IPF, a vesicular uptake inhibitory protein factor, can reduce the Ca(2+)-dependent, evoked release of glutamate, GABA and serotonin

Ivermectin ameliorates bleomycin-induced lung fibrosis in male rats by inhibiting the inflammation and oxidative stress

BACKGROUND

Idiopathic pulmonary fibrosis (IPF) is a pulmonary fibrotic disease characterized by a poor prognosis, which its pathogenesis involves the accumulation of abnormal fibrous tissue, inflammation, and oxidative stress. Ivermectin, a positive allosteric modulator of GABAA receptor, exerts anti-inflammatory and antioxidant properties in preclinical studies. The present study investigates the potential protective effects of ivermectin treatment in rats against bleomycin-induced IPF.

MATERIALS AND METHODS

The present study involved 42 male Wistar rats, which were divided into five groups: control (without induction of IPF), bleomycin (IPF-induced by bleomycin 2.5 mg/kg, by intratracheal administration), and three fibrosis groups receiving ivermectin (0.5, 1, and 3 mg/kg). lung tissues were harvested for measurement of oxidative stress [via myeloperoxidase (MPO), superoxide dismutase (SOD), glutathione (GSH)] and inflammatory markers (tumor necrosis factor-α [TNF-α], interleukin-1β [IL-1β], and transforming growth factor-β [TGF-β]). Histological assessments of tissue damage were performed using hematoxylin-eosin (H&E) and Masson's trichrome staining methods.

RESULTS

The induction of fibrosis via bleomycin was found to increase levels of MPO as well as TNF-α, IL-1β, and TGF-β while decrease SOD activity and GSH level. Treatment with ivermectin at a dosage of 3 mg/kg was able to reverse the effects of bleomycin-induced fibrosis on these markers. In addition, results from H&E and Masson's trichrome staining showed that ivermectin treatment at this same dose reduced tissue damage and pulmonary fibrosis.

CONCLUSION

The data obtained from this study indicate that ivermectin may have therapeutic benefits for IPF, likely due to its ability to reduce inflammation and mitigate oxidative stress-induced toxicity.

Enriched Alternative Splicing in Islets of Diabetes-Susceptible Mice

Dysfunctional islets of Langerhans are a hallmark of type 2 diabetes (T2D). We hypothesize that differences in islet gene expression alternative splicing which can contribute to altered protein function also participate in islet dysfunction. RNA sequencing (RNAseq) data from islets of obese diabetes-resistant and diabetes-susceptible mice were analyzed for alternative splicing and its putative genetic and epigenetic modulators. We focused on the expression levels of chromatin modifiers and SNPs in regulatory sequences. We identified alternative splicing events in islets of diabetes-susceptible mice amongst others in genes linked to insulin secretion, endocytosis or ubiquitin-mediated proteolysis pathways. The expression pattern of 54 histones and chromatin modifiers, which may modulate splicing, were markedly downregulated in islets of diabetic animals. Furthermore, diabetes-susceptible mice carry SNPs in RNA-binding protein motifs and in splice sites potentially responsible for alternative splicing events. They also exhibit a larger exon skipping rate, e.g., in the diabetes gene Abcc8, which might affect protein function. Expression of the neuronal splicing factor Srrm4 which mediates inclusion of microexons in mRNA transcripts was markedly lower in islets of diabetes-prone compared to diabetes-resistant mice, correlating with a preferential skipping of SRRM4 target exons. The repression of Srrm4 expression is presumably mediated via a higher expression of miR-326-3p and miR-3547-3p in islets of diabetic mice. Thus, our study suggests that an altered splicing pattern in islets of diabetes-susceptible mice may contribute to an elevated T2D risk.

The mechanism and regulation of vesicular glutamate transport: Coordination with the synaptic vesicle cycle

The transport of classical neurotransmitters into synaptic vesicles generally relies on a H⁺ electrochemical gradient (∆μ_H+). Synaptic vesicle uptake of glutamate depends primarily on the electrical component ∆ψ as the driving force, rather than the chemical component ∆pH. However, the vesicular glutamate transporters (VGLUTs) belong to the solute carrier 17 (SLC17) family, which includes closely related members that function as H⁺ cotransporters. Recent work has also shown that the VGLUTs undergo allosteric regulation by H⁺ and Cl^-, and exhibit an associated Cl^- conductance. These properties appear to coordinate VGLUT activity with the large ionic shifts that accompany the rapid recycling of synaptic vesicles driven by neural activity. Recent structural information also suggests common mechanisms that underlie the apparently divergent function of SLC17 family members, and that confer allosteric regulation.

