New cellular mechanisms of gap junction degradation and recycling.

Connexins (Cx), the constitutive proteins of gap junctions, are key actors of many physiological processes. Therefore alterations of Cx expression and degradation lead to the development of physiopathological disorders. Due to the formation of a double membrane vesicle termed annular gap junction (AGJ), gap junction degradation is a unique physiological process for which many cellular aspects remain unclear. By using a combination of time-lapse fluorescence microscopy and high resolution transmission electron microscopy we evidenced new specific cellular events concerning gap junction degradation and recycling. Indeed, by time lapse video microscopy we demonstrated, for the first time to our knowledge, that an entire annular gap junction can be fully recycled back to the plasma membrane. Moreover, we dissected the degradative processes of gap junction by electron microscopy approaches. Interestingly, in addition to canonical autophagy and heterophagy pathways, previously described, we discovered that both pathways could sometimes intermingle. Strikingly, our results also highlighted a new lysosome-based autophagy pathway that could play a pivotal role in common autophagy degradation. The present investigation reveals that AGJ degradation is a more complex process that it was previously thought. First, a complete recycling of the gap junction plaque after its internalization could occur. Second, the degradation of this peculiar double membrane structure is possible through autophagy, heterophagy, hetero-autophagy or by lysosomal-based autophagy. Altogether, this work underlines novel aspects of gap junction degradation that could be extended to other cell biology processes. This article is protected by copyright. All rights reserved.