Clonal distribution of melanocytes in piebald-spotted and variegated mice

Slc20a1b is essential for hematopoietic stem/progenitor cell expansion in zebrafish

Hematopoietic stem and progenitor cells (HSPCs) are able to self-renew and can give rise to all blood lineages throughout their lifetime, yet the mechanisms regulating HSPC development have yet to be discovered. In this study, we characterized a hematopoiesis defective zebrafish mutant line named smu07, which was obtained from our previous forward genetic screening, and found the HSPC expansion deficiency in the mutant. Positional cloning identified that slc20a1b, which encodes a sodium phosphate cotransporter, contributed to the smu07 blood phenotype. Further analysis demonstrated that mutation of slc20a1b affects HSPC expansion through cell cycle arrest at G2/M phases in a cell-autonomous manner. Our study shows that slc20a1b is a vital regulator for HSPC proliferation in zebrafish early hematopoiesis and provides valuable insights into HSPC development.

Regulation of microtubule dynamics, mechanics and function through the growing tip

Microtubule dynamics and their control are essential for the normal function and division of all eukaryotic cells. This plethora of functions is, in large part, supported by dynamic microtubule tips, which can bind to various intracellular targets, generate mechanical forces and couple with actin microfilaments. Here, we review progress in the understanding of microtubule assembly and dynamics, focusing on new information about the structure of microtubule tips. First, we discuss evidence for the widely accepted GTP cap model of microtubule dynamics. Next, we address microtubule dynamic instability in the context of structural information about assembly intermediates at microtubule tips. Three currently discussed models of microtubule assembly and dynamics are reviewed. These are considered in the context of established facts and recent data, which suggest that some long-held views must be re-evaluated. Finally, we review structural observations about the tips of microtubules in cells and describe their implications for understanding the mechanisms of microtubule regulation by associated proteins, by mechanical forces and by microtubule-targeting drugs, prominently including cancer chemotherapeutics.

Multiple splice variants within the bovine silver homologue (SILV) gene affecting coat color in cattle indicate a function additional to fibril formation in melanophores

Background

The silver homologue(SILV) gene plays a major role in melanosome development. SILV is a target for studies concerning melanoma diagnostics and therapy in humans as well as on skin and coat color pigmentation in many species ranging from zebra fish to mammals. However, the precise functional cellular mechanisms, in which SILV is involved, are still not completely understood. While there are many studies addressing SILV function upon a eumelaneic pigment background, there is a substantial lack of information regarding the further relevance of SILV, e.g. for phaeomelanosome development.

Results

In contrast to previous results in other species reporting SILV expression exclusively in pigmented tissues, our experiments provide evidence that the bovine SILV gene is expressed in a variety of tissues independent of pigmentation. Our data show that the bovine SILV gene generates an unexpectedly large number of different transcripts occurring in skin as well as in non-pigmented tissues, e.g. liver or mammary gland. The alternative splice sites are generated by internal splicing and primarily remove complete exons. Alternative splicing predominantly affects the repeat domain of the protein, which has a functional key role in fibril formation during eumelanosome development.

Conclusion

The expression of the bovine SILV gene independent of pigmentation suggests SILV functions exceeding melanosome development in cattle. This hypothesis is further supported by transcript variants lacking functional key elements of the SILV protein relevant for eumelanosome development. Thus, the bovine SILV gene can serve as a model for the investigation of the putative additional functions of SILV. Furthermore, the splice variants of the bovine SILV gene represent a comprehensive natural model to refine the knowledge about functional domains in the SILV protein. Our study exemplifies that the extent of alternative splicing is presumably much higher than previously estimated and that alternatively spliced transcripts presumably can generate molecules of deviating function compared to their constitutive counterpart.

