Biomedical subjects

Craig B Neylon

Publications and source records attributed to Craig B Neylon.

12 recordsLinked to original sources

PubMed2006

Endothelial coordination of cerebral vasomotion via myoendothelial gap junctions containing connexins 37 and 40.

Control of cerebral vasculature differs from that of systemic vessels outside the blood-brain barrier. The hypothesis that the endothelium modulates vasomotion via direct myoendothelial coupling was investigated in a small vessel of the cerebral circulation. In the primary branch of the rat basilar artery, membrane potential, diameter, and calcium dynamics associated with vasomotion were examined using selective inhibitors of endothelial function in intact and endothelium-denuded arteries. Vessel anatomy, protein, and mRNA expression were studied using conventional electron microscopy high-resolution ultrastructural and confocal immunohistochemistry and quantitative PCR. Membrane potential oscillations were present in both endothelial cells and smooth muscle cells (SMCs), and these preceded rhythmical contractions during which adjacent SMC intracellular calcium concentration ([Ca(2+)](i)) waves were synchronized. Endothelium removal abolished vasomotion and desynchronized adjacent smooth muscle cell [Ca(2+)](i) waves. N(G)-nitro-l-arginine methyl ester (10 microM) did not mimic this effect, and dibutyryl cGMP (300 muM) failed to resynchronize [Ca(2+)](i) waves in endothelium-denuded arteries. Combined charybdotoxin and apamin abolished vasomotion and depolarized and constricted vessels, even in absence of endothelium. Separately, (37,43)Gap27 and (40)Gap27 abolished vasomotion. Extensive myoendothelial gap junctions (3 per endothelial cell) composed of connexins 37 and 40 connected the endothelial cell and SMC layers. Synchronized vasomotion in rat basilar artery is endothelium dependent, with [Ca(2+)](i) waves generated within SMCs being coordinated by electrical coupling via myoendothelial gap junctions.

Animals↗

PubMed2006

Regulation of the slow afterhyperpolarization in enteric neurons by protein kinase A.

The slow after-hyperpolarization (sAHP) following the action potential is an important determinant of the firing patterns of enteric neurons. The channel responsible for the sAHP thus serves as a critical control point at which neurotransmitters and inflammatory mediators modulate gut motility. Many of these receptor-evoked pathways are known to inhibit the sAHP and, thus, excite enteric neurons. They act through protein kinase A (PKA) which is a strong inhibitor of the sAHP current while protein phosphatases enhance the current. Increasing evidence suggests that the sAHP is mediated by the opening of intermediate-conductance Ca-activated potassium (IK) channels. This neuronal IK channel, previously known to be expressed in a variety of non-excitable cells, is strongly influenced by protein kinases. Investigation of the molecular basis for the modulation of IK channels by protein phosphorylation indicates that there are multiple mechanisms of channel control. Inhibition of channel activity by PKA involves phosphorylation sites located within the calmodulin-binding domain of the channel. The localization of these sites within the region involved in Ca2+ activation suggests that PKA-mediated phosphorylation of the channel opposes the conformational changes caused by binding of Ca/calmodulin, which would otherwise lead to opening of the channel. We suggest that the channel exists as a macromolecular complex involving calmodulin, protein kinases, protein phosphatase and possibly other proteins. The regulation of the channel through kinases and phophatases results in exquisite control of neuronal firing and subsequent modulation of enteric reflexes.

Animals↗

PubMed2006

Evidence for involvement of both IKCa and SKCa channels in hyperpolarizing responses of the rat middle cerebral artery.

BACKGROUND AND PURPOSE: Endothelium-derived hyperpolarizing factor responses in the rat middle cerebral artery are blocked by inhibiting IKCa channels alone, contrasting with peripheral vessels where block of both IKCa and SKCa is required. As the contribution of IKCa and SKCa to endothelium-dependent hyperpolarization differs in peripheral arteries, depending on the level of arterial constriction, we investigated the possibility that SKCa might contribute to equivalent hyperpolarization in cerebral arteries under certain conditions. METHODS: Rat middle cerebral arteries (approximately 175 microm) were mounted in a wire myograph. The effect of KCa channel blockers on endothelium-dependent responses to the protease-activated receptor 2 agonist, SLIGRL (20 micromol/L), were then assessed as simultaneous changes in tension and membrane potential. These data were correlated with the distribution of arterial KCa channels revealed with immunohistochemistry. RESULTS: SLIGRL hyperpolarized and relaxed cerebral arteries undergoing variable levels of stretch-induced tone. The relaxation was unaffected by specific inhibitors of IKCa (TRAM-34, 1 micromol/L) or SKCa (apamin, 50 nmol/L) alone or in combination. In contrast, the associated smooth-muscle hyperpolarization was inhibited, but only with these blockers in combination. Blocking nitric oxide synthase (NOS) or guanylyl cyclase evoked smooth-muscle depolarization and constriction, with both hyperpolarization and relaxation to SLIGRL being abolished by TRAM-34 alone, whereas apamin had no effect. Immunolabeling showed SKCa and IKCa within the endothelium. CONCLUSIONS: In the absence of NO, IKCa underpins endothelium-dependent hyperpolarization and relaxation in cerebral arteries. However, when NOS is active SKCa contributes to hyperpolarization, whatever the extent of background contraction. These changes may have relevance in vascular disease states where NO release is compromised and when the levels of SKCa expression may be altered.

