What is Cytoplasm?

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00:00:07.13 Hi. My name is Margaret Gardel and I run a biophysics lab at the
00:00:11.13 University of Chicago. Today I am going to tell you about my favorite
00:00:14.20 problem at the interface of biology and physics.
00:00:17.03 When you first learn about cells, you think about the structure and
00:00:21.06 function of individual organelles. What you learn much less about is
00:00:25.12 the stuff that takes up the rest of the space within the cell known as the
00:00:29.12 cytoplasm. They cytoplasm is about 50% water. The rest of the
00:00:34.16 volume is filled with densely packed globular protein, small organelles,
00:00:39.20 and dense filamentous networks known as the actin, microtubules and
00:00:44.07 intermediate filament cytoskeletons. So this complicated material
00:00:49.10 can be thought of as a complex fluid or visco-elastic solid.
00:00:53.08 The fact that you could call it either liquid or a solid depends.
00:00:56.26 It is said that it's mechanics on both the length and time scales you are
00:01:00.27 considering. This field of soft condensed matter physics is well equipped
00:01:06.22 to build physical models of such complicated materials.
00:01:11.00 The fact that the cytoplasm is made up of macromolecules
00:01:15.11 means that thermal effects are going to be important.
00:01:18.03 If we now go down to the scale of individual proteins, we can, in this
00:01:24.28 animation, can start to see the effects of thermal energy at this
00:01:29.13 nanometer length scale. You see these randomized motions of
00:01:33.02 individual macromolecules known as Brownian motion
00:01:35.14 drives random agitation and collisions between macromolecules.
00:01:40.25 You also get a sense of how crowed the cytoplasm is.
00:01:44.03 All soft material have this type of thermal effects at play
00:01:49.11 and so we have really built physical models of such soft
00:01:54.07 materials in the physical world.
00:01:57.19 What makes the cytoplasm unique is it also contains an abundance
00:02:02.02 of mechanoenzymes. That is proteins that can convert chemical energy
00:02:06.15 to do mechanical work. An excellent example of this is molecular motors.
00:02:10.24 Here is a movie and animation of a motor protein, kinesin, walking
00:02:15.00 along a microtubule track. Right, the kinesin is taking in a chemical
00:02:20.06 energy source and is using that to do mechanical work, to drive its
00:02:24.25 motility and generate local stresses. The fact that the cytoplasm
00:02:29.08 contains all of these mechanoenzymes means that there is an
00:02:31.17 external energy source in addition to just thermal effects. This drives
00:02:37.11 the cytoplasm out of thermodynamic equilibrium
00:02:39.17 and makes it a very unique material.
00:02:43.13 So the cytoplasm is this soft material that is driven far from
00:02:47.08 thermodynamic equilibrium. It has no analogs of
00:02:49.29 similar types of material in the physical world. This means that new
00:02:53.28 physics is required understand the physics of the cytoplasm.
00:02:57.01 I'm going to talk you briefly about two examples
00:03:02.10 of where physics can play an important role in understanding
00:03:04.24 aspects of cell physiology. The first is intercellular transport.
00:03:08.27 For many different aspects of life process in a cell, they have to
00:03:13.11 move organelles and molecules from one side of the cell to the other.
00:03:17.13 The types of cargo that need to be transported range from
00:03:21.27 small molecules like water to up to micron size organelles.
00:03:26.14 Now if we just think about thermal forces and diffusion
00:03:30.16 it's incapable of moving even the smallest cargo over very
00:03:34.09 large distances in a reasonable time.
00:03:36.13 Therefore you need motor proteins transport this cargo
00:03:40.21 along the filamentous tracks. Now if the cytoplasm
00:03:44.07 were just simply water, the effective drag felt on any
00:03:48.05 of these cargo would be negligible and motor proteins
00:03:51.10 could do this quite easily. However the cytoplasm is this
00:03:54.17 very complex environment and these cargo are going
00:03:57.29 to feel very large resistances. How do motor protein overcome
00:04:02.16 this problem to really effectively move cargo?
00:04:05.15 So if we just think about cargo moving in the cytoplasm,
