UNIT 4 – Intermediate Filaments (Q&A) | MZO-001 MSCZOO | IGNOU

SAQ 1 

Fill in the blanks: 
a) The fibrous and rope-like structures refer to .................... .
Answers: Intermediate filaments

b) A protein that provides the mechanical strength and integrity to skin and hair is called ..................... .
Answers: Keratin 

c) How many types of polypeptides are present in intermediate filaments?

Intermediate filaments (IFs) are a key component of the cytoskeleton in eukaryotic cells, providing essential structural support and maintaining the shape and stability of cells. These filaments are composed of polypeptides, which are long chains of amino acids that fold into a functional structure. The polypeptides in intermediate filaments polymerize to form filamentous structures that help to anchor organelles, support cell shape and provide mechanical strength. Each type of intermediate filament is made from a specific group of polypeptides that are tissue-specific and vary in their amino acid composition and function.

There are five major types of polypeptides found in intermediate filaments, each with distinct characteristics and functions in different cell types.

[Note- In addition to these five main types, recent research has led to the reclassification of one protein, Nestin, into a new category as the sixth type.]

Type I - Acidic Keratins

  • Type I includes acidic keratins. These are cytoplasmic proteins primarily found in epithelial cells and epithelial tissues like the skin, hair and nails.
  • They are crucial for providing structural stability to these tissues and help form a strong and flexible matrix.
  • Type I keratins pair with Type II keratins (basic or neutral keratins), forming heterodimers. These dimers then assemble into keratin filaments, which are part of the intermediate filament system.

Type II - Basic Keratins

  • Type II consists of basic keratins. They are primarily present in epithelial tissues, especially in the epidermis, hair, nails and other keratinized tissues.
  • These proteins work together with Type I keratins, forming heterodimers that eventually polymerize to form intermediate filaments.
  • They play a major role in mechanical strength and provide protection against physical stress in the skin and other epithelial structures.

Type III - Vimentin-like Proteins

  • Type III includes proteins like vimentin, desmin, GFAP (Glial Fibrillary Acidic Protein), peripherin and syncoilin.
  • These proteins are mainly expressed in mesenchymal cells, fibroblasts, muscle cells and astrocytes.
  • Vimentin is widely distributed in many cells and plays a crucial role in maintaining the cytoskeletal network.
  • Desmin, found in muscle cells, is responsible for sarcomere organization.
  • Syncoilin is found in smooth muscle cells and helps in maintaining the structural integrity of muscle fibers.

Type IV - Neurofilaments and Related Proteins

  • Type IV consists of proteins like neurofilaments (NF-L, NF-M, NF-H), alpha-internexin, synemin and Nestin.
  • Neurofilaments are critical in neuronal cells, particularly in axons, where they provide structural stability and support for neuron function.
  • Alpha-internexin and synemin are expressed in neurons and muscle cells, playing essential roles in neuronal development and axonal function.
  • Nestin is especially important in neural development, functioning in neural stem cells and muscle progenitor cells.
[Note- Nestin was originally classified under Type IV because it was associated with neurofilament proteins and was believed to function in a similar way to other type IV intermediate filaments. However, recent research has shown that Nestin plays a specific and crucial role in neural stem cells and muscle progenitor cells. Its unique involvement in neurogenesis and muscle regeneration led to its reclassification as Type VI. This change reflects its distinct role, which sets it apart from other intermediate filaments.]

Type V - Nuclear Lamins

  • Type V is composed of lamins that form the nuclear lamina.
  • These include Lamin A, Lamin C, Lamin B1 and Lamin B2, which are primarily involved in maintaining the shape and structure of the nucleus.
  • Lamins also play a significant role in nuclear transport, DNA replication and cell division. They help anchor nuclear pores and regulate processes like chromatin organization.
There are five major types of polypeptides found in intermediate filaments, each with distinct characteristics and functions in different cell types. [Note- In addition to these five main types, recent research has led to the reclassification of one protein, Nestin, into a new category as the sixth type.]

