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Aberrant lysosomal carbohydrate storage accompanies endocytic defects and neurodegeneration in Drosophila benchwarmer.

Dermaut B, Norga KK, Kania A, Verstreken P, Pan H, Zhou Y, Callaerts P, Bellen HJ - J. Cell Biol. (2005)

Bottom Line: Here, we report that loss of Drosophila benchwarmer (bnch), a predicted lysosomal sugar carrier, leads to carbohydrate storage in yolk spheres during oogenesis and results in widespread accumulation of enlarged lysosomal and late endosomal inclusions.Finally, we find that loss of bnch strongly enhances tau neurotoxicity in a dose-dependent manner.We hypothesize that, in bnch, defective lysosomal carbohydrate efflux leads to endocytic defects with functional consequences in synaptic strength, neuronal viability, and tau neurotoxicity.

View Article: PubMed Central - PubMed

Affiliation: Department of Molecular and Human Genetics, Baylor College of Medicine, Houston, TX 77030, USA.

ABSTRACT
Lysosomal storage is the most common cause of neurodegenerative brain disease in preadulthood. However, the underlying cellular mechanisms that lead to neuronal dysfunction are unknown. Here, we report that loss of Drosophila benchwarmer (bnch), a predicted lysosomal sugar carrier, leads to carbohydrate storage in yolk spheres during oogenesis and results in widespread accumulation of enlarged lysosomal and late endosomal inclusions. At the bnch larval neuromuscular junction, we observe similar inclusions and find defects in synaptic vesicle recycling at the level of endocytosis. In addition, loss of bnch slows endosome-to-lysosome trafficking in larval garland cells. In adult bnch flies, we observe age-dependent synaptic dysfunction and neuronal degeneration. Finally, we find that loss of bnch strongly enhances tau neurotoxicity in a dose-dependent manner. We hypothesize that, in bnch, defective lysosomal carbohydrate efflux leads to endocytic defects with functional consequences in synaptic strength, neuronal viability, and tau neurotoxicity.

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Endosome-to-lysosome trafficking is reduced in bnch mutant garland cells. Endocytosed A-avidin is labeled in green and Lysotracker in red. After 20 min, strong colabeling of avidin positive endosomes and Lysotracker was observed in wild-type cells (A–C), whereas in bnch mutant cells the vast majority of A-avidin positive endosomal compartments was detected outside Lysotracker-positive compartments (D–F). After 90 min A-avidin– and Lysotracker-positive compartments nearly completely overlapped in both wild-type (G–I) and bnch cells (K–M).
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fig6: Endosome-to-lysosome trafficking is reduced in bnch mutant garland cells. Endocytosed A-avidin is labeled in green and Lysotracker in red. After 20 min, strong colabeling of avidin positive endosomes and Lysotracker was observed in wild-type cells (A–C), whereas in bnch mutant cells the vast majority of A-avidin positive endosomal compartments was detected outside Lysotracker-positive compartments (D–F). After 90 min A-avidin– and Lysotracker-positive compartments nearly completely overlapped in both wild-type (G–I) and bnch cells (K–M).

Mentions: Our ultrastructural analysis in the visual system reveals a dramatic accumulation of abnormal inclusions suggestive of a primary lysosomal defect. Moreover, the accumulation of MVBs in mutant photoreceptors is consistent with a decreased rate of endosome-to-lysosome trafficking, and at bnch mutant neuromuscular synapses we find defects consistent with impaired synaptic vesicle recycling at the level of endocytosis. To further test if bnch mutations affect dynamic aspects of endocytosis, we performed endocytic tracer uptake experiments in live larval garland cells (Fig. 6). Garland cells are similar to nephrocytes, have a rapid rate of fluid phase endocytosis, and are an established system to study endocytic dynamics (Kosaka and Ikeda, 1983). Endocytic uptake of Alexa 488–conjugated avidin (A-avidin) in garland cells labeled with Lysotracker was monitored over time. Although strong colabeling of A-avidin–positive endosomes and Lysotracker was already observed after 20 min in wild-type garland cells, A-avidin–positive endosomes were still easily detectable outside the Lysotracker-positive compartments in bnch mutant cells. After 90 min, A-avidin– and Lysotracker-positive compartments nearly completely overlapped in both bnch and wild-type cells. These results suggest that, although endosome-to-lysosome trafficking is not inhibited, the rate of this process is reduced in bnch-deficient larval garland cells, in agreement with the previous observations.


