A systematic scoping review of the neurological effects of COVID-19

BACKGROUND

The global coronavirus 2019 (COVID-19) pandemic began in early 2020, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). In mid-2020 the CIAO (Modelling the Pathogenesis of COVID-19 Using the Adverse Outcome Pathway Framework) project was established, bringing together over 75 interdisciplinary scientists worldwide to collaboratively investigate the underlying biological mechanisms of COVID-19 and consolidate the data using the Adverse Outcome Pathway (AOP) Framework. Neurological symptoms such as anosmia and encephalitis have been frequently reported to be associated with infection with SARS-CoV-2.

OBJECTIVE

Within CIAO, a working group was formed to conduct a systematic scoping review of COVID-19 and its related neurological symptoms to determine which key events and modulating factors are most commonly reported and to identify knowledge gaps.

DESIGN

LitCOVID was used to retrieve 86,075 papers of which 10,244 contained relevant keywords. After title and abstract screening, 2,328 remained and their full texts were reviewed based on predefined inclusion and exclusion criteria. 991 studies fulfilled the inclusion criteria and were retrieved to conduct knowledge synthesis.

RESULTS

The majority of publications reported human observational studies. Early key events were less likely to be reported compared to middle and late key events/adverse outcomes. The majority of modulating factors described related to age or sex. Less recognised COVID-19 associated AO or neurological effects of COVID-19 were also identified including multiple sclerosis/demyelination, neurodegeneration/cognitive effects and peripheral neuronal effects.

CONCLUSION

There were many methodological and reporting issues noted in the reviewed studies. In particular, publication abstracts would benefit from clearer reporting of the methods and endpoints used and the key findings, to ensure relevant papers are included when systematic reviews are conducted. The information extracted from the scoping review may be useful in understanding the mechanisms of neurological effects of COVID-19 and to further develop or support existing AOPs linking COVID-19 and its neurological key events and adverse outcomes. Further evaluation of the less recognised COVID-19 effects is needed.

The Adverse Outcome Pathway Framework Applied to Neurological Symptoms of COVID-19

Several reports have shown that the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has the potential to also be neurotropic. However, the mechanisms by which SARS-CoV-2 induces neurologic injury, including neurological and/or psychological symptoms, remain unclear. In this review, the available knowledge on the neurobiological mechanisms underlying COVID-19 was organized using the AOP framework. Four AOPs leading to neurological adverse outcomes (AO), anosmia, encephalitis, stroke, and seizure, were developed. Biological key events (KEs) identified to induce these AOs included binding to ACE2, blood–brain barrier (BBB) disruption, hypoxia, neuroinflammation, and oxidative stress. The modularity of AOPs allows the construction of AOP networks to visualize core pathways and recognize neuroinflammation and BBB disruption as shared mechanisms. Furthermore, the impact on the neurological AOPs of COVID-19 by modulating and multiscale factors such as age, psychological stress, nutrition, poverty, and food insecurity was discussed. Organizing the existing knowledge along an AOP framework can represent a valuable tool to understand disease mechanisms and identify data gaps and potentially contribute to treatment, and prevention. This AOP-aligned approach also facilitates synergy between experts from different backgrounds, while the fast-evolving and disruptive nature of COVID-19 emphasizes the need for interdisciplinarity and cross-community research.

Organophosphates modulate tissue transglutaminase activity in differentiated C6 neural cells

OBJECTIVE

The organophosphate compounds chlorpyrifos (O, O-diethyl O-[3,5,6-trichloro-2-pyridinyl] phosphorothioate, CPF) and phenyl saligenin phosphate (PSP) have been widely implicated in developmental neurotoxicity and neurodegeneration. However, the underlying mechanism remains unclear. Transglutaminase (TG)2 is a calcium ion (Ca2+)-dependent enzyme with an important role in neuronal cell outgrowth and differentiation and in neurotoxin activity and is modulated by organophosphates.

MATERIALS AND METHODS

We studied TG2 activity modulation by CPO and PSP during differentiation in C6 glioma cells. We studied the effects of CPO or PSP treatment with or without the TG2 inhibitor Z-DON and identified potential TG2 protein substrates via mass spectrometry.

RESULTS

PSP and CPO did not affect cell viability but affected TG2 activity in differentiating cells. Our results indicate that the organophosphate-induced amine incorporation activity of TG2 may have a direct effect on neuronal outgrowth, differentiation, and cell survival by modifying several essential microtubule proteins, including tubulin. Inhibiting TG2 reduced neurite length but not cell survival.

CONCLUSIONS

TG2 inhibitors can protect against organophosphate-induced neuropathy and could be used for developing novel therapeutic strategies for treating brain cancer and neurodegenerative disorders.

COVID-19 through Adverse Outcome Pathways: Building networks to better understand the disease - 3rd CIAO AOP Design Workshop

On April 28-29, 2021, 50 scientists from different fields of expertise met for the 3rd online CIAO workshop. The CIAO project “Modelling the Pathogenesis of COVID-19 using the Adverse Outcome Pathway (AOP) framework” aims at building a holistic assembly of the available scientific knowledge on COVID-19 using the AOP framework. An individual AOP depicts the disease progression from the initial contact with the SARS-CoV-2 virus through biological key events (KE) toward an adverse outcome such as respiratory distress, anosmia or multiorgan failure. Assembling the individual AOPs into a network highlights shared KEs as central biological nodes involved in multiple outcomes observed in COVID-19 patients. During the workshop, the KEs and AOPs established so far by the CIAO members were presented and posi­tioned on a timeline of the disease course. Modulating factors influencing the progression and severity of the disease were also addressed as well as factors beyond purely biological phenomena. CIAO relies on an interdisciplinary crowd­sourcing effort, therefore, approaches to expand the CIAO network by widening the crowd and reaching stakeholders were also discussed. To conclude the workshop, it was decided that the AOPs/KEs will be further consolidated, inte­grating virus variants and long COVID when relevant, while an outreach campaign will be launched to broaden the CIAO scientific crowd.

