Human peripheral monocytes capture elements of the state of microglial activation in the brain

Despite a growing focus on neuroimmune mechanisms of Alzheimer’s disease (AD), the role of peripheral monocytes remains largely unknown. Circulating monocytes communicate with the brain’s resident myeloid cells, microglia, via chemical signaling and can directly infiltrate the brain parenchyma.1 Thus, molecular signatures of monocytes may serve as indicators of neuropathological events unfolding in the CNS.2–5 However, no studies have yet directly tested the association of monocyte gene expression on longitudinal cognitive decline or postmortem neuropathology and brain gene expression in aging. Here we present a resource of RNA sequencing of purified CD14+ human monocytes - including an eQTL map - from over 200 elderly individuals, most with accompanying bulk brain RNA sequencing profiles, longitudinal cognitive assessments, and detailed postmortem neuropathological examinations. We tested the direct correlation of gene expression between monocytes and bulk brain tissue, finding very few significant signals driven largely by genetic variation. However, we did identify sets of monocyte-expressed genes that were highly predictive of postmortem microglial activation, diffuse amyloid plaque deposition, and cerebrovascular disease. Our findings prioritize potential blood-based molecular biomarkers for AD; they also reveal the previously unknown architecture of shared gene expression between the CNS and peripheral immune system in aging.

Accelerated Aging Characterizes the Early Stage of Alzheimer’s Disease

For Alzheimer’s disease (AD), aging is the main risk factor, but whether cognitive impairments due to aging resemble early AD deficits is not yet defined. When working with mouse models of AD, the situation is just as complicated, because only a few studies track the progression of the disease at different ages, and most ignore how the aging process affects control mice. In this work, we addressed this problem by comparing the aging process of PS2APP (AD) and wild-type (WT) mice at the level of spontaneous brain electrical activity under anesthesia. Using local field potential recordings, obtained with a linear probe that traverses the posterior parietal cortex and the entire hippocampus, we analyzed how multiple electrical parameters are modified by aging in AD and WT mice. With this approach, we highlighted AD specific features that appear in young AD mice prior to plaque deposition or that are delayed at 12 and 16 months of age. Furthermore, we identified aging characteristics present in WT mice but also occurring prematurely in young AD mice. In short, we found that reduction in the relative power of slow oscillations (SO) and Low/High power imbalance are linked to an AD phenotype at its onset. The loss of SO connectivity and cortico-hippocampal coupling between SO and higher frequencies as well as the increase in UP-state and burst durations are found in young AD and old WT mice. We show evidence that the aging process is accelerated by the mutant PS2 itself and discuss such changes in relation to amyloidosis and gliosis.

Loss of Awareness or Urinary Dysfunction? Investigating Amyloidosis and Urinary Physiology in a Transgenic Mouse

Alzheimer’s disease (AD) is a devastating disorder primarily affecting older adults and is the most common neurodegenerative disease in the US. More than one in three AD patients experience AD-associated urinary dysfunction (ADUD), which directly contributes to their institutionalization. While ADUD has been clinically regarded as a result of poor cognitive control over urinary function, the physiology underlying loss of urinary control remains unknown. We hypothesize that amyloidosis in the CNS results in pathologic changes in urinary structure and function. Tg-APP/PS1DE9 mice were used before plaque deposition (4-6 months) and after plaque accumulation (8-10 months) and compared to WT littermates. Behavioral assays (open field testing and voiding spot assays) were performed to assess cortical function. Pressure-flow cystometry was conducted under urethane anesthesia to assess autonomic control of urinary function without cortical influence. Pharmacomyography of bladder strips was used to determine tissue-level changes in the absence of CNS input. In Tg-APP/PS1DE9 mice, plaque accumulation resulted in significant cystometric changes to voiding phase parameters, but not storage phase parameters. Pharmacologic studies showed decreased sensitivity to adrenergic stimulation without change in muscarinic sensitivity. Behavioral assays demonstrated significant differences between transgenic animals and WT in locomotion and voiding spot sizes. We interpret our data to support AD-related pathology of Aβ accumulation results in a distinct urinary phenotype in our model, analogous to the ADUD observed in AD patients. Establishing and verifying models of ADUD may improve the efficacy of treating ADUD and increase quality of life for patients and their caregivers.

