Paper
Module
: 06 Animal Physiology
: 02 Neuronal Cytoskeleton
Development Team
Principal Investigator
:
Prof. Neeta Sehgal
Department of Zoology, University of Delhi
Co-Principal Investigator :
Prof. D.K. Singh
Department of Zoology, University of Delhi
Paper Coordinator
:
Prof. Rakesh Kumar Seth
Department of Zoology, University of Delhi
Content Writer
: Dr. Sarita Nanda1, Dr. Varsha Baweja2, Dr. Anju Jain1
1. Daulat Ram College;
2. Deshbandhu College
University of Delhi
: Prof. Neeta Sehgal
Department of Zoology, University of Delhi
Content Reviewer
ZOOLOGY
Animal Physiology
Neuronal Cytoskeleton
Description of Module
Subject Name
ZOOLOGY
Paper Name
Zool 006 Animal Physiology
Module Name/Title
Organization and Evolution of Nervous system
Module Id
M02 Neuronal Cytoskeleton
Keywords
Microtubule, MAP, Tau, Microfilament, Intermediate filament, Motor
proteins, Kinesin, Dynein, Myosin
Contents
1.
Learning Objectives
2.
Introduction
3.
Microtubule
3.1. Characteristics of Microtubules in neuronal cells
3.2. Microtubule associated protein (MAP) in neuronal cells
4.
Intermediate Filaments
5.
Microfilaments
6.
Molecular Motors
6.1. Motor proteins associated with microfilaments in neurons
6.2. Motor proteins associated with microtubules in neurons
7.
Roles of molecular motors in neurons
8.
Interaction of cytoskeleton elements
8.1. Interaction in neuron-neuron connections
8.1.1. Neuron – Neuron Interaction
8.1.2. Neuron-glial cells Interaction
9.
Changes in cytoskeleton elements in the neuropathology
9.1. Changes in cytoskeleton elements after injury and during regeneration
9.2. Changes in cytoskeleton elements in neuropathology
10. Regulation of Function of cytoskeletal proteins
11. Summary
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Animal Physiology
Neuronal Cytoskeleton
1. Learning Objectives
At the end of this module the students will learn about
 The cytoskeleton elements present in the cells of the nervous system
 The details of microtubules and proteins associated with them
 The details of intermediate filament
 The details of microfilaments and the proteins associated with them.
 The Motor proteins associated with microtubule and microfilament
 Role of Motor Proteins
 Interaction within cytoskeleton element
 Changes of cytoskeleton in regeneration and injury
 Regulation of function of cytoskeleton element
2. Introduction
Neurons are one of the largest cells present in the human body. They have unique structure
and function. Like all other cells the shape and structure of the neuron is dependent on the
framework of cytoskeleton elements present in these cells. This cytoskeleton framework also
provides the pathways to move organelles and neurotransmitters within the cell. These
cytoskeleton elements include all the essential elements like microtubules, intermediate
filaments and the microfilaments (Fig 1). These elements are stabilized by association with
proteins like microtubule associated protein (MAP), tau etc. The movement of organelles
and neurotransmitters are facilitated with motor protein molecules like myosin, kinesin and
dynein.
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Animal Physiology
Neuronal Cytoskeleton
3. Microtubule
3.1. Characteristics of Microtubules in neuronal cells
The microtubules (MT) in the axon and dendrites are different from other cells as they lie free
in the cell and are not seen emanating from the microtubule originating centre (MTOC).
The organization of polarity of MT is different in the axons and dendrites. All the MTs in the
axon are arranged with their plus ends away from the cell body. On the other hand, half of
the plus end MTs in the dendrites face away from the body and other half show plus end
towards the cell body (Fig 2)
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Animal Physiology
Neuronal Cytoskeleton
The MT is composed of subunit alpha, beta, gamma-tubulins as in other neuronal cells but
they exist in isoforms which are exclusive to the brain. These also exhibit modifications in
form of acetylation, tyrosination, phosphorylations etc. These neuronal MTs exhibit greater
stability as they do not depolymerise easily and maintain longer length than any other cell.
