Phosphorylation of neuroligin-2 by PKA regulates its cell surface abundance and synaptic stabilization

The trans-synaptic adhesion molecule neuroligin-2 (NL2) is essential for the development and function of inhibitory synapses. NL2 recruits the postsynaptic scaffold protein gephyrin, which, in turn, stabilizes γ-aminobutyric acid type A receptors (GABA_ARs) in the postsynaptic domain. Thus, the amount of NL2 at the synapse can control synaptic GABA_AR concentration to tune inhibitory neurotransmission efficacy. Here, using biochemistry, imaging, single-particle tracking, and electrophysiology, we uncovered a key role for cAMP-dependent protein kinase (PKA) in the synaptic stabilization of NL2. We found that PKA-mediated phosphorylation of NL2 at Ser⁷¹⁴ caused its dispersal from the synapse and reduced NL2 surface amounts, leading to a loss of synaptic GABA_ARs. Conversely, enhancing the stability of NL2 at synapses by abolishing PKA-mediated phosphorylation led to increased inhibitory signaling. Thus, PKA plays a key role in regulating NL2 function and GABA-mediated synaptic inhibition.

AKAP79 enables calcineurin to directly suppress protein kinase A activity

Interplay between the second messengers cAMP and Ca2+ is a hallmark of dynamic cellular processes. A common motif is the opposition of the Ca2+-sensitive phosphatase calcineurin and the major cAMP receptor, protein kinase A (PKA). Calcineurin dephosphorylates sites primed by PKA to bring about changes including synaptic long-term depression (LTD). AKAP79 supports signaling of this type by anchoring PKA and calcineurin in tandem. In this study, we discovered that AKAP79 increases the rate of calcineurin dephosphorylation of type II PKA regulatory subunits by an order of magnitude. Fluorescent PKA activity reporter assays, supported by kinetic modeling, show how AKAP79-enhanced calcineurin activity enables suppression of PKA without altering cAMP levels by increasing PKA catalytic subunit capture rate. Experiments with hippocampal neurons indicate that this mechanism contributes toward LTD. This non-canonical mode of PKA regulation may underlie many other cellular processes.

GABAAR isoform and subunit structural motifs determine synaptic and extrasynaptic receptor localisation

GABA_A receptors (GABA_ARs) are the principal inhibitory neurotransmitter receptors in the central nervous system. They control neuronal excitability by synaptic and tonic forms of inhibition mostly mediated by different receptor subtypes located in specific cell membrane subdomains. A consensus suggests that α1-3βγ comprise synaptic GABA_ARs, whilst extrasynaptic α4βδ, α5βγ and αβ isoforms largely underlie tonic inhibition. Although some structural features that enable the spatial segregation of receptors are known, the mobility of key synaptic and extrasynaptic GABA_ARs are less understood, and yet this is a key determinant of the efficacy of GABA inhibition. To address this aspect, we have incorporated functionally silent α-bungarotoxin binding sites (BBS) into prominent hippocampal GABA_AR subunits which mediate synaptic and tonic inhibition. Using single particle tracking with quantum dots we demonstrate that GABA_ARs that are traditionally considered to mediate synaptic or tonic inhibition are all able to access inhibitory synapses. These isoforms have variable diffusion rates and are differentially retained upon entering the synaptic membrane subdomain. Interestingly, α2 and α4 subunits reside longer at synapses compared to α5 and δ subunits. Furthermore, a high proportion of extrasynaptic δ-containing receptors exhibited slower diffusion compared to δ subunits at synapses. A chimera formed from δ-subunits, with the intracellular domain of γ2L, reversed this behaviour. In addition, we observed that receptor activation affected the diffusion of extrasynaptic, but not of synaptic GABA_ARs. Overall, we conclude that the differential mobility profiles of key synaptic and extrasynaptic GABA_ARs are determined by receptor subunit composition and intracellular structural motifs. This article is part of the special issue entitled 'Mobility and trafficking of neuronal membrane proteins'.

