Neuron cytoskeleton: parts and functions
These are the different parts of the cytoskeleton of neurons, elements that give them structure.
The cytoskeleton is a three-dimensional structure in all eukaryotic cells and, therefore, can be found in neurons.
Although it does not differ much from the rest of somatic cells, the cytoskeleton of neurons has some characteristics of its own.The cytoskeleton of neurons is important when they have defects, as is the case in Alzheimer's disease.
Below we will see the three types of filaments that make up this structure, its particularities with respect to the rest of cytoskeletons and how it is affected in Alzheimer's disease.
The cytoskeleton of the neuron.
The cytoskeleton is one of the defining elements of eukaryotic cells.The nuclei are those that have a defined nucleus, a structure that can be observed in animal and plant cells. This structure is, in essence, the internal scaffold on which the organelles are supported, organizing the cytosol and the vesicles found in it, such as lysosomes.
Neurons are eukaryotic cells specialized to form connections with others and constitute the nervous system and, as with any other eukaryotic cell, neurons possess a cytoskeleton. The neuron cytoskeleton, structurally speaking, is not very different from that of any other cell, possessing microtubules, intermediate filaments and actin filaments.
We will now look at 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 found in other cells of the body. Their main structure consists of a polymer of 50-kDa tubulin subunits, which coils in such a way as to form a hollow tube with a diameter of 25 nanometers.tubulin subunits, which coils 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 sequential similarity close to 40%. It is these proteins that constitute the hollow tube, by forming protofilaments that join laterally, thus forming the microtubule.
Tubulin is an important substance, since its dimers are the ones that its dimers are responsible for joining two molecules of guanosine triphosphate (GTP), dimers which have the ability to form the microtubule.The tubulin dimers have the capacity 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 modifiability to the cytoskeletal structure.
The microtubules of the axon and dendrites are not continuous to the cell body, nor are they associated with any MHC.nor are they associated with any visible MTOC (microtubule organizing center). Axonal microtubules can be 100 μm long, but have a uniform polarity. In contrast, dendrite microtubules are shorter, exhibiting mixed polarity, with only 50% of their microtubules oriented toward the distal termination to the cell body.
Although the microtubules of neurons are composed of the same components that can be found in the rest of the cells, it should be noted that they may present some differences. Brain microtubules contain tubulins of different isotypes, and with a variety of proteins associated with them. In addition, the composition of microtubules varies depending on the location within the neuron, the composition of microtubules varies depending on the location within the neuron, such as axons or dendrites.such as axons or dendrites. This suggests that brain microtubules may specialize for different tasks, depending on the unique environments provided by the neuron.
Intermediate filaments
As with microtubules, intermediate filaments are as much a component of neuronal cytostructure as they are of any other cell. These filaments play a very interesting role in determining the degree of cell specificity, as well as being used as markers of cellularas well as being used as markers of cell differentiation. In appearance, these filaments resemble a string.
In the organism there are up to five types of intermediate filaments, ordered from I to V, some of which can be found in the neuron:
Type I and II intermediate filaments are keratinic in nature and can be found in various combinations with epithelial cells in the body.. In contrast, type III filaments can be found in less differentiated cells, such as glia cells or neuronal precursors, although they have also been seen in more fully formed cells, such as those comprising smooth Muscle tissue and in mature astrocytes.
Type IV intermediate filaments are specific to neurons, presenting a common pattern between exons and introns, which differ significantly from those of type IV intermediate filaments.which differ significantly from those of the three previous types. Type V filaments are those found in the nuclear lamins, forming the part that envelops the cell nucleus.
Although these five different types of intermediate filaments are more or less specific to certain cells, it is worth mentioning that the nervous system contains a diversity of them. Despite their molecular heterogeneity, all intermediate filaments in eukaryotic cells appear, as mentioned above, as fibers resembling a rope, between 8 and 12 nanometers in diameter.
Neuronal filaments can be hundreds of micrometers long and have projections in the form of lateral arms.. In contrast, in other somatic cells, such as glia and non-neuronal cells, these filaments are shorter, lacking lateral arms.
The main type of intermediate filament that can be found in the myelinated axons of the neuron consists of three protein subunits, forming a triplet: a high molecular weight subunit (NFH, 180 to 200 kDa), a medium molecular weight subunit (NFM, 130 to 170 kDa) and a low molecular weight subunit (NFL, 60 to 70 kDa). Each protein subunit is encoded by a separate gene. These proteins are the ones that make up the type IV filaments, which are expressed only in neurons and have a characteristic structure.
However, although the type IV filaments are those specific to the nervous system, other filaments can also be found in the nervous system. Vimentin is one of the proteins that make up the type III filaments, which are present in a great variety of cells.It is present in a wide variety of cells, including fibroblasts, microglia and smooth muscle cells. They are also found in embryonic cells, as precursors of glia and neurons. Astrocytes and Schwann cells contain glial fibrillary acidic protein, which constitutes type III filaments.
Actin microfilaments
Actin microfilaments are the oldest components of the cytoskeleton.. They consist of 43-kDa actin monomers, which are arranged like two strings of pearls, 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 disease?
It has been discovered a relationship between the presence of beta-amyloid peptides, components of the plaques that accumulate in the brain in Alzheimer's disease, and the presence of beta-amyloid peptides in the brain in Alzheimer's disease.The rapid loss of dynamics of the neuronal cytoskeleton, especially in the dendrites, where the nerve impulse is received. As this part is less dynamic, the transmission of information becomes less efficient, and synaptic activity decreases.
In a healthy neuron, its cytoskeleton is composed of actin filaments which, although anchored, have a certain 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. However, if cofilin 1 is phosphorylated, i.e., a phosphorus atom is added, it stops functioning correctly.
It has been shown that exposure to beta-amyloid peptides induces increased phosphorylation of cofilin 1. This causes the cytoskeleton to lose dynamism, since the actin filaments are stabilized, and the structure loses flexibility. The dendritic spines lose function.
One of the causes of phosphorylation of cofilin 1 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.
Bibliographic references:
- Molina, Y.. (2017). Cytoskeleton and neurotransmission. Molecular basis and protein interactions of vesicular transport and fusion in a neuroendocrine model. UMH PhD Journal. 2. 4. 10.21134/doctumh.v2i1.1263.
- 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
(Updated at Apr 13 / 2024)