dictyNews

Electronic Edition

Volume 44, number 20

July 20, 2018



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Abstracts

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Mitochondrial HTRA2 Plays a Positive, Protective Role in Dictyostelium 

discoideum but Is Cytotoxic When Overexpressed



Suwei Chen 1,2, Oana Sanislav 1, Sarah J. Annesley 1 and 

Paul R. Fisher 1,*



1 Discipline of Microbiology, Department of Physiology Anatomy and 

Microbiology, La Trobe University, VIC 3086, Australia

2 School of Modern Agriculture and Biological Science and 

Technology, Ankang University, Shaanxi 725000, China

* Author to whom correspondence should be addressed.





Genes 2018, 9(7), 355   https://doi.org/10.3390/genes9070355

(This article belongs to the Special Issue Mitochondria and Aging)



HTRA2 is a mitochondrial protein, mutations in which are associated 

with autosomal dominant late-onset Parkinson’s disease (PD). The 

mechanisms by which HTRA2 mutations result in PD are poorly 

understood. HTRA2 is proposed to play a proteolytic role in protein 

quality control and homeostasis in the mitochondrial intermembrane 

space. Its loss has been reported to result in accumulation of 

unfolded and misfolded proteins. However, in at least one case, 

PD-associated HTRA2 mutation can cause its hyperphosphorylation, 

possibly resulting in protease hyperactivity. The consequences of 

overactive mitochondrial HTRA2 are not clear. Dictyostelium 

discoideum provides a well-established model for studying 

mitochondrial dysfunction, such as has been implicated in the 

pathology of PD. We identified a single homologue of human HTRA2 

encoded in the Dictyostelium discoideum genome and showed that it 

is localized to the mitochondria where it plays a cytoprotective role. 

Knockdown of HTRA2 expression caused defective morphogenesis 

in the multicellular phases of the Dictyostelium life cycle. In vegetative 

cells, it did not impair mitochondrial respiration but nonetheless caused 

slow growth (particularly when the cells were utilizing a bacterial food 

source), unaccompanied by significant defects in the requisite 

endocytic pathways. Despite its protective roles, we could not 

ectopically overexpress wild type HTRA2, suggesting that 

mitochondrial HTRA2 hyperactivity is lethal. This toxicity was 

abolished by replacing the essential catalytic serine S300 with alanine 

to ablate serine protease activity. Overexpression of protease-dead 

HTRA2 phenocopied the effects of knockdown, suggesting that the 

mutant protein competitively inhibits interactions between wild type 

HTRA2 and its binding partners. Our results show that cytopathological 

dysfunction can be caused either by too little or too much HTRA2 

activity in the mitochondria and suggest that either could be a cause 

of PD.





submitted by:  Paul Fisher [[log in to unmask]]

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Secretion and function of Cln5 during the early stages of Dictyostelium 

development



Robert J. Huber and Sabateeshan Mathavarajah



Department of Biology, Trent University, Peterborough, Ontario, 

Canada





BBA Molecular Cell Research, accepted



Mutations in CLN5 cause neuronal ceroid lipofuscinosis (NCL), a 

currently untreatable neurodegenerative disorder commonly known 

as Batten disease. Several genetic models have been generated to 

study the function of CLN5, but one limitation has been the lack of a 

homolog in lower eukaryotic model systems. Our previous work 

revealed a homolog of CLN5 in the social amoeba Dictyostelium 

discoideum. We used a Cln5-GFP fusion protein to show that the 

protein is secreted and functions as a glycoside hydrolase in 

Dictyostelium. Importantly, we also revealed this to be the molecular 

function of human CLN5. In this study, we generated an antibody 

against Cln5 to show that the endogenous protein is secreted during 

the early stages of Dictyostelium development. Like human CLN5, 

the Dictyostelium homolog is glycosylated and requires this post-

translational modification for secretion. Cln5 secretion bypasses the 

Golgi complex, and instead, occurs via an unconventional pathway 

linked to autophagy. Interestingly, we observed co-localization of 

Cln5 and GFP-Cln3 as well as increased secretion of Cln5 and 

Cln5-GFP in cln3- cells. Loss of Cln5 causes defects in adhesion 

and chemotaxis, which intriguingly, has also been reported for 

Dictyostelium cells lacking Cln3. Finally, autofluorescence was 

detected in cln5- cells, which is consistent with observations in 

mammalian systems. Together, our data support a function for Cln5 

during the early stages of multicellular development, provides further 

evidence for the molecular networking of NCL proteins, and provides 

insight into the mechanisms that may underlie CLN5 function in 

humans.



submitted by:  Robert Huber  [[log in to unmask]]

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[End dictyNews, volume 44, number 20]