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Specialized Cortex Glial Cells Accumulate Lipid Droplets in Drosophila melanogaster.

Kis V, Barti B, Lippai M, Sass M - PLoS ONE (2015)

Bottom Line: Although the central nervous system contains the highest relative amount and the largest number of different lipid species, neither the spatial nor the temporal distribution of LDs has been described.Superficial cortex glial cells, combined with subperineurial glial cells, express the Drosophila fatty acid binding protein (Dfabp), as we have demonstrated through light- and electron microscopic immunocytochemistry.To the best of our best knowledge this is the first study that describes LD neuroanatomy in the Drosophila larval brain.

View Article: PubMed Central - PubMed

Affiliation: Department of Anatomy, Cell and Developmental Biology, Eötvös Loránd University, Budapest, Hungary.

ABSTRACT
Lipid droplets (LDs) are common organelles of the majority of eukaryotic cell types. Their biological significance has been extensively studied in mammalian liver cells and white adipose tissue. Although the central nervous system contains the highest relative amount and the largest number of different lipid species, neither the spatial nor the temporal distribution of LDs has been described. In this study, we used the brain of the fruitfly, Drosophila melanogaster, to investigate the neuroanatomy of LDs. We demonstrated that LDs are exclusively localised in glial cells but not in neurons in the larval nervous system. We showed that the brain's LD pool, rather than being constant, changes dynamically during development and reaches its highest value at the beginning of metamorphosis. LDs are particularly enriched in cortex glial cells located close to the brain surface. These specialized superficial cortex glial cells contain the highest amount of LDs among glial cell types and encapsulate neuroblasts and their daughter cells. Superficial cortex glial cells, combined with subperineurial glial cells, express the Drosophila fatty acid binding protein (Dfabp), as we have demonstrated through light- and electron microscopic immunocytochemistry. To the best of our best knowledge this is the first study that describes LD neuroanatomy in the Drosophila larval brain.

No MeSH data available.


Related in: MedlinePlus

Representative electron micrographs of particular glial cell types.LDs are marked by arrows. (A) Perineurial (PG) and subperineurial cells (SPG) located on the surface of the brain. (B) A superficial glial cell located in the outer layer of the brain cortex, containing high amount of LDs. (C) A neuropil glia (NP) located at the cortex-neuropil boundary ensheating axons. (D) A deeper cortex glia (CG) found close to the cortex-neuropil boundary encapsulating a large peptiderg neuron (PN) and several other neurons (N) with its processes. No LDs seen in the cytoplasm of such a cortex glia. Note the presence of large clusters of neurosecretory vesicles (arrowheads) in the cytoplasm of PN. Scalebar: 2μm.
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pone.0131250.g003: Representative electron micrographs of particular glial cell types.LDs are marked by arrows. (A) Perineurial (PG) and subperineurial cells (SPG) located on the surface of the brain. (B) A superficial glial cell located in the outer layer of the brain cortex, containing high amount of LDs. (C) A neuropil glia (NP) located at the cortex-neuropil boundary ensheating axons. (D) A deeper cortex glia (CG) found close to the cortex-neuropil boundary encapsulating a large peptiderg neuron (PN) and several other neurons (N) with its processes. No LDs seen in the cytoplasm of such a cortex glia. Note the presence of large clusters of neurosecretory vesicles (arrowheads) in the cytoplasm of PN. Scalebar: 2μm.

