Synaptophysin and synapsin I as tools for the study of the exo-endocytotic cycle

Motor neuron translatome reveals deregulation of SYNGR4 and PLEKHB1 in mutant TDP-43 amyotrophic lateral sclerosis models

Amyotrophic lateral sclerosis (ALS) is an incurable neurological disease with progressive loss of motor neuron (MN) function in the brain and spinal cord. Mutations in TARDBP, encoding the RNA-binding protein TDP-43, are one cause of ALS, and TDP-43 mislocalization in MNs is a key pathological feature of >95% of ALS cases. While numerous studies support altered RNA regulation by TDP-43 as a major cause of disease, specific changes within MNs that trigger disease onset remain unclear. Here, we combined translating ribosome affinity purification (TRAP) with RNA sequencing to identify molecular changes in spinal MNs of TDP-43–driven ALS at motor symptom onset. By comparing the MN translatome of hTDP-43^A315T mice to littermate controls and to mice expressing wild type hTDP-43, we identified hundreds of mRNAs that were selectively up- or downregulated in MNs. We validated the deregulated candidates Tex26, Syngr4, and Plekhb1 mRNAs in an independent TRAP experiment. Moreover, by quantitative immunostaining of spinal cord MNs, we found corresponding protein level changes for SYNGR4 and PLEKHB1. We also observed these changes in spinal MNs of an independent ALS mouse model caused by a different patient mutant allele of TDP-43, suggesting that they are general features of TDP-43-driven ALS. Thus, we identified SYNGR4 and PLEKHB1 to be deregulated in MNs at motor symptom onset in TDP-43-driven ALS models. This spatial and temporal pattern suggests that these proteins could be functionally important for driving the transition to the symptomatic phase of the disease.

The antibiotic doxycycline mimics the NGF signaling in PC12 cells: A relevant mechanism for neuroprotection

Doxycycline has been used as antibiotic since the 1960s. Recently, studies have shown that doxycycline is neuroprotective in models of neurodegenerative diseases and brain injuries, mainly due to anti-inflammatory and anti-apoptotic effects. However, it is not known if doxycycline has neurotrophic potential, which is relevant, considering the role of axonal degeneration at the early stages of neurodegeneration in Alzheimer's disease, Amyotrophic Lateral Sclerosis and Parkinson's disease as well as in normal aging. Axons are preceded by the formation of neurites, the hallmark of the neuronal differentiation induced by neurotrophins like NGF. Therefore, the modulation of neurotrophin receptors aimed at formation and regeneration of axons has been proposed as a strategy to delay the progression of neurodegeneration and has gained relevance as new techniques for early diagnosis arise. Based on these premises, we investigated the potential of doxycycline to mimic the effects of Nerve Growth Factor (NGF) with focus on the signaling pathways and neuronal modulators of neurite initiation, growth and branching. We used PC12 cells, a neuronal model widely employed to study the neurotrophic pathways and mechanisms induced by NGF. Results showed that doxycycline induced neurite outgrowth via activation of the trkA receptor and the downstream signaling pathways, PI3K/Akt and MAPK/ERK, without inducing the expression of NGF. Doxycycline also increased the expression of GAP-43, synapsin I and NF200, proteins involved in axonal and synaptic plasticity. Altogether, these data demonstrate, for the first time, the neurotrophic potential of doxycycline, which might be useful to restore the neuronal connectivity lost at the initial phase of neurodegeneration.

Highly Efficient Conversion of Motor Neuron-Like NSC-34 Cells into Functional Motor Neurons by Prostaglandin E2

Motor neuron diseases are a group of progressive neurological disorders that degenerate motor neurons. The neuroblastoma × spinal cord hybrid cell line NSC-34 is widely used as an experimental model in studies of motor neuron diseases. However, the differentiation efficiency of NSC-34 cells to neurons is not always sufficient. We have found that prostaglandin E₂ (PGE₂) induces morphological differentiation in NSC-34 cells. The present study investigated the functional properties of PGE₂-differentiated NSC-34 cells. Retinoic acid (RA), a widely-used agent inducing cell differentiation, facilitated neuritogenesis, which peaked on day 7, whereas PGE₂-induced neuritogenesis took only 2 days to reach the same level. Whole-cell patch-clamp recordings showed that the current threshold of PGE₂-treated cell action potentials was lower than that of RA-treated cells. PGE₂ and RA increased the protein expression levels of neuronal differentiation markers, microtubule-associated protein 2c and synaptophysin, and to the same extent, motor neuron-specific markers HB9 and Islet-1. On the other hand, protein levels of choline acetyltransferase and basal release of acetylcholine in PGE₂-treated cells were higher than in RA-treated cells. These results suggest that PGE₂ is a rapid and efficient differentiation-inducing factor for the preparation of functionally mature motor neurons from NSC-34 cells.

