Effects of the neuronal phosphoprotein synapsin I on actin polymerization. II. Analytical interpretation of kinetic curves.

The general features of the kinetics of actin polymerization are investigated by mathematical models, with the aim of identifying the kinetically relevant parameters in the process and detecting and interpreting the alterations occurring in actin polymerization under various experimental conditions. Polymerization curves, obtained by following the increase in fluorescence of actin derivatized with N-(1-pyrenyl) iodoacetamide, are fitted using analytical equations derived from biochemical models of the actin polymerization process. Particular attention is given to the evaluation of the effects of the neuronal phosphoprotein synapsin I. The models obtained under various ionic conditions reveal that synapsin I interacts with actin in a very complex fashion, sharing some of the properties of classical nucleating proteins but displaying also actions not described previously for other actin-binding proteins. Synapsin I appears to bind G-actin with a very high stoichiometry (1:2-4), and the complex behaves as an F-actin nucleus, producing actin filaments under conditions where spontaneous polymerization is negligible. These actions of synapsin I are markedly affected by site-specific phosphorylation of the protein. An original transformation of the fluorescence data, which estimates the disappearance rate of actin monomer toward the critical concentration, is presented and shown to be of general usefulness for the study of actin-binding proteins

Effects of the neuronal phosphoprotein synapsin I on actin polymerization. I. Evidence for a phosphorylation-dependent nucleating effect.

Synapsin I is a synaptic vesicle-specific phosphoprotein which is able to bind and bundle actin filaments in a phosphorylation-dependent fashion. In the present paper we have analyzed the effects of synapsin I on the kinetics of actin polymerization and their modulation by site-specific phosphorylation of synapsin I. We found that dephosphorylated synapsin I accelerates the initial rate of actin polymerization and decreases the rate of filament elongation. The effect was observed at both low and high ionic strength, was specific for synapsin I, and was still present when polymerization was triggered by F-actin seeds. Dephosphorylated synapsin I was also able to induce actin polymerization and bundle formation in the absence of KCl and MgCl2. The effects of synapsin I were strongly decreased after its phosphorylation by Ca2+/calmodulin-dependent protein kinase II. These observations suggest that synapsin I has a phosphorylation-dependent nucleating effect on actin polymerization. The data are compatible with the view that changes in the phosphorylation state of synapsin I play a functional role in regulating the interactions between the nerve terminal cytoskeleton and synaptic vesicles in various stages of the exoendocytotic cycle

Two Conserved Cysteine Triads in Human Ero1α Cooperate for Efficient Disulfide Bond Formation in the Endoplasmic Reticulum

Human Ero1alpha is an endoplasmic reticulum (ER)-resident protein responsible for protein disulfide isomerase (PDI) oxidation. To clarify the molecular mechanisms underlying its function, we generated a panel of cysteine replacement mutants and analyzed their capability of: 1) complementing a temperature-sensitive yeast Ero1 mutant, 2) favoring oxidative folding in mammalian cells, 3) forming mixed disulfides with PDI and ERp44, and 4) adopting characteristic redox-dependent conformations. Our results reveal that two essential cysteine triads (Cys85-Cys94-Cys99 and Cys391-Cys394-Cys397) cooperate in electron transfer, with Cys94 likely forming mixed disulfides with PDI. Dominant negative phenotypes arise when critical residues within the triads are mutated (Cys394, Cys397, and to a lesser extent Cys99). Replacing the first cysteine in either triad (Cys85 or Cys391) generates mutants with weaker activity. In addition, mutating either Cys85 or Cys391, but not Cys397, reverts the dominant negative phenotype of the C394A mutant. These findings suggest that interactions between the two triads, dependent on Cys85 and Cys391, are important for Ero1alpha function, possibly stabilizing a platform for efficient PDI oxidation

Synapsins contribute to the dynamic spatial organization of synaptic vesicles in an activity-dependent manner.

