Murine features of neurogenesis in the human hippocampus across the lifespan from 0 to 100 years.

BACKGROUND: Essentially all knowledge about adult hippocampal neurogenesis in humans still comes from one seminal study by Eriksson et al. in 1998, although several others have provided suggestive findings. But only little information has been available in how far the situation in animal models would reflect the conditions in the adult and aging human brain. We therefore here mapped numerous features associated with adult neurogenesis in rodents in samples from human hippocampus across the entire lifespan. Such data would not offer proof of adult neurogenesis in humans, because it is based on the assumption that humans and rodents share marker expression patterns in adult neurogenesis. Nevertheless, together the data provide valuable information at least about the presence of markers, for which a link to adult neurogenesis might more reasonably be assumed than for others, in the adult human brain and their change with increasing age. METHODS AND FINDINGS: In rodents, doublecortin (DCX) is transiently expressed during adult neurogenesis and within the neurogenic niche of the dentate gyrus can serve as a valuable marker. We validated DCX as marker of granule cell development in fetal human tissue and used DCX expression as seed to examine the dentate gyrus for additional neurogenesis-associated features across the lifespan. We studied 54 individuals and detected DCX expression between birth and 100 years of age. Caveats for post-mortem analyses of human tissues apply but all samples were free of signs of ischemia and activated caspase-3. Fourteen markers related to adult hippocampal neurogenesis in rodents were assessed in DCX-positive cells. Total numbers of DCX expressing cells declined exponentially with increasing age, and co-expression of DCX with the other markers decreased. This argued against a non-specific re-appearance of immature markers in specimen from old brains. Early postnatally all 14 markers were co-expressed in DCX-positive cells. Until 30 to 40 years of age, for example, an overlap of DCX with Ki67, Mcm2, Sox2, Nestin, Prox1, PSA-NCAM, Calretinin, NeuN, and others was detected, and some key markers (Nestin, Sox2, Prox1) remained co-expressed into oldest age. CONCLUSIONS: Our data suggest that in the adult human hippocampus neurogenesis-associated features that have been identified in rodents show patterns, as well as qualitative and quantitative age-related changes, that are similar to the course of adult hippocampal neurogenesis in rodents. Consequently, although further validation as well as the application of independent methodology (e.g. electron microscopy and cell culture work) is desirable, our data will help to devise the framework for specific research on cellular plasticity in the aging human hippocampus.

Neurogranin is expressed by principal cells but not interneurons in the rodent and monkey neocortex and hippocampus.

As a substrate of protein kinase C (PKC), neurogranin (NG) is involved in the regulation of calcium signaling and activity-dependent plasticity. Recently, we have shown that, in the rodent cerebellum, NG is exclusively expressed by gamma-aminobutyric acidergic Golgi cells, whereas, in the monkey cerebellum, brush cells were the only neuronal population expressing NG (Singec et al. [2003] J. Comp. Neurol. 459:278-289). In the present study, we analyzed the neocortical and hippocampal expression patterns of NG in adult mouse (C57Bl/6), rat (Wistar), and monkey (Cercopithecus aetiops). By using immunocytochemistry and nonradioactive in situ hybridization, we demonstrate strong NG expression by principal cells in different neocortical layers and in the hippocampus by granule cells of the dentate gyrus and pyramidal neurons of CA1-CA3. In contrast, double-labeling experiments in rodents revealed that neocortical and hippocampal interneurons expressing glutamate decarboxylase 67 (GAD67) were consistently devoid of NG. In addition, by using antibodies against parvalbumin, calbindin, and calretinin, we could demonstrate the absence of NG in interneurons of monkey frontal cortex and hippocampus. Together these findings corroborate the idea of different calcium signaling pathways in excitatory and inhibitory cells that may contribute to different modes of synaptic plasticity in these neurons.

Neurogranin expression by cerebellar neurons in rodents and non-human primates.

Neurogranin (NG) is a brain-specific protein kinase C substrate involved in the regulation of calcium signaling and neuronal plasticity. A rostrocaudal expression profile, with large amounts in telencephalic brain regions and low expression levels in phylogenetically older brain structures, was reported previously. In the cerebellum, expression of NG has not been described. By using immunocytochemistry and in situ hybridization, we found that NG is expressed in the mouse (C57Bl/6), rat (Wistar), and monkey (Cercopithecus aetiops) cerebella. In the mouse cerebellum, Golgi cells were strongly immunoreactive for NG, whereas other cerebellar neurons were devoid of this protein. Cell counts showed 1.6-fold more immunopositive Golgi cells in the hemispheres (61.1 +/- 8.0 cells/mm(2)) than in the vermis (37.5 +/- 3.3 cells/mm(2)). Developmental studies showed detectable NG in the mouse cerebellum as early as on postnatal day 10 (P10). In contrast to the mouse, in the rat cerebellum we found only a few Golgi cells containing NG (hemispheres, 2.4 +/- 0.5 cells/mm(2); vermis, 1.5 +/- 0.3 cells/mm(2)). In the monkey cerebellum, unipolar brush cells, localized in the granular layer, were heavily labeled, whereas Golgi cells were devoid of NG. This study demonstrated that NG is strongly expressed in specific gamma-aminobutyric acidergic neurons in the rodent cerebellum. In addition, NG expression in the primate cerebellum by brush cells, which are excitatory, showed remarkable cell type-specific and species-specific expression patterns of a postsynaptic protein mediating calcium signaling mechanisms.

Synaptic vesicle protein synaptoporin is differently expressed by subpopulations of mouse hippocampal neurons.

In the hippocampus, the synaptic vesicle protein synaptoporin (SPO) has been reported to be exclusively enriched in the granule cell axons, the mossy fibers. In this study, we show that in adult rats and mice SPO immunoreactivity (IR) is also detectable in strata oriens, radiatum, and lacunosum-moleculare of CA1-CA3, as well as perisomatically in the hippocampus proper and fascia dentata. In situ hybridization confirmed that SPO mRNA was present in granule cells and CA3 pyramidal cells but not in CA1 pyramidal cells. Importantly, cells scattered throughout the hippocampal layers resembling the distribution of interneurons were found to synthesize high amounts of SPO mRNA, too. Thus, these findings indicate that SPO expression in the hippocampus was underestimated until now. Moreover, double-labeling immunohistochemistry and confocal microscopy revealed selective colocalization of SPO and glutamate decarboxylase (GAD 65), a marker for gamma-aminobutyric acid (GABA)ergic terminals. To identify SPO expressing interneurons, in situ hybridization was combined with immunocytochemistry against parvalbumin (PV), calbindin (CB), calretinin (CR), cholecystokinin (CCK), and vasoactive intestinal polypeptide (VIP). We found that SPO transcripts were differentially expressed by various interneuron subpopulations in the hippocampus of C57Bl/6 mice (PV 44.2%, CB 46.3%, CR 19.3%, CCK 38.6%, VIP 59.9%). Immunoelectron microscopy for SPO labeled synaptic vesicle profiles in distinct symmetric and asymmetric synapses. In conclusion, our data demonstrate that hippocampal principal cells and interneurons display a variety of synaptic vesicles that are likely to contribute to the functional characteristics of their output synapses.