Listen to Your Gut

When considering the function of the ear, the gut doesn’t automatically come to mind as a research model. After all, what could two such different tissues have in common? Unexpectedly, the answer lies in some of the most vital functional components of each organ – namely, the gut microvilli and the stereocilia of the ear. These two structures behave in very different ways; while the job of the microvilli is to add surface area to the intestinal epithelium and function in nutrient absorption, stereocilia are responsible for hearing and balance. But what they share is the protein complex that organizes them. Although ear and gut use different complexes, each protein subunit in one has an exact counterpart in the other – one of which even derives from the same gene in both tissues. And as if to mirror this duplication of complexes, two groups published similar findings at the same time in the journal Developmental Cell (1,2). We spoke to Scott Crawley from Vanderbilt University and Jianchao Li from Hong Kong University of Science and Technology to find out more about the organizational proteins of both gut and ear.

Why study the gut microvilli?

Jianchao Li: We were initially interested in inherited deafness, especially in Usher syndrome, a rare genetic disorder caused by genetic mutations that lead to impairments in both vision and hearing. Previous studies by scientists in the gut field suggested that there might be some kind of linkage between gut and ear, given that the microscopic structures are quite similar — the stereocilia in the ear hair cells and the microvilli in the gut epithelia. The more we looked into these structures, the more we realized that this was true.

Scott Crawley: The ultrastructure of gut microvilli has long been a fascinating research subject. When scientists first started looking at them, they named them the “brush border” – a very apt term, because they collectively resemble a scrub brush. Although there are countless interesting aspects of the brush border, I think the thing that drew me to study this aspect of biology is the amazing order exhibited by the system. How exactly an intestinal epithelial cell creates upwards of 1,000 finger-like membrane protrusions on its surface (the “bristles” of the brush), all of which exhibit uniform length, is a remarkable feat of biological engineering. We wanted to know what proteins contribute to this process.

Can you describe the protein complex that organizes the microvilli?

JL: Gut microvilli are the finger-like protrusions that line the intestines. The backbones of these protrusions are made of actin proteins. What our research has revealed is how the key protein components interact with each other to assemble the microvilli – and, along with that, the fact that these protein components are strikingly similar to those essential for the assembly of stereocilia (hair-like mechanosensory organelles) in the inner ear.

SC: We found that, during the formation of the brush border, microvilli are physically connected to one another through small, thread-like links at their distal tips. We discovered that these thread-like links are composed of a pair of cadherin adhesion molecules – a class of proteins that essentially act like “molecular Velcro.” Cadherins glue opposing membrane surfaces together, and that’s exactly what they do in the brush border: they glue neighboring microvilli together. Specifically, we found that the thread-like links are made of protocadherin-24 and mucin-like protocadherin. Both cadherins localize to the distal tips of microvilli through interactions with a myosin motor protein (myosin-7B) and two scaffolding molecules (harmonin-a and ANKS4B). We termed this complex the intermicrovillar adhesion complex (IMAC).

Interestingly, the IMAC is very similar to an adhesion complex that connects the stereocilia of the inner ear. Each protein component of the IMAC has a functional counterpart in the inner ear adhesion complex (known as the Usher complex, because genetic defects in its components cause Type I Usher syndrome, the most common form of deaf-blindness). All of the proteins in these two complexes come from different genes, with the exception of harmonin, which uses different splice variants of the same gene. Interestingly, Usher syndrome patients with defects in the harmonin gene also suffer from intestinal disease. So, although the gut and the inner ear are two completely different organs (with completely different functions!), their epithelial cells use a common adhesion mechanism to remodel their apical surfaces just after the cells are born.

What are the implications of this discovery?

JL: This information provides a starting point for those who are studying gut microvilli to look for functional alterations that may lead to disease. Considering systems with similar structures (like the gut, the ear and the kidney), it’s possible that changes to a single gene might cause problems in several of these systems.

SC: I think studying Usher syndrome has been particularly challenging for researchers because there’s no cell culture model system for the inner ear epithelium. When researchers want to study whether and how a particular mutation causes Usher syndrome, they typically have to make a genetically modified mouse to see how the mutation affects the formation of stereocilia – an expensive and time-consuming process. In contrast, we have excellent cell culture model options for the intestine.

Now that we know the IMAC and the Usher complexes are so highly homologous, we can use the IMAC as a substitute to investigate how Usher syndrome mutations cause disease. We can compare the two complexes, make the equivalent Usher syndrome mutation in the IMAC component, and watch how it affects brush border formation. I think this will give us an unprecedented opportunity to understand the molecular pathology of Usher syndrome, and it may just bring us one step closer to finding effective treatments.

How will this affect the diagnosis and treatment of intestinal disorders?

JL: In contrast to the corresponding genes in Usher syndrome, very little human genomic data so far has linked the IMAC genes to intestinal disorders. Our work on elucidating the protein-based organization of the brush border may help predict which mutations are likely to alter function, bringing genetic diagnoses into clinicians’ sights. Digestive disorders are likely to be polygenic, so one genetic mutation alone may not lead directly to disorders – but changes to even a single gene can make the gut system more vulnerable.

SC: We know that Usher syndrome patients with defects in the harmonin gene can suffer from intestinal disease along with the usual deaf-blindness. This leads us to believe that genetic defects in the other IMAC components might cause similar intestinal disease. Knowing that IMAC genes could potentially cause disease should allow us to rapidly screen patients with undiagnosed intestinal illness for potential mutations in their IMAC components. That would give us a starting point to understanding why these patients are sick – and hopefully, in the future, lead to solutions.

What are the next steps for your research?

JL: Another puzzle left to solve is why a single mutation can cause a disease as debilitating as Usher syndrome, while corresponding mutations in gut microvilli don’t lead to noticeable symptoms. If the gut system is older in evolutionary time and the digestive function more crucial for life, it could be that the gut system has developed more robust backup systems, or that the symptoms are associated with additional genetic factors we have yet to discover.

SC: I’m interested in trying to understand how the IMAC and Usher complexes assemble in epithelial cells. Our latest research suggests that assembly of these complexes inside the cell is a highly regulated process. By obtaining a better understanding of how they assemble, I think we will garner a lot of insight into how disease-causing mutations disrupt the normal function of these adhesion complexes.