Spotlight on spotted mice: a review of white spotting mouse mutants and associated human pigmentation disorders

Mutation of genes that regulate neural crest-derived melanoblast development and survival can result in reduction and/or loss of mature melanocytes. The reduction in melanocyte number in the skin and hair follicles manifests itself as areas of hypopigmentation, commonly described as white spotting in mice. To date ten genes have been identified which are associated with white-spotting phenotypes in mouse. Seven of these genes are associated with neural crest and melanocyte disorders in humans. This review summarizes the phenotypes associated with mutation of these genes in both mouse and man. We describe our current understanding of how these genes function in development, and explore their complex roles regulating the various stages of melanocyte development.

Piebald lethal (sl) acts early to disrupt the development of neural crest-derived melanocytes

Mice homozygous for the piebald lethal (sl) mutation have a predominantly white coat due to the absence of neural crest-derived melanocytes in the hair follicles. To investigate the time in embryonic development when the s1 gene affects the melanocyte lineage, we compared the distribution of melanocyte precursors in wild-type and mutant embryos, using an antibody specific for tyrosinase-related protein 2 (TRP-2). TRP-2 positive cells were first observed adjacent to the anterior cardinal vein in 10.5-day postcoitem wild-type embryos. From 11.5 to 13.5 days postcoitem, there was a nonuniform distribution of TRP-2 positive cells along the anterior-posterior axis, with the highest density of cells in the head and tail regions. Along the dorsal-ventral axis, the cells were restricted to positions lateral, but never dorsal, to the neural tube. In homozygous sl/sl embryos TRP-2 staining was restricted to the non-neural crest-derived melanocytes of the pigmented retinal epithelium and the telencephalon. Few positive cells were seen in areas that will form neural crest-derived melanocytes in the inner ear, skin, hair follicles, leg musculature, or heart. We conclude that the piebald lethal mutation acts prior to the onset of TRP-2 expression to disrupt the development of neural crest-derived melanocytes. The non-uniform distribution of melanoblasts in wild-type mice suggests that piebald acts stochastically to affect melanocyte development.

Effects of the white allele of the mi locus on coat pigmentation in chimeric mice

To investigate the cellular action of the Miwh allele in the mouse with regard to its effects upon coat color patterns, we generated a series of aggregation chimeras, using embryos that differ in their mi locus genotype. We have obtained 11 chimeras Miwh/+C/C<==>+/+ c/c and 8 chimeras +/+ C/C<==>+/+ c/c. Chimerism was determined by coat and retinal pigment epithelium mosaicism and by the electrophoretic analysis of GPI-1 isoenzymes. In Miwh/+ C/C<==>+/+ c/c mice white coat color prevailed due to the higher percentage of unpigmented areas and the higher percentage of unpigmented hairs in pigmented areas. Our data indicate that a single Miwh gene dose decreases the melanoblast proliferative activity, causing the lightening of coat pigmentation. In Miwh/+ C/C<==>+/+ c/c mice a few pigmented hairs were often detected on the belly where Miwh/+ mice always had a white spot. This suggests that in the chimeras the presence of some non-Miwh cells in the skin of the belly allows pigment cells to develop. Using embryos of two substrains of Miwh/Miwh mice that differ in their Gpi-1 locus genotype we have produced 8 Miwh/+<==>Miwh/Miwh chimeras. In all these chimeras coat color patterns resembled those of Miwh/+ heterozygotes despite the higher percentage of the Miwh/Miwh component in three chimeras. Mosaic hairs were absent in the chimeras. This shows that the chimeras have only one Miwh/+ melanoblast population which actively proliferates and colonizes almost all hair follicles. Thus the Miwh/Miwh dermis and epidermis do not suppress proliferation and differentiation of the Miwh/+ melanoblasts except the certain area on the belly.

