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Perineuronal net

Perineuronal net is a biology topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Perineuronal net rather than just read about it. In short: Perineuronal nets (PNNs) are specialized extracellular matrix structures responsible for synaptic stabilization in the adult brain. PNNs are found around certain neuron cell bodies and proximal neurites in the central nervous system.

Perineuronal net — main illustration
Perineuronal net — illustration

Key takeaways

  • Perineuronal net belongs to biology; place it in that map before memorising details.
  • Learn the definition first, then one example that makes the definition concrete.
  • Connect Perineuronal net to a quantity you can measure, compute or draw — that is where exam questions come from.
  • Reproduce the core statement of Perineuronal net from memory before moving on to harder problems.

Reference excerpt

Perineuronal nets (PNNs) are specialized extracellular matrix structures responsible for synaptic stabilization in the adult brain. PNNs are found around certain neuron cell bodies and proximal neurites in the central nervous system. PNNs play a critical role in the closure of the childhood critical period, and their digestion can cause restored critical period-like synaptic plasticity in the adult brain. They are largely negatively charged and composed of chondroitin sulfate proteoglycans, molecules that play a key role in development and plasticity during postnatal development and in the adult. PNNs appear to be mainly present in the cortex, hippocampus, thalamus, brainstem, and the spinal cord. Studies of the rat brain have shown that the cortex contains high numbers of PNNs in the motor and primary sensory areas and relatively fewer in the association and limbic cortices. In the cortex, PNNs are associated mostly with inhibitory interneurons and are thought to be responsible for maintaining the excitatory/inhibitory balance in the adult brain.

History The existence of PNNs has been inferred by Golgi, Lugaro, Donaggio, Martinotti, Ramón y Cajal and Meyer. However, Ramón y Cajal credits Golgi with the discovery of PNNs because he was the first to draw attention to them and gave the first precise description in 1893. Moreover, Golgi brought interest to the subject due to his opinion that the PNN was not a neuronal structure but rather a "kind of corset of neurokeratin which impeded the spread of current from cell to cell". Despite debating the topic, Ramón y Cajal claimed that the perineuronal net was simply a staining artifact derived from the coagulation of extracellular fluids. Due to his influential opinion at the time, interest in the topic subsided. Interest arose in the 1960s when several authors drew attention to the presence of periodic-acid-Schiff-positive (PAS-positive) material surrounding nerve cells. This PAS-positive material was suspected of being composed of negatively charged substances, such as chondroitin sulfate proteoglycans (CSPGs). However, the authors clung to the idea that the material was intricately connected to the blood–brain barrier and failed to see the similarities it had with the perineuronal net described by Golgi. Interest only rose again in the past few decades when it was discovered that PNNs constitute markers for physiologically mature neurons.

Composition PNNs are composed of a condensed matrix of chondroitin sulfate proteoglycans, molecules that consist of a core protein and a glycosaminoglycan (GAG) chain. The CS-GAG chains associated with PNNs differs from those found floating in the extra-cellular matrix in a noncondensed form. PNNs are composed of brevican, neurocan, versican, aggrecan, phosphacan, hyaluronan, tenascin-R and various link proteins. The CSPGs aggrecan, versican, neurocan, brevican, and phosphacan are bound to hyaluronan. Many of the components of PNNs are also expressed in other forms of ECM in the brain. Aggrecan is selectively expressed in PNNs and is essential for the construction and maintenance of PNNs. PNNs found in both the brain and the spinal cord have the same composition. Chondroitinase ABC (ChABC), a bacterial enzyme routinely used to digest CSPGs, works by catalyzing the removal of the CS-GAG chains of CSPGs, thus it is not selective to PNNs. Mutant mice deficient in tenascin-R or link protein have attenuated PNNs. In the cortex and other subcortical areas, PNNs preferentially surround GABAergic interneurons containing the calcium-binding protein parvalbumin. The onset of the critical period corresponds closely to the emergence of parvalbumin-positive cells. Parvalbumin-positive cells synapse onto α1-subunit-containing GABAA receptors. The α1-subunit-containing GABAA receptors have been shown to be the only GABAA receptors that drive cortical plasticity. For this reason, PNNs were first thought to have a strong role in the closure of the critical period.