Phenyl Saligenin Phosphate Disrupts Cell Morphology and the Actin Cytoskeleton in Differentiating H9c2 Cardiomyoblasts and Human-Induced Pluripotent Stem-Cell-Derived Cardiomyocyte Progenitor Cells

We have previously shown that phenyl saligenin phosphate (PSP), an organophosphorus compound which is classed as a weak inhibitor of acetylcholinesterase, triggered cytotoxicity in mitotic and differentiated H9c2 cardiomyoblasts. The aim of this study was to assess whether sublethal concentrations of PSP could disrupt the morphology of differentiating rat H9c2 cardiomyoblasts and human-induced pluripotent stem-cell-derived cardiomyocyte progenitor cells (hiPSC-CMs) and to assess the underlying cytoskeletal changes. PSP-induced changes in protein expression were monitored via Western blotting, immunocytochemistry, and proteomic analysis. PSP-mediated cytotoxicity was determined by measuring MTT reduction, LDH release, and caspase-3 activity. Sublethal exposure to PSP (3 μM) induced morphological changes in differentiating H9c2 cells (7, 9, and 13 days), reflected by reduced numbers of spindle-shaped cells. Moreover, this treatment (7 days) attenuated the expression of the cytoskeletal proteins cardiac troponin I, tropomyosin-1, and α-actin. Further proteomic analysis identified nine proteins (e.g., heat shock protein 90-β and calumenin) which were down-regulated by PSP exposure in H9c2 cells. To assess the cytotoxic effects of organophosphorus compounds in a human cell model, we determined their effects on human-induced pluripotent stem-cell-derived cardiomyocyte progenitor cells. Chlorpyrifos and diazinon-induced cytotoxicity (48 h) was evident only at concentrations >100 μM. By contrast, PSP exhibited cytotoxicity in hiPSC-CMs at a concentration of 25 μM following 48 h exposure. Finally, sublethal exposure to PSP (3 μM; 7 days) induced morphological changes and decreased the expression of cardiac troponin I, tropomyosin-1, and α-actin in hiPSC-CMs. In summary, our data suggest cardiomyocyte morphology is disrupted in both cell models by sublethal concentrations of PSP via modulation of cytoskeletal protein expression.

Neurite outgrowth inhibitory levels of organophosphates induce tissue transglutaminase activity in differentiating N2a cells: evidence for covalent adduct formation

Organophosphate compounds (OPs) induce both acute and delayed neurotoxic effects, the latter of which is believed to involve their interaction with proteins other than acetylcholinesterase. However, few OP-binding proteins have been identified that may have a direct role in OP-induced delayed neurotoxicity. Given their ability to disrupt Ca²⁺ homeostasis, a key aim of the current work was to investigate the effects of sub-lethal neurite outgrowth inhibitory levels of OPs on the Ca²⁺-dependent enzyme tissue transglutaminase (TG2). At 1-10 µM, the OPs phenyl saligenin phosphate (PSP) and chlorpyrifos oxon (CPO) had no effect cell viability but induced concentration-dependent decreases in neurite outgrowth in differentiating N2a neuroblastoma cells. The activity of TG2 increased in cell lysates of differentiating cells exposed for 24 h to PSP and chlorpyrifos oxon CPO (10 µM), as determined by biotin-cadaverine incorporation assays. Exposure to both OPs (3 and/or 10 µM) also enhanced in situ incorporation of the membrane permeable substrate biotin-X-cadaverine, as indicated by Western blot analysis of treated cell lysates probed with ExtrAvidin peroxidase and fluorescence microscopy of cell monolayers incubated with FITC-streptavidin. Both OPs (10 µM) stimulated the activity of human and mouse recombinant TG2 and covalent labelling of TG2 with dansylamine-labelled PSP was demonstrated by fluorescence imaging following SDS-PAGE. A number of TG2 substrates were tentatively identified by mass spectrometry, including cytoskeletal proteins, chaperones and proteins involved protein synthesis and gene regulation. We propose that the elevated TG2 activity observed is due to the formation of a novel covalent adduct between TG2 and OPs.

A co-culture nanofibre scaffold model of neural cell degeneration in relevance to Parkinson's disease

Current therapeutic strategies for Parkinson’s disease (PD) aim to delay progression or replace damaged neurons by restoring the original neuronal structures. The poor regenerative capacity of neural tissue highlights the need for the development of cellular environments to model the pathogenesis of PD. In the current work, we have characterised the growth, survival and response to PD mimetics of human SH-SY5Y neuroblastoma and U-87MG glioblastoma cell lines cultured on polyacrylonitrile (PAN) and Jeffamine® doped polyacrylonitrile (PJ) nano-scaffolds. Differentiation induced by a range of agents was evaluated by immunoassays of neural protein biomarkers. PAN and PJ nanofibre scaffolds provided suitable three-dimensional (3D) environment to support the growth, differentiation and network formation of dopaminergic neuron- and astrocyte-like cell populations, respectively. The scaffolds selectively supported the survival and differentiation of both cell populations with prolonged neuronal survival when exposed to PD mimetics in the presence of astrocytes in a co-culture model. Such 3D nanoscaffold-based assays could aid our understanding of the molecular basis of PD mimetic-induced Parkinsonism and the discovery of neuroprotective agents.

