Haplosufficiency of PAX3 for melanoma development in Tyr: NRASQ61K; Cdkn2a-/- mice allows identification and sorting of melanoma cells using a Pax3GFP reporter allele.

The role of the Pax3 gene in embryonic development of pigment cells is well characterized. By contrast, the function of Pax3 in melanoma development is controversial. Indeed, data obtained from cultured cells suggest that PAX3 may contribute to melanomagenesis. PAX3 is found to be overexpressed in melanomas and also in nevi compared with normal skin samples. Pax3 homozygous loss of function is embryonic lethal. To assess the role of Pax3 in melanoma development in vivo, we analyzed Pax3 haploinsufficiency in a mouse model of melanoma predisposition. The Pax3(GFP/+) knock-in reporter system was combined with the Tyr::NRAS(Q61K); Cdkn2a(-/-) mouse melanoma model. Melanoma development was followed over 18 months. Histopathological, immunohistochemical, and molecular analyses of lesions at different stages of melanoma progression were carried out. Fluorescence-activated cell sorting on GFP of cells from primary or metastatic melanoma was followed by ex-vivo transformation tests and in-vivo passaging. We report here that Tyr::NRAS(Q61K); Cdkn2a(-/-); Pax3(GFP/+) mice developed metastasizing melanoma as their Tyr::NRAS(Q61K); Cdkn2a(-/-); littermates. Histopathology showed no differences between the two genotypes, although Pax3 mRNA and PAX3 protein levels in Pax3(GFP/+) lesions were reduced by half. The Pax3(GFP) allele proved to be a convenient marker to identify and directly sort heterogeneous populations of melanoma cells within the tumor bulk at each stage of melanoma progression. This new mouse model represents an accurate and reproducible means for identifying melanoma cells in vivo to study the mechanisms of melanoma development.

HACD1, a regulator of membrane composition and fluidity, promotes myoblast fusion and skeletal muscle growth.

The reduced diameter of skeletal myofibres is a hallmark of several congenital myopathies, yet the underlying cellular and molecular mechanisms remain elusive. In this study, we investigate the role of HACD1/PTPLA, which is involved in the elongation of the very long chain fatty acids, in muscle fibre formation. In humans and dogs, HACD1 deficiency leads to a congenital myopathy with fibre size disproportion associated with a generalized muscle weakness. Through analysis of HACD1-deficient Labradors, Hacd1-knockout mice, and Hacd1-deficient myoblasts, we provide evidence that HACD1 promotes myoblast fusion during muscle development and regeneration. We further demonstrate that in normal differentiating myoblasts, expression of the catalytically active HACD1 isoform, which is encoded by a muscle-enriched splice variant, yields decreased lysophosphatidylcholine content, a potent inhibitor of myoblast fusion, and increased concentrations of ≥ C18 and monounsaturated fatty acids of phospholipids. These lipid modifications correlate with a reduction in plasma membrane rigidity. In conclusion, we propose that fusion impairment constitutes a novel, non-exclusive pathological mechanism operating in congenital myopathies and reveal that HACD1 is a key regulator of a lipid-dependent muscle fibre growth mechanism.

Thrombopoietin-increased DNA-PK-dependent DNA repair limits hematopoietic stem and progenitor cell mutagenesis in response to DNA damage.

DNA double-strand breaks (DSBs) represent a serious threat for hematopoietic stem cells (HSCs). How cytokines and environmental signals integrate the DNA damage response and contribute to HSC-intrinsic DNA repair processes remains unknown. Thrombopoietin (TPO) and its receptor, Mpl, are critical factors supporting HSC self-renewal and expansion. Here, we uncover an unknown function for TPO-Mpl in the regulation of DNA damage response. We show that DNA repair following γ-irradiation (γ-IR) or the action of topoisomerase-II inhibitors is defective in Mpl(-/-) and in wild-type mouse or human hematopoietic stem and progenitor cells treated in the absence of TPO. TPO stimulates DNA repair in vitro and in vivo by increasing DNA-PK-dependent nonhomologous end-joining efficiency. This ensures HSC chromosomal integrity and limits their long-term injury in response to IR. This shows that niche factors can modulate the HSC DSB repair machinery and opens new avenues for administration of TPO agonists for minimizing radiotherapy-induced HSC injury and mutagenesis.

Centronuclear myopathy in Labrador retrievers: a recent founder mutation in the PTPLA gene has rapidly disseminated worldwide.

Centronuclear myopathies (CNM) are inherited congenital disorders characterized by an excessive number of internalized nuclei. In humans, CNM results from ~70 mutations in three major genes from the myotubularin, dynamin and amphiphysin families. Analysis of animal models with altered expression of these genes revealed common defects in all forms of CNM, paving the way for unified pathogenic and therapeutic mechanisms. Despite these efforts, some CNM cases remain genetically unresolved. We previously identified an autosomal recessive form of CNM in French Labrador retrievers from an experimental pedigree, and showed that a loss-of-function mutation in the protein tyrosine phosphatase-like A (PTPLA) gene segregated with CNM. Around the world, client-owned Labrador retrievers with a similar clinical presentation and histopathological changes in muscle biopsies have been described. We hypothesized that these Labradors share the same PTPLA(cnm) mutation. Genotyping of an international panel of 7,426 Labradors led to the identification of PTPLA(cnm) carriers in 13 countries. Haplotype analysis demonstrated that the PTPLA(cnm) allele resulted from a single and recent mutational event that may have rapidly disseminated through the extensive use of popular sires. PTPLA-deficient Labradors will help define the integrated role of PTPLA in the existing CNM gene network. They will be valuable complementary large animal models to test innovative therapies in CNM.

