Epigenetic cell fate in Candida albicans is controlled by transcription factor condensates acting at super-enhancer-like elements

Microbial Synthetic Epigenetic Tools Design and Applications

Microbial synthetic epigenetics offers significant opportunities for the design of synthetic biology tools by leveraging reversible gene control mechanisms without altering DNA sequences. However, limited understanding and a lack of technologies for thorough analysis of the mechanisms behind epigenetic modifications have hampered their utilization in biotechnological applications. In this review, we explore advancements in developing epigenetic-based synthetic gene regulatory tools at both transcriptional and post-transcriptional levels. Furthermore, we examine strategies developed to construct epigenetic-based circuits that provide controllable and stable gene regulation, aiming to boost the performance of microbial chassis cells. Finally, we discuss the current challenges and perspectives in the development of synthetic epigenetic tools.

The Heat Shock Response as a Condensate Cascade

The heat shock response (HSR) is a gene regulatory program controlling expression of molecular chaperones implicated in aging, cancer, and neurodegenerative disease. Long presumed to be activated by toxic protein aggregates, recent work suggests a new functional paradigm for the HSR in yeast. Rather than toxic aggregates, adaptive biomolecular condensates comprised of orphan ribosomal proteins (oRP) and stress granule components have been shown to be physiological chaperone clients. By titrating away the chaperones Sis1 and Hsp70 from the transcription factor Hsf1, these condensates activate the HSR. Upon release from Hsp70, Hsf1 forms spatially distinct transcriptional condensates that drive high expression of HSR genes. In this manner, the negative feedback loop controlling HSR activity - in which Hsf1 induces Hsp70 expression and Hsp70 represses Hsf1 activity - is embedded in the biophysics of the system. By analogy to phosphorylation cascades that transmit information via the dynamic activity of kinases, we propose that the HSR is organized as a condensate cascade that transmits information via the localized activity of molecular chaperones.

Structure and interactions of prion-like domains in transcription factor Efg1 phase separation

Candida albicans, a prominent member of the human microbiome, can make an opportunistic switch from commensal coexistence to pathogenicity accompanied by an epigenetic shift between the white and opaque cell states. This transcriptional switch is under precise regulation by a set of transcription factors (TFs), with Enhanced Filamentous Growth Protein 1 (Efg1) playing a central role. Previous research has emphasized the importance of Efg1's prion-like domain (PrLD) and the protein's ability to undergo phase separation for the white-to-opaque transition of C. albicans. However, the underlying molecular mechanisms of Efg1 phase separation have remained underexplored. In this study, we delved into the biophysical basis of Efg1 phase separation, revealing the significant contribution of both N-terminal (N) and C-terminal (C) PrLDs. Through NMR structural analysis, we found that Efg1 N-PrLD and C-PrLD are mostly disordered but have prominent partial α-helical secondary structures in both domains. NMR titration experiments suggest that the partially helical structures in N-PrLD act as hubs for self-interaction as well as Efg1 interaction with RNA. Using condensed-phase NMR spectroscopy, we uncovered diverse amino acid interactions underlying Efg1 phase separation. Particularly, we highlight the indispensable role of tyrosine residues within the transient α-helical structures of PrLDs particularly in the N-PrLD compared to the C-PrLD in stabilizing phase separation. Our study provides evidence that the transient α-helical structure is present in the phase-separated state and highlights the particular importance of aromatic residues within these structures for phase separation. Together, these results enhance the understanding of C. albicans transcription factor interactions that lead to virulence and provide a crucial foundation for potential antifungal therapies targeting the transcriptional switch.

Heterotypic interactions can drive selective co-condensation of prion-like low-complexity domains of FET proteins and mammalian SWI/SNF complex

Prion-like domains (PLDs) are low-complexity protein sequences enriched within nucleic acid-binding proteins including those involved in transcription and RNA processing. PLDs of FUS and EWSR1 play key roles in recruiting chromatin remodeler mammalian SWI/SNF (mSWI/SNF) complex to oncogenic FET fusion protein condensates. Here, we show that disordered low-complexity domains of multiple SWI/SNF subunits are prion-like with a strong propensity to undergo intracellular phase separation. These PLDs engage in sequence-specific heterotypic interactions with the PLD of FUS in the dilute phase at sub-saturation conditions, leading to the formation of PLD co-condensates. In the dense phase, homotypic and heterotypic PLD interactions are highly cooperative, resulting in the co-mixing of individual PLD phases and forming spatially homogeneous condensates. Heterotypic PLD-mediated positive cooperativity in protein-protein interaction networks is likely to play key roles in the co-phase separation of mSWI/SNF complex with transcription factors containing homologous low-complexity domains.

