Endoplasmic reticulum as a source of Ca2+ in neurotransmitter secretion

Protein nanoparticle-induced osmotic pressure gradients modify pulmonary edema through hyperpermeability in acute respiratory distress syndrome

Acute respiratory distress syndrome (ARDS), caused by noncardiogenic pulmonary edema (PE), contributes significantly to Coronavirus 2019 (COVID-19)-associated morbidity and mortality. We explored the effect of transmembrane osmotic pressure (OP) gradients in PE using a fluorescence resonance energy transfer-based Intermediate filament (IF) tension optical probe. Angiotensin-II- and bradykinin-induced increases in intracellular protein nanoparticle (PN)-OP were associated with inflammasome production and cytoskeletal depolymerization. Intracellular protein nanoparticle production also resulted in cytomembrane hyperpolarization and L-VGCC-induced calcium signals, which differed from diacylglycerol-induced calcium increment via TRPC6 activation. Both pathways involve voltage-dependent cation influx and OP upregulation via SUR1-TRPM4 channels. Meanwhile, intra/extracellular PN-induced OP gradients across membranes upregulated pulmonary endothelial and alveolar barrier permeability. Attenuation of intracellular PN, calcium signals, and cation influx by drug combinations effectively relieved intracellular OP and pulmonary endothelial nonselective permeability, and improved epithelial fluid absorption and PE. Thus, PN-OP is pivotal in pulmonary edema in ARDS and COVID-19, and transmembrane OP recovery could be used to treat pulmonary edema and develop new drug targets in pulmonary injury.

Graphical Abstract

Nature and Action of Antibodies in Myasthenia Gravis

This article discusses antibodies associated with immune-mediated myasthenia gravis and the pathologic action of these antibodies at the neuromuscular junctions of skeletal muscle. To explain how these antibodies act, we consider the physiology of neuromuscular transmission with emphasis on 4 features: the structure of the neuromuscular junction; the roles of postsynaptic acetylcholine receptors and voltage-gated Na⁺ channels and in converting the chemical signal from the nerve terminal into a propagated action potential on the muscle fiber that triggers muscle contraction; the safety factor for neuromuscular transmission; and how the safety factor is reduced in different forms of autoimmune myasthenia gravis.

Increased vesicle recycling in response to osmotic cell swelling. Cause and consequence of hypotonicity-provoked ATP release

Osmotic swelling of Intestine 407 cells leads to an immediate increase in cell surface membrane area as determined using the fluorescent membrane dye FM 1-43. In addition, as measured by tetramethylrhodamine isothiocyanate (TRITC)-dextran uptake, a robust (>100-fold) increase in the rate of endocytosis was observed, starting after a discrete lag time of 2-3 min and lasting for approximately 10-15 min. The hypotonicity-induced increase in membrane surface area, like the cell swelling-induced release of ATP (Van der Wijk, T., De Jonge, H. R., and Tilly, B. C. (1999) Biochem. J. 343, 579-586), was diminished after 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-acetoxymethyl ester loading or cytochalasin B treatment. Uptake of TRITC-dextrans, however, was not affected. Treatment of the cells with the vesicle-soluble N-ethylmaleimide-sensitive factor attachment protein receptor-specific protease Clostridium botulinum toxin F not only nearly eliminated the hypotonicity-induced increase in membrane surface area but also strongly diminished the release of ATP, indicating the involvement of regulated exocytosis. Both the ATP hydrolase apyrase and the MEK inhibitor PD098059 diminished the osmotic swelling-induced increase in membrane surface area as well as the subsequent uptake of TRITC-dextrans. Taken together, the results indicate that extracellular ATP is required for the hypotonicity-induced vesicle recycling and suggest that a positive feedback loop, involving purinergic activation of the Erk-1/2 pathway, may contribute to the release of ATP from hypo-osmotically stimulated cells.

Extraocular muscle responses to orbital cooling (ice test) for ocular myasthenia gravis diagnosis

BACKGROUND

As a result of clinical and laboratory investigations of temperature correlates of myasthenia gravis, orbital cooling (ice test) has been developed as a reliable test for ocular myasthenia diagnosis through blepharoptosis response. The test has not been utilized in a prospective manner for myasthenia diagnosis through extraocular muscle responses.

METHODS

Fifteen patients with acquired motility disorders were studied with the use of orbital cooling and other tests for myasthenia gravis. Orbital cooling was performed in a standard fashion for all patients. In 14 of 15 patients, the diagnosis of myasthenia was not established at the time the ice test was performed. Fifteen non-myasthenic patients with acquired motility disorders were also studied with use of the ice test. Temperatures during orbital cooling were measured in the superior cul-de-sac of one patient and between the lateral rectus muscle and globe in 3 patients.

RESULTS

All patients subsequently proven to have myasthenia gravis by other tests and by response to myasthenia therapy had a positive (diagnostic of myasthenia) response to the ice test. No patient had a false-positive or a paradoxical response to the ice test. No control patient had a positive blepharoptosis or motility response to orbital cooling. Temperature measurements demonstrated significant cooling effects in the superotemporal cul-de-sac and beneath the lateral rectus muscles after orbital cooling for 5 minutes.

CONCLUSIONS

Orbital cooling, within certain parameters, can be a useful clinical test for myasthenia diagnosis through motility response, as well as blepharoptosis response.

