first few

Cell fate

Results from a series of responses to signals, involving transcriptional changes and protein changes, that eventually produce a new cellular identity

Transcriptional changes

Altered gene transcription (RNA synthesis) in response to a signal; part of the slow response pathway

Protein changes

Altered protein synthesis (translation) that follows transcriptional changes, contributing to altered cytoplasmic machinery and behavior

3 Stages from stem cell to cell type

Developmental potential; commitment/determination; differentiation

Developmental potential

Initial stage where stem cells are undifferentiated and have numerous potential fates

Commitment/Determination

Progression of changes that limits a cell's identity options

Differentiation

Progression of changes to a specific cell type; once fate is determined it must be maintained in the cell and its progeny

Terminally differentiated cell

A cell that has reached its final fate and can no longer divide

Differentiation properties

Progressive; temporally regulated; spatially regulated; requires coordination of expression of many gene sets to specify each cell type

Dental papilla cells

Small, undifferentiated cells committed to only a few fates; the first stage in odontoblast differentiation

Preodontoblasts

Enlarged and elongated cells; the second stage in odontoblast differentiation

Odontoblasts

Columnar cells with a prominent nucleus and large cytoplasm for synthesizing organic matrix to be secreted; the third/final stage of differentiation

Concept 1

Cell fate results from regulation of gene expression using more than one type of regulatory control, appropriate for time, place, and eventual fate

Where growth factors act in histodifferentiation

Time: cap/bell stage; Place: begins at the enamel knot and proceeds apically toward the CEJ

7 Steps for regulation of gene expression

1. Transcriptional control 2. RNA-processing control 3. RNA transport and localization control 4. mRNA degradation control 5. Translational control 6. Protein degradation control 7. Protein activity control

Predominant gene regulation mechanism

Transcriptional control

Transcriptional regulators

Determine the time, place, quantity, and which mRNAs are made

Promoter

Specific DNA sequence where RNA polymerase binds to initiate transcription

Transcription factors

Proteins that recognize and bind DNA; can activate/induce or repress/inhibit RNA transcription

Cis-regulatory sequence

A DNA sequence (roughly 5-10 nucleotides) upstream of the transcription start site that helps regulate transcription

Gene control region

Consists of a promoter plus many cis-regulatory DNA sequences in a complex arrangement upstream of the coding region

Why both repressors and activators are needed

To finetune the transcription of each gene

Combinatorial transcriptional regulation

Cell fate is determined by combinations of different regulatory proteins present in a cell, so distinct combinations produce distinct cell types from one precursor

Concept 1 summary skills

Explain how transcription factors and external signals regulate gene expression; remember each step needs to be appropriate for time, place, and fate

Concept 2

Signal transduction output depends on the activities of prior and upstream signals

Positive feedback

When a later product of a pathway acts on an earlier step to stimulate its own continued production or activity

Negative feedback

When a later product of a pathway (feedback inhibitor) acts on an earlier step to inhibit continued production or activity

Master transcription regulator (positive feedback example)

A transient signal turns on synthesis of protein A; protein A acts as a transcription factor that binds its own promoter and makes more of itself, maintaining gene expression even after the initial signal is gone

Why feedback regulation matters for signaling

It allows a cell to respond differently to the same signal at a later point in development

Crosstalk

When one signaling pathway affects another, often via a shared component, allowing integration of multiple signals and rigorous control of gene expression

Concept 2 summary skills

Describe positive and negative feedback; understand crosstalk in signal transduction

Concept 3

Cell fate is the end product of a series of signaling events and responses

Reciprocal induction

The process where epithelial cells signal ectomesenchymal cells and ectomesenchymal cells signal epithelial cells at every step of odontogenesis

Stages of tooth development

Dental lamina, bud, cap, bell, late bell

Enamel knot

Signaling center in the cap stage that coordinates morphogenesis of the tooth crown

Secondary enamel knots

Signaling centers involved in differentiation and mineralization

FGF signaling receptor type

Receptor tyrosine kinases (RTK)

