Ünal Publications
2022
Meiotic Nuclear Pore Complex Remodeling Provides Key Insights into Nuclear Basket Organization. Journal of Cell Biology
Grant A King*, Rahel Wettstein*, Joseph M Warberg, Keerthana Chetlapalli, Madison E Walsh, Ludovic Gillet, Claudia Hernandez-Armenta, Pedro Beltrao, Ruedi Aebersold, Sue L Jaspersen, Joao Matos# and Elçin Ünal# (*co-first, # co-corresponding)
Nuclear pore complexes (NPCs) are large proteinaceous assemblies that mediate nuclear compartmentalization. NPCs undergo largescale structural rearrangements during mitosis in metazoans and some fungi. However, our understanding of NPC remodeling beyond mitosis remains limited. Using time-lapse fluorescence microscopy, we discovered that NPCs undergo two mechanistically-separable remodeling events during budding yeast meiosis whereby parts or all of the nuclear basket transiently dissociate from the NPC core during meiosis I and II, respectively. Meiosis I detachment, observed for Nup60 and Nup2, is driven by Polo kinase-mediated phosphorylation of Nup60 at its interface with the Y-complex. Subsequent reattachment of Nup60-Nup2 to the NPC core is mediated by a lipid-binding amphipathic helix in Nup60. Preventing Nup60-Nup2 reattachment causes misorganization of the entire nuclear basket in gametes. Strikingly, meiotic nuclear basket remodeling also occurs in the distantly related fission yeast, Schizosaccharomyces pombe. Our study reveals a conserved and developmentally programmed aspect of NPC plasticity, providing key mechanistic insights into nuclear basket organization.
Gametogenesis: Exploring an Endogenous Rejuvenation Program to Understand Cellular Aging and Quality Control. Annual Review of Genetics
Tina L Sing, Gloria A Brar and Elçin Ünal
Gametogenesis is a conserved developmental program whereby a diploid progenitor cell differentiates into haploid gametes, the precursors for sexually reproducing organisms. In addition to ploidy reduction and extensive organelle remodeling, gametogenesis naturally rejuvenates the ensuing gametes, leading to resetting of life span. Excitingly, ectopic expression of the gametogenesis-specific transcription factor Ndt80 is sufficient to extend life span in mitotically dividing budding yeast, suggesting that meiotic rejuvenation pathways can be repurposed outside of their natural context. In this review, we highlight recent studies of gametogenesis that provide emerging insight into natural quality control, organelle remodeling, and rejuvenation strategies that exist within a cell. These include selective inheritance, programmed degradation, and de novo synthesis, all of which are governed by the meiotic gene expression program entailing many forms of noncanonical gene regulation. Finally, we highlight critical questions that remain in the field and provide perspective on the implications of gametogenesis research on human health span.
Meiotic cDNA Libraries Reveal Gene Truncations and Mitochondrial Proteins Important for Competitive Fitness in Saccharomyces Cerevisiae. GENETICS
Tina L Sing, Katie Conlon, Stephanie H Lu, Nicole Madrazo, Kaitlin Morse, Juliet C Barker, Ina Hollerer, Gloria A Brar, Peter H Sudmant# and Elçin Ünal# (# co-corresponding)
Gametogenesis is an evolutionarily conserved developmental program whereby a diploid progenitor cell undergoes meiosis and cellular remodeling to differentiate into haploid gametes, the precursors for sexual reproduction. Even in the simple eukaryotic organism Saccharomyces cerevisiae, the meiotic transcriptome is very rich and complex, thereby necessitating new tools for functional studies. Here, we report the construction of 5 stage-specific, inducible complementary DNA libraries from meiotic cells that represent over 84% of the genes found in the budding yeast genome. We employed computational strategies to detect endogenous meiotic transcript isoforms as well as library-specific gene truncations. Furthermore, we developed a robust screening pipeline to test the effect of each complementary DNA on competitive fitness. Our multiday proof-of-principle time course revealed 877 complementary DNAs that were detrimental for competitive fitness when overexpressed. The list included mitochondrial proteins that cause dose-dependent disruption of cellular respiration as well as library-specific gene truncations that expose a dominant negative effect on competitive growth. Together, these high-quality complementary DNA libraries provide an important tool for systematically identifying meiotic genes, transcript isoforms, and protein domains that are important for a specific biological function.