SAQ 2 

a) Desmin facilitates ....................... in muscles. 
Answer: Contraction

b) Neurofilaments present in............... .
Answer: Neurons

c) List the steps of assembly of intermediate filaments.

Intermediate filaments (IFs) are strong, rope-like structures in the cytoskeleton of animal cells. They provide mechanical strength and help maintain the shape and stability of the cell. Unlike microtubules and actin filaments, intermediate filaments are not involved in movement but are mainly known for their structural support. The process of their assembly is step-wise and highly organized.

There are six main steps involved in the assembly of intermediate filaments.

Step 1: Formation of Monomers

The basic building blocks of intermediate filaments are protein monomers. Each monomer has a long central alpha-helical rod domain and short non-helical head and tail regions. These monomers are soluble in the cytoplasm and exist freely before assembly begins.

Step 2: Formation of Coiled-Coil Dimers

Two monomers come together and wrap around each other in parallel to form a coiled-coil dimer. In this dimer, both monomers are aligned in the same direction (parallel) and their alpha-helices interact to form a stable structure.

Step 3: Formation of Tetramers

Next, two dimers come together in an antiparallel and staggered arrangement to form a tetramer. This is a very important step because this tetramer is the main unit used for further filament formation. Each tetramer has four polypeptide chains, arranged in such a way that the ends of the filaments do not have polarity, unlike microtubules.

Step 4: Formation of Protofilaments

Several tetramers align side by side, overlapping slightly, to form protofilaments. These are short linear structures made from multiple tetramers. The bonding between the tetramers is mainly through weak non-covalent interactions.

Step 5: Formation of Protofibrils

Four protofilaments laterally associate with each other to form a protofibril. This lateral packing provides strength and thickness to the structure.

Step 6: Formation of Mature Intermediate Filament

Finally, about eight protofilaments twist around each other to form a mature intermediate filament. This final filament is about 10 nanometers in diameter and is very strong, flexible and resistant to mechanical stress. This is the complete and functional form found in the cytoplasm.

Throughout this process, no ATP or GTP is required, which makes intermediate filament assembly different from microtubules or actin filaments. Also, since the tetramers are antiparallel, the final filament has no polarity.
There are six main steps involved in the assembly of intermediate filaments. Step 1: Formation of Monomers The basic building blocks of intermediate filaments are protein monomers. Each monomer has a long central alpha-helical rod domain and short non-helical head and tail regions. These monomers are soluble in the cytoplasm and exist freely before assembly begins. Step 2: Formation of Coiled-Coil Dimers

SAQ 3 

a) Pseudofolliculitis barbae causes due to the mutation in ………...... Gene. 
Answer: K6hf

b) …...........…., ….........….. and ……......….. are the most well-known IFAPs.
Answer: Plectin, desmoplakin and filaggrin

c) ………..... and ……....…. can control the activity of downstream effectors that promote or inhibit filament assembly.
Answer: RhoA and Cdc42

TERMINAL QUESTION 

1. What do you mean by Hutchinson–Gilford progeria? 

Hutchinson–Gilford Progeria Syndrome (HGPS) is a very rare genetic disorder in children, where the body ages much faster than normal. The word "progeria" comes from Greek, where "pro" means "before" and "geras" means "old age." This disease was first explained by Dr. Jonathan Hutchinson in 1886 and later by Dr. Hastings Gilford in 1897, which is why it is called Hutchinson–Gilford progeria. It is caused by a mutation in the LMNA gene, which gives instructions to make a protein called lamin A. This protein supports the structure of the nucleus in cells. In HGPS, the mutation forms an abnormal protein called progerin, which weakens the nuclear envelope, leading to faster cell damage and aging. The mutation is usually not inherited, meaning the parents are normal and the change happens suddenly during early development.

