Franca Codazzi

Control of astrocyte Ca2+ oscillations and waves by oscillating translocation and activation of protein kinase C

Abstract Background: Glutamate-induced Ca 2+ oscillations and waves coordinate astrocyte signaling responses, which in turn regulate neuronal excitability. Recent studies have suggested that the generation of these Ca 2+ oscillations requires a negative feedback that involves the activation of conventional protein kinase C (cPKC). Here, we use total internal reflection fluorescence (TIRF) microscopy to investigate if and how periodic plasma membrane translocation of cPKC is used to generate Ca 2+ oscillations and waves. Results: Glutamate stimulation of astrocytes triggered highly localized GFP-PKCγ plasma membrane translocation events, induced rapid oscillations in GFP-PKCγ translocation, and generated GFP-PKCγ translocation waves that propagated across and between cells. These translocation responses were primarily mediated by the Ca 2+ -sensitive C2 domains of PKCγ and were driven by localized Ca 2+ spikes, by oscillations in Ca 2+ concentration, and by propagating Ca 2+ waves, respectively. Interestingly, GFP-conjugated C1 domains from PKCγ or PKCδ that have been shown to bind diacylglycerol (DAG) also oscillated between the cytosol and the plasma membrane after glutamate stimulation, suggesting that PKC is repetitively activated by combined oscillating increases in Ca 2+ and DAG concentrations. The expression of C1 domains, which increases the DAG buffering capacity and thereby delays changes in DAG concentrations, led to a marked prolongation of Ca 2+ spikes, suggesting that PKC activation is involved in terminating individual Ca 2+ spikes and waves and in defining the time period between Ca 2+ spikes. Conclusions: Our study suggests that cPKCs have a negative feedback role on Ca 2+ oscillations and waves that is mediated by their repetitive activation by oscillating DAG and Ca 2+ concentrations. Periodic translocation and activation of cPKC can be a rapid and markedly localized signaling event that can limit the duration of individual Ca 2+ spikes and waves and can define the Ca 2+ spike and wave frequencies.

Synergistic Control of Protein Kinase Cγ Activity by Ionotropic and Metabotropic Glutamate Receptor Inputs in Hippocampal Neurons

Conventional protein kinase C (PKC) isoforms are abundant neuronal signaling proteins with important roles in regulating synaptic plasticity and other neuronal processes. Here, we investigate the role of ionotropic and metabotropic glutamate receptor (iGluR and mGluR, respectively) activation on the generation of Ca2+ and diacylglycerol (DAG) signals and the subsequent activation of the neuron-specific PKCγ isoform in hippocampal neurons. By combining Ca2+ imaging with total internal reflection microscopy analysis of specific biosensors, we show that elevation of both Ca2+ and DAG is necessary for sustained translocation and activation of EGFP (enhanced green fluorescent protein)-PKCγ. Both DAG production and PKCγ translocation were localized processes, typically observed within discrete microdomains along the dendritic branches. Markedly, intermediate-strength NMDA receptor (NMDAR) activation or moderate electrical stimulation generated Ca2+ but no DAG signals, whereas mGluR activation generated DAG but no Ca2+ signals. Both receptors were needed for PKCγ activation. This suggests that a coincidence detection process exists between iGluRs and mGluRs that relies on a molecular coincidence detection process based on the corequirement of Ca2+ and DAG for PKCγ activation. Nevertheless, the requirement for costimulation with mGluRs could be overcome for maximal NMDAR stimulation through a direct production of DAG via activation of the Ca2+-sensitive PLCδ (phospholipase Cδ) isoform. In a second important exception, mGluRs were sufficient for PKCγ activation in neurons in which Ca2+ stores were loaded by previous electrical activity. Together, the dual activation requirement for PKCγ provides a plausible molecular interpretation for different synergistic contributions of mGluRs to long-term potentiation and other synaptic plasticity processes.

Iron handling in hippocampal neurons: activity-dependent iron entry and mitochondria-mediated neurotoxicity

The characterization of iron handling in neurons is still lacking, with contradictory and incomplete results. In particular, the relevance of non‐transferrin‐bound iron (NTBI), under physiologic conditions, during aging and in neurodegenerative disorders, is undetermined. This study investigates the mechanisms underlying NTBI entry into primary hippocampal neurons and evaluates the consequence of iron elevation on neuronal viability. Fluorescence‐based single cell analysis revealed that an increase in extracellular free Fe2+ (the main component of NTBI pool) is sufficient to promote Fe2+ entry and that activation of either N‐methyl‐d‐aspartate receptors (NMDARs) or voltage operated calcium channels (VOCCs) significantly potentiates this pathway, independently of changes in intracellular Ca2+ concentration ([Ca2+]i). The enhancement of Fe2+ influx was accompanied by a corresponding elevation of reactive oxygen species (ROS) production and higher susceptibility of neurons to death. Interestingly, iron vulnerability increased in aged cultures. Scavenging of mitochondrial ROS was the most powerful protective treatment against iron overload, being able to preserve the mitochondrial membrane potential and to safeguard the morphologic integrity of these organelles. Overall, we demonstrate for the first time that Fe2+ and Ca2+ compete for common routes (i.e. NMDARs and different types of VOCCs) to enter primary neurons. These iron entry pathways are not controlled by the intracellular iron level and can be harmful for neurons during aging and in conditions of elevated NTBI levels. Finally, our data draw the attention to mitochondria as a potential target for the treatment of the neurodegenerative processes induced by iron dysmetabolism.