Long Undecoded Transcript Isoform (LUTI) Detection in Meiotic Budding Yeast by Direct RNA and Transcript Leader Sequencing. STAR Protocols
Amy Tresenrider, Minghao Chia, Folkert J van Werven and Elçin Ünal
LUTIs (Long Undecoded Transcript Isoforms) are 5'-extended and poorly translated mRNAs that can downregulate transcription from promoters more proximal to a gene's coding sequence (CDS). In this protocol, polyA RNA is extracted from budding yeast cells undergoing highly synchronized meiosis. Using a combination of long-read direct RNA sequencing and transcript leader sequencing (TL-seq), meiosis-specific LUTIs are systematically identified. Following identification, TL-seq is used to quantify the abundance of both LUTI and the more canonical gene-proximal (PROX) transcripts. For complete details on the use and execution of this protocol, please refer to Tresenrider et al. (2021).
2021
The Power of Perpetual Collaboration: An Interview with Elçin Ünal and Gloria Brar. Molecular Cell
Here, Elçin Ünal and Gloria Brar tell us how the Br-Ün Lab came to be, the cons, but mostly the pros, of running a joint lab and things to consider, as well as their philosophies in research and mentoring a diverse group of scientists.
Integrated Genomic Analysis Reveals Key Features of Long Undecoded Transcript Isoform-based Gene Repression. Molecular Cell
Amy Tresenrider, Kate Morse, Victoria Jorgensen, Minghao Chia, Hanna Liao, Folkert J van Werven and Elçin Ünal
Long undecoded transcript isoforms (LUTIs) represent a class of non-canonical mRNAs that downregulate gene expression through the combined act of transcriptional and translational repression. While single gene studies revealed important aspects of LUTI-based repression, how these features affect gene regulation on a global scale is unknown. Using transcript leader and direct RNA sequencing, here, we identify 74 LUTI candidates that are specifically induced in meiotic prophase. Translational repression of these candidates appears to be ubiquitous and is dependent on upstream open reading frames. However, LUTI-based transcriptional repression is variable. In only 50% of the cases, LUTI transcription causes downregulation of the protein-coding transcript isoform. Higher LUTI expression, enrichment of histone 3 lysine 36 trimethylation, and changes in nucleosome position are the strongest predictors of LUTI-based transcriptional repression. We conclude that LUTIs downregulate gene expression in a manner that integrates translational repression, chromatin state changes, and the magnitude of LUTI expression.
Meiotic Regulation of the Ndc80 Complex Composition and Function. Current Genetics
Jingxun Chen and Elçin Ünal
This review describes the current models for how the subunit abundance of the Ndc80 complex, a key kinetochore component, is regulated in budding yeast and metazoan meiosis. The past decades of kinetochore research have established the Ndc80 complex to be a key microtubule interactor and a central hub for regulating chromosome segregation. Recent studies further demonstrate that Ndc80 is the limiting kinetochore subunit that dictates the timing of kinetochore activation in budding yeast meiosis. Here, we discuss the molecular circuits that regulate Ndc80 protein synthesis and degradation in budding yeast meiosis and compare the findings with those from metazoans. We envision the regulatory principles discovered in budding yeast to be conserved in metazoans, thereby providing guidance into future investigations on kinetochore regulation in human health and disease.