Aberrant lysosomal carbohydrate storage accompanies endocytic defects and neurodegeneration in Drosophila benchwarmer.

Dermaut B, Norga KK, Kania A, Verstreken P, Pan H, Zhou Y, Callaerts P, Bellen HJ - J. Cell Biol. (2005)

Endosome-to-lysosome trafficking is reduced in bnch mutant garland cells. Endocytosed A-avidin is labeled in green and Lysotracker in red. After 20 min, strong colabeling of avidin positive endosomes and Lysotracker was observed in wild-type cells (A–C), whereas in bnch mutant cells the vast majority of A-avidin positive endosomal compartments was detected outside Lysotracker-positive compartments (D–F). After 90 min A-avidin– and Lysotracker-positive compartments nearly completely overlapped in both wild-type (G–I) and bnch cells (K–M).
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Related In: Results  -  Collection

Show All Figures
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fig6: Endosome-to-lysosome trafficking is reduced in bnch mutant garland cells. Endocytosed A-avidin is labeled in green and Lysotracker in red. After 20 min, strong colabeling of avidin positive endosomes and Lysotracker was observed in wild-type cells (A–C), whereas in bnch mutant cells the vast majority of A-avidin positive endosomal compartments was detected outside Lysotracker-positive compartments (D–F). After 90 min A-avidin– and Lysotracker-positive compartments nearly completely overlapped in both wild-type (G–I) and bnch cells (K–M).
Mentions: Our ultrastructural analysis in the visual system reveals a dramatic accumulation of abnormal inclusions suggestive of a primary lysosomal defect. Moreover, the accumulation of MVBs in mutant photoreceptors is consistent with a decreased rate of endosome-to-lysosome trafficking, and at bnch mutant neuromuscular synapses we find defects consistent with impaired synaptic vesicle recycling at the level of endocytosis. To further test if bnch mutations affect dynamic aspects of endocytosis, we performed endocytic tracer uptake experiments in live larval garland cells (Fig. 6). Garland cells are similar to nephrocytes, have a rapid rate of fluid phase endocytosis, and are an established system to study endocytic dynamics (Kosaka and Ikeda, 1983). Endocytic uptake of Alexa 488–conjugated avidin (A-avidin) in garland cells labeled with Lysotracker was monitored over time. Although strong colabeling of A-avidin–positive endosomes and Lysotracker was already observed after 20 min in wild-type garland cells, A-avidin–positive endosomes were still easily detectable outside the Lysotracker-positive compartments in bnch mutant cells. After 90 min, A-avidin– and Lysotracker-positive compartments nearly completely overlapped in both bnch and wild-type cells. These results suggest that, although endosome-to-lysosome trafficking is not inhibited, the rate of this process is reduced in bnch-deficient larval garland cells, in agreement with the previous observations.

Bottom Line: Here, we report that loss of Drosophila benchwarmer (bnch), a predicted lysosomal sugar carrier, leads to carbohydrate storage in yolk spheres during oogenesis and results in widespread accumulation of enlarged lysosomal and late endosomal inclusions.Finally, we find that loss of bnch strongly enhances tau neurotoxicity in a dose-dependent manner.We hypothesize that, in bnch, defective lysosomal carbohydrate efflux leads to endocytic defects with functional consequences in synaptic strength, neuronal viability, and tau neurotoxicity.

View Article: PubMed Central - PubMed

Affiliation: Department of Molecular and Human Genetics, Baylor College of Medicine, Houston, TX 77030, USA.

ABSTRACT
Lysosomal storage is the most common cause of neurodegenerative brain disease in preadulthood. However, the underlying cellular mechanisms that lead to neuronal dysfunction are unknown. Here, we report that loss of Drosophila benchwarmer (bnch), a predicted lysosomal sugar carrier, leads to carbohydrate storage in yolk spheres during oogenesis and results in widespread accumulation of enlarged lysosomal and late endosomal inclusions. At the bnch larval neuromuscular junction, we observe similar inclusions and find defects in synaptic vesicle recycling at the level of endocytosis. In addition, loss of bnch slows endosome-to-lysosome trafficking in larval garland cells. In adult bnch flies, we observe age-dependent synaptic dysfunction and neuronal degeneration. Finally, we find that loss of bnch strongly enhances tau neurotoxicity in a dose-dependent manner. We hypothesize that, in bnch, defective lysosomal carbohydrate efflux leads to endocytic defects with functional consequences in synaptic strength, neuronal viability, and tau neurotoxicity.

Show MeSH
Related in: MedlinePlus