Microglial Expression of the Wnt Signaling Modulator DKK2 Differs between Human Alzheimer's Disease Brains and Mouse Neurodegeneration Models.

Wnt signaling is crucial for synapse and cognitive function. Indeed, deficient Wnt signaling is causally related to increased expression of DKK1, an endogenous negative Wnt regulator, and synapse loss, both of which likely contribute to cognitive decline in Alzheimer's disease (AD). Increasingly, AD research efforts have probed the neuroinflammatory role of microglia, the resident immune cells of the CNS, which have furthermore been shown to be modulated by Wnt signaling. The DKK1 homolog DKK2 has been previously identified as an activated response and/or disease-associated microglia (DAM/ARM) gene in a mouse model of AD. Here, we performed a detailed analysis of DKK2 in mouse models of neurodegeneration, and in human AD brain. In APP/PS1 and APPNL-G-F AD mouse model brains as well as in SOD1G93A ALS mouse model spinal cords, but not in control littermates, we demonstrated significant microgliosis and microglial Dkk2 mRNA upregulation in a disease-stage-dependent manner. In the AD models, these DAM/ARM Dkk2+ microglia preferentially accumulated close to ßAmyloid plaques. Furthermore, recombinant DKK2 treatment of rat hippocampal primary neurons blocked WNT7a-induced dendritic spine and synapse formation, indicative of an anti-synaptic effect similar to that of DKK1. In stark contrast, no such microglial DKK2 upregulation was detected in the postmortem human frontal cortex from individuals diagnosed with AD or pathologic aging. In summary, the difference in microglial expression of the DAM/ARM gene DKK2 between mouse models and human AD brain highlights the increasingly recognized limitations of using mouse models to recapitulate facets of human neurodegenerative disease.

A primary rodent triculture model to investigate the role of glia-neuron crosstalk in regulation of neuronal activity.

Neuroinflammation and hyperexcitability have been implicated in the pathogenesis of neurodegenerative disease, and new models are required to investigate the cellular crosstalk involved in these processes. We developed an approach to generate a quantitative and reproducible triculture system that is suitable for pharmacological studies. While primary rat cells were previously grown in a coculture medium formulated to support only neurons and astrocytes, we now optimised a protocol to generate tricultures containing neurons, astrocytes and microglia by culturing in a medium designed to support all three cell types and adding exogenous microglia to cocultures. Immunocytochemistry was used to confirm the intended cell types were present. The percentage of ramified microglia in the tricultures decreases as the number of microglia present increases. Multi-electrode array recordings indicate that microglia in the triculture model suppress neuronal activity in a dose-dependent manner. Neurons in both cocultures and tricultures are responsive to the potassium channel blocker 4-aminopyridine, suggesting that neurons remained viable and functional in the triculture model. Furthermore, suppressed neuronal activity in tricultures correlates with decreased densities of dendritic spines and of the postsynaptic protein Homer1 along dendrites, indicative of a direct or indirect effect of microglia on synapse function. We thus present a functional triculture model, which, due to its more complete cellular composition, is a more relevant model than standard cocultures. The model can be used to probe glia-neuron interactions and subsequently aid the development of assays for drug discovery, using neuronal excitability as a functional endpoint.

Design of a Potent, Selective, and Brain-Penetrant Inhibitor of Wnt-Deactivating Enzyme Notum by Optimization of a Crystallographic Fragment Hit.

Notum is a carboxylesterase that suppresses Wnt signaling through deacylation of an essential palmitoleate group on Wnt proteins. There is a growing understanding of the role Notum plays in human diseases such as colorectal cancer and Alzheimer's disease, supporting the need to discover improved inhibitors, especially for use in models of neurodegeneration. Here, we have described the discovery and profile of 8l (ARUK3001185) as a potent, selective, and brain-penetrant inhibitor of Notum activity suitable for oral dosing in rodent models of disease. Crystallographic fragment screening of the Diamond-SGC Poised Library for binding to Notum, supported by a biochemical enzyme assay to rank inhibition activity, identified 6a and 6b as a pair of outstanding hits. Fragment development of 6 delivered 8l that restored Wnt signaling in the presence of Notum in a cell-based reporter assay. Assessment in pharmacology screens showed 8l to be selective against serine hydrolases, kinases, and drug targets.

Epigenetic repression of Wnt receptors in AD: a role for Sirtuin2-induced H4K16ac deacetylation of Frizzled1 and Frizzled7 promoters.

