Review and Development

Cell proliferation general review

The Cell Cycle

According to Cooper in “The Cell: a molecular approach” the cell cycle is a series of events which takes place in a cell as it grows and divides. The cell cycle is divided into three periods which include interphase, mitotic phase, and cytokinesis. About 95% of the cell cycle is spent in interphase, which is the period of time between mitosis events. Interphase can be further broken down into three more phases: G₁, S, and G₂. G₁ is known as the gap phase, it is the phase which corresponds to the interval or gap between mitosis and initiation of DNA replication. During G1, the cell is growing but does not replicate its DNA. After G₁ comes the S or synthesis phase. During the S phase, DNA replication takes place. After DNA synthesis has concluded, the cell goes into the G₂ or gap 2 phase. During G₂, the cell continues to grow and proteins are synthesized to prepare for mitosis. If a cell will never again divide, the cell is said to be in the G₀ phase, the cell will function but never divide. One example of cells in the G₀ phase are neurons in the central nervous system.  

http://www.ncbi.nlm.nih.gov/books/NBK9876/

 Regulation of the cell cycle

Cooper in “The Cell: a molecular approach” states that the progression through the cell cycle is regulated primarily by extracellular growth factors that signal the cell to proliferate. This is regulated in the G₁ phase of the cell cycle. In the latter half of the G₁ phase the cell reaches a restriction point. Only after receiving appropriate signals is a cell allowed to enter the S phase. However, if the needed factors are not present then the cell arrests. Arrested cells which enter a quiescent stage are said to be in the G₀ phase. Cells can remain in G₀ for long periods of time without proliferating.  Even though G₀ cells are metabolically active, they cease growth and may have reduced rates of protein synthesis. Many cells remain in G₀ until they are stimulated to divide by growth factors or other extra cellular signals. For example, skin fibroblast cells are arrested in G₀ until they are stimulated to divide as required to repair damage resulting from a wound. This proliferation is triggered by platelet derived growth factor.

http://www.ncbi.nlm.nih.gov/books/NBK9876/figure/A2441/?report=objectonly

Cell cycle checkpoints

During cell division the cell must strictly control its division activities in order to ensure proper division. The cell can accomplish this through several check points. If these check points are not successfully passed then the cell cannot divide. Cooper in “The Cell: a molecular approach” describes this process in detail. There are several checkpoints within the cell cycle to ensure that incomplete or damaged chromosomes are not replicated and passed to daughter cells. One well known checkpoint in the cell cycle occurs in G₂ and prevents the initiation of mitosis until all DNA replication has been completed. The G2 checkpoint is able to sense unreplicated DNA and generate a signal which leads to cell cycle arrest. This cell cycle arrest prevents the initiation of M phase so that the cell remains in G₂ until the genome has been properly replicated. The cell can also become arrested in response to DNA damage. The arrest in this case allows time for the DNA to be repaired so that the damaged DNA is not passed on to daughter cells. DNA damage can also slow the progression of cells through the S phase and arrest the cell cycle progression at a checkpoint in G_1. which can allow the DNA to be repaired before it enters the S phase. The G₁ arrest is regulated by the protein p53 which is rapidly induced in response to DNA damage. Mutations of the p53 gene are among the most common genetic alteration in human cancers.
For more information on the cell cycle, click here.

http://www.ncbi.nlm.nih.gov/books/NBK9876/figure/A2445/?report=objectonly

Cell cycle review video

Important pathways in cell proliferation and development

When studying cell proliferation it is very important to understand what is happening on a molecular level. There are many signaling events and pathways which lead to cell proliferation, cells must be able to receive a signal and be able to respond to that signal. The hedgehog, wnt, and SMAD pathways are a few which play a role in cell proliferation.

