SubSol-HIe is an AMPK-dependent hypoxia-responsive subnucleus of the nucleus tractus solitarius that coordinates the hypoxic ventilatory response and protects against apnoea in mice

Functional magnetic resonance imaging (fMRI) suggests that the hypoxic ventilatory response is facilitated by the AMP-activated protein kinase (AMPK), not at the carotid bodies, but within a subnucleus (Bregma -7.5 to -7.1 mm) of the nucleus tractus solitarius that exhibits right-sided bilateral asymmetry. Here, we map this subnucleus using cFos expression as a surrogate for neuronal activation and mice in which the genes encoding the AMPK-α1 (Prkaa1) and AMPK-α2 (Prkaa2) catalytic subunits were deleted in catecholaminergic cells by Cre expression via the tyrosine hydroxylase promoter. Comparative analysis of brainstem sections, relative to controls, revealed that AMPK-α1/α2 deletion inhibited, with right-sided bilateral asymmetry, cFos expression in and thus activation of a neuronal cluster that partially spanned three interconnected anatomical nuclei adjacent to the area postrema: SolDL (Bregma -7.44 mm to -7.48 mm), SolDM (Bregma -7.44 mm to -7.48 mm) and SubP (Bregma -7.48 mm to -7.56 mm). This approximates the volume identified by fMRI. Moreover, these nuclei are known to be in receipt of carotid body afferent inputs, and catecholaminergic neurons of SubP and SolDL innervate aspects of the ventrolateral medulla responsible for respiratory rhythmogenesis. Accordingly, AMPK-α1/α2 deletion attenuated hypoxia-evoked increases in minute ventilation (normalised to metabolism), reductions in expiration time, and increases sigh frequency, but increased apnoea frequency during hypoxia. The metabolic response to hypoxia in AMPK-α1/α2 knockout mice and the brainstem and spinal cord catecholamine levels were equivalent to controls. We conclude that within the brainstem an AMPK-dependent, hypoxia-responsive subnucleus partially spans SubP, SolDM and SolDL, namely SubSol-HIe, and is critical to coordination of active expiration, the hypoxic ventilatory response and defence against apnoea.

Of Mice and Men and Plethysmography Systems: Does LKB1 Determine the Set Point of Carotid Body Chemosensitivity and the Hypoxic Ventilatory Response?

Our recent studies suggest that the level of liver kinase B1 (LKB1) expression in some way determines carotid body afferent discharge during hypoxia and to a lesser extent during hypercapnia. In short, phosphorylation by LKB1 of an as yet unidentified target(s) determines a set point for carotid body chemosensitivity. LKB1 is the principal kinase that activates the AMP-activated protein kinase (AMPK) during metabolic stresses, but conditional deletion of AMPK in catecholaminergic cells, including therein carotid body type I cells, has little or no effect on carotid body responses to hypoxia or hypercapnia. With AMPK excluded, the most likely target of LKB1 is one or other of the 12 AMPK-related kinases, which are constitutively phosphorylated by LKB1 and, in general, regulate gene expression. By contrast, the hypoxic ventilatory response is attenuated by either LKB1 or AMPK deletion in catecholaminergic cells, precipitating hypoventilation and apnea during hypoxia rather than hyperventilation. Moreover, LKB1, but not AMPK, deficiency causes Cheyne-Stokes-like breathing. This chapter will explore further the possible mechanisms that determine these outcomes.

AMPK facilitates the hypoxic ventilatory response through non-adrenergic mechanisms at the brainstem

We recently demonstrated that the hypoxic ventilatory response (HVR) is facilitated by the AMP-activated protein kinase (AMPK) in catecholaminergic neural networks that likely lie downstream of the carotid bodies within the caudal brainstem. Here, we further subcategorise the neurons involved, by cross-comparison of mice in which the genes encoding the AMPK-α1 (Prkaa1) and AMPK-α2 (Prkaa2) catalytic subunits were deleted in catecholaminergic (TH-Cre) or adrenergic (PNMT-Cre) neurons. As expected, the HVR was markedly attenuated in mice with AMPK-α1/α2 deletion in catecholaminergic neurons, but surprisingly was modestly augmented in mice with AMPK-α1/α2 deletion in adrenergic neurons when compared against a variety of controls (TH-Cre, PNMT-Cre, AMPK-α1/α2 floxed). Moreover, AMPK-α1/α2 deletion in catecholaminergic neurons precipitated marked hypoventilation and apnoea during poikilocapnic hypoxia, relative to controls, while mice with AMPK-α1/α2 deletion in adrenergic neurons entered relative hyperventilation with reduced apnoea frequency and duration. We conclude, therefore, that AMPK-dependent modulation of non-adrenergic networks may facilitate increases in ventilatory drive that shape the classical HVR, whereas AMPK-dependent modulation of adrenergic networks may provide some form of negative feedback or inhibitory input to moderate HVR, which could, for example, protect against hyperventilation-induced hypocapnia and respiratory alkalosis.

AMPK deficiency in smooth muscles causes persistent pulmonary hypertension of the new-born and premature death

AMPK has been reported to facilitate hypoxic pulmonary vasoconstriction but, paradoxically, its deficiency precipitates pulmonary hypertension. Here we show that AMPK-α1/α2 deficiency in smooth muscles promotes persistent pulmonary hypertension of the new-born. Accordingly, dual AMPK-α1/α2 deletion in smooth muscles causes premature death of mice after birth, associated with increased muscularisation and remodeling throughout the pulmonary arterial tree, reduced alveolar numbers and alveolar membrane thickening, but with no oedema. Spectral Doppler ultrasound indicates pulmonary hypertension and attenuated hypoxic pulmonary vasoconstriction. Age-dependent right ventricular pressure elevation, dilation and reduced cardiac output was also evident. K_V1.5 potassium currents of pulmonary arterial myocytes were markedly smaller under normoxia, which is known to facilitate pulmonary hypertension. Mitochondrial fragmentation and reactive oxygen species accumulation was also evident. Importantly, there was no evidence of systemic vasculopathy or hypertension in these mice. Moreover, hypoxic pulmonary vasoconstriction was attenuated by AMPK-α1 or AMPK-α2 deletion without triggering pulmonary hypertension.

On a Magical Mystery Tour with 8-Bromo-Cyclic ADP-Ribose: From All-or-None Block to Nanojunctions and the Cell-Wide Web

A plethora of cellular functions are controlled by calcium signals, that are greatly coordinated by calcium release from intracellular stores, the principal component of which is the sarco/endooplasmic reticulum (S/ER). In 1997 it was generally accepted that activation of various G protein-coupled receptors facilitated inositol-1,4,5-trisphosphate (IP3) production, activation of IP3 receptors and thus calcium release from S/ER. Adding to this, it was evident that S/ER resident ryanodine receptors (RyRs) could support two opposing cellular functions by delivering either highly localised calcium signals, such as calcium sparks, or by carrying propagating, global calcium waves. Coincidentally, it was reported that RyRs in mammalian cardiac myocytes might be regulated by a novel calcium mobilising messenger, cyclic adenosine diphosphate-ribose (cADPR), that had recently been discovered by HC Lee in sea urchin eggs. A reputedly selective and competitive cADPR antagonist, 8-bromo-cADPR, had been developed and was made available to us. We used 8-bromo-cADPR to further explore our observation that S/ER calcium release via RyRs could mediate two opposing functions, namely pulmonary artery dilation and constriction, in a manner seemingly independent of IP3Rs or calcium influx pathways. Importantly, the work of others had shown that, unlike skeletal and cardiac muscles, smooth muscles might express all three RyR subtypes. If this were the case in our experimental system and cADPR played a role, then 8-bromo-cADPR would surely block one of the opposing RyR-dependent functions identified, or the other, but certainly not both. The latter seemingly implausible scenario was confirmed. How could this be, do cells hold multiple, segregated SR stores that incorporate different RyR subtypes in receipt of spatially segregated signals carried by cADPR? The pharmacological profile of 8-bromo-cADPR action supported not only this, but also indicated that intracellular calcium signals were delivered across intracellular junctions formed by the S/ER. Not just one, at least two. This article retraces the steps along this journey, from the curious pharmacological profile of 8-bromo-cADPR to the discovery of the cell-wide web, a diverse network of cytoplasmic nanocourses demarcated by S/ER nanojunctions, which direct site-specific calcium flux and may thus coordinate the full panoply of cellular processes.