Calcium signaling in the ER: its role in neuronal plasticity and neurodegenerative disorders

Endoplasmic reticulum (ER) is a multifaceted organelle that regulates protein synthesis and trafficking, cellular responses to stress, and intracellular Ca2+ levels. In neurons, it is distributed between the cellular compartments that regulate plasticity and survival, which include axons, dendrites, growth cones and synaptic terminals. Intriguing communication networks between ER, mitochondria and plasma membrane are being revealed that provide mechanisms for the precise regulation of temporal and spatial aspects of Ca2+ signaling. Alterations in Ca2+ homeostasis in ER contribute to neuronal apoptosis and excitotoxicity, and are being linked to the pathogenesis of several different neurodegenerative disorders, including Alzheimer's disease and stroke.

Disorders of neuromuscular junction ion channels

Ion channel defects produce a clinically diverse set of disorders that range from cystic fibrosis and some forms of migraine to renal tubular defects and episodic ataxias. This review discusses diseases related to impaired function of the skeletal muscle acetylcholine receptor and calcium channels of the motor nerve terminal. Myasthenia gravis is an autoimmune disease caused by antibodies directed toward the skeletal muscle acetylcholine receptor that compromise neuromuscular transmission. Congenital myasthenias are genetic disorders, a subset of which are caused by mutations of the acetylcholine receptor. Lambert-Eaton myasthenic syndrome is an immune disorder characterized by impaired synaptic vesicle release likely related to a defect of calcium influx. The disorders will illustrate new insights into synaptic transmission and ion channel structure that are relevant for all ion channel disorders.

Kinetics, Ca2+ dependence, and biophysical properties of integrin-mediated mechanical modulation of transmitter release from frog motor nerve terminals

Neurotransmitter release from frog motor nerve terminals is strongly modulated by change in muscle length. Over the physiological range, there is an approximately 10% increase in spontaneous and evoked release per 1% muscle stretch. Because many muscle fibers do not receive suprathreshold synaptic inputs at rest length, this stretch-induced enhancement of release constitutes a strong peripheral amplifier of the spinal stretch reflex. The stretch modulation of release is inhibited by peptides that block integrin binding of natural ligands. The modulation varies linearly with length, with a delay of no more than approximately 1-2 msec and is maintained constant at the new length. Moreover, the stretch modulation persists in a zero Ca2+ Ringer and, hence, is not dependent on Ca2+ influx through stretch activated channels. Eliminating transmembrane Ca2+ gradients and buffering intraterminal Ca2+ to approximately normal resting levels does not eliminate the modulation, suggesting that it is not the result of release of Ca2+ from internal stores. Finally, changes in temperature have no detectable effect on the kinetics of stretch-induced changes in endplate potential (EPP) amplitude or miniature EPP (mEPP) frequency. We conclude, therefore, that stretch does not act via second messenger pathways or a chemical modification of molecules involved in the release pathway. Instead, there is direct mechanical modulation of release. We postulate that tension on integrins in the presynaptic membrane is transduced mechanically into changes in the position or conformation of one or more molecules involved in neurotransmitter release, altering sensitivity to Ca2+ or the equilibrium for a critical reaction leading to vesicle fusion.

Subcellular localization of ryanodine receptors in rat brain

Subcellular fractions of rat brain were used to determine the subcellular localization of ryanodine receptors. [3H]Ryanodine binding in purified cortical, cerebellar and hippocampal synaptosomes was up to 3.6-fold higher compared with mitochondrial fractions. The density of sites (Bmax) in hippocampal mossy fiber synaptosomes (249 fmol/mg protein) was 3.7-fold greater than in cortical synaptosomes (67 fmol/mg protein) and binding affinity was approximately 2-fold greater in the former (KD, 6.1 nM) than the latter (KD, 3.1 nM). At single sub-saturating concentrations of [3H]ryanodine, binding was 1.6-fold higher in mossy fibers compared with total hippocampal synaptosomes. [3H]Ryanodine binding sites were distributed similarly in subfractions of cortical synaptosomes and microsomes from discontinuous sucrose density gradients. An enrichment of sites was found in the lightest fractions containing the lowest activities of plasma membrane (5'-nucleotidase) and endoplasmic reticulum (glucose 6-phosphatase) enzyme markers when data for microsomal and synaptosomal subfractions were expressed as activity/binding per mg protein and when data for synaptosomal subfractions were expressed as a percentage of total activity/binding in collected fractions. Thus, ryanodine receptors appear to be concentrated in presynaptic terminals where they may play a major role in neurotransmitter release, and appear to be localized either in a specialized endoplasmic reticulum subcompartment or a distinct subcellular organelle.

Calcium-channel blockers and gastrointestinal motility: basic and clinical aspects

Several calcium-channel blockers currently in use for the treatment of cardiovascular disorders have recently been tested for their effects on gastrointestinal motility. The rationale for this approach centers on the concept that calcium-channel blockers are at least as potent in inhibiting intestinal smooth muscle as in relaxing vascular smooth muscle. This review will give an outline of the most recent findings on the role of calcium and calcium channels in smooth muscle and neuronal function in the digestive system. It will also consider the mechanisms by which calcium-channel blockers may affect gastrointestinal motility and assess potential clinical applications in gastroenterology. The main goal for researchers in this field will be the development of gut-selective agents, with no cardiovascular side effects.