Transcription factor paralogs orchestrate alternative gene regulatory networks by context-dependent cooperation with multiple cofactors

Widespread variation in molecular interactions and regulatory properties among transcription factor isoforms

Most human Transcription factors (TFs) genes encode multiple protein isoforms differing in DNA binding domains, effector domains, or other protein regions. The global extent to which this results in functional differences between isoforms remains unknown. Here, we systematically compared 693 isoforms of 246 TF genes, assessing DNA binding, protein binding, transcriptional activation, subcellular localization, and condensate formation. Relative to reference isoforms, two-thirds of alternative TF isoforms exhibit differences in one or more molecular activities, which often could not be predicted from sequence. We observed two primary categories of alternative TF isoforms: "rewirers" and "negative regulators", both of which were associated with differentiation and cancer. Our results support a model wherein the relative expression levels of, and interactions involving, TF isoforms add an understudied layer of complexity to gene regulatory networks, demonstrating the importance of isoform-aware characterization of TF functions and providing a rich resource for further studies.

Multimodal Hox5 activity generates motor neuron diversity

Motor neurons (MNs) are the final output of circuits driving fundamental behaviors, such as respiration and locomotion. Hox proteins are essential in generating the MN diversity required for accomplishing these functions, but the transcriptional mechanisms that enable Hox paralogs to assign distinct MN subtype identities despite their promiscuous DNA binding motif are not well understood. Here we show that Hoxa5 controls chromatin accessibility in all mouse spinal cervical MN subtypes and engages TALE co-factors to directly bind and regulate subtype-specific genes. We identify a paralog-specific interaction of Hoxa5 with the phrenic MN-specific transcription factor Scip and show that heterologous expression of Hoxa5 and Scip is sufficient to suppress limb-innervating MN identity. We also demonstrate that phrenic MN identity is stable after Hoxa5 downregulation and identify Klf proteins as potential regulators of phrenic MN maintenance. Our data identify multiple modes of Hoxa5 action that converge to induce and maintain MN identity.

Predicting the DNA binding specificity of mutated transcription factors using family-level biophysically interpretable machine learning

Sequence-specific interactions of transcription factors (TFs) with genomic DNA underlie many cellular processes. High-throughput in vitro binding assays coupled with computational analysis have made it possible to accurately define such sequence recognition in a biophysically interpretable yet mechanism-agonistic way for individual TFs. The fact that such sequence-to-affinity models are now available for hundreds of TFs provides new avenues for predicting how the DNA binding specificity of a TF changes when its protein sequence is mutated. To this end, we developed an analytical framework based on a tetrahedron embedding that can be applied at the level of a given structural TF family. Using bHLH as a test case, we demonstrate that we can systematically map dependencies between the protein sequence of a TF and base preference within the DNA binding site. We also develop a regression approach to predict the quantitative energetic impact of mutations in the DNA binding domain of a TF on its DNA binding specificity, and perform SELEX-seq assays on mutated TFs to experimentally validate our results. Our results point to the feasibility of predicting the functional impact of disease mutations and allelic variation in the cell-wide TF repertoire by leveraging high-quality functional information across sets of homologous wild-type proteins.

Cis-regulatory modes of Ultrabithorax inactivation in butterfly forewings

Hox gene clusters encode transcription factors that drive regional specialization during animal development: for example the Hox factor Ubx is expressed in the insect metathoracic (T3) wing appendages and differentiates them from T2 mesothoracic identities. Hox transcriptional regulation requires silencing activities that prevent spurious activation and regulatory crosstalks in the wrong tissues, but this has seldom been studied in insects other than Drosophila, which shows a derived Hox dislocation into two genomic clusters that disjoined Antennapedia (Antp) and Ultrabithorax (Ubx). Here, we investigated how Ubx is restricted to the hindwing in butterflies, amidst a contiguous Hox cluster. By analysing Hi-C and ATAC-seq data in the butterfly Junonia coenia, we show that a Topologically Associated Domain (TAD) maintains a hindwing-enriched profile of chromatin opening around Ubx. This TAD is bordered by a Boundary Element (BE) that separates it from a region of joined wing activity around the Antp locus. CRISPR mutational perturbation of this BE releases ectopic Ubx expression in forewings, inducing homeotic clones with hindwing identities. Further mutational interrogation of two non-coding RNA encoding regions and one putative cis-regulatory module within the Ubx TAD cause rare homeotic transformations in both directions, indicating the presence of both activating and repressing chromatin features. We also describe a series of spontaneous forewing homeotic phenotypes obtained in Heliconius butterflies, and discuss their possible mutational basis. By leveraging the extensive wing specialization found in butterflies, our initial exploration of Ubx regulation demonstrates the existence of silencing and insulating sequences that prevent its spurious expression in forewings.