Incident depression in patients diagnosed with multiple sclerosis: a multi-database study

BACKGROUND AND PURPOSE

Data on rates of newly diagnosed depression after multiple sclerosis (MS) diagnosis are sparse. Here, incident, treated depression in MS patients after diagnosis compared with matched non-MS patients is described.

METHODS

A matched cohort study was conducted in two separate electronic medical databases: the US Department of Defense (US-DOD) military healthcare system and the UK's Clinical Practice Research Datalink GOLD (UK-CPRD). The study population included all patients with a first recorded diagnosis of MS and matched non-MS patients. Patients with a history of treated depression were excluded. Incidence rates and incidence rate ratios with 95% confidence intervals for treated depression after MS diagnosis/matched date were estimated.

RESULTS

Incidence rate ratios of treated depression amongst MS patients compared with non-MS patients were 3.20 (95% confidence interval 3.05-3.35) in the US-DOD and 1.90 (95% confidence interval 1.74-2.06) in the UK-CPRD. Incidence rate ratios were elevated across age and sex. Rates were higher in females than males but, compared to non-MS patients, males with MS had a higher relative risk than females with MS.

CONCLUSIONS

Multiple sclerosis patients in the UK and the USA have a two- to three-fold increased risk of new, treated depression compared to matched non-MS patients.

AMPK and the Need to Breathe and Feed: What's the Matter with Oxygen?

We live and to do so we must breathe and eat, so are we a combination of what we eat and breathe? Here, we will consider this question, and the role in this respect of the AMP-activated protein kinase (AMPK). Emerging evidence suggests that AMPK facilitates central and peripheral reflexes that coordinate breathing and oxygen supply, and contributes to the central regulation of feeding and food choice. We propose, therefore, that oxygen supply to the body is aligned with not only the quantity we eat, but also nutrient-based diet selection, and that the cell-specific expression pattern of AMPK subunit isoforms is critical to appropriate system alignment in this respect. Currently available information on how oxygen supply may be aligned with feeding and food choice, or vice versa, through our motivation to breathe and select particular nutrients is sparse, fragmented and lacks any integrated understanding. By addressing this, we aim to provide the foundations for a clinical perspective that reveals untapped potential, by highlighting how aberrant cell-specific changes in the expression of AMPK subunit isoforms could give rise, in part, to known associations between metabolic disease, such as obesity and type 2 diabetes, sleep-disordered breathing, pulmonary hypertension and acute respiratory distress syndrome.

Infections in patients diagnosed with multiple sclerosis: A multi-database study

BACKGROUND

Recent data on the rates of infections among patients with multiple sclerosis (MS) are sparse. The objective of this study was to quantify incidence of infections in patients with MS compared with a matched sample of patients without MS (non-MS).

METHODS

This study was conducted in two separate electronic medical databases: the United States Department of Defense (US-DOD) military health care system and the United Kingdom's Clinical Practice Research Datalink GOLD (UK-CPRD). We identified patients with a first recorded diagnosis of MS between 2001 and 2016 (UK-CPRD) or 2004 and 2017 (US-DOD) and matched non-MS patients. We identified infections recorded after the MS diagnosis date (or the matched date in non-MS patients) and calculated incidence rates (IRs) and incidence rate ratios (IRRs) with 95% confidence intervals (CIs) by infection site and type.

RESULTS

Relative to non-MS patients, MS patients had higher rates of any infection (US-DOD IRR 1.76; 95% CI 1.72-1.80 and UK-CPRD IRR 1.25; 95% CI 1.21-1.29) and a two-fold higher rate of hospitalized infections (US-DOD IRR 2.43; 95% CI 2.23-2.63 and UK-CPRD IRR 2.00; 95% CI 1.84-2.17). IRs of any infection were higher in females compared with males in both MS and non-MS patients, while IRs of hospitalized infections were similar between sexes in both MS and non-MS patients. The IR of first urinary tract or kidney infection was nearly two-fold higher in MS compared with non-MS patients (US-DOD IRR 1.88; 95% CI 1.81-1.95 and UK-CPRD IRR 1.97; 95% CI 1.86-2.09) with higher rates in females compared with males. IRs for any opportunistic infection, candidiasis and any herpes virus were increased between 20 and 52% among MS patients compared with non-MS patients. IRs of meningitis, tuberculosis, hepatitis B and C were all low.

CONCLUSION

MS patients have an increased risk of infection, notably infections of the renal tract, and a two-fold increased risk of hospitalized infections compared with non-MS patients.

The cell-wide web coordinates cellular processes by directing site-specific Ca2+ flux across cytoplasmic nanocourses

Ca²⁺ coordinates diverse cellular processes, yet how function-specific signals arise is enigmatic. We describe a cell-wide network of distinct cytoplasmic nanocourses with the nucleus at its centre, demarcated by sarcoplasmic reticulum (SR) junctions (≤400 nm across) that restrict Ca²⁺ diffusion and by nanocourse-specific Ca²⁺-pumps that facilitate signal segregation. Ryanodine receptor subtype 1 (RyR1) supports relaxation of arterial myocytes by unloading Ca²⁺ into peripheral nanocourses delimited by plasmalemma-SR junctions, fed by sarco/endoplasmic reticulum Ca²⁺ ATPase 2b (SERCA2b). Conversely, stimulus-specified increases in Ca²⁺ flux through RyR2/3 clusters selects for rapid propagation of Ca²⁺ signals throughout deeper extraperinuclear nanocourses and thus myocyte contraction. Nuclear envelope invaginations incorporating SERCA1 in their outer nuclear membranes demarcate further diverse networks of cytoplasmic nanocourses that receive Ca²⁺ signals through discrete RyR1 clusters, impacting gene expression through epigenetic marks segregated by their associated invaginations. Critically, this circuit is not hardwired and remodels for different outputs during cell proliferation.

Although calcium signals are known to be critical for many cellular processes, how signaling elicits specific functions remains unclear. In visually striking work, Duan et al. reveal that networks of cytoplasmic nanocourses orchestrate cell activity by directing site-specific calcium signals.

The LKB1-AMPK-α1 signaling pathway triggers hypoxic pulmonary vasoconstriction downstream of mitochondria

Hypoxic pulmonary vasoconstriction (HPV), which aids ventilation-perfusion matching in the lungs, is triggered by mechanisms intrinsic to pulmonary arterial smooth muscles. The unique sensitivity of these muscles to hypoxia is conferred by mitochondrial cytochrome c oxidase subunit 4 isoform 2, the inhibition of which has been proposed to trigger HPV through increased generation of mitochondrial reactive oxygen species. Contrary to this model, we have shown that the LKB1-AMPK-α1 signaling pathway is critical to HPV. Spectral Doppler ultrasound revealed that deletion of the AMPK-α1 catalytic subunit blocked HPV in mice during mild (8% O₂) and severe (5% O₂) hypoxia, whereas AMPK-α2 deletion attenuated HPV only during severe hypoxia. By contrast, neither of these genetic manipulations affected serotonin-induced reductions in pulmonary vascular flow. HPV was also attenuated by reduced expression of LKB1, a kinase that activates AMPK during energy stress, but not after deletion of CaMKK2, a kinase that activates AMPK in response to increases in cytoplasmic Ca²⁺ Fluorescence imaging of acutely isolated pulmonary arterial myocytes revealed that AMPK-α1 or AMPK-α2 deletion did not affect mitochondrial membrane potential during normoxia or hypoxia. However, deletion of AMPK-α1, but not of AMPK-α2, blocked hypoxia from inhibiting K_V1.5, the classical "oxygen-sensing" K⁺ channel in pulmonary arterial myocytes. We conclude that LKB1-AMPK-α1 signaling pathways downstream of mitochondria are critical for the induction of HPV, in a manner also supported by AMPK-α2 during severe hypoxia.

