Astrocytes are star-shaped glial cells in the brain and spinal cord. They perform many functions, including biochemical support of endothelial cells that form the blood–brain barrier, provision of nutrients to the nervous tissue, maintenance of extracellular ion balance, and a role in the repair and scarring process of the brain and spinal cord following traumatic injuries. However, while there is decades of data in mice about these nervous system support cells; the functional and molecular similarities between human and murine astrocytes are poorly understood. Now, Stanford researchers present the first ever functional and molecular comparison of human and mouse astrocytes. The team state that their findings suggest that human astrocytes, in contrast to mouse astrocytes, are better at detecting neuroactivity and adjusting their functions in response. The opensource study is published in the journal Neuron.
Previous studies from postmortem human tissues have revealed that human astrocytes are much larger and more complex than their murine counterparts. More recently, transplantation of human glial progenitors into mouse brains has been shown to improve learning and memory. These observations raise questions about how murine astrocyte physiology and function might extend to humans. However, the investigation of human astrocytes has faced issues related to access, as samples of living tissue must be obtained from brain cancer or epilepsy surgeries or fetal tissue. Purification has also proved difficult in the past, with the separation of astrocytes from other cells often killing them and causing many experiments to end in failure. The current study overcame the technical challenges by developing an antibody-driven protocol that isolates astrocytes and keeps them alive in culture.
The current study used an immunopanning-based technique that utilizes an antibody targeted against a surface protein expressed by human astrocytes, to generate purified cultures of primary human astrocytes. With this method, the lab purified astrocytes from over 20 fetal, juvenile, and adult human subjects and then performed transcriptome profiling and functional studies to compare developmental and interspecies differences. The researchers state that their method allowed, for the first time, direct investigation the functions of human astrocytes. Like mouse astrocytes, the team observed that human astrocytes strongly promote neuronal survival, synapse formation, and engulf synapses. Results show that human astrocytes retain their larger size in vitro and that adult human astrocytes demonstrate robust mGluR5-mediated calcium responses to glutamate stimulation.
The group then performed RNA-seq transcriptome profiling of purified human neurons, astrocytes, oligodendrocytes, microglia, and endothelial cells and established a database that serves as a road map for understanding cell-type-specific gene function in the human brain. By comparing human and mouse astrocyte transcriptome profiles, findings show that large numbers of genes are shared by astrocytes of both species, however, human and mouse astrocytes have many unique genomic and functional traits.
Results show that astrocytes come in two distinct stages, progenitor and mature, with early-stage astrocytes and brain cancer closely resembling each other. The team state that this brings up the possibility that brain cancer cells that originate from glial cells can be forced into a mature state, where they are unable to divide. The researchers stress that this finding couldn’t have been made without the use of fetal tissue. They go on to conclude that studies cannot guess the biology of human brains and neurodevelopmental disorders by studying mouse brains alone.
The team surmise that their study identified hundreds of genes expressed exclusively by human astrocytes, and more investigation will likely reveal additional biological differences. They go on to add that, potentially, this work will help recognize the role of these cells in biological disorders. For the future, the researchers state that with their new method they plan to begin looking at the unique properties of human astrocyte cells in a range of disease types, including Alzheimer’s, ALS, stroke, injury, autism, and schizophrenia.