Lipids exert diverse functions in living organisms. They form cellular membranes, store and transport energy and play signalling roles. Some lipid species function in all of these processes, making them ideal candidates to coordinate metabolism with cellular homeostasis and animal development. This theme was central to Suzanne Eaton's research in the fruit fly, Drosophila. Here, we discuss her work on membrane lipid homeostasis in changing environments and on functions for lipids in the Hedgehog signalling pathway. We further highlight lipoproteins as inter-organ carriers of lipids and lipid-linked morphogens, which communicate dietary and developmental signals throughout the organism.

Scientific innovation tends to arise from the combination of different concepts and approaches, and here Suzanne Eaton (1959-2019) was in a class of her own. Widely recognized for her work at the intersection of developmental and cell biology, Suzanne's contributions to linking the fields of developmental biology with lipid metabolism were no less ground-breaking. At a time when membrane biochemistry relied on cell culture systems, Suzanne realized that the combination of powerful genetics and a comparably simple tissue architecture made Drosophila the ideal model in which to investigate cellular membrane lipids within a complex animal. Pushing the tiny fruit fly as a system for biochemical experiments certainly gave her students some long days at the dissecting microscope but those efforts were rewarded. The results yielded profound insights into the homeostasis of membrane lipids, its coordination between different tissues and its response to dietary changes. Moreover, the study of lipid metabolism in a developing animal set the stage for the discovery that lipoproteins are not merely systemic nutrient carriers. Rather, by moving signalling lipids and lipid-linked morphogens between cells and organs, lipoproteins help coordinate animal development with metabolism. In light of the sweeping breadth of Suzanne's research on the interplay between animal development and metabolism, the following is but an attempt to outline the most important discoveries from her lab.

In the 1990s, studies in cultured mammalian cells suggested that plasma membrane lipids are not uniformly miscible, but rather form phase-separated domains referred to as lipid rafts (Simons and Ikonen, 1997). In her first senior author paper, Suzanne took up the challenge to prove the existence of lipid rafts in animal cell membranes in vivo. She and her colleagues showed that, indeed, detergent-resistant membrane domains could successfully be isolated from Drosophila embryos and were found to be enriched in sterols and sphingolipids, reminiscent of lipid rafts in the plasma membrane of mammalian cells (Rietveld et al., 1999). Unexpectedly, analysis of the proteins enriched in lipid rafts identified a set of lipid-modified peripheral membrane proteins: glycosylphosphatidylinositol (GPI)-linked proteins and the cholesterol-modified morphogen Hedgehog, which patterns the developing embryo (Rietveld et al., 1999). This early work already demonstrated that subcellular biochemical processes could be studied within the animal organism and that these studies could yield important insights into developmental biology.

Each animal cell comprises a complex mixture of hundreds of different lipid species, the collective behaviour of which determines biophysical membrane properties such as fluidity, thickness and lipid-protein interactions (Harayama and Riezman, 2018). So how do cells coordinate cell-autonomous lipid synthesis with exogenous lipid delivery from lipogenic organs and dietary sources to sustain membrane homeostasis? Drosophila turned out to be the ideal model for tackling this question, allowing time- and tissue-dependent genetic manipulations and dietary intervention to control lipid supply to different organs. Joining forces with Andrej Shevchenko (Max Planck Institute of Molecular Cell Biology and Genetics), who was developing shotgun lipidomics approaches, Suzanne published a series of papers that dissected how tissue lipidomes respond to perturbed lipoprotein metabolism, dietary alterations and developmental stages (Carvalho et al., 2010, 2012; Palm et al., 2012; Brankatschk et al., 2018).

Drosophila recapitulates many features of mammalian lipid metabolism: the fat body bears functional analogy to the liver and adipose tissue; the intestine absorbs dietary lipids; and the mechanisms of lipid storage and inter-organ transport are largely conserved on a molecular level (Musselman and Kühnlein, 2018). However, the comparatively simple lipoprotein system of Drosophila centres around one main lipoprotein called Lipophorin, which is scaffolded by apoLipophorin, a homologue of mammalian apolipoprotein B, allowing complete genetic suppression of specific inter-organ lipid fluxes (Olofsson and Borèn, 2005; Palm et al., 2012; Panáková et al., 2005). When the delivery of lipids from fat or intestine is blocked, some tissues, such as the brain, maintain lipid composition autonomously (Palm et al., 2012). By contrast, other tissues, such as the imaginal disc epithelium, have lost the ability to synthesize adequate amounts of membrane lipids and require lipid supply from the circulation.

