Miz1 is required for hair follicle structure and hair morphogenesis

Regulation of BCR-mediated Ca2+ mobilization by MIZ1-TMBIM4 safeguards IgG1+ GC B cell-positive selection

The transition from immunoglobulin M (IgM) to affinity-matured IgG antibodies is vital for effective humoral immunity. This is facilitated by germinal centers (GCs) through affinity maturation and preferential maintenance of IgG+ B cells over IgM+ B cells. However, it is not known whether the positive selection of the different Ig isotypes within GCs is dependent on specific transcriptional mechanisms. Here, we explored IgG1+ GC B cell transcription factor dependency using a CRISPR-Cas9 screen and conditional mouse genetics. We found that MIZ1 was specifically required for IgG1+ GC B cell survival during positive selection, whereas IgM+ GC B cells were largely independent. Mechanistically, MIZ1 induced TMBIM4, an ancestral anti-apoptotic protein that regulated inositol trisphosphate receptor (IP3R)-mediated calcium (Ca2+) mobilization downstream of B cell receptor (BCR) signaling in IgG1+ B cells. The MIZ1-TMBIM4 axis prevented mitochondrial dysfunction-induced IgG1+ GC cell death caused by excessive Ca2+ accumulation. This study uncovers a unique Ig isotype-specific dependency on a hitherto unidentified mechanism in GC-positive selection.

SP1 and KROX20 Regulate the Proliferation of Dermal Papilla Cells and Target the CUX1 Gene

Previous studies have demonstrated that CUX1 could contribute to the proliferation of DPCs in vitro, but the upstream transcriptional regulatory mechanisms of CUX1 remain largely unknown. This study aimed to investigate the upstream transcriptional regulators of CUX1 to enhance our comprehension of the mechanism of action of the CUX1 gene in ovine DPCs. Initially, the JASPAR (2024) software was used to predict the upstream target transcription factors for the CUX1 gene. Subsequently, through RT-qPCR and a double luciferase reporter assay, the interaction between SP1, KROX20, and CUX1 was established, respectively. The results indicated that SP1 and KROX20 were two highly reliable upstream transcription regulators for the CUX1 gene. Additionally, we found that SP1 promoted the proliferation of DPCs by overexpressing SP1 in DPCs, and KROX20 inhibited the proliferation of DPCs by overexpressing KROX20 in DPCs. These findings are also consistent with the transcriptional regulation of CUX1 by SP1 and KROX20, respectively. This study suggests that the effect of DPC proliferation in vitro by CUX1 may regulated by the transcription factors SP1 and KROX20.

The zinc finger protein Miz1 suppresses liver tumorigenesis by restricting hepatocyte-driven macrophage activation and inflammation

Chronic inflammation plays a central role in hepatocellular carcinoma (HCC), but the contribution of hepatocytes to tumor-associated inflammation is not clear. Here, we report that the zinc finger transcription factor Miz1 restricted hepatocyte-driven inflammation to suppress HCC, independently of its transcriptional activity. Miz1 was downregulated in HCC mouse models and a substantial fraction of HCC patients. Hepatocyte-specific Miz1 deletion in mice generated a distinct sub-group of hepatocytes that produced pro-inflammatory cytokines and chemokines, which skewed the polarization of the tumor-infiltrating macrophages toward pro-inflammatory phenotypes to promote HCC. Mechanistically, Miz1 sequestrated the oncoprotein metadherin (MTDH), preventing MTDH from promoting transcription factor nuclear factor κB (NF-κB) activation. A distinct sub-group of pro-inflammatory cytokine-producing hepatocytes was also seen in a subset of HCC patients. In addition, Miz1 expression inversely correated with disease recurrence and poor prognosis in HCC patients. Our findings identify Miz1 as a tumor suppressor that prevents hepatocytes from driving inflammation in HCC.

