Tissue transglutaminase in astrocytes is enhanced by inflammatory mediators and is involved in the formation of fibronectin fibril-like structures

Nathaly Espitia Pinzón,Anne-Marie Van Dam,Wia Baron,Benjamin Drukarch,John J P Brevé,John G J M Bol

doi:10.1186/s12974-017-1031-2

Nathaly Espitia Pinzón, Anne-Marie Van Dam + Show 4 more

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https://doi.org/10.1186/s12974-017-1031-2

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Abstract

BackgroundDuring multiple sclerosis (MS) lesion formation, inflammatory mediators are produced by microglial cells and invading leukocytes. Subsequently, hypertrophic astrocytes fill the lesion and produce extracellular matrix (ECM) proteins that together form the astroglial scar. This is beneficial because it seals off the site of central nervous system (CNS) damage. However, astroglial scarring also forms an obstacle that inhibits remyelination of brain lesions. This is possibly an important cause for incomplete remyelination of the CNS in early stage MS patients and for failure of remyelination when the disease progresses. Tissue transglutaminase (TG2), a Ca2+-dependent enzyme that can cross-link proteins, appears in astrocytes in inflammatory MS lesions and may contribute to the rearrangement of ECM protein deposition and aggregation.MethodsThe effect of different inflammatory mediators on TG2 and fibronectin, an ECM protein, protein levels was examined in primary rat microglia and astrocytes by western blotting. Also, TG2 activity was analyzed in primary rat astrocytes by a TG activity assay. To determine the role of TG2 in the deposition and cross-linking of fibronectin, a TG2 inhibitor and TG2 knockdown astrocytes were used.ResultsOur data show that under inflammatory conditions in vitro, TG2 production is enhanced in astrocytes and microglia. We observed that in particular, astrocytes produce fibronectin that can be cross-linked and aggregated by exogenous TG2. Moreover, inflammatory stimulus-induced endogenously produced TG2 is involved in the appearance of morphological fibril-like fibronectin deposits but does not lead to cross-linked fibronectin aggregates.ConclusionsOur in vitro observations suggest that during MS lesion formation, when inflammatory mediators are produced, astrocyte-derived TG2 may contribute to ECM rearrangement, and subsequent astroglial scarring.

Highlights

During multiple sclerosis (MS) lesion formation, inflammatory mediators are produced by microglial cells and invading leukocytes
Upon subsequent treatment of astrocytes and microglia with a variety of inflammatory mediators, Tissue transglutaminase (TG2) protein levels were significantly increased after LPS, Tumor necrosis factor-alpha (TNF-α)+Interleukin-1 beta (IL-1β), or IL-4 treatment (p < 0.001 for all, Fig. 1a, b)
Considering the minimal amount of fibronectin detected in microglia (Fig. 1d), we decided to continue our study with a focus on astrocytes

Summary

Introduction

During multiple sclerosis (MS) lesion formation, inflammatory mediators are produced by microglial cells and invading leukocytes. Hypertrophic astrocytes fill the lesion and produce extracellular matrix (ECM) proteins that together form the astroglial scar. This is beneficial because it seals off the site of central nervous system (CNS) damage. Astroglial scarring forms an obstacle that inhibits remyelination of brain lesions This is possibly an important cause for incomplete remyelination of the CNS in early stage MS patients and for failure of remyelination when the disease progresses. In multiple sclerosis (MS), a chronic inflammatory, demyelinating disease of the central nervous system (CNS), various classes of inflammatory white matter lesions can be identified [1, 2]. The ECM proteins fibronectin and laminin have been shown to be more expressed by astrocytes [25,26,27,28] and aggregated in chronically demyelinated multiple sclerosis lesions [29], possibly contributing to the non-regenerative nature of these lesions [30, 31]

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