Noradrenaline induces CX3CL1 production and release by neurons

Highlights

• Noradrenaline induces CX3CL1 expression and synthesis in neurons.

• Noradrenaline inhibits ADAM10 and ADAM17 activity and synthesis.

• Noradrenaline increases neuronal dendritic arborization.

Abstract

CX3CL1 is a chemokine for which neurons constitute its primary source within the brain. Besides acting as a chemokine, CX3CL1 regulates multiple processes and is known to inhibit microglial activation. Because of this, CX3CL1 is considered as a messenger used by neurons to communicate with microglia. Similarly, the neurotransmitter noradrenaline reduces microglial activation and production of neurotoxic agents. Based on this, the regulation of neuronal CX3CXL1 by noradrenaline was analyzed.

In primary cortical neurons, noradrenaline induced the accumulation of CX3CL1 protein and mRNA. Noradrenaline also increased CX3CL1 in its soluble form despite the inhibition of the activity and synthesis of ADAM10 and ADAM17, the main proteases known to cleave CX3CL1 from the neuronal membrane. Noradrenaline-treated neurons displayed a higher degree of dendritic arborization and a characteristic accumulation of CX3CL1 in the dendritic bifurcation zones. The soluble CX3CL1 produced by neurons after noradrenaline treatment, reduced the accumulation of nitrites in microglia. These findings indicate that NA anti-inflammatory actions are mediated by neuronal CX3CL1. In addition, CX3CL1 seems to be involved in the development of neuronal processes stimulated by noradrenaline.

Introduction

The reduced production of noradrenaline (NA) in the locus coeruleus is known to be associated with certain neurodegenerative processes such as Alzheimer's (Heneka et al., 2002) or Parkinson's diseases (Delaville et al., 2011). NA depletion, more than a mere disease-associated alteration, seems to be a factor contributing to the progression of such diseases. This is confirmed by the anti-inflammatory and neuroprotective actions observed for NA in vivo and in vitro (Chalermpalanupap et al., 2013, Feinstein et al., 2002, Kalinin et al., 2012).

Based on this, we studied different mechanisms through which NA may exert its neuroprotective actions. One of these seems to be the regulation of astrocytic production of certain chemokines (Hinojosa et al., 2013, Madrigal et al., 2009). Therefore, we have focused on CX3CL1 (also known as fractalkine or neurotactin), a chemokine whose anti-neuroinflammatory actions overlap with some of those described for NA (Limatola and Ransohoff, 2014). This way, we observed NA ability to regulate astrocytic production of CX3CL1 (Hinojosa et al., 2013). However, differently from other chemokines, CX3CL1 is mainly expressed in the CNS by neurons, while astrocytes constitute a less relevant source of it (Nishiyori et al., 1998). Based on this, we analyze here NA effects on neuronal expression and synthesis of CX3CL1.

CX3CL1 is expressed as a membrane-bound form which can be shed by different enzymes, mainly by the disintegrin-and-metalloproteinases ADAM10 and ADAM17 (Garton et al., 2001, Hundhausen et al., 2003). Consequently, CX3CL1 can function in a membrane-bound or in a soluble form. The first is known to promote the adhesion of immune cells (Fong et al., 1998) and the last one exhibits more chemotactic activity (Bazan et al., 1997), but a complete description of all their functions or a precise differentiation between the roles of both CX3CL1 forms are not available yet.

CX3CR1, the only receptor described for CX3CL1, is mainly expressed by microglia within the CNS (Nishiyori et al., 1998). This fact, combined with the preeminent position of neurons among CX3CL1 producers, has led many authors to consider this chemokine as a messenger used by neurons to communicate with microglia and repress their activation (Harrison et al., 1998, Nishiyori et al., 1998).

We show here that NA stimulates CX3CL1 expression by cultured neurons, elevating its concentration inside and outside these cells, despite of NA inhibition of neuronal ADAM10 and ADAM17 activity and synthesis. In addition, CX3CL1 neutralization allowed us to confirm its involvement in the microglia inhibition stimulated by NA.