Table 1: Microtubule associated proteins of the neurons
Protein present
Form present
Alpha Tubulin Isoform
Beta Tubulin
Isoform
Gamma Tubulin Normal
Map 1a
Map 1 b
Map 2a
Appears later
Appears early but
declines later
High Molecular weight
Map 2b
High Molecular weight
Tau
Isoforms
Location
All cells
Modification
Acetylation,
tyrosination
Phosphorylation
All Cells
All Cells, Microtubular
Organising Centre (MTOC)
All cells
Phosphorylation
Present in axons
Phosphorylation
Present in dendrites of
phosphorylation
mature neurons
Present throughout lifetime Phosphorylation
in dendrites
Present in axons
Phosphorylation
3.2. Microtubule associated protein (MAP) in neuronal cells
MTs are associated with proteins called Microtubule Associated Protein (MAP)which
maintain their stability, interaction with other elements and in their functions. The MAPS of
the brain are MAP 1A, MAP 1 B, MAP 2A, MAP 2B, Tau protein, MAP 3, MAP 4 m,
molecular motor proteins kinesin and dynein, Structural MAPs. MAPs help to enable the MT
to assemble and disassemble. The molecular motors help in transport of organelles and
neurotransmitters. The structural MAPs help to interact with other cytoskeleton proteins.
The distribution of these MAPs is different in various parts of the neurons. The soma and
dendrites show the presence of MAP2 whereas the axons are rich with Tau. MAP1a and
MAP1b are present in the entire neuron- soma, dendrite and axon but MAP 1b is
phosphorylated in axon only. Six types of tau isoforms have been identified. (Table 1)
MTs act as tracks on which the membrane-bound organelles and vesicles of neurotransmitters
are transported in retrograde and anterograde movement. In the anterograde movement, the
ZOOLOGY
Animal Physiology
Neuronal Cytoskeleton
movement is away from the cell body whereas in retrograde the movement is in reverse
direction from tail to head.
4. Intermediate Filaments
Intermediate filaments (IF) are typical in each tissue and are therefore used as biomarkers.
The intermediates in the neuron are called the Neurofilament (NF) and those in the glial cell
are called Glial Filament (GF). The NFs and the GFs possess several types of intermediate
filaments which maintain their characteristic shape and the function. The NFs have
projections from the sides unlike the IFs of the other cells and they form a loose network. On
the other hand, the GF do not show any projections like the cells of other parts of the body
and they form a closely spaced network.
The NFs are made up of three protein subunits:
High molecular weight protein NFH (180-200 kDa), Middle molecular weight protein NFM
(130kDa-170 kDa) and low molecular weight NFL (60-70 kDa) protein. All three proteins
forms intertwine to form a core protein at the amino terminal end. The NFH and the NFM
possess a long terminal carboxyl end which projects out as side arms. These side arms can be
phosphorylated which increases their charge density and repulsion.
Vimentin is another type of IF present in the microglial cells, neuronal and glial precursors.
GFAP- Glial Fibrillar Acid Protein are the other IF proteins present in the astrocyte and
Schwann cells. GFAP is tightly packed as it doesn't have side arms. Some other IF like Alpha
internexin, peripherin and nestin are present in some neurons and neuron precursors. Alpha
internexin is present in both central Nervous system (CNS) and peripheral nervous system
(PNS). And they are observed in developing brain rather than mature brain. Peripherin is
seen only in PNS. Nestin is associated with cells participating in the development of the
brain. Therefore, it can act as marker of neurogenesis. Some of the mature oligodendrocytes
lack IF.
IF are not only essential for cell survival but help to maintain the shape and structure as they
are one of the most stable cytoskeletal element.
ZOOLOGY
Animal Physiology
Neuronal Cytoskeleton
5. Microfilaments
The microfilaments are abundant in cells of the nervous system. But their location is more
concentrated near their plasma membrane and the presynaptic terminal, spines of the
dendrites and growth cones. They form a network and are prominent cytoskeletal elements in
the mature neurons. MFs are of varying lengths like they are short in neurons, longer in
growth cones and forming bundles in filopodia and lamellipodia. They do not form stress
fibers like other cells of the body.
There are several MF associated proteins maintain the structure and stability of the proteins.