Identification of the transcription factor Miz1 as an essential regulator of diphthamide biosynthesis using a CRISPR-mediated genome-wide screen

Diphthamide is a unique post-translationally modified histidine residue (His715 in all mammals) found only in eukaryotic elongation factor-2 (eEF-2). The biosynthesis of diphthamide represents one of the most complex modifications, executed by protein factors conserved from yeast to humans. Diphthamide is not only essential for normal physiology (such as ensuring fidelity of mRNA translation), but is also exploited by bacterial ADP-ribosylating toxins (e.g., diphtheria toxin) as their molecular target in pathogenesis. Taking advantage of the observation that cells defective in diphthamide biosynthesis are resistant to ADP-ribosylating toxins, in the past four decades, seven essential genes (Dph1 to Dph7) have been identified for diphthamide biosynthesis. These technically unsaturated screens raise the question as to whether additional genes are required for diphthamide biosynthesis. In this study, we performed two independent, saturating, genome-wide CRISPR knockout screens in human cells. These screens identified all previously known Dph genes, as well as further identifying the BTB/POZ domain-containing transcription factor Miz1. We found that Miz1 is absolutely required for diphthamide biosynthesis via its role in the transcriptional regulation of Dph1 expression. Mechanistically, Miz1 binds to the Dph1 proximal promoter via an evolutionarily conserved consensus binding site to activate Dph1 transcription. Therefore, this work demonstrates that Dph1-7, along with the newly identified Miz1 transcription factor, are likely to represent the essential protein factors required for diphthamide modification on eEF2.

Murine dorsal hair type is genetically determined by polymorphisms in candidate genes that influence BMP and WNT signalling

Mouse dorsal coat hair types, guard, awl, auchene and zigzag, develop in three consecutive waves. To date, it is unclear if these hair types are determined genetically through expression of specific factors or can change based on their mesenchymal environment. We undertook a novel approach to this question by studying individual hair type in 67 Collaborative Cross (CC) mouse lines and found significant variation in the proportion of each type between strains. Variation in the proportion of zigzag, awl and auchene, but not guard hair, was largely due to germline genetic variation. We utilised this variation to map a quantitative trait locus (QTL) on chromosome 12 that appears to influence a decision point switch controlling the propensity for either second (awl and auchene) or third wave (zigzag) hairs to develop. This locus contains two strong candidates, Sostdc1 and Twist1, each of which carry several ENCODE regulatory variants, specific to the causal allele, that can influence gene expression, are expressed in the developing hair follicle, and have been previously reported to be involved in regulating human and murine hair behaviour, but not hair subtype determination. Both of these genes are likely to play a part in hair type determination via regulation of BMP and/or WNT signalling.

Keratinocyte Cytokine Networks Associated with Human Melanocytic Nevus Development

Melanocytes can group together in nevi, commonly thought to form because of intrinsic somatic mutations involving MAPK pathway activation. However, the role of the microenvironment, in particular keratinocytes, in nevogenesis is rarely studied. Melanocytes proliferate during the hair follicle growth phase and in some basal cell carcinomas, allowing us to construct keratinocyte gene expression clusters correlated with melanocyte activation. We asked whether such correlations are evident in the more subtle context of regulation of melanocyte behavior in normal skin. We considered genes which, when mutated in keratinocytes in mice, lead to nevogenesis. Across the human GTEx normal skin database, their expression was correlated with that of keratinocyte cytokines KITLG, HGF, FGF2, EDN1, and melanocyte markers. These cytokines have pleiotropic effects on melanocyte-specific and pigmentation genes and also influence mast cell gene expression. We show five classes of keratinocyte genes that, via germline genetic variation, influence melanocyte activity. These include genes involved in SHH signaling, structural keratins, ribosomal biogenesis, and stem cell governance. In agreement with the finding of KITLG linked to nevogenesis in human genome-wide association studies, we provide evidence that specific keratinocyte cytokines are components of networks that may drive or exacerbate nevus development.

