Cell Signaling

Cell Signaling for the MCAT

Part 1 of 4 — Signaling Logic & Receptor Classes

Modes of Intercellular Signaling

Ligand Chemistry Determines Receptor Location

• Hydrophilic ligands (peptides, catecholamines like epinephrine): cannot cross the membrane → bind cell-surface receptors → fast responses via second messengers; the hormone is the "first messenger"
• Hydrophobic ligands (steroids, thyroid hormone, vitamin D, retinoic acid): cross the bilayer → bind intracellular receptors that act as ligand-activated transcription factors → slower responses (hours) requiring new transcription/translation
• Thyroid hormone is the classic trap: it is derived from tyrosine (an amino acid) but is lipophilic and uses a nuclear receptor

The Four Surface Receptor Families

Core Principles Every Pathway Shares

1. Specificity: only cells with the receptor respond — the same hormone can trigger different responses in different tissues (epinephrine relaxes airway smooth muscle via beta-2 but constricts vessels via alpha-1)
2. Amplification: one ligand → many G proteins → many cAMP → many PKA → thousands of phosphorylated targets; cascades multiply the signal at each tier
3. Integration: pathways cross-talk; a cell sums its inputs
4. Termination: every "on" switch has a dedicated "off" switch (Part 4) — a signal that cannot be turned off is a disease

Signaling Fundamentals 🎯

Receptor Binding Is Saturable and Specific — Like Enzymes, Minus Catalysis

Ligand-receptor binding obeys the same math as enzyme kinetics:

$[LR] = \frac{[R]_{total}[L]}{K_d + [L]}$

• $K_d$ = ligand concentration at which half the receptors are occupied; lower $K_d$ = higher affinity
• A competitive antagonist shifts the dose-response curve right (more agonist needed) without lowering the maximal response
• A noncompetitive antagonist (or receptor loss) lowers the maximal response
• Agonist = binds and activates; antagonist = binds and blocks; partial agonist = activates submaximally even at full occupancy

Nitric Oxide — The Gas That Breaks the Rules

• NO is made from arginine by nitric oxide synthase in endothelial cells
• As a small nonpolar gas, it needs no receptor on the membrane: it diffuses into adjacent smooth muscle and binds soluble guanylyl cyclase → cGMP → smooth muscle relaxation
• Nitroglycerin (angina) is an NO donor; sildenafil inhibits the phosphodiesterase (PDE5) that degrades cGMP — two drugs, same pathway, different nodes
• NO is a paracrine signal with a lifetime of seconds — range limited by its rapid reaction with hemoglobin and oxygen

Reading Dose-Response Curves (Passage Skill)

Receptors & Dose-Response 🎯

Key Takeaways — Part 1

• Signaling ranges: endocrine (blood), paracrine (local), autocrine (self), juxtacrine (contact), synaptic
• Hydrophilic ligands → surface receptors + second messengers (fast); lipophilic ligands (steroids, thyroid hormone) → intracellular receptors → transcription (slow)
• Four surface receptor classes: ligand-gated channels (ms), GPCRs (s), RTKs (min), JAK-STAT/cytokine receptors
• Response identity lives in the target cell: one ligand, different receptors/subtypes, different outcomes
• Binding math mirrors enzymes: $K_d$ = half-occupancy concentration; competitive antagonist = right shift, max preserved; noncompetitive = max lowered
• Amplification: each catalytic tier multiplies the signal — one hormone molecule can mobilize millions of product molecules
• NO: gas, no membrane receptor, activates soluble guanylyl cyclase → cGMP → vasodilation (nitroglycerin adds NO; sildenafil spares cGMP)

Cell Signaling for the MCAT

Part 2 of 4 — GPCRs & Second Messengers

The G Protein Cycle — A Molecular Timer

A GPCR has 7 transmembrane helices and couples to a heterotrimeric G protein (subunits alpha, beta, gamma):

1. Resting state: G-alpha holds GDP and stays bound to beta-gamma
2. Ligand binds the GPCR → the receptor acts as a GEF (guanine nucleotide exchange factor): G-alpha releases GDP and picks up GTP (GTP is simply more abundant — the receptor does not phosphorylate anything)
3. G-alpha-GTP dissociates from beta-gamma; both halves can regulate effectors
4. Built-in off switch: G-alpha slowly hydrolyzes its own GTP → GDP (intrinsic GTPase), then reassociates with beta-gamma

The signal's duration is set by the GTPase timer — a theme that returns with Ras in Part 3 and with toxins in Part 4.