Long-term dynamics of aberrant neuronal activity in awake Alzheimer’s disease transgenic mice

Alzheimer’s disease (AD) is associated with aberrant neuronal activity, which is believed to critically determine disease symptoms. How these activity alterations emerge, how stable they are over time, and whether cellular activity dynamics are affected by the amyloid plaque pathology remains incompletely understood. We here repeatedly recorded the activity from identified neurons in cortex of awake APPPS1 transgenic mice over four weeks during the early phase of plaque deposition using in vivo two-photon calcium imaging. We found that aberrant activity during this stage largely persisted over the observation time. Novel highly active neurons slowly emerged from former intermediately active neurons. Furthermore, activity fluctuations were independent of plaque proximity, but aberrant activity was more likely to persist close to plaques. These results support the notion that neuronal network pathology observed in models of cerebral amyloidosis is the consequence of persistent single cell aberrant neuronal activity, a finding of potential diagnostic and therapeutic relevance for AD.

A New Tool for the Analysis of the Effect of Intracerebrally Injected Anti-Amyloid-β Compounds

Background: A wide range of techniques has been developed over the past decades to characterize amyloid-β (Aβ) pathology in mice. Until now, no method has been established to quantify spatial changes in Aβ plaque deposition due to targeted delivery of substances using ALZET ® pumps. Objective: Development of a methodology to quantify the local distribution of Aβ plaques after intracerebral infusion of compounds. Methods: We have developed a toolbox to quantify Aβ plaques in relation to intracerebral injection channels using Zeiss AxioVision ® and Microsoft Excel ® software. For the proof of concept, intracerebral stereotactic surgery was performed in 50-day-old APP-transgenic mice injected with PBS. At the age of 100 days, brains were collected for immunhistological analysis. Results: The toolbox can be used to analyze and evaluate Aβ plaques (number, size, and coverage) in specific brain areas based on their location relative to the point of the injection or the injection channel. The tool provides classification of Aβ plaques in pre-defined distance groups using two different approaches. Conclusion: This new analytic toolbox facilitates the analysis of long-term continuous intracerebral experimental compound infusions using ALZET ® pumps. This method generates reliable data for Aβ deposition characterization in relation to the distribution of experimental compounds.

Longitudinal increase of CSF soluble TREM2 is driven by early aggregation of Aβ42 and associates with slower amyloid deposition and clinical decline in autosomal-dominant Alzheimer’s disease

Therapeutic modulation of TREM2-dependent microglial function provides an additional strategy to slow progression of Alzheimer disease (AD). Although studies on animal models suggest that TREM2 is protective, the trigger of increased TREM2 expression during disease progression and its clinical and pathological consequences in AD remain unclear. We measured longitudinally soluble TREM2 (sTREM2) as a surrogate marker for protective TREM2-signalling in cerebrospinal fluid (CSF) from participants in the Dominantly Inherited Alzheimer Network (DIAN) observational study. In mutation carriers (MC), the longitudinal sTREM2 increase followed the earliest aggregation of Aβ42 captured by CSF-Aβ42 decrease, but not yet by Pittsburg compound-B Positron Emission Tomography (PiB-PET). Higher sTREM2 increase rates provided protection from Aβ-deposition, whereas lower rates enhanced p-tau increase associated with PiB-PET increase. Moreover, presymptomatic MC with high or low sTREM2 increase rates have opposite associations between CSF Aβ42 and PiB-PET longitudinal changes, suggesting that TREM2 modifies Aβ plaque deposition and compaction. Finally, higher sTREM2 increase rates protected from cortical shrinkage and cognitive decline. Our findings position the TREM2 response within the amyloid cascade right after the first pathological changes in Aβ42 aggregation, support ongoing efforts to develop TREM2 modulating therapies, and predict a very early window for therapeutic intervention.

Acute Trem2 reduction triggers increased microglial phagocytosis, slowing amyloid deposition in mice

Heterozygous genetic variants within the TREM2 gene show a strong association with increased Alzheimer’s disease (AD) risk. Amyloid beta–depositing mouse models haploinsufficient or null for Trem2 have identified important relationships among TREM2, microglia, and AD pathology; however, results are challenging to interpret in the context of varying microglial phenotypes and disease progression. We hypothesized that acute Trem2 reduction may alter amyloid pathology and microglial responses independent of genetic Trem2 deletion in mouse models. We developed antisense oligonucleotides (ASOs) that potently but transiently lower Trem2 messenger RNA throughout the brain and administered them to APP/PS1 mice at varying stages of plaque pathology. Late-stage ASO-mediated Trem2 knockdown significantly reduced plaque deposition and attenuated microglial association around plaque deposits when evaluated 1 mo after ASO injection. Changes in microglial gene signatures 1 wk after ASO administration and phagocytosis measured in ASO-treated cells together indicate that microglia may be activated with short-term Trem2 reduction. These results suggest a time- and/or dose-dependent role for TREM2 in mediating plaque deposition and microglial responses in which loss of TREM2 function may be beneficial for microglial activation and plaque removal in an acute context.