Partial genetic suppression of a loss of function mutant of the Neuronal Ceroid Lipofuscinosis-associated protease TPP1 in Dictyostelium discoideum

Abstract

Abstract Neuronal Ceroid Lipofuscinosis (NCL) is the most common childhood-onset neurodegenerative disease. NCL is inevitably fatal, and there is no current treatment. Children with NCL show progressive decline in movement, vision, and mental abilities and accumulation of autofluorescent deposits in neurons and other cell types. Late-infantile NCL is caused by mutations in the lysosomal protease tripeptdyl 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 is not found in Drosophila, C. elegans, or S. cerevisiae. Here, we characterize ddTpp1, a TPP1 ortholog present in the social amoeba Dictyostelium discoideum. Lysates from cells lacking ddTpp1 show 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 lacking 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 may be to limit autophagy. Cells lacking ddTpp1 show strongly impaired development in the presence of the lysosome-perturbing drug chloroquine, and this phenotype can be suppressed by a secondary mutation in the gene stpA, which encodes a protein with some similarity to mammalian oxysterol-binding proteins (OSBPs). Together, these results suggest that targeting specific proteins may be a viable way to suppress the effects of loss of TPP1 function.