VGLUT substrates and inhibitors: A computational viewpoint

The vesicular glutamate transporters (VGLUTs) bind and move glutamate (Glu) from the cytosol into the lumen of synaptic vesicles using a H⁺-electrochemical gradient (ΔpH and Δψ) generated by the vesicular H⁺-ATPase. VGLUTs show very low Glu binding and to date, no three-dimensional structure has been elucidated. Prior studies have attempted to identify the key residues involved in binding VGLUT substrates and inhibitors using homology models and docking experiments. Recently, the inward and outward oriented crystal structures of d-galactonate transporter (DgoT) emerged as possible structure templates for VGLUT. In this review, a new homology model for VGLUT2 based on DgoT has been developed and used to conduct docking experiments to identify and differentiate residues and binding orientations involved in ligand interactions. This review describes small molecule-ligand interactions including docking using a VGLUT2 homology model derived from DgoT.

Presynaptic Molecular Determinants of Quantal Size

The quantal hypothesis for the release of neurotransmitters at the chemical synapse has gained wide acceptance since it was first worked out at the motor endplate in frog skeletal muscle in the 1950’s. Considering the morphological identification of synaptic vesicles (SVs) at the nerve terminals that appeared to be homogeneous in size, the hypothesis proposed that signal transduction at synapses is mediated by the release of neurotransmitters packed in SVs that are individually uniform in size; the amount of transmitter in a synaptic vesicle is called a quantum. Although quantal size—the amplitude of the postsynaptic response elicited by the release of neurotransmitters from a single vesicle—clearly depends on the number and sensitivity of the postsynaptic receptors, accumulating evidence has also indicated that the amount of neurotransmitters stored in SVs can be altered by various presynaptic factors. Here, I provide an overview of the concepts and underlying presynaptic molecular underpinnings that may regulate quantal size.

Glutamate Release

Our aim was to review the processes of glutamate release from both biochemical and neurophysiological points of view. A large body of evidence now indicates that glutamate is specifically accumulated into synaptic vesicles, which provides strong support for the concept that glutamate is released from synaptic vesicles and is the major excitatory neurotransmitter. Evidence suggests the notion that synaptic vesicles, in order to sustain the neurotransmitter pool of glutamate, are endowed with an efficient mechanism for vesicular filling of glutamate. Glutamate-loaded vesicles undergo removal of Synapsin I by CaM kinase II-mediated phosphorylation, transforming to the release-ready pool. Vesicle docking to and fusion with the presynaptic plasma membrane are thought to be mediated by the SNARE complex. The Ca(2+)-dependent step in exocytosis is proposed to be mediated by synaptotagmin.

Cryoloading: introducing large molecules into live synaptosomes

Neurons communicate with their target cells primarily by the release of chemical transmitters from presynaptic nerve terminals. The study of CNS presynaptic nerve terminals, isolated as synaptosomes (SSMs) has, however, been hampered by the typical small size of these structures that precludes the introduction of non-membrane permeable test substances such as peptides and drugs. We have developed a method to introduce large alien compounds of at least 150 kDa into functional synaptosomes. Purified synaptosomes are frozen in cryo-preserving buffer containing the alien compound. Upon defrosting, many of the SSMs contain the alien compound presumably admitted by bulk buffer-transfer through the surface membranes that crack and reseal during the freeze/thaw cycle. ~80% of the cryoloaded synaptosomes were functional and recycled synaptic vesicles (SVs), as assessed by a standard styryl dye uptake assay. Access of the cryoloaded compound into the cytoplasm and biological activity were confirmed by block of depolarization-induced SV recycling with membrane-impermeant BAPTA (a rapid Ca(2+)-scavenger), or botulinum A light chain (which cleaves the soluble NSF attachment protein receptor (SNARE) protein SNAP25). A major advantage of the method is that loaded frozen synaptosomes can be stored virtually indefinitely for later experimentation. We also demonstrate that individual synaptosome types can be identified by immunostaining of receptors associated with its scab of attached postsynaptic membrane. Thus, cryoloading and scab-staining permits the examination of SV recycling in identified individual CNS presynaptic nerve terminals.