Cell surface expression of homomeric GABAA receptors depends on single residues in subunit transmembrane domains

Cell surface expression of type A GABA receptors (GABA_ARs) is a critical determinant of the efficacy of inhibitory neurotransmission. Pentameric GABA_ARs are assembled from a large pool of subunits according to precise co-assembly rules that limit the extent of receptor structural diversity. These rules ensure that particular subunits, such as ρ1 and β3, form functional cell surface ion channels when expressed alone in heterologous systems, whereas other brain-abundant subunits, such as α and γ, are retained within intracellular compartments. Why some of the most abundant GABA_AR subunits fail to form homomeric ion channels is unknown. Normally, surface expression of α and γ subunits requires co-assembly with β subunits via interactions between their N-terminal sequences in the endoplasmic reticulum. Here, using molecular biology, imaging, and electrophysiology with GABA_AR chimeras, we have identified two critical residues in the transmembrane domains of α and γ subunits, which, when substituted for their ρ1 counterparts, permit cell surface expression as homomers. Consistent with this, substitution of the ρ1 transmembrane residues for the α subunit equivalents reduced surface expression and altered channel gating, highlighting their importance for GABA_AR trafficking and signaling. Although not ligand-gated, the formation of α and γ homomeric ion channels at the cell surface was revealed by incorporating a mutation that imparts the functional signature of spontaneous channel activity. Our study identifies two single transmembrane residues that enable homomeric GABA_AR subunit cell surface trafficking and demonstrates that α and γ subunits can form functional ion channels.

Wnt Signaling Mediates LTP-Dependent Spine Plasticity and AMPAR Localization through Frizzled-7 Receptors

The structural and functional plasticity of synapses is critical for learning and memory. Long-term potentiation (LTP) induction promotes spine growth and AMPAR accumulation at excitatory synapses, leading to increased synaptic strength. Glutamate initiates these processes, but the contribution from extracellular modulators is not fully established. Wnts are required for spine formation; however, their impact on activity-mediated spine plasticity and AMPAR localization is unknown. We found that LTP induction rapidly increased synaptic Wnt7a/b protein levels. Acute blockade of endogenous Wnts or loss of postsynaptic Frizzled-7 (Fz7) receptors impaired LTP-mediated synaptic strength, spine growth, and AMPAR localization at synapses. Live imaging of SEP-GluA1 and single-particle tracking revealed that Wnt7a rapidly promoted synaptic AMPAR recruitment and trapping. Wnt7a, through Fz7, induced CaMKII-dependent loss of SynGAP from spines and increased extrasynaptic AMPARs by PKA phosphorylation. We identify a critical role for Wnt-Fz7 signaling in LTP-mediated synaptic accumulation of AMPARs and spine plasticity.

Phospho-dependent Accumulation of GABABRs at Presynaptic Terminals after NMDAR Activation

Here, we uncover a mechanism for regulating the number of active presynaptic GABAB receptors (GABABRs) at nerve terminals, an important determinant of neurotransmitter release. We find that GABABRs gain access to axon terminals by lateral diffusion in the membrane. Their relative accumulation is dependent upon agonist activation and the presence of the two distinct sushi domains that are found only in alternatively spliced GABABR1a subunits. Following brief activation of NMDA receptors (NMDARs) using glutamate, GABABR diffusion is reduced, causing accumulation at presynaptic terminals in a Ca(2+)-dependent manner that involves phosphorylation of GABABR2 subunits at Ser783. This signaling cascade indicates how synaptically released glutamate can initiate, via a feedback mechanism, increased levels of presynaptic GABABRs that limit further glutamate release and excitotoxicity.

Brief report: isogenic induced pluripotent stem cell lines from an adult with mosaic down syndrome model accelerated neuronal ageing and neurodegeneration

