lecture 5: multicellular organization and hydra...

Lecture 5:

Multicellular Organization

and Hydra Regeneration

Jordi Soriano Fradera

Dept. Física de la Matèria Condensada, Universitat de Barcelona

UB Institute of Complex Systems

September 2016

■ Why do we age?

■ Why we cannot regenerate?

▫ These two questions have obsessed humanity for thousands of years.

▫ Their understanding (and practical application) are among the major medical

objectives of the 21st century.

The capability of regeneration has decreased along evolution as a response

to the higher complexity and higher sexual reproduction efficiency.

1. Framework: regeneration

■ Why do we age?

■ Why we cannot regenerate?

▫ These two questions have obsessed humanity for thousands of years.

▫ Their understanding (and practical application) are among the major medical

objectives of the 21st century

The capability of regeneration has decreased along evolution as a response

to the higher complexity and higher sexual reproduction efficiency.

1. Framework: regeneration

▫ In mammals, only few tissues can regenerate, e.g. :

- Fingerprints in humans,

- Antlers in male deer,

- Parts of the ear in rabbits and some mice.

Problem: regeneration implies the formation of new stem cells, that

differentiate to form the new tissues. Normally, this only occurs during

embryonic development.

■ Key points:

- Multicellular organisms are extremely complex. Tissues have to be placed

properly (and their development coordinated) during embryogenesis and

growth.

- Axis establishment is the first and most critical step during embryogenesis.

Its failure stops further development.

- It is still not fully understood the complete set of strategies that Nature has

developed for axis establishment and body plan maintenance.

2. Animal complexity

Egg Sperm

Oocyte

Embryo

- Symmetry-breaking

- Organizer formation

- Body plan organization

- Patterning

- Development

Animal

Cell division

Today we begin to understand the whole picture.

New experimental tools allow accurate cell tracking.

2. Animal complexity

+

Egg Sperm+

Embryo

- Symmetry-breaking

- Organizer formation

- Body plan organization

- Patterning

- Development

Animal

Cell division

Oocyte

2. Animal complexity

Embryo

- Symmetry-breaking

- Organizer formation

- Body plan organization

- Patterning

- Development

Animal

Spemann/Mangold Organizer Experiment

Egg Sperm+

Cell division

Oocyte

2. Animal complexity

- Symmetry-breaking

- Organizer formation

- Body plan organization

- Patterning

- Development

Animal

Embryo

Egg Sperm+

Cell division

Oocyte

2. Animal complexity

Embryo

- Symmetry-breaking

- Organizer formation

- Body plan organization

- Patterning

- Development

Animal- Turing patterns [static]

-Turing + spatio-temporal forcing

(e.g. traveling waves) [dynamic]

2. Animal complexity

Egg Sperm+

Cell division

Oocyte

Which are the simplest biophysical scenarios

to understand development?

- Hydra.

- Drosophila.

- Zebrafish.

- Salamander.

Tentacles

Head

Hypostome

Foot

Bud

■ State of constant growth and tissue

replacement.

■ Development controlled by one

organizer (located at the hypostome).

■ Complex patterning involved in...

* asexual reproduction,

* body structure maintenance

and regeneration.

Permanent, immortal “embryo”

Astonishing regeneration

capabilities

Great model system!

3. Hydra: Simple and complex alike

3. Hydra: Simple and complex alike

Permanent, immortal “embryo”

Astonishing regeneration

capabilities

Great model system!

■ State of constant growth and tissue

replacement.

■ Development controlled by one

organizer (located at the hypostome).

■ Complex patterning involved in...

* asexual reproduction,

* body structure maintenance

and regeneration.

■ State of constant growth and tissue

replacement. Permanent, immortal “embryo”

3. Hydra: Simple and complex alike

Cells migrate from

the center towards

the tentacles, replacing

the old tissue.

Isotropic configuration broken symmetry

■ Hydra allows to study spontaneous symmetry breaking

(higher animals: initial asymmetries in the egg define axis)

1 % of tissue makes a normal Hydra!

