Astrocytes Are the Source of TNF Mediating Homeostatic Synaptic Plasticity

Tumor necrosis factor α (TNF) mediates homeostatic synaptic plasticity (HSP) in response to chronic activity blockade, and prior work has established that it is released from glia. Here we demonstrate that astrocytes are the necessary source of TNF during HSP. Hippocampal cultures from rats of both sexes depleted of microglia still will increase TNF levels following activity deprivation and still express TTX-driven HSP. Slice cultures from mice of either sex with a conditional deletion of TNF from microglia also express HSP, but critically, slice cultures with a conditional deletion of TNF from astrocytes do not. In astrocytes, glutamate signaling is sufficient to reduce NFκB signaling and TNF mRNA levels. Further, chronic TTX treatment increases TNF in an NFκB-dependent manner, although NFκB signaling is dispensable for the neuronal response to TTX-driven HSP. Thus, astrocytes can sense neuronal activity through glutamate spillover and increase TNF production when activity falls, to drive HSP through the production of TNF.

Sustained TNF signaling is required for the synaptic and anxiety-like behavioral response to acute stress

Acute stress triggers plasticity of forebrain synapses as well as behavioral changes. Here we reveal that Tumor Necrosis Factor α (TNF) is a required downstream mediator of the stress response in mice, necessary for stress-induced synaptic potentiation in the ventral hippocampus and for an increase in anxiety-like behaviour. Acute stress is sufficient to activate microglia, triggering the long-term release of TNF. Critically, on-going TNF signaling specifically in the ventral hippocampus is necessary to sustain both the stress-induced synaptic and behavioral changes, as these could be reversed hours after induction by antagonizing TNF signaling. This demonstrates that TNF maintains the synaptic and behavioral stress response in vivo, making TNF a potential novel therapeutic target for stress disorders.

Astrocytic TNFα regulates the behavioral response to antidepressants

Recent studies have suggested that cytokines, and in particular tumor necrosis factor alpha (TNFα), have a role in modulating antidepressant efficacy. To directly test this idea, we compared the response of TNFα(-/-) mice and astrocyte-specific TNFα(-/-) mice to the antidepressants fluoxetine and desipramine. Using standard behavior models for measuring antidepressant efficacy, the forced swim test (FST) and tail suspension test (TST), we determined that TNFα(-/-) mice were essentially normal in basal behavior in the FST and TST. However, TNFα(-/-) mice showed no behavioral response to a standard dose of chronic antidepressant treatment, in sharp contrast to wildtype mice. Similar results were seen with acute antidepressant treatment, but TNFα(-/-) mice did respond to a very high-dose acute antidepressant treatment. We also assessed in vitro and in vivo effects of fluoxetine on TNFα expression. Glia responded to serotonin in vitro and fluoxetine in vivo by upregulating TNFα mRNA. Consistent with this source of TNFα, mice with an astrocyte-specific deletion of TNFα also did not respond to standard chronic antidepressant treatment. These data suggest that astrocytic TNFα is important to the sensitivity of the behavioral response to administration of antidepressants.

Residues contributing to the Na(+)-binding pocket of the SLC24 Na(+)/Ca(2+)-K(+) Exchanger NCKX2

Na(+)/Ca(2+)-K(+) exchangers (NCKX; gene family SLC24) are plasma membrane Ca(2+) transporters that mediate the extrusion of one Ca(2+) ion and one K(+) ion in exchange for four Na(+) ions. NCKX is modeled to have two sets of five transmembrane segments separated by a large cytosolic loop; within each set of transmembrane segments are regions of internal symmetry termed alpha(1) and alpha(2) repeats. The central residues that are important for Ca(2+) and K(+) liganding and transport have been identified in NCKX2, and they comprise three central acidic residues, Glu(188) in alpha(1) and Asp(548) and Asp(575) in alpha(2), as well as Ser/Thr residues one-helical turn away from these residues. In this study, we have scanned through more than 100 single-residue substitutions of NCKX2 for shifts in Na(+) affinity using a fluorescence assay to monitor changes in free Ca(2+) in HEK293 cells treated with gramicidin to control intracellular Na(+). We have identified 31 residues that, when substituted, result in shifts in Na(+) affinity, either toward higher or lower K(m) values when compared with wild type NCKX2 (K(m) for Na(+) 58 mm). These residues include the central acidic residues Glu(188), Asp(548), and Asp(575), and their neighboring residues in alpha(1) and alpha(2), in addition to a number of newly investigated residues in transmembrane segment 3. Our results relate the identification of residues important for Na(+) transport in this study to those previously identified as important in the counter-transport of Ca(2+) and K(+), lending support to the alternating access model of transmembrane transport.

Examining Ca2+ extrusion of Na+/Ca2+-K+ exchangers

Na+/Ca2+-K+ exchangers (NCKX) are plasma membrane transporters that are thought to mainly mediate Ca2+ extrusion (along with K+) at the expense of the Na+ electrochemical gradient. However, because they are bidirectional, most assays have relied on measuring their activity in the reverse (Ca2+ import) mode. Herein we describe a method to control intracellular ionic conditions, and examine the forward (Ca2+ extrusion) mode of exchange of NCKX2.

Structure-function relationships of the NCKX2 Na+/Ca2+-K+ exchanger

K+-dependent Na+/Ca2+ exchangers (NCKX) have been shown to play important roles in physiological processes as diverse as phototransduction in rod photoreceptors, motor learning and memory in mice, and skin pigmentation in humans. Most structure-function studies on NCKX proteins have been carried out on the NCKX2 isoform, but sequence similarity suggests that the results obtained with the NCKX2 isoform are likely to apply to all NCKX1-5 members of the human SLC24 gene family. Here we review our recent work on the NCKX2 protein concerning the topological arrangement of transmembrane segments carrying out cation transport, and concerning residues important for transport function and cation binding.

Na+-dependent inactivation of the retinal cone/brain Na+/Ca2+-K+ exchanger NCKX2

The SLC24 gene family Na+/Ca2+-K+ exchangers (NCKX) are bidirectional plasma membrane transporters whose main function is the extrusion of Ca2+ from the cytosol. In this study, we used human embryonic kidney 293 cells expressing human retinal cone/brain NCKX2 to examine its Na+ affinity and kinetic parameters of Ca2+ transport. With the use of the ionophore gramicidin to control alkali cation concentrations across the plasma membrane, application of high intracellular Na+ promoted large NCKX2-mediated increases in intracellular free Ca2+ in the 15-20 microm range; this also resulted in inactivation of NCKX2 transport, the first description of this novel kinetic state. The affinity of NCKX2 for internal Na+ was found to be sigmoidal, with a Hill coefficient of 2.6 and Kd = 50 mm. The time-dependent (t(1/2) approximately 40s) inactivation of NCKX2 required high intracellular Na+ levels (Kd > 50 mm) as well as high occupancy of the extracellular Ca2+-binding site. Also reported are two residues whose substitution resulted in an increase in internal Na+ affinity to values of approximately 19 mm; these mutants also displayed enhanced inactivation, suggesting that inactivation requires binding of Na+ to its intracellular transport sites. These findings are the first report of a regulatory kinetic state of Ca2+ transport via NCKX2 Na+/Ca2+-K+ exchangers that may play a prominent role in regulation of Ca2+ extrusion in cellular environments such as neuronal synapses that experience frequent and dynamic Ca2+ fluxes.