Animals↗

PubMed2006

Spatial separation of endothelial small- and intermediate-conductance calcium-activated potassium channels (K(Ca)) and connexins: possible relationship to vasodilator function?

Activation of endothelial cell small- (S) and intermediate- (I) conductance calcium-activated potassium channels (K(Ca)) and current or molecular transfer via myoendothelial gap junctions underlies endothelium-derived hyperpolarization leading to vasodilation. The mechanism underlying the K(Ca) component of vasodilator activity and the characteristics of gap junctions are targets for the selective control of vascular function. In the rat mesenteric artery, where myoendothelial gap junctions and connexin (Cx) 40 are critical for the transmission of the endothelial cell hyperpolarization to the smooth muscle, SK(Ca) and IK(Ca) provide different facets of the endothelium-derived hyperpolarization response, being critical for the hyperpolarization and repolarization phases, respectively. The present study addressed the question of whether this functional separation of responses may be related to the spatial localization of the associated channels? The distribution of endothelial SK(Ca) and IK(Ca) and Cx subtype(s) were examined in the rat mesenteric artery using conventional confocal and high-resolution ultrastructural immunohistochemistry. At the internal elastic lamina-smooth muscle cell interface at internal elastic lamina holes (as potential myoendothelial gap junction sites), strong punctate IK(Ca), Cx37 and Cx40 expression was present. SK(Ca), Cx37, Cx40 and Cx43 were localized to adjacent endothelial cell gap junctions. High-resolution immunohistochemistry demonstrated IK(Ca) and Cx37-conjugated gold to myoendothelial gap junction-associated endothelial cell projections. Clear co-localization of K(Ca) and Cxs suggests a causal relationship between their activity and the previously described differential functional activation of SK(Ca) and IK(Ca). Such precise localizations may represent a selective target for control of vasodilator function and vascular tone.

Animals↗

PubMed2005

Downregulated REST transcription factor is a switch enabling critical potassium channel expression and cell proliferation.

Induction of K(Ca)3.1 (IKCa) potassium channel plays an important role in vascular smooth muscle cell proliferation. Here, we report that the gene encoding K(Ca)3.1 (KCNN4) contains a functional repressor element 1-silencing transcription factor (REST or NRSF) binding site and is repressed by REST. Although not previously associated with vascular smooth muscle cells, REST is present and recruited to the KCNN4 gene in situ. Significantly, expression of REST declines when there is cellular proliferation, showing an inverse relationship with functional K(Ca)3.1. Downregulated REST and upregulated K(Ca)3.1 are also evident in smooth muscle cells of human neointimal hyperplasia grown in organ culture. Furthermore, inhibition of K(Ca)3.1 suppresses neointimal formation, and exogenous REST reduces the functional impact of K(Ca)3.1. Here, we show REST plays a previously unrecognized role as a switch regulating potassium channel expression and consequently the phenotype of vascular smooth muscle cells and human vascular disease.

Animals↗

PubMed2004

Protein kinase A inhibits intermediate conductance Ca2+-activated K+ channels expressed in Xenopus oocytes.

Intermediate-conductance (IK) Ca(2+)-activated K(+) channels are expressed in many different cell types where they perform a variety of functions including cell volume regulation, transepithelial secretion, lymphocyte activation and cell cycle progression. IK channels are thought to be regulated by phosphorylation; however, whether kinases act directly on the channel is unclear. Using IK channels heterologously expressed in Xenopus oocytes, we demonstrate that IK channels are potently inhibited (60%) by the catalytic subunit of protein kinase A (PKA). Inhibition of IK channel current by PKA is abolished by mutation of four phosphorylation residues (S312, T327, S332, and T348) in the putative calmodulin-binding region of the channel. Evidence for direct modulation of the IK channel by PKA was further demonstrated using GST fusion proteins. The major site of phosphorylation was found to be serine 332; however, other residues were also phosphorylated. We conclude that IK channels can be directly regulated by the cAMP second-messenger system. The mechanism appears to involve direct phosphorylation by PKA of a modulatory locus in the cytoplasmic region of the channel, the site at which calmodulin is thought to interact. Modulation of IK channels by protein kinases may be an important mechanism regulating cell function.

