Bacterial Cytolysin during Meningitis Disrupts the Regulation of Glutamate in the Brain, Leading to Synaptic Damage

Carolin Wippel,Jiangtao Ma,Timothy J Mitchell,Roland Nau,Christina Förtsch,Wolfgang Brück,Sabrina Hupp,Stephanie Bunkowski,Asparouh I Iliev,Alexandra Bohl,Jana Maurer,Paul M Sullam

doi:10.1371/journal.ppat.1003380

Abstract

Streptococcus pneumoniae (pneumococcal) meningitis is a common bacterial infection of the brain. The cholesterol-dependent cytolysin pneumolysin represents a key factor, determining the neuropathogenic potential of the pneumococci. Here, we demonstrate selective synaptic loss within the superficial layers of the frontal neocortex of post-mortem brain samples from individuals with pneumococcal meningitis. A similar effect was observed in mice with pneumococcal meningitis only when the bacteria expressed the pore-forming cholesterol-dependent cytolysin pneumolysin. Exposure of acute mouse brain slices to only pore-competent pneumolysin at disease-relevant, non-lytic concentrations caused permanent dendritic swelling, dendritic spine elimination and synaptic loss. The NMDA glutamate receptor antagonists MK801 and D-AP5 reduced this pathology. Pneumolysin increased glutamate levels within the mouse brain slices. In mouse astrocytes, pneumolysin initiated the release of glutamate in a calcium-dependent manner. We propose that pneumolysin plays a significant synapto- and dendritotoxic role in pneumococcal meningitis by initiating glutamate release from astrocytes, leading to subsequent glutamate-dependent synaptic damage. We outline for the first time the occurrence of synaptic pathology in pneumococcal meningitis and demonstrate that a bacterial cytolysin can dysregulate the control of glutamate in the brain, inducing excitotoxic damage.

Highlights

IntroductionStreptococcus pneumoniae (pneumococcal) meningitis is the most common form of bacterial meningitis [1]
Streptococcus pneumoniae meningitis is the most common form of bacterial meningitis [1]
We found that the brain cortex of patients with meningitis demonstrated a loss of synapses, and we identified the major pneumococcal neurotoxin pneumolysin as a sufficient cause of this loss

Summary

Introduction

Streptococcus pneumoniae (pneumococcal) meningitis is the most common form of bacterial meningitis [1]. Pneumolysin (PLY) is a critical pneumococcal pathogenic factor that belongs to the cholesterol-dependent cytolysin (CDC) group. This 54-kD protein contains four domains and targets plasmalemmal cholesterol through its fourth domain [4]. To obtain tools to study the cellular effects of pore formation by PLY, the delta mutant form of PLY has been created [9], which lacks the amino acids alanine 146 and arginine 147. To obtain tools to study the cellular effects of pore formation by PLY, the delta mutant form of PLY has been created [9], which lacks the amino acids alanine and arginine This mutation makes the refolding of domain 3, and pore formation, impossible [9]. There is some experimental evidence from a rabbit model, that argues against the role of PLY in meningitis [13], raising the question of the specificity of different animal models

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