The Caenorhabditis elegans Shugoshin regulates TAC-1 in cilia

The conserved Shugoshin (SGO) protein family is essential for mediating proper chromosome segregation from yeast to humans but has also been implicated in diverse roles outside of the nucleus. SGO's roles include inhibiting incorrect spindle attachment in the kinetochore, regulating the spindle assembly checkpoint (SAC), and ensuring centriole cohesion in the centrosome, all functions that involve different microtubule scaffolding structures in the cell. In Caenorhabditis elegans, a species with holocentric chromosomes, SGO-1 is not required for cohesin protection or spindle attachment but appears important for licensing meiotic recombination. Here we provide the first functional evidence that in C. elegans, Shugoshin functions in another extranuclear, microtubule-based structure, the primary cilium. We identify the centrosomal and microtubule-regulating transforming acidic coiled-coil protein, TACC/TAC-1, which also localizes to the basal body, as an SGO-1 binding protein. Genetic analyses indicate that TAC-1 activity must be maintained below a threshold at the ciliary base for correct cilia function, and that SGO-1 likely participates in constraining TAC-1 to the basal body by influencing the function of the transition zone 'ciliary gate'. This research expands our understanding of cellular functions of Shugoshin proteins and contributes to the growing examples of overlap between kinetochore, centrosome and cilia proteomes.

Defining function of wild-type and three patient-specific TP53 mutations in a zebrafish model of embryonal rhabdomyosarcoma

In embryonal rhabdomyosarcoma (ERMS) and generally in sarcomas, the role of wild-type and loss- or gain-of-function TP53 mutations remains largely undefined. Eliminating mutant or restoring wild-type p53 is challenging; nevertheless, understanding p53 variant effects on tumorigenesis remains central to realizing better treatment outcomes. In ERMS, >70% of patients retain wild-type TP53, yet mutations when present are associated with worse prognosis. Employing a kRASG12D-driven ERMS tumor model and tp53 null (tp53-/-) zebrafish, we define wild-type and patient-specific TP53 mutant effects on tumorigenesis. We demonstrate that tp53 is a major suppressor of tumorigenesis, where tp53 loss expands tumor initiation from <35% to >97% of animals. Characterizing three patient-specific alleles reveals that TP53C176F partially retains wild-type p53 apoptotic activity that can be exploited, whereas TP53P153Δ and TP53Y220C encode two structurally related proteins with gain-of-function effects that predispose to head musculature ERMS. TP53P153Δ unexpectedly also predisposes to hedgehog-expressing medulloblastomas in the kRASG12D-driven ERMS-model.

The RNA Demethylase ALKBH5 Maintains Endoplasmic Reticulum Homeostasis by Regulating UPR, Autophagy, and Mitochondrial Function

Eukaryotic cells maintain cellular fitness by employing well-coordinated and evolutionarily conserved processes that negotiate stress induced by internal or external environments. These processes include the unfolded protein response, autophagy, endoplasmic reticulum-associated degradation (ERAD) of unfolded proteins and altered mitochondrial functions that together constitute the ER stress response. Here, we show that the RNA demethylase ALKBH5 regulates the crosstalk among these processes to maintain normal ER function. We demonstrate that ALKBH5 regulates ER homeostasis by controlling the expression of ER lipid raft associated 1 (ERLIN1), which binds to the activated inositol 1, 4, 5,-triphosphate receptor and facilitates its degradation via ERAD to maintain the calcium flux between the ER and mitochondria. Using functional studies and electron microscopy, we show that ALKBH5-ERLIN-IP3R-dependent calcium signaling modulates the activity of AMP kinase, and consequently, mitochondrial biogenesis. Thus, these findings reveal that ALKBH5 serves an important role in maintaining ER homeostasis and cellular fitness.

