Elsevier

Neurobiology of Disease

Volume 83, November 2015, Pages 1-15
Neurobiology of Disease

Inflammation without neuronal death triggers striatal neurogenesis comparable to stroke

https://doi.org/10.1016/j.nbd.2015.08.013Get rights and content

Highlights

  • LPS-induced inflammation without neuronal loss triggers striatal neurogenesis

  • Magnitude of striatal neurogenesis is similar to that after stroke-induced injury.

  • Microarray on sorted microglia identifies factors potentially regulating neurogenesis.

  • CXCL13 is upregulated in microglia and increases neuroblast migration in vitro.

  • Microglia-derived CXCL13, acting through CXCR5, might be neuroprotective in stroke.

Abstract

Ischemic stroke triggers neurogenesis from neural stem/progenitor cells (NSPCs) in the subventricular zone (SVZ) and migration of newly formed neuroblasts toward the damaged striatum where they differentiate to mature neurons. Whether it is the injury per se or the associated inflammation that gives rise to this endogenous neurogenic response is unknown. Here we showed that inflammation without corresponding neuronal loss caused by intrastriatal lipopolysaccharide (LPS) injection leads to striatal neurogenesis in rats comparable to that after a 30 min middle cerebral artery occlusion, as characterized by striatal DCX + neuroblast recruitment and mature NeuN +/BrdU + neuron formation. Using global gene expression analysis, changes in several factors that could potentially regulate striatal neurogenesis were identified in microglia sorted from SVZ and striatum of LPS-injected and stroke-subjected rats. Among the upregulated factors, one chemokine, CXCL13, was found to promote neuroblast migration from neonatal mouse SVZ explants in vitro. However, neuroblast migration to the striatum was not affected in constitutive CXCL13 receptor CXCR5−/− mice subjected to stroke. Infarct volume and pro-inflammatory M1 microglia/macrophage density were increased in CXCR5−/− mice, suggesting that microglia-derived CXCL13, acting through CXCR5, might be involved in neuroprotection following stroke. Our findings raise the possibility that the inflammation accompanying an ischemic insult is the major inducer of striatal neurogenesis after stroke.

Introduction

Ischemic stroke following cerebral artery occlusion is a leading cause of chronic disability in humans, and effective treatment to promote recovery is lacking. It is well established that neural stem/progenitor cells (NSPCs) in the subventricular zone (SVZ) of adult rodents continuously produce new neuroblasts that migrate into the injured striatum for several months after stroke (Arvidsson et al., 2002, Thored et al., 2006, Thored et al., 2007, Parent et al., 2002). These neuroblasts differentiate to mature neurons, become integrated (Yamashita et al., 2006), project to substantia nigra (Sun et al., 2012), and seem to be functional (Hou et al., 2008). There is also evidence for enhanced SVZ cell proliferation and neuroblast formation after stroke in humans (Macas et al., 2006, Marti-Fabregas et al., 2010, Minger et al., 2007). The interest in this potential self-repair mechanism was further increased by a recent report showing that neuroblasts from SVZ enter striatum and become interneurons in adult humans under normal conditions (Ernst et al., 2014). However, in rats, only a fraction of stroke-induced neurons survive long-term (Arvidsson et al., 2002), and it is unclear whether they contribute to the spontaneous functional recovery after the insult (Lagace, 2012).

Stroke is associated with inflammation, which exerts a complex influence on several steps of striatal neurogenesis (Tobin et al., 2014). Several months following stroke in rats, activated microglia/macrophages continue to be localized in the ipsilateral SVZ concomitant with the continuous production of new neuroblasts migrating into the striatum (Thored et al., 2009). Factors released from activated microglia/macrophages can either stimulate NSPC proliferation in the SVZ, as with IGF-1 (Thored et al., 2009, Yan et al., 2006) and IL-15 (Gomez-Nicola et al., 2011), or in the case of TNF-α signaling through TNF-R1, suppress it (Iosif et al., 2008). Activated microglia/macrophages are also involved in directing neuroblasts to the damaged area by secreting CXCL12 (Robin et al., 2006), MCP-1 (Yan et al., 2007), and osteopontin (Yan et al., 2009). Finally, one study indicates that inflammation associated with stroke contributes to the poor survival of the new striatal neurons (Hoehn et al., 2005), similar to what has been described for new hippocampal neurons in other inflammatory environments (Ekdahl et al., 2003, Monje et al., 2003). The number of recruited neuroblasts correlates with the volume of striatal injury after stroke of different severities (Thored et al., 2006), but whether it is the injury per se or the associated inflammation that induces striatal neurogenesis is unknown.

We show here that inflammation without neuronal death, evoked by intrastriatal LPS-injection, is sufficient to trigger striatal neurogenesis similar to that after stroke in rats. Using global gene expression analysis on sorted rat microglia, we identified several potential regulators of this response with changes observed both after LPS and middle cerebral artery occlusion (MCAO). One of the upregulated factors, CXCL13, improved migration of mouse SVZ neuroblasts in vitro. However, knockout (KO) mice for CXCR5, the receptor for CXCL13, showed no impairment of neuroblast migration after stroke but more extensive injury and pro-inflammatory phenotype of microglia/macrophages.

Section snippets

Animals and experimental design

All procedures were carried out in accordance with the guidelines set by the Malmö-Lund Ethical Committee for the use of laboratory animals, and were conducted in accordance with the European Union directive on the subject of animal rights. Procedures were carried out on male Wistar rats (250–300 g, Charles River, Germany), male C57BL/6 J mice (25–30 g, Charles River, Germany), and male CXCR5−/− mice (25–30 g, The Jackson Laboratory, http://jaxmice.jax.org/strain/006659.html), housed under 12 h

Intrastriatal LPS injection causes massive inflammation without neuronal loss in rat striatum

We first determined whether intracerebral administration of LPS could induce microglia/macrophage activation without causing neuronal cell death. In a pilot experiment, we injected different doses of LPS ranging from 0.01 to 100 μg into mouse striatum. Substantial microglia/macrophage activation without any neuronal death was found in the striatum 2 weeks after injection of 10 μg LPS in mice (data not shown). Because the neurogenic response after stroke is more robust in rats, we wanted to explore

Discussion

We show here that brain inflammation is sufficient for inducing a regenerative response from NSPCs located in the SVZ of the adult rodent brain. LPS-induced striatal inflammation without neuronal loss triggered the migration of neuroblasts into the striatum and their differentiation into mature neurons. The magnitude of neuroblast production in response to inflammation was similar to that observed after a 30 min MCAO, which caused major neuronal loss in the striatum. Stroke and LPS-induced

Conflict of interest disclosure

The authors declare no conflicts of interests.

Acknowledgments

We thank Stefan Lang for assisting with microarray analysis, Zhi Ma and Teona Roschupkina for help with cell sorting, Zhaolu Wang for help with cell counting, and Linda Jansson for technical assistance. This work was supported by the Swedish Research Council, the European Union project TargetBraIn (279017), AFA Foundation (100231), Torsten Söderberg Foundation and Swedish Government Initiative for Strategic Research Areas (StemTherapy).

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  • Cited by (0)

    1

    K.Z.C. and R.G. contributed equally to this work.

    2

    O.L. and Z.K. have shared senior authorship.

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