Melanoblast generation

I would like to write a brief review that covers the processes of pigment generation in chicken. Because of my limit time and effort, this review will be divided into several blogs that posted at most one blog each week.

When we are talking about melanoblast generation, neural crest has to be mentioned. Neural crest is derived from neural tube after gastrula stage. Or more precisely, neural crest cells are specified at the border of the neural plate and the non-neural ectoderm. The first group of cell that differentiated into neural crest cell will migrate ventrolaterally and become neurons and glial. In chicken embryo, one day later (Reedy et al., 1998), the following groups of neural crest cells become melanoblast then migrate dorsolaterally.

So, our first question is what causes the neural tube cells becoming into neural crest cells. At least we know Wnt signaling is in charge of this process, Wnt6 induces neural crest production through the noncanonical signaling pathway and Wnt1 inhibits neural crest induction through canonical signaling pathway. One of the targets of these pathways is FOXD3 (Forkhead box transcriptional repressor, Corina et al., 2007), which is necessary for neural/glial precursors and also can inhibit MITF (Microphthalmia-associated transcription factor, key transcriptional factor for melanoblast formation) expression. So, FOXD3 acts like a switch between neural/glial precursors and melanoblast derived from neural tube, and Wnt6 active FOXD3 while Wnt1 inactive FOXD3. Bone morphogenetic proteins (BMP) also induce FOXD3 during the initial generation of neural crest (Taneyhill and Bronner-Fraser, 2005). Evidence shows that neural crest induction is underway during gastrulation (By expression of the paired box transcription factor PAX7) and well before proper neural plate appearance (when Wnt6 and BMP expressed). But there are other results indicate that the expression of PAX7 is regulated by Wnt6 and BMP (Martı´n L et al., 2006). So, the relationship between PAX7, Wnt6 and BMP in the initiation of neural crest seems unclear.

MITF, a key transcriptional factor for melanoblast formation, is mentioned above. It is activated by PAX3, SOX10 and WNT3A which are exist in neural tube before the formation of neural crest. So MITF should be depressed in other way otherwise the first group of neural crest cell should be melanoblast. That is through the binding of FOXD3 with MITF-M (the melanocyte-specific isoform of MITF) promoter which prevents the binding of PAX3 with this promoter. As FOXD3 is expressed exclusively in the neural/glial precursors, it acts like a key that regulates the lineage switch between neural crest derived glial cells and pigment cells (Aaron et al., 2009). Besides, BMP-4 is expressed in the dorsal neural tube throughout the time when neural/glial precursors are migrating, but is decreased coincident with the timing of melanoblast migration later. This expression pattern suggests that BMP-4 antagonizes melanogenesis (Jin et al., 2001) in conjunction with FOXD3. This lineage switch occurs while the neural crest precursors are still resident in the neural tube.

 

Citations:

Reedy, M. V., Faraco, C. D. and Erickson, C. A. The delayed entry of thoracic neural crest cells into the dorsolateral path is a consequence of the late emigration of melanogenic neural crest cells from the neural tube. Dev. Biol. 1998. 200, 234-246.

Corina Schmidt, Imelda M. McGonnell, Steve Allen, Anthony Otto, and Ketan Patel. Wnt6 Controls Amniote Neural Crest Induction Through the Non-canonical Signaling Pathway. Developmental Dynamics. 2007. 236:2502–2511.

Taneyhill, L. A. and Bronner-Fraser, M. Dynamic alterations in gene expression after Wnt-mediated induction of avian neural crest. Mol. Biol. Cell. 2005. 16, 5283-5293.

Martı´n L. Basch, Marianne Bronner-Fraser, Martı´n. Garcı´a-Castro. Specification of the neural crest occurs during gastrulation and requires Pax7. Nature. 2006. Vol 441, 11, 218-222.

Aaron J. Thomas, Carol A. Erickson. FOXD3 regulates the lineage switch between neural crest derived glial cells and pigment cells by repressing MITF through a non-canonical mechanism, Development 136, 1849-1858 (2009).

Jin, E. J., Erickson, C. A., Takada, S. and Burrus, L. W. Wnt and BMP signaling govern lineage segregation of melanocytes in the avian embryo. Dev. Biol. 2001. 233, 22-37.

Countershading is a kind of pattern in animal coloration which means the dorsal hair or feather color is darker than the ventral hair or feather color. Most of us think it is a kind of crypsis for animals not to be found easily by others. But in fact, people are still debating about why countershading and we still have little understanding about this common phenomenon in many species. There are two aspects for the question of why countershading, the function of countershading on animals and the molecular mechanism of countershading development.

