Protrudin and PDZD8 contribute to neuronal integrity by promoting lipid extraction required for endosome maturation

Abstract

Endosome maturation depends on membrane contact sites (MCSs) formed between endoplasmic reticulum (ER) and endolysosomes (LyLEs). The mechanism underlying lipid supply for this process and its pathophysiological relevance remains unclear, however. Here, we identify PDZD8—the mammalian ortholog of a yeast ERMES subunit—as a protein that interacts with protrudin, which is located at ER-LyLE MCSs. Protrudin and PDZD8 promote the formation of ER-LyLE MCSs, and PDZD8 shows the ability to extract various lipids from the ER. Overexpression of both protrudin and PDZD8 in HeLa cells, as well as their depletion in mouse primary neurons, impairs endosomal homeostasis by inducing the formation of abnormal large vacuoles reminiscent of those apparent in spastin- or REEP1-deficient neurons. The protrudin-PDZD8 system is also essential for the establishment of neuronal polarity. Our results suggest that protrudin and PDZD8 cooperatively promote endosome maturation by mediating ER-LyLE tethering and lipid extraction at MCSs, thereby maintaining neuronal polarity and integrity.

Lipid Transfer–Dependent Endosome Maturation Mediated by Protrudin and PDZD8 in Neurons

“…A differential proteomics analysis of brain extracts from wild-type and protrudin-deficient mice was performed to identify proteins that might function cooperatively with protrudin at ER-endosome MCSs. This analysis uncovered PDZD8, in addition to VAP-A and VAP-B, as a key binding partner of protrudin ( Elbaz-Alon et al, 2020 ; Shirane et al, 2020b ). An independent study also identified protrudin as a binding partner of PDZD8 ( Elbaz-Alon et al, 2020 ).…”

“…Neurons with HSP-associated mutations of the genes for spastin or REEP1 were recently found to manifest abnormal enlargement of LEs and lysosomal dysfunction as a result of defects in ER-endosome MCSs and impaired endosomal homeostasis ( Allison et al, 2017 ; Lee et al, 2020 ). As described in more detail below, disruption of the protrudin-PDZD8 complex has also been shown to result in the formation of abnormal LEs as well as in disturbance of neuronal polarity and axonal degeneration ( Shirane et al, 2020b ). These findings implicate the protrudin-PDZD8 complex in regulation of endosome maturation at ER-endosome MCSs.…”

“…This complex mediates interaction between the ER and mitochondria and contributes to the biosynthesis of phospholipids by mediating lipid transfer in a manner dependent on the synaptotagmin-like mitochondrial lipid–binding protein (SMP) domain of Mmm1 ( Figure 4 ; Kornmann et al, 2009 ). Although PDZD8 was also known to tether the ER and mitochondria and to regulate Ca 2+ dynamics in neurons ( Hirabayashi et al, 2017 ), it was only shown to possess lipid transfer activity after its identification as a binding partner of protrudin ( Shirane et al, 2020b ). Protrudin and PDZD8 form a stable complex at the ER membrane, with the abundance of protrudin being greatly diminished in the brain of PDZD8-deficient mice.…”

“…Protrudin and PDZD8 form a stable complex at the ER membrane, with the abundance of protrudin being greatly diminished in the brain of PDZD8-deficient mice. Knockdown of PDZD8 in PC12 cells also resulted in a loss of protrudin that was dependent on the proteasome ( Shirane et al, 2020b ).…”

“…HSP-related proteins have recently been implicated in the regulation of endosome maturation at ER-endosome MCSs ( Allison et al, 2017 ), although the physiological role and regulatory mechanisms of such maturation remain to be fully elucidated. A new study has now revealed that protrudin-dependent lipid transfer from the ER to endosomes promotes endosome maturation at ER-endosome MCSs ( Shirane et al, 2020b ). Protrudin-deficient mice show no signs of axonopathy, however, but instead manifest an abnormal behavioral phenotype ( Shirane et al, 2020a ), suggesting that protrudin might play an important role in normal neuronal development and behavior.…”