Signal transduction

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signal transduction pathways diagram receptor second messenger

This pathophysiology diagram illustrates G-protein-coupled receptor (GPCR) signal transduction pathways across a plasma membrane. The schematic details four primary Gα subunit cascades. The Gαs pathway activates adenylate cyclase (AC) to convert ATP into cAMP, which stimulates Protein Kinase A (PKA). Conversely, Gαi/o inhibits AC, reducing cAMP and inhibiting PKA activity. The Gαq/11 pathway activates Phospholipase C (PLC), which cleaves PIP2 into diacylglycerol (DAG) and inositol triphosphate (IP3). DAG subsequently activates Protein Kinase C (PKC), while IP3 triggers calcium (Ca2+) release from the endoplasmic reticulum (ER) to serve as a second messenger. The Gα12/13 pathway is shown activating GTPases, involved in cytoskeleton and signal regulation. Additionally, the diagram depicts GPCR desensitization through arrestins and the dissociation of the Gβγ complex. The visual highlights the molecular mechanisms by which extracellular ligands regulate metabolic pathways, gene expression, and enzyme activity in human cellular physiology.

This pathophysiology diagram compares the intracellular signaling pathways of the full-length growth hormone receptor (flGHR) and the exon 3-deleted growth hormone receptor (d3GHR). Both pathways illustrate Growth Hormone (GH) binding to the extracellular domain of the GHR, leading to the recruitment and phosphorylation of Janus kinase 2 (JAK2) in the cytoplasm. Downstream signal transduction involves the activation of SHC, IRS, STAT, PI3K, and MAPK pathways. In the d3GHR variant, the diagram depicts hyperactivation of these signaling cascades, visually indicated by 'burst' icons around the cytoplasmic proteins (STAT, PI3K, IRS, MAPK), multiple phosphorylation markers on JAK2, and red upward arrows. This hyperactivation in the d3GHR pathway is associated with increased sensitivity of the JAK-STAT pathway, resulting in enhanced transcription of GH target genes compared to the standard flGHR isoform. The illustration serves as an educational tool for endocrinology and molecular biology to demonstrate how genetic variations in receptor structure influence hormonal signal intensity and gene expression.

This pathophysiology diagram illustrates the Receptor Tyrosine Kinase (RTK) and RAS signaling pathway, highlighting mechanisms of intracellular signal transduction. The process initiates at the plasma membrane where 'Growth factors' bind to the extracellular domain of an RTK. This triggers intracellular autophosphorylation, represented by green 'P' markers, which recruits adapter proteins GRB2 and SHP2. The central component is the RAS molecular switch, shown cycling between an 'ON' state (GTP-bound) and an 'OFF' state (GDP-bound). Transition to the active 'ON' state is facilitated by Guanine Nucleotide Exchange Factors (GEF), while inactivation is mediated by GTPase-activating proteins (GAP). Activated RAS triggers two primary downstream effector cascades: the PI3K/AKT/mTOR pathway and the BRAF/MEK/ERK (MAPK) pathway. The diagram links these pathways to critical biological outcomes including 'Tumor growth', 'Survival', and 'Proliferation'. This educational visual is designed to explain oncogenic signaling and cellular regulation at a level suitable for medical and biomedical students.

This medical illustration depicts the Fibroblast Growth Factor Receptor (FGFR) signaling pathway, a key mechanism in oncology, particularly breast cancer. The diagram is organized vertically, showing signal transduction from the extracellular matrix, through the cell membrane and cytoplasm, to the nucleus. At the top, Fibroblast Growth Factors (FGFs) bind to the transmembrane FGFR, inducing receptor dimerization. FGFR inhibitors are shown targeting this initial step. Intracellularly, the signal branches into four major cascades: 1. PLCγ leading to Protein Kinase C (PKC) activation. 2. JAK activating STAT proteins. 3. FRS2/GRB2/SOS activating the PI3K/AKT/mTOR pathway. 4. The RAS/RAF/MEK/ERK1/2 (MAPK) pathway. All four pathways converge on the nucleus to induce 'Gene expression changes,' illustrated by a DNA double helix. These genomic changes drive downstream oncogenic cellular responses including differentiation, proliferation, survival, and angiogenesis. The diagram uses arrows to indicate activation and T-bars to represent therapeutic inhibition, serving as an educational model for targeted cancer therapy.