Meiotic Cells Counteract Programmed Retrotransposon Activation via RNA-Binding Translational Repressor Assemblies. Developmental Cell
Raphaelle Laureau, Annie Dyatel, Gizem Dursuk, Samantha Brown, Hannah Adeoye, Jia-Xing Yue, Matteo De Chiara, Anthony Harris, Elçin Ünal, Gianni Liti, Ian R Adams and Luke E Berchowitz
Retrotransposon proliferation poses a threat to germline integrity. While retrotransposons must be activated in developing germ cells in order to survive and propagate, how they are selectively activated in the context of meiosis is unclear. We demonstrate that the transcriptional activation of Ty3/Gypsy retrotransposons and host defense are controlled by master meiotic regulators. We show that budding yeast Ty3/Gypsy co-opts binding sites of the essential meiotic transcription factor Ndt80 upstream of the integration site, thereby tightly linking its transcriptional activation to meiotic progression. We also elucidate how yeast cells thwart Ty3/Gypsy proliferation by blocking translation of the retrotransposon mRNA using amyloid-like assemblies of the RNA-binding protein Rim4. In mammals, several inactive Ty3/Gypsy elements are undergoing domestication. We show that mammals utilize equivalent master meiotic regulators (Stra8, Mybl1, Dazl) to regulate Ty3/Gypsy-derived genes in developing gametes. Our findings inform how genes that are evolving from retrotransposons can build upon existing regulatory networks during domestication.
2020
Angelika Amon. Developmental Cell
Gloria A Brar and Elçin Ünal
In 2015, Angelika Amon presented a seminar as part of the prestigious Harvey Society lecture series, describing the breakthrough that cemented her iconic status in the cell cycle field. Together with her postdoctoral fellow Rosella Visintin, Angelika had shown that exit from mitosis and return to G1 was mediated by the regulation of the phosphatase Cdc14, which antagonizes cyclin-dependent kinase (CDK) activity and resets CDK-based marks when it is released from inhibitory tethering to a protein partner in the nucleolus. In this lecture, she explained the results of two seminal papers concisely and brilliantly: they reduced the abstract idea of mitotic exit to a simple problem of regulating one protein-protein interaction. Much of the “mitotic exit network” (MEN) regulation that her lab led the way in defining, based on these initial inroads, would subsequently be found to be conserved in the Hippo pathway, which controls tissue growth and size control in animals. This set of findings, and her description of them, encapsulate Angelika’s science. Over her far too short 23-year career as an independent PI, she tackled complex but fundamental questions with rigor and enthusiasm, carefully unearthing clear principles that became foundations for emergent fields. Her ability to derive and communicate clarity from complex biological systems was unparalleled, and her loss will be felt in many facets of biological research.
Cellular Quality Control during Gametogenesis. Experimental Cell Research
Jay S Goodman, Grant A King and Elçin Ünal
A hallmark of aging is the progressive accumulation of cellular damage. Age-induced damage arises due to a decrease in organelle function along with a decline in protein quality control. Although somatic tissues deteriorate with age, the germline must maintain cellular homeostasis in order to ensure the production of healthy progeny. While germline quality control has been primarily studied in multicellular organisms, recent evidence suggests the existence of gametogenesis-specific quality control mechanisms in unicellular eukaryotes, highlighting the evolutionary conservation of meiotic events beyond chromosome morphogenesis. Notably, budding yeast eliminates age-induced damage during meiotic differentiation, employing novel organelle and protein quality control mechanisms to produce young and healthy gametes. Similarly, organelle and protein quality control is present in metazoan gametogenesis; however, whether and how these mechanisms contribute to cellular rejuvenation requires further investigation. Here, we summarize recent findings that describe organelle and protein quality control in budding yeast gametogenesis, examine similar quality control mechanisms in metazoan development, and identify research directions that will improve our understanding of meiotic cellular rejuvenation.
