Gene expression profile of the third pharyngeal pouch reveals role of mesenchymal MafB in embryonic thymus development

Exploring Large MAF Transcription Factors: Functions, Pathology, and Mouse Models with Point Mutations

Large musculoaponeurotic fibrosarcoma (MAF) transcription factors contain acidic, basic, and leucine zipper regions. Four types of MAF have been elucidated in mice and humans, namely c-MAF, MAFA, MAFB, and NRL. This review aimed to elaborate on the functions of MAF transcription factors that have been studied in vivo so far, as well as describe the pathology of human patients and corresponding mouse models with c-MAF, MAFA, and MAFB point mutations. To identify the functions of MAF transcription factors in vivo, we generated genetically modified mice lacking c-MAF, MAFA, and MAFB and analyzed their phenotypes. Further, in recent years, c-MAF, MAFA, and MAFB have been identified as causative genes underpinning many rare diseases. Careful observation of human patients and animal models is important to examine the pathophysiological mechanisms underlying these conditions for targeted therapies. Murine models exhibit phenotypes similar to those of human patients with c-MAF, MAFA, and MAFB mutations. Therefore, generating these animal models emphasizes their usefulness for research uncovering the pathophysiology of point mutations in MAF transcription factors and the development of etiology-based therapies.

Mesenchymal cell replacement corrects thymic hypoplasia in murine models of 22q11.2 deletion syndrome

22q11.2 deletion syndrome (22q11.2DS) is the most common human chromosomal microdeletion, causing developmentally linked congenital malformations, thymic hypoplasia, hypoparathyroidism, and/or cardiac defects. Thymic hypoplasia leads to T cell lymphopenia, which most often results in mild SCID. Despite decades of research, the molecular underpinnings leading to thymic hypoplasia in 22q11.2DS remain unknown. Comparison of embryonic thymuses from mouse models of 22q11.2DS (Tbx1neo2/neo2) revealed proportions of mesenchymal, epithelial, and hematopoietic cell types similar to those of control thymuses. Yet, the small thymuses were growth restricted in fetal organ cultures. Replacement of Tbx1neo2/neo2 thymic mesenchymal cells with normal ones restored tissue growth. Comparative single-cell RNA-Seq of embryonic thymuses uncovered 17 distinct cell subsets, with transcriptome differences predominant in the 5 mesenchymal subsets from the Tbx1neo2/neo2 cell line. The transcripts affected included those for extracellular matrix proteins, consistent with the increased collagen deposition we observed in the small thymuses. Attenuating collagen cross-links with minoxidil restored thymic tissue expansion for hypoplastic lobes. In colony-forming assays, the Tbx1neo2/neo2-derived mesenchymal cells had reduced expansion potential, in contrast to the normal growth of thymic epithelial cells. These findings suggest that mesenchymal cells were causal to the small embryonic thymuses in the 22q11.2DS mouse models, which was correctable by substitution with normal mesenchyme.

Limits to in vivo fate changes of epithelia in thymus and parathyroid by ectopic expression of transcription factors Gcm2 and Foxn1

The development of the parathyroid and the thymus from the third pharyngeal pouch depends on the activities of the Gcm2 and Foxn1 transcription factors, respectively, whose expression domains sharply demarcate two regions in the developing third pharyngeal pouch. Here, we have generated novel mouse models to examine whether ectopic co-expression of Gcm2 in the thymic epithelium and of Foxn1 in the parathyroid perturbs the establishment of organ fates in vivo. Expression of Gcm2 in the thymic rudiment does not activate a parathyroid-specific expression programme, even in the absence of Foxn1 activity. Co-expression of Foxn1 in the parathyroid fails to impose thymopoietic capacity. We conclude that the actions of Foxn1 and Gcm2 transcription factors are cell context-dependent and that they each require permissive transcription factor landscapes in order to successfully interfere with organ-specific cell fate.

