The selective and neuronal activity-dependent degradation of synaptic proteins appears to be crucial for long-term synaptic plasticity. One such protein is activity-regulated cytoskeleton-associated protein (Arc), which regulates the synaptic content of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPAR), excitatory synapse strength and dendritic spine morphology. The levels of Arc protein are tightly regulated, and its removal occurs via proteasome-mediated degradation that requires prior ubiquitination. Glycogen synthase kinases α and β (GSK3α, GSKβ; collectively named GSK3α/β) are serine-threonine kinases with abundant expression in the central nervous system. Both GSK3 isozymes are tonically active under basal conditions, but their activity is regulated by intra- and extracellular factors, intimately involved in neuronal activity. Similar to Arc, GSK3α and GSK3β contribute to synaptic plasticity and the structural plasticity of dendritic spines. The present study identified Arc as a GSK3α/β substrate and showed that GSKβ promotes Arc degradation under conditions that induce de novo Arc synthesis. We also found that GSK3α/β inhibition potentiated spine head thinning that was caused by the prolonged stimulation of N-methyl-D-aspartate receptors (NMDAR). Furthermore, overexpression of Arc mutants that were resistant to GSK3β-mediated phosphorylation or ubiquitination resulted in a stronger reduction of dendritic spine width than wildtype Arc overexpression. Thus, GSK3β terminates Arc expression and limits its effect on dendritic spine morphology. Taken together, the results identify GSK3α/β-catalyzed Arc phosphorylation and degradation as a novel mechanism for controlling the duration of Arc expression and function.

Cystathionine β-synthase (CBS) deficiency, a genetic disorder in homocysteine (Hcy) metabolism in humans, elevates plasma Hcy-thiolactone and leads to connective tissue abnormalities that affect the cardiovascular and skeletal systems. However, the underlying mechanism of these abnormalities is not understood. Hcy-thiolactone has the ability to form isopeptide bonds with protein lysine residues, which generates N-homocysteinylated protein. Because lysine residues are involved in collagen cross-linking, N-homocysteinylation of these lysines should impair cross-linking. Using a Tg-I278T Cbs mouse model of hyperhomocysteinemia (HHcy) which replicates the connective tissue abnormalities observed in CBS-deficient patients, we found that N-Hcy-collagen was elevated in bone, tail, and heart of Cbs mice, whereas pyridinoline cross-links were significantly reduced. Plasma deoxypyridinoline cross-link and cross-linked carboxyterminal telopeptide of type I collagen were also significantly reduced in the Cbs mice. Lysine oxidase activity and mRNA level were not reduced by the Cbs genotype. We also showed that collagen carries S-linked Hcy bound to the thiol of N-linked Hcy. In vitro experiments showed that Hcy-thiolactone modifies lysine residues in collagen type I α-1 chain. Residue K, located in the nonhelical N-telopeptide region and involved in pyridinoline cross-link formation, was also N-homocysteinylated in vivo Taken together, our findings showed that N-homocysteinylation of collagen in Cbs mice impairs its cross-linking. These findings explain, at least in part, connective tissue abnormalities observed in HHcy.-Perła-Kajan, J., Utyro, O., Rusek, M., Malinowska, A., Sitkiewicz, E., Jakubowski, H. N-Homocysteinylation impairs collagen cross-linking in cystathionine β-synthase-deficient mice: a novel mechanism of connective tissue abnormalities.

No abstract

Oligomeric forms of Aβ peptide are most likely the main synaptotoxic and neurotoxic agent in Alzheimer’s disease. Toxicity of various Aβ oligomeric forms has been confirmed in vivo and also in vitro. However, in vitro preparations were found to be orders of magnitude less toxic than oligomers obtained from in vivo sources. This difference can be explained by the presence of a covalent cross-link, which would stabilize the oligomer. In the present work, we have characterized the structural properties of Aβ dimers and trimers stabilized by di- and tri-tyrosine cross-links. Using ion mobility mass spectrometry we have compared the collisional cross-section of non-cross-linked and cross-linked species. We have found that the presence of cross-links does not generate new unique forms but rather shifts the equilibrium towards more compact oligomer types that can also be detected for non-cross-linked peptide. In consequence, more extended forms, probable precursors of off-pathway oligomeric species, become relatively destabilized in cross-linked oligomers and the pathway of oligomer evolution becomes redirected towards fibrillar structures.

Activation of the receptor for advanced glycation end products (RAGE) leads to a chronic proinflammatory signal, affecting patients with a variety of diseases. Potentially beneficial modification of RAGE activity requires understanding the signal transduction mechanism at the molecular level. The ligand binding domain is structurally uncoupled from the cytoplasmic domain, suggesting receptor oligomerization is a requirement for receptor activation. In this study, we used hydrogen-deuterium exchange and mass spectrometry to map structural differences between the monomeric and oligomeric forms of RAGE. Our results indicated the presence of a region shielded from exchange in the oligomeric form of RAGE and led to the identification of a new oligomerization interface localized at the linker region between domains C1 and C2. Based on this finding, a model of a RAGE dimer and higher oligomeric state was constructed.