Transcription Factors in the Pathogenesis of Lupus Nephritis and Their Targeted Therapy

Systemic lupus erythematosus (SLE) is a prototype inflammatory autoimmune disease, characterized by breakdown of immunotolerance to self-antigens. Renal involvement, known as lupus nephritis (LN), is one of the leading causes of morbidity and a significant contributor to mortality in SLE. Despite current pathophysiological advances, further studies are needed to fully understand complex mechanisms underlying the development and progression of LN. Transcription factors (TFs) are proteins that regulate the expression of genes and play a crucial role in the development and progression of LN. The mechanisms of TF promoting or inhibiting gene expression are complex, and studies have just begun to reveal the pathological roles of TFs in LN. Understanding TFs in the pathogenesis of LN can provide valuable insights into this disease's mechanisms and potentially lead to the development of targeted therapies for its management. This review will focus on recent findings on TFs in the pathogenesis of LN and newly developed TF-targeted therapy in renal inflammation.

TALE transcription factors: Cofactors no more

Exd/PBX, Hth/MEIS and PREP proteins belong to the TALE (three-amino-acid loop extension) superclass of transcription factors (TFs) with an atypical homedomain (HD). Originally discovered as "cofactors" to HOX proteins, revisiting their traditional role in light of genome-wide experiments reveals a strong and reproducible pattern of HOX and TALE co-occupancy across diverse embryonic tissues. While confirming that TALE increases HOX specificity and selectivity in vivo, this wider outlook also reveals novel aspects of HOX:TALE collaboration, namely that HOX TFs generally require pre-bound TALE factors to access their functional binding sites in vivo. In contrast to the restricted expression domains of HOX TFs, TALE factors are largely ubiquitous, and PBX and PREP are expressed at the earliest developmental stages. PBX and MEIS control development of many organs and tissues and their dysregulation is associated with congenital disease and cancer. Accordingly, many instances of TALE cooperation with non HOX TFs have been documented in various systems. The model that emerges from these studies is that TALE TFs create a permissive chromatin platform that is selected by tissue-restricted TFs for binding. In turn, HOX and other tissue-restricted TFs selectively convert a ubiquitous pool of low affinity TALE binding events into high confidence, tissue-restricted binding events associated with transcriptional activation. As a result, TALE:TF complexes are associated with active chromatin and domain/lineage-specific gene activity. TALE ubiquitous expression and broad genomic occupancy, as well as the increasing examples of TALE tissue-specific partners, reveal a universal and obligatory role for TALE in the control of tissue and lineage-specific transcriptional programs, beyond their initial discovery as HOX co-factors.

Homeodomain complex formation and biomolecular condensates in Hox gene regulation

Hox genes are a family of homeodomain transcription factors that regulate specialized morphological structures along the anterior-posterior axis of metazoans. Over the past few decades, researchers have focused on defining how Hox factors with similar in vitro DNA binding activities achieve sufficient target specificity to regulate distinct cell fates in vivo. In this review, we highlight how protein interactions with other transcription factors, many of which are also homeodomain proteins, result in the formation of transcription factor complexes with enhanced DNA binding specificity. These findings suggest that Hox-regulated enhancers utilize distinct combinations of homeodomain binding sites, many of which are low-affinity, to recruit specific Hox complexes. However, low-affinity sites can only yield reproducible responses with high transcription factor concentrations. To overcome this limitation, recent studies revealed how transcription factors, including Hox factors, use intrinsically disordered domains (IDRs) to form biomolecular condensates that increase protein concentrations. Moreover, Hox factors with altered IDRs have been associated with altered transcriptional activity and human disease states, demonstrating the importance of IDRs in mediating essential Hox output. Collectively, these studies highlight how Hox factors use their DNA binding domains, protein-protein interaction domains, and IDRs to form specific transcription factor complexes that yield accurate gene expression.