AMPK-α1 or AMPK-α2 Deletion in Smooth Muscles Does Not Affect the Hypoxic Ventilatory Response or Systemic Arterial Blood Pressure Regulation During Hypoxia

The hypoxic ventilatory response (HVR) is markedly attenuated by AMPK-α1 deletion conditional on the expression of Cre-recombinase in tyrosine hydroxylase (TH) expressing cells, precipitating marked increases in apnea frequency and duration. It was concluded that ventilatory dysfunction caused by AMPK deficiency was driven by neurogenic mechanisms. However, TH is transiently expressed in other cell types during development, and it is evident that central respiratory depression can also be triggered by myogenic mechanisms that impact blood supply to the brain. We therefore assessed the effect on the HVR and systemic arterial blood pressure of AMPK deletion in vascular smooth muscles. There was no difference in minute ventilation during normoxia. However, increases in minute ventilation during severe hypoxia (8% O₂) were, if affected at all, augmented by AMPK-α1 and AMPK-α2 deletion in smooth muscles; despite the fact that hypoxia (8% O₂) evoked falls in arterial SpO₂ comparable with controls. Surprisingly, these mice exhibited no difference in systolic, diastolic or mean arterial blood pressure during normoxia or hypoxia. We conclude that neither AMPK-α1 nor AMPK-α2 are required in smooth muscle for the regulation of systemic arterial blood pressure during hypoxia, and that AMPK-α1 deficiency does not impact the HVR by myogenic mechanisms.

mTORC1 controls lysosomal Ca2+ release through the two-pore channel TPC2

Two-pore segment channel 2 (TPC2) is a ubiquitously expressed, lysosomally targeted ion channel that aids in terminating autophagy and is inhibited upon its association with mechanistic target of rapamycin (mTOR). It is controversial whether TPC2 mediates lysosomal Ca²⁺ release or selectively conducts Na⁺ and whether the binding of nicotinic acid adenine dinucleotide phosphate (NAADP) or phosphatidylinositol 3,5-bisphosphate [PI(3,5)P₂] is required for the activity of this ion channel. We show that TPC2 is required for intracellular Ca²⁺ signaling in response to NAADP or to mTOR inhibition by rapamycin. In pulmonary arterial myocytes, rapamycin and NAADP evoked global Ca²⁺ transients that were blocked by depletion of lysosomal Ca²⁺ stores. Preincubation of cells with high concentrations of rapamycin resulted in desensitization and blocked NAADP-evoked Ca²⁺ signals. Moreover, rapamycin and NAADP did not evoke discernable Ca²⁺ transients in myocytes derived from Tpcn2 knockout mice, which showed normal responses to other Ca²⁺-mobilizing signals. In HEK293 cells stably overexpressing human TPC2, shRNA-mediated knockdown of mTOR blocked rapamycin- and NAADP-evoked Ca²⁺ signals. Confocal imaging of a genetically encoded Ca²⁺ indicator fused to TPC2 demonstrated that rapamycin-evoked Ca²⁺ signals localized to lysosomes and were in close proximity to TPC2. Therefore, inactivation of mTOR may activate TPC2 and consequently lysosomal Ca²⁺ release.

Genotoxic Damage Activates the AMPK-α1 Isoform in the Nucleus via Ca2+/CaMKK2 Signaling to Enhance Tumor Cell Survival

Many genotoxic cancer treatments activate AMP-activated protein kinase (AMPK), but the mechanisms of AMPK activation in response to DNA damage, and its downstream consequences, have been unclear. In this study, etoposide activates the α1 but not the α2 isoform of AMPK, primarily within the nucleus. AMPK activation is independent of ataxia-telangiectasia mutated (ATM), a DNA damage-activated kinase, and the principal upstream kinase for AMPK, LKB1, but correlates with increased nuclear Ca2+ and requires the Ca2+/calmodulin-dependent kinase, CaMKK2. Intriguingly, Ca2+-dependent activation of AMPK in two different LKB1-null cancer cell lines caused G1-phase cell-cycle arrest, and enhanced cell viability/survival after etoposide treatment, with both effects being abolished by knockout of AMPK-α1 and α2. The CDK4/6 inhibitor palbociclib also caused G1 arrest in G361 but not HeLa cells and, consistent with this, enhanced cell survival after etoposide treatment only in G361 cells. These results suggest that AMPK activation protects cells against etoposide by limiting entry into S-phase, where cells would be more vulnerable to genotoxic stress.Implications: These results reveal that the α1 isoform of AMPK promotes tumorigenesis by protecting cells against genotoxic stress, which may explain findings that the gene encoding AMPK-α1 (but not -α2) is amplified in some human cancers. Furthermore, α1-selective inhibitors might enhance the anticancer effects of genotoxic-based therapies. Mol Cancer Res; 16(2); 345-57. ©2017 AACR.

AMP-activated Protein Kinase Deficiency Blocks the Hypoxic Ventilatory Response and Thus Precipitates Hypoventilation and Apnea

RATIONALE

Modulation of breathing by hypoxia accommodates variations in oxygen demand and supply during, for example, sleep and ascent to altitude, but the precise molecular mechanisms of this phenomenon remain controversial. Among the genes influenced by natural selection in high-altitude populations is one for the adenosine monophosphate-activated protein kinase (AMPK) α1-catalytic subunit, which governs cell-autonomous adaptations during metabolic stress.

OBJECTIVES

We investigated whether AMPK-α1 and/or AMPK-α2 are required for the hypoxic ventilatory response and the mechanism of ventilatory dysfunctions arising from AMPK deficiency.

METHODS

We used plethysmography, electrophysiology, functional magnetic resonance imaging, and immediate early gene (c-fos) expression to assess the hypoxic ventilatory response of mice with conditional deletion of the AMPK-α1 and/or AMPK-α2 genes in catecholaminergic cells, which compose the hypoxia-responsive respiratory network from carotid body to brainstem.

MEASUREMENTS AND MAIN RESULTS

AMPK-α1 and AMPK-α2 deletion virtually abolished the hypoxic ventilatory response, and ventilatory depression during hypoxia was exacerbated under anesthesia. Rather than hyperventilating, mice lacking AMPK-α1 and AMPK-α2 exhibited hypoventilation and apnea during hypoxia, with the primary precipitant being loss of AMPK-α1 expression. However, the carotid bodies of AMPK-knockout mice remained exquisitely sensitive to hypoxia, contrary to the view that the hypoxic ventilatory response is determined solely by increased carotid body afferent input to the brainstem. Regardless, functional magnetic resonance imaging and c-fos expression revealed reduced activation by hypoxia of well-defined dorsal and ventral brainstem nuclei.

CONCLUSIONS

AMPK is required to coordinate the activation by hypoxia of brainstem respiratory networks, and deficiencies in AMPK expression precipitate hypoventilation and apnea, even when carotid body afferent input is normal.

Registered Replication Report: Rand, Greene, and Nowak (2012)

In an anonymous 4-person economic game, participants contributed more money to a common project (i.e., cooperated) when required to decide quickly than when forced to delay their decision (Rand, Greene & Nowak, 2012), a pattern consistent with the social heuristics hypothesis proposed by Rand and colleagues. The results of studies using time pressure have been mixed, with some replication attempts observing similar patterns (e.g., Rand et al., 2014) and others observing null effects (e.g., Tinghög et al., 2013; Verkoeijen & Bouwmeester, 2014). This Registered Replication Report (RRR) assessed the size and variability of the effect of time pressure on cooperative decisions by combining 21 separate, preregistered replications of the critical conditions from Study 7 of the original article (Rand et al., 2012). The primary planned analysis used data from all participants who were randomly assigned to conditions and who met the protocol inclusion criteria (an intent-to-treat approach that included the 65.9% of participants in the time-pressure condition and 7.5% in the forced-delay condition who did not adhere to the time constraints), and we observed a difference in contributions of -0.37 percentage points compared with an 8.6 percentage point difference calculated from the original data. Analyzing the data as the original article did, including data only for participants who complied with the time constraints, the RRR observed a 10.37 percentage point difference in contributions compared with a 15.31 percentage point difference in the original study. In combination, the results of the intent-to-treat analysis and the compliant-only analysis are consistent with the presence of selection biases and the absence of a causal effect of time pressure on cooperation.

Nanojunctions of the Sarcoplasmic Reticulum Deliver Site- and Function-Specific Calcium Signaling in Vascular Smooth Muscles

Vasoactive agents may induce myocyte contraction, dilation, and the switch from a contractile to a migratory-proliferative phenotype(s), which requires changes in gene expression. These processes are directed, in part, by Ca²⁺ signals, but how different Ca²⁺ signals are generated to select each function is enigmatic. We have previously proposed that the strategic positioning of Ca²⁺ pumps and release channels at membrane-membrane junctions of the sarcoplasmic reticulum (SR) demarcates cytoplasmic nanodomains, within which site- and function-specific Ca²⁺ signals arise. This chapter will describe how nanojunctions of the SR may: (1) define cytoplasmic nanospaces about the plasma membrane, mitochondria, contractile myofilaments, lysosomes, and the nucleus; (2) provide for functional segregation by restricting passive diffusion and by coordinating active ion transfer within a given nanospace via resident Ca²⁺ pumps and release channels; (3) select for contraction, relaxation, and/or changes in gene expression; and (4) facilitate the switch in myocyte phenotype through junctional reorganization. This should serve to highlight the need for further exploration of cellular nanojunctions and the mechanisms by which they operate, that will undoubtedly open up new therapeutic horizons.

