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Unpublished draft

Pathway macrostates

Mammalian Cell Signaling Pathway Macrostates: discrete, named, experimentally interpretable operating regimes

The table below catalogs major mammalian signaling pathways by their experimentally observable macrostate(s) — the coarse-grained, discrete operating regimes that go beyond simple “pathway is on/off” and are distinguished by measurable dynamical signatures. For each, I describe the recognizable states and the experimental readout used to discriminate them.

Summary Table of Discrete Signaling Macrostates

PathwayNamed macrostates (discrete regimes)What the states areHow to tell which state you’re in (experimental readout)
NF-κB (TNFα → IKK → IκB → NF-κB)(1) OFF (cytoplasmic, inactive) (2) DIGITAL ON (single pulse, variable amplitude) (3) SUSTAINED ON (persistent nuclear) (4) OSCILLATORY (damped/sustained nuclear cycling)OFF: p50/p65 heterodimer sequestered in cytoplasm by IκBα. DIGITAL ON: a single spike of nuclear NF-κB, with peak amplitude encoding stimulus strength. SUSTAINED ON: IκB degraded and NF-κB stays in nucleus with no recovery. OSCILLATORY: cycles of nuclear translocation (typically ~100-min period).Live-cell imaging of NF-κB–GFP (p65-GFP) nuclear-to-cytoplasmic ratio over time [1–3]. Single-cell microscopy reveals that at low TNF doses, only a fraction of cells activate (digital “all-or-none” at the single-cell level) [1]; at higher doses, all cells activate with oscillatory or sustained patterns [2,3].
ERK/MAPK (RTK → Ras → Raf → MEK → ERK)(1) OFF (2) TRANSIENT ON (pulse, minutes) (3) SUSTAINED ON (plateau) (4) OSCILLATORY (repeated pulses)OFF: ERK unphosphorylated. TRANSIENT ON: ERK-PP peak at 5–10 min, decays within 1 hr. SUSTAINED ON: persistent phosphorylation (e.g., >4 hr in PC12 cells → differentiation). OSCILLATORY: repeated ~15–20 min pulses (e.g., in response to EGF with negative feedback).Phospho-ERK (ppERK) immunoblotting or immunostaining; live-cell FRET biosensors (e.g., EKAREV) [4–6]. Duration coding: sustained ERK → differentiation (PC12), transient → proliferation. Negative feedback from ERK to RAF creates oscillations and switch-like responses [7].
p53 (DNA damage → ATM/Chk2 → p53 → Mdm2)(1) OFF (low, basal) (2) DIGITAL PULSES (fixed-amplitude oscillations) (3) SUSTAINED HIGH (apoptotic)OFF: p53 levels low, Mdm2-mediated degradation unimpeded. DIGITAL PULSES: repeated fixed-amplitude p53 pulses (period ~5–6 hr) with increasing frequency as DNA damage increases (frequency-modulated coding). SUSTAINED HIGH: monotonic elevated p53, associated with apoptosis.Live-cell p53-GFP imaging; single-cell luminescence reporters. Pulse count scales with DNA damage; sustained high p53 occurs under severe/unreparable damage [8,9]. The p53–Mdm2 negative feedback loop is the core oscillator [10].
Wnt/β-catenin(1) OFF (β-catenin degraded) (2) ON (β-catenin stabilized, nuclear)OFF: Destruction complex (AXIN, APC, GSK3β) phosphorylates β-catenin → proteasomal degradation. TCF/LEF bound to Groucho repressors, transcription silenced. ON: Wnt ligand → LRP/Fz → Dishevelled → destruction complex inactivated → β-catenin accumulates → translocates to nucleus → displaces Groucho → TCF activates transcription.β-catenin accumulation by immunofluorescence (nuclear vs. cytoplasmic); TOPFlash/FOPFlash luciferase reporter. The OFF→ON transition is switch-like due to double-negative feedback loops [11].
Notch(1) OFF (uncleaved) (2) ON (NICD liberated, nuclear)OFF: Full-length Notch at plasma membrane. ON: Sequential proteolysis (ADAM10 → γ-secretase) → NICD (Notch Intracellular Domain) released → translocates to nucleus → binds CSL/RBP-J → recruits co-activators (MAML) → transcription ON.Nuclear NICD by immunofluorescence (e.g., cleaved Notch1 Val1744 antibody); NICD-GFP translocation; HES1 luciferase reporter. Notch is classically a binary fate switch — single threshold, all-or-none activation [12,13].
