Abstract
Gonadotropin-releasing hormone (GnRH) neurons are the central regulators of reproduction in vertebrates. GnRH cell bodies reside in the basal forebrain, and most extend long neurites in the caudal direction to terminate in the median eminence (ME) within the medial basal hypothalamus (MBH), the site of hormone secretion. Using immunohistochemistry, in vitro culture and in vivo genetic deletion strategies, my thesis investigated the role of a morphogen, Sonic Hedgehog (Shh) in the growth of GnRH neurites to the MBH across development. Dual label immunohistochemistry revealed that Shh was present in the basal forebrain, specifically in the preoptic area (POA), ventral hypothalamic neuroepithelium (VHN) and ventromedial hypothalamus (VMH) between embryonic day 14.5 (E14.5) and E16.5, when GnRH neurite extension is at its peak. At E14.5, both GnRH cell bodies and GnRH neurites were found to be within or close to the Shh-expressing domain in the basal forebrain. The coincidence of Shh and GnRH neurons in the basal forebrain suggests that Shh may signal to these embryonic GnRH neurons to regulate their development.
To investigate this, a transgenic mouse line with the Shh receptor, Smoothened (Smo) deleted specifically from GnRH neurons (GnRH-Smo) was generated by using Cre-loxP technology. GnRH-Smo mice displayed normal GnRH cell body distributions, as there was no change in GnRH cell count or distribution pattern along the anterior- posterior or the medio-lateral axis in the basal forebrain between WT and KO mice. However, GnRH-Smo mice showed a significant decrease in GnRH innervation of the MBH at E18.5, but the innervation density was restored to WT levels by P0. This suggests that GnRH neurons suffer a delay in neurite growth when Shh/Smo signaling is abolished during their development. An acute brain slice preparation was adapted to examine the downstream signaling mechanisms by which Shh affects GnRH neurite growth. Preliminary data indicate that Shh activates the non-canonical pathway via phospho-activation of Src family kinase in developing GnRH neurons. This observation is consistent with a local role of Shh signaling in the growth of neurites, where it is envisioned that Shh acts via Smo to activate a Src-dependent signaling cascade to stimulate GnRH neurite growth.
Finally, I tested the functional significance of the Smo-deletion-dependent perturbed GnRH neurite development on the activity of the GnRH neuronal network. Female GnRH-Smo mice underwent puberty despite an approximately 30% delay in GnRH innervation of the MBH during development. This is not surprising since GnRH innervation of MBH was restored by birth in GnRH-Smo KO mice and a previous study has shown that only ~25% of the normal number of GnRH neurons is sufficient to drive the reproduction axis.
Taken together, the experiments performed for this thesis demonstrate that embryonic GnRH neurons use Shh for their development, and that Shh stimulates GnRH neurite growth by potentially acting through a non-canonical Shh signaling pathway. These studies provide further understanding of the molecular mechanisms by which GnRH nerve terminals arrive at the MBH, and identify an additional neuronal population whose neurites utilise Shh/Smo signaling for their development.