RACK1, a clue to the diagnosis of cutaneous melanomas in horses.

BACKGROUND: Melanocytic proliferations are common in horses but the diagnosis of malignancy is not always straightforward. To improve diagnosis and prognosis, markers of malignancy are needed. Receptor for activated C kinase 1 (RACK1) protein may be such a marker. RACK1 was originally found to characterize malignant melanocytic lesions in the Melanoblastoma-bearing Libechov minipig (MeLiM) and, later, in human patients. Our purpose was to investigate the value of RACK1 in the classification of cutaneous melanocytic proliferations in horses. RESULTS: Using immunofluorescence, we report here that both MITF (Microphthalmia-associated transcription factor) and PAX3 (Paired box 3) allow the identification of melanocytic cells in horse skin samples. Importantly, RACK1 was detected in melanocytic lesions but not in healthy skin melanocytes. Finally, we found that RACK1 labeling can be used in horses to distinguish benign melanocytic tumors from melanomas. Indeed, RACK1 labeling appeared more informative to assess malignancy than individual histomorphological features. CONCLUSIONS: This study confirms that horses provide an interesting model for melanoma genesis studies. It establishes MITF and PAX3 as markers of horse melanocytic cells. RACK1 emerges as an important marker of malignancy which may contribute to progress in the diagnosis of melanomas in both human and veterinary medicine.

Pax3( GFP ) , a new reporter for the melanocyte lineage, highlights novel aspects of PAX3 expression in the skin.

The paired box gene 3 (Pax3) is expressed during pigment cell development. We tested whether the targeted allele Pax3(GFP) can be used as a reporter gene for pigment cells in the mouse. We found that enhanced green fluorescent protein (GFP) can be seen readily in every melanoblast and melanocyte in the epidermis and hair follicles of Pax3(GFP/+) heterozygotes. The GFP was detected at all differentiation stages, including melanocyte stem cells. In the dermis, Schwann cells and nestin-positive cells of the piloneural collars resembling the nestin-positive hair follicle multipotent stem cells exhibited a weaker GFP signal. Pigment cells could be purified by fluorescent activated cell sorting and grown in vitro without feeder cells, giving pure cultures of melanocytes. The Schwann cells and nestin-positive cells of the piloneural collars were FACS-isolated based on their weak expression of GFP. Thus Pax3(GFP) can discriminate distinct populations of cells in the skin.

Notch signaling and the developing hair follicle.

Notch function in the hair follicle has been mainly studied by use of transgenic mice carrying either loss or gain of function mutations in various members of the pathway. These studies revealed that whereas embryonic development of the hair follicle can be achieved without Notch, its postnatal development requires an intact Notch signaling in the hair bulb and the outer root sheath. Among the many roles played by Notch in the hair follicle, two can be highlighted: in the bulge, Notch controls a cell fate switch in hair follicle stem cells or their progenitors, preventing them from adopting an epidermal fate. In the hair bulb, Notch controls cell differentiation, ensuring the proper development of every layer of the hair shaft and inner root sheath. Notch function in the hair follicle is both cell autonomous and cell non autonomous and involves intercellular communication between adjacent layers.

A canine Arylsulfatase G (ARSG) mutation leading to a sulfatase deficiency is associated with neuronal ceroid lipofuscinosis.

Neuronal ceroid lipofuscinoses (NCLs) represent the most common group of inherited progressive encephalopathies in children. They are characterized by progressive loss of vision, mental and motor deterioration, epileptic seizures, and premature death. Rare adult forms of NCL with late onset are known as Kufs' disease. Loci underlying these adult forms remain unknown due to the small number of patients and genetic heterogeneity. Here we confirm that a late-onset form of NCL recessively segregates in US and French pedigrees of American Staffordshire Terrier (AST) dogs. Through combined association, linkage, and haplotype analyses, we mapped the disease locus to a single region of canine chromosome 9. We eventually identified a worldwide breed-specific variant in exon 2 of the Arylsulfatase G (ARSG) gene, which causes a p.R99H substitution in the vicinity of the catalytic domain of the enzyme. In transfected cells or leukocytes from affected dogs, the missense change leads to a 75% decrease in sulfatase activity, providing a functional confirmation that the variant might be the NCL-causing mutation. Our results uncover a protein involved in neuronal homeostasis, identify a family of candidate genes to be screened in patients with Kufs' disease, and suggest that a deficiency in sulfatase is part of the NCL pathogenesis.

Uterine contractions depend on KIT-positive interstitial cells in the mouse: genetic and pharmacological evidence.

