Interleukin-1β mediates proliferation and differentiation of multipotent neural precursor cells through the activation of SAPK/JNK pathway

Abstract

Neural precursor cells (NPCs) have been experimentally used to repair the damaged nervous system either by exogenous transplantation or by endogenous activation. In post-injury inflammation, an array of cytokines including interleukin-1β (IL-1β) are released by host as well as invading immune cells and increased markedly. In the present study, we investigated the effects of IL-1β on the survival, proliferation, differentiation and migration of NPCs as well as underlying intracellular signaling pathways. NPCs derived from the E16 rat brain were expanded in neurospheres that were found to express IL-1β, IL-1RI and IL-1RII, but not IL-1α and IL-1ra. IL-1β inhibited the proliferation of NPCs in a dose-dependent manner, an effect that can be reversed by IL-1ra, an antagonist for IL-1 receptor. This inhibitory effect of IL-1β on NPCs proliferation resulted in part from its effect on increased apoptosis of NPCs. Moreover, IL-1ra did not affect NPCs lineage fate but rather inhibited GFAP expression in differentiated astrocytes. We also found that IL-1ra had no effect on the transmigration of NPCs in vitro. Finally, we showed that the effect of IL-1β on NPCs proliferation and differentiation appeared to be mediated by SAPK/JNK, but not ERK, P38MAPK nor NF-κB pathways. These findings collectively suggest that the inflammatory environment following CNS injuries may influence the ability of NPCs to repair the damage.

Introduction

Interleukin-1 (IL-1) designates two mediators, IL-1α and IL-1β, having less than 30% sequence homology but similar three-dimensional structure and biological activities. The action of IL-1 is regulated by different mechanisms involving a membrane-associated true receptor (type I interleukin-1 receptor, IL-1RI), a decoy (type II interleukin-1 receptor, IL-1RII) and a specific receptor antagonist (IL-1ra). IL-1α and IL-1β activities are mediated via the IL-1RI since IL-1RII has no signaling activity (Sims et al., 1993). IL-1ra, on the other hand, competitively binds to IL-1RI but fails to trigger signaling and blocks the activities of IL-1α and IL-1β, creating an efficient feedback mechanism (Seckinger et al., 1987).

In the CNS, IL-1 is expressed at high levels during prenatal and postnatal development (Dziegielewska et al., 2000, Giulian et al., 1988). IL-1 expression declines to low constitutive levels in the normal adult (Vitkovic et al., 2000) but markedly increases after injury (Pan et al., 2002, Pineau and Lacroix, 2007, Ricci-Vitiani et al., 2006, Rothwell and Luheshi, 2000, Wang et al., 1997, Yang et al., 2004, Yang et al., 2005). Astrocytes and microglia are the main intrinsic sources of IL-1, and IL-1 can induce the secretion of multiple proinflammatory cytokines, chemokines and prostaglandins by activating microglia and astrocytes (Aloisi et al., 1992, Degousee et al., 2001). Therefore, IL-1 is considered as a candidate neurotoxin that contributes to the progressive CNS disorders (Vela et al., 2002). Administration of IL-1ra or IL-1β blocking antibodies reduces neuronal death subsequent to ischemia (Loddick and Rothwell, 1996, Yamasaki et al., 1995) and disruption of IL-1 signaling also confers protection to hosts in other inflammatory disorders such as bacterial infection and endotoxin administration (Boelens et al., 2000, Labow et al., 1997). However, IL-1 can also exert beneficial effects, particularly when released in modest concentrations. IL-1 can enhance the survival of primary neurons in vitro by directly and indirectly inducing nerve growth factor from astrocytes (Brenneman et al., 1992, Spranger et al., 1990). IL-1 also induces the production of fibroblast growth factor-2 (FGF-2), which can act as a trophic factor for motor neurons as well as for basal forebrain neurons (Albrecht et al., 2002, Ho and Blum, 1997).

In light of the diverse actions of IL-1 on different cell types, the mechanisms underlying IL-1 actions are not well understood. Many signaling pathways have been suggested to mediate IL-1 actions in different cell types. The most well-characterized pathway involves a kinase cascade leading to the activation of a transcription factor, the nuclear factor-κB (NF-κB) (Cao et al., 1996, Malinin et al., 1997). For example, NF-κB activation in glial cells mediates the IL-1 regulation of cytokines and growth factors (Friedman et al., 1996, Heese et al., 1998, Malinin et al., 1997). However, other signaling pathways can also be activated by IL-1, including the mitogen-activated protein kinase (MAPK) cascades involving p38 MAPK, stress-activated protein kinase/c-Jun N-terminal kinase (SAPK/JNK) and the classical MAPK extracellular-signal regulated kinase (ERK44/42) (Dunne and O'Neill, 2003, O'Neill, 2002, O'Neill and Greene, 1998). For instance, activation of p38 has been suggested to regulate the effects of IL-1β on tau phosphorylation in cortical neurons (Li et al., 2003). Thus, the activation of specific pathways may differ in distinct cell types and that may mediate distinct biological consequences of IL-1 actions.

Neural precursor cells (NPCs) have been experimentally used to repair the damaged nervous system (Ben-Hur et al., 2005), either by transplantation of cells grown in vitro or by activation of endogenous NPCs. Although IL-1 has been shown to affect bioactivity of microglia, astrocytes and neurons, as mentioned above, little is known about its effect on NPCs. Here we report that IL-1β has effects on the survival, proliferation and differentiation of NPCs. We provide evidence that the effect of IL-1β on NPCs requires the phosphorylation of SAPK/JNK signaling pathway.