David J. Read

Botulinum neurotoxin C initiates two different programs for neurite degeneration and neuronal apoptosis

Clostridial neurotoxins are bacterial endopeptidases that cleave the major SNARE proteins in peripheral motorneurons. Here, we show that disruption of synaptic architecture by botulinum neurotoxin C1 (BoNT/C) in central nervous system neurons activates distinct neurodegenerative programs in the axo-dendritic network and in the cell bodies. Neurites degenerate at an early stage by an active caspase-independent fragmentation characterized by segregation of energy competent mitochondria. Later, the cell body mitochondria release cytochrome c, which is followed by caspase activation, apoptotic nuclear condensation, loss of membrane potential, and, finally, cell swelling and lysis. Recognition and scavenging of dying processes by glia also precede the removal of apoptotic cell bodies, in line with a temporal and spatial segregation of different degenerative processes. Our results suggest that, in response to widespread synaptic damage, neurons first dismantle their connections and finally undergo apoptosis, when their spatial relationships are lost.

Neuropathy Target Esterase Is Required for Adult Vertebrate Axon Maintenance

The enzyme neuropathy target esterase (NTE) is present in neurons and deacylates the major membrane phospholipid, phosphatidylcholine (PtdCho). Mutation of the NTE gene or poisoning by neuropathic organophosphates—chemical inhibitors of NTE—causes distal degeneration of long spinal axons in humans. However, analogous neuropathological changes have not been reported in nestin-cre:NTEfl/fl mice with NTE-deficient neural tissue. Furthermore, altered PtdCho homeostasis has not been detected in NTE-deficient vertebrates. Here, we describe distal degeneration of the longest spinal axons in ∼3-week-old nestin-cre:NTEfl/fl mice and in adult C57BL/6J mice after acute dosing with a neuropathic organophosphate: in both groups early degenerative lesions were followed by swellings comprising accumulated axoplasmic material. In mice dosed acutely with organophosphate, maximal numbers of lesions, in the longest spinal sensory axon tract, were attained within days and were preceded by a transient rise in neural PtdCho. In nestin-cre:NTEfl/fl mice, sustained elevation of PtdCho over many months was accompanied by progressive degeneration and massive swelling of axons in sensory and motor spinal tracts and by increasing hindlimb dysfunction. Axonal lesion distribution closely resembled that in hereditary spastic paraplegia (HSP). The importance of defective membrane trafficking in HSP and the association of NTE with the endoplasmic reticulum—the starting point for the constitutive secretory pathway and transport of neuronal materials into axons—prompted investigation for a role of NTE in secretion. Cultured NTE-deficient neurons displayed modestly impaired secretion, consistent with neuronal viability and damage in vivo initially restricted to distal parts of the longest axons.

Regulation of Kv channel expression and neuronal excitability in rat medial nucleus of the trapezoid body maintained in organotypic culture.

Principal neurons of the medial nucleus of the trapezoid body (MNTB) express a spectrum of voltage‐dependent K+ conductances mediated by Kv1–Kv4 channels, which shape action potential (AP) firing and regulate intrinsic excitability. Postsynaptic factors influencing expression of Kv channels were explored using organotypic cultures of brainstem prepared from P9–P12 rats and maintained in either low (5 mm, low‐K) or high (25 mm, high‐K) [K+]o medium. Whole cell patch‐clamp recordings were made after 7–28 days in vitro. MNTB neurons cultured in high‐K medium maintained a single AP firing phenotype, while low‐K cultures had smaller K+ currents, enhanced excitability and fired multiple APs. The calyx of Held inputs degenerated within 3 days in culture, having lost their major afferent input; this preparation of calyx‐free MNTB neurons allowed the effects of postsynaptic depolarisation to be studied with minimal synaptic activity. The depolarization caused by the high‐K aCSF only transiently increased spontaneous AP firing (<2 min) and did not measurably increase synaptic activity. Chronic depolarization in high‐K cultures raised basal levels of [Ca2+]i, increased Kv3 currents and shortened AP half‐widths. These events relied on raised [Ca2+]i, mediated by influx through voltage‐gated calcium channels (VGCCs) and release from intracellular stores, causing an increase in cAMP‐response element binding protein (CREB) phosphorylation. Block of VGCCs or of CREB function suppressed Kv3 currents, increased AP duration, and reduced Kv3.3 and c‐fos expression. Real‐time PCR revealed higher Kv3.3 and Kv1.1 mRNA in high‐K compared to low‐K cultures, although the increased Kv1.1 mRNA was mediated by a CREB‐independent mechanism. We conclude that Kv channel expression and hence the intrinsic membrane properties of MNTB neurons are homeostatically regulated by [Ca2+]i‐dependent mechanisms and influenced by sustained depolarization of the resting membrane potential.

