Hippocampal astrogenesis and astrocyte dynamic - from development to adult and aging stages

Astrocytes are highly abundant in the mammalian brain and their functions are of vital importance for all aspects of development, adaption and aging of the central nervous system (CNS). Mounting evidence indicates the astrocytes’ cardinal contribution to a wide range of neuropathies, still, our understanding of astrocyte development significantly lags behind those of other CNS cells. For this project, we combined immunohistochemical approaches with genetic fate-mapping, behavioural paradigms, single cell transcriptomics and in vivo two-photon imaging, to comprehensively assess the generation and proliferation of astrocytes in the DG across the lifespan of a mouse. Astrogenesis in the DG is initiated by radial glia-like neural stem cells giving rise to locally dividing astrocytes that enlarge the astrocyte compartment in an outside-in-pattern. Also in the adult DG, the vast majority of astrogenesis is mediated through proliferation of local astrocytes. Interestingly, locally dividing astrocytes revealed an unexpected plasticity and were able to adapt their proliferation to environmental and behavioural stimuli. Our study establishes astrocytes as enduring plastic elements in DG circuits, implicating a vital contribution of astrocyte dynamics to hippocampal plasticity.

 

Hippocampal astrocyte diversity – from morphology to function

Neuronal heterogeneity is a well-described phenomenon and has been established as a pillar of higher CNS function. In contrast, glial heterogeneity and its implication for neural circuit function are poorly understood. We present evidence that the adult mouse DG of the hippocampus is populated by molecularly distinct astrocyte subtypes that are associated with distinct DG layers. Employing a combinatorial code of markers for the identification of astrocyte subtypes we show that different DG regions harbor distinct astrocyte subtypes. Astrocytes localized to different DG compartments furthermore exhibited subtype-specific morphologies highlighting that astrocyte intraregional diversity is reflected by distinct molecular as well as structural features. Physiologically, astrocytes associated to upper DG layers (molecular layer and granule zone) form large astrocytic syncytia, while those located to lower DG compartments (subgranular zone and hilus) constitute smaller networks suggesting region-specific functional diversification of astrocyte subtypes. Supporting the notion of subtype-specific physiological and functional features we show that astrocyte subtypes differentially express glutamate transporters, which is associated with different amplitudes of glutamate transporter-mediated currents. Importantly, key molecular and morphological features of astrocyte diversity in the murine DG were conserved in humans, indicating that also functional diversity of astrocytes is a general feature potentially relevant for human DG physiology. These findings suggest that diversity of astrocytes goes beyond the broad scale of developmental ancestry and affects - equivalent to what has been shown for neurons - also regional networks. This adds another layer of complexity to our understanding of neural network composition and function, which will be crucial for further studies on astrocytes in health and disease.

 

Role of astrocytes in synapse formation in adult-born hippocampal neurons

In a project embedded in the DFG Research Training Group GRK 2162 "Neurodevelopment and Vulnerability of the CNS" we are investigating the contribution of astrocyte to synapse formation of adult-born hippocampal granule neurons. Specifically, we will focus on the role of Neurexin1, which we identified by single cell transcriptomics to be highly expressed in astrocytes that are directly associated with dendrites of DG neurons. In the developing brain, astrocytes have been shown to contribute to synapse formation via the Neurexin/Neuroligin system. We will now determine the function of Neurexin1 in astrocytes of the adult hippocampal stem cell niche by an in vivo mouse genetic approach in combination with electrophysiological recordings (in collaboration with Prof. Christina Alzheimer’s group at the Institute of Physiology, FAU Erlangen).

 

Factors involved in the regulation of hippocampal neurogenesis

The fixed balance between neuro- and astrogenesis in the adult hippocampus even upon external stimuli implies the existence of a mechanism that tightly regulates the fate decision of adult NSCs. This leads to the question, which factors are responsible in controlling the neuron-to-astrocyte ratio? The transcription factor Sox9 emerged as a master-regulator of the neuron/glial switch in NSCs during development. In order to reveal mechanisms by which the balance between adult hippocampal neurogenesis and astrogenesis is mediated, we are applying mouse genetics (overexpression and conditional knock out) to assess if Sox9 is involved in fate decision of adult hippocampal NSCs.

 

In the adult DG, intra-regional astrocyte diversity manifested in three major astrocyte subtypes, which are associated to distinct DG compartments and can be distinguished by their morphology and by differential marker gene expressions. We hypothesize that these morphological, positional, and molecular differences account for different functional properties of astrocyte subtypes during the process of adult neurogenesis. To functionally test our hypothesis, we will establish an in vitro system to assess if NSPCs proliferation capacity as well as differentiation and maturation of progeny may be affected by co-cultures with different astrocyte subtypes.

The aim of these projects is to promote our understanding of intrinsic and extrinsic regulatory mechanism that control adult NSC behavior and hence hippocampal plasticity. This presents an important prerequisite to better understand glio- and neuropathological phenotypes and their contribution to brain disease.

 

Astrocyte diversity and astrogenesis under pathological conditions

Highlighting their critical role, an increasing number of neurodegenerative and neuropsychiatric disorders have been associated with deficits in astrocyte function. However, the role of astrocytes in pathophysiology and treatment of those diseases are poorly understood until now. Recently we started to investigate the pathology of niche astrocytes, their structural heterogeneity and their dynamics in mouse models of neuropsychiatric diseases (depressive disorder) and neurodegenerative diseases (Parkinson's disease) together with collaboration partners from the University Clinics Erlangen (Prof. Kornhuber, Dept. of Psychiatry, and Prof. Winkler, Dept. of Molecular Neurology, respectively).