P0113 Maintained microglial homeostasis and adult neurogenesis despite chronic ileocolitis
Authors: Mundlos H , Masanetz R K , Gauer C , Stolzer I , Günther C , Winkler J , Süß P 
Published: January 2025

Abstract

Background
Inflammatory bowel diseases (IBD), such as Crohn‘s disease and ulcerative colitis, are associated with neuropsychiatric comorbidities, including depression, anxiety, and Parkinson’s disease [1]. These are presumably mediated along the gut-immune-brain axis by the transition of inflammation across gastrointestinal and CNS-surrounding barriers, causing the activation of microglia [1]. Adult hippocampal neurogenesis, the generation of new neurons in the hippocampus throughout life, is impaired by peripheral inflammation [2] and linked to affective and neurodegenerative disorders [3]. In the context of IBD, adult hippocampal neurogenesis has only been investigated in chemically induced animal models mimicking ulcerative colitis, leading to heterogeneous results [1].

Methods
In this study, we used the Casp8∆IEC ileocolitis mouse model which is characterized by a specific knockout of caspase 8 in intestinal epithelial cells, leading to ileocolitis that resembles Crohn’s disease [4]. We investigated the effects of ileocolitis on microglia and on adult neurogenesis in the hippocampal dentate gyrus (DG). The hippocampi of 14- and 24-week-old Casp8∆IEC mice were compared to wild-type (wt) mice using immunohistochemistry and immunofluorescence analyses. Microglial density and proliferation were assessed by quantifying ionized calcium-binding adapter molecule 1 (Iba1)+ and Iba1+Mcm2+ myeloid cells, respectively. Additionally, microglial phagocytic activation was examined by co-staining Iba1 with the lysosomal marker CD68. To evaluate adult hippocampal neurogenesis, progenitor cell proliferation and maturation were analyzed by quantifying Mcm2+ proliferating cells and doublecortin (DCX)+ neuroblasts in the DG. Cells undergoing cell death during maturation were quantified by co-staining DCX with terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL).

Results
Microglial density and proliferation in the dentate gyrus region were preserved between Casp8∆IEC and wt mice at both 14 and 24 weeks of age. Likewise, microglial activation state and phagocytosis remained unchanged between the two groups at both time points
Adult hippocampal neurogenesis was maintained between the two groups. The proliferation and maturation of progenitor cells, as well as the rate of cell death during maturation in the dentate gyrus, remained similar between Casp8∆IEC and wt mice.

Conclusion
These results demonstrate that key aspects of hippocampal immune function and adult hippocampal neurogenesis are resilient to experimental ileocolitis induced by the deletion of Casp8 in intestinal epithelial cells.

References
(1)Masanetz RK, Winkler J, Winner B, Günther C, Süß P. The Gut-Immune-Brain Axis: An Important Route for Neuropsychiatric Morbidity in Inflammatory Bowel Disease. Int J Mol Sci. 2022;23(19):11111. Published 2022 Sep 21. doi:10.3390/ijms231911111
(2)Ekdahl CT, Claasen JH, Bonde S, Kokaia Z, Lindvall O. Inflammation is detrimental for neurogenesis in adult brain. Proc Natl Acad Sci U S A. 2003;100(23):13632-13637. doi:10.1073/pnas.2234031100
(3)Toda T, Parylak SL, Linker SB, Gage FH. The role of adult hippocampal neurogenesis in brain health and disease. Mol Psychiatry. 2019;24(1):67-87. doi:10.1038/s41380-018-0036-2
(4)Günther C, Martini E, Wittkopf N, et al. Caspase-8 regulates TNF-α-induced epithelial necroptosis and terminal ileitis. Nature. 2011;477(7364):335-339. Published 2011 Sep 14. doi:10.1038/nature10400