Primary astrocytes are a type of glial cell, or support cell, found in the central nervous system, which includes the brain and spinal cord. These cells do not transmit electrical signals like neurons. Astrocytes earned their name from their characteristic star-like shape, with “astro” meaning star and “cyte” meaning cell. When scientists refer to “primary” astrocytes, they are talking about cells directly isolated from living tissue, such as a neonatal rodent brain, for study in a laboratory setting. This approach differs from using immortalized cell lines, which are genetically modified to multiply indefinitely and may not fully represent natural cell behavior.
The Fundamental Roles of Astrocytes
Astrocytes carry out many functions to maintain a stable environment within the brain, supporting the proper operation of neurons. They provide structural support, acting as physical scaffolding for neurons and contributing to the overall architecture of brain tissue. Astrocytes also supply energy to neurons, converting glucose to lactate within astrocytes and then transporting it to neurons for fuel. This metabolic cooperation helps meet the high energy demands of active neurons.
These cells play a part in maintaining the blood-brain barrier (BBB), a highly selective border that protects the brain from harmful substances in the blood. Astrocyte end-feet, specialized extensions, wrap around blood vessels and interact with endothelial cells, contributing to the barrier’s integrity and regulating the passage of molecules. Astrocytes also regulate communication between neurons at synapses, the junctions where signals are passed. They clear excess neurotransmitters like glutamate from the synaptic space, preventing overstimulation that could harm neurons.
Astrocytes participate in synaptic pruning, a process where unnecessary synaptic connections are removed during brain development and learning. They are also involved in maintaining the balance of ions and water in the brain’s extracellular space. This includes buffering extracellular potassium ions, which accumulate during intense neuronal activity, to prevent neuronal hyperexcitability. This “spatial potassium buffering” involves astrocytes taking up potassium and distributing it through their interconnected networks via gap junctions.
Isolating and Culturing Primary Astrocytes
Obtaining primary astrocytes for scientific research involves a precise series of laboratory steps. Brain tissue, typically from neonatal rodents, is carefully removed under sterile conditions. The meninges, protective membranes surrounding the brain, are meticulously detached to avoid contaminating the astrocyte culture with other cell types like fibroblasts.
After removal, the brain tissue is cut into smaller pieces and then subjected to dissociation. This involves breaking down the tissue into individual cells, often using enzymatic treatments such as trypsin and DNase, combined with gentle mechanical trituration. The resulting cell suspension is then filtered through cell strainers to remove any remaining tissue clumps and ensure a single-cell suspension.
Once dissociated, the cells are placed into culture dishes coated with substances like poly-D-lysine, which promotes cell attachment and growth, in a nutrient-rich medium. To achieve a purer astrocyte culture, a common technique involves shaking the culture flasks. This shaking detaches less adherent cells, such as microglia and oligodendrocyte progenitor cells, leaving a more enriched population of astrocytes attached to the flask surface.
Astrocytes in Brain Health and Disease
When the brain experiences injury or disease, astrocytes undergo a transformation known as reactive astrogliosis, a spectrum of changes in their morphology, function, and gene expression. This response can be multifaceted, sometimes offering protective benefits and other times contributing to detrimental effects. For instance, reactive astrocytes can form a glial scar around the injury site, which acts as a physical barrier to contain damage and limit the spread of inflammation. However, this scar can also hinder axonal regeneration, impeding the brain’s ability to repair itself.
In neurodegenerative conditions like Alzheimer’s disease, astrocyte dysfunction is increasingly recognized as a contributing factor. Astrocytes are involved in the processing of amyloid-beta (Aβ) plaques, both in their production and clearance. Impaired astrocyte function, including reduced ability to clear excess glutamate, is observed in Alzheimer’s brains, potentially contributing to neuronal damage. Astrocytes also play a role in tau pathology, internalizing and accumulating tau protein, which can lead to their own death and contribute to synaptic dysfunction.
In Parkinson’s disease, astrocytes contribute to neuroinflammation and impaired glutamate metabolism. Dysfunctional astrocytes can lose their supportive functions and contribute to neurotoxicity. Astrocytes also internalize alpha-synuclein, a protein that aggregates in Parkinson’s disease, and their improper processing can trigger an inflammatory response.
Following acute events like stroke, reactive astrocytes form glial scars to isolate the injured area, yet they can also release molecules that inhibit axon regrowth. In epilepsy, dysfunctional astrocytes can fail to properly buffer extracellular potassium ions and clear glutamate from synapses. This leads to increased neuronal excitability and contributes to seizure activity.
The Primary Astrocyte Research Model
Primary astrocyte cultures serve as a tool in neuroscience research, allowing scientists to investigate astrocyte biology in a controlled environment. An advantage of using primary cells is their greater biological relevance compared to immortalized cell lines. This model enables researchers to conduct experiments on a single cell type, isolating specific astrocyte functions that would be difficult to study in the complex environment of a living organism.
Despite these benefits, primary astrocyte cultures have inherent limitations. When grown in a laboratory, these cells typically exist in a two-dimensional (2D) environment, which differs significantly from the intricate three-dimensional (3D) structure of the brain. This artificial flatness can alter their morphology and gene expression patterns, making them less representative of their natural state. The process of isolating primary astrocytes can also induce changes in their gene expression, further impacting how accurately they reflect astrocytes in their native tissue. Primary cultures often lack the complex interactions with other brain cells, such as neurons and microglia, which are essential for many astrocyte functions in a living brain.