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10 ZBED4, A Novel Retinal Protein Expressed in Cones and Müller Cells

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Sections of human retina used for in situ hybridization with a ZBED4 antisense riboprobe showed a positive signal in the inner segment of cone photoreceptor cells as expected. No signal was seen after hybridization with the control ZBED4 sense riboprobe (data not shown).

10.2.3ZBED4 is Expressed Both in Nuclei and Cytoplasm of Human Cones

Immunohistochemistry studies were performed on sections of different donor human retinas and also on sections of cow retina using the polyclonal antibody generated against a ZBED4 N-terminus synthetic peptide. ZBED4 is present in the nucleus and in the inner segment of cones. The protein can also be observed in the pedicle of cones and in the innermost retinal layer (Fig. 10.3a). ZBED4 is not detectable in the outer segments. An oblique section of human retina (Fig. 10.3b) clearly shows the presence of ZBED4 in the cytoplasm of the inner segments. The same localization of ZBED4 can be seen in cow retina (Fig. 10.3c). Sections stained with pre-immune serum and with antibody against ZBED4 that had been pre-absorbed with the peptide used to generate it showed no positive signal.

10.2.3.1 Human ZBED4 is Also Expressed in Müller Cells Endfeet

In order to determine whether the anti-ZBED4 staining of the innermost retinal cell layer was specific for ganglion cells or the endfeet of Müller cells, we used markers for these cell types in double labeling experiments. Only the antibody against vimentin, a Müller glial cell marker, showed co-localization with ZBED4 at the Müller cell endfeet.

10.2.4Human ZBED4 is Distributed Between Nuclear and Cytoplasmic Retinal Fractions

Cellular fractionation of human retinas was performed to separate nuclear and cytoplasmic extracts, followed by SDS-PAGE of the samples and immunoblotting. Anti ZBED4 polyclonal antibody specifically identified a protein with an apparent molecular mass of 135-kDa in both the nuclear and cytosolic extracts of human retina.

10.2.5Subcellular Localization of ZBED4 in Stably Transfected Cells

To study the subcellular localization of ZBED4, its complete coding region was subcloned into the mammalian expression vector, pcDNA4/HisMax, tagged at the

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Fig. 10.3 Localization of the ZBED4 protein in human and bovine retinas; Human and bovine retinal sections were double-stained with N-terminus ZBED4 antibody followed by FITC-conjugated secondary antibody, rhodamine-conjugated PNA and DAPI. Individual images were obtained for the staining of nuclei (DAPI, left panels) and anti-ZBED4 (right panels) with the use of appropriate filters. (a) Human retinal section showing staining of cone nuclei (arrows) and cone inner segments (arrowheads). Note also the anti-ZBED4 staining of the innermost layer of the retina and of cone pedicles (open arrowhead). Magnification: 400X. (b) An obliquely cut retinal section shows the cone inner segment localization of ZBED4 (arrows). (c) Bovine retinal section, cone inner segment (arrowheads) are stained and to a lesser degree the endfeet of Müller cells. Magnification: 600X. OS, outer segments; IS, inner segments; ONL, outer nuclear layer; OPL, outer plexiform layer; INL, inner nuclear layer; IPL, inner plexiform layer; GCL, ganglion cell layer

10 ZBED4, A Novel Retinal Protein Expressed in Cones and Müller Cells

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N-terminus with both Xpress and poly-histidine (6-His) epitopes. HEK293 cells were stably transfected with this construct and the expressed fusion protein was double immunostained using an Anti-Xpress-FITC mouse monoclonal antibody and a rabbit polyclonal peptide antibody (aa 8–25) against ZBED4. Using fluorescence microscopy, the fusion protein was localized to the nuclei of transfected cells (Fig. 10.4) by both antibodies.

Fig. 10.4 Intracellular distribution of the ZBED4 protein in HEK293 cells stably transfected with a ZBED4 expression vector; Cells were double stained with antibodies against ZBED4 and Xpress, and with DAPI for localization of nuclei. The images were obtained by fluorescence microscopy. Both antibodies detected nuclear localization of expressed ZBED4

10.2.6Purification of His-Tagged ZBED4 and Its Dimerization In Vivo

A nickel column was used to purify the His-tagged ZBED4 from protein extracts corresponding to the nuclear fraction of the stably transfected HEK293 cells. Protein extracts from untransfected HEK 293 cells, stably transfected HEK 293 cells, and the purified His-tagged ZBED4 were then separated by SDS-PAGE on an inverted gradient gel. The presence of both the endogenous (135 kDa) and recombinant (139 kDa) ZBED4 proteins on the Western blot of the purified sample from the nickel column suggests self-association of these endogenous and recombinant ZBED4 proteins. The hATC domain of ZBED4 may be responsible for this dimerization. This is supported by studies that have shown that the conserved hydrophobic amino acids in the hATC domain of proteins function in self association, an essential feature required for nuclear accumulation (seen as puncta) and DNA binding (Yamashita et al. 2007).

10.2.7Mass Spectrometry Identifies Putative Proteins Interacting with ZBED4

The enriched recombinant ZBED4 protein was subjected to SDS-PAGE on an inverted gradient gel and stained with SYPRO Ruby. The destained area of the gel that corresponded to the ZBED4 band on the Western blot was excised,