lecture iv. mechanisms of neural development delve… · lecture iv. mechanisms of neural...

Bio 3411 Lecture IV. Mechanisms of Neural Development

September 10, 2012

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Lecture IV. Mechanisms of Neural Development

Bio 3411 Monday

September 10, 2012

September 10, 2012 Lecture IV. Mechanisms of Neural Development

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Readings NEUROSCIENCE: 5th ed, pp 477-506 (sorta)

4th ed, pp 545-575 (sorta) References†: †Fainsod, A., Steinbeisser, H., & De Robertis, E. M. (1994). EMBO J, 13(21), 5015-5025. †Hemmati-Brivanlou, A., & Melton, D. (1997). Annu Rev Neurosci, 20, 43-60. †Melton, D. A. (1987). Nature, 328(6125), 80-82. †Sasai, Y., & De Robertis, E. M. (1997). Dev Biol, 182(1), 5-20. †Smith, W. C., & Harland, R. M. (1992). Cell, 70(5), 829-840. †Weeks, D. L., & Melton, D. A. (1987). Proc Natl Acad Sci U S A, 84(9), 2798-2802. †Wilson, P. A., & Hemmati-Brivanlou, A. (1995). Nature, 376(6538), 331-333. †Xanthos, J. B., Kofron, M., Wylie, C., & Heasman, J. (2001). Development, 128(2), 167-180. †Zimmerman, L. B., de Jesus-Escobar, J. M., & Harland, R. M. (1996). Cell, 86(4), 599-606. ______________________

†(pdfs on course websites: [[http://www.nslc.wustl.edu/courses/Bio3411/bio3411.html]

September 10, 2012 Lecture IV. Mechanisms of Neural

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Embryogenesis 1. Maternal cytoplasmic determinants. 2. Fertilization creates dorsal-ventral axis. 3. Cell division. 4. Blastula created. 6. Ectoderm, mesoderm, endoderm created. by molecular signals along the Animal/Vegetal axis. 5. Gastrulation. 6. Spemann organizer creates anterior-posterior axis. 7. Notocord induces the Neural Plate. 8. Neurulation forms the Neural Tube. 9. Neural crest cells form the PNS. 10. Segmentation & Cephalization (anterior enlargement) September 10, 2012 Lecture IV. Mechanisms of Neural

Development 3

1)  Cell Signaling

2)  Discovery of the Organizer

3)  How Could this Work?

4) The Answer

5) Blockers

6) Current View

7) Summary


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Cell Signaling


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Neuroinduction


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Intracellular Signaling through a Kinase Cascade; Signal Amplification (Suppression) and Multiple Control Points


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Endoderm and Mesoderm involute with gastrulation: Induction of the Neural Plate from Neuroectoderm,

by the underlying, closely apposed Mesoderm.


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Discovery of the Organizer


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Hilde Mangold and Hans Spemann

•  Key experiments performed in 1921-1923 at the University of Freiburg, Germany. •  Hilde Mangold was a 24 year old graduate student when she performed these experiments. She died tragically in an accidental alcohol heater explosion. •  Hans Spemann was awarded the Nobel Prize in 1935.


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Mangold –Spemann Experiments (1924)


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How Could this Work?


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Explant Experiments with Animal Caps from Amphibian Blastula: Puzzling Results…

!


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Isolating Inducing Factors that Promote Neuronal Differentiation; “Sigma Catalog” Experiments

Result in Further Confusion…

+ Candidate Neuroinducing

Factors ?

(Intact)

(Many positives, including apparently non-biological factors!)


