Blimp1 regulates development of the posterior forelimb, caudal pharyngeal arches, heart and sensory vibrissae in mice

A new Prdm1-Cre line is suitable for studying the second heart field development

The second heart field (SHF) plays a pivotal role in heart development, particularly in outflow tract (OFT) morphogenesis and septation, as well as in the expansion of the right ventricle (RV). Two mouse Cre lines, the Mef2c-AHF-Cre (Mef2c-Cre) and Isl1-Cre, have been widely used to study the SHF development. However, Cre activity is triggered not only in the SHF but also in the RV in the Mef2c-Cre mice, and in the Isl1-Cre mice, Cre activation is not SHF-specific. Therefore, a more suitable SHF-Cre line is desirable for better understanding SHF development. Here, we generated and characterized the Prdm1-Cre knock-in mice. In comparison with Mef2c-Cre mice, the Cre activity is similar in the pharyngeal and splanchnic mesoderm, and in the OFT of the Prdm1-Cre mice. Nonetheless, it was noticed that Cre expression is largely reduced in the RV of Prdm1-Cre mice compared to the Mef2c-Cre mice. Furthermore, we deleted Hand2, Nkx2-5, Pdk1 and Tbx20 using both Mef2c-Cre and Prdm1-Cre mice to study OFT morphogenesis and septation, making a comparison between these two Cre lines. New insights were obtained in understanding SHF development including differentiation into cardiomyocytes in the OFT using Prdm1-Cre mice. In conclusion, we found that Prdm1-Cre mouse line is a more appropriate tool to monitor SHF development, while the Mef2c-Cre mice are excellent in studying the role and function of the SHF in OFT morphogenesis and septation.

Unlocking Genetic Mysteries during the Epic Sperm Journey toward Fertilization: Further Expanding Cre Mouse Lines

The spatiotemporal expression patterns of genes are crucial for maintaining normal physiological functions in animals. Conditional gene knockout using the cyclization recombination enzyme (Cre)/locus of crossover of P1 (Cre/LoxP) strategy has been extensively employed for functional assays at specific tissue or developmental stages. This approach aids in uncovering the associations between phenotypes and gene regulation while minimizing interference among distinct tissues. Various Cre-engineered mouse models have been utilized in the male reproductive system, including Dppa3-MERCre for primordial germ cells, Ddx4-Cre and Stra8-Cre for spermatogonia, Prm1-Cre and Acrv1-iCre for haploid spermatids, Cyp17a1-iCre for the Leydig cell, Sox9-Cre for the Sertoli cell, and Lcn5/8/9-Cre for differentiated segments of the epididymis. Notably, the specificity and functioning stage of Cre recombinases vary, and the efficiency of recombination driven by Cre depends on endogenous promoters with different sequences as well as the constructed Cre vectors, even when controlled by an identical promoter. Cre mouse models generated via traditional recombination or CRISPR/Cas9 also exhibit distinct knockout properties. This review focuses on Cre-engineered mouse models applied to the male reproductive system, including Cre-targeting strategies, mouse model screening, and practical challenges encountered, particularly with novel mouse strains over the past decade. It aims to provide valuable references for studies conducted on the male reproductive system.

Human split hand/foot variants are not as functional as wildtype human PRDM1 in the rescue of craniofacial defects

BACKGROUND

Split hand/foot malformation (SHFM) is a congenital limb disorder presenting with limb anomalies, such as missing, hypoplastic, or fused digits, and often craniofacial defects, including a cleft lip/palate, microdontia, micrognathia, or maxillary hypoplasia. We previously identified three novel variants in the transcription factor, PRDM1, that are associated with SHFM phenotypes. One individual also presented with a high arch palate. Studies in vertebrates indicate that PRDM1 is important for development of the skull; however, prior to our study, human variants in PRDM1 had not been associated with craniofacial anomalies.

METHODS

Using transient mRNA overexpression assays in prdm1a-/- mutant zebrafish, we tested whether the PRDM1 SHFM variants were functional and could lead to a rescue of the craniofacial defects observed in prdm1a-/- mutants. We also mined previously published CUT&RUN and RNA-seq datasets that sorted EGFP-positive cells from a Tg(Mmu:Prx1-EGFP) transgenic line that labels the pectoral fin, pharyngeal arches, and dorsal part of the head to examine Prdm1a binding and the effect of Prdm1a loss on craniofacial genes.

RESULTS

The prdm1a-/- mutants exhibit craniofacial defects including a hypoplastic neurocranium, a loss of posterior ceratobranchial arches, a shorter palatoquadrate, and an inverted ceratohyal. Injection of wildtype (WT) hPRDM1 in prdm1a-/- mutants partially rescues the palatoquadrate phenotype. However, injection of each of the three SHFM variants fails to rescue this skeletal defect. Loss of prdm1a leads to a decreased expression of important craniofacial genes by RNA-seq, including emilin3a, confirmed by hybridization chain reaction expression. Other genes including dlx5a/dlx6a, hand2, sox9b, col2a1a, and hoxb genes are also reduced. Validation by real-time quantitative PCR in the anterior half of zebrafish embryos failed to confirm the expression changes suggesting that the differences are enriched in prx1 expressing cells.

CONCLUSION

These data suggest that the three SHFM variants are likely not functional and may be associated with the craniofacial defects observed in the humans. Finally, they demonstrate how Prdm1a can directly bind and regulate genes involved in craniofacial development.

Molecular Pathways and Animal Models of Truncus Arteriosus

In normal cardiovascular development in birds and mammals, the outflow tract of the heart is divided into two distinct channels to separate the oxygenated systemic blood flow from the deoxygenated pulmonary circulation. When the process of outflow tract septation fails, a single common outflow vessel persists resulting in a serious clinical condition known as persistent truncus arteriosus or common arterial trunk. In this chapter, we will review molecular pathways and the cells that are known to play a role in the formation and development of the outflow tract and how genetic manipulation of these pathways in animal models can result in common arterial trunk.

Molecular Pathways and Animal Models of Semilunar Valve and Aortic Arch Anomalies

The great arteries of the vertebrate carry blood from the heart to the systemic circulation and are derived from the pharyngeal arch arteries. In higher vertebrates, the pharyngeal arch arteries are a symmetrical series of blood vessels that rapidly remodel during development to become the asymmetric aortic arch arteries carrying oxygenated blood from the left ventricle via the outflow tract. At the base of the aorta, as well as the pulmonary trunk, are the semilunar valves. These valves each have three leaflets and prevent the backflow of blood into the heart. During development, the process of aortic arch and valve formation may go wrong, resulting in cardiovascular defects, and these may, at least in part, be caused by genetic mutations. In this chapter, we will review models harboring genetic mutations that result in cardiovascular defects affecting the great arteries and the semilunar valves.

