Can damaged lung tissue regenerate? If so, which cells contribute to this regeneration? Can we isolate these regenerative cells and make them available to people with failing lungs?
These are all pointed questions, and associate professor of medicine at the University of San Francisco, Thiennu H. Vu, has published a recent paper in the Journal of Clinical Investigation that partially answers these questions.
The lung consists of a large quantity of tubes that conduct the air to the bloodstream. These tubes, the trachea, bronchi, bronchioles and terminal bronchioles, constitute the “conducting zone” of the lungs. They serve to deliver the air from outside our bodies to the bloodstream. The actual site of gas exchange or “respiratory zone” occurs at the “alveoli.” The terminal bronchioles end in an inflation that resembles a tiny sac. This sac, the alveolus, is very thin; one cell thick.
The cells that compose the alveolus are called alveolar epithelial cells (AECs). There are two types of AECs: flat “type I pneumocytes,” which typically are unable to divide and die off they are damaged by toxins, and “type II pneumocytes,” also known as “great alveolar cells” or “septal cells.”
Type II pneumocytes are usually found near the junctions between alveoli and the septae that separate the alveoli. It is thought that type II cells can divide and replace type I cells if the type I cells are destroyed. Type II cells also secrete large quantities of “surfactant” which is a chemical that keeps the alveolar surfaces from sticking together as they expand and contract. Are type II pneumocytes the primary healing cell in the lung? Vu’s group set out to address this question.
Vu and her co-workers had an indication that mice that lack a particular surface molecule called “integrin beta4” could not repair their lungs after lung damage. Integrins are cell adhesion molecules that help cells stick to the substratum. If we think of lung cells as having a head (the apical surface), and a foot (the basal surface), the foot part of the cell stands on a foundation and this foundation in lungs is something called the “basement membrane.”
Basement membranes are common to other types of cells, but basement membranes in the lung are rich in a protein called “laminin,” and the beta4 integrin, with help from another integrin subunit called alpha6, binds tightly to laminin and keeps the lung cells lock to the foundational basement membrane.
Since the cells that contained alpha6/beta4 on their surfaces seemed to the cells responsible for regenerating the lung after the lung was damaged, Vu and her colleagues stained lung tissue with antibodies against the beta4 integrin. What they discovered surprised them: The beta4-expressing cells did NOT overlap with those cells that made surfactant (type II cells). Furthermore, when they tried to correlate the presence of the beta4 integrin with the available lung cell types (type I AECs, ciliated bronchial cells, type II AECs, and Clara cells), they were not able to show that these beta4 cells corresponded to any known lung cell type.
Next, Vu and others cultured lung cells in artificial media and the beta4 integrin-containing cells grew extremely well, but the other lung cells failed to grow. The growing beta4-positive cells also proved to be a mixed population and had the beta4 integrin in common, but little else.
The next experiment utilized a culture system that Vu’s lab helped develop whereby extirpated lung tissues are used to grow mini-lung-like organs when transplanted into a “nude” mouse (a mouse whose immune system does not work properly). By using a nude mouse, the implanted cells will form the mini-lung without the mouse’s immune system destroying it. By using their mini-lung growing system, Vu and her colleagues were able to grow the mini-lungs effectively if they used whole, macerated lung tissue. The growing lungs went through the various embryonic stages of lung development, thus showing that this assay is an excellent way to study lung development. Next they tried to grow the mini-lungs by using only integrin beta4-containing lung cells plus some embryonic cells. The beta4-positive cells grew into mini-lungs and formed a wide variety of lung-specific cell types. The integrin beta4-containing cells also directed the embryonic epithelial cells to form proper sac-like alveoli. This assay definitively showed that the beta4-positive cells could form type I and type II pneumocytes.
Finally, they injured the lungs of mice with a drug called bleomycin and looked at the cells in the lungs to see if the quantity of beta4-containing cells increased. The results were crystal clear; the beta4-positive cells increased many fold. Then they asked if the type II pneumocytes were dividing in the damaged lungs. They used genetically engineered mice that would express green fluorescent protein in their type II pneumocytes. Then they injured the lungs of these mice and asked if the type II cells increased their numbers. The answer was a clear NO. The regeneration that created new type II pneumocytes created cells that did not express green fluorescent protein, which means that the new type II cells were made from cells that did not originally express green fluorescent protein. Therefore, the beta4-positive cells were the cells regenerating the lung and not the type II cells. The type II cells that were dividing had been derived from the beta4-positive cells.
Vu and her colleagues end this paper with this modest understatement: “Understanding the determinants of β4+ AEC population size and how these cells expand, self-organize, and differentiate along particular lineages should provide further insights into the processes of lung repair, the foundation for better therapeutics.”
I’ll say. If these cells can be found and characterized in humans, they could revolutionize lung treatments. That would be a revolutionary treatment.