Trisomy 21 (T21), Down Syndrome (DS) is the most common genetic cause of dementia and intellectual disability. Modeling DS is beginning to yield pharmaceutical therapeutic interventions for amelioration of intellectual disability, which are currently being tested in clinical trials. DS is also a unique genetic system for investigation of pathological and protective mechanisms for accelerated ageing, neurodegeneration, dementia, cancer, and other important common diseases. New drugs could be identified and disease mechanisms better understood by establishment of well-controlled cell model systems. We have developed a first nonintegration-reprogrammed isogenic human induced pluripotent stem cell (iPSC) model of DS by reprogramming the skin fibroblasts from an adult individual with constitutional mosaicism for DS and separately cloning multiple isogenic T21 and euploid (D21) iPSC lines. Our model shows a very low number of reprogramming rearrangements as assessed by a high-resolution whole genome CGH-array hybridization, and it reproduces several cellular pathologies seen in primary human DS cells, as assessed by automated high-content microscopic analysis. Early differentiation shows an imbalance of the lineage-specific stem/progenitor cell compartments: T21 causes slower proliferation of neural and faster expansion of hematopoietic lineage. T21 iPSC-derived neurons show increased production of amyloid peptide-containing material, a decrease in mitochondrial membrane potential, and an increased number and abnormal appearance of mitochondria. Finally, T21-derived neurons show significantly higher number of DNA double-strand breaks than isogenic D21 controls. Our fully isogenic system therefore opens possibilities for modeling mechanisms of developmental, accelerated ageing, and neurodegenerative pathologies caused by T21.

Snake neurotoxin α-bungarotoxin is an antagonist at native GABA(A) receptors

The snake neurotoxin α-bungarotoxin (α-Bgtx) is a competitive antagonist at nicotinic acetylcholine receptors (nAChRs) and is widely used to study their function and cell-surface expression. Increasingly, α-Bgtx is also used as an imaging tool for fluorophore-labelling studies, and given the structural conservation within the pentameric ligand-gated ion channel family, we assessed whether α-Bgtx could bind to recombinant and native γ-aminobutyric type-A receptors (GABA_ARs). Applying fluorophore-linked α-Bgtx to recombinant αxβ1/2γ2 GABA_ARs expressed in HEK-293 cells enabled clear cell-surface labelling of α2β1/2γ2 contrasting with the weaker staining of α1/4β1/2γ2, and no labelling for α3/5/6β1/2γ2. The labelling of α2β2γ2 was abolished by bicuculline, a competitive antagonist at GABA_ARs, and by d-tubocurarine (d-Tc), which acts in a similar manner at nAChRs and GABA_ARs. Labelling by α-Bgtx was also reduced by GABA, suggesting that the GABA binding site at the receptor β–α subunit interface forms part of the α-Bgtx binding site. Using whole-cell recording, high concentrations of α-Bgtx (20 μM) inhibited GABA-activated currents at all αxβ2γ2 receptors examined, but at lower concentrations (5 μM), α-Bgtx was selective for α2β2γ2. Using α-Bgtx, at low concentrations, permitted the selective inhibition of α2 subunit-containing GABA_ARs in hippocampal dentate gyrus granule cells, reducing synaptic current amplitudes without affecting the GABA-mediated tonic current. In conclusion, α-Bgtx can act as an inhibitor at recombinant and native GABA_ARs and may be used as a selective tool to inhibit phasic but not tonic currents in the hippocampus.

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Recombinant GABA_A receptors are inhibited by α-bungarotoxin

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The β–α subunit interface of GABA_A receptors forms the α-bungarotoxin binding site.

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α-bungarotoxin can selectively inhibit α2 subunit-containing GABA_A receptors (α2β2γ2).

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α-bungarotoxin inhibits GABA synaptic currents in the hippocampus.

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GABA-mediated tonic currents are unaffected by α-bungarotoxin

Photo-antagonism of the GABAA receptor

Neurotransmitter receptor trafficking is fundamentally important for synaptic transmission and neural network activity. GABAA receptors and inhibitory synapses are vital components of brain function, yet much of our knowledge regarding receptor mobility and function at inhibitory synapses is derived indirectly from using recombinant receptors, antibody-tagged native receptors and pharmacological treatments. Here we describe the use of a set of research tools that can irreversibly bind to and affect the function of recombinant and neuronal GABAA receptors following ultraviolet photoactivation. These compounds are based on the competitive antagonist gabazine and incorporate a variety of photoactive groups. By using site-directed mutagenesis and ligand-docking studies, they reveal new areas of the GABA binding site at the interface between receptor β and α subunits. These compounds enable the selected inactivation of native GABAA receptor populations providing new insight into the function of inhibitory synapses and extrasynaptic receptors in controlling neuronal excitation.

