Homeostatic regulation of axonal Kv1.1 channels accounts for both synaptic and intrinsic modifications in the hippocampal CA3 circuit

Homeostatic plasticity of intrinsic excitability goes hand in hand with homeostatic plasticity of synaptic transmission. However, the mechanisms linking the two forms of homeostatic regulation have not been identified so far. Using electrophysiological, imaging, and immunohistochemical techniques, we show here that blockade of excitatory synaptic receptors for 2 to 3 d induces an up-regulation of both synaptic transmission at CA3-CA3 connections and intrinsic excitability of CA3 pyramidal neurons. Intrinsic plasticity was found to be mediated by a reduction of Kv1.1 channel density at the axon initial segment. In activity-deprived circuits, CA3-CA3 synapses were found to express a high release probability, an insensitivity to dendrotoxin, and a lack of depolarization-induced presynaptic facilitation, indicating a reduction in presynaptic Kv1.1 function. Further support for the down-regulation of axonal Kv1.1 channels in activity-deprived neurons was the broadening of action potentials measured in the axon. We conclude that regulation of the axonal Kv1.1 channel constitutes a major mechanism linking intrinsic excitability and synaptic strength that accounts for the functional synergy existing between homeostatic regulation of intrinsic excitability and synaptic transmission.

Magnetic microscopy and topological stability of homochiral Néel domain walls in a Pt/Co/AlOx trilayer

The microscopic magnetization variation in magnetic domain walls in thin films is a crucial property when considering the torques driving their dynamic behaviour. For films possessing out-of-plane anisotropy normally the presence of Néel walls is not favoured due to magnetostatic considerations. However, they have the right structure to respond to the torques exerted by the spin Hall effect. Their existence is an indicator of the interfacial Dzyaloshinskii-Moriya interaction (DMI). Here we present direct imaging of Néel domain walls with a fixed chirality in device-ready Pt/Co/AlOx films using Lorentz transmission electron and Kerr microscopies. It is shown that any independently nucleated pair of walls in our films form winding pairs when they meet that are difficult to annihilate with field, confirming that they all possess the same topological winding number. The latter is enforced by the DMI. The field required to annihilate these winding wall pairs is used to give a measure of the DMI strength. Such domain walls, which are robust against collisions with each other, are good candidates for dense data storage.

Dysfunction of the PI3K-Akt-GSK-3 pathway is a common feature in cell culture and in vivo models of prion disease

AIMS

Transmissible spongiform encephalopathies, also called prion diseases, are characterized by the cerebral accumulation of misfolded prion protein (PrP(SC) ) and subsequent neurodegeneration. However, despite considerable research effort, the molecular mechanisms underlying prion-induced neurodegeneration are poorly understood. Here, we explore the hypothesis that prions induce dysfunction of the PI3K/Akt/GSK-3 signalling pathway.

METHODS

We employed two parallel approaches. Using cell cultures derived from mouse primary neurones and from a human neuronal cell line, we identified common elements that were modified by the neurotoxic fragment of PrP(106-126) . These studies were then complemented by comparative analyses in a mouse model of prion infection.

RESULTS

The presence of a polymerized fragment of the prion protein (PrP(106-126) ) or of a prion strain altered PI3K-mediated signalling, as evidenced by Akt inhibition and GSK-3 activation. PI3K activation by the addition of insulin or the expression of a constitutively active Akt mutant restored normal levels of Akt and GSK-3 activity. These changes were correlated with a reduction in caspase activity and an increase in neuronal survival. Moreover, we found that activation of caspase 3, Erk and GSK-3 are common features of PrP(106-126) -mediated neurotoxicity in cellular systems and prion infection in the mouse cerebellum, while activation of caspase 12 and JNK was observed in cellular models.

CONCLUSIONS

Our findings in cell culture and in vivo models of prion disease demonstrate marked alterations to the PI3K/Akt/GSK-3 pathway and suggest that two additional pathways contribute to PrP-induced neurotoxicity as responsible of JNK and caspase 12 activation.

