The endocannabinoid system modulates a transient TNF pathway that induces neural stem cell proliferation

Abstract

Evidence is emerging that the tumour necrosis factor (TNF-α) is a potent signal that induces neural stem cell proliferation and migration. We show that NSC self-renewal is controlled by bi-directional cross-talk between the endocannabinoid system and the TNF signalling pathway. By blocking endogenous TNF-α activity, we demonstrate that the TNF system is critical for the proliferation of NSC. Furthermore, we show that pharmacological blockade of the CB1/CB2 cannabinoid receptors dramatically suppresses TNF-α-induced NSC proliferation. Interestingly, we found that CB1 or CB2 agonists induce NSC proliferation coupled to a significant increase in both TACE/ADAM 17 and TNF-α levels. Overall these data suggest a novel mode of action for the endocannabinoid system in NSC proliferation that is coupled to TNF signalling and that may be of therapeutic interest in the emerging field of brain repair.

Introduction

It has long been accepted that our brain tissue is incapable of regenerating and that from birth, neurodegeneration is a long trip towards senility. However, in the adult mammalian brains some discrete regions do retain the capacity to generate functional neurons, such as the subventricular zone (SVZ) and the subgranular zone (SGZ) of the hippocampal dentate gyrus. In these regions adult neural stem cells (NSC) continue to proliferate, differentiate, and undergo neurogenesis (Alvarez-Buylla and Lim, 2004).

Recent reports suggest that there is a synergy between the immune system and NSC to promote functional recovery, since immune cells help to maintain neurogenesis in germinal centres of the adult CNS even under non-pathological conditions (Ziv et al., 2006a, Ziv et al., 2006b, Butovsky et al., 2006). Similarly, there is substantial evidence that neural stem cell proliferation in the SVZ is modulated in various neurodegenerative diseases (Curtis et al., 2007). Therefore, some cross-talk between elements involved in the neuroinflammatory response and those that interact with NSC might occur and to some extent, may activate endogenous brain repair (Rolls et al., 2007).

The pleiotropic cytokine, TNF-α, is a key part of the inflammatory response that modulates the proliferation of neural progenitors in the subventricular zone of the adult rat brain, acting through two different receptor subtypes, TNF-RI and TNF-RII (Wu et al., 2000, Allan and Rothwell, 2001). Indeed, evidence from TNF receptor knock-out (KO) mice suggests that signalling through TNF-RI suppresses neural progenitor proliferation and neurogenesis in vivo, while signalling through TNF-RII enhances neurogenesis under basal conditions or in neurodegenerative disorders (Iosif et al., 2006). More recent studies confirmed that under pathological conditions such as stroke, activity of the TNF-α-converting enzyme (TACE/ADAM 17) protease induces neural progenitor proliferation in the SVZ (Katakowski et al., 2007).

In the brain, the endocannabinoid system is composed of two well characterized lipid mediators, N-arachidonoylethanolamine (anandamide) and 2-arachidonoylglycerol (2-AG), ligands of the CB1 and CB2 cannabinoid receptors (Di Marzo et al., 2004). Both cannabinoid receptors are expressed by NSC/precursor cells in the SVZ, and impaired neurogenesis has been found in mice that lack the CB1 receptor (Arévalo-Martín et al., 2007, Jin et al., 2004). Two closely related sn-1 diacylglycerol lipases α and β (DAGL α and β) are responsible for the synthesis of 2-AG from diacylglycerol (DAG), (Bisogno et al., 2003, Williams et al., 2003). Notably, DAGL α is expressed in NSC and endocannabinoids are produced by neurospheres in culture (Molina-Holgado et al., 2007, Aguado et al., 2005). Moreover, TNF-α induces the rapid and transient production of DAG, the substrate for DAGL α and β, through the activation of phospholipase C (Schütze et al., 1991). Although, TNF-α secretion occurs during chronic inflammation, it is likely that cells are not continuously exposed to stimulating conditions, but rather experience transient pulses. Similarly, prolonged treatment of neurosphere cultures with TNF-α promotes proliferation while a moderate apoptosis was detected (Widera et al., 2006). Hence, we have adopted a short term, 15 min, stimulation protocol to search for a possible interaction between TNF-α and the endocannabinoid system in the induction of neural stem cell proliferation. Our data suggest that there is bi-directional cross-talk between endocannabinoids and TNF-α signalling which modulates the proliferation of neural stem cells.