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Signal Transduction

Signal transduction is the process by which a cell detects an extracellular signal and converts it into an intracellular response - ultimately altering gene expression, enzyme activity, ion flux, or cytoskeletal organization. Even single-cell organisms rely on it; multicellular life made it indispensable for coordinating embryonic development, immunity, and integrated physiology.

1. Modes of Cell Communication

Before a receptor can act, the signal must reach its target cell. Four delivery modes exist:

Mode	Mechanism	Example
Endocrine	Ligand enters circulation, acts on distant organ	Insulin from pancreas acting on muscle
Paracrine	Ligand acts on neighboring cells in same tissue	Cytokines in a lymph node
Autocrine	Cell signals itself	Growth factors in some tumors
Juxtacrine	Direct cell-to-cell or cell-matrix contact	Notch-Delta signaling in development

Paracrine signals are kept local by rapid endocytosis, extracellular enzyme degradation (e.g., acetylcholinesterase at the neuromuscular junction), or immobilization by the extracellular matrix.

Medical Physiology, p. (block 1, lines 1464-1482)

2. Types of Receptors

Receptors are grouped by the signaling mechanism they use (Robbins Fig. 1.13, shown above):

A. Receptor Tyrosine Kinases (RTKs)

Integral membrane proteins (e.g., for EGF, insulin, PDGF). Ligand-induced cross-linking activates intrinsic tyrosine kinase in their cytoplasmic tails, phosphorylating tyrosine residues on the receptor itself and downstream targets. The phosphorylated receptor then recruits adaptor proteins (e.g., GRB2 via SH2 domains) that couple to RAS and downstream cascades.

B. Non-Receptor Tyrosine Kinases (e.g., Src-family)

Receptors without intrinsic catalytic activity (immune receptors, some cytokine receptors, integrins) recruit separate intracellular kinases. Src is the prototype - it contains SH2 domains (bind phosphorylated receptors) and SH3 domains (bind proline-rich motifs) to aggregate signaling complexes.

C. G Protein-Coupled Receptors (GPCRs)

The largest family (>1,500 members), with seven transmembrane segments. Ligand binding causes the GPCR to associate with a heterotrimeric G protein (Gα-GDP·Gβγ). The interaction exchanges GDP for GTP, activating Gα:

Gαs - activates adenylate cyclase → ↑cAMP → activates PKA
Gαi/o - inhibits adenylate cyclase → ↓cAMP → inhibits PKA
Gαq/11 - activates phospholipase C (PLC) → cleaves PIP2 into IP3 (triggers Ca²⁺ release from ER) + DAG (activates PKC)
Gα12/13 - activates Rho GTPases → cytoskeletal remodeling

GPCRs desensitize through arrestin binding and receptor internalization.

D. Nuclear Receptors

Lipid-soluble ligands (steroid hormones, thyroid hormone, vitamin D, retinoic acid) diffuse across the plasma membrane and bind intracellular receptors, forming ligand-receptor complexes that directly bind nuclear DNA and activate or repress gene transcription.

E. Notch Receptors

Ligand binding (from an adjacent cell) triggers proteolytic cleavage of Notch; the intracellular domain (IC Notch) translocates to the nucleus and activates transcription of Notch target genes.

F. Wnt/Frizzled Pathway

Wnt binds the Frizzled receptor (a distinct GPCR class) and co-receptor LRP5/6. This recruits Dishevelled, which disrupts the β-catenin degradation complex. Stabilized β-catenin translocates to the nucleus as a transcription factor. Without Wnt, β-catenin is continuously targeted by ubiquitin-directed proteasomal degradation.

