Characteristics of Human Astrocytes
Characterized by their star-like shape astrocytes, also known as astroglia, represent the most abundant cell type in the brain. Closely linked to neurons with pivotal roles in synaptic activity and blood-brain barrier function, the study of astrocytes using in vitro co-culture models is becoming increasingly important in neuroscience research.
Astrocytes support the metabolic and trophic development of neurons. These cells serve a variety a well-established stage-specific functions in synaptogenesis, myelination and neuronal migration as they mature.
Human astrocytes are over 20 times larger by volume and have a vastly increased number of synapses in comparison to individual rodent astrocytes. It is thought this could contribute to advanced human cognitive ability and complexity.
Astrogenesis
In the developing neural system, the three major cell types namely neurons, astrocytes and oligodendrocytes share common ancestors, neural stem cells or progenitor cells. In the initial phase of neural development, neural stem cells contribute to neurogenesis followed by a wave of gliogenesis. Two of the major signaling pathways known to contribute to activation of astrogenesis are the JAK/STAT and Notch. Upstream extrinsic factors lead to the activation of these pathways. The major players are IL-6 cytokine family members, CNTF, LIF and CT-1, which along with BMP and Notch signaling lead to stimulation of astrogenesis.
One major impact of these factors is STAT3 activation. STAT3 is essential in the induction of expression of astrocyte-lineage specific genes including GFAP and S100B at the transcriptional level. STAT3 is a core component of the STAT3:p300/CBP:SMAD1 transcriptional complex modulating activation of factors involved in astrogenesis. Other transcription factors involved in the process include SOX9 and HES1. Furthermore, chromatin modifications such as demethylation of the GFAP promoter must occur for astrocyte development to proceed.
Astrocytes in human neurodevelopmental disorders
It is becoming increasingly apparent that astrocytes are highly significant when there is disruption of the choreography of neural development, leading to disease pathogenesis in neurodevelopmental disorders.
Lessons from monogenetic forms of autism have shown that primary astrocyte dysfunction can underlie these disorders. For example, Rett syndrome, caused by mutations in methyl CpG binding protein 2 (MECP2), features abnormal dendritic morphology and altered synapse physiology. Patients with schizophrenia express increased levels of glial fibrillary acidic protein (GFAP), which is a feature associated with astrogliosis.
Understanding the impact of astrocyte dysfunction and behavior, including the impact on surrounding neurons, is of high importance in understanding the underlying causes of such disorders and will pave the way towards future therapies.
Astrocytes and neurodegeneration
Neurodegenerative disorders such as Alzheimer’s and Parkinson’s diseases have long been studied from the viewpoint of degeneration of neuronal populations. Evidence suggests that glial atrophy often appears at early disease stages, for example, in amyotrophic lateral sclerosis (ALS), degeneration of astrocytes occurs prior to symptoms caused by motor-neuron defects. Moreover, activation of astrocytes has been linked to production of amyloid beta in Alzheimer’s disease therefore providing a contribution to amyloid plaque formation.
Other potential areas of study
Study of astrocyte-mediated neurotoxicity
– Neurogenesis research
– Study of inflammation and healing in the brain
– Glial scar formation
– Neuron and astrocyte co-culture to examine surface molecule expression and trophic factor release
– Blood-brain barrier
– Drug development for neurodegenerative disorders
– Stroke
Human Induced Pluripotent Stem Cell (iPSC)-Derived Astrocytes in Disease Modeling & Drug Discovery
The availability of human induced pluripotent stem cell (iPSC)-derived astrocytes and culture reagents such as those offered by Axol, enable researchers to generate pure cell populations for use in drug discovery and disease modeling. Biologically relevant cultures such as this could be used to determine the direct effect of compounds on these specific cells, or elucidate the mechanisms that occur in neuronal development or precede the onset of neurological conditions. Alternatively, they may be co-cultured for more complex analysis of the central nervous system (CNS) in vitro. Further precision may be achieved by generating isogenic cell lines with disease-specific mutations using technologies such as CRISPR/Cas9 gene editing. This enables a direct comparison of the cells without any confounding environmental or genetic factors that might otherwise be present in samples derived from different individuals.
Astrocyte Immunocytochemistry (ICC) Markers
It is important to ensure that the in vitro differentiation of iPSCs mimics the in vivo process. Hence, marker expression should be examined to determine the developmental stage of the cell. GFAP is the prototypical marker used to confirm astrocyte phenotype and differentiate them from other glial cells.
Astrocytes will have high levels of GFAP and low levels of neuronal class III ß-tubulin (TUJ1), a microtubule marker of neuronal lineage. Axol iPSC-Derived Astrocyte populations typically contain ≥ 80% GFAP-positive astrocytes and ≤15% TUJ1-positive neurons.
