KNOCK ‐ OUT IN CELL LINES VIA TALEN TECHNOLOGY
III.1. C HARACTERIZATION OF THE HUMAN A LDO ‐ K ETO
III.1.5. D ETECTION OF ENDOGENOUS AKR1B15 ISOFORMS
Chapter III.1.1.3 demonstrated that AKR1B15 is expressed on RNA level in distinct human tissues as well as in the BeWo cell line and the SGBS cell strain. In addition, the subcellular localization studies showed that AKR1B15.1 is a mitochondrial protein whereas AKR1B15.2 is a cytosolic one [III.1.3]. To analyze whether the RNA is translated into protein in vivo and to verify the mitochondrial localization of AKR1B15.1 in an immunocytochemical independent manner, different approaches were performed.
WE S T E R N B L O T T I N G W I T H H U M A N T O T A L T I S S U E A ND C E L L L YS A T E S In the first instance, the abundance of AKR1B15 isoforms in different total protein tissues lysates (adipose tissue, placenta, prostate, testis, skeletal muscle, and thymus) as well as total cell lysates (BeWo and SGBS) on protein level was analyzed by Western blotting. For this, the monoclonal rat‐anti‐AKR1B15 clone 9A5 and mouse‐anti‐AKR1B15.2 clone 29D4 antibody supernatants were used as primary antibodies.
The Western blots, performed with the monoclonal rat‐anti‐AKR1B15 clone 9A5 and mouse‐
anti‐AKR1B15.2 clone 29D4 primary antibodies, revealed no bands via ECL detection [data not shown] but showed some more or less intense bands via the more sensitive infrared fluorescence (IR) detection, using the Licor technology [Figure III‐20]. Exemplary results gained from Western blots with total tissue and cell lysates using IR‐dye labeled secondary antibodies are shown in Figure III‐20.
Although AKR1B15.1 and AKR1B15.2 were easily detectable when transiently overexpressed in HEK‐293 cells (AKR1B15.1/15.2) by using the monoclonal rat‐anti‐AKR1B15 clone 9A5 and goat‐anti‐rat‐AlexaFluor 790 antibody pair, either no protein bands or protein bands with a divergent molecular weight were visible in tissues [Figure III‐20B]. Here, the overexpressed AKR1B15.1 appeared as double band possessing a more intense band at the expected 36.5 kDa and a weaker 0.5‐1 kDa smaller side band. The rat‐anti‐AKR1B15 clone 9A5 primary antibody acted quite specifically in the HEK‐293 cell background since, with the exception of one endogenous protein of approximately 45 kDa, no further endogenous protein bands were visible in HEK‐293 cell samples [Figure III‐20B]. No protein bands corresponding to the molecular weight of the in HEK‐293 cells overexpressed, untagged AKR1B15 isoforms were detectable in tissue samples [Figure III‐20B]. Adipose tissue and placenta samples revealed one to three very weak signals. Among the tissues tested, skeletal muscle showed the most prominent signals at approximately 20 kDa and 45 kDa and some weaker signals at approximately 33 kDa, 35 kDa, and 42 kDa. The 20 kDa band, which is far from the expected molecular weight of the AKR1B15 isoforms, could be found in both placenta and skeletal muscle samples [Figure III‐20B]. In contrast, the approximately 35 kDa and 45 kDa bands in the skeletal muscle sample were also visible in the control blot (without primary antibody) and negative control (HEK‐293), respectively, and thus seemed unlikely to correspond to an AKR1B15 protein band [Figure III‐20A, B].
The monoclonal mouse‐anti‐AKR1B15.2 clone 29D4 and IRDye 800CW goat‐anti‐mouse antibody pair which should specifically detect the longer AKR1B15.2 isoform recognized, beside the overexpressed AKR1B15.2 (approx. 40 kDa), several endogenous proteins in both tissue lysates and HEK‐293 cell lysates [Figure III‐20C]. Some of these signals were also present in the blots detected via the monoclonal rat‐anti‐AKR1B15 clone 9A5 antibody and secondary goat‐anti‐rat‐AlexaFluor 790 antibody pair but also in the IRDye 800CW goat‐anti‐
mouse antibody control blot (without primary antibody) and differed from the expected molecular weight for AKR1B15.2 [Figure III‐20B, C, D].
