The NG2 Proteoglycan Protects Oligodendrocyte Precursor Cells against Oxidative Stress via Interaction with OMI/HtrA2.

Frank Maus,Eva-Maria Krämer-Albers,Colin Watts,Dominik Sakry,Judith Stegmüller,Hauke B Werner,Fabien Binamé,Richard Heywood,Khalad Karram,Klaus-Armin Nave,Krishnaraj Rajalingam,Rejko Krüger,Jacqueline Trotter

doi:10.1371/journal.pone.0137311

Abstract

The NG2 proteoglycan is characteristically expressed by oligodendrocyte progenitor cells (OPC) and also by aggressive brain tumours highly resistant to chemo- and radiation therapy. Oligodendrocyte-lineage cells are particularly sensitive to stress resulting in cell death in white matter after hypoxic or ischemic insults of premature infants and destruction of OPC in some types of Multiple Sclerosis lesions. Here we show that the NG2 proteoglycan binds OMI/HtrA2, a mitochondrial serine protease which is released from damaged mitochondria into the cytosol in response to stress. In the cytosol, OMI/HtrA2 initiates apoptosis by proteolytic degradation of anti-apoptotic factors. OPC in which NG2 has been downregulated by siRNA, or OPC from the NG2-knockout mouse show an increased sensitivity to oxidative stress evidenced by increased cell death. The proapoptotic protease activity of OMI/HtrA2 in the cytosol can be reduced by the interaction with NG2. Human glioma expressing high levels of NG2 are less sensitive to oxidative stress than those with lower NG2 expression and reducing NG2 expression by siRNA increases cell death in response to oxidative stress. Binding of NG2 to OMI/HtrA2 may thus help protect cells against oxidative stress-induced cell death. This interaction is likely to contribute to the high chemo- and radioresistance of glioma.

Highlights

Oligodendrocyte precursor cells (OPC) in the CNS are characterised by expression of Nerveglial antigen 2 protein (NG2, termed chondroitin sulfate proteoglycan 4 (CSPG4)), a type 1-transmembrane protein and chondroitin sulfate proteoglycan. [1,2]
Together with the Y2H analysis, these results demonstrate that OMI/HtrA2 can bind to the NG2 proteoglycan
When the OMI protease is inhibited with UCF101, the difference in viability between control and NG2 Small interfering RNA (siRNA) treated cells disappears (Fig 6D), showing that the protective function of NG2 is dependent on OMI protease activity, similar to the effects observed in OPC

Summary

Introduction

Oligodendrocyte precursor cells (OPC) in the CNS are characterised by expression of Nerveglial antigen 2 protein (NG2, termed chondroitin sulfate proteoglycan 4 (CSPG4)), a type 1-transmembrane protein and chondroitin sulfate proteoglycan. [1,2]. OMI/HtrA2 binds to the inhibitors of apoptosis proteins (IAPs) and degrades them via the OMI/HtrA2 protease activity, resulting in caspase activation and induction of apoptosis [19,20]. We report that expression of NG2 has a protective effect in OPC under oxidative stress conditions through binding and sequestering OMI/HtrA2. This interaction reduces the protease activity of OMI/HtrA2. Human glioma cells expressing high levels of NG2 are more resistant to induction of cell death by oxidative stress: reduction of NG2 levels by siRNA decreases their resistance. Expression of NG2 by OPC may aid in protecting OPC against induction of cell death by oxidative stress. The interaction is likely to contribute to resistance to chemo- and radiation therapy

Ethics Statement

Results

Discussion