The transcription factor Ofi1 is critical for white-opaque switching in natural MTLa/α isolates of Candida albicans

Candida albicans, an opportunistic fungal pathogen, is able to switch between two distinct cell types: white and opaque. While white-to-opaque switching is typically repressed by the a1/α2 heterodimer in MTLa/α cells, it was recently reported that switching can also occur in some natural MTLa/α strains under certain environmental conditions. However, the regulatory program governing white-opaque switching in MTLa/α cells is not fully understood. Here, we collected 90 clinical isolates of C. albicans, 16 of which possess the ability to form opaque colonies. Among the known regulators implicated in white-opaque switching, only OFI1 exhibited significantly higher expression in these 16 strains compared to the reference strain SC5314. Importantly, ectopic expression of OFI1 in both clinical isolates and laboratory strains promoted switching frequency even in the absence of N-acetylglucosamine and high CO2 , the optimal condition for white-to-opaque switching in MTLa/α strains. Deleting OFI1 resulted in a reduction in opaque-formation frequency and the stability of the opaque cell in MTLa/α cells. Ofi1 binds to the promoters of WOR1 and WOR3 to induce their expression, which facilitates white-to-opaque switching. Ofi1 is conserved across the CTG species. Altogether, our study reported the identification of a transcription factor Ofi1 as the critical regulator that promotes white-to-opaque switching in natural MTLa/α isolates of C. albicans.

Macromolecular crowding: Sensing without a sensor

All living cells are crowded with macromolecules. Crowding can directly modulate biochemical reactions to various degrees depending on the sizes, shapes, and binding affinities of the reactants. Here, we explore the possibility that cells can sense and adapt to changes in crowding through the widespread modulation of biochemical reactions without the need for a dedicated sensor. Additionally, we explore phase separation as a general physicochemical response to changes in crowding, and a mechanism to both transduce information and physically restore crowding homeostasis.

Filamentation and biofilm formation are regulated by the phase-separation capacity of network transcription factors in Candida albicans

The ability of the fungus Candida albicans to filament and form biofilms contributes to its burden as a leading cause of hospital-acquired infections. Biofilm development involves an interconnected transcriptional regulatory network (TRN) consisting of nine transcription factors (TFs) that bind both to their own regulatory regions and to those of the other network TFs. Here, we show that seven of the nine TFs in the C. albicans biofilm network contain prion-like domains (PrLDs) that have been linked to the ability to form phase-separated condensates. Construction of PrLD mutants in four biofilm TFs reveals that these domains are essential for filamentation and biofilm formation in C. albicans. Moreover, biofilm PrLDs promote the formation of phase-separated condensates in the nuclei of live cells, and PrLD mutations that abolish phase separation (such as the removal of aromatic residues) also prevent biofilm formation. Biofilm TF condensates can selectively recruit other TFs through PrLD-PrLD interactions and can co-recruit RNA polymerase II, implicating condensate formation in the assembly of active transcriptional complexes. Finally, we show that PrLD mutations that block the phase separation of biofilm TFs also prevent filamentation in an in vivo model of gastrointestinal colonization. Together, these studies associate transcriptional condensates with the regulation of filamentation and biofilm formation in C. albicans, and highlight how targeting of PrLD-PrLD interactions could prevent pathogenesis by this species.

Heterotypic interactions in the dilute phase can drive co-condensation of prion-like low-complexity domains of FET proteins and mammalian SWI/SNF complex

Prion-like domains (PLDs) are low-complexity protein sequences enriched within nucleic acid-binding proteins including those involved in transcription and RNA processing. PLDs of FUS and EWSR1 play key roles in recruiting chromatin remodeler mammalian SWI/SNF complex to oncogenic FET fusion protein condensates. Here, we show that disordered low-complexity domains of multiple SWI/SNF subunits are prion-like with a strong propensity to undergo intracellular phase separation. These PLDs engage in sequence-specific heterotypic interactions with the PLD of FUS in the dilute phase at sub-saturation conditions, leading to the formation of PLD co-condensates. In the dense phase, homotypic and heterotypic PLD interactions are highly cooperative, resulting in the co-mixing of individual PLD phases and forming spatially homogeneous co-condensates. Heterotypic PLD-mediated positive cooperativity in protein-protein interaction networks is likely to play key roles in the co-phase separation of mSWI/SNF complex with transcription factors containing homologous low-complexity domains.