Activation of transglutaminase 2 by nerve growth factor in differentiating neuroblastoma cells: A role in cell survival and neurite outgrowth

NGF (nerve growth factor) and tissue transglutaminase (TG2) play important roles in neurite outgrowth and modulation of neuronal cell survival. In this study, we investigated the regulation of TG2 transamidase activity by NGF in retinoic acid-induced differentiating mouse N2a and human SH-SY5Y neuroblastoma cells. TG2 transamidase activity was determined using an amine incorporation and a peptide cross linking assay. In situ TG2 activity was assessed by visualising the incorporation of biotin-X-cadaverine using confocal microscopy. The role of TG2 in NGF-induced cytoprotection and neurite outgrowth was investigated by monitoring hypoxia-induced cell death and appearance of axonal-like processes, respectively. The amine incorporation and protein crosslinking activity of TG2 increased in a time and concentration-dependent manner following stimulation with NGF in N2a and SH-SY5Y cells. NGF mediated increases in TG2 activity were abolished by the TG2 inhibitors Z-DON (Z-ZON-Val-Pro-Leu-OMe; Benzyloxycarbonyl-(6-Diazo-5-oxonorleucinyl)-l-valinyl-l-prolinyl-l-leucinmethylester) and R283 (1,3,dimethyl-2[2-oxo-propyl]thio)imidazole chloride) and by pharmacological inhibition of extracellular signal-regulated kinases 1 and 2 (ERK1/2), protein kinase B (PKB) and protein kinase C (PKC), and removal of extracellular Ca²⁺. Fluorescence microscopy demonstrated NGF induced in situ TG2 activity. TG2 inhibition blocked NGF-induced attenuation of hypoxia-induced cell death and neurite outgrowth in both cell lines. Together, these results demonstrate that NGF stimulates TG2 transamidase activity via a ERK1/2, PKB and PKC-dependent pathway in differentiating mouse N2a and human SH-SY5Y neuroblastoma cells. Furthermore, NGF-induced cytoprotection and neurite outgrowth are dependent upon TG2. These results suggest a novel and important role of TG2 in the cellular functions of NGF.

Role of transglutaminase 2 in PAC1 receptor mediated protection against hypoxia-induced cell death and neurite outgrowth in differentiating N2a neuroblastoma cells

The PAC₁ receptor and tissue transglutaminase (TG2) play important roles in neurite outgrowth and modulation of neuronal cell survival. In this study, we investigated the regulation of TG2 activity by the PAC₁ receptor in retinoic acid-induced differentiating N2a neuroblastoma cells. TG2 transamidase activity was determined using an amine incorporation and a peptide cross linking assay. In situ TG2 activity was assessed by visualising the incorporation of biotin-X-cadaverine using confocal microscopy. TG2 phosphorylation was monitored via immunoprecipitation and Western blotting. The role of TG2 in PAC₁ receptor-induced cytoprotection and neurite outgrowth was investigated by monitoring hypoxia-induced cell death and appearance of axonal-like processes, respectively. The amine incorporation and protein crosslinking activity of TG2 increased in a time and concentration-dependent manner following stimulation with pituitary adenylate cyclase-activating polypeptide-27 (PACAP-27). PACAP-27 mediated increases in TG2 activity were abolished by the TG2 inhibitors Z-DON and R283 and by pharmacological inhibition of protein kinase A (KT 5720 and Rp-cAMPs), protein kinase C (Ro 31-8220), MEK1/2 (PD 98059), and removal of extracellular Ca²⁺. Fluorescence microscopy demonstrated PACAP-27 induced in situ TG2 activity. TG2 inhibition blocked PACAP-27 induced attenuation of hypoxia-induced cell death and outgrowth of axon-like processes. TG2 activation and cytoprotection were also observed in human SH-SY5Y cells. Together, these results demonstrate that TG2 activity was stimulated downstream of the PAC₁ receptor via a multi protein kinase dependent pathway. Furthermore, PAC₁ receptor-induced cytoprotection and neurite outgrowth are dependent upon TG2. These results highlight the importance of TG2 in the cellular functions of the PAC₁ receptor.

Phenyl Saligenin Phosphate Induced Caspase-3 and c-Jun N-Terminal Kinase Activation in Cardiomyocyte-Like Cells

At present, little is known about the effect(s) of organophosphorous compounds (OPs) on cardiomyocytes. In this study, we have investigated the effects of phenyl saligenin phosphate (PSP), two organophosphorothioate insecticides (diazinon and chlorpyrifos), and their acutely toxic metabolites (diazoxon and chlorpyrifos oxon) on mitotic and differentiated H9c2 cardiomyoblasts. OP-induced cytotoxicity was assessed by monitoring MTT reduction, LDH release, and caspase-3 activity. Cytotoxicity was not observed with diazinon, diazoxon, or chlorpyrifos oxon (48 h exposure; 200 μM). Chlorpyrifos-induced cytotoxicity was only evident at concentrations >100 μM. In marked contrast, PSP displayed pronounced cytotoxicity toward mitotic and differentiated H9c2 cells. PSP triggered the activation of JNK1/2 but not ERK1/2, p38 MAPK, or PKB, suggesting a role for this pro-apoptotic protein kinase in PSP-induced cell death. The JNK1/2 inhibitor SP 600125 attenuated PSP-induced caspase-3 and JNK1/2 activation, confirming the role of JNK1/2 in PSP-induced cytotoxicity. Fluorescently labeled PSP (dansylated PSP) was used to identify novel PSP binding proteins. Dansylated PSP displayed cytotoxicity toward differentiated H9c2 cells. 2D-gel electrophoresis profiles of cells treated with dansylated PSP (25 μM) were used to identify proteins fluorescently labeled with dansylated PSP. Proteomic analysis identified tropomyosin, heat shock protein β-1, and nucleolar protein 58 as novel protein targets for PSP. In summary, PSP triggers cytotoxicity in differentiated H9c2 cardiomyoblasts via JNK1/2-mediated activation of caspase-3. Further studies are required to investigate whether the identified novel protein targets of PSP play a role in the cytotoxicity of this OP, which is usually associated with the development of OP-induced delayed neuropathy.

Modulation of transglutaminase 2 activity in H9c2 cells by PKC and PKA signalling: a role for transglutaminase 2 in cytoprotection

BACKGROUND AND PURPOSE

Tissue transglutaminase (TG2) has been shown to mediate cell survival in many cell types. In this study, we investigated whether the role of TG2 in cytoprotection was mediated by the activation of PKA and PKC in cardiomyocyte-like H9c2 cells.

EXPERIMENTAL APPROACH

H9c2 cells were extracted following stimulation with phorbol-12-myristate-13-acetate (PMA) and forskolin. Transglutaminase activity was determined using an amine incorporating and a protein crosslinking assay. The presence of TG isoforms (TG1, 2, 3) was determined using Western blot analysis. The role of TG2 in PMA- and forskolin-induced cytoprotection was investigated by monitoring H₂O₂-induced oxidative stress in H9c2 cells.