Amino Acid Sequence↗

PubMed2004

Intermediate conductance potassium (IK) channels occur in human enteric neurons.

IK channels, which had been previously found in hemopoetically derived cells (including erythrocytes and lymphocytes) and epithelial cells, where they regulate proliferation, cell volume regulation and secretion, have only recently been discovered in neurons, where they had previously been claimed not to occur. Based on immunohistochemical detection of IK channel-like immunoreactivity, it has been reported that IK channel expression in enteric neurons is suppressed in Crohn's disease. In the present work we have investigated whether authentic IK channels are expressed by enteric neurons. Human and mouse tissue was investigated by immunohistochemistry, Western blot and RT-PCR. Immunohistochemical studies revealed IK channel-like immunoreactivity in large myenteric neurons, but not in other cell types in the external muscle layers. Many of these nerve cells had calbindin immunoreactivity. Western blots from the external muscle revealed an immunoreactive band at the molecular weight of the IK channel. Using RT-PCR, we detected a transcript corresponding to the IK channel gene in extracts from the ganglion containing layer. The sequence obtained from the RT-PCR product was identical to that previously published for the IK channel. We conclude that IK channels are expressed by human enteric neurons, including large smooth surfaced neurons that are possibly the human equivalent of the Dogiel type II neurons that express these channels in small mammals.

Adult↗

PubMed2004

Intermediate-conductance calcium-activated potassium channels in enteric neurones of the mouse: pharmacological, molecular and immunochemical evidence for their role in mediating the slow afterhyperpolarization.

Calcium-activated potassium channels are critically important in modulating neuronal cell excitability. One member of the family, the intermediate-conductance potassium (IK) channel, is not thought to play a role in neurones because of its predominant expression in non-excitable cells such as erythrocytes and lymphocytes, in smooth muscle tissues, and its lack of apparent expression in brain. In the present study, we demonstrate that IK channels are localized on specific neurones in the mouse enteric nervous system where they mediate the slow afterhyperpolarization following an action potential. IK channels were localized by immunohistochemistry on intrinsic primary afferent neurones, identified by their characteristic Dogiel type II morphology. The slow afterhyperpolarization recorded from these cells was abolished by the IK channel blocker clotrimazole. RT-PCR and western analysis of extracts from the colon revealed an IK channel transcript and protein identical to the IK channel expressed in other cell types. These results indicate that IK channels are expressed in neurones where they play an important role in modulating firing properties.

Animals↗

PubMed2003

Expression of intermediate conductance potassium channel immunoreactivity in neurons and epithelial cells of the rat gastrointestinal tract.

Recent functional evidence suggests that intermediate conductance calcium-activated potassium channels (IK channels) occur in neurons in the small intestine and in mucosal epithelial cells in the colon. This study was undertaken to investigate whether IK channel immunoreactivity occurs at these and at other sites in the gastrointestinal tract of the rat. IK channel immunoreactivity was found in nerve cell bodies throughout the gastrointestinal tract, from the esophagus to the rectum. It was revealed in the initial segments of the axons, but not in axon terminals. The majority of immunoreactive neurons had Dogiel type II morphology and in the myenteric plexus of the ileum all immunoreactive neurons were of this shape. Intrinsic primary afferent neurons in the rat small intestine are Dogiel type II neurons that are immunoreactive for calretinin, and it was found that almost all the IK channel immunoreactive neurons were also calretinin immunoreactive. IK channel immunoreactivity also occurred in calretinin-immunoreactive, Dogiel type II neurons in the caecum. Epithelial cells of the mucosal lining were immunoreactive in the esophagus, stomach, small and large intestines. In the intestines, the immunoreactivity occurred in transporting enterocytes, but not in mucous cells. Immunoreactivity was at both the apical and basolateral surfaces. A small proportion of mucosal endocrine cells was immunoreactive in the duodenum, ileum and caecum, but not in the stomach, proximal colon, distal colon or rectum. There was immunoreactivity of vascular endothelial cells. It is concluded that IK channels are located on cell bodies and proximal parts of axons of intrinsic primary afferent neurons, where, from functional studies, they would be predicted to lower neuronal excitability when opened in response to calcium entry. In the mucosa of the small and large intestine, IK channels are probably involved in control of potassium exchange, and in the esophageal and gastric mucosa they are possibly involved in control of cell volume in response to osmotic challenge.