Abstract 3528: ALKBH5 promotes cancer growth by regulating ER homeostasis via UPR, autophagy, and mitochondrial function

Background: N6-methyladenosine (m6A) RNA methylation is a dynamic reversible epitranscriptomic modification that includes methyl transferases (writes m6A), demethylases (erases m6A) and reader proteins, which proofreads m6A marks of a specific site of transcripts. Recent studies have suggested that RNA methylation affects several fundamental cellular and molecular functions including mRNA splicing, stability, export, stem cell fate, circadian rhythms, DNA repair and cell survival. The objective of this study is to establish the mechanisms by which RNA demethylase ALKBH5 (AlkB homolog 5) facilitates tumor growth and progression.

Methods: To establish the significance of RNA methylation in children’s cancers, we performed siRNA screen targeting m6A writers, erasers, and readers in osteosarcoma (OS). We used several OS cell lines including 143B, MG63, SaOS2, U2OS and multiple patient derived OS cell lines. Mechanistic studies were conducted using ALKBH5 KO and knockdown cells and by measuring the status of Autophagy and UPR associated proteins using Western blot analysis, confocal and electron microscopy.

Results: Our results revealed that depletion of ALKBH5, a demethylase that erases the m6A mark from the target gene, altered the autophagy in OS cells. Interestingly, we found that level of LC3, which is the universal marker for autophagy, was significantly increased in OS cells. RNA seq analysis showed that depletion of ALKBH5 significantly altered several autophagy related genes in the OS cells. To better understand the molecular mechanism by which ALKBH5 regulates autophagy, we investigated the Endoplasmic Reticulum (ER) stress-induced Unfolded Protein Response (UPR) pathway, which is a known activator of autophagy. We discovered that ER stress induced UPR signaling pathway is highly activated in ALKBH5 depleted cancer cells. Further, mechanistic studies suggested that ALKBH5 promoted ER homeostasis by controlling the expression of ER lipid raft associated 1 (ERLIN1), which binds to the activated inositol 1, 4, 5,-triphosphate receptor and facilitates its degradation via ERAD to maintain calcium flux between ER and mitochondria. Using functional studies and electron microscopy, we show that ALKBH5-ERLIN1-IP3R-dependent calcium signaling modulates the activity of AMP kinase, and consequently mitochondrial biogenesis. These findings thus reveal that ALKBH5 serves an important role in maintaining ER homeostasis and cellular fitness.

Conclusion: These findings provide novel insight into how m6A may promote cancer cell growth by regulating the crosstalk among ER signaling, UPR, autophagy, and mitochondrial function. Our study is the first to show that RNA methylation plays an important role in osteosarcoma by regulating autophagy via UPR.

Citation Format: Panneerdoss Subbarayalu, Daisy Medina, Pooja Yadav, Santosh Timilsina, Kunal Baxi, Ratna Vadlamudi, Yidong Chen, Manjeet Rao. ALKBH5 promotes cancer growth by regulating ER homeostasis via UPR, autophagy, and mitochondrial function. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 3528.

Vertical Inhibition of the RAF-MEK-ERK Cascade Induces Myogenic Differentiation, Apoptosis and Tumor Regression in H/NRAS Q61X-mutant Rhabdomyosarcoma

