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So far I have been talking to you about the biochemical components

within the cells that are involved in biochemical signaling.

These involve proteins, lipids, other small

molecules that chemically react with one another

to produce chemical signals, and the

transmission is all through chemical, biochemical pathways.

In this lecture, I will talk to you about two

of the kinds of signals signals that are made up

of physical forces or electrical activity and how protein, protein

interactions and protein functions lead to this kinds of signals.

And how these signals are integrated along with

biochemical signals in the cell to produce physiologic functions.

The focus, these are very large areas of studies

and in 20 minutes or so I will not be

able to even give you a full introduction to the biophysics of

forces that control biological functions or electrical activity.

But I will touch on a few salient features of brute force forces

in biology, physical forces in biology, as well as electrical activity in biology.

And refer you to the mm cell biology cha, chapters in the book

on cell biology that will give you a simple overview of these kinds

of signals in biological systems.

So let us get started.

The Intracellular Cytoskeleton - Proteins to filaments.

So the intracellular cytoskeleton is largely made up

of two kinds of filaments, if you want to call it that, or actin

filaments and microtubules made up of tubulin.

Both these filaments assemble from monomer proteins in a vectorial manner.

That is, the beginning and the ends look different and this is sort of an important

aspect of filament assembly, disassembly that is controlled in response to signal.

Filaments are dynamically unstable as they're continuously being formed

at one end by the addition of monomer, and dissolving

if you want to think of it that way,

at the other end due to the shedding of monomers.

And this process of growing at one end and

then dissolving at the other end is called treadmilling.

this, the formation and the sort of

dissolution of these filament filaments at the two ends are controlled biochemically.

And monomeric actin and tubulin have either ATP or GTP bound

to them and upon association into a polymer, the nucleotide is

hydrolyzed AD such that ADP or GDP remain bound to actin or tubulin.

The ability of actin or tubulin to go between the ATP and ADP bound states

allowed them to stay as either part of a filament, or fall off and be monomers.

So you can immediately see that by controlling nucleotide hydrolysis

one can form and the sub filaments eh, and this allows

for a variety of control systems to operate on the lifetime of the filaments.

Intermediate filaments, so that suggests those by desmen, and others pro

rigidity to maintain the cell shape, and in contrast to actin and my microtubules,

they are not often directly regulated by more intrinsically

regulated by such nucleotide hydrolysis.

Force gen.

So, one of the consequences of this formation and this, sort

of dissolution of the filaments is the generation of force.

And there is a very famous model mathematical model, called elastin,

elastic Brownian Ratchet model, that was developed by Alex Mogliner and

George Oster in the early 2000, published in a paper in 2003.

it, it sort of built on a model that developed back

in the late 90s, and so, and I will briefly describe this

model because this is a very good example of how analytical representation

can give you deep understanding of a very important biological process.

This model studies the movement of this bacteria called listeria

that is studied as a prototype for force generation.

And the dep and force generation is

dependent on actin polymerization at the plasma

membrane with the back end of the

actin filaments being anchored in the cytoskeleton.

In this model what what is shown is how the filament can be attached

and then detached or bent, as is shown here, allowing for

the addition of actin monomers right near the membrane.

And disallowing this addition of the monomer to a growing

filament pushes the membrane forward and this sort of pushing

activity due to the addition of monomer at the leading

edge of the actin filament allows the bacteria itself to move.

Not just one filament, but many, many, many at the leading edge.

The relationship between force and velocity for a

single filament is governed by the equation given.

Eh, that Alex Mogliner and George Oster,

developed and it reflects both the free polymerization

velocity, the force exerted by the filament by the

by the growing filament, the length of the filament,

the thermal energy and the depolymerization,

for a given surface load.

At steady state with a constant rate the way

the this equation can be written is to a

provides a velocity and you can see the terms

up here and I won't repeat all the terms.

But this what this equation allows you do is to calculate

velocity as a function of the work done per filament

work done, work done per working filament in

breaking an attachment and the rate of velocity.

The, the one sort of entity I want to point out is there is a limit

velocity which which is the maximum velocity the attachment which is v zero.

Is the velocity at which the attachment bond stretches

to its length to cover the lifetime of the bond.

This is the bond attachment of the filament to the membrane.

So as this filament bends and produces a little bit of extra space

that can that can allow you to add sort of an, that can allow you to add an

actin filament to push actin monomer at the cell surface.

You can then push the membrane forward, and this

kind of pushing allows the entire cell to move.

So this is sort of an example of analytical model of how

the actin polymerization at the plasma

membrane can drive the movement of bacteria.

So in this particular case that I just described the generation of

force was really due to polymerization of the filament active filaments.

Another way or a more common way actually, of how force is

generated within cells is contra is by motor proteins.

Motor proteins such as myosin use ATP hydrolysis to

generate to move along actin filaments to generate force.

This set of actin myosin coupling is

the basis by which muscles fibers develop tension.

