Cranial Dermal Cell Development in Mouse

The Dermis

  • The skin consists of the epidermis, derived from the surface ectoderm, and the underlying dermis. (1) Interactions between the epidermis and underlying dermis induce development of the epidermal appendages, such as hair follicles and glands of the skin. (1)
  • The somites give rise to axial skeleton, striated muscle, and dorsal dermis. (1)
  • It was shown in 2006 that in the mouse En1-expressing cells of the central dermomyotome give rise to the dorsal dermis, epaxial muscle, and interscapular brown fat. (1)
    • En1 is a homeobox transcription factor gene expressed in the central dermomytome. (1)
    • It was also shown that dorsal dermal fate specification from the central somite indicate that Wnts, via β-catenin, provide an instructive signal for dermal fate. (2)
  • In a 2008 study performed exploring the possible role of β-catenin in the survival and specification of ventral dermis revealed a new role of Wnt Signaling/β-catenin in  ventral dermal progenitor cell survival while maintaining a functionally conserved role in dorsal and ventral dermal cell fate specification. It was also found that the ventral dermis arises from the lateral plate mesoderm and that β-catenin is required for ventral dermal development. (2)
  • The dorsal and ventral trunk dermis originate from the somites and lateral plate mesoderm, respectively. By contrast, mouse craniofacial dermis originates from ectoderm-derived cranial neural crest and paraxial mesoderm. (3)

Canonical Wnt Pathway

  • This pathway is involved in early embryonic patterning, cell fate specification, proliferation, and the maintenance of stem cell compartments. (2)
  • β-catenin is a key transducer of the Wnt signaling pathway.
    • Embryos fail to gastrulate in the absence of β-catenin activity.
    • Unregulated β-catenin activity leads to cancer in adults
  • In the absence of Wnt signaling , the β-catenin proteins are phosphorylated and marked for degradation. In the presence of Wnt signaling, the unphosphorylated form of β-catenin accumulates in the cytoplasm, translocates to the nucleus, binds to the TCF-Lef family transcription factors, and promotes the transcription of Wnt genes. (2)
  • Please follow the link for more information and a very useful diagram depicting the Canonical Wnt Pathway.

What’s Next?

  • Recent research exploring the role of canonical Wnt signaling/β-catenin via Dermo1 in cranial dermal cell development.
  • Dermo1 is the earliest dermal progenitor marker, around E11.5. (2)
  • Future applications of this research.

Introduction

  • The cranial dermis develops from cephalic mesoderm and neural crest cells, the signals specifying the dermal lineage is unclear. A complete understanding of the signaling cues involved cranial dermal cell fate induction and differentiation is a significant unresolved issue. (3)
  • In the study conducted by Tran et al (2010) focusing on the role of canonical Wnt signaling/β-catenin via Dermo1 in cranial dermal cell development they tested the requirement for canonical Wnt signal transduction for dermal lineage specification in the supraorbital mesenchyme. The goal being to identify one of the developmental origins of cranial dermal precursors and demonstrate the necessity of Wnt signaling in cranial dermal cell fate selection. (3)
    • It was also found that Wnt signaling is required to suppress a latent cartilage cell fate in craniofacial and ventral trunk dermal precursors. (3)

Results

Wnt signal transduction is required for craniofacial dermal development

Figure 1: Extensive contribution of Dermo1 lineages and roles of Wnt signaling/B-catenin in cranial dermis and bones. (A-D)X-gal stained coronal sections of E16.5 embryos. Dermo1Cre; R26R marked cells contributed to the entire cranial dermis, frontal and parietal bones, and the cartilaginous base of the skull (A,C). Dermo1Cre; R26R; β-cateninflox/flox mutants lacked dermis and ossified cranial bones, and had ectopic cartilage tissue that was positive for Alcian Blue below the epidermis in the cranium (B,D). hf, hair follicles. (3)

  • In order to determine whether Wnt signal transduction is required for craniofacial dermal cell fate selection the researchers conditionally deleted β-catenin in an early mesenchymal progenitor population. Since Dermo1/Twist2 mRNA expression is restricted to the sub-ectodermal cells that will become craniofacial dermal progenitors this line was used to genetically eliminate  β-catenin. In the resulting β-catenin mutants the cranial dermis tissue and ossified bones were both absent. Researchers found cartilage tissue in this absence, suggesting a role of Wnt signaling in cranial dermal and bone developement. (3)

Loss of Wnt signaling transduction in engrailed 1 lineage also leads to loss of cranial dermis

Figure 2: Absence of cranial dermis and ectopic cartilage in the conditional En1Cre; β-catenin loss-of-function mutants. Please select the image for more information.

  • It was found that in the absence of Wnt signaling/β-catenin in the En1 lineage, sub-ectodermal β-gal+ cartilage nodules developed instead of cranial dermis and cranial bones. (3) This lineage was used because it is more restricted to the dermal fate and would further prove the need of Wnt signal transduction in dermal development.
  • Though most of the cranial mesenchyme became cartilage, not all of it had the same differentiation. It was found that a  few intervening cells between cartilage nodules, that still lacked β-catenin and Dermo1, were producing Runx2 cranial bone precursors until E15.5, after which the amount of cranial dermal-specific markers decrease. Showing that Wnt signal transduction was not required for generating Runx2 cranial bone precursor cells, but subsequently was required for cranial bone differentiation. (3) See the above Figure 2.

The engrailed 1 domain in the supraorbital region includes some cranial bone and dermal progenitors in mouse embryos

  • In order to visualize the activity of the engrailed 1 (En1) promoter, researchers analyzed En1LacZ knock-in embryos by whole-mount X-gal staining (Figure 3A).