The precise subcellular organization of synaptic vesicles (SVs) at presynaptic sites allows for rapid and spatially restricted exocytotic release of neurotransmitter. The synapsins (Syns) are a family of presynaptic proteins that control the availability of SVs for exocytosis by reversibly tethering them to each other and to the actin cytoskeleton in a phosphorylation-dependent manner. Syn ablation leads to reduction in the density of SV proteins in nerve terminals and increased synaptic fatigue under high-frequency stimulation, accompanied by the development of an epileptic phenotype. We analyzed cultured neurons from wild-type and Syn I,II,III(-/-) triple knock-out (TKO) mice and found that SVs were severely dispersed in the absence of Syns. Vesicle dispersion did not affect the readily releasable pool of SVs, whereas the total number of SVs was considerably reduced at synapses of TKO mice. Interestingly, dispersion apparently involved exocytosis-competent SVs as well; it was not affected by stimulation but was reversed by chronic neuronal activity blockade. Altogether, these findings indicate that Syns are essential to maintain the dynamic structural organization of synapses and the size of the reserve pool of SVs during intense SV recycling, whereas an additional Syn-independent mechanism, whose molecular substrate remains to be clarified, targets SVs to synaptic boutons at rest and might be outpaced by activity

Can selectivity be functionally modulated in ion channels?

In a two-electrode voltage-clamp study on the rat sympathetic neuron, the properties of the subsynaptic native neuronal AChR (nAChR) in response to the physiologically released ACh were shown to be modified within a few hours after denervation (Sacchi et al., 2008), suggesting that the nAChR ion selectivity switched from preferential permeability to potassium ions to scarce selectivity between K+ and Na+; the changes regarded synaptic, but not extrasynaptic, receptors and revealed an unexpected flexibility of the nicotinic channel in its permeation properties. Subsequently, a number of quite simple experimental procedures in intact ganglia were also shown to produce changes in conductance and ion selectivity properties of the nAChR; unlike denervation, such procedures (resting membrane potential shifts within a voltage range of physiological interest, ionic modifications, and the action of \u3b1-bungarotoxin) were very unlikely to acutely produce modifications in nAChR subunit composition or steric conformation (Sacchi et al., 2011). Posttranslational modifications of the channel protein might have occurred, but the consistency of those results with the idea that impermeant Cl 12 ions might affect cation binding and/or penetration into the pore raised the aforementioned question of whether extrinsic factors might contribute, together with the structural organization of the pore, to determine the permeability and ion selectivity of the channel. Actually, any change in the potential profile along the pore (and/or at its mouths) is bound to affect both the thermodynamic aspects of ion permeation (local field profile and ion binding to sites within the pore) and the kinetic aspects (ease of ion displacement and traveling among subsequent sites). These appear to constitute two distinct factors in determining ion selectivity, but the two aspects are strictly related. The electrical field across the membrane, determined by the membrane potential (Vm), may well be constant and produce a linear change in free energy along the pore. However, the thermodynamic profiles in ion-selective channels display nonlinear variations in the local potential seen by each ion, so that the energetic profile, \u394G(x) = G(x) 12 G(0) (where 0 refers to the extracellular bulk solution), is not simply a result of the presence of the membrane potential. At equilibrium, the probability for an ion to be located at x, p(x), is proportional to exp(\u2013\u394G(x)/RT); this determines the ratio between forward and backward velocities. As a purely qualitative, numerical example, an arbitrary pore energy profile has been simulated for the Na+ and K+ ions. The profiles for the ions are arbitrarily designed, based on the suggested differential coordination of Na+ and K+ with the charges lining the pore (Nimigean and Allen, 2011). From the same energy profile in the presence of a constant electrical field (Vm = \u20133RT/zF 48 \u201375 mV) the relative occupancy along the pore for Na+ and K+ has been computed, and the resulting permeability derived. It is shown that simple modifications of the profiles by Ca- concentration could account for the observed changes in cation selectivit

Neural precursor cells tune striatal connectivity through the release of IGFBPL1

The adult brain retains over life endogenous neural stem/precursor cells (eNPCs) within the subventricular zone (SVZ). Whether or not these cells exert physiological functions is still unclear. In the present work, we provide evidence that SVZ-eNPCs tune structural, electrophysiological, and behavioural aspects of striatal function via secretion of insulin-like growth factor binding protein-like 1 (IGFBPL1). In mice, selective ablation of SVZ-eNPCs or selective abrogation of IGFBPL1 determined an impairment of striatal medium spiny neuron morphology, a higher failure rate in GABAergic transmission mediated by fast-spiking interneurons, and striatum-related behavioural dysfunctions. We also found IGFBPL1 expression in the human SVZ, foetal and induced-pluripotent stem cell-derived NPCs. Finally, we found a significant correlation between SVZ damage, reduction of striatum volume, and impairment of information processing speed in neurological patients. Our results highlight the physiological role of adult SVZ-eNPCs in supporting cognitive functions by regulating striatal neuronal activity

La cellula come sistema biologico integrato

in "Fisiologia Medica". F. Conti Ed