Structure and position-specific interactions of prion-like domains in transcription factor Efg1 phase separation

Candida albicans, a prominent member of the human microbiome, can make an opportunistic switch from commensal coexistence to pathogenicity accompanied by an epigenetic shift between the white and opaque cell states. This transcriptional switch is under precise regulation by a set of transcription factors (TFs), with Enhanced Filamentous Growth Protein 1 (Efg1) playing a central role. Previous research has emphasized the importance of Egf1's prion-like domain (PrLD) and the protein's ability to undergo phase separation for the white-to-opaque transition of C. albicans. However, the underlying molecular mechanisms of Efg1 phase separation have remained underexplored. In this study, we delved into the biophysical basis of Efg1 phase separation, revealing the significant contribution of both N-terminal (N) and C-terminal (C) PrLDs. Through NMR structural analysis, we found that Efg1 N-PrLD and C-PrLD are mostly disordered though have prominent partial α-helical secondary structures in both domains. NMR titration experiments suggest that the partially helical structures in N-PrLD act as hubs for self-interaction as well as Efg1 interaction with RNA. Using condensed-phase NMR spectroscopy, we uncovered diverse amino acid interactions underlying Efg1 phase separation. Particularly, we highlight the indispensable role of tyrosine residues within the transient α-helical structures of PrLDs particularly in the N-PrLD compared to the C-PrLD in stabilizing phase separation. Our study provides evidence that the transient α-helical structure is present in the phase separated state and highlights the particular importance of aromatic residues within these structures for phase separation. Together, these results enhance the understanding of C. albicans TF interactions that lead to virulence and provide a crucial foundation for potential antifungal therapies targeting the transcriptional switch.

[Role of Liquid-Liquid Phase Separation in Cell Fate Transition and Diseases]

Liquid-liquid phase separation (LLPS), a novel mechanism of the organization and formation of cellular structures, plays a vital role in regulating cell fate transitions and disease pathogenesis and is gaining widespread attention. LLPS may lead to the assemblage of cellular structures with liquid-like fluidity, such as germ granules, stress granules, and nucleoli, which are classic membraneless organelles. These structures are typically formed through the high-concentration liquid aggregation of biomacromolecules driven by weak multivalent interactions. LLPS is involved in regulating various intracellular life activities and its dysregulation may cause the disruption of cellular functions, thereby contributing to the pathogenesis and development of neurodegenerative diseases, infectious diseases, cancers, etc. Herein, we summarized published findings on the LLPS dynamics of membraneless organelles in physiological and pathological cell fate transition, revealing their crucial roles in cell differentiation, development, and various pathogenic processes. This paper provides a fresh theoretical framework and potential therapeutic targets for LLPS-related studies, opening new avenues for future research.

Multifactor transcriptional control of alternative oxidase induction integrates diverse environmental inputs to enable fungal virulence

Metabolic flexibility enables fungi to invade challenging host environments. In Candida albicans, a common cause of life-threatening infections in humans, an important contributor to flexibility is alternative oxidase (Aox) activity. Dramatic induction of this activity occurs under respiratory-stress conditions, which impair the classical electron transport chain (ETC). Here, we show that deletion of the inducible AOX2 gene cripples C. albicans virulence in mice by increasing immune recognition. To investigate further, we examined transcriptional regulation of AOX2 in molecular detail under host-relevant, ETC-inhibitory conditions. We found that multiple transcription factors, including Rtg1/Rtg3, Cwt1/Zcf11, and Zcf2, bind and regulate the AOX2 promoter, conferring thousand-fold levels of inducibility to AOX2 in response to distinct environmental stressors. Further dissection of this complex promoter revealed how integration of stimuli ranging from reactive species of oxygen, nitrogen, and sulfur to reduced copper availability is achieved at the transcriptional level to regulate AOX2 induction and enable pathogenesis.