Tunable Transcriptional Interference at the Endogenous Alcohol Dehydrogenase Gene Locus in Drosophila Melanogaster. G3
Victoria Jorgensen, Jingxun Chen*, Helen Vander Wende*, Devon E Harris, Alicia McCarthy, Shane Breznak, Siu Wah Wong-Deyrup, Yuzhang Chen, Prashanth Rangan, Gloria A Brar, Eric M Sawyer#, Leon Y Chan#, Elçin Ünal# (* co-second, # co-corresponding)
Neighboring sequences of a gene can influence its expression. In the phenomenon known as transcriptional interference, transcription at one region in the genome can repress transcription at a nearby region in cis Transcriptional interference occurs at a number of eukaryotic loci, including the alcohol dehydrogenase (Adh) gene in Drosophila melanogaster Adh is regulated by two promoters, which are distinct in their developmental timing of activation. It has been shown using transgene insertion that when the promoter distal from the Adh start codon is deleted, transcription from the proximal promoter becomes de-regulated. As a result, the Adh proximal promoter, which is normally active only during the early larval stages, becomes abnormally activated in adults. Whether this type of regulation occurs in the endogenous Adh context, however, remains unclear. Here, we employed the CRISPR/Cas9 system to edit the endogenous Adh locus and found that removal of the distal promoter also resulted in the untimely expression of the proximal promoter-driven mRNA isoform in adults, albeit at lower levels than previously reported. Importantly, transcription from the distal promoter was sufficient to repress proximal transcription in larvae, and the degree of this repression was dependent on the degree of distal promoter activity. Finally, upregulation of the distal Adh transcript led to the enrichment of histone 3 lysine 36 trimethylation over the Adh proximal promoter. We conclude that the endogenous Adh locus is developmentally regulated by transcriptional interference in a tunable manner.
Aurora B-dependent Ndc80 Degradation Regulates Kinetochore Composition in Meiosis . Genes & Development
Jingxun Chen, Andrew Liao, Emily N Powers, Hanna Liao, Lori A Kohlstaedt, Rena Evans, Ryan M Holly, Jenny Kim Kim, Marko Jovanovic and Elçin Ünal
The kinetochore complex is a conserved machinery that connects chromosomes to spindle microtubules. During meiosis, the kinetochore is restructured to accommodate a specialized chromosome segregation pattern. In budding yeast, meiotic kinetochore remodeling is mediated by the temporal changes in the abundance of a single subunit called Ndc80. We previously described the regulatory events that control the timely synthesis of Ndc80. Here, we report that Ndc80 turnover is also tightly regulated in meiosis: Ndc80 degradation is active in meiotic prophase, but not in metaphase I. Ndc80 degradation depends on the ubiquitin ligase APCAma1 and is mediated by the proteasome. Importantly, Aurora B-dependent Ndc80 phosphorylation, a mark that has been previously implicated in correcting erroneous microtubule-kinetochore attachments, is essential for Ndc80 degradation in a microtubule-independent manner. The N terminus of Ndc80, including a 27-residue sequence and Aurora B phosphorylation sites, is both necessary and sufficient for kinetochore protein degradation. Finally, defects in Ndc80 turnover predispose meiotic cells to chromosome mis-segregation. Our study elucidates the mechanism by which meiotic cells modulate their kinetochore composition through regulated Ndc80 degradation, and demonstrates that Aurora B-dependent regulation of kinetochores extends beyond altering microtubule attachments.
The Dynamic Nuclear Periphery as a Facilitator of Gamete Health and Rejuvenation. Current Genetics
Grant A King and Elçin Ünal
The nuclear periphery is a hotspot for the accumulation of age-induced damage in eukaryotic cells. The types of damage that occur at the periphery and their phenotypic consequences have begun to be characterized; however, the mechanisms by which cells repair or eliminate nuclear damage remain poorly understood. Using budding yeast meiosis as a natural system to study cellular rejuvenation, we recently discovered a novel nuclear quality control event, in which age-induced damage is sequestered away from dividing chromosomes to a discarded nuclear compartment that we term the GUNC (for "Gametogenesis Uninherited Nuclear Compartment"). Interestingly, extensive nuclear remodeling occurs even in young cells, including a surprising modularity of the nuclear pore complex, suggesting a general contribution to gamete fitness. In this review, we discuss these findings in the context of recent evidence that the nuclear periphery is a highly dynamic region critical for cellular health.
