Weeping lacrimal glands from the laboratory

Lacrimal organoid

Cross-section through the lacrimal organoid of a mouse. (Image: Yorick Post / Hubrecht Institute)

In order to keep our eyes moist and to protect them, the lacrimal glands produce a watery secretion, the tear fluid. Although various diseases are associated with impaired tear production, little is known about the exact function of the tear glands. Researchers have now grown mini tear glands from stem cells. As a model system, these so-called organoids can help to find new treatment options for patients with dry eyes in the future. In mice, the researchers have also shown that transplants of the lacrimal glands from the laboratory are basically possible.

The tear glands provide our eyes with a protective layer of fluid. With every blink, the tear fluid is distributed on the cornea and conjunctiva and prevents the eye from drying out. In addition, the tears carry away foreign bodies and have an antibacterial effect. Various diseases, including Sjogren’s syndrome, a poorly understood autoimmune disorder, are associated with a disorder of tear production or secretion. “This can have serious consequences, such as dryness of the eye and even ulcers on the cornea,” says Rachel Kalmann from the University Medical Center Utrecht in the Netherlands.

Mini tear glands as a model system

So that such diseases can be better researched in the future, she and a research team led by Marie Bannier-Hélaouët from the Hubrecht Institute in Utrecht have developed a model system: In the laboratory, the researchers cultivated miniature human tear glands from adult stem cells. “At least five percent of the adult population suffer from dry eyes, which is mostly related to a defect in tear production by the lacrimal gland,” says Bannier-Hélaouët’s colleague Yorick Post. “But treatment options are limited because there was no complete understanding of biology and no reliable, long-term in vitro model for studying the lacrimal gland.”

The model that the researchers have now developed corresponds in structure and function to the tissue in human lacrimal glands. To do this, the researchers cultivated the human stem cells in a special nutrient medium with growth factors that ensure the correct differentiation of the cells. “The challenge was to make the organoids cry, as this is a hallmark of the lacrimal gland,” says Bannier-Hélaouët. “We had to modify the cocktail of factors in which the organoids are grown so that they become the mature cells that we have in our lacrimal glands and that are able to cry.”

Crying in the laboratory bowl

When we burst into tears – be it because of pain or strong emotions – the lacrimal glands react to certain messenger substances such as the stress hormone norepinephrine. In the laboratory, too, the researchers were able to make the tear gland organoids cry with norepinephrine. Since the organoids have no tear ducts, they pour the tears inside and swell up like a balloon. From the size of the organoid, the researchers can tell whether the crying worked. “Further experiments showed that different cells in the lacrimal gland produce different components of tears. And these cells react differently to tear-inducing stimuli, ”says Post.

For further research, the team also grew lacrimal organoids from mouse stem cells. They used this to investigate the role played by the Pax6 gene, which is involved in eye development. Using Crispr / Cas9 technology, they switched off the gene in some of the mouse organoids. Without the gene, the lacrimal glands could not mature properly and important receptors for tear-inducing stimuli were missing. In addition, genes that are responsible for certain components of the tear fluid were downregulated. Since a disorder of Pax6 was also observed in patients with Sjörgren’s syndrome, the model system could provide insights into causes and treatment options.

Transplants conceivable in the future

The researchers also see potential for regenerative medicine: To test the basic feasibility, they transplanted human organoid cells into the lacrimal glands of mice. In fact, the transplanted cells began to organize themselves in the new environment and form duct-like structures. As in the laboratory, the organoids were also able to produce tears after the transplant.

“There is still a long way to go before these mini-organoids can be used for regenerative therapies,” says Bannier-Hélaouët’s colleague Hans Clevers. One restriction so far, for example, is that the cultured organoids predominantly consist of only one of two important cell types of the lacrimal glands. In the future, the researchers want to try to breed the other cell type too, so that one day they will be able to produce complete lacrimal glands. However, the current model can already be a valuable aid for research. It can help to further investigate the functioning of the lacrimal glands, to track down disease mechanisms and to test new drugs. “Organoids represent a worthwhile model for better understanding the biology of the lacrimal gland and for studying the pathologies of lacrimal flow,” the researchers say.

Source: Marie Bannier-Hélaouët (Hubrecht Institute, Utrecht, Netherlands), Cell Stem Cell, doi: 1016 / j.stem.2021.02.024

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