00:04:09.15 depending on your size, you are going to feel a different local
00:04:12.23 environment. Small molecules are going to be able to diffuse
00:04:16.07 rapidly and go in between spaces of globular proteins
00:04:20.12 whereas large cargo are going to be effectively trapped.
00:04:23.23 This means that the type of material you are considering
00:04:27.19 in the cytoplasm really depends on the landscape you are looking at.
00:04:31.01 It will mostly be liquid like at small length scales and it will appear
00:04:35.12 like a solid at long length scales. This transition between a liquid and
00:04:39.12 solid occurs at length scales relevant to organelles. Most proteins
00:04:44.25 will experience a liquid like environment whereas chromosomes
00:04:47.26 will experience a more solid like environment.
00:04:50.26 Now how does a cell stay well mixed to be able to transport all of its objects and
00:04:58.00 and get them in the right place in the right time?
00:04:59.19 Well one recent idea is that the enzymatic activity within the cytoplasm
00:05:05.26 actually serves to mix the cytoplasm up. This is akin to
00:05:11.09 the convective mixing you see a pot of water on a stove top.
00:05:17.03 Right, that eviction really mixes the water in a way that much
00:05:20.04 better than diffusion. So this act of stirring is then thought to liquidize
00:05:25.12 the cytoplasm over all the relevant length scales
00:05:29.09 so the chromosomes also feel a liquid like environment.
00:05:31.24 Now the question is does the enzymatic activity really stir the cytoplasm
00:05:37.16 and is it possible that this enzymatic activity could be regulated
00:05:41.11 to regulate the amount of stirring during different times of the
00:05:46.00 cell metabolism. That is could a cell effectively transition
00:05:49.25 itself from being liquid like to solid like depending on its life stage?
00:05:53.12 The second example I would like to talk about is in cell morphogenesis.
00:05:58.01 At the outer cell membrane, there is a dense network of
00:06:02.02 filamentous proteins known as the actin cortex and this actin cortex
00:06:08.03 is responsible for cell shape changes you see during cell division,
00:06:12.03 cell migration and tissue morphogenesis.
00:06:14.22 So what is the mechanics of this outer polymer network
00:06:19.11 that enables a cell to do these robust shape changes?
00:06:22.13 So to get an idea of what this actin cortex looks like
00:06:26.00 you can see this fluorescence microscopy image of a reconstituted actin cortex
00:06:30.22 where actin filaments are labeled in red. You can see it is quite dense.
00:06:35.10 Over a 100 micromolar. And it is a very thin, dense sheet of actin at
00:06:40.24 at the plasma membrane. And so what are the mechanics of
00:06:44.09 this thin sheet?
00:06:45.24 Well actin networks are semi-flexible polymers.
00:06:49.12 And what semi-flexible means is that their banding rigidity is comparable to
00:06:53.12 thermal energy and the ratio between these two components
00:06:58.26 gives you a length scale that tells you the persistence length,
00:07:02.05 the length scales for which the polymer maintains its rigidity
00:07:05.11 and since this length scale is 17 microns, that means a actin filament
00:07:09.13 will sometimes behave flexible over larger length scales and
00:07:14.09 behave more rigid over shorter length scales.
00:07:17.06 Semi-flexible polymers are almost exclusively found in biological polymers
00:07:22.05 and confer the networks they form with very unique behaviors not seen
00:07:25.25 in physical polymer networks and this area of trying to understand how this
00:07:29.22 semi-flexible affects force transmission is an active area of research.
00:07:33.12 Secondly, we know that actin filaments are not static but they are actively assembling
00:07:38.19 and disassembling and so how does this active assembly and disassembly of
00:07:43.09 the polymers affect their mechanical response to the networks?
00:07:46.29 We simply just don't have the models that integrate polymerization dynamics
00:07:51.17 with the mechanics. Some speculation is the polymerization dynamics are
00:07:55.24 essential for fluidizing the cortex and enabling large shape changes while
00:08:01.01 maintaining a uniform density during processes like cell division.
00:08:04.25 So in addition to the actin filaments in the cortex, the main force generators
00:08:09.26 in the cortex are myosin 2 motors. In this fluorescence image, myosin 2
