To examine the presence of theDrosophilaprotein Glutactin in Clone 8+ cells anti-Dm-glutactin antibody was kindly provided by John Fessler.
Anti-α-tubulin antibody which decorates the microtubule cytoskeleton and phalloidin, which stains for the f-actin cytoskeleton were also employed in order to assess the association of glutactin with the cytoskeleton.
7.2.1:Immunohistochemistry
Sterile coverslips were placed in Petri dishes and Clone 8+ cells were seeded at 3.5 x 106per 5ml in CSM or in CSM with ecdysone added to give a final
concentration of 10ng/ml and incubated at 25°C. After 24 hours the medium was removed and the cells were rinsed carefully with PBS. As the majority of OCl.8+ cells and ZfeCl.8+ cells were suspended in the medium, rinsing with PBS was not always carried out and the medium was removed as carefully as possible.
7.2.2:Glutactin and f-actin
Staining for filamentous actin was performed with phalloidin conjugated to a red-fluorescent dye, Alexa Fluor 568 phalloidin (Molecular probes). In order to preserve the actin filaments, cells were fixed with 4% paraformaldehyde for
20 minutes (this procedure was carried out in a fume hood) followed by gentle rinsing in PBS four x five minutes. 0.1% Glycine in PBS was added to the cells for 5 minutes in order to quench any aldehyde fluorescence. Cells were rinsed in PBS three x 5 minutes and incubated with 0.1% Triton-X in PBS for 1 minute to permeabilise the cells. This was followed by rinsing in PBS three x5 minutes. The cells were then incubated for 1 hour 20 minutes at room temperature with phalloidin, then rinsed gently 4 x 5 minutes in PBS. This was followed by incubation for 2 hours at 25°C with primary antibody, rabbit anti-Dm-Glutactin used at 1:1500 dilutedin PBS. Cells were rinsed gently 4 x 5 minutes in PBS followed by incubation withsecondary antibodyanti-rabbit IgG conjugated withfluorescein 1:500 for 2 hrs at 25°C, after which cells were rinsed gently 4 x 5 minutes in PBS.
In some preparations cells were also incubated for 2 hours at 25°C with monoclonal mouse anti-α-tubulin (Sigma T9026) diluted 1:1000 in PBS. Cells were rinsed with gentle agitation 4 x 5 minutes in PBS and incubated withsecondary antibodyanti-mouseIgG conjugated withfluorescein, Texas red or AMCA, 1:250 for 2 hrs at 25°C.
7.2.3:
α
-tubulin and GlutactinCells were fixed in ice-cold methanol, 0°C, for 4 minutes and rinsed gently in PBS three x 5 minutes. They were then incubated for 2 hours at 25°C with primary antibody rabbit anti-Dm-glutactin, used at 1:1500 diluted in PBS, followed by rinsing with gentle agitation 4 x 5 minutes in PBS. Incubation of the cells with secondary antibody anti-rabbit IgG conjugated with fluorescein or Texas red 1:500 was carried out for 2 hrs at 25°C. Following this cells were rinsed with agitation 4 x 5 minutes in PBS and incubated for 2 hours at 25°C
with monoclonal mouse anti-α-tubulin antibody diluted 1:1000 in PBS. Cells were rinsed with gentle agitation 4 x 5 minutes in PBS and incubated with secondary antibody anti-mouse IgG conjugated fluorescein or Texas red 1:250 for 2 hrs at 25°C. Finally the cells were rinsed with gentle agitation for at least 4 x 5 minutes in PBS.
Controls: Cells were fixed only to monitor autofluorescence
Cells were incubated with secondary antibodies to assess non-specific binding of the secondary
Anti-actin antibody to confirm the presence of f-actin.
Nuclear DNA was stained by incubation with 0.125µg/ml DAPI solution for 10 minutes at room temperature, followed by rinsing in PBS, with gentle agitation, 4 x 5 minutes.
Cells were kept covered at all times and the procedures were carried out in a darkened room. Primary and secondary antibody preparations were
centrifuged for 20 seconds and the supernatant was used. To confirm the presence of glutactin in the wing disc, third instar larvae were dissected and anti-Dm-glutactin antibody was used to stain the disc tissue (figure 7.1, pg 105).
