MOLECULAR EMBRYOLOGY 3

1. MOLECULAR EMBRYOLOGY 3

2. Activin as a morphogen Morphogen is a diffusible molecule controlling the differentiation of other cells in a dosage-dependent manner Gurdon experiment: Beads releasing activin were surrounded by Xenopus animal cup cells. No activin induces the cells to become ectoderm. Low Activin activates Brachyury and the surrounding cells became Ventral Mesoderm. High levels of Activin induces Goosecoid and the cells became Dorsal Mesoderm

3. Dorsal-Ventral Axis in the Neural Tube The notochord produces Shh and induces the neural tube to become floor plate and to produce Shh The ectodermal cells produce BMP4, 7, 5, Dorsalin and Activin and induce the dorsal neural tube to become roof plate and to start to produce the same proteins Two gradients are created the TGF-B family and the Shh gradients Different concentrations of these proteins activate the expression of different sets of genes These genes are responsible for the different cell fate of the neural cells along the D/V axis in sensory neurons, interneurons and motorneurons

4. Neural Crest Cells • Neural crest cells are formed at the juxtaposition between the prospective neural plate and the ectoderm • They migrate to different locations • They originate many cell types such as neurons and glia of the sensory, sympathetic, parasympathetic nervous system and the pigment containing cells of the epidermis

5. Neural Crest Cell Induction FoxD3, Slug Wnt6, ectoderm BMPs

6. Neural Crest Cell Migration • Slug activates factors inducing the dissociation of tight junctions • Migrating cells follow clues from the EM • One set of proteins (fibronectin, laminin) promote migration while ephrins impede migration

7. Induction during Development • Induction means that a cell or a group of cells secrete a substance that can affect the fate and the behavior of neighboring cells • Membrane proteins on one cell interact with receptor proteins on adjacent cell surfaces (justacrine interactions) • Proteins synthesized by one cell diffuse over a small distance to induce changes in neighbouring cells (paracrine interaction)

8. Paracrine Interaction Paracrine Factors Proteins synthesized by one cell diffuse over a small distance to induce changes in neighbouring cells. Embryos have a complex tool kit of paracrine pathways: the FGF, the Wnt, the Hedgehog families and the TGF-β superfamily

9. A Paracrine Pathway When a ligand (paracrine factor) binds its receptor induces a conformational change in its structure This in turn uncovers a Kinase activity The active receptor phosphorilates downstream proteins The cascade of phosphorilations activates a transcription factor that activates or represses a set of target genes NUCLEUS-TRANSCRIPTION

10. Activation of the Casein Gene during Milk Production Prolactin binds to its receptors causing them to dimerize Two Jack2 proteins are brought together and they can phosphorilate the receptors The active receptor phosphorilates inactive STAT proteins and cause them to dimerize STAT dimers move to the nucleus and they activate the expression of the casein gene

11. The Hh Pathway In the absence of Hh, the Ci protein is tethered to the MTs by the Cos2 and Fused proteins This binding allow PKA and Slimb to cleave Ci into a transcriptional repressor Ci enters the nucleus and it represses Hh-responsive genes In the absence of Hh, Patched inhibits Smoothened

12. The Hh Pathway When Hh binds to its receptor Patched, induces a conformational change in the receptor In so doing, it releases the inhibition of Smoothened protein and Smoothened inactivates the cleavage proteins, PKA and Slimb This also releases Ci from the MTs probably because of the additon of more phosphate groups to Fused and Cos2 Ci enters the nucleus, binds to CBP, a co-activator, and activates the transcription of Hh-target genes

13. The Sonic Hedgehog Pathway • The Shh pathway is important in vertebrate limb development and neural differentiation • Mice mutants for Shh pathway have major limb abnormalities as well as Cyclopia, a single eye in the middle of the forehead • Cholesterol binds to the N-terminus of the Shh protein and allows this paracrine factors to diffuse over a range of few micrometers

14. “The amazing thing about development is not that sometimes it goes wrong, but that it ever succeeds”

15. Teratology • Teratology is the branch of embryology and pathology that deals with abnormal development and congenital (present at birth) malformations • Congenital abnormalities can have intrinsic and extrinsic causes • Abnormalities caused by genetic events (mutations, aneuploidies, translocations) are called malformations.

16. Teratology • Our molecular understanding of mammalian development has expanded exponentially over the last few years Congenital abnormalities can have intrinsic and extrinsic causes • It is now possible to link a malformation to a genetic defect • This raises the possibility of finding a therapy

17. Teratogenesis and Shh This lamb has the Cyclopia phenotype as the ewe has eaten a plant Veratrum californicum This plant contains chemical products such as jervine and cyclopamine that can block cholesterol synthesis This blocks the Shh pathway and causes Cyclopia phenotype Holoprosencephaly is a human disease where humans are born with the Cyclopia phenotype Patients affected by this disease have mutations in the Shh gene

18. Teratogenesis and Environment • Teratogens ( Greek, monster-formers) are environmental factors that can disrupt development • Most teratogens produce their effects during critical periods of development • Teratogenic agents can be naturally present in the environment or they can be artificial compounds

19. Teratogenesis and Environment • Heavy methals (Zinc, lead and mercury) are potent teratogens • Lead and mercury damage the developing nervous system (small brains and eyes) • In highly industrialized societies heavy metals are found in drinking water and vegetables. Mercury can be transmitted through the placenta and the mother’s milk

20. Retinoic acid as a teratogen • RA can become a teratogen if present in large amounts or at particular times during development • It is a secreted molecule involved in A/P axis formation in mammals and in forming the jaws • It affects Hox gene expression in A/P axis determination and it inhibits neural crest cell migration from the cranial region of the neural tube

21. RA as a Teratogen RA causes a characteristic pattern of anomalies: extra or missing limbs or misplaced eyes, deformed jaws and heart and gut malformations In the US methoprenoic acid used to fight mosquitos induced RA-like pattern of malformations in frogs. In the picture a frog and a tadpole (B) with eye abnormalities. MA is structurally similar to RA

22. Testicular Feminization • TF is a type of male pseudo-hermafroditism characterized by female external genitalia incompletely developed and testes present within the abdomen • Androgens and estrogens are formed but target tissues are unresponsive to androgens • It is due to mutations in the androgen receptor gene on the chromosome X

23. Anthelmintics as Teratogens • Benzimidazole compounds are used as anthelmintics to cure parasite infections of the intestine • They are teratogens in domestic and experimental animals • Many embryos will be aborted and the survivors exhibit skeletal, muscle and renal malformations

24. Freemartinism It is one of the most severe forms of sexual abnormalities in cattle. When a female is twin with a male they share the placental membrane connecting the foetuses with the mother A joining of the placental membranes occurs at fortieth day of pregnancy and the fluids of the foetuses are mixed When hormones are mixed, every twin develops with characteristics of the other sex The male exhibit only reduced fertility while the female is completely infertile. A freemartin is genetically a female but it has an abnormal reproductive anatomy with female external genitalia while internal genitalia consist of a variable array of phenotypically male organs