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Drosophila lipophorin receptors mediate the uptake of neutral lipids in oocytes and imaginal disc cells by an endocytosis-independent mechanism.

Parra-Peralbo E, Culi J - PLoS Genet. (2011)

Bottom Line: Furthermore, our data indicate that endocytosis of the lipophorin receptors is not required to mediate the uptake of neutral lipids.These findings suggest a model where lipophorin receptors promote the extracellular lipolysis of lipophorins.This model is reminiscent of the lipolytic processing of triglyceride-rich lipoproteins that occurs at the mammalian capillary endothelium, suggesting an ancient role for LDLR-like proteins in this process.

View Article: PubMed Central - PubMed

Affiliation: Centro Andaluz de Biología del Desarrollo (CSIC-UPO), Universidad Pablo de Olavide, Sevilla, Spain.

ABSTRACT
Lipids are constantly shuttled through the body to redistribute energy and metabolites between sites of absorption, storage, and catabolism in a complex homeostatic equilibrium. In Drosophila, lipids are transported through the hemolymph in the form of lipoprotein particles, known as lipophorins. The mechanisms by which cells interact with circulating lipophorins and acquire their lipidic cargo are poorly understood. We have found that lipophorin receptor 1 and 2 (lpr1 and lpr2), two partially redundant genes belonging to the Low Density Lipoprotein Receptor (LDLR) family, are essential for the efficient uptake and accumulation of neutral lipids by oocytes and cells of the imaginal discs. Females lacking the lpr2 gene lay eggs with low lipid content and have reduced fertility, revealing a central role for lpr2 in mediating Drosophila vitellogenesis. lpr1 and lpr2 are transcribed into multiple isoforms. Interestingly, only a subset of these isoforms containing a particular LDLR type A module mediate neutral lipid uptake. Expression of these isoforms induces the extracellular stabilization of lipophorins. Furthermore, our data indicate that endocytosis of the lipophorin receptors is not required to mediate the uptake of neutral lipids. These findings suggest a model where lipophorin receptors promote the extracellular lipolysis of lipophorins. This model is reminiscent of the lipolytic processing of triglyceride-rich lipoproteins that occurs at the mammalian capillary endothelium, suggesting an ancient role for LDLR-like proteins in this process.

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Uptake of neutral lipids is mediated by a subset of lpr1 and lpr2 isoforms.(A–I) Wing imaginal discs of Df(3R)lpr1/2 genotype in which the indicated lpr1 and lpr2 isoforms and chimeras were overexpressed in the posterior compartment using the en-gal4 driver. The relevant protein domains of the overexpressed isoforms are depicted in the drawings, which follow the code shown in Figure 1. Neutral lipids are shown in green and in grey in a separate channel. The overexpressed Lpr1 and Lpr2 proteins were detected by immunostaining using an antibody that recognizes the HA tag except in panel D in which α-Lpr1 was used. Lpr2E (A), Lpr1H (C), Lpr1J (E), Lpr2F+LA1+NCN (F) and Lpr2F+LA1 (G) proteins rescued lipid uptake in the posterior compartment whereas Lpr2F (B), Lpr1D (D), Lpr2F+NCN (H) and Lpr2F+LA2 (I) did not. Note that the presence of LA-1 defines the isoform ability to rescue neutral lipid uptake. Scale bar: 100 µm. All panels are shown at the same magnification.
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pgen-1001297-g004: Uptake of neutral lipids is mediated by a subset of lpr1 and lpr2 isoforms.(A–I) Wing imaginal discs of Df(3R)lpr1/2 genotype in which the indicated lpr1 and lpr2 isoforms and chimeras were overexpressed in the posterior compartment using the en-gal4 driver. The relevant protein domains of the overexpressed isoforms are depicted in the drawings, which follow the code shown in Figure 1. Neutral lipids are shown in green and in grey in a separate channel. The overexpressed Lpr1 and Lpr2 proteins were detected by immunostaining using an antibody that recognizes the HA tag except in panel D in which α-Lpr1 was used. Lpr2E (A), Lpr1H (C), Lpr1J (E), Lpr2F+LA1+NCN (F) and Lpr2F+LA1 (G) proteins rescued lipid uptake in the posterior compartment whereas Lpr2F (B), Lpr1D (D), Lpr2F+NCN (H) and Lpr2F+LA2 (I) did not. Note that the presence of LA-1 defines the isoform ability to rescue neutral lipid uptake. Scale bar: 100 µm. All panels are shown at the same magnification.

