Cytoskeleton 2

Describe the common structure of the diverse intermediate filament isoforms found in mammalian cells.
Discuss the molecular basis for the mechanical properties of intermediate filaments.
Explain the central role of intermediate filaments in the mechanical stability of cells and tissues.
Explain the importance of the lamin intermediate filaments and the Nuclear Lamina.
Describe diseases related to intermediate filament defects.

Intermediate Filaments Characteristics:
- Highly flexible
- High tensile strength
Molecular Assembly:
- Made from several coiled-coil proteins
- Forms a resilient, filamentous network capable of managing mechanical stress
Distensibility:
- Intermediate filaments can be stretched to >300% of their original length without breaking.

General Description:
- Monomers are highly elongated fibrous proteins.
Tripartite Structure:
- Central α-helical rod domain.
- An N-terminal ‘head’.
- A C-terminal ‘tail’.
Rod Domain Composition:
- Extended α-helical region containing long tandem repeats called heptad repeats (consisting of 7 amino acids).

From Monomers to Dimers:
- Monomers twist around each other forming a parallel coiled-coil dimer.
Dimer to Tetramer:
- Pair of parallel dimers associate in a staggered anti-parallel fashion to produce a tetramer.
- Tetramers are soluble subunits of intermediate filaments.
From Tetramers to Unit-Length Filaments:
- Chains of tetramers form proto-filaments.
- Eight proto-filaments pack together in a helical array to form a unit-length filament.
Final Filament Formation:
- Multiple unit-length filaments bind together to create the final filament, which contains 32 polypeptide chains.

Key Properties:
- Flexibility and strength due to a unique structure.
Close Packing and Lateral Associations:
- Structural integrity and breaking resistance are due to the close packing of coiled-coil structures.
Comparison to Rope:
- Analogy to the traditional rope-making process which intertwines natural fibers to create strength, paralleling the assembly of intermediate filaments.

Uniformity in Formation:
- Though formation is uniform, differences in head/tail domains define distinct intermediate filament families.

Keratin's Role in Skin:
- Found in cells enduring significant mechanical stress such as those in skin.
Genetic Diseases Affecting Keratin:
- Mutations in genes associated with keratin lead to defective keratin protein production, resulting in:
- Formation of blisters due to weak cytoskeleton causing basal epidermal cell fragility.
- Example: Epidermolysis bullosa simplex (EBS).

Description and Function:
- The nuclear lamina is a meshwork of intermediate filament proteins known as lamins.
- Lines the inside of the inner nuclear membrane providing structural support to the nucleus.
- Thickness: 10-20 nm, interrupted at nuclear pores.

Location:
- Lamins are primarily located in the nucleus; other intermediate filaments are found in the cytoplasm.
Structural Differences:
- Lamins have a longer central rod than many other intermediate filaments.
Nuclear Transport Signal:
- Lamins have a nuclear transport signal in their C-terminal tail domain.
Assembly Comparison:
- Lamins form a stable 2D meshwork (nuclear lamina); other intermediate filaments form a 1D network.

Chromatin Organization:
- Assist in organizing chromosomes within the nucleus.
Gene Expression Regulation:
- Play a role in controlling gene expression through cellular interactions.
Cell Cycle Management:
- Help reassemble the nucleus after cell division.
Nuclear Pore Complexes:
- Anchor nuclear pore complexes that facilitate molecular transport in and out of the cell.

Role of A-type Lamins:
- Essential for telomere structure, length, and function.
Consequences of Mutation/Loss:
- Leads to destabilization of 53BP1, important in DNA damage response and double-strand break repair.
- Alters the nuclear distribution of telomeres resulting in telomere shortening and genomic instability.

Definition:
- Genetic diseases associated with mutations in A-type lamins (LMNA) and telomere dysfunction.
Common Diseases:
- Examples of laminopathies include:
- Dunnigan-type Familial Partial Lipodystrophy
- Hutchinson-Gilford Progeria Syndrome
- Emery-Dreifuss Muscular Dystrophy
- Adult-onset Autosomal Dominant Leukodystrophy

Overview:
- Caused by a point mutation in the LMNA gene resulting in abnormal A-type lamin (progerin).
Effects of Progerin:
- Integrates into the nuclear lamina leading to nuclear abnormalities and disrupted cell function.

Protein Composition:
- Affected structure includes globular head domain, α-helical coiled-coil domain, and globular tail domain.
- Notable mutations such as p.Met540Thr and c.1824C>T contribute to disease progression.
Age of Onset Variability:
- Varied presentation of symptoms at different age milestones from age 2 to 37.

Cause and Consequences:
- Mutations in LMNA leading to instability in nuclear lamina.
- This instability affects signaling pathways and gene regulation, particularly impacting muscle function leading to muscle weakness characteristic of EDME.

Intermediate filaments provide flexibility and strength via coiled-coil protein assembly.
Largely found in mechanically stressed cells (e.g., skin).
Monomer composition includes a tripartite structure: α-helical rod domain, N-terminal head, C-terminal tail.
Assembly Process: Monomers ➜ Dimers ➜ Tetramers ➜ Unit-length filaments ➜ Final filament.
Notable examples: α-keratin in skin, lamins in the nuclear lamina.
Genetic mutations in keratin lead to skin diseases; mutations in LMNA cause laminopathies.