Actin, Microtubules, and Intermediate Filaments

  >   Rahul's Noteblog   >   Notes on Histology   >   Actin, Microtubules, and Intermediate Filaments

Actin Filaments:

• 7-8 nm wide 5-10% of protein in non-muscle cells.

• Over 20 % in muscle.

• Each actin monomer binds 2 molecules of calcium and one ATP.

• Polar with a fast (+) growing and a slow (-) growing end.

G Actin:

G actin (globular) is the monomeric form F actin (filamentous) is the polymerized form The following favor assembly: Sufficient monomer concentration NaCl Calcium ATP.

Drugs that affect actin:

Drugs affect assembly / disassembly:

Cytochalasins:

• Fungal metabolites inhibit addition of monomer to the + end.

Phalloidin:

• Stabilizes F actin by preventing depolymerization.

Actin affects cell shape:

Actin generally occurs as bundles or networks that support the plasmalemma thus affecting cell shape. Networks can be 2D and planar as in lamellipodia or 3D as within the cell producing a gel.

Actin-associated Proteins:

A partial selected list of Actin Associated Proteins (AAP): Actin sequestering proteins: PROFILIN forms a stable 1:1 complex with G actin but does not depolymerize F actin. Actin capping and severing proteins: GELSOLIN fragments actin in the presence of calcium, no known function without calcium.

Bundle-Forming Proteins:

• FASCIN and FIMBRIN.

Gel Forming Proteins:

• FILAMIN

Contraction Producing Proteins:

Non-muscle myosin Active as the motive force in the contractile ring of cytokinesis at the end of mitosis.

RBC Membrane Support:

Spectrin, ankyrin, band 4.1 and other proteins support the RBC membrane and allow it to deform and spring back.

Microvillus:

• The microvillus is a highly ordered structure primarily composed of very ordered actin and AAP.

• ~40 actin filaments per MV.

• Villin and fimbrin bundle actin.

• The base of MV are embedded in the terminal web.

Terminal Web:

The terminal web is a complex meshwork of several proteins including intermediate filaments (keratin), myosin, filamin, tropomyosin and others. It provides tonicity to the cell apex.

Dystrophin:

Dystrophin anchors a cortical actin network to the extracellular matrix (ECM). Dystrophin is found on the cytosolic side of the plasmalemma in striated and cardiac muscle cells. It (1) cross links actin with a network of proteins and (2) attaches the network to a glycoprotein complex in the membrane. The membrane complex in turn attaches to laminin and agrin of the ECM. Dystrophin is lacking in patients with Duchene muscular dystrophy (DMD). DMD is a fatal, degenerative, sex-linked genetic disease.

Actin Stress Fibers:

In cultured cells, and presumably in vivo as well, actin bundles associate with the plasmalemma in a manner that enables them to pull against the substratum or another cell. In cultured cells, bundles of actin form associations called stress fibers that terminate in cell substrate contact zones called adhesion plaques or focal contacts. Vinculin is enriched where actin terminates in adhesion plaques.

Vinculin:

Vinculin does not bind directly with actin but does so via several intermediate proteins to functionally connect the actin cytoskeleton to fibronectin in the ECM via fibronectin receptors.

Actin Dynamics:

Actin addition at the + end of the lamellipodium extends the cell process and actin-myosin interaction along with cross-linking proteins moves the membrane. Actin depolymerizes at the trailing edge.

Cell Adhesion:

Cell-cell and cell-matrix adhesion are similar in that they both use specific receptors to link to the ECM (integrins) or to other cells (cadherins).

Intermediate Filaments:

Intermediate Filaments are a class of highly insoluble, tough, durable fibrous proteins, average diameter 8-10 nm, that provide mechanical support for the plasmalemma where it contacts other cells or the ECM. Unlike microtubules and MF, intermediate filaments do not play a role in motility and there are no known motor proteins that move along intermediate filaments.

Intermediate Filaments in Cells:

• Intermediate filaments do not occur in all cell types.

• Intermediate filaments are prominent in cells subjected to mechanical stress.

• Intermediate filaments generally form an extensive network that surrounds the nucleus and emanates out into the cytoplasm to the plasmalemma.

• Generally intermediate filaments remain polymerized in terminally differentiated cells in which they occur. One exception, nuclear lamins.

Major Intermediate Filaments:

Keratins:

• Associated with junctional complexes in epithelial cells.

Vimentin:

• Expressed in blood vessel endothelium and some epithelial cells. Found in many different cell types early in development, in some mature cells like fibroblasts and nearly all cultured cells.

Desmin:

• Links myofibrils into bundles in smooth and skeletal muscle.

Glial Acidic fibrillary protein (GAFP):

• Found in glial cells (astrocytes, Schwann cells and oligodendrocytes.

