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.

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

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

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)

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

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

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

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

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

Hox genes pattern the rostro-caudal axis

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

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

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

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

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

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

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

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

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.

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

Electroporation of plasmid DNA into chick embryo neural tube

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

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

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