Oncogenic RAS signaling is an attractive target for fusion-negative rhabdomyosarcoma (FN-RMS). Our study validates the role of the ERK MAPK effector pathway in mediating RAS dependency in a panel of H/NRASQ61X-mutant RMS cells and correlates in vivo efficacy of the MEK inhibitor trametinib with pharmacodynamics of ERK activity. A screen is used to identify trametinib-sensitizing targets and combinations are evaluated in cells and tumor xenografts. We find that the ERK MAPK pathway is central to H/NRASQ61X-dependency in RMS cells, however there is poor in vivo response to clinically relevant exposures with trametinib, which correlates with inefficient suppression of ERK activity. CRISPR screening points to vertical inhibition of the RAF-MEK-ERK cascade by co-suppression of MEK and either CRAF or ERK. CRAF is central to rebound pathway activation following MEK or ERK inhibition. Concurrent CRAF suppression and MEK or ERK inhibition, or concurrent pan-RAF and MEK/ERK inhibition (pan-RAFi + MEKi/ERKi), or concurrent MEK and ERK inhibition (MEKi + ERKi) all synergistically block ERK activity and induce myogenic differentiation and apoptosis. In vivo assessment of pan-RAFi + ERKi or MEKi + ERKi potently suppress growth of H/NRASQ61X RMS tumor xenografts, with pan-RAFi + ERKi being more effective and better tolerated. We conclude that CRAF reactivation limits the activity of single agent MEK/ERK inhibitors in FN-RMS. Vertical targeting of the RAF-MEK-ERK cascade, and particularly co-targeting of CRAF and MEK or ERK, or the combination of pan-RAF inhibitors with MEK or ERK inhibitors, have synergistic activity and potently suppress H/NRASQ61X-mutant RMS tumor growth.

SNAI2-Mediated Repression of BIM Protects Rhabdomyosarcoma from Ionizing Radiation

Ionizing radiation (IR) and chemotherapy are mainstays of treatment for patients with rhabdomyosarcoma, yet the molecular mechanisms that underlie the success or failure of radiotherapy remain unclear. The transcriptional repressor SNAI2 was previously identified as a key regulator of IR sensitivity in normal and malignant stem cells through its repression of the proapoptotic BH3-only gene PUMA/BBC3. Here, we demonstrate a clear correlation between SNAI2 expression levels and radiosensitivity across multiple rhabdomyosarcoma cell lines. Modulating SNAI2 levels in rhabdomyosarcoma cells through its overexpression or knockdown altered radiosensitivity in vitro and in vivo. SNAI2 expression reliably promoted overall cell growth and inhibited mitochondrial apoptosis following exposure to IR, with either variable or minimal effects on differentiation and senescence, respectively. Importantly, SNAI2 knockdown increased expression of the proapoptotic BH3-only gene BIM, and chromatin immunoprecipitation sequencing experiments established that SNAI2 is a direct repressor of BIM/BCL2L11. Because the p53 pathway is nonfunctional in the rhabdomyosarcoma cells used in this study, we have identified a new, p53-independent SNAI2/BIM signaling axis that could potentially predict clinical responses to IR treatment and be exploited to improve rhabdomyosarcoma therapy. SIGNIFICANCE: SNAI2 is identified as a major regulator of radiation-induced apoptosis in rhabdomyosarcoma through previously unknown mechanisms independent of p53.

Abstract LB251: Vertical inhibition of the RAF MEK ERK cascade induces myogenic differentiation, apoptosis and tumor regression in H/NRAS Q61X mutant rhabdomyosarcoma