Figure III‐20: The existence of endogenous AKR1B15 isoforms in vivo can only be speculated by Western blot analysis of total human tissue samples.
Shown are exemplary Western blots for the detection of endogenous AKR1B15 isoforms in human tissue samples. The Western blots were performed with total tissue lysate samples of human adipose tissue, placenta, and skeletal muscle. Lysates of non‐transfected HEK‐293 cells (negative control) and a mixture of HEK‐293 cells transiently transfected with either pcDNA3.1‐AKR1B15.1 or pcDNA3.1‐AKR1B15.2 (AKR1B15.1/15.2) served as negative and positive controls, respectively. (A, D) Controls without primary antibody for the identification of nonspecific secondary antibody signals. The control membranes were only incubated in the respective secondary antibody dilution: goat‐anti‐rat‐
AlexaFluor 790 (1:200000) (A) or IRDye 800CW goat‐anti‐mouse (1:20000) (D). (B, C) Membranes incubated with primary and secondary antibodies for the detection of endogenous AKR1B15 isoforms.
For the detection of both AKR1B15 isoforms, the membranes were stained via the monoclonal rat‐anti‐
AKR1B15 clone 9A5 primary antibody dilution (1:25) and the respective goat‐anti‐rat‐AlexaFluor 790 secondary antibody dilution (1:200000) (B). The longer AKR1B15.2 was selectively detected by the monoclonal mouse‐anti‐AKR1B15.2 clone 29D4 primary (1:25 diluted) and the IRDye 800CW goat‐anti‐
mouse secondary antibody (1:20000 diluted) pair (C).
Protein bands which possibly resulted from endogenous AKR1B15 isoforms are marked by double daggers; overexpressed AKR1B15.1 and AKR1B15.2 protein bands are marked by stars and arrow heads, respectively.
So far, none of the protein bands resulting from the Western blots with the monoclonal rat‐anti‐AKR1B15 clone 9A5 and mouse‐anti‐AKR1B15.2 clone 29D4 antibodies could have been directly assigned to the AKR1B15 isoforms in total tissue and cell lysates because none of the detected signals did accord with the theoretical molecular weight of AKR1B15.1 or AKR1B15.2 and the signals of the overexpressed AKR1B15 isoforms [Figure III‐20, data not shown]. However, due to the high specificity of the monoclonal rat‐anti‐AKR1B15 clone 9A5 antibody and the assumption that the antibodies were simply not sensitive enough to detect endogenous AKR1B15 isoforms in the crowded environment of total tissue lysates, the rat‐
anti‐AKR1B15 clone 9A5 antibody was further used in Western blots with enriched samples.
PR E D I C T I O N O F P O S T‐T R A N S L A T I O N A L M O D I F I C A T I O N S I T E S I N AKR1B15 IS O F O R M S
Since it was not possible to ambiguously detect endogenous AKR1B15 isoforms in human total tissue or cell lysates, both AKR1B15 isoform sequences were analyzed in silico for sites of post‐translational modifications, perhaps preventing or diminishing the binding of antibodies or shifting the molecular weight of the endogenous isoforms.
Figure III‐21: Several post‐translational modification sites are predicted in AKR1B15 isoforms.
Summarized are the results from in silico predictions of post‐translational modification sites in AKR1B15 isoforms.
Please note that the prediction of residues which are surface exposed is based on the structure of the highly related AKR1B10. Residues of the catalytic tetrad are highlighted in bold. The epitopes targeted by AKB‐1, AKB‐2, and AKB‐3 antibodies are colored in green, yellow, and red, respectively.
Here it was found that various phosphorylation sites as well as some glycation, SUMOylation, and ubiquitination sites are predicted for the AKR1B15 isoforms [Figure III‐21]. Although predicted, a glycosylation of the proteins seems unlikely since no signal peptide for ER or Golgi apparatus import was detectable in any of both isoforms. In addition, no sulfation site was predicted by the Sulfinator server (ExPASy). Based on the structure of the highly identical AKR1B10, a portion of the predicted post‐translational modification sites do not face the surface, e.g., the catalytic Lys106 and Lys78 in AKR1B15.2 and AKR1B15.1, respectively [Figure III‐21, highlighted in bright gray]. Thus, these sites represent most probably false positive predictions. However, some predicted sites are located within the epitopes targeted by the generated antibodies and might prevent the binding of antibodies to the AKR1B15 isoforms. Yet, the presence of post‐translational modifications within the endogenous AKR1B15 isoforms needs to be analyzed by molecular biological methods in the future.