7.2.4:Microscopy
Coverslips were mounted in Vectamount (Vectashield) and left overnight in the refrigerator and viewed the next day. Immunofluorescence microscopy was carried out and the images were contrast enhanced using Openlab (v2.1.5). In an aid to examine areas of colocalisation, images were sharpened using Adobe Photoshop (v5.5). The letters l and o were not used to label the images.
To confirm the presence of glutactin in the wing disc, third instar larvae were dissected and anti-Dm-glutactin antibody was used to stain the disc tissue (figure 7.1).
Figure 7.1: Wild-type third instar imaginal wing disc A DIC image showing an imaginal disc
dissected from a late third instar larva
shortly after culture initiation. The process of evagination has begun and the disc is
beginning to curl upwards. The folds of the disc proper which forms the adult
appendage can be seen. Dissected larval tissue is also present.
d, imaginal disc; f, folds; l, larval tissue Scale bar represents:200µm
Figure 7.2: 3rd instar wing imaginal disc, anti-Dm-Glutactin
Immunofluorescence image showing the imaginal disc in figure 7.1 after antibody staining with the protein anti-Dm-Glutactin. Glutactin surrounds the whole wing disc and appears like an amorphous layer in some areas. Another imaginal disc is present which has also been positively stained. The larval tissue which can be seen in figure 7.1 has not stained for glutactin.
g, glutactin
7.3:Results
The presence of the protein,Drosophilaglutactin, was detected in all Clone 8+ cell lines, but not in all cells probed with anti-Drosophila-glutactin antibody.
7.3.1:24 hours after culture initiation
24 hours after culture initiation glutactin was detected in the cytoplasm of single YCl.8+, Cl.8R and ZfeCl.8+ cells which had formed an attachment to the substrate revealing a punctate expression within all cells (figure 7a(i)).
Visualisation of the glutactin by immunofluorescence microscopy indicated that glutactin was expressed mainly in the uppermost region of the cell as opposed to the lower region attached to the substrate. Small vesicles of secreted glutactin were visualised, the immunofluorescence detected was brighter in these vesicles (figure 7a(i)). Long and short processes were
observed emanating from cells in contact with the substrate. There appeared to be a thinner layer of cell membrane or secreted extracellular matrix
associated with some cells and the substrate (figure 7a(ii)). Secreted glutactin vesicles were detected along cell processes and close to areas where the cells had aggregated and in association with the thin layers of putative
extracellular matrix (figures 7b, 7c(i,ii)). Secreted glutactin was also detected on the apical cell surface in the majority of YCl.8+ and Cl.8R cells (figure 7d(i,ii,iii)).
7.3.2:The cytoskeleton
At 24 hours after culture initiation anti
-α
-tubulin antibody which decorates the microtubule cytoskeleton and phalloidin which detects f-actin for the actincytoskeleton were both detected in the cytoplasm of all Clone 8+ cells.
α
- tubulin was detected underlying the membrane of cells which had formed an attachment to the substrate. An amorphous layer of glutactin was alsodetected surrounding some cells.
α
-tubulin did not colocalise withglutactin although close association was observed (figure 7e(i,ii,)). f-actin was detected close to the nucleus at the uppermost part of Clone 8+ cells and colocalisation of f-actin with glutactin was detected (image overlay of glutactin and f-actin expression produced orange and/or yellow immunofluorescence in areas where glutactin and f-actin were colocalised) (figure 7f(i,ii,)).Glutactin was detected in the cytoplasm and as a secreted molecule in a small number of ZfeCl.8+ cells which had formed an attachment to the substrate as in the YCl8+ and Cl.8R cells described above (figure 7a(i)). Cells of this type were not observed in OCl.8+ preparations. Large membrane bound vesicles were observed closely associated to the cell membrane in the majority of ZfeCl.8+ cells attached to the substrate. Glutactin and f-actin were detected in association with these large vesicles (figures 7g (i,ii,iii)). Cells of this type were not observed in OCl.8+ preparations. Large vesicles which stained for
glutactin and f-actin were also detected at the basolateral region of cells
attached to the substrate (these were also present in the YCl.8+ and Cl.8R cells attached to the substrate but not shown). No secreted vesicles of glutactin were observed.
α
-tubulin was prominent in all rounded cells which had been fixed whilst undergoing mitosis and a greater proportion of cells were observedundergoing cell division in the OCl.8+ and ZfeCl.8+ preparations (figure 7h). Glutactin was detected in the cytoplasm and surrounding YCl.8+ and Cl.8R cells. Colocalisation with f-actin was observed in the cytoplasm (figure 7i).