Synaptogyrin-2 Promotes Replication of a Novel Tick-borne Bunyavirus through Interacting with Viral Nonstructural Protein NSs

Synaptogyrin-2 is a non-neuronal member of the synaptogyrin family involved in synaptic vesicle biogenesis and trafficking. Little is known about the function of synaptogyrin-2. Severe fever with thrombocytopenia syndrome (SFTS) is an emerging infectious disease characterized by high fever, thrombocytopenia, and leukocytopenia with high mortality, caused by a novel tick-borne phlebovirus in the family Bunyaviridae. Our previous studies have shown that the viral nonstructural protein NSs forms inclusion bodies (IBs) that are involved in viral immune evasion, as well as viral RNA replication. In this study, we sought to elucidate the mechanism by which NSs formed the IBs, a lipid droplet-based structure confirmed by NSs co-localization with perilipin A and adipose differentiation-related protein (ADRP). Through a high throughput screening, we identified synaptogyrin-2 to be highly up-regulated in response to SFTS bunyavirus (SFTSV) infection and to be a promoter of viral replication. We demonstrated that synaptogyrin-2 interacted with NSs and was translocated into the IBs, which were reconstructed from lipid droplets into large structures in infection. Viral RNA replication decreased, and infectious virus titers were lowered significantly when synaptogyrin-2 was silenced in specific shRNA-expressing cells, which correlated with the reduced number of the large IBs restructured from regular lipid droplets. We hypothesize that synaptogyrin-2 is essential to promoting the formation of the IBs to become virus factories for viral RNA replication through its interaction with NSs. These findings unveil the function of synaptogyrin-2 as an enhancer in viral infection.

Citalopram Ameliorates Synaptic Plasticity Deficits in Different Cognition-Associated Brain Regions Induced by Social Isolation in Middle-Aged Rats

Our previous experiments demonstrated that social isolation (SI) caused AD-like tau hyperphosphorylation and spatial memory deficits in middle-aged rats. However, the underlying mechanisms of SI-induced spatial memory deficits remain elusive. Middle-aged rats (10 months) were group or isolation reared for 8 weeks. Following the initial 4-week period of rearing, citalopram (10 mg/kg i.p.) was administered for 28 days. Then, pathophysiological changes were assessed by performing behavioral, biochemical, and pathological analyses. We found that SI could cause cognitive dysfunction and decrease synaptic protein (synaptophysin or PSD93) expression in different brain regions associated with cognition, such as the prefrontal cortex, dorsal hippocampus, ventral hippocampus, amygdala, and caudal putamen, but not in the entorhinal cortex or posterior cingulate. Citalopram could significantly improve learning and memory and partially restore synaptophysin or PSD93 expression in the prefrontal cortex, hippocampus, and amygdala in SI rats. Moreover, SI decreased the number of dendritic spines in the prefrontal cortex, dorsal hippocampus, and ventral hippocampus, which could be reversed by citalopram. Furthermore, SI reduced the levels of BDNF, serine-473-phosphorylated Akt (active form), and serine-9-phosphorylated GSK-3β (inactive form) with no significant changes in the levels of total GSK-3β and Akt in the dorsal hippocampus, but not in the posterior cingulate. Our results suggest that decreased synaptic plasticity in cognition-associated regions might contribute to SI-induced cognitive deficits, and citalopram could ameliorate these deficits by promoting synaptic plasticity mainly in the prefrontal cortex, dorsal hippocampus, and ventral hippocampus. The BDNF/Akt/GSK-3β pathway plays an important role in regulating synaptic plasticity in SI rats.