Rhabdomyosarcoma (RMS) is the most commonly diagnosed soft tissue sarcoma in children. While the 5-year event-free survival for localized rhabdomyosarcoma is over 70%, it is less than 30% for patients presenting with metastatic disease. Given that the current treatment modalities for RMS patients remain cytotoxic therapies with profound life-long lasting complications, identification of less toxic and more effective therapies is highly desired. Whole-genome and RNA sequencing of human rhabdomyosarcoma have identified frequent RAS genes oncogenic mutations in fusion-negative rhabdomyosarcoma (FN-RMS), which comprises 65% of RMS. This high incidence of RAS mutations present opportunity for design of targeted therapeutic strategies for FN-RMS. By siRNA-mediated knock down of individual RAS isoforms we showed that H/NRAS Q61X-mutant FN-RMS cell lines exhibited oncogenic RAS dependency and that knockdown of the Q61X-mutant H/NRAS inhibited ERK MAPK, but not PI3K-AKT pathway activity. Knockdown of the Q61X-mutant H/NRAS phenocopied ERK inhibitor treatment by inducing myogenic differentiation. Further, as compared to RAS-wild type RMS cells, H/NRAS-Q61X FN-RMS cell lines were preferentially more sensitive to MEK or ERK, but not PI3K or AKT inhibitors. RNA sequencing analysis revealed significant overlap of genes signatures associated with NRAS-Q61H knockdown and ERK inhibitor treatment in the FN-RMS cell line RD. These results indicated a central role of the ERK MAPK pathway in mediating H/NRAS Q61X-dependency in FN-RMS. However, in vivo evaluation of clinically relevant exposers with the MEK inhibitor trametinib demonstrated poor response, which correlated with inefficient ERK inhibition in pharmacodynamic assays. A trametinib-sensitizing CRISPR screen pointed to vertical inhibition of the RAF-MEK-ERK cascade by co-suppression of MEK and either CRAF or ERK. Concurrent CRAF suppression and MEK or ERK inhibition, or concurrent pan-RAF and MEK/ERK inhibition (pan-RAFi + MEKi/ERKi), or concurrent MEK and ERK inhibition (MEKi + ERKi) all synergistically blocked ERK activity and induced cell cycle arrest, myogenic differentiation and cell death. In vivo evaluation of low dose pan-RAFi + ERKi or MEKi + ERKi potently suppressed H/NRAS Q61X RMS tumor growth, with pan-RAFi + ERKi being more effective and better tolerated, suggesting wider therapeutic window. These results support the clinical translation of pan-RAFi + MEKi/ERKi for H/NRAS Q61X-mutant FN-RMS.

Citation Format: Natalia Garcia, Vanessa Del-Pozo, Marielle Yohe, Craig Goodwin, Terry Shackleford, Long Wang, Kunal Baxi, Yidong Chen, Myron Ignatius, Kris Wood, Peter Houghton, Angelina Vaseva. Vertical inhibition of the RAF MEK ERK cascade induces myogenic differentiation, apoptosis and tumor regression in H/NRAS Q61X mutant rhabdomyosarcoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr LB251.

Zebrafish Tumor Graft Transplantation to Grow Tumors In Vivo That Engraft Poorly as Single Cell Suspensions

Angiosarcoma is a clinically aggressive tumor with a high rate of mortality. It can arise in vascular or lymphatic tissues, involve any part of the body, and aggressively spread locally or metastasize. Angiosarcomas spontaneously develop in the tp53 deleted (tp53^del/del) zebrafish mutant. However, established protocols for tumor dissection and transplantation of single cell suspensions of angiosarcoma tumors result in inferior implantation rates. To resolve these complications, we developed a new tumor grafting technique for engraftment of angiosarcoma and similar tumors in zebrafish, which maintains the tumor microenvironment and has superior rates of engraftment.

Interaction between SNAI2 and MYOD enhances oncogenesis and suppresses differentiation in Fusion Negative Rhabdomyosarcoma

Rhabdomyosarcoma (RMS) is an aggressive pediatric malignancy of the muscle, that includes Fusion Positive (FP)-RMS harboring PAX3/7-FOXO1 and Fusion Negative (FN)-RMS commonly with RAS pathway mutations. RMS express myogenic master transcription factors MYOD and MYOG yet are unable to terminally differentiate. Here, we report that SNAI2 is highly expressed in FN-RMS, is oncogenic, blocks myogenic differentiation, and promotes growth. MYOD activates SNAI2 transcription via super enhancers with striped 3D contact architecture. Genome wide chromatin binding analysis demonstrates that SNAI2 preferentially binds enhancer elements and competes with MYOD at a subset of myogenic enhancers required for terminal differentiation. SNAI2 also suppresses expression of a muscle differentiation program modulated by MYOG, MEF2, and CDKN1A. Further, RAS/MEK-signaling modulates SNAI2 levels and binding to chromatin, suggesting that the differentiation blockade by oncogenic RAS is mediated in part by SNAI2. Thus, an interplay between SNAI2, MYOD, and RAS prevents myogenic differentiation and promotes tumorigenesis.