Depending on lifestyle and food availability, animals acquire vastly different complements of lipids from the diet. Because Drosophila is a sterol auxotroph, the nature and abundance of its membrane sterols are dictated by dietary availability. In fact, despite accumulating substantial amounts of sterols in its membranes, Drosophila tolerates a more than fivefold reduction of bulk membrane sterols (Carvalho et al., 2010). Surprisingly, membrane phospholipids and sphingolipids also reflect dietary lipid composition to a striking degree, although Drosophila possesses the biosynthetic capacity to generate non-sterol lipids. In the wild, fruit flies feed on the yeast that grows on decomposing plant fruits, two food sources that greatly differ in sterol and fatty acid content. In the lab, membrane lipids of Drosophila larvae raised on yeast or plant food become more similar to the respective food source (Carvalho et al., 2012). Despite these differences in tissue lipidomes, cellular membranes must support the same molecular functions. This suggests that cells tolerate substantial differences in the nature and abundance of individual lipid species and can sustain ensemble membrane properties.

The study of Drosophila tissue lipidomes has intriguing implications for the evolution of membrane lipid composition. Several cell types in flies and mammals share certain lipid signatures; for example, the brain is rich in long, polyunsaturated phospholipids, the intestine in hydroxylated sphingolipids (Carvalho et al., 2012). These observations suggest that such lipid classes play similar roles in functionally analogous cell types of flies and mammals, despite differences in general membrane lipid composition between these organisms. The major head group of phospholipids and sphingolipids has changed during animal evolution – ethanolamine in flies as opposed to choline in mammals (Carvalho et al., 2012; Rietveld et al., 1999). At the same time, Drosophila membrane lipids contain shorter fatty acid chains than their mammalian counterparts. Conceivably, phospholipid head group and fatty acid chain length evolved concertedly to sustain membrane biophysical properties.

In one of her last papers, Suzanne turned to yet another level of complexity: the interplay between metabolic homeostasis and environmental influences, i.e. how does a cold-blooded animal maintain membrane fluidity to thrive across a range of temperatures? The perhaps unexpected answer was a change in feeding behaviour: when exposed to low temperature, the food preference of Drosophila shifts from yeast to plants (Brankatschk et al., 2018). Increased acquisition and membrane lipid incorporation of the unsaturated fatty acids abundant in a plant diet sustains membrane fluidity at low temperatures. As a consequence, Drosophila development, lifespan and motor coordination are all improved in the cold. Thus, a change in diet can support cellular and organismal homeostasis in a shifting environment.

Encouraged by her work on membrane organization in vivo, Suzanne became interested in the mechanistic basis of cell polarity in vivo, focussing on the wing imaginal disc epithelium of the developing Drosophila larva. She and her co-workers showed that GPI-linked GFP, first developed as a tool in Madin–Darby canine kidney cells, when expressed in the wing imaginal disc allowed the labelling of basolateral membranes. Surprisingly, some GPI-linked GFP was detected at a distance from its source in non-expressing cells where it colocalized with the endocytic compartment (Greco et al., 2001). Further work suggested that membrane-associated GFP-GPI spreads through the epithelium via exovesicles. Importantly, the lipid-linked morphogen Wingless (a Drosophila Wnt), which is endogenously produced in a subset of imaginal disc cells, was also found to colocalize with exovesicles in non-producing cells, providing early evidence that exovesicles participate in communication between cells (Greco et al., 2001). These exovesicles were named ‘argosomes’ because they constituted a vehicle for lipid-linked proteins to travel, reminiscent of the ship Argo in Greek mythology on which Jason and the Argonauts sailed from Iolcos to Colchis to retrieve the Golden Fleece.

The discovery of argosomes sparked one of Suzanne's subsequent major research interests: the spreading and signalling of the morphogens Hedgehog and Wingless. Morphogens are secreted signalling proteins that regulate growth and patterning of developing animal tissues. They emanate from localized morphogen-producing cells and spread through tissues over a distance to regulate the fate of morphogen-receiving cells. Because morphogens are secreted proteins, it was puzzling for developmental biologists that Hedgehog and Wingless are covalently lipid-modified and hence tightly associated with cellular membranes (Nusse, 2003). The discovery of argosomes suggested a molecular mechanism for the membrane release of lipid-linked morphogens in a soluble transport form (Greco et al., 2001).