Synaptic degeneration in rat brain after prolonged oral exposure to silver nanoparticles

Neurotoxicity of silver nanoparticles has been confirmed in both in vitro and in vivo studies. However, the mechanisms of the toxic action have not been fully clarified. Since nanoparticles are likely to have the ability to enter the brain and significantly accumulate in this organ, it is important to investigate their neurotoxic mechanisms. Here we examine the effect of prolonged exposure of rats to small (10nm) citrate-stabilized silver nanoparticles (as opposed to the ionic silver) on synapse ultrastructure and specific proteins. Administration of both nanosilver and ionic silver over a two-week period resulted in ultrastructural changes including blurred synapse structure and strongly enhanced density of synaptic vesicles clustering in the center of the presynaptic part. Disturbed synaptic membrane leading to liberation of synaptic vesicles into neuropil, which testifies for strong synaptic degeneration, was characteristic feature observed under AgNPs exposure. Also a noteworthy finding was the presence of myelin-like structures derived from fragmented membranes and organelles which are associated with neurodegenerative processes. Additionally, we observed significantly decreased levels of the presynaptic proteins synapsin I and synaptophysin, as well as PSD-95 protein which is an indicator of postsynaptic densities. The present study demonstrates that exposure of adult rats to both forms of silver leads to ultrastructural changes in synapses. However, it seems that small AgNPs lead to more severe synaptic degeneration, mainly in the hippocampal region of brain. The observations may indicate impairment of nerve function and, in the case of hippocampus, may predict impairment of cognitive processes.

Human A53T α-synuclein causes reversible deficits in mitochondrial function and dynamics in primary mouse cortical neurons

Parkinson's disease (PD) is the second most common neurodegenerative disease. A key pathological feature of PD is Lewy bodies, of which the major protein component is α-synuclein (α-syn). Human genetic studies have shown that mutations (A53T, A30P, E46K) and multiplication of the α-syn gene are linked to familial PD. Mice overexpressing the human A53T mutant α-syn gene develop severe movement disorders. However, the molecular mechanisms of α-syn toxicity are not well understood. Recently, mitochondrial dysfunction has been linked with multiple neurodegenerative diseases including Parkinson's disease. Here we investigated whether mitochondrial motility, dynamics and respiratory function are affected in primary neurons from a mouse model expressing the human A53T mutation. We found that mitochondrial motility was selectively inhibited in A53T neurons while transport of other organelles was not affected. In addition, A53T expressing neurons showed impairment in mitochondrial membrane potential and mitochondrial respiratory function. Furthermore, we found that rapamycin, an autophagy inducer, rescued the decreased mitochondrial mobility. Taken together, these data demonstrate that A53T α-syn impairs mitochondrial function and dynamics and the deficit of mitochondrial transport is reversible, providing further understanding of the disease pathogenesis and a potential therapeutic strategy for PD.

Expression of Nav1.7 in DRG neurons extends from peripheral terminals in the skin to central preterminal branches and terminals in the dorsal horn

BACKGROUND

Sodium channel Nav1.7 has emerged as a target of considerable interest in pain research, since loss-of-function mutations in SCN9A, the gene that encodes Nav1.7, are associated with a syndrome of congenital insensitivity to pain, gain-of-function mutations are linked to the debiliting chronic pain conditions erythromelalgia and paroxysmal extreme pain disorder, and upregulated expression of Nav1.7 accompanies pain in diabetes and inflammation. Since Nav1.7 has been implicated as playing a critical role in pain pathways, we examined by immunocytochemical methods the expression and distribution of Nav1.7 in rat dorsal root ganglia neurons, from peripheral terminals in the skin to central terminals in the spinal cord dorsal horn.

RESULTS

Nav1.7 is robustly expressed within the somata of peptidergic and non-peptidergic DRG neurons, and along the peripherally- and centrally-directed C-fibers of these cells. Nav1.7 is also expressed at nodes of Ranvier in a subpopulation of Aδ-fibers within sciatic nerve and dorsal root. The peripheral terminals of DRG neurons within skin, intraepidermal nerve fibers (IENF), exhibit robust Nav1.7 immunolabeling. The central projections of DRG neurons in the superficial lamina of spinal cord dorsal horn also display Nav1.7 immunoreactivity which extends to presynaptic terminals.

