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Intermediate filaments are composed of a family of related proteins that share structural and sequence features. They were initially designated 'intermediate' because their average diameter is between those of narrower microfilaments and wider myosin filaments found in muscle cells. The cytoskeleton of a cell is made up of microtubules, actin filaments, and intermediate filaments. These structures give the cell its shape and help organize the cell's parts, including muscle cells.
| Characteristics | Values |
|---|---|
| Diameter | 6-12 nm |
| Composition | Family of related proteins |
| Types | Type III, Type IV, Type V, Type VI |
| Examples of Type III IF Proteins | Vimentin, Desmin, Glial fibrillary acidic protein (GFAP), Peripherin |
| Examples of Type IV IF Proteins | Neurofilaments (NF-L, NF-M, NF-H) |
| Muscle Cells with Intermediate Filaments | Smooth muscle cells, striated muscle cells, heart muscle cells |
| Functions | Structural support, mechanical strength, cell migration, cell contraction |
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What You'll Learn
Desmin filaments are found in muscle cells
Intermediate filaments are composed of a family of related proteins that share common structural and sequence features. They are named "intermediate" because their average diameter (10 nm) is between those of narrower microfilaments (actin) and wider myosin filaments found in muscle cells.
Desmin is a type III intermediate filament protein that is specifically expressed in muscle cells. It is a muscle-specific protein and a key subunit of the intermediate filament in cardiac, skeletal, and smooth muscle tissues. Desmin filaments are found in muscle cells, where they play a critical role in maintaining the structural and mechanical integrity of the contractile apparatus in muscle tissues.
In adult muscle, desmin forms a scaffold around the Z-disk of the sarcomere and connects the Z-disk to the subsarcolemmal cytoskeleton. It links the myofibrils laterally by connecting the Z-disks. Through its connection to the sarcomere, desmin connects the contractile apparatus to the cell nucleus, mitochondria, and post-synaptic areas of motor endplates. These connections maintain the structural and mechanical integrity of the cell during contraction while also assisting in force transmission and longitudinal load-bearing.
Desmin also connects the individual actin-myosin assemblies of muscle cells to one another and to the plasma membrane, thereby linking the actions of individual contractile elements. It is present in Z-discs and intercalated discs in cardiac muscle. In addition, desmin may play a role in signalling between the extracellular matrix (ECM) and the sarcomere, which could help regulate muscle contraction and movement.
When desmin is not functioning properly, there is an improper mitochondrial distribution, number, morphology, and function. Desmin-related myopathies are associated with mutations in the gene that codes for desmin, which can lead to dilated cardiomyopathy, skeletal myopathy, and smooth muscle defects.
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Vimentin filaments are found in smooth muscle cells
Intermediate filaments are composed of a family of related proteins that share structural and sequence features. They are called "intermediate" because their average diameter of 10 nm is between the diameters of narrower microfilaments (actin) and wider myosin filaments found in muscle cells. Animal intermediate filaments are subcategorized into six types based on similarities in amino acid sequence and protein structure.
Vimentin is a type III intermediate filament protein that is found in a variety of different cell types, including fibroblasts, leukocytes, blood vessel endothelial cells, and smooth muscle cells. It is the most widely distributed of all IF proteins. In smooth muscle cells, vimentin filaments play an important role in mediating active force development and passive tension, which is not regulated by myosin light chain phosphorylation.
Vimentin filaments undergo spatial reorganization in cultured smooth muscle cells in response to contractile activation. They are a major intermediate filament protein in airway and vascular smooth muscle. For example, aortic smooth muscle cells contain vimentin instead of desmin. Smooth muscle cells of the digestive, respiratory, and urogenital tracts contain abundant amounts of vimentin.
Vimentin tunes cell stress by assisting in both actomyosin force transmission and reinforcement of microtubule networks under compression. The competition between these two opposing effects of vimentin is regulated by the microenvironment stiffness. Vimentin can alter nuclear architecture and chromatin distribution, and the liberation of vimentin heads by HIV-1 protease may play a role in HIV-1-associated cytopathogenesis and carcinogenesis.
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Neurofilaments are found in nerve cells
Neurofilaments are a type of intermediate filament found in nerve cells. They are composed of neurofilament proteins, which are classified as type IV intermediate filament proteins. These proteins are designated NF-L (low molecular weight), NF-M (medium molecular weight), and NF-H (high molecular weight). Neurofilaments are the major intermediate filaments in most mature neurons and are particularly abundant in the long axons of motor neurons.