To understand the mechanisms by which argosomes form and function, Suzanne and colleagues turned to the biochemical approach of purifying such vesicles from wing imaginal discs. However, in yet another unexpected twist, GPI-linked proteins as well as Hedgehog and Wingless were not recovered on exovesicles; rather, they co-purified with the lipoprotein Lipophorin and were shown to be colocalized with Lipophorin in the wing imaginal disc (Panáková et al., 2005). Thus, one possible mechanism for the mobilization and spread of lipid-linked Hedgehog and Wingless turned out to be lipoprotein particles, which originate from systemic circulation and act locally within the imaginal disc epithelium (Panáková et al., 2005). Lipophorin also associates with glypicans, a class of heparan sulphate proteoglycans (HSPGs) that exert various functions in the signalling and spreading of Hedgehog and other morphogens. This suggests that lipoproteins may serve as assembly platforms for multiple components of the Hedgehog signalling pathway (Eugster et al., 2007). Interestingly, a recent study showed that the glypican Dally-like protein can also mobilize Wingless from cell membranes directly by structurally shielding the hydrophobic lipid moiety of Wingless from the aqueous environment (McGough et al., 2020). Similar to lipid-linked morphogens, ectopically expressed fluorescent proteins engineered to carry a variety of lipid anchors are released on lipoproteins from imaginal disc membranes (Brankatschk and Eaton, 2010). Moreover, tissue culture cells can secrete the mammalian Hedgehog protein sonic hedgehog on exogenously added lipoproteins from either humans or flies (Palm et al., 2013). Thus, secretion of lipid-linked proteins on lipoproteins is an evolutionarily conserved mechanism that is likely driven by lipid-lipid rather than lipid-protein interactions. By combining genetic, biochemical and cell biology approaches, these elegant papers yielded important insights into the mechanisms by which lipid-linked proteins spread through tissues.

The function of lipoproteins as inter-organ lipid carriers raised the intriguing possibility that lipoproteins could transport lipid-linked morphogens not only locally within imaginal discs, but also systematically through circulation. Conceivably, by integrating morphogen signalling and lipid metabolism, lipoproteins could play a role in the systemic coordination of animal development and metabolism. In support of this idea, lipoprotein-associated Hedgehog was shown to be present not only in imaginal discs but also in the haemolymph, the invertebrate equivalent to blood (Palm et al., 2013; Rodenfels et al., 2014). However, haemolymph Hedgehog does not derive from imaginal discs; rather, this pool of Hedgehog is secreted by the intestine. In response to nutrient availability, Drosophila larvae regulate intestinal production of Hedgehog, which is secreted on lipoproteins into circulation to tune growth rates and developmental timing through signalling to distant organs. This process is especially important during starvation when intestinally derived Hedgehog triggers the mobilization of triacylglycerol stores in the fat body. Thus, Hedgehog can act as an endocrine hormone that coordinates the developmental and metabolic response of multiple tissues with nutrient availability (Rodenfels et al., 2014).

A central function of lipoproteins is the systemic transport of nutritional lipids such as triglycerides and sterols, but lipoproteins also contain a variety of less abundant lipid classes. This stimulated Suzanne to ask whether lipoproteins might transport signalling lipids between different organs, in functional analogy to their role in transporting lipid-linked signalling proteins. She and her colleagues revealed that, in addition to Lipophorin, Drosophila produces a second lipoprotein referred to as Lipid Transfer Particle (LTP), which exports lipids from the intestine onto the major circulating lipid carrier, Lipophorin (Palm et al., 2012). Both Lipophorin and LTP can cross the blood-brain barrier (Brankatschk and Eaton, 2010; Brankatschk et al., 2014). In the Drosophila brain, LTP accumulates on specific neurons that contact insulin-producing cells, which regulate growth in response to nutrient availability by secreting several insulin-like peptides. It therefore appeared that LTP conveys information about dietary lipid composition to the brain to regulate the secretion of insulin-like peptides, either directly by delivering a signalling lipid from the intestine or indirectly by promoting lipid transfer from Lipophorin and thus increasing lipid transfer to these cells (Brankatschk et al., 2014).