Molecular characterization of the B lymphocyte-induced maturation protein-1 (blimp1) gene of common carp (Cyprinus carpio) and its transcription repression involves recruitment of histone deacetylase HDAC3

Blimp1 is the master regulator of B cell terminal differentiation in mammals, it inhibits expression of many transcription factors including bcl6, which provides the basis for promoting further development of activated B lymphocytes into plasma cells. Blimp-1 is thought to act as a sequence-specific recruitment factor for chromatin-modifying enzymes including histone deacetylases (HDAC) and methyltransferases to repress target genes. The cDNA of Ccblimp1a (Cyprinus carpio) open reading frame is 2337 bp encoding a protein of 777 amino acids. CcBlimp1a contains a SET domain, two Proline Rich domains, and five ZnF_C2H2 domains. Blimp1 are conserved in vertebrate species. Ccblimp1a transcripts were detected in common carp larvae from 1 dpf (day post fertilization)to 31 dpf. Ccblimp1a expression was up-regulated in peripheral blood leukocytes (PBL) and spleen leukocytes (SPL) of common carp stimulated by intraperitoneal lipopolysaccharide (LPS) injection. Ccblimp1a expression in PBL and SPL of common carp was induced by TNP-LPS and TNP-KLH. The results indicated TNP-LPS induced a rapid response in PBL and TNP-KLH induced much stronger response in SPL and PBL. IHC results showed that CcBlimp1 positive cells were distributed in the head kidney, trunk kidney, liver, and gut. Immunofluorescence stain results showed that CcBlimp1 was expressed in IgM + lymphocytes. The subcellular localization of CcBlimp1 in the nuclei indicated CcBlimp1 may be involved in the differentiation of IgM + lymphocytes. Further study focusing on the function of CcBlimp1 transcriptional repression was performed using dual luciferase assay. The results showed that the transcription repression of CcBlimp1 on bcl6aa promoter was affected by the histone deacetylation inhibitor and was synergized with histone deacetylase 3 (HDAC3). The results of Co-IP in HEK293T and immunoprecipitation in SPL indicated that CcBlimp1 recruited HDAC3 and might be involved in the formation of complexes. These results suggest that CcBlimp1 is an important transcription factor in common carp lymphocytes. Histone deacetylation modification mediated by HDAC3 may have important roles in CcBlimp1 transcriptional repression during the differentiation of lymphocytes.

PRDM1 DNA-binding zinc finger domain is required for normal limb development and is disrupted in split hand/foot malformation

Split hand/foot malformation (SHFM) is a rare limb abnormality with clefting of the fingers and/or toes. For many individuals, the genetic etiology is unknown. Through whole-exome and targeted sequencing, we detected three novel variants in a gene encoding a transcription factor, PRDM1, that arose de novo in families with SHFM or segregated with the phenotype. PRDM1 is required for limb development; however, its role is not well understood and it is unclear how the PRDM1 variants affect protein function. Using transient and stable overexpression rescue experiments in zebrafish, we show that the variants disrupt the proline/serine-rich and DNA-binding zinc finger domains, resulting in a dominant-negative effect. Through gene expression assays, RNA sequencing, and CUT&RUN in isolated pectoral fin cells, we demonstrate that Prdm1a directly binds to and regulates genes required for fin induction, outgrowth and anterior/posterior patterning, such as fgfr1a, dlx5a, dlx6a and smo. Taken together, these results improve our understanding of the role of PRDM1 in the limb gene regulatory network and identified novel PRDM1 variants that link to SHFM in humans.

The Transcriptional Regulator Prdm1 Is Essential for the Early Development of the Sensory Whisker Follicle and Is Linked to the Beta-Catenin First Dermal Signal

Prdm1 mutant mice are one of the rare mutant strains that do not develop whisker hair follicles while still displaying a pelage. Here, we show that Prdm1 is expressed at the earliest stage of whisker development in clusters of mesenchymal cells before placode formation. Its conditional knockout in the murine soma leads to the loss of expression of Bmp2, Shh, Bmp4, Krt17, Edar, and Gli1, though leaving the β-catenin-driven first dermal signal intact. Furthermore, we show that Prdm1 expressing cells not only act as a signaling center but also as a multipotent progenitor population contributing to the several lineages of the adult whisker. We confirm by genetic ablation experiments that the absence of macro vibrissae reverberates on the organization of nerve wiring in the mystacial pads and leads to the reorganization of the barrel cortex. We demonstrate that Lef1 acts upstream of Prdm1 and identify a primate-specific deletion of a Lef1 enhancer named Leaf. This loss may have been significant in the evolutionary process, leading to the progressive defunctionalization and disappearance of vibrissae in primates.

Conserved and Unique Functions of Blimp1 in Immune Cells

B-lymphocyte-induced maturation protein-1 (Blimp1), is an evolutionarily conserved transcriptional regulator originally described as a repressor of gene transcription. Blimp1 crucially regulates embryonic development and terminal differentiation in numerous cell lineages, including immune cells. Initial investigations of Blimp1’s role in immunity established its non-redundant role in lymphocytic terminal effector differentiation and function. In B cells, Blimp1 drives plasmablast formation and antibody secretion, whereas in T cells, Blimp1 regulates functional differentiation, including cytokine gene expression. These studies established Blimp1 as an essential transcriptional regulator that promotes efficient and controlled adaptive immunity. Recent studies have also demonstrated important roles for Blimp1 in innate immune cells, specifically myeloid cells, and Blimp1 has been established as an intrinsic regulator of dendritic cell maturation and T cell priming. Emerging studies have determined both conserved and unique functions of Blimp1 in different immune cell subsets, including the unique direct activation of the igh gene transcription in B cells and a conserved antagonism with BCL6 in B cells, T cells, and myeloid cells. Moreover, polymorphisms associated with the gene encoding Blimp1 (PRDM1) have been linked to numerous chronic inflammatory conditions in humans. Blimp1 has been shown to regulate target gene expression by either competing with other transcription factors for binding to the target loci, and/or by recruiting various chromatin-modifying co-factors that promote suppressive chromatin structure, such as histone de-acetylases and methyl-transferases. Further, Blimp1 function has been shown to be essentially dose and context-dependent, which adds to Blimp1’s versatility as a regulator of gene expression. Here, we review Blimp1’s complex roles in immunity and highlight specific gaps in the understanding of the biology of this transcriptional regulator, with a major focus on aspects that could foster the description and understanding of novel pathways regulated by Blimp1 in the immune system.

Limb development genes underlie variation in human fingerprint patterns

Fingerprints are of long-standing practical and cultural interest, but little is known about the mechanisms that underlie their variation. Using genome-wide scans in Han Chinese cohorts, we identified 18 loci associated with fingerprint type across the digits, including a genetic basis for the long-recognized “pattern-block” correlations among the middle three digits. In particular, we identified a variant near EVI1 that alters regulatory activity and established a role for EVI1 in dermatoglyph patterning in mice. Dynamic EVI1 expression during human development supports its role in shaping the limbs and digits, rather than influencing skin patterning directly. Trans-ethnic meta-analysis identified 43 fingerprint-associated loci, with nearby genes being strongly enriched for general limb development pathways. We also found that fingerprint patterns were genetically correlated with hand proportions. Taken together, these findings support the key role of limb development genes in influencing the outcome of fingerprint patterning.

•

GWAS identifies variants associated with fingerprint type across all digits

•

Fingerprint-associated genes are strongly enriched for limb development functions

•

Evi1 alters dermatoglyphs in mice by modulating limb rather than skin development

•

Fingerprint patterns are genetically correlated with hand and finger proportions

FOXO1 represses lymphatic valve formation and maintenance via PRDM1

Intraluminal lymphatic valves (LVs) contribute to the prevention of lymph backflow and maintain circulatory homeostasis. Several reports have investigated the molecular mechanisms which promote LV formation; however, the way in which they are suppressed is not completely clear. We show that the forkhead transcription factor FOXO1 is a suppressor of LV formation and maintenance in lymphatic endothelial cells. Oscillatory shear stress by bidirectional flow inactivates FOXO1 via Akt phosphorylation, resulting in the upregulation of a subset of LV-specific genes mediated by downregulation of a transcriptional repressor, PRDM1. Mice with an endothelial-specific Foxo1 deletion have an increase in LVs, and overexpression of Foxo1 in mice produces a decrease in LVs. Genetic reduction of PRDM1 rescues the decrease in LV by Foxo1 overexpression. In conclusion, FOXO1 plays a critical role in lymph flow homeostasis by preventing excess LV formation. This gene might be a therapeutic target for lymphatic circulatory abnormalities.