00:08:14.05 is seen as an elongated rod and at the molecular scale, these
00:08:18.22 elongated rods are actually bipolar filaments of myosin 2
00:08:22.18 motors that drive antiparallel sliding of actin filaments. So these myosin 2 motors
00:08:28.02 generate local stresses. In contrast to the highly organized circameric structure
00:08:33.21 actomyosin you seen in striated myofibrils in muscle cells, in non muscle cells
00:08:40.04 you can see actomyosin is very disorganized. This means the myosin 2 is going to
00:08:46.01 generate forces that vary in space and time. So we can understand. We can start to
00:08:52.00 explore how forces get transmitted in this very disorganized network
00:08:56.18 by reconstituting again these are actin filaments with myosin 2 motors on
00:08:59.25 top of it and we can start to see these random motions of myosin 2.
00:09:04.28 So because the actin cytoskeleton has so many
00:09:09.08 can be controlled spatially, we think that the architecture of the actin cytoskeleton is
00:09:14.09 regulating the activity of the myosin 2 motors but we simply lack the
00:09:17.28 models to understand what is driving these types of responses.
00:09:21.26 And likewise, you can see that the myosin 2 is forming these dense aggregates.
00:09:27.03 This type of aggregation is not seen in wild type cells and so what in the cytoplasm is
00:09:32.01 is allowing myosin 2 contractile forces to not concentrate the cytoplasm
00:09:38.08 in certain regions of the cell.
00:09:40.14 So I’ve given you two examples of outstanding questions in cell mechanics
00:09:46.28 but I think there is even a greater challenge to be had at this interface.
00:09:51.15 For instance, we currently now just build models that are specific to
00:09:56.19 individual aspects of cell physiology. That is we try to understand shape changes during
00:10:00.25 cell division or cell morphogenesis or cell migration but could we build a universal
00:10:05.29 model of cell mechanics that guides a cell in all its different shape changes
00:10:11.13 through its entire life span? Can we really understand all the versatility
00:10:16.18 in the cytoplasm that a cell experiences. Secondly, we have increasing knowledge of
00:10:23.18 the molecular details of cellular processes however these molecular details
00:10:27.15 do not confer the physical control parameters that are guiding the physics of
00:10:33.00 the cytoplasm. So we don't have these control parameters and
00:10:37.13 currently different models confer different amounts of importance to
00:10:44.03 different parameters and so these need to be rectified and we need to have
00:10:48.18 predictive theories that can then be experimentally tested
00:10:51.25 to see which one of these are correct.
00:10:53.25 Hopefully once we know the right control parameters,
00:10:57.20 it will give us increased understanding of how the cytoplasm confers
00:11:01.16 very robust responses in cells. That is, there ability to respond to a large range
00:11:08.02 environmental stimuli and undergo very dramatic shape changes.
00:11:12.10 It may even tell us how these very complicated cytoplasmic structures
00:11:17.07 evolved to give us such a beautiful mechanical responses.
00:11:24.26 And so these and many other questions at the interface of
00:11:28.18 cell biology and physics will give us lots of challenges for years to come.

This Talk

Speaker: Margaret Gardel

Audience:

Educators of Adv. Undergrad / Grad
Researcher

Recorded: May 2014

Talk Overview

Margaret Gardel overviews what is cytoplasm and explains that the cytoplasm is more than just a simple solution inside the cell. It’s a complex, densely packed fluid with unique physical properties ripe for exploration. While explaining what is cytoplasm, she ponders, for example, how motor proteins overcome the physical resistance of carrying cargo, and how cells undergo shape changes in response to different stimuli. To help address these questions, she argues that new theories and approaches are required.

Speaker Bio

Margaret Gardel

Margaret Gardel is an Associate Professor in the Department of Physics, James Franck Institute, and Institute for Biophysical Dynamics at the University of Chicago. She draws from the fields of condensed matter physics and cell biology to study the mechanics of physical behaviors of cells, including adhesion, shape changes, and force generation. Continue Reading