Robbins, Cotran & Kumar, pp. 32-35

3. Core Second Messenger Systems

Second Messenger	Source	Key Effectors
cAMP	Adenylate cyclase from ATP	PKA → CREB (transcription), phosphorylase kinase, ion channels
IP3	PLC from PIP2	Binds IP3R on ER → Ca²⁺ release
DAG	PLC from PIP2	Activates PKC (with Ca²⁺)
Ca²⁺	ER (via IP3) or extracellular	Calmodulin → CaM kinases, MLCK, calpain
cGMP	Guanylate cyclase (NO-stimulated)	PKG, PDE inhibition, smooth muscle relaxation

4. Major Downstream Pathways

RAS-MAPK Pathway

Growth factor → RTK autophosphorylation → adaptor proteins (GRB2/SOS) → RAS exchanges GDP for GTP (activated) → RAF (MAPKKK) → MEK (MAPKK) → ERK/MAPK → nuclear transcription factors (MYC, JUN) → cell proliferation and survival. RAS is inactivated when its intrinsic GTPase activity (enhanced by GAPs) hydrolyzes GTP back to GDP.

PI3K-AKT-mTOR Pathway

Activated RTK recruits PI3K → phosphorylates PIP2 to PIP3 → recruits AKT to membrane → AKT activates mTOR → protein synthesis, cell survival, glucose metabolism. PTEN phosphatase opposes this pathway by converting PIP3 back to PIP2 (PTEN is a major tumor suppressor).

JAK-STAT Pathway

Type I and II cytokine receptors activate associated Janus kinases (JAKs). JAKs phosphorylate cytokine receptor subunits → STATs (signal transducers and activators of transcription) are recruited and phosphorylated → STAT dimers translocate to the nucleus → cytokine-responsive gene expression. This pathway is targeted therapeutically by JAK inhibitors (e.g., tofacitinib in rheumatoid arthritis).

NF-κB Pathway

Pro-inflammatory signals → IKK complex phosphorylates IκB → ubiquitin-mediated degradation of IκB → free NF-κB translocates to nucleus → genes for inflammation, survival, proliferation.

Firestein & Kelley's Textbook of Rheumatology, pp. 1330-1634

5. Signaling Networks, Nodes, and Hubs

The old "linear cascade" view is an oversimplification. Phosphorylation of a single protein can simultaneously cause:

Enzyme activation or inactivation
Nuclear/cytoplasmic shuttling of transcription factors
Actin polymerization or depolymerization
Protein stabilization or degradation
Feedback loop activation

Adaptor proteins (containing SH2/SH3 domains) organize complexes by physically linking enzymes - they determine which proteins are recruited and thus which downstream events occur. By analogy to computer networks, these protein-protein complexes are "nodes" and the biochemical events are "hubs." This network complexity is studied by systems biology / computational biology.

Robbins, Cotran & Kumar, p. 34

6. Receptor Diagrams

The Robbins Fig. 1.13 diagram above shows the major receptor categories (A) and the RAS/PI3K downstream cascade in detail (B).

Here is the GPCR pathway diagram:

And the RTK / RAS signaling cascade:

7. Clinical Relevance

Disease/Drug	Pathway Involved
Cancer (many types)	Mutated RAS (constitutively active), loss of PTEN, amplified RTKs (EGFR in lung, HER2 in breast)
Rheumatoid Arthritis	JAK-STAT pathway; targeted by JAK inhibitors (tofacitinib, baricitinib)
Bipolar Disorder	Lithium inhibits GSK3β and phosphoinositide enzymes upstream of PKC
Diabetes (Type 2)	Defective insulin receptor (RTK) or post-receptor PI3K-AKT signaling
Hirschsprung disease	Loss-of-function mutations in RET receptor tyrosine kinase
Wnt pathway cancers	β-catenin accumulation (colorectal cancer - APC mutations)
Targeted therapies	Imatinib (BCR-ABL kinase), erlotinib (EGFR), vemurafenib (BRAF V600E)

Harrison's Principles of Internal Medicine 22E (2025), p. 3696

Summary

Signal transduction follows a general logic:

Reception - extracellular ligand binds a receptor
Transduction - receptor activates intracellular intermediaries (G proteins, kinase cascades, second messengers)
Amplification - kinase cascades exponentially amplify the signal
Integration - multiple pathways converge on transcription factors
Response - altered gene expression, enzyme activity, cell behavior
Termination - phosphatases, GTPase activity, receptor internalization, and feedback loops shut off the signal

The 518 known protein kinases in the human genome, acting in redundant and cross-talking networks, give cells extraordinary capacity to fine-tune responses to their environment - and explain why dysregulation of these pathways underlies so much human disease.

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