Mononuclear phagocyte regulation by the transcription factor Blimp-1 in health and disease

B lymphocyte‐induced maturation protein‐1 (Blimp‐1), the transcription factor encoded by the gene Prdm1, plays a number of crucial roles in the adaptive immune system, which result in the maintenance of key effector functions of B‐ and T‐cells. Emerging clinical data, as well as mechanistic evidence from mouse studies, have additionally identified critical functions of Blimp‐1 in the maintenance of immune homeostasis by the mononuclear phagocyte (MNP) system. Blimp‐1 regulation of gene expression affects various aspects of MNP biology, including developmental programmes such as fate decisions of monocytes entering peripheral tissue, and functional programmes such as activation, antigen presentation and secretion of soluble inflammatory mediators. The highly tissue‐, subset‐ and state‐specific regulation of Blimp‐1 expression in MNPs suggests that Blimp‐1 is a dynamic regulator of immune activation, integrating environmental cues to fine‐tune the function of innate cells. In this review, we will discuss the current knowledge regarding Blimp‐1 regulation and function in macrophages and dendritic cells.

An updated classification of hair follicle morphogenesis

Hair follicle (HF) formation in developing embryonic skin requires stepwise signalling between the epithelial epidermis and mesenchymal dermis, and their specialized derivatives, the placode/germ/peg and dermal condensate/papilla, respectively. Classically, distinct stages of HF morphogenesis have been defined, in the mouse model, based on (a) changes in cell morphology and aggregation; (b) expression of few known molecular markers; (c) the extent of follicle downgrowth; and (d) the presence of differentiating cell types. Refined genetic strategies and recent emerging technologies, such as live imaging and transcriptome analyses of isolated cell populations or single cells, have enabled a closer dissection of the signalling requirements at different stages of HF formation, particularly early on. They have also led to the discovery of precursor cells for placode, dermal condensate and future bulge stem cells that, combined with molecular insights into their fate specification and subsequent formation, serve as novel landmarks for early HF morphogenetic events and studies of the signalling networks mediating these processes. In this review, we integrate the emergence of HF precursor cell states and novel molecular markers of fate and formation to update the widely used 20-year-old seminal classification guide of HF morphogenetic stages by Paus et al. We then temporally describe the latest insights into the early cellular and molecular events and signalling requirements for HF morphogenesis in relation to one another in a holistic manner.

NonO Is a Novel Co-factor of PRDM1 and Regulates Inflammatory Response in Monocyte Derived-Dendritic Cells

Proper expression of the transcription factor, Positive regulatory domain 1 (PRDM1), is required for maintaining homeostasis of human monocyte derived-dendritic cells (MO-DCs). The molecular mechanisms and gene targets of PRDM1 in B and T lymphocytes have been identified. However, the function of PRDM1 in dendritic cells (DCs) remains unclear. We investigate co-regulators of PRDM1 in MO-DCs identified by mass spectrometry (MS) and co-immunoprecipitation (Co-IP). Notably, non-POU domain-containing octamer-binding protein (NonO) was found to be a PRDM1 binding protein in the nucleus of MO-DCs. NonO is recruited to the PRDM1 binding site in the promoter region of IL-6. Knockdown of NonO expression by siRNA lessened suppression of IL-6 promoter activity by PRMD1 following LPS stimulation. While NonO binding to PRDM1 was observed in human myeloma cell lines, an effect of NonO on IL-6 expression was not observed. Thus, loss of NonO interrupted the inhibitory effect of PRDM1 on IL-6 expression in MO-DCs, but not plasma cells. Moreover, MO-DCs with low expression of PRDM1 or NonO induce an increased number of IL-21-producing T_FH-like cells in vitro. These data suggest that low level of PRDM1 and NonO lead to enhanced activation of MO-DCs and the regulation of MO-DC function by PRDM1 is mediated through cell lineage-specific mechanisms.

The conserved and divergent roles of Prdm3 and Prdm16 in zebrafish and mouse craniofacial development

The formation of the craniofacial skeleton is a highly dynamic process that requires proper orchestration of various cellular processes in cranial neural crest cell (cNCC) development, including cell migration, proliferation, differentiation, polarity and cell death. Alterations that occur during cNCC development result in congenital birth defects and craniofacial abnormalities such as cleft lip with or without cleft palate. While the gene regulatory networks facilitating neural crest development have been extensively studied, the epigenetic mechanisms by which these pathways are activated or repressed in a temporal and spatially regulated manner remain largely unknown. Chromatin modifiers can precisely modify gene expression through a variety of mechanisms including histone modifications such as methylation. Here, we investigated the role of two members of the PRDM (Positive regulatory domain) histone methyltransferase family, Prdm3 and Prdm16 in craniofacial development using genetic models in zebrafish and mice. Loss of prdm3 or prdm16 in zebrafish causes craniofacial defects including hypoplasia of the craniofacial cartilage elements, undefined posterior ceratobranchials, and decreased mineralization of the parasphenoid. In mice, while conditional loss of Prdm3 in the early embryo proper causes mid-gestation lethality, loss of Prdm16 caused craniofacial defects including anterior mandibular hypoplasia, clefting in the secondary palate and severe middle ear defects. In zebrafish, prdm3 and prdm16 compensate for each other as well as a third Prdm family member, prdm1a. Combinatorial loss of prdm1a, prdm3, and prdm16 alleles results in severe hypoplasia of the anterior cartilage elements, abnormal formation of the jaw joint, complete loss of the posterior ceratobranchials, and clefting of the ethmoid plate. We further determined that loss of prdm3 and prdm16 reduces methylation of histone 3 lysine 9 (repression) and histone 3 lysine 4 (activation) in zebrafish. In mice, loss of Prdm16 significantly decreased histone 3 lysine 9 methylation in the palatal shelves but surprisingly did not change histone 3 lysine 4 methylation. Taken together, Prdm3 and Prdm16 play an important role in craniofacial development by maintaining temporal and spatial regulation of gene regulatory networks necessary for proper cNCC development and these functions are both conserved and divergent across vertebrates.

Blimp-1 Functions as a Molecular Switch to Prevent Inflammatory Activity in Foxp3+RORγt+ Regulatory T Cells

Foxp3⁺ regulatory T cells (Treg) are essential modulators of immune responses, but the molecular mechanisms underlying their function are not fully understood. Here we show that the transcription factor Blimp-1 is a crucial regulator of the Foxp3⁺RORγt⁺ Treg subset. The intrinsic expression of Blimp-1 in these cells is required to prevent production of Th17-associated cytokines. Direct binding of Blimp-1 to the Il17 locus in Treg is associated with inhibitory histone modifications but unaltered binding of RORgt. In the absence of Blimp-1, the Il17 locus is activated, with increased occupancy of the co-activator p300 and abundant binding of the transcriptional regulator IRF4, which is required, along with RORγt, for IL-17 expression in the absence of Blimp-1. We also show that despite their sustained expression of Foxp3, Blimp-1^−/− RORγt⁺IL-17-producing Treg lose suppressor function and can promote intestinal inflammation, indicating that repression of Th17-associated cytokines by Blimp-1 is a crucial requirement for RORγt⁺ Treg function.

Ogawa et al. demonstrate that the transcription factor Blimp-1 is required to prevent production of Th17-associated cytokines and inflammatory activity of microbiota-specific Foxp3⁺RORγt⁺ Treg. These findings uncover a critical role for Blimp-1 in Foxp3⁺Treg function and shed light on the intricate mechanisms underlying Treg phenotypic stability.

