Sean J. Mulligan

Calcium transients in astrocyte endfeet cause cerebrovascular constrictions

Cerebral blood flow (CBF) is coupled to neuronal activity and is imaged in vivo to map brain activation. CBF is also modified by afferent projection fibres that release vasoactive neurotransmitters in the perivascular region, principally on the astrocyte endfeet that outline cerebral blood vessels. However, the role of astrocytes in the regulation of cerebrovascular tone remains uncertain. Here we determine the impact of intracellular Ca2+ concentrations ([Ca2+]i) in astrocytes on the diameter of small arterioles by using two-photon Ca2+ uncaging to increase [Ca2+]i. Vascular constrictions occurred when Ca2+ waves evoked by uncaging propagated into the astrocyte endfeet and caused large increases in [Ca2+]i. The vasoactive neurotransmitter noradrenaline increased [Ca2+]i in the astrocyte endfeet, the peak of which preceded the onset of arteriole constriction. Depressing increases in astrocyte [Ca2+]i with BAPTA inhibited the vascular constrictions in noradrenaline. We find that constrictions induced in the cerebrovasculature by increased [Ca2+]i in astrocyte endfeet are generated through the phospholipase A2–arachidonic acid pathway and 20-hydroxyeicosatetraenoic acid production. Vasoconstriction by astrocytes is a previously unknown mechanism for the regulation of CBF.

Astrocyte control of the cerebrovasculature

The control of cerebral vessel diameter is of fundamental importance in maintaining healthy brain function because it is critical to match cerebral blood flow (CBF) to the metabolic demand of active neurons. Recent studies have shown that astrocytes are critical players in the regulation of cerebral blood vessel diameter and that there are several molecular pathways through which astrocytes can elicit these changes. Increased intracellular Ca2+ in astrocytes has demonstrated a dichotomy in vasomotor responses by causing the constriction as well as the dilation of neighboring blood vessels. The production of arachidonic acid (AA) in astrocytes by Ca2+ sensitive phospholipase A2 (PLA2) has been shown to be common to both constriction and dilation mechanisms. Constriction results from the conversion of AA to 20‐hydroxyeicosatetraenoic acid (20‐HETE) and dilation from the production of prostaglandin E2 (PGE2) or epoxyeicosatrienoic acid (EET) and the level of nitric oxide (NO) appears to dictate which of these two pathways is recruited. In addition the activation of Ca2+ activated K+ channels in astrocyte endfeet and the efflux of K+ has also been suggested to modify vascular tone by hyperpolarization and relaxation of smooth muscle cells (SMCs). The wide range of putative pathways indicates that more work is needed to clarify the contributions of astrocytes to vascular dynamics under different cellular conditions. Nonetheless it is clear that astrocytes are important albeit complicated regulators of CBF.

Microglia processes block the spread of damage in the brain and require functional chloride channels

Microglia cells exhibit two forms of motility, constant movement of filopodia probing surrounding brain tissue, and outgrowth of larger processes in response to nearby damage. The mechanisms and functions of filopodia sensing and process outgrowth are not well characterized but are likely critical for normal immune function in the brain. Using two photon laser scanning microscopy we investigated microglia process outgrowth in response to damage, and explored the relationship between process outgrowth and filopodia movement. Further, we examined the roles of Cl− or K+ channel activation, as well as actin polymerization in these two distinct processes, because mechanistic understanding could provide a strategy to modulate microglia function. We found that volume sensitive Cl− channel blockers (NPPB, tamoxifen, DIDS) prevented the rapid process outgrowth of microglia observed in response to damage. In contrast, filopodia extension during sensing was resistant to Cl− channel inhibitors, indicating that these motile processes have different cellular mechanisms. However, both filopodia sensing and rapid process outgrowth were blocked by inhibition of actin polymerization. Following lesion formation under control conditions, rapidly outgrowing processes contacted the damaged area and this was associated with a 37% decrease in lesion volume. Inhibition of process outgrowth by Cl− channel block, prevention of actin polymerization, or by selectively ablating microglia all allowed lesion volume to increase and spread into the surrounding tissue. Therefore, process outgrowth in response to focal brain damage is beneficial by preventing lesion expansion and suggests microglia represent a front line defence against damage in the brain.

D1 Receptors Physically Interact with N-Type Calcium Channels to Regulate Channel Distribution and Dendritic Calcium Entry

Dopamine signaling through D1 receptors in the prefrontal cortex (PFC) plays a critical role in the maintenance of higher cognitive functions, such as working memory. At the cellular level, these functions are predicated to involve alterations in neuronal calcium levels. The dendrites of PFC neurons express D1 receptors and N-type calcium channels, yet little information exists regarding their coupling. Here, we show that D1 receptors potently inhibit N-type channels in dendrites of rat PFC neurons. Using coimmunoprecipitation, we demonstrate the existence of a D1 receptor-N-type channel signaling complex in this region, and we provide evidence for a direct receptor-channel interaction. Finally, we demonstrate the importance of this complex to receptor-channel colocalization in heterologous systems and in PFC neurons. Our data indicate that the N-type calcium channel is an important physiological target of D1 receptors and reveal a mechanism for D1 receptor-mediated regulation of cognitive function in the PFC.