WE S T E R N B L O T T I N G W I T H E NR IC H E D M I T O C H O N D R I A F R O M BEWO C E L L L I N E
Despite the fact that the rat‐anti‐AKR1B15 clone 9A5 antibody was not very useful for the detection of endogenous AKR1B15 isoforms in total cell or tissue lysates (probably due to a limited sensitivity), it was also used in Western blots with enriched mitochondria which were isolated from BeWo cell line cultures. These Western blots pursued more or less two objectives: Primary, the detection of endogenous AKR1B15 isoforms in enriched fractions and secondary, the verification of the mitochondrial subcellular localization of AKR1B15.1.
Figure III‐22: The procedure for the isolation of mitochondria from BeWo cell line cultures yielded sound mitochondria.
Shown are exemplary Rhodamine 123 fluorescence time curves for the determination of the integrity of isolated BeWo mitochondria via the relative membrane potential ΔΨm.
The relative membrane potential of 50 ng isolated mitochondria (protein mass) was calculated according to Equation II‐4 from the intensity of Rhodamine 123 fluorescence prior (‐FCCP) and after (+FCCP) the decoupling of the respiratory chain by the addition of FCCP in technical triplicates. Immediate addition of FCCP at the beginning of the measurements served as negative control.
FCCP, carbonyl cyanide‐4‐(trifluoromethoxy)‐phenylhydrazone.
Five confluent cultures in 75 cm2 culture flasks were required to harvest a total number of
2x107 BeWo cells, which were needed per isolation batch. The procedure described in II.3.4
resulted in a mitochondrial fraction of 360 μg protein per 2x107 BeWo cells. The integrity of the mitochondrial fraction was analyzed via the membrane potential of intact mitochondria using Rhodamine 123 fluorescence quenching. Figure III‐22 illustrates that the mitochondrial fraction (9000xg pellet) contained intact mitochondria with sufficient purity by showing a relative membrane potential ΔΨm of 1.7.
Western blots with the 800xg and 9000xg pellets, resulting from the isolation of mitochondria from BeWo cells, demonstrated the endogenous existence of AKR1B15.1 and AKR1B15.2 on protein level in the BeWo cell line [Figure III‐23]. Here, both AKR1B15 isoforms could be detected in the 800xg pellet (including nuclei, unbroken cells, and cell debris) as well as the 9000xg pellet (including mitochondria) by using the monoclonal rat‐anti‐AKR1B15 clone 9A5 supernatant and the polyclonal goat‐anti‐rat‐AlexaFluor 790 antibody as primary and secondary antibody, respectively [Figure III‐23B].
As a result, both AKR1B15 transcripts, AKR1B15.1 and AKR1B15.2, are translated into low amounts of protein in vivo since weak protein bands were detected in Western blots with BeWo cell fractions. The fact that both AKR1B15 isoforms were only visible in enriched fractions but not in total protein tissue lysate could be explained by a poor expression of AKR1B15, which was also seen on RNA level [III.1.1.3], and a reduced sensitivity of the monoclonal antibodies due to possible post‐translational modifications at the epitope sites.
Figure III‐23: Endogenous AKR1B15.1 and AKR1B15.2 could be detected in BeWo cell fractions.
Shown are Western blots for the detection of endogenous AKR1B15 isoforms. The Western blots were carried out with fractions (800xg and 9000xg pellets) resulting from the isolation of mitochondria from BeWo cells. Lysates of non‐transfected HEK‐293 cells (negative control) and a mixture of HEK‐293 cells transiently transfected with pcDNA3.1‐AKR1B15.1 or pcDNA3.1‐AKR1B15.2 (AKR1B15.1/15.2) served as negative and positive control, respectively. Membranes were stained using either only the goat‐anti‐rat‐AlexaFluor 790 secondary antibody in a final dilution of 1:200000 (A) or the monoclonal rat‐anti‐AKR1B15 clone 9A5 primary (1:25 diluted) and goat‐anti‐rat‐AlexaFluor 790 secondary antibody (1:200000 diluted) pair (B).