Altered lysosomal positioning affects lysosomal functions in a cellular model of Huntington's disease

Abstract

Huntington's disease (HD) is a hereditary and devastating neurodegenerative disorder caused by a mutation in the huntingtin protein. Understanding the functions of normal and mutant huntingtin protein is the key to reveal the pathogenesis of HD and develop therapeutic targets. Huntingtin plays an important role in vesicular and organelle trafficking. Lysosomes are dynamic organelles that integrate several degradative pathways and regulate the activity of mammalian target of rapamycin complex 1 (mTORC1). In the present study, we show that the perinuclear accumulation of lysosomes is increased in a cellular model of HD derived from HD knock-in mice and primary fibroblasts from a HD patient. This perinuclear lysosomal accumulation can be reversed when normal huntingtin is overexpressed in HD cells. To further investigate the functional significance of the increased perinuclear lysosomal accumulation in HD cells, we demonstrate subsequently that basal mTORC1 activity is increased in HD cells. In addition, autophagic influx is also increased in HD cells in response to serum deprivation, which leads to a premature fusion of lysosomes with autophagosomes. Taken together, our data suggest that the increased perinuclear accumulation of lysosomes may play an important role in HD pathogenesis by altering lysosomal-dependent functions.

Mechanisms and functions of lysosome positioning

“…For example, cytosol acidification causes dispersal of the perinuclear lysosome population, whereas subsequent alkalinization returns them to their central location (Heuser, 1989;Parton et al, 1991). Other perturbations, such as aggresome formation (Zaarur et al, 2014), starvation (Korolchuk et al, 2011), drug-induced apoptosis (Yu et al, 2016), expression of pathogenic mutant forms of huntingtin (Erie et al, 2015) or leucine-rich repeat kinase 2 (LRRK2) (Dodson et al, 2012) and LSDs (Uusi-Rauva et al, 2012;Li et al, 2016b), result in perinuclear clustering of lysosomes. During movement, lysosomes sometimes tubulate in a process that might involve attachment to both kinesins and dynein pulling in opposite directions (Mrakovic et al, 2012;Li et al, 2016b).…”

“…For example, expression of mutant huntingtin (the causal agent in Huntington's disease) or mutant leucine-rich repeat kinase 2 (LRRK2) (the most common causal agent in familial Parkinson's disease) promote accumulation of lysosomes in the juxtanuclear area of the cell (Caviston et al, 2011;Erie et al, 2015). These effects involve changes in mTORC1, dynein and Rab7 (Caviston et al, 2011;Erie et al, 2015), although the exact mechanisms remain to be elucidated.…”

Structure/Function Analysis of the Huntingtin N-terminus Encoded by Htt Exon 1 Using Knock-In Mouse Models

“…It is therefore not surprising that a mHTT-mediated transport defect can result in an accumulation of autophagosomes and reduced clearance of cellular waste [31]. In addition to changes in autophagosome generation and transport, HD mouse models also have altered lysosomal subcellular positioning, which can significantly alter autophagosome-lysosome fusion dynamics [32]. No significant differences between any of the genotypes were observed during the reversal task at 3 (Fig.…”

“…Interestingly, lysosomes maintain a diffuse distribution throughout the cell body of wild type cells, but in HD cells, lysosomes adopt an aberrant perinuclear distribution [32]. Because autophagosomes that are generated in the distal tips of axons mature as they are transported retrogradely towards the cell body for fusion with lysosomes and eventual degradation [33], inefficient transport of autophagosomes and the altered localization of lysosomes can lead to defective clearance of aggregates, proteins, and organelles in HD [31,32].…”

“…mHTT causes significant changes in macroautophagic flux and efficiency in HD patients and mouse models [30][31][32]151]. An increased number of autophagosomes within neurons, and increased mRNA levels of key autophagy pathway proteins LC3A, ULK1, and LAMP2A in the brains of HD patients and HD mouse models has been described, suggesting that autophagy is upregulated in response to mHTT expression [17,[152][153][154].…”