Although a role in intestine-to-brain communication suggested novel functions for lipoproteins as signalling particles, lipoprotein-derived lipids also turned out to play a direct role in local Hedgehog signalling in the wing imaginal disc. In the early 2000s, multiple lines of evidence suggested a key role for small molecules in regulating the Hedgehog signalling pathway. When Hedgehog binds to and inhibits its receptor, Patched, the downstream effector Smoothened becomes de-repressed and in turn activates the transcription factor Cubitus interruptus (Ci). Patched shares similarities with bacterial proteins that use proton gradients to transport lipophilic molecules across membranes and represses Smoothened activity sub-stoichiometrically. This suggested a model in which Patched regulates the availability of small molecules that subsequently act on Smoothened (Taipale et al., 2002). Interestingly, the Eaton group showed that genetic depletion of Lipophorin restricts the range of Hedgehog signalling in the wing imaginal disc, but also leads to partial pathway activation in the whole disc epithelium by stabilizing Ci (Khaliullina et al., 2009). This effect of Lipophorin on the Hedgehog signalling pathway is independent of Hedgehog itself. Rather, lipoprotein-derived lipids directly contribute to Patched-mediated repression of Smoothened. Biochemical fractionation identified these lipids as endocannabinoids, which are present in Drosophila lipoproteins and repress Hedgehog signalling at physiological concentrations (Khaliullina et al., 2015). The association of Hedgehog with lipoproteins blocks the repressive function of lipoprotein lipids on downstream signalling (Palm et al., 2013). Taken together, this work identified roles for systemic Hedgehog signalling and lipoprotein metabolism during animal development.

Suzanne Eaton's research into the connections between lipid metabolism and animal development generated a trove of unexpected findings and observations (Fig. 1). The analysis of tissue lipidomes in response to organismal and environmental alterations revealed a remarkable tolerance of Drosophila cells to changes in membrane lipid composition. It remains to be established whether these principles are evolutionarily conserved or contingent on Drosophila physiology. Unlike flies, mammals control their body temperature within a narrow range and can synthesize their major membrane lipids, including cholesterol. Nevertheless, all animal cells presumably must exert exquisite control over the biophysical properties of their membranes. The underlying molecular pathways of lipid sensing and homeostatic response largely remain to be discovered. By allowing great experimental control over membrane lipidomes, Drosophila continues to be an excellent model in which to address this problem. The tremendous variation in diet between human individuals further suggests that it would be worthwhile to determine the extent to which human cells incorporate dietary lipids into their membranes.

Fig. 1.

Lipoproteins in animal development and homeostasis. (A) Schematic highlighting the composition of the Drosophia lipoprotein Lipophorin. Lipophorin particles contain a neutral core (yellow) of hydrophobic lipids surrounded by a layer of polar lipids (blue). They are scaffolded by the apolipoprotein apoLipophorin (green), a homologue of mammalian apolipoprotein B, which scaffolds very low- and low-density lipoproteins. Lipophorin can associate with morphogens (pink oval) and glypicans (orange). (B) Drosophila Lipophorin functions as an inter-organ transporter of nutritional lipids. Moreover, Lipophorin carries the morphogen Hedgehog as well as lipids that regulate the Hedgehog signalling pathway, which regulate developmental patterning. Thereby, Lipophorin communicates dietary and environmental signals throughout the organism to coordinate metabolic homeostasis with development.

Fig. 1.

Lipoproteins in animal development and homeostasis. (A) Schematic highlighting the composition of the Drosophia lipoprotein Lipophorin. Lipophorin particles contain a neutral core (yellow) of hydrophobic lipids surrounded by a layer of polar lipids (blue). They are scaffolded by the apolipoprotein apoLipophorin (green), a homologue of mammalian apolipoprotein B, which scaffolds very low- and low-density lipoproteins. Lipophorin can associate with morphogens (pink oval) and glypicans (orange). (B) Drosophila Lipophorin functions as an inter-organ transporter of nutritional lipids. Moreover, Lipophorin carries the morphogen Hedgehog as well as lipids that regulate the Hedgehog signalling pathway, which regulate developmental patterning. Thereby, Lipophorin communicates dietary and environmental signals throughout the organism to coordinate metabolic homeostasis with development.

