Spatiotemporal pattern of the postnatal astrogliogenesis in the rat hippocampal formation

Astroglia in Autism Spectrum Disorder

Autism spectrum disorder (ASD) is an umbrella term encompassing several neurodevelopmental disorders such as Asperger syndrome or autism. It is characterised by the occurrence of distinct deficits in social behaviour and communication and repetitive patterns of behaviour. The symptoms may be of different intensity and may vary in types. Risk factors for ASD include disturbed brain homeostasis, genetic predispositions, or inflammation during the prenatal period caused by viruses or bacteria. The number of diagnosed cases is growing, but the main cause and mechanism leading to ASD is still uncertain. Recent findings from animal models and human cases highlight the contribution of glia to the ASD pathophysiology. It is known that glia cells are not only "gluing" neurons together but are key players participating in different processes crucial for proper brain functioning, including neurogenesis, synaptogenesis, inflammation, myelination, proper glutamate processing and many others. Despite the prerequisites for the involvement of glia in the processes related to the onset of autism, there are far too little data regarding the engagement of these cells in the development of ASD.

Differential Timing and Coordination of Neurogenesis and Astrogenesis in Developing Mouse Hippocampal Subregions

Neocortical development has been extensively studied and therefore is the basis of our understanding of mammalian brain development. One fundamental principle of neocortical development is that neurogenesis and gliogenesis are temporally segregated processes. However, it is unclear how neurogenesis and gliogenesis are coordinated in non-neocortical regions of the cerebral cortex, such as the hippocampus, also known as the archicortex. Here, we show that the timing of neurogenesis and astrogenesis in the Cornu Ammonis (CA) 1 and CA3 regions of mouse hippocampus mirrors that of the neocortex; neurogenesis occurs embryonically, followed by astrogenesis during early postnatal development. In contrast, we find that neurogenesis in the dentate gyrus begins embryonically but is a protracted process which peaks neonatally and continues at low levels postnatally. As a result, astrogenesis, which occurs during early postnatal development, overlaps with the process of neurogenesis in the dentate gyrus. During all stages, neurogenesis overwhelms astrogenesis in the dentate gyrus. In addition, we find that the timing of peak astrogenesis varies by hippocampal subregion. Together, our results show differential timing and coordination of neurogenesis and astrogenesis in developing mouse hippocampal subregions and suggest that neurogenesis and gliogenesis occur simultaneously during dentate gyrus development, challenging the conventional principle that neurogenesis and gliogenesis are temporally separated processes.

The involvement of astrocytes in early-life adversity induced programming of the brain

Early‐life adversity (ELA) in the form of stress, inflammation, or malnutrition, can increase the risk of developing psychopathology or cognitive problems in adulthood. The neurobiological substrates underlying this process remain unclear. While neuronal dysfunction and microglial contribution have been studied in this context, only recently the role of astrocytes in early‐life programming of the brain has been appreciated. Astrocytes serve many basic roles for brain functioning (e.g., synaptogenesis, glutamate recycling), and are unique in their capacity of sensing and integrating environmental signals, as they are the first cells to encounter signals from the blood, including hormonal changes (e.g., glucocorticoids), immune signals, and nutritional information. Integration of these signals is especially important during early development, and therefore we propose that astrocytes contribute to ELA induced changes in the brain by sensing and integrating environmental signals and by modulating neuronal development and function. Studies in rodents have already shown that ELA can impact astrocytes on the short and long term, however, a critical review of these results is currently lacking. Here, we will discuss the developmental trajectory of astrocytes, their ability to integrate stress, immune, and nutritional signals from the early environment, and we will review how different types of early adversity impact astrocytes.

Maturation of astrocyte morphology and the establishment of astrocyte domains during postnatal hippocampal development

Mature protoplasmic astrocytes exhibit an extremely dense ramification of fine processes, yielding a 'spongiform' morphology. This complex morphology enables protoplasmic astrocytes to maintain intimate relationships with many elements of the brain parenchyma, most notably synapses. Recently, it has been demonstrated that astrocytes establish individual cellular-level domains within the neuropil, with limited overlap occurring between the extents of neighboring astrocytes. The highly ramified nature of protoplasmic astrocytes is closely associated with their ability to create such domains. This study was an attempt to characterize the development of spongiform processes and the establishment of astrocyte domains. A combination of immunolabeling for the astrocyte-specific markers glial fibrillary acidic protein and S100beta with intracellular dye labeling in fixed tissue slices allowed for the identification of immature astrocytes and the elucidation of their complete, well-preserved morphologies. We find that during the first two postnatal weeks astrocytes extend stringy, filopodial processes. Fine, spongiform processes appear during the third week. Protoplasmic astrocytes are quite heterogeneous in morphology at 1-week postnatum, but there is a remarkable consistency in morphology by 2 weeks of age. Finally, protoplasmic astrocytes initially extend long, overlapping processes during the first two postnatal weeks. The subsequent elaboration of spongiform processes results in the development of boundaries between neighboring astrocyte domains. Stray processes that encroach on neighboring domains are eventually pruned by 1 month of age. These observations suggest that domain formation is largely the consequence of competition between astrocyte processes, similar to the well-studied competitive interactions between certain neuronal dendritic fields.

Hippocampal granule neuron production and population size are regulated by levels of bFGF

Numerous studies of the proliferative effects of basic fibroblast growth factor (bFGF) in culture, including neonatal and adult hippocampal precursors, suggest that the factor plays a ubiquitous and life-long role in neurogenesis. In contrast, in vivo, bFGF is devoid of effects on neurons in mature hippocampus, raising the possibility that bFGF exhibits developmental stage-specific activity in the complex animal environment. To define neurogenetic effects in the newborn, a single subcutaneous injection of bFGF (20 ng/gm) was administered to postnatal day 1 (P1) rats, and hippocampal DNA content was quantified: bFGF elicited an increase in total DNA throughout adulthood, by 48% at P4, 25% at P22, and 17% at P180, suggesting that bFGF increases hippocampal cell number. To define mechanisms, bromodeoxyuridine (BrdU) was injected at P1 and mitotically labelled cells were assessed at P22: there was a twofold increase in BrdU-positive cells in the dentate granule cell layer (GCL), indicating that bFGF enhanced the generation of neurons, or neuronogenesis, from a cohort of precursors. Moreover, enhanced mitosis and survival led to a 33% increase in absolute GCL neuron number, suggesting that neuron production depends on environmental levels of bFGF. To evaluate this possibility, bFGF-knockout mice were analyzed: hippocampal DNA content was decreased at all ages examined (P3, -42%; P21, -28%; P360, -18%), and total GCL neuron and glial fibrillary acidic protein (GFAP)-positive cell number were decreased by 30%, indicating that bFGF is necessary for normal hippocampal neurogenesis. We conclude that environmental levels of bFGF regulate neonatal hippocampal neurogenesis. As adult hippocampal neuronogenesis was unresponsive to bFGF manipulation in our previous study [Wagner, J.P., Black, I.B. & DiCicco-Bloom, E. (1999) J. Neurosci., 19, 6006], these observations suggest distinct, stage-specific roles of bFGF in the dentate gyrus granule cell lineage.