The precise composition of neurofilaments in a given nerve cell depends on the relative expression levels of the neurofilament proteins in the cell at a particular time. For example, NF-H expression is low in developing neurons and increases postnatally in neurons with myelinated axons. In mature myelinated axons, neurofilaments can be the single most abundant cytoplasmic structure, occupying most of the axonal cross-sectional area.
Neurofilaments are believed to function primarily to provide structural support for axons and to regulate axon diameter, which influences nerve conduction velocity. The assembly of neurofilaments prior to their entry into the axon is a critical process that involves the dephosphorylation of the head domain by protein phosphatase 2A (PP2A). Neurofilaments are also involved in axonal transport, with most neurofilament proteins being synthesized within the cell body and travelling long distances along axons to reach their sites of function.
Neurofilaments play an important role in the pathogenesis of motor neuron diseases such as amyotrophic lateral sclerosis (ALS) and other neurological diseases. Accumulation and abnormal assembly of neurofilaments are characteristic features of ALS, and experiments in transgenic mice have suggested that these abnormalities may contribute to the death of affected neurons. Neurofilament antibodies are commonly used in diagnostic neuropathology to distinguish neurons from glia and to detect axonal damage in diseases affecting the central nervous system.
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Keratin filaments are found in epithelial cells
Keratin filaments are indeed found in epithelial cells. Keratin is a structural fibrous protein, also known as scleroprotein, and is a key structural material in hair, skin, nails, feathers, horns, claws, and hooves in vertebrates. Keratin is also responsible for protecting epithelial cells from damage or stress.
Keratin is a type of intermediate filament (IF). IFs are composed of related proteins that share structural and sequence features. They were initially designated 'intermediate' because their average diameter (10 nm) is between the diameters of narrower microfilaments (actin) and wider myosin filaments found in muscle cells. Keratin filaments are formed by pairing type I and type II molecules, which are two of the four types of cytoplasmic IFs.
Keratin filaments are important for the mechanical stability and integrity of epithelial cells and tissues. They also have regulatory functions and are involved in intracellular signaling pathways, such as protection from stress, wound healing, and apoptosis. Keratin filaments are also involved in epithelial cell migration, which is fundamental to multiple developmental processes, epithelial tissue morphogenesis, and maintenance.
The human genome encodes 54 functional keratin genes, located on chromosomes 12 and 17. These genes are expressed in highly specific patterns related to the epithelial type and stage of cellular differentiation. Keratin filaments are abundant in keratinocytes in the hornified layer of the epidermis and are also present in epithelial cells in general.
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Actin filaments are found in muscle cells
Actin is a globular protein that forms microfilaments in the cytoskeleton and thin filaments in muscle fibrils. It is present in almost all eukaryotic cells, including muscle cells, where it makes up around 10% of the total protein mass.
Actin filaments are essential for muscle contraction, cell motility, cell division, and cytokinesis. In muscle cells, actin filaments are organised into regular arrays that complement thicker filaments formed from another protein called myosin. Together, these two proteins create the force responsible for muscle contraction.
Actin filaments in muscle cells are activated when a signal to contract is sent along a nerve. Myosin, a molecular motor, then converts chemical energy in the form of ATP to mechanical energy, generating force and movement. This interaction between actin and myosin is responsible for muscle contraction and plays a central role in cell biology.
There are three distinct types of muscle cells in vertebrates: skeletal muscle, cardiac muscle, and smooth muscle. Skeletal muscle is responsible for voluntary movements, cardiac muscle pumps blood from the heart, and smooth muscle is responsible for involuntary movements in organs such as the stomach and intestines. In both skeletal and cardiac muscle, the contractile elements of the cytoskeleton are arranged in highly organised arrays, contributing to our understanding of muscle contraction at the molecular level.
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Frequently asked questions
Intermediate filaments are found in heart muscle cells, smooth muscle cells, and striated muscle cells.
There are two types of intermediate filaments found in muscle cells: desmin and vimentin.
Desmin intermediate filaments are structural components of the sarcomeres in muscle cells. They connect different cell organelles like the desmosomes with the cytoskeleton.
Vimentin intermediate filaments are found in a variety of cell types, including smooth muscle cells, fibroblasts, leukocytes, and blood vessel endothelial cells. They support the cellular membranes and transmit membrane receptor signals to the nucleus.