The emerging role of AMPK in the regulation of breathing and oxygen supply

Regulation of breathing is critical to our capacity to accommodate deficits in oxygen availability and demand during, for example, sleep and ascent to altitude. It is generally accepted that a fall in arterial oxygen increases afferent discharge from the carotid bodies to the brainstem and thus delivers increased ventilatory drive, which restores oxygen supply and protects against hypoventilation and apnoea. However, the precise molecular mechanisms involved remain unclear. We recently identified as critical to this process the AMP-activated protein kinase (AMPK), which is key to the cell-autonomous regulation of metabolic homoeostasis. This observation is significant for many reasons, not least because recent studies suggest that the gene for the AMPK-α1 catalytic subunit has been subjected to natural selection in high-altitude populations. It would appear, therefore, that evolutionary pressures have led to AMPK being utilized to regulate oxygen delivery and thus energy supply to the body in the short, medium and longer term. Contrary to current consensus, however, our findings suggest that AMPK regulates ventilation at the level of the caudal brainstem, even when afferent input responses from the carotid body are normal. We therefore hypothesize that AMPK integrates local hypoxic stress at defined loci within the brainstem respiratory network with an index of peripheral hypoxic status, namely afferent chemosensory inputs. Allied to this, AMPK is critical to the control of hypoxic pulmonary vasoconstriction and thus ventilation-perfusion matching at the lungs and may also determine oxygen supply to the foetus by, for example, modulating utero-placental blood flow.

AMP-activated protein kinase inhibits Kv 1.5 channel currents of pulmonary arterial myocytes in response to hypoxia and inhibition of mitochondrial oxidative phosphorylation

Key points

Progression of hypoxic pulmonary hypertension is thought to be due, in part, to suppression of voltage‐gated potassium channels (K_v) in pulmonary arterial smooth muscle by hypoxia, although the precise molecular mechanisms have been unclear.

AMP‐activated protein kinase (AMPK) has been proposed to couple inhibition of mitochondrial metabolism by hypoxia to acute hypoxic pulmonary vasoconstriction and progression of pulmonary hypertension.

Inhibition of complex I of the mitochondrial electron transport chain activated AMPK and inhibited K_v1.5 channels in pulmonary arterial myocytes.

AMPK activation by 5‐aminoimidazole‐4‐carboxamide riboside, A769662 or C13 attenuated K_v1.5 currents in pulmonary arterial myocytes, and this effect was non‐additive with respect to K_v1.5 inhibition by hypoxia and mitochondrial poisons.

Recombinant AMPK phosphorylated recombinant human K_v1.5 channels in cell‐free assays, and inhibited K⁺ currents when introduced into HEK 293 cells stably expressing K_v1.5.

These results suggest that AMPK is the primary mediator of reductions in K_v1.5 channels following inhibition of mitochondrial oxidative phosphorylation during hypoxia and by mitochondrial poisons.

Abstract

Progression of hypoxic pulmonary hypertension is thought to be due, in part, to suppression of voltage‐gated potassium channels (K_v) in pulmonary arterial smooth muscle cells that is mediated by the inhibition of mitochondrial oxidative phosphorylation. We sought to determine the role in this process of the AMP‐activated protein kinase (AMPK), which is intimately coupled to mitochondrial function due to its activation by LKB1‐dependent phosphorylation in response to increases in the cellular AMP:ATP and/or ADP:ATP ratios. Inhibition of complex I of the mitochondrial electron transport chain using phenformin activated AMPK and inhibited K_v currents in pulmonary arterial myocytes, consistent with previously reported effects of mitochondrial inhibitors. Myocyte K_v currents were also markedly inhibited upon AMPK activation by A769662, 5‐aminoimidazole‐4‐carboxamide riboside and C13 and by intracellular dialysis from a patch‐pipette of activated (thiophosphorylated) recombinant AMPK heterotrimers (α2β2γ1 or α1β1γ1). Hypoxia and inhibitors of mitochondrial oxidative phosphorylation reduced AMPK‐sensitive K⁺ currents, which were also blocked by the selective K_v1.5 channel inhibitor diphenyl phosphine oxide‐1 but unaffected by the presence of the BK_Ca channel blocker paxilline. Moreover, recombinant human K_v1.5 channels were phosphorylated by AMPK in cell‐free assays, and K⁺ currents carried by K_v1.5 stably expressed in HEK 293 cells were inhibited by intracellular dialysis of AMPK heterotrimers and by A769662, the effects of which were blocked by compound C. We conclude that AMPK mediates K_v channel inhibition by hypoxia in pulmonary arterial myocytes, at least in part, through phosphorylation of K_v1.5 and/or an associated protein.

Progression of hypoxic pulmonary hypertension is thought to be due, in part, to suppression of voltage‐gated potassium channels (K_v) in pulmonary arterial smooth muscle by hypoxia, although the precise molecular mechanisms have been unclear.

AMP‐activated protein kinase (AMPK) has been proposed to couple inhibition of mitochondrial metabolism by hypoxia to acute hypoxic pulmonary vasoconstriction and progression of pulmonary hypertension.

Inhibition of complex I of the mitochondrial electron transport chain activated AMPK and inhibited K_v1.5 channels in pulmonary arterial myocytes.

AMPK activation by 5‐aminoimidazole‐4‐carboxamide riboside, A769662 or C13 attenuated K_v1.5 currents in pulmonary arterial myocytes, and this effect was non‐additive with respect to K_v1.5 inhibition by hypoxia and mitochondrial poisons.

Recombinant AMPK phosphorylated recombinant human K_v1.5 channels in cell‐free assays, and inhibited K⁺ currents when introduced into HEK 293 cells stably expressing K_v1.5.

These results suggest that AMPK is the primary mediator of reductions in K_v1.5 channels following inhibition of mitochondrial oxidative phosphorylation during hypoxia and by mitochondrial poisons.

Exome Sequencing and the Management of Neurometabolic Disorders

BACKGROUND

Whole-exome sequencing has transformed gene discovery and diagnosis in rare diseases. Translation into disease-modifying treatments is challenging, particularly for intellectual developmental disorder. However, the exception is inborn errors of metabolism, since many of these disorders are responsive to therapy that targets pathophysiological features at the molecular or cellular level.

METHODS

To uncover the genetic basis of potentially treatable inborn errors of metabolism, we combined deep clinical phenotyping (the comprehensive characterization of the discrete components of a patient's clinical and biochemical phenotype) with whole-exome sequencing analysis through a semiautomated bioinformatics pipeline in consecutively enrolled patients with intellectual developmental disorder and unexplained metabolic phenotypes.

RESULTS

We performed whole-exome sequencing on samples obtained from 47 probands. Of these patients, 6 were excluded, including 1 who withdrew from the study. The remaining 41 probands had been born to predominantly nonconsanguineous parents of European descent. In 37 probands, we identified variants in 2 genes newly implicated in disease, 9 candidate genes, 22 known genes with newly identified phenotypes, and 9 genes with expected phenotypes; in most of the genes, the variants were classified as either pathogenic or probably pathogenic. Complex phenotypes of patients in five families were explained by coexisting monogenic conditions. We obtained a diagnosis in 28 of 41 probands (68%) who were evaluated. A test of a targeted intervention was performed in 18 patients (44%).

CONCLUSIONS

Deep phenotyping and whole-exome sequencing in 41 probands with intellectual developmental disorder and unexplained metabolic abnormalities led to a diagnosis in 68%, the identification of 11 candidate genes newly implicated in neurometabolic disease, and a change in treatment beyond genetic counseling in 44%. (Funded by BC Children's Hospital Foundation and others.).