Hsp90 activity is necessary to acquire a proper neuronal polarization

Chaperones are critical for the folding and regulation of a wide array of cellular proteins. Heat Shock Proteins (Hsps) are the most representative group of chaperones. Hsp90 represents up to 1-2% of soluble protein. Although the Hsp90 role is being studied in neurodegenerative diseases, its role in neuronal differentiation remains mostly unknown. Since neuronal polarity mechanisms depend on local stability and degradation, we asked whether Hsp90 could be a regulator of axonal polarity and growth. Thus, we studied the role of Hsp90 activity in a well established model of cultured hippocampal neurons using an Hsp90 specific inhibitor, 17-AAG. Our present data shows that Hsp90 inhibition at different developmental stages disturbs neuronal polarity formation or axonal elongation. Hsp90 inhibition during the first 3h in culture promotes multiple axon morphology, while this inhibition after 3h slows down axonal elongation. Hsp90 inhibition was accompanied by decreased Akt and GSK3 expression, as well as, a reduced Akt activity. In parallel, we detected an alteration of kinesin-1 subcellular distribution. Moreover, these effects were seconded by changes in Hsp70/Hsc70 subcellular localization that seem to compensate the lack of Hsp90 activity. In conclusion, our data strongly suggests that Hsp90 activity is necessary to control the expression, activity or location of specific kinases and motor proteins during the axon specification and axon elongation processes. Even more, our data demonstrate the existence of a "time-window" for axon specification in this model of cultured neurons after which the inhibition of Hsp90 only affects axonal elongation mechanisms.

Self-assembled iron oxide nanoparticle multilayer: x-ray and polarized neutron reflectivity

We have investigated the structure and magnetism of self-assembled, 20 nm diameter iron oxide nanoparticles covered by an oleic acid shell for scrutinizing their structural and magnetic correlations. The nanoparticles were spin-coated on an Si substrate as a single monolayer and as a stack of 5 ML forming a multilayer. X-ray scattering (reflectivity and grazing incidence small-angle scattering) confirms high in-plane hexagonal correlation and a good layering property of the nanoparticles. Using polarized neutron reflectivity we have also determined the long range magnetic correlations parallel and perpendicular to the layers in addition to the structural ones. In a field of 5 kOe we determine a magnetization value of about 80% of the saturation value. At remanence the global magnetization is close to zero. However, polarized neutron reflectivity reveals the existence of regions in which magnetic moments of nanoparticles are well aligned, while losing order over longer distances. These findings confirm that in the nanoparticle assembly the magnetic dipole-dipole interaction is rather strong, dominating the collective magnetic properties at room temperature.

Structural and magnetic characterization of self-assembled iron oxide nanoparticle arrays

We report about a combined structural and magnetometric characterization of self-assembled magnetic nanoparticle arrays. Monodisperse iron oxide nanoparticles with a diameter of 20 nm were synthesized by thermal decomposition. The nanoparticle suspension was spin-coated on Si substrates to achieve self-organized arrays of particles and subsequently annealed at various conditions. The samples were characterized by x-ray diffraction, and bright and dark field high resolution transmission electron microscopy. The structural analysis is compared to magnetization measurements obtained by superconducting quantum interference device magnetometry. We can identify either multi-phase Fe(x)O/γ-Fe(2)O(3) or multi-phase Fe(x)O/Fe(3)O(4) nanoparticles. The Fe(x)O/γ-Fe(2)O(3) system shows a pronounced exchange bias effect which explains the peculiar magnetization data found for this system.

Evidence for core-shell magnetic behavior in antiferromagnetic Co3O4 nanowires

Employing magnetometry measurements, we have studied Co3O4 nanowires focusing on the core-shell behavior. We find two magnetic contributions, i.e., a regular antiferromagnetic and an additional irreversible one. The first contribution can be attributed to the antiferromagnetically ordered wire cores. The nature of the second one can be identified using thermoremanent and isothermoremanent magnetizaton curves as magnetic fingerprints of the irreversible magnetization. We conclude that the nanowire shell behaves like a two-dimensional diluted antiferromagnet in a field.

Pharmacological inhibition of GSK-3 is not strictly correlated with a decrease in tyrosine phosphorylation of residues 216/279

Recent evidence suggests that intramolecular autophosphorylation is responsible for the tyrosine phosphorylation (pY) of residues 279 or 216 of glycogen synthase kinase-3 (GSK-3alpha or beta), an event that appears to play an important role in regulating this kinase. This provocative hypothesis was based on the capacity of certain nonselective GSK-3 inhibitors to alter both the activity of GSK-3 and its pY. Inhibitors of GSK-3 are not always capable of preventing this tyrosine phosphorylation, which may require an extended period of time. For example, although lithium chloride inhibits GSK-3 activity, this inhibition does not alter its pY content. Furthermore, even when GSK-3 activity is impaired, GSK-3 pY can still be modified by physiological or pharmacological agents. Taken together, these data indicate that GSK-3 kinase activity is not necessarily correlated with the extent of GSK-3 pY. We hypothesized that some as-yet-unidentified tyrosine kinases and phosphatases may also regulate this kinase.