PRDM1 controls the sequential activation of neural, neural crest and sensory progenitor determinants

During early embryogenesis, the ectoderm is rapidly subdivided into neural, neural crest and sensory progenitors. How the onset of lineage determinants and the loss of pluripotency markers are temporally and spatially coordinated in vivo is still debated. Here, we identify a crucial role for the transcription factor PRDM1 in the orderly transition from epiblast to defined neural lineages in chick. PRDM1 is initially expressed broadly in the entire epiblast, but becomes gradually restricted as cell fates are specified. We find that PRDM1 is required for the loss of some pluripotency markers and the onset of neural, neural crest and sensory progenitor specifier genes. PRDM1 directly activates their expression by binding to their promoter regions and recruiting the histone demethylase Kdm4a to remove repressive histone marks. However, once neural lineage determinants become expressed, they in turn repress PRDM1, whereas prolonged PRDM1 expression inhibits neural, neural crest and sensory progenitor genes, suggesting that its downregulation is necessary for cells to maintain their identity. Therefore, PRDM1 plays multiple roles during ectodermal cell fate allocation.

Hedgehog Signal and Genetic Disorders

The hedgehog (Hh) family comprises sonic hedgehog (Shh), Indian hedgehog (Ihh), and desert hedgehog (Dhh), which are versatile signaling molecules involved in a wide spectrum of biological events including cell differentiation, proliferation, and survival; establishment of the vertebrate body plan; and aging. These molecules play critical roles from embryogenesis to adult stages; therefore, alterations such as abnormal expression or mutations of the genes involved and their downstream factors cause a variety of genetic disorders at different stages. The Hh family involves many signaling mediators and functions through complex mechanisms, and achieving a comprehensive understanding of the entire signaling system is challenging. This review discusses the signaling mediators of the Hh pathway and their functions at the cellular and organismal levels. We first focus on the roles of Hh signaling mediators in signal transduction at the cellular level and the networks formed by these factors. Then, we analyze the spatiotemporal pattern of expression of Hh pathway molecules in tissues and organs, and describe the phenotypes of mutant mice. Finally, we discuss the genetic disorders caused by malfunction of Hh signaling-related molecules in humans.

Genetic basis for primordial germ cells specification in mouse and human: Conserved and divergent roles of PRDM and SOX transcription factors

Primordial germ cells (PGCs) are embryonic precursors of sperm and egg that pass on genetic and epigenetic information from one generation to the next. In mammals, they are induced from a subset of cells in peri-implantation epiblast by BMP signaling from the surrounding tissues. PGCs then initiate a unique developmental program that involves comprehensive epigenetic resetting and repression of somatic genes. This is orchestrated by a set of signaling molecules and transcription factors that promote germ cell identity. Here we review significant findings on mammalian PGC biology, in particular, the genetic basis for PGC specification in mice and human, which has revealed an evolutionary divergence between the two species. We discuss the importance and potential basis for these differences and focus on several examples to illustrate the conserved and divergent roles of critical transcription factors in mouse and human germline.

Fibroblast state switching orchestrates dermal maturation and wound healing

Murine dermis contains functionally and spatially distinct fibroblast lineages that cease to proliferate in early postnatal life. Here, we propose a model in which a negative feedback loop between extracellular matrix (ECM) deposition and fibroblast proliferation determines dermal architecture. Virtual-tissue simulations of our model faithfully recapitulate dermal maturation, predicting a loss of spatial segregation of fibroblast lineages and dictating that fibroblast migration is only required for wound healing. To test this, we performed in vivo live imaging of dermal fibroblasts, which revealed that homeostatic tissue architecture is achieved without active cell migration. In contrast, both fibroblast proliferation and migration are key determinants of tissue repair following wounding. The results show that tissue-scale coordination is driven by the interdependence of cell proliferation and ECM deposition, paving the way for identifying new therapeutic strategies to enhance skin regeneration.

Roles of microRNAs in mammalian reproduction: from the commitment of germ cells to peri-implantation embryos

MicroRNAs (miRNAs) are active regulators of numerous biological and physiological processes including most of the events of mammalian reproduction. Understanding the biological functions of miRNAs in the context of mammalian reproduction will allow a better and comparative understanding of fertility and sterility in male and female mammals. Herein, we summarize recent progress in miRNA‐mediated regulation of mammalian reproduction and highlight the significance of miRNAs in different aspects of mammalian reproduction including the biogenesis of germ cells, the functionality of reproductive organs, and the development of early embryos. Furthermore, we focus on the gene expression regulatory feedback loops involving hormones and miRNA expression to increase our understanding of germ cell commitment and the functioning of reproductive organs. Finally, we discuss the influence of miRNAs on male and female reproductive failure, and provide perspectives for future studies on this topic.

Blimp-1/PRDM1 is a critical regulator of Type III Interferon responses in mammary epithelial cells

The transcriptional repressor Blimp-1 originally cloned as a silencer of type I interferon (IFN)-β gene expression controls cell fate decisions in multiple tissue contexts. Conditional inactivation in the mammary gland was recently shown to disrupt epithelial cell architecture. Here we report that Blimp-1 regulates expression of viral defense, IFN signaling and MHC class I pathways, and directly targets the transcriptional activator Stat1. Blimp-1 functional loss in 3D cultures of mammary epithelial cells (MECs) results in accumulation of dsRNA and expression of type III IFN-λ. Cultures treated with IFN lambda similarly display defective lumen formation. These results demonstrate that type III IFN-λ profoundly influences the behavior of MECs and identify Blimp-1 as a critical regulator of IFN signaling cascades.

Principles for the regulation of multiple developmental pathways by a versatile transcriptional factor, BLIMP1

Single transcription factors (TFs) regulate multiple developmental pathways, but the underlying mechanisms remain unclear. Here, we quantitatively characterized the genome-wide occupancy profiles of BLIMP1, a key transcriptional regulator for diverse developmental processes, during the development of three germ-layer derivatives (photoreceptor precursors, embryonic intestinal epithelium and plasmablasts) and the germ cell lineage (primordial germ cells). We identified BLIMP1-binding sites shared among multiple developmental processes, and such sites were highly occupied by BLIMP1 with a stringent recognition motif and were located predominantly in promoter proximities. A subset of bindings common to all the lineages exhibited a new, strong recognition sequence, a GGGAAA repeat. Paradoxically, however, the shared/common bindings had only a slight impact on the associated gene expression. In contrast, BLIMP1 occupied more distal sites in a cell type-specific manner; despite lower occupancy and flexible sequence recognitions, such bindings contributed effectively to the repression of the associated genes. Recognition motifs of other key TFs in BLIMP1-binding sites had little impact on the expression-level changes. These findings suggest that the shared/common sites might serve as potential reservoirs of BLIMP1 that functions at the specific sites, providing the foundation for a unified understanding of the genome regulation by BLIMP1, and, possibly, TFs in general.

Long-lived unipotent Blimp1-positive luminal stem cells drive mammary gland organogenesis throughout adult life

The hierarchical relationships between various stem and progenitor cell subpopulations driving mammary gland morphogenesis and homoeostasis are poorly understood. Conditional inactivation experiments previously demonstrated that expression of the zinc finger transcriptional repressor Blimp1/PRDM1 is essential for the establishment of epithelial cell polarity and functional maturation of alveolar cells. Here we exploit a Prdm1.CreERT2-LacZ reporter allele for lineage tracing experiments. Blimp1 expression marks a rare subpopulation of unipotent luminal stem cells that initially appear in the embryonic mammary gland at around E17.5 coincident with the segregation of the luminal and basal compartments. Fate mapping at multiple time points in combination with whole-mount confocal imaging revealed these long-lived unipotent luminal stem cells survive consecutive involutions and retain their identity throughout adult life. Blimp1⁺ luminal stem cells give rise to Blimp1⁻ progeny that are invariably Elf5⁺ERα⁻PR⁻. Thus, Blimp1 expression defines a mammary stem cell subpopulation with unique functional characteristics.

