The lung of an air-breathing animal is a complex organ formed by a series of highly branched, hollow tubes terminating in tiny sacs where gas exchange with blood occurs. In mammals, most of these structures develop while the animal is still in utero. In a paper published May 20, 2024 in the journal Developmental Cell, Princeton scientists Danelle Devenport, Celeste Nelson, and colleagues detail the surprising ways in which a protein called Vangl contributes to the penultimate stage of lung development, called sacculation.

At the time that sacculation begins, the lung has already attained its highly branched structure but does not yet possess the tiny sacs from which oxygen transfers to the bloodstream once the animal is born and begins to breathe air. During sacculation, the ends of the airways fill with fluid, causing them to inflate like a balloon, and bringing about a dramatic increase in surface area that is required before the sacs can form.

Very little is known about the genes, proteins or processes involved in sacculation, but several recent studies have suggested that a network of proteins called the planar cell polarity complex (PCP) might be involved. PCP proteins are best known for the functions they exhibit in epithelia, the single-cell-deep sheet of cells that form the external boundaries of many tissues, including the gas-exchange sacs of the lung. In epithelial tissues such as those found in the early mouse embryo, the PCP protein Vangl2 is found at one side of the cell whereas a different PCP protein is found on the opposite side. This pattern tells epithelial cells where their “forward” and “rear” ends are, which helps them coordinate with their neighbors during the large-scale tissue movements that characterize many stages of embryonic development.

“We often think of lung morphogenesis as being driven by changes intrinsic to the lung epithelium itself. Similarly, PCP is best understood in epithelia so it seemed reasonable to expect that PCP would function in the epithelial compartment of the lung,” said Devenport, associate professor in the Princeton Department of Molecular Biology at Princeton.

Graduate student Sarah Paramore, working under the guidance of Devenport and fellow Princeton faculty member Nelson, set out to study which PCP proteins affect sacculation, and how they might do so. As a first step, Paramore and colleagues examined which members of the PCP complex are present in airway termini and their neighboring tissues when sacculation takes place. They were quite surprised to find that PCP proteins were largely absent from the epithelia of growing lung saccules. On the other hand, the PCP protein Vangl2 and other closely related Vangl proteins were abundantly present within a different tissue called lung mesenchyme that surrounds the growing saccules. Compared to epithelia, mesenchyme is a less specialized and less well-organized tissue that is not thought to form the type of highly structured tissues that epithelia do. Nonetheless, the team found that mice lacking Vangl2 specifically in lung mesenchyme had severe defects in sacculation.

These observations indicate that PCP proteins do not affect sacculation through the well-described mechanisms at play in epithelial tissues. Instead, Vangl proteins appear to be moonlighting on their own, doing something not in the epithelium but in the mesenchyme to help sacculation along. In agreement with this idea, subsequent experiments indicated that in lung mesenchyme Vangl2 interacts with an entirely different network of proteins, separate from its usual partners in the PCP network, to assist with sacculation.

How might Vangl proteins in the mesenchyme help guide the formation of lung saccules? Carolina Trenado-Yuste, a postdoctoral fellow in the Nelson lab, built a computational model which indicated that as saccules expand in response to pressure, the neighboring mesenchyme accommodates that expansion by behaving like a fluid. Examination of lung mesenchyme showed that neighboring cells disperse throughout the tissue over time, tending to make the mesenchyme more fluid, just as the model predicted. Observation of mesenchyme cells lacking Vangl2 suggested the protein may help this fluidification by driving the cells to adopt a slippery, spindly shape. Devenport and colleagues are already planning additional studies that should help us better understand how this pathway works.

“It is a big surprise that Vangl acts in the mesenchyme in the absence of other key PCP components,” said Devenport. “These proteins usually function in complexes with each other but Vangl is doing something different in this tissue that is essential for preparing the lung for that first breath after birth.”

Citation: Sarah V. Paramore, Carolina Trenado-Yuste, Rishabh Sharan, Celeste M. Nelson and Danelle Devenport. Vangl-dependent mesenchymal thinning shapes the distal lung during murine sacculation. Developmental Cell. 2024. DOI: 10.1016/j.devcel.2024.03.010

Funding: Work described in this story was supported by the National Institutes of Health (HL110335, HL118532, HL120142, HL164861, HD099030, HD111539, AR066070, AR068320); the Genetics and Molecular Biology Training Grant of the Molecular Biology Department at Princeton University (T32 GM007388); the Camille & Henry Dreyfus Foundation, and a Faculty Scholars Award from the Howard Hughes Medical Institute; Ruth L. Kirschstein (F31) Fellowship; NJCCR; and the NSF Graduate Research Fellowship Program. 

Funders: National Institutes of Health, Princeton Molecular Biology, Camille & Henry Dreyfus Foundation, Howard Hughes Medical Institute, Ruth L. Kirschstein Fellowship, NJCCR, National Science Foundation

Grants: HL110335, HL118532, HL120142, HL164861, HD099030, HD111539, AR066070, AR068320, T32 GM007388