Lecture 5: Multicellular Organization and Hydra Regeneration

Transcription

1 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

2 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.

3 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. 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.

4 2. Animal complexity 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.

5 2. Animal complexity Egg + Sperm Oocyte Embryo Animal Cell division - Symmetry-breaking - Organizer formation - Body plan organization - Patterning - Development Today we begin to understand the whole picture. New experimental tools allow accurate cell tracking.

6 2. Animal complexity Egg + Sperm Oocyte Embryo Animal Cell division - Symmetry-breaking - Organizer formation - Body plan organization - Patterning - Development

7 2. Animal complexity Egg + Sperm Oocyte Spemann/Mangold Organizer Experiment Embryo Animal Cell division - Symmetry-breaking - Organizer formation - Body plan organization - Patterning - Development

8 2. Animal complexity Egg + Sperm Oocyte Embryo Animal Cell division - Symmetry-breaking - Organizer formation - Body plan organization - Patterning - Development

9 2. Animal complexity Egg + Sperm Oocyte Embryo Cell division Animal - Symmetry-breaking - Organizer formation - Body plan organization - Patterning - Development - Turing patterns [static] -Turing + spatio-temporal forcing (e.g. traveling waves) [dynamic]

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11 Which are the simplest biophysical scenarios to understand development? - Hydra. - Drosophila. - Zebrafish. - Salamander.

12 3. Hydra: Simple and complex alike State of constant growth and tissue replacement. Permanent, immortal embryo Tentacles Astonishing regeneration capabilities Development controlled by one organizer (located at the hypostome). Complex patterning involved in... * asexual reproduction, * body structure maintenance and regeneration. Hypostome Foot Head Bud Great model system!

13 3. Hydra: Simple and complex alike State of constant growth and tissue replacement. Permanent, immortal embryo Astonishing regeneration capabilities Development controlled by one organizer (located at the hypostome). Complex patterning involved in... * asexual reproduction, * body structure maintenance and regeneration. Great model system!

14 3. Hydra: Simple and complex alike State of constant growth and tissue replacement. Permanent, immortal embryo Cells migrate from the center towards the tentacles, replacing the old tissue.

15 3. Hydra: Simple and complex alike Hydra allows to study spontaneous symmetry breaking (higher animals: initial asymmetries in the egg define axis) Isotropic configuration broken symmetry organizer 1 % of tissue makes a normal Hydra!

16 3. Hydra: Simple and complex alike Hydra allows to study spontaneous symmetry breaking (higher animals: initial asymmetries in the egg define axis) Isotropic configuration broken symmetry Pattern formation through RD? self--organization?

17 4. Experiments in regenerating Hydra H: H: Head B: Body column F: Foot Example of the closing process. First 40 min. 75X real time. 1 mm

18 4. Experiments in regenerating Hydra H: H: Head B: Body column F: Foot 1 mm 3000X real time.

19 4. Experiments in regenerating Hydra 150 mm

20 1 mm

21 head Coexistence of different stable structures, early feet and heads Reaction-diffusion at play! foot 1 mm

22 Axis and organizer How do we understand it?

23 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) Additional evidences for organizer s dominance in development: - big fragments (old axis preserved) - buds (new axis since the beginning) - organizer inserted (new axis imposed)

24 7. Simplest RD model for Hydra 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). a i very important! Matlab exercise!

25 7. Simplest RD model for Hydra 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

26 7. Simplest RD model for Hydra 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

27 8. Elaborated RD model for Hydra 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.

28 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

29 8. Elaborated RD model for Hydra The model reads: Head: Foot: Tentacles: Positiona value:

30 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. Require energy to be maintained! self-organization self-assembly (convection, biochemical patterns) (crystals, colloids) Recent ideas consider that self-organization in biology imply: Minimum energy of the biological system. Minimum entropy, i.e. minimum number of admissible states. entropy set of admissible states entropy Optimal information exchange among system s elements.

31 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 criticality Some observable D characterizing the system goes as D ~ s -g. What (our) experiments say?

32 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

33 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 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.

34 9. Regeneration through self-organization In situ hybridization patterns: 50 mm Early development 50 mm After axis formation 1 mm Adult / fully regenerated - Apply color threshold to get black and white contours. - Quantify spots size s distribution P(s). - We studied ~100 animals / patterns.

35 KS-1 property area (a.u.) (%) Results: 1) Total ks-1 area along development and other quantities: Development time (h) ks -1 area

36 KS-1 property area (a.u.) (%) Results: 1) Total ks-1 area along development and other quantities: Development time (h) ks -1 area stiffness

37 KS-1 property area (a.u.) (%) Results: 1) Total ks-1 area along development and other quantities: Development time (h) ks -1 area stiffness freq. oscillations

38 9. Regeneration through self-organization

39 9. Regeneration through self-organization

40 X: ks-1 promoting factor. n: production rate (increases with mechanical stress). c: discharged fraction. Avalanche-like dynamics. Long range cell-cell communication.

41 9. Other observations: role of external perturbations External perturbations may define axis only if applied before S.B. Evidence: temperature gradient experiment const T T = 0.6 C 240 T = 0.9 C T = 0.6 C after symmetry-breaking

42 End of lecture 5

43 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. 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.

44 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: (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).