THYROIDITIS DEVELOPING POST INITIATION OF OCTREOTIDE IN A CASE OF TSH SECRETING TUMOR

Thyroiditis developing post pituitary surgery in a case of TSH secreting tumor has been reported, albeit rarely. A 46 year old female was treated as a case of hypothyroidism for almost five years, however, TSH levels remained unsuppressed in spite of increasing thyroxine doses. A cyclic pattern of TSH with T3 secretion was observed after stopping thyroxine, though T4 levels were normal. T3 suppression test revealed a non suppressed TSH and MRI sella revealed a pituitary mass with Grade 1 cavernous sinus invasion. With a diagnosis of TSH secreting tumor, the patient was initiated on octreotide therapy. TFT normalized within 4 days of initiation of octreotide, tumor size reduced by about 30% within 2 months of therapy and goiter size reduced to almost half with octreotide therapy. Anti TPO levels which were initially negative became positive within a month of octreotide therapy and FNAC thyroid revealed thyroiditis. To the best of our knowledge, this is the second case report of development of thyroiditis after octreotide therapy. The immunomodulatory role of TSH and somatostatin may have a role in the development of thyroiditis in this case.

Abstract B35: Liaison between SNAI2 and MYOD enhances oncogenesis and suppresses differentiation in fusion-negative rhabdomyosarcoma

Rhabdomyosarcoma (RMS) is a pediatric malignancy of the muscle, and patients with high-risk fusion-negative RMS (FN-RMS), the major subtype of this disease, are associated with RAS pathway activating mutations and have a poor survival rate of &lt;30%. RMS cells express the myogenic master transcription factors MYOD and MYOG and yet are unable to differentiate. Here, we report an oncogenic role for SNAI2, which is highly expressed in FN-RMS, that blocks differentiation and promotes tumor growth through inhibition of a MYOD, MYOG, and MEF2 program. HiC analyses of the tridimensional-structure around the SNAI2 locus in IMR-90 cells find that it is regulated by a 1.2 mb enhancer regulatory region. In RMS tumors, MYOD can engage multiple SNAI2 enhancers and directly induce SNAI2 expression, while SNAI2 knockdown in RMS RD and JR1 results in increased expression of MYOD, indicating that SNAI2 in turn can repress MYOD expression. Employing two validated shRNAs to knock down SNAI2 in FN-RMS RD, JR1, and RD18 cells, we find that SNAI2 plays an oncogenic role by blocking myogenic differentiation and promoting growth both in vitro and in vivo in murine xenograft experiments. SNAI2 knockdown potentiates vincristine treatments and expands differentiation and greatly reduces tumor growth in vivo. In order to understand molecularly how SNAI2 blocks differentiation and promotes growth, we optimized ChIP-seq experiments in FN-RMS RD, JR1, and SMS-CTR cells to define genome-wide chromatin binding of SNAI2 and MYOD, with or without SNAI2 knockdown. Our ChIP-sequencing experiments confirmed that SNAI2 binds EBoxes, including the SNAI2 motif and motifs that MYOD and MYOG engage. Importantly, combining ChIPseq and RNAseq analyses, we discovered that SNAI2 preferentially binds EBox elements associated with enhancer elements and regulates gene expression by dampening enhancer function. SNAI2 competes with MYOD at a subset of myogenic enhancers associated with terminal differentiation, thus blocking differentiation in FN-RMS cells while potentially enabling progrowth effects of MYOD. SNAI2 also downregulates the expression of a MYOG, MEF2A/C/D, and CDKN1A differentiation program that is suppressed in FN-RMS cells, which upon reactivation along with MYOD drives robust differentiation and inhibits tumor xenograft growth in mice. Finally, we establish that SNAI2 function is downstream of the RAS program involved in blocking differentiation. In summary, SNAI2, through inhibition of a MYOD, MYOG, MEF2, and CDNK1A program, blocks tumor differentiation and promotes growth in FN-RMS.

