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Team Publications
Biogenesis and Functions of Lysosome-Related Organelles
Year of publication 2011
Francesca Giordano, Sabrina Simoes, Graça Raposo (2011 Jul 7)
The ocular albinism type 1 (OA1) GPCR is ubiquitinated and its traffic requires
endosomal sorting complex responsible for transport (ESCRT) function.
Proceedings of the National Academy of Sciences of the United States of America : 11906-11 :
DOI : 10.1073/pnas.1103381108
Summary
The function of signaling receptors is tightly controlled by their intracellular trafficking. One
major regulatory mechanism within the endo-lysosomal system required for receptor
localization and down-regulation is protein modification by ubiquitination and downstream
interactions with the endosomal sorting complex responsible for transport (ESCRT)
machinery. Whether and how these mechanisms operate to regulate endosomal sorting of
mammalian G protein-coupled receptors (GPCRs) remains unclear. Here, we explore the
involvement of ubiquitin and ESCRTs in the trafficking of OA1, a pigment cell-specific GPCR,
target of mutations in Ocular Albinism type 1, which localizes intracellularly to melanosomes
to regulate their biogenesis. Using biochemical and morphological methods in combination
with overexpression and inactivation approaches we show that OA1 is ubiquitinated and that
its intracellular sorting and down-regulation requires functional ESCRT components.
Depletion or overexpression of subunits of ESCRT-0, -I, and -III markedly inhibits OA1
degradation with concomitant retention within the modified endosomal system. Our data
further show that OA1 ubiquitination is uniquely required for targeting to the intralumenal
vesicles of multivesicular endosomes, thereby regulating the balance between downregulation and delivery to melanosomes. This study highlights the role of ubiquitination and
the ESCRT machinery in the intracellular trafficking of mammalian GPCRs and has
implications for the physiopathology of ocular albinism type 1.
Claudia G Almeida, Ayako Yamada, Danièle Tenza, Daniel Louvard, Graça Raposo, Evelyne
Coudrier (2011 Jun 14)
Myosin 1b promotes the formation of post-Golgi carriers by regulating actin
assembly and membrane remodelling at the trans-Golgi network.
Nature cell biology : 779-89 : DOI : 10.1038/ncb2262
Summary
The function of organelles is intimately associated with rapid changes in membrane shape.
By exerting force on membranes, the cytoskeleton and its associated motors have an
important role in membrane remodelling. Actin and myosin 1 have been implicated in the
invagination of the plasma membrane during endocytosis. However, whether myosin 1 and
actin contribute to the membrane deformation that gives rise to the formation of post-Golgi
carriers is unknown. Here we report that myosin 1b regulates the actin-dependent post-Golgi
traffic of cargo, generates force that controls the assembly of F-actin foci and, together with
the actin cytoskeleton, promotes the formation of tubules at the TGN. Our results provide
INSTITUT CURIE, 20 rue d’Ulm, 75248 Paris Cedex 05, France | 1
Team Publications
Biogenesis and Functions of Lysosome-Related Organelles
evidence that actin and myosin 1 regulate organelle shape and uncover an important
function for myosin 1b in the initiation of post-Golgi carrier formation by regulating actin
assembly and remodelling TGN membranes.
Angélique Bobrie, Marina Colombo, Graça Raposo, Clotilde Théry (2011 Jun 8)
Exosome secretion: molecular mechanisms and roles in immune responses.
Traffic (Copenhagen, Denmark) : 1659-68 : DOI : 10.1111/j.1600-0854.2011.01225.x
Summary
Exosomes are small membrane vesicles, secreted by most cell types from multivesicular
endosomes, and thought to play important roles in intercellular communications. Initially
described in 1983, as specifically secreted by reticulocytes, exosomes became of interest for
immunologists in 1996, when they were proposed to play a role in antigen presentation.
More recently, the finding that exosomes carry genetic materials, mRNA and miRNA, has
been a major breakthrough in the field, unveiling their capacity to vehicle genetic messages.
It is now clear that not only immune cells but probably all cell types are able to secrete
exosomes: their range of possible functions expands well beyond immunology to
neurobiology, stem cell and tumor biology, and their use in clinical applications as
biomarkers or as therapeutic tools is an extensive area of research. Despite intensive efforts
to understand their functions, two issues remain to be solved in the future: (i) what are the
physiological function(s) of exosomes in vivo and (ii) what are the relative contributions of
exosomes and of other secreted membrane vesicles in these proposed functions? Here, we
will focus on the current ideas on exosomes and immune responses, but also on their
mechanisms of secretion and the use of this knowledge to elucidate the latter issue.
Toine ten Broeke, Guillaume van Niel, Marca H M Wauben, Richard Wubbolts, Willem Stoorvogel
(2011 Apr 27)
Endosomally stored MHC class II does not contribute to antigen presentation by
dendritic cells at inflammatory conditions.
Traffic (Copenhagen, Denmark) : 1025-36 : DOI : 10.1111/j.1600-0854.2011.01212.x
Summary
Major histocompatibility complex (MHC) class II (MHCII) is constitutively expressed by
immature dendritic cells (DC), but has a short half-life as a consequence of its transport to
and degradation in lysosomes. For its transfer to lysosomes, MHCII is actively sorted to the
intraluminal vesicles (ILV) of multivesicular bodies (MVB), a process driven by its
ubiquitination. ILV have, besides their role as an intermediate compartment in lysosomal
transfer, also been proposed to function as a site for MHCII antigen loading and temporal
storage. In that scenario, DC would recruit antigen-loaded MHCII to the cell surface in
response to a maturation stimulus by allowing ILV to fuse back with the MVB delimiting
membrane. Other studies, however, explained the increase in cell surface expression during
DC maturation by transient upregulation of MHCII synthesis and reduced sorting of newly
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Team Publications
Biogenesis and Functions of Lysosome-Related Organelles
synthesized MHCII to lysosomes. Here, we have characterized the relative contributions from
the biosynthetic and endocytic pathways and found that the vast majority of antigen-loaded
MHCII that is stably expressed at the plasma membrane by mature DC is synthesized after
exposure to inflammatory stimuli. Pre-existing endosomal MHCII contributed only when it
was not yet sorted to ILV at the moment of DC activation. Together with previous records,
our current data are consistent with a model in which passage of MHCII through ILV is not
required for antigen loading in maturing DC and in which sorting to ILV in immature DC
provides a one-way ticket for lysosomal degradation.
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