“We looked at what happens in a healthy heart as it ages under standardized conditions,” explains Dr. Sascha Sauer, who heads the Genomics Platform at the Max Delbrück Center for Molecular Medicine (MDC) in Berlin, and is also a scientist at the German Center for Cardiovascular Research (DZHK ). The results reveal where the aging heart is particularly vulnerable, and thus also provide a possible basis for new approaches to treatment.
Using single-cell RNA sequencing, the scientists were able to analyze each cell separately to find out which genes in its genetic material were being expressed at that moment in time. Together with Professor Stefanie Dimmeler’s research group from Goethe University Frankfurt, the Berlin researchers used this method to investigate a total of around 28,000 heart cells – taken from young and old mice. The result is a comprehensive cell atlas of gene activity in aged mammalian hearts, which the team has now presented in the journal JCI
Genes lose their synchronicity
The most striking differences between young and old hearts were observed in connective tissue cells known as fibroblasts. The fibroblasts from the hearts of young mice displayed similarly active genes in each cell. In the older mice, this gene activity was not so consistent from cell to cell. “If you look at a few cells, the pattern is still halfway okay,” says Sauer. “But all these small deviations show that, overall, old cells become more and more differentiated and therefore no longer work together as well. As a result, the highly complex heart system becomes somewhat confused.”
In the outermost layer, or epicardium, of the aged hearts they investigated, Sauer, Dimmeler and their colleagues also found a group of fibroblast cells in which genes associated with calcification were expressed. “We already knew that vessels become increasingly calcified with age,” explains Sauer. “But we didn’t know that healthy hearts contain a certain subtype of aging fibroblasts that contribute to this process.”
A negative influence
In aged hearts, fibroblasts also send out proteins known as serpins to endothelial cells – the cells that line the inside of blood vessels. Serpins make it harder for the endothelial cells to fuse, which can have a negative impact on the blood vessel lining. When the researchers used antibodies against the serpins, they were able to reverse these effects. The scientists suspect that the interaction between fibroblasts and endothelial cells is actually far more complex, as other genes responsible for cell communication are also regulated differently in old fibroblasts than in young ones.
High specialization is lost
The researchers only found age-related differences in the heart muscle cells when they looked for genes that play an important role in muscle contraction. These genes behave very differently in aged heart muscle cells. Other genes – for example, those that are responsible for metabolic pathways and that are identical in all cells – are equally active in old and young heart muscle cells. “If you think about how an organism develops, highly specialized cells tend to emerge later. And it seems the last ones in are also the first ones out. In other words, late-developing specializations are very susceptible to aging,” explains Sauer.
Sauer and his team now hope to find out why gene activity in individual cells becomes so variable with age. To do this, they are looking at markers on the DNA, or methylations. These determine whether a gene is expressed or not.
Ramon Vidal et al. (2019): „Transcriptional heterogeneity of fibroblasts is a hallmark of the aging heart”, JCI Insight, DOI: .
Caution: Parallel analysis of gene activity of individual cells in miniaturized formats. Photo: David Ausserhofer / MDC