Cellular Evaluation of Postnatal Hippocampus in Down Syndrome

Down Syndrome (DS), caused by the trisomy 21, is the most common intellectual developmental disorder. Children with DS display deficits in ample memory tasks attributed to alterations in memory-related brain structures, including the hippocampus. Although, many studies in DS focused on development of the brain during prenatal stages, little is known about the cellular evolution of the hippocampus in postnatal periods in DS. Therefore, here we examined the neurochemical spatiotemporal development of neuronal profiles in pediatric postnatal hippocampus in DS and neurotypical developing (NTD) controls. A quantitative and qualitative neuronal distribution was performed in hippocampal sections containing the proper hippocampus, dentate gyrus (DG) and subiculum obtained at autopsy from 1 day to 3 year-old infants in DS and NTD age-matched controls using antibodies against the non-phosphorylated high-molecular-weight neurofilament, a marker of differentiated neurons (SMI-32), the calcium binding protein calbindin D-28k (CAB), and the migration neuronal marker microtubule-associated protein doublecortin (DCX). In addition, Aβ and phosphorylated tau was also immunohistochemically examined in the hippocampus using 6E10, Aβ1-42 and the phosphorylated CP-13 and AT8 tau antibodies, respectively. We found APP/Aβ immunoreactivity, but not Aβ1-42, in diffuse-like plaques in the hippocampus from 1 day to 3 year old infants and young children in DS and NTD cases. By contrast, phosphorylated fetal tau was not immunodetected in the hippocampus at any age in both groups. SMI-32 immunolabeled neurons were observed in the hilus, CA2 field and subiculum in early postnatal cases in DS and NTD. The number of SMI-32 immunoreactive (ir) granule cells in the DG were significantly decreased in DS compared to NTD. While a strong DCX immunoreactivity was observed in the granule cells of the DG in the hippocampus in both groups at early postnatal stages, a more accelerated reduction was observed in DS. CAB-ir neuronal distribution in the postnatal hippocampus was comparable between the youngest and the oldest infants in NTD and DS. In addition, strong positive correlations were observed between DG-DCX-ir cells numbers and both DG-CAB-ir and DG-SMI-32-ir values as well as negative correlations between the brain weight and DG granule cell-ir numbers for all markers in DS. These findings suggest that neuronal maturation and migration in the hippocampus are compromised in early postnatal stages of the development in DS and may contribute to the intellectual disabilities observed in this group.