Human piebald trait resulting from a dominant negative mutant allele of the c-kit membrane receptor gene

Human piebald trait is an autosomal dominant defect in melanocyte development characterized by patches of hypopigmented skin and hair. Although the molecular basis of piebaldism has been unclear, a phenotypically similar "dominant spotting" of mice is caused by mutations in the murine c-kit protooncogene. In this regard, one piebald case with a point mutation and another with a deletion of c-kit have been reported, although a polymorphism or the involvement of a closely linked gene could not be excluded. To confirm the hypothesis that piebaldism results from mutations in the human gene, c-kit exons were amplified by polymerase chain reaction from the DNA of 10 affected subjects and screened for nucleotide changes by single-stranded conformation polymorphism analysis. In one subject with a variant single-stranded conformation polymorphism pattern for the first exon encoding the kinase domain, DNA sequencing demonstrated a missense mutation (Glu583----Lys). This mutation is identical to the mouse W37 mutation which abolishes autophosphorylation of the protein product and causes more extensive depigmentation than "null" mutations. In accord with this "dominant negative" effect, the identical mutation in this human kindred is associated with unusually extensive depigmentation. Thus, the finding of a piebald subject with a mutation that impairs receptor activity strongly implicates the c-kit gene in the molecular pathogenesis of this human developmental defect.

Control of melanocyte proliferation and differentiation in the mouse epidermis

Melanocyte-stimulating hormone plays an important role in the regulation of melanocyte differentiation in the mouse epidermis by inducing tyrosinase activity, melanosome formation, translocation of melanosomes, and increased dendritogenesis. The proliferative activity of differentiating epidermal melanocytes of newborn mice during the healing of skin wounds is regulated by semi-dominant genes, suggesting that the genes are involved in regulating the proliferative activity of epidermal melanocytes during differentiation. From the results of serum-free culture of epidermal cell suspensions from neonatal mouse skin, basic fibroblast growth factor is shown to stimulate the sustained proliferation of melanoblasts in the presence of dibutyryl adenosine 3',5'-cyclic monophosphate and keratinocyte-derived factors. Moreover, each step of melanocyte differentiation is controlled by numerous coat color genes. These genes control melanocyte differentiation by regulating the differentiation of neural crest cells into melanoblasts in embryonic skin, or by regulating the differentiation of neural crest cells into melanoblasts in embryonic skin, or by regulating transcription and/or translation of the tyrosinase gene in the differentiating melanocytes. These results suggest that melanocyte proliferation and differentiation in the mouse epidermis are controlled by both genetic factors and local tissue environment.

Deletion of the c-kit protooncogene in the human developmental defect piebald trait

The protooncogene c-kit is critical for development of hematopoietic stem cells, germ cells, and melanoblasts in the mouse. Homozygous mutations of this gene in the mouse cause anemia, infertility, and albinism, whereas heterozygous mutant mice usually exhibit only a white forehead blaze and depigmentation of the ventral body, tail, and feet. The heterozygous mouse phenotype is very similar to human piebald trait, which is characterized by a congenital white hair forelock and ventral and extremity depigmentation. To investigate the possibility that alterations in the human c-kit gene may be a cause of piebald trait, DNA from seven unrelated affected individuals was examined by Southern blot analysis. One subject, although cytogenetically normal, has a heterozygous deletion of the c-kit protooncogene. This deletion encompasses the entire coding region for c-kit and also involves the closely linked gene for platelet-derived growth factor receptor alpha. Fluorescence in situ hybridization of genomic c-kit probes to metaphase chromosomes independently confirmed the deletion in this case. These findings provide molecular evidence mapping piebald trait to the c-kit locus on chromosome 4. Although we cannot exclude the involvement of other closely linked genes, the demonstration of a genomic c-kit deletion in one subject with piebald trait and the marked concordance of the human and mouse phenotypes provide strong evidence for the role of c-kit in the development of human melanocytes and in the pathogenesis of piebald trait.