Mentions: Since we found LDs exclusively in glial cells, we sought to determine whether LDs are distributed equally between glial cell types, or if each cell type has a different LD content. To answer this question, we generated genetically labelled single glial cells in which glial membranes were highlighted with myr-RFP and LD content was labeled with Lsd2-GFP (Fig 2A). Lsd2 (Lipid storage droplet 2) is the Drosophila orthologue of the mammalian perilipin2, a widely used marker of LDs [27, 28]. This double labeling technique allowed us to identify glial subtypes through their morphology, and to determine their LD content simultaneously in the same cell. Drosophila larval glial cells can be classified into four types: perineurial, subperineurial, cortex and neuropil glia [29]. The smallest are perineurial cells (PG,), filopodial shaped glial cells located at the surface of the brain. Subperineurial cells (SPG) are very large, flat shaped, polyploid cells (>150μm in width) located just below the perineurial layer. SPGs are connected to each other through septate junctions, through which they form the blood-brain barrier in the Drosophila brain [30,31]. Cortex glial cells (CGs) are multipolar, form a delicate network in the cortex of the brain, and insulate neuronal cell bodies with very thin processes. Neuropil glia (NP) has two subclasses: the ensheathing glia which has flattened cell bodies and lacks processes penetrating into the neuropil; and the reticular glia, which has many processes that invade the neuropil and encapsulate axon terminals and dendrites [20,24]. NP cell bodies can be located at the cortex-neuropil boundary or in the neuropil. Each glia type described above contained different amounts of LDs during the clonal analysis but the cortex glial cells located in the superficial region of the brain cortex accumulated significantly more LDs in comparison to the other types (Fig 2A and 2B). These lipid accumulating cortex glial cells had characteristic processes encapsulating unlabeled large areas, possibly cell bodies of neurons. Based on the location and the size of these unlabeled profiles, we propose that they could be neuroblasts, known to be located close to the brain surface. Interestingly, we found some cortex glial cells that contained no LDs (Fig 2A, Fig 3D). These cells had very thin filopodial processes and were located in the deeper regions of the brain cortex (S2 Fig). To validate our results obtained with single cell labeling, we also generated myr-RFP labeled clone cells on a genetic background where all the cortex glial membranes were highlighted with a GFP tagged version of the glial specific Na+,K+-ATPase—Nrv2 [32]. The morphology of cortex glial clones labeled this way was identical to the lipid accumulating cells observed in the superficial cortex during LD analysis (Fig 2C). We also compared the distribution of LDs with the pattern of specific glial subtypes and found that the signal of fluorescently labelled LDs (Lsd2-GFP expressed in glial cells) strongly overlapped with the pattern of cortex glia in whole mount larval brains (Fig 2D). We quantified this observation on EM sections where glial cell types can be identified through their soma location, the morphology of their processes, and the neural elements to which they are attached (Fig 2E). We found that cortex glial cells located close to the brain surface possessed the highest LD number/cell area ratio (11,8±4,6), while other glial types contained notably less LDs: PG (2,6±2,5), SPG (4,1±3,5), NP (2,1±1,7). For the represesentative EM images of each glial type listed above, see Fig 3. In the EM, we also found cells in the deeper cortex insulating neuronal cell bodies, containing no LDs (Fig 3D). These cells may represent a specific subclass of cortex glial cells. These data suggest that (superficial) cortex glial cells are the main storage depositories of LDs in the larval brain.


Specialized Cortex Glial Cells Accumulate Lipid Droplets in Drosophila melanogaster.

Kis V, Barti B, Lippai M, Sass M - PLoS ONE (2015)

Representative electron micrographs of particular glial cell types.LDs are marked by arrows. (A) Perineurial (PG) and subperineurial cells (SPG) located on the surface of the brain. (B) A superficial glial cell located in the outer layer of the brain cortex, containing high amount of LDs. (C) A neuropil glia (NP) located at the cortex-neuropil boundary ensheating axons. (D) A deeper cortex glia (CG) found close to the cortex-neuropil boundary encapsulating a large peptiderg neuron (PN) and several other neurons (N) with its processes. No LDs seen in the cytoplasm of such a cortex glia. Note the presence of large clusters of neurosecretory vesicles (arrowheads) in the cytoplasm of PN. Scalebar: 2μm.
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Related In: Results  -  Collection