Hypoxia, AMPK activation and uterine artery vasoreactivity

Genes near adenosine monophosphate-activated protein kinase-α1 (PRKAA1) have been implicated in the greater uterine artery (UtA) blood flow and relative protection from fetal growth restriction seen in altitude-adapted Andean populations. Adenosine monophosphate-activated protein kinase (AMPK) activation vasodilates multiple vessels but whether AMPK is present in UtA or placental tissue and influences UtA vasoreactivity during normal or hypoxic pregnancy remains unknown. We studied isolated UtA and placenta from near-term C57BL/6J mice housed in normoxia (n = 8) or hypoxia (10% oxygen, n = 7-9) from day 14 to day 19, and placentas from non-labouring sea level (n = 3) or 3100 m (n = 3) women. Hypoxia increased AMPK immunostaining in near-term murine UtA and placental tissue. RT-PCR products for AMPK-α1 and -α2 isoforms and liver kinase B1 (LKB1; the upstream kinase activating AMPK) were present in murine and human placenta, and hypoxia increased LKB1 and AMPK-α1 and -α2 expression in the high- compared with low-altitude human placentas. Pharmacological AMPK activation by A769662 caused phenylephrine pre-constricted UtA from normoxic or hypoxic pregnant mice to dilate and this dilatation was partially reversed by the NOS inhibitor l-NAME. Hypoxic pregnancy sufficient to restrict fetal growth markedly augmented the UtA vasodilator effect of AMPK activation in opposition to PE constriction as the result of both NO-dependent and NO-independent mechanisms. We conclude that AMPK is activated during hypoxic pregnancy and that AMPK activation vasodilates the UtA, especially in hypoxic pregnancy. AMPK activation may be playing an adaptive role by limiting cellular energy depletion and helping to maintain utero-placental blood flow in hypoxic pregnancy.

Inactivation of corticotropin-releasing hormone-induced insulinotropic role by high-altitude hypoxia

We have shown that hypoxia reduces plasma insulin, which correlates with corticotropin-releasing hormone (CRH) receptor 1 (CRHR1) in rats, but the mechanism remains unclear. Here, we report that hypobaric hypoxia at an altitude of 5,000 m for 8 h enhances rat plasma CRH, corticosterone, and glucose levels, whereas the plasma insulin and pancreatic ATP/ADP ratio is reduced. In islets cultured under normoxia, CRH stimulated insulin release in a glucose- and CRH-level-dependent manner by activating CRHR1 and thus the cAMP-dependent protein kinase pathway and calcium influx through L-type channels. In islets cultured under hypoxia, however, the insulinotropic effect of CRH was inactivated due to reduced ATP and cAMP and coincident loss of intracellular calcium oscillations. Serum and glucocorticoid-inducible kinase 1 (SGK1) also played an inhibitory role. In human volunteers rapidly ascended to 3,860 m, plasma CRH and glucose levels increased without a detectable change in plasma insulin. By contrast, volunteers with acute mountain sickness (AMS) exhibited a marked decrease in HOMA insulin sensitivity (HOMA-IS) and enhanced plasma CRH. In conclusion, hypoxia may attenuate the CRH-insulinotropic effect by reducing cellular ATP/ADP ratio, cAMP and calcium influx, and upregulated SGK1. Hypoxia may not affect HOMA-IS in healthy volunteers but reduces it in AMS volunteers.

Modulation of the LKB1-AMPK Signalling Pathway Underpins Hypoxic Pulmonary Vasoconstriction and Pulmonary Hypertension

Perhaps the defining characteristic of pulmonary arteries is the process of hypoxic pulmonary vasoconstriction (HPV) which, under physiological conditions, supports ventilation-perfusion matching in the lung by diverting blood flow away from oxygen deprived areas of the lung to oxygen rich regions. However, when alveolar hypoxia is more widespread, either at altitude or with disease (e.g., cystic fibrosis), HPV may lead to hypoxic pulmonary hypertension. HPV is driven by the intrinsic response to hypoxia of pulmonary arterial smooth muscle and endothelial cells, which are acutely sensitive to relatively small changes in pO2 and have evolved to monitor oxygen supply and thus address ventilation-perfusion mismatch. There is now a consensus that the inhibition by hypoxia of mitochondrial oxidative phosphorylation represents a key step towards the induction of HPV, but the precise nature of the signalling pathway(s) engaged thereafter remains open to debate. We will consider the role of the AMP-activated protein kinase (AMPK) and liver kinase B1 (LKB1), an upstream kinase through which AMPK is intimately coupled to changes in oxygen supply via mitochondrial metabolism. A growing body of evidence, from our laboratory and others, suggests that modulation of the LKB1-AMPK signalling pathway underpins both hypoxic pulmonary vasoconstriction and the development of pulmonary hypertension.

Lysosomal two-pore channel subtype 2 (TPC2) regulates skeletal muscle autophagic signaling

Postnatal skeletal muscle mass is regulated by the balance between anabolic protein synthesis and catabolic protein degradation, and muscle atrophy occurs when protein homeostasis is disrupted. Autophagy has emerged as critical in clearing dysfunctional organelles and thus in regulating protein turnover. Here we show that endolysosomal two-pore channel subtype 2 (TPC2) contributes to autophagy signaling and protein homeostasis in skeletal muscle. Muscles derived from Tpcn2^−/− mice exhibit an atrophic phenotype with exacerbated autophagy under starvation. Compared with wild types, animals lacking TPC2 demonstrated an enhanced autophagy flux characterized by increased accumulation of autophagosomes upon combined stress induction by starvation and colchicine treatment. In addition, deletion of TPC2 in muscle caused aberrant lysosomal pH homeostasis and reduced lysosomal protease activity. Association between mammalian target of rapamycin and TPC2 was detected in skeletal muscle, allowing for appropriate adjustments to cellular metabolic states and subsequent execution of autophagy. TPC2 therefore impacts mammalian target of rapamycin reactivation during the process of autophagy and contributes to maintenance of muscle homeostasis.

Organelle-specific subunit interactions of the vertebrate two-pore channel family

The organellar targeting of two-pore channels (TPCs) and their capacity to associate as homo- and heterodimers may be critical to endolysosomal signaling. A more detailed understanding of the functional association of vertebrate TPC1-3 is therefore necessary. We report here that when stably expressed in HEK293 cells, human (h) TPC1 and chicken (c) TPC3 were specifically targeted to different subpopulations of endosomes, hTPC2 was specifically targeted to lysosomes, and rabbit (r) TPC3 was specifically targeted to both endosomes and lysosomes. Intracellular dialysis of NAADP evoked a Ca(2+) transient in HEK293 cells that stably overexpressed hTPC1, hTPC2, and rTPC3, but not in cells that stably expressed cTPC3. The Ca(2+) transients induced in cells that overexpressed endosome-targeted hTPC1 were abolished upon depletion of acidic Ca(2+) stores by bafilomycin A1, but remained unaffected following depletion of endoplasmic reticulum stores by thapsigargin. In contrast, Ca(2+) transients induced via lysosome-targeted hTPC2 and endolysosome-targeted rTPC3 were abolished by bafilomycin A1 and markedly attenuated by thapsigargin. NAADP induced marked Ca(2+) transients in HEK293 cells that stably coexpressed hTPC2 with hTPC1 or cTPC3, but failed to evoke any such response in cells that coexpressed interacting hTPC2 and rTPC3 subunits. We therefore conclude that 1) all three TPC subtypes may support Ca(2+) signaling from their designate acidic stores, and 2) lysosome-targeted (but not endosome-targeted) TPCs support coupling to the endoplasmic reticulum.

Cytoplasmic nanojunctions between lysosomes and sarcoplasmic reticulum are required for specific calcium signaling

Herein we demonstrate how nanojunctions between lysosomes and sarcoplasmic reticulum (L-SR junctions) serve to couple lysosomal activation to regenerative, ryanodine receptor-mediated cellular Ca ²⁺ waves. In pulmonary artery smooth muscle cells (PASMCs) it has been proposed that nicotinic acid adenine dinucleotide phosphate (NAADP) triggers increases in cytoplasmic Ca ²⁺ via L-SR junctions, in a manner that requires initial Ca ²⁺ release from lysosomes and subsequent Ca ²⁺-induced Ca ²⁺ release (CICR) via ryanodine receptor (RyR) subtype 3 on the SR membrane proximal to lysosomes. L-SR junction membrane separation has been estimated to be < 400 nm and thus beyond the resolution of light microscopy, which has restricted detailed investigations of the junctional coupling process. The present study utilizes standard and tomographic transmission electron microscopy to provide a thorough ultrastructural characterization of the L-SR junctions in PASMCs. We show that L-SR nanojunctions are prominent features within these cells and estimate that the junctional membrane separation and extension are about 15 nm and 300 nm, respectively. Furthermore, we develop a quantitative model of the L-SR junction using these measurements, prior kinetic and specific Ca ²⁺ signal information as input data. Simulations of NAADP-dependent junctional Ca ²⁺ transients demonstrate that the magnitude of these signals can breach the threshold for CICR via RyR3. By correlation analysis of live cell Ca ²⁺ signals and simulated Ca ²⁺ transients within L-SR junctions, we estimate that “trigger zones” comprising 60–100 junctions are required to confer a signal of similar magnitude. This is compatible with the 110 lysosomes/cell estimated from our ultrastructural observations. Most importantly, our model shows that increasing the L-SR junctional width above 50 nm lowers the magnitude of junctional [Ca ²⁺] such that there is a failure to breach the threshold for CICR via RyR3. L-SR junctions are therefore a pre-requisite for efficient Ca ²⁺signal coupling and may contribute to cellular function in health and disease.