Two of them are spectrin and ankyrin. Different isozymes are present in dendrites and axons.
Fimbrins help to form actin bundles in growth cone. Similarly, other actin based proteins like
Gelsolin, Profilin, beta Thymosin etc are seen to associated with neuro- cytoskeleton.
The microfilaments interact with plasma proteins and they restrict the movement of proteins
axons and dendrites. This maintains the characteristic of protein distribution in axons and
dendrites. They also help the neurons and the glial cells to interact with extracellular matrix
and cell-cell interaction through tight junction and focal adhesions. They also help to hold
the cell organelles in the cell cortex. They also help in myelination, lamellipodia and
filopodia formation during cell movement. The regenerating and developing neurons have
fast growing ends which are rich in actin microfilaments
6. Molecular Motors
Molecular motors help to transport proteins, neurotransmitters and organelles in both
anterograde (movement from soma to axon) and retrograde manner (movement from axon
towards soma). These molecular motors are various forms of myosin which work on actin
tracks. Dynein and kinesin are the molecular motors which work on microtubular tracks.
6.1. Motor proteins associated with microfilaments in neurons
Several types of myosin proteins work with microfilaments. These have been found to be
Myosin 1b, II b, V a, VI, VII a, IX a, and X. They are similar in structure to motor proteins
ZOOLOGY
Animal Physiology
Neuronal Cytoskeleton
present with cytoskeleton present in other parts of the body. The cargo is carried by the tail
portion of the cargo.
6.2. Motor proteins associated with microtubules in neurons
The microtubules of neurons are associated with kinesin and dynein as in other cells. The
kinesins associated with microtubules are Kinesin I, II, III, IV, XIII, XIV. They help in both
anterograde and retrograde movements. Kinesin I, II, III, IV are types which have head
towards their n terminal end and they show movement towards the positive end of the
microtubule. Kinesin XIII has head in the middle and these can destabilize the tracks.
Kinesin XIV have their head towards the C terminal end and these show movement towards
the negative end of the microtubule.
Another motor protein dynein is also associated with microtubule. It is similar to dynein
present in any other tissue. These are large molecules which have their motor domain located
on the C terminal end. It carries cargo on its tail towards the negative end of the microtubule.
Dynein is associated with dynactin which is an adapter protein linking the dynein with
microtubule.
Dynactin have shown to be important in the dendritic polarization and
neurogenesis.
7. Roles of molecular motors in neurons
The molecular motors help in some of the following functions related with neurons (Fig 3)
1. Movement of mitochondria within the cell.
2. Movement of cytoskeletal elements within the cell
3. Polarisation related factors transport
4. Internalisation of Growth factors like Nerve growth factors (NGF), Brain derived growth
factors (BDGF), Neurotrophin 3 and 4 (NT3, NT 4) and their receptors after their
interaction on the cell surface with dynein to the cell nucleus where they induce gene
expression to promote growth.
ZOOLOGY
Animal Physiology
Neuronal Cytoskeleton
NGF with its receptor TrKA interact at the plasma membrane. Their message move in
the retrograde manner toward cell nucleus with the help of dynein. This helps in cell
survival. On the other hand, disrupting the movement of dynein through a molecule
called dymantin promotes atrophy and cell death of neurons. Similarly, BDGF promotes
dendrites development after its interaction with its receptor Trk B. Here the endosome is
internalized by dynein after its interaction with snapin.
5. The neurotransmitter (NT) release at the synapse also requires the participation of
molecular motors. The movement of vesicles to the presynapse requires the participation
of synaptophysin, synaptotagmin, Rab 3A, Myosin for the anterograde movement. The
post synaptic retrograde movement requires the participation of Myosin Va and VI.
6. The sensory organs like ears require myosin I c, Myosin III for the functioning of
stereocilia.
8. Interaction of cytoskeleton elements
A close interaction has been observed in the process of neurogenesis, interaction of neurons
with the glial cells as well as in injury, regeneration and neuropathology.
