High resolution live cell imaging reveals novel cyclin A2 degradation foci involving autophagy

Abstract

Cyclin A2 is a key player in the regulation of the cell cycle. Its degradation in mid-mitosis relies on the ubiquitin-proteasome system (UPS). Using high-resolution microscopic imaging, we find that cyclin A2 persists beyond metaphase. Indeed, we identify a novel cyclin-A2-containing compartment that forms dynamic foci. Fö rster (or fluorescence) resonance energy transfer (FRET) and fluorescence lifetime imaging microscopy (FLIM) analyses show that cyclin A2 ubiquitylation takes place predominantly in these foci before spreading throughout the cell. Moreover, inhibition of autophagy in proliferating cells induces the stabilisation of a subset of cyclin A2, whereas induction of autophagy accelerates the degradation of cyclin A2, thus showing that autophagy is a novel regulator of cyclin A2 degradation.

Autophagy and the Cell Cycle: A Complex Landscape

“…In accordance with the reported ongoing mitophagy in arrested mitotic cells (38), LC3 puncta have been observed in mitotic cells, although at a significantly decreased level compared to interphase cells (28, 29, 39, 40). While these may also represent inefficient autophagy inhibition, Loukil et al observed LC3, p62, and lysosomal markers colocalizing with Cyclin A2 foci during mitosis and found that autophagy partially contributes to mediating mitotic Cyclin A2 degradation (40).…”

“…While these may also represent inefficient autophagy inhibition, Loukil et al observed LC3, p62, and lysosomal markers colocalizing with Cyclin A2 foci during mitosis and found that autophagy partially contributes to mediating mitotic Cyclin A2 degradation (40). Thus, an intriguing although highly controversial theory is the existence of distinct sites of active autophagy during cell division.…”

“…Furthermore, an alternative function for cyclin A2 in potentiating RhoA GTP loading by its GEFs has also been described (130). While the majority of cyclin A2 is degraded by the proteasome in prometaphase (131–133), a small fraction of cyclin A2 was shown to persist in foci later in mitosis and appeared to be subjected to autophagic degradation (40). It is therefore possible that autophagy may have a composite function in controlling appropriate RhoA protein levels and activity at the cytokinesis midzone, by mediating RhoA and Cyclin A2 degradation in late mitosis.…”

Hepatocellular carcinoma–related cyclin D1 is selectively regulated by autophagy degradation system

“…Cyclins and CDKs are ubiquitinated and degraded through the proteasomal pathway to maintain cell‐cycle progression . Loukil et al reported that autophagy is a regulator of cyclin A2 degradation beyond the metaphase of the cell cycle in proliferating cells . We show that both of the autophagic and proteasomal degradation systems degrade cyclin D1, as demonstrated by MG132 and CQ treatment.…”

“…(22) Loukil et al reported that autophagy is a regulator of cyclin A2 degradation beyond the metaphase of the cell cycle in proliferating cells. (23) We show that both of the autophagic and proteasomal degradation systems degrade cyclin D1, as demonstrated by MG132 and CQ treatment. However, the subsequent suppression of the cell-cycle progression was only rescued by CQ treatment, but not by MG132 (Fig.…”