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

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

Marmigère and Ernfors Nature Reviews Neuroscience 8, 114–127 (February 2007)

TrkB et/ou c-Ret

TrkC

TrkA ou c-Ret

Moelle épinière

Ganglion Rachidien Dorsaux (DRG)

Neurones prorioceptifs

Neurones mécanoceptifs

Neurones nociceptifs

Spinal cord Dorsal root ganglion (DRG)

ProprioceptorsTrkC/NT-3

Nociceptors

ThermoceptorsTrkA/NGF

Ret/GDNF

MechanoreceptorsTrkB/BDNF

Somatosensory neurons are highly diverse

Neurotrophins and sensory neuron development

> survival

> maturation

> axonal projections

Summary

HypothesisSpecies 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, maturationand 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.

ResultsWe 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.

ConclusionWe 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)

1. Search for transcriptional targets of NGF different between birds and mammals

i. Genes enriched in nociceptorsii. NGF-regulated genes expressed in nociceptors in vivoiii. 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

Results1. 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-mammalianvertebrates e.g. birds

Fig. 2

Abnormal expression of Mrgbp4 in peptidergic neuronsin 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 nociceptiveaxonal 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 MouseLoss of NGFor 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