Chronic up-regulation of the SHH pathway normalizes some developmental effects of trisomy in Ts65Dn mice

Abstract

Down Syndrome (DS) is a highly complex developmental genetic disorder caused by trisomy for human chromosome 21 (Hsa21). All individuals with DS exhibit some degree of brain structural changes and cognitive impairment; mouse models such as Ts65Dn have been instrumental in understanding the underlying mechanisms. Several phenotypes of DS might arise from a reduced response of trisomic cells to the Sonic Hedgehog (SHH) growth factor. If all trisomic cells show a similar reduced response to SHH, then up-regulation of the pathway in trisomic cells might ameliorate multiple DS phenotypes. We crossed Ptch1tm1Mps/+ mice, in which the canonical SHH pathway is expected to be up-regulated in every SHH-responsive cell due to the loss of function of one allele of the pathway suppressor, Ptch1, to the Ts65Dn DS model and assessed the progeny for possible rescue of multiple DS-related phenotypes. Down-regulation of Ptch produced several previously unreported effects on development by itself, complicating interpretation of some phenotypes, and a number structural or behavioral effects of trisomy were not compensated by SHH signaling. However, a deficit in a nest-building task was partially restored in Ts;Ptch+/− mice, as were structural anomalies of the cerebellum in Ts65Dn mice. These results extend the body of evidence indicating that reduced response to SHH in trisomic cells and tissues contributes to various aspects of the trisomic phenotype.

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

“…We crossed Ts65Dn and Ptch1 tm1Mps/+ mice to generate four genotypes: Ts65Dn mice (Ts;WT); Ts65Dn mice haploinsufficient for Ptch1 (Ts; Ptch +/− ); euploid control (Eu;WT); and euploid haploinsufficient for Ptch1 allele (Eu; Ptch +/− ) (Table ) (Dutka et al, ). Three‐dimensional coordinates of biological landmarks measured on the skulls of adult mice were collected from micro‐computed tomography (micro‐CT) images and analyzed (Fig.…”

“…The multivariate regression analysis (Fig. ) does capture some size differences between the Eu;WT and Eu; Ptch +/− mice, with the latter showing a marked increase in size (on the higher end of the centroid size axis), suggesting that haploinsufficiency for Ptch1 does, to an extent, affect craniofacial size in the euploid mice (Dutka et al, ). This analysis also indicates that size related changes in cranial shape (i.e., allometry) are different between the euploid and trisomic mice.…”

“…The B6;129‐ Ptch1 tm1Mps / J mice were also obtained from the Jackson Laboratory, backcrossed for five generations onto a B6 background, and then crossed with C3H mice to create an F1 generation of B6C3H mice. 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 ).…”

“…Following the premise that if all trisomic cells have a similarly attenuated response to SHH signaling, up‐regulation of the HH pathway might normalize the developmental, including craniofacial, anomalies associated with DS, Dutka et al () crossed Ptch1 tm1Mps /+ with Ts65Dn mice to yield Ts; Ptch +/− mice (Table ). The resulting increase of SHH signaling in Ts; Ptch +/− mice improved nest building activities and corrected structural defects in the cerebellum compared with Ts65Dn (Dutka et al, ), partly replicating the results previously reported for trisomic mice acutely exposed to the SHH agonist, SAG (Roper et al, ; Das et al, ). 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, ).…”

Sonic Hedgehog Signaling and Hippocampal Neuroplasticity

“…However, a deficit in a cerebellum-dependent motor learning task (phase reversal adaptation and consolidation of the vestibulo-ocular reflex) is not ameliorated by postnatal SAG treatment in Ts65Dn mice, demonstrating brain region/neuronal circuit-dependent effects of Shh signaling in this DS model [53]. Consistent with complex roles for Shh in DS are data showing that crossing of Ts65Dn mice with mice with reduced Ptch expression (which increases Shh signaling) normalizes some, but not all, of the brain structural and behavioral phenotypes in the DS mice [54]. …”