Short tandem repeats bind transcription factors to tune eukaryotic gene expression

Short tandem repeats (STRs) are enriched in eukaryotic cis-regulatory elements and alter gene expression, yet how they regulate transcription remains unknown. We found that STRs modulate transcription factor (TF)-DNA affinities and apparent on-rates by about 70-fold by directly binding TF DNA-binding domains, with energetic impacts exceeding many consensus motif mutations. STRs maximize the number of weakly preferred microstates near target sites, thereby increasing TF density, with impacts well predicted by statistical mechanics. Confirming that STRs also affect TF binding in cells, neural networks trained only on in vivo occupancies predicted effects identical to those observed in vitro. Approximately 90% of TFs preferentially bound STRs that need not resemble known motifs, providing a cis-regulatory mechanism to target TFs to genomic sites.

A bioinformatics screen reveals hox and chromatin remodeling factors at the Drosophila histone locus

BACKGROUND

Cells orchestrate histone biogenesis with strict temporal and quantitative control. To efficiently regulate histone biogenesis, the repetitive Drosophila melanogaster replication-dependent histone genes are arrayed and clustered at a single locus. Regulatory factors concentrate in a nuclear body known as the histone locus body (HLB), which forms around the locus. Historically, HLB factors are largely discovered by chance, and few are known to interact directly with DNA. It is therefore unclear how the histone genes are specifically targeted for unique and coordinated regulation.

RESULTS

To expand the list of known HLB factors, we performed a candidate-based screen by mapping 30 publicly available ChIP datasets of 27 unique factors to the Drosophila histone gene array. We identified novel transcription factor candidates, including the Drosophila Hox proteins Ultrabithorax (Ubx), Abdominal-A (Abd-A), and Abdominal-B (Abd-B), suggesting a new pathway for these factors in influencing body plan morphogenesis. Additionally, we identified six other factors that target the histone gene array: JIL-1, hormone-like receptor 78 (Hr78), the long isoform of female sterile homeotic (1) (fs(1)h) as well as the general transcription factors TBP associated factor 1 (TAF-1), Transcription Factor IIB (TFIIB), and Transcription Factor IIF (TFIIF).

CONCLUSIONS

Our foundational screen provides several candidates for future studies into factors that may influence histone biogenesis. Further, our study emphasizes the powerful reservoir of publicly available datasets, which can be mined as a primary screening technique.

Combinatorial transcription factor binding encodes cis-regulatory wiring of forebrain GABAergic neurogenesis

Transcription factors (TFs) bind combinatorially to genomic cis-regulatory elements (cREs), orchestrating transcription programs. While studies of chromatin state and chromosomal interactions have revealed dynamic neurodevelopmental cRE landscapes, parallel understanding of the underlying TF binding lags. To elucidate the combinatorial TF-cRE interactions driving mouse basal ganglia development, we integrated ChIP-seq for twelve TFs, H3K4me3-associated enhancer-promoter interactions, chromatin and transcriptional state, and transgenic enhancer assays. We identified TF-cREs modules with distinct chromatin features and enhancer activity that have complementary roles driving GABAergic neurogenesis and suppressing other developmental fates. While the majority of distal cREs were bound by one or two TFs, a small proportion were extensively bound, and these enhancers also exhibited exceptional evolutionary conservation, motif density, and complex chromosomal interactions. Our results provide new insights into how modules of combinatorial TF-cRE interactions activate and repress developmental expression programs and demonstrate the value of TF binding data in modeling gene regulatory wiring.

