Partial genetic suppression of a loss-of-function mutant of the neuronal ceroid lipofuscinosis-associated protease TPP1 in Dictyostelium discoideum

摘要

Neuronal ceroid lipofuscinosis (NCL) is the most common childhood-onset neurodegenerative disease. NCL is inevitably fatal, and there is currently no treatment available. Children with NCL show a progressive decline in movement, vision and mental abilities, and an accumulation of autofluorescent deposits in neurons and other cell types. Late-infantile NCL is caused by mutations in the lysosomal protease tripeptidyl peptidase 1 ( TPP1 ). TPP1 cleaves tripeptides from the N-terminus of proteins in vitro, but little is known about the physiological function of TPP1. TPP1 shows wide conservation in vertebrates but it is not found in Drosophila , Caenorhabditis elegans or Saccharomyces cerevisiae . Here, we characterize ddTpp1, a TPP1 ortholog present in the social amoeba Dictyostelium discoideum . Lysates from cells lacking ddTpp1 show a reduced but not abolished ability to cleave a TPP1 substrate, suggesting that other Dictyostelium enzymes can perform this cleavage. ddTpp1 and human TPP1 localize to the lysosome in Dictyostelium , indicating conserved function and trafficking. Cells that lack ddTpp1 show precocious multicellular development and a reduced ability to form spores during development. When cultured in autophagy-stimulating conditions, cells lacking ddTpp1 rapidly decrease in size and are less viable than wild-type cells, suggesting that one function of ddTpp1 could be to limit autophagy. Cells that lack ddTpp1 exhibit strongly impaired development in the presence of the lysosome-perturbing drug chloroquine, and this phenotype can be suppressed through a secondary mutation in the gene that we name suppressor of tpp1 ? A ( stpA ), which encodes a protein with some similarity to mammalian oxysterol-binding proteins (OSBPs). Taken together, these results suggest that targeting specific proteins could be a viable way to suppress the effects of loss of TPP1 function.