Growing evidence supports a role for deficient Wnt signalling in Alzheimer's disease (AD). First, the Wnt antagonist DKK1 is elevated in AD brains and is required for amyloid-ß-induced synapse loss. Second, LRP6 Wnt co-receptor is required for synapse integrity and three variants of this receptor are linked to late-onset AD. However, the expression/role of other Wnt signalling components remain poorly explored in AD. Wnt receptors Frizzled1 (Fzd1), Fzd5, Fzd7 and Fzd9 are of interest due to their role in synapse formation/plasticity. Our analyses showed reduced FZD1 and FZD7 mRNA levels in the hippocampus of human early AD stages and in the hAPPNLGF/NLGF mouse model. This transcriptional downregulation was accompanied by reduced levels of the pro-transcriptional histone mark H4K16ac and a concomitant increase of its deacetylase Sirt2 at Fzd1 and Fzd7 promoters in AD. In vitro and in vivo inhibition of Sirt2 rescued Fzd1 and Fzd7 mRNA expression and H4K16ac levels at their promoters. In addition, we showed that Sirt2 recruitment to Fzd1 and Fzd7 promoters is dependent on FoxO1 activity in AD, thus acting as a co-repressor. Finally, we found reduced levels of SIRT2 inhibitory phosphorylation in nuclear samples from human early AD stages with a concomitant increase in the SIRT2 phosphatase PP2C. This results in hyperactive nuclear Sirt2 and favours Fzd1 and Fzd7 repression in AD. Collectively, our findings define a novel role for nuclear hyperactivated SIRT2 in repressing Fzd1 and Fzd7 expression via H4K16ac deacetylation in AD. We propose SIRT2 as an attractive target to ameliorate AD pathology.

Small-molecule inhibitors of carboxylesterase Notum.

Notum has recently been identified as a negative regulator of Wnt signaling through the removal of an essential palmitoleate group from Wnt proteins. There are emerging reports that Notum plays a role in human disease, with published data suggesting that targeting Notum could represent a new therapeutic approach for treating cancer, osteoporosis and neurodegenerative disorders. Complementary hit-finding strategies have been applied with successful approaches that include high-throughput screening, activity-based protein profiling, screening of fragment libraries and virtual screening campaigns. Structural studies are accelerating the discovery of new inhibitors of Notum. Three fit-for-purpose examples are LP-922056, ABC99 and ARUK3001185. The application of these small-molecule inhibitors is helping to further advance an understanding of the role Notum plays in human disease.

Single-Cell Quantification of mRNA Expression in The Human Brain.

RNA analysis at the cellular resolution in the human brain is challenging. Here, we describe an optimised approach for detecting single RNA transcripts in a cell-type specific manner in frozen human brain tissue using multiplexed fluorescent RNAscope probes. We developed a new robust analytical approach for RNAscope quantification. Our method shows that low RNA integrity does not significantly affect RNAscope signal, recapitulates bulk RNA analysis and provides spatial context to transcriptomic analysis of human post-mortem brain at single-cell resolution. In summary, our optimised method allows the usage of frozen human samples from brain banks to perform quantitative RNAscope analysis.

G protein-coupled receptor 37-like 1 modulates astrocyte glutamate transporters and neuronal NMDA receptors and is neuroprotective in ischemia.

We show that the G protein-coupled receptor GPR37-like 1 (GPR37L1) is expressed in most astrocytes and some oligodendrocyte precursors in the mouse central nervous system. This contrasts with GPR37, which is mainly in mature oligodendrocytes. Comparison of wild type and Gpr37l1-/- mice showed that loss of GPR37L1 did not affect the input resistance or resting potential of astrocytes or neurons in the hippocampus. However, GPR37L1-mediated signalling inhibited astrocyte glutamate transporters and - surprisingly, given its lack of expression in neurons - reduced neuronal NMDA receptor (NMDAR) activity during prolonged activation of the receptors as occurs in ischemia. This effect on NMDAR signalling was not mediated by a change in the release of D-serine or TNF-α, two astrocyte-derived agents known to modulate NMDAR function. After middle cerebral artery occlusion, Gpr37l1 expression was increased around the lesion. Neuronal death was increased by ∼40% in Gpr37l1-/- brain compared to wild type in an in vitro model of ischemia. Thus, GPR37L1 protects neurons during ischemia, presumably by modulating extracellular glutamate concentration and NMDAR activation.

Combining Double Fluorescence In Situ Hybridization with Immunolabelling for Detection of the Expression of Three Genes in Mouse Brain Sections.