Pan-junctional sarcoplasmic reticulum in vascular smooth muscle: nanospace Ca2+ transport for site- and function-specific Ca2+ signalling

This review focuses on how smooth muscle sarcoplasmic reticulum (SR), the major releasable Ca(2+) store in these cells, performs its many functions by communicating with the plasma membrane (PM) and other organelles across cytoplasmic nanospaces, defined by membrane-membrane junctions less than 50 nm across. In spite of accumulating evidence in favour of the view that cytoplasmic nanospaces are a prerequisite for effective control of diverse cellular functions, our current understanding of how smooth muscle cells accomplish site- and function-specific Ca(2+) signalling remains in its infancy. We first present evidence in support of the view that effective Ca(2+) signalling depends on the restricted diffusion of Ca(2+) within cytoplasmic nanospaces. We then develop an evidence-based model of the smooth muscle SR - the 'pan-junctional SR' model - that incorporates a network of tubules and quilts that are capable of auto-regulating their Ca(2+) content and determining junctional [Ca(2+)]i through loading and unloading at membrane-membrane nanojunctions. Thereby, we provide a novel working hypothesis in order to inform future investigation into the control of a variety of cellular functions by local Ca(2+) signals at junctional nanospaces, from contraction and energy metabolism to nuclear transcription. Based on the current literature, we discuss the molecular mechanisms whereby the SR mediates these multiple functions through the interaction of ion channels and pumps embedded in apposing membranes within inter-organellar junctions. We finally highlight the fact that although most current hypotheses are qualitatively supported by experimental data, solid quantitative simulations are seriously lacking. Considering that at physiological concentrations the number of calcium ions in a typical junctional nanospace between the PM and SR is of the order of 1, ion concentration variability plays a major role as the currency of information transfer and stochastic quantitative modelling will be required to both test and develop working hypotheses.

Ion channel regulation by the LKB1-AMPK signalling pathway: the key to carotid body activation by hypoxia and metabolic homeostasis at the whole body level

Our recent investigations provide further support for the proposal that, consequent to inhibition of mitochondrial oxidative phosphorylation, activation of AMP-activated protein kinase (AMPK) mediates carotid body excitation by hypoxia. Consistent with the effects of hypoxia, intracellular dialysis from a patch pipette of an active (thiophosphorylated) recombinant AMPK heterotrimer (α2β2γ1) or application of the AMPK activators AICAR and A769662: (1) Inhibited BK(Ca) currents and TASK K(+) currents in rat carotid body type I cells; (2) Inhibited whole-cell currents carried by KCa1.1 and TASK3, but not TASK1 channels expressed in HEK293 cells; (3) Triggered carotid body activation. Furthermore, preliminary studies using mice with conditional knockout in type I cells of the primary upstream kinase that activates AMPK in response to metabolic stresses, LKB1, appear to confirm our working hypothesis. Studies on mice with knockout of the catalytic α1 subunit and α2 subunits of AMPK, respectively, have proved equally consistent. Accumulating evidence therefore suggests that the LKB1-AMPK signalling pathway is necessary for hypoxia-response coupling by the carotid body, and serves to regulate oxygen and therefore energy supply at the whole body level.

Two-pore channels for integrative Ca signaling

Two-pore channels (TPCs) are related to voltage-gated Ca(2+) and Na(+) channels. They most likely work as dimers with each of the two TPC protein subunits containing two pore-forming domains. Recent studies suggest that TPCs are expressed on the membranes of endosomes and lysosomes where they form receptors for nicotinic acid adenine dinucleotide phosphate (NAADP), the most potent Ca(2+) mobilizing messenger inside cells. Upon activation by NAADP, Ca(2+) release from endolysosomal stores through TPCs triggers cytoplasmic Ca(2+) signals. Because of discrete localizations of these acidic vesicles and their small, albeit variable, sizes, the Ca(2+) signals from endolysosomes are local and, perhaps, represent unique elementary Ca(2+) events. These localized signals can be converted into regenerative global Ca(2+) waves by triggering Ca(2+)-induced Ca(2+) release from endoplasmic reticulum. We will discuss the implications of these findings and the significance of TPCs in integrative Ca(2+) signaling in animal cells.

Long-chain acylcarnitine deficiency in patients with chronic fatigue syndrome. Potential involvement of altered carnitine palmitoyltransferase-I activity

OBJECTIVE

The underlying aetiology of chronic fatigue syndrome is currently unknown; however, in the light of carnitine's critical role in mitochondrial energy production, it has been suggested that chronic fatigue syndrome may be associated with altered carnitine homeostasis. This study was conducted to comparatively examine full endogenous carnitine profiles in patients with chronic fatigue syndrome and healthy controls.

DESIGN

A cross-sectional, observational study.

SETTING AND SUBJECTS

Forty-four patients with chronic fatigue syndrome and 49 age- and gender-matched healthy controls were recruited from the community and studied at the School of Pharmacy & Medical Sciences, University of South Australia.

MAIN OUTCOME MEASURES

All participants completed a fatigue severity scale questionnaire and had a single fasting blood sample collected which was analysed for l-carnitine and 35 individual acylcarnitine concentrations in plasma by LC-MS/MS.

RESULTS

Patients with chronic fatigue syndrome exhibited significantly altered concentrations of C8:1, C12DC, C14, C16:1, C18, C18:1, C18:2 and C18:1-OH acylcarnitines; of particular note, oleyl-L-carnitine (C18:1) and linoleyl-L-carnitine (C18:2) were, on average, 30-40% lower in patients than controls (P < 0.0001). Significant correlations between acylcarnitine concentrations and clinical symptomology were also demonstrated.

CONCLUSIONS

It is proposed that this disturbance in carnitine homeostasis is reflective of a reduction in carnitine palmitoyltransferase-I (CPT-I) activity, possibly a result of the accumulation of omega-6 fatty acids previously observed in this patient population. It is hypothesized that the administration of omega-3 fatty acids in combination with l-carnitine would increase CPT-I activity and improve chronic fatigue syndrome symptomology.

A Cochrane review of the evidence for non-surgical interventions for flexible pediatric flat feet

The pediatric flat foot is a frequent presentation in clinical practice, a common concern to parents and continues to be debated within professional ranks. As an entity, it is confused by varied classifications, the notion of well-intended prevention and unsubstantiated, if common, treatment. The available prevalence estimates are all limited by variable sampling, assessment measures and age groups and hence result in disparate findings (0.6-77.9%). Consistently, flat foot has been found to normally reduce with age. The normal findings of flat foot versus children's age estimates that approximately 45% of preschool children, and 15% of older children (average age 10 years) have flat feet. Few flexible flat feet have been found to be symptomatic. Joint hypermobility and increased weight or obesity may increase flat foot prevalence, independently of age. Most attempts at classification of flat foot morphology include the arch, heel position and foot flexibility. Usual assessment methods are footprint measures, X-rays and visual (scaled) observations. There is no standardized framework from which to evaluate the pediatric flat foot. The pediatric flat foot is often unnecessarily treated, being ill-defined and of uncertain prognosis. Contemporary management of the pediatric flat foot is directed algorithmically within this review, according to pain, age, flexibility; considering gender, weight, and joint hypermobility. When foot orthoses are indicated, inexpensive generic appliances will usually suffice. Customised foot orthoses should be reserved for children with foot pain and arthritis, for unusual morphology, or unresponsive cases. Surgery is rarely indicated for pediatric flat foot (unless rigid) and only at the failure of thorough conservative management. The assessment of the pediatric flatfoot needs to be considered with reference to the epidemiological findings, where there is consensus that pediatric flexible flat foot reduces with age and that most children are asymptomatic. Globally, there is need for a standard by which the pediatric flat foot is assessed classified and managed. Until then, assessment should utilize the available evidence-based management model, the p-FFP Future research needs to evaluate the pediatric flat foot from representative samples, of healthy and known disease-group children prospectively, and using validated assessment instruments. The preliminary findings of the benefits of foot exercises, and discrete investigation into the effects of shoes and footwear use are also warranted.

