Toxins 2010, 2, 683-737; doi:10.3390/toxins2040683 OPEN ACCESS toxins ISSN 2072–6651 www.mdpi.com/journal/toxins Review Bacterial Toxins and the Nervous System: Neurotoxins and Multipotential Toxins Interacting with Neuronal Cells Michel R. Popoff 1,* and Bernard Poulain 2 1 Institut Pasteur, Bactéries anaérobies et Toxines, 25 rue du Dr Roux, F-757254, PARIS cedex 15, France 2 Neurotransmission et Sécrétion Neuroendocrine, CNRS UPR 2356 IFR 37 - Neurosciences, Centre de Neurochimie, 5, rue Blaise Pascal, F-67084 STRASBOURG cedex, France; E-Mail: [email protected] * Author to whom correspondence should be addressed; E-Mail: [email protected]. Received: 21 February 2010; in revised form: 18 March 2010 / Accepted: 7 April 2010 / Published: 15 April 2010 Abstract: Toxins are potent molecules used by various bacteria to interact with a host organism. Some of them specifically act on neuronal cells (clostridial neurotoxins) leading to characteristics neurological affections. But many other toxins are multifunctional and recognize a wider range of cell types including neuronal cells. Various enterotoxins interact with the enteric nervous system, for example by stimulating afferent neurons or inducing neurotransmitter release from enterochromaffin cells which result either in vomiting, in amplification of the diarrhea, or in intestinal inflammation process. Other toxins can pass the blood brain barrier and directly act on specific neurons. Keywords: toxin; neurotoxin; enterotoxin; nervous system; actin cytoskeleton; small gtpases; neurotransmitter 1. Introduction Pathogenic bacteria use various strategies to interact with target tissues and cells of the host organism. Among them, toxin production constitutes an efficient way to alter specific functions of target cells. Toxins can act locally or at a distance from the infectious site and are responsible for severe diseases in man and animals. While some toxins, called cytotoxins, cause disruption of all cell types, thus permitting the pathogen access to nutrients, other toxins are only active on specific cells such as enterotoxins active Toxins 2010, 2 684 on epithelial intestinal cells and neurotoxins targeting neuronal cells. This is achieved by the recognition of specific cell surface receptor(s) and/or specific intracellular target(s). When bound to the receptor, toxins can unleash their toxic program at the cell membrane by interfering with signal transduction pathways, pore formation, or enzymatic activities towards membrane compounds. In contrast, other toxins enter the cytosol, recognize, and modify specific intracellular targets. According to the nature of the target and the type of modification, intracellular active toxins cause a dramatic alteration of cellular functions such as protein synthesis, cell homeostasis, cell cycle progression, vesicular traffic, and actin cytoskeletal rearrangements. The nervous system which is diffused through all the organism is one of the main target of bacterial toxins. If neurotoxins, like clostridial neurotoxins, exclusively interact with neuronal cells from the central or peripheral nervous system inducing specific neurological symptoms, other toxins recognize a broader range of cell types including neuronal cells. For example, cytotoxins can trigger necrosis or apoptosis in various cell types as well as in neuronal cells. Thereby various toxins, in addition to their specific activity on some cell types, affect certain neuronal cells, directly or indirectly, leading to neurological symptoms associated with typical clinical signs resulting from the other affected cells (Table 1, and Figure 1). This review is focused on the various modes of interaction with neuronal cells of bacterial neurotoxins and other toxins affecting the nervous system. 2. The Cellular and Molecular Mechanisms Involved in Neuroexocytosis: An Overview 2.1. An overview of neurotransmission Transfer of information or command between neurons, or neurons and target cells (muscle fibers, endocrine cells, etc.) is most often chemical in nature and occurs at highly specialized contact sites termed synapses. Here, the release of neurotransmitter molecules by the presynaptic elements enables activation of receptors localized on the postsynaptic target. Neurotransmitter molecules are comprised of small organic molecules as acetylcholine (ACh), catecholamines like dopamine or noradrenaline, serotonin (5-HT), glutamate, gamma-aminobutyric acid (GABA), glycine, adenosine-triphosphate (ATP), and numerous peptides such as vaso-intestinal peptide (VIP), substance P (SP), and calcitonin gene-related protein (CGRP). Released transmitter substance(s) can activate ligand-gated ionic channels or metabotropic receptors, thus mediating either transmembrane ionic fluxes