The computational capabilities of many-to-many protein interaction networks

Many biological circuits comprise sets of protein variants that interact with one another in a many-to-many, or promiscuous, fashion. These architectures can provide powerful computational capabilities that are especially critical in multicellular organisms. Understanding the principles of biochemical computations in these circuits could allow more precise control of cellular behaviors. However, these systems are inherently difficult to analyze, due to their large number of interacting molecular components, partial redundancies, and cell context dependence. Here, we discuss recent experimental and theoretical advances that are beginning to reveal how promiscuous circuits compute, what roles those computations play in natural biological contexts, and how promiscuous architectures can be applied for the design of synthetic multicellular behaviors.

Characterisation of the role and regulation of Ultrabithorax in sculpting fine-scale leg morphology

Hox genes are expressed during embryogenesis and determine the regional identity of animal bodies along the antero-posterior axis. However, they also function post-embryonically to sculpt fine-scale morphology. To better understand how Hox genes are integrated into post-embryonic gene regulatory networks, we further analysed the role and regulation of Ultrabithorax (Ubx) during leg development in Drosophila melanogaster. Ubx regulates several aspects of bristle and trichome patterning on the femurs of the second (T2) and third (T3) leg pairs. We found that repression of trichomes in the proximal posterior region of the T2 femur by Ubx is likely mediated by activation of the expression of microRNA-92a and microRNA-92b by this Hox protein. Furthermore, we identified a novel enhancer of Ubx that recapitulates the temporal and regional activity of this gene in T2 and T3 legs. We then used transcription factor (TF) binding motif analysis in regions of accessible chromatin in T2 leg cells to predict and functionally test TFs that may regulate the Ubx leg enhancer. We also tested the role of the Ubx co-factors Homothorax (Hth) and Extradenticle (Exd) in T2 and T3 femurs. We found several TFs that may act upstream or in concert with Ubx to modulate trichome patterning along the proximo-distal axis of developing femurs and that the repression of trichomes also requires Hth and Exd. Taken together our results provide insights into how Ubx is integrated into a post-embryonic gene regulatory network to determine fine-scale leg morphology.

A bioinformatics screen reveals Hox and chromatin remodeling factors at the Drosophila histone locus

Cells orchestrate histone biogenesis with strict temporal and quantitative control. To efficiently regulate histone biogenesis, the repetitive Drosophila melanogaster replication-dependent histone genes are arrayed and clustered at a single locus. Regulatory factors concentrate in a nuclear body known as the histone locus body (HLB), which forms around the locus. Historically, HLB factors are largely discovered by chance, and few are known to interact directly with DNA. It is therefore unclear how the histone genes are specifically targeted for unique and coordinated regulation. To expand the list of known HLB factors, we performed a candidate-based screen by mapping 30 publicly available ChIP datasets and 27 factors to the Drosophila histone gene array. We identified novel transcription factor candidates, including the Drosophila Hox proteins Ultrabithorax, Abdominal-A and Abdominal-B, suggesting a new pathway for these factors in influencing body plan morphogenesis. Additionally, we identified six other transcription factors that target the histone gene array: JIL-1, Hr78, the long isoform of fs(1)h as well as the generalized transcription factors TAF-1, TFIIB, and TFIIF. Our foundational screen provides several candidates for future studies into factors that may influence histone biogenesis. Further, our study emphasizes the powerful reservoir of publicly available datasets, which can be mined as a primary screening technique.

The pioneering function of the hox transcription factors

Ever since the discovery that the Hox family of transcription factors establish morphological diversity in the developing embryo, major efforts have been directed towards understanding Hox-dependent patterning. This has led to important discoveries, notably on the mechanisms underlying the collinear expression of Hox genes and Hox binding specificity. More recently, several studies have provided evidence that Hox factors have the capacity to bind their targets in an inaccessible chromatin context and trigger the switch to an accessible, transcriptional permissive, chromatin state. In this review, we provide an overview of the evidences supporting that Hox factors behave as pioneer factors and discuss the potential mechanisms implicated in Hox pioneer activity as well as the significance of this functional property in Hox-dependent patterning.