Acute hyperbilirubinaemia induces presynaptic neurodegeneration at a central glutamatergic synapse

There is a well‐established link between hyperbilirubinaemia and hearing loss in paediatrics, but the cellular mechanisms have not been elucidated. Here we used the Gunn rat model of hyperbilirubinaemia to investigate bilirubin‐induced hearing loss. In vivo auditory brainstem responses revealed that Gunn rats have severe auditory deficits within 18 h of exposure to high bilirubin levels. Using an in vitro preparation of the auditory brainstem from these rats, extracellular multi‐electrode array recording from the medial nucleus of the trapezoid body (MNTB) showed longer latency and decreased amplitude of evoked field potentials following bilirubin exposure, suggestive of transmission failure at this synaptic relay. Whole‐cell patch‐clamp recordings confirmed that the electrophysiological properties of the postsynaptic MNTB neurons were unaffected by bilirubin, with no change in action potential waveforms or current–voltage relationships. However, stimulation of the trapezoid body was unable to elicit large calyceal EPSCs in MNTB neurons of hyperbilirubinaemic rats, indicative of damage at a presynaptic site. Multi‐photon imaging of anterograde‐labelled calyceal projections revealed axonal staining and presynaptic profiles around MNTB principal neuron somata. Following induction of hyperbilirubinaemia the giant synapses were largely destroyed. Electron microscopy confirmed loss of presynaptic calyceal terminals and supported the electrophysiological evidence for healthy postsynaptic neurons. MNTB neurons express high levels of neuronal nitric oxide synthase (nNOS). Nitric oxide has been implicated in mechanisms of bilirubin toxicity elsewhere in the brain, and antagonism of nNOS by 7‐nitroindazole protected hearing during bilirubin exposure. We conclude that bilirubin‐induced deafness is caused by degeneration of excitatory synaptic terminals in the auditory brainstem.

Organophosphates induce distal axonal damage, but not brain oedema, by inactivating neuropathy target esterase

Single doses of organophosphorus compounds (OP) which covalently inhibit neuropathy target esterase (NTE) can induce lower-limb paralysis and distal damage in long nerve axons. Clinical signs of neuropathy are evident 3weeks post-OP dose in humans, cats and chickens. By contrast, clinical neuropathy in mice following acute dosing with OPs or any other toxic compound has never been reported. Moreover, dosing mice with ethyloctylphosphonofluoridate (EOPF) - an extremely potent NTE inhibitor - causes a different (subacute) neurotoxicity with brain oedema. These observations have raised the possibility that mice are intrinsically resistant to neuropathies induced by acute toxic insult, but may incur brain oedema, rather than distal axonal damage, when NTE is inactivated. Here we provide the first report that hind-limb dysfunction and extensive axonal damage can occur in mice 3weeks after acute dosing with a toxic compound, bromophenylacetylurea. Three weeks after acutely dosing mice with neuropathic OPs no clinical signs were observed, but distal lesions were present in the longest spinal sensory axons. Similar lesions were evident in undosed nestin-cre:NTEfl/fl mice in which NTE had been genetically-deleted from neural tissue. The extent of OP-induced axonal damage in mice was related to the duration of NTE inactivation and, as reported in chickens, was promoted by post-dosing with phenylmethanesulfonylfluoride. However, phenyldipentylphosphinate, another promoting compound in chickens, itself induced in mice lesions different from the neuropathic OP type. Finally, EOPF induced subacute neurotoxicity with brain oedema in both wild-type and nestin-cre:NTEfl/fl mice indicating that the molecular target for this effect is not neural NTE.

Molecular cloning of neuropathy target esterase (NTE).

Covalent modification of NTE, a neuronal protein with serine esterase activity, by certain organophosphates (OP) initiates degeneration of long axons in the peripheral and central nervous system. Simple inhibition of NTE esterase activity does not initiate neuropathy; the latter requires aging of the OP bound to the catalytic serine residue so that a negatively-charged species is left attached to the active site. This may indicate that a non-esterase function of NTE is important for axonal maintenance. We have recently cloned NTE and shown that it is unrelated to any known serine hydrolases but contains a novel C-terminal domain which is conserved from bacteria to man. Furthermore, the catalytic serine is located within this domain at the centre of a helical hydrophobic segment of the polypeptides secondary structure. The integrity of NTE would be severely compromised by the presence of a negatively-charged organophosphate moiety at this site. Implications for possible higher-order structures and functions for NTE are discussed.

