Suppression of Microgliosis With the Colony-Stimulating Factor 1 Receptor Inhibitor PLX3397 Does Not Attenuate Memory Defects During Epileptogenesis in the Rat

Abstract

Events of status epilepticus (SE) trigger the development of temporal lobe epilepsy (TLE), a type of focal epilepsy that is commonly drug-resistant and is highly comorbid with cognitive deficits. While SE-induced hippocampal injury, accompanied by gliosis and neuronal loss, typically disrupts cognitive functions resulting in memory defects, it is not definitively known how. Our previous studies revealed extensive hippocampal microgliosis that peaked between 2 and 3 weeks after SE and paralleled the development of cognitive impairments, suggesting a role for reactive microglia in this pathophysiology. Microglial survival and proliferation are regulated by the colony-stimulating factor 1 receptor (CSF1R). The CSF1R inhibitor PLX3397 has been shown to reduce/deplete microglial populations and improve cognitive performance in models of neurodegenerative disorders. Therefore, we hypothesized that suppression of microgliosis with PLX3397 during epileptogenesis may attenuate the hippocampal-dependent spatial learning and memory deficits in the rat pilocarpine model of SE and acquired TLE. Different groups of control and SE rats were fed standard chow (SC) or chow with PLX3397 starting immediately after SE and for 3 weeks. Novel object recognition (NOR) and Barnes maze (BM) were performed to determine memory function between 2 and 3 weeks after SE. Then microglial populations were assessed using immunohistochemistry. Control rats fed with either SC or PLX3397 performed similarly in both NOR and BM tests, differentiating novel vs. familiar objects in NOR, and rapidly learning the location of the hidden platform in BM. In contrast, both SE groups (SC and PLX3397) showed significant deficits in both NOR and BM tests compared to controls. Both PLX3397-treated control and SE groups had significantly decreased numbers of microglia in the hippocampus (60%) compared to those in SC. In parallel, we found that PLX3397 treatment also reduced SE-induced hippocampal astrogliosis. Thus, despite drastic reductions in microglial cells, memory was unaffected in the PLX3397-treated groups compared to those in SC, suggesting that remaining microglia may be sufficient to help maintain hippocampal functions. In sum, PLX3397 did not improve or worsen the memory deficits in rats that sustained pilocarpine-induced SE. Further research is required to determine whether microglia play a role in cognitive decline during epileptogenesis.

What microglia depletion approaches tell us about the role of microglia on synaptic function and behavior

“…Microglia depletion, either by hippocampal clodronate infusion or PLX3397 (1 week), had a transitory negative effect on BM acquisition, not evident after 3 weeks of depletion (Torres et al, 2016). Accordingly, 2-3-weeks of PLX did not affect spatial learning and memory in young mice (Elmore et al, 2014;Dagher et al, 2015;Willis et al, 2020) and rats (Wyatt-Johnson et al, 2021). A noticeable exception was found in aged mice, where PLX5622 (3 weeks) impaired memory retention especially in females (Unger et al, 2018).…”

“…In contrast, in adult animals a large set of studies consistently found no effect of microglia depletion, either pharmacological or genetic, in recognition memory in the NOR and NOL tasks (Acharya et al, 2016;Feng et al, 2016;Gibson et al, 2019;Kakae et al, 2019;Allen et al, 2020;Crapser et al, 2020;De Luca et al, 2020;Worthen et al, 2020;Witcher et al, 2021;Wyatt-Johnson et al, 2021). No effects have been observed upon microglia repopulation either (Elmore et al, 2015;De Luca et al, 2020;Henry et al, 2020).…”

“…The improved behavioral outcomes were associated with increased neuronal survival (Li et al, 2020), dendritic spine densities (Rice et al, 2015) and neuronal connectivity (Witcher et al, 2021), suggesting that circuit remodeling by microglia after damage may be detrimental for behavior and cognition. However, two studies reported that microglia depletion failed to restore memory deficits in a model of pilocarpine-induced epilepsy (Wyatt-Johnson et al, 2021) and TBI (Willis et al, 2020).…”

Unlike Brief Inhibition of Microglia Proliferation after Spinal Cord Injury, Long-Term Treatment Does Not Improve Motor Recovery

“…Similarly, duration of microglia eradication in other diseases led to various outcomes. A 3-week PLX3397-treatment in a mouse model of temporal lobe epilepsy had no effect on memory deficits [ 30 ], whereas a 3-month treatment in a mouse model of Alzheimer’s disease reduced amyloid plaque formation [ 31 ]. Differences in the functional outcome observed after a complete or selective subset of microglia depletion in neurological diseases and trauma may also reflect a molecular and functional heterogeneity of microglia and their responses, depending on the location in the CNS (for review see [ 32 ]), as well as the type of damage.…”

Macrophage subsets and their role: co-relation with colony-stimulating factor-1 receptor and clinical relevance

“…However, there is evidence that suggests the opposite, emphasizing the neuroprotective role of CSF-1/CSF-1R. CSF-1R inhibitor PLX3397 has been reported to significantly reduce the number of microglia which resulted in exacerbated brain infarction and vividly increased the production of inflammatory mediators by astrocytes [ 177 , 178 ] (Fig. 4 ) (Table 4 ).…”