CD90 Marks a Mesenchymal Program in Human Thymic Epithelial Cells In Vitro and In Vivo

Thymic epithelium is critical for the structural integrity of the thymus and for T cell development. Within the fully formed thymus, large numbers of hematopoietic cells shape the thymic epithelium into a scaffold-like structure which bears little similarity to classical epithelial layers, such as those observed in the skin, intestine or pancreas. Here, we show that human thymic epithelial cells (TECs) possess an epithelial identity that also incorporates the expression of mesenchymal cell associated genes, whose expression levels vary between medullary and cortical TECs (m/cTECs). Using pluripotent stem cell (PSC) differentiation systems, we identified a unique population of cells that co-expressed the master TEC transcription factor FOXN1, as well as the epithelial associated marker EPCAM and the mesenchymal associated gene CD90. Using the same serum free culture conditions, we also observed co-expression of EPCAM and CD90 on cultured TECs derived from neonatal human thymus in vitro. Single cell RNA-sequencing revealed these cultured TECs possessed an immature mTEC phenotype and expressed epithelial and mesenchymal associated genes, such as EPCAM, CLDN4, CD90 and COL1A1. Importantly, flow cytometry and single cell RNA-sequencing analysis further confirmed the presence of an EPCAM+CD90+ population in the CD45- fraction of neonatal human thymic stromal cells in vivo. Using the human thymus cell atlas, we found that cTECs displayed more pronounced mesenchymal characteristics than mTECs during embryonic development. Collectively, these results suggest human TECs possess a hybrid gene expression program comprising both epithelial and mesenchymal elements, and provide a basis for the further exploration of thymus development from primary tissues and from the in vitro differentiation of PSCs.

The fibroblast: An emerging key player in thymic T cell selection

Fibroblasts have recently attracted attention as a key stromal component that controls the immune responses in lymphoid tissues. The thymus has a unique microenvironment comprised of a variety of stromal cells, including fibroblasts and thymic epithelial cells (TECs), the latter of which is known to be important for T cell development because of their ability to express self‐antigens. Thymic fibroblasts contribute to thymus organogenesis during embryogenesis and form the capsule and medullary reticular network in the adult thymus. However, the immunological significance of thymic fibroblasts has thus far only been poorly elucidated. In this review, we will summarize the current views on the development and functions of thymic fibroblasts as revealed by new technologies such as multicolor flow cytometry and single cell–based transcriptome profiling. Furthermore, the recently discovered role of medullary fibroblasts in the establishment of T cell tolerance by producing a unique set of self‐antigens will be highlighted.

Radiation inducible MafB gene is required for thymic regeneration

The thymus facilitates mature T cell production by providing a suitable stromal microenvironment. This microenvironment is impaired by radiation and aging which lead to immune system disturbances known as thymic involution. Young adult thymus shows thymic recovery after such involution. Although various genes have been reported for thymocytes and thymic epithelial cells in such processes, the roles of stromal transcription factors in these remain incompletely understood. MafB (v-maf musculoaponeurotic fibrosarcoma oncogene homolog B) is a transcription factor expressed in thymic stroma and its expression was induced a day after radiation exposure. Hence, the roles of mesenchymal MafB in the process of thymic regeneration offers an intriguing research topic also for radiation biology. The current study investigated whether MafB plays roles in the adult thymus. MafB/green fluorescent protein knock-in mutant (MafB^+/GFP) mice showed impaired thymic regeneration after the sublethal irradiation, judged by reduced thymus size, total thymocyte number and medullary complexity. Furthermore, IL4 was induced after irradiation and such induction was reduced in mutant mice. The mutants also displayed signs of accelerated age-related thymic involution. Altogether, these results suggest possible functions of MafB in the processes of thymic recovery after irradiation, and maintenance during aging.

Functional analysis of large MAF transcription factors and elucidation of their relationships with human diseases

The large MAF transcription factor group is a group of transcription factors with an acidic region, a basic region, and a leucine zipper region. Four types of MAF, MAFA, MAFB, c-MAF, and NRL, have been identified in humans and mice. In order to elucidate the functions of the large MAF transcription factor group in vivo, our research group created genetically modified MAFA-, MAFB-, and c-MAF-deficient mice and analyzed their phenotypes. MAFA is expressed in pancreatic β cells and is essential for insulin transcription and secretion. MAFB is essential for the development of pancreatic endocrine cells, formation of inner ears, podocyte function in the kidneys, and functional differentiation of macrophages. c-MAF is essential for lens formation and osteoblast differentiation. Furthermore, a single-base mutation in genes encoding the large MAF transcription factor group causes congenital renal disease, eye disease, bone disease, diabetes, and tumors in humans. This review describes the functions of large MAF transcription factors in vivo and their relationships with human diseases.

