Models of aging tissue can be hard to devise if the collection of tissue samples would require an invasive procedure. Such is the case with cardiovascular tissue. Although blood vessel cells could be generated through the reprogramming of skin cells, this approach would take the cells through an embryonic-like state. That is, with this approach, the markers of aging would be erased. However, there is another possibility: direct reprogramming. It bypasses the embryonic-like state, leaving markers of aging intact.
Direct reprogramming has been used this way before. Back in 2015, researchers based at the Salk Institute showed that fibroblasts could be directly reprogrammed into neurons, skipping the induced pluripotent stem cell stage that would have erased the cells’ aging signatures. The resulting brain cells retained their markers of age, letting researchers study how neurons change with age.
In new research, Salk Institute scientists led by Martin Hetzer, PhD, used direct conversion to create two types of vasculature cells. The scientists started with fibroblasts collected from donors of different ages and from patients suffering Hutchinson-Gilford Progeria Syndrome (HGPS), a disorder of accelerated, premature aging. Applying direct conversion, the scientists generated induced vascular endothelial cells (iVECs) and smooth muscle cells (iSMCs). And so, the scientists were able to model vascular aging and HGPS in vitro.
Details of this work appeared September 8 in the journal eLife, in an article titled, “Direct reprogramming of human smooth muscle and vascular endothelial cells reveals defects associated with aging and Hutchinson-Gilford progeria syndrome.” The “defects” noted in the article’s title appear to correspond to therapeutic targets.
“iVECs induced from old donors revealed upregulation of GSTM1 and PALD1, genes linked to oxidative stress, inflammation, and endothelial junction stability, as vascular aging markers,” the article’s authors wrote. “We found that iSMCs from HGPS donors overexpressed bone morphogenetic protein (BMP)−4, which plays a key role in both vascular calcification and endothelial barrier damage observed in HGPS.”
“A functional assay performed on PALD1 KD VECs demonstrated a recovery in vascular permeability,” they continued. “Furthermore, targeting BMP4 with blocking antibody recovered the functionality of the vascular barrier in vitro, hence representing a potential future therapeutic strategy to limit cardiovascular dysfunction in HGPS.”
Essentially, the study revealed clues as to why blood vessels tend to become leaky and hardened with aging. Moreover, it identified targets that could be hit by new drugs intended to slow aging in vascular cells.
“The vasculature is extremely important for aging, but its impact has been underestimated because it has been difficult to study how blood vessel cells age,” said Hetzer, the paper’s senior author and Salk’s vice president and chief science officer.
Most knowledge about how blood vessel cells age comes from observations of how the blood vessels themselves change over time: veins and arteries become less elastic, thickening and stiffening. These changes can contribute to blood pressure increases and a heightened risk of heart disease with age.
In the current study, 21 genes were expressed at different levels in the iSMCs from old and young people, including genes related to the calcification of blood vessels. Nine genes were expressed differently according to age in the iVECs, including genes related to inflammation. In patients with HGPS, some genes reflected the same expression patterns usually seen in older people, while other patterns were unique.
In particular, levels of BMP-4 protein, which is known to play a role in the calcification of blood vessel, were slightly higher in aged cells compared to younger cells, but more significantly higher in smooth muscle cells from progeria patients. This suggests that the protein is particularly important in accelerated aging.
“One of the biggest theoretical implications of this study is that we now know we can longitudinally study a single patient during aging or during the course of a treatment and study how their vasculature is changing and how we might be able to target that,” remarked Simone Bersini, PhD, a Salk postdoctoral fellow and co-first author of the paper.
“We are among the first to use this technique to study the aging of the vascular system,” noted Roberta Schulte, PhD, the Hetzer lab coordinator and co-first author of the paper. “The idea of developing both of these cell types from fibroblasts was out there, but we tweaked the techniques to suit our needs.”
“By repeating what was done with neurons, we’ve demonstrated that this direct reprogramming is a powerful tool that can likely be applied to many cell types to study aging mechanisms in all sorts of other human tissues,” added Hetzer.
Hetzer’s team is planning future studies to probe the exact molecular mechanisms by which some of the genes they found to change with age control the changes seen in the vasculature.
