Acute upregulation of hedgehog signaling in mice causes differential effects on cranial morphology

Abstract

Hedgehog (HH) signaling, and particularly signaling by sonic hedgehog (SHH), is implicated in several essential activities during morphogenesis, and its misexpression causes a number of developmental disorders in humans. In particular, a reduced mitogenic response of cerebellar granule cell precursors to SHH signaling in a mouse model for Down syndrome (DS), Ts65Dn, is substantially responsible for reduced cerebellar size. A single treatment of newborn trisomic mice with an agonist of the SHH pathway (SAG) normalizes cerebellar morphology and restores some cognitive deficits, suggesting a possible therapeutic application of SAG for treating the cognitive impairments of DS. Although the beneficial effects on the cerebellum are compelling, inappropriate activation of the HH pathway causes anomalies elsewhere in the head, particularly in the formation and patterning of the craniofacial skeleton. To determine whether an acute treatment of SAG has an effect on craniofacial morphology, we quantitatively analyzed the cranial form of adult euploid and Ts65Dn mice that were injected with either SAG or vehicle at birth. We found significant deformation of adult craniofacial shape in some animals that had received SAG at birth. The most pronounced differences between the treated and untreated mice were in the midline structures of the facial skeleton. The SAG-driven craniofacial dysmorphogenesis was dose-dependent and possibly incompletely penetrant at lower concentrations. Our findings illustrate that activation of HH signaling, even with an acute postnatal stimulation, can lead to localized dysmorphology of the skull by generating modular shape changes in the facial skeleton. These observations have important implications for translating HH-agonist-based treatments for DS.

Chronic up‐regulation of sonic hedgehog has little effect on postnatal craniofacial morphology of euploid and trisomic mice

“…Eu; Ptch +/− mice also have slightly raised and rounded cranial vaults compared with the “flatter” vaults of Eu;WT mice, suggesting that, while subtle, the cranial changes caused by the absence of one Ptch allele are similar in both the Eu; Ptch +/− and Ts; Ptch +/− . Given that substantial perturbations in SHH signaling can cause serious craniofacial defects (Hu and Helms, ; Hu et al, ; Tapadia et al, ; Cordero et al, ; Singh et al, ), our results are surprising in that we did not find overt changes in the facial morphology of the Eu; Ptch1 +/− mice compared with Eu;WT.…”

“…The subset of nine SAG treated euploid mice (orange dots on the positive end of PC1) shared a snout morphology distinct from all the other mice. These nine animals had retracted and depressed nasal bones, medio‐laterally expanded snouts and prominent ridging along the lateral aspects of the fronto‐nasal‐premaxillary junction (Singh et al, ); none of the Ptch +/− mice (euploid or trisomic) showed shape deformations similar to the changes exhibited by the affected SAG‐treated mice. PC2 (18.9%) separated all the trisomic mice, regardless of genotype, from all the euploid mice (Fig.…”

“…The Ptch1 tm1Mps/+ mice were maintained in the Reeves' laboratory colony as a B6 × C3H advanced intercross (Dutka et al, ). The B6C3H SAG and Vehicle (Veh) treated euploid mice were also maintained in the Reeves' laboratory and alternate litters were injected with either 20 μg/g of SAG or Veh, subcutaneously at the back of the skull within hours after birth (Das et al, ; Singh et al, ) (Table ). Between 10 and 18 weeks of age, all the mice in the dataset were anesthetized with isoflourane and euthanized by cervical dislocation or perfusion.…”

“…However, in contrast to trisomic mice that were treated with SAG, the Ts; Ptch +/− mice did not show improvement in behavioral activities mediated by the hippocampus (Das et al, ; Dutka et al, ). Moreover, despite the beneficial effects of SAG on cerebellar morphology of Ts65Dn mice, further examination of SAG‐treated euploid mice (controls) revealed that acute up‐regulation of the pathway caused dysmorphology in the midline structures of the face in some treated mice (Singh et al, ).…”

Facial shape differences between rats selected for tame and aggressive behaviors

“…In this regard, experimental models for domestication can provide ways to test the role of NCC and other developmental pathways in generating traits most commonly found in domesticated animals. For example, perturbations in the sonic hedgehog pathway [39–43], fibroblast growth factors and their respective receptors [44–46] and bone morphogenetic proteins [47–49] affect growth and development of the craniofacial form, but the role of NCC in regulating these important developmental networks still needs further investigation [50]. …”