Mentions: lpr1 and lpr2 are transcribed as multiple isoforms (Figure 1B), raising the issue of whether they share similar properties regarding lipid uptake. To answer this question, we first compared the isoforms transcribed from the corresponding distal versus proximal promoters. We generated two HA-tagged transgenes that allowed controlled expression of Lpr2F isoform (UAS-lpr2F) and Lpr2E isoform (UAS-lpr2E) and examined their ability to rescue lipid uptake in Df(3R)lpr1/2 animals. Expression of UAS-lpr2E in the posterior compartment of the wing imaginal disc driven by en-gal4 completely rescued lipid accumulation in that compartment (Figure 4A). In contrast, expression of UAS-lpr2F did not rescue lipid uptake (Figure 4B). In a similar assay, we found that whereas Lpr1H isoform rescued lipid uptake, expression of Lpr1D did not (Figure 4C, 4D). Equivalent results were obtained when we examined the role of these isoforms during vitellogenesis. Germ-line expression of UASp-lpr1J or UASp-lpr2E, driven byV32-gal4, rescued oogenesis and fertility of Df(3R)lpr1/2 females. The amount of lipid droplets accumulated in nurse cells was similar to the wild-type (Figure 5C, 5E, compare to wild-type in Figure 2G) and the number of degenerating egg chambers was dramatically reduced (Figure 5D, 5F and Figure S3B). In contrast, expression of UASp-lpr2F did not rescue fertility, lipid uptake nor egg chamber degeneration (Figure 5A, 5B).


Drosophila lipophorin receptors mediate the uptake of neutral lipids in oocytes and imaginal disc cells by an endocytosis-independent mechanism.

Parra-Peralbo E, Culi J - PLoS Genet. (2011)

Uptake of neutral lipids is mediated by a subset of lpr1 and lpr2 isoforms.(A–I) Wing imaginal discs of Df(3R)lpr1/2 genotype in which the indicated lpr1 and lpr2 isoforms and chimeras were overexpressed in the posterior compartment using the en-gal4 driver. The relevant protein domains of the overexpressed isoforms are depicted in the drawings, which follow the code shown in Figure 1. Neutral lipids are shown in green and in grey in a separate channel. The overexpressed Lpr1 and Lpr2 proteins were detected by immunostaining using an antibody that recognizes the HA tag except in panel D in which α-Lpr1 was used. Lpr2E (A), Lpr1H (C), Lpr1J (E), Lpr2F+LA1+NCN (F) and Lpr2F+LA1 (G) proteins rescued lipid uptake in the posterior compartment whereas Lpr2F (B), Lpr1D (D), Lpr2F+NCN (H) and Lpr2F+LA2 (I) did not. Note that the presence of LA-1 defines the isoform ability to rescue neutral lipid uptake. Scale bar: 100 µm. All panels are shown at the same magnification.
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC3037410&req=5