Tracking cell surface mobility of GPCRs using α-bungarotoxin-linked fluorophores

GABA(B) receptors are G-protein-coupled receptors (GPCRs) that are activated by GABA, the principal inhibitory neurotransmitter in the central nervous system. Cell surface mobility of GABA(B) receptors is a key determinant of the efficacy of slow and prolonged synaptic inhibition initiated by GABA. Therefore, experimentally monitoring receptor mobility and how this can be regulated is of primary importance for understanding the roles of GABA(B) receptors in the brain, and how they may be therapeutically exploited. Unusually for a GPCR, heterodimerization between the R1 and R2 subunits is required for the cell surface expression and signaling by prototypical GABA(B) receptors. Here, we describe a minimal epitope-tagging method, based on the incorporation of an α-bungarotoxin binding site (BBS) into the GABA(B) receptor, to study receptor internalization in live cells using a range of imaging approaches. We demonstrate how this technique can be adapted by modifying the BBS to monitor the simultaneous movement of both R1 and R2 subunits, revealing that GABA(B) receptors are internalized as heteromers.

Gamma-aminobutyric acid type B (GABA(B)) receptor internalization is regulated by the R2 subunit

γ-Aminobutyric acid type B (GABA(B)) receptors are important for slow synaptic inhibition in the CNS. The efficacy of inhibition is directly related to the stability of cell surface receptors. For GABA(B) receptors, heterodimerization between R1 and R2 subunits is critical for cell surface expression and signaling, but how this determines the rate and extent of receptor internalization is unknown. Here, we insert a high affinity α-bungarotoxin binding site into the N terminus of the R2 subunit and reveal its dominant role in regulating the internalization of GABA(B) receptors in live cells. To simultaneously study R1a and R2 trafficking, a new α-bungarotoxin binding site-labeling technique was used, allowing α-bungarotoxin conjugated to different fluorophores to selectively label R1a and R2 subunits. This approach demonstrated that R1a and R2 are internalized as dimers. In heterologous expression systems and neurons, the rates and extents of internalization for R1aR2 heteromers and R2 homomers are similar, suggesting a regulatory role for R2 in determining cell surface receptor stability. The fast internalization rate of R1a, which has been engineered to exit the endoplasmic reticulum, was slowed to that of R2 by truncating the R1a C-terminal tail or by removing a dileucine motif in its coiled-coil domain. Slowing the rate of internalization by co-assembly with R2 represents a novel role for GPCR heterodimerization whereby R2 subunits, via their C terminus coiled-coil domain, mask a dileucine motif on R1a subunits to determine the surface stability of the GABA(B) receptor.

Transfer of antibiotic resistance by transformation with eDNA within oral biofilms

We demonstrate that live donor Veillonella dispar cells can transfer the conjugative transposon Tn916 to four different Streptococcus spp. recipients in a multispecies oral consortium growing as a biofilm in a constant depth film fermentor. Additionally, we demonstrate that purified V. dispar DNA can transform Streptococcus mitis to tetracycline resistance in this experimental system. These data show that transfer of conjugative transposon-encoded antibiotic resistance can occur by transformation in addition to conjugation in biofilms.

An analysis of the CFH Y402H genotype in AMD patients and controls from the UK, and response to PDT treatment

AIM

Mutation in the complement factor H (CFH) gene is an important risk factor for age-related macular degeneration (AMD). In this study, we identified the strength of the CFH Y402H gene variant association in a UK AMD cohort and tested the hypothesis that this variant may influence the biological response of choroidal neovascularisation (CNV) following photodynamic therapy (PDT) for CNV.

METHODS

A total of 557 cases with AMD and 551 normal controls were genotyped for the CFH Y402H (1277 C/T) variant using the 5' nuclease TaqMan assay for allelic discrimination. The CFH gene association for AMD, for the different CNV subtypes and for patients needing PDT was estimated. Twenty-seven PDT-treated patients were followed up for 15 months with ETDRS-derived vision, clinical examination, and fundus angiography. Individuals with different CFH genotypes were then analysed for any association with visual change following PDT.