Functions

Neuroprotection A fine regulation of axonal and dendritic growth is required in the adult CNS to preserve important connections while still allowing for structural plasticity. This function has been recognized to be mediated by several myelin-associated proteins and CSPGs. In order to assess the physiological role of PNNs in the undamaged CNS, ChABC was injected in the healthy cerebellum of adult rats. In the site of ChABC injections, there was profuse outgrowth of the terminal branches of Purkinje cell neurons. However, myelinated axon segments were not affected and remained normal. Purkinje axon sprouting was first evident four days after the degradation of CSPGs. Within 42 days, the expression of CSPGs gradually recovered, at which point axon outgrowth regressed, indicating that there was no significant formation of stable synaptic contacts. While CSPGs are very important in neuroprotection, this indicates that CSPGs may not be the only molecules important for the preservation of anatomical plasticity.

Restriction of AMPA receptor mobility Cell surface proteins, including neurotransmitter receptors, are highly mobile in the plasma membrane due to lateral diffusion. Fast movements of AMPA-type glutamate receptors (AMPARs) are involved in the modulation of synaptic transmission. As a receptor is used, it becomes desensitized and unable to operate efficiently for a short period of time. Diffusion of the desensitized receptor for the exchange of a naive functional one increases synaptic fidelity during fast repetitive stimulation. PNNs compartmentalize the neuronal surface and act as lateral diffusion barriers for AMPARs, limiting synaptic exchange. This may be part of the reason that synaptic plasticity is limited once PNNs become upregulated.

… excerpt ends here. Continue reading the full article.

Illustrations

Perineuronal net: A. Perineuronal nets are made of chondroitin sulfate proteoglycans (CSPGs). Here, the CSPGs neurocan, versican, brevican, and aggrecan are noncovalently bonded to hyaluronan. Associations occur between other CSPGs through tenascin (T, triangles). Tenascin, in turn, binds to CS glycosaminoglycans (red lines) as well as cell surface CSPGs. Phosphacan can also bind to cell surface receptors such as NCAM. B. Application of chondroitinase ABC (ChABC) degrades all the CS glycosaminoglycans (red lines) as well as hyaluronan (pink line), causing major disruptions in the structure of the perineuronal net. These disruptions may allow axons to penetrate the vacated space and permit restoration of neural plasticity.
A. Perineuronal nets are made of chondroitin sulfate proteoglycans (CSPGs). Here, the CSPGs neurocan, versican, brevican, and aggrecan are noncovalently bonded to hyaluronan. Associations occur between other CSPGs through tenascin (T, triangles). Tenascin, in turn, binds to CS glycosaminoglycans (red lines) as well as cell surface CSPGs. Phosphacan can also bind to cell surface receptors such as NCAM. B. Application of chondroitinase ABC (ChABC) degrades all the CS glycosaminoglycans (red lines) as well as hyaluronan (pink line), causing major disruptions in the structure of the perineuronal net. These disruptions may allow axons to penetrate the vacated space and permit restoration of neural plasticity.

Worked examples

Example 1 — a first encounter with Perineuronal net

Start with the simplest possible case. Write down what Perineuronal net claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In biology, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Perineuronal net before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Perineuronal net ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Perineuronal net

In research
Perineuronal net appears in biology research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry
Engineering practice reuses Perineuronal net in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom
Perineuronal net is common in secondary-school and first-year university syllabi. It links to neighbouring topics Neuroplasticity, so understanding it makes those chapters shorter.
In everyday life
Look for Perineuronal net outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Perineuronal net in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
  2. Close the page and write down what Perineuronal net means in your own words.
  3. Compare your version with the excerpt and mark what you missed.
  4. Work through the three examples above with pen and paper.
  5. Explain Perineuronal net out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Perineuronal net in simple terms?

Perineuronal nets (PNNs) are specialized extracellular matrix structures responsible for synaptic stabilization in the adult brain. PNNs are found around certain neuron cell bodies and proximal neurites in the central nervous system.

Why does Perineuronal net matter?

Because it connects several biology ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Perineuronal net?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Perineuronal net.

Tags

  • Neuroplasticity

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