Abstract :
[en] Microglia regulate hippocampal neurogenesis during chronic neurodegeneration
Hippocampal neurogenesis is an active process essential for the neuronal plasticity of the CNS. It occurs in the subgranular layer (SGL) of the dentate gyrus (DG) and gives rise to new granule cells throughout life. This neurogenic niche is influenced by many physiological and pathological stimuli such as inflammatory factors (Parent, 2003). Neuroinflammation is an important factor shared by many neurodegenerative diseases and is known to alter hippocampal neurogenesis in various conditions (Ekdahl et al., 2003). Although microglia are the main inflammatory actors in these diseases, their exact function in regulating neurogenesis is still largely unclear. Their actions seem to depend on their activation profile, suggesting a large complexity in the immune mechanisms of microglial cells on adult neurogenesis (Ekdahl et al., 2009).
In previous related studies, our group demonstrated that prion disease is marked by an enhancement of both hippocampal neurogenesis and microglial proliferation, suggesting a link between these 2 processes (Gomez-Nicola et al., 2013 and 2014). We also found that the activation of the CSF1R pathway promotes the expansion of microglia and its pro-inflammatory state during the disease. Targeting CSF1R activation with drugs is therefore an effective tool to study the contribution of microglia in many processes like adult neurogenesis. With this view, this project aims to investigate whether microglia can directly regulate hippocampal neurogenesis in chronic neurodegeneration such as prion disease and to define the molecular basis of that response.
Our study was based on an experimental model of prion disease (i.e. ME7) induced in macgreen mice (c-fms-EGFP) allowing to label microglial cells with green fluorescence. The analysis of the sub-regional microglial expansion in the different layers of the DG by IHC with the biomarker Iba1 revealed a homogeneous expansion of microglial population in the ME7 DG with no change in microglial percentage distribution between diseased and control (NBH) mice. The modulation of the CSF1R pathway with agonists (i.e. IL34, CSF1R) or inhibitors (i.e. GW2580) allowed us to explore the effects of proliferating microglia on SGL neurogenesis during prion disease. Our DCX immunostains revealed that the activation of microglial proliferation stimulates hippocampal neurogenesis as seen by the increased number of neural precursor cells (DCX+ cells). In contrast, the selective inhibition of microglial proliferation by GW2580 prevents the enhancement of DCX+ cells during ME7 disease. These data support a direct correlation between the increased hippocampal neurogenesis and the expansion of microglia during prion disease. By retroviral tracing with the red fluorescent protein m-Cherry, a morphological analysis was realized on 4 week-old newborn granule neurons and revealed some alterations (i.e. hyper-ramification and shortening of the dendritic tree) in the neuronal differentiation during ME7 disease that were prevented by the inhibition of microglial proliferation with GW2580.
As our results demonstrate a direct impact of microglia on neurogenesis, we next analyzed the molecular players potentially implicated by using a gene screening strategy. After having selected several candidate genes involved in both neurogenesis and inflammation to compare their relative expression between ME7 and NBH mice, TGFβ, CNTF and IGF1 were identified as candidate molecular actors regulating the pro-neurogenic actions of microglia thanks to qPCR experiments realized on DG microdissected samples. The expression of these genes was decreased in response to GW2580 proving a direct relation with the expansion of microglia. We then focused on the effects of TGFβ pathway alterations on hippocampal neurogenesis by using AdDcn adenovirus to deliver Decorin, a natural inhibitor of the TGFβ pathway. The blockade of TGFβ activity in ME7 mice mimics the effects of microglial inhibition as it reduces the number of DCX+ cells supporting a stimulating effect of TGFβ on the hippocampal neurogenic pool during prion disease.
In conclusion, our study demonstrates an important pro-neurogenic role of microglia in prion disease and highlights the role of TGFβ signaling in regulating this process. Our results link inflammation and neurogenesis in the context of chronic neurodegeneration and provide new clues into the control of the self-repairing mechanisms of the brain suggesting microglia as a potential promising target for the design of new neuroprotective immunomodulatory therapies.