The Three G-alpha Families to Know Cold

The cAMP Pathway (Gs)

$\text{ATP} \xrightarrow{\text{adenylyl cyclase}} \text{cAMP} \xrightarrow{\text{PDE}} \text{AMP}$

• cAMP binds the regulatory subunits of protein kinase A (PKA), releasing active catalytic subunits
• PKA phosphorylates serine/threonine residues on target enzymes — e.g., in liver, PKA phosphorylation activates glycogen phosphorylase kinase (glycogen breakdown ON) and inactivates glycogen synthase (synthesis OFF): one kinase flips both switches in the same metabolic direction
• PKA can also enter the nucleus and phosphorylate CREB, linking a fast cytosolic pathway to slower gene expression

The Phosphoinositide Pathway (Gq)

PLC cleaves the membrane lipid PIP$_2$ into two messengers:

• IP$_3$ (soluble): diffuses to the ER and opens IP$_3$-gated Ca$^{2+}$ channels → cytosolic Ca$^{2+}$ spikes (from ~100 nM toward micromolar)
• DAG (stays in the membrane): with Ca$^{2+}$, activates protein kinase C (PKC)
• Ca$^{2+}$ also binds calmodulin; Ca$^{2+}$-calmodulin activates CaM kinases and myosin light-chain kinase (smooth muscle contraction — why alpha-1/Gq stimulation constricts vessels)

Amplification in Numbers

One epinephrine molecule → ~100 G proteins → each adenylyl cyclase makes many cAMP → PKA → phosphorylase cascade → millions of glucose-1-phosphate molecules. Each catalytic tier multiplies; only stoichiometric (1:1) steps do not.

GPCR Mechanics 🎯

Calcium — The Ion That Acts Like a Hormone

Cytosolic free Ca$^{2+}$ is kept near $10^{-7}$ M against ~$2.5 \times 10^{-3}$ M outside (SERCA and PMCA pumps, Na$^+$/Ca$^{2+}$ exchanger). Because the resting level is so low, a small influx produces a large fold change — ideal for switching:

• Calmodulin is the universal decoder for Ca$^{2+}$ from all three sources

• Named calmodulin targets: CaM kinase II (learning/memory) and calcineurin (a phosphatase — the immunosuppressants cyclosporine/tacrolimus block it). Ca$^{2+}$ signals are often oscillatory, and frequency encodes information (CaMKII acts as a frequency decoder). At the nerve terminal, the Ca$^{2+}$ sensor that triggers vesicle fusion is synaptotagmin.

Second Messengers at a Glance

Caffeine and theophylline are nonselective PDE inhibitors — they prolong cAMP signals, which is why methylxanthines mimic mild adrenergic stimulation.

Vision — A GPCR Pathway That Runs Backwards

• Rhodopsin (a GPCR with covalently bound retinal) absorbs a photon → activates the G protein transducin (Gt) → activates a cGMP phosphodiesterase → cGMP falls → cGMP-gated Na$^+$/Ca$^{2+}$ channels close → the photoreceptor hyperpolarizes and releases less glutamate
• Light = channel closing: the rare pathway where the stimulus lowers a second messenger; amplification is enormous (1 photon → hundreds of thousands of cGMP hydrolyzed)

Second Messengers 🎯

Key Takeaways — Part 2

• G protein cycle: receptor = GEF (GDP → GTP exchange turns G-alpha ON); intrinsic GTPase turns it OFF — activity duration is a hydrolysis timer
• Gs → adenylyl cyclase up → cAMP → PKA; Gi → cyclase down (+ beta-gamma opens K$^+$ channels, e.g., M2 slowing the heart); Gq → PLC → IP$_3$ (ER Ca$^{2+}$ release) + DAG (PKC)
• PKA coordinates programs by phosphorylating many substrates with opposite effects (phosphorylase ON, glycogen synthase OFF) — no futile cycles
• Ca$^{2+}$ is a second messenger released, not synthesized; ~10,000-fold gradient makes small fluxes loud; calmodulin decodes it (CaM kinases, MLCK)
• Messenger removal is a drug target: PDE inhibitors (caffeine, sildenafil) prolong cyclic-nucleotide signals
• Vision inverts the logic: light lowers cGMP → channels close → hyperpolarization
• Amplification occurs only at catalytic steps; a locked-on G-alpha (GTPase-dead, cholera toxin) means a signal that outlives its ligand

• Low-yield extras: CaMKII (learning/memory, a Ca$^{2+}$-frequency decoder), calcineurin and its blockers cyclosporine/tacrolimus, oscillatory Ca$^{2+}$ coding, and synaptotagmin as the Ca$^{2+}$ sensor for vesicle fusion

Cell Signaling for the MCAT

Part 3 of 4 — RTKs, Ras-MAPK, PI3K & JAK-STAT

Receptor Tyrosine Kinases — Growth-Factor Logic

RTKs (insulin receptor, EGF receptor, PDGF receptor) follow one canonical activation sequence:

1. Ligand binding → receptor dimerization (the insulin receptor is a pre-formed disulfide-linked dimer that changes conformation instead)
2. Trans-autophosphorylation: each kinase domain phosphorylates tyrosines on its partner
3. Phosphotyrosines become docking sites for proteins with SH2 domains (Grb2, PI3K's regulatory subunit, PLC-gamma)
4. Docked adaptors launch parallel downstream cascades

An RTK is receptor + enzyme in one protein — contrast with GPCRs, which have no catalytic activity and borrow a G protein.

