Ca2+ influx into leech neuropile glial cells mediated by nicotinic acetylcholine receptors

An annotated CNS transcriptome of the medicinal leech, Hirudo verbana: De novo sequencing to characterize genes associated with nervous system activity

The medicinal leech is one of the most venerated model systems for the study of fundamental nervous system principles, ranging from single-cell excitability to complex sensorimotor integration. Yet, molecular analyses have yet to be extensively applied to complement the rich history of electrophysiological study that this animal has received. Here, we generated the first de novo transcriptome assembly from the entire central nervous system of Hirudo verbana, with the goal of providing a molecular resource, as well as to lay the foundation for a comprehensive discovery of genes fundamentally important for neural function. Our assembly generated 107,704 contigs from over 900 million raw reads. Of these 107,704 contigs, 39,047 (36%) were annotated using NCBI's validated RefSeq sequence database. From this annotated central nervous system transcriptome, we began the process of curating genes related to nervous system function by identifying and characterizing 126 unique ion channel, receptor, transporter, and enzyme contigs. Additionally, we generated sequence counts to estimate the relative abundance of each identified ion channel and receptor contig in the transcriptome through Kallisto mapping. This transcriptome will serve as a valuable community resource for studies investigating the molecular underpinnings of neural function in leech and provide a reference for comparative analyses.

Ca2+ transients in myenteric glial cells during the colonic migrating motor complex in the isolated murine large intestine

Enteric glia cells (EGCs) form a dense network around myenteric neurons in a ganglia and are likely to have not only a supportive role but may also regulate or be regulated by neural activity. Our aims were to determine if EGCs are activated during the colonic migrating motor complex (CMMC) in the isolated murine colon. Strips of longitudinal muscle were removed and Ca(2+) imaging (Fluo-4) used to study activity in EGCs within myenteric ganglia during CMMCs, followed by post hoc S100 staining to reveal EGCs. The cell bodies of EGCs and their processes formed caps and halos, respectively, around some neighbouring myenteric neurons. Some EGCs (36%), which were largely quiescent between CMMCs, exhibited prolonged tetrodotoxin (TTX; 1 μm)-sensitive Ca(2+) transients that peaked ∼39 s following a mucosal stimulus that generated the CMMC, and often outlasted the CMMC (duration ∼23 s). Ca(2+) transients in EGCs often varied in duration within a ganglion; however, the duration of these transients was closely matched by activity in closely apposed nerve varicosities, suggesting EGCs were not only innervated but the effective innervation was localized. Furthermore, all EGCs, even those that were quiescent, responded with robust Ca(2+) transients to KCl, caffeine, nicotine, substance P and GR 64349 (an NK2 agonist), suggesting they were adequately loaded with indicator and that some EGCs may be inhibited by substances released by neighbouring neurons. Intracellular Ca(2+) waves were visualised propagating between closely apposed glia and from glial cell processes to the soma (velocity 12 μm s(-1)) where they produced an accumulative rise in Ca(2+), suggesting that the soma acts as an integrator of Ca(2+) activity. In conclusion, Ca(2+) transients in EGCs occur secondary to nerve activity; their activation is driven by intrinsic excitatory nerve pathways that generate the CMMC.

Coupling of ionic events to protein kinase signaling cascades upon activation of alpha7 nicotinic receptor: cooperative regulation of alpha2-integrin expression and Rho kinase activity

Defining the signaling mechanisms and effector proteins mediating phenotypic and mechanical plasticity of keratinocytes (KCs) during wound epithelialization is one of the major goals in epithelial cell biology. The acetylcholine (ACh)-gated ion channels, or nicotinic ACh receptors (nAChRs), mediate the nicotinergic signaling that controls crawling locomotion of KCs. To elucidate relative contributions of the ionic and protein kinase-mediated events elicited due to activation of alpha7 nAChRs, we quantitated expression of alpha2-integrin gene at the mRNA and protein levels and also measured Rho kinase activity in KCs stimulated with the alpha7 agonist AR-R17779 while blocking the Na+ or Ca2+ entry and/or inhibiting signaling kinases. The results demonstrated the existence of the two-component signaling systems coupling the ionic events and protein kinase signaling cascades downstream of alpha7 nAChR to simultaneous up-regulation of alpha2-integrin expression and activation of Rho kinase. The Raf/MEK1/ERK1/2 cascade up-regulating alpha2-integrin was activated due to both Ca2+-dependent recruitment of Ca2+/calmodulin-dependent protein kinase II and protein kinase C and Ca2+-independent activation of Ras. Likewise the phosphatidylinositol 3-kinase-mediated activation of Rho kinase was elicited due to both Ca2+ entry-dependent involvement of Ca2+/calmodulin-dependent protein kinase II and Ca2+-independent activation of Jak2. Thus, although the initial signals emanating from activated alpha7 nAChR are different in nature the pathways intersect at common effector molecules providing for a common end point effect. This novel paradigm of nAChR-mediated coordination of the ionic and metabolic signaling events can allow an auto/paracrine ACh to simultaneously alter gene expression and induce reciprocal changes in the cytoskeleton and contractile system of KCs required to compete a particular step of wound epithelialization.