Super-enhancer landscape rewiring in cancer: The epigenetic control at distal sites

Super-enhancers evolve as elements at the top of the hierarchical control of gene expression. They are important end-gatherers of signaling pathways that control stemness, differentiation or adaptive responses. Many epigenetic regulations focus on these regions, and not surprisingly, during the process of tumorigenesis, various alterations can account for their dysfunction. Super-enhancers are emerging as key drivers of the aberrant gene expression landscape that sustain the aggressiveness of cancer cells. In this review, we will describe and discuss about the structure of super-enhancers, their epigenetic regulation, and the major changes affecting their functionality in cancer.

Integrated analysis of SR-like protein kinases Sky1 and Sky2 links signaling networks with transcriptional regulation in Candida albicans

Fungal infections are a major global health burden where Candida albicans is among the most common fungal pathogen in humans and is a common cause of invasive candidiasis. Fungal phenotypes, such as those related to morphology, proliferation and virulence are mainly driven by gene expression, which is primarily regulated by kinase signaling cascades. Serine-arginine (SR) protein kinases are highly conserved among eukaryotes and are involved in major transcriptional processes in human and S. cerevisiae. Candida albicans harbors two SR protein kinases, while Sky2 is important for metabolic adaptation, Sky1 has similar functions as in S. cerevisiae. To investigate the role of these SR kinases for the regulation of transcriptional responses in C. albicans, we performed RNA sequencing of sky1Δ and sky2Δ and integrated a comprehensive phosphoproteome dataset of these mutants. Using a Systems Biology approach, we study transcriptional regulation in the context of kinase signaling networks. Transcriptomic enrichment analysis indicates that pathways involved in the regulation of gene expression are downregulated and mitochondrial processes are upregulated in sky1Δ. In sky2Δ, primarily metabolic processes are affected, especially for arginine, and we observed that arginine-induced hyphae formation is impaired in sky2Δ. In addition, our analysis identifies several transcription factors as potential drivers of the transcriptional response. Among these, a core set is shared between both kinase knockouts, but it appears to regulate different subsets of target genes. To elucidate these diverse regulatory patterns, we created network modules by integrating the data of site-specific protein phosphorylation and gene expression with kinase-substrate predictions and protein-protein interactions. These integrated signaling modules reveal shared parts but also highlight specific patterns characteristic for each kinase. Interestingly, the modules contain many proteins involved in fungal morphogenesis and stress response. Accordingly, experimental phenotyping shows a higher resistance to Hygromycin B for sky1Δ. Thus, our study demonstrates that a combination of computational approaches with integration of experimental data can offer a new systems biological perspective on the complex network of signaling and transcription. With that, the investigation of the interface between signaling and transcriptional regulation in C. albicans provides a deeper insight into how cellular mechanisms can shape the phenotype.

Phase separation in fungi

Phase separation, in which macromolecules partition into a concentrated phase that is immiscible with a dilute phase, is involved with fundamental cellular processes across the tree of life. We review the principles of phase separation and highlight how it impacts diverse processes in the fungal kingdom. These include the regulation of autophagy, cell signalling pathways, transcriptional circuits and the establishment of asymmetry in fungal cells. We describe examples of stable, phase-separated assemblies including membraneless organelles such as the nucleolus as well as transient condensates that also arise through phase separation and enable cells to rapidly and reversibly respond to important environmental cues. We showcase how research into phase separation in model yeasts, such as Saccharomyces cerevisiae and Schizosaccharomyces pombe, in conjunction with that in plant and human fungal pathogens, such as Ashbya gossypii and Candida albicans, is continuing to enrich our understanding of fundamental molecular processes.

Architectural groups of a subtelomeric gene family evolve along distinct paths in Candida albicans

Subtelomeres are dynamic genomic regions shaped by elevated rates of recombination, mutation, and gene birth/death. These processes contribute to formation of lineage-specific gene family expansions that commonly occupy subtelomeres across eukaryotes. Investigating the evolution of subtelomeric gene families is complicated by the presence of repetitive DNA and high sequence similarity among gene family members that prevents accurate assembly from whole genome sequences. Here, we investigated the evolution of the telomere-associated (TLO) gene family in Candida albicans using 189 complete coding sequences retrieved from 23 genetically diverse strains across the species. Tlo genes conformed to the 3 major architectural groups (α/β/γ) previously defined in the genome reference strain but significantly differed in the degree of within-group diversity. One group, Tloβ, was always found at the same chromosome arm with strong sequence similarity among all strains. In contrast, diverse Tloα sequences have proliferated among chromosome arms. Tloγ genes formed 7 primary clades that included each of the previously identified Tloγ genes from the genome reference strain with 3 Tloγ genes always found on the same chromosome arm among strains. Architectural groups displayed regions of high conservation that resolved newly identified functional motifs, providing insight into potential regulatory mechanisms that distinguish groups. Thus, by resolving intraspecies subtelomeric gene variation, it is possible to identify previously unknown gene family complexity that may underpin adaptive functional variation.