or activation of intracellular signaling pathway(s). For example, in the central nervous system, depending on the ion species flowing through the channel, activation of ligand-gated ionic channels can cause depolarization (i.e., excitation) or hyperpolarization (i.e., inhibition) of the postsynaptic plasma membrane, respectively, and the excitation/inhibition net balance determines eventual initiation of action potentials propagated in the neuron until the next synapses. The evoked endplate potential at muscle fibers following stimulation of the motor nerve (and subsequent ACh release) is a depolarization that may reach the threshold for initiating muscle action potential, which itself propagates along muscle fiber ultimately triggering its contraction. In the enteric nervous system, release of neurotransmitter molecules (as VIP) by mucosal nerve endings directly contacting the enterocytes, or indirectly via the activation of entero-chrommaffin cells releasing 5-HT, can lead to activation of metabotropic receptors, intracellular activation of the adenylate cyclase and downstream cAMP-dependent pathways, resulting in an active co-transport of ions species (Na+, K+, Cl−) and water efflux for osmotic compensation in the intestine lumen (viz the molecular mechanisms of diarrhoea [5]). Toxins 2010, 2 685 Table 1. Bacterial neurotoxins and other toxins interacting with the nervous system. Target neuronal MLD Toxin Bacteria Structure Receptor Activity Effects cell (μg/kg)1 Toxins inhibiting the neuroexocytosis proteolysis of gangliosides inhibiton of SNARE proteins Botulinum C. botulinum single chain protein (GD , GT ) acetylcholine motoneurons 1b 1b (VAMP, 0.0003 neurotoxins C. baratii (150 kDa) synaptotagmin, release (flaccid SNAP25, C. butyricum SV2 paralysis) syntaxin) inhibition of gangliosides proteolysis of neurotransmitter Tetanus single chain protein inhibitory (GD , GT ) 1b 1b SNARE protein release (GABA, 0.001 neurotoxin C. tetani (150 kDa) interneurons GPI-anchored (VAMP) glycine) (spastic protein paralysis) inactivation of inhibition of single chain protein potentially all Rho and Ras- Lethal toxin unknown neurotransmitter 0.1 C. sordellii (250 kDa) neurons GTPases release (glucosylation) inactivation of inhibition of single chain protein potentially all Toxin B unknown Rho-GTPases neurotransmitter 32 C. difficile (250 kDa) neurons (glucosylation) release neuronal single chain protein hippocampal pore-forming Pneumolysin cholesterol apoptosis S. pneumoniae (53 kDa) neurons activity (meningitis) single chain protein enterocyte pore-forming Enterotoxin 80 C. perfringens (36 kDa) neurons activity Toxins stimulating neurosecretion stimulation of single chain protein hippocampal pore-forming glutamate Epsilon toxin unknown 0.1 C. perfringens (36 kDa) neurons activity release (excitation) Toxins 2010, 2 686 Table 1. Cont. inactivation of enterochrompaffin Gsα and 5-HT release Cholera toxin V. cholerae AB5 structure cells and enteric ganglioside GM1 250 activation of (diarrhea) neurons adenylate cyclase inactivation of enterochromaffin Heat labile Gsα and 5-HT release E. coli AB5 structure cells and enteric ganglioside GM1 250 enterotoxin activation of (diarrhea) neurons adenylate cyclase release of inactivation of inflammatory single chain protein enterocytes membrane Rho-GTPases, Toxin A C. difficile mediators and 0.35 (300 kDa) enteric neurons glycoprotein other neuropeptides mechanism? (diarrhea) stimulation of GMP increase, Heat stable short peptide enterocyte c enteric nervous E. coli guanylate cyclase other enterotoxin (2–5 kDa) enteric neurons? system mechanism? (diarrhea) 5-HT release histocompatibility superantigen Staphylococcal single chain protein enterochromaffin stimulation of 20 S. aureus complex class II other enterotoxins (25–30 kDa) cells, vagal nerve 5-HT receptor (monkey) molecules mechanism? 3 (emesis) cyclic stimulation of + Cereulide B. cereus dodecadepsipeptide vagal nerve 5HT3 receptor K ionophore 5-HT3 receptor (1.2 kDa) (emesis) 1 Mouse lethal doses per kg of body weight according to [1–4]. Toxins 2010, 2 687 Figure 1. Schematic representation of bacterial toxins active on the nervous system. In nerve terminals, transmitter molecules are stored into at least two different classes of secretory organelles: the small lucent synaptic vesicles, and large dense core vesicles or granules. The small lucent vesicles, approximately 50 nm in diameter, contain small organic molecules and are formed by either budding from the early endosome or recycling of empty vesicles [6,7]. The large dense core Toxins 2010, 2 688 vesicles are analogous to the secretory granules present in endocrine and exocrine cells and have a biogenesis different from that of the synaptic vesicles. Similar to many cargo