BMP signaling receptor type

Receptor serine/threonine kinases (RSTK)

Pax9

A transcription factor activated by FGF8 signaling and inhibited by BMP4/BMP2 signaling

How FGF8 and BMP pattern the odontogenic placode

FGF8 activates Pax9 transcription ("on"), driving cells toward an odontogenic placode fate; BMP4/BMP2 inhibit Pax9 transcription ("off"), so those cells do not acquire a tooth fate

TGF-beta/Smad signaling pathway

Pathway activated by BMP4 or BMP2 binding their receptor; involves SMAD proteins forming a complex with SMAD4, passing through the nuclear pore to change gene transcription (e.g., of Pax9)

Concept 3 summary skills

Discuss how a series of responses results in a new cell type; explain how neighboring cells obtain different fates

Cell Signaling

The ability of a cell to receive, process, and transmit signals with its environment and with itself

Function of cell signaling

Sensing stimulus; cell-cell communication; information processing; decision making

Ligand

A signal molecule (extracellular signal) that binds to a receptor

Receptor

A protein with outside and inside components that binds a ligand; can be on the membrane or inside the cell

Non-target cell

A cell with no receptor for a given ligand, so it cannot respond to that signal

5 Key elements of cell signaling

Signals; Receptors; Signal transducers; Effector proteins; Functional changes

Endocrine signaling

Hormone released into bloodstream and circulates to the whole body

Paracrine signaling

Local mediator released and acts locally on nearby cells

Synaptic signaling

Neurotransmitter released across a synapse and acts locally

Contact-dependent signaling

Membrane-bound signal molecule not released; requires direct cell-cell contact; very localized

Fast cellular response

Post-translational modification / altered protein function; occurs in seconds to minutes

Slow cellular response

Transcriptional changes / altered protein synthesis; occurs in minutes to hours

Examples of extracellular signals

Proteins, peptides (insulin), amino acids, nucleotides (GTP, ATP), steroids, fatty acid derivatives, dissolved gas (nitric oxide)

Examples of intracellular signals

Sex hormones, thyroid hormone, steroids, nucleotides (DNA, RNA), small hydrophobic molecules, metabolites, vitamins, dissolved gas (NO)

Why intracellular signals are predominantly slow

Because they typically act by changing transcription

Cortisol

A steroid hormone that passes through the plasma membrane, binds a receptor in the cytoplasm, enters the nucleus, and promotes/inhibits transcription of target genes

Nuclear hormone receptors

Receptors for molecules (like cortisol) that can cross the membrane, so the receptor can be inside the cell

3 types of cell surface receptors

Ion channel-linked receptors; enzyme-linked receptors; G protein-coupled receptors

Ion channel-linked receptor

Receptor where ligand binding opens/closes a channel, changing membrane potential to trigger a cellular response

Enzyme-linked receptor

Receptor with a ligand-binding domain outside and enzyme activity (e.g., tyrosine kinase domain) inside the cell

G protein-coupled receptor (GPCR)

Receptor with an extracellular ligand-binding region and a cytosolic region that interacts with a G protein (GDP/GTP-binding)

Receptor tyrosine kinase (RTK)

The largest enzyme-coupled receptor family; the receptor itself is a kinase that phosphorylates tyrosines

Examples of RTKs

EGF receptor, insulin/IGF1 receptor, NGF receptor, PDGF/MCSF receptor, FGF receptor, VEGF receptor, Eph receptor

RTK activation steps

Ligand binds → receptors dimerize → receptor kinase activated → signal transduction proteins dock at phosphorylated tyrosines

Why the RTK signaling complex is transient

It must be transient and constantly available so the receptor can receive signal again; phosphatases reverse the phosphorylation

Protein kinase

Enzyme that adds a phosphate group (using ATP → ADP) to turn a protein signal ON

Protein phosphatase

Enzyme that removes a phosphate group (hydrolysis, releasing Pi) to turn a protein signal OFF

Why constitutive activation is problematic

An "always on" signal (e.g., unregulated growth signal) is not properly switched off, which can lead to cancer