Developmental interactions in the pigmentary system of the tip of the mouse tail: effects of coat-color genes on the expression of a tail-spotting gene

The tails of agouti C3H/HeJmsHir mice are completely pigmented, whereas the tails of black C57BL/10JHir animals possess unpigmented tips. Genetic analysis indicates that white tail-tipping is due to an autosomal recessive gene, with incomplete penetrance, that segregates independently from the gene for agouti with a maternal influence in the F1 generation. To analyze the influence of specific coat-color genes on the expression of tail-spotting in mice, five congenic lines of C57BL/10JHir with different coat colors were prepared. No influence was observed on the occurrence of tail-spotting in agouti (A/A) or dilute (d/d) mice or in F1 mice from crosses between black and albino (c/c), or in F1 mice from crosses between black and pink-eyed dilution (p/p). However, the frequency of tail-spotting was dramatically decreased in brown (b/b) mice. These results suggest that the mutant allele (b) at the brown locus is involved in determining the extent of pigmented areas in the tail tips of mice through an interaction with the tail-spotting gene.

Site of beige (bg) and leaden (ln) pigment gene expression determined by recombinant embryonic skin grafts and aggregation mouse chimaeras employing sash (Wsh) homozygotes

Aggregation chimaeras were constructed by fusing embryos homozygous for sash (Wsh) with fuzzy, leaden beige (fz,ln,bg) homozygotes to investigate the site of action of the beige and leaden loci. The genotype of the hair follicle was identified by fuzzy alleles (+ +fzorfzfz). All melanocytes were derived from the fuzzy leaden beige population, as sash homozygotes do not produce functional melanocytes. Reciprocal recombinant epidermal/ /dermal skin grafts were constructed from 14-day embryonic skin of homozygousfzlnbg and either albino (aa cc) or pink-eyed dilution (pp) embryos to test for any dermal expression of leaden or beige, since the epidermal and dermal genotype of the chimaeric hair follicles could differ.

Patches of fuzzy and non-fuzzy hairs were distributed throughout the coats of two of the three chimaeras obtained. The pigmented regions were blue grey, typical of the leaden beige interactive phenotype. Large abnormal beige granules were found in fuzzy and non-fuzzy hairs. Melanocytes in both classes of growing follicles were nucleopetal, typical of leaden. Similarly, the results of the 14-day skin grafts showed that the beige and leaden loci are melanocyte-autonomous.

The chimaeras showed a pigment distribution resembling the heterozygous sash phenotype. Thus the 1:1 gene dosage of sash: wild type in heterozygotes and chimaeras has an overall effect on pigment pattern that overrides the predicted random distribution of the melanocyte precursors.

Quail melanoblast migration in two breeds of fowl and in their hybrids: evidence for a dominant genic control of the mesodermal pigment cell pattern through the tissue environment

In the Silkie fowl large numbers of melanocytes invade most internal tissues and organs. The factors involved in this internal pigment cell pattern were studied by grafting quail neural tube segments into White Leghorn, White Silkie, and F1 hybrids (White Silkie male X White Leghorn female). Sections of quail neural tube five somites long, excised at the level of the last formed somites, were grafted isotopically and ischoronically. Various tissues and organs (mesenteries, muscles, testis, ovary, mesonephros, metanephros, and adrenals) excised from the internal region corresponding to the peripheral transverse strip of quail melanocytes, were studied after staining by the Feulgen-Rossenbeck technique. Despite some variations in pigment cell density, Silkie and hybrid grafted embryos exhibited an extensive quail internal pigmentation similar to the melanocyte distribution in the Silkie breed. In white Leghorn host embryos, the internal pigmentation remained limited. These results show the part played by tissular factors in the expression of the Silkie pigment phenotype and that this genetic tissular character is dominant. On the contrary, White Leghorn embryos, grafted with Silkie neural tube segments, never exhibited any internal pigmentation; the melanocytes deriving from the grafted Silkie neural tube were only localized at the dermoepidermal level. Thus, the migrating and/or differentiating capabilities of the Silkie premelanoblasts are different from those of quail premelanoblasts. The sex-linked inhibitor of the White Leghorn tissue interferes at the level of the pigment cells of chickens but not of quails.

