Intermediate Filaments

Intermediate Filaments - 1

MEDICAL CELL BIOLOGY

INTERMEDIATE FILAMENTSOctober 15, 2010

Thomas J. Schmidt, Ph.D.Department of Moluecular Physiology and Biophysics

6-452 BSB, 335-7847

Reading Assignment: Alberts; Molecular Biology of the Cell (5th Ed.), Chapter 16

Key Concepts:

1. Intermediate filaments are strong polymers of fibrous polypeptidesthat resist stretch and play a structural function.

2. The four major classes of intermediate filaments include: keratinfilaments, neurofilaments, vimentin-containing filaments and nuclear lamins.

3. The first three classes of intermediate filaments are expressed in atissue-specific manner.

4. The morphology of these elongated molecules includes: a subunit-specific NH2-terminus (head) of variable size; a homologous central-helical region (rod domain); and a subunit-specific COOH-terminus (tail).

5. Intermediate filaments resist mechanical force, are a major component of the protective outer layer of skin cells and strengthen epithelia undergoingshape changes during morphogenesis.

Key Terms:

intermediate filaments glial fibrillary acidic proteinkeratin desminhemidesmosomes peripherindesmosomes nuclear lamins

progerianeurofilaments epidermolysis bullosa simplexamyotrophic lateral sclerosis vimentin

Medical Cell Biology Intermediate Filaments 1Thomas J. Schmidt, Ph.D. Email: [email protected] October 15, 2010


Clinical Update:

Title: “IF-pathies”: A broad spectrum of intermediate filament-associated diseases

M.B. Omary, J Clin Invest, 119(7):1756-1762, 2009

What are examples of intermediate filament-associated diseases?

Examples of these diseases include amyotrophic lateral sclerosis, progeria, and epidermolysis bullosa simplex.

Clinical Correlation: situation

Mary Smith, a 1 week-old female infant, was brought to the Pediatric Newborn Clinic. The infant was born at term and seemed normal at birth. Birth weight was 7 lbs. 4 oz. The pregnancy was normal. The mother was concerned because her infant had numerous skin blisters, predominantly in areas where her diaper had contact with her skin, but also elsewhere on her body. Discussions with the mother did not prove any evidence of child abuse.



Lecture Outline:

II. Intermediate Filaments

A. Definition - strong polymers of fibrous polypeptides that resiststretch and play a structural (tension-bearing) function

B. Found in most, but not all cell types

C. Most important function is to provide mechanical support for plasma membrane where it comes into contact with other cells or extracellular matrix.

D. Reinforce cells and organization of cells into tissues

E. Form a tight weave around the nucleus and then spread toward plasma membrane

F. Form a tightly woven basketwork, the nuclear lamina, inside the nuclear envelope

G. Characteristics

1. intermediate diameter (10nm)

2. subunits are fibrous proteins and almost all are incorporatedinto stable intermediate filaments

3. ATP or GTP hydrolysis is not requied for intermediate filamentpolymerization

4. no polarity of intermediate filaments

5. insoluble fiber system

6. tissue-specific forms of intermediate filaments contain different subunits (Table 16-1)



TABLE 16-1 Major Types of Intermediate Filament Proteins in Vertebrate CellsTypes of IF Component Polypeptides Cellular LocationNuclear lamins A, B, and C nuclear lamina (inner lining

of nuclear envelopeVimentin-like vimentin many cells of mesenchymal

origindesmin muscleglial fibrillary acidic protein glial cells (astrocytes and

some Schwann cellsperipherin some neurons

Epithelial type I keratins (acidic) epithelial cells and their derivatives (e.g. hair and nails

type II keratins (basic)

Axonal neurofilament proteins neurons (NF-L, NF-M, and NF-H)

a. keratin filaments (most diverse family of subunits) are found in epithelial cells

i. type I (acidic)

ii. type II (neutral/basic)

iii. heteropolymers formed from 1:1 ratio of type I and type II polypeptides

iv. hard keratins - nails, hair etc.