Swelling-activated chloride channels in leech Retzius neurons

During periods of high activity neurons are expected to swell due to the uptake of Cl(-). To find out whether leech Retzius neurons possess swelling-activated Cl(-) channels that facilitate Cl(-) efflux and, hence, volume recovery, we exposed the cells to hypotonic solutions. In hypotonic solutions, the cells slowly swelled but did not undergo a regulatory volume decrease. However, the cell volume increased less than predicted for an ideal osmometer, suggesting the action of a compensatory mechanism. The cell swelling was paralleled by a marked decrease in the input resistance as well as by the activation of a membrane current with a reversal potential close to the Cl(-) equilibrium potential. This current was substantially diminished by removing bath Cl(-), by applying the Cl(-) channel blocker DIDS, or by treating the cells with the tubulin polymerization inhibitor colchicine. Furthermore, in the presence of colchicine or vinblastine, the cell swelling was substantially increased. It is concluded that leech Retzius neurons possess swelling-activated Cl(-) channels that require an intact microtubule system for activation. The channels may help to restore cell volume after periods of high neuronal activity.

The role of inflammation in Alzheimer's disease

Considerable evidence gained over the past decade has supported the conclusion that neuroinflammation is associated with Alzheimer's disease (AD) pathology. Inflammatory components related to AD neuroinflammation include brain cells such as microglia and astrocytes, the classic and alternate pathways of the complement system, the pentraxin acute-phase proteins, neuronal-type nicotinic acetylcholine receptors (AChRs), peroxisomal proliferators-activated receptors (PPARs), as well as cytokines and chemokines. Both the microglia and astrocytes have been shown to generate beta-amyloid protein (Abeta), one of the main pathologic features of AD. Abeta itself has been shown to act as a pro-inflammatory agent causing the activation of many of the inflammatory components. Further substantiation for the role of neuroinflammation in AD has come from studies that demonstrate patients who took non-steroidal anti-inflammatory drugs had a lower risk of AD than those who did not. These same results have led to increased interest in pursuing anti-inflammatory therapy for AD but with poor results. On the other hand, increasing amount of data suggest that AChRs and PPARs are involved in AD-induced neuroinflammation and in this regard, future therapy may focus on their specific targeting in the AD brain.

Ca2+ influx into identified leech neurons induced by 5-hydroxytryptamine

The role of 5-hydroxytryptamine (5-HT, serotonin) in the control of leech behavior is well established and has been analyzed extensively on the cellular level; however, hitherto little is known about the effect of 5-HT on the cytosolic free calcium concentration ([Ca(2+)](i)) in leech neurons. As [Ca(2+)](i) plays a pivotal role in numerous cellular processes, we investigated the effect of 5-HT on [Ca(2+)](i) (measured by Fura-2) in identified leech neurons under different experimental conditions, such as changed extracellular ion composition and blockade of excitatory synaptic transmission. In pressure (P), lateral nociceptive (N1), and Leydig neurons, 5-HT induced a [Ca(2+)](i) increase which was predominantly due to Ca(2+) influx since it was abolished in Ca(2+)-free solution. The 5-HT-induced Ca(2+) influx occurred only if the cells depolarized sufficiently, indicating that it was mediated by voltage-dependent Ca(2+) channels. In P and N1 neurons, the membrane depolarization was due to Na(+) influx through cation channels coupled to 5-HT receptors, whereby the dose-dependency suggests an involvement in excitatory synaptic transmission. In Leydig neurons, 5-HT receptor-coupled cation channels seem to be absent. In these cells, the membrane depolarization activating the voltage-dependent Ca(2+) channels was evoked by 5-HT-triggered excitatory glutamatergic input. In Retzius, anterior pagoda (AP), annulus erector (AE), and median nociceptive (N2) neurons, 5-HT had no effect on [Ca(2+)](i).