Citation Format: Silvia Pomella, Prethish Sreenivas, Berkley E. Gryder, Long Wang, Matteo Cassandri, Kunal Baxi, Nicole R. Hensch, Elena Carcarino, Young Song, Marielle Yohe, Bruno Amadio, Ignazio Caruana, Cristiano De Stefanis, Rita De Vito, Franco Locatelli, Yidong Chen, Eleanor Y. Chen, Peter Houghton, Javed Khan, Rossella Rota, Myron S. Ignatius. Liaison between SNAI2 and MYOD enhances oncogenesis and suppresses differentiation in fusion-negative rhabdomyosarcoma [abstract]. In: Proceedings of the AACR Special Conference on the Advances in Pediatric Cancer Research; 2019 Sep 17-20; Montreal, QC, Canada. Philadelphia (PA): AACR; Cancer Res 2020;80(14 Suppl):Abstract nr B35.

tp53 deficiency causes a wide tumor spectrum and increases embryonal rhabdomyosarcoma metastasis in zebrafish

The TP53 tumor-suppressor gene is mutated in >50% of human tumors and Li-Fraumeni patients with germ line inactivation are predisposed to developing cancer. Here, we generated tp53 deleted zebrafish that spontaneously develop malignant peripheral nerve-sheath tumors, angiosarcomas, germ cell tumors, and an aggressive Natural Killer cell-like leukemia for which no animal model has been developed. Because the tp53 deletion was generated in syngeneic zebrafish, engraftment of fluorescent-labeled tumors could be dynamically visualized over time. Importantly, engrafted tumors shared gene expression signatures with predicted cells of origin in human tissue. Finally, we showed that tp53^del/del enhanced invasion and metastasis in kRAS^G12D-induced embryonal rhabdomyosarcoma (ERMS), but did not alter the overall frequency of cancer stem cells, suggesting novel pro-metastatic roles for TP53 loss-of-function in human muscle tumors. In summary, we have developed a Li-Fraumeni zebrafish model that is amenable to large-scale transplantation and direct visualization of tumor growth in live animals.

Assessing Lysosomal Alkalinization in the Intestine of Live Caenorhabditis elegans

The nematode Caenorhabditis elegans (C. elegans) is a model system that is widely used to study longevity and developmental pathways. Such studies are facilitated by the transparency of the animal, the ability to do forward and reverse genetic assays, the relative ease of generating fluorescently labeled proteins, and the use of fluorescent dyes that can either be microinjected into the early embryo or incorporated into its food (E. coli strain OP50) to label cellular organelles (e.g. 9-diethylamino-5H-benzo(a)phenoxazine-5-one and (3-{2-[(1H,1'H-2,2'-bipyrrol-5-yl-kappaN(1))methylidene]-2H-pyrrol-5-yl-kappaN}-N-[2-(dimethylamino)ethyl]propanamidato)(difluoro)boron). Here, we present the use of a fluorescent pH-sensitive dye that stains intestinal lysosomes, providing a visual readout of dynamic, physiological changes in lysosomal acidity in live worms. This protocol does not measure lysosomal pH, but rather aims to establish a reliable method of assessing physiological relevant variations in lysosomal acidity. cDCFDA is a cell-permeant compound that is converted to the fluorescent fluorophore 5-(and-6)-carboxy-2',7'-dichlorofluorescein (cDCF) upon hydrolysis by intracellular esterases. Protonation inside lysosomes traps cDCF in these organelles, where it accumulates. Due to its low pKa of 4.8, this dye has been used as a pH sensor in yeast. Here we describe the use of cDCFDA as a food supplement to assess the acidity of intestinal lysosomes in C. elegans. This technique allows for the detection of alkalinizing lysosomes in live animals, and has a broad range of experimental applications including studies on aging, autophagy, and lysosomal biogenesis.