3. Hydra: Simple and complex alike

organizer

Isotropic configuration broken symmetry

■ Hydra allows to study spontaneous symmetry breaking

(higher animals: initial asymmetries in the egg define axis)

3. Hydra: Simple and complex alike

self--organization?

Pattern formation

through RD?

H:

75X real time.1 mm

H: Head

B: Body column

F: Foot

Example of the closing process.

First 40 min.

4. Experiments in regenerating Hydra

H:

3000X real time.1 mm

H: Head

B: Body column

F: Foot

4. Experiments in regenerating Hydra

4. Experiments in regenerating Hydra

150 mm

1 mm

head

foot

Coexistence of different stable structures,

early “feet” and “heads”

Reaction-diffusion at play!

1 mm

Axis and organizer

How do we understand it?

■ Additional evidences for organizer’s dominance in development:

- big fragments (old axis preserved)

- buds (new axis since the beginning)

- organizer inserted (new axis imposed)

6. The importance of the organizer

■ The organizer is biologically well stablished:

- The organizer is a group of ~10 cells located at the head.

- The organizer is always the first structure to be restored

(e.g. fragments or beheaded adult Hydra).

- An organizer does not allow the presence of another one too close (e.g. big

aggregates, grafting experiments)

■ Minimal model (1972) able to generate patterns:

1) Two morphogens: activator and inhibitor.

(expressed at the head organizer, HO)

2) Activator: Short range (or slow diffusion).

3) Inhibitor: Long range (or fast diffusion).

7. Simplest RD model for Hydra

Matlab exercise!

very important!

a

i

■ The Meinhardt model explains well:

- Spontaneous symmetry-breaking in embryos.

- Head regeneration in Hydra.

- Stability of the body plan.

Biological data shows that the head organizer produces both

the activator (a) and inhibitor (i).

7. Simplest RD model for Hydra

a

i a

i

■ The Meinhardt model explains well:

- Spontaneous symmetry-breaking in embryos.

- Head regeneration in Hydra.

- Stability of the body plan.

Biological data shows that the head organizer produces both

the activator (a) and inhibitor (i).

7. Simplest RD model for Hydra

However… budding cannot be explained

More elaborated models? a

i

Bud

■ The new Meinhardt model (1993) introduces:

- Activator and inhibitor for head, foot and tentacles.

- A global positional value (PV) that links all structures. The structures are

activated according to the local PV concentration.

- The range of activators and inhibitors change dynamically to allocate all the

structures.

- PV is a tissue property.

Do not diffuse, but scales with size.

8. Elaborated RD model for Hydra

■ The head organizer may be the source of the global positional value.

PV

8. Elaborated RD model for Hydra

■ The positional value can vary locally to accommodate new structures.

PV

■ The model reads:

8. Elaborated RD model for Hydra

Head: Foot:

Tentacles:

Positiona value:

9. Regeneration through self-organization

Self-organization is a process by which a system (formed by elements

and their interactions) becomes ordered in space and/or time.

■ Recent ideas consider that self-organization in biology imply:

▫ Minimum energy of the biological system.

▫ Minimum entropy, i.e. minimum number of admissible states.

▫ Optimal information exchange among system’s elements.

self-organization

self-assembly

Require energy

to be maintained!

(crystals, colloids)

(convection, biochemical patterns)

set of admissible statesentropy

entropy

9. Regeneration through self-organization

▫ It seems that Hydra cells first self-organize to build a functional

hollow structure to later activate RD mechanisms.

Fluctuations may help driving the system

towards the configuration with minimum energy.

▫ Activation of RD mechanisms require symmetry-breaking and the

growth of fluctuations. This may be activated by cell-cell communication,

gene expression…

▫ And Hydra may drive itself towards

a critical state where the system is

scale-invariant and correlations maximum.

self-organized criticalitySome observable D

characterizing the system

goes as D ~ s-g.

What (our) experiments say?

9. Regeneration through self-organization

Possible model:

1) Initial swelling is passive, but provides mechanical stimulation

to cells and activates molecular cues.

S Evidence: no oscillations no axis formation no regeneration

Possible model:

1) Initial swelling is passive, but provides mechanical stimulation

to cells and activates molecular cues.

