The mechanisms of neurodegeneration remain poorly understood, with investigations hampered by the combination of complex genetic and environmental factors implicated in neurodegenerative diseases. Therefore, monogenic disorders such as the neuronal ceroid lipofuscinoses (NCLs) can provide important clues to the molecular events underlying neurodegeneration. Juvenile NCL (JNCL) has the latest onset of all the childhood NCLs, and is characterised by blindness, seizures, and mental and physical decline, leading to death in early adulthood. The NCLs are usually autosomal recessive, and mutations in the CLN3 gene cause JNCL. No disease-modifying therapies are available for JNCL, and since CLN3 is a membrane protein, existing therapeutic strategies for soluble proteins are not applicable.
Here, the fission yeast Schizosaccharomyces pombe is used as a model organism for studying the currently undetermined function of CLN3. Fission yeast cells are rod shaped and grow in a strict bipolar manner; therefore, abnormal cell morphology can indicate underlying defects in molecular pathways. Additionally, each fission yeast cell has many vacuoles, which are the yeast equivalents of the mammalian lysosome, thus making them particularly suitable for studying lysosomal storage diseases such as the NCLs.
The orthologue of CLN3 in fission yeast is btn1. All disease-causing missense mutations of CLN3 were modelled in Btn1p, and their ability to rescue four defects of fission yeast cells lacking btn1 was assessed. Fission yeast cells deleted for btn1 have multiple defects, including larger vacuoles, a cytokinesis delay under normal growth conditions, cell curving and a failure to initiate polarised growth during growth at higher temperatures. None of the mutations associated with disease rescued all of the phenotypes assayed here, although many had a significant effect on one or more phenotype. Mutations of residues on the lumenal face of the protein almost completely ablated protein function. One of the defects, cell curving, was rescued only by the mutant corresponding to the disease-causing missense mutation p.Glu295Lys. Patients with this mutation have the mildest disease phenotype known for CLN3, with onset of blindness at the normal age but very delayed onset of other symptoms.
Implications and future directions
This study further establishes the use of fission yeast to model JNCL. It also demonstrates this organism’s potential in identifying disease-modifying factors for JNCL and other neurodegenerative diseases. In particular, the correlation between mild disease progression and the rescue of the cell curving phenotype suggests a potential therapeutic target to delay onset of later symptoms. Further studies using fission yeast can identify the molecular defects underlying the mutant phenotypes, and thus highlight targets for novel therapeutic agents.