Cyclic adenosine diphosphate ribose activates ryanodine receptors, whereas NAADP activates two-pore domain channels

The mechanism by which cyclic adenosine diphosphate ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP) mobilize intracellular Ca(2+) stores remains controversial. It is open to question whether cADPR regulates ryanodine receptors (RyRs) directly, as originally proposed, or indirectly by promoting Ca(2+) uptake into the sarco/endoplasmic reticulum by sarco/endoplasmic reticulum Ca(2+)-ATPases. Conversely, although we have proposed that NAADP mobilizes endolysosomal Ca(2+) stores by activating two-pore domain channels (TPCs), others suggest that NAADP directly activates RyRs. We therefore assessed Ca(2+) signals evoked by intracellular dialysis from a patch pipette of cADPR and NAADP into HEK293 cells that stably overexpress either TPC1, TPC2, RyR1, or RyR3. No change in intracellular Ca(2+) concentration was triggered by cADPR in either wild-type HEK293 cells (which are devoid of RyRs) or in cells that stably overexpress TPC1 and TPC2, respectively. By contrast, a marked Ca(2+) transient was triggered by cADPR in HEK293 cells that stably expressed RyR1 and RyR3. The Ca(2+) transient was abolished following depletion of endoplasmic reticulum stores by thapsigargin and block of RyRs by dantrolene but not following depletion of acidic Ca(2+) stores by bafilomycin. By contrast, NAADP failed to evoke a Ca(2+) transient in HEK293 cells that expressed RyR1 or RyR3, but it induced robust Ca(2+) transients in cells that stably overexpressed TPC1 or TPC2 and in a manner that was blocked following depletion of acidic stores by bafilomycin. We conclude that cADPR triggers Ca(2+) release by activating RyRs but not TPCs, whereas NAADP activates TPCs but not RyRs.

Cyclic adenosine diphosphate ribose activates ryanodine receptors, whereas NAADP activates two-pore domain channels

The mechanism by which cyclic adenosine diphosphate ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP) mobilize intracellular Ca(2+) stores remains controversial. It is open to question whether cADPR regulates ryanodine receptors (RyRs) directly, as originally proposed, or indirectly by promoting Ca(2+) uptake into the sarco/endoplasmic reticulum by sarco/endoplasmic reticulum Ca(2+)-ATPases. Conversely, although we have proposed that NAADP mobilizes endolysosomal Ca(2+) stores by activating two-pore domain channels (TPCs), others suggest that NAADP directly activates RyRs. We therefore assessed Ca(2+) signals evoked by intracellular dialysis from a patch pipette of cADPR and NAADP into HEK293 cells that stably overexpress either TPC1, TPC2, RyR1, or RyR3. No change in intracellular Ca(2+) concentration was triggered by cADPR in either wild-type HEK293 cells (which are devoid of RyRs) or in cells that stably overexpress TPC1 and TPC2, respectively. By contrast, a marked Ca(2+) transient was triggered by cADPR in HEK293 cells that stably expressed RyR1 and RyR3. The Ca(2+) transient was abolished following depletion of endoplasmic reticulum stores by thapsigargin and block of RyRs by dantrolene but not following depletion of acidic Ca(2+) stores by bafilomycin. By contrast, NAADP failed to evoke a Ca(2+) transient in HEK293 cells that expressed RyR1 or RyR3, but it induced robust Ca(2+) transients in cells that stably overexpressed TPC1 or TPC2 and in a manner that was blocked following depletion of acidic stores by bafilomycin. We conclude that cADPR triggers Ca(2+) release by activating RyRs but not TPCs, whereas NAADP activates TPCs but not RyRs.

Selective expression in carotid body type I cells of a single splice variant of the large conductance calcium- and voltage-activated potassium channel confers regulation by AMP-activated protein kinase

Inhibition of large conductance calcium-activated potassium (BK_Ca) channels mediates, in part, oxygen sensing by carotid body type I cells. However, BK_Ca channels remain active in cells that do not serve to monitor oxygen supply. Using a novel, bacterially derived AMP-activated protein kinase (AMPK), we show that AMPK phosphorylates and inhibits BK_Ca channels in a splice variant-specific manner. Inclusion of the stress-regulated exon within BK_Ca channel α subunits increased the stoichiometry of phosphorylation by AMPK when compared with channels lacking this exon. Surprisingly, however, the increased phosphorylation conferred by the stress-regulated exon abolished BK_Ca channel inhibition by AMPK. Point mutation of a single serine (Ser-657) within this exon reduced channel phosphorylation and restored channel inhibition by AMPK. Significantly, RT-PCR showed that rat carotid body type I cells express only the variant of BK_Ca that lacks the stress-regulated exon, and intracellular dialysis of bacterially expressed AMPK markedly attenuated BK_Ca currents in these cells. Conditional regulation of BK_Ca channel splice variants by AMPK may therefore determine the response of carotid body type I cells to hypoxia.

An emerging role for NAADP-mediated Ca2+ signaling in the pancreatic β-cell

Several recent reports, including one in this journal, have reignited the debate about whether the calcium-mobilizing messenger, nicotinic adenine nucleotide diphosphate (NAADP) plays a central role in the regulation of calcium signalling in pancreatic β-cell. These studies have highlighted a role for NAADP-induced Ca(2+) mobilization not only in mediating the effects of the incretin, GLP-1 and the autocrine proliferative effects of insulin, but also possibly a fundamental role in glucose-mediated insulin secretion in the pancreatic β-cell.

Mechanisms for acute oxygen sensing in the carotid body

Hypoxic chemotransduction in the carotid body requires release of excitatory transmitters from type I cells that activate afferent sensory neurones. Transmitter release is dependent on voltage-gated Ca2+ entry which is evoked by membrane depolarization. This excitatory response to hypoxia is initiated by inhibition of specific O2 sensitive K+ channels, of which several types have been reported. Here, we discuss mechanisms which have been put forward to account for hypoxic inhibition of type I cell K+ channels. Whilst evidence indicates that one O2 sensitive K+ channel, BKCa, may be regulated by gasotransmitters (CO and H2S) in an O2-dependent manner, other studies now indicate that activation of AMP-activated protein kinase (AMPK) accounts for inhibition of both BKCa and 'leak' O2 sensitive K+ channels, and perhaps also other O2 sensitive K+ channels reported in different species. We propose that type I cell AMPK activation occurs as a result of inhibition of mitochondrial oxidative phosphorylation, and does not require increased production of reactive oxygen species. Thus, AMPK activation provides the basis for unifying the 'membrane' and 'mitochondrial' hypotheses, previously regarded as disparate, to account for hypoxic chemotransduction.

Use of cells expressing gamma subunit variants to identify diverse mechanisms of AMPK activation

A wide variety of agents activate AMPK, but in many cases the mechanisms remain unclear. We generated isogenic cell lines stably expressing AMPK complexes containing AMP-sensitive (wild-type, WT) or AMP-insensitive (R531G) gamma2 variants. Mitochondrial poisons such as oligomycin and dinitrophenol only activated AMPK in WT cells, as did AICAR, 2-deoxyglucose, hydrogen peroxide, metformin, phenformin, galegine, troglitazone, phenobarbital, resveratrol, and berberine. Excluding AICAR, all of these also inhibited cellular energy metabolism, shown by increases in ADP:ATP ratio and/or by decreases in cellular oxygen uptake measured using an extracellular flux analyzer. By contrast, A769662, the Ca(2+) ionophore, A23187, osmotic stress, and quercetin activated both variants to varying extents. A23187 and osmotic stress also increased cytoplasmic Ca(2+), and their effects were inhibited by STO609, a CaMKK inhibitor. Our approaches distinguish at least six different mechanisms for AMPK activation and confirm that the widely used antidiabetic drug metformin activates AMPK by inhibiting mitochondrial respiration.

