Cell identity and positional information

How does a neuron find its target?

Snail Salivary Neuron

Snail Salivary neuron B4

Snail Esophageal Neuron

A) Normal B5 axonal projections

C) Normal B4 axonal projections

Neuron B4

Salivary glandcell

Normal After ET crush After regeneration

Axonal Regeneration

Neuron B5 is not prohibited from crossing the buccal commissure,But within the ET B5 axons make The proper choices. However Neuron B4 sends axons Indiscriminantly down all ET branches.

What does this tell us?

• There are guidance signals in the ET branches,

• two uniquely identified neurons respond differently to these signals.

• Either The signals, or the responses to the signals, in a regenerating adult are partially similar and partially different from those in the embryo.

Chick embryo-Reversal of spinal cord segments

Motor neuronsStill reach their proper targets

• Two big questions: How does a neuron get a proper identity?

• How does an axon navigate through an embryo to find a proper target?

• 1)• Positional information• Gene regulation• 2)• Molecular guidance signals

The Central Dogma

• About 3% to 5% of the total genome is expressed in a given cell at a given time.

• Regulatory Proteins and Transcription• Proteins called transcription factors function by

binding to the promoter and to another region called the enhancer. The enhancer region may be located at a distance from the gene. These transcription factors are necessary for RNA polymerase to attach. Transcription begins when the factors at the promoter region bind with the factors at the enhancer region creating a loop in the DNA. 

• In the diagram below, transcription factors are represented by the green rectangle and the red oval.

• Hundreds of different transcription factors have been discovered; each recognizes and binds with a specific nucleotide sequence in the DNA. A specific combination of transcription factors is necessary to activate a gene.

• Transcription factors are regulated by signals produced from other molecules. For example, hormones activate transcription factors and thus enable transcription. Hormones therefore activate certain genes.

Blastula Chordin and noggin Expressing

Contains siamois(dorsalizing transcription factor)Induced by Wnt growth factor Can induce the

Spemann Organizer

Inhibits dorsaling,Induces ventralizing genes

twisted

TwistedGastrulation(chordin Cofactor)

For epidermal development

Dorsalizing (neuralizing) &

ventralizing (epidermalizing

) pathways

Dorsalizing (neuralizing) & ventralizing (epidermalizing) pathways

For epidermal development

• In 1822 Geoffroy St. Hillaire suggested that vertebrates were just dorso ventral inversions of invertebrates and represented an inversion of polarity.

• This is confirmed by molecular biology.

Short gastrulation

Chordindecapentaplegic

Anterior-Posterior Polarity

• There is already polarity in the egg.• Gradients of “morphogens” provide

positional information and cascades of diferential activation of transcription factors in different cells.

Evidence for two positional information gradients in

insect eggs

Ligate early embryo

Normal leafhoper embryo

2 partial embryos

Ligate blastoderm

More segments but missing middle

Ligate blastoderm& transplant posterior pole

Hence gradients of positional information sources at each pole. Each turns on one set of genes and represses another.K. Sander

1975

• Figure 1   Sander's ligature experiments on the embryo of the leafhopper insect Euscelis. (A) Normal embryo seen in ventral view. The black ball at the bottom represents a cluster of symbiotic bacteria that marks the posterior pole. (B) After ligating the early embryo, partial embryos form, but the head and thoracic segments are missing from both embryos. (C) When ligated later (at the blastoderm stage), more of the missing segments are formed, but the embryos still lack the most central segments. (D) When the posterior pole cytoplasm is transplanted into an embryo ligated at the blastoderm stage, a small but complete embryo forms in the anterior half, while the posterior half forms an inverted partial embryo. These results can be explained in terms of gradients at the poles of the embryo that turn on one set of structures and repress the formation of others. (After Sander, 1960, and French, 1988.)

The first maternal morphogen found--Bicoid

• Drosophila—A maternal morphogen called Bicoid (a transcription factor) is concentrated in what will become the anterior end. It diffuses out and forms a gradient and dependent upon its concentration activates different subsets of other transcription factors in different cells that subdivides the body axis.

_inhibits

Inhibits --

Double gradient of 4 maternal RNAs for transcription factors sets up the Ant-post axis in Drosophila

Anterior end:Bicoid and Hunchback

Caudal and Nanos-Posterior end

From Sanes,DH,Et al, 2000

• Homeobox genes (HOX genes) have a homeodomain that codes for transcription factors

Human

From Purves, et al, 2004

HOX genes have sequential overlapping expression

domains

From Squire, et al, 2003