KEY RESULTS

Western blotting showed TG2 >> TG1 protein expression but no detectable TG3. The amine incorporating activity of TG2 in H9c2 cells increased in a time and concentration-dependent manner following stimulation with PMA and forskolin. PMA and forskolin-induced TG2 activity was blocked by PKC (Ro 31-8220) and PKA (KT 5720 and Rp-8-Cl-cAMPS) inhibitors respectively. The PMA- and forskolin-induced increases in TG2 activity were attenuated by the TG2 inhibitors Z-DON and R283. Immunocytochemistry revealed TG2-mediated biotin-X-cadaverine incorporation into proteins and proteomic analysis identified known (β-tubulin) and novel (α-actinin) protein substrates for TG2. Pretreatment with PMA and forskolin reversed H2 O2 -induced decrease in MTT reduction and release of LDH. TG2 inhibitors R283 and Z-DON blocked PMA- and forskolin-induced cytoprotection.

CONCLUSIONS AND IMPLICATIONS

TG2 activity was stimulated via PKA- and PKC-dependent signalling pathways in H9c2 cells These results suggest a role for TG2 in cytoprotection induced by these kinases.

Cardioprotective and cardiotoxic effects of quercetin and two of its in vivo metabolites on differentiated h9c2 cardiomyocytes

Whilst mitotic rat embryonic cardiomyoblast-derived H9c2 cells have been widely used as a model system to study the protective mechanisms associated with flavonoids, they are not fully differentiated cardiac cells. Hence, the aim of this study was to investigate the cardioprotective and cardiotoxic actions of quercetin and two of its major in vivo metabolites, quercetin 3-glucuronide and 3'-O-methyl quercetin, using differentiated H9c2 cells. The differentiated cardiomyocyte-like phenotype was confirmed by monitoring expression of cardiac troponin 1 after 7 days of culture in reduced serum medium containing 10 nM all-trans retinoic acid. Quercetin-induced cardiotoxicity was assessed by monitoring MTT reduction, lactate dehydrogenase (LDH) release, caspase 3 activity and reactive oxygen species production after prolonged flavonoid exposure (72 hr). Cardiotoxicity was observed with quercetin and 3'-O-methyl quercetin, but not quercetin 3-glucuronide. Cardioprotection was assessed by pre-treating differentiated H9c2 cells with quercetin or its metabolites for 24 hr prior to 2-hr exposure to 600 μM H2 O2, after which oxidative stress-induced cell damage was assessed by measuring MTT reduction and LDH release. Cardioprotection was observed with quercetin and 3'-O-methyl quercetin, but not with quercetin 3-glucuronide. Quercetin attenuated H2 O2 -induced activation of ERK1/2, PKB, p38 MAPK and JNK, but inhibitors of these kinases did not modulate quercetin-induced protection or H2 O2 -induced cell death. In summary, quercetin triggers cardioprotection against oxidative stress-induced cell death and cardiotoxicity after prolonged exposure. Further studies are required to investigate the complex interplay between the numerous signalling pathways that are modulated by quercetin and which may contribute to the cardioprotective and cardiotoxic effects of this important flavonoid.

Alterations in the mitochondrial proteome of neuroblastoma cells in response to complex 1 inhibition

Increasing evidence points to mitochondrial dysfunction in Parkinson's disease (PD) associated with complex I dysfunction, but the exact pathways which lead to cell death have not been resolved. 2D-gel electrophoresis profiles of isolated mitochondria from neuroblastoma cells treated with subcytotoxic concentrations of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), a well-characterized complex I inhibitor, were assessed to identify associated targets. Up to 27 differentially expressed proteins were observed, of which 16 were identified using peptide mass fingerprinting. Changes in protein levels were validated by immunoprobing 1D blots, confirming increases in heat shock cognate 71 kDa (Hsc70), 60 kDa heat shock protein (Hsp60), fumarase, glutamate oxaloacetate transaminase 2, ATP synthase subunit d, and voltage-dependent anion-channel 1 (VDAC1). Immunoprobing of 2D blots revealed isoform changes in Hsc70, Hsp60, and VDAC1. Subcytotoxic concentrations of MPTP modulated a host of mitochondrial proteins including chaperones, metabolic enzymes, oxidative phosphorylation-related proteins, an inner mitochondrial protein (mitofilin), and an outer mitochondrial membrane protein (VDAC1). Early changes in chaperones suggest a regulated link between complex 1 inhibition and protein folding. VDAC1, a multifunctional protein, may have a key role in signaling between mitochondria and the rest of the cell prior to cell death. Our work provides new important information of relevance to PD.

Neuroprotection from diazinon-induced toxicity in differentiating murine N2a neuroblastoma cells

In previous work, the outgrowth of axon-like processes by differentiating mouse N2a neuroblastoma cells was shown to be inhibited by exposure to 10 microM diazinon. In the present work, N2a cells were induced to differentiate for 24 h in the presence and absence of 10 microM diazinon and 20% (v/v) conditioned medium derived from differentiating rat C6 glioma cells. Cells were then stained or lysed for morphological and biochemical analyses, respectively. The data showed that co-treatment with conditioned medium prevented the neurite inhibitory effect of diazinon. Furthermore, a significant recovery was also observed in the reduced levels of neurofilament heavy chain (NFH), heat shock protein-70 (HSP-70) and growth-associated protein-43 (GAP-43) observed as a result of diazinon treatment in the absence of conditioned medium, as seen by densitometric analysis of Western blots of cell lysates probed with monoclonal antibodies N52, BRM-22 and GAP-7B10. By contrast, no significant change was noted in the reactivity of cell lysates with antibodies against alpha- and beta-tubulin under any condition tested. After pre-incubation with a polyclonal anti-glial cell line-derived neurotrophic factor (GDNF) antibody, conditioned medium derived from rat C6 glioma cells lost its ability to protect N2a cells against the neurite inhibitory effects of diazinon. In conclusion, these data demonstrate that C6 conditioned medium protects N2a cells from the neurite inhibitory effects of diazinon by blocking molecular events leading to axon damage and that GDNF is implicated in these effects.

