New, Fundamental Biological Process Identified in Batten Disease

A new, fundamental biological process in neuronal ceroid lipofuscinosis 8 (CLN8) disease—a form of the rare lysosomal storage disorder, Batten disease—has been discovered by a team of investigators from Baylor College of Medicine.

“My lab is inspired by 2 complementary tasks: Understanding the functioning of the cell, and of the lysosome in particular, and identifying entry points that can be used to develop therapies for neurodegenerative diseases,” corresponding author Marco Sardiello, PhD, assistant professor of molecular and human genetics at Baylor, said in an interview with Rare Disease Report^®.

Although investigators knew CNL8 disease was associated with defects in the CLN8 protein, but since it is not located in the lysosome, (instead, it is located in the endoplasmic reticulum), they were puzzled as to why a lysosomal storage disorder could occur from mutations in a protein that is not in the lysosome.

“In this study, the 2 lines of research converged towards the understanding of an unexpected mechanism that the cell employs to generate lysosomes,” Sardiello said. “This mechanism offers a new entry point—namely, delivery of lysosomal enzymes—that could be leveraged for the development of therapies for a family of neurodegenerative diseases, the Neuronal Ceroid Lipofuscinoses, also called Batten disease.”

To solve the mystery, investigators focused their attention on proteins that would assist in the exit of lysosomal enzymes from the endoplasmic reticulum en route to the lysosomes.

“We narrowed it down to 4 candidates, and CLN8 was one of them,” said first author, Alberto di Ronza, PhD, who was a postdoctoral researcher in the Sardiello lab while he was working on this project, in a recent statement. “It was the only 1 that interacted with two-thirds of the lysosomal enzymes we tested.”

Subsequently, the team worked with a mouse model that recapitulated many of the characteristics of the disease observed in humans. Specifically, the team worked with mice carrying defective CLN8 molecules. Upon analysis, they found that fewer lysosomal enzymes were in lysosomes of the studied mice.

“We discovered that the exit from the endoplasmic reticulum of newly synthesized lysosomal enzymes requires the protein CLN8,” Sardiello said. “In a nutshell, CLN8 functions as a ‘cargo receptor’ that selectively recognizes and transfers newly synthesized lysosomal enzymes from the endoplasmic reticulum to the Golgi complex, where the enzymes are modified before their final delivery to the lysosome.”

The team also found that a specific piece of CLN8 catches lysosomal enzymes and facilitates their exit from the endoplasmic reticulum, thus acting like a hook. Molecular signals that help CLN8 move from the endoplasmic reticulum to its destination and back.

“We also characterized the subdomain of CLN8 that interacts with the enzymes (a sort of ‘hook’ for the enzymes) and identified other parts of the CLN8 that are important for its correct trafficking from the endoplasmic reticulum to the Golgi complex, and back,” Sardiello explained.

Furthermore, the team found that interaction with the COPII and COPI machineries via specific export and retrieval signals localized in the cytosolic carboxy terminus of CLN8 is required for endoplasmic reticulum (ER)-to-Golgi trafficking of CLN8.

“When CLN8 is defective or absent due to genetic mutations, this process is interrupted, and lysosomal enzymes do not traffic efficiently outside of the endoplasmic reticulum,” Sardiello continued. “Thus, the lysosome contains fewer degradative enzymes and, over time, it accumulates substances that are toxic to the cell.

Impaired lysosome biogenesis is caused by the depletion of soluble enzymes in the lysosome, which results from CLN8 deficiency. The second luminal loop of CLN8 is required for binding to lysosomal enzymes to take place, which is terminated by some disease-causing mutations within this region.

The discoveries from this study open the door for further research of new, potential therapeutic therapies for CLN8 disease.

“Clinically, the most relevant result is the identification of lysosomal enzyme depletion as the probable cause of lysosomal dysfunction (and disease symptoms) in patients who harbor mutations in the CLN8 gene,” Sardiello said. “It would be important to identify alternative ways to improve the trafficking and maturation of lysosomal enzymes when CLN8 is defective or absent. Also, it could be tested whether or not supplying the most relevant enzymes (an approach called "enzyme replacement therapy") would help overcome the symptoms.”

From the data, study authors concluded that impaired transport of lysosomal enzymes underlies Batten disease caused by mutations in CLN8 are underlying via the example of an ER receptor serving the biogenesis of an organelle.

“It remains to be established if CLN8 works with other partners, how it responds to the cellular degradative needs, and how it recognizes the enzymes that need to be transported to the lysosome,” Sardiello said, looking forward. “Also, it will be important to identify a way to pharmacologically enhance the trafficking of lysosomal enzymes when CLN8 is defective or absent.

The study “CLN8 is an endoplasmic reticulum cargo receptor that regulates lysosome biogenesis” was originally published in Nature Cell Biology.