The latest medical research on Molecular Genetic Pathology

The accumulation of β-amyloid (Aβ) peptides into insoluble plaques is an early pathological feature of Alzheimer's disease (AD). BACE1 is the sole β-secretase for Aβ generation, making it an attractive therapeutic target for AD therapy. While BACE1 inhibitors have been shown to reduce Aβ levels in people with AD, clinical trials targeting BACE1 have failed due to unwanted synaptic deficits. Understanding the physiological role of BACE1 in individual cell types is essential for developing effective BACE inhibitors for the treatment of AD. Recent single-cell RNA transcriptomic assays revealed that oligodendrocytes are enriched with genes required for generating Aβ. However, the contribution of oligodendrocytes to amyloid plaque burden in AD and the side effects of oligodendrocyte-specific Bace1 deletion remain to be explored.

We generated an oligodendrocyte-specific Bace1 knockout model (Bace1fl/fl;Olig2-Cre) to monitor potential disruptions in myelination using standard electron microscopy. Long-term potentiation (LTP) was monitored to measure synaptic integrity. We crossed the Bace1fl/fl;Olig2-Cre model with heterozygous AppNL-G-F/wt knock-in AD mice to generate AD mice lacking oligodendrocyte Bace1 (Bace1fl/fl;Olig2-Cre; AppNL-G-F/wt) and examined amyloid plaque number and insoluble Aβ levels and gliosis in these animals. Single nuclei RNA sequencing experiments were conducted to examine molecular changes in response to Bace1 deficiency in oligodendrocytes in the wild type or APP knock-in background.

Bace1 deletion in oligodendrocytes caused no change in myelin thickness in the corpus callosum but a marginal reduction in myelin sheath thickness of the optic nerve. Synaptic strength measured by LTP was not different between Bace1fl/fl;Olig2-Cre and age-matched Bace1fl/fl control animals, suggesting no major effect on synaptic plasticity. Intriguingly, deletion of Bace1 in 12-month-old heterozygous AD knock-in mice (Bace1fl/fl;Olig2-Cre; AppNL-G-F/wt mice) caused a significant reduction of amyloid plaques by ~ 33% in the hippocampus and ~ 29% in the cortex compared to age-matched AD mice (Bace1fl/fl;AppNL-G-F/wt). Insoluble Aβ1-40 and Aβ1-42 levels were reduced comparably while more astrocytes and microglia were observed in surrounding amyloid plaques. Unbiased single-nuclei RNA sequencing results revealed that deletion of oligodendrocyte Bace1 in APPNL-G-F/wt knock-in mice increased expression of genes associated with Aβ generation and clearance such as ADAM10, Ano4, ApoE, Il33, and Sort1.

Our results provide compelling evidence that the amyloidogenic pathway in oligodendrocytes contributes to Aβ plaque formation in the AD brain. While specifically targeting BACE1 inhibition in oligodendrocytes for reducing Aβ pathology in AD is likely challenging, this is a potentially explorable strategy in future studies.

In Parkinson's patients, intestinal dysbiosis can occur years before clinical diagnosis, implicating the gut and its microbiota in the disease. Recent evidence suggests the gut microbiota may trigger body-first Parkinson Disease (PD), yet the underlying mechanisms remain unclear. This study aims to elucidate how a dysbiotic microbiome through intestinal immune alterations triggers PD-related neurodegeneration.

To determine the impact of gut dysbiosis on the development and progression of PD pathology, wild-type male C57BL/6 mice were transplanted with fecal material from PD patients and age-matched healthy donors to challenge the gut-immune-brain axis.

This study demonstrates that patient-derived intestinal microbiota caused midbrain tyrosine hydroxylase positive (TH +) cell loss and motor dysfunction. Ileum-associated microbiota remodeling correlates with a decrease in Th17 homeostatic cells. This event led to an increase in gut inflammation and intestinal barrier disruption. In this regard, we found a decrease in CD4 + cells and an increase in pro-inflammatory cytokines in the blood of PD transplanted mice that could contribute to an increase in the permeabilization of the blood-brain-barrier, observed by an increase in mesencephalic Ig-G-positive microvascular leaks and by an increase of mesencephalic IL-17 levels, compatible with systemic inflammation. Furthermore, alpha-synuclein aggregates can spread caudo-rostrally, causing fragmentation of neuronal mitochondria. This mitochondrial damage subsequently activates innate immune responses in neurons and triggers microglial activation.

We propose that the dysbiotic gut microbiome (dysbiome) in PD can disrupt a healthy microbiome and Th17 homeostatic immunity in the ileum mucosa, leading to a cascade effect that propagates to the brain, ultimately contributing to PD pathophysiology. Our landmark study has successfully identified new peripheral biomarkers that could be used to develop highly effective strategies to prevent the progression of PD into the brain.

Anti-amyloid-β (Aβ) immunotherapy trials have revealed amyloid-related imaging abnormalities (ARIA) as the most prevalent and serious adverse events linked to pathological changes in cerebral vasculature. Recent studies underscore the critical involvement of perivascular macrophages and the infiltration of peripheral immune cells in regulating cerebrovascular damage. Specifically, Aβ antibodies engaged at cerebral amyloid angiopathy (CAA) deposits trigger perivascular macrophage activation and the upregulation of genes associated with vascular permeability. Nevertheless, further research is needed to understand the immediate downstream consequences of macrophage activation, potentially exacerbating CAA-related vascular permeability and microhemorrhages linked to Aβ immunotherapy.

This study investigates immune responses induced by amyloid-targeting antibodies and CAA-induced microhemorrhages using RNA in situ hybridization, histology and digital spatial profiling in an Alzheimer's disease (AD) mouse model of microhemorrhage.

In the present study, we have demonstrated that bapineuzumab murine surrogate (3D6) induces profound vascular damage, leading to smooth muscle cell loss and blood-brain barrier (BBB) breakdown. In addition, digital spatial profiling (DSP) reveals that distinct immune responses contribute to vascular damage with peripheral immune responses and perivascular macrophage activation linked to smooth muscle cell loss and vascular fibrosis, respectively. Finally, RNA in situ hybridization identifies two distinct subsets of Trem2+ macrophages representing tissue-resident and monocyte-derived macrophages around vascular amyloid deposits. Overall, these findings highlight multifaceted roles of immune activation and vascular damage in driving the development of microhemorrhage.

In summary, our study has established a significant link between CAA-Aβ antibody immune complex formation, immune activation and vascular damage leading to smooth muscle cell loss. However, the full implications of this cascade on the development of microhemorrhages requires further exploration. Additional investigations are warranted to unravel the precise molecular mechanisms leading to microhemorrhage, the interplay of diverse immune populations and the functional roles played by various Trem2+ macrophage populations in response to Aβ immunotherapy.