CD4+ Th-APC with Acquired Peptide/MHC Class I and II Complexes Stimulate Type 1 Helper CD4+ and Central Memory CD8+ T Cell Responses

T cell-T cell Ag presentation is increasingly attracting attention. We previously showed that the in vitro OVA-pulsed dendritic cell (DCOVA)-activated CD4+ Th cells acquired OVA peptide/MHC (pMHC) class I and costimulatory molecules such as CD54 and CD80 from DCOVA and acted as CD4+ Th-APC capable of stimulating OVA-specific CD8+ CTL responses. In this study, we further applied the OVA-specific TCR-transgenic OT I and OT II mice with deficiency of various cytokines or costimulatory molecule genes useful for studying the molecular mechanisms underlying in Th-APC’s stimulatory effect. We demonstrated that DCOVA-stimulated OT II CD4+ Th-APC also acquired costimulatory molecules such as CD40, OX40L, and 4-1BBL and the functional pMHC II complexes by DCOVA activation. CD4+ Th-APC with acquired pMHC II and I were capable of stimulating CD4+ Th1 and central memory CD8+44+CD62LhighIL-7R+ T cell responses leading to antitumor immunity against OVA-expressing mouse B16 melanoma. Their stimulatory effect on CD8+ CTL responses and antitumor immunity is mediated by IL-2 secretion, CD40L, and CD80 signaling and is specifically targeted to CD8+ T cells in vivo via acquired pMHC I. In addition, CD4+ Th-APC expressing OVA-specific TCR, FasL, and perforin were able to kill DCOVA and neighboring Th-APC expressing endogenous and acquired pMHC II. Taken together, we show that CD4+ Th-APC can modulate immune responses by stimulating CD4+ Th1 and central memory CD8+ T cell responses and eliminating DCOVA and neighboring Th-APC. Therefore, our findings may have great impacts in not only the antitumor immunity, but also the regulatory T cell-dependent immune tolerance in vivo.

VRACs CARVe a Path for Novel Mechanisms of Communication in the CNS

Because the brain is encased by the skull, the ability to control cell volume in the brain is crucial, and pathological conditions that disturb cell volume homeostasis may severely compromise neural function and survival. Astrocytes are the main cell type to show swelling in response to pathological conditions. More recently, a role for swelling-induced neurotransmitter release from astrocytes under nonpathological conditions has been reported. Astrocytes express a volume-regulated anion channel (VRAC) that is involved in volume homeostasis. In addition to transporting chloride, VRACs allow the efflux of chloride and amino acids such as taurine, glutamate, and aspartate. Glutamate and aspartate are potent activators of neuronal glutamate receptors. Therefore, this nonsynaptic form of cellular communication may modulate neuronal excitability and synaptic activity.

Direct in vivo evidence of CD4+ T cell requirement for CTL response and memory via pMHC-I targeting and CD40L signaling

CD4+ T cell help contributes critically to DC‐induced CD8+ CTL immunity. However, precisely how these three cell populations interact and how CD4+ T cell signals are delivered to CD8+ T cells in vivo have been unclear. In this study, we developed a novel, two‐step approach, wherein CD4+ T cells and antigen‐presenting DCs productively engaged one another in vivo in the absence of cognate CD8+ T cells, after which, we selectively depleted the previously engaged CD4+ T cells or DCs before allowing interactions of either population alone with naïve CD8+ T cells. This protocol thus allows us to clearly document the importance of CD4+ T‐licensed DCs and DC‐primed CD4+ T cells in CTL immunity. Here, we provide direct in vivo evidence that primed CD4+ T cells or licensed DCs can stimulate CTL response and memory, independent of DC‐CD4+ T cell clusters. Our results suggest that primed CD4+ T cells with acquired pMHC‐I from DCs represent crucial “immune intermediates” for rapid induction of CTL responses and for functional memory via CD40L signaling. Importantly, intravital, two‐photon microscopy elegantly provide unequivocal in vivo evidence for direct CD4‐CD8+ T cell interactions via pMHC‐I engagement. This study corroborates the coexistence of direct and indirect mechanisms of T cell help for a CTL response in noninflammatory situations. These data suggest a new “dynamic model of three‐cell interactions” for CTL immunity derived from stimulation by dissociated, licensed DCs, primed CD4+ T cells, and DC‐CD4+ T cell clusters and may have significant implications for autoimmunity and vaccine design.

Dynamic volume changes in astrocytes are an intrinsic phenomenon mediated by bicarbonate ion flux.