Hox dosage and morphological diversification during development and evolution

Hox genes encode for evolutionary conserved transcription factors that have long fascinated biologists since the observation of the first homeotic transformations in flies. Hox genes are developmental architects that instruct the formation of various and precise morphologies along the body axes in cnidarian and bilaterian species. In contrast to these highly specific developmental functions, Hox genes encode for proteins that display poorly selective DNA-binding properties in vitro. This "Hox paradox" has been partially solved with the discovery of the TALE-class cofactors, which interact with all Hox members and form versatile Hox/TALE protein complexes on DNA. Here, we describe the role of the Hox dosage as an additional molecular strategy contributing to further resolve the Hox paradox. We present several cases where the Hox dosage is involved in the formation of different morphologies in invertebrates and vertebrates, with a particular emphasis on flight appendages in insects. We also discuss how the Hox dosage could be interpreted in different types of target enhancers within the nuclear environment in vivo. Altogether our survey underlines the Hox dosage as a key mechanism for shaping Hox molecular function during development and evolution.

Hox function and specificity – A tissue centric view

Since their discovery, the Hox genes, with their incredible power to reprogram the identity of complete body regions, a phenomenon called homeosis, have captured the fascination of many biologists. Recent research has provided new insights into the function of Hox proteins in different germ layers and the mechanisms they employ to control tissue morphogenesis. We focus in this review on the ectoderm and mesoderm to highlight new findings and discuss them with regards to established concepts of Hox target gene regulation. Furthermore, we highlight the molecular mechanisms involved the transcriptional repression of specific groups of Hox target genes, and summarize the role of Hox mediated gene silencing in tissue development. Finally, we reflect on recent findings identifying a large number of tissue-specific Hox interactor partners, which open up new avenues and directions towards a better understanding of Hox function and specificity in different tissues.

Hox genes in spiders: Their significance for development and evolution

Hox genes are known for their role in the specification of typical body plan features in animals. Evolutionary changes in Hox gene function are believed to be involved in the emergence of the diverse body plans we observe in animals today. Spiders share many body plan features with other arthropods, but also have numerous unique traits of their own. Studies of spider Hox genes have already provided insights into evolutionarily conserved and derived features of the spider body plan and their genetic basis. However, many aspects of Hox gene biology have been insufficiently studied in spiders so far. In this review, we highlight previous comparative studies of Hox genes in spiders and their significance for our understanding of the evolution of the spider body plan. We also identify aspects of Hox gene biology that need to be studied in greater detail. Many spider Hox genes have not been investigated beyond their mRNA expression patterns, and the role of Hox genes with apparently plesiomorphic or dual functions, like ftz and Hox3 is still unclear. Spiders have a duplicated Hox gene cluster, but possible sub- or neofunctionalisation of duplicates have not yet been studied systematically. Future research should therefore focus on these issues, in addition to the role of Polycomb and trithorax-mediated regulation, the identification of regulatory regions, cofactors or spider-specific target genes, and the significance of non-coding RNAs transcribed from within the Hox cluster and even from the antisense strand of particular Hox genes.

Specificity of the Hox member Deformed is determined by transcription factor levels and binding site affinities

Hox proteins have similar binding specificities in vitro, yet they control different morphologies in vivo. This paradox has been partially solved with the identification of Hox low-affinity binding sites. However, anterior Hox proteins are more promiscuous than posterior Hox proteins, raising the question how anterior Hox proteins achieve specificity. We use the AP2x enhancer, which is activated in the maxillary head segment by the Hox TF Deformed (Dfd). This enhancer lacks canonical Dfd-Exd sites but contains several predicted low-affinity sites. Unexpectedly, these sites are strongly bound by Dfd-Exd complexes and their conversion into optimal Dfd-Exd sites results only in a modest increase in binding strength. These small variations in affinity change the sensitivity of the enhancer to different Dfd levels, resulting in perturbed AP-2 expression and maxillary morphogenesis. Thus, Hox-regulated morphogenesis seems to result from the co-evolution of Hox binding affinity and Hox dosage for precise target gene regulation.

Despite the central role of Hox genes in controlling morphogenesis, the DNA binding of different Hox members is relatively similar. Here they show that specificity of Hox member Dfd relies on a precise balance of transcription factors and binding site affinities.