2019
Meiotic Cellular Rejuvenation is Coupled to Nuclear Remodeling in Budding Yeast. eLife
Grant A King*, Jay S Goodman*, Jennifer G Schick, Keerthana Chetlapalli, Danielle M Jorgens, Kent L McDonald and Elçin Ünal (* co-first)
Production of healthy gametes in meiosis relies on the quality control and proper distribution of both nuclear and cytoplasmic contents. Meiotic differentiation naturally eliminates age-induced cellular damage by an unknown mechanism. Using time-lapse fluorescence microscopy in budding yeast, we found that nuclear senescence factors - including protein aggregates, extrachromosomal ribosomal DNA circles, and abnormal nucleolar material - are sequestered away from chromosomes during meiosis II and subsequently eliminated. A similar sequestration and elimination process occurs for the core subunits of the nuclear pore complex in both young and aged cells. Nuclear envelope remodeling drives the formation of a membranous compartment containing the sequestered material. Importantly, de novo generation of plasma membrane is required for the sequestration event, preventing the inheritance of long-lived nucleoporins and senescence factors into the newly formed gametes. Our study uncovers a new mechanism of nuclear quality control and provides insight into its function in meiotic cellular rejuvenation.
Evidence for an Integrated Gene Repression Mechanism Based on mRNA Isoform Toggling in Human Cells. G3
Ina Hollerer, Juliet C Barker, Victoria Jorgensen, Amy Tresenrider, Claire Dugast-Darzacq, Leon Y Chan, Xavier Darzacq, Robert Tjian, Elçin Ünal# and Gloria A Brar# (# co-corresponding)
We recently described an unconventional mode of gene regulation in budding yeast by which transcriptional and translational interference collaborate to down-regulate protein expression. Developmentally timed transcriptional interference inhibited production of a well translated mRNA isoform and resulted in the production of an mRNA isoform containing inhibitory upstream open reading frames (uORFs) that prevented translation of the main ORF. Transcriptional interference and uORF-based translational repression are established mechanisms outside of yeast, but whether this type of integrated regulation was conserved was unknown. Here we find that, indeed, a similar type of regulation occurs at the locus for the human oncogene MDM2 We observe evidence of transcriptional interference between the two MDM2 promoters, which produce a poorly translated distal promoter-derived uORF-containing mRNA isoform and a well-translated proximal promoter-derived transcript. Down-regulation of distal promoter activity markedly up-regulates proximal promoter-driven expression and results in local reduction of histone H3K36 trimethylation. Moreover, we observe that this transcript toggling between the two MDM2 isoforms naturally occurs during human embryonic stem cell differentiation programs.
Developmental Regulation of an Organelle Tether Coordinates Mitochondrial Remodeling in Meiosis. Journal of Cell Biology
Eric M Sawyer, Pallavi R Joshi, Victoria Jorgensen, Julius Yunus, Luke E Berchowitz and Elçin Ünal
Cellular differentiation involves remodeling cellular architecture to transform one cell type to another. By investigating mitochondrial dynamics during meiotic differentiation in budding yeast, we sought to understand how organelle morphogenesis is developmentally controlled in a system where regulators of differentiation and organelle architecture are known, but the interface between them remains unexplored. We analyzed the regulation of mitochondrial detachment from the cell cortex, a known meiotic alteration to mitochondrial morphology. We found that mitochondrial detachment is enabled by the programmed destruction of the mitochondria-endoplasmic reticulum-cortex anchor (MECA), an organelle tether that bridges mitochondria and the plasma membrane. MECA regulation is governed by a meiotic transcription factor, Ndt80, which promotes the activation of a conserved kinase, Ime2. We further present evidence for Ime2-dependent phosphorylation and degradation of MECA in a temporally controlled manner. Our study defines a key mechanism that coordinates mitochondrial morphogenesis with the landmark events of meiosis and demonstrates that cells can developmentally regulate tethering to induce organelle remodeling.
2018
Single Molecule Fluorescence In Situ Hybridization (smFISH) Analysis in Budding Yeast Vegetative Growth and Meiosis. Journal of Visual Experimentation
Single molecule fluorescence in situ hybridization (smFISH) is a powerful technique to study gene expression in single cells due to its ability to detect and count individual RNA molecules. Complementary to deep sequencing-based methods, smFISH provides information about the cell-to-cell variation in transcript abundance and the subcellular localization of a given RNA. Recently, we have used smFISH to study the expression of the gene NDC80 during meiosis in budding yeast, in which two transcript isoforms exist and the short transcript isoform has its entire sequence shared with the long isoform. To confidently identify each transcript isoform, we optimized known smFISH protocols and obtained high consistency and quality of smFISH data for the samples acquired during budding yeast meiosis. Here, we describe this optimized protocol, the criteria that we use to determine whether high quality of smFISH data is obtained, and some tips for implementing this protocol in other yeast strains and growth conditions.
