MERFISH-based pooled genetic screening reveals cell non-autonomous phenotypes in intact tissue

Cells do not live in isolation. Even within a single organ, they exist in a crowded, structured neighborhood where their position, shape, and constant communication with neighbors shape their behavior. For decades, scientists have been able to study how changing a single gene affects a cell, but doing so in a way that preserves the cell's original location and its relationship to the surrounding tissue has been a major hurdle. Traditional methods require taking cells out of the body, breaking them apart, and reading their genetic code in a test tube. This process destroys the map of where the cell was and who its neighbors were. To truly understand how a genetic change ripples through a living tissue, researchers needed a way to look inside the intact organ, identify which cells had been altered, and see how those changes affected the cells right next to them, all without losing the spatial context.

A team of researchers at Harvard University and the Howard Hughes Medical Institute has developed a new imaging technique that solves this problem. They created a method to perform large-scale genetic screens directly inside intact mouse liver tissue. By using a high-resolution imaging system called MERFISH, they were able to track the genetic identity of individual cells, measure their internal molecular state, and map their exact location relative to neighbors. The key innovation was a way to read the genetic "barcodes" that identify which cells had been altered, without using complex chemical steps to amplify the signal. This simplification allowed them to detect far more genetic markers and cellular messages than previous methods, enabling them to profile 1.5 million cells in a single experiment. The result is a detailed, three-dimensional map showing not just how a modified cell changes itself, but how it reshapes the entire local environment around it.

The researchers began by refining their tools to work without the need for enzymatic amplification, a chemical process often used to boost weak signals in imaging. In their previous work, they had to use enzymes to multiply the genetic barcodes attached to the cells they wanted to study, but this added steps that sometimes reduced the clarity of the data. They discovered that by designing shorter, more compact genetic barcodes and relying on the natural abundance of these molecules inside the cell, they could detect them directly through simple hybridization. This direct approach proved to be significantly more efficient. It recovered nearly ten times as many genetic barcodes and two and a half times as many cellular messages per cell compared to their older method. This improvement meant they could confidently assign a genetic identity to far more cells and see subtle changes in gene activity that were previously invisible.

With this improved system in hand, the team turned to the mouse liver, a complex organ made up of many different cell types working together. They introduced a library of genetic instructions into the livers of young mice, targeting about 200 specific genes in liver cells known as hepatocytes. Each liver cell received a unique genetic barcode that acted as a label, allowing the researchers to know exactly which gene had been altered in that specific cell. After the mice grew and the genetic changes took effect, the researchers sliced the liver tissue and used their imaging system to scan the entire organ. They identified the genetic identity of each cell, measured the activity of hundreds of genes within it, and recorded its precise position in the tissue. In total, they analyzed more than 1.5 million cells, creating a massive dataset that linked genetic changes to cellular states and spatial locations.

The study revealed two distinct types of effects. First, the researchers confirmed that altering a gene in a liver cell caused that specific cell to change its own behavior, a phenomenon known as a cell-autonomous response. For example, when they disrupted a gene involved in protein folding, the affected liver cells showed a clear stress response, turning on genes to deal with the damage while turning off genes related to their normal liver functions. These changes were confined to the altered cells, with neighboring cells remaining unaffected. This confirmed that the new method could accurately capture the direct consequences of genetic perturbations within the complex environment of a living organ.

More surprisingly, the researchers found that changing a single gene in a liver cell could also trigger changes in the cells immediately surrounding it, a phenomenon called a cell non-autonomous response. The genetic alteration in one cell sent signals that reshaped the local neighborhood. In one striking example, when they disrupted a gene called Vhl in liver cells, those cells began to act as if they were in a low-oxygen environment. This change triggered a cascade of effects in the nearby cells. The blood vessel cells and support cells right next to the altered liver cells started to express genes associated with building new blood vessels and activating their own growth programs. These changes were strongest in the cells closest to the altered liver cell and faded as the distance increased, showing a clear gradient of influence radiating from the source.

In another discovery, the team found that disrupting a gene involved in a cellular pump called V-ATPase led to a localized accumulation of a specific type of immune cell. When liver cells had impaired V-ATPase function, a type of immune cell resembling a plasmacytoid dendritic cell gathered in high numbers right next to them. This accumulation was not seen with other genetic changes, suggesting a specific link between the failure of this cellular pump and the recruitment of these immune cells. The researchers observed that these immune cells were concentrated within a very short distance of the altered liver cells, forming a distinct cluster that was not present in the surrounding healthy tissue.

The power of this new approach lies in its ability to connect the dots between a genetic change, the cell's own reaction, and the reaction of its neighbors, all while preserving the physical layout of the tissue. By mapping these interactions in a living organ, the researchers showed that genetic perturbations do not just happen in a vacuum; they ripple outward, altering the composition and behavior of the local cellular community. The study provides a simplified and scalable way to investigate how genetically altered cells reshape their microenvironments, offering a new window into the complex social life of cells within an intact tissue. This method opens the door to understanding how diseases might spread or be contained within an organ, and how the physical arrangement of cells influences their function, without the need to break the tissue apart.