CONCLUSIONS

The expression of Nav1.7 in DRG neurons extends from peripheral terminals in the skin to preterminal central branches and terminals in the dorsal horn. These data support a major contribution for Nav1.7 in pain pathways, including action potential electrogenesis, conduction along axonal trunks and depolarization/invasion of presynaptic axons. The findings presented here may be important for pharmaceutical development, where target engagement in the right compartment is essential.

Cyclin-dependent kinase 5, Munc18a and Munc18-interacting protein 1/X11α protein up-regulation in Alzheimer’s disease

Besides formation of neurofibrillary tangles and neuron loss, the Alzheimer's disease brain is characterized by neuritic plaques consisting of beta-amyloid peptide deposits and impaired neurotransmission. The proteins Munc18a, Munc18-interacting protein 1 and Munc18-interacting protein 2 mediate exocytosis and decrease beta-amyloid peptide formation. Cyclin-dependent kinase 5 and its activator p35 disrupt Munc18a-syntaxin 1 binding, thereby promoting synaptic vesicle fusion during exocytosis. We investigated protein levels of the signaling pathway: p35, cyclin-dependent kinase 5, Munc18a, syntaxin 1A and 1B, Munc18-interacting protein 1 and Munc18-interacting protein 2 in Alzheimer's disease cortex and found that this pathway was up-regulated in the Alzheimer's disease parietal and occipital cortex. In the cortex of transgenic Tg2576 mice over-expressing human beta-amyloid precursor protein with the Swedish mutation known to lead to familial Alzheimer's disease, which have substantial levels of beta-amyloid peptide but lack neurofibrillary tangles and neuron loss, no alterations of protein levels were detected. These data suggest that the pathway is enhanced in dying or surviving neurons and might serve a protective role by compensating for decreased neurotransmission and decreasing beta-amyloid peptide levels early during the progression of Alzheimer's disease.

Dynamic responses of presynaptic terminal membrane pools following KCl and sucrose stimulation

The cholinergic presynaptic terminals of Torpedo electric organ have been examined morphometrically following stimulation by KCI and sucrose. The objective was to confirm correlations predicted by the vesicle hypothesis between miniature end-plate potentials (MEPPs) and morphometric changes in terminal ultrastructure. Both secretegogues generated high frequencies of MEPPs and also distinctive though differing ultrastructural changes. The synaptic vesicles show classes of 68 and 90 nm diameters and both store acetylcholine (ACh). KCl stimulation depleted the 90 nm class first whereas sucrose reversed the order of depletion. Very few instances of actual vesicle fusion were seen. Dose-response correlations between vesicle density and secretegogue strength (mM) and duration were higher with sucrose. Both secretegogues produced declines in vesicle numbers and densities and yielded multimodal distributions of large vesicles with an average 160 nm mean diameter. No meaningful correlations were detected between numbers of MEPPs and vesicles and little evidence was found to indicate that vesicles were fusing to terminal plasma membrane in numbers approximating MEPP release. Linear regression analysis was used to quantitatively examine relationships between the vesicle membrane pool and other pools of the putative exo/endocytotic pathway. Correlation coefficients between vesicle and terminal plasma membrane pools were non-significant and of positive sign, indicating independent, similar responses. Non-significant, negative coefficients were obtained when vacuole and 160 nm vesicle membrane values were included. These tests further argue against claims that vesicles are actively fusing with the plasma membrane. These conflicting findings for both secretegogues preclude meaningful correlations between vesicle changes and numbers of MEPPs generated and again emphasize the difficulty of validating the vesicle hypothesis by ultrastructural means. On the other hand, the study shows that vesicular, vacuolar and terminal membrane pools are dynamically changing during transmitter release, presumably interacting with cytosolic membrane constituents. A dynamical release process therefore has been proposed to account for the two classes of MEPPs, the rapid changes in class ratio and the mutable characteristics of the bell-MEPP that presently challenge the quantal-vesicular claims of prepackaged, immutable, exocytotically released packets of transmitter. This model features a state for each MEPP class with class and size determined at moment of release. For example, a single flicker of a channel would generate the sub-MEPP (defined subunit of an MEPP) and 7-20 flickering channels would generate the bell-MEPP.