A new VGLUT-specific potent inhibitor: pharmacophore of Brilliant Yellow

The increased concentration of glutamate in synaptic vesicles, mediated by the vesicular glutamate transporter (VGLUT), is an initial vital step in glutamate synaptic transmission. Evidence indicates that aberrant overexpression of VGLUT is involved in certain pathophysiologies of the central nervous system. VGLUT is subject to inhibition by various types of agents. The most potent VGLUT-specific inhibitor currently known is Trypan Blue, which is highly charged, hence membrane-impermeable. We have sought a potent, VGLUT-specific agent amenable to easy modification to a membrane-permeable analog. We provide evidence that Brilliant Yellow exhibits potent, VGLUT-specific inhibition, with a Ki value of 12 nM. Based upon structure-activity relationship studies and molecular modeling, we have defined the potent inhibitory pharmacophore of Brilliant Yellow. This study provides new insight into development of a membrane-permeable agent to lead to specific blockade, with high potency, of accumulation of glutamate into synaptic vesicles in neurons.

VMAT2: a dynamic regulator of brain monoaminergic neuronal function interacting with drugs of abuse

The monoaminergic neuron, in particular the dopaminergic neuron, is central to mediating the hedonic and addictive properties of drugs of abuse. The effects of amphetamine (AMPH) and cocaine (COC), for example, depend on the ability to increase dopamine in the synapse, by effects on either the plasma membrane transporter DAT or the vesicular transporter for monoamine storage, VMAT2. The potential role of DAT as a target for AMPH and COC has been reviewed extensively. Here, we present VMAT2 as a target that enables the rewarding and addictive actions of these drugs, based on imaging, neurochemical, biochemical, cell biological, genetic, and immunohistochemical evidence. The presence of VMAT2 in noradrenergic, serotoninergic, histaminergic, and potentially trace aminergic neurons invites consideration of a wider role for aminergic neurotransmission in AMPH and COC abuse and addiction.

The Neurotransmitter Cycle and Quantal Size

Changes in the response to release of a single synaptic vesicle have generally been attributed to postsynaptic modification of receptor sensitivity, but considerable evidence now demonstrates that alterations in vesicle filling also contribute to changes in quantal size. Receptors are not saturated at many synapses, and changes in the amount of transmitter per vesicle contribute to the physiological regulation of release. On the other hand, the presynaptic factors that determine quantal size remain poorly understood. Aside from regulation of the fusion pore, these mechanisms fall into two general categories: those that affect the accumulation of transmitter inside a vesicle and those that affect vesicle size. This review will summarize current understanding of the neurotransmitter cycle and indicate basic, unanswered questions about the presynaptic regulation of quantal size.

Molecular pharmacology of glutamate transporters, EAATs and VGLUTs

L-Glutamate serves as a major excitatory neurotransmitter in the mammalian central nervous system (CNS) and is stored in synaptic vesicles by an uptake system that is dependent on the proton electrochemical gradient (VGLUTs). Following its exocytotic release, glutamate activates fast-acting, excitatory ionotropic receptors and slower-acting metabotropic receptors to mediate neurotransmission. Na+-dependent glutamate transporters (EAATs) located on the plasma membrane of neurons and glial cells rapidly terminate the action of glutamate and maintain its extracellular concentration below excitotoxic levels. Thus far, five Na+-dependent glutamate transporters (EAATs 1-5) and three vesicular glutamate transporters (VGLUTs 1-3) have been identified. Examination of EAATs and VGLUTs in brain preparations and by heterologous expression of the various cloned subtypes shows these two transporter families differ in many of their functional properties including substrate specificity and ion requirements. Alterations in the function and/or expression of these carriers have been implicated in a range of psychiatric and neurological disorders. EAATs have been implicated in cerebral stroke, epilepsy, Alzheimer's disease, HIV-associated dementia, Huntington's disease, amyotrophic lateral sclerosis (ALS) and malignant glioma, while VGLUTs have been implicated in schizophrenia. To examine the physiological role of glutamate transporters in more detail, several classes of transportable and non-transportable inhibitors have been developed, many of which are derivatives of the natural amino acids, aspartate and glutamate. This review summarizes the development of these indispensable pharmacological tools, which have been critical to our understanding of normal and abnormal synaptic transmission.

