Lecture 5: Multicellular Organization and Hydra Regenerationdeim.urv.cat/~alephsys/IBERSINC/courses/05-HYDRA.pdf · The capability of regeneration has decreased along evolution as

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.


+

Egg Sperm+

Embryo

- Symmetry-breaking



- Patterning

- Development

Animal

Cell division

Oocyte


Embryo

- Symmetry-breaking



- Patterning

- Development

Animal

Spemann/Mangold Organizer Experiment

Egg Sperm+

Cell division

Oocyte


- Symmetry-breaking



- Patterning

- Development

Animal

Embryo

Egg Sperm+

Cell division

Oocyte


Embryo

- Symmetry-breaking



- Patterning

- Development

Animal- Turing patterns [static]

-Turing + spatio-temporal forcing

(e.g. traveling waves) [dynamic]


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


Permanent, immortal “embryo”

Astonishing regeneration

capabilities

Great model system!


replacement.

■ Development controlled by one

organizer (located at the hypostome).

■ Complex patterning involved in...

* asexual reproduction,

* body structure maintenance

and regeneration.


replacement. Permanent, immortal “embryo”


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!


organizer

Isotropic configuration broken symmetry

■ Hydra allows to study spontaneous symmetry breaking

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


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



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).


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).


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


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

PV

■ The model reads:


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


▫ 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?


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.


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:


…

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.)


pro

pert

y (

%)

0

100

stiffness

ks -1 area


Results:

…


Results:

0 10 20 30 40 50 60 70 80

0

5

10

KS

-1 a

rea

(a

.u.)


pro

pert

y (

%)

0

100

stiffness

ks -1 area

freq. oscillations



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).

Documents

Lecture 5: Multicellular Organization and Hydra Regenerationdeim.urv.cat/~alephsys/IBERSINC/courses/05-HYDRA.pdf · The capability of regeneration has decreased along evolution as