Glutactin was detected in the cytoplasm of OCl.8+ and ZfeCl.8+ round cells which had been suspended in the medium (no attachments to the substrate were observed in these cells). The intensity of the glutactin was variable, some cells contained an abundance of cytoplasmic glutactin whereas glutactin was not detected at all in some cells (figure 7j(i,ii)).
α
-tubulin was detected just beneath the plasma membrane of these round cells and did not colocalise with glutactin.Four distinct OCl.8+ rounded cell types were associated with glutactin and f- actin expression. Firstly cells in which glutactin was located throughout the cytoplasm and f-actin was located just beneath the plasma membrane, colocalisation was observed (figure 7j(i)). In others, glutactin was located at one pole of the cell and f-actin was located at the other and colocalisation was observed in the mid region (data not shown). In cells where glutactin was located throughout the cytoplasm no f-actin was observed (figure 7j(i)). Finally cells were found where f-actin was located throughout the cytoplasm, but no glutactin was observed (figure 7j(ii)).
7.3.3:Filopodia and cell aggregates
Glutactin was evident between cell-cell contacts of all round cell aggregates. DIC microscopy revealed that these cells were connected by processes (figures 7k(i,ii)). In the Cl.8R cells longer, thicker processes (filopodia) were present. These processes stained intensely with the anti-glutactin antibody. The
processes appeared to emanate from single cells, were conspicuous at cell-cell contacts, and connected groups of cells (figures 7m(i,ii)). After 24 hours large aggregates of cells attached to the substrate were only found in the YCl.8+ and Cl.8R cell preparations (figures 7n(i,ii)). Glutactin enveloped the cells and was present as an amorphous deposit. Glutactin colocalised with f-actin around
the cell membrane (figure 7n(ii)). Aggregates were also connected by processes which stained for f-actin (figure 7p).
7.3.4:Day 5 after culture initiation
By day five after culture initiation, YCl.8+ and Cl.8R cells had formed the aggregates of cells normally associated with YCl.8+ at this time (Chapter 5). Examination of the cells by DIC microscopy revealed processes underlying layers, on the tops and between cells. Cytoplasmic and secreted glutactin was detected in cells and along cell processes (figures 7q(ii)). Glutactin was
colocalised with f-actin along cell processes (figures 7q(iii). Many cells
appeared to be pentagonal in shape when visualised with f-actin (figure 7r(i)). An abundance of glutactin vesicles were observed secreted from Cl.8R
covering the tops of cells (figures 7r(ii)).
After five days glutactin was detected completely surrounding the ZfeCl.8+ suspended cells (figure 7s(i,ii)). Colocalisation of glutactin with f-actin andα- tubulin was not observed. High background staining of glutactin was
observed due to fixation of residual medium. OCl.8+ cell aggregates were not found in cell preparations.
7.3.5: After ecdysone treatment
After ecdysone treatment glutactin and f-actin were detected in small
membrane vesicles which were present in the extracellular space, fixed to the coverslip, in YCl.8+ fixed preparations (figure 7t(i)). Membrane bound
vesicles of up to 2μm in diameter were also observed in close association with the YCl.8+ cells where glutactin was detected (figures 7t(ii,iii,iv)). The vesicles appeared to be in close association with cells via processes and at times were
attached to the cell membrane by stalk-like protruberances or fine cytoplasmic threads which appeared to be budding off (figure 7tiii).
There were no apparent changes in the expression of glutactin in the Cl.8R cells.
After ecdysone treatment glutactin was detected in a large number of
membrane bound vesicles in ZfeCl.8+ cells. Vesicles were detected within the cell cytoplasm and as secreted vesicles in close association with the cell membrane (figure 7u(i)). Staining for f-actin and glutactin showed
colocalisation in these vesicles. Further examination showed that the vesicles were still bound to the cell membrane (figure 7u(ii,iii,iv)). No vesicles were observed in the extracellular space fixed to the coverslip.
In OCl.8+ round cells one large area of glutactin was detected in the cell cytoplasm. Many of the OCl.8+ cells appeared to be damaged (figure 7v(i)). The extruded contents of the OCl8+ cell cytoplasm stained intensely for glutactin and f-actin. Small vesicles of glutactin were surrounded by the f-actin and glutactin (figure 7v(ii)).