Inhibition of extension outgrowth in differentiating rat C6 glioma cells by chlorpyrifos and chlorpyrifos oxon: effects on microtubule proteins

The aim of this work was to assess the toxic effects of the phosphorothionate insecticide chlorpyrifos (CPF) and its major in vivo metabolite chlorpyrifos oxon (CPO) on differentiating rat C6 glioma cells. At sublethal concentrations (1-10 microM), both compounds were able to inhibit the development of extensions from C6 cells induced to differentiate by sodium butyrate. Western blot analysis of C6 cell lysates revealed that 4 h exposure to CPF was associated with decreased levels of the cytoskeletal protein MAP1B compared to controls, whereas the levels of the cytoskeletal proteins tubulin and MAP2c were not significantly affected. Western blot analysis of extracts of cells treated with CPO showed a significant, concentration-dependent decrease in the levels of tubulin after 24 h. MAP-1B levels were also significantly decreased. The above changes were not temporally related to acetylcholinesterase (AChE) inhibition. These results suggest that both CPF and CPO can exert toxic effects directly on glial cell differentiation and that the latter compound has a potent effect on the microtubule network.

Reduced tubulin tyrosination as an early marker of mercury toxicity in differentiating N2a cells

The aims of this work were to compare the effects of methyl mercury chloride and thimerosal on neurite/process outgrowth and microtubule proteins in differentiating mouse N2a neuroblastoma and rat C6 glioma cells. Exposure for 4h to sublethal concentrations of both compounds inhibited neurite outgrowth to a similar extent in both cells lines compared to controls. In the case of N2a cells, this inhibitory effect by both compounds was associated with a fall in the reactivity of western blots of cell extracts with monoclonal antibody T1A2, which recognises C-terminally tyrosinated alpha-tubulin. By contrast, reactivity with monoclonal antibody B512 (which recognises total alpha-tubulin) was unaffected at the same time point. These findings suggest that decreased tubulin tyrosination represents a neuron-specific early marker of mercury toxicity associated with impaired neurite outgrowth.

The effects of diazinon and cypermethrin on the differentiation of neuronal and glial cell lines

Diazinon and cypermethrin are pesticides extensively used in sheep dipping. Diazinon is a known anti-cholinesterase, but there is limited information regarding its molecular mechanism of action. This paper describes the effects of diazinon and cypermethrin at a morphological and molecular level on differentiating mouse N2a neuroblastoma and rat C6 glioma cell lines. Concentrations up to 10 microM of both compounds and their mixture had no effect on the viability of either cell line, as determined by methyl blue tetrazolium reduction and total protein assays. Microscopic analysis revealed that 1 microM and 10 microM diazinon but not cypermethrin inhibited the outgrowth of axon-like processes in N2a cells after a 24-h exposure but neither compound affected process outgrowth by differentiating C6 cells at these concentrations. Under these conditions, 10 microM diazinon inhibited AChE slightly compared to the control after a 4-h exposure but not after 24 h. Western blotting analysis showed that morphological changes were associated with reduced cross-reactivity with antibodies that recognize the neurofilament heavy chain (NFH), microtubule associated protein MAP 1B and HSP-70 compared to control cell extracts, whereas reactivity with anti-alpha-tubulin antibodies was unchanged. Aggregation of NFH was observed in cell bodies of diazinon-treated N2a cells, as determined by indirect immunofluorescence staining. These data demonstrate that diazinon specifically targets neurite outgrowth in neuronal cells and that this effect is associated with disruption of axonal cytoskeleton proteins, whereas cypermethrin has no effect on the same parameters.

Effects of phenyl saligenin phosphate on phosphorylation of pig brain tubulin in vitro

Phenyl saligenin phosphate (PSP) induces a characteristic neuropathy (OPIDN), the molecular basis of which has not been precisely defined. This study examined the in vitro effects of PSP on the phosphorylation of serine and threonine residues of proteins in porcine brain cytosol. Quantitative analysis of Western blots probed with antibodies recognizing phosphorylated serine residues demonstrated that 100μM PSP induced a significant increase in the phosphorylation of serine residues of a 50kDa protein. This protein was identified as the α- and β-tubulin subunits by probing Western blots of extracts separated by two-dimensional polyacrylamide gel electrophoresis with anti-phosphoserine and anti-tubulin antibodies. By contrast, threonine phosphorylation of the 50kDa polypeptide and other proteins detected on Western blots probed with anti-phosphothreonine antibodies, was not significantly affected by PSP. These data indicate that PSP is able to induce increased phosphorylation of tubulin in serine residues, consistent with a possible role for this phenomenon in OPIDN induction.

Inhibition of neurite outgrowth in differentiating mouse N2a neuroblastoma cells by phenyl saligenin phosphate: Effects on MAP kinase (ERK 1/2) activation, neurofilament heavy chain phosphorylation and neuropathy target esterase activity

Sub-lethal concentrations of the organophosphate phenyl saligenin phosphate (PSP) inhibited the outgrowth of axon-like processes in differentiating mouse N2a neuroblastoma cells (IC(50) 2.5 microM). A transient rise in the phosphorylation state of neurofilament heavy chain (NFH) was detected on Western blots of cell extracts treated with 2.5 microM PSP for 4 h compared to untreated controls, as determined by a relative increase in reactivity with monoclonal antibody Ta51 (anti-phosphorylated NFH) compared to N52 (anti-total NFH). However, cross-reactivity of PSP-treated cell extracts was lower than that of untreated controls after 24 h exposure, as indicated by decreased reactivity with both antibodies. Indirect immunofluorescence analysis with these antibodies revealed the appearance of neurofilament aggregates in the cell bodies of treated cells and reduced axonal staining compared to controls. By contrast, there was no significant change in reactivity with anti-alpha-tubulin antibody B512 at either time point. The activation state of the MAP kinase ERK 1/2 increased significantly after PSP treatment compared to controls, particularly at 4 h, as indicated by increased reactivity with monoclonal antibody E-4 (anti-phosphorylated MAP kinase) but not with polyclonal antibody K-23 (anti-total MAP kinase). The observed early changes were concomitant with almost complete inhibition of the activity of neuropathy target esterase (NTE), one of the proposed early molecular targets in organophosphate-induced delayed neuropathy (OPIDN).