pgen-1001297-g004: Uptake of neutral lipids is mediated by a subset of lpr1 and lpr2 isoforms.(A–I) Wing imaginal discs of Df(3R)lpr1/2 genotype in which the indicated lpr1 and lpr2 isoforms and chimeras were overexpressed in the posterior compartment using the en-gal4 driver. The relevant protein domains of the overexpressed isoforms are depicted in the drawings, which follow the code shown in Figure 1. Neutral lipids are shown in green and in grey in a separate channel. The overexpressed Lpr1 and Lpr2 proteins were detected by immunostaining using an antibody that recognizes the HA tag except in panel D in which α-Lpr1 was used. Lpr2E (A), Lpr1H (C), Lpr1J (E), Lpr2F+LA1+NCN (F) and Lpr2F+LA1 (G) proteins rescued lipid uptake in the posterior compartment whereas Lpr2F (B), Lpr1D (D), Lpr2F+NCN (H) and Lpr2F+LA2 (I) did not. Note that the presence of LA-1 defines the isoform ability to rescue neutral lipid uptake. Scale bar: 100 µm. All panels are shown at the same magnification.
Mentions: lpr1 and lpr2 are transcribed as multiple isoforms (Figure 1B), raising the issue of whether they share similar properties regarding lipid uptake. To answer this question, we first compared the isoforms transcribed from the corresponding distal versus proximal promoters. We generated two HA-tagged transgenes that allowed controlled expression of Lpr2F isoform (UAS-lpr2F) and Lpr2E isoform (UAS-lpr2E) and examined their ability to rescue lipid uptake in Df(3R)lpr1/2 animals. Expression of UAS-lpr2E in the posterior compartment of the wing imaginal disc driven by en-gal4 completely rescued lipid accumulation in that compartment (Figure 4A). In contrast, expression of UAS-lpr2F did not rescue lipid uptake (Figure 4B). In a similar assay, we found that whereas Lpr1H isoform rescued lipid uptake, expression of Lpr1D did not (Figure 4C, 4D). Equivalent results were obtained when we examined the role of these isoforms during vitellogenesis. Germ-line expression of UASp-lpr1J or UASp-lpr2E, driven byV32-gal4, rescued oogenesis and fertility of Df(3R)lpr1/2 females. The amount of lipid droplets accumulated in nurse cells was similar to the wild-type (Figure 5C, 5E, compare to wild-type in Figure 2G) and the number of degenerating egg chambers was dramatically reduced (Figure 5D, 5F and Figure S3B). In contrast, expression of UASp-lpr2F did not rescue fertility, lipid uptake nor egg chamber degeneration (Figure 5A, 5B).

Bottom Line: Furthermore, our data indicate that endocytosis of the lipophorin receptors is not required to mediate the uptake of neutral lipids.These findings suggest a model where lipophorin receptors promote the extracellular lipolysis of lipophorins.This model is reminiscent of the lipolytic processing of triglyceride-rich lipoproteins that occurs at the mammalian capillary endothelium, suggesting an ancient role for LDLR-like proteins in this process.

View Article: PubMed Central - PubMed

Affiliation: Centro Andaluz de Biología del Desarrollo (CSIC-UPO), Universidad Pablo de Olavide, Sevilla, Spain.

ABSTRACT
Lipids are constantly shuttled through the body to redistribute energy and metabolites between sites of absorption, storage, and catabolism in a complex homeostatic equilibrium. In Drosophila, lipids are transported through the hemolymph in the form of lipoprotein particles, known as lipophorins. The mechanisms by which cells interact with circulating lipophorins and acquire their lipidic cargo are poorly understood. We have found that lipophorin receptor 1 and 2 (lpr1 and lpr2), two partially redundant genes belonging to the Low Density Lipoprotein Receptor (LDLR) family, are essential for the efficient uptake and accumulation of neutral lipids by oocytes and cells of the imaginal discs. Females lacking the lpr2 gene lay eggs with low lipid content and have reduced fertility, revealing a central role for lpr2 in mediating Drosophila vitellogenesis. lpr1 and lpr2 are transcribed into multiple isoforms. Interestingly, only a subset of these isoforms containing a particular LDLR type A module mediate neutral lipid uptake. Expression of these isoforms induces the extracellular stabilization of lipophorins. Furthermore, our data indicate that endocytosis of the lipophorin receptors is not required to mediate the uptake of neutral lipids. These findings suggest a model where lipophorin receptors promote the extracellular lipolysis of lipophorins. This model is reminiscent of the lipolytic processing of triglyceride-rich lipoproteins that occurs at the mammalian capillary endothelium, suggesting an ancient role for LDLR-like proteins in this process.

Show MeSH
Related in: MedlinePlus