The role of stem/progenitor cell populations in mammary gland morphogenesis is not well understood. Here, the authors show that a transcriptional repressor, Blimp1, is expressed in a rare luminal stem cell population, which contribute to duct formation, and survive multiple rounds of pregnancy and involution.

John Saunders' ZPA, Sonic hedgehog and digit identity - How does it really all work?

Among John Saunders' many seminal contributions to developmental biology, his discovery of the limb 'zone of polarizing activity' (ZPA) is arguably one of the most memorable and ground-breaking. This discovery introduced the limb as a premier model for understanding developmental patterning and promoted the concept of patterning by a morphogen gradient. In the 50 years since the discovery of the ZPA, Sonic hedgehog (Shh) has been identified as the ZPA factor and the basic components of the signaling pathway and many aspects of its regulation have been elucidated. Although much has also been learned about how it regulates growth, the mechanism by which Shh patterns the limb, how it acts to instruct digit 'identity', nevertheless remains an enigma. This review focuses on what has been learned about Shh function in the limb and the outstanding puzzles that remain to be solved.

The transcriptional repressor Blimp1 is expressed in rare luminal progenitors and is essential for mammary gland development

Mammary gland morphogenesis depends on a tight balance between cell proliferation, differentiation and apoptosis, to create a defined functional hierarchy within the epithelia. The limited availability of stem cell/progenitor markers has made it challenging to decipher lineage relationships. Here, we identify a rare subset of luminal progenitors that express the zinc finger transcriptional repressor Blimp1, and demonstrate that this subset of highly clonogenic luminal progenitors is required for mammary gland development. Conditional inactivation experiments using K14-Cre and WAPi-Cre deleter strains revealed essential functions at multiple developmental stages. Thus, Blimp1 regulates proliferation, apoptosis and alveolar cell maturation during puberty and pregnancy. Loss of Blimp1 disrupts epithelial architecture and lumen formation both in vivo and in three-dimensional (3D) primary cell cultures. Collectively, these results demonstrate that Blimp1 is required to maintain a highly proliferative luminal subset necessary for mammary gland development and homeostasis.

Highlighted article: In the mouse mammary gland, Blimp1 marks a rare progenitor population, and is required for cell proliferation and polarity as well as efficient milk production.

New insights into Blimp-1 in T lymphocytes: a divergent regulator of cell destiny and effector function

B lymphocyte-induced maturation protein-1 (Blimp-1) serves as a master regulator of the development and function of antibody-producing B cells. Given that its function in T lymphocytes has been identified within the past decade, we review recent findings with emphasis on its role in coordinated control of gene expression during the development, differentiation, and function of T cells. Expression of Blimp-1 is mainly confined to activated T cells and is essential for the production of interleukin (IL)-10 by a subset of forkhead box (Fox)p3⁺ regulatory T cells with an effector phenotype. Blimp-1 is also required to induce cell elimination in the thymus and critically modulates peripheral T cell activation and proliferation. In addition, Blimp-1 promotes T helper (Th) 2 lineage commitment and limits Th1, Th17 and follicular helper T cell differentiation. Furthermore, Blimp-1 coordinates with other transcription factors to regulate expression of IL-2, IL-21 and IL-10 in effector T lymphocytes. In CD8⁺ T cells, Blimp-1 expression is distinct in heterogeneous populations at the stages of clonal expansion, differentiation, contraction and memory formation when they encounter antigens. Moreover, Blimp-1 plays a fundamental role in coordinating cytokine receptor signaling networks and transcriptional programs to regulate diverse aspects of the formation and function of effector and memory CD8⁺ T cells and their exhaustion. Blimp-1 also functions as a gatekeeper of T cell activation and suppression to prevent or dampen autoimmune disease, antiviral responses and antitumor immunity. In this review, we discuss the emerging roles of Blimp-1 in the complex regulation of gene networks that regulate the destiny and effector function of T cells and provide a Blimp-1-dominated transcriptional framework for T lymphocyte homeostasis.

Characterization and functional analysis of the Paralichthys olivaceus prdm1 gene promoter

PR domain containing protein 1 (Prdm1) is a transcriptional repressor identified in various species and plays multiple important roles in immune response and embryonic development. However, little is known about the transcriptional regulation of the prdm1 gene. This study aims to characterize the promoter of Paralichthys olivaceus prdm1 (Po-prdm1) gene and determine the regulatory mechanism of Po-prdm1 expression. A 2000bp-long 5'-flanking region (translation initiation site designated as +1) of the Po-prdm1 gene was isolated and characterized. The regulatory elements in this fragment were then investigated and many putative transcription factor (TF) binding sites involved in immunity and multiple tissue development were identified. A 5'-deletion analysis was then conducted, and the ability of the deletion mutants to promote luciferase and green fluorescent protein (GFP) expression in a flounder gill cell line was examined. The results revealed that the minimal promoter is located in the region between -446 and -13bp, and the region between -1415 and -13bp enhanced the promoter activity. Site-directed mutation analysis was subsequently performed on the putative regulatory elements sites, and the results indicated that FOXP1, MSX and BCL6 binding sites play negative functional roles in the regulation of the Po-prdm1 expression in FG cells. In vivo analysis demonstrated that a GFP reporter gene containing 1.4kb-long promoter fragment (-1415/-13) was expressed in the head and trunk muscle fibres of transient transgenic zebrafish embryos. Our study provided the basic information for the exploration of Po-prdm1 regulation and expression.

Dermal Blimp1 Acts Downstream of Epidermal TGFβ and Wnt/β-Catenin to Regulate Hair Follicle Formation and Growth

B-lymphocyte-induced maturation protein 1 (Blimp1) is a transcriptional repressor that regulates cell growth and differentiation in multiple tissues, including skin. Although in the epidermis Blimp1 is important for keratinocyte and sebocyte differentiation, its role in dermal fibroblasts is unclear. Here we show that Blimp1 is dynamically regulated in dermal papilla cells during hair follicle (HF) morphogenesis and the postnatal hair cycle, preceding dermal Wnt/β-catenin activation. Blimp1 ablation in E12.5 mouse dermal fibroblasts delayed HF morphogenesis and growth and prevented new HF formation after wounding. By combining targeted quantitative PCR screens with bioinformatic analysis and experimental validation we demonstrated that Blimp1 is both a target and a mediator of key dermal papilla inductive signaling pathways including transforming growth factor-β and Wnt/β-catenin. Epidermal overexpression of stabilized β-catenin was able to override the HF defects in Blimp1 mutant mice, underlining the close reciprocal relationship between the dermal papilla and adjacent HF epithelial cells. Overall, our study reveals the functional role of Blimp1 in promoting the dermal papilla inductive signaling cascade that initiates HF growth.

Principles of early human development and germ cell program from conserved model systems

Human primordial germ cells (hPGCs), the precursors of sperm and eggs, originate during weeks 2-3 of early post-implantation development. Using in vitro models of hPGC induction, recent studies have suggested that there are marked mechanistic differences in the specification of human and mouse PGCs. This may be due in part to the divergence in their pluripotency networks and early post-implantation development. As early human embryos are not accessible for direct study, we considered alternatives including porcine embryos that, as in humans, develop as bilaminar embryonic discs. Here we show that porcine PGCs originate from the posterior pre-primitive-streak competent epiblast by sequential upregulation of SOX17 and BLIMP1 in response to WNT and BMP signalling. We use this model together with human and monkey in vitro models simulating peri-gastrulation development to show the conserved principles of epiblast development for competency for primordial germ cell fate. This process is followed by initiation of the epigenetic program and regulated by a balanced SOX17-BLIMP1 gene dosage. Our combinatorial approach using human, porcine and monkey in vivo and in vitro models provides synthetic insights into early human development.

