A Trem2R47H mouse model without cryptic splicing drives age- and disease-dependent tissue damage and synaptic loss in response to plaques

Abstract

Background The TREM2 R47H variant is one of the strongest genetic risk factors for late-onset Alzheimer’s Disease (AD). Unfortunately, many current Trem2R47H mouse models are associated with cryptic mRNA splicing of the mutant allele that produces a confounding reduction in protein product. To overcome this issue, we developed the Trem2R47H NSS (Normal Splice Site) mouse model in which the Trem2 allele is expressed at a similar level to the wild-type Trem2 allele without evidence of cryptic splicing products. Methods Trem2R47H NSS mice were treated with the demyelinating agent cuprizone, or crossed with the 5xFAD mouse model of amyloidosis, to explore the impact of the TREM2 R47H variant on inflammatory responses to demyelination, plaque development, and the brain’s response to plaques. Results Trem2R47H NSS mice display an appropriate inflammatory response to cuprizone challenge, and do not recapitulate the null allele in terms of impeded inflammatory responses to demyelination. Utilizing the 5xFAD mouse model, we report age- and disease-dependent changes in Trem2R47H NSS mice in response to development of AD-like pathology. At an early (4-month-old) disease stage, hemizygous 5xFAD/homozygous Trem2R47H NSS (5xFAD/Trem2R47H NSS) mice have reduced size and number of microglia that display impaired interaction with plaques compared to microglia in age-matched 5xFAD hemizygous controls. This is associated with a suppressed inflammatory response but increased dystrophic neurites and axonal damage as measured by plasma neurofilament light chain (NfL) level. Homozygosity for Trem2R47H NSS suppressed LTP deficits and loss of presynaptic puncta caused by the 5xFAD transgene array in 4-month-old mice. At a more advanced (12-month-old) disease stage 5xFAD/Trem2R47H NSS mice no longer display impaired plaque-microglia interaction or suppressed inflammatory gene expression, although NfL levels remain elevated, and a unique interferon-related gene expression signature is seen. Twelve-month old Trem2R47H NSS mice also display LTP deficits and postsynaptic loss. Conclusions The Trem2R47H NSS mouse is a valuable model that can be used to investigate age-dependent effects of the AD-risk R47H mutation on TREM2 and microglial function including its effects on plaque development, microglial-plaque interaction, production of a unique interferon signature and associated tissue damage.

iPSC‐derived microglia carrying the TREM2 R47H/+ mutation are proinflammatory and promote synapse loss

“…While TREM2 knockout mice exhibit impaired developmental synaptic pruning (Filipello et al, 2018), our transplantation experiments show that human microglia carrying the TREM2 R47H/+ mutation can promote synapse loss in the mouse brain (Figure 5). These findings are broadly consistent with recent observations that TREM2 R47H mutations can promote neuron and synapse loss in vitro and in aged mice (Popescu et al, 2022;Tran et al, 2023). It should be noted, however, that Popescu et al (2022) also observed increased in vitro synaptosome uptake by TREM2 R47H/+ iPSC microglia, in contrast to our findings (Figure 3).…”

“…Inflammatory responses by microglia in amyloid and tau models are generally dampened by the loss of TREM2 (Griciuc et al, 2019; Jay et al, 2017; Leyns et al, 2017). Studies examining TREM2 R47H mutations have often reported similar effects to knockout mutations, with R47H microglia in amyloid models exhibiting impaired plaque localization and response (Cheng‐Hathaway et al, 2018; Song et al, 2018; Tran et al, 2023). The apparent effects of the R47H mutation in tau mouse models have varied, with reports of increased or decreased tau seeding, microglial inflammation, and synaptic/cognitive effects (Gratuze et al, 2020; Leyns et al, 2019; Sayed et al, 2021; Tran et al, 2023).…”

Genetic diversity promotes resilience in a mouse model of Alzheimer's disease

“…NfL, a marker for axonal injury/degeneration, has emerged as a potential biomarker for human AD 48 . Recent studies have also shown that plasma NfL concentration correlates well with plaque load in AD mouse models 49,50 . To gain insight about the influence of genetic diversity on axonal injury/disease progression, we quantified NfL in both the plasma and cortical insoluble fraction of mice of WT‐ and 5x‐CC strains at 4 and 12 months of age.…”

Single cell spatial transcriptomics reveals distinct patterns of dysregulation in non-neuronal and neuronal cells induced by the Trem2R47H Alzheimer’s risk gene mutation

“…Recent efforts have produced a mouse model of the Trem2 R47H mutation in which bulk RNA-seq analysis has identi ed a unique Trem2 R47H induced interferon signature believed to be associated with microglia in response to Aβ pathology [22]. Activation of microglia signi cantly impacts neuronal function and can be neurotoxic [23], but resolving the cell type speci c impacts of this mutation on neuronal populations requires single-cell analysis.…”