Characterization of fluo-3 labelling of dense bodies at the hair cell's presynaptic active zone

The presynaptic active zone is the critical region of a chemical synapse at which Ca2+ entry provokes neurotransmitter release by exocytotic fusion of synaptic vesicles. To facilitate investigations of synaptic function, we have identified a group of fluorescent substances that label individual active zones in living hair cells. The Ca2+ indicator fluo-3, the compound studied in most detail, binds to the presynaptic dense bodies that are characteristic of active zones in hair cells and other cells that tonically release transmitter. The indicator's binding is reversible, with a dissociation constant of approximately 350 microM. Because fluo-3 that is bound to a presynaptic dense body continues to detect Ca2+ with an unaltered dissociation constant, the binding of this substance provides a valuable tool for exploration of the Ca2+ concentration at the site of vesicle fusion.

Effects of decentralization on the levels of GAP-43 and p38 (synaptophysin) in sympathetic adrenergic neurons: a semi-quantitative study using immunofluorescence and confocal laser scanning microscopy

The distribution of GAP-43 in superior cervical ganglion (SCG) and iris were studied in normal animals and following decentralization using immunofluorescence and confocal laser scanning microscopy (CLSM). GAP-43-like immunoreactivity (LI) was compared with p38 (synaptophysin)-LI, and tyrosin hydroxylase (TH)-LI. In the control SCG, GAP-43-LI and p38-LI were mainly localized in nerve terminals around the principal neurons. The neuronal perikarya were negative for GAP-43, but positive for p38 in a perinuclear zone, as well as positive for TH. SIF cells (Small Intensely Fluorescent cells, ganglionic interneurons) were positive for GAP-43, TH and p38. One day after decentralization, GAP-43-LI and p38-LI in nerve terminals around principal neurons had disappeared. Some of the principal neurons showed a weak GAP-43-immunoreactivity. Three days post-decentralization, GAP-43- and p38-positive nerve terminals around the neurons had reappeared in considerable numbers and the intra-ganglionic nerve bundles were positive for both antibodies. In the control irides, GAP-43-LI and p38-LI were distributed in a varicose pattern in the nerve bundles, around blood vessels and in the network of terminals. Double labelling studies showed that GAP-43-LI was colocalized with TH-LI and p38-LI. The network of terminals in the dilator plate of the irides was quantified by measuring the fluorescence intensity of randomly selected areas, using CLSM. Three days after decentralization the intensity of GAP-43-LI and p38-LI had significantly increased. TH-LI had decreased 8 days after decentralization. The results indicate that GAP-43-LI and p38-LI are normally present in the nerve fibers and terminals of both pre- and post-ganglionic neurons in adult rats. The expression of GAP-43-LI and p38-LI in post-ganglionic neurons is preganglionically regulated, as indicated by the increased expression after decentralization. The expression of p38 in these neurons is probably regulated via mechanisms that are separate from those which regulate GAP-43, since it showed a different time course than that of GAP-43-LI.

Morphological aberrations in therapy-resistant partial epilepsy (TRPE). Confocal laser scanning and 3D reconstructions of Lucifer Yellow injected atypical pyramidal neurons in epileptic human cortex

Epileptic temporal and parietal cortices, removed from 6 patients with therapy-resistant (intractable) partial epilepsy (TRPE) during neurosurgery, were studied. Neurons (40-50 in each slice) in laminae I-VI and white matter were injected with Lucifer Yellow (LY). Samples were examined in a confocal laser scanning microscope (BioRad [Richmond, CA] MRC 600), and individual cells were scanned at 0.1-2 microns incremental levels. 2D maximal linear projection was used for overview. Frames (50-60) of scanned neurons were transformed into 3D volumes, using VoxelView software on a Silicone Graphics workstation, and rotated. All samples contained pyramidal neurons with duplicated apical dendrites, additional basal dendrites, or were misplaced in a horizontal position in the white matter. Rarely were such cells observed in normal cases. The relation between the observations and the disease is discussed. The attempt to simultaneously apply immunofluorescence was successful concerning synaptic vesicle antigens. This approach will be used for a detailed study of the synaptology of this disease.