Overexpression of the White Opaque Switching Master Regulator Wor1 Alters Lipid Metabolism and Mitochondrial Function in Candida albicans

Candida albicans is a commensal yeast that inhabits the gastrointestinal tract of humans; increased colonization of this yeast in this niche has implicated the master regulator of the white-opaque transition, Wor1, by mechanisms not completely understood. We have addressed the role that this transcription factor has on commensalism by the characterization of strains overexpressing this gene. We show that WOR1 overexpression causes an alteration of the total lipid content of the fungal cell and significantly alters the composition of structural and reserve molecular species lipids as determined by lipidomic analysis. These cells are hypersensitive to membrane-disturbing agents such as SDS, have increased tolerance to azoles, an augmented number of peroxisomes, and increased phospholipase activity. WOR1 overexpression also decreases mitochondrial activity and results in altered susceptibility to certain oxidants. All together, these changes reflect drastic alterations in the cellular physiology that facilitate adaptation to the gastrointestinal tract environment.

Collaboration between Antagonistic Cell Type Regulators Governs Natural Variation in the Candida albicans Biofilm and Hyphal Gene Expression Network

Candida albicans is among the most significant human fungal pathogens. However, the vast majority of C. albicans studies have focused on a single clinical isolate and its marked derivatives. We investigated natural variation among clinical C. albicans isolates in gene regulatory control of biofilm formation, a process crucial to virulence. The transcription factor Efg1 is required for biofilm-associated gene expression and biofilm formation. Previously, we found extensive variation in Efg1-responsive gene expression among 5 diverse clinical isolates. However, chromatin immunoprecipitation sequencing analysis showed that Efg1 binding to genomic loci was uniform among the isolates. Functional dissection of strain differences identified three transcription factors, Brg1, Tec1, and Wor1, for which small changes in expression levels reshaped the Efg1 regulatory network. Brg1 and Tec1 are known biofilm activators, and their role in Efg1 network variation may be expected. However, Wor1 is a known repressor of EFG1 expression and an inhibitor of biofilm formation. In contrast, we found that a modest increase in WOR1 RNA levels, reflecting the expression differences between C. albicans strains, could augment biofilm formation and expression of biofilm-related genes. The analysis of natural variation here reveals a novel function for a well-characterized gene and illustrates that strain diversity offers a unique resource for elucidation of network interactions.

Multiple molecular events underlie stochastic switching between 2 heritable cell states in fungi

Eukaryotic transcriptional networks are often large and contain several levels of feedback regulation. Many of these networks have the ability to generate and maintain several distinct transcriptional states across multiple cell divisions and to switch between them. In certain instances, switching between cell states is stochastic, occurring in a small subset of cells of an isogenic population in a seemingly homogenous environment. Given the scarcity and unpredictability of switching in these cases, investigating the determining molecular events is challenging. White-opaque switching in the fungal species Candida albicans is an example of stably inherited cell states that are determined by a complex transcriptional network and can serve as an experimentally accessible model system to study characteristics important for stochastic cell fate switching in eukaryotes. In standard lab media, genetically identical cells maintain their cellular identity (either "white" or "opaque") through thousands of cell divisions, and switching between the states is rare and stochastic. By isolating populations of white or opaque cells, previous studies have elucidated the many differences between the 2 stable cell states and identified a set of transcriptional regulators needed for cell type switching and maintenance of the 2 cell types. Yet, little is known about the molecular events that determine the rare, stochastic switching events that occur in single cells. We use microfluidics combined with fluorescent reporters to directly observe rare switching events between the white and opaque states. We investigate the stochastic nature of switching by beginning with white cells and monitoring the activation of Wor1, a master regulator and marker for the opaque state, in single cells and throughout cell pedigrees. Our results indicate that switching requires 2 stochastic steps; first an event occurs that predisposes a lineage of cells to switch. In the second step, some, but not all, of those predisposed cells rapidly express high levels of Wor1 and commit to the opaque state. To further understand the rapid rise in Wor1, we used a synthetic inducible system in Saccharomyces cerevisiae into which a controllable C. albicans Wor1 and a reporter for its transcriptional control region have been introduced. We document that Wor1 positive autoregulation is highly cooperative (Hill coefficient > 3), leading to rapid activation and producing an "all or none" rather than a graded response. Taken together, our results suggest that reaching a threshold level of a master regulator is sufficient to drive cell type switching in single cells and that an earlier molecular event increases the probability of reaching that threshold in certain small lineages of cells. Quantitative molecular analysis of the white-opaque circuit can serve as a model for the general understanding of complex circuits.

