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In 2007, a genome-wide siRNA screen identified a requirement for the Tao1 kinase (also known as microtubule affinity-regulation kinase kinase (MARKK), Timm et al., 2003; Johne et al., 2008) in the spindle checkpoint (Draviam et al., 2007). Most interestingly, Tao1 depletion by siRNA was reported to cause a selective loss of Mad2 but not Mad1 from KTs (Draviam et al., 2007), very similar to the early data obtained after PICH siRNA (Baumann et al., 2007). Intrigued by this similarity, we originally suspected that PICH and Tao1 cooperate in a regulatory step to control Mad2 localization. However, in line with the obtained evidence that the apparent requirement for PICH in Mad2 localization is likely to reflect an off-target effect, we analyzed the Tao1 siRNA phenotype more carefully. To this end, different Tao1- directed siRNA oligonucleotides were compared for their efficiency to deplete Tao1, and their effects on Mad2 localization, protein and transcript levels, as well as their ability to abolish

SAC activity. This survey included the most effective previously published Tao1 siRNA oligonucleotide (Tao1-NCB3) as well four new oligonucleotides (Tao1-2 to Tao1-5) and a smart pool comprising Tao1-2 to Tao1-5 (Tao1-SP). Western blotting revealed that all Tao1- directed siRNAs exhibited a similar ability to deplete Tao1 from HeLa cells, and curiously, PICH-1 siRNA also lowered Tao1 levels (Fig. 39a). Surprisingly but most importantly, the already published siTao1-NCB3 oligonucleotide additionally caused a significant reduction of Mad2 protein, comparable to the reduction of Mad2 seen after PICH-1 or PICH-CC siRNA (Fig. 39a). In contrast, Mad2 levels remained unaffected upon treatment of cells with the other Tao1 siRNAs, except for Tao1-4, which produced a marginal effect. Similar results were obtained upon depletion of Tao1 from U2OS and RPE-1 cells (Fig. 39b). Quantitative Western blot analysis (three independent experiments; normalized to α-Tubulin) revealed that Tao1-NCB3 siRNA substantially lowered Mad2 protein levels. The other Tao1-directed siRNAs barely affected Mad2, although they depleted Tao1 with similar efficiency as Tao1- NCB3 (Fig. 39c). We observed that the KT localization of Mad2 was abolished only in cells treated with the Tao1-NCB3 siRNA but not in cells treated with any of the newly designed Tao1-directed siRNA oligonucleotides (Fig. 39d+e). Supporting our findings, a recently published study shows that Tao1 is unlikely to function as a spindle checkpoint component (Westhorpe et al., 2010). Currently we cannot explain the discrepancy between this observation and the previous report, indicating that Tao1 siRNA does not lower Mad2 levels (Draviam et al., 2007).

Figure 39: Evaluation of protein knockdown efficiency, Mad2 localization, and SAC phenotype for different siRNA oligonucleotides targeting the Tao1 kinase.a HeLa cells were transfected with the indicated

siRNA duplexes, and lysates were probed by Western blotting with the indicated antibodies. Asterisks indicate

the reduction of Mad2 levels by some but not all Tao1- and PICH-directed siRNAs. b Effects on Mad2 protein

levels after depletion of PICH and Tao1 from U2OS and RPE-1 cells. U2OS (upper panel) and RPE-1 cells

(lower panel) were treated with the indicated PICH- and Tao1-directed siRNA duplexes for 48 h, and the

abundance of both the primary targets and Mad2 protein was analyzed by Western blotting. The asterisks

indicate significant reduction of Mad2 protein levels. c Western blots from three individual experiments (in

HeLa cells) were used for quantification of Mad2 and Tao1 protein levels. Band intensities were measured using the Aida Image Analyzer software. The histograms show the mean band intensities normalized against

background and α-Tubulin, and error bars indicate standard errors. Note that Mad2 protein levels were

significantly reduced in lysates from siPICH-1-, siPICH-CC-, and siTao1-NCB3-treated cells (marked by

asterisks). d Cells were treated with different Tao1 siRNAs and stained with antibodies directed against Mad2

response to Tao1-NCB3, but not other Tao1-directed duplexes. e Quantitative analysis of Mad2 localization in

cells depleted of Tao1. HeLa cells were treated as described in d. The graph shows the percentage of

prometaphase cells in which Mad2 localizes correctly to KTs; shown are the averages from three independent experiments with standard errors (n > 150).

Downregulation of Mad2 by PICH-1 and the Tao1-NCB3 siRNA oligonucleotides was independent of cell cycle stages as Mad2 protein was lost in thymidine-arrested interphase as well as MG132-arrested mitotic cell lysates (Fig. 40).