Identification of functionally segregated sarcoplasmic reticulum calcium stores in pulmonary arterial smooth muscle

In pulmonary arterial smooth muscle, Ca(2+) release from the sarcoplasmic reticulum (SR) via ryanodine receptors (RyRs) may induce constriction and dilation in a manner that is not mutually exclusive. We show here that the targeting of different sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPases (SERCA) and RyR subtypes to discrete SR regions explains this paradox. Western blots identified protein bands for SERCA2a and SERCA2b, whereas immunofluorescence labeling of isolated pulmonary arterial smooth muscle cells revealed striking differences in the spatial distribution of SERCA2a and SERCA2b and RyR1, RyR2, and RyR3, respectively. Almost all SERCA2a and RyR3 labeling was restricted to a region within 1.5 microm of the nucleus. In marked contrast, SERCA2b labeling was primarily found within 1.5 microm of the plasma membrane, where labeling for RyR1 was maximal. The majority of labeling for RyR2 lay in between these two regions of the cell. Application of the vasoconstrictor endothelin-1 induced global Ca(2+) waves in pulmonary arterial smooth muscle cells, which were markedly attenuated upon depletion of SR Ca(2+) stores by preincubation of cells with the SERCA inhibitor thapsigargin but remained unaffected after preincubation of cells with a second SERCA antagonist, cyclopiazonic acid. We conclude that functionally segregated SR Ca(2+) stores exist within pulmonary arterial smooth muscle cells. One sits proximal to the plasma membrane, receives Ca(2+) via SERCA2b, and likely releases Ca(2+) via RyR1 to mediate vasodilation. The other is located centrally, receives Ca(2+) via SERCA2a, and likely releases Ca(2+) via RyR3 and RyR2 to initiate vasoconstriction.

Effect of ketoprofen and its enantiomers on the renal disposition of methotrexate in the isolated perfused rat kidney

Non-steroidal anti-inflammatory drugs (NSAIDs) have been shown to inhibit the renal tubular secretion of methotrexate. However, the relative contribution of the active S- and inactive R-enantiomers is unknown. This study examined the effect of racemic ketoprofen and its enantiomers on the renal disposition of methotrexate in the isolated perfused rat kidney (IPK). Nineteen kidneys were divided between a control and three treatment groups. Controls were perfused with methotrexate alone (25 μg mL−1, n = 5) over three 30-min periods. Treatment groups were perfused with methotrexate (25 μg mL−1) for the first period, followed by a second period of methotrexate (25 μg mL−1) plus R- (n = 5), S- (n = 5) or RS-ketoprofen (n = 4) at 25 μg mL−1, and a third period of methotrexate (25 μg mL−1) plus R-, S- or RS-ketoprofen (50 μg mL−1). Perfusate and urine were collected over 10-min intervals. Methotrexate was measured by HPLC and its binding in perfusate by ultrafiltration. The clearance ratio (CR) for methotrexate was obtained by dividing the renal clearance by the product of its fraction unbound and the glomerular filtration rate. During control experiments, there was no significant change in the CR over 90 min. R-, S- and RS-ketoprofen at 50 μg mL−1 reduced the CR of methotrexate significantly, but there was no difference between the three groups. While the enantiomers of ketoprofen reduced the renal excretion of methotrexate, the interaction was not enantioselective.

TPCs: Endolysosomal channels for Ca2+ mobilization from acidic organelles triggered by NAADP

Two-pore channels (TPCs or TPCNs) are novel members of the large superfamily of voltage-gated cation channels with slightly higher sequence homology to the pore-forming subunits of voltage-gated Ca(2+) and Na(+) channels than most other members. Recent studies demonstrate that TPCs locate to endosomes and lysosomes and form Ca(2+) release channels that respond to activation by the Ca(2+) mobilizing messenger, nicotinic acid adenine dinucleotide phosphate (NAADP). With multiple endolysosomal targeted NAADP receptors now identified, important new insights into the regulation of endolysosomal function in health and disease will therefore be unveiled.

Calcium signaling via two-pore channels: local or global, that is the question

Recently, we identified, for the first time, two-pore channels (TPCs, TPCN for gene name) as a novel family of nicotinic acid adenine dinucleotide phosphate (NAADP)-gated, endolysosome-targeted calcium release channels. Significantly, three subtypes of TPCs have been characterized, TPC1-3, with each being targeted to discrete acidic calcium stores, namely lysosomes (TPC2) and endosomes (TPC1 and TPC3). That TPCs act as NAADP-gated calcium release channels is clear, given that NAADP binds to high- and low-affinity sites associated with TPC2 and thereby induces calcium release and homologous desensitization, as observed in the case of endogenous NAADP receptors. Moreover, NAADP-evoked calcium signals via TPC2 are ablated by short hairpin RNA knockdown of TPC2 and by depletion of acidic calcium stores with bafilomycin. Importantly, however, NAADP-evoked calcium signals were biphasic in nature, with an initial phase of calcium release from lysosomes via TPC2, being subsequently amplified by calcium-induced calcium release (CICR) from the endoplasmic reticulum (ER). In marked contrast, calcium release via endosome-targeted TPC1 induced only spatially restricted calcium signals that were not amplified by CICR from the ER. These findings provide new insights into the mechanisms that cells may utilize to "filter" calcium signals via junctional complexes to determine whether a given signal remains local or is converted into a propagating global signal. Essentially, endosomes and lysosomes represent vesicular calcium stores, quite unlike the ER network, and TPCs do not themselves support CICR or, therefore, propagating regenerative calcium waves. Thus "quantal" vesicular calcium release via TPCs must subsequently recruit inositol 1,4,5-trisphoshpate receptors and/or ryanodine receptors on the ER by CICR to evoke a propagating calcium wave. This may call for a revision of current views on the mechanisms of intracellular calcium signaling. The purpose of this review is, therefore, to provide an appropriate framework for future studies in this area.

Ion channel regulation by AMPK: the route of hypoxia-response coupling in thecarotid body and pulmonary artery

Vital homeostatic mechanisms monitor O2 supply and adjust respiratory and circulatory function to meet demand. The pulmonary arteries and carotid bodies are key systems in this respect. Hypoxic pulmonary vasoconstriction (HPV) aids ventilation-perfusion matching in the lung by diverting blood flow from areas with an O2 deficit to those rich in O2, while a fall in arterial pO2 increases sensory afferent discharge from the carotid body to elicit corrective changes in breathing patterns. We discuss here the new concept that hypoxia, by inhibiting oxidative phosphorylation, activates AMP-activated protein kinase (AMPK) leading to consequent phosphorylation of target proteins, such as ion channels, which initiate pulmonary artery constriction and carotid body activation. Consistent with this view, AMPK knockout mice exhibit an impaired ventilatory response to hypoxia. Thus, AMPK may be sufficient and necessary for hypoxia-response coupling and may regulate O2 and thereby energy (ATP) supply at the whole body as well as the cellular level.

Modulation of O(2) sensitive K (+) channels by AMP-activated protein kinase

Hypoxic inhibition of K(+) channels in type I cells is believed to be of central importance in carotid body chemotransduction. We have recently suggested that hypoxic channel inhibition is mediated by AMP-activated protein kinase (AMPK). Here, we have further explored the modulation by AMPK of recombinant K(+) channels (expressed in HEK293 cells) whose native counterparts are considered O(2)-sensitive in the rat carotid body. Inhibition of maxiK channels by AMPK activation with AICAR was found to be independent of [Ca(2+)](i) and occurred regardless of whether the alpha subunit was co-expressed with an auxiliary beta subunit. All effects of AICAR were fully reversed by the AMPK inhibitor compound C. MaxiK channels were also inhibited by the novel AMPK activator A-769662 and by intracellular dialysis with the constitutively active, truncated AMPK mutant, T172D. The molecular identity of the O(2)-sensitive leak K(+) conductance in rat type I cells remains unclear, but shares similarities with TASK-1 and TASK-3. Recombinant TASK-1 was insensitive to AICAR. However, TASK-3 was inhibited by either AICAR or A-769662 in a manner which was reversed by compound C. These data highlight a role for AMPK in the modulation of two proposed O(2) sensitive K(+) channels found in the carotid body.

The acid test: the discovery of two-pore channels (TPCs) as NAADP-gated endolysosomal Ca(2+) release channels

In this review, we describe the background and implications of our recent discovery that two-pore channels (TPCs) comprise a novel class of calcium release channels gated by the intracellular messenger nicotinic acid adenine dinucleotide phosphate (NAADP). Their localisation to the endolysosomal system highlights a new function for these organelles as targets for NAADP-mediated Ca(2+) mobilisation. In addition, we describe how TPCs may also trigger further Ca(2+) release by coupling to the endoplasmic reticular stores through activation of IP(3) receptors and ryanodine receptors.