Redundant functions of I-BAR family members, IRSp53 and IRTKS, are essential for embryonic development

The insulin receptor substrate of 53 kDa, IRSp53, is an adaptor protein that works with activated GTPases, Cdc42 and Rac, to modulate actin dynamics and generate membrane protrusions in response to cell signaling. Adult mice that lack IRSp53 fail to regulate synaptic plasticity and exhibit hippocampus-associated learning deficiencies. Here, we show that 60% of IRSp53 null embryos die at mid to late gestation, indicating a vital IRSp53 function in embryonic development. We find that IRSp53 KO embryos displayed pleiotropic phenotypes such as developmental delay, oligodactyly and subcutaneous edema, and died of severely impaired cardiac and placental development. We further show that double knockout of IRSp53 and its closest family member, IRTKS, resulted in exacerbated placental abnormalities, particularly in spongiotrophoblast differentiation and development, giving rise to complete embryonic lethality. Hence, our findings demonstrate a hitherto under-appreciated IRSp53 function in embryonic development, and further establish an essential genetic interaction between IRSp53 and IRTKS in placental formation.

PRDM1/BLIMP1 is widely distributed to the nascent fetal-placental interface in the mouse gastrula

BACKGROUND

PRDM1 is a transcriptional repressor that contributes to primordial germ cell (PGC) development. During early gastrulation, epiblast-derived PRDM1 is thought to be restricted to a lineage-segregated germ line in the allantois. However, given recent findings that PGCs overlap an allantoic progenitor pool that contributes widely to the fetal-umbilical interface, posterior PRDM1 may also contribute to soma.

RESULTS

Within the posterior mouse gastrula (early streak, 12-s stages, embryonic days ∼6.75-9.0), PRDM1 localized to all tissues containing putative PGCs; however, PRDM1 was also found in all three primary germ layers, their derivatives, and two presumptive growth centers, the allantoic core domain and ventral ectodermal ridge. While PRDM1 and STELLA colocalized predominantly within the hindgut, where putative PGCs reside, other colocalizing cells were found in non-PGC sites. Additional PRDM1 and STELLA cells were found independent of each other throughout the posterior region, including the hindgut. The Prdm1-Cre-driven reporter supported PRDM1 localization in the majority of sites; however, some Prdm1 descendants were found in sites independent of PRDM1 protein, including allantoic mesothelium and hindgut endoderm.

CONCLUSIONS

Posterior PRDM1 contributes more broadly to the developing fetal-maternal connection than previously recognized, and PRDM1 and STELLA, while overlapping in putative PGCs, also co-localize in several other tissues. Developmental Dynamics 246:50-71, 2017. © 2016 Wiley Periodicals, Inc.

PRMT5 is essential for the maintenance of chondrogenic progenitor cells in the limb bud

During embryonic development, undifferentiated progenitor cells balance the generation of additional progenitor cells with differentiation. Within the developing limb, cartilage cells differentiate from mesodermal progenitors in an ordered process that results in the specification of the correct number of appropriately sized skeletal elements. The internal pathways by which these cells maintain an undifferentiated state while preserving their capacity to differentiate is unknown. Here, we report that the arginine methyltransferase PRMT5 has a crucial role in maintaining progenitor cells. Mouse embryonic buds lacking PRMT5 have severely truncated bones with wispy digits lacking joints. This novel phenotype is caused by widespread cell death that includes mesodermal progenitor cells that have begun to precociously differentiate into cartilage cells. We propose that PRMT5 maintains progenitor cells through its regulation of Bmp4 Intriguingly, adult and embryonic stem cells also require PRMT5 for maintaining pluripotency, suggesting that similar mechanisms might regulate lineage-restricted progenitor cells during organogenesis.

FACS-sorted putative oogonial stem cells from the ovary are neither DDX4-positive nor germ cells

Whether the adult mammalian ovary contains oogonial stem cells (OSCs) is controversial. They have been isolated by a live-cell sorting method using the germ cell marker DDX4, which has previously been assumed to be cytoplasmic, not surface-bound. Furthermore their stem cell and germ cell characteristics remain disputed. Here we show that although OSC-like cells can be isolated from the ovary using an antibody to DDX4, there is no good in silico modelling to support the existence of a surface-bound DDX4. Furthermore these cells when isolated were not expressing DDX4, and did not initially possess germline identity. Despite these unremarkable beginnings, they acquired some pre-meiotic markers in culture, including DDX4, but critically never expressed oocyte-specific markers, and furthermore were not immortal but died after a few months. Our results suggest that freshly isolated OSCs are not germ stem cells, and are not being isolated by their DDX4 expression. However it may be that culture induces some pre-meiotic markers. In summary the present study offers weight to the dogma that the adult ovary is populated by a fixed number of oocytes and that adult de novo production is a rare or insignificant event.

Zinc fingers 1, 2, 5 and 6 of transcriptional regulator, PRDM4, are required for its nuclear localisation

PRDM4 is a member of the PRDM family of transcriptional regulators which control various aspects of cellular differentiation and proliferation. PRDM proteins exert their biological functions both in the cytosol and the nucleus of cells. All PRDM proteins are characterised by the presence of two distinct structural motifs, the PR/SET domain and the zinc finger (ZF) motifs. We previously observed that deletion of all six zinc fingers found in PRDM4 leads to its accumulation in the cytosol, whereas overexpressed full length PRDM4 is found predominantly in the nucleus. Here, we investigated the requirements for single zinc fingers in the nuclear localisation of PRDM4. We demonstrate that ZF's 1, 2, 5 and 6 contribute to the accumulation of PRDM4 in the nucleus. Their effect is additive as deleting either ZF1-2 or ZF 5-6 redistributes PRDM4 protein from being almost exclusively nuclear to cytosolic and nuclear. We investigated the potential mechanism of nuclear shuttling of PRDM4 via the importin α/β-mediated pathway and find that PRDM4 nuclear targeting is independent of α/β-mediated nuclear import.

Sequences analyses and expression profiles in tissues and embryos of Japanese flounder (Paralichthys olivaceus) PRDM1

PRDM1 (PRDI-BF1-RIZ1 homologous domain containing 1) appears to be a pleiotropic regulatory factor in various processes. It contains a PR (PRDI-BF1-RIZ1 homologous) domain protein and five zinc fingers. In the present study, a gene coding the homolog of prdm1 and the 5' regulatory region of prdm1 was identified from the Paralichthys olivaceus (denoted Po-prdm1). Results of real-time quantitative polymerase chain reaction amplification (RT-qPCR) and in situ hybridization (ISH) in embryos revealed that Po-prdm1 was highly expressed between the early gastrula and tail bud stages, with its expression peaking in the mid-gastrula stage, whereas the results of RT-qPCR and ISH in tissues demonstrated that Po-prdm1 transcripts were ubiquitously detected in all tissues, which indicates its pleiotropic function in multiple processes. ISH of gonadal tissues revealed that the transcripts were located in the nucleus and cytoplasm of the oocytes in the ovaries but only in the spermatogonia and not in the spermatocytes in the testes. The Po-prdm1 transcription factor binding sites and their conserved binding region among vertebrates were analyzed in this study. The combined results suggest that Po-PRDM1 has a conserved function in teleosts and mammals.