M1 muscarinic allosteric modulators slow prion neurodegeneration and restore memory loss

The current frontline symptomatic treatment for Alzheimer’s disease (AD) is whole-body upregulation of cholinergic transmission via inhibition of acetylcholinesterase. This approach leads to profound dose-related adverse effects. An alternative strategy is to selectively target muscarinic acetylcholine receptors, particularly the M1 muscarinic acetylcholine receptor (M1 mAChR), which was previously shown to have procognitive activity. However, developing M1 mAChR–selective orthosteric ligands has proven challenging. Here, we have shown that mouse prion disease shows many of the hallmarks of human AD, including progressive terminal neurodegeneration and memory deficits due to a disruption of hippocampal cholinergic innervation. The fact that we also show that muscarinic signaling is maintained in both AD and mouse prion disease points to the latter as an excellent model for testing the efficacy of muscarinic pharmacological entities. The memory deficits we observed in mouse prion disease were completely restored by treatment with benzyl quinolone carboxylic acid (BQCA) and benzoquinazoline-12 (BQZ-12), two highly selective positive allosteric modulators (PAMs) of M1 mAChRs. Furthermore, prolonged exposure to BQCA markedly extended the lifespan of diseased mice. Thus, enhancing hippocampal muscarinic signaling using M1 mAChR PAMs restored memory loss and slowed the progression of mouse prion disease, indicating that this ligand type may have clinical benefit in diseases showing defective cholinergic transmission, such as AD.

Stereo-specific degradation of the R-(+) isomer of O-n-hexyl-S-methylphosphorothioamidate catalysed by rabbit serum.

Resolved isomers of O-n-hexyl-S-methylphosphorothioamidate (HXM) which had been synthesised by separate stereospecific routes were analysed by chiral glc: about 2-3% of R-(+) isomer was found in the S-(-) sample and accounted for nearly all the inhibitory power against neuropathy target esterase. Incubation of racemic HXM with rabbit serum led to slow but very specific disposal of R-(+) isomer to undetectable levels with very slight loss of S-(-): the rate of disposal was roughly estimated to be about 1% of the published rate of hydrolysis of paraoxon. Incubation with crystalline chymotrypsin caused a preferential but not totally selective disposal of S-(-) isomer.

Molecular cloning of neuropathy target esterase

A single ingestion of certain organophosphorus esters (OPs) can cause a syndrome known as Organophosphate Induced Delayed Polyneuropathy (OPIDP), a paralysing neuropathy with degeneration of long axons, developing after a latent period of approximately one to three weeks. The primary target of these OPs has been shown to be a 155kDa neural protein designated Neuropathy Target Esterase (NTE), and the toxic effects apparently due to the covalent inhibition and subsequent secondary modification of this protein. Recently NTE has been purified to apparent homogeneity using a novel biotinylated OP and sufficient pure protein was produced for limited protein sequencing. The aim of this project was to clone NTE cDNA using peptide sequence data. Initially, these sequences were used to design degenerate oligonucleotide primers for amplifying sections of brain cDNA by polymerase chain reaction (PCR). These approaches were unsuccessful. Subsequently, database searching with the peptide sequences identified a number of Expressed Sequence Tags (ESTs); these could be aligned to form an initial contig of 2.2kbp which encoded the 3’ end of NTE cDNA. The 5’ end of NTE cDNA, comprising a further 2.2kbp, was obtained by a PCR-based technique. The final 4.4kbp contig encoded a 1327 residue polypeptide predicted to have a molecular mass of 146kDa and at least one transmembrane domain. A novel serine esterase domain of approximately 200 residues was present near the C-terminus. NTE is unrelated to any known serine hydrolases but homologous proteins are predicted to be present in diverse prokaryotic and eukaryotic organisms. The homologue in Drosophila is associated with the swisscheese phenotype, an age-dependent neurodegeneration of the brain. NTE was also mapped to chromosome 19p13.3 between markers D19216 and D19S413 (using the UniGene database) and an OMIM search reveals that this is near the locus of cerebellar ataxia (Cayman type). Abstract ii Table of contents iii List of Figures v Abbreviations ixii Table of contents iii List of Figures v Abbreviations ix Chapter

Screening of O-ethyl O-4-nitrophenyl phosphoramidate (ENPP) for delayed neuropathic potential

O-Ethyl-O-4-nitrophenylphosphoramidate is a short-acting anticholinesterase and a possible candidate for a prophylactic agent against nerve agents since human acetylcholinesterase inhibited by this agent undergoes rapid spontaneous reactivation which can be accelerated further, if necessary, by treatment with oximes. Doses of the agent > 1 mg/kg (s.c.) given to unprotected rats were fatal in a short time but 2 rats and one hen given 0.5 mg/kg survived. Hens given 2.5 or 4 mg/kg s.c. 20 min after prophylactic physostigmine + atropine survived acute effects and were killed 4.5 or 24 h later. Brain and spinal cord neuropathy target esterase levels of these hens were depressed only 4-10% compared with levels in brains from hens given only oxime + atropine or of undosed animals. Clinical signs of neuropathy were not seen in surviving birds observed for 3 weeks. It appears there would be negligible delayed neuropathic hazard associated with administration of O-ethyl-O-4-nitrophenylphosphoramidate at subacute doses.