1. Species-specific neuronal circuits directed by neurotrophic factor
control of transcriptional programmes
> novel neural circuits arise during evolution to encode unique
behaviors among different animal species.

2. Chick
Mouse
> hopping gait = sautiller
> feathers
> alternating locomotion
> hairs, a cold adaptation necessary
for the prevention of heat loss.
Expect species specific differences in spinal circuitry

3. elaborate nerve endings
Different innervation of skin in mammals and birds
elaborate nerve endings
in the epidermis
no nerve endings
in the epidermis

4. Diversité des neurones sensoriels périphériques
des ganglions rachidiens
Ganglion Rachidien Dorsaux (DRG)
TrkA ou c-Ret
TrkC
TrkB et/ou c-Ret
De part leur diversité, les neurones sensoriels des ganglions rachidiens représentent un modèle de choix pour répondre à ces questions fondamentales. Ces neurones peuvent être classé de différente manière selon leur fonction, leur taille, les marqueurs spécifiques qu’ils expriment, notament les récepteurs des neurotrophines, ou encore par leur projections centrales et périphériques. De manière simplifiée, 3 grandes classes de neurones sensoriels se détachent:
Les nocicepteurs et thermorécepteurs qui transmettent les stimuli douloureux et qui expriment soit le récepteur du NGF TrkA soit le récepteur du GDNF c-Ret
Les propriocepteurs qui expriment le récepteur de la neurotrophine 3 TrkC, se projettent sur les muscles et évalue la tension musculaire. Enfin, les mechanorecepteurs constitue une classe bien moins caractérisée. Ils expriment le récepteur du BDNF TrkB ou c-Ret et sont responsable des sensations tactiles.
Mise à part ces caractéristiques, chaque classe de neurones sensoriels présentent une connectivité centrale très caractéristique et hautement reproductible. L’ensemble des données actuelles concernant diversification et établissement des projections suggèrent que les deux processus sont coordonnés par un même groupe de facteurs de transcription à différents stade du développement.
Moelle épinière
Neurones prorioceptifs
Neurones mécanoceptifs
Neurones nociceptifs
Marmigère and Ernfors Nature Reviews Neuroscience 8, 114–127 (February 2007)

6. Dorsal root ganglion (DRG)
Somatosensory neurons are highly diverse
Spinal cord
Dorsal root ganglion (DRG)
Proprioceptors
TrkC/NT-3
Mechanoreceptors
TrkB/BDNF
AIM OF MY PROJECT IS:
The neurons of the dorsal root ganglion are the first relay in the somato-sensory system. The sensory neurons of the DRG allow us to perceive signals from environment. The cell bodies of these neurons are situated in the spinal ganglion along the spinal cord. Different sub-types of sensory neurons are responsable for different sensory modalities e.g. nociceptors …
Specific subtypes of neurons, that innervate different peripheral targets send information to the spinal cord. Thus, the muscle proprioceptors ….. Etc. Myelinated, large-diameter cell bodies etc. Neurotrophic factor requirements ….
The nociceptors sense noxious stimuli which are percieved as pain. Their axons are unmyelinated or thinly myelinated.
The different types of sensory neurons project to different regions of the spinal cord
1. Find molecular markers to help us to understand physiology
2. Molecular changes after lesion of the a peripheral nerve…… since this type of nerve injury can lead to several pathological conditions such as …
To do this, we have establisehd a multidisciplinary approach combining molecular analysis with physiological analyses in intereaction with F. Scamps group. I will show you an example of this approach before exposing the main project.
The establishment of this system involves a complex interplay between specification factors, neurotrophic factors that regulate the survival and differentiation of these neurons, transcription factors that regulate targeting of central processes ….
Nociceptors
Thermoceptors
TrkA/NGF
Ret/GDNF

