Forebrain Shh overexpression improves cognitive function and locomotor hyperactivity in an aneuploid mouse model of Down syndrome and its euploid littermates

Abstract

Down syndrome (DS) is the leading genetic cause of intellectual disability and causes early-onset dementia and cerebellar hypoplasia. The prevalence of attention deficit hyperactivity disorder is elevated in children with DS. The aneuploid DS mouse model “Ts65Dn” shows prominent brain phenotypes, including learning and memory deficits, cerebellar hypoplasia, and locomotor hyperactivity. Previous studies indicate that impaired Sonic hedgehog (Shh) signaling contributes to neurological phenotypes associated with DS and neurodegenerative diseases. However, because of a lack of working inducible Shh knock-in mice, brain region-specific Shh overexpression and its effects on cognitive function have not been studied in vivo. Here, with Gli1-LacZ reporter mice, we demonstrated that Ts65Dn had reduced levels of Gli1, a sensitive readout of Shh signaling, in both hippocampus and cerebellum at postnatal day 6. Through site-specific transgenesis, we generated an inducible human Shh knock-in mouse, TRE-bi-hShh-Zsgreen1 (TRE-hShh), simultaneously expressing dually-lipidated Shh-Np and Zsgreen1 marker in the presence of transactivator (tTA). Double transgenic mice “Camk2a-tTA;TRE-hShh” and “Pcp2-tTA;TRE-hShh” induced Shh overexpression and activated Shh signaling in a forebrain and cerebellum, respectively, specific manner from the perinatal period. Camk2a-tTA;TRE-hShh normalized locomotor hyperactivity and improved learning and memory in 3-month-old Ts65Dn, mitigated early-onset severe cognitive impairment in 7-month-old Ts65Dn, and enhanced spatial cognition in euploid mice. Camk2a-tTA;TRE-hShh cohort maintained until 600days old showed that chronic overexpression of Shh in forebrain from the perinatal period had no effect on longevity of euploid or Ts65Dn. Pcp2-tTA;TRE-hShh did not affect cognition but mitigated the phenotype of cerebellar hypoplasia in Ts65Dn. Our study provides the first in vivo evidence that Shh overexpression from the perinatal period protects DS brain integrity and enhances learning and memory in normal mice, indicating the broad therapeutic potential of Shh ligand for other neurological conditions. Moreover, the first inducible hShh site-specific knock-in mouse could be widely used for spatiotemporal Shh signaling regulation.

Overexpression screen of chromosome 21 genes reveals modulators of Sonic hedgehog signaling relevant to Down syndrome

“…Our results also provide evidence for why Down syndrome mouse models present with variable severities of cerebellar hypoplasia. Ts65Dn mice and Ts1Cje mice possess a similar reduction of cerebellar volume normalized to total brain volume ( 7, 11, 12, 46-49 ). Ms1Cje/Ts65Dn mice do not show substantial cerebellar hypoplasia, although only three trisomic animals were analyzed ( 46 ).…”

“…Our results also provide evidence for why Down syndrome mouse models present with variable severities of cerebellar hypoplasia. Ts65Dn mice and Ts1Cje mice possess a similar reduction of cerebellar volume normalized to total brain volume (7,11,12,(46)(47)(48)(49).…”

“…1). Ts65Dn mice and Ts1Cje mice possess a similar reduction of cerebellar volume normalized to total brain volume (7,11,12,(46)(47)(48)(49).…”

“…Consistent with this hypothesis, a single treatment with the SHH pathway agonist SAG on the day of birth rescues adult cerebellar morphology, performance on the Morris water maze, and some aspects of hippocampal long-term potentiation in Ts65Dn mice ( 11 ). Similarly, overexpression of a SHH transgene in the forebrain improves learning and memory in Ts65Dn mice, whereas overexpression of this SHH transgene in cerebellar Purkinje cells increases cerebellar volume but does not improve performance on learning and memory tasks ( 12 ). Another recent study showed that increasing the concentration of SAG normalizes expression of OLIG2 , a chromosome 21 gene critical for oligodendrocyte development, in “brain-like” neural progenitors derived from human trisomy 21 iPSCs ( 13 ).…”

The first transchromosomic rat model with human chromosome 21 shows robust Down syndrome features

“…Drugs for treating central nervous system disorders have low approval rates 17 . The use of trisomic DS mouse models has identified several potential therapeutic candidates to improve learning and memory in people with trisomy 21 [18][19][20][21][22] . Animal models that support more robust physiological and behavioral analysis will provide an essential layer of preclinical validation, with improved outcomes for drug development.…”