Inhibition of neurite outgrowth in N2a cells by leptophos and carbaryl: effects on neurofilament heavy chain, GAP-43 and HSP-70

The neurodegenerative properties of the organophosphate ester leptophos (LEP) and the carbamate ester carbaryl (CB), both of which can cause neuropathic effects in animals, were investigated in differentiating mouse N2a neuroblastoma cells. At a sublethal concentration of 3 microM, both LEP and CB were able to inhibit the outgrowth of axon-like processes from N2a cells after only 4 h of exposure. Extracts of cells exposed to LEP showed decreased cross-reactivities with monoclonal antibodies that recognise the neurofilament heavy chain (NFH) and the growth-associated protein GAP-43. However, they exhibited increased cross-reactivity with a monoclonal antibody that recognises the heat shock protein HSP-70. In contrast, no changes were noted in the levels of antibody binding in blots of extracts of cells exposed to CB. It is concluded that, although both LEP and CB inhibit the formation of axons in vitro, the early biochemical changes underlying the neurodegenerative effects of the two compounds are different.

The recovery of Arcobacter butzleri NCTC 12481 from various temperature treatments

AIMS

The aim of this study was to determine the growth and survival characteristics for Arcobacter butzleri NCTC 12481.

METHODS AND RESULTS

The temperature and pH growth ranges were 15-39 degrees C and pH 6.0-8.0, as determined using impedance microbiology. The maximum specific growth rate was 00.57 h(-1) at 30 degrees C, pH 7.0. Arcobacter butzleri harvested from the exponential phase was more resistant to heat treatment than stationary phase cells (D55 1.1 and 0.4 min, respectively). Fluorescent dye uptake, and the release of UV-absorbing material, increased in heat-treated cells. After 21 d storage at 4 and -20 degrees C, A. butzleri was recovered on blood agar, but not on the isolation media CAT or CCDA.

CONCLUSION

Arcobacter butzleri cells from the exponential phase were less heat sensitive than those from the stationary phase. The organism was able to survive cold storage for at least 3 weeks.

SIGNIFICANCE AND IMPACT OF THE STUDY

The growth and survival characteristics have been quantified thus providing a greater understanding of this newly emerging pathogen.

The toxicity of chlorpyrifos towards differentiating mouse N2a neuroblastoma cells

The aim of this work was to study the effects of chlorpyrifos (CPF) on the outgrowth of axons by differentiating mouse N2a neuroblastoma cells. This was achieved by morphological, Western blotting and enzymatic analyses of cells induced to differentiate in the presence and absence of CPF added either at the same time (co-differentiation) or 16 h after (post-differentiation) the induction of cell differentiation. The outgrowth of axon-like processes was impaired following 4 or 8 h exposure to CPF in both co- and post-differentiation experiments. Western blotting analysis revealed reduced levels of neurofilament heavy chain (NF-H) following 8 h of exposure but no significant effect at 4 h under both co- and post-differentiation conditions. By contrast, levels of the heat shock protein HSP-70 were raised at both time points, but only in co-differentiation experiments. Neuropathy target esterase (NTE) activity was lower than controls following 4 or 8 h of exposure under co-differentiation conditions, but not under any post-differentiation conditions. The results suggest that the inhibition of axon production and maintenance by CPF in differentiating N2a cells may involve multiple targets, which are different under co- and post-differentiation conditions.

Effects of neuropathic and non-neuropathic isomers of tricresyl phosphate and their microsomal activation on the production of axon-like processes by differentiating mouse N2a neuroblastoma cells

The aim of this work was to investigate the sublethal neuropathic effects of tricresyl phosphate (TCP: mixed isomers), triorthocresyl phosphate (TO:CP) and triparacresyl phosphate (TP:CP) on differentiating mouse N2a neuroblastoma cells. This was achieved by a combination of measurements of cell viability, axon outgrowth and the levels of cytoskeletal proteins detectable on western blots of extracts from cells induced to differentiate in the presence and absence of the compounds. In a time-course experiment TCP inhibited the outgrowth of axon-like processes following exposure times of 24 h or longer. Dose-response experiments indicated that TCP and TO:CP exhibited similar sustained levels of toxicity following both 24 and 48 h exposure, with no significant difference between their respective IC(50) values. By contrast, TP:CP demonstrated a transient effect on the outgrowth of axon-like processes, which was detectable after 24 but not 48 h of exposure. Isomer-specific patterns of toxicity were also evident at earlier time-points, with only the ortho isomer showing significant levels of inhibition of axon outgrowth following 4-8 h exposure. Probing of western blots with antibodies against cytoskeletal proteins indicated that the inhibition of axon outgrowth by these compounds was associated with a sustained reduction in the levels of phosphorylated neurofilament heavy chain. The inhibitory effect on axon outgrowth of TO:CP but not TP:CP was enhanced in the presence of a microsomal activation system. Since TO:CP is the most neuropathic of the isomers of TCP in vivo, differentiating N2a cells provide a useful cellular system for mechanistic studies of the neurodegenerative effects of this organophosphate.

