Molecular Genetics of Development Topics   

C. elegans as a Model for Cell Fate determination

Vulval Development: Overview

• The fate of all 959 cells oc C. elegans is mapped and can be followed througout development.
• C. elegans is a great model organism because of its short life cycle, nearly transparent anatomy, relative simple structure, ease of culture, and the availability of molecular and genetic databases.
• Mutations in vulval development can lead to a multiple vulva (MUV) phenotype, or a no vulva present (VUL) phenotype. Witout a vulva, offspring hatch inside the mother creating a "sack of worms". This obvious result have made functional genomics possible in C. elegans.
• Many fascinating experiments can be done with C. elegans, including mosaic and ablation experiment. Mosaic experiments allow the worm to develop with some cells expressing certain proteins while others cells do not. Ablation experiments allow for a particular cell to be destroyed while the remaining organism development continues.
• Vulval development is controlled by the anchor cell, which signals the Pn.p cells by an EGF-like ligand (lin-3).
• The Pn.p cells can take on any of three fates as signaled by the Anchor cell and neighboring cells. This includes 1°, 2°, or 3°. Only 1° and 2° contribute to actual vulval development.
• A canonical RAS/MAPK pathway is initiated in P6.p cells due to the lin-3 signal from the ANchor cell, leading to it becoming the 1° cell.
• The 1° P6.p cell sends a lateral signal to the surrounding 05.p and p7.p cells causing them to take 2° identity by a lin-12/Notch pathway. This signal was recently discovered after decades of searching and may be a ligand called DSL-1.
• A long standing issue has been whether lin-3 is a graded signal that alone is required for cell fate determination. It has just been shown that the RAS pathway is activated in P5.p and P7.p as well but negative regulators from the lin-12/Notch pathway inhibit RAS causing them to take on the 2° fate.
• Studying vulval development has led to much information on the EGF pathway, which encodes several proto-oncogenes.

bla blah

Take Quiz: [Q1] [Q2] [Q3]

Back to Basics: Topic 1
                        Topic 2

Advance Topics: Topic 1
                         Topic 2

Vulval Develpment: The experimental model

Caenorhabditis elegans has all the common characteristics you want in a model organism: short lifespan, easy to grow, simple anatomy, etc. It can be a hermaphrodite or male.

C. elegans has a very specific pattern of cell development. The origins of all 959 cells can be traced and have been mapped: extensive genetic information is known, the genome has been sequenced.

Several important types of experiments can be done with C. elegans to provide key information: ablation experiments, mosaic experiments, and Mul or Vul phenotype experiments. Mosaic experiments allow the worm to develop with some cells expressing certain proteins while others cells do not. Ablation experiments use a laser to kill a specific cell while the rest of the animal is unharmed, so development continues. The only reason we can do mosaic and ablation experiments is that we know the worms development is invariant.

Vulva development in C. elegans is a good developmental model because it is precise, invariant and highly reproducible. No vulva (Vul) or multiple vulva (Mul) mutants have been shown to be important in elucidating cell fate determination. Vulva organogenesis happens from L3 stage to adult in three phases:

• Phase I : Execution and short-range migrations. 22 epithelial cells are produced and group to form a short tube of seven rings.
• Phase II : Cell Fusion. Vulval muscle attachments happen at L4 stage.
• Phase III : Eversion. Vulva partially turns inside out and the lumen closes off until it is time for eggs to pass.

Lets look at Phase I in more detail. The fate of six vulval precursor cells (VPCs), P3.p to P8.p, to either fate 1°, 2° or 3° is determined by signaling from two sources: the Anchor cell and neighboring cells. The cell closest to the Anchor cell becomes 1°, the two cells on either side of 1° become 2°, and the next cells by the 2° cells become 3° .

In an ablation experiment were the Anchor cell is killed, the vulval precursor cells all become 3° and no vulva develops (Vul phenotype). If the Anchor cell is moved one cell over in any direction, the order of 1°, 2° and 3° cells also moves so the new cells closest to the Anchor cell is 1°. If all VPCs are killed except one, that cell will still acquire the fate according to its position relative to the Anchor cell. Thus there is some kind of morphogenic signaling originating from the Anchor cell that controls the fate of VPCs.

Take Quiz: [Q1] [Q2] [Q3]

Back to Basics: Topic 1
                        Topic 2

Advance Topics: Topic 1
                         Topic 2

Vulval Develpment: EGF and lin-12/Notch Signaling

Looking at developmental mutants has led to the discovery of many developmental pathways, most notable the EGF (Epidermal Growth Factor) and lin-12/Notch pathways. cont...

Take Quiz: [Q1] [Q2] [Q3]

Back to Basics: Topic 1
                        Topic 2

Advance Topics: Topic 1
                         Topic 2

Apoptosis: Overview

• Role of mitochondria in apoptosis: release of cytochrome c, enabling formation of apoptosome
• There are redundant pathways for phagocytosis in C. elegans, conserved from C. elegans to mammals.
• Pro- vs. anti-apoptotic BCL-2 family proteins compete
• Different tissues use the same cell death machinery but may have different activating signals
• The apoptosome is made of Apaf-1, cytochrome C and caspase
• The BCL-2 family proteins are important regulators of apoptosis.
• Activation --> Core Cell Death Machinery --> Degradation --> Phagocytosis
• Core Cell Death Machinery: Thanatin --> Inhibitor --> Adaptor --> Executioner
  

Take Quiz: [Q1] [Q2] [Q3]

Back to Basics: Topic 1
                        Topic 2

Advance Topics: Topic 1
                         Topic 2

Apoptosis:

C. elegans development generates 1090 somatic cells but the adult organism has only 959 somatic cells, the rest are removed by apoptosis. Specific cells always die.

The apoptosis pathway has similar elements in nematodes and vertebrates. Executioner proteins (CED-3 in nematodes, caspases in vertebrates) chop-up the cell after being activated by adaptor proteins (CED-4 or Apaf-1). Inhibitor proteins like CED-9 or Bcl-2/Bcl-XL prevent the adaptor and executioner from interacting. A thanatin (?) protein (EGL-1 or BH-3 proteins) prevents the inhibitor proteins from acting.

etc...

Take Quiz: [Q1] [Q2] [Q3]

Back to Basics: Topic 1
                        Topic 2

Advance Topics: Topic 1
                         Topic 2

Continue to "nest topic" or take a test: [T1] [T2] [T3].

Need more practice? Answer the following review questions:

Questions not yet available