Schwann cell Myc-interacting zinc-finger protein 1 without pox virus and zinc finger: epigenetic implications in a peripheral neuropathy

Functionality of adult peripheral nerves essentially relies on differentiation of Schwann cells during postnatal development, as well as fine-tuned re- and transdifferentiation in response to peripheral nerve injury. Epigenetic histone modifications play a major role during the differentiation of embryonic stem cells and diverse organ specific progenitor cells, yet only little is known about the epigenetic regulation of Schwann cells. Just recently, Fuhrmann et al. reported how the transcription factor Myc-interacting zinc-finger protein 1 (Miz1) might contribute to Schwann cell differentiation through repression of the histone demethylase Kdm8. Here, we discuss the potential novel role of Miz1 in Schwann cell differentiation and give a short overview about previously reported histone modifications underlying peripheral nerve development and response to injury.

Miz1 Controls Schwann Cell Proliferation via H3K36me2 Demethylase Kdm8 to Prevent Peripheral Nerve Demyelination

Schwann cell differentiation and myelination depends on chromatin remodeling, histone acetylation, and methylation, which all affect Schwann cell proliferation. We previously reported that the deletion of the POZ (POxvirus and Zinc finger) domain of the transcription factor Miz1 (Myc-interacting zinc finger protein; encoded by Zbtb17) in mouse Schwann cells (Miz1ΔPOZ) causes a neuropathy at 90 d after birth [postnatal day (P) 90], with a subsequent spontaneous regeneration. Here we show that RNA sequencing from Miz1ΔPOZ and control animals at P30 revealed a set of upregulated genes with a strong correlation to cell-cycle regulation. Consistently, a subset of Schwann cells did not exit the cell cycle as observed in control animals and the growth fraction increased over time. From the RNAseq gene list, two direct Miz1 target genes were identified, one of which encodes the histone H3K36^me2 demethylase Kdm8. We show that the expression of Kdm8 is repressed by Miz1 and that its release in Miz1ΔPOZ cells induces a decrease of H3K36^me2, especially in deregulated cell-cycle-related genes. The linkage between elevated Kdm8 expression, hypomethylation of H3K36 at cell-cycle-relevant genes, and the subsequent re-entering of adult Schwann cells into the cell cycle suggests that the release of Kdm8 repression in the absence of a functional Miz1 is a central issue in the development of the Miz1ΔPOZ phenotype.SIGNIFICANCE STATEMENT The deletion of the Miz1 (Myc-interacting zinc finger protein 1) POZ (POxvirus and Zinc finger) domain in Schwann cells causes a neuropathy. Here we report sustained Schwann cell proliferation caused by an increased expression of the direct Miz1 target gene Kdm8, encoding a H3K36me2 demethylase. Hence, the demethylation of H3K36 is linked to the pathogenesis of a neuropathy.

Altered glycogen metabolism causes hepatomegaly following an Atg7 deletion

Autophagy is a lysosomal degradation process involved in the turnover of organelles or other cell constituents, in providing sources for energy production under starving conditions and in cell metabolism. A key protein in the macroautophagic machinery is the autophagy-related protein (Atg) 7. Constitutive deletion of Atg7 is lethal at birth. A conditional deletion of Atg7 in hepatocytes leads to hepatomegaly and in aged animals to liver tumors. With this study, we aim at analyzing the hepatomegaly development in more detail. The 3- to 4-fold enlargement of the liver takes place between days 25 and 35 after birth (P25-P35) and persists at least until P90. This is accompanied by a change in the expression of enzymes involved in the glycogen/glucose metabolism. While glycogen synthesis is inhibited, glucose is preferentially kept as glucose-6-phosphate inside the cells, inducing a swelling of the cells caused by hyperosmolarity. An increase of lipogenic enzymes suggests that glucose-6-phosphate is delivered to lipogenic pathways, which is supported by the occurrence of a steatosis around P30. The development of hepatomegaly is accompanied by a polyploidisation of hepatocytes, an enhanced expression of genes related to inflammatory processes and an infiltration of macrophages and granulocytes. Our data provide evidence that the attenuation of macroautophagy in hepatocytes leads to a glucose retention that causes cell swelling. The resulting hepatomegaly, which develops in a time interval of about 10 days, perturbs liver perfusion and induces an inflammatory reaction together with polyploidisation.

