A two-layer safeguard for stem cell identity

Every cell in the body contains the same DNA. What makes a stem cell special is that it can become many different cell types. To keep that potential, it must avoid switching on genes that push it too early toward a specific fate. How cells maintain this delicate balance has remained unclear.

Inside the nucleus, DNA is wrapped around proteins, forming a compact structure often compared to beads on a string. This packaging is not just structural. It helps decide which genes are active and which stay silent.

Cells can swap some of these packaging proteins for slightly different versions. One such variant, called H2A.Z, is common in stem cells and is enriched near active and poised regulatory regions, including genes involved in development. Another key player is a large protein complex built around a protein named SRCAP, which inserts H2A.Z into DNA packaging. Until now, most researchers assumed that SRCAP’s main role was simply to load this variant.

A team led by Armelle Tollenaere and David Suter at EPFL set out to separate the roles of SRCAP and H2A.Z in mouse pluripotent stem cells.

The researchers engineered stem cells so that SRCAP could be rapidly degraded on demand. This allowed them to observe what happened to DNA packaging and gene activity almost in real time.

They also created a modified version of SRCAP that could no longer insert H2A.Z. By comparing cells with normal SRCAP and mutant SRCAP, they could determine which effects required H2A.Z and which did not.

As expected, when they removed SRCAP from the cells, H2A.Z quickly disappeared from many key regulatory regions. They found that H2A.Z itself mainly acted as a brake on the expression of many lineage-specific genes, including transcription factors involved in differentiation. But SRCAP also turned out to have a second, H2A.Z-independent function: limiting how often important developmental regulators, known as transcription factors, could bind to DNA.

In simple terms, H2A.Z helped restrain the expression of lineage-specific genes, while SRCAP physically restricted how often transcription factors could access their binding sites in DNA.

The combination of several approaches including live-cell single molecule imaging, pointed to a model in which the large SRCAP complex physically hinders transcription factor access to DNA, preventing excessive or inappropriate binding.

Together, SRCAP and H2A.Z form a coordinated two-layer safeguard system. The H2A.Z layer controls whether developmental regulators are expressed, while the SRCAP layer limits how easily these regulators can attach to DNA and activate gene programs.

By acting on the same regulators at two different levels, production and access to DNA, this pair helps keep pluripotent stem cells stable and prevents erratic activation of differentiation pathways. At the same time, the system remains flexible enough to allow change when the right signals arrive.

This work reveals a previously unrecognized mechanism by which cells protect their identity while preserving their potential. By clarifying how stem cells prevent inappropriate activation of developmental programs, it provides new insight into how cell identity is maintained during early development and how its breakdown could contribute to disease or faulty differentiation.

Other contributors

• Max-Planck Institute of Biochemistry
• Ludwig Maximilian University of Munich
• Ulm University