Nervous system evolution 3: Evolution across the DV axis: pattern, proliferation and the neural crest

Question 1

DV Patterning

Answer 1

• D and V extremes of the neural tube are adjacent to two other
  tissues: ectoderm and
  notochord
• proliferation occurs in
  the ventricular region
• cells move laterally
  to differentiate

Question 2

motor neurones

Answer 2

• ## ventral spinal cord

Question 3

Ventral spinal cord

Answer 3

• lots of other cell types,
  mostly interneurons

Question 4

DV Pattern

Answer 4

• progenitor cells lie by
  the ventricle lumen in
  clear ‘zones’
• produce a suite of DV
  restricted cell types

Question 5

Floor plate:

Answer 5

• dorsal and ventral glial populations
• signal source: Shh

Question 6

Shh

Answer 6

• controls ventral patterning
• expressed by notochord and
  floor plate cells
• sufficient: ectopic expression
• necessary: KO

Question 7

Shh as a morphogen

Answer 7

• early, naïve neural plate cultured with different concentrations of
  Shh protein adopts different DV character
• cell type formations reflects distance away from floor plate and notochord
• time is an important developmental axis: quantity of signal received over time

Question 8

Shh pathway

Answer 8

• Shh binds Ptc
• Smo released from Ptc repression
• Smo prevents Gli->GliR
• Gli -> GliA
• GliA -> nucleus
• ratio of GliA/GliR in nucleus determines transcriptional outcomes
• GliA upregulates Ptc, Gli1/2
• signal is quenched in nearby cells, less diffuses as a EC signalling gradient

Question 9

smoothened (Smo), patched (Ptc)

Answer 9

TM

Question 10

Gli

Answer 10

• zinc finger TF
• GliA: activator
• GliR repressor

Question 11

GliA/R

Answer 11

• Gli1, 2, 3 isoforms expressed early in the CNS
• Shh signal: Gli3 down (indirectly, secondarily), Gli1 and 2 up
• dynamic system (incl. feedback loops)

Question 12

Gli 1 and 2

Answer 12

• make more GliA
• ventrally expressed

Question 13

Gli3

Answer 13

• makes potent GliR
• dorsally expressed

Question 14

Dorsal signals

Answer 14

• Shh
• TGFβ
• GDF7

Question 15

TGFβ

Answer 15

• Bmp’s
• Gdf7
• Dorsalin-1 (dsl-1)

Question 16

Shh

Answer 16

in the floor plate

Question 17

GDF7

Answer 17

in the roof plate

Question 18

Roof plate signals

Answer 18

• TGFβ
• Wnt
• varies by species

Question 19

Wnt signals in the roof plate

Answer 19

• Wnt1
• Wnt3a

Question 20

Bmps

Answer 20

• 2/4/7
• probably morphogenic

Question 21

Bmp in dorsal signalling

Answer 21

• DNA construct: Bmp4 ORF downstream of CMV
  viral promoter
• electroporated into neural
  tube cells on one side only
• impact assessed by examining anatomy/gene expression
• sufficient: both progenitors and progeny converted
• LOF hard to do at genetic level because of pleiotropy: early developmental phenotype; neural tube doesn’t form

Question 22

CMV

Answer 22

• cytomegalovirus
• strong
• works in any vertebrate cell type

Question 23

Bmp pleiotropy

Answer 23

• lots of expression domains
• expressed earlier in development for formation of the neural plate

Question 24

Wnt in dorsal signalling

Answer 24

• electroporate into dorsal neural tube in early embryo
• general domain size reshaping
• ## pattern remains the same

Question 25

Wnt signalling mechanism

Answer 25

- Wnt signalling feeds through beta-catenin to regulate TCF4 - opposes ventral Shh signal - "dorsal tension"

Question 26

TCF4

Answer 26

- TF - regulates Gli3

Question 27

Ventral Bmp repression

Answer 27

- chordin extracellular inhibitor produced by notochord

Question 28

Making precise progenitor zones

Answer 28

- Gli-regulated TFs occupy distinct progenitor territories - mostly homeodomain - cross-repression (mutually exclusive) - create distinct patterns and circuits - Dbx1 and Nkx6.2 are both regulated by Gli, but Dbx1 needs less GliA

Question 29

Notch signalling

Answer 29

- keeps cells in proliferation - cells that move laterally lose exposure to Notch; start to differentiate. - cell type they form depends on three things: 1) progenitor zone they are in (Dbx, Nkx6 etc) 2) when 3) where they are along the AP axis (= Hox)

Question 30

Evo perspective

Answer 30

- worked out in vertebrate model systems - largely shared - some derived differences - esp. in zebrafish

Question 31

Amphioxus and Tunicates have:

Answer 31

- dorsal neural tube: forms by neurulation - enlarged head CNS; some neurons and sensory cells - tail neural tube: single cell layer in cross section - some similar cell types to those in vertebrates

Question 32

Amphioxus and Tunicates differ from vertebrates as:

Answer 32

- far fewer cells in brain and spinal cord - probably fewer cell types - lack of obvious CNS structural organisation (segments etc)

Question 33

Genomic innovations between Amphioxus and Tunicates and Vertebrates

Answer 33

2x WGDs: - 1x basal to all vertebrates - 1x specific to jawed vertebrates

Question 34

Developmental innovations between Amphioxus and Tunicates and Vertebrates

Answer 34

- multi-layered spinal cord? - DV progenitor zones in spinal cord? - Brain secondary organisers? - Rhombomeres? - A well-defined hindbrain?

Question 35

DV progenitor zones in Amphioxus and Tunicates

Answer 35

- TALEN-based KO of amphioxus Shh - Shh drives cells directly into differentiation (not via a proliferative progenitor stage) * genes involves in establishing vertebrate DV zones are expressed in amphioxus CNS, but downstream cross-repressive interactions that form the vertebrate zones are not there

Question 36

Explaining the DV progenitor zones in Amphioxus and Tunicates

Answer 36

- maybe the WGDs played a role - some of the DV zones regulatory genes are paralogues descended from this - Gsx1/2, Dbx1/2, Nkx6.1/6.2

Question 37

Brain and spinal cord innovation are paralleled by

Answer 37

- PNS innovation for sensory control - motor-muscle-skeletal systems for locomotion - ecological transition from filter feeders with simple sensory systems and limited motor control/behaviours to predators with sophisticated senses and complex behaviours

Question 38

neural crest

Answer 38

- vertebrate dorsal neural tube cells + neural plate border - brain neurones, ganglia - skeletal elements including many of those in the head; predatory apparatus: bones, skull, jaws - axon insulation; support rapid communication: Schwann cells -transduction and transmission of nerve inputs - retain or reactivate high level of potency: express Yamanaka TFs

Question 39

Does neural crest explain innovations in vertebrate PNS?

Question 40

PNS

Answer 40

includes enteric nervous system and parts of the PNS ganglia* in trunk and head

Question 41

ganglia

Answer 41

- concentrations of neural cell bodies. - DRG, CSG

Question 42

Dorsal Root Ganglia

Answer 42

- DRG - feed trunk sensory input to the spinal cord

Question 43

Cranial Sensory Ganglia

Answer 43

- CSG - in the head - feed in cranial sensation from touch, pain, gustation, hearing/vibration

Question 45

Structure:

Answer 45

1. Define 2. Ventral: Shh 3. Dorsal: roof plate 4. Making precise progenitor zones 5. Notch signalling 6. Amphioxus and tunicates (developmental innovations, ecological transitions and neural crest)