Control of Cell Number - csus.edu

B. Control of Cell Number

1. Review of the mitotic cell cycle and cell death

2. Regulation of cell number and quality during mitosis

3. Regulation of cell type during mitosis

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Numbers in a Cell Population

• Cell number is a combination of....

• Cell divisions – Cell deaths (necrotic + programmed)

• Necrosis is premature cell death– disease, injury, starvation, toxicity, excitotoxicity

• Programmed cell death is death by design– apoptosis, anoikis, cornification, autophagy

• Same for an organism, system, organ or tissue, and for single cell populations in an ecosystem

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Figure 18-13 Molecular Biology of the Cell (© Garland Science 2008)

Programmed cell death is an essential component

It guarantees appropriate ennervation patterns

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Figure Q18-1 Molecular Biology of the Cell (© Garland Science 2008)

Even gives us our fingers and toes!

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Figure 17-51a Molecular Biology of the Cell (© Garland Science 2008)

Overview of the Mitotic Cell Cycle and Cell Division.Diploid cells duplicate the contents of their cytosol and nucleus prior to splitting to form two genetically exact daughter cells.

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We’ve learned to both control it.........

A mutation in a signal molecule that limits muscle cell division has been bred in.

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Fig. 11‐19

2 µm

And fear it.........

A normal cell next to a tumor derived from uncontrolled cell divisions

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Like everything else, the process of cell division has evolved over time

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Fig. 12‐11‐1

Origin ofreplication

Two copiesof origin

E. coli cellBacterialchromosome

Plasmamembrane

Cell wall

Prokaryotic Divisionis called binary fission.

Two copies of circular DNA, beginning at the origin of replication, actively move apart from each other.

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Fig. 12‐11‐2


Two copiesof origin


Plasmamembrane

Cell wall

Origin Origin

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Fig. 12‐11‐4


Two copiesof origin


Plasmamembrane

Cell wall

Origin Origin

When the daughter chromosomes reach the opposite poles, the cell separates into two daughter cells

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Fig. 12‐12

(a) Bacteria

Bacterialchromosome

Chromosomes

Microtubules

Intact nuclearenvelope

(b) Dinoflagellates

Kinetochoremicrotubule

Intact nuclearenvelope

(c) Diatoms and yeasts

Kinetochoremicrotubule

Fragments ofnuclear envelope

(d) Most eukaryotes

Since prokaryotes evolved before eukaryotes, mitosis probably evolved from binary fission.

Certain protists exhibit types of cell division that seem intermediate between binary fission and mitosis.

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The complex mitotic cell cycle is used by the organism to control:

cell numbercell qualitycell type

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TargetMechanisms

CellNumber

CellType

CellQuality

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Figure 17-62 (part 1 of 3) Molecular Biology of the Cell (© Garland Science 2008)

We’ve even looked at one pathway already

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Fig. 12‐5

S(DNA synthesis)

G1

G2

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Interphase does the real work of mitosis

• Gap 1 (G1) is when the cell gets ready for DNA synthesis– Need DNA synthase complexes, repair enzymes, histones, etc..

• S‐phase is when the cell synthesizes and edits chromosomes– Synthesis and error editing are the primary objectives.

• Gap 2 (G2) is when the cell gets ready for cell division– Need organelles, cytoskeletal proteins, molecular motors,

metabolic enzymes, etc.– Kinetochores are refined and finalized.

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Fig. 12‐UN1

Telophase andCytokinesis

Anaphase

Metaphase

Prometaphase

Prophase

MITOTIC (M) PHASE

Cytokinesis

Mitosis

SG1

G2

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M‐Phase

• Prophase: chromosomes condense with kinetichores, centrisomes move to poles, nuclear membrane disintegrates

• Metaphase: spindle fibers attach and push to midline

• Anaphase: kinetichores pull sister chromatids to poles

• Telophase: the reversal of prophase activities

• Cytokinesis: actin‐based separation of cytosol into daughters

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Sister chromatids are attached to each other at centromeres andeach is individually attached to a kinetochore from the opposite pole

metaphase

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Figure 17-43b Molecular Biology of the Cell (© Garland Science 2008)

anaphase

Sister chromatids separate from each other, producing 92 individual chromosomes in humans, and are pulled by kinetichores to the poles.

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dynein motors

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Cytokinesis results from the assembly of an actin‐myosin ring that gets smaller and smaller as mysoin pulls actin along actin. Ring assembly is microtubule‐dependent and myosin light chain kinase must be phosphorylated to begin.