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pone.0131250.g003: Representative electron micrographs of particular glial cell types.LDs are marked by arrows. (A) Perineurial (PG) and subperineurial cells (SPG) located on the surface of the brain. (B) A superficial glial cell located in the outer layer of the brain cortex, containing high amount of LDs. (C) A neuropil glia (NP) located at the cortex-neuropil boundary ensheating axons. (D) A deeper cortex glia (CG) found close to the cortex-neuropil boundary encapsulating a large peptiderg neuron (PN) and several other neurons (N) with its processes. No LDs seen in the cytoplasm of such a cortex glia. Note the presence of large clusters of neurosecretory vesicles (arrowheads) in the cytoplasm of PN. Scalebar: 2μm.
Mentions: Since we found LDs exclusively in glial cells, we sought to determine whether LDs are distributed equally between glial cell types, or if each cell type has a different LD content. To answer this question, we generated genetically labelled single glial cells in which glial membranes were highlighted with myr-RFP and LD content was labeled with Lsd2-GFP (Fig 2A). Lsd2 (Lipid storage droplet 2) is the Drosophila orthologue of the mammalian perilipin2, a widely used marker of LDs [27, 28]. This double labeling technique allowed us to identify glial subtypes through their morphology, and to determine their LD content simultaneously in the same cell. Drosophila larval glial cells can be classified into four types: perineurial, subperineurial, cortex and neuropil glia [29]. The smallest are perineurial cells (PG,), filopodial shaped glial cells located at the surface of the brain. Subperineurial cells (SPG) are very large, flat shaped, polyploid cells (>150μm in width) located just below the perineurial layer. SPGs are connected to each other through septate junctions, through which they form the blood-brain barrier in the Drosophila brain [30,31]. Cortex glial cells (CGs) are multipolar, form a delicate network in the cortex of the brain, and insulate neuronal cell bodies with very thin processes. Neuropil glia (NP) has two subclasses: the ensheathing glia which has flattened cell bodies and lacks processes penetrating into the neuropil; and the reticular glia, which has many processes that invade the neuropil and encapsulate axon terminals and dendrites [20,24]. NP cell bodies can be located at the cortex-neuropil boundary or in the neuropil. Each glia type described above contained different amounts of LDs during the clonal analysis but the cortex glial cells located in the superficial region of the brain cortex accumulated significantly more LDs in comparison to the other types (Fig 2A and 2B). These lipid accumulating cortex glial cells had characteristic processes encapsulating unlabeled large areas, possibly cell bodies of neurons. Based on the location and the size of these unlabeled profiles, we propose that they could be neuroblasts, known to be located close to the brain surface. Interestingly, we found some cortex glial cells that contained no LDs (Fig 2A, Fig 3D). These cells had very thin filopodial processes and were located in the deeper regions of the brain cortex (S2 Fig). To validate our results obtained with single cell labeling, we also generated myr-RFP labeled clone cells on a genetic background where all the cortex glial membranes were highlighted with a GFP tagged version of the glial specific Na+,K+-ATPase—Nrv2 [32]. The morphology of cortex glial clones labeled this way was identical to the lipid accumulating cells observed in the superficial cortex during LD analysis (Fig 2C). We also compared the distribution of LDs with the pattern of specific glial subtypes and found that the signal of fluorescently labelled LDs (Lsd2-GFP expressed in glial cells) strongly overlapped with the pattern of cortex glia in whole mount larval brains (Fig 2D). We quantified this observation on EM sections where glial cell types can be identified through their soma location, the morphology of their processes, and the neural elements to which they are attached (Fig 2E). We found that cortex glial cells located close to the brain surface possessed the highest LD number/cell area ratio (11,8±4,6), while other glial types contained notably less LDs: PG (2,6±2,5), SPG (4,1±3,5), NP (2,1±1,7). For the represesentative EM images of each glial type listed above, see Fig 3. In the EM, we also found cells in the deeper cortex insulating neuronal cell bodies, containing no LDs (Fig 3D). These cells may represent a specific subclass of cortex glial cells. These data suggest that (superficial) cortex glial cells are the main storage depositories of LDs in the larval brain.

Bottom Line: Although the central nervous system contains the highest relative amount and the largest number of different lipid species, neither the spatial nor the temporal distribution of LDs has been described.Superficial cortex glial cells, combined with subperineurial glial cells, express the Drosophila fatty acid binding protein (Dfabp), as we have demonstrated through light- and electron microscopic immunocytochemistry.To the best of our best knowledge this is the first study that describes LD neuroanatomy in the Drosophila larval brain.

View Article: PubMed Central - PubMed

Affiliation: Department of Anatomy, Cell and Developmental Biology, Eötvös Loránd University, Budapest, Hungary.

ABSTRACT
Lipid droplets (LDs) are common organelles of the majority of eukaryotic cell types. Their biological significance has been extensively studied in mammalian liver cells and white adipose tissue. Although the central nervous system contains the highest relative amount and the largest number of different lipid species, neither the spatial nor the temporal distribution of LDs has been described. In this study, we used the brain of the fruitfly, Drosophila melanogaster, to investigate the neuroanatomy of LDs. We demonstrated that LDs are exclusively localised in glial cells but not in neurons in the larval nervous system. We showed that the brain's LD pool, rather than being constant, changes dynamically during development and reaches its highest value at the beginning of metamorphosis. LDs are particularly enriched in cortex glial cells located close to the brain surface. These specialized superficial cortex glial cells contain the highest amount of LDs among glial cell types and encapsulate neuroblasts and their daughter cells. Superficial cortex glial cells, combined with subperineurial glial cells, express the Drosophila fatty acid binding protein (Dfabp), as we have demonstrated through light- and electron microscopic immunocytochemistry. To the best of our best knowledge this is the first study that describes LD neuroanatomy in the Drosophila larval brain.

No MeSH data available.


Related in: MedlinePlus