RESULTS

The risk association for AMD with the CFH CC genotype (odd ratio (OR)=3.62, Pc<0.0001) was similar to that reported in other Caucasian cohorts. The magnitude and strength of this association was stronger in AREDS stages 2-4 (ORs=4.48, 2.69, and 5.17). ORs for the risk of predominantly classic CNV were significantly raised for both the CC (OR=17.87, P<0.0001) and CT (OR=9.06, P=0.0002) genotypes. The number of patients carrying the high-risk C allele was 70.4% in those requiring PDT as compared to 52.3% in the non-PDT group (OR=2.16, P=0.011), and presence of the CC genotype significantly increased the risk of PDT (OR=5.48, P=0.015). The degree of visual loss following PDT was significantly higher in the CFH CC genotype group (P=0.038); 50% of CC cases (n=13) and 45% of the CT cases (n=12) lost 15 or more ETDRS letters at final follow-up.

CONCLUSION

In this UK cohort of AMD patients, the CFH Y402H variant was significantly enriched in patients with predominantly classic CNV. Patients homozygous for the CFH Y402H genotype seem to have worse visual acuity after PDT.

Assessment of effect of nasal continuous positive pressure on laryngeal opening using fibre optic laryngoscopy

AIM

To assess the effect of nasal continuous positive airways pressure (CPAP) on the dimensions of the laryngeal opening.

METHODS

Nine preterm infants who had previously received ventilatory support for respiratory distress syndrome (RDS) were studied. All were receiving nasal CPAP. The laryngeal opening was visualised using a fibre optic video camera system. The ratio of width to length of the opening was measured on and off CPAP.

RESULTS

In eight of the infants the width: length ratio increased on CPAP; mean change for group +24.4% (95% CI +11.9 to +37.9).

CONCLUSIONS

Nasal CPAP seems to dilate the larynx. This may explain the selective beneficial effects of CPAP on mixed and obstructive apnoea.

Endocarditis in the 80s in a general hospital in Auckland, New Zealand

The clinical and investigative features of 102 episodes of infective endocarditis were analysed retrospectively. The most frequent presenting symptoms (malaise, fever, sweats, myalgia, weight loss) were non-specific. Fever, cardiac murmur, tachycardia, vascular phenomena and a change in mental state were the most common physical signs at admission. Anaemia was present in half the episodes and renal and liver dysfunction in about one-third. Streptococci (61) and staphylococci (31) were the causative organisms in all but 10 episodes. The commonest predisposing factors were underlying cardiac disease (52 per cent) and a preceding focus of infection (14.6 per cent). Left ventricular failure (33 per cent) and focal neurological disease (29 per cent) occurred frequently. Valvular surgery was performed in 20 episodes, with two in-hospital deaths. Overall hospital mortality was 27.5 per cent and death was most commonly neurological (11/28). A higher mortality was associated with elevated total white blood count, microscopic haematuria, renal or liver dysfunction at admission, S. aureus endocarditis, the development of left ventricular failure or focal neurological disease, age greater than or equal to 60 years and persistence of fever after one week of antibiotic therapy. The absence of both renal dysfunction at admission and subsequent microscopic haematuria identified a group with a very low hospital mortality (4.7 per cent). The three-year mortality of the entire group was 43.5 per cent.

Preventive treatment of asymptomatic left ventricular dysfunction following myocardial infarction

Progressive asymptomatic ventricular dilatation can occur following myocardial infarction and severe ventricular dysfunction is often present by the time clinical congestive heart failure occurs. In a randomized, double-blind trial, the effects of captopril 25 mg tid, frusemide 40 mg daily and placebo were studied in 90 patients with asymptomatic left ventricular dysfunction (ejection fraction less than 45%) 1 week following Q wave myocardial infarction. Left ventricular volumes and function were assessed at intervals during the subsequent year using two-dimensional echocardiography. The frusemide and placebo groups showed significant increases in ventricular volumes with stroke volume index unchanged and ejection fraction slightly reduced, whereas the captopril group showed a significant reduction in left ventricular end-systolic volume index with stroke volume index and ejection fraction increased. At 12 months the difference in the change in ejection fraction from baseline between the captopril and frusemide groups was 10.5% and captopril and placebo groups 9.6% (both P less than 0.0001). There was a significant difference in occurrence of clinical heart failure in the placebo group compared with the other groups (P less than 0.05). Blood pressure increased significantly within the normal range in the placebo and frusemide groups whereas there was essentially no change from baseline in the captopril group. There was no significant correlation between baseline left ventricular volumes and function and subsequent change, and anterior and inferior infarct subgroups showed similar responses within the treatment groups. In conclusion, captopril improves asymptomatic ventricular dysfunction and prevents clinical heart failure during the year following myocardial infarction. Frusemide may also prevent clinical heart failure but not progressive ventricular dilatation.

