Local perfusion of capillaries reveals disrupted beta-amyloid homeostasis at the blood-brain barrier in Tg2576 murine Alzheimer's model

by Selleckchem | Feb 04 2024

Background: Parenchymal accumulation of beta-amyloid (Aβ) characterizes Alzheimer's disease (AD). Aβ homeostasis is maintained by two ATP-binding cassette (ABC) transporters (ABCC1 and ABCB1) mediating efflux, and the receptor for advanced glycation end products (RAGE) mediating influx across the blood-brain barrier (BBB). Altered transporter levels and disruption of tight junctions (TJ) were linked to AD. However, Aβ transport and the activity of ABCC1, ABCB1 and RAGE as well as the functionality of TJ in AD are unclear.

Methods: ISMICAP, a BBB model involving microperfusion of capillaries, was used to assess BBB properties in acute cortical brain slices from Tg2576 mice compared to wild-type (WT) controls using two-photon microscopy. TJ integrity was tested by vascularly perfusing biocytin-tetramethylrhodamine (TMR) and quantifying its extravascular diffusion as well as the diffusion of FM1-43 from luminal to abluminal membranes of endothelial cells (ECs). To assess ABCC1 and ABCB1 activity, calcein-AM was perfused, which is converted to fluorescent calcein in ECs and gets actively extruded by both transporters. To probe which transporter is involved, probenecid or Elacridar were applied, individually or combined, to block ABCC1 and ABCB1, respectively. To assess RAGE activity, the binding of 5-FAM-tagged Aβ by ECs was quantified with or without applying FPS-ZM1, a RAGE antagonist.

Results: In Tg2576 mouse brain, extravascular TMR was 1.8-fold that in WT mice, indicating increased paracellular leakage. FM1-43 staining of abluminal membranes in Tg2576 capillaries was 1.7-fold that in WT mice, indicating reduced TJ integrity in AD. While calcein was undetectable in WT mice, its accumulation was significant in Tg2576 mice, suggesting lower calcein extrusion in AD. Incubation with probenecid or Elacridar in WT mice resulted in a marked calcein accumulation, yet probenecid alone had no effect in Tg2576 mice, implying the absence of probenecid-sensitive ABC transporters. In WT mice, Aβ accumulated along the luminal membranes, which was undetectable after applying FPS-ZM1. In contrast, marginal Aβ fluorescence was observed in Tg2576 vessels, and FPS-ZM1 was without effect, suggesting reduced RAGE binding activity.

Conclusions: Disrupted TJ integrity, reduced ABCC1 functionality and decreased RAGE binding were identified as BBB alterations in Tg2576 mice, with the latter finding challenging the current concepts. Our results suggest to manage AD by including modulation of TJ proteins and Aβ-RAGE binding.

Comments:

This study delves deep into the mechanisms of Alzheimer's disease (AD) and the specific alterations in the blood-brain barrier (BBB) that contribute to the accumulation of beta-amyloid (Aβ), a hallmark of AD. They utilized a BBB model called ISMICAP and various imaging techniques to understand how the BBB properties differ between AD mice (Tg2576) and wild-type (WT) mice.

Here's a breakdown of the key findings:

1. **Increased Paracellular Leakage:** Tg2576 mice showed higher extravascular diffusion of biocytin-tetramethylrhodamine (TMR), indicating increased paracellular leakage, suggesting disrupted tight junction (TJ) integrity in AD.

2. **Reduced TJ Integrity:** The staining of abluminal membranes with FM1-43 in Tg2576 capillaries was higher, indicating reduced TJ integrity compared to WT mice.

3. **Altered Transporter Activity:** Lower calcein extrusion was observed in Tg2576 mice, suggesting reduced functionality of ATP-binding cassette (ABC) transporters (ABCC1 and ABCB1) responsible for Aβ efflux. Probenecid or Elacridar, which block these transporters, had different effects between Tg2576 and WT mice, indicating altered transporter activity in AD.

4. **Decreased RAGE Binding:** In Tg2576 vessels, reduced binding of 5-FAM-tagged Aβ by endothelial cells (ECs) was observed compared to WT mice. The application of FPS-ZM1, a RAGE antagonist, had minimal effect in Tg2576 mice, suggesting decreased activity of the receptor for advanced glycation end products (RAGE) in AD.

The conclusions drawn from these findings propose potential avenues for managing AD:

- Modulating TJ proteins could be beneficial in addressing BBB integrity issues observed in AD.
- Considering the reduced Aβ-RAGE binding, strategies involving modulation of RAGE activity might be less effective or require alternative approaches in AD treatment.

This study challenges existing concepts and sheds light on specific BBB alterations in AD, offering insights that could potentially influence therapeutic approaches for managing the disease.