Intercellular Communication and Cell Signalling Networks

Introduction to Intercellular Communication

Multicellular life requires exquisitely precise intercellular communication enabling coordinated development from a single fertilised cell to a trillion-cell organism with hundreds of distinct cell types, tissue-level responses to injury, physiological homeostasis across organ systems, and coordinated immune responses. Cells communicate through four modalities: endocrine signalling (hormones released into the bloodstream reaching distant targets); paracrine signalling (locally diffusing signals acting on nearby cells); juxtacrine/contact-dependent signalling (cell surface ligands activating receptors on directly adjacent cells); and synaptic signalling (rapid point-to-point electrochemical transmission at neuronal synapses). Each modality is used in distinct biological contexts determined by the required speed, spatial specificity, and amplitude of the response.

Signal transduction—the intracellular translation of extracellular signals into cellular responses—involves receptor activation, second messenger generation, kinase cascade relay, transcription factor nuclear translocation, and gene expression change. Networks of signalling pathways interact through crosstalk, feedback loops, and shared second messengers, creating complex non-linear input-output relationships. Systems approaches using phosphoproteomics, signalling reporter gating, and computational modelling reveal emergent properties of signalling networks—bistable switches, ultrasensitive responses, oscillations, and cell fate decision circuits—that underlie developmental transitions and cancer signalling rewiring.

Major Intercellular Signalling Pathways

Notch Signalling

Notch receptors (Notch 1-4) are transmembrane receptors activated by DSL ligands (Delta-like 1/3/4, Jagged 1/2) on adjacent cells—strictly requiring direct cell-cell contact. Upon ligand binding, gamma-secretase cleaves Notch intracellularly, releasing NICD (Notch intracellular domain) which translocates to the nucleus and displaces the HDAC co-repressor from CSL/RBPJ transcription factor, switching it to activating NICD-MAML complex expression of Hes/Hey family repressors and cyclin D1. Notch drives cell fate decisions via lateral inhibition (a cell expressing high Delta inhibits Notch in neighbours, reinforcing its own identity while specifying different identity in neighbours—mechanisms establishing checkerboard patterns of intestinal secretory vs. absorptive cells, neuronal vs. epithelial fates). Gain-of-function NOTCH1 mutations drive T-ALL leukaemia (~60% of cases)—validated therapeutic target for gamma-secretase inhibitors.

Hedgehog Signalling

Hedgehog (Hh) ligands (Sonic Hh—SHH, Desert Hh—DHH, Indian Hh—IHH) signal through the Patched (PTCH1/2)/Smoothened (SMO) receptor system. Without Hh, PTCH1 inhibits SMO; Hh binding relieves PTCH1 inhibition of SMO; active SMO activates GLI transcription factors (GLI1—activator, GLI2—activator, GLI3—repressor) by releasing them from Suppressor of Fused (SUFU) inhibition, allowing nuclear translocation and target gene expression (PTCH1, BCL2, N-Myc, Cyclin D). Hh signalling patterns limb digit identity (SHH from the Zone of Polarising Activity specifies posterior digit identity in a concentration-dependent morphogen gradient), is essential for neural tube dorsoventral patterning, and mediates intestinal stem cell niche regulation. SMO inhibitors (vismodegib, sonidegib) are approved for basal cell carcinoma—where PTCH1 mutations constitutively activate SMO—and for Gorlin syndrome-associated BCCs.

Gap Junctions and Chemical Synapses

Gap Junctions

Gap junctions are intercellular channels formed by hexameric connexin hemichannels (connexons) from adjacent cells aligning to form an aqueous pore coupling cytoplasms directly. They allow passage of ions, second messengers (IP3, cAMP, Ca2+), and small metabolites (up to ~1 kDa) enabling electrical coupling (cardiac and smooth muscle coordinated contraction), calcium wave propagation in glia, and metabolic cooperation between coupled cells. Connexin 26 (GJB2) and connexin 30 (GJB6) mutations are the most common cause of non-syndromic hereditary deafness—impairing potassium recycling through cochlear gap junction network required for hair cell electromotility. Connexin 43 (GJA1) is the most widely expressed—its loss causes cardiac arrhythmias and developmental defects. Gap junction-mediated coupling is regulated by pH, Ca2+, and phosphorylation, providing dynamic control of metabolic coupling.

