Calmodulin binds the N-terminus of the functional amyloid Orb2A inhibiting fibril formation.

The cytoplasmic polyadenylation element-binding protein Orb2 is a key regulator of long-term memory (LTM) in Drosophila. The N-terminus of the Orb2 isoform A is required for LTM and forms cross-ß fibrils on its own. However, this N-terminus is not part of the core found in ex vivo fibrils. We previously showed that besides forming cross-ß fibrils, the N-terminus of Orb2A binds anionic lipid membranes as an amphipathic helix. Here, we show that the Orb2A N-terminus can similarly interact with calcium activated calmodulin (CaM) and that this interaction prevents fibril formation. Because CaM is a known regulator of LTM, this interaction could potentially explain the regulatory role of Orb2A in LTM.

Metal Binding Properties of the N-Terminus of the Functional Amyloid Orb2.

The cytoplasmic polyadenylation element binding protein (CPEB) homologue Orb2 is a functional amyloid that plays a key regulatory role for long-term memory in Drosophila. Orb2 has a glutamine, histidine-rich (Q/H-rich) domain that resembles the Q/H-rich, metal binding domain of the Hpn-like protein (Hpnl) found in Helicobacter pylori. In the present study, we used chromatography and isothermal titration calorimetry (ITC) to show that the Q/H-rich domain of Orb2 binds Ni2+ and other transition metals ions with µM affinity. Using site directed mutagenesis, we show that several histidine residues are important for binding. In particular, the H61Y mutation, which was previously shown to affect the aggregation of Orb2 in cell culture, completely inhibited metal binding of Orb2. Finally, we used thioflavin T fluorescence and electron microscopy images to show that Ni2+ binding induces the aggregating of Orb2 into structures that are distinct from the amyloid fibrils formed in the absence of Ni2+. These data suggest that transition metal binding might be important for the function of Orb2 and potentially long-term memory in Drosophila.

The Functional Amyloid Orb2A Binds to Lipid Membranes.

Lipid membranes interact with and influence the aggregation of many amyloid-forming proteins. Orb2 is a cytoplasmic polyadenylation element-binding protein homolog in Drosophila melanogaster that forms functional amyloids necessary for long-term memory. One isoform, Orb2A, has a unique N-terminus that has been shown to be important for the formation of amyloid-like aggregates and long-term memory in vivo. Orb2A is also found enriched in the synaptic membrane fraction. Our sequence and hydropathy analysis suggests that it can form an amphipathic helix, which is ideal for lipid membrane interaction. We used circular dichroism and site-directed spin labeling coupled with electron paramagnetic resonance to test the first 88 amino acids of Orb2A for lipid interaction. We show that Orb2A1-88 interacts with anionic lipid membranes using an amphipathic helix at its unique N-terminus. This interaction depends on the charge of the lipid membrane and the degree of membrane curvature. We used transmission electron microscopy and electron paramagnetic resonance to show that the presence of anionic small unilamellar vesicles inhibits amyloid fibril formation by Orb2A. This inhibition by anionic membranes could be a potential mechanism regulating Orb2A amyloid formation in vivo.

Identification and Structural Characterization of the N-terminal Amyloid Core of Orb2 isoform A.

Orb2 is a functional amyloid that plays a key role in Drosophila long-term memory formation. Orb2 has two isoforms that differ in their N-termini. The N-terminus of the A isoform (Orb2A) that precedes its Q-rich prion-like domain has been shown to be important for Orb2 aggregation and long-term memory. However, besides the fact that it forms fibrillar aggregates, structural information of Orb2 is largely absent. To understand the importance of the N-terminus of Orb2A and its relation to the fibril core, we recorded solid-state NMR and EPR data on fibrils formed by the first 88 residues of Orb2A (Orb2A88). These data show that the N-terminus of Orb2A not only promotes the formation of fibrils, but also forms the fibril core of Orb2A88. This fibril core has an in-register parallel ß-sheet structure and does not include the Q-rich, prion-like domain of Orb2. The Q-rich domain is part of the unstructured region, which becomes increasingly dynamic towards the C-terminus.

Differential effects of the antioxidant n-acetylcysteine on the production of catabolic mediators in IL-1beta-stimulated human osteoarthritic synoviocytes and chondrocytes.

Oxidative stress may play a relevant role in synovial inflammation and subsequently on cartilage damage during osteoarthritis development. We have assessed how the antioxidant N-acetylcysteine (NAC) affects the expression of different proinflammatory and structural mediators in human stimulated osteoarthritic synoviocytes and chondrocytes. Synovial membrane and articular cartilage were obtained from the osteoarthritis knees of patients who underwent joint replacement surgery. In cells stimulated with IL-1beta (10U/mL), the effects of NAC (2mmol/L) were tested at the mean peak plasma level following oral administration of therapeutic doses and its influence on prostaglandin E2 (PGE(2)) production, cyclooxigenase-2 (COX-2) expression, nitric oxide (NO) synthesis, nuclear factor kappa B (NF-kappaB) activation, and metalloproteinase-1 (MMP-1) and metalloproteinase-13 (MMP-13) expression were evaluated. While NAC significantly diminished PGE(2) release and the expression of both COX-2 and MMP-13 protein in IL-1beta-stimulated synoviocytes, it failed to modify their production in stimulated chondrocytes. Likewise, NAC only inhibited IL-1beta-stimulated NF-kappaB activation in synoviocytes. No inhibition of IL-1beta-induced NO synthesis by chondrocytes was observed following NAC incubation, while synoviocytes did not release detectable levels of NO. NAC did not induce changes in MMP-1 protein expression in either IL-1beta-stimulated synoviocytes or chondrocytes. Thus, NAC decreases the synthesis of several catabolic mediators by osteoarthritic synoviocytes, whereas, it failed to produce the same effects in osteoarthritic chondrocytes. Our results suggest that NAC inhibits some oxygen-dependent mechanisms in synoviocytes but not in chondrocytes during osteoarthritis. Therefore, NAC might have a symptomatic effect on the synovium rather than a structural effect on the cartilage in osteoarthritis.