Epidermal β-catenin activation remodels the dermis via paracrine signalling to distinct fibroblast lineages

Sustained epidermal Wnt/β-catenin signalling expands the stem cell compartment and induces ectopic hair follicles (EFs). This is accompanied by extensive fibroblast proliferation and extracellular matrix (ECM) remodelling in the underlying dermis. Here we show that epidermal Hedgehog (Hh) and Transforming growth factor-beta (TGF-β) signalling mediate the dermal changes. Pharmacological inhibition or genetic deletion of these pathways prevents β-catenin-induced dermal reprogramming and EF formation. Epidermal Shh stimulates proliferation of the papillary fibroblast lineage, whereas TGF-β2 controls proliferation, differentiation and ECM production by reticular fibroblasts. Hh inhibitors do not affect TGF-β target gene expression in reticular fibroblasts, and TGF-β inhibition does not prevent Hh target gene induction in papillary fibroblasts. However, when Hh signalling is inhibited the reticular dermis does not respond to epidermal β-catenin activation. We conclude that the dermal response to epidermal Wnt/β-catenin signalling depends on distinct fibroblast lineages responding to different paracrine signals.

The molecular mechanisms regulating skin dermal changes are unclear. Here, the authors show that deletion of Hedgehog (Hh) in the upper dermis alters the response to epidermal Wnt signalling, which, together with changes in extracellular matrix production, influences distinct fibroblast lineages differently.

The transcriptional repressor Blimp-1 acts downstream of BMP signaling to generate primordial germ cells in the cricket Gryllus bimaculatus

Segregation of the germ line from the soma is an essential event for transmission of genetic information across generations in all sexually reproducing animals. Although some well-studied systems such as Drosophila and Xenopus use maternally inherited germ determinants to specify germ cells, most animals, including mice, appear to utilize zygotic inductive cell signals to specify germ cells during later embryogenesis. Such inductive germ cell specification is thought to be an ancestral trait of Bilateria, but major questions remain as to the nature of an ancestral mechanism to induce germ cells, and how that mechanism evolved. We previously reported that BMP signaling-based germ cell induction is conserved in both the mouse Mus musculus and the cricket Gryllus bimaculatus, which is an emerging model organism for functional studies of induction-based germ cell formation. In order to gain further insight into the functional evolution of germ cell specification, here we examined the Gryllus ortholog of the transcription factor Blimp-1 (also known as Prdm1), which is a widely conserved bilaterian gene known to play a crucial role in the specification of germ cells in mice. Our functional analyses of the Gryllus Blimp-1 ortholog revealed that it is essential for Gryllus primordial germ cell development, and is regulated by upstream input from the BMP signaling pathway. This functional conservation of the epistatic relationship between BMP signaling and Blimp-1 in inductive germ cell specification between mouse and cricket supports the hypothesis that this molecular mechanism regulated primordial germ cell specification in a last common bilaterian ancestor.

An emerging role for prdm family genes in dorsoventral patterning of the vertebrate nervous system

The embryonic vertebrate neural tube is divided along its dorsoventral (DV) axis into eleven molecularly discrete progenitor domains. Each of these domains gives rise to distinct neuronal cell types; the ventral-most six domains contribute to motor circuits, while the five dorsal domains contribute to sensory circuits. Following the initial neurogenesis step, these domains also generate glial cell types—either astrocytes or oligodendrocytes. This DV pattern is initiated by two morphogens—Sonic Hedgehog released from notochord and floor plate and Bone Morphogenetic Protein produced in the roof plate—that act in concentration gradients to induce expression of genes along the DV axis. Subsequently, these DV-restricted genes cooperate to define progenitor domains and to control neuronal cell fate specification and differentiation in each domain. Many genes involved in this process have been identified, but significant gaps remain in our understanding of the underlying genetic program. Here we review recent work identifying members of the Prdm gene family as novel regulators of DV patterning in the neural tube. Many Prdm proteins regulate transcription by controlling histone modifications (either via intrinsic histone methyltransferase activity, or by recruiting histone modifying enzymes). Prdm genes are expressed in spatially restricted domains along the DV axis of the neural tube and play important roles in the specification of progenitor domains, as well as in the subsequent differentiation of motor neurons and various types of interneurons. Strikingly, Prdm proteins appear to function by binding to, and modulating the activity of, other transcription factors (particularly bHLH proteins). The identity of key transcription factors in DV patterning of the neural tube has been elucidated previously (e.g. the nkx, bHLH and pax families), but it now appears that an additional family is also required and that it acts in a potentially novel manner.

Blimp1/Prdm1 Functions in Opposition to Irf1 to Maintain Neonatal Tolerance during Postnatal Intestinal Maturation

The neonatal intestine is a very complex and dynamic organ that must rapidly adapt and remodel in response to a barrage of environmental stimuli during the first few postnatal weeks. Recent studies demonstrate that the zinc finger transcriptional repressor Blimp1/Prdm1 plays an essential role governing postnatal reprogramming of intestinal enterocytes during this period. Functional loss results in global changes in gene expression patterns, particularly in genes associated with metabolic function. Here we engineered a knock-in allele expressing an eGFP-tagged fusion protein under control of the endogenous regulatory elements and performed genome wide ChIP-seq analysis to identify direct Blimp1 targets and further elucidate the function of Blimp1 in intestinal development. Comparison with published human and mouse datasets revealed a highly conserved core set of genes including interferon-inducible promoters. Here we show that the interferon-inducible transcriptional activator Irf1 is constitutively expressed throughout fetal and postnatal intestinal epithelium development. ChIP-seq demonstrates closely overlapping Blimp1 and Irf1 peaks at key components of the MHC class I pathway in fetal enterocytes. The onset of MHC class I expression coincides with down-regulated Blimp1 expression during the suckling to weaning transition. Collectively, these experiments strongly suggest that in addition to regulating the enterocyte metabolic switch, Blimp1 functions as a gatekeeper in opposition to Irf1 to prevent premature activation of the MHC class I pathway in villus epithelium to maintain tolerance in the neonatal intestine.

The transcriptional repressor Blimp1/Prdm1 plays a pivotal role in the metabolic switch that occurs in the small intestine during the suckling to weaning transition. Notably, expression profiling of perinatal Blimp1-deficient small intestine revealed premature activation of metabolic genes normally restricted to post-weaning enterocytes. To further elucidate the function of Blimp1 in intestinal development, we engineered a novel Blimp1-eGFP-fusion knock-in mouse strain to perform ChIP-seq analysis. In addition to identifying which metabolic genes are direct Blimp1 targets, ChIP-seq analysis revealed a highly conserved Blimp1/Irf-1 overlapping sites that function to control MHC class I antigen processing during acquisition of neonatal tolerance in the first weeks after birth during early colonization of the intestinal tract by commensal microorganisms. Moreover, immunohistochemical analysis of human fetal intestine suggests that a BLIMP1/IRF-1 axis may also function in human intestinal epithelium development.

Elaboration and Innervation of the Vibrissal System in the Rock Hyrax (Procavia capensis)

Mammalian tactile hairs are commonly found on specific, restricted regions of the body, but Florida manatees represent a unique exception, exhibiting follicle-sinus complexes (FSCs, also known as vibrissae or tactile hairs) on their entire body. The orders Sirenia (including manatees and dugongs) and Hyracoidea (hyraxes) are thought to have diverged approximately 60 million years ago, yet hyraxes are among the closest relatives to sirenians. We investigated the possibility that hyraxes, like manatees, are tactile specialists with vibrissae that cover the entire postfacial body. Previous studies suggested that rock hyraxes possess postfacial vibrissae in addition to pelage hair, but this observation was not verified through histological examination. Using a detailed immunohistochemical analysis, we characterized the gross morphology, innervation and mechanoreceptors present in FSCs sampled from facial and postfacial vibrissae body regions to determine that the long postfacial hairs on the hyrax body are in fact true vibrissae. The types and relative densities of mechanoreceptors associated with each FSC also appeared to be relatively consistent between facial and postfacial FSCs. The presence of vibrissae covering the hyrax body presumably facilitates navigation in the dark caves and rocky crevices of the hyrax's environment where visual cues are limited, and may alert the animal to predatory or conspecific threats approaching the body. Furthermore, the presence of vibrissae on the postfacial body in both manatees and hyraxes indicates that this distribution may represent the ancestral condition for the supraorder Paenungulata.