Intracellular signal transducers

A cascade of molecules (signaling proteins and second messengers) in a tightly regulated network that relay a signal from receptor to effector proteins

Second messenger

A small intracellular signaling molecule that amplifies and relays a signal (e.g., cAMP, IP3, DAG)

Ras

A small monomeric GTP-binding protein anchored to the membrane by a lipid tail; transmits RTK signals

Ras-GEF

Guanine nucleotide exchange factor; activates Ras by exchanging GDP for GTP

GAP

GTPase-activating protein; inactivates a GTPase (like Ras) by stimulating GTP hydrolysis, leaving it bound to GDP

Ras: active vs inactive state

Inactive when bound to GDP; active when bound to GTP

Ras mutation in cancer

A mutation in Ras's GTPase activity can prevent it from shutting itself off, causing constitutive growth signaling

MAP kinase cascade

Ras activates MAPKKK (MAP3K) → phosphorylates MAPKK (MAP2K) → phosphorylates MAPK → phosphorylates proteins/transcription regulators, coordinating cellular changes

MAPKKK (MAP3K)

First kinase in the MAP kinase cascade, activated by Ras

MAPKK (MAP2K)

Second kinase in the cascade; phosphorylated by MAPKKK

MAPK

Third kinase in the cascade; phosphorylated by MAPKK; phosphorylates proteins and transcription regulators

PI 3-kinase (PI3K) pathway

Pathway used by growth factors like IGF-1; PI3K phosphorylates inositol phospholipids in the membrane, allowing signaling proteins to dock

Akt

A protein kinase activated downstream of PI3K signaling; relays the signal onward (e.g., to Tor) affecting cell growth

Insulin receptor signaling result

Activates PI3K/Akt pathway leading to GLUT4 glucose transporter translocation and cell growth/survival

GPCR structure

A single polypeptide that crosses the membrane seven times ("seven-transmembrane receptor"), coupled to a heterotrimeric G protein

Heterotrimeric G protein

G protein made of three subunits (alpha, beta, gamma); alpha and gamma have lipid anchors in the membrane; alpha binds GDP or GTP

GPCR activation steps

Signal binds receptor → conformational change → G protein alpha subunit swaps GDP for GTP → beta-gamma dimer dissociates → both activate effectors

How GPCR signaling turns off

Hydrolysis of GTP to GDP on the alpha subunit returns the G protein to its inactive heterotrimeric state

Adenylyl cyclase

Enzyme activated by G proteins that converts ATP into cyclic AMP (cAMP)

Phospholipase C

Enzyme activated by G proteins that converts phosphatidylinositol into DAG and IP3

cAMP phosphodiesterase

Enzyme that degrades cAMP into AMP, turning off the cAMP signal

Caffeine's mechanism

Inhibits cAMP phosphodiesterase, preventing cAMP breakdown and prolonging the signal

Protein Kinase A (PKA)

Kinase activated by cAMP; has fast effects (activating glycogen phosphorylase for glycogen breakdown) and slow effects (activating transcription regulators)

PKA fast effect

Activates phosphorylase kinase → activates glycogen phosphorylase → glycogen breakdown (altered metabolism)

PKA slow effect

Enters nucleus and activates a transcription regulator → transcription of target gene (altered gene expression)

Hedgehog receptor pathway

Structurally similar to GPCRs; ligand Sonic Hedgehog (Shh) binds receptor Patched (Ptch), co-receptor Smoothened (Smo), activating transcription factor Gli, leading to cell proliferation/differentiation

Cross-talk

When two different receptor pathways influence each other's activity, modifying the overall output, often via shared signal transduction components

What are the two main categories of fluoride's cariostatic action?

Anticariogenic effect (crystal solubility, demineralization, remineralization) and Antimicrobial effect

How are calcium ions arranged in the hydroxyapatite crystal?

Two ways: columnar (along the column of the HA crystal) and triangular (surrounds the OH- ion)

Why does the outer surface of the HA crystal attract acid attack?

PO4 on the outside gives the crystal a negative charge, making the tooth structure surface negative