Cellular Evaluation of Postnatal Frontal Cortex in Down Syndrome

Down syndrome (DS) is a common genetic developmental disorder characterized by the trisomy of chromosome 21 (Hsa21). All individuals with DS have some kind of intellectual disability, associated with dysfunction in cognition-related structures, including the frontal cortex. Studies have examined developmental changes in the frontal cortex during prenatal stages in DS, however little is known about cortical lamination and neuronal differentiation in postnatal periods in this neurodevelopmental disorder. Therefore, we examined the quantitative and qualitative distribution of neuronal profiles containing the neuronal migration protein doublecortin (DCX), the non-phosphorylated high-molecular-weight neurofilament SMI-32, the calcium-binding proteins calbindin D-28K (Calb), calretinin (Calr), and parvalbumin (Parv), as well as human β-amyloid and APP (6E10), Aβ1-42, and phospho-tau (CP-13) in the supragranular (SG, II/III) and infragranular (IG, V/VI) layers in the DS postnatal frontal cortex compared to neurotypically developing (NTD) controls from ages 28 weeks to 196.4 weeks using immunohistochemistry. Furthermore, cortical lamination was evaluated using thionin, a Nissl stain. We found DCX-immunoreactive (-ir) cells in both the SG and IG layers in younger cases, but not in the oldest cases in both groups. Strong expression of SMI-32 immunoreactivity was observed in pyramidal cells in layers III and V in the oldest cases in both groups, however SMI-32-ir cells appeared much earlier in NTD compared to DS. We found small and fusiform Calb-ir cells in the younger cases (28 to 44 weeks), while in the oldest cases, Calb immunoreactivity was also found in pyramidal cells. Calr-ir cells appeared earlier in DS at 32 weeks compared to NTD at 44 weeks, however both groups showed large bipolar fusiform-shaped Calr-ir cells in the oldest cases. Diffuse APP/Aβ-ir plaque-like accumulations were found in the frontal cortex grey and white matter at all ages, but no Aβ1-42 immunoreactivity was detected in any case. Furthermore, neuropil (but not cellular) granular CP-13 immunostaining was seen in layer I only at 41 weeks NTD and 33 weeks DS. Cell counts show a significantly higher cell number in SG compared to IG for all the neuronal markers in both groups, except in Calb and SMI-32. In NTD, age and brain weight showed the strongest correlations with all cellular counts, except in thionin where DS had a stronger negative correlation with age and brain weight compared to NTD. In addition, height and body weight showed a strong negative correlation in NTD with the migration and neurogenesis marker DCX. These findings suggest that trisomy 21 affects the postnatal frontal cortex lamination, neuronal migration<br/>eurogenesis, and differentiation of projection pyramidal cells and interneurons, which contribute to the disruption of the local and projection inhibitory and excitatory circuitries that may underlie the cognitive disabilities in DS.