Our data agree with previous evidence that inflammation is detrimental to hippocampal neurogenesis in the adult brain. We found that focal, sustained hippocampal inflammation causes severe depletion of developing neuroblasts and skews the fate of neural progenitors in the SGZ away from a neuronal lineage and toward an astroglial fate in the adult brain. Interestingly, if offspring from neural progenitors are allowed to mature, they are resistant to the effects of sustained expression of IL-1β. This resistance suggests that the blockade caused by IL-1β occurs prior to the commitment of neural progenitors to a neuronal fate. In contrast, Mathieu and colleagues saw a decrease in BrdU-labeled cells that developed into neurons but no difference in astroglial differentiation after 14 days using an adenovirus expressing human IL-1β in rats (Mathieu et al., 2010
). Belarbi and colleagues recently showed no alteration in total numbers of BrdU labeled neurons in rats pretreated with 28 days of intraventricular infusion of lipopolysaccharide (Belarbi et al., 2011
). Differences between our models of neuroinflammation as well as timing may account for these conflicting results.
Our study does not address how sustained IL-1β affects other cell populations. For example, the effect of sustained IL-1β on NPCs in the SGZ of the hippocampus is unknown. Common markers for NPCs, such as nestin and GFAP, are upregulated in reactive astrocytes due to inflammation and distinguishing between NPCs and reactive astrocytes becomes difficult. Pulse labeling with BrdU should label dividing NPCs prior to the onset of inflammation but would not indicate their presence or absence a month or more later. Other more specific methods of examining NPC survival will be needed for further analysis.
This work shows that inflammation inhibits neurogenesis but the mechanism remains unclear. The absence of DCX+
cells is confined to the inflamed hippocampus illustrating a change in the microenvironment. One potential alteration may be disruption of nutrients being supplied by the microvasculature or exposure to CSF. For example, vascular endothelial growth factor has been shown to increase adult hippocampal neurogenesis (Cao et al., 2004
). Another possibility is a cell-mediated disruption of DCX+
cells. We did not observe any direct association of DCX+
cells with microglia, astrocytes, T cells, neutrophils, or MHC-II+
cells. This finding contrasts with results from Sierra and colleagues who recently demonstrated that microglia phagocytose apoptotic neural progenitors as part of normal clearance (Sierra et al., 2010
). Unfortunately, the late time points in the current study are insufficient to draw any clear conclusions about whether clearance by microglia is occurring. In addition, at the time points examined in this study, surviving DCX+
cells were rare and found distal to the FIV(Cre) injection site in sections less populated with inflammatory cells. Given the complexity of the inflammatory environment, multiple factors may contribute to the inflammatory induced suppression of neurogenesis.
Our findings also show that voluntary exercise increases neurogenesis only in the absence of neuroinflammation. The mechanism of exercise-induced increase in neurogenesis is unknown, but our results suggest that it does not act or is insufficient in the presence of inflammation. This contrasts with a recent study demonstrating forced running prevented reduction in BrdU-labeled DCX+
cells in the SGZ following repetitive i.p. lipopolysaccharide injections in adult mice (Wu et al., 2007
). Some of the differences between our study and the study by Wu and colleagues may be related to distinctions in central versus peripheral inflammatory models as well as duration of inflammation. It is interesting that we see a beneficial effect of voluntary running in a non-inflamed region of the brain in animals that have focal inflammation. Differences between animals regarding access to the running wheel, distance traveled, and running speed may have existed. We purposely housed only two animals per running wheel to allow each animal as much access as possible and to avoid social isolation that can downregulate neurogenesis. We visually observed running by all animals but did not monitor distance traveled or running speed which may be possible confounders. We also controlled for the presence of the running wheel as a source of environmental enrichment. Due to the pronounced negative effect of sustained IL-1β on the DCX+
cell population, a floor effect is likely to have occurred where the sustained inflammation was so severe that it could not be overcome by exercise. Interestingly, in a model of peripheral E. coli
infection, running was shown to reduce IL-1β in aged rats (Barrientos et al., 2011
). Because we saw no improvement in DCX+
cell survival due to exercise in the presence of human IL-1β, we did not examine any molecular changes in endogenous markers such as levels of caspase-1, IL-1Ra, murine IL-1β, etc. Further studies examining levels of endogenous cytokines could be utilized to address this issue.
Our results about the negative effect of increased IL-1β on DCX+
neuroblasts in the SGZ are consistent with previous reports of decreased adult hippocampal neurogenesis due to IL-1β caused by stress or i.c.v. infusion (Goshen et al., 2008
; Koo and Duman, 2008
). Our model differs by the injection of virus directly into the hippocampus that may contribute to decreased neurogenesis. Indeed, WT animals that received a similar viral injection exhibited a modest decrease in DCX+
cells. However, the effect with virus that activated IL-1β expression was much greater. We were able to prevent this outcome by disrupting IL-1β signaling using animals deficient in IL-1R1; similarly, other groups have shown that IL-1β is less harmful in IL-1R1 knockouts or in the presence of exogenous IL-1Ra (Goshen et al., 2008
; Koo and Duman, 2008
). In addition, IL-1β may be responsible for the decrease in neurogenesis seen with aging (Kuzumaki et al., 2010
) since this can be alleviated by pharmacologic inhibition of caspase-1, which is necessary for IL-1β production (Gemma et al., 2007
). Overall, these results support IL-1β’s negative role in regulating adult hippocampal neurogenesis.
While our results show that sustained hippocampal IL-1β expression drastically reduces adult neurogenesis in the SGZ, the mechanism remains unclear. One possibility is that IL-1β acts directly on NPCs. While some observers have not been able to detect any effect of IL-1β on NPCs in vitro (Mathieu et al., 2010
; Monje et al., 2003
), Koo and Duman recently showed that NPCs in the hippocampus express IL-1R1 in vivo, and IL-1β decreases neurogenesis by causing cell cycle arrest that is dependent on NF-κB activation in vitro (Koo and Duman, 2008
). Therefore, IL-1β may prevent differentiation to mature neurons without causing cell death of NPCs. Future experiments will have to address the survival of NPCs during sustained IL-1β expression and whether IL-1β is acting directly on them. While hIL-1β initiates and maintains sustained inflammation, we know that TNF-α, IL-6, mIL-1β, and mIL-1α are all elevated in our model (Hein et al., 2010
; Shaftel et al., 2007b
). Both TNF-α and IL-6 are believed to be detrimental to adult hippocampal neurogenesis (Iosif et al., 2006
; Monje et al., 2003
; Vallieres et al., 2002
). Given the complexity of inflammation in vivo, it is likely that multiple factors contribute to the effect that we see in our IL-1βXAT
model. By understanding the effect of sustained inflammation on adult neurogenesis, more appropriate therapies can be developed for inflammatory CNS disorders that alter the neurogenic regions of the brain.