• The majority of brain tumors are derived from astrocytes and contain GAFP.

Function of Neurofilaments in Neurons:

Stabilize neuron geometry.

Diseases Related to Neurofilament Abnormalities:

Alzheimer's Disease:

• Many neurons have large intermediate filaments tangles.

Amyotropic Lateral Sclerosis (Lou Gherig's Disease):

• Large swollen initial axon segments filled with NF early in disease.

Aluminum and Acrylamide Poisoning:

• Exposure to solvents like methyl n-butyl ketone and n-hexane by industrial workers and glue vapors result in abnormal pileups of NF in terminals and cell bodies.

Kuru:

• Inherited degenerative nervous system disorder with death within 1 year and other slow virus infections characterized by abnormal NF.

Neural Proteins:

Peripherin:

• Found in peripheral and CNS neurons.

Internexin:

• Found in developing CNS.

Nestin:

• Found in neuroepithelial stem cells.

Types of Lamins:

Lamins A, B and C comprise the nuclear lamina or fibrous dense lamina adherent to the inner surface of the nuclear envelope and also binds to heterochromatin.

Functions of Intermediate Filaments in Cells:

Intermediate filament associated proteins (intermediate filamentsAP) generally link intermediate filaments to membranes and are plentiful in epithelial cells.

Intermediate filaments participate in epithelial adhesions in cell-cell attachments (desmosomes) and cell-ECM attachments (hemidesmosomes)

The Cell Cycle and Related Phases:

Cell cycle is a repeating sequence of biochemical and morphological events that result in the exact replication of a cell and it's DNA. Cycle has 4 phases G1, S, G2 and mitosis. Nuclear division and segregation (karyokinesis) followed by division of the cell body (cytokinesis).

Interphase:

Interphase includes G1, S and G2: G1 growth period or post-mitotic gap 1: synthesis of RNA and protein synthesis goes into high gear S - synthesis of DNA (2N to 4N) G2 pre-mitotic gap 2, preparation for mitosis M or mitosis or division

Centriole Cycle:

Centrioles are duplicated by S G 0 is a stationary phase in which cells do not divide again, such as in neurons.

Mitotic spindle:

The mitotic spindle is a diamond-shaped structure composed of microtubules, MAPs, KRPs, dyenin, ABP and other as yet to be identified proteins.

Metaphase Spindle:

Microtubules are polarized with - end at the poles (microtubules).

Replicated chromosomes are attached to the + end of microtubules by the kinetochore. Such microtubules are termed kmicrotubules (kinetochore microtubules). Daughter chromatids are joined by the centromere.

Assembly of tubulin occurs at or near the kinetochore. The kinetochore also appears to serve as a cap for the + end thereby stabilizing kmicrotubules.

Chromosome capture:

In prometaphase when the nuclear membrane breaks down, microtubules nucleated at the poles attach to kinetochores laterally and shift to the tip. This may be due to dyenin. As the kinetochore moves nearer the pole it encounters a higher density of microtubules and the side-on attachments are converted to plus-end-attachments to microtubules. The opposite kinetochore gets captured on the other side and metaphase tension is produced.

Hypothesis:

Two forces may separate chromatids in anaphase: Microtubules shorten at both ends but mainly at the kinetochore and/or A - motor protein at the kinetochore uses ATP to move along the microtubules, with the + end of the microtubules depolymerizing as it becomes exposed. Both ideas may operate simultaneously.

Polar microtubules continuously undergo addition at the + end and lengthen to spindle.

Metaphase:

New microtubules are added at plus end and removed from minus end. Anaphase - centromere attachment of chromatids is lost and microtubules shortens by tubulin loss at both ends.

Anaphase:

Two separate forces contribute to Anaphase B: + motor KRP cross link adjacent nKmicrotubules and slide microtubules past each other and push the poles apart - motor proteins bind the cell cortex to astral microtubules and pull the poles apart.

Video of Muscle Contraction:

Additional Reading:

Basic Histology

1. Introduction to Histology
2. Basic Cell Physiology
3. Actin, Microtubules, and Intermediate Filaments
4. Mitochondria, Nucleus, Endoplasmic Reticulum, Golgi
5. Epithelium (Epithelial Tissue)
6. Connective and Adipose Tissue
7. Types of Cartilage
8. Osteogenesis
9. Nervous Tissue
10. Muscle Tissue
11. Cardiovascular System
12. Blood and Hematopoiesis
13. Lymphoid Tissue
14. Digestive Tract I: Oral Cavity
15. Digestive Tract II: Esophagus through Intestines
16. Liver, Pancreas, and Gall Bladder
17. Respiratory System
18. Integument
19. Urinary System
20. Endocrine System
21. Male Reproductive System
22. Female Reproductive System
23. Eye and Ear

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