ZBTB17 (MIZ1) Is Important for the Cardiac Stress Response and a Novel Candidate Gene for Cardiomyopathy and Heart Failure

BACKGROUND

Mutations in sarcomeric and cytoskeletal proteins are a major cause of hereditary cardiomyopathies, but our knowledge remains incomplete as to how the genetic defects execute their effects.

METHODS AND RESULTS

We used cysteine and glycine-rich protein 3, a known cardiomyopathy gene, in a yeast 2-hybrid screen and identified zinc-finger and BTB domain-containing protein 17 (ZBTB17) as a novel interacting partner. ZBTB17 is a transcription factor that contains the peak association signal (rs10927875) at the replicated 1p36 cardiomyopathy locus. ZBTB17 expression protected cardiac myocytes from apoptosis in vitro and in a mouse model with cardiac myocyte-specific deletion of Zbtb17, which develops cardiomyopathy and fibrosis after biomechanical stress. ZBTB17 also regulated cardiac myocyte hypertrophy in vitro and in vivo in a calcineurin-dependent manner.

CONCLUSIONS

We revealed new functions for ZBTB17 in the heart, a transcription factor that may play a role as a novel cardiomyopathy gene.

Late onset neuropathy with spontaneous clinical remission in mice lacking the POZ domain of the transcription factor Myc-interacting zinc finger protein 1 (Miz1) in Schwann cells

The transcription factor Miz1 (Myc-interacting zinc finger 1) is a known regulator of the cell cycle but also has cell cycle-independent functions. Here we analyzed the role of Miz1 in the peripheral nervous system, using an early embryonic conditional knock-out model in which the Miz1 POZ domain is ablated in Schwann cells. Although the development of myelinated nerve fibers was not impaired, Miz1ΔPOZ mice acquired behavioral signs of a peripheral neuropathy at the age of 3 months. At this time, ultrastructural analysis of the sciatic nerve showed de- and dysmyelination of fibers, with massive outfoldings and a focal infiltration of macrophages. Although the expression of genes encoding structural myelin proteins, such as periaxin, myelin basic protein, and myelin protein zero, was decreased, genes associated with a negative regulation of myelination, including c-Jun, Sox2, and Id2, were up-regulated in Miz1ΔPOZ mice compared with controls. In animals older than 4 months, the motor disabilities vanished, and the ultrastructure of the sciatic nerve exhibited numerous tomacula and remyelinated fibers, as indicated by thinner myelin. No second acute attack was observed up to the age of 1 year. Thus, the deletion of the Miz1 POZ domain in Schwann cells induces an acute neuropathy with a subsequent regeneration in which there is ongoing balancing between de- and remyelination. Miz1ΔPOZ mice are impaired in the maintenance of myelinated fibers and are a promising model for studying remyelination in adult peripheral nerves.

Miz-1 activates gene expression via a novel consensus DNA binding motif

The transcription factor Miz-1 can either activate or repress gene expression in concert with binding partners including the Myc oncoprotein. The genomic binding of Miz-1 includes both core promoters and more distal sites, but the preferred DNA binding motif of Miz-1 has been unclear. We used a high-throughput in vitro technique, Bind-n-Seq, to identify two Miz-1 consensus DNA binding motif sequences—ATCGGTAATC and ATCGAT (Mizm1 and Mizm2)—bound by full-length Miz-1 and its zinc finger domain, respectively. We validated these sequences directly as high affinity Miz-1 binding motifs. Competition assays using mutant probes indicated that the binding affinity of Miz-1 for Mizm1 and Mizm2 is highly sequence-specific. Miz-1 strongly activates gene expression through the motifs in a Myc-independent manner. MEME-ChIP analysis of Miz-1 ChIP-seq data in two different cell types reveals a long motif with a central core sequence highly similar to the Mizm1 motif identified by Bind-n-Seq, validating the in vivo relevance of the findings. Miz-1 ChIP-seq peaks containing the long motif are predominantly located outside of proximal promoter regions, in contrast to peaks without the motif, which are highly concentrated within 1.5 kb of the nearest transcription start site. Overall, our results indicate that Miz-1 may be directed in vivo to the novel motif sequences we have identified, where it can recruit its specific binding partners to control gene expression and ultimately regulate cell fate.