One main problem about crypsis theory is that why ventral hair or feather color is lighter? We will all agree with that when looking upward, it will be hard to find the fish because their ventral color is light and very close to the color of the sky, same thing about looking fish downward. But is that situation the same in some mammals? It seems no chance for others to look at a rodent, a felid or a Artiodactyla upward. If the hair colors of these animals are all dark in dorsal and in ventral, it seems to be a better crypsis. So, there are several alternative theories to explain the function of lighter ventral hair color, like illuminate the food under the body, protection from ultraviolet light; thermoregulation; and protection from abrasion. Other theory says the lighter color in ventral help the cubs finding the papilla easier which convinced me more.

While looking at birds, most of the birds have countershading and the main function of that may be crypsis because their situation is similar with fish rather than mammals. But another phenomenon confused me is the sexual dichromatism, which means the pattern of males and females are different.

 

Pic.1 Cardinal: left female, right male. (Photo from Google)

 

Pic.2 Penacook: left female, right male. (Photo from Google)

Pic.3 Mallard: left female, right male. (Photo from Google)

Pic.4 Red Jungle fowl (ancestor of all domestic chickens): left male, right female. (Photo from Google)

In these birds above, we can find sexual dichromatism and countershading only happened in females not in males. Here is a problem; sexual dichromatism is more common in birds rather than mammals. If the lighter color in ventral helping the cubs finding the papilla, this sexual dichromatism is more possible to happen in mammals rather than in birds as there is no lactation in birds. In other words, if the countershading only serves as crypsis, is that means male birds need less protection than females? I would like to say yes to this question but more evidence is needed.

So, the real function of countershading needs to be discovered in the future. But we have a better understanding on the molecular mechanism of countershading development. The main pigment in mammals and in birds is melanin, which can form to kinds of particles: eumelanin and pheomelanin. Eumelanin shows a black/brown color and pheomelanin shows a yellow/red color. Melanocortin 1 receptor (MC1R) plays a critical role in the synthesis of melanin. MC1R is a G protein-coupled receptor that activates the cAMP signaling pathway, after binding with α -melanocyte-stimulating hormone (α-MSH) or adrenocorticotropic hormone (ACTH), eumelanin will be synthesized. The agouti signaling protein(ASIP) has a competitive effect on bingding MC1R, when MC1R is coupled with ASIP, the synthesis of eumelanin will be repressed and pheomelanin will be synthesized. In mice, there are five kinds of ASIP mRNA variants and are controlled by two kinds of promoters: the hair cycle-specific promoter and the ventral-specific promoter. The hair cycle-specific promoter acts at the midpoint of the hair growth cycle to produce hairs with a black base. The ventral-specific promoter directs expression throughout the entire hair growth cycle of ventral but not dorsal hair follicles. In this way countershading is developed. In rabbits, only two kinds of ASIP mRNA variants are expressed by the action of two promoters to produce countershading pattern.

Scientists focused on the function of ASIP just in recent years. The chicken feather follicles express at least seven kinds of ASIP mRNA variants using three promoters. And the chicken ASIP gene is expressed in a wide variety of tissues, which contrasts with the expression of the wild-type mouse agouti (the murine ASIP) gene which is limited only to skin which suggests that ASIP in birds may play more roles than that in mammals. In 2012, scientists found that the distal ASIP promoter not only acts to produce countershading in chicks and adult females, but also plays an important role for creating sexual plumage dichromatism controlled by estrogen. What surprised me is that the countershading and sex dichromatism is associated again in molecular level! I believe this is kind of evidence that no countershading in male birds is a kind of benefit for the whole population in evolution. If we understand this benefit, we will be able to solve the problem of why countershading.

In addition, it is the melanocytes that produce the melanin, so the presence of melanocytes is also a key factor that determines the pattern of coloration. What I can tell now is that the melanoblasts (precursor of melanocytes) are derived from neural crest cells (NCCs), a multipotent population of cells emigrating from the dorsal neural tube. Considering countershading, I believe it is a benefit at least in chicken that the melanoblasts migrate from dorsal to ventral not ventral to dorsal. Because some time the migration will not complete at the time of hatching (See Pic5), so countershading is formed. In other words, the migration from dorsal to ventral gives the pigment to the chick at least in the dorsal part and somehow protects the chick (although the countershading function is still in debate). I would like to talk about more about NCCs and chicken pigmentation next week after digging deeper in the articles.

Pic.4 Chick display countershading(Photo taken by me).

Citations:

H. Vrieling, D.M. Duhl, S.E. Millar, K.A. Miller, G.S. Barsh, Differences in dorsal and ventral pigmentation result from regional expression of the mouse agouti gene, Proc. Natl. Acad. Sci. USA 91 (1994) 5667–5671.

L. Fontanesi, L. Forestier, D. Allain, E. Scotti, F. Beretti, S. Deretz-Picoulet, E. Pecchioli, C. Vernesi, T.J. Robinson, J.L. Malaney, V. Russo, A. Oulmouden, Characterization of the rabbit agouti signaling protein (ASIP) gene: transcripts and phylogenetic analyses and identification of the causative mutation of the nonagouti black coat colour, Genomics 95 (2010) 166–175.