Animals↗

PubMed2003

Oligophrenin-1, a Rho GTPase-activating protein (RhoGAP) involved in X-linked mental retardation, is expressed in the enteric nervous system.

Oligophrenin-1 is a RhoGTPase-activating protein (RhoGAP) that is involved in the regulation of shape changes in dendritic spines, and outgrowth of axons and dendrites in the brain. These changes in neuronal morphology are central to the mechanisms of plasticity, learning, and memory. Although the enteric nervous system also exhibits long-term changes in neuronal function, the expression and involvement of oligophrenin-1 has not previously been investigated. We show by RT-PCR analysis that oligophrenin-1 mRNA is expressed in the myenteric plexus (MP) of the guinea pig ileum. Sequencing of RT-PCR products showed that guinea pig oligophrenin-1 mRNA is 98% and 87% homologous to human and mouse oligophrenin-1, respectively, except that a 42 bp sequence is absent from the guinea pig mRNA. This 42 bp sequence codes for a sequence of 14 amino acids located near the carboxy-terminal end of the RhoGAP domain in the human sequence. An antibody that recognizes human oligophrenin-1 identified a 91 kDa protein band in rat and mouse brain lysates and in guinea pig sciatic nerve, and a 36 kDa protein band in both purified enteric ganglion cell and brain lysate from guinea pig. Oligophrenin-1 is localized specifically to neurons and varicose axons in the MPs and submucosal plexuses (SMPs) of the guinea pig and rat, but is not detectable in glial cells, smooth muscle, or other cell types. These findings indicate that oligophrenin-1 is expressed in the enteric nervous system, where it may regulate morphological changes in axons and dendrites, and thus modulate neuronal connectivity.

Animals↗

PubMed2002

Potassium channels and vascular proliferation.

Potassium channels are currently the focus of much attention because of their recently discovered role in the regulation of vascular smooth muscle growth. Dramatic alterations in the expression and activity of K+ channels causing marked changes in the cell's electrical properties accompany enhanced growth of smooth muscle cells (SMCs). These findings indicate that alterations in K+ channel function are important for SMC proliferation. However, the mechanisms by which changes in K+ channel activity influence cellular growth pathways are poorly understood. The emergent electrical properties caused by modulation of K+ channels are associated with marked differences in the spatial and temporal organization of Ca2+ signaling. Thus, changes in K+ channel function may represent a universal mechanism by which Ca2+ signals are targeted towards activation of gene expression and cell growth. As enhanced growth of smooth muscle underlies many cardiovascular diseases and clinical pathologies, the identification of an important role for K+ channels in SMC proliferation indicates a new source of therapeutic targets to regulate proliferative vascular disorders.

Animals↗

PubMed2002

Regulation of K+ channels underlying the slow afterhyperpolarization in enteric afterhyperpolarization-generating myenteric neurons: role of calcium and phosphorylation.

1. Myenteric afterhyperpolarization-generating myenteric (AH) neurons serve as intrinsic primary afferent neurons of the enteric nervous system and generate prolonged or slow afterhyperpolarizing potentials (slow AHP). The slow AHP is generated by an increase in a Ca2+-activated K+ conductance (gK-Ca) and is inhibited by enteric neurotransmitters leading to increased excitability. 2. Using cell-attached patch-clamp recordings from AH neurons, we have shown that K+ channels with an intermediate unitary conductance (IK channels) open following action potential firing. 3. In excised patches from AH neurons, we have identified an IK-like channel that can be activated by submicromolar levels of cytoplasmic Ca2+ and is not voltage dependent. 4. Application of the catalytic subunit of cAMP-dependent protein kinase to the cytoplasmic surface of inside-out patches inhibits the opening of IK-like channels previously activated by Ca2+. 5. The IK-like channels are resistant to external tetraethylammonium (5 mmol/L) and apamin (0.3-1 micro mol/L), but are inhibited by clotrimazole (10 micro mol/L). 6. Our present data support the idea that an increase in the open probability of IK-like channels in AH neurons following an increase in cytoplasmic [Ca2+] is responsible for the slow AHP and their opening is modulated by kinases.

Action Potentials↗