Miz1 is required to maintain autophagic flux

Miz1 is a zinc finger protein that regulates the expression of cell cycle inhibitors as part of a complex with Myc. Cell cycle-independent functions of Miz1 are poorly understood. Here we use a Nestin-Cre transgene to delete an essential domain of Miz1 in the central nervous system (Miz1(ΔPOZNes)). Miz1(ΔPOZNes) mice display cerebellar neurodegeneration characterized by the progressive loss of Purkinje cells. Chromatin immunoprecipitation sequencing and biochemical analyses show that Miz1 activates transcription upon binding to a non-palindromic sequence present in core promoters. Target genes of Miz1 encode regulators of autophagy and proteins involved in vesicular transport that are required for autophagy. Miz1(ΔPOZ) neuronal progenitors and fibroblasts show reduced autophagic flux. Consistently, polyubiquitinated proteins and p62/Sqtm1 accumulate in the cerebella of Miz1(ΔPOZNes) mice, characteristic features of defective autophagy. Our data suggest that Miz1 may link cell growth and ribosome biogenesis to the transcriptional regulation of vesicular transport and autophagy.

Miz1 deficiency in the mammary gland causes a lactation defect by attenuated Stat5 expression and phosphorylation

Miz1 is a zinc finger transcription factor with an N-terminal POZ domain. Complexes with Myc, Bcl-6 or Gfi-1 repress expression of genes like Cdkn2b (p15^Ink4) or Cdkn1a (p21^Cip1). The role of Miz1 in normal mammary gland development has not been addressed so far. Conditional knockout of the Miz1 POZ domain in luminal cells during pregnancy caused a lactation defect with a transient reduction of glandular tissue, reduced proliferation and attenuated differentiation. This was recapitulated in vitro using mouse mammary gland derived HC11 cells. Further analysis revealed decreased Stat5 activity in Miz1ΔPOZ mammary glands and an attenuated expression of Stat5 targets. Gene expression of the Prolactin receptor (PrlR) and ErbB4, both critical for Stat5 phosphorylation (pStat5) or pStat5 nuclear translocation, was decreased in Miz1ΔPOZ females. Microarray, ChIP-Seq and gene set enrichment analysis revealed a down-regulation of Miz1 target genes being involved in vesicular transport processes. Our data suggest that deranged intracellular transport and localization of PrlR and ErbB4 disrupt the Stat5 signalling pathway in mutant glands and cause the observed lactation phenotype.

Biphasic influence of Miz1 on neural crest development by regulating cell survival and apical adhesion complex formation in the developing neural tube

Myc interacting zinc finger protein-1 (Miz1) is a transcription factor known to regulate cell cycle- and cell adhesion-related genes in cancer. Here we show that Miz1 also plays a critical role in neural crest development. In the chick, Miz1 is expressed throughout the neural plate and closing neural tube. Its morpholino-mediated knockdown affects neural crest precursor survival, leading to reduction of neural plate border and neural crest specifier genes Msx-1, Pax7, FoxD3, and Sox10. Of interest, Miz1 loss also causes marked reduction of adhesion molecules (N-cadherin, cadherin6B, and α1-catenin) with a concomitant increase of E-cadherin in the neural folds, likely leading to delayed and decreased neural crest emigration. Conversely, Miz1 overexpression results in up-regulation of cadherin6B and FoxD3 expression in the neural folds/neural tube, leading to premature neural crest emigration and increased number of migratory crest cells. Although Miz1 loss effects cell survival and proliferation throughout the neural plate, the neural progenitor marker Sox2 was unaffected, suggesting a neural crest-selective effect. The results suggest that Miz1 is important not only for survival of neural crest precursors, but also for maintenance of integrity of the neural folds and tube, via correct formation of the apical adhesion complex therein.