Biological activities of Bv8 analogues

The small protein Bv8, secreted by the skin of the frog Bombina variegata, belongs to a novel family of secreted proteins whose orthologues have been identified in snakes (MIT) and in mammals (prokineticins (PKs)). A characteristic feature of this protein family is the same N‐terminal sequence, AVITGA, and the presence of 10 cysteines with identical spacing in the C‐terminal domain. Two closely related G protein‐coupled receptors that mediate signal transduction of Bv8/PKs have been cloned (PK‐R1 and PK‐R2). In mammals, the Bv8/PK protein family is involved in a number of biological activities such as ingestive behaviours, circadian rhythms, angiogenesis and pain sensitization. In an attempt to identify the structural determinants required for the pronociceptive activity of Bv8, we prepared Bv8 derivatives lacking one (des‐Ala‐Bv8) or two (des‐Ala‐Val‐Bv8) residues from the N‐terminus. des‐Ala‐Bv8 displayed a receptor affinity five times lower than that of Bv8, it was five times less potent in inducing [Ca2+]i transients and in causing p42/p44 MAPK phosphorylation in CHO‐cells expressing PK‐R1 and PK‐R2. Moreover, dA‐Bv8 was about 20 times less potent than Bv8 in inducing hyperalgesia in rats. The deletion of the first two amino acids of Bv8 abolished any biological activity both ‘in vitro’ and ‘in vivo’; however, des‐AlaVal‐Bv8 is able to antagonize the Bv8‐induced hyperalgesia, binding the PK‐Rs on peripheral and central projections of the primary sensitive neurons.

HIV‐1 gp120 Glycoprotein Induces [Ca2+]i Responses not only in Type‐2 but also Type‐1 Astrocytes and Oligodendrocytes of the Rat Cerebellum

Cultures of cerebellar cortex cells were exposed to the HIV‐1 envelope glycoprotein, gp120, and investigated for cytosolic Ca2+ ion concentration ([Ca2+]i) changes by the fura‐2 ratio videoimaging technique while bathed in complete, Na+‐free or Mg2+‐free Krebs‐Ringer media. At the end of the [Ca2+]i experiments the cells were fixed and immunoidentified through the revelation of markers specific for neurons (microtubule associated protein‐2), type‐2 (A2B5) or all (glial fibrillary acidic protein) astrocytes, oligodendrocytes (galactocerebroside) or microglia (F4/80 antibody). In complete medium, rapid biphasic (spike‐plateau) responses induced by gp120 (0.1–1 nM) were observed in a subpopulation of type‐2 astrocytes. In addition, slow but progressive responses were observed in other type‐2 cells and oligodendrocytes, whereas type‐1 astrocytes showed small responses, if any, and granule neurons did not respond at all. Use of Na+‐free medium (a condition that blocked another gp120‐induced response, cytosolic alkalinization) resulted in an increase in [Ca2+]i response that was appreciable not only in type‐2 but also in most type‐1 astrocytes, possibly because of the inhibition of the Na+/Ca2+ exchanger and the ensuing decrease in Ca2+ extrusion. Granule neurons, including those in direct contact with responsive astrocytes, remained unresponsive, even when the experiments were carried out in Mg2+‐free medium supplemented with glycine, a condition that favours activation of the glutamatergic N‐methyl‐D‐aspartate (NMDA) receptor. The results obtained demonstrate that sensitivity to gp120 is a property of not only a few type‐2 astrocytes but of the majority of cerebellar glial cells, which, however, do not respond to the protein with glutamate release, as indicated by the negative results obtained with NMDA‐receptor‐expressing granule neurons. Single glial cell [Ca2+]i increase, the faster and most sensitive effect of gp120 revealed in the brain so far, could be ultimately employed to reveal CD4‐independent transmembrane signalling machanisms of the viral protein that, at the moment, remain almost entirely unknown.

