Endothelial NO Shifts Aggregates from Toxic to Protective States to Modulate Arterial Stiffness

Hypothesis

In aged endothelial cells, nitric oxide (NO) bioavailability determines whether protein aggregates act as toxic deposits or as protective, amyloid‑like scaffolds that sequester oxidized proteins and reduce arterial stiffness.

Mechanistic Rationale

NO signaling influences redox balance and S‑nitrosylation of chaperones and proteasome subunits, thereby tuning the cell’s proteostatic capacity. When NO declines, oxidative stress rises, increasing the load of misfolded, oxidized proteins. In this setting, the endothelium may up‑regulate a backup aggregation pathway akin to the SAPA system described in C. elegans, diverting newly synthesized or damaged proteins into insoluble, ordered assemblies. These assemblies could functionalize as extracellular matrix‑associated amyloids that safely isolate harmful species, similar to ApoE aggregates that neutralize bacterial toxins in skin. Conversely, sufficient NO maintains chaperone activity and proteasome flux, preventing aggregation and favoring degradation or refolding. Thus, NO acts as a molecular switch that shifts the equilibrium between deleterious aggregate accumulation and beneficial, sequestering aggregates.

Testable Predictions

1. Endothelial cells with pharmacologically or genetically elevated NO will show lower levels of insoluble, ubiquitin‑positive aggregates and higher soluble oxidized protein levels compared with NO‑deficient counterparts.
2. Inducing aggregation (e.g., with low‑dose proteasome inhibitor) in NO‑rich endothelium will produce aggregates that co‑localize with markers of oxidized proteins and exhibit reduced cytotoxicity, whereas the same stimulus in NO‑poor cells yields aggregates that correlate with increased ROS and cell death.
3. Restoring NO in aged mouse aortas will decrease pulse wave velocity (PWV) and concurrently shift the biochemical fraction of vascular proteins from the SDS‑insoluble to the soluble fraction, without a change in total protein carbonylation.
4. Inhibiting the putative SAPA‑like aggregation factor (e.g., knocking down a homolog of the C. elegans SAPA component) in NO‑deficient endothelium will abolish the protective effect of aggregates, leading to heightened endothelial permeability and stiffness despite low NO.

Experimental Approach

• Use human umbilical vein endothelial cells (HUVECs) treated with L‑NAME to inhibit NO synthase or with an NO donor (e.g., DETA‑NONOate) to elevate NO. Measure soluble vs. insoluble protein fractions by sequential extraction and Western blot for ubiquitin, p62, and carbonyl modifications.
• Assess aggregate morphology via Thioflavin T staining and electron microscopy to determine amyloid‑like characteristics.
• Evaluate functional outcomes: monocyte adhesion, permeability (FITC‑dextran flux), and ROS production.
• In vivo, administer NO donors or eNOS‑gene‑aged mice and perform PWV measurements, immunohistochemical staining for aggregates in the aorta, and biochemical fractionation.
• Apply CRISPRi to target candidate SAPA‑like genes (identified via transcriptomic upregulation in NO‑deficient endothelium) and repeat the above assays.

If aggregates prove to be NO‑regulated protective sinks, therapies that boost NO might not only improve vasodilation but also fine‑tune proteostatic remodeling to mitigate age‑related arterial stiffening without globally suppressing aggregation—a nuanced shift from viewing aggregates solely as pathology to recognizing them as conditionally adaptive structures.