The role of MIZ-1 in MYC-dependent tumorigenesis

A hallmark of MYC-transformed cells is their aberrant response to antimitogenic signals. Key examples include the inability of MYC-transformed cells to arrest proliferation in response to antimitogenic signals such as TGF-β or DNA damage and their inability to differentiate into adipocytes in response to hormonal stimuli. Given the plethora of antimitogenic signals to which a tumor cell is exposed, it is likely that the ability to confer resistance to these signals is central to the transforming properties of MYC in vivo. At the same time, the inability of MYC-transformed cells to halt cell-cycle progression on stress may establish a dependence on mutations that impair or disable apoptosis. We propose that the interaction of MYC with the zinc finger protein MIZ-1 mediates resistance to antimitogenic signals. In contrast to other interactions of MYC, there is currently little evidence that MIZ-1 associates with MYC in normal, unperturbed cells. The functional interaction of both proteins becomes apparent at oncogenic expression levels of MYC and association with MIZ-1 mediates both oncogenic functions of MYC as well as tumor-suppressive responses to oncogenic levels of MYC.

Mule/Huwe1/Arf-BP1 suppresses Ras-driven tumorigenesis by preventing c-Myc/Miz1-mediated down-regulation of p21 and p15

Tumorigenesis results from dysregulation of oncogenes and tumor suppressors that influence cellular proliferation, differentiation, apoptosis, and/or senescence. Many gene products involved in these processes are substrates of the E3 ubiquitin ligase Mule/Huwe1/Arf-BP1 (Mule), but whether Mule acts as an oncogene or tumor suppressor in vivo remains controversial. We generated K14Cre;Mule(flox/flox(y)) (Mule kKO) mice and subjected them to DMBA/PMA-induced skin carcinogenesis, which depends on oncogenic Ras signaling. Mule deficiency resulted in increased penetrance, number, and severity of skin tumors, which could be reversed by concomitant genetic knockout of c-Myc but not by knockout of p53 or p19Arf. Notably, in the absence of Mule, c-Myc/Miz1 transcriptional complexes accumulated, and levels of p21CDKN1A (p21) and p15INK4B (p15) were down-regulated. In vitro, Mule-deficient primary keratinocytes exhibited increased proliferation that could be reversed by Miz1 knockdown. Transfer of Mule-deficient transformed cells to nude mice resulted in enhanced tumor growth that again could be abrogated by Miz1 knockdown. Our data demonstrate in vivo that Mule suppresses Ras-mediated tumorigenesis by preventing an accumulation of c-Myc/Miz1 complexes that mediates p21 and p15 down-regulation.

Mutations in KCTD1 cause scalp-ear-nipple syndrome

Scalp-ear-nipple (SEN) syndrome is a rare, autosomal-dominant disorder characterized by cutis aplasia of the scalp; minor anomalies of the external ears, digits, and nails; and malformations of the breast. We used linkage analysis and exome sequencing of a multiplex family affected by SEN syndrome to identify potassium-channel tetramerization-domain-containing 1 (KCTD1) mutations that cause SEN syndrome. Evaluation of a total of ten families affected by SEN syndrome revealed KCTD1 missense mutations in each family tested. All of the mutations occurred in a KCTD1 region encoding a highly conserved bric-a-brac, tram track, and broad complex (BTB) domain that is required for transcriptional repressor activity. KCTD1 inhibits the transactivation of the transcription factor AP-2α (TFAP2A) via its BTB domain, and mutations in TFAP2A cause cutis aplasia in individuals with branchiooculofacial syndrome (BOFS), suggesting a potential overlap in the pathogenesis of SEN syndrome and BOFS. The identification of KCTD1 mutations in SEN syndrome reveals a role for this BTB-domain-containing transcriptional repressor during ectodermal development.

