Tight junctions' crucial role in embryo model development

A group of chemicals control the cell division, identity development, and spatial positioning of the developing human embryo's cells. These signalling chemicals direct a single layer of embryonic stem cells to divide into three layers of various cell types that would subsequently develop into various body parts in a process known as gastrulation. have demonstrated that tight connections between cells may be crucial to the process of gastrulation in human embryos.

Shinya Yamanaka, MD, PhD, senior scientist at Gladstone and a senior author on the paper, adds, "This study has exciting implications for the design of gastrulation models and other lab techniques for differentiating stem cells into specialised cell types."

The easier it will be to reproduce these events in reliable, repeatable ways, the greater our understanding of the signalling pathways in embryos will be. The group is already using its findings to create fresh methods for converting stem cells cultured in a dish into human egg cells—a process that might one day be employed for in vitro fertilisation.

Learning on the Brink

The foundation for the growth of the entire human body is laid by gastrulation. Starting with a layer of induced pluripotent stem cells, or iPS cells—adult cells that have been reprogrammed to resemble embryonic stem cells, meaning they can differentiate into any cell type in the body—researchers have discovered ways to recreate a simplified version of this fundamental process in a dish.

A protein called BMP4, a crucial signalling molecule in gastrulation, is then added by researchers, causing the cells in the dish to start dividing into the three layers of cells present in the embryo. It is unclear why some cells differentiate into one cell type while others do not because all of the cells seem to receive the same BMP4 signal.

While others develop into other cell types.

Ivana Vasic, PhD, the study's principal author and a former Gladstone postdoctoral researcher, notes that "this has been a bit of a puzzle in the field." The same message from BMP4 is either being interpreted differently by each of these cells, or they aren't actually receiving the same cue.

Vasic discovered that the iPS cells grouped together in the dish have proteins that are the building blocks for tight junctions, which are barriers between cells while constructing a lab model of gastrulation. She did, however, observe that the tight connections don't usually actually come together.

The tight connections could build consistently when the cells were grown in a less congested environment, according to Yamanaka, Vasic, and their team. Only cells at the edge of the cluster received enough BMP4 to activate molecular pathways that would push them to differentiate into various layer cell types, which was their "aha" moment when they introduced BMP4 to the unconfined cells.

According to Vasic, tight connections between neighbouring cells appear to render them resistant to BMP4 signals. The edge cells, however, are receiving the strongest instructions from BMP4 since they lack a partner to make a tight connection with on their outer side.

TJP1, a protein essential for creating tight junctions in iPS cells, was suppressed using CRISPR genome editing technology by the researchers to demonstrate the significance of tight junctions in gastrulation. They found that all cells, not only the edge cells, were stimulated when BMP4 was given to cells missing the TJP1 protein.

Yamanaka, a professor of anatomy at UC San Francisco and director emeritus and professor at the Centre for iPS Cell Research and Application (CiRA), Kyoto University in Japan, claims, "We showed that removing the tight junctions made all the cells respond to BMP4." This shows that, more fundamentally, the structure of cells is exceedingly complex and that tight junctions prevent cells from reacting to signals in gastrulation models.

According to Todd McDevitt, PhD, a former senior scientist at Gladstone and a senior author of the paper, "Generally speaking, this study demonstrates how perturbations to innate properties of iPS cells can modulate their sensitivity to extracellular cues and alter their cell fate trajectory." The ability of iPS cells to produce more uniform populations of differentiated cells for therapeutic purposes may finally be unlocked thanks to this approach.

Egg Cell Production in a Dish

The group then looked more closely at which cells had been triggered by BMP4 after disrupting tight junction development.

We made a rather surprising discovery, adds Vasic: "It turns out that we could create a special kind of cell called a primordial germ cell-like cell." These lab-made stem cells mirror the sperm and egg cell progenitors found in humans.

The production of primordial germ cell-like cells from iPS cells has proven challenging, despite the fact that scientists have long searched for a dependable method. Vasic and her coworkers had identified that inhibiting TJP1 could serve as the foundation of an innovative technique to effectively manufacture these special cells.

In order to use this technology in a potential new strategy for treating women's infertility, Vasic has now formed a new business, Vitra Labs.

To produce eggs that individuals could utilise for in vitro fertilisation, researchers are essentially attempting to mimic the biological process of egg formation.