Iron uptake in quiescent and inflammation-activated astrocytes: A potentially neuroprotective control of iron burden

Astrocytes play a crucial role in proper iron handling within the central nervous system. This competence can be fundamental, particularly during neuroinflammation, and neurodegenerative processes, where an increase in iron content can favor oxidative stress, thereby worsening disease progression. Under these pathological conditions, astrocytes undergo a process of activation that confers them either a beneficial or a detrimental role on neuronal survival. Our work investigates the mechanisms of iron entry in cultures of quiescent and activated hippocampal astrocytes. Our data confirm that the main source of iron is the non-transferrin-bound iron (NTBI) and show the involvement of two different routes for its entry: the resident transient receptor potential (TRP) channels in quiescent astrocytes and the de novo expressed divalent metal transporter 1 (DMT1) in activated astrocytes, which accounts for a potentiation of iron entry. Overall, our data suggest that at rest, but even more after activation, astrocytes have the potential to buffer the excess of iron, thereby protecting neurons from iron overload. These findings further extend our understanding of the protective role of astrocytes under the conditions of iron-mediated oxidative stress observed in several neurodegenerative conditions.

Expression of divalent metal transporter 1 in primary hippocampal neurons: reconsidering its role in non‐transferrin‐bound iron influx

J. Neurochem. (2012) 120, 269–278.

Purkinje neuron Ca2+ influx reduction rescues ataxia in SCA28 model

Spinocerebellar ataxia type 28 (SCA28) is a neurodegenerative disease caused by mutations of the mitochondrial protease AFG3L2. The SCA28 mouse model, which is haploinsufficient for Afg3l2, exhibits a progressive decline in motor function and displays dark degeneration of Purkinje cells (PC-DCD) of mitochondrial origin. Here, we determined that mitochondria in cultured Afg3l2-deficient PCs ineffectively buffer evoked Ca²⁺ peaks, resulting in enhanced cytoplasmic Ca²⁺ concentrations, which subsequently triggers PC-DCD. This Ca²⁺-handling defect is the result of negative synergism between mitochondrial depolarization and altered organelle trafficking to PC dendrites in Afg3l2-mutant cells. In SCA28 mice, partial genetic silencing of the metabotropic glutamate receptor mGluR1 decreased Ca²⁺ influx in PCs and reversed the ataxic phenotype. Moreover, administration of the β-lactam antibiotic ceftriaxone, which promotes synaptic glutamate clearance, thereby reducing Ca²⁺ influx, improved ataxia-associated phenotypes in SCA28 mice when given either prior to or after symptom onset. Together, the results of this study indicate that ineffective mitochondrial Ca²⁺ handling in PCs underlies SCA28 pathogenesis and suggest that strategies that lower glutamate stimulation of PCs should be further explored as a potential treatment for SCA28 patients.

Inhibition of lipopolysaccharide-induced microglia activation by calcitonin gene related peptide and adrenomedullin

Calcitonin gene related peptide (CGRP) and adrenomedullin are potent biologically active peptides that have been proposed to play an important role in vascular and inflammatory diseases. Their function in the central nervous system is still unclear since they have been proposed as either pro-inflammatory or neuroprotective factors. We investigated the effects of the two peptides on astrocytes and microglia, cells of the central nervous system that exert a strong modulatory activity in the neuroinflammatory processes. In particular, we studied the ability of CGRP and adrenomedullin to modulate microglia activation, i.e. its competence of producing and releasing pro-inflammatory cytokines/chemokines that are known to play a crucial role in neuroinflammation. In this work we show that the two neuropeptides exert a potent inhibitory effect on lipopolysaccharide-induced microglia activation in vitro, with strong inhibition of the release of pro-inflammatory mediators (such as NO, cytokines and chemokines). Both CGRP and adrenomedullin are known to promote cAMP elevation, this second messenger cannot fully account for the observed inhibitory effects, thereby suggesting that other signaling pathways are involved. Interestingly, the inhibitory effect of CGRP and adrenomedullin appears to be stimulus specific, since direct activation with pro-inflammatory cytokines was not affected. Our findings clarify aspects of microglia activation, and contribute to the comprehension of the switch from reparative to detrimental function that occurs when glia is exposed to different conditions. Moreover, they draw the attention to potential targets for novel pharmacological intervention in pathologies characterized by glia activation and neuroinflammation.

β-Secretase activity in rat astrocytes: Translational block of BACE1 and modulation of BACE2 expression

BACE1 and BACE2 are two closely related membrane‐bound aspartic proteases. BACE1 is widely recognized as the neuronal β‐secretase that cleaves the amyloid‐β precursor protein, thus allowing the production of amyloid‐β, i.e. the peptide that has been proposed to trigger the neurodegenerative process in Alzheimer’s disease. BACE2 has ubiquitous expression and its physiological and pathological role is still unclear. In light of a possible role of glial cells in the accumulation of amyloid‐β in brain, we have investigated the expression of these two enzymes in primary cultures of astrocytes. We show that astrocytes possess β‐secretase activity and produce amyloid‐β because of the activity of BACE2, but not BACE1, the expression of which is blocked at the translational level. Finally, our data demonstrate that changes in the astrocytic phenotype during neuroinflammation can produce both a negative as well as a positive modulation of β‐secretase activity, also depending on the differential responsivity of the brain regions.