As can been seen in the following figure, the engrailed 1-expressing supraorbital mesenchyme harbors a heterogenous population of cranial bone and dermal precursors that were derived from both the cranial neural crest and paraxial mesoderm.

Figure 3: Expression of engrailed 1 during mouse craniofacial development. Please select the image for more information.

  • Further experiments showed that the onset of cranial dermal precursor marker expression coincides with the timing of canonical Wnt signal transduction. It was observed, at E11.5, that cells of the En1 lineage were processing Wnt signaling, nuclear β-catenin localization, and Wnt signaling reporter activity; while simultaneously expressing the earliest dermal progenitor marker, Dermo1. (3)

Figure 4: Wnt signaling is necessary and sufficient for dermal specification of cranial dermis at E11.5. Please select the image for more information.

Canonical Wnt signaling activity is necessary and sufficient for expression of cranial dermal progenitor marker

  • Further experiments exploring manipulations of Wnt signaling activity levels in supraorbital mesenchyme found a lineage-specific requirement of Wnt signal transduction for the Dermo1 dermal progenitor but not for the Runx2 bone progenitor marker expression; which was still present in spite of the lack of Wnt signal transduction. The researchers also looked into the possibility of whether the loss of Dermo1 expression in the β-catenin mutant could be due to a reduction in cell survival and proliferation prior to E12.5. Thorough analysis leads to the conclusion that loss of dermal marker expression was probably not due to a small decrease in proliferation. Also tested whether forced activation of Wnt was capable of inducing Dermo1 expression, confirming that conditional activation of Wnt signal transduction was sufficient for Dermo1 marker expression. (3) See Figure 4 above.

Absence of Wnt signaling leads to loss of cranial dermal cell fate specification and gain of cartilage cell fate

  • The researchers then tested whether β-catenin mutants in the supraorbital region became specified to the cartilage fate in the absence of the Wnt signaling. Since Sox9 is downstream of Dermo1 and is the earliest cartilage cell fate marker, expression would represent a cartilage fate. At various times, the absence of Wnt signaling resulted in the expression of Sox9 as a result of the lack of dermal fate specification. (3) Please see the figure below.

Figure 5: Changes of early marker expression in sub-ectodermal cells in the absence of Wnt signaling/β-catenin at E12.5. Please select the image for more information.

  • Since Dermo1 can also act as a transcriptional repressor, the researchers suggest that it is a candidate target factor mediating the repression of Sox9 in cranial and trunk dermal precursors. Through the use of an expression vector for mouse Dermo1 and analysis via quantitative real-time PCR relative to mock-transfected cells, it was confirmed that Dermo1 may mediate some of the effects of Wnt signaling/β-catenin in suppressing the cartilage cell fate in skull and trunk precursors. (3) Please select Figure 5 for data and details.
  • The mechanism by which Dermo1 is regulated in undifferentiated mesenchyme or dermal precursors is unknown. However, researchers are beginning to believe that several Tcf-Lef-binding motifs near located near or within Dermo1 could possibly serve as potential upstream enhancers. (3)

Wrap Up

The above study is incredibly interesting in highlighting the crucial role that Wnt/β-catenin signaling plays in dermal cell fate selection and development. It was revealed that without the Wnt signaling cue, a cartilage cell fate is prompted at the expense of dermal and bone lineages. (3) The study also identified Dermo1 as a mediator of Wnt signaling in specifying dermal cell fate and suppressing the cartilage cell fate. (3) This research was conducted in the Atit lab and has some very promising prospectives in the world of scleroderma. In gaining an understanding of the mechanisms underlying dermal development the Atit lab hopes to understand dermal thickening and fibrosis in the mutant mouse and possibly one day make use of the Wnt signaling pathway as a new potential therapeutic target for treating skin fibrosis in scerloderma. (4) While the study described above was incredibly thorough; in the furture, there must be further exploration into the understanding of Dermo1 as an important dermal target gene, especially in the craniofacial region. Also future genetic analysis of the possible dermal-specific promoters and enhancers would further help to complete the understanding of the genetic program for cranial dermal development. (3) These studies could have far-reaching applications for skin tissue engineering and healing.

References

  1. Atit, Radhika, Sema K. Sgaier, Othman A. Mohamed, Makoto M. Taketo, Daniel Dufort, Alexandra L. Joyner, Lee Niswander, and Ronald A. Conlon. “Beta-catenin Activation Is Necessary and Sufficient to Specify the Dorsal Dermal Fate in the Mouse.” Developmental Biology (2006): 164-76. Elsevier. Web.
  2. Ohtola, Jennifer, John Myers, Batool Akhtar-Zaidi, Diana Zuzindlak, Pooja Sandesara, Karen Yeh, Susan Mackem, and Radhika Atit. “Beta-catenin Has Sequential Roles in the Survival and Specification of Ventral Dermis.” Development (2008): 2321-329. PubMed. Web.
  3. Tran, Thu H., Andrew Jarrell, Gabriel E. Zentner, Adrienne Welsh, Isaac Brownell, Peter C. Scacheri, and Radhika Atit. “Role of Canonical Wnt Signaling/B-catenin via Dermo1 in Cranial Dermal Cell Development.” Development (2010). PubMed. Web.
  4. Atit, Radhika P. “Forced Activation of Wnt Signaling in Dermal Fibroblasts: A Genetic Model for Scleroderma.” Scleroderma Research Foundation. Case Western Reserve University. Web. <http://www.srfcure.org/research/funded-research/2224-regulation-chang>

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