This intracellular force can be used to move a variety of

intracellular components, including actin filaments, organelles, and sometimes

even cargoes within to different parts of the cells.

Motor proteins, of course, also contribute to whole cell motility, because by pushing

the filaments forward they can move these, they can move cells as well.

So myosin motors, and there are other

motor proteins such as dianine, and others.

That hydrolyse ATP to generate force.

So this is another major way in

which motor proteins can generate force within cells.

Okay, so it's sort of has become clear by the way I'm describing now is that

biochemical reactions give rise to force within cells.

And you can sort of think of this similar to the way that biochemical reactions give

rise to chemical signals that can be secreted

out of cells for another cell to recognize.

So similarly, if this force can be sort of manifested outside the

cell, or even within the cell, how can this force be sensed?

So does the cell know, how did the cell know

that a certain amount of force exists in its environment?

And what, and it needs to respond to this needs to respond to this, force.

The process by which force ascends is called mechanotransduction.

And this is really not, a conceptual, be at a conceptual level, not very different

from the way in which, chemical signals are transduced to the plasma membranes.

Interactions between say the extracellular mode matrix and integrins

and actin filaments, deforms the either extracellular matrix and generates force.

And this is shown here in terms of the matrix

proteins and the filaments out here, and it is the

integrin pulling between the matrix protein and the filaments that generates

force This force activates channels that are mechano sensitive channels.

Receptors such as integrin and a variety of signalling enzymes such

as tyrosine kinases and phosphatases and proteins

adapter proteins such as cas by altering their structures.

So typically the application of force to a protein will allow many bonds to

sort of be broken is opening or altering the structure of the protein and

this opening or altering the structure leads to exposure of domains that may not

have been, protein domains that may not

have been previously accessible to interaction partners.

And making these interaction domains accessible allows them to control

Allows them to pra, convert the mechanical signal into a biochemical signal.

Once a biochemical signal is sort of initiated and this is shown in the lower

picture, it can then lower schematic, it can then in turn activate

small G proteins by regulating their GEFs and GAPs.

And these G proteins can in turn, can transmit signal to

cytoskeletal regulators, forming a feedback loop,

where these G proteins, like I

showed you in the previous slides, can connect to biochemical and transcriptional

pathways to affect gene regulation or metabolic processes within the cell.

So one can actually use the standard biochemical approach of

connecting a variety of signaling components to form a network.

And this picture that although it looks a little

complicated really shows how an extracellular matrix protein, by

interacting with an integrin can go through a series of,

cell signalling molecules at the cell surface in cytoplasm.

To regulate proteins that in turn control actin filament

growth and branching the, the, the target proteins are

shown somewhere out here, and, and these target proteins

in turn can reg, regulate filament gro, growth and

capping and, it does control cell movement.

In this paper that a graduate student in my lab had developed

a model for as part of a thesis research, Our partnering was able

to show that the filament growth

process can be monitored computationally, using the

hybrid models that can ta, that

combines deterministic model of the signalling reactions.

That is, how the signal flow from say, here to here, or from here to here.

To to control the growth of filaments and the, and the stochastic modelling

of the growth of the actin filament network is shown here in a series of

still pictures of how from starting from the cell spreading on a fibronectin

surface, how the filaments grow as series of time, to allow the cell to spread.

So the biochemistry and the mechanical forces can be captured

so that one can really understand control of both

force generation, force sensing, and the utility

of the force that is generated in terms of cellular behaviors.

So the take home points in this

part on bioforce and biological, cell biological systems are the following.

Interactions between specific components lead to

the formation of specific proteins lead to

the formation of filaments and other structures

that are capable of generating mechanical forces.

Force generation within cells can be analytically modeled and the

Mogilner-Oster model for the Elastic Brownian Ratchet shows how the modeling of

this force generation and propagation can be used to,

to can be used to sort of describe how a bacterium can move.

Or other mammalian cells as well.

The interaction between molecular motors and the filaments is

another major way in which force is generated within cells.

And molecular motors are very important for a variety of cellular functions

including secretion or movement of the molecules to the

to various parts of a neuron, the generation of force and tension

in muscle cells, and so on.

Force generation by actin filaments is regulated by cell-signaling network, and,

conversely, force signals can be, sensed by the signaling networks.

And, the sensing is called mechanotransduction.

And, so mechanotransduction plays a very important part in

balancing the, both the generation of force and the sensing of force, so that

cells live in an environment that is ver, that is homeostatic or

balanced control of force in relationship to other activities of the cells.

Hybrid models, and we'll deal with what hybrid models

are in the coming slides, which combines both deterministic and

stochastic models, can be used to study cell-signaling

networks, regulation of cell spreading through the control of force generation.

So here one can go from sort of biochemical and elementary force

reactions to wholesale behavior using sort of quantitative approaches for this.

Thank you.

Mechanical Forces in Cell Biology

系统生物学导论

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