ZOOLOGY
Animal Physiology
Neuronal Cytoskeleton
8.1. Interaction in neuron-neuron connections
8.1.1. Neuron – Neuron Interaction
When synapse has to be formed between two neurons the growth cone present at the end of
terminal ends grow towards the adjoining neuron. The neurite projecting from the growth
cone senses the direction in which it should grow. These growing ends has to rearrange the
cytoskeleton elements to achieve growth in the particular direction. While rearranging these
elements it may form pointed projections, or web like projections like lamellipodia or
filopodia.
8.1.2. Neuron-glial cells Interaction
The myelinated axons are rich in neurofilaments widely separated by spacer arms on the
sides. The spaces are occupied by microtubules, organelles and short microfilaments. It is
hypothesized that during myelination there is reorganization of these elements. The NFS are
signaled to pack closely due to the reduction of phosphorylation of NFH and NFM tail
domain.
9. Changes in cytoskeleton elements in the neuropathology
9.1. Changes in cytoskeleton elements after injury and during regeneration
It has been observed that PNS neurons repair much faster and more effectively than CNS
neurons and in some incidences CNS neurons may not repair at all. Following the injury in
the CNS the astrocytes proliferate to repair the area and may result in forming a scar. This
area is observed to be full of GFAP IF bundles. This may help to repair but the scar may
obstruct neuronal elongation and repair. Gene expression studies during regeneration of
neurons have shown greater synthesis of MT and MF proteins rather than NFs indicating
greater participation of MT and MF in regeneration.
9.2. Changes in cytoskeleton elements in neuropathology
Initially the neuropathological conditions may not be associated with any change in structure
and function of cytoskeleton elements but latter on as these functions get disrupted the
neuronal function gets disturbed. Many neuropathological diseases like Amyotrophic lateral
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Neuronal Cytoskeleton
sclerosis are associated with changes in NFs present in the axons. Alzheimers and parkinsons
disease is associated with aggregation of cytoskeleton elements.
10. Regulation of Function of cytoskeletal proteins
One of the popular method of regulation of cytoskeletal elements is to phosphorylate NFs and
tau proteins in axons. Similarly, Phosphorylation has also been observed in MAPs and MF
associated proteins. This phosphorylation will all be affected by regulations of kinases and
phosphotases present around.
11. Summary
Neurons have unique structure which is supported by neuroskeleton made up of microtubule
(MT), microfilament (MF) and intermediate filaments (IF). These elements give it structure,
function and stability of the cells. These microtubules are similar to non neuronal cells
except for few modifications. Some isoforms present in the neurons are specific for the brain
and these may acquire modifications by acetylations, phosphorylation etc. Microtubules are
stabilized by microtubular associated proteins (MAPs) as in other non-neuronal cells but their
isoforms and their arrangements differ in dendrites and axons. The microtubules in the axon
are organized in a specific manner with their positive ends away from the soma whereas the
dendrites have the microtubules arranged in mixed order i.e. positive end either away or
towards the soma.
Their stabilizing MAPs are also differentially distributed with Map 1 a, 1b and various
isoforms of Tau found in the axons whereas Map 2a and 2b found in the dendrites. The
intermediates filaments of neuron are of three kinds NFL, NFM and NFH whereas vimentin
and GFAP in Schwann cells and in the glial cells which can be used as biomarkers of the
brain tissue. The microfilaments are similar to non-neuronal cells which help in neurogenesis,
plasticity, cell –cell junctions, restriction of movement of proteins in axons and dendrites.
The motor proteins associated with cytoskeleton are similar to non-neuronal cells which
participate in transport of proteins, organelles, neurotransmitters etc. There are several kinds
of kinesin-like N- kinesin, M Kinesin and C kinesin and dynein which travel on microtubular
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Animal Physiology
Neuronal Cytoskeleton
tracks. The dyneins support retrograde movement whereas the kinesins support anterograde
and retrograde movements. On the other hand, different kinds of myosins like Myosin 1b, II
b, V a, VI, VII a, IX a, and X act as motor proteins which carry cargo of proteins,
cytoskeleton elements on the Polarity: The organization of cytoskeleton elements with N
terminal end acting as positive end and C terminal end acting as the negative end.
ZOOLOGY
Animal Physiology
Neuronal Cytoskeleton