v. each epithelial cell type expresses a characteristic heteropolymer of type I and type II keratins – clinically useful in diagnosis of epiththelial cancers



vi. role of keratin filaments in normal cells – desmosomes (bond neighboring cells) and hemidesmosomes (anchor cells to underlying basal lamina

desmoglein (major protein in skin desmosomes) – autoantibodies disrupt adhesion between cells causing blistering of skin and mucous membranes

demoplakin – recessive mutations in gene causes skin disorder and cardiac arrhythmias that can lead to sudden cardiac death

b. neurofilaments - found in neurons and formcytoskeleton of axons and dendrites

i. heteropolymers of three different neurofilament polypeptides

ii. number of neurofilaments determines diameter of axon-speed of impulse

iii. neurodegenerative disease – amyotrophic lateral sclerosis (ALS, Lou Gehrig’s Disease)



associated with an accumulation and abnormal assembly of neurofilaments in motor neuron cell bodies and axons

degeneration of axons leads to muscle weakness and atrophy

c. filaments composed of vimentin-like proteins

i. vimentin - present in many cells of mesenchymal origin including fibroblasts, endothelial cells and white blood cells.

ii. Frequently associated with microtubules

iii. glial fibrillary acidic protein - glial cells

iv. desmin - muscle cells (stabilizes sarcomeres)

mutations prevent polymerization and formation of filamentous network

slowly progressive respiratory insufficiency can lead to disability and death

v. peripherin – neurons

d. nuclear lamins - form two-dimensional sheet like lattice of intermediate filaments (nuclear lamina) on inner surface of nuclear envelope

i. lamin B receptor – integral protein located on inner nuclear membrane (mutations of this protein can result in altered nuclear morphology)



ii. expression of nuclear lamins and/or lamin B receptor varies during differentiation (increased flexibility of nuclear envelope)

iii. nuclear lamins differ from cytoplasmic intermediate filaments: nuclear transport signals; dynamic structure (less stable)

iv. the nuclear lamina network disassembles at start ofmitosis and then reassembles at the end of mitosis

v. mediated by phosphorylation/dephosphorylation of serine residues on nuclear lamin polypeptides

vi. progeria – disease characterized by premature aging

extremely rare genetic disorder with no known cure

patients usually die around 13 years of age

development of symptoms is comparable to aging at a rate 8-10 times faster than normal



mutation in lamin A gene results in defective protein (missing 50 amino acids) that cannot support

the nucleus

mechanism(s) linking misshapen nuclei to aging symptoms is not known

G. Morphology of elongated fibrous intermediate filament molecules (Fig. 16-19)

1. subunit-specific NH2-terminus (head) of variable size

2. homologous central -helical region (rod domain) containslong tandem repeats of distinctive amino acid sequencecalled “heptad repeat”

3. subunit-specific COOH-terminus (tail) of variable size

4. variable regions facilitate attachment to other cytoskeletalstructures

a. plectin makes bundles of vimentin



b. plectin links intermediate filaments to microtubules and actin filament bundles

c. plectin attaches intermediate filaments to plasma membrane

d. mutations in plectin gene cause a devastating human disease

5. assembly of intermediate filament - bundles of keratin filaments held together by filaggrin

6. in most cells intermediate filament protein molecules are in fully polymerized state, with very little free tetramer.

H. Functions

1. some keratins strengthen epithelia undergoing shape changes during morphogenesis

2. some keratins are very abundant in the tough protective outer layer of skin cells

3. resist mechanical stress as demonstrated by blistering of skin caused by mutant keratin gene (example: epidermolysis bullosasimplex) (Fig. 16-21)



REVIEW QUESTIONS

1. Explain why pathologists often find it very useful to determine which pattern of intermediate filament proteins are expressed in a tumor biopsy.

2. Explain how the nuclear lamins differ from other intermediate filaments.

3. Explain at the cellular level the underlying defect in amyotrophic lateral sclerosis.

4. Explain at the cellular level the underlying defect in epidermolysis bullosa simplex.


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Intermediate Filaments