[Note- LMNA Gene: The LMNA gene is responsible for producing Lamin A and Lamin C proteins, which are crucial for maintaining the structural integrity of the nucleus. It is located on chromosome 1 (specifically at 1q21.2).]

Cause of the Disease:

HGPS is caused by a mutation in the LMNA gene present on chromosome 1. This gene makes lamin A, a protein that maintains the shape and stability of the nucleus. Due to the mutation, a defective protein called progerin is produced. Progerin gets stuck in the nuclear membrane and causes the nucleus to become abnormal in shape. This affects the function of the cell and leads to early aging of the body.

Symptoms:

The symptoms of HGPS start appearing within the first 1 to 2 years of life. Children appear normal at birth but then start showing early signs of aging such as:
  • Growth failure and short height
  • Loss of hair (alopecia)
  • Thin skin with visible veins
  • A small face with a pointed nose and small jaw
  • Stiff joints and weak bones
  • Loss of body fat and muscles
  • Aged facial appearance
  • Hardening of arteries (atherosclerosis), leading to heart problems
  • Importantly, the mental development of affected children remains normal.

Diagnosis:

HGPS is usually suspected by physical symptoms. The final confirmation is done through genetic testing, especially testing the LMNA gene to detect the specific mutation responsible for producing progerin.

Treatment:

There is no permanent cure for progeria, but treatments can help improve quality of life. A drug called lonafarnib, which is a farnesyltransferase inhibitor, has been approved and shows benefits in slowing disease progression. Other supportive treatments include:
  • Medications for heart problems
  • Physical therapy
  • Healthy, high-energy diet
  • Regular monitoring of cardiovascular health

2. Mention the types and functions of intermediate filaments.

Intermediate filaments (IFs) are one of the three major components of the cytoskeleton, along with microtubules and microfilaments. These filaments are made up of fibrous polypeptides and are about 10 nm in diameter, which is intermediate between actin filaments (thinner) and microtubules (thicker). They mainly provide mechanical strength and help maintain the shape of the cell. Based on the types of proteins (polypeptides) that form them, intermediate filaments are classified into five major types (according to traditional classification). However, based on new research, a sixth type has also been added recently.

Type I: Acidic Keratins

These are found in epithelial cells and are rich in acidic amino acids. They always form heterodimers with type II keratins to build stable filaments.

Function: Provide mechanical support to epithelial tissues and help in forming hair, nails and skin.

Type II: Basic Keratins

These are also found in epithelial cells but are basic in nature. They pair with type I keratins to form heterodimers.

Function: Together with type I keratins, they maintain cell shape and resist mechanical stress in epithelial cells.

Type III: Vimentin-like Filaments

This group includes vimentin (in mesenchymal cells), desmin (in muscle cells), glial fibrillary acidic protein or GFAP (in astrocytes), peripherin (in peripheral neurons) and syncoilin (also in muscle cells).

Function: These filaments anchor organelles like the nucleus and maintain cellular integrity and positioning.

Type IV: Neurofilaments

These are mainly found in neurons and include neurofilament proteins (NF-L, NF-M, NF-H), α-internexin, nestin and synemin.

Function: Provide structural support to axons and maintain their diameter which helps in nerve signal transmission.

Type V: Nuclear Lamins

This group includes Lamin A, Lamin C, Lamin B1 and Lamin B2, which form a mesh-like layer called the nuclear lamina beneath the inner nuclear membrane.

Function: Provide structural support to the nucleus and regulate important nuclear activities like DNA replication and RNA transcription.

Note (About Sixth Type):

Recent research has identified Nestin (previously grouped under Type IV) as being significantly different in function and structure. It is now classified under a new sixth type of intermediate filaments because of its development-specific expression in neural stem cells and its highly dynamic behavior, which differs from other stable neurofilaments.
There are five major types of polypeptides found in intermediate filaments, each with distinct characteristics and functions in different cell types. [Note- In addition to these five main types, recent research has led to the reclassification of one protein, Nestin, into a new category as the sixth type.]