Cytokine Signalling Networks

Cytokines—a diverse superfamily including interleukins, interferons, TNF family, CSF, CXCL chemokines—mediate immune cell communication and are primary drivers of inflammatory responses, haematopoiesis, and anti-viral immunity. Cytokine receptor signalling is primarily through the JAK-STAT pathway: constitutively associated JAKs (JAK1/2/3, TYK2) transphosphorylate each other and activate receptor-recruited STAT proteins (STAT1-6) which dimerize and translocate to the nucleus activating target gene transcription. JAK inhibitors (tofacitinib, baricitinib, ruxolitinib) are approved for RA, psoriatic arthritis, myelofibrosis, polycythaemia vera, and GVHD—by interrupting JAK-STAT signalling from multiple inflammatory cytokines simultaneously. Anti-cytokine biologics (anti-TNF, anti-IL-6R, anti-IL-17A, anti-IL-23) provide pathway-specific targeting with excellent efficacy in autoimmune diseases where specific cytokine loops drive pathology.

Examples and Applications

Example 1: Wnt Gradient in Intestinal Stem Cell Niche

The intestinal crypt is organised by a Wnt gradient highest at the crypt base (from Paneth cells producing Wnt3 and Wnt11, and subepithelial fibroblasts producing Wnt2b) gradually decreasing up the crypt-villus axis. Lgr5+ intestinal stem cells at the crypt base maintain stem identity through high Wnt/beta-catenin activity; cells moving up the crypt differentiate as Wnt activity decreases and BMP activity (from villus stroma) increases. Wnt activators (CHIR-99021, R-spondin1) enable intestinal organoid culture by maintaining crypt stem cells in vitro. APC mutations (familial adenomatous polyposis, FAP) constitutively activate Wnt/beta-catenin in intestinal epithelial cells—the initiating event in 80% of colorectal cancers—established by identifying APC (adenomatous polyposis coli protein as a beta-catenin destruction complex component).

Example 2: NF-kappaB Signalling in Inflammation

NF-kappaB (Nuclear Factor kappa-light-chain-enhancer of activated B cells) is a transcription factor family (RelA/p65, RelB, c-Rel, p50, p52) activated by inflammatory stimuli (TNF, IL-1, LPS/TLR4, RANKL [bone remodelling], T cell receptor), regulating hundreds of inflammatory, anti-apoptotic, and cell cycle target genes. Canonical NF-kappaB: IKKbeta phosphorylates IkBalpha (inhibitor) causing its ubiquitination/proteasomal degradation, releasing p50-p65 to translocate to nucleus. Non-canonical pathway: NIK activates IKKalpha phosphorylating p100, triggering p100-to-p52 processing. Negative feedback: NF-kappaB activates IkBalpha transcription, resynthesizing the inhibitor. NF-kappaB hyperactivation drives tumour survival (DLBCL, MM), autoimmunity (RA, IBD). Thalidomide/IMiDs degrade IKZF1/3 CRL ubiquitin ligase substrates in myeloma through NF-kappaB-adjacent mechanisms—clinical translation of NF-kappaB pathway biology.

Example 3: Ephrin-Eph Receptor Signalling

Ephrins (membrane-anchored ligands on one cell) and Eph receptors (receptor tyrosine kinases on adjacent cells) mediate bidirectional contact-dependent signalling—unique in that both ligand-bearing and receptor-bearing cells transduce signals simultaneously. Eph-ephrin interactions guide axon pathfinding (anterior-posterior topographic mapping in the visual system—EphA-ephrinA graded opposing gradients cause retinal ganglion cell axons to project to specific tectum locations based on concentration matching), establish tissue boundary formation in somites and rhombomeres (preventing cell mixing between adjacent segments), and regulate intestinal crypt-villus positions (EphB2/3 forward signalling drives stem cell positioning at crypt base; ephrinB1 reverse signalling guides Paneth cell positioning). EphA2 overexpression in cancer promotes invasion; EphB receptor loss in colorectal cancer allows invasive phenotype through loss of ephrin-mediated boundary restriction.

Example 4: VEGF Signalling in Angiogenesis

Vascular endothelial growth factor (VEGF-A, VEGF-B, VEGF-C/D, PIGF) signals through receptor tyrosine kinases (VEGFR1/FLT1, VEGFR2/KDR, VEGFR3) to control vascular development, permeability, and angiogenesis (sprouting of new vessels from existing vasculature). Tip cells at the angiogenic front extend filopodia guided by VEGF gradients; stalk cells proliferate behind tip cells extending the vessel; phalanx cells form quiescent stable vessels. VEGF is upregulated by HIF-1 in hypoxia, promoting tumour vascularisation—the rationale for anti-VEGF therapy in cancer. Bevacizumab (anti-VEGF-A), ramucirumab (anti-VEGFR2), aflibercept (VEGF trap for all VEGF isoforms), and TKIs targeting VEGFR1-3 (sunitinib, sorafenib, axitinib) are widely approved for metastatic colorectal, lung, renal, and liver cancers, exploiting the tumour's angiogenic dependence.

