Chapter 11: Neural Crest Cell and Axonal Speciticity

Neural crest cells and axon growth cones migrate far from their source of origin to specific places in the embryo.

• NCC originate at dorsal most region of neural tube
• Recognize cues --> migrate --> respond to local signals --> differentiate

Neural crest is a transient structure dorsal to neural tube.

Neural crest cells (NCC) undergo epithelial-mesenchyme transition and start migrating.

NCC have the ability to self-renewal (stem cell-like property)

Specification of NCC from neural plate boundary is a multi-step process.

Neural Plate border specifiers (BMPs, Wnts and FGFs) --> induce expression of Neural Crest specifiers.

Neural crest cells generate many cell types Fate depends on where they migrate to and settle

Four Major functional domains of NCC

•Cranial (cephalic) neural crest cells – produce craniofacial mesenchyme (cartilage and bone tissue of face)

• •Trunk neural crest cells
•     oPigment synthesizing melanocytes
•     oDorsal root ganglia forming cells

• •Vagal and Sacral neural crest cells
•     oGenerate parasympathetic ganglia of gut

•Cardiac neural crest cellsoDevelop musculo-connective tissue of the walls of large arteries and septa

Ventral pathway --> form sensory and sympathetic neurons, adrenomedullary cells and Schwann cells

Dorsolateral pathway --> travel through dermis --> colonize skin and hair follicles --> produce melanin (mealnocytes)

• Presumptive ectoderm produces BMP 4 and 7 --> expression of Slug protein and RhoB protein in the cells destined to become neural crest --> start migration 
•      Slug protein --> dissociation of E-cadherin  
•      Rho B –promotes migration

Recognition of extracellular matrix - Path of migration is controlled by extracellular matrices 

Migration promoting proteins – fibronectin, laminin, tenascin, collagens, proteoglycans  Thrombospondin, an ECM protein found only in the anterior side , promotes NCC migration

Migration impeding proteins – Ephrin proteins, Semaphorin-3F

Chick NCCs express a4b1 integrin --> help in locomotion of NCC [NCC without a4b1 integrin --> disorientation --> apoptosis]

Thrombospondin – found only in the anterior portion of sclerotome of somites --> promotes NCC migration

Ephrin proteins are responsible for positioning of dorsal root ganglia --> segmentation, patterning of peripheral nervous system

Ephrin proteins – expressed in the posterior side of the sclerotome.

Neural crest cells express Eph receptors.

Eph binding with its receptor interferes with migration.

These NCCs remain in neural tube till the dermis is formed by dermotome.

In contrast to ventral pathway, Ephrin stimulates the migration of NCC in dorsal pathway

Dorso-laterally migrating NCC --> melanocytes

Ultimately settle in the hair follicle bulge and become Melanocyte stem cells --> provide pigment to the hair shaft

     •Depletion of these stem cells à gray hair

•Trunk neural crest cells can differentiate into many cell types

•     oNeck region – they form parasympathetic (cholinergic) neurons
•   
•     oThoracic region – they form sympathetic (adrenergic) neurons

    oIf exchanged (transplanted) --> can form position specific neurons

Final differentiation is determined by local environment

oBMP2 (secreted by heart, lung) --> cholinergic neurons

oGlial growth factor --> glial cells

oGlucocorticoids (in adrenal) --> adrenomedullary cells

• 
• “Face” – initially is the art-work of Cranial neural crest cells

Hind brain is segmented by Rhombomeres

Cranial NCC follow 3 major pathways

1st pathway – cells from r1 and r2 --> form jawbones, incus and malleus and fronto-nasal process

2nd pathway – cells from r4 --> hyoid cartilage of neck and Stapes

3rd pathway – cells from r6 --> Thymus, Thyroid and Parathyroid

•Located between Cranial and Trunk NCC

•Cardiac NCCs produce entire musculo-connective tissues of large arteries (Kirby et al. 2000)

    •Heart actually comes from lateral plate mesoderm

In mice – Ca NCCs express Pax3

•Mutations in Pax3 (Conway et al. 2002)

   •persistent truncus arteriosus (failure to separate aortas and pulmonary artery)

   •defects in thymus, thyroid and parathyroid glands

•Specification of type of neurons is determined by position

   •Dorsal side --> sensory neurons (induced by BMPs)

   •Ventro-lateral margin of neural tube --> motor neurons (induced by Shh)

•Target specificity is determined by birthday of neurons and mediated by Lim genes (structurally related to Hox genes)

•Younger motor neurons --> migrate to periphery compared to older motor neurons

Function of Nervous system depends specificity of axonal connections, which occurs in 3 steps:

Pathway selection – axons travel along a route that leads to a particular region of embryo

Target selection – axons when reach correct area, recognize and bind to cells

Address selection – internal patterns are refined and axons “fine tune” their connection to target cells

•Growth cone ‘feels” its way along the substrate, moves by elongation and contraction of filopodia called microspikes

•Each microspike samples the microenvironment --> sends signals back to soma --> differentiate

•Growth cone:  Locomotory organelle of the growing neuron that senses environmental cues

• •Contact guidance: Extracellular environment provides navigational information
•     •Growth cones prefer to migrate on adhesive surfaces – glycoprotein Laminin paves the path cells – a growth promoting substance
•    •Guidance is also provided by adhesion molecules, N-CAM, L1 etc
•    •Semaphorin proteins also guide growth cones by selective repulsion

Labeled Pathways: Contact guidance provides only general information

    -A given neuron can recognize the surface of another neurons that has grown out before it

•Once neuron reaches the group of cells containing potential target, they depend on neurotrophins secreted by target cells for guidance.

•Nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin 3 (NT-3) – are short range chemotactic or chemorepulsive factors

• 
• Synapse - specialized junction between axon and its target

A. growth cone approaches developing muscle cells (has acetyl choline receptors)

B. Axon releases Agrin --> clustering of ACh receptors

C. Neurotransmitter vesicle enters axonal terminal – extracellular matrix connects the synaptic space

D. Other axons converge on the same synaptic site

E. Completion of muscle innervation