3. Explain different types of keratin-related disorders.

Keratins are a large family of structural proteins that form intermediate filaments in epithelial cells. They are mainly classified into Type I (acidic keratins) and Type II (basic keratins). These keratins work together to form stable filaments that give strength and resilience to epithelial tissues like skin, hair and nails. When mutations occur in keratin genes, the structure of intermediate filaments gets disrupted. This causes the cells to become fragile, leading to a group of genetic disorders known as keratinopathies or keratin-related disorders. Most keratin disorders are inherited in an autosomal dominant manner and each disorder shows specific symptoms depending on the tissue where the keratin is expressed.

There are many keratin-related disorders, but the most well-known include the following:

1. Epidermolysis Bullosa Simplex (EBS)

EBS is caused by mutations in the KRT5 or KRT14 gene. These keratins are present in the basal layer of the epidermis. Due to the mutation, the skin becomes very fragile and forms blisters easily after minor injuries, especially on the hands and feet. In severe forms, blistering can occur over large parts of the body. The disease mainly affects the stability of basal keratinocytes, which are responsible for maintaining the strength of the skin's lower layer.

2. Epidermolytic Hyperkeratosis (EHK)

EHK results from mutations in either the KRT1 or KRT10 gene. These keratins are expressed in the suprabasal layer of the epidermis. The main symptoms are thickened and scaly skin along with blistering, which is often present from birth or early infancy. The mutation affects the connection between skin cells, making the upper layers of the skin weak and prone to damage, leading to a loss of normal skin integrity.

3. Pachyonychia Congenita (PC)

This disorder is caused by mutations in one of the following genes: KRT6A, KRT6B, KRT16, or KRT17. It primarily affects the nails, palms, soles and oral mucosa. Individuals suffering from PC show nail dystrophy, painful thickened skin on palms and soles, white patches in the mouth and sometimes cysts under the skin. The disorder results from poor keratinocyte differentiation, leading to weak structural support in affected epithelial areas.

4. Monilethrix

Monilethrix is associated with mutations in KRT81, KRT83, or KRT86 genes which are mainly found in hair shaft keratin. The main symptom of this disorder is brittle, beaded hair that breaks easily, leading to sparse hair on the scalp. The mutation affects the strength and continuity of the hair shaft due to poor keratin fiber formation, making hair highly breakable and uneven.

5. Steatocystoma Multiplex

This condition occurs due to a mutation in the KRT17 gene and affects the sebaceous glands of the skin. Individuals with this disorder develop multiple sebaceous cysts, especially on the chest, arms and trunk. The mutation affects the proper development of the sebaceous duct lining cells, leading to cyst formation due to abnormal keratin accumulation.

4. What benefits do intermediate filaments offer? Specify how it affects cell division.

Intermediate filaments (IFs) are one of the three main types of cytoskeletal fibers found in eukaryotic cells. Unlike microtubules and actin filaments, they are not involved in rapid transport or motility, but they provide strong mechanical support and maintain the structural integrity of cells. These filaments are composed of different proteins depending on the cell type, such as keratin, vimentin, desmin, neurofilament proteins and nuclear lamins.

Benefits Offered by Intermediate Filaments

Intermediate filaments provide several critical functions to the cell, contributing to its stability and protection:
  • Mechanical Strength:
    • Intermediate filaments form a robust internal framework that helps cells withstand mechanical stress such as stretching, compression and shear. This is especially important in tissues like skin, muscles and nerves.
  • Cell Shape and Stability:
    • They help maintain the overall shape of the cell and resist mechanical deformation, making cells more stable.
  • Organelle Positioning:
    • They play a vital role in the positioning of organelles, such as anchoring the nucleus within the cell and helping organize other cellular structures.
  • Support for Cell Junctions:
    • They provide structural support to cell junctions like desmosomes and hemidesmosomes, which are crucial for holding cells together and attaching cells to the extracellular matrix.
  • Tissue-Specific Functions:
    • For example, keratins are found in epithelial cells and form protective layers of skin. In neurons, neurofilaments help in maintaining axon structure.