The Ras-MAPK Cascade (Proliferation Axis)

$\text{RTK} \to \text{Grb2} \to \text{SOS} \to \text{Ras} \to \text{Raf} \to \text{MEK} \to \text{ERK}$

• Grb2 (adaptor, no catalytic activity) recruits SOS, a GEF that loads Ras with GTP
• Ras is a small monomeric G protein (no beta-gamma): GTP-bound = ON; intrinsic GTPase (helped by GAPs) = OFF — the same timer logic as G-alpha
• Raf (a MAP kinase kinase kinase) phosphorylates MEK, which phosphorylates ERK (a MAP kinase); ERK enters the nucleus and phosphorylates transcription factors driving cyclin D and cell-cycle entry
• A three-kinase tier gives amplification, sharp switch-like output, and multiple regulatory nodes

The PI3K-Akt Cascade (Survival & Metabolism Axis)

• PI3K phosphorylates the membrane lipid PIP$_2$ to PIP$_3$ (a membrane-embedded docking signal, not a soluble messenger)
• PIP$_3$ recruits Akt (PKB) to the membrane, where it is activated by phosphorylation
• Akt promotes survival (inhibits apoptotic proteins), growth (mTOR), and metabolism — in muscle/adipose, insulin → PI3K → Akt drives GLUT4 vesicle translocation
• PTEN is the phosphatase that removes PIP$_3$'s 3-phosphate — a tumor suppressor because it shuts this pathway off

JAK-STAT — The Shortcut to the Nucleus

Cytokine receptors (erythropoietin, growth hormone, interferons) have no kinase domain of their own:

1. Ligand → receptor dimerization → associated JAK kinases trans-phosphorylate the receptor tails
2. STAT proteins dock via SH2 domains, get phosphorylated, dimerize, and translocate to the nucleus as ready-made transcription factors
3. Only two protein tiers between membrane and DNA — fast transcriptional responses, less amplification hardware

RTK Pathways 🎯

Insulin Signaling — One Receptor, Two Timescales

The insulin receptor illustrates how a single RTK produces both acute and long-term effects:

• Akt activates glycogen synthase indirectly: it phosphorylates and inhibits GSK-3, the kinase that would otherwise keep glycogen synthase switched off.

Insulin and glucagon meet at cAMP: glucagon (Gs) raises it, insulin lowers it. Hormone pairs that converge on one node with opposite signs are classic passage material.

Kinase Cascades as Signal Processors

Why stack three kinases (Raf → MEK → ERK) instead of one?

• Amplification: each active kinase phosphorylates many downstream molecules
• Ultrasensitivity: multi-step cascades convert graded inputs into switch-like outputs
• Cross-talk nodes: PKA can phosphorylate and inhibit Raf — a cAMP pathway vetoing a growth pathway
• Scaffold proteins hold Raf/MEK/ERK together, insulating parallel MAPK cascades that share components

• The parallel MAPK cascades a scaffold keeps apart are named ERK, JNK and p38 (the latter two respond mainly to stress and cytokines).

Experimental Dissection Toolkit

• The standard GTPase-dead Ras used in the lab is G12V (glycine 12 replaced by valine) — the same codon most often mutated in human tumors.

Epistasis logic: a constitutively active MEK rescues signaling in Ras-inhibited cells (MEK is downstream), but a constitutively active receptor does not rescue MEK inhibition.

Pathway Dissection 🎯

Key Takeaways — Part 3

• RTK activation: ligand → dimerization → trans-autophosphorylation → phosphotyrosine docking sites for SH2-domain proteins
• Ras-MAPK: Grb2 (adaptor) → SOS (GEF) → Ras-GTP → Raf → MEK → ERK → nucleus → proliferation genes; Ras uses the same GTP on/off timer as G-alpha
• PI3K makes PIP$_3$ (membrane docking site) → Akt → survival, mTOR growth signaling, GLUT4 translocation; PTEN erases PIP$_3$ and is a major tumor suppressor
• JAK-STAT: receptor has no kinase; JAKs phosphorylate, STATs dimerize and go straight to DNA (cytokines, EPO, interferons)
• Insulin signals on two timescales: acute Akt effects (GLUT4, glycogen synthase, cAMP lowering) and slower MAPK-driven transcription
• Adaptors and scaffolds signal by proximity, not catalysis; cascades add amplification and switch-like behavior
• Experimental logic: constitutively active mutant = sufficiency (and bypasses upstream blocks); dominant-negative = necessity; epistasis orders the pathway

• Low-yield extras: Akt activates glycogen synthase by inhibiting GSK-3; the parallel MAPK cascades are ERK, JNK and p38; the lab's GTPase-dead Ras is G12V

Cell Signaling for the MCAT

Part 4 of 4 — Termination, Toxins & MCAT Integration

Every ON Switch Has a Dedicated OFF Switch

• The GAPs that accelerate G-alpha's GTPase are the RGS proteins (regulators of G-protein signaling).