EFG1, Everyone’s Favorite Gene in Candida albicans: A Comprehensive Literature Review

Candida sp. are among the most common fungal commensals found in the human microbiome. Although Candida can be found residing harmlessly on the surface of the skin and mucosal membranes, these opportunistic fungi have the potential to cause superficial skin, nail, and mucus membrane infections as well as life threatening systemic infections. Severity of infection is dependent on both fungal and host factors including the immune status of the host. Virulence factors associated with Candida sp. pathogenicity include adhesin proteins, degradative enzymes, phenotypic switching, and morphogenesis. A central transcriptional regulator of morphogenesis, the transcription factor Efg1 was first characterized in Candida albicans in 1997. Since then, EFG1 has been referenced in the Candida literature over three thousand times, with the number of citations growing daily. Arguably one of the most well studied genes in Candida albicans, EFG1 has been referenced in nearly all contexts of Candida biology from the development of novel therapeutics to white opaque switching, hyphae morphology to immunology. In the review that follows we will synthesize the research that has been performed on this extensively studied transcription factor and highlight several important unanswered questions.

Role of the WOR1 Promoter of Candida albicans in Opaque Commitment

During induced differentiation, the process often involves a commitment event, after which induced cells, when returned to noninducing conditions, continue to differentiate. The commitment event is rarely identified. Candida albicans differentiates from the white to opaque phenotype, a prerequisite for mating and a process accompanying colonization of the lower gastrointestinal tract and skin. In analyses of white cell populations induced to synchronously differentiate from the white to opaque phenotype, opaque commitment occurs at approximately the same time as evagination and chitin ring formation in the process of daughter cell formation, several hours after the master switch gene WOR1 is upregulated. Mutational analyses of transcription factor binding regions P1, P2, P3, P4, and P6 of the WOR1 promoter reveal that individual deletion of any of the five transcription factor binding regions does not eliminate morphological differentiation to the opaque cell phenotype under opaque-inducing conditions, but individual deletion of P2, P3, or P4, blocks opaque commitment and maintenance of the opaque phenotype after transition to noninducing conditions. These results suggest that commitment occurs at the level of the WOR1 promoter and that morphological differentiation can be dissociated from phenotypic commitment.

Integrative multi-omics profiling reveals cAMP-independent mechanisms regulating hyphal morphogenesis in Candida albicans

Microbial pathogens grow in a wide range of different morphologies that provide distinct advantages for virulence. In the fungal pathogen Candida albicans, adenylyl cyclase (Cyr1) is thought to be a master regulator of the switch to invasive hyphal morphogenesis and biofilm formation. However, faster growing cyr1Δ/Δ pseudorevertant (PR) mutants were identified that form hyphae in the absence of cAMP. Isolation of additional PR mutants revealed that their improved growth was due to loss of one copy of BCY1, the negative regulatory subunit of protein kinase A (PKA) from the left arm of chromosome 2. Furthermore, hyphal morphogenesis was improved in some of PR mutants by multigenic haploinsufficiency resulting from loss of large regions of the left arm of chromosome 2, including global transcriptional regulators. Interestingly, hyphal-associated genes were also induced in a manner that was independent of cAMP. This indicates that basal protein kinase A activity is an important prerequisite to induce hyphae, but activation of adenylyl cyclase is not needed. Instead, phosphoproteomic analysis indicated that the Cdc28 cyclin-dependent kinase and the casein kinase 1 family member Yck2 play key roles in promoting polarized growth. In addition, integrating transcriptomic and proteomic data reveals hyphal stimuli induce increased production of key transcription factors that contribute to polarized morphogenesis.

