Background: The multidrug-resistance-1 (MDR1) gene encodes an ATP-dependent efflux transporter that is highly expressed in the colon. In mice, loss of MDR1 function results in colitis with similarities to human IBD. Recently, we observed a marked accumulation of dysmorphic mitochondria within the mdr1a-deficient colonic epithelial cells (CEC) using electron microscopy (EM). We hypothesise that loss of MDR1 results in intestinal mitochondrial dysfunction, a relevant process which drives the development of colitis in IBD.
Methods: We characterised mitochondrial function in mdr1a-deficiency mouse model and in shRNA-knockdown of MDR1 in T84 CECs. In vivo, we tested if induced gut mitochondrial dysfunction can potentiate colitis, by using direct colonic administration of rotenone and MitoQ10 (mitochondrial reactive oxygen species, mtROS inducer and inhibitor respectively) in mdr1a−/−; and in acute + recovery DSS-colitis. Furthermore, we generated a novel mouse model with intestinal-epithelial specific deletion (IECΔ) of superoxide dismutase-2 (SOD2) gene responsible for mtROS detoxification to directly test the role of mitochondria in CECs. Finally, we analysed current GWA datasets (42 992 IBD/53 536 controls) to determine the clinical significance of mitochondrial homoestasis in IBD.
Results: Damaged mitochondria accumulate in mdr1a−/− CECs vs. ileum/liver/lung; and vs. wild-type and il-10−/− CECs. Mdr1a−/− CECs have increased expression of p62, LC3 (general autophagy), PINK (specific mitophagy) and SOD2 protein expressions and impaired cellular energetics with reduced baseline respiration. Isolated Mdr1a−/− mitochondria have lower threshold to induced damage and produced more mtROS, which are replicated in vitro in T84 shMDR1 CECs. In vivo, colonic rotenone acccelerated spontaneous mdr1a−/− colitis, increased the severity of acute DSS-colitis in mdr1a−/− and in WT mice. Inhibition of mROS using MitoQ10 attenuated the severity and promoted the recovery from DSS colitis. SOD2-IECΔ mice displayed analagous dysmorphic mitochondria in CECs and are highly susceptible to DSS colitis. We showed that 29 (5.0%) of 574 IBD susceptibility genes (p<5×10–8) have direct roles in mitochondria function (GO term: 0005739). MDR1 and SOD2 genes showed associations with p=3.19×10–3 and 3.04×10–3 respectively.
Conclusions: MDR1 has an important protective role for the mitochondria in the colon. Given that many IBD susceptibility genes are involved in the regulation of mitochondrial health, our findings suggest that mitochondrial toxin + genetic susceptibility interaction leading to mitochondrial dysfunction is a novel pathogenic mechanism that could offer many new therapeutic opportunities for IBD.