7. Neurotrophins and sensory
neuron development
> survival
> maturation
> axonal projections

9. Summary
Hypothesis
Species are endowed with unique sensory capabilities encoded by divergent neural circuits.
One potential explanation for how divergent circuits have evolved is that conserved
extrinsic signals are differentially interpreted by developing neurons of different species
to yield unique patterns of axonal connections. Although NGF controls survival, maturation
and axonal projections of nociceptors of different vertebrates, whether the NGF signal is
differentially transduced in different species to yield unique features of nociceptor circuits
is unclear.
Results
We identified a species-specific signaling module induced by NGF and mediated by a
rapidly evolving Hox transcription factor, Hoxd1. Mice lacking Hoxd1 display altered
nociceptor circuitry which resembles that normally found in chicks. Conversely, ectopic
expression of Hoxd1 in developing chick nociceptors promotes a pattern of axonal
projections reminiscent of the mouse.
Conclusion
We propose that conserved growth factors control divergent neuronal transcriptional
events which mediate interspecies differences in neural circuits and the behaviors
they control.
The form of a scientific paper

10. NGF and sensory neuron development
Nociceptors
- pain
- temperature
- touch
- itch
> survival
> maturation
> axonal projections

11. Hox genes in development
Hox genes pattern the rostro-caudal axis

12. Hox genes pattern the rostro-caudal axis

13. HoxD family – role in limb patterning (medio-lateral axis)
Fig. 2   Hoxd genes in limb development. (a) Schematic diagram of part of the Hoxd cluster, showing the arrangement of six of the nine Hoxd genes along a single chromosome. Although there are many different families of developmental genes, only Hox clusters show such close spatial relations. (b) Chick embryo, showing the developing limb. As the limb-bud grows, it exhibits a dynamic, overlapping pattern of Hoxd expression, which correlates with the spatial arrangement of the genes along the chromosome. This subdivides the developing limb into regions, which then go on to produce distinctive structures. For example, each of the three regions at the apex of the developing limb produce one of the three fingers seen in the adult chicken

15. 1. Search for transcriptional targets of NGF different between birds and mammals
i. Genes enriched in nociceptors
ii. NGF-regulated genes expressed in nociceptors in vivo
iii. mouse DRG explants grown in the presence or absence of NGF for identification
of NGF-dependent genes expressed in nociceptors in vitro.
> NGF dependent genes in mouse nociceptors
Mouse DRG culture Chick DRG culture
+ NGF NGF
> differentially regulated genes detected by Q-PCR
HoxD1

16. Results
1. A screen for genes controlled by NGF signaling in mammalian nociceptors
HoxD1 is an NGF regulated in mouse, but not in chick

17. Fig. 1
HoxD1 expressed in mouse nociceptors, but not in chick

18. 2. Developmental expression of Hoxd1 in different vertebrate speciesWT
HoxD1 -/-
Nociceptive innervation of the skin of Hoxd1−/− mice resembles that of non-mammalian
vertebrates e.g. birds

19. Fig. 2
Abnormal expression of
Mrgbp4 in peptidergic neurons
in HoxD1-/- mice

20. 3. Hoxd1 instructs nociceptor central axonal projections within the mammalian spinal cord
Mirroring species-specific differences in innervation of the skin, the patterns of nociceptive
axonal projections within the spinal cords of mammals and birds are also distinct
> ectopic innervation of deep layers of the spinal cord

21. Different patterns of nociceptor innervation of the spinal cord in mouse and chick

22. 4. Hoxd1 instructs nociceptor central axonal projections within the mammalian spinal cord
- suggests that Hoxd1 mediates NGF-dependent suppression of nociceptor projections
into deep layers of the spinal cord.

23. In Mouse
Loss of NGF or
Loss of HoxD1
> chick-type spinal cord innervation
What is effect of expressing HoxD1 in chick DRG neurons?
The gain-of-function experiment

24. Electroporation of plasmid DNA into chick embryo neural tube

25. 5. Ectopic Hoxd1 expression in chick nociceptors induces mammal-like traits
> HoxD1 expression in chick suppressed deep layer nociceptor innervation in chick

26. 6. Hoxd1−/− mice have deficits in cold sensitivity

27. Conclusion
> Hoxd1 instructs development of mammal-specific features of nociceptive
neural circuitry.
> behavioral sensitivity to extreme cold is markedly compromised
in Hoxd1 mutant mice
> suggests HoxD1 was co-opted by nociceptors in mammals for cold sensation