Effects of ATP and derivatives on neuropile glial cells of the leech central nervous system

We investigated the effects of ATP (adenosine 5'-triphosphate) and derivatives on leech neuropile glial cells, focusing on exposed glial cells. ATP dose-dependently depolarized or hyperpolarized neuropile glial cells in situ as well as exposed neuropile glial cells. These potential shifts varied among cells and repetitive ATP application did not change their amplitude, duration or direction. In exposed neuropile glial cells, ATP most frequently induced a Na(+)-dependent depolarization and decreased the input resistance. The agonist potency ATP > ADP (adenosine 5'-diphosphate) > AMP (adenosine 5'-monophosphate) > adenosine indicates that P2 purinoceptors mediate this depolarization. The P2Y agonist 2-methylthio-ATP mimicked the ATP-induced depolarization, whereas the P2Y antagonist PPADS (pyridoxal-phosphate-6-azophenyl-2', 4'-disulphonic acid) reduced it. P2X agonists were without effect. Because the P1 antagonist 8-SPT (8-(p-sulphophenyl)-theophylline) also depressed ATP-induced depolarizations and some ATP-insensitive glial cells responded to adenosine, we suggest coexpression of metabotropic P2Y and P1 purinoceptors. The ATP-induced depolarization requires activation of Na(+) channels or nonselective cation channels, whereas the ATP-induced hyperpolarization indicates activation of K(+) channels. ATP also increased the intracellular Ca(2+) concentration ([Ca(2+)](i)), that is independent of Ca(2+) influx but reflects intracellular Ca(2+) release possibly triggered by IP(3) formation. ADP and AMP also increased [Ca(2+)](i), but were less efficient than ATP; adenosine and 2-methylthio-ATP did not affect [Ca(2+)](i). In view of the mobilization of intracellular Ca(2+), ATP is clearly different from other leech neurotransmitters, because it enables intracellular Ca(2+) signaling without causing prominent changes in glial membrane potential. Thus disturbance of the extracellular microenvironment and the demand for metabolic energy are minimized.

Ionic mechanism of 4-aminopyridine action on leech neuropile glial cells

Extracellular 4-aminopyridine (4-AP), tetraethylammonium chloride (TEA) and quinine depolarized the neuropile glial cell membrane and decreased its input resistance. As 4-AP induced the most pronounced effects, we focused on the action of 4-AP and clarified the ionic mechanisms involved. 4-AP did not only block glial K+ channels, but also induced Na+ and Ca2+ influx via other than voltage-gated channels. The reversal potential of the 4-AP-induced current was -5 mV. Application of 5 mM Ni2+ or 0.1 mM d-tubocurarine reduced the 4-AP-induced depolarization and the associated decrease in input resistance. We therefore suggest that 4-AP mediates neuronal acetylcholine release, apparently by a presynaptic mechanism. Activation of glial nicotinic acetylcholine receptors contributes to the depolarization, the decrease in input resistance, and the 4-AP-induced inward current. Furthermore, the 4-AP-induced depolarization activates additional voltage-sensitive K+ and Cl- channels and 4-AP-induced Ca2+ influx could activate Ca2+-sensitive K+ and Cl- channels. Together these effects compensate and even exceed the 4-AP-mediated reduction in K+ conductance. Therefore, the 4-AP-induced depolarization was paralleled by a decreasing input resistance.

Macroscopic and single-channel chloride currents in neuropile glial cells of the leech central nervous system

In patch-clamp experiments we characterized four Cl- channels (42 pS, 70 pS, 80 pS and 229 pS) underlying the large Cl- conductance of leech neuropile glial cells. They differed with respect to their gating, their rectification and their activity in the cell-attached configuration, showed the selectivity sequence I->Cl->/=Br->F- and were impermeable to SO42-. The four channels were blocked by NPPB, DPC, niflumic acid and DIDS and exhibited either three or four sublevel states. The outward rectifying 42 pS, 70 pS and 80 pS Cl- channels were classified as intermediate conductance Cl- channels and they could contribute to the high Cl- conductance of the glial membrane, which stabilizes the glial membrane potential. The inward rectifying 229 pS Cl- channel is very similar to vertebrate high conductance Cl- channels, which are assumed to be part of an emergency system that is activated under pathophysiological conditions. In voltage-clamp experiments we calculated that the Cl- conductance amounts to one-third of the total membrane conductance. Reduction of this Cl- conductance by Cl- channel inhibitors markedly depolarized the glial cell membrane. These prominent depolarizations depended on Na+ influx and in most cases the glial cells failed to regulate their membrane potential following wash-out of the inhibitors.