FUS oncofusion protein condensates recruit mSWI/SNF chromatin remodeler via heterotypic interactions between prion-like domains

Fusion transcription factors generated by genomic translocations are common drivers of several types of cancers including sarcomas and leukemias. Oncofusions of the FET (FUS, EWSR1, and TAF15) family proteins result from the fusion of the prion-like domain (PLD) of FET proteins to the DNA-binding domain (DBD) of certain transcription regulators and are implicated in aberrant transcriptional programs through interactions with chromatin remodelers. Here, we show that FUS-DDIT3, a FET oncofusion protein, undergoes PLD-mediated phase separation into liquid-like condensates. Nuclear FUS-DDIT3 condensates can recruit essential components of the global transcriptional machinery such as the chromatin remodeler SWI/SNF. The recruitment of mammalian SWI/SNF (mSWI/SNF) is driven by heterotypic PLD-PLD interactions between FUS-DDIT3 and core subunits of SWI/SNF, such as the catalytic component BRG1. Further experiments with single-molecule correlative force-fluorescence microscopy support a model wherein the fusion protein forms condensates on DNA surface and enrich BRG1 to activate transcription by ectopic chromatin remodeling. Similar PLD-driven co-condensation of mSWI/SNF with transcription factors can be employed by other oncogenic fusion proteins with a generic PLD-DBD domain architecture for global transcriptional reprogramming.

Biochemical Timekeeping Via Reentrant Phase Transitions

Appreciation for the role of liquid-liquid phase separation in the functional organization of cellular matter has exploded in recent years. More recently there has been a growing effort to understand the principles of heterotypic phase separation, the demixing of multiple proteins and nucleic acids into a single functional condensate. A phase transition is termed reentrant if it involves the transformation of a system from one state into a macroscopically similar or identical state via at least two phase transitions elicited by variation of a single parameter. Reentrant liquid-liquid phase separation can occur when the condensation of one species is tuned by another. Reentrant phase transitions have been modeled in vitro using protein and RNA mixtures. These biochemical studies reveal two features of reentrant phase separation that are likely important to functional cellular condensates: (1) the ability to generate condensates with layered functional topologies, and (2) the ability to generate condensates whose composition and duration are self-limiting to enable a form of biochemical timekeeping. We relate these biochemical studies to potential cellular examples and discuss how layered topologies and self-regulation may impact key biological processes.

The microbial and host factors that govern Candida gastrointestinal colonization and dissemination

Candida species are among the most prevalent and abundant members of the gut mycobiota, with Candida albicans (CA) being the most prominent member. CA colonizes numerous mucosal surfaces, most notably the gastrointestinal (GI) and genitourinary tracts. In a healthy host, CA is a pathobiont that exists as a commensal but can become pathogenic if the host's immune system becomes suppressed. The microbial and/or host factors that dictate CA's ability to colonize mucosal surfaces and its ability to disseminate remain of great interest. Here, we review the recent advances and insights regarding Candida colonization and dissemination of the mammalian GI tract.

Recording of DNA-binding events reveals the importance of a repurposed Candida albicans regulatory network for gut commensalism

Candida albicans is a fungal component of the human gut microbiota and an opportunistic pathogen. C. albicans transcription factors (TFs), Wor1 and Efg1, are master regulators of an epigenetic switch required for fungal mating that also control colonization of the mammalian gut. We show that additional mating regulators, WOR2, WOR3, WOR4, AHR1, CZF1, and SSN6, also influence gut commensalism. Using Calling Card-seq to record Candida TF DNA-binding events in the host, we examine the role and relationships of these regulators during murine gut colonization. By comparing in-host transcriptomes of regulatory mutants with enhanced versus diminished commensal fitness, we also identify a set of candidate commensalism effectors. These include Cht2, a GPI-linked chitinase whose gene is bound by Wor1, Czf1, and Efg1 in vivo, that we show promotes commensalism. Thus, the network required for a C. albicans sexual switch is biochemically active in the host intestine and repurposed to direct commensalism.

Moving beyond disease to function: Physiological roles for polyglutamine-rich sequences in cell decisions

Glutamine-rich tracts, also known as polyQ domains, have received a great deal of attention for their role in multiple neurodegenerative diseases, including Huntington's disease (HD), spinocerebellar ataxia (SCA), and others [22], [27]. Expansions in the normal polyQ tracts are thus commonly linked to disease, but polyQ domains themselves play multiple important functional roles in cells that are being increasingly appreciated. The biochemical nature of these domains allows them to adopt a number of different structures and form large assemblies that enable environmental responsiveness, localized signaling, and cellular memory. In many cases, these involve the formation of condensates that have varied material states. In this review, we highlight known and emerging functional roles for polyQ tracts in normal cell physiology.