Ensuring Fidelity of Chromosome Segregation. Molecular Biology of the Cell
Elçin Ünal and Jorge Z Torres
At the first joint ASCB EMBO Meeting, the fields of cell division, cell cycle, and cell death took the center stage in three separate Minisymposia. In this review, we highlight some of the topics presented in the Minisymposium entitled “Ensuring Fidelity of Chromosome Segregation.”
One-two Punch Mechanism of Gene Repression: a Fresh Perspective on Gene Regulation. Current Genetics
Amy Tresenrider and Elçin Ünal
Cellular differentiation depends on temporally controlled waves of gene activation and inactivation that ultimately transform one cell type into another. It is well established that transcription factor cascades coordinate the timely activation of gene expression clusters during development. In comparison, much less is understood about how gene repression events are coordinated with the transcription factor-driven waves of gene activation and how this repression is achieved at a mechanistic level. Using budding yeast as a model, we recently discovered a new gene regulatory event, whereby a central meiotic transcription factor induces the expression of an mRNA isoform to repress gene expression through an integrated transcriptional and translational mechanism. This new model could explain how gene activation and inactivation waves can be temporally coordinated. In this review, we discuss our findings and their potential implications.
2017
Kinetochore Inactivation by Expression of a Repressive mRNA. eLife
Jingxun Chen*, Amy Tresenrider*, Minghao Chia, David T McSwiggen, Gianpiero Spedale, Victoria Jorgensen, Hanna Liao, Folkert J van Werven# and Elçin Ünal# (* co-first, # co-corresponding)
Differentiation programs such as meiosis depend on extensive gene regulation to mediate cellular morphogenesis. Meiosis requires transient removal of the outer kinetochore, the complex that connects microtubules to chromosomes. How the meiotic gene expression program temporally restricts kinetochore function is unknown. We discovered that in budding yeast, kinetochore inactivation occurs by reducing the abundance of a limiting subunit, Ndc80. Furthermore, we uncovered an integrated mechanism that acts at the transcriptional and translational level to repress NDC80 expression. Central to this mechanism is the developmentally controlled transcription of an alternate NDC80 mRNA isoform, which itself cannot produce protein due to regulatory upstream ORFs in its extended 5' leader. Instead, transcription of this isoform represses the canonical NDC80 mRNA expression in cis, thereby inhibiting Ndc80 protein synthesis. This model of gene regulation raises the intriguing notion that transcription of an mRNA, despite carrying a canonical coding sequence, can directly cause gene repression.
Transcription of a 5' Extended mRNA Isoform Directs Dynamic Chromatin Changes and Interference of a Downstream Promoter. eLife
Minghao Chia, Amy Tresenrider, Jingxun Chen, Gianpiero Spedale, Victoria Jorgensen, Elçin Ünal# and Folkert J van Werven# (# co-corresponding)
Cell differentiation programs require dynamic regulation of gene expression. During meiotic prophase in Saccharomyces cerevisiae, expression of the kinetochore complex subunit Ndc80 is downregulated by a 5' extended long undecoded NDC80 transcript isoform. Here we demonstrate a transcriptional interference mechanism that is responsible for inhibiting expression of the coding NDC80 mRNA isoform. Transcription from a distal NDC80 promoter directs Set1-dependent histone H3K4 dimethylation and Set2-dependent H3K36 trimethylation to establish a repressive chromatin state in the downstream canonical NDC80 promoter. As a consequence, NDC80 expression is repressed during meiotic prophase. The transcriptional mechanism described here is rapidly reversible, adaptable to fine-tune gene expression, and relies on Set2 and the Set3 histone deacetylase complex. Thus, expression of a 5' extended mRNA isoform causes transcriptional interference at the downstream promoter. We demonstrate that this is an effective mechanism to promote dynamic changes in gene expression during cell differentiation.