How Intermediate Filaments Affect Cell Division

Disruption or alteration in intermediate filaments can negatively impact cell division. Here are the following ways in which intermediate filaments can affect or impact cell division:
  • Impact on Chromosome Alignment
    • Intermediate filaments, particularly keratin filaments in epithelial cells, assist in organizing the cytoskeleton, which is essential for proper chromosome alignment during metaphase. When these filaments are disrupted, chromosomes may not align correctly, leading to errors in cell division.
  • Disassembly of Nuclear Lamins During Mitosis
    • Nuclear lamins, which are a type of intermediate filament found in the nuclear envelope, undergo disassembly during the onset of mitosis. This process is crucial for the nuclear envelope to break down, allowing the chromosomes to separate during cell division. Without proper disassembly, mitosis can be stalled, leading to defects in division.
  • Improper Formation of the Mitotic Spindle
    • Intermediate filaments, such as vimentin in fibroblasts, are involved in anchoring and stabilizing the mitotic spindle. The spindle apparatus is responsible for segregating chromosomes. Disruption of vimentin filaments can lead to improper formation or positioning of the mitotic spindle, affecting chromosome segregation.
  • Influence the Process of Cytokinesis and Cell Cleavage
    • Intermediate filaments also influence the process of cytokinesis, where the cell divides into two daughter cells. The proper formation of the cleavage furrow depends on the interaction between actin filaments and intermediate filaments. Disruption of these filaments can lead to improper cleavage and incomplete cell division, resulting in multinucleated cells.

5. Describe the pattern of intermediate filaments' intracellular arrangement.

Intermediate filaments (IFs) are one of the three main components of the cytoskeleton, along with microtubules and actin filaments. They have a very distinct and well-organized intracellular pattern which provides mechanical support, maintains cell shape and stabilizes organelle position.

The pattern of their arrangement is highly regulated and follows a characteristic intracellular network that can be described as follows:

1. Perinuclear Concentration

Intermediate filaments are usually concentrated around the nucleus. This region is known as the perinuclear region. The filaments often radiate from this area toward the cell periphery. Nuclear lamins, a type of intermediate filament, form a dense and organized meshwork just beneath the inner nuclear membrane called the nuclear lamina. This lamina provides structural support to the nucleus and helps in organizing chromatin.

2. Radiating Toward the Cell Periphery

From the perinuclear region, intermediate filaments extend outward throughout the cytoplasm and reach the cell cortex (area beneath the plasma membrane). They form a fine network or scaffold that fills the space between the nucleus and the cell membrane, often crossing the cytoplasm in a highly branched pattern. This radiating arrangement gives the cell strong internal support and maintains spatial organization.

3. Anchoring at Desmosomes and Hemidesmosomes

In epithelial cells, intermediate filaments like cytokeratins are attached to desmosomes (cell–cell junctions) and hemidesmosomes (cell–matrix junctions). These junctions anchor the intermediate filaments at the cell surface, creating a mechanical continuum between cells and their substrate. This network resists mechanical stress and maintains tissue integrity.

4. Cell Type-Specific Arrangement

The exact pattern of intermediate filament arrangement varies depending on the cell type and the specific type of intermediate filament protein. For example:
  • In epithelial cells, keratin filaments form dense networks attached to desmosomes.
  • In neurons, neurofilaments are arranged longitudinally along axons for structural support and axonal transport.
  • In muscle cells, desmin filaments align with the contractile apparatus and help maintain sarcomere integrity.

5. Association with Organelles

Intermediate filaments also interact with organelles like mitochondria, Golgi bodies and lysosomes. They help in stabilizing their position within the cytoplasm and sometimes influence their function. For example, vimentin filaments often surround and support the positioning of mitochondria and the Golgi apparatus.






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