Protection from MPTP-induced neurotoxicity in differentiating mouse N2a neuroblastoma cells

We have shown previously that subcytotoxic concentrations of MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) inhibit axon outgrowth and are associated with increased neurofilament heavy chain (NF-H) phosphorylation in differentiating mouse N2a neuroblastoma cells while higher doses (> 100 microM) cause cell death. In this work we assessed the ability of potential neuroprotective agents to alleviate both MPTP-induced cell death (cytotoxicity) and MPTP-induced NF-H phosphorylation/reduction in axon outgrowth (neurotoxicity) in N2a cells induced to differentiate by dbcAMP. The neurotoxic effects of MPTP occurred in the absence of significant alterations in energy status or mitochondrial membrane potential. The hormone oestradiol (100 microM) reduced the cytotoxic effect of MPTP, but blocked di-butyryl cyclic AMP (dbcAMP)-induced differentiation, i.e. axon outgrowth. Both the cytotoxic and neurotoxic effects of MPTP were reduced by the monoamine oxidase (MAO) inhibitors deprenyl and, to a lesser extent, clorgyline. Alleviation of both neurotoxicity and cytotoxicity was also achieved by conditioned medium derived from rat C6 glioma cells. In contrast, whilst the p38 MAP kinase inhibitor, SB202190, protected cells against MPTP-induced neurotoxicity, it could not maintain cell viability at high MPTP exposures. In each case neuroprotection involved maintenance of the differentiating phenotype linked with attenuation of NF-H hyper-phosphorylation; the latter may represent a mechanism by which neuronal cells can moderate MPTP-induced neurotoxicity. The use of a simplified neuronal cell model, which expresses subtle biochemical changes following neurotoxic insult, could therefore provide a valuable tool for the identification of potential neuroprotective agents.

Effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine on differentiating mouse N2a neuroblastoma cells

The effect of the neurotoxin 1-methyl-4-phenyl-1,2,3, 6-tetrahydropyridine (MPTP) was investigated in mouse N2a neuroblastoma cells, induced to differentiate by serum withdrawal and addition of dibutyryl cyclic AMP, over a 24-h period. Addition of MPTP (10 microM) during differentiation caused a change in cell morphology characterised by an inhibition of axon outgrowth, in the absence of cell death. Biochemical characterisation by western blotting revealed that MPTP had no significant effects on the levels of actin, alpha-tubulin, or total heavy-chain neurofilament (NF-H). However, NF-H phosphorylation appeared to increase following MPTP treatment when blots were probed with the phosphorylation state-specific antibodies RMd09 and Ta51. In addition, indirect immunofluorescence analysis revealed an accumulation of phosphorylated NF-H in the cell perikaryon, suggesting that altered NF-H distribution was associated with the observed effects of MPTP on cell morphology. These changes may represent a useful in vitro marker of MPTP neurotoxicity within a simple differentiating neuronal cell model system.

The effects of carbaryl and trichlorphon on differentiating mouse N2a neuroblastoma cells

The ability of the carbamate pesticide carbaryl (CB) and the organophosphate pesticide trichlorphon (TCL) to inhibit the outgrowth of axon-like processes was studied using mouse N2a neuroblastoma cells induced to differentiate by serum withdrawal. At concentrations of 1 and 2 microg/ml (4.97 and 9.94 microM), CB did not cause cell death but inhibited the outgrowth of axon-like processes from N2a cells. This effect was noted as early as 24 h after exposure of the cells to CB. A similar effect was observed with TCL at concentrations of 1 and 2 microg/ml (3.89 and 7.78 microM). Western blot analysis of cell extracts treated with the pesticides showed decreased cross reactivities with the monoclonal antibody RMd09 compared to control extracts. The results indicate that CB and TCL are both able to inhibit axon development and that this effect is associated with reduced levels of the neurofilament high molecular weight protein subunit (NFH).

Tricresyl phosphate inhibits the formation of axon-like processes and disrupts neurofilaments in cultured mouse N2a and rat PC12 cells

Tricresyl phosphate (1 microg/ml) inhibited the outgrowth of axon-like processes in mouse N2a neuroblastoma and rat PC12 pheochromocytoma cell lines induced to differentiate by serum withdrawal and nerve growth factor addition, respectively. By contrast, it had no effect on the outgrowth of processes by rat C6 glioma cells induced to differentiate with sodium butyrate. The effect on axon outgrowth in the two neuronal cell lines correlated with altered distribution of neurofilament proteins, as determined by indirect immunofluorescence with monoclonal antibody RMd09. Western blots of neuronal cell extracts probed with the same antibody revealed decreased cross-reactivity after exposure to tricresyl phosphate. The results suggest that tricresyl phosphate has a selective effect on neuronal cell differentiation, which involves impaired axon outgrowth, reduced levels of the neurofilament heavy chain and disruption of the neurofilament network.

The disruption of brain microtubules in vitro by the phospholipase inhibitor p-bromophenacyl bromide

The influence of p-bromophenacyl bromide (pBPAB) and structural analogues on the assembly and Ca2+ sensitivity of porcine brain microtubules (MTs) was studied by spectrophotometric measurements in vitro. MT assembly was inhibited by 36 microM pBPAB but not by the structural analogues p-chlorophenacyl chloride or acetophenone. In the presence of pBPAB, but not structural analogues, the addition of 10 mM Ca2+ induced aggregation of polymerized MT protein, whereas a decrease in turbidity (due to MT disassembly) was observed in controls. The effects of pBPAB on both MT assembly and Ca2+ sensitivity were blocked by glutathione, but not by N-acetyl L-cysteine, N-acetyl L-lysine nor L-tyrosine, indicating that a highly reduced sulphydryl group(s) may be involved. Western blotting analyses of drug-treated MTs revealed a form of tubulin with altered electrophoretic characteristics, probably caused by a covalent interaction with pBPAB. MT preparations polymerized in the presence of the drug contained fewer MTs than control samples, the predominant structures being identified as amorphous aggregates of MT proteins. The fact that pBPAB affects MT integrity at an effective anti-inflammatory dose in vitro may reflect the involvement of MT disruption in some of the pharmacological effects of this drug. pBPAB is not therefore a suitable tool for studying the specific involvement of phospholipase A2 in cellular events.

