The Adapter Protein ASC Promotes the Deposition of Systemic AA Amyloid in Mice

Core Concepts

The adapter protein ASC (Apoptosis-associated speck-like protein containing a caspase recruitment domain) interacts with the amyloid precursor protein serum amyloid A (SAA) and promotes the deposition of systemic AA amyloid in a mouse model of inflammation-associated amyloidosis.

Abstract

The content explores the role of the ASC inflammasome in the pathogenesis of inflammation-associated systemic AA amyloidosis. Key findings include:

ASC colocalizes with SAA in cardiac tissue from a patient with AA amyloidosis, suggesting a potential interaction between the two proteins.

In vitro experiments show that recombinant ASC specks accelerate the fibrillation of SAA, and limited proteolysis-mass spectrometry reveals that ASC interacts with SAA via its pyrin domain.

In a mouse model of AA amyloidosis, the absence of ASC (Pycard-/- mice) leads to a significant reduction in splenic amyloid deposition compared to wild-type (Pycard+/+) mice. This is not due to altered SAA induction by the inflammatory stimulus.

The reduced amyloid load in Pycard-/- mice is not a consequence of enhanced amyloid phagocytosis by macrophages, as SAA-stimulated phagocytic activity is impaired in Pycard-/- bone marrow-derived macrophages.

Treatment with anti-ASC antibodies decreases the amyloid load in wild-type mice with AA amyloidosis, suggesting that anti-ASC immunotherapy may be a potential therapeutic approach.

A population-wide screen of over 19,000 hospital patients reveals that natural anti-ASC autoantibodies are exceedingly rare (<0.01%), indicating a high tolerance of the human immune system to ASC.

In summary, the content demonstrates that the ASC inflammasome plays a crucial role in the pathogenesis of inflammation-associated systemic AA amyloidosis by promoting the aggregation and deposition of SAA amyloid.

Stats

Serum SAA concentrations were significantly higher in Pycard+/+ AA+ mice compared to Pycard-/- AA+ mice at 24 hours post injection.
Quantification of splenic amyloid load showed a 2.3-fold and 3.4-fold reduction in Pycard-/- AA+ mice compared to Pycard+/+ AA+ mice at day 16 and day 30, respectively.
Western blot analysis revealed a 1.9-fold reduction in total splenic SAA in Pycard-/- AA+ mice compared to Pycard+/+ AA+ mice.
Anti-ASC antibody treatment reduced the splenic amyloid load by 2.5-fold in Pycard+/+ AA+ mice compared to untreated controls.

Quotes

"ASC colocalizes tightly with SAA in human AA amyloidosis."
"Purified recombinant ASC specks accelerated SAA fibril formation in vitro."
"In a murine model of inflammation-associated AA amyloidosis, splenic AA amyloid load was conspicuously decreased in Pycardtm1Vmd/tm1Vmd mice which lack ASC."
"Treatment with anti-ASC antibodies decreased the amyloid loads in wild-type mice suffering from AA amyloidosis."

Key Insights Distilled From

The ASC inflammasome adapter controls the extent of peripheral protein aggregate deposition in inflammation-associated amyloidosis

by Losa,M., Sch... at www.biorxiv.org 05-02-2021

https://www.biorxiv.org/content/10.1101/2021.05.01.442282v3

Deeper Inquiries

How might the interaction between ASC and SAA be leveraged for the development of novel therapeutic strategies beyond anti-ASC antibodies?

The interaction between ASC and SAA opens up possibilities for the development of novel therapeutic strategies beyond anti-ASC antibodies. One approach could involve targeting the specific epitopes in the pyrin domain of ASC that interact with SAA. By designing small molecules or peptides that disrupt this interaction, it may be possible to inhibit the acceleration of SAA fibril formation and reduce amyloid deposition. This targeted approach could potentially be more specific and effective than broad immunotherapy with anti-ASC antibodies.
Another strategy could involve modulating the downstream signaling pathways activated by the ASC-SAA interaction. Since ASC is a key component of the NLRP3, NLRC4, and AIM2 inflammasomes, targeting these pathways could potentially disrupt the inflammatory response that leads to amyloidosis. By developing inhibitors or modulators of these pathways, it may be possible to reduce the production of SAA and inhibit the formation of amyloid fibrils.
Furthermore, understanding the structural details of the ASC-SAA interaction could lead to the development of novel therapeutics that specifically target this interaction. For example, designing peptides or small molecules that mimic the binding interface between ASC and SAA could competitively inhibit their interaction and prevent amyloid formation. This targeted approach could offer a more precise and effective way to intervene in the disease process.

How might the interaction between ASC and SAA be leveraged for the development of novel therapeutic strategies beyond anti-ASC antibodies?

The interaction between ASC and SAA is not limited to AA amyloidosis and Alzheimer's disease. Other protein aggregation diseases that might be influenced by the ASC inflammasome include systemic lupus erythematosus (SLE), type 2 diabetes, and Parkinson's disease. In SLE, ASC has been implicated in the activation of the NLRP3 inflammasome, leading to the production of pro-inflammatory cytokines and autoantibodies. Targeting ASC in SLE could help modulate the inflammatory response and reduce disease severity.
In type 2 diabetes, ASC has been shown to play a role in the activation of the NLRP3 inflammasome in response to high glucose levels. This leads to the production of IL-1β and IL-18, contributing to insulin resistance and beta-cell dysfunction. By targeting ASC in type 2 diabetes, it may be possible to reduce inflammation and improve metabolic outcomes.
In Parkinson's disease, ASC has been implicated in the activation of the NLRP3 inflammasome in microglia, leading to the production of pro-inflammatory cytokines and neuronal damage. Targeting ASC in Parkinson's disease could help reduce neuroinflammation and protect against neurodegeneration. By understanding the role of ASC in these diseases, novel therapeutic strategies could be developed to target the ASC inflammasome and modulate the inflammatory response.

Given the high tolerance of the human immune system to ASC, what other innate immune components or pathways could be targeted to modulate inflammation-associated amyloidosis and other protein misfolding diseases?

In addition to targeting ASC, other innate immune components or pathways could be targeted to modulate inflammation-associated amyloidosis and other protein misfolding diseases. One potential target is the NLRP3 inflammasome itself, which is activated by ASC in response to various danger signals. By developing inhibitors of NLRP3 or modulators of its activation, it may be possible to reduce the production of pro-inflammatory cytokines and inhibit the formation of amyloid fibrils.
Another target could be the P2X7 receptor, which is involved in the activation of the NLRP3 inflammasome by SAA. By blocking the P2X7 receptor or modulating its activity, it may be possible to disrupt the ASC-SAA interaction and prevent the formation of amyloid deposits. Targeting the P2X7 receptor could offer a more specific approach to modulating inflammation-associated amyloidosis.
Furthermore, targeting the downstream signaling pathways activated by the inflammasome, such as the NF-κB pathway or the JAK-STAT pathway, could also be a viable strategy. By developing inhibitors of these pathways, it may be possible to reduce the production of pro-inflammatory cytokines and inhibit the inflammatory response that leads to amyloidosis. This multi-target approach could offer a comprehensive strategy to modulate inflammation-associated amyloidosis and other protein misfolding diseases.