Receptor Desensitization — Three Depths of Shutdown

1. Rapid desensitization (seconds-minutes): GRK (G-protein receptor kinase) phosphorylates the agonist-occupied GPCR; beta-arrestin binds, sterically uncoupling it from G proteins
2. Internalization (minutes): arrestin links the receptor to clathrin pits; endosomal receptors can recycle back (resensitization) or proceed onward
3. Downregulation (hours): sustained stimulation routes receptors to lysosomes and lowers receptor gene expression — total receptor number falls

Because GRKs prefer occupied receptors, desensitization is use-dependent: the cell turns down exactly the signal it is hearing. This underlies drug tolerance (e.g., chronic beta-agonists in asthma).

Bacterial Toxins — Natural Pathway Mutants

Both cholera and pertussis raise cAMP — one by jamming the accelerator ON, one by cutting the brake. Passages expect you to distinguish the mechanisms, not just the outcome.

When Termination Fails: Cancer Signatures

• Oncogenic Ras (~30% of human cancers): point mutations destroy GTPase activity and GAP sensitivity → permanent proliferation signal; a gain-of-function change needing only ONE mutant allele
• HER2 amplification (breast cancer): so many RTKs that they dimerize without ligand — trastuzumab targets the extracellular domain
• BCR-ABL (chronic myeloid leukemia): translocation fuses ABL kinase into a constitutively active form — imatinib occupies its ATP site
• Loss of PTEN or NF1: an eraser/GAP is deleted — loss-of-function, typically requiring both alleles (contrast with oncogenes)

• Oncogenic Ras mutations most often hit codon 12.

Termination & Toxins 🎯

The MCAT Signaling Passage Toolkit

Choosing the Right Control

• Membrane-permeant analogs (of cAMP, or of DAG for PKC) let you activate a pathway mid-stream — the "chemical epistasis" trick

• The standard analogs are dibutyryl-cAMP and phorbol esters (which bind PKC's DAG site). Phorbol esters activate PKC persistently because, unlike DAG, they are not metabolized — chronic PKC activation is why they are tumor promoters.

• A receptor-negative cell line transfected with the receptor of interest is the cleanest test of receptor sufficiency

Integration Across Topics

• Signaling ↔ transport: ADH → GPCR/cAMP → aquaporin-2 insertion; insulin → RTK/PI3K → GLUT4 insertion — the two canonical "hormone inserts a transporter" stories
• Signaling ↔ cell cycle: ERK drives cyclin D; PDGF released at wounds pushes fibroblasts past the restriction point (Part 2 of the cell-cycle lesson)
• Signaling ↔ apoptosis: growth-factor withdrawal lowers Akt activity, releasing pro-apoptotic proteins — "death by neglect"
• Signaling ↔ endocrine: every hormone in the endocrine chapters uses one of these four receptor classes; classify the ligand's chemistry first and the mechanism follows

MCAT Integration 🎯

Key Takeaways — Part 4

• Termination machinery is specific and essential: GTPase (+ GAP acceleration), phosphatases, PDEs, Ca$^{2+}$ pumps, and receptor desensitization
• GPCR shutdown ladder: GRK phosphorylates occupied receptors → beta-arrestin uncouples → internalization → lysosomal downregulation; explains drug tolerance
• Cholera locks Gs ON (GTPase blocked); pertussis locks Gi OFF (coupling blocked) — different lesions, both raise cAMP
• Oncogene logic: gain-of-function (oncogenic Ras, HER2 amplification, BCR-ABL) needs one allele; tumor-suppressor logic: loss-of-function (PTEN, NF1) needs both
• GEF loss = cannot turn on; GAP loss = cannot turn off — keep the on/off steps straight
• GTP-gamma-S mimicking hormone places a G protein in the pathway; membrane-permeant cAMP or DAG analogs activate pathways mid-stream for epistasis mapping
• Inhibitor panels order a pathway: a driver lesion is downstream of every inhibitor that fails and at or upstream of every inhibitor that works

• Low-yield extras: RGS proteins (GAPs for G-alpha); Ras codon 12; dibutyryl-cAMP and phorbol esters, the persistent non-metabolized DAG mimics that act as tumor promoters