Mouse primordial germ cells: a reappraisal

Current dogma is that mouse primordial germ cells (PGCs) segregate within the allantois, or source of the umbilical cord, and translocate to the gonads, differentiating there into sperm and eggs. In light of emerging data on the posterior embryonic-extraembryonic interface, and the poorly studied but vital fetal-umbilical connection, we have reviewed the past century of experiments on mammalian PGCs and their relation to the allantois. We demonstrate that, despite best efforts and valuable data on the pluripotent state, what is and is not a PGC in vivo is obscure. Furthermore, sufficient experimental evidence has yet to be provided either for an extragonadal origin of mammalian PGCs or for their segregation within the posterior region. Rather, most evidence points to an alternative hypothesis that PGCs in the mouse allantois are part of a stem/progenitor cell pool that exhibits all known PGC "markers" and that builds/reinforces the fetal-umbilical interface, common to amniotes. We conclude by suggesting experiments to distinguish the mammalian germ line from the soma.

BLIMP1 is required for postnatal epidermal homeostasis but does not define a sebaceous gland progenitor under steady-state conditions

B-lymphocyte-induced nuclear maturation protein 1 (BLIMP1) was previously reported to define a sebaceous gland (SG) progenitor population in the epidermis. However, the recent identification of multiple stem cell populations in the hair follicle junctional zone has led us to re-evaluate its function. We show, in agreement with previous studies, that BLIMP1 is expressed by postmitotic, terminally differentiated epidermal cells within the SG, interfollicular epidermis, and hair follicle. Epidermal overexpression of c-Myc results in loss of BLIMP1(+) cells, an effect modulated by androgen signaling. Epidermal-specific deletion of Blimp1 causes multiple differentiation defects in the epidermis in addition to SG enlargement. In culture, BLIMP1(+) sebocytes have no greater clonogenic potential than BLIMP1(-) sebocytes. Finally, lineage-tracing experiments reveal that, under steady-state conditions, BLIMP1-expressing cells do not divide. Thus, rather than defining a sebocyte progenitor population, BLIMP1 functions in terminally differentiated cells to maintain homeostasis in multiple epidermal compartments.

Prdm1 functions in the mesoderm of the second heart field, where it interacts genetically with Tbx1, during outflow tract morphogenesis in the mouse embryo

Congenital heart defects affect at least 0.8% of newborn children and are a major cause of lethality prior to birth. Malformations of the arterial pole are particularly frequent. The myocardium at the base of the pulmonary trunk and aorta and the arterial tree associated with these great arteries are derived from splanchnic mesoderm of the second heart field (SHF), an important source of cardiac progenitor cells. These cells are controlled by a gene regulatory network that includes Fgf8, Fgf10 and Tbx1. Prdm1 encodes a transcriptional repressor that we show is also expressed in the SHF. In mouse embryos, mutation of Prdm1 affects branchial arch development and leads to persistent truncus arteriosus (PTA), indicative of neural crest dysfunction. Using conditional mutants, we show that this is not due to a direct function of Prdm1 in neural crest cells. Mutation of Prdm1 in the SHF does not result in PTA, but leads to arterial pole defects, characterized by mis-alignment or reduction of the aorta and pulmonary trunk, and abnormalities in the arterial tree, defects that are preceded by a reduction in outflow tract size and loss of caudal pharyngeal arch arteries. These defects are associated with a reduction in proliferation of progenitor cells in the SHF. We have investigated genetic interactions with Fgf8 and Tbx1, and show that on a Tbx1 heterozygote background, conditional Prdm1 mutants have more pronounced arterial pole defects, now including PTA. Our results identify PRDM1 as a potential modifier of phenotypic severity in TBX1 haploinsufficient DiGeorge syndrome patients.

Expression patterns of prdm1 during chicken embryonic and germline development

PRDM1 (PR domain containing 1) is a transcriptional repressor that has been identified in various species and is crucial for cell growth, differentiation and development. However, the expression pattern and role of PRDM1 in development has not been sufficiently established in birds. We therefore investigate the spatio-temporal expression of PRDM1 in various tissues, especially in the germline, during chicken development, providing the basis for functional study. Our results show that prdm1 mRNA was expressed in blastodermal cells (BCs) at stage X and in various tissues including the liver, skin, lung, kidney, eye, bursa of fabricius, spleen, proventriculus, gizzard, intestine, testis, ovary, tongue, feathers and thymus but was not or was only sparcely present in the heart, brain and skeletal muscle. The level of prdm1 mRNA was highest in the BCs among all tissues tested and significantly changed during development in many tissues, such as the blastoderm, bursa of fabricius, spleen, feathers and germline. Furthermore, the expression of the PRDM1 protein generally paralleled the mRNA results, except for in the gizzard. Immunohistochemistry also revealed that PRDM1 was localized in the smooth muscle. In addition, during germline development, PRDM1 was found to be continuously expressed in the presumptive primordial germ cells (PGCs) at stage X, the circulating PGCs in blood and the germ cells in the gonads from embryonic day 6 to adult in both males and females. The expression pattern of PRDM1 in chicken thus suggests that this protein plays an important role during chicken development, such as in BC differentiation, feather formation and germ cell specification.

Distinct fibroblast lineages determine dermal architecture in skin development and repair

Fibroblasts are the major mesenchymal cell type in connective tissue and deposit the collagen and elastic fibres of the extracellular matrix (ECM). Even within a single tissue, fibroblasts exhibit considerable functional diversity, but it is not known whether this reflects the existence of a differentiation hierarchy or is a response to different environmental factors. Here we show, using transplantation assays and lineage tracing in mice, that the fibroblasts of skin connective tissue arise from two distinct lineages. One forms the upper dermis, including the dermal papilla that regulates hair growth and the arrector pili muscle, which controls piloerection. The other forms the lower dermis, including the reticular fibroblasts that synthesize the bulk of the fibrillar ECM, and the preadipocytes and adipocytes of the hypodermis. The upper lineage is required for hair follicle formation. In wounded adult skin, the initial wave of dermal repair is mediated by the lower lineage and upper dermal fibroblasts are recruited only during re-epithelialization. Epidermal β-catenin activation stimulates the expansion of the upper dermal lineage, rendering wounds permissive for hair follicle formation. Our findings explain why wounding is linked to formation of ECM-rich scar tissue that lacks hair follicles. They also form a platform for discovering fibroblast lineages in other tissues and for examining fibroblast changes in ageing and disease.

A replicative self-renewal model for long-lived plasma cells: questioning irreversible cell cycle exit

Plasma cells are heterogenous in terms of their origins, secretory products, and lifespan. A current paradigm is that cell cycle exit in plasma cell differentiation is irreversible, following a pattern familiar in short-lived effector populations in other hemopoietic lineages. This paradigm no doubt holds true for many plasma cells whose lifespan can be measured in days following the completion of differentiation. Whether this holds true for long-lived bone marrow plasma cells that are potentially maintained for the lifespan of the organism is less apparent. Added to this the mechanisms that establish and maintain cell cycle quiescence in plasma cells are incompletely defined. Gene expression profiling indicates that in the transition of human plasmablasts to long-lived plasma cells a range of cell cycle regulators are induced in a pattern that suggests a quiescence program with potential for cell cycle re-entry. Here a model of relative quiescence with the potential for replicative self-renewal amongst long-lived plasma cells is explored. The implications of such a mechanism would be diverse, and the argument is made here that current evidence is not sufficiently strong that the possibility should be disregarded.