NeuroPSI – Université Paris-Saclay

Identification and characterization of Sonic Hedgehog signal for brain maintenance and repair

Résumé
In the adult mouse brain, Sonic Hedgehog (Shh) signaling regulates stem and progenitor cell maintenance, neuronal and glial circuitry and brain repair. However, the sources of Shh remain poorly characterized.

Using single-molecule fluorescent in situ hybridization, I discovered Shh expression in a wider population of adult mouse neurons than initially reported. Besides GABAergic neurons in the ventral pallidum and cholinergic neurons in the motor nuclei, Shh transcripts were observed in midbrain dopaminergic neurons and nNOS nitrergic neurons in different brain areas, such as the molecular layer of the cerebellum, cortical layers, the CA3 pyramidal cell layer of the hippocampus, and in hypothalamic nuclei. These data underline the importance of nNOS neurons in regulating Shh availability to brain cells.

I also detected Shh mRNAs in a subset of oligodendrocytes (OLs) expressing Olig2 and Sox10 mRNAs across almost all brain areas. Interestingly, by using the Shh-C9C5 monoclonal antibody I reported a broad Shh expression in a subpopulation of CC1+ mature OLs that do not express PDGFRα, an OL progenitor cells (OPCs) marker. These data suggest a subset of mature OLs as a new source of Shh in physiological conditions.

Importantly, I showed that Shh-C9C5 expression increases in the postnatal mouse brain, in parallel with the Myelin Basic Protein (MBP). Using primary culture of cortical rodent OLs, I revealed that both Shh mRNA and protein are expressed at earlier division time points (day in vitro 1, D1) and are upregulated during OLs maturation, preceding MBP, while myelinating cells are co-expressing Shh-C9C5 and MBP at D3 and D6. Interestingly, by using the LPC focal demyelination mouse model, I unraveled a rapid induction of both Shh mRNA and peptide in PDGFRα+ OPCs located into the corpus callosum (cc) during remyelination. Moreover, by invalidating Shh expression, I showed a decrease in MBP and myelin proteolipid protein (PLP) expression at 10 and 20 days post lesion (dpl), suggesting a role of Shh in guiding OLs maturation and myelin production. These data suggest the increase of Shh production by OLs as a new pharmacological target for remyelination.

My work provides the most robust central map of Shh-expressing cells to date and suggests a framework for new studies aimed at understanding new mechanisms implicated in brain maintenance and disease. Moreover, these data provide insight in using Shh modulators to promote OPC recruitment and maturation, facilitating remyelination and functional recovery in demyelinating diseases.

Composition du Jury

• • Dr. Elisabeth Traiffort (President, Paris-Saclay University)
  • Dr. Carlos Parras (Reviewer, Institut du Cerveau)
  • Dr. Marie-Ange Renault (Reviewer, University of Bordeaux)
  • Dr. Paolo Giacobini (Examiner, University of Lille)
  • Dr. Francois Boussin (Examiner, Institut François Jacob, CEA)

et

• Dr. Martial Ruat (Institut NeuroPSI, Saclay)