Autophagy and lysosomal degradation ensure accurate chromosomal segregation to prevent genomic instability Authors: Eugènia Almacellas (1, 2), Joffrey Pelletier (2), Charles Day (3, 4), Santiago Ambrosio (5), Albert Tauler (1, 2), Caroline Mauvezin (2) Affiliations: 1: Department of Biochemistry and Physiology, Faculty of Pharmacy, University of Barcelona, Barcelona, Spain. 2: Metabolism and Cancer Laboratory, Molecular Mechanisms and Experimental Therapy in Oncology Program (Oncobell, Institut d'Investigació Biomèdica de Bellvitge - IDIBELL, L'Hospitalet de Llobregat, Spain. 3: Hormel Institute, University of Minnesota, Austin, MN, USA. 4: Neuro-Oncology Program, Mayo Clinic, Rochester, MN, USA. 5: Department of Physiological Sciences, Faculty of Medicine and Health Sciences, University of Barcelona, Barcelona, Spain. ABSTRACT Lysosomes are cytosolic organelles responsible for the degradation of substrates coming from different converging pathways, including macroautophagy. To date, the degradative function of the lysosomes has been mainly studied in interphase cells, while their role during mitosis remains controversial. Indeed, some studies propose that they are inactive during cell division to protect the genetic material from degradation, and others indicate certain activity of selective autophagy at specific mitotic phases. Mitosis dictates the faithful transmission of genetic material among generations, and perturbations of mitotic division lead to chromosomal instability, a hallmark of cancer. Heretofore, correct mitotic progression relies on the orchestrated degradation of mitotic factors, which was mainly attributed to ubiquitin-triggered proteasome-dependent degradation. We undertook to study different cancer cell lines and found that lysosomes and autophagy are active during mitosis and are necessary for the process. We showed that mitotic transition also relies on lysosome-dependent degradation, as impairment of lysosomes increased mitotic timing and led to mitotic errors, thus promoting chromosomal instability. Furthermore, using proteomic approach, we identified more than 100 novel putative lysosomal substrates in mitotic cells. Among them, WAPL, a cohesin regulatory protein, emerged as a novel SQSTM1-interacting protein for targeted lysosomal degradation. Understanding the role of lysosomes and autophagy in mitotic progression led to another discovery. Indeed, cells that have suffered errors during mitotic progression, either due to alterations of lysosomal function or by other stresses, present a nucleus with a toroidal morphology, with the appearance of a perforated nucleus, after they divided. Until now, the only biomarker used for the detection of chromosomal instability was the micronucleus. We propose that the toroidal nucleus represents a complementary new biomarker for the identification of cells with chromosomal instability, inherent in cancer cells. Our results establish a connection between two influential fields in cancer research: autophagy and chromosomal instability. Our findings serve as precedent for the characterization of the regulating mechanisms involving autophagy and lysosomes to maintain chromosomal stability.