Visualizing the inside of cells at previously impossible resolutions provides vivid insight into how they work

All life consists of cells that are several sizes smaller than a grain of salt. Their apparently simple structures mask the intricate and complex molecular activity that enables them to perform the functions that sustain life. Scientists are beginning to be able to visualize this activity to a level of detail they have not been able to before.

Biological structures can be visualized by either starting at the level of the whole organism and working down, or starting at the level of individual atoms and working up. However, there has been a resolution gap between a cell’s smallest structures, such as the cytoskeleton that supports the cell’s shape, and its largest structures, such as ribosomes that make proteins in the cells.

By analogy with Google Maps, while researchers have been able to see entire cities and individual houses, they did not have the tools to see how the houses came together to make up neighborhoods. Seeing these details at the neighborhood level is crucial to understanding how individual components work together in the environment of a cell.

New tools are constantly bridging this gap. And ongoing development of one particular technique, cryo-electron tomography, or cryo-ET, has the potential to deepen how scientists study and understand how cells function in health and disease.

As former editor-in-chief of Science magazine and as a researcher who has studied difficult to visualize large protein structures for decades, I have witnessed astonishing advances in the development of tools that can determine biological structures in detail. Just as it becomes easier to understand how complicated systems work when you know what they look like, understanding how biological structures fit together in a cell is the key to understanding how organisms work.

A brief history of microscopy

In the 17th century, light microscopy first revealed the existence of cells. In the 20th century, electron microscopy offered even greater detail, revealing the elaborate structures of cells, including organelles such as the endoplasmic reticulum, a complex network of membranes that play key roles in protein synthesis and transport.

From the 1940s to the 1960s, biochemists worked to separate cells into their molecular components and learn to determine the 3D structures of proteins and other macromolecules at or near atomic resolution. This was first done using X-ray crystallography to visualize the structure of myoglobin, a protein that supplies oxygen to muscles.

Over the past decade, techniques based on nuclear magnetic resonance, which produces images based on how atoms interact in a magnetic field, and cryo-electron microscopy have rapidly increased the number and complexity of the structures scientists can visualize.

What are Cryo-EM and Cryo-ET?

Cryo-electron microscopy, or cryo-EM, uses a camera to detect how an electron beam is deflected as the electrons pass through a sample to visualize structures at the molecular level. Samples are quickly frozen to protect them from radiation damage. Detailed models of the structure of interest are created by taking multiple images of individual molecules and slicing them into a 3D structure.

Cryo-ET shares similar components with cryo-EM, but uses different methods. Because most cells are too thick to image clearly, an area of ‚Äč‚Äčinterest within a cell is first thinned out using an ion beam. The sample is then tilted to take multiple images of it at different angles, analogous to a CT scan of a body part (although in this case the imaging system itself is tilted, rather than the patient). These images are then combined by a computer to produce a 3D image of part of the cell.

The resolution of this image is high enough for researchers (or computer programs) to identify the individual components of different structures in a cell. Researchers have used this approach, for example, to show how proteins move and break down inside an algal cell.

Many of the steps researchers once had to do manually to determine the structures of cells are being automated, allowing researchers to identify new structures at much higher speeds. For example, combining cryo-EM with artificial intelligence programs such as AlphaFold can facilitate image interpretation by predicting protein structures that have not yet been characterized.

Understand cell structure and function

As imaging methods and workflows improve, researchers will be able to tackle some key questions in cell biology with different strategies.

The first step is to decide which cells and which regions within those cells will be studied. Another visualization technique called correlated light and electron microscopy, or CLEM, uses fluorescent tags to locate areas where interesting processes are taking place in living cells.

Comparing the genetic difference between cells can provide further insight. Scientists can look at cells that are unable to perform certain functions and see how this is reflected in their structure. This approach can also help researchers study how cells interact with each other.

Cryo-ET is likely to remain a specialized tool for some time. But further technological developments and increasing availability will allow the scientific community to investigate the relationship between cellular structure and function at previously inaccessible levels of detail. I expect to see new theories of how we understand cells, moving from disorganized bags of molecules to intricately organized and dynamic systems.

This article has been republished from The conversation under a Creative Commons license. Read the original article.

Image credit: Nanography, CC BY-SA

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