Depletion of BBSome Subunits Alters Receptor Endocytosis and Promotes EMT via TGF-β Signalling

Inside nearly every cell of the human body lies a tiny, hair-like antenna called a primary cilium. Though microscopic, this structure acts as a crucial sensory hub, detecting signals from the outside world and translating them into instructions that tell the cell how to grow, move, or communicate. When the machinery that builds or maintains these antennas malfunctions, the result is a group of disorders known as ciliopathies. One of the most complex of these is Bardet-Biedl syndrome, a genetic condition that affects multiple organs, causing obesity, kidney problems, and, most notably, a progressive loss of vision. The blindness stems from the degeneration of the light-sensitive cells at the back of the eye, a process driven by the failure of these cellular antennas to manage the flow of information correctly.

Scientists have long suspected that the root of this blindness involves a specific protein complex called the BBSome, which functions as a sort of traffic controller for the cilium. Its job is to help move molecules in and out of the antenna, ensuring that receptors—the cell's surface sensors—arrive where they are needed and are removed when their job is done. If this traffic system breaks down, receptors can pile up on the cell surface, sending constant, confusing signals that eventually damage the tissue. A new study by researchers at the Universidade de Vigo and their international collaborators has peeled back the layers of this mystery, revealing exactly how the loss of two key BBSome proteins disrupts this delicate balance in retinal cells and triggers a chain reaction that leads to cell death and tissue breakdown.

The researchers focused on two specific genes, BBS1 and BBS4, which are frequently mutated in patients with Bardet-Biedl syndrome. To understand what happens when these genes are missing, they created laboratory models using human retinal pigment epithelial cells, a type of cell that supports the light-detecting neurons in the eye. They used a precise gene-editing tool to switch off either the BBS1 or the BBS4 gene, creating two distinct versions of cells that lacked these critical proteins. Their first observation confirmed what was known from animal models: without these proteins, the cells still grew their cilia, but the antennas were significantly shorter. In the cells missing BBS4, the antennas were not only short but also appeared much less frequently, suggesting that this protein plays a broader role in the initial assembly of the structure.

The team then turned their attention to the surface of these cells to see what receptors were behaving strangely. They performed a detailed inventory of the proteins sitting on the cell membrane and found a striking accumulation of a specific sensor called TGFBR1. This receptor is part of a signaling pathway known as TGF-β, which normally helps cells communicate and maintain their shape. In healthy cells, this receptor is constantly being pulled inside the cell, recycled, or broken down to keep the signal in check. In the cells lacking BBS1 or BBS4, however, the receptor got stuck on the surface, unable to be properly internalized. This buildup meant the cells were receiving a continuous, unregulated signal, which the researchers suspected was pushing the cells toward a dangerous state called epithelial-to-mesenchymal transition. This is a process where organized, stationary cells lose their structure and begin to behave like wandering, disorganized cells, a shift that is often a precursor to tissue degeneration.

To understand how the BBSome was failing, the scientists investigated whether these proteins physically grabbed onto the cellular machinery responsible for moving receptors inside the cell. They tested for direct connections between the BBSome and the main transport vesicles, the small bubbles that carry cargo within the cell. Using both biochemical experiments and advanced computer modeling, they found no evidence of a stable, physical handshake between the BBSome and these transport components. This was a significant discovery because it ruled out the idea that the BBSome acts as a permanent part of the transport truck. Instead, the data suggested that the BBSome influences traffic indirectly, perhaps by organizing the environment or by transiently interacting with other partners, rather than by being a fixed part of the delivery system itself.

The researchers then watched the journey of the stuck TGFBR1 receptor in real time to see where it went wrong. In healthy cells, the receptor is quickly pulled inside, sorted into a recycling bin to be sent back to the surface, or sent to a degradation bin to be destroyed. In the cells missing BBS1, the receptor lingered in the early sorting area and then got stuck in the recycling loop, returning to the surface repeatedly without ever being broken down. This created a cycle of excessive signaling. In contrast, the cells missing BBS4 showed a different problem: the receptor was not sent to the recycling bin at all, yet the signaling pathway still became hyperactive. This revealed that the two missing proteins cause distinct traffic jams that lead to the same dangerous outcome: an overactive signal that tells the cell to change its identity.

This overactive signaling had a direct impact on the cell's behavior. The researchers observed that the cells missing BBS1, in particular, began to change their shape and started moving more freely, a hallmark of the epithelial-to-mesenchymal transition. They also found that these cells had a reduced ability to close wounds in a laboratory setting, mirroring the tissue fragility seen in patients. Interestingly, while both types of defective cells showed signs of this transition, the cells missing BBS1 exhibited a much more dramatic shift in their identity and movement capabilities compared to those missing BBS4. This difference suggests that while the loss of either protein disrupts the system, the specific nature of the disruption determines how severely the cell's behavior is altered.

The study also uncovered a surprising link to the cell's internal cleanup system, known as autophagy. In healthy cells, this system acts like a recycling plant, breaking down damaged parts. The researchers found that the cells missing BBS1 had a very low level of this cleanup activity at rest, which spiked when they were stimulated. Conversely, the cells missing BBS4 had a high level of cleanup activity that dropped when stimulated. This opposing behavior suggests that the two proteins regulate the cell's internal maintenance in completely different ways, yet both failures contribute to the buildup of signaling proteins that drive the cell toward degeneration.

Ultimately, this work provides a clearer picture of why the retina fails in Bardet-Biedl syndrome. It shows that the loss of BBSome proteins does not simply stop the cell from building cilia; it fundamentally breaks the rules of how the cell manages its surface receptors. By preventing the proper removal and recycling of the TGF-β receptor, the BBSome dysfunction forces the cell into a state of constant, high-alert signaling. This signal pushes the retinal cells to lose their organized structure and begin to migrate, a process that undermines the integrity of the eye's tissue. The findings suggest that the path to blindness in these patients is not just a structural failure but a signaling crisis, where the cell's inability to clear its sensors leads to a cascade of changes that destroy the tissue. This insight opens the door to new therapeutic strategies, suggesting that targeting the signaling pathways themselves, rather than just trying to fix the cilia, might offer a way to slow down the degeneration and preserve vision in patients with this condition.