S Evidence: no oscillations no axis formation no regeneration

2) Correlations in the system grow driven by molecular signaling,

inducing organizer formation and activation or RD mechanisms.

S Evidence: gene expression patterns along regeneration.

3) Organizer locks axis and controls further development.

9. Regeneration through self-organization

Early development After axis formation

Adult / fully regenerated

1 mm50 mm

50 mm

- Apply color threshold to get black and white contours.

- Quantify spots size s distribution P(s).

- We studied ~100 animals / patterns.

In situ hybridization patterns:

9. Regeneration through self-organization

…

0 10 20 30 40 50 60 70 80

0

5

10

KS

-1 a

rea

(a

.u.)

Development time (h)

pro

pert

y (

%)

0

100

ks -1 area

1) Total ks-1 area along development and other quantities:

Results:

…

0 10 20 30 40 50 60 70 80

0

5

10

KS

-1 a

rea

(a

.u.)

Development time (h)

pro

pert

y (

%)

0

100

stiffness

ks -1 area

1) Total ks-1 area along development and other quantities:

Results:

…

1) Total ks-1 area along development and other quantities:

Results:

0 10 20 30 40 50 60 70 80

0

5

10

KS

-1 a

rea

(a

.u.)

Development time (h)

pro

pert

y (

%)

0

100

stiffness

ks -1 area

freq. oscillations

9. Regeneration through self-organization

9. Regeneration through self-organization

X: ks-1 promoting factor.

n: production rate

(increases with mechanical stress).

c: discharged fraction.

▫ Avalanche-like dynamics.

▫ Long range cell-cell communication.

■ External perturbations may define axis only if applied before S.B.

Evidence: temperature gradient experiment.

0

30

60

90

120

150

180

210

240

270

300

330

const T

T = 0.6 °C

T = 0.9 °C

T = 0.6 °C after symmetry-breaking

9. Other observations: role of external perturbations

End of lecture 5

Questions and discussion aspects:

- How far are we from regenerative medicine?

- Coupled RD systems could fully describe human

embryogenesis? How many RD systems would be required?

- What do you know about self-organized criticality? A major

controversy is that power laws may appear too easily.

TAKE HOME MESSAGE:

- Hydra is one the most versatile systems to study regeneration and

pattern formation. It shares features observed in higher animals.

- Embryogenesis and regeneration from identical cells involves the

formation of a main foot-head axis and the activation of patterning

mechanisms.

- Patterning can be fully understood by using reaction-diffusion models.

However, other ideas coexist, inspired in self-organized criticality.

References

▫ C.B.Kimmel et al., “Stages of embryonic development of the zebrafish”, Developmental

Dyn. (1995).

▫ B. Peña et al., “Transverse instabilities in chemical Turing patterns of stripes”,

Phys. Rev. E (2003).

▫ A. M. Turing, Philos. Trans. R. Soc. London, Ser. B (1952).

▫ A. Gierer and H. Meinhardt, “A theory of biological pattern formation”, Kybernetik

(1972).

▫ A. Gierer, “Generation of biological patterns and form: Some physical, mathematical,

and logical aspects”, Progr. Biophys. molec. Biol. (1981).

▫ H. Meinhardt, “A Model for Pattern Formation of Hypostome, Tentacles, and Foot in

Hydra: How to Form Structures Close to Each Other, How to Form Them at a Distance”,

Developmental Biology (1993).

D.E. Turcotte, “Self-organized criticality”, Rep. Prog. Phys. (1999).

▫ V.I. Yukalov, “Self-organization in complex systems as decision making”,

arXiv:1408.1529 (2014).

▫ J. Soriano et al., “Hydra Molecular Network Reaches Criticality at the Symmetry-

Breaking Axis-Defining Moment”, Phys. Rev. Lett. (2006).

▫ A. Gamba et al., “Critical Behavior and Axis Defining Symmetry Breaking in Hydra

Embryonic Development”, Phys. Rev. Lett. (2012).

▫ J. Soriano et al., “Mechanogenetic Coupling of Hydra Symmetry Breaking and Driven

Turing Instability Model”, Biophysical Journal (2009).