Non-Epithelial Stromal Cells in Thymus Development and Function

The thymus supports T-cell development via specialized microenvironments that ensure a diverse, functional and self-tolerant T-cell population. These microenvironments are classically defined as distinct cortex and medulla regions that each contain specialized subsets of stromal cells. Extensive research on thymic epithelial cells (TEC) within the cortex and medulla has defined their essential roles during T-cell development. Significantly, there are additional non-epithelial stromal cells (NES) that exist alongside TEC within thymic microenvironments, including multiple subsets of mesenchymal and endothelial cells. In contrast to our current understanding of TEC biology, the developmental origins, lineage relationships, and functional properties, of NES remain poorly understood. However, experimental evidence suggests these cells are important for thymus function by either directly influencing T-cell development, or by indirectly regulating TEC development and/or function. Here, we focus attention on the contribution of NES to thymic microenvironments, including their phenotypic identification and functional classification, and explore their impact on thymus function.

Non-Epithelial Thymic Stromal Cells: Unsung Heroes in Thymus Organogenesis and T Cell Development

The stromal microenvironment in the thymus is essential for generating a functional T cell repertoire. Thymic epithelial cells (TECs) are numerically and phenotypically one of the most prominent stromal cell types in the thymus, and have been recognized as one of most unusual cell types in the body by virtue of their unique functions in the course of the positive and negative selection of developing T cells. In addition to TECs, there are other stromal cell types of mesenchymal origin, such as fibroblasts and endothelial cells. These mesenchymal stromal cells are not only components of the parenchymal and vascular architecture, but also have a pivotal role in controlling TEC development, although their functions have been less extensively explored than TECs. Here, we review both the historical studies on and recent advances in our understanding of the contribution of such non-TEC stromal cells to thymic organogenesis and T cell development. In particular, we highlight the recently discovered functional effect of thymic fibroblasts on T cell repertoire selection.

Phenotypic analysis of mice carrying human-type MAFB p.Leu239Pro mutation

The transcription factor, MafB, plays important role in the differentiation and functional maintenance of various cells and tissues, such as the inner ear, kidney podocyte, parathyroid gland, pancreatic islet, and macrophages. The rare heterozygous substitution (p.Leu239Pro) of the DNA binding domain in MAFB is the cause of Focal Segmental Glomerulosclerosis associated with Duane Retraction Syndrome, which is characterized by impaired horizontal eye movement due to cranial nerve maldevelopment in humans. In this research, we generated mice carrying MafB p.Leu239Pro (Mafb^mt/mt) and retrieved their tissues for analysis. As a result, we found that the phenotype of Mafb^mt/mt mouse was similar to that of the conventional Mafb deficient mouse. This finding suggests that the Leucine residue at 239 in the DNA binding domain plays a key role in MafB function and could contribute to the diagnosis or development of treatment for patients carrying the MafB p.Leu239Pro missense variant.

Mice harboring an MCTO mutation exhibit renal failure resembling nephropathy in human patients

Multicentric carpotarsal osteolysis (MCTO) is a condition involving progressive osteolysis of the carpal and tarsal bones that is associated with glomerular sclerosis and renal failure (MCTO nephropathy). Previous work identified an autosomal dominant missense mutation in the transactivation domain of the transcription factor MAFB as the cause of MCTO. Several methods are currently used for MCTO nephropathy treatment, but these methods are invasive and lead to severe side effects, limiting their use. Therefore, the development of alternative treatments for MCTO nephropathy is required; however, the pathogenesis of MCTO in vivo is unclear without access to a mouse model. Here, we report the generation of an MCTO mouse model using the CRISPR/Cas9 system. These mice exhibit nephropathy symptoms that are similar to those observed in MCTO patients. Mafb^MCTO/MCTO mice show developmental defects in body weight from postnatal day 0, which persist as they age. They also exhibit high urine albumin creatinine levels from a young age, mimicking the nephropathic symptoms of MCTO patients. Characteristics of glomerular sclerosis reported in human patients are also observed, such as histological evidence of focal segmental glomerulosclerosis (FSGS), podocyte foot process microvillus transformation and podocyte foot process effacement. Therefore, this study contributes to the development of an alternative treatment for MCTO nephropathy by providing a viable mouse model.

MafB is a critical regulator of complement component C1q

The transcription factor MafB is expressed by monocytes and macrophages. Efferocytosis (apoptotic cell uptake) by macrophages is important for inhibiting the development of autoimmune diseases, and is greatly reduced in Mafb-deficient macrophages. Here, we show the expression of the first protein in the classical complement pathway C1q is important for mediating efferocytosis and is reduced in Mafb-deficient macrophages. The efferocytosis defect in Mafb-deficient macrophages can be rescued by adding serum from wild-type mice, but not by adding serum from C1q-deficient mice. By hemolysis assay we also show that activation of the classical complement pathway is decreased in Mafb-deficient mice. In addition, MafB overexpression induces C1q-dependent gene expression and signals that induce C1q genes are less effective in the absence of MafB. We also show that Mafb-deficiency can increase glomerular autoimmunity, including anti-nuclear antibody deposition. These results show that MafB is an important regulator of C1q.