Visualization and molecular analysis of nuclear import of protein kinase CK2 subunits in living cells

We have generated fusion proteins between the subunits of CK2 and GFP and characterized their behaviour in living cells. The expressed fusion proteins were functional and interacted with endogenous CK2. Imaging of NIH3T3 cells expressing low level of GFP-CK2alpha or GFP-CK2beta showed that both proteins were mostly nuclear in interphase. Both CK2 subunits contain nuclear localization domains that target them independently to the nucleus. Once in the nucleus, both subunits diffused rapidly in the nucleoplasm. In mitotic cells, CK2 subunits were dispersed throughout the cytoplasm and were not associated to chromatin. Our data are compatible with the idea that each subunit can translocate individually to the nucleus to interact with each other or with important cellular partners. Understanding the molecular mechanisms which regulate the dynamic localization of CK2 subunits will be of central importance.

Crystal structure of the human protein kinase CK2 regulatory subunit reveals its zinc finger-mediated dimerization

Protein kinase CK2 is a tetramer composed of two alpha catalytic subunits and two beta regulatory subunits. The structure of a C-terminal truncated form of the human beta subunit has been determined by X-ray crystallography to 1.7 A resolution. One dimer is observed in the asymmetric unit of the crystal. The most striking feature of the structure is the presence of a zinc finger mediating the dimerization. The monomer structure consists of two domains, one entirely alpha-helical and one including the zinc finger. The dimer has a crescent shape holding a highly acidic region at both ends. We propose that this acidic region is involved in the interactions with the polyamines and/or catalytic subunits. Interestingly, conserved amino acid residues among beta subunit sequences are clustered along one linear ridge that wraps around the entire dimer. This feature suggests that protein partners may interact with the dimer through a stretch of residues in an extended conformation.

Binding of polylysine to protein kinase CK2, measured by Surface Plasmon Resonance

The interaction between protein kinase CK2 and polylysine has been studied by Surface Plasmon Resonance (SPR). The binding process has a very low energy of activation, it is irreversible, and too slow as to explain the enzyme activity stimulation as a direct consequence of the polylysine binding. The polylysine interaction with a peptide substrate and with casein are faster, and in agreement with a substrate-mediated mechanism of activity stimulation. After several hours of incubation, the binding of polylysine to CK2 produces the loss of enzymatic activity.

Kinetics of the reaction between 5,5'-dithiobis[2-nitrobenzoic acid] and the sulphydryl group in Zn(2+)-dependent beta-lactamase II

The kinetics of the reaction of the thiol residue in Zn(2+)-dependent beta-lactamase II with 5,5'-dithiobis[2-nitrobenzoic acid], and the concomitant inactivation revealed that both events take place at the same rate. The inactivation could not be reverted by incubation with Zn2+ or by using a substrate concentration about eight times the Km of the enzyme. EDTA incubation also produced inactivation of the enzyme, although it was reverted by increasing the substrate concentration in the assay. A dual role is proposed for Zn2+ in beta-lactamase. The kinetic analysis of the thiol modification and the concomitant inactivation is in agreement with previous reports on the implication of the metal ion in catalysis. A role in stabilizing the native structure of the enzyme is also suggested.

Degradation of beta-lactam antibiotics in the presence of Zn2+ and 2-amino-2-hydroxymethylpropane-1,3-diol (Tris). A hypothetical non-enzymic model of beta-lactamases

The system composed of 2-amino-2-hydroxymethylpropane-1,3-diol (Tris) and Zn2+ catalyses the degradation of cephalosporins. The beta-lactam opening fits to a first-order process, with a constant directly proportional to the zinc ion concentration. The pH and Tris concentration dependency displayed by the first-order constant, as well as the nature of the degradation products point to a mechanism that can be considered as an extension of that proposed for the benzylpenicillin degradation. The mechanism proposed here, and the values of the kinetic constants calculated, as compared with those of beta-lactamases, lead to the conclusion that the Tris-Zn2+ system simulates the catalytic action of the serine beta-lactamases rather than the action of the Zn(2+)-dependent type of enzymes.

Comparative study of various hydrogen ion buffers to assay Zn(2+)-dependent beta-lactamases

The low Zn2+ complex formation constants, the capacity to degrade penicillin G in combination with Zn2+, and UV absorbance make 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid, 3-[N-tris(hydroxymethyl)methylamino]-2-hydroxypropanesulfonic acid, 1,3-bis[tris(hydroxymethyl)methylamino]propane, and 1,4-piperazinediethanesulfonic acid convenient buffers to study Zn(2+)-dependent beta-lactamases.