Astrocytes, the major type of non-neuronal cells in the brain, play an important functional role in extracellular potassium ([K+]o) and pH homeostasis. Pathological brain states that result in [K+]o and pH dysregulation have been shown to cause astrocyte swelling. However, whether astrocyte volume changes occur under physiological conditions is not known. In this study we used two-photon imaging to visualize real-time astrocyte volume changes in the stratum radiatum of the hippocampus CA1 region. Astrocytes were observed to swell by 19.0±0.9% in response to a small physiological increase in the concentration of [K+]o (3 mM). Astrocyte swelling was mediated by the influx of bicarbonate (HCO3−) ions as swelling was significantly decreased when the influx of HCO3− was reduced. We found: 1) in HCO3− free extracellular solution astrocytes swelled by 5.4±0.7%, 2) when the activity of the sodium-bicarbonate cotransporter (NBC) was blocked the astrocytes swelled by 8.3±0.7%, and 3) in the presence of an extracellular carbonic anhydrase (CA) inhibitor astrocytes swelled by 11.4±0.6%. Because a significant HCO3− efflux is known to occur through the γ-amino-butyric acid (GABA) channel, we performed a series of experiments to determine if astrocytes were capable of HCO3− mediated volume shrinkage with GABA channel activation. Astrocytes were found to shrink −7.7±0.5% of control in response to the GABAA channel agonist muscimol. Astrocyte shrinkage from GABAA channel activation was significantly decreased to −5.0±0.6% of control in the presence of the membrane-permeant CA inhibitor acetazolamide (ACTZ). These dynamic astrocyte volume changes may represent a previously unappreciated yet fundamental mechanism by which astrocytes regulate physiological brain functioning.

A Distinct Role of CD4 + Th17- and Th17-Stimulated CD8 + CTL in the Pathogenesis of Type 1 Diabetes and Experimental Autoimmune Encephalomyelitis

Both CD4+ Th17-cells and CD8+ cytotoxic T lymphocytes (CTLs) are involved in type 1 diabetes and experimental autoimmune encephalomyelitis (EAE). However, their relationship in pathogenesis of these autoimmune diseases is still elusive. We generated ovalbumin (OVA)- or myelin oligodendrocyte glycoprotein (MOG)-specific Th17 cells expressing RORγt and IL-17 by in vitro co-culturing OVA-pulsed and MOG35-55 peptide-pulsed dendritic cells (DCOVA and DCMOG) with CD4+ T cells derived from transgenic OTII and MOG-T cell receptor mice, respectively. We found that these Th17 cells when transferred into C57BL/6 mice stimulated OVA- and MOG-specific CTL responses, respectively. To assess the above question, we adoptively transferred OVA-specific Th17 cells into transgenic rat insulin promoter (RIP)-mOVA mice or RIP-mOVA mice treated with anti-CD8 antibody to deplete Th17-stimulated CD8+ T cells. We demonstrated that OVA-specific Th17-stimulated CTLs, but not Th17 cells themselves, induced diabetes in RIP-mOVA. We also transferred MOG-specific Th17 cells into C57BL/6 mice and H-2Kb−/− mice lacking of the ability to generate Th17-stimulated CTLs. We further found that MOG-specific Th17 cells, but not Th17-activated CTLs induced EAE in C57BL/6 mice. Taken together, our data indicate a distinct role of Th17 cells and Th17-stimulated CTLs in the pathogenesis of TID and EAE, which may have great impact on the overall understanding of Th17 cells in the pathogenesis of autoimmune diseases.

Sumatriptan inhibition of N-type calcium channel mediated signaling in dural CGRP terminal fibres

The selective 5-HT₁ receptor agonist sumatriptan is an effective therapeutic for migraine pain yet the antimigraine mechanisms of action remain controversial. Pain-responsive fibres containing calcitonin gene-related peptide (CGRP) densely innervating the cranial dura mater are widely believed to be an essential anatomical substrate for the development of migraine pain. 5-HT₁ receptors in the dura colocalize with CGRP fibres in high density and thus provide a possible peripheral site of action for sumatriptan. In the present study, we used high-resolution optical imaging selectively within individual mouse dural CGRP nociceptive fibre terminations and found that application of sumatriptan caused a rapid, reversible dose-dependent inhibition in the amplitude of single action potential evoked Ca²⁺ transients. Pre-application of the 5-HT₁ antagonist GR 127935 or the selective 5-HT(1D) antagonist BRL 15572 prevented inhibition while the selective 5-HT(1B) antagonist SB 224289 did not, suggesting this effect was mediated selectively through the 5-HT(1D) receptor subtype. Sumatriptan inhibition of the action potential evoked Ca²⁺ signaling was mediated selectively through N-type Ca²⁺ channels. Although the T-type Ca²⁺ channel accounted for a greater proportion of the Ca²⁺ signal it did not mediate any of the sumatriptan inhibition. Our findings support a peripheral site of action for sumatriptan in inhibiting the activity of dural pain fibres selectively through a single Ca²⁺ channel subtype. This finding adds to our understanding of the mechanisms that underlie the clinical effectiveness of 5-HT₁ receptor agonists such as sumatriptan and may provide insight for the development of novel peripherally targeted therapeutics for mitigating the pain of migraine.