Transcription factor MafB may play an important role in secondary hyperparathyroidism

The transcription factor MafB is essential for development of the parathyroid glands, the expression of which persists after morphogenesis and in adult parathyroid glands. However, the function of MafB in adult parathyroid tissue is unclear. To investigate this, we induced chronic kidney disease (CKD) in wild-type and MafB heterozygote (MafB+/-) mice by feeding them an adenine-supplemented diet, leading to secondary hyperparathyroidism. The elevated serum creatinine and blood urea nitrogen levels in heterozygous and wild-type mice fed the adenine-supplemented diet were similar. Interestingly, secondary hyperparathyroidism, characterized by serum parathyroid hormone elevation and enlargement of parathyroid glands, was suppressed in MafB+/- mice fed the adenine-supplemented diet compared to similarly fed wild-type littermates. Quantitative RT-PCR and immunohistochemical analyses showed that the increased expression of parathyroid hormone and cyclin D2 in mice with CKD was suppressed in the parathyroid glands of heterozygous CKD mice. A reporter assay indicated that MafB directly regulated parathyroid hormone and cyclin D2 expression. To exclude an effect of a developmental anomaly in MafB+/- mice, we analyzed MafB tamoxifen-induced global knockout mice. Hypocalcemia-stimulated parathyroid hormone secretion was significantly impaired in MafB knockout mice. RNA-sequencing analysis indicated PTH, Gata3 and Gcm2 depletion in the parathyroid glands of MafB knockout mice. Thus, MafB appears to play an important role in secondary hyperparathyroidism by regulation of parathyroid hormone and cyclin D2 expression. Hence, MafB may represent a new therapeutic target in secondary hyperparathyroidism.

Mafb lineage tracing to distinguish macrophages from other immune lineages reveals dual identity of Langerhans cells

Current systems for conditional gene deletion within mouse macrophage lineages are limited by ectopic activity or low efficiency. In this study, we generated a Mafb-driven Cre strain to determine whether any dendritic cells (DCs) identified by Zbtb46-GFP expression originate from a Mafb-expressing population. Lineage tracing distinguished macrophages from classical DCs, neutrophils, and B cells in all organs examined. At steady state, Langerhans cells (LCs) were lineage traced but also expressed Zbtb46-GFP, a phenotype not observed in any other population. After exposure to house dust mite antigen, Zbtb46-negative CD64⁺ inflammatory cells infiltrating the lung were substantially lineage traced, but Zbtb46-positive CD64^- cells were not. These results provide new evidence for the unique identity of LCs and challenge the notion that some inflammatory cells are a population of monocyte-derived DCs.

MafB deficiency accelerates the development of obesity in mice

MafB, a transcription factor expressed selectively in macrophages, has important roles in some macrophage-related diseases, especially in atherosclerosis. In this study, we investigated the mechanism by which hematopoietic-specific MafB deficiency induces the development of obesity. Wild-type and hematopoietic cell-specific Mafb-deficient mice were fed a high-fat diet for 10 weeks. The Mafb-deficient mice exhibited higher body weights and faster rates of body weight increase than control mice. The Mafb-deficient mice also had a higher percentage of body fat than the wild-type mice, due to increased adipocyte size and serum cholesterol levels. Reverse transcription-PCR analysis showed a reduction in apoptosis inhibitor of macrophage (AIM) in Mafb-deficient adipose tissue. AIM is known as an inhibitor of lipogenesis in adipocytes and is expressed in adipose tissue macrophages. Collectively, our data suggest that Mafb deficiency in hematopoietic cells accelerates the development of obesity.

Neuro-immune modulation of the thymus microenvironment (review)

The thymus is the primary site for T-cell lympho-poiesis. Its function includes the maturation and selection of antigen specific T cells and selective release of these cells to the periphery. These highly complex processes require precise parenchymal organization and compartmentation where a plethora of signalling pathways occur, performing strict control on the maturation and selection processes of T lymphocytes. In this review, the main morphological characteristics of the thymus microenvironment, with particular emphasis on nerve fibers and neuropeptides were assessed, as both are responsible for neuro-immune‑modulation functions. Among several neurotransmitters that affect thymus function, we highlight the dopaminergic system as only recently has its importance on thymus function and lymphocyte physiology come to light.

