Fukuda, R. | Search bioRxiv

bioRxiv2018

Transcytosis maintains CFTR apical polarity in the face of constitutive and mutation-induced basolateral missorting

Apical polarity of cystic fibrosis transmembrane conductance regulator (CFTR) is essential for solute and water transport in secretory epithelia and can be impaired in human diseases. Maintenance of apical polarity in the face of CFTR non-polarized delivery and compromised apical retention of mutant CFTRs lacking PDZ-domain protein (NHERF1) interaction, remains enigmatic. Here we show that basolateral CFTR delivery originates from biosynthetic (~35%) and endocytic (~65%) recycling missorting. Basolateral channels are retrieved via basolateral-to-apical transcytosis, enhancing CFTR apical expression by two-fold and suppressing its degradation. CFTR transcytosis is microtubule-dependent but independent of Myo5B-, Rab11- and NHERF1 binding to its C-terminal DTRL motif in airway epithelia. Increased basolateral delivery due to compromised apical recycling and accelerated internalization upon impaired NHERF1-CFTR association is largely counterbalanced by CFTR efficient basolateral internalization and apical transcytosis. Thus, transcytosis represents a previously unrecognized but indispensable mechanism for maintaining CFTR apical polarity by attenuating its constitutive and mutation-induced basolateral missorting.

cell biology↗

bioRxiv2018

Genetic compensation is triggered by mutant mRNA degradation

Genetic compensation by transcriptional modulation of related gene(s) (also known as transcriptional adaptation) has been reported in numerous systems 1-3; however, whether and how such a response can be activated in the absence of protein feedback loops is unknown. Here, we develop and analyze several models of transcriptional adaptation in zebrafish and mouse that we show are not caused by loss of protein function. We find that the increase in transcript levels is due to enhanced transcription, and observe a correlation between the levels of mutant mRNA decay and transcriptional upregulation of related genes. To assess the role of mutant mRNA degradation in triggering transcriptional adaptation, we use genetic and pharmacological approaches and find that mRNA degradation is indeed required for this process. Notably, uncapped RNAs, themselves subjected to rapid degradation, can also induce transcriptional adaptation. Next, we generate alleles that fail to transcribe the mutated gene and find that they do not show transcriptional adaptation, and exhibit more severe phenotypes than those observed in alleles displaying mutant mRNA decay. Transcriptome analysis of these different alleles reveals the upregulation of hundreds of genes with enrichment for those showing sequence similarity with the mutated genes mRNA, suggesting a model whereby mRNA degradation products induce the response via sequence similarity. These results expand the role of the mRNA surveillance machinery in buffering against mutations by triggering the transcriptional upregulation of related genes. Besides implications for our understanding of disease-causing mutations, our findings will help design mutant alleles with minimal transcriptional adaptation-derived compensation.

genetics↗