How can the cytoskeleton of a cell be described?

In this post we are going to answer the question ‘’How can the cytoskeleton of a cell be described?’’ We will explain what the cytoskeleton is, what it is made of and its main functions within the neuron.

How can the cytoskeleton of a cell be described?

A cell’s cytoskeleton is composed of microtubules, filaments of actin, and intermediate filaments.

Neurofilaments are a type of intermediate filaments 7 nanometers thick present in the cytoplasm of neurons. They are involved in the maintenance of neuronal structure and axonal transport.

Sometimes biological structures hold many more secrets than we initially think. In the world of nature, knowledge is practically infinite, since it encompasses layers and morphological layers until it reaches the most basic compounds of any living being, the amino acids and the chemical elements that make them up. To what level do we want to reach in this search for knowledge?

On the one hand, we have neurons with their delimited sections (axon, dendrites and soma), the communication between them through synapses, neurotransmitters and their effects on the brain.

All of these topics have already been extensively covered, but we can still take a finer thread. In this opportunity, we take the opportunity to show you everything you need to know about neurofilaments.

Neurofilaments: the neuronal skeleton

It is incredible to know that the skeleton of living beings is made up of cells, but that these also need their own “skeletal structure” to maintain their shape and functionality. That is, we find complex organization even in the most basic functional unit that life gives us.

Since we cannot address the role of neurofilaments without first understanding the structural organization of a cell, we are going to stop for a moment on the cytoskeleton and its function.

About the cytoskeleton

The cytoskeleton is defined as a three-dimensional network of proteins that provides internal support in cells but also intervenes in the transport of compounds, organization and cell division.

Making an analogue with the observable macroscopic world, this complex network would act like the beams of a building, but also like the elevator and the stairs. Incredible true?

The cytoskeleton is made up of three main compounds:

• Microfilaments: made up of two actin chains, a globular protein. They maintain the shape of the cell.
• Intermediate filaments: made up of a more heterogeneous family of proteins, they provide stability to cellular organelles due to their strong bonds.
• Microtubules: made up of alba and beta-tubulin, they are responsible for the movement of substances within the cell and their division.

It should be noted that the structure and dynamics of the cytoskeleton depend on the way in which the cell relates to the outside (that is, the extracellular matrix) and the stresses of tension, rigidity and compression that it experiences throughout its development.

We are facing a dynamic and not rigid framework that adapts exquisitely to the process that the cell is undergoing at any given moment. Now, how are neurofilaments related to all of the above?

The cytoskeleton of the neuron

The cytoskeleton is one of the defining elements of eukaryotic cells, that is, those that have a defined nucleus, a structure which can be observed in animal and plant cells.

This structure is, in essence, the internal scaffold on which the organelles are based, organizing the cytosol and the vesicles that are found in it, such as lysosomes.

Neurons are eukaryotic cells specialized in forming connections with others and constituting the nervous system and, as with any other eukaryotic cell, neurons have a cytoskeleton.

The cytoskeleton of the neuron, structurally speaking, is not very different from that of any other cell, having microtubules, intermediate filaments and actin filaments.

Below we will see each of these three types of filaments or tubes, specifying how the cytoskeleton of the neuron differs from that of other somatic cells.

Microtubules

The microtubules of the neuron are not very different from those that can be found in other cells of the body. Its main structure consists of a 50-kDa tubulin subunit polymer, which is twisted in such a way that it forms a hollow tube with a diameter of 25 nanometers.

There are two types of tubulin: alpha and beta. Both are proteins not very different from each other, with a sequence similarity close to 40%.

 It is these proteins that constitute the hollow tube, through the formation of protofilaments that come together laterally, thus forming the microtubule.

Tubulin is an important substance, since its dimers are responsible for joining two molecules of guanosine triphosphate (GTP), dimers which have the ability to perform enzymatic activity on these same molecules.

It is through this GTPase activity that is involved in the formation (assembly) and disassembly (disassembly) of the microtubules themselves, giving flexibility and the ability to modify the cytoskeletal structure.

The axon microtubules and dendrites are not continuous with the cell body, nor are they associated with any visible MTOC (microtubule organizing center). Axonal microtubules can be 100 µm in length, but have uniform polarity.

In contrast, the microtubules of the dendrites are shorter, presenting mixed polarity, with only 50% of their microtubules oriented towards the termination distal to the cell body.

Although the microtubules of neurons are made up of the same components that can be found in other cells, it should be noted that they may present some differences. Microtubules of the brain contain tubulins of different isotypes, and with a variety of proteins associated with them.

Furthermore, the composition of microtubules varies depending on the location within the neuron, such as axons or dendrites. This suggests that the microtubules in the brain could specialize in different tasks, depending on the unique environments that the neuron provides.

Intermediate filaments

As with microtubules, intermediate filaments are components as much of the neuronal cytostructure as that of any other cell. These filaments play a very interesting role in determining the degree of specificity of the cell, in addition to being used as markers of cell differentiation. In appearance, these filaments resemble a rope.

In the body there are up to five types of intermediate filaments, ordered from I to V and, some of them being those that can be found in the neuron:

Type I and II intermediate filaments are keratin in nature and can be found in various combinations with epithelial cells of the body.

In contrast, type III cells can be found in less differentiated cells, such as glial cells or neuronal precursors, although they have also been seen in more formed cells, such as those that make up smooth muscle tissue and in astrocytes. mature.