Regulation of vesicular neurotransmitter transporters

Neurotransmitters are key molecules of neurotransmission. They are concentrated first in the cytosol and then in small synaptic vesicles of presynaptic terminals by the activity of specific neurotransmitter transporters of the plasma and the vesicular membrane, respectively. It has been shown that postsynaptic responses to single neurotransmitter packets vary over a wide range, which may be due to a regulation of vesicular neurotransmitter filling. Vesicular filling depends on the availability of transmitter molecules in the cytoplasm and the active transport into secretory vesicles relying on a proton gradient. In addition, it is modulated by vesicle-associated heterotrimeric G proteins, Galphao2 and Galphaq, which regulate VMAT activities in brain and platelets, respectively, and may also be involved in the regulation of VGLUTs. It appears that the vesicular content activates the G protein, suggesting a signal transduction form the luminal site which might be mediated by a vesicular G-protein coupled receptor or, as an alternative, possibly by the transporter itself. These novel functions of G proteins in the control of transmitter storage may link regulation of the vesicular content to intracellular signal cascades.

[Pharmacology of excitatory amino acid transporters (EAATs and VGLUTs)]

L-Glutamate is a major excitatory neurotransmitter in the mammalian central nervous system (CNS). It contributes not only to fast synaptic neurotransmission but also to complex physiological processes like plasticity, learning, and memory. Glutamate is synthesized in the cytoplasm and stored in synaptic vesicles by a proton gradient-dependent uptake system (VGLUTs). Following its exocytotic release, glutamate activates different kinds of glutamate receptors and mediates excitatory neurotransmission. To terminate the action of glutamate and maintain its extracellular concentration below excitotoxic levels, glutamate is quickly removed by Na(+)-dependent glutamate transporters (EAATs). Recently, three vesicular glutamate transporters (VGLUT1-3) and five Na(+)-dependent glutamate transporters (EAAT1-5) were identified. VGLUTs and EAATs are thought to play important roles in neuronal disorders, such as amyotrophic lateral sclerosis, Alzheimer's disease, cerebral ischemia, and Huntington's disease. In this review, the development of new compounds to regulate the function of VGLUTs and EAATs will be described.

Aberrant reduction of an inhibitory protein factor in a rat epileptic model

Certain forms of seizure involve excessive glutamate transmission. We have recently identified a protein, referred to as the inhibitory protein factor (IPF), which potently inhibits glutamate uptake into isolated synaptic vesicles. In an effort to understand the mechanism underlying excessive glutamate transmission associated with seizure, we have analyzed IPF content in various brain regions of the spontaneously epileptic rat, SER (tm/tm, zi/zi), the absence-seizure tremor rat, TM (tm/tm), and the seizure-free control rats zitter ZI (zi/zi) and Wistar tremor control, each at 13 weeks of age. IPF content was found to be markedly reduced in the hippocampus, but not in the other brain regions, of SER, compared to the control and TM rats. TM rats also exhibited reduced IPF content compared to seizure-free controls. These changes appear developmentally regulated; no such alteration was observed in 8-week-old rats, which rarely show seizure. These observations indicate that an aberrant decrease in IPF is associated with certain forms of seizure; this decrease could lead to an abnormal increase in the amount of exocytotically released glutamate through its excessive accumulation in synaptic vesicles.

Prolonged depolarization of rat cerebral synaptosomes leads to an increase in vesicular glutamate content

Glutamate accumulation into synaptic vesicles is a vital step in glutamate synaptic transmission. In this study, we have explored the possibility that vesicular glutamate storage may be subject to some regulation. Synaptosomes were depolarized and subjected to [3H] glutamate under non-depolarizing conditions, and vesicular [3H] glutamate content was determined by a filter-based assay. We present evidence here that prolonged depolarization of synaptosomes leads to an increase in vesicular glutamate content. Induction of this enhanced state is time- and temperature-dependent. The enhanced state has two components, one readily reversible and the other long-lasting. The up-regulation of glutamate storage capacity could lead to an increase in quantal size and play a role in modulation of glutamate transmission efficiency.