Effect of reducing atrial pressure on atrial natriuretic factor and vasoactive hormones in congestive heart failure secondary to ischemic and nonischemic dilated cardiomyopathy

The effect of an acute and sustained reduction in atrial pressure on atrial natriuretic factor (ANF) and vasoactive hormone secretion was studied in 9 patients with congestive heart failure (CHF). Intravenous nitroglycerin was titrated to reduce the pulmonary artery wedge pressure by 30 to 50% and maintain this reduction for 4 hours. After 60 minutes of nitroglycerin administration, the mean decrement in wedge pressure was 10.0 +/- 1.7 (standard error) mm Hg (35%) and plasma ANF was 65.3 +/- 13.9 pmol/liter (35%). The initial decrease, sustained reduction and later increase in plasma ANF levels closely paralleled the changes in pulmonary arterial wedge (r = 0.94, p less than 0.0001) and right atrial pressures (r = 0.91, p less than 0.0001) during and immediately after the nitroglycerin infusion. Plasma aldosterone and cortisol levels increased during the first 2 hours of the nitroglycerin infusion, but there was little change in plasma norepinephrine or plasma renin activity. Although levels were elevated in CHF, plasma ANF still responded rapidly to changes in atrial pressure. A sustained reduction in pressure produced a sustained reduction in ANF levels. These findings provide further support for a regulatory role of ANF, even in chronic CHF.

Treatment of patients with symptomless left ventricular dysfunction after myocardial infarction

In a randomised, double-blind trial 60 patients with left ventricular dysfunction (ejection fraction less than 45%) but without clinical evidence of heart failure 1 week after Q wave myocardial infarction were given captopril 25 mg thrice a day, frusemide 40 mg daily, or placebo. Left ventricular volumes were measured at baseline and at 1, 3, 6, 9, and 12 months with cross-sectional echocardiography and Simpson's rule analysis of standardised apical views. The captopril group showed no significant change in left ventricular end-diastolic volume index but left ventricular end-systolic volume index was significantly reduced and stroke volume index and ejection fraction were significantly increased from 1 month on. In contrast, the frusemide and placebo groups showed significant increases in ventricular volumes, with stroke volume index unchanged and ejection fraction slightly reduced. The changes in the captopril group were significantly different from those in the other groups.

Simple test of intestinal calcium absorption measured by stable strontium

A clinical test of intestinal calcium absorption has been developed using non-radioactive stable strontium as a calcium tracer. In nine elderly subjects there was a close correlation between the fractional absorption of strontium and radioactive calcium (45Ca) during a five hour period after the simultaneous oral administration of the two tracers. Comparable precision was achieved with each tracer in six subjects in whom the test was repeated after two weeks. The effect of food on strontium absorption was examined in a further 33 normal subjects (age 21-60 years), and the administration of the strontium with a standard breakfast was shown to reduce the variance at individual time points. A simplified test in which serum strontium concentration was measured four hours after the oral dose given with a standard breakfast was adopted as the routine procedure. The normal range (mean (2 SD], established over 97 tests in 53 patients, was 7.0-18.0% of the dose in the extracellular fluid. A further 30 patients with possible disorders of calcium absorption (10 with primary hyperparathyroidism and 20 with coeliac disease) were studied by this standard test. In both groups of patients the mean four hour strontium values were significantly different from normal. This standard strontium absorption test allows assessment of calcium absorption with sufficient sensitivity and precision to have a wide application in clinical practice.

Aspects of dermatology practice in Auckland

In June 1984, a survey of outpatient attendances at the Auckland Hospital skin clinic was carried out. Similar data was obtained about patients attending a dermatologist in private in Auckland. There were 869 visits to the skin clinic, 36% of them new patients, and 296 visits in the private clinic, 48% new. Twenty-eight percent of patients were over 65 years old. The most common conditions seen were cutaneous malignancies (20%), solar keratoses (17%), and eczema (17%).