Figure 40: PICH-1 and Tao1-NCB3 siRNA oligonucleotides reduce Mad2 protein levels independent of the cell cycle stage. HeLa cells were transfected with PICH-1, Tao1-NCB3 or GL2 (control) siRNA for 48 h and arrested in interphase by a thymidine block for the last 24 h of siRNA treatment or released for 10 h from the thymidine block and treated with MG132 for the last 2 h of release before mitotic shake off. Cell lysates were subjected to Western blotting and analyzed with indicated antibodies.

Next, we used qRT-PCR to analyze the mRNA levels of Mad1 (control), Mad2, and Tao1 after treatment of HeLa cells with different siRNAs targeting Tao1 or PICH. Mad1 mRNA remained constant in all cases as expected. Tao1 mRNA levels were similarly reduced by all Tao1 siRNAs and, to a lesser extent, PICH-1 siRNA. Most importantly, Mad2 mRNA was significantly reduced upon treatment with siPICH-1 and siTao1-NCB3 siRNAs and, to a lesser extent, Tao1-4 siRNA. No changes in Mad2 mRNA levels were detectable in cells treated with siPICH-3, siTao1-2, siTao1-3, or control GL2 siRNA (Fig. 41).

Figure 41: Histogram illustrating mRNA levels after Tao1 siRNA treatment. After treatment of HeLa cells

with the indicated siRNAs, RNA was extracted for qRT-PCR measurements. Bars indicate relative mRNA

expression levels of Mad1 (blue), Mad2 (red), and Tao1 (green; in AU); shown are averages from three

independent experiments and standard errors. Significant reduction of Mad2 mRNA in PICH-1 and Tao1-NCB3 siRNA-treated cells is marked by asterisks.

In addition, the cellular phenotype of cells depleted of Tao1 by different oligonucleotides was analyzed. Cells that had been treated with the Tao1-NCB3 siRNA duplexes went through mitosis more rapidly (NEBD to anaphase; average 14 min) than siGL2-treated control cells (average 33 min). Compared to the other Tao1 siRNAs which did not show acceleration from NEBD to anaphase, the mitotic timing was slightly reduced in cells treated with the Tao1-4 siRNA (average 23 min; Fig. 42a). Parallel experiments showed an override of the nocodazole-induced SAC arrest in response to siTao1-NCB3 (average duration of arrest 25 min), consistent with the original report implicating Tao1 in SAC function (Draviam et al., 2007). A robust checkpoint arrest (often followed by cell death after 12-15 h) was seen upon depletion of Tao1 by siTao1-2 or siTao1-3. Cells treated with Tao1-4 siRNA showed a partial phenotype by usually exiting mitosis after about 4 h (Fig. 42b).

Figure 42: Histogram illustrating the mitotic timing, and nocodazole response after Tao1-directed siRNA treatment. HeLa cells stably expressing histone H2B-GFP were transfected with the indicated siRNAs and

analyzed by live-cell imaging. a The histogram illustrates the elapsed average time from NEBD to anaphase

onset (40 cells for each treatment), and error bars indicate standard errors. bHistogram summarizes the results

of live-cell time-lapse microscopy performed on cells that had been depleted of Tao1 by 48 h treatments with different Tao1 siRNA duplexes and then treated with nocodazole during the last 16 h. Error bars indicate

We noticed that the oligonucleotides giving the off-target effect were purchased from a different company than the newly designed siRNA duplexes. To rule out a company specific modification on the duplexes used, we compared identical oligonucleotide sequences purchased from the two companies (Qiagen and Dharmacon) by Western blotting (Fig. 43). The results confirmed that the off-target effect is sequence specific and can be observed in siRNA oligonucleotides purchased from both companies.

Figure 43: Western Blot analysis of siRNA oligonucleotides from Qiagen and Dharmacon. HeLa cells were treated with indicated siRNA oligonucleotides for 48 h and analyzed for PICH and Mad2 protein levels by Western blotting. -Tubulin serves as loading control.

Collectively, we interpret the above data to suggest that the apparent role of Tao1 kinase in SAC signaling might in reality reflect an off-target effect. This effect is produced by particular Tao1-directed siRNA oligonucleotides either directly or indirectly on Mad2, similar to the situation described for certain PICH siRNA duplexes. Therefore, our data and the results published recently by Steven Taylor and coworkers (Westhorpe et al., 2010) do not support Tao1 being a component of the spindle checkpoint.

2.7. Rescue of PICH and Tao1 siRNA phenotypes by Mad2 expression from a bacterial