The effects of urolithin A on poly I:C-induced microglial activation.

Neuroinflammation can be triggered by various stimuli, including viral infections. Viruses can directly invade the brain and infect neuronal cells or indirectly trigger a "cytokine storm" in the periphery that eventually leads to microglial activation in the brain. While this initial activation of microglial cells is important for viral clearance, chronic activation leads to excessive inflammation and oxidative stress, which can be neurotoxic. Remarkebly, recent studies have shown that certain viruses such as influenza A virus, coronavirus, herpes virus and Epstein-Barr virus may be involved in the development of neurodegenerative diseases such as Parkinson's disease, Alzheimer's disease, and multiple sclerosis. Therefore, it is important to find therapeutic strategies against chronic neuroinflammation triggered by viral infections. Here, we investigated the effects of urolithin A (UA) on microglial activation in vitro induced by a viral mimetic, poly I:C, in a triple co-culture system of neurons, astrocytes and microglial cells. Immunocytochemistry was used to perform a comprehensive single-cell analysis of the morphological changes of microglia as an indicator of their reactive state. Treatment with UA significantly prevented the poly I:C-induced reactive state of microglia, which was characterized by increased expression of the microglial activation markers CD68 and IBA-1. UA restored the poly I:C-induced morphology by restoring microglial ramification. In addition, UA was able to reduce the release of the pro-inflammatory mediators CCL2, TNF-α, and IL-1ß and showed a trend toward attenuation of cellular ROS production in poly I:C-treated cultures. Overall, this study suggests that UA as a component of a healthy diet may help prevent virus-induced neuroinflammation and may have therapeutic potential for future studies to prevent or treat neurodegenerative diseases by targeting the associated neuroinflammatory processes.

Repeated performance of spatial memory tasks ameliorates cognitive decline in APP/PS1 mice.

BACKGROUND: Alzheimer's disease (AD) is a burden on the public health system because it is a neurodegenerative disease that is incurable and for which there is no successful treatment. AD patients suffer from symptoms for many years, with progressive loss of cognitive and functional abilities. In addition to the features of AD, described as amyloid plaques and neurofibrillary tangles, neuroinflammatory processes, genetic factors, and lifestyle also play important roles. Increasing evidence for lifestyle factors includes possible changes due to smoking, social engagement, and physical activity. METHODS: Morris water maze behavioral tasks were performed to analyze the formation of spatial memory. APPswe/PS1dE9 mice with a remarkable increase in amyloid-ß production associated with certain behavioral abnormalities comparable to AD symptoms and age-matched wild-type littermates were trained several times at 3, 6, 9, and 12 months of age and compared with untrained groups at 9 and 12 months of age. Performance during the acquisition phase, in the reference memory test, and in searching strategies were analyzed. RESULTS: 9- and 12-month-old APP/PS1 mice showed cognitive impairment, especially in the reference memory test and searching strategies. This cognitive deterioration was reversed in 9- and 12-month-old APP/PS1 mice that had been previously trained several times. Even in the reversal test, in which memory formation must be adapted to the new platform position, several trained APP/PS1 mice performed better. CONCLUSION: Repeated spatial memory training in the water maze showed positive effects on memory formation in APP/PS1 mice. Interestingly, the cohort that had been previously trained several times was able to use increased hippocampus-dependent strategies, similar to the WT mice. This may suggest that cognitively demanding and physically active tasks can improve cognitive function.

IL-37 expression reduces acute and chronic neuroinflammation and rescues cognitive impairment in an Alzheimer's disease mouse model.

The anti-inflammatory cytokine interleukin-37 (IL-37) belongs to the IL-1 family but is not expressed in mice. We used a human IL-37 (hIL-37tg) expressing mouse, which has been subjected to various models of local and systemic inflammation as well as immunological challenges. Previous studies reveal an immunomodulatory role of IL-37, which can be characterized as an important suppressor of innate immunity. Here, we examined the functions of IL-37 in the central nervous system and explored the effects of IL-37 on neuronal architecture and function, microglial phenotype, cytokine production and behavior after inflammatory challenge by intraperitoneal LPS-injection. In wild-type mice, decreased spine density, activated microglial phenotype and impaired long-term potentiation (LTP) were observed after LPS injection, whereas hIL-37tg mice showed no impairment. In addition, we crossed the hIL-37tg mouse with an animal model of Alzheimer's disease (APP/PS1) to investigate the anti-inflammatory properties of IL-37 under chronic neuroinflammatory conditions. Our results show that expression of IL-37 is able to limit inflammation in the brain after acute inflammatory events and prevent loss of cognitive abilities in a mouse model of AD.

The NLRP3 inflammasome inhibitor OLT1177 rescues cognitive impairment in a mouse model of Alzheimer's disease.