Hedgehog (Shh → Ptch → Smo → Gli)(1) OFF (GliR state) (2) ON (GliA state)OFF: Ptch inhibits Smo → Gli2/3 proteolytically processed into repressor (GliR, truncated) → enters nucleus → represses targets. ON: Shh binds Ptch → releases Smo → Gli processing blocked → full-length Gli activators (GliA) accumulate → activate targets.Gli1 mRNA (itself a target); Gli2/3 proteolytic fragments by Western blot [14,15]; Gil-luciferase reporters. The OFF→ON transition involves a switch in Gli from a truncated repressor to a full-length activator state.
Hippo (MST1/2 → LATS1/2 → YAP/TAZ)(1) OFF (YAP/TAZ cytoplasmic, degraded) (2) ON (YAP/TAZ nuclear)OFF: Active MST1/2→LATS1/2→phosphorylates YAP/TAZ (S127/S89) → 14-3-3 binding → cytoplasmic retention + degradation. ON: Kinase cascade OFF → YAP/TAZ unphosphorylated → nuclear → bind TEAD → proliferation/survival genes ON.YAP phospho-S127 and total YAP immunofluorescence (nuclear/cytoplasmic ratio); TEAD-luciferase reporter. This is effectively a rheostat — graded nuclear YAP levels scale with tissue stiffness and cell density [16,17].
TGF-β/Smad(1) OFF (R-Smad cytoplasmic) (2) ON (R-Smad nuclear complex)OFF: R-Smads (Smad2/3) bound to SARA in cytoplasm, receptor kinase off. ON: TGF-β→TβRII→TβRI→phosphorylates R-Smads→form Smad2/3-Smad4 trimer→nuclear accumulation→transcription.Smad2/3 phospho-immunofluorescence (nuclear translocation); CAGA-luciferase reporter. Duration of nuclear Smad determines response (transient vs. sustained nuclear) [18].
JAK-STAT (Cytokine receptor → JAK → STAT)(1) OFF (STAT cytoplasmic) (2) ON (STAT nuclear, cycling)OFF: STAT monomers in cytoplasm. ON: Ligand→receptor dimerization→JAKs trans-phosphorylate→STAT recruited→phosphorylated→dimerize→nuclear import→transcription→export. The system exhibits nucleocytoplasmic cycling with a period of ~30–90 min (STAT3/STAT5).STAT phospho-Y (e.g., pY-STAT3) by IF/Western; STAT-GFP live-cell imaging reveals pulsed translocation [19]. The cycling is due to delayed negative feedback (SOCS proteins) and dephosphorylation.
Ca²⁺ (IP₃ → ER Ca²⁺ release)(1) BASAL (low nM) (2) SPIKING (discrete transients) (3) BURSTING (clustered spikes) (4) WAVE (propagating front) (5) SUSTAINED PLATEAUBASAL: [Ca²⁺]c ~100 nM. SPIKING: single transient peaks (frequency encodes stimulus strength — frequency-modulated signaling). BURSTING: groups of spikes separated by quiescence. WAVE: regenerative Ca²⁺ wave across the cell or tissue. SUSTAINED PLATEAU: tonic elevated Ca²⁺ (often cytotoxic).Ca²⁺-sensitive fluorescent dyes (Fluo-4, Fura-2 ratio imaging); genetically encoded indicators (GCaMP). The frequency of Ca²⁺ oscillations (spikes/min) encodes agonist concentration [20,21].
GPCRs (heptahelical → G protein/β-arrestin)(1) INACTIVE (R) (2) ACTIVE (R*) — further subdivided into: - G-protein–biased (Rʸ) - β-arrestin–biased (Rᵝ) - Constitutively active (R*)INACTIVE: receptor unliganded, heterotrimeric Gₐᵦᵧ intact. ACTIVE: agonist bound, conformational change, Gₐᵦᵧ dissociates. Biased states: different ligands stabilize different active conformations that preferentially couple to G-protein vs. β-arrestin pathways [22,23]. Single receptors can sample multiple active states.BRET/FRET biosensors for G-protein activation (e.g., Gₛ, Gₐ, Gₐ); β-arrestin recruitment (Tango assay, bioluminescence); receptor conformational sensors (intramolecular FRET). Different ligands produce different active-state signatures [22,24].
Integrins (αβ heterodimers)(1) BENT/CLOSED (low affinity) (2) EXTENDED/CLOSED (intermediate) (3) EXTENDED/OPEN (high affinity)BENT/CLOSED: ectodomain bent at knee, legs together, low affinity for ECM. EXTENDED/CLOSED: legs extended but headpiece closed (intermediate). EXTENDED/OPEN: legs extended, headpiece open, high-affinity ligand binding → outside-in signaling.Conformation-specific antibodies (e.g., PAC-1 for activated αIIbβ3; mAb24 for β₂); FRET between integrin α and β cytoplasmic tails (closed→open = loss of FRET) [25,26].
mTORC1/mTORC2(1) OFF (inactive complex) (2) ON (active, lysosomal/perinuclear)OFF: mTORC1 disassembled or inhibited by rapamycin/amino acid starvation; mTORC2 off upon growth factor withdrawal. ON: mTORC1 — active at lysosomal surface (Rheb-GTP + Rag GTPases); phosphorylates S6K/4E-BP1. mTORC2 — active at plasma membrane; phosphorylates AKT (S473).Phospho-S6K (T389), phospho-4E-BP1 (T37/46) for mTORC1; phospho-AKT (S473) for mTORC2