The Hedgehog pathway

Gilbert, 2006 discusses how the proteins of the hedgehog family are thought of as paracrine factors. These paracrine factors are often used by the developing embryo to induce particular cell types and create boundaries between tissues. The Hedgehog pathway is also important in vertebrate lib development, neural differentiation, and facial morphogenesis. It is also needed for hair development in mammals.

http://www.nature.com/nrg/journal/v7/n11/box/nrg1969_BX1.html

Proteins of the hedgehog family bind to a receptor called patched. Patched is not a signal transducer but it prevents the smoothened protein from functioning. When Hedgehog is not present, smoothened is inactive and the Cubitus interruptus (Ci) protein (or Gli in vertebrates) is tethered to the microtubules of the cell. While on the microtubules, Gli is cleaved in such a way that portion of it enters the nucleus and acts as a transcription repressor. However, when Hedgehog binds to Patched, the Patched protein shape is changed so that it no longer inhibits smoothened. Smoothened then acts to release Ci from the microtubules which prevents it from being cleaved. The whole Ci protein can now enter the nucleus where it can act as a transcription activator of the same genes it was previously repressing. One good example of how hedgehog signaling is used in cell proliferation and differentiation is in the mesenchymal cells in the mouse metanephric kidney.  It has been demonstrated that sonic hedgehog promotes mesenchymal cell proliferation, regulates differentiation of smooth muscle progenitor cells, and aids in patterning mesenchymal differentiation through inhibition of smooth muscle formation (Jing et al., 2002). This is very important in developing smooth muscle for uretal structures.  

The Wnt Family

Gilbert, 2006 describes Wnts as a family of cysteine rich glycoproteins and there are at least 15 of this gene family in vertebrates. Wnt proteins are critical in establishing the polarity of insect and vertebrate limbs, promoting proliferation of stem cells, and in several steps of the mammalian urogenital system development. 

http://www.wormbook.org/chapters/www_wntsignaling/wntsignaling.html

The Canonical Wnt Pathway

Wnt family proteins usually interact with a pair of transmembrane receptor proteins, one is called frizzled and the other is called LRP5/6. In the absence of Wnt, the transcriptional cofactor β-catinin is constantly being degraded by a protein complex which contains axin, APC, and GSK3 (Glycogen synthase kinase 3). GSK3 phosphorlates β-catinin so that it will be recognized and degraded by the proteasome. Wnt responsive genes are also repressed by the LEF/TCF transcription factor, which binds to histone deacetylases. However, when Wnt comes into contact with the cell membrane, it brings together the Frizzled and LRP5/6 receptors. This causes LRP5/6 to bind axin and GSK3 which allows the Frizzled protein to bind to disheveled. Disheveled stabilizes the axin, which keeps GSK3 bound to the cell membrane, preventing it from phosphorylating β-catinin. Due to this, β-catinin accumulates and enters the nucleus where it binds to the LEF/TCF transcription factor. This displaces the histone deacetylase and activated transcription. The binding of β-catinin turns the repressor into a transcriptional activator and activates Wnt responsive genes. The WNT pathway is very important in cell proliferation and differentiation. One example of WNT in proliferation is in neural development and establishment of the embryonic cerebellum. WNT is responsible for stimulating neural stem cells to proliferate (Pei et al., 2012).

The TGF-β superfamily

There are more than 30 related members of the TGF-β superfamily and they regulate many important interactions in development. This family includes the nodal and activin families as well as the bone morphogenetic proteins (BMP) to name a few. TGF-β 1, 2, 3, and 5 are involved in regulating formation of ECM between cells and regulating cell division/ proliferation. BMPs are also very important as they regulate cell division, apoptosis, cell migration, and differentiation. They are also needed to specify epidermis in development. Nodal and activin proteins are very important in specifying the different regions of the mesoderm and for specifying left and right axis in the vertebrate body.

http://www.motifolio.com/5111158.html

The Smad pathway

The Smad pathway, as described by Gilbert 2016, is activated by TGF-β super family ligands. First, an activation complex is formed by the binding of the ligand by the type I and type II receptors. This allows the type II receptor to phosphorylate the type I receptor on specific serine or threonine residues. After the type I receptor is phosphorylated, it can phosphorylate Smad proteins. Receptors which bind TGF-β family proteins or members of the activin family phosphorylate Smads 2 and 3. Receptors which bind BMP proteins phosphorylate Smads 1 and 5. Once Smad is phosphorylated, it can complex with Smad4 to form transcription factors which can enhance or repress gene transcription. Smad activation is known to regulate a broad array of transcription factors which are important in cell proliferation. TGF-β is a quite potent anti-mitogenic and pro-apoptotic ligand which is mediated by smad protiens. It is because of this that TGF-β and SMAD are very important regulators of cell proliferation (Dijke et al., 2002).

Development of the Epidermis

The epidermis

The skin is a tough, elastic, water impermeable membrane which is the largest organ in our body. Skin is constantly being renewed thanks to the regenerative ability of the population of epidermal stem cells which remain active throughout life. 

Mammalian skin as three major components:

1)      A stratified epidermis

2)      An underlying dermis composed of loosely packed fibroblasts

3)      Neural crest derived melanocytes that reside in the basal epidermis and hair follicles.

https://cnx.org/contents/FPtK1zmh@6.12:RxywCGkA@5/Layers-of-the-Skin

Origin of the Epidermis

As described by Gilbert 2016, the epidermis originates from the ectodermal cells which cover the embryo just after neurulation (the folding process in embryos which includes the transformation of the neural plate into the neural tube). http://biology.kenyon.edu/courses/biol114/Chap14/Chapter_14.html

This surface ectoderm is induced to form epidermis instead of neural tissue by the signaling action of BMPs. The BMPs promote epidermal specification and induce transcription factors that block the pathway responsible for forming neural tissue. This is a good example of how specification of one tissue blocks the specification of an alternate tissue. At first, the epidermis is only one cell layer thick. However, it later becomes a two layered structure. The outer layer gives rise to a temporary covering called the periderm. The periderm is shed once the inner layer differentiates to form a true epidermis. The inner layer is called the basal layer or stratum germinativum. The basal layer contains epidermal stem cells which help replenish the epidermal cells. These epidermal skin cells divide asymmetrically. The daughter cell stays attached to the basal lamina and remains a stem cell while the other cell leaves the basal layer, migrates out, and starts differentiating. This differentiation is positively regulated by the notch pathway. Without notch, there is hyperproliferation of the dividing cells. The notch signal promotes the synthesis of keratins.

Cell division in the skin

When cells divide in the basal layer, it produces younger cells which push the older cells to the boarder of the skin. After synthesis of the differentiated products, the cells cease transcription and metabolic activity halts. These cells are known as keratinocytes and are bound tightly together to produce a water impermeable seal. As the dead cells reach the surface of the epidermis, they are flattened sacs of keratin protein. These cells from the stratum corneum. The dead cells of this layer are constantly shed and replenished. BMPs help initiate epidermal production by inducing the p63 transcription in the basal layer. This p63 transcription factor is required for keratinocyte proliferation. It is also believed that p63 stimulates a notch ligand called jagged. Jagged is a protein in the basal cells that activates the notch protein on the cells above, which activates the keratinocyte differentiation pathway and prevents further cell divisions (Gilbert, 2006).

Literature Cited

Cell cycle Genetics Home Reference. N.p., 2016. Web. 12 Mar. 2016.
https://ghr.nlm.nih.gov/glossary=cellcycle

Cooper, Geoffrey. "The Eukaryotic Cell Cycle". Sinauer Associates (2000): n. pag. Web. 12 Mar. 2016.
http://www.ncbi.nlm.nih.gov/books/NBK9876/

Gilbert, Scott F. Developmental Biology. Sunderland, Mass.: Sinauer Associates Publishers, 2006. Print.

Yu, j., TJ, carroll., and AP, McMahon (2002) Sonic hedgehog regulates proliferation and differentiation of mesenchymal cells in the mouse metanephric kidney, Development 5301-12.http://www.ncbi.nlm.nih.gov/pubmed/12399320

Pei, Y., Brun, SN., Markant, SL., Lento, W., Gison, P., Taketo, MM., Goivannini, M., Gilbertson, RJ., Wechesler-Reya, RJ., (2012) WNT signaling increases proliferation and impairs differentiation of stem cells in the developing cerebellum. Development 1724-33.
http://www.ncbi.nlm.nih.gov/pubmed/22461560

Dijke, p., Goumans, MJ., Itoh, F., Itoh, S., (2002) Regulation of cell proliferation by smad proteins. Journal of cell physiology 1-16.

http://www.ncbi.nlm.nih.gov/pubmed/11920677