C. Yoshihara, A. Fukao, K. Ando, Y. Tashiro, S. Taniuchi, S. Takahashi, S. Takeuchi, Elaborate color patterns of individual chicken feathers may be formed by the agouti signaling protein, General and Comparative Endocrinology 175 (2012) 495–499.

Eri Oribe, Ayaka Fukao, Chihiro Yoshihara, Misa Mendori, Karen G. Rosal, Sumio Takahashi, Sakae Takeuchi, Conserved distal promoter of the agouti signaling protein (ASIP) gene controls sexual dichromatism in chickens, General and Comparative Endocrinology 177 (2012) 231–237.

Ben Dorshorst, Anna-Maja Molin, Carl-Johan Rubin, Anna M. Johansson, Lina Stromstedt, Manh-Hung Pham, Chih-Feng Chen, Finn Hallbook, Chris Ashwell, Leif Andersson, A Complex Genomic Rearrangement Involving the Endothelin 3 Locus Causes Dermal Hyperpigmentation in the Chicken, PLoS Genetics, 12, 2011, Volume 7, Issue 12, e1002412.

Wikipedia – Countershading http://en.wikipedia.org/wiki/Countershading#cite_note-42

 

 

This is a true story told by Dr. Liwu Li yesterday. It is really amazing so I would like to share this with you.

Even the topic I am using is so attractive. We know genomic equivalence, which means in one organism, each cell has the same sets of genes. Mutation may happen in single cell but should not be tissue-specific except in embryo stage. But how can tissue-specific mutation just happen in a 25 years old human being?  I can’t imagine this before knowing the following story, the answer to this question is cancer. Then, more and more people are curious in curing of inborn immune deficiency. It is a big problem in medicine and in biology as this kind of deficiency is determined by genes. But if we can find a way to repair the genes even in adult cells, it must be great improvement in medicine and biology. Here comes a possible way.

Once a day, a physician encountered a boy with a very strange disease. The boy’s skin is full of ulcer everywhere because all of his neutrophil (a kind of leukocyte) do not work. Of cause the boy is very easily to be sick. After study, it is just a single mutation in the end of chromosome 13 causes this symptom. So it is a kind of inborn immune deficiency. As I mentioned above, till now we have nothing to do to cure inborn immune deficiency. That physician did not give up, he explored the articles and find one single paper in 1960s reports one patient who has the almost the same symptom as the boy. But the results are the same, no way to cure that. However, very coincidently, the physician found that the patient described in that paper is the mother of this boy! Why the physician didn’t notice this at the beginning? Because the mother does not have the symptom and she even forgot she used to have. When she was 25, the symptom suddenly and spontaneity disappeared without any medicine or treatment! And that happened several years before she gave birth to this boy. By sequencing the mothers’ different tissue cells, the physician found that the mother does have the single mutation in different tissue cells except in neutrophil. This can explain that the mother doesn’t have the symptom but it can be inherited to her son. But why this happened? It is answered by a very simple experiment, observing the neutrophil’s chromosome by microscope: the whole part of the end of chromosome 13 which contains that SNP is missing.

Chromosome breakage causes the chromosome deletion. There are three possible results of chromosome breakage, first on is been repaired. There are repair mechanisms to relink our chromosome which happened every day in out body. But there is very little possibility that we fail to repair a broken chromosome or relink chromosome in a wrong way. Within this situation, most of such cells will encounter apoptosis, very little of them will become cancer cell because cancer gene is somehow be activated by chromosome breakage or falsely relinking. What’s more, our body has variety of mechanism to recognize the cancer cell and kill them before they develop. So, back to our story, the mother’s neutrophil is the results of such an extremely rare event, suffer chromosome breakage but still alive, even functions normally. First, the region of chromosome 13 that the neutrophil lost contains many genes including the single mutation causing that syndrome, but luckily, none of them are useful to neutrophil. Second, if only one neutrophil has such broken chromosome, it will not make any difference, but it is impossible that multi neutrophils have the same broken chromosome at the same time. What’s miraculously is this chromosome breakage also triggers the cancerization of that single neutrophil. And furthermore, it didn’t develop into the kind of invasive spreading cancer but a little bit aggressive that killed other neutrophils and become dominant. As the result, most of the neutrophils are daughter cells of that single neutrophil which has the normal immune function.

That’s all about the story that I heard, but it will never come an end. It may open a whole new gate for medicine and biology. If we can find the ways to change different tissue’s single cell into a kind of cancer cell which is aggressive but not invasive spreading and still functions normally like the neutrophil in the above story, it may be a wonderful way to cure inborn genetic deficiency. On the other hand, this extremely rare event (chromosome breakage in that specific region in that specific cell) happened in that mother. Is this really a random event? By suspecting this question, we may discover other great things.