Type IV intermediate filaments are specific to neurons, presenting a common pattern between exons and introns, which differ significantly from those of the three previous types. Type V are those found in the nuclear laminae, forming the part that surrounds the cell nucleus.

Although these five different types of intermediate filaments are more or less specific to certain cells, it should be mentioned that the nervous system contains a diversity of these.

Despite their molecular heterogeneity, all the intermediate filaments in eukaryotic cells appear, as we have mentioned, as fibers that resemble a rope, with a diameter between 8 and 12 nanometers.

Neural filaments can be hundreds of microns long, as well as having projections in the form of lateral arms. In contrast, in other somatic cells, such as those of the glia and non-neuronal cells, these filaments are shorter, lacking lateral arms.

But although those of the nervous system are type IV, other filaments can also be found in it. Vimentin is one of the proteins that make up type III filaments, present in a wide variety of cells, including fibroblasts, microglia, and smooth muscle cells.

They are also found in embryonic cells, as precursors to glia and neurons. Astrocytes and Schwann cells contain acidic fibrillar glial protein, which constitutes type III filaments.

Actin microfilaments

Actin microfilaments are the oldest components of the cytoskeleton. They are made up of 43-kDa actin monomers, which are arranged like two strings of beads, with diameters of 4 to 6 nanometers.

Actin microfilaments can be found in neurons and glial cells, but are especially concentrated in presynaptic terminals, dendritic spines, and neural growth cones.

What role does the neuronal cytoskeleton play in Alzheimer’s?

A relationship has been discovered between the presence of beta-amyloid peptides, components of the plaques that accumulate in the brain in Alzheimer’s disease, and the rapid loss of dynamics of the neuronal cytoskeleton, especially in dendrites, where the impulse is received.

As this part is less dynamic, the transmission of information becomes less efficient, in addition to decreasing synaptic activity.

In a healthy neuron, its cytoskeleton is made up of actin filaments that, although anchored, have some flexibility.

For the necessary dynamism to occur so that the neuron can adapt to the demands of the environment, there is a protein, cofilin 1, which is responsible for cutting the actin filaments and separating their units.

Thus, the structure changes shape, however, if cofilin 1 is phosphorylated, that is, a phosphorous atom is added, it stops working correctly.

Exposure to beta-amyloid peptides has been shown to induce greater phosphorylation of cofilin 1. This causes the cytoskeleton to lose dynamism, as the actin filaments stabilize, and the structure loses flexibility. Dendritic spines lose function.

One of the causes that make cofilin 1 phosphorylate is when the enzyme ROCK (Rho-kinase) acts on it.

This enzyme phosphorylates molecules, inducing or deactivating their activity, and would be one of the causes of Alzheimer’s symptoms, since it deactivates cofilin 1.

To avoid this effect, especially during the early stages of the disease, there is the drug Fasucil, which inhibits the action of this enzyme and prevents cofilin 1 from losing its function.

Summary

As we have seen, the world of neurofilaments is not only reduced to a structural protein framework.

We move on nanoscopic scales, but clearly, the effects of the abundance of these essential components of the neuronal cytoskeleton are expressed at the behavioral and physiological level in living beings.

This highlights the importance of each of the elements that make up our cells. Who was going to tell us that a greater abundance of a particular filament could be an indicator of the early stages of a disease such as Alzheimer’s?

In the end, each small component is one more piece of the puzzle that gives rise to the sophisticated machine that is the human body. If one of them fails, the effect can reach heights much larger than the few micrometers or nanometers that this structure can occupy in a physical space.

FAQS: How can the cytoskeleton of a cell be described?

How can the cytoskeleton of a cell be described quizlet?

The cytoskeleton protects and shapes a cell, allows organelles to position and transport, provides strength, helps in cell division, and helps cell movement

What describes a cytoskeleton?

In the cytoplasm, the network of protein filaments and microtubules that governs cell structure, maintains intracellular organization, and is involved in the movement of cells.

Does the cytoskeleton surround the cell?

No, that’s the cytoplasm. 

What role does the cytoskeleton play in a cell?

The basic functions of the cytoskeleton are to modulate the shape of the cell, to provide mechanical strength and integrity, to enable cells to move and to promote the intracellular transport of supramolecular structures, vesicles and even organelles.

What are the 3 functions of the cytoskeleton?

1) give shape to cells that lack the wall of a cell; 

2) allow cell movement, e.g. The crawling motion or contraction of muscle cells of white blood cells and amoebas; 

3) organelle movement inside the cell and endocytosis;

In this post we answered the question ‘’How can the cytoskeleton of a cell be described?’’ We explained what the cytoskeleton is, what it is made of and its main functions within the neuron.

If you have any questions or comments please let us know!

References

Kirkpatrick LL, Brady ST. Molecular Components of the Neuronal Cytoskeleton. In: Siegel GJ, Agranoff BW, Albers RW, et al., editors. Basic Neurochemistry: Molecular, Cellular and Medical Aspects. 6th edition. Philadelphia: Lippincott-Raven; 1999. Available from: https://www.ncbi.nlm.nih.gov/books/NBK28122/

Rush, T. et al (2018) Synaptotoxicity in Alzheimer’s disease involved a dysregulation of actin cytoskeleton dynamics through cofilin 1 phosphorylation The Journal of Neuroscience doi: 10.1523/JNEUROSCI.1409-18.2018