Cell Differentiation: Cell interactions in Development

1. Cell Differentiation: Cell interactions in Development Patricia Zuk, PhD Research Director Regenerative Biogengineering and Repair (REBAR) Lab Department of Surgery David Geffen School of Medicine at UCLA

2. so cells can interact with each other and with their environment • this interaction turns specific signaling paths ON or OFF • these pathways become important for mediating proliferation, differentiation and apoptosis • all three are crucial to development

3. Differentiation: Stem cells • so fertilization of the egg takes place in the oviduct • the fertilizes zygote travels to the uterus for implantation • along the way – the zygote begins to divide (mitosis) • 2-cell, 4-cell, 8-cell embryonic stages etc…. • the embryo reaches a stage called the morula = ball of small cells (embryo has not enlargened) • by the end of the first week the second embryonic stage – the blastocyst - forms

4. Differentiation: Embryonic Stem cells • the blastocyst is a hollow ball of cells containing an outer rings of progenitor cells = trophoblast and an inner mass of cells at one end of the embryo = inner cell mass • it is these ICM cells that are the source for the derivation of embryonic stem (ES) cells • the ES cells are said to be totipotent – have the ability to specialize or differentiate into ALL cells of the embryo • the blastocyst then begins a process of differentiation and these ES cells form populations of stem cells with more restricted potentials • the ES cells first differentiate into two layers called the embryonic disc – divides the blastocyst cavity into an amniotic cavity and a yolk sac (primitive hematopoietic organ) • these two layers then continue to differentiate into the three germ layers of the embyro • ectoderm, mesoderm and endoderm • the formation of these germ layers marks the gastrula embryonic stage

5. Germ Layers • the ectoderm, mesoderm and endoderm are thought to be made up of stem cells with a more restricted phenotype when compared to ES cells BUT still capable of forming multiple cell types within that lineage • e.g. pluripotent stem cells • interactions between signaling molecules produced by these germ layers and with the developing ECM around these tissues results in specific developmental events = patterning • patterning requires the exposure of cells to a succession of signals and subsequent activation of their associated pathways

6. Cellular interactions in development: Induction • interactions between the cells of the germ layers influence the fate of the stem cells within these layers • can affect their differentiation paths • induction = mechanism where one cell population influences the development of neighbouring cells • e.g. mesoderm induces the overlying ectoderm to form neural tissue • embryonic development is a series of inductive events • binary – have a choice between one fate or another (presence of one signal – development down one path, absence of signal – development down another path • gradient – multiple fates may result – dependent upon the level or threshold of the signaling molecule (these signaling molecules are called morphogens) • relay – a signal induces a cascade which determines the fate of cells in proximity – these cells than produce additional signals which affect the fate of their neighbours

7. Cellular Interactions: Patterning and TGFb superfamily signaling • a number of extracellular signaling molecules play roles in determining the fates of developing cells and where these cells will develop • TGFb superfamily: TGFb, BMPs, activin (BMP2 and BMP4) • wide role in development • mediate proliferation of stem cells • can determine stem cell fate – e.g. BMP4 = mesoderm • also mediate the production of CAMs, growth factors, ECM molecules – all which play roles in cell signaling and resulting differentiation responses • homologs found in multiple vertebrates • binding of these homologs to their receptors results in their activation • this activation phosphorylates/activates downstream adaptors called Smads • a cascade of Smads are activated resulting in eventual nuclear translocation and regulation of gene expression • these Smads are specific to the TGFb superfamily growth factor (Smad 1 = BMP2, Smad 2 = TGFb) • this signaling path is fine-tuned by the action of several adaptor proteins (chordin, xolloid, noggin) • this fine tuning can result in the induction of neural tissue as opposed to mesodermal tissue and results in the patterning of tissues within the embryo

8. Other Patterning Signal Paths • numerous other patterning pathways comprised of multiple signaling proteins • e.g. hedgehog pathway – patterning of local tissues and determination of cell fates • initial studies in Drosophila • homologs now characterized in vertebrates • hedgehog (sonic and indian hh in vertebrates), wingless (Wnt in vertebrates), patched, smoothened, dishevelled • limb patterning – FGF, sonic hedghog, • neural tube differentiation – sonic hedgehog gradients

9. Want to know how ridiculous this gets?? • other signaling molecules in cell differentiation/tissue fate determination • brachyury – mesoderm • goosecoid • toll, spaetzle, cactus & dorsal • dorsal targets: snail, twist, rhomboid • delta, notch, jagged, suppressor of hairless • hunchback, Kruppel • even-skipped, fushi-tarazu