Example 5: Endocrine Disruption

Endocrine disruptors—exogenous chemicals interfering with hormone signalling—include industrial chemicals (polychlorinated biphenyls, dioxins, BPA), pesticides (DDT, atrazine), pharmaceuticals (DES, ethinyl oestradiol), and heavy metals (lead, mercury). BPA (bisphenol A) from plastics binds oestrogen receptors ERalpha and ERbeta with low affinity and membrane-associated oestrogen receptor GPER1 with higher affinity—causing non-monotonic dose-response curves (effects at low concentrations not predicted by high-dose toxicology studies). Prenatal oestrogen exposure (DES, prescribed 1940s-70s) caused vaginal clear cell adenocarcinoma in daughters—demonstrating transplacental endocrine disruption causing cancer in offspring. Epigenetic mechanisms (DNA methylation changes from early-life endocrine disruptor exposure) may transmit effects across generations—transgenerational endocrine disruption epigenetics is an active research area.

Example 6: The mTOR Signalling Hub

mTOR (mechanistic target of rapamycin) kinase forms two complexes: mTORC1 (rapamycin-sensitive, including Raptor—integresses PI3K/Akt growth signals, amino acid sensing through Rag/Ragulator, energy sensing through AMPK, and oxygen sensing through HIF) when active phosphorylates S6K1 and 4E-BP1 driving ribosome biogenesis and protein synthesis, lipid synthesis (via SREBP1), and autophagy suppression; mTORC2 (Rictor-containing, rapamycin-insensitive) phosphorylates Akt-S473, SGK, and PKC for cytoskeletal organisation. mTOR hyperactivation through PI3K/PTEN alterations, TSC1/2 and LKB1 mutations drives cancer growth and is exploited by tumours. mTOR inhibitors (everolimus, temsirolimus) approved in renal cell carcinoma, breast cancer, pancreatic NET, and tuberous sclerosis complex (TSC) lesions. Rapamycin (sirolimus) extends lifespan in multiple organisms—targeting mTOR as an ageing intervention target.

Example 7: Hormone Receptor Signalling in Breast Cancer

Oestrogen receptor alpha (ESR1) is expressed in ~75% of breast cancers; ER+ tumours are driven by oestrogen binding ESR1, causing receptor dimerisation, nuclear translocation, and gene activation of proliferative genes (PR, cyclin D1, BRCA1, GATA3 targets). Endocrine therapy (tamoxifen—ER antagonist; aromatase inhibitors letrozole, anastrozole, exemestane reducing oestrogen synthesis; fulvestrant—ER degrader) prevents ER-driven transcription effectively treating early and metastatic ER+ breast cancer. Activating ESR1 mutations (Y537S, D538G—most common) arise under aromatase inhibitor therapy, constitutively activating ER without oestrogen binding—conferring resistance to AIs but retaining sensitivity to selective ER degraders (SERDs: fulvestrant, elacestrant, giredestrant). CDK4/6 inhibitors (palbociclib, ribociclib, abemaciclib) combined with aromatase inhibitors or SERDs prevent ER-driven cell cycle entry providing additive benefit.

Example 8: Paracrine Senescent Cell Signalling—the SASP

Cellular senescence—irreversible cell cycle arrest—accompanies a major shift in secretome called the SASP (senescence-associated secretory phenotype) comprising dozens of cytokines (IL-6, IL-8), chemokines (CXCL1/2, CCL2), matrix metalloproteinases (MMP1/3/9), growth factors (VEGF, HGF, EGF), and non-coding RNAs. The SASP exerts paracrine effects on surrounding tissue: recruiting immune cells for clearance of senescent cells (beneficial); promoting chronic inflammation that impairs tissue homeostasis and function (pathological in chronic senescence accumulation); paradoxically promoting tumour growth and metastasis in pre-neoplastic microenvironments. The NF-kappaB and C/EBP-beta transcription factors drive SASP gene expression, activated by cytoplasmic chromatin fragments activating cGAS-STING. Senolytic drugs eliminating senescent cells (ABT-263 navitoclax, dasatinib+quercetin) reduce SASP-driven inflammation with early clinical evidence suggesting benefit in idiopathic pulmonary fibrosis and other fibrotic diseases.

Try it live

Everything above runs in your browser — open Intercellular Signalling Networks and change the parameters while it is running. Nothing is installed, nothing is uploaded, the whole model lives in one tab.