New neurons in old brains: implications of age in the analysis of neurogenesis in post-mortem tissue

Abstract

Adult neurogenesis, the proliferation and integration of newly generated neurons, has been observed in the adult mammalian hippocampus of many species. Numerous studies have also found adult neurogenesis in the human hippocampus, but several recent high-profile studies have suggested that this process is considerably reduced in humans, occurring in children but not in adults. In comparison, rodent studies also show age-related decline but a greater degree of proliferation of new neurons in adult animals. These differences may represent biological species differences or could alternatively be explained by methodological differences in tissue handling and fixation. Here, we examine whether differences in the post-mortem interval between death and tissue fixation might impact subsequent detection of adult neurogenesis due to increased tissue degradation. Because there are fewer new neurons present in older subjects to begin with we hypothesized that, subject age might interact significantly with post-mortem interval in the detection of adult neurogenesis. We analyzed neurogenesis in the hippocampus of rats that were either perfusion-fixed or the brains extracted and immersion-fixed at various post-mortem intervals. We observed an interaction between animal age and the time delay between death and tissue fixation. While similar levels of neurogenesis were observed in young rats regardless of fixation, older rats had significantly fewer labeled neurons when fixation was not immediate. Furthermore, the morphological detail of the labeled neurons was significantly reduced in the delayed fixation conditions at all ages. This study highlights critical concerns that must be considered when using post-mortem tissue to quantify adult neurogenesis.

Methods to study adult hippocampal neurogenesis in humans and across the phylogeny

“…For instance, although NR2A and NR2B subunits appear to be rapidly degraded after death (Wang et al, 2000), most subunits of N‐methyl‐ d ‐aspartate (NMDA) and α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionic acid (AMPA)‐type glutamate receptors are stable at ~18 h PMD. Studies performed by Boekhoorn et al (2006) and Terstege et al (2022) suggested enhanced susceptibility of DCX to postmortem degradation. These authors observed that, in rat samples with artificially induced prolonged PMDs (≥8 h), DCX staining disappears from the dendrites and is restricted to the soma and nucleus of immature DGCs.…”

“…However, subsequent research revealed that, although certain proteins may show particularly rapid postmortem degradation (Boekhoorn et al, 2006;Sorensen, 1984;Terstege et al, 2022), especially in certain regions of the brain (Siew et al, 2004), the immunodetection of most proteins and enzymes is moderately resistant to this phenomenon (Blair et al, 2016;Ritchie et al, 1986;Schut et al, 2017;Wang et al, 2000). For instance, although NR2A and NR2B subunits appear to be rapidly degraded after death (Wang et al, 2000), most subunits of N-methyl-Daspartate (NMDA) and α-amino-3-hydroxy-5-methyl- reported that this phenomenon is accentuated in aged animals (Terstege et al, 2022). DCX is detected in the human DG at prolonged PMD intervals (Flor-Garcia et al, 2020;Moreno-Jimenez et al, 2019Terreros-Roncal et al, 2021).…”

“…4-isoxazolepropionic acid (AMPA)-type glutamate receptors are stable at $18 h PMD. Studies performed byBoekhoorn et al (2006) andTerstege et al (2022) suggested enhanced susceptibility of DCX to postmortem degradation. These authors observed that, in rat samples with artificially induced prolonged PMDs (≥8 h), DCX staining disappears from the dendrites and is restricted to the soma and nucleus of immature DGCs.Terstege et al …”

Analysis of Adult Hippocampal Neurogenesis in Neurodegenerative disease

“…Interestingly, from their published images, similar DCX + cell shapes were observed. PMI is another key element for Human IHC work (Terstege et al, 2022). Fresh tissue is reported to allow for the detection of DCX via IHC (Terstege et al, 2022).…”

“…PMI is another key element for Human IHC work (Terstege et al, 2022). Fresh tissue is reported to allow for the detection of DCX via IHC (Terstege et al, 2022). Moreno-Jimenez et al selected tissues which only have 2.5 hours to 10 hours of PMI (Moreno-Jimé nez et al, 2019).…”