Abstract
To determine whether neurons lacking huntingtin can participate in development and survive in postnatal brain, we used two approaches in an effort to create mice consisting of wild-type cells and cells without huntingtin. In one approach, chimeras were created by aggregating the 4-8 cell embryos from matings of Hdh (+/-) mice with wild-type 4-8 cell embryos. No chimeric offspring that possessed homozygous Hdh (-/-) cells were obtained thereby, although statistical considerations suggest that such chimeras should have been created. By contrast, Hdh (-/-) ES cells injected into blastocysts yielded offspring that were born and in adulthood were found to have Hdh (-/-) neurons throughout brain. The Hdh (-/-) cells were, however, 5-10 times more common in hypothalamus, midbrain, and hindbrain than in telencephalon and thalamus. Chimeric animals tended to be smaller than wild-type littermates, and chimeric mice rich in Hdh (-/-) cells tended to show motor abnormalities. Nonetheless, no brain malformations or pathologies were evident. The apparent failure of aggregation chimeras possessing Hdh (-/-) cells to survive to birth is likely attributable to the previously demonstrated critical role of huntingtin in extraembryonic membranes. That Hdh (-/-) cells in chimeric mice created by blastocyst injection are under-represented in adult telencephalon and thalamus implies a role for huntingtin in the development of these regions, whereas the neurological dysfunction in brains enriched in Hdh (-/-) cells suggests a role for huntingtin in adult brain. Nonetheless, the lengthy survival of Hdh (-/-) cells in adult chimeric mice indicates that individual neurons in many brain regions do not require huntingtin to participate in normal brain development and to survive.
Highlights
To determine whether neurons lacking huntingtin can participate in development and survive in postnatal brain, we used two approaches in an effort to create mice consisting of wildtype cells and cells without huntingtin
To distinguish chimeras bearing Hdh Ϫ/Ϫ cells from those bearing Hdh ϩ/Ϫ cells, we believed that PCR genotyping of DNA isolated from a tail biopsy would be inadequate, because wild-type cells in the tail biopsy would hide evidence of homozygous Hdh knock-out cells
It is of note that, of the 21 chimeric mice created by aggregation methods that were born, the percentage of contribution from the embryos derived from the Hdh ϩ/Ϫ cross ranged from 5 to 95%
Summary
To determine whether neurons lacking huntingtin can participate in development and survive in postnatal brain, we used two approaches in an effort to create mice consisting of wildtype cells and cells without huntingtin. Chimeras were created by aggregating the 4–8 cell embryos from matings of Hdh ϩ/Ϫ mice with wild-type 4–8 cell embryos. That Hdh Ϫ/Ϫ cells in chimeric mice created by blastocyst injection are under-represented in adult telencephalon and thalamus implies a role for huntingtin in the development of these regions, whereas the neurological dysfunction in brains enriched in Hdh Ϫ/Ϫ cells suggests a role for huntingtin in adult brain. The lengthy survival of Hdh Ϫ/Ϫ cells in adult chimeric mice indicates that individual neurons in many brain regions do not require huntingtin to participate in normal brain development and to survive.
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