Numerous studies demonstrate that neuroinflammation is a key player in the progression of Alzheimer's disease (AD). Interleukin (IL)-1ß is a main inducer of inflammation and therefore a prime target for therapeutic options. The inactive IL-1ß precursor requires processing by the the nucleotide-binding oligomerization domain-like receptor family, pyrin domain containing 3 (NLRP3) inflammasome into a mature and active form. Studies have shown that IL-1ß is up-regulated in brains of patients with AD, and that genetic inactivation of the NLRP3 inflammasome improves behavioral tests and synaptic plasticity phenotypes in a murine model of the disease. In the present study, we analyzed the effect of pharmacological inhibition of the NLRP3 inflammasome using dapansutrile (OLT1177), an oral NLRP3-specific inhibitor that is safe in humans. Six-month-old WT and APP/PS1 mice were fed with standard mouse chow or OLT1177-enriched chow for 3 mo. The Morris water maze test revealed an impaired learning and memory ability of 9-mo-old APP/PS1 mice (P = 0.001), which was completely rescued by OLT1177 fed to mice (P = 0.008 to untreated APP/PS1). Furthermore, our findings revealed that 3 mo of OLT1177 diet can rescue synaptic plasticity in this mouse model of AD (P = 0.007 to untreated APP/PS1). In addition, microglia were less activated (P = 0.07) and the number of plaques was reduced in the cortex (P = 0.03) following NLRP3 inhibition with OLT1177 administration. We also observed an OLT1177 dose-dependent normalization of plasma metabolic markers of AD to those of WT mice. This study suggests the therapeutic potential of treating neuroinflammation with an oral inhibitor of the NLRP3 inflammasome.

Enduring Changes in Neuronal Function upon Systemic Inflammation Are NLRP3 Inflammasome Dependent.

Neuroinflammation can be caused by various insults to the brain and represents an important pathologic hallmark of neurodegenerative diseases including Alzheimer's disease (AD). Infection-triggered acute systemic inflammation is able to induce neuroinflammation and may negatively affect neuronal morphology, synaptic plasticity, and cognitive function. In contrast to acute effects, persisting consequences for the brain on systemic immune stimulation remain largely unexplored. Here, we report an age-dependent vulnerability of wild-type (WT) mice of either sex toward a systemic immune stimulation by Salmonella typhimurium lipopolysaccharide (LPS). Decreased neuronal complexity three months after peripheral immune stimulation is accompanied by impairment in long-term potentiation (LTP) and spatial learning. Aged APP/PS1 mice reveal an increased sensitivity also to LPS of Escherichia coli, which had no effect in WT mice. We further report that these effects are mediated by NOD-like receptor family, pyrin domain containing 3 (NLRP3) inflammasome activation, since the genetic ablation and pharmacological inhibition using the NLRP3 inhibitor MCC950 rescue the morphological and electrophysiological phenotype.SIGNIFICANCE STATEMENT Acute peripheral immune stimulation has been shown to have both positive and negative effects on Aß deposition. Improvements or worsening may be possible in acute inflammation. However, there is still no evidence of effects longer than a month after stimulation. The data are pointing to an important role of the NOD-like receptor family, pyrin domain containing 3 (NLRP3) inflammasome for mediating the long-term consequences of systemic immune stimulation, which in addition turns out to be age dependent.

Immune Challenge Alters Reactivity of Hippocampal Noradrenergic System in Prenatally Stressed Aged Mice.

Prenatal stress (PS) has long-term sequelae for the morphological and functional status of the central nervous system of the progeny. A PS-induced proinflammatory status of the organism may result in an impairment of both hippocampal synaptic plasticity and hippocampus-dependent memory formation in adults. We addressed here the question of how PS-induced alterations in the immune response in young and old mice may contribute to changes in hippocampal function in aging. Immune stimulation (via LPS injection) significantly affected the ability of the hippocampal CA3-CA1 synapse of PS mice to undergo long-term potentiation (LTP). Elevated corticosterone level in the blood of aged PS mice that is known to influence LTP magnitude indicates a chronic activation of the HPA axis due to the in utero stress exposure. We investigated the contribution of adrenergic receptors to the modulation of hippocampal synaptic plasticity of aged mice and found that impaired LTP in the PS-LPS group was indeed rescued by application of isoproterenol (a nonspecific noradrenergic agonist). Further exploration of the mechanisms of the observed phenomena will add to our understanding of the interaction between PS and proinflammatory immune activation and its contribution to the functional and structural integrity of the aging brain.

Nogo-A regulates spatial learning as well as memory formation and modulates structural plasticity in the adult mouse hippocampus.

Behavioral learning has been shown to involve changes in the function and structure of synaptic connections of the central nervous system (CNS). On the other hand, the neuronal circuitry in the mature brain is characterized by a high degree of stability possibly providing a correlate for long-term storage of information. This observation indicates the requirement for a set of molecules inhibiting plasticity and promoting stability thereby providing temporal and spatial specificity to plastic processes. Indeed, signaling of Nogo-A via its receptors has been shown to play a crucial role in restricting activity-dependent functional and structural plasticity in the adult CNS. However, whether Nogo-A controls learning and memory formation and what are the cellular and molecular mechanisms underlying this function is still unclear. Here we show that Nogo-A signaling controls spatial learning and reference memory formation upon training in the Morris water maze and negatively modulates structural changes at spines in the mouse hippocampus. Learning processes and the correlated structural plasticity have been shown to involve changes in excitatory as well as in inhibitory neuronal connections. We show here that Nogo-A is highly expressed not only in excitatory, but also in inhibitory, Parvalbumin positive neurons in the adult hippocampus. By this means our current and previous data indicate that Nogo-A loss-of-function positively influences spatial learning by priming the neuronal structure to a higher plasticity level. Taken together our results link the role of Nogo-A in negatively regulating plastic processes to a physiological function in controlling learning and memory processes in the mature hippocampus and open the interesting possibility that it might mainly act by controlling the function of the hippocampal inhibitory circuitry.