In this issue, we highlight a study from Ingrid Fleming's group at Goethe University Frankfurt (first author Mauro Siragusa), published in Cardiovascular Research, on a largely unexplored layer of the endothelial proteome: microproteins.
Why this paper matters
Our genome contains thousands of small stretches of DNA, often overlapping better-known genes, that were long assumed not to code for anything. We now know many of them are in fact translated into small functional proteins, called microproteins. Whether endothelial cells make use of this hidden protein layer, and whether it changes with inflammation, was unknown. Given how central the endothelium is to vascular inflammation and atherosclerosis, this is a meaningful gap, and closing it required combining two demanding techniques: a method to capture exactly what endothelial cells are actively translating (RiboTag sequencing), and a proteomic approach specifically optimised to detect very small proteins, which are normally missed (low molecular weight mass spectrometry).
Main findings
- Using this combined approach in both mouse tissues and cultured human endothelial cells, the authors confidently identified around 2700 microproteins in mice and about 2200 in human endothelial cells, each one seen in at least two independent samples.
- Inflammation changed which microproteins were produced. In humans, IL-1beta (a key inflammatory messenger in atherosclerosis) altered the endothelial microprotein profile, and around 350 microproteins were detectable in patients' blood, with roughly 30 rising and 25 falling within hours of an induced controlled heart injury (during a septal ablation procedure used to treat hypertrophic cardiomyopathy).
- One specific microprotein, encoded within the gene PSTPIP2, was studied more closely with a custom antibody: it increased with inflammation in cultured endothelial cells, and was found in both early atherosclerotic lesions in mice and advanced atherosclerotic plaques from patients.
- A genetic screen, designed to isolate the effect of the microproteins themselves from that of the genes they overlap, showed that 250 of them influence endothelial cell growth or survival.
A couple of caveats are worth keeping in mind. Only a small fraction of these microproteins (114 out of thousands) have a clear counterpart in both mice and humans, meaning most of them are quite species-specific; this limits how much mouse data can tell us about human biology. Also, solid functional evidence is still available for only a handful of these microproteins (PSTPIP2 being the best example), while the biological role of most of the catalogue remains to be tested individually rather than assumed.
Potential applications
The authors themselves frame this as a resource rather than a finished story, but two translational directions stand out. First, since some microproteins are secreted and detectable in blood, and their levels shift after cardiac injury, they are candidate biomarkers worth exploring for vascular and cardiac disease, alongside established markers. Second, given their very small size, microproteins are, in principle, easier to target with drugs than larger proteins, which makes this catalogue a starting point for identifying new therapeutic targets in inflammation-driven vascular disease, once individual candidates such as miP-PSTPIP2 are functionally validated.