Suppression of inflammation and acute lung injury by Miz1 via repression of C/EBP-δ

Inflammation is essential for host defense but can cause tissue damage and organ failure if unchecked. How the inflammation is resolved remains elusive. Here we report that the transcription factor Miz1 was required for terminating lipopolysaccharide (LPS)-induced inflammation. Genetic disruption of the Miz1 POZ domain, which is essential for its transactivation or repression activity, resulted in hyper-inflammation, lung injury and increased mortality in LPS-treated mice while reduced bacterial load and mortality in mice with Pseudomonas aeruginosa pneumonia. Loss of the Miz1 POZ domain prolonged pro-inflammatory cytokine expression. Upon stimulation, Miz1 was phosphorylated at Ser178, which is required for recruiting histone deacetylase 1 to repress transcription of C/EBP-δ, an amplifier of inflammation. Our data provide a long-sought mechanism underlying resolution of LPS-induced inflammation.

The BTB-ZF transcription factors

The BTB-ZF (broad-complex, tramtrack and bric-à-brac--zinc finger) proteins are encoded by at least 49 genes in mouse and man and commonly serve as sequence-specific silencers of gene expression. This review will focus on the known physiological functions of mammalian BTB-ZF proteins, which include essential roles in the development of the immune system. We discuss their function in terminally differentiated lymphocytes and the progenitors that give rise to them, their action in hematopoietic malignancy and roles beyond the immune system.

Miz1 is a critical repressor of cdkn1a during skin tumorigenesis

The transcription factor Miz1 forms repressive DNA-binding complexes with the Myc, Gfi-1 and Bcl-6 oncoproteins. Known target genes of these complexes encode the cyclin-dependent kinase inhibitors (CKIs) cdkn2b (p15^Ink4), cdkn1a (p21^Cip1), and cdkn1c (p57^Kip2). Whether Miz1-mediated repression is important for control of cell proliferation in vivo and for tumor formation is unknown. Here we show that deletion of the Miz1 POZ domain, which is critical for Miz1 function, restrains the development of skin tumors in a model of chemically-induced, Ras-dependent tumorigenesis. While the stem cell compartment appears unaffected, interfollicular keratinocytes lacking functional Miz1 exhibit a reduced proliferation and an accelerated differentiation of the epidermis in response to the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA). Tumorigenesis, proliferation and normal differentiation are restored in animals lacking cdkn1a, but not in those lacking cdkn2b. Our data demonstrate that Miz1-mediated attenuation of cell cycle arrest pathways via repression of cdkn1a has a critical role during tumorigenesis in the skin.

Transcriptional repression: the dark side of myc

The (c-)Myc oncoprotein and its cousins, the N-Myc and L-Myc proteins, show all hallmarks of transcriptional activator proteins: Myc carries a carboxy-terminal DNA binding domain, which mediates sequence-specific binding to DNA. At its amino-terminus, Myc carries a transcriptional regulatory domain that strongly activates transcription when fused to an ectopic DNA binding domain; moreover, the strength of activation of different members of the Myc family correlates with their ability to transform rodent cells. Furthermore, activation of conditional alleles of Myc, either tetracycline or estrogen inducible, upregulates expression of a large number of genes, both in tissue culture and in transgenic animals. Indeed, many of these genes have essential roles in cell proliferation, cell growth, and metabolism; two of them, odc, encoding ornithine decarboxylase, a rate-limiting enzyme of polyamine biosynthesis, and rpl24, encoding a constituent of the large ribosomal subunit, are haploinsufficient for Myc-induced lymphomagenesis but not for normal development, arguing very strongly that upregulation of both genes is critical for Myc-dependent tumor formation. Undoubtedly, therefore, Myc exerts part of its biological activities via transcriptional upregulation of a large number of target genes. One of the key issues in the field is whether there are additional biochemical activities of the Myc protein and, if so, whether and how they contribute to Myc biology. This review summarizes evidence demonstrating that Myc has the ability to repress transcription and that this may be an important function during oncogenic transformation.