The Roles of Chromatin Accessibility in Regulating the Candida albicans White-Opaque Phenotypic Switch

Candida albicans, a diploid polymorphic fungus, has evolved a unique heritable epigenetic program that enables reversible phenotypic switching between two cell types, referred to as “white” and “opaque”. These cell types are established and maintained by distinct transcriptional programs that lead to differences in metabolic preferences, mating competencies, cellular morphologies, responses to environmental signals, interactions with the host innate immune system, and expression of approximately 20% of genes in the genome. Transcription factors (defined as sequence specific DNA-binding proteins) that regulate the establishment and heritable maintenance of the white and opaque cell types have been a primary focus of investigation in the field; however, other factors that impact chromatin accessibility, such as histone modifying enzymes, chromatin remodelers, and histone chaperone complexes, also modulate the dynamics of the white-opaque switch and have been much less studied to date. Overall, the white-opaque switch represents an attractive and relatively “simple” model system for understanding the logic and regulatory mechanisms by which heritable cell fate decisions are determined in higher eukaryotes. Here we review recent discoveries on the roles of chromatin accessibility in regulating the C. albicans white-opaque phenotypic switch.

Transcriptional Circuits Regulating Developmental Processes in Candida albicans

Candida albicans is a commensal member of the human microbiota that colonizes multiple niches in the body including the skin, oral cavity, and gastrointestinal and genitourinary tracts of healthy individuals. It is also the most common human fungal pathogen isolated from patients in clinical settings. C. albicans can cause a number of superficial and invasive infections, especially in immunocompromised individuals. The ability of C. albicans to succeed as both a commensal and a pathogen, and to thrive in a wide range of environmental niches within the host, requires sophisticated transcriptional regulatory programs that can integrate and respond to host specific environmental signals. Identifying and characterizing the transcriptional regulatory networks that control important developmental processes in C. albicans will shed new light on the strategies used by C. albicans to colonize and infect its host. Here, we discuss the transcriptional regulatory circuits controlling three major developmental processes in C. albicans: biofilm formation, the white-opaque phenotypic switch, and the commensal-pathogen transition. Each of these three circuits are tightly knit and, through our analyses, we show that they are integrated together by extensive regulatory crosstalk between the core regulators that comprise each circuit.

Intrinsic disorder is an essential characteristic of components in the conserved circadian circuit

INTRODUCTION

The circadian circuit, a roughly 24 h molecular feedback loop, or clock, is conserved from bacteria to animals and allows for enhanced organismal survival by facilitating the anticipation of the day/night cycle. With circadian regulation reportedly impacting as high as 80% of protein coding genes in higher eukaryotes, the protein-based circadian clock broadly regulates physiology and behavior. Due to the extensive interconnection between the clock and other cellular systems, chronic disruption of these molecular rhythms leads to a decrease in organismal fitness as well as an increase of disease rates in humans. Importantly, recent research has demonstrated that proteins comprising the circadian clock network display a significant amount of intrinsic disorder.

MAIN BODY

In this work, we focus on the extent of intrinsic disorder in the circadian clock and its potential mechanistic role in circadian timing. We highlight the conservation of disorder by quantifying the extent of computationally-predicted protein disorder in the core clock of the key eukaryotic circadian model organisms Drosophila melanogaster, Neurospora crassa, and Mus musculus. We further examine previously published work, as well as feature novel experimental evidence, demonstrating that the core negative arm circadian period drivers FREQUENCY (Neurospora crassa) and PERIOD-2 (PER2) (Mus musculus), possess biochemical characteristics of intrinsically disordered proteins. Finally, we discuss the potential contributions of the inherent biophysical principals of intrinsically disordered proteins that may explain the vital mechanistic roles they play in the clock to drive their broad evolutionary conservation in circadian timekeeping.

CONCLUSION

The pervasive conservation of disorder amongst the clock in the crown eukaryotes suggests that disorder is essential for optimal circadian timing from fungi to animals, providing vital homeostatic cellular maintenance and coordinating organismal physiology across phylogenetic kingdoms. Video abstract.