A focused in situ hybridization screen identifies candidate transcriptional regulators of thymic epithelial cell development and function

Background

Thymic epithelial cells (TECs) are necessary for normal T cell development. Currently, one transcription factor, Foxn1 is known to be necessary for the progression of fetal TEC differentiation. However, some aspects of fetal TEC differentiation occur in Foxn1 mutants, suggesting the existence of additional transcriptional regulators of TEC differentiation. The goal of this study was to identify some of the additional candidate transcription factors that may be involved in the specification and/or differentiation of TECs during fetal development.

Methodology/Principal Findings

We identified candidate fetal TEC transcriptional regulators via data and text mining. From our data mining we selected the transcription factors Foxg1, Isl1, Gata3, Nkx2-5, Nkx2-6 and Sox2 for further studies. Whole mount in situ hybridizations confirmed the expression of these transcription factors within subdomains of the third pharyngeal pouch from E9.5–E10.5. By E11.5 days Foxg1 and Isl1 transcripts were the only mRNAs from this group of genes detected exclusively within the thymus domain of the third pouch. Based on this initial in situ hybridization analysis, we focused on defining the expression of Foxg1 and Isl1 during multiple stages of thymus development and TEC differentiation. We found that Foxg1 and Isl1 are specifically expressed in differentiating TECs during fetal and postnatal stages of thymus development. In addition, we found differential expression of Islet1 and Foxn1 within the fetal and postnatal TEC population.

Conclusions/Significance

Our studies have identified two developmental transcription factors that are excellent candidate regulators of thymic epithelial cell specification and differentiation during fetal development. Our results suggest that Foxg1 and Isl1 may play a role in the regulation of TEC differentiation during fetal and postnatal stages. Our results also demonstrate heterogeneity of TECs marked by the differential expression of transcription factors, potentially providing new insights into the regulation of TEC differentiation.

Intrinsic innervation and dopaminergic markers after experimental denervation in rat thymus

The aim of this study was to examine rat thymus innervation using denervation techniques and to explore the related microanatomical localization of dopamine, D1, D2 receptors and dopamine membrane transporter (DAT).

In the thymus subcapsular region, the parenchymal cholinergic fibers belong exclusively to phrenic nerve branching. No somatic phrenic nerve branching was detected in any other analysed thymus lobule regions.

In rats subjected to sympathetic or parasympathetic ablation, it was observed that catecholaminergic and cholinergic nerve fibers respectively contributed to forming plexuses along vessel walls.

In the subcapsular and septal region, no parenchymal nerve branching, belonging to sympathetic or parasympathetic nervous system was noted. Instead, in the deep cortical region, cortico-medullary junction (CM-j) and medulla, catecholaminergic and cholinergic nerve fibers were detected along the vessels and parenchyma.

Dopamine and dopamine receptors were widely diffused in the lobular cortico-medullary junction region and in the medulla, where the final steps of thymocyte maturation and their trafficking take place.

No variation in dopamine and DAT immune reaction was observed following total or partial parasympathectomy or phrenic nerve cutting. After chemical or surgical sympathectomy however, neither dopamine nor DAT immune reaction was noted again. Instead, D1 and D2 dopamine receptor expression was not affected by thymus denervation.

In rats subjected to specific denervation, it was observed the direct intraparenchymal branching of the phrenic nerve and sympathetic and parasympathetic fibers into thymus parenchyma along vessels. These findings on the dopaminergic system highlight the importance of neurotransmitter receptor expression in the homeostasis of neuroimmune modulation.

Thymic epithelial cells: the multi-tasking framework of the T cell "cradle"

The thymus provides the anatomical "cradle" that fosters developing thymocytes. Thymic epithelial cells (TECs) are specialized cellular components that may be viewed as a multifunctional "frame" to nurture distinct stages of thymopoiesis. A symbiotic relationship between TECs and thymocytes exists because reciprocal interactions are required to achieve complete maturation of both cell types. Here, we propose that crucial instructive signals delivered by developing thymocytes negatively regulate functional attributes of immature TECs (including the expression of Delta-like 4 (DLL4) and interleukin-7 (IL-7)) that are required during early stages of thymopoiesis, while promoting the diversification of more mature TEC subsets. Thus, the division of labour among TECs may be coordinated directly by local cellular feedback mechanisms operating within distinct thymic niches.