Site of gene action of the white allele (Miwh) of the microphthalmia locus: a dermal-epidermal recombination study

The white allele Miwh of the microphthalmia locus, when homozygous, causes a complete absence of neural crest-derived melanocytes in skin and internal organs. The site(s) of gene action of Miwh on melanoblast differentiation were examined by making dermal-epidermal recombinant grafts with skin from 13-14-day embryos of the following genotypes: normal (+/+), heterozygous white (Miwh/+), homozygous white (Miwh/Miwh), and dominant spotting (W/W). Based on results from these grafts, the following conclusions were reached: Miwh/Miwh epidermis is as effective as W/W epidermis in supporting the differentiation of +/+ follicular melanocytes. Miwh/Miwh dermis is completely permissive to in situ differentiation of +/+ follicular melanocytes. Thirteen-fourteen-day embryonic Miwh/Miwh skin, both epidermis and dermis, alters of blocks the differentiation of +/+ melanoblasts into dermal melanocytes. By 13 days of development, melanoblasts from both Miwh/+ and Miwh/Miwh skin, even when presented with a permissive W/W environment, are irreversibly redirected into abnormal developmental pathways resulting in either a reduction in follicular pigmentation (Miwh/+) or a complete absence of follicular melanocytes (Miwh/Miwh).

Effect of selection for spot size on reproduction and body weight in mice

Spot size in descendants from the Goodale white-spotted stock of mice responded to selection for increased spot size. The realized heritability estimate was 0.52. However, no correlated response of reproduction to spot size selection was found in the present study, nor was there any correlated response among body weight variables.

Interactions between normal and pigment cell populations mutant at the dominant-spotting (W) and steel (Sl) loci in the mouse

Through the use of dermal-epidermal recombination methods a competition between mouse embryo melanoblasts of the genotype Wv/w C/C, w/w c/c, Sld/sl C/C and sl/sl c/c was established. Control combinations were made between C/C and c/d components. The extent of pigment found in hair of grafts after three weeks growth in mouse testes was used as evidence of an interaction between populations. Normal and albino melanoblasts were found to be similar in viability, whereas melanoblasts of the genotype Wv/w C/C were largely excluded from hair follicles when placed in competition with w/w c/c melanoblasts. No difference in competitive advantage was observed between Sld/sl C/C and sl/sl c/c populations. These results confirm that the W and sl loci act at different sites. In addition they suggest that Wv/w melanoblasts are marginally viable cells that cannot compete with normal melanoblasts when the popuolations interact. The Wv/w melanoblast failure can also explain the spotting pattern and pigment dilution characteristic of dominant-spotting heterozygous mice.

Position effect variegation in the mouse

Position effect variegation has been studied in female mice heterozygous for thefleckedX-autosome translocation, T(7;X)Ct. Some of these carried the spotting gene (s) which clarifies the variegated patterns. Others carried a second X-autosome translocation, T(X; 16)16H, which suppresses the randomness of X-chromosome activity.

It was found the position effect variegation stems primarily from early occurring events which lead to the formation of clones of cells with different phenotypes. In this respect the phenomenon appears to parallel that found inDrosophila. However, in the mouse, late-occurring events are also found which can only be readily accounted for by the reactivation of previously inactive loci. They occur, not only during foetal development, but throughout the life-time of the animals and in a manner which suggests they derive from a progressive retreat of the inactivating influence of the heterochromaticXchromosome back along the attached autosome towards the breakpoint. It is proposed that the early occurring events do not lay down fixed programmes of gene suppression, as proposed forDrosophila, but that, like the later-occurring events, they represent the reactivation of previously inactivated loci. The possibility that this might also be true forDrosophilais discussed.

The study also provided evidence favouring the view that theX-chromosome controlling element, Xce, modifies the heterozygous phenotypes ofX-linked genes by biasing the randomness of the X-inactivation process, rather than by operating through cell selection mechanisms. The data also support and extend Mintz's (1967) concept of pigment pattern differentiation.