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The cleavage furrow

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Cell Division in Plants

In some plants growth continues over the life of the organism

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Regulation of the Cell Cycle

• There are four major ‘checkpoints’ that monitor sensor systems and trigger molecular switches to get to next stage– The G1/S checkpoint – to enter the cycle or not– The S‐checkpoint – to synthesize DNA or not– The G2/M checkpoint – to divide the cell or not– The M‐checkpoint – to shift from metaphase to anaphase

• The sensor systems are focused outside the cell as well as inside the cell to make sure conditions are appropriate

• When ‘off’, the molecular switches halt progression, when ‘on’ they biochemically start the next stage

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Fig. 12‐14

SG1

M checkpoint

G2M

Controlsystem

G1 checkpoint

G2 checkpoint

S checkpoint

The book claims 3 and describes 4, so we’ll go with 4

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• For many cells, the G1 checkpoint seems to be the most important one

• If a cell receives a go‐ahead signal at the G1 checkpoint, it will usually complete the S, G2, and M phases and divide

• If the cell does not receive the go‐ahead signal, it will exit the cycle, switching into a nondividing state called the G0 phase

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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Fig. 12‐15

G1

G0

G1 checkpoint

(a) Cell receives a go‐aheadsignal

G1

(b) Cell does not receive ago‐ahead signal

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Stop and Go Signals at the G1 Checkpoint

• In multicellular organisms, the classic required external signals are growth factors, cytokines and hormones

• Most cells also exhibit anchorage dependence, in which they must be attached in order to divide

• Most cells also exhibit density‐dependent inhibition, in which crowded cells stop dividing

• Many single cells also monitor external nutrient availability and will not divide if it is inadequate

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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The Cell Cycle Switches at the First Three Checkpoints are Molecules Called Cyclins and Cyclin‐Dependent Kinases

Cyclins are expressed and degraded, when present the cell moves forward

1. G1 and G1/S cyclins regulate the G1 checkpoint

2. S cyclins regulate S checkpoint

3. M cyclins regulate G2/M checkpoint

Cyclin‐dependent kinases are always present and are activated by the binding of their appropriate cyclin

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Cyclin isdegraded

Cdk

MPF

G2checkpoint

Cyclin

Cyclinaccum

ulation

1. Cdk is always there, 2. if conditions are OK cyclin is expressed,3. the combination is an active kinase, and 4. activity is lost with cyclin

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What does cyclin‐cdk phosphorylate?

• G1‐cdk– Rb protein: loses inhibition of E2F

– E2F then activates transcription of S‐cyclin

• M‐cdk– histones: chromosome condensation

– lamins: nuclear membrane fragmentation

– myosin: purse‐string cytokinesis

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So, back to our growth factor example.....

What does immediate early gene expression cause?

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Immediate early gene, Myc, expression activates the expression of G1 cyclin

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M Checkpoint Regulation

• If any kinetochores are not attached to spindle microtubules during metaphase they send a molecular signal that delays cycle progression

• The signal activates the anaphase‐promoting complex, or cyclosome, known as APC/C

• APC/C is a protease that destroys:– cyclins: stopping cell cycle– securin: which drives sister chromatid separation

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Apoptosis During Mitosis Controls Cell Quality

• Every cell cycle in multicellular organisms are molecularly hardwired with apoptosis as a potential outcome if things aren’t just right

• The primary cause is imperfect DNA copying

• This is one of our primary defenses against erroneous or uncontrolled proliferation

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Characteristics: 1. Cessation of DNA repair mechanisms2. Cell shrinkage3. Nuclear membrane blebbing

4. DNA fragmentation5. Death

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Can result from bad copies during DNA replication

p%#Mdm2 will cause p53 degradation

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p21 has been shown to block cyclin/cdk activity at G1‐S, S and G2‐M

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The Roles of p21 and PCNA

• The apoptotic clock:

– PCNA = the proliferating cell nuclear antigen

– If [p21] rises to 5X [PCNA] apoptosis will happen

– p21 will reach binding affinity for PCNA

– The dimer will shut off DNA repair mechanisms

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When apoptosis is inactive, Bcl2 is bound to APAF‐1 and the dimer acts to block assemblage of mitochondrial membrane

channel proteins, BH123 (Bax and Bac) ,

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The loss of DNA repair mechanisms:

1. Activates BH3‐only proteins

2. Signal Bax translocation to the mitochondria

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apoptosisinducing factor

Bax/Bac dimers

BH3‐only inactivates Bcl2/APAF‐1 inhibition and along with Bax dimerization with Bac forms active channels

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Release of cytochrome C from the mitochondria outcompetes Bcl2 for APAf‐1 and activates assembly of the apoptosome

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(8, 9, 10)

(3, 6, 7)

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Characteristics: 1. Cessation of DNA repair mechanisms

2. Cell shrinkage3. Nuclear membrane blebbing

4. DNA fragmentation5. Death

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Extrinsic apoptotic cascade also uses the common caspase cascade

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3. Regulation of cell type during mitosis

• Review – Symmetric vs. Asymmetric Cell Division

– Stem Cell and Embryonic Cell Divisions

• Regulation– Control in the Cytosol

– Control in the Nucleus

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Common Cell Divisions Produce Daughters Like the Parent to Replenish the Population

Common during growth and the repair of damaged tissues

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The Stem Cell Concept

• Asymmetric division of stem cells produces one new stem cell and one differentiated daughter– Many stem cells in the embryo and adult

• In some organs: frequent replenishing divisions– gut, epidermis, bone marrow

– example: billions of blood cells are destroyed by the spleen every hour

• In others, they only divide in response to stress or the need to repair the organ– heart, prostate

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Stem Cell Mitosis

Each division produces daughter cell(s) unlike the parent cell:

‐ The first two are asymmetricand produce one stem cell

‐ The last one produces two like daughters unlike parent

HSC

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Stem Cell Types: Embryonic Stem Cells

The Inner Cell Mass

Produces all cells of the embryo

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Stem Cell Types: Adult Stem Cells

• Committed stem cells with limited potential

– hematopietic stem cells ‐ hair stem cells

– mesenchymal stem cells ‐melanocyte stem cells

– epidermal stem cells ‐muscle stem cells

– neural stem cells ‐ tooth stem cells

– gut stem cells ‐ germline stem cells

– mammary stem cells

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The hair shaft is composed of keratinocytes, lubricated by sebaceous secretions, with melanin for color.

‐ “Bulge” is the stem cell niche for hair basal cells,sebocytes and melanocytes.

‐ The first two arise from a common stem cell butmelanocytes arise from a committed stem cell.

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Hematopoietic stem cells in the bone marrow are the kings of differentiation choices

Progenitor cells will produce two daughters like each other but not like the parent, with the choice being determined by the current, local signaling combination

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Control of Assymetrical Division in the Cytosol

• G2 is the key to differentiation events

• The daughter cells can be built differently– DNA is the same but....

– RNA’s can be different

– Proteins can be different

– Cytoskeleton and organelles can even be different

• The parent cell must focus its placement of these components on either side of the furrow

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Isolation of transcription factors across midline

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Fig. 18‐15a

Two differentcytoplasmicdeterminants

Unfertilized egg cell

Sperm

Fertilization

Zygote

Mitoticcell division

Two‐celledembryoshave twodifferentcell types

NucleusThe unfertilized egg is the queen of cytoplasmicisolation of cell fate determinants

All stem cells do it effectively

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We use the embryonic process of cleavage to disperse the careful distribution of cytoplasmicdeterminants laid down in the egg.

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The G‐phases of somatic mitosis allow for cytoplasmic growth so that the daughter cells are equal in size to the parent cell.

In cleavage we want to use the egg cytoplasmic material so we just skip the G‐phases all together.

It makes the cell cycle go very fast!‐ Frogs can make 37,000 cells in 43 hours.‐ Fruit flies can make 50,000 in 12 hours (10 min!)

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They don’t even bother to make plasmamembranes until later!

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Control of asymmetric division in the nucleus

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Two DNA methyltransferases are important in modifying DNA

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Development of pluripotency in the inner cell mass depends on methylation pattern

Both parent gametes have methylation patterns that must be removed for all genes to be available to the developing organism.

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One of the big hurdles to somatic cell nuclear transfer cloning was overcoming the adult methylation pattern

It took hundreds of failed attempts before the successful cloning of Dolly from adult mammary epithelium.

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Each step of differentiation of a given cell type depends on changing methylation patterns

These genes maximize effectiveness when coding for transcription factors, SNuRPs, signaling receptors, etc.

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Control of Cell Number - csus.edu