Cytoskeleton Filaments

Original Editor - Lucinda hampton Top Contributors - Lucinda hampton and Vidya Acharya  

Introduction

Neurons (blue): cytoskeletons shown in red and green.

Cytoskeleton is a network of protein filaments in the cytoplasm within all eukaryotic cells. Cytoskeleton is not a fixed structure; it is adaptive and dynamic and its componennts are is in constant flux.

The complex mesh of protein filaments and motor proteins aid in cell movement, stabilizing the cell, maintaining shape, internal structure and mechanical support of the cell and enabling cells to carry out basic functions like division and movement. By means of this cytoskeletal network, the cell integrates numerous signals in order to coordinate its behavior, communicate with other cells, and adjust to its environment.

Understanding the mechanics of cytoskeletal networks and the roles of cytoskeleton have in many cell biological processes gives rise to extensive cellular behaviours that depend upon them,

Genetic defects in cytoskeletal proteins are associated with mechanical defects in cells and tissues, leading to eg Focal Segmental Glomerulosclerosis, Epidermolysis Bullosa, Muscular Dystrophy, Intermediate filament gene mutations cause Charcot–Marie–Tooth disease and motor neurone disease.

Filament Types

A molecular motor “walking” on a MT

There are three types of filaments make up the cytoskeleton. All are arranged into networks that resists deformation. But they can reorganize when subjected to external forces.

1. Actin filaments: They are the most abundant proteins in eukaryotes. They occur in a cell in the form of meshwork or bundles of parallel fibres. Actin filaments are semiflexible on the length-scale of the cell. Thaye rapidly reorganise enabling cell migration and change in shape and sustain mechanical stress.
2. Microtubules (MT) are the most rigid of the cytoskeleton filaments. They orient vertically. longer filaments that are constantly assembling and disassembling. MTs play a crucial role in e.g. moving the daughter chromosomes to the newly forming daughter cells during mitosis. Bundles of MTs form the cilia and flagella found in protozoans and in the cells of some multicellular animals. Largest filaments, 24 nm in diameter, composed of a protein called tubulin.
3. Intermediate filaments: are the least stiff of all. They resist tensile forces better than compressive load.

Filaments in Neurons

Mitosis diagram

Tau (a micro tubular protein) tangles

All the types of cytoskeleton filaments work together to guarantee correct formation of the nervous system during the embryonic development and to assure its function in adulthood. Both cytoskeletal filaments and motor proteins are required for axonal transport. Important neuronal events where the function of cytoskeletal filaments are required are

1. During embryonic development: the cytoskeleton participating in the growth and guidance of axons
2. Adult life: the cytoskeleton essential for maintaining neuronal homeostasis and neuronal plasticity
3. Injury repair: peripheral axon needs to regenerate after being injured, peripheral neurons requiring a specific set of cytoskeletal proteins to ensure nerve regeneration upon damage.

Given the importance of cytoskeleton for neurons, it is easy to see how neurological disorders involve changes either in the expression, dynamics, and and/or the stability of cytoskeletal proteins or their mutations. e.g. tauopathies, all share the common denominator of the altered structure or function of the microtubule-associated protein tau .

Striated Muscle Filaments

The cytoskeleton fibres in striated muscle have diverse roles as mentioned above, including as functioning as a structure for inter- and intracellular signaling. It does this by linking the complex functioning units of thee sarcomeres, enabling them to function effectively, and in turn provide a connection for these structures in turn to the sarcolemma, cell–cell junctions, the mitochondria, and nucleus. When not functioning correctly pathologies occur as in e.g. myopathies.