Post-translational control of Myc function during differentiation

Myc proteins are deeply involved in multiple biological processes including cell proliferation, growth, metabolism, apoptosis, differentiation, and tumorigenesis. Paradoxically, Myc proteins have been found to be capable of both inhibiting and facilitating differentiation depending on the biological context. Recently we identified a new mode of Myc regulation in differentiating muscle cells in which c-Myc protein is proteolytically cleaved by calcium-dependent calpains in the cytoplasm. This cleavage serves two purposes. First, it inactivates the transcriptional function of Myc by removing its C-terminus, a region responsible for the interaction of Myc with Max and DNA. Second, it alters cytoskeletal architecture and accelerates muscle differentiation through the activity of the remaining N-terminal cleavage product (termed Myc-nick). Here we discuss the roles and regulation of full-length Myc and Myc-nick in terminal differentiation and propose a model in which calpain-mediated cleavage of Myc operates as a functional switch.

Epidermal patterning and induction of different hair types during mouse embryonic development

An intriguing question in developmental biology is how epidermal pattern formation processes are established and what are the molecular mechanisms involved in these events. The establishment of the pattern is concomitant with the formation of ectodermal appendages, which involves complex interactions between the epithelium and the underlying mesenchyme. Among ectodermal appendages, hair follicles are the "mini organs" that produce hair shafts. Several developmental and structural features are common to all hair follicles and to the hair shaft they produce. However, many different hair types are produced in a single organism. Also, different characteristics can be observed depending on the part of the body where the hair follicle is formed. Here, we review the mechanisms involved in the patterning of different hair types during mouse embryonic development as well as the influence of the body axes on hair patterning.

The transcriptional repressor RP58 is crucial for cell-division patterning and neuronal survival in the developing cortex

The neocortex and the hippocampus comprise several specific layers containing distinct neurons that originate from progenitors at specific development times, under the control of an adequate cell-division patterning mechanism. Although many molecules are known to regulate this cell-division patterning process, its details are not well understood. Here, we show that, in the developing cerebral cortex, the RP58 transcription repressor protein was expressed both in postmitotic glutamatergic projection neurons and in their progenitor cells, but not in GABAergic interneurons. Targeted deletion of the RP58 gene led to dysplasia of the neocortex and of the hippocampus, reduction of the number of mature cortical neurons, and defects of laminar organization, which reflect abnormal neuronal migration within the cortical plate. We demonstrate an impairment of the cell-division patterning during the late embryonic stage and an enhancement of apoptosis of the postmitotic neurons in the RP58-deficient cortex. These results suggest that RP58 controls cell division of progenitor cells and regulates the survival of postmitotic cortical neurons.

Miz1 and HectH9 regulate the stability of the checkpoint protein, TopBP1

The Myc-associated zinc-finger protein, Miz1, activates transcription of the p21cip1 gene in response to UV irradiation. Miz1 associates with topoisomerase II binding protein1 (TopBP1), an essential activator of the Atr kinase. We show here that Miz1 is required for the recruitment of a fraction of TopBP1 to chromatin, for the protection of TopBP1 from proteasomal degradation and for Atr-dependent signal transduction. TopBP1 that is not bound to chromatin is degraded by the HectH9 (Mule, ARF-BP1 and HUWE1) ubiquitin ligase. Myc antagonizes the binding of TopBP1 to Miz1; as a result, expression of Myc leads to dissociation of TopBP1 from chromatin, reduces the amount of total TopBP1 and attenuates Atr-dependent signal transduction. Our data show that Miz1 and Myc affect the activity of the Atr checkpoint through their effect on TopBP1 chromatin association and stability.

MYC in mammalian epidermis: how can an oncogene stimulate differentiation?

MYC in human epidermal stem cells can stimulate differentiation rather than uncontrolled proliferation. This discovery was, understandably, greeted with scepticism by researchers. However, subsequent studies have confirmed that MYC can stimulate epidermal stem cells to differentiate and have shed light on the underlying mechanisms. Two concepts that are relevant to cancer have emerged: first, MYC regulates similar genes in different cell types, but the biological consequences are context-dependent; and second, MYC activation is not a simple 'on/off' switch - the cellular response depends on the strength and duration of MYC activity, which in turn is affected by the many cofactors and regulatory pathways with which MYC interacts.