Effect of the sonic hedgehog receptor smoothened on the survival and function of dopaminergic neurons

Abstract

Objective To determine the influence of the sonic hedgehog (shh) pathway and its receptor smoothened (smo), on the survival and functionality of dopaminergic (DA) neurons. Background During early development, shh induces the differentiation of DA neurons. However, it is unknown whether shh signaling is required in the maturation or maintenance of DA neurons during later development and adulthood due to the lethality of traditional shh knockout models. Methods We utilized the cre-loxP system to achieve late developmental stage and cell type-specific deletion of the shh receptor, smo, in DA neurons by crossing DATcre (dopamine transporter) mice with SmoloxP/loxP mice. We assessed for differences between knockout (ko) and wildtype (wt) mice using combined histochemistry, gene expression analysis, and behavioral evaluation. Number and size of DA neurons in ventral midbrain and the DA neural terminal density in striatum were measured using unbiased stereological quantification. The survival of DA neurons under neurotoxin challenge was examined in the unilateral 6-hydroxydopamine (6-OHDA) Parkinson's disease animal model and the more subtle function under challenge of the dopaminergic system was examined by methamphetamine single- and repeated challenge in wt and ko mice. Results Tyrosine hydroxylase (TH) positive neuronal counts and neuronal size in substantia nigra (SN) and ventral tegmental area (VTA) showed no difference between wt and DAT-Smo ko mice in young (5 months) or aged (22 months) mice. There was also no difference in the striatal DA projections between wt and ko mice in both age groups. In unilateral striatal 6-OHDA lesions modeling Parkinson's disease, using stereotaxic injection of 6-OHDA intrastriatally led to loss of dopaminergic neurons in SN and diminished TH positive projections in striatum. However, there was no differences in survival of DA neurons between wt and ko mice. DAT-Smo ko mice demonstrated hyperactivity compared to wt mice at 5 months, but showed no difference in activity at 22 months. When injected with a one-time bolus of methamphetamine (METH), despite the higher basal locomotion activity, DAT-Smo ko mice showed a diminished response to a single METH challenge. In METH sensitization testing, ko mice showed decreased sensitization compared to wt mice without evidence of a delayed shift in dynamics of sensitization. Gene expression analysis showed decreased gene expression of smo, Gli 1 (known target gene of smo) and BDNF (brain-derived neurotrophic factor) in the SN. Gene expression was not altered in striatum for the genes examined in this study including dopamine receptor genes, neurotropic genes such as Glial cell line-derived neurotrophic factor (GDNF), and bone morphogenetic protein 7 (BMP7). Conclusion Our study showed the smo receptor function is not required for the maturation and survival of DA neurons during late development, aging or under stress challenge. However, smo function has an influence on behavior in young adult mice and in responses of mice to a drug ...

Substantia nigra dopaminergic neurons and striatal interneurons are engaged in three parallel but interdependent postnatal neurotrophic circuits

“…In the absence of Shh during development, striatal interneurons are disturbed (Gonzalez‐Reyes et al, 2012); however, after this critical developmental period, our results demonstrate that the absence of Shh signaling is less dramatic to postnatal striatal interneurons. Similarly, Shh signaling is not required for the survival of DA SNpc in a cell‐autonomous manner after E16 in mice (Zhou et al, 2016), but it is required at this late developmental time for functional specification of DA SNpc (Zhou et al, 2016). Interestingly, Shh has a distinct role over other cholinergic neurons, as an enhancer of postmitotic survival of basal forebrain cholinergic neurons (Reilly, Karavanova, Williams, Mahanthappa, & Allendoerfer, 2002) and as differentiation factor for motor neurons (Ericson, Morton, Kawakami, Roelink, & Jessell, 1996).…”

Expression patterns of key Sonic Hedgehog signaling pathway components in the developing and adult mouse midbrain and in the MN9D cell line

“…Other studies have proposed the scenario that SHH is crucial for the development of mDA neurons exclusively during the time window E8.5–E11, acting as an early patterning molecule of the floor plate and midbrain area (Mesman et al 2014 ; Blaess et al 2006 ), thus supporting the view that SHH is only indirectly involved in the differentiation of mDA neurons. In line with this notion, the smoothened receptor function is apparently not required for the maturation and survival of mDA neurons during late development, aging or under stress challenge (Zhou et al 2016 ). In the study of Hayes et al ( 2011 ), the authors investigated the contribution of the SHH-secreting versus the SHH-responding cells to the mDA neuron domains and proposed that the developmental order from the anterior to the posterior domain is established within the progenitors before they terminally differentiate and that the timing of gene expression of multiple lineages, including SHH and Gli1, is the underlying event for mDA neuron heterogeneity.…”

Heterozygous mutation of Sonic Hedgehog receptor (Ptch) drives cerebellar overgrowth and sex-specifically alters hippocampal and cortical layer structure, activity, and social behavior in female mice

“…Thus, alterations in dopamine release within the mPFC might explain the increased activity and altered social behavior observed in female Ptch +/- mice. Supporting this possibility, mice with conditional inactivation of Smo specifically in dopamine cells are hyperactive (Zhou et al, 2016), whereas dopamine receptor D1 blockade in the mPFC of rats reduces the distance traveled in an open-field task (Hall et al, 2009). Those studies suggest that cerebellar-related increases in mPFC dopamine release might mediate the alterations in activity and social observed in the Ptch1 +/- females.…”