eprintid: 32058
rev_number: 15
eprint_status: archive
userid: 6870
dir: disk0/00/03/20/58
datestamp: 2022-08-08 13:03:54
lastmod: 2022-08-25 17:30:27
status_changed: 2022-08-08 13:03:54
type: doctoralThesis
metadata_visibility: show
creators_name: Hempelmann, Pia
title: STARD3: A Sphingolipid Transporter at Lysosome-ER Contact Sites
divisions: i-140001
adv_faculty: af-14
abstract: Sphingolipid transport between organelles has been increasingly studied during the last 
years, and a large body of evidence points to protein-mediated sphingolipid transfer at 
organelle contact sites. This is well characterized for sphingolipid transfer along the
biosynthetic pathway from the endoplasmic reticulum (ER) towards the plasma 
membrane. However, the catabolic pathway is far less well studied. Here, a crucial but 
unexplored step is the recycling of the sphingosine backbone upon lysosomal 
sphingolipid degradation and its reintegration into the biosynthetic pathway at the ER. 
In this study, I propose that the lysosomal cholesterol transporter STARD3 acts as a 
sphingosine transfer protein at lysosome-ER contact sites to facilitate sphingosine entry 
into the recycling pathway.
STARD3 has been previously described to tether lysosomes to the ER via its FFAT motif 
that binds to ER-resident proteins. At this contact, its START-domain transfers cholesterol 
from the ER to lysosomes to support endosome maturation and to maintain cholesterol 
levels at the ER. I employed functionalized, photocrosslinkable sphingosine in intact cells 
to show that STARD3 also binds to sphingosine. Functionally, I could show that 
overexpression of STARD3 drives the sphingosine metabolism towards biogenesis of 
higher sphingolipid species, such as sphingomyelin, while depleting cellular STARD3
levels results in a delayed sphingosine metabolism, which indicates STARD3 as a 
lysosomal sphingosine exporter. This transfer is dependent on a functional FFAT motif, 
consistent with sphingosine transfer taking place at the lysosome-ER contact site.
STARD3 FFAT mutation or drug-induced prevention of sphingosine entry into the 
recycling pathway switches the sphingolipid metabolism towards the degradation 
pathway. Finally, molecular simulations, mutational analyses, and cholesterol 
competition studies revealed the START domain as a sphingosine transfer domain that 
can either transfer cholesterol or sphingosine at lysosome-ER contact sites.
Overall, I hypothesize that STARD3 transfers sphingosine from the lysosome towards the 
ER along a subcellular lipid gradient, potentially in exchange for the transfer of 
cholesterol. As such, STARD3 is the first protein able to facilitate sphingosine exit from 
lysosomes towards other organelles. This represents an important connection between 
the catabolic and anabolic sphingolipid pathways and has wide-ranging implications for 
future studies relating to lysosomal lipid efflux and sphingolipid-mediated processes in 
health and disease.
date: 2022
id_scheme: DOI
id_number: 10.11588/heidok.00032058
ppn_swb: 1815209992
own_urn: urn:nbn:de:bsz:16-heidok-320581
date_accepted: 2022-07-19
advisor: HASH(0x56009aab2248)
language: eng
bibsort: HEMPELMANNSTARD3ASPH20220803
full_text_status: public
place_of_pub: Heidelberg
citation:   Hempelmann, Pia  (2022) STARD3: A Sphingolipid Transporter at Lysosome-ER Contact Sites.  [Dissertation]     
document_url: https://archiv.ub.uni-heidelberg.de/volltextserver/32058/1/Dissertation_Pia_Hempelmann.pdf