Glial calcium: homeostasis and signaling function

Glial cells respond to various electrical, mechanical, and chemical stimuli, including neurotransmitters, neuromodulators, and hormones, with an increase in intracellular Ca2+ concentration ([Ca2+]i). The increases exhibit a variety of temporal and spatial patterns. These [Ca2+]i responses result from the coordinated activity of a number of molecular cascades responsible for Ca2+ movement into or out of the cytoplasm either by way of the extracellular space or intracellular stores. Transplasmalemmal Ca2+ movements may be controlled by several types of voltage- and ligand-gated Ca(2+)-permeable channels as well as Ca2+ pumps and a Na+/Ca2+ exchanger. In addition, glial cells express various metabotropic receptors coupled to intracellular Ca2+ stores through the intracellular messenger inositol 1,4,5-triphosphate. The interplay of different molecular cascades enables the development of agonist-specific patterns of Ca2+ responses. Such agonist specificity may provide a means for intracellular and intercellular information coding. Calcium signals can traverse gap junctions between glial cells without decrement. These waves can serve as a substrate for integration of glial activity. By controlling gap junction conductance, Ca2+ waves may define the limits of functional glial networks. Neuronal activity can trigger [Ca2+]i signals in apposed glial cells, and moreover, there is some evidence that glial [Ca2+]i waves can affect neurons. Glial Ca2+ signaling can be regarded as a form of glial excitability.

Single potassium channels in neuropile glial cells of the leech central nervous system

We performed patch-clamp experiments to identify distinct K+ channels underlying the high K+ conductance and K+ uptake mechanism of the neuropile glial cell membrane on the single-channel level. In the soma membrane four different types of K+ channels were characterized, which were found to be distributed in clusters. Since no other types of K+ channels were observed, these appear to be the complete repertoire of K+ channels expressed in the soma region of this cell type. The outward rectifying 42 pS K+ channel could markedly contribute to the high K+ conductance and the maintenance of the membrane potential, since it shows the highest open probability of all channels. The channel gating occurred in bursts and patch excision decreased the open probability. The outward rectifying 74 pS K+ channel was rarely active in the cell-attached configuration; however, patch excision enhanced its open probability considerably. This type of channel may be involved in neuron-glial crosstalk, since it is activated by both depolarizations and increases in the intracellular Ca2+ concentration, which are known to be induced by neurotransmitter release following the activation of neurons. The 40 pS and 83 pS K+ channels showed inward rectifying properties, suggesting their involvement in the regulation of the extracellular K+ content. The 40 pS K+ channel could only be observed in the inside-out configuration. The 83 pS channel was activated following patch excision. At membrane potentials more negative than -60 mV, flickering events indicated voltage-dependent gating.

Extracellular Ca2+ changes during transmitter application in the leech central nervous system

Changes in extracellular Ca2+ concentration ([Ca2+]e) evoked by transmitters and transmitter agonists, respectively, and by elevation of bath K+ concentration were recorded in isolated segmental ganglia of the leech Hirudo medicinalis using Ca(2+)-selective microelectrodes. A 1-min bath application of kainate (10 microM), glutamate (1 mM), aspartate (1 mM), or carbachol (200 microM) decreased [Ca2+]e by up to 1 mM, whereas the inhibitory transmitters gamma-amino butyric acid (GABA, 100 microM) and serotonin (5-HT, 100 microM) did not change [Ca2+]e. The amplitude of the kainate-induced changes in [Ca2+]e increased with repetitive applications, and changes were blocked by 6-cyano-7-dinitroquinozaline-2,3-dione (CNQX). Elevation of bath K+ concentration from 4 to 40 mM led to a Ni(2+)-sensitive decrease in [Ca2+]e by 0.9 mM. Our results suggest that excitatory transmission in the leech central nervous system might be accompanied by substantial decreases in [Ca2+]e.