Regulation of Lysosomal Function by the DAF-16 Forkhead Transcription Factor Couples Reproduction to Aging in Caenorhabditis elegans

Aging in eukaryotes is accompanied by widespread deterioration of the somatic tissue. Yet, abolishing germ cells delays the age-dependent somatic decline in Caenorhabditis elegans In adult worms lacking germ cells, the activation of the DAF-9/DAF-12 steroid signaling pathway in the gonad recruits DAF-16 activity in the intestine to promote longevity-associated phenotypes. However, the impact of this pathway on the fitness of normally reproducing animals is less clear. Here, we explore the link between progeny production and somatic aging and identify the loss of lysosomal acidity-a critical regulator of the proteolytic output of these organelles-as a novel biomarker of aging in C. elegans The increase in lysosomal pH in older worms is not a passive consequence of aging, but instead is timed with the cessation of reproduction, and correlates with the reduction in proteostasis in early adult life. Our results further implicate the steroid signaling pathway and DAF-16 in dynamically regulating lysosomal pH in the intestine of wild-type worms in response to the reproductive cycle. In the intestine of reproducing worms, DAF-16 promotes acidic lysosomes by upregulating the expression of v-ATPase genes. These findings support a model in which protein clearance in the soma is linked to reproduction in the gonad via the active regulation of lysosomal acidification.

A Genome Scale Screen for Mutants with Delayed Exit from Mitosis: Ire1-Independent Induction of Autophagy Integrates ER Homeostasis into Mitotic Lifespan

Proliferating eukaryotic cells undergo a finite number of cell divisions before irreversibly exiting mitosis. Yet pathways that normally limit the number of cell divisions remain poorly characterized. Here we describe a screen of a collection of 3762 single gene mutants in the yeast Saccharomyces cerevisiae, accounting for 2/3 of annotated yeast ORFs, to search for mutants that undergo an atypically high number of cell divisions. Many of the potential longevity genes map to cellular processes not previously implicated in mitotic senescence, suggesting that regulatory mechanisms governing mitotic exit may be broader than currently anticipated. We focused on an ER-Golgi gene cluster isolated in this screen to determine how these ubiquitous organelles integrate into mitotic longevity. We report that a chronic increase in ER protein load signals an expansion in the assembly of autophagosomes in an Ire1-independent manner, accelerates trafficking of high molecular weight protein aggregates from the cytoplasm to the vacuoles, and leads to a profound enhancement of daughter cell production. We demonstrate that this catabolic network is evolutionarily conserved, as it also extends reproductive lifespan in the nematode Caenorhabditis elegans. Our data provide evidence that catabolism of protein aggregates, a natural byproduct of high protein synthesis and turn over in dividing cells, is among the drivers of mitotic longevity in eukaryotes.

High throughput studies have yielded large collections of genes that together govern post-mitotic longevity in eukaryotic cells. However, it is also clear that mitotic lifespan is subject to regulation via intricate mechanisms that facilitate exit from mitosis. Elucidating these mechanisms has been the subject of intensive research in part because failure to exit mitosis is associated with cell immortalization, a hallmark of neoplastic growth. Yet, to date mechanisms driving mitotic lifespan remain poorly characterized largely due to the absence of a feasible high throughput screening platform. Here we describe a large-scale screen in yeast Saccharomyces cerevisiae for mutants that undergo an atypically high number of cell divisions before exiting mitosis. We report an intricate cross talk between Endoplasmic Reticulum (ER) homeostasis and mitotic longevity. Autophagy, activated in response to ER stress, delays mitotic senescence in part by removing high molecular weight cytoplasmic protein aggregates. This evolutionarily conserved catabolic network similarly extends reproductive lifespan in the nematode Caenorhabditis elegans. Our data highlight that, similar to its role in extending post-mitotic lifespan, catabolism of protein aggregates is among the drivers of mitotic longevity in eukaryotes.