Different molecular mechanisms involved in spontaneous and oxidative stress-induced mitochondrial fragmentation in tripeptidyl peptidase-1 (TPP-1)-deficient fibroblasts

Abstract

NCLs (neuronal ceroid lipofuscinoses) form a group of eight inherited autosomal recessive diseases characterized by the intralysosomal accumulation of autofluorescent pigments, called ceroids. Recent data suggest that the pathogenesis of NCL is associated with the appearance of fragmented mitochondria with altered functions. However, even if an impairement in the autophagic pathway has often been evoked, the molecular mechanisms leading to mitochondrial fragmentation in response to a lysosomal dysfunction are still poorly understood. In this study, we show that fibroblasts that are deficient for the TPP-1 (tripeptidyl peptidase-1), a lysosomal hydrolase encoded by the gene mutated in the LINCL (late infantile NCL, CLN2 form) also exhibit a fragmented mitochondrial network. This morphological alteration is accompanied by an increase in the expression of the protein BNIP3 (Bcl2/adenovirus E1B 19 kDa interacting protein 3) as well as a decrease in the abundance of mitofusins 1 and 2, two proteins involved in mitochondrial fusion. Using RNAi (RNA interference) and quantitative analysis of the mitochondrial morphology, we show that the inhibition of BNIP3 expression does not result in an increase in the reticulation of the mitochondrial population in LINCL cells. However, this protein seems to play a key role in cell response to mitochondrial oxidative stress as it sensitizes mitochondria to antimycin A-induced fragmentation. To our knowledge, our results bring the first evidence of a mechanism that links TPP-1 deficiency and oxidative stress-induced changes in mitochondrial morphology.

Mutation update: Review of TPP1 gene variants associated with neuronal ceroid lipofuscinosis CLN2 disease

“…Several common pathogenic cascades have been identified in lysosomal storage disorders, including altered lipid trafficking, autophagy, altered calcium homeostasis and oxidative stress (Vitner, Platt, & Futerman, ). Specifically, in vitro studies have linked TPP1 deficiency to oxidative stress and changes in mitochondrial morphology (Van Beersel et al, ). Regardless of the initiating mechanisms, the uniform neuropathological features of the NCLs may suggest the existence of shared pathogenic pathways for NCL proteins (Haltia, ; Palmer et al, ).…”

Effect of acute hypoxic shock on the rat brain morphology and tripeptidyl peptidase I activity

“…Moreover, TPPI can activate Bid throughout a proteolytic cleavage thus participating in the receptor-dependent apoptosis. On the other hand, fibroblasts, deficient in TPPI exhibit much higher levels of active BNIP3 than the normal fibroblasts (Van Beersel et al, 2013). Although such studies have not been performed on nerve cells, it is possible that the low TPPI levels contribute to the elevated activation of BNIP3 and neuronal death by necrosis.…”

“…TPPI is shown to participate in receptor-mediated apoptosis throughout proteolytic cleavage of Bid in isolated fibroblasts (Autefage et al, 2009). On the other hand, it is demonstrated (Van Beersel et al, 2013) that TPPIdeficient fibroblasts exhibit an enhanced BNIP3 (Bcl2/adenovirus E1 B 19 kDa interacting protein 3) activation. BNIP3 is activated upon oxidative stress (Kubli et al, 2008).…”

ROS and intracellular ion channels

“…If, for example, TRPML are inhibited by ROS then the membrane traffic processes controlled by these channels are likely to be inhibited by oxidative stress. Indeed, autophagy inhibition by oxidative stress has been shown [7, 34, 128]. This would suggest that beyond damaging the organelles, ROS delay the cellular repair by suppressing autophagy, lysosomal exocytosis (including pathogen expulsion) and other clearance processes that depend on the membrane traffic events driven by TRPML [121, 122, 129–133].…”