Ion exchanger in the brain: Quantitative analysis of perineuronally fixed anionic binding sites suggests diffusion barriers with ion sorting properties.

Markus Morawski,Thomas Arendt,Friedrich E Wagner,Gert Brückner,Wolfram Meyer-Klaucke,Wolfgang Tröger,Carsten Jäger,Tilo Reinert,Anja Reinert

doi:10.1038/srep16471

Abstract

Perineuronal nets (PNs) are a specialized form of brain extracellular matrix, consisting of negatively charged glycosaminoglycans, glycoproteins and proteoglycans in the direct microenvironment of neurons. Still, locally immobilized charges in the tissue have not been accessible so far to direct observations and quantifications. Here, we present a new approach to visualize and quantify fixed charge-densities on brain slices using a focused proton-beam microprobe in combination with ionic metallic probes. For the first time, we can provide quantitative data on the distribution and net amount of pericellularly fixed charge-densities, which, determined at 0.4–0.5 M, is much higher than previously assumed. PNs, thus, represent an immobilized ion exchanger with ion sorting properties high enough to partition mobile ions in accord with Donnan-equilibrium. We propose that fixed charge-densities in the brain are involved in regulating ion mobility, the volume fraction of extracellular space and the viscosity of matrix components.

Highlights

The extracellular matrix (ECM) is composed of long-chain macromolecules many of which are linked to cell surfaces while others float in the extracellular space
A potential Fe-Cl-cluster could be excluded because the PIXE maps showed no elevated Cl concentrations at the iron loaded perineuronal net (PN). It is the structure and composition of the neuronal microenvironment in form of the extracellular space that determines how molecules migrate through the brain
Recent estimates indicate that the extracellular space occupies at least 15% to 20% of brain tissue, with a large regional variation of its actual width[19]

Summary

Introduction

The ECM is composed of long-chain macromolecules many of which are linked to cell surfaces while others float in the extracellular space. The PN consists of long-chain polyelectrolytes with three main components: glycosaminoglycans (e.g. hyaluronan, chondroitin sulfate and heparin sulfate), glycoproteins (e.g. tenascin), and proteoglycans (e.g. lectican family)[14]. Due to their glycosaminoglycan components, PNs may form highly negatively charged fixed structures in the direct microenvironment of neurons[15,16]. Through electrostatic interactions, they may, significantly alter the diffusion properties and local homeostasis of physiologically relevant mobile ions. We provide quantitative results on the pericellularly fixed negative-charge density in the extracellular space based on measured concentrations of the bound cationic probe ion

Methods

Results

Conclusion