Over the years, multiple mechanisms have been identified through which the lipid-modified morphogens Hedgehog and Wingless can travel through tissues, including exovesicles/argosomes, lipoproteins, multimeres, glycipan-shielded morphogen complexes, and membrane protrusions (Ayers et al., 2010; Callejo et al., 2011; Gradilla et al., 2014; Greco et al., 2001; Matusek et al., 2014; McGough et al., 2020; Panáková et al., 2005; Vyas et al., 2008). The precise functions of lipoprotein-associated Hedgehog and other Hedgehog secretion forms in patterning imaginal discs remain to be clarified (Petrov et al., 2017; Thérond, 2012). Presumably, the form in which Hedgehog proteins are secreted depends on cellular and tissue context. However, lipoproteins are unique in this regard, because they can transport Hedgehog into circulation and carry lipids that repress the Hedgehog pathway. Lipoprotein-associated Hedgehog may thus be particularly important in coordinating inter-organ signalling and systemic responses. In mammals, the association of Hedgehog and Wnt with lipoproteins has been reported in cell culture systems (Neumann et al., 2009; Palm et al., 2013), but in vivo evidence is still missing. Considering the focus of mammalian lipoprotein research on systemic lipid transport and associated metabolic diseases, and the experimental challenges for investigating lipoproteins in the developing embryo, a pioneer of Suzanne's calibre may be required to address this. It also remains unknown whether mammalian Hedgehog proteins have systemic functions. Interestingly, Hedgehog signalling has been implicated in the regulation of metabolic processes such as fat storage and glucose uptake in mouse adipocytes (Pospisilik et al., 2010; Teperino et al., 2012). The source of the relevant Hedgehog ligand has not been identified, but it may well be borne from circulation on lipoproteins.

Suzanne Eaton was a visionary and inspirational scientist, as well as a great mentor and person. The central themes of Suzanne's work concerning the interplay of metabolism and development emerged in her first papers and resurfaced time and again, bringing new facets to a stunning array of seemingly distant but deeply interconnected biological processes. She will remain a shining example for a collaborative and interdisciplinary approach to science that embraces life in all its complexities.

This article is part of a collection that commemorates the work of Suzanne Eaton. See also Dahmann and Classen (2020), Mlodzik (2020) and Prince et al. (2020) in this issue.

We both had the privilege to pursue our PhDs in Suzanne's group at the MPI-CBG in Dresden. Being part of the Eaton Lab meant learning to enjoy science in its truest form, and we are grateful for Suzanne's unlimited support and mentoring during those years and beyond. She will be missed.

