Researchers at the University of Michigan have discovered that a rare disease called cystinosis shares the same molecular mechanism as found in a form of cystic fibrosis. This novel finding, reported in the Journal of Clinical Investigation sheds light on how mutated proteins are cleared by the mechanism, impacting the buildup of cystine crystals and leading to severe effects on organs like the kidneys and eyes.
The root problem originates from a malfunction in the lysosome, a cellular organelle often referred to as the cell’s recycling center. Lysosomes takes in cellular waste, breaks it down into reusable components, and transports these materials back into the cell. But when the protein that transports one of the recycled amino acids back into the cell mutates and fails, a cellular mechanism cleans up the faulty protein and allows amino acid, or cystine, to build up in the lysosome leading to cystinosis.
“If cystinosis not treated at an early age, some of the effects are irreversible and it could include impaired growth, kidney failure, and neurological problems,” said Varsha Venkatarangan, graduate student in the U-M Department of Molecular, Cellular and Developmental Biology (MCDB) and lead author of the study. “Typically, the symptoms of the disease are treated rather than the root problem. So we have been wondering what could be the possible cellular mechanism of this disease.”
To help unravel the mechanism at play, Venkatarangan worked with fibroblasts derived from patients with the disease and determined that the disease mechanism called endoplasmic-reticulum-associated degradation (ERAD)—the same mechanism behind other diseases such as a major form of cystic fibrosis—degrades a mutated version of the lysosome cystine transporter.
Using this information, the investigators were able to show that using a previously identified drug molecule helped stabilize the transporter protein. These findings could have implications for drug repurposing or as a targeted therapy for cystinosis.
“This drug molecule is a small chemical that can somehow assist the protein folding of the mutant and protect it from being degraded,” said MCDB professor Ming Li. “This chemical chaperone helps it achieve the right formation so that it can be functional.”
Researchers in Li’s lab collected nearly 40 mutations that caused cystinosis to study their protein stability. While doing this painstaking work, the researchers identified one mutation that had a very short half-life compared to the non-mutated form of the protein.
“We were interested about the fact that this disease mutant was degrading so rapidly, “said Venkatarangan. “What was the potential mechanism by which it was degrading, and the machinery involved.”
By conducting genetic and chemical tests to inhibit various protein degradation mechanisms, Venkatarangan determined that ERAD was the key player of the rare disease molecular mechanism. ERAD is a cellular pathway situated in the endoplasmic reticulum, a network responsible for transporting proteins within the cell. It targets misfolded proteins and marks them for degradation.
Crucially, chemical chaperones, already known to work against ERAD in cystic fibrosis, were shown to stabilize the protein linked to cystinosis. These chaperones aided the protein’s correct folding, preserving its functionality. Further testing, conducted in collaboration with University of California, San Diego (UCSD) researchers revealed a significant reduction in cystine concentration within lysosomes when the functional transporter was present.
As it turns out, the investigators found that the connection between the endoplasmic reticulum and the lysosome membrane proteins is essential. Lysosome membrane proteins travel from the endoplasmic reticulum to the Golgi apparatus, which packages them for delivery to the lysosome. Proper folding during this process is crucial to prevent premature degradation by quality control systems like ERAD. Mutant proteins were found to be prematurely degraded during the trafficking process, resulting in their disappearance.
The researchers noted that research in this area is still in its early stages and the findings were based on experiments that only used cultured patient cells. But the results are encouraging enough to progress this information to testing in animals and humans to build evidence as to whether the existing drug is effective.