Tyrosination state of alpha-tubulin in regenerating peripheral nerve

Certain modifications of the neuronal cytoskeleton that are associated with development also occur during regeneration of adult mammalian peripheral nerve. The aim of the present study was to examine one such modification, the tyrosination of alpha-tubulin. Adult rats were anaesthetized and the left or right sciatic nerve randomly selected and crushed to induce regeneration. In certain instances nerves were crushed then ligatured about the crush, to prevent regeneration. Five days later the rats were killed and the regenerating (or ligatured) and the contralateral (control) nerves were removed. Quantitative immunoblotting of nerve homogenates with antibodies that recognize tyrosinated alpha-tubulin and total alpha-tubulin revealed a significant increase (p < 0.01) in the proportion of alpha-tubulin that was tyrosinated in nerve pieces distal (peripheral) to a nerve crush and to uncrushed nerve. No such difference occurred in ligatured (crushed but nonregenerating) nerve, implying that the increase was related to the presence of regenerating fibres; nor was there any gradient in tyrosination of alpha-tubulin in control nerves. This effect was confirmed by cytofluorimetric scanning and fluorescence confocal laser scanning microscopy of fixed sections of control and regenerating nerve, stained with antibodies directed against tyrosinated alpha-tubulin. When nerves were separated into fractions containing assembled and nonassembled tubulin, a significant (p < 0.01) increase was found in the proportion of tyrosinated alpha-tubulin in the nonassembled tubulin fraction in nerve pieces containing regenerating fibres. This occurred in the absence of a change in the proportion of assembled and nonassembled tubulin.(ABSTRACT TRUNCATED AT 250 WORDS)

The toxicity of organophosphate compounds towards cultured PC12 cells

The effects of three representative organophosphates (OPs), tricresyl phosphate (TCP), triphenyl phosphite (TPP) and paraoxon (POX) on the proliferation and viability of rat PC12 pheochromocytoma cells were studied. With respect to its IC50, TCP was at least an order of magnitude more potent in its antiproliferative activity than both TPP and POX. All test OPs were cytotoxic at concentrations inhibiting cell proliferation. No compound inhibited cell growth below 10 micrograms/ml. For TCP and TPP the estimated IC50 values from proliferation assays were lower than published LD50 values in vivo, whereas paraoxon was much less toxic in vitro than in vivo. Subcytotoxic levels of TCP (1 micrograms/ml) were found to inhibit the maintenance of neurites on cells grown in the presence of nerve growth factor.

Subcellular distribution and immunological detection of retrograde axonally transported proteins in acrylamide and diabetic neuropathies

Neuropathies produced by both acrylamide- and streptozotocin-induced diabetes in the rat are accompanied by a deficit in the retrograde axonal transport of a defined group of proteins that can be visualized on two-dimensional polyacrylamide gels. In this work, these proteins are identified as being primarily soluble and being absent from rat brain. They are not immunologically related to the major retrogradely transported protein synaptophysin. Polyclonal antiserum to the proteins was produced in mice and used to confirm the reduction in their retrograde transport in sciatic nerve of diabetic and acrylamide-treated rats.

Glycation of brain actin in experimental diabetes

Actin is a neuronal protein involved in axonal transport and nerve regeneration, both of which are known to be impaired in experimental diabetes. To determine if actin is subject to glycation, we rendered rats diabetic by injection of streptozotocin. Two or 6 weeks later brains were removed and a preparation of cytoskeletal proteins was analyzed by two-dimensional polyacrylamide gel electrophoresis. Brains from diabetic animals contained an extra polypeptide that migrated close to actin and reacted with monoclonal antibody C4 against actin. It was also found in a preparation of soluble synaptic proteins from diabetic rat brain, indicating that it was at least partly neuronal in origin. This polypeptide could be produced by incubation of cytoskeletal proteins from brains of nondiabetic rats with glucose-6-phosphate in vitro. The appearance of this glycated actin in diabetic animals was prevented by administration of insulin for a period of 6 weeks. We could not detect any effect of glycation in vitro on the ability of muscle G-actin to form F-actin filaments and its significance for the function of actin remains to be determined. The finding that glycation of platelet-derived actin from diabetic patients was significantly increased implies that the abnormality may also occur in clinical diabetes.

Reconstitution of intermediate filaments from a higher plant

Immunological studies have shown that plants contain intermediate-filament antigens, but it is not known whether these proteins are capable in themselves of forming filaments. To address this problem, a detergent-resistant and high-salt-insoluble fraction from carrot (Daucus carota L.) suspension cells was solubilized with 9 M-urea and then subjected to a two-step dialysis procedure, devised for the reconstitution of animal intermediate filaments. This induced the self-assembly of 10 nm filaments and large bundles of filaments. The predominant components of reconstituted material were polypeptides with apparent molecular masses between 58 and 62 kDa. These polypeptides immunoblotted with two monoclonal antibodies known to show broad cross-reactivity with intermediate filaments across the phylogenetic spectrum. This establishes that the antigens are able to self-assemble into intermediate-sized filaments.

Subcellular localization of iodinated thyroid tubulin

Subcellular fractions enriched in mitochondria, plasma membranes, microsomes and Golgi apparatus were obtained from thyroid glands of rats injected with I125. Autoradiography of SDS-polyacrylamide gels revealed the presence of a number of radiolabelled proteins in each membrane fraction. One polypeptide, with the same electrophoretic mobility as brain tubulin, was found in all fractions except the plasma membranes and was immunoprecipitated with commercial anti-tubulin monoclonal antibodies. Hydrolysis of Asp-Pro linkages of I125 labelled tubulin with formic acid indicated that there were iodination sites in both the carboxy terminal one third and the amino terminal two thirds of the molecule. These results, together with the absence of iodinated tubulin from the cytosolic fraction, are consistent with the idea that a population of thyroid membrane tubulin is iodinated at multiple sites either just before or after insertion into intracellular membranes where it may act as an anchorage point for microtubule-membrane interactions.

Sodium butyrate induces major morphological changes in C6 glioma cells that are correlated with increased synthesis of a spectrin-like protein

Butyrate induced flattening and development of cell processes in rat glioma (C6) cells and this change was correlated with an increase in the synthesis of a polypeptide doublet with an apparent molecular weight of about 200 kDa. Blot analysis revealed that at least one of these polypeptides was a spectrin-like protein. Indirect immunofluorescence studies with the spectrin antiserum indicated that the antigen was present in the cell bodies, and also in the cell processes. Thus fodrin may be one the major targets for the action of butyrate on C6 cells.