Ayers
,
K. L.
,
Gallet
,
A.
,
Staccini-Lavenant
,
L.
and
Thérond
,
P. P.
(
2010
).
The long-range activity of Hedgehog is regulated in the apical extracellular space by the glypican dally and the hydrolase Notum
.
Dev. Cell
18
,
605
-
620
.
Brankatschk
,
M.
and
Eaton
,
S.
(
2010
).
Lipoprotein particles cross the blood-brain barrier in Drosophila
.
J. Neurosci.
30
,
10441
-
10447
.
Brankatschk
,
M.
,
Dunst
,
S.
,
Nemetschke
,
L.
and
Eaton
,
S.
(
2014
).
Delivery of circulating lipoproteins to specific neurons in the Drosophila brain regulates systemic insulin signaling
.
eLife
3
,
e02862
.
Brankatschk
,
M.
,
Gutmann
,
T.
,
Knittelfelder
,
O.
,
Palladini
,
A.
,
Prince
,
E.
,
Grzybek
,
M.
,
Brankatschk
,
B.
,
Shevchenko
,
A.
,
Coskun
,
Ü.
and
Eaton
,
S.
(
2018
).
A Temperature-dependent switch in feeding preference improves Drosophila development and survival in the cold
.
Dev. Cell
46
,
781
-
793.e4
.
Callejo
,
A.
,
Bilioni
,
A.
,
Mollica
,
E.
,
Gorfinkiel
,
N.
,
Andrés
,
G.
,
Ibáñez
,
C.
,
Torroja
,
C.
,
Doglio
,
L.
,
Sierra
,
J.
and
Guerrero
,
I.
(
2011
).
Dispatched mediates Hedgehog basolateral release to form the long-range morphogenetic gradient in the Drosophila wing disk epithelium
.
Proc. Natl. Acad. Sci. USA
108
,
12591
-
12598
.
Carvalho
,
M.
,
Schwudke
,
D.
,
Sampaio
,
J. L.
,
Palm
,
W.
,
Riezman
,
I.
,
Dey
,
G.
,
Gupta
,
G. D.
,
Mayor
,
S.
,
Riezman
,
H.
,
Shevchenko
,
A.
, et al. 
(
2010
).
Survival strategies of a sterol auxotroph
.
Development
137
,
3675
-
3685
.
Carvalho
,
M.
,
Sampaio
,
J. L.
,
Palm
,
W.
,
Brankatschk
,
M.
,
Eaton
,
S.
and
Shevchenko
,
A.
(
2012
).
Effects of diet and development on the Drosophila lipidome
.
Mol. Syst. Biol.
8
,
600
.
Dahmann
,
C.
and
Classen
,
A.-K.
(
2020
).
Charting the unknown currents of cellular flows and forces. Development 147, dev186403.
Eugster
,
C.
,
Panáková
,
D.
,
Mahmoud
,
A.
and
Eaton
,
S.
(
2007
).
Lipoprotein-heparan sulfate interactions in the Hh pathway
.
Dev. Cell
13
,
57
-
71
.
Gradilla
,
A.-C.
,
González
,
E.
,
Seijo
,
I.
,
Andrés
,
G.
,
Bischoff
,
M.
,
González-Mendez
,
L.
,
Sánchez
,
V.
,
Callejo
,
A.
,
Ibáñez
,
C.
,
Guerra
,
M.
, et al. 
(
2014
).
Exosomes as Hedgehog carriers in cytoneme-mediated transport and secretion
.
Nat. Commun.
5
,
5649
.
Greco
,
V.
,
Hannus
,
M.
and
Eaton
,
S.
(
2001
).
Argosomes: a potential vehicle for the spread of morphogens through epithelia
.
Cell
106
,
633
-
645
.
Harayama
,
T.
and
Riezman
,
H.
(
2018
).
Understanding the diversity of membrane lipid composition
.
Nat. Rev. Mol. Cell Biol.
19
,
281
-
296
.
Khaliullina
,
H.
,
Panáková
,
D.
,
Eugster
,
C.
,
Riedel
,
F.
,
Carvalho
,
M.
and
Eaton
,
S.
(
2009
).
Patched regulates smoothened trafficking using lipoprotein-derived lipids
.
Development
136
,
4111
-
4121
.
Khaliullina
,
H.
,
Bilgin
,
M.
,
Sampaio
,
J. L.
,
Shevchenko
,
A.
and
Eaton
,
S.
(
2015
).
Endocannabinoids are conserved inhibitors of the Hedgehog pathway
.
Proc. Natl. Acad. Sci. USA
112
,
3415
-
3420
.
Matusek
,
T.
,
Wendler
,
F.
,
Polès
,
S.
,
Pizette
,
S.
,
D'Angelo
,
G.
,
Fürthauer
,
M.
and
Thérond
,
P. P.
(
2014
).
The ESCRT machinery regulates the secretion and long-range activity of Hedgehog
.
Nature
516
,
99
-
103
.
McGough
,
I. J.
,
Vecchia
,
L.
,
Bishop
,
B.
,
Malinauskas
,
T.
,
Beckett
,
K.
,
Joshi
,
D.
,
O'Reilly
,
N.
,
Siebold
,
C.
,
Jones
,
E. Y.
and
Vincent
,
J.-P.
(
2020
).
Glypicans shield the Wnt lipid moiety to enable signalling at a distance
.
Nature
585
,
85
-
90
.
Mlodzik
,
M.
(
2020
).
Planar cell polarity: moving from single cells to tissue-scale biology. Development 147, dev186346.