Tyrosine phosphorylation and activation of mitogen-activated protein kinase in the rat brain following transient cerebral ischemia

Activation of trophic factor receptors stimulates tyrosine phosphorylation on proteins and supports neuronal survival. We report that in the recovery phase following reversible cerebral ischemia, tyrosine phosphorylation increases in the membrane fraction of the resistant hippocampal CA3/dentate gyrus (DG) region, whereas in the sensitive CA1 region or striatum, tyrosine phosphorylation is less marked or decreases. In the cytosolic fractions, a 42-kDa protein, identified as mitogen-activated protein (MAP) kinase, is markedly phosphorylated and activated immediately following ischemia, in particular in CA3/DG, but not in striatum. In the CA1 region, phosphorylation of MAP kinase is less intense and decreases later during reperfusion, which could explain the delay of neuronal degeneration in this structure. The data suggest that in ischemia-resistant neurons the growth factor receptor-coupled signaling cascade is stimulated and, through its effects on DNA transcription and mRNA translation, supports neuronal survival.

The role of synaptic proteins in the pathogenesis of disorders of the central nervous system

Complex sets of nervous system functions are dependent on proper working of the synaptic apparatus, and these functions are regulated by diverse synaptic proteins that are distributed in various subcellular compartments of the synapse. The most extensively studied synaptic proteins are synaptophysin, the synapsins, growth associated protein 43 (GAP-43), SV-2, and p65. Moreover, synaptic terminals contain a great number of other proteins involved in calcium transport, neurotransmission, signaling, growth and plasticity. Probes against various synaptic proteins have recently been used to study synaptic alterations in human disease, as well as in experimental models of neurological disorders. Such probes are useful markers of synaptic function and synaptic population density in the nervous system. For the present, we will review the role of synaptic proteins in the following conditions: Alzheimer's disease (AD) and other disorders including ischemia, disorders where synapse-associated proteins are abnormally accumulated in the nerve terminals, synaptic proteins altered after denervation, and synaptic proteins as markers in neoplastic disorders. The study of the molecular alterations of the synapses and of plasticity might yield important clues as to the mechanisms of neurodegeneration in AD, and of the patterns of presynaptic and dendritic damage under diverse pathological conditions.

Neuronal protein NP185 is developmentally regulated, initially expressed during synaptogenesis, and localized in synaptic terminals

Evidence is presented here that demonstrates the presence of NP185 (AP3) in neuronal cells, specifically within syn-aptic terminals of the central nervous system and in the peripheral nervous system, particularly in the neuro-muscular junction of adult chicken muscle. Biochemical results obtained in our laboratories indicate that NP185 is associated with brain synaptic vesicles, with clathrin-coated vesicles, and with the synaptosomal plasma membrane. Also, NP185 binds to tubulin and clathrin light chains and the binding is regulated by phosphorylation (Su et al., 1991). Based on these properties and the data reported here, we advance the postulate that NP185 fulfills multiple functions in synaptic terminals. One function is that of a plasma membrane docking or channel protein, another of a signaling molecule for brain vesicles to reach the synaptic terminal region, and a third is that of a recycling molecule by binding to protein components on the lipid bilayer of the synaptic plasma membrane during the process of endocytosis. In support of these premises, a thorough study of NP185 using the developing chick brain, adult mouse brain, and chicken straited muscle was begun by temporally and spatially mapping the expression and localization of NP185 in evolving and mature nerve endings. To achieve these objectives, monoclonal antibodies to NP185 were used for immunocytochemistry in tissue sections of chicken and mouse cerebella. The distribution of NP185 was compared with those of other cytoskeletal and cytoplasmic proteins of axons and synapses, namely synaptophysin, vimentin, neurofilament NF68, and the intermediate filaments of glial cells (GFAP). The data indicate that expression of NP185 temporally coincides with synaptogenesis, and that the distribution of this protein is specific for synaptic terminal buttons of the CNS and the PNS.

Purification of an Arg-Gly-Asp selective matrix receptor from brain synaptic plasma membranes

Brain synaptic plasma membranes specifically associated with matrix protein monolayers containing the Arg-Gly-Asp sequence recognized by integrin-type adhesion receptors. Experiments using fibronectin affinity chromatography to identify the synaptosomal receptors responsible for this interaction led to the purification of a 55 kDa Arg-Gly-Asp recognition protein that is labelled by antibodies against the alpha 5 beta 1 integrin.