Detection of gene expression in different types of brain cells e.g., neurons, astrocytes, oligodendrocytes, oligodendrocyte precursors and microglia, can be hampered by the lack of specific primary or secondary antibodies for immunostaining. Here we describe a protocol to detect the expression of three different genes in the same brain section using double fluorescence in situ hybridization with two gene-specific probes followed by immunostaining with an antibody of high specificity directed against the protein encoded by a third gene. The Aspartoacyclase (ASPA) gene, mutations of which can lead to a rare human white matter disease - Canavan disease - is thought to be expressed in oligodendrocytes and microglia but not in astrocytes and neurons. However, the precise expression pattern of ASPA in the brain has yet to be established. This protocol has allowed us to determine that ASPA is expressed in a subset of mature oligodendrocytes and it can be generally applied to a wide range of gene expression pattern studies.

RORalpha, a key to the development and functioning of the brain.

Studies of staggerer mice, in which retinoid-related orphan receptor-alpha (RORα) is mutated, have provided new insights into the critical functions of RORα in various physiological processes in peripheral tissues and in the brain. Staggerer mice present an ataxic phenotype caused by a massive neurodegeneration in the cerebellum. As a result, most of studies have focused on the role of RORα in the development of the cerebellum. Recent studies have expanded the role of RORα to other structures and functions in the brain. RORα was considered to be exclusively expressed in neurons in the brain. Recently, it has been shown that, in addition to its neuronal expression, RORα is expressed in glial cells and particularly in astrocytes in different brain regions. Moreover, RORα has been implicated in the regulation of some astrocyte functions such as the inflammatory function. Several reports have also presented evidence for a role of RORα in diverse pathological processes including oxidative stress-induced apoptosis and cerebral hypoxia. This review therefore focuses on the emerging roles of RORα in the brain and particularly in astrocytes.

Cell-autonomous and non-cell-autonomous neuroprotective functions of RORα in neurons and astrocytes during hypoxia.

There is increasing evidence to suggest that the neuronal response to hypoxia is regulated through their interactions with astrocytes. However, the hypoxia-induced molecular mechanisms within astrocytes which influence neuronal death have yet to be characterized. In this study, we investigated the roles of the nuclear receptor RORα (retinoid-related orphan receptor-α) respectively in neurons and astrocytes during hypoxia using cultures and cocultures of neurons and astrocytes obtained from RORα-deficient mice. We found that loss of RORα function in neuronal cultures increases neuronal death after hypoxia, suggesting a cell-autonomous neuroprotective effect of RORα. Moreover, wild-type neurons cocultured with RORα-deficient astrocytes are characterized by a higher death rate after hypoxia than neurons cocultured with wild-type astrocytes, suggesting that RORα also has a non-cell-autonomous action. By using cocultures of neurons and astrocytes of different genotypes, we showed that this neuroprotective effect of RORα in astrocytes is additive to its effect in neurons, and is mediated in part by cell-to-cell interactions between neurons and astrocytes. We also found that RORα is upregulated by hypoxia in both neurons and astrocytes. Furthermore, our data showed that RORα does not alter oxidative mechanisms during hypoxia but regulates hypoxic inducible factor 1α (HIF-1α) expression, a major regulator of hypoxia sensing, in a cell-specific manner. Indeed, the neuroprotective function of RORα in astrocytes correlates with a downregulation of HIF-1α selectively in these cells. Altogether, our results show that RORα is a key molecular player in hypoxia, protecting neurons through its dual action in neurons and astrocytes.

The nuclear receptor ROR(alpha) exerts a bi-directional regulation of IL-6 in resting and reactive astrocytes.

Astrocytes and one of their products, IL-6, not only support neurons but also mediate inflammation in the brain. Retinoid-related orphan receptor-alpha (RORalpha) transcription factor has related roles, being neuro-protective and, in peripheral tissues, anti-inflammatory. We examined the relation of ROR(alpha) to astrocytes and IL-6 using normal and ROR(alpha) loss-of-function mutant mice. We have shown ROR(alpha) expression in astrocytes and its up-regulation by pro-inflammatory cytokines. We have also demonstrated that ROR(alpha) directly trans-activates the Il-6 gene. We suggest that this direct control is necessary to maintain IL-6 basal level in the brain and may be a link between the neuro-supportive roles of ROR(alpha), IL-6, and astrocytes. Furthermore, after inflammatory stimulation, the absence of ROR(alpha) results in excessive IL-6 up-regulation, indicating that ROR(alpha) exerts an indirect repression probably via the inhibition of the NF-kappaB signaling. Thus, our findings indicate that ROR(alpha) is a pluripotent molecular player in constitutive and adaptive astrocyte physiology.