Musselman
,
L. P.
and
Kühnlein
,
R. P.
(
2018
).
Drosophila as a model to study obesity and metabolic disease
.
J. Exp. Biol.
221
,
jeb163881
.
Neumann
,
S.
,
Coudreuse
,
D. Y. M.
,
van der Westhuyzen
,
D. R.
,
Eckhardt
,
E. R. M.
,
Korswagen
,
H. C.
,
Schmitz
,
G.
and
Sprong
,
H.
(
2009
).
Mammalian Wnt3a is released on lipoprotein particles
.
Traffic
10
,
334
-
343
.
Nusse
,
R.
(
2003
).
Wnts and Hedgehogs: lipid-modified proteins and similarities in signaling mechanisms at the cell surface
.
Development
130
,
5297
-
5305
.
Olofsson
,
S.-O.
and
Borèn
,
J.
(
2005
).
Apolipoprotein B: a clinically important apolipoprotein which assembles atherogenic lipoproteins and promotes the development of atherosclerosis
.
J. Intern. Med.
258
,
395
-
410
.
Palm
,
W.
,
Sampaio
,
J. L.
,
Brankatschk
,
M.
,
Carvalho
,
M.
,
Mahmoud
,
A.
,
Shevchenko
,
A.
and
Eaton
,
S.
(
2012
).
Lipoproteins in Drosophila melanogaster—assembly, function, and influence on tissue lipid composition
.
PLoS Genet.
8
,
e1002828
.
Palm
,
W.
,
Swierczynska
,
M. M.
,
Kumari
,
V.
,
Ehrhart-Bornstein
,
M.
,
Bornstein
,
S. R.
and
Eaton
,
S.
(
2013
).
Secretion and signaling activities of lipoprotein-associated hedgehog and non-sterol-modified hedgehog in flies and mammals
.
PLoS Biol.
11
,
e1001505
.
Panáková
,
D.
,
Sprong
,
H.
,
Marois
,
E.
,
Thiele
,
C.
and
Eaton
,
S.
(
2005
).
Lipoprotein particles are required for Hedgehog and wingless signalling
.
Nature
435
,
58
-
65
.
Petrov
,
K.
,
Wierbowski
,
B. M.
and
Salic
,
A.
(
2017
).
Sending and receiving Hedgehog signals
.
Annu. Rev. Cell Dev. Biol.
33
,
145
-
168
.
Pospisilik
,
J. A.
,
Schramek
,
D.
,
Schnidar
,
H.
,
Cronin
,
S. J. F.
,
Nehme
,
N. T.
,
Zhang
,
X.
,
Knauf
,
C.
,
Cani
,
P. D.
,
Aumayr
,
K.
,
Todoric
,
J.
, et al. 
(
2010
).
Drosophila genome-wide obesity screen reveals hedgehog as a determinant of brown versus white adipose cell fate
.
Cell
140
,
148
-
160
.
Prince
,
E.
,
Marcetteau
,
J.
and
Thérond
,
P. P.
(
2020
).
Circulating Hedgehog: a fresh view of a classic morphogen. Development 147, dev186395.
Rietveld
,
A.
,
Neutz
,
S.
,
Simons
,
K.
and
Eaton
,
S.
(
1999
).
Association of sterol- and glycosylphosphatidylinositol-linked proteins with Drosophila raft lipid microdomains
.
J. Biol. Chem.
274
,
12049
-
12054
.
Rodenfels
,
J.
,
Lavrynenko
,
O.
,
Ayciriex
,
S.
,
Sampaio
,
J. L.
,
Carvalho
,
M.
,
Shevchenko
,
A.
and
Eaton
,
S.
(
2014
).
Production of systemically circulating Hedgehog by the intestine couples nutrition to growth and development
.
Genes Dev.
28
,
2636
-
2651
.
Simons
,
K.
and
Ikonen
,
E.
(
1997
).
Functional rafts in cell membranes
.
Nature
387
,
569
-
572
.
Taipale
,
J.
,
Cooper
,
M. K.
,
Maiti
,
T.
and
Beachy
,
P. A.
(
2002
).
Patched acts catalytically to suppress the activity of smoothened
.
Nature
418
,
892
-
896
.
Teperino
,
R.
,
Amann
,
S.
,
Bayer
,
M.
,
McGee
,
S. L.
,
Loipetzberger
,
A.
,
Connor
,
T.
,
Jaeger
,
C.
,
Kammerer
,
B.
,
Winter
,
L.
,
Wiche
,
G.
, et al. 
(
2012
).
Hedgehog partial agonism drives warburg-like metabolism in muscle and brown fat
.
Cell
151
,
414
-
426
.
Thérond
,
P. P.
(
2012
).
Release and transportation of Hedgehog molecules
.
Curr. Opin. Cell Biol.
24
,
173
-
180
.
Vyas
,
N.
,
Goswami
,
D.
,
Manonmani
,
A.
,
Sharma
,
P.
,
Ranganath
,
H. A.
,
VijayRaghavan
,
K.
,
Shashidhara
,
L. S.
,
Sowdhamini
,
R.
and
Mayor
,
S.
(
2008
).
Nanoscale organization of hedgehog is essential for long-range signaling
.
Cell
133
,
1214
-
1227
.

Competing interests

The authors declare no competing or financial interests.