Question

Enzyme glutamine synthase is not found in a. astrocytes b. oligodendrocytes c. none of the above is true d. all are true

Short answer

Answer: b. Oligodendrocytes

Step-by-step solution

Understanding the concept

Glutamine synthase is an enzyme that plays a crucial role in the synthesis of glutamine in cells. Glutamine is an essential amino acid, which is used by various cells in the body. It is important to know which cell types contain this enzyme.

Analyzing the options

a. Astrocytes: Astrocytes are a type of glial cell in the central nervous system. They have various functions, including the control of the synapse, metabolic support for neurons, uptake and recycling of neurotransmitters, and many others. Importantly, astrocytes express the enzyme glutamine synthase and play a role in the glutamate-glutamine cycle. b. Oligodendrocytes: Oligodendrocytes are another type of glial cell found in the central nervous system. They are responsible for producing the myelin sheath, which insulates the axons of neurons and facilitates the conduction of electrical signals. Oligodendrocytes do not have a significant expression of glutamine synthase. c. None of the above is true: If neither of the above options is correct, this option would be the answer. d. All are true: If both astrocytes and oligodendrocytes do not have glutamine synthase, this option would be the answer.

Identifying the correct answer

As mentioned earlier, astrocytes express the enzyme glutamine synthase, while oligodendrocytes do not. This means that option (a) is incorrect and option (b) is correct. Since option (b) is true, neither option (c) nor option (d) can be correct. So, the correct answer is: b. Oligodendrocytes

Key concepts

Glutamine synthase

Glutamine synthase is a vital enzyme involved in the synthesis of glutamine from glutamate and ammonia. This enzyme is exceptionally important for maintaining the nitrogen balance in cells. The reaction it catalyzes is particularly crucial in the brain, where it helps regulate the levels of neurotransmitters.
This regulation is part of what is known as the glutamate-glutamine cycle.
In this cycle, astrocytes use glutamine synthase to convert glutamate, a major excitatory neurotransmitter, into glutamine.

• Glutamine can then be released and taken up by neurons.
• Neurons convert glutamine back to glutamate, restarting the cycle.

This process highlights the enzyme's role in neurotransmitter recycling, helping to prevent excessive neuronal stimulation and potential excitotoxicity.

Astrocytes

Astrocytes are star-shaped glial cells that play a significant role in the central nervous system (CNS). They were once thought to merely support neurons, but recent research reveals they have many crucial functions.
These include:

• Maintaining the blood-brain barrier, which protects the CNS from harmful substances.
• Providing metabolic support to neurons by supplying nutrients and removing waste products.
• Regulating neurotransmitter levels, particularly through the uptake and recycling of glutamate.

The presence of glutamine synthase in astrocytes emphasizes their role in neurotransmitter cycling.
By converting glutamate to glutamine, astrocytes help prevent excessive accumulation of glutamate, which could lead to excitotoxic damage in neurons.

Oligodendrocytes

Oligodendrocytes are another type of glial cell, distinct in their primary function, which is the formation of the myelin sheath around axons in the CNS. This sheath insulates the axons and accelerates the transmission of electrical signals, essential for rapid and efficient neural communication.
Interestingly, oligodendrocytes are not significantly involved in neurotransmitter cycling, unlike astrocytes.

• These cells do not express high levels of glutamine synthase.
• Instead, their main contribution to neuronal function is through insulation and support.

Thus, while they play a crucial role in maintaining the rapid signaling capabilities of the CNS, they do not directly partake in the glutamate-glutamine cycle.
Their specialized function differentiates them from other glial cells like astrocytes, underlining the diversity of roles within the CNS.