinating oligodendrocytes caused severe hypomyelination and premature death without overt loss of oligodendrocytes. Through live imaging, we discovered that VAMP2/3-mediated exocytosis drives membrane expansion within myelin sheaths to initiate wrapping and power sheath elongation. In conjunction with membrane expansion, mass spectrometry of oligodendrocyte surface proteins revealed that VAMP2/3 incorporates axon-myelin adhesion proteins that are collectively required to form nodes of Ranvier. Together, our results demonstrate that VAMP2/3-mediated membrane expansion in oligodendrocytes is indispensable for myelin formation, uncovering a cellular pathway that could sculpt myelination patterns in response to activity-dependent signals or be therapeutically targeted to promote regeneration in disease.

Allosteric regulation of γ-secretase activity by a phenylimidazole-type γ-secretase modulator.

γ-Secretase is an intramembrane-cleaving protease responsible for the generation of amyloid-ß (Aß) peptides. Recently, a series of compounds called γ-secretase modulators (GSMs) has been shown to decrease the levels of long toxic Aß species (i.e., Aß42), with a concomitant elevation of the production of shorter Aß species. In this study, we show that a phenylimidazole-type GSM allosterically induces conformational changes in the catalytic site of γ-secretase to augment the proteolytic activity. Analyses using the photoaffinity labeling technique and systematic mutational studies revealed that the phenylimidazole-type GSM targets a previously unidentified extracellular binding pocket within the N-terminal fragment of presenilin (PS). Collectively, we provide a model for the mechanism of action of the phenylimidazole-type GSM in which binding at the luminal side of PS induces a conformational change in the catalytic center of γ-secretase to modulate Aß production.

New photocleavable linker: α-thioacetophenone-type linker.

Photocleavable linkers are advantageous over the common linkers because they could be cleaved without using reagents. A novel photocleavable linker with an α-thioacetophenone moiety has been developed. This linker, which can be cleaved upon irradiation at 365 nm via the Norrish type II reaction, is applicable to a protein affinity purification system, allowing target proteins to be effectively isolated. This novel linker would serve as an effective tool in chemical biology.

FTY720/fingolimod, a sphingosine analogue, reduces amyloid-ß production in neurons.

Sphingosine-1-phosphate (S1P) is a pluripotent lipophilic mediator working as a ligand for G-protein coupled S1P receptors (S1PR), which is currently highlighted as a therapeutic target for autoimmune diseases including relapsing forms of multiple sclerosis. Sphingosine related compounds, FTY720 and KRP203 known as S1PR modulators, are phosphorylated by sphingosine kinase 2 (SphK2) to yield the active metabolites FTY720-P and KRP203-P, which work as functional antagonists for S1PRs. Here we report that FTY720 and KRP203 decreased production of Amyloid-ß peptide (Aß), a pathogenic proteins causative for Alzheimer disease (AD), in cultured neuronal cells. Pharmacological analyses suggested that the mechanism of FTY720-mediated Aß decrease in cells was independent of known downstream signaling pathways of S1PRs. Unexpectedly, 6-days treatment of APP transgenic mice with FTY720 resulted in a decrease in Aß40, but an increase in Aß42 levels in brains. These results suggest that S1PR modulators are novel type of regulators for Aß metabolisms that are active in vitro and in vivo.

Contribution of the γ-secretase subunits to the formation of catalytic pore of presenilin 1 protein.

γ-Secretase is an intramembrane-cleaving protease related to the etiology of Alzheimer disease. γ-Secretase is a membrane protein complex composed of presenilin (PS) and three indispensable subunits: nicastrin, Aph-1, and Pen-2. PS functions as a protease subunit forming a hydrophilic catalytic pore structure within the lipid bilayer. However, it remains unclear how other subunits are involved in the pore formation. Here, we show that the hydrophilic pore adopted with an open conformation has already been formed by PS within the immature γ-secretase complex. The binding of the subunits induces the close proximity between transmembrane domains facing the catalytic pore. We propose a model in which the γ-secretase subunits restrict the arrangement of the transmembrane domains of PS during the formation of the functional structure of the catalytic pore.