A monoclonal antibody raised against bacterially expressed MPV17 sequences shows peroxisomal, endosomal and lysosomal localisation in U2OS cells

Abstract

Recessive mutations in the MPV17 gene cause mitochondrial DNA depletion syndrome, a fatal infantile genetic liver disease in humans. Loss of function in mice leads to glomerulosclerosis and sensineural deafness accompanied with mitochondrial DNA depletion. Mutations in the yeast homolog Sym1, and in the zebra fish homolog tra cause interesting, but not obviously related phenotypes, although the human gene can complement the yeast Sym1 mutation. The MPV17 protein is a hydrophobic membrane protein of 176 amino acids and unknown function. Initially localised in murine peroxisomes, it was later reported to be a mitochondrial inner membrane protein in humans and in yeast. To resolve this contradiction we tested two new mouse monoclonal antibodies directed against the human MPV17 protein in Western blots and immunohistochemistry on human U2OS cells. One of these monoclonal antibodies showed specific reactivity to a protein of 20 kD absent in MPV17 negative mouse cells. Immunofluorescence studies revealed colocalisation with peroxisomal, endosomal and lysosomal markers, but not with mitochondria. This data reveal a novel connection between a possible peroxisomal/endosomal/lysosomal function and mitochondrial DNA depletion.

Fig. 1

Western blot immunoreactivity of the polyclonal anti-MPV17 antibody Ab 93374 (left) and the monoclonal anti- human MPV17 antibodies 6F5 and 5D2 on extracts of human and murine cells of different MPV17 genotypes. Left extracts from human U2OS cells, untransfected or transfected with the MPV17 expression clone SC118652 (origene), were probed with the antibody ab 93374 (abcam). Center untransfected U2OS cells with MPV17 +/+ endogenous genotype probed with the monoclonal antibodies 6F5 and 5D2. Right Murine embryo fibroblast (MEF) cells of MPV17 +/+ or MPV17-/- genotype probed with the monoclonal antibodies 6F5 and 5D2. Indicated marker sizes were transferred from Ponceau stained marker bands run in parallel. Loading was controlled as indicated by reaction of the filters with an anti-ß-actin antibody. 10 μg of protein were separated on SDS gels and analysed by Western blot according to [16] (left panel), and Kinkley et al. [10] respectively

Fig. 2

Ab93374 (Abcam) recognizes MPV17 specific structures in U2OS cells transiently transfected with an MPV17 expression construct in immunofluorescence analysis. Ab93374 signal (green) is largely missing in nontransfected cells. The immunoreactivity does colocalise with peroxisomal (anti-catalase, mouse polyclonal, Abcam ab88650) and mitochondrial (anti-complex IV mitochondrial subunit I, mouse monoclonal, Invitrogen 459600) markers (red). Immunofluorescence (IF) method was according to Pircher et al. [16] using a MicroRadiance confocal scanning system (Bio-Rad) in combination with a Zeiss Axiophot microscope. Colocalisation analysis was performed using the Fiji software of Image J and is illustrated by white dots

Fig. 3

In U2OS cells, 5D2 antibody detects a punctate pattern colocalising with a peroxisomal (PMP70) but not with a mitochondrial (MitoTracker) marker. Top Rabbit anti PMP70 antibody (decorated green) was a gift from W. Just, Heidelberg. Secondary antibodies: Alexa Fluor 555 anti-mouse and Alexa Fluor 488 anti-rabbit. Bottom Mitotracker 7510 (Invitrogen) was used as recommended by the supplier. Secondary antibody: Alexa 488 anti-mouse. IF method and microscopy according to Kinkley et al. [10] using a Zeiss LSM confocal microscope. Mathematical colocalisation analysis was performed using the Fiji software of Image J and is illustrated by white dots

Fig. 4

5D2 colocalisation with a marker of early endosomes (EEA1) and lysosomes (LAMP1) in U2OS cells. Top Partial colocalisation of 5D2 with EEA1 (rabbit anti-human). Bottom colocalisation of 5D2 with LAMP1 in U2OS cells. The IF method, secondary antibodies, and Image J analysis were used as described in the legend of Fig. 3

Fig. 5

Colocalisation of 5D2 and LAMP1 mediated immunofluorescence in a U2OS single cell. IF and analysis of 5D2 and Colocalisation of 5D2 with LAMP1 in U2OS cells. The IF method, secondary antibodies, and Image J analysis were used as described in the legend of Fig. 3. In addition a merge of the two immunofluorescence pictures is depicted