In the gastrointestinal tract, interstitial cells of Cajal (ICCs) generate a pacemaker activity. They produce electric slow waves that trigger and coordinate gut smooth muscle contractions. Interstitial cells of Cajal's slender shape is revealed by KIT immunostaining. Based on several features, including KIT expression and KIT dependence, ICC-like cells were identified in nongastrointestinal tissues. Here, we investigated in the mouse whether uterine contractions depend on ICC-like cells' activity. By labeling KIT-expressing cells, we found putative ICC-like cells in the uterus, observed as KIT-positive interstitial, long spindle-shaped cells with fine branched cytoplasm processes, distributed in muscular layers and in subepithelial connective tissue. We then checked the potential KIT dependence of ex vivo contractile activity of the uterus by combining genetic and pharmacological approaches, using the Kit W-v hypomorphic mutation, and imatinib as a KIT noncompetitive inhibitor. We found a significant reduction in frequency of longitudinal uterine contractions in Kit W-v/Kit W-v compared with Kit+/+ mice, whereas amplitude was unaffected. There was no difference in frequency or amplitude of circular uterine contractions between Kit W-v/Kit W-v and Kit+/+ mice. Ex vivo treatment of Kit+/+ uterine horns with imatinib resulted in a dose-dependent reduction of the frequency and amplitude of longitudinal myometrial contractions. Amplitude and frequency of circular contractions were unaffected in presence of imatinib. These concurrent results suggest that longitudinal contractions of the uterus depend on a KIT signaling pathway of ICC-like cells. The existence of ICC-like cells in the myometrium may enhance our understanding of uterine spontaneous contractile activity and suggest new approaches for treatment of uterine contractility disorders.

Melanoblasts' proper location and timed differentiation depend on Notch/RBP-J signaling in postnatal hair follicles.

The Notch/RBP-J pathway is involved in a variety of developmental processes and in tissue homeostasis. In the melanocyte lineage, it has been shown that Notch signaling acts through Hes1 to maintain the melanocyte stem cell population in the hair follicle. This study was designed to determine whether Notch signaling is implicated in other steps of melanocyte-lineage postnatal development. For this purpose, we developed mice in which the RBP-J gene was conditionally ablated in the melanocyte lineage and used the Dct-lacZ reporter transgene to track melanocytes and their precursors in individual hair follicles. We determine that Notch/RBP-J-deficient melanoblasts are in reduced number within the hair follicle and gather within its lower permanent part. Moreover, our results show that Notch signaling is necessary to prevent differentiation of melanocyte stem cells and of melanoblasts before they reach the hair bulb. Finally, our data show that Notch signaling is involved in proper location of melanoblasts in the outer root sheath and of melanocytes in the hair matrix. These findings reveal previously unrecognized roles for Notch signaling in the melanocyte lineage.

[Melanocyte stem cells in adults]. / Les cellules souches des mélanocytes de l'adulte.

Melanocyte stem cells have been recently localized in mice, in the outer root sheath of the lower permanent portion of the hair follicle. Specific depletion of melanocyte stem cell population is responsible for natural hair greying in aging mice and humans. Melanocyte stem cells also seem to drive the growth of malignant melanomas. A few mutations, either spontaneous or genetically engineered, accelerate the natural process of hair greying with age. These mutations allowed the identification of genes and signalling pathways controlling emergence, maintenance and/or differentiation of melanocyte stem cells. This review summarizes recent studies on the melanocyte stem cells and defines a few major unanswered questions in the field.

Accelerated intestinal transit in inbred mice with an increased number of interstitial cells of Cajal.

The interstitial cells of Cajal (ICC) play an important role in coordinating intestinal motility, and structural alterations in ICC are found in several human digestive diseases. Mouse models with defects in ICC allow a better understanding of their functions. We investigated the pattern of intestinal motility and the distribution of ICC in the PRM/Alf inbred mouse strain, characterized by a selective intestinal lengthening. In PRM/Alf mice, the digestive transit time, evaluated by using thermophilic Bacillus subtilis spores, was normal, indicating accelerated transit. The contractility and slow-wave frequency, recorded on isolated segments from the proximal small intestine, were significantly increased. The number of ICC was also significantly higher along the small intestine and the colon. The concomitant increase of the contractility, the slow-wave frequency, and the number of ICC is consistent with the proposal of a role of ICC number increase in the higher intestinal transit speed. The PRM/Alf model should be useful to further investigate the roles of ICC in the control of digestive motility.

Genetic interaction between a maternal factor and the zygotic genome controls the intestine length in PRM/Alf mice.

Postoperative management of small and large bowel resections would be helped by use of intestinotrophic molecules. Here, we present a mouse inbred strain called PRM/Alf that is characterized by a selective intestinal lengthening. We show that PRM/Alf intestine is one-third longer compared with other inbred strains. The phenotype is acquired mostly during the postnatal period, before weaning. Its genetic determinism is polygenic, and involves a strong maternal effect. Cross-fostering experiments revealed that the dam's genotype acts synergistically with the offspring's genotype to confer the longest intestine. Moreover, genes in the offspring have a direct effect on intestine length. Possible involvement of milk growth factors and identification of candidate genes are discussed.