http://www.stanford.edu/group/nusselab/cgi-bin/wnt/

How Do Cells Differentiate?

By: Melissa Mulvany, Azman Rashid, Michael Schanz


Powerpoint on:

https://docs.google.com/presentation/d/1fwGnJR4x23Nio83dby4rsdg1A8Yi20eVTgWAbPRvb2I/edit

What is a Stem Cell?


  • Stem cells are a variety of cell that is present in multi-cellular organisms.These cells, like all body cells, have all the required genetic information present, but unlike body cells, somatic cells, stem cells have the ability to differentiate and become a variety of somatic cell with a specific biological function. The DNA of the stems cells is less compressed than differentiated cells. The DNA in differentiated cells is structured through histones, proteins found in eukaryotes that package the cells into nucleosomes. With in this set up the genes that the differentiated cell does not use for its specific function are shut down and tightly wrapped around the histones. In contrast, the stem cell’s DNA is wrapped loosely around the histones creating a more opened structure. This loose packaging of DNA allows all genes present able to be expressed.Through chemical cues, the stem cells begins to shut down unnecessary genes and begins structuring the DNA to fit the necessary requirements. This is the basic process of differentiation, which will be discussed in greater detail in a moment. (10)

external image histone_wind.jpg
  • Stem cells are characterized by their ability to divide and differentiate while maintaining the population of stem cells. This process is known as obligatory asymmetric replication. In this process the stem cell divides and produces a father cell that is a exact clone of the original. The other cell formed through the division, known as the daughter cell, becomes forms into a differentiated cell. Another process to maintain the population of stem cell is known as stochastic renewal. Stochastic renewal is the process of a stem cell dividing into two differentiated cells or going through mitosis, while a second stem cell goes through the opposite division. This maintains the number of viable stem cells in the organism. (11)

Stem Cell Performing Obligatory Asymmetric  Replication (16)
Stem Cell Performing Obligatory Asymmetric Replication (16)

  • The ability to differentiate is defined by the potency of the the stem cells. The most potent stem cells are totipotent stem cells. These cells are pre-embryonic cells formed at fertilization and early development. These stem cells can differentiate into embryonic or extra-embryonic cells and can form a viable organism if isolated. In plants totipotent cells are common, but in animal cells totipotent cells are nearly nonexistent after zygote formation. The next potent variation of stem cells are known as pluripotent stem cells. These cells are derived from the three germ layers formed during early development. Pluripotent stem cells differentiate into the body cells of an organism. As the organism forms the stem cells become less potent and specialize into differentiating into only a set of specific cells specialized for the different structures in an organism, these stem cells are known as multipotent stem cells. As the organism continues to develop, these multipotent stem cells differentiate further and form unipotent stem cells that can only produce a single specific type of cell.

  • Once an organism is fully developed the Pluripotent stem cells become more specialized and are characterized into two separate subsets of adult stem cells, somatic stem cells and germline stem cells. Somatic stem cells are the stem cells found throughout an organisms body found in different tissues. These somatic stem cells replenish an organism’s tissue cells as the cells die off. A clear example of somatic stem cells is found when looking at the immune system of mammals. Pluripotent stem cells in bone marrow divide into lymphoid stem cells, a type of somatic cell. Lymphoid stem cells, when differentiating, become B cells and T cells to maintain the defense against antigens. (See diagram below).
    Pluripotent Stem Cell Differentiation (8)
    Pluripotent Stem Cell Differentiation (8)

  • Germ line stem cells are the stem cells used in creating gametes. A characteristic of germline cells is their ability to duplicate indefinitely because of the presence of the telomerase enzyme that prevents chromosome fusion and other negative affects.

What is Differentiation?


  • In multi-cellular organisms specific tissues made of specialized cells fulfill specific functions to guarantee continued survival of the organism. As these specialized cells are destroyed or damaged they are replaced by cells of the same type either through cell division, mitosis, or through stem cells dividing and producing another stem cell and the specialized cell. Depending on the tissue, either method of production could be present. Differentiation is the ladder method.

  • Differentiation is the process of stem cells creating a new specialized cell either in a specific tissue or during the development of an organism. In stem cells, as stated prior, the DNA is loosely wrapped around the histones, and all the genes are exposed and have the potential to stay active or be turned off. The genes that are left active define the differentiated cell because the active genes are able to support transcription and protein production. These processes of gene modification is caused by many factors including small molecules, secreted proteins from neighboring cells, temperature, and oxygen levels. After development it is neighboring cells that dictate gene expression and differentiation. Following the packaging of the DNA and deactivation of unnecessary genes the cells are further specialized. This continence of differentiation is caused by chemical signalling that causes DNA methylation, the addition of a methyl group to a region of the DNA, and histones modification. Both DNA methylation and histone modification are processes of cellular memory increasing or decreasing gene expression. Both DNA methylation and histones modification do not change the DNA, but can be inherited through cell division. These two modification factors are necessary attributes to continue cellular differentiation in higher level organisms. (12)

How is it Initially Differentiated?


  • The major differentiation in an organism occurs during development after fertilization. During cleavage, the zygote cells are all totipotent, meaning that each cell has the ability to divide into all the differentiated cells that are in an organism. This is caused through the presence of IQ-1, a small molecule. IQ-1 inhibits the coactivator p300, which initiated cell differentiation. At the same time, IQ-1 enhances the interaction between coactivator CBP and B-catenin, and through this interaction the cells are told to continue to divide while remaining as fully potent stem cells. After a multiple cell cycles the cells begin to differentiate as p300 levels increase (6). They begin to differentiate by the cytoplasmic determinants, transcription factors, RNA, and other proteins, present in the cytoplasm of the egg cell, contributed by the mother. Each cell in the embryo differentiates differently based on the environment it is exposed to. The environment contains different signals This differentiation prompts the formation of the blastocyst. The inner cellular mass of the blastocyst become pluripotent stem cells. The pluripotent stem cells continue to differentiate and form the three germ layers, the ectoderm, endoderm, and mesoderm. As development continues, the germ layers and stem cells form the organs and tissues of the organism. (17)







(15)
  • Positional information given by information from chemical signals allows for the cell to tell its location relative to the body axis and neighboring cells. In the organism, Drosophilia, scientists studied that the positional information of the cells operates on a finer and finer scale, as the embryo differentiates, to establish the correct segment and position of each cell. The research on the Drosophila can be applied to human development and how each stem cell differentiates differently as it gets more specialized. (17)

  • Researchers have recently discovered that how embryonic stem cells develop has a lot to do with how the genome of the cell is expressed. This means that parts of the genome (like the non-coding portions of the DNA and lineage-specific/tissue-specific genes, etc) that aren't normally expressed in cells are able to be expressed. After any embryonic cell is differentiated, the undergo a process that "silences" these portions of the gene, but until that begins the embryonic stem cell has a fully open and active genome. This causes the embryonic stem cells to be able to develop into any cell because this maintains the flexibility of the cell, and allows it to become any cell type. Once the silencing occurs, after differentiation, this ability is gone. (18)

How Cell Signaling is Involved in Stem Cell Development:


  • The IQ-1 molecule uses signaling pathways within the organism to control how the stem cells develop. The IQ-1 molecule functions in the way that is explained above, but it uses cell signaling pathways to work this way. While it inhibits coactivator p300, it also blocks one arm of a signaling pathway called the Wnt pathway, while increasing the strength of the signal from the other arm of the Wnt pathway. This signaling pathway has two effects on stem cells; a proliferative effect and a differentiative effect. More simply, it has one effect that increases the size of the cell, and another effect that causes the cell to actually differentiate. The IQ-1 molecule is the major signaling molecule of stem cells, it's the molecule that is responsible for actually signaling for the cell to keep dividing. (6)

  • The Wnt pathway is formed of Wnt proteins, and much of what we know about the Wnt pathway has been discovered while studying Drosophila. This pathway is the main pathway used for cell signaling during embryogenesis and up until the cell differentiates. Many developmental defects are a result of a mutation of Wnt proteins, or abnormal signaling along the Wnt pathway. (19)

wnt_pathway.gif

(20)

The Role of Proteins in the Differentiation of Somatic Cells:


Muscle cells:

MyoD is a transcription factor found in cells committed to be muscle cells. A transcription factor is a protein that binds to specific parts of the DNA, controlling the flow of transcription. MyoD is also known to change fully differentiated fat and liver cells into muscle cells, but not other types of cells (17). An embryonic precursor cell, a unipotent cell, creates MyoD from the master control gene. The MyoD would be an internal signal to increase the rate of transcription in another part of the DNA. More Transcription factors would be produced as a result of MyoD. The factors would increase the rate of muscle-related protein production. This process would transform the precursor cell to a myoblast, a determined muscle cell.

Screen_Shot_2012-03-12_at_12.05.56_AM.jpg(17)

Liver cells:

According to Colorado State University, The liver cell or hepatocyte is not a terminally differentiated cell. With the right transcription factors, the cell can differentiate and proliferate to reform the liver after a liver transplant or surgery. Transcription factors NF-KB and STAT3 along with mitogens allow for the regeneration of the liver. A mitogen is a chemical substance, usually a protien, that triggers cell division. However, the picture from the Journal of Biological Chemistry believes that haematopoietic stem cells are used in the differentiation process. The Journal also has published research on different proteins created by different cells that aid in the regeneration of the liver. (9)

Diagram of Liver Regeneration
Diagram of Liver Regeneration
(9)

Neurons:

GRP, gastrin-releasing peptide and NRP, nonribosomal pepdite affect the CNS neural stem cell determining what type of cell it becomes relating to the nervous system (5).

external image differentiation.jpg

(5)

Skin Cells:

β-catenin controls skin stem cell differentiation to allow the cell to differentiate into follicular keratinocytes, cells forming the protective barrier of the skin from the outside environment. Without the protein, the cell becomes a regular epidermal cell in the skin. (13)

external image Epidermis-delimited.JPG(14)

A final video



Essay:


A Embryonic Totipotent Stem Cell is inserted to the human muscle.
a) Compare the stem cell and it's neighboring cells
b) How does the stem cell position and function like it's neighboring cells?
(10 pt)


Questions:




1. What is the initial cause of differentiation during development?
A) The fertilization of the ova and the formation of the zygote
B) High levels of p300
C) Presence of IQ-1 molecules
D) Formation of the Blastocyst
E) It can not be determined

2. What is difference about the DNA in a stem cell?
A) The DNA is loosely wrapped around histones
B) The DNA is tightly wrapped around histones
C) There is a methyl group attached to the DNA
D) There is no difference
E) All genes are activated

3. What does it mean for a stem cell to be totipotent?
A) It is in the process of differentiating
B) It is present at the early stages of development
C) It is differentiating into a pluripotent stem cell
D) It is a common stem cell in plants and animals
E) It has the potential to turn into any cell or an individual

4. If a zygote has formed into a blastocyst, then what type(s) of stem cell(s) is/are present?
A) Totipotent
B) Pluripotent
C) Multipotent
D) A & B
E) B & C

5. What is Obligatory Asymmetric replication?
A) A function of somatic cells
B) A function of germ line cells
C) A method to create more stem cells
D) A function to maintain stem cells
E) A method that creates two body cells

6. Cytoplasmic Determinants include...
A) strands of RNA
B) proteins and lipids
C) proteins
D) a and b
E) a and c

7. The Positional information of the cell operates at a finer and finer scale until correctly segmented. Scientists discovered this in the
A) Drosophilia
B) ameoba
C) biophilia
D) C. Elegans
E) homo sapien

8. Which type of somatic cell does MyoD have an effect on when introduced to its environment?
A) muscle cell
B) neuron
C) fat cell
D) leukocyte
E) hemoglobin

9. The _ allows the CNS neural stem cell to differentiate into the neuron
A) nonribosomal peptide
B) androgen-producing peptide
C) gastrin-producing peptide
D) myosin
E) actin

10. An IQ-1 molecule plays a role in cell signaling between stem cells by
A) opening up the one signaling pathway (the Wnt pathway)
B) opening up all the cell signaling pathways
C) inhibiting one arm of the Wnt pathway and strengthening the other arm of the pathway
D) it's not involved in cell signaling between stem cells
E) None of the above

Sources:

Neural stem cell picture (5)

Information on IQ-1 molecules -(6)

Blood cell picture from stem cell

http://www.pathologyoutlines.com/images/marrow/001.jpg - Pluripotent Stem Cell Differentiation photo (8)

Regeneration of the liver (9)

http://learn.genetics.utah.edu/content/tech/stemcells/ips/ - Histones and DNA photo and information on Stem cell DNA structure (10)

http://jbiol.com/content/8/8/70 - Information on stem cell renewal (11)

http://www.nature.com/scitable/topicpage/cell-differentiation-and-tissue-14046412 - information on stem cell differentiation (12)

β-Catenin and stem cell differentiation in skin (13)

Eipidermis picture (14)

The video on cell differentiation - (15)

http://www.nature.com/scitable/content/ne0000/ne0000/ne0000/ne0000/14707620/U2CP3-4B_RegulProteinsLoop_ksm.jpg - picture of Obligatory asymmetric replication (16)

Campbell, Neil A., and Jane B. Reece. "The Genetic Bases of Development." Biology. Vol. 6. San Francisco: Benjamin Cummings, 2002. Print. (17)

Information on development of embryonic stem cells (18)

Information on the Wnt pathway (19)

Wnt picture (20)