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Models for Neural Induction

+”Epidermal factor”

+”Neuronal factor”

Presumptive Neuroectoderm

Epidermis

Neurons

Model 1:


(“default”) Presumptive Neuroectoderm

Epidermis

Neurons

Model 2:


(“default”)


Epidermis

Neurons

Model 3:


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TGF-β Proteins Signal Through Heterodimeric Receptors and Smad Transcription Factors


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The Answer


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A Dominant-Negative Receptor Subunit Blocks Activation of the Signaling Pathway

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(Hemmati-Brivanlou and Melton, 1992)


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Blocking TGF-β Signaling by a Dominant-Negative Receptor Causes

Isolated Neuroectoderm to Become Neuronal

Animal Cap (Intact)

(Intact)

+ TGF-β Signaling

(+Dominant-Negative Type II Receptor cRNA)

TFG-β Signaling Blocked by expression of Dom-Neg Type II Receptor Subunit Animal Cap

(Intact)

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+ TGF-β Signaling


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TGF-β: Transforming Growth Factor - β BMP-4: Bone Morphogenic Protein - 4

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BMP-4 (TGF-β) Signaling Results in “Neural Epidermal Induction”


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September 10, 2012




Epidermis

Neurons

Model 1:


(“default”) Presumptive Neuroectoderm

Epidermis

Neurons

Model 2:

(“default”)


Epidermis

Neurons

Model 3:

Models for Neural Induction

+BMP-4


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BMP-4 (Secreted by Neuroectodermal Cells) Inhibits Neuronal Fate and Promotes Epidermal Fate.

Tissue Dissociation dilutes BMP-4 activity

(Endogenous BMP-4 Diluted)

(Wilson and Hemmati-Brivanlou, 1995)

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[BMP-4]

Neural

+ BMP-4

Epidermal


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Recombinant BMP-4 Promotes Epidermal Fate and Inhibits Neuronal Fate

(Wilson and Hemmati-Brivanlou, 1995)


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BMP-4 mRNA is Expressed in Presumptive Ectoderm


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Blockers


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Are there native anatgonists of BMP-4? Secreted from underlying mesoderm? Yes… chordin / noggin / follistatin. And they are enriched in the Spemann-Mangold Organizer!


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Differential Substractive Screen yields Chordin, a BMP-4 antagonist (1994)


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Functional Expression Cloning yields noggin, a BMP-4 anatagonist (1992)


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Chordin/Noggin/Follistatin directly bind to and inactivate BMP-4


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Structure of Noggin-BMP complex


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Molecular Mechanism of Neuralization


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Current View


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TGF-β proteins signal through heterodimeric receptors and Smad transcription factors


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Neural induction mechanisms are conserved:

Ligand

Receptor

Antagonist

Transcription Factor

BMP-4

Type I Type II Type III

noggin chordin

follistatin

Smad1 Smad2 Smad3 Smad4 Smad5

Vertebrates decapentaplegic (dpp)

punt

thick veins (tkv), saxophone (sax)

Short-gastrulation (sog)

Mothers against decapentaplegic (MAD)

Medea

Drosophila


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BMP-4 is only one member of the large evolutionarily conserved TGF-β gene family, which mediates many different tissue inductive events.

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Relationships between members of the TGF-β super family. (After Hogan, 1996)


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Summary


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1. Neuroectodermal cells choose either a neuronal or epidermal fate. 2. Interactions between mesoderm and neuroectoderm induce neuroectoderm to

adopt the neural fate. 3. Induction is signaled by Bone Morphogenic Protein-4 (BMP-4), a protein made

and secreted by neuroectodermal cells. 4. BMP-4 inhibits neuralization and promotes the epidermal fate in neighboring cells. 5. Mesodermal cells secrete proteins (Chordin, Noggin, Follistatin) which directly

bind and antagonizes BMP-4 activity.

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Neurogenesis: Inductive Mechanisms


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6. Neuroectodermal cells become neurons by suppression of BMP-4 activity by secreted antagonists from underlying mesodermal cells. 7. The “default” state of neuroectodermal cells is neuronal. 8. This mechanism is conserved between vertebrates and invertebrates. 9. BMP-4 is a member of the Transforming Growth Factor (TGF-β) family of signaling molecules. 10. Similar signaling events in the nervous system mediate changes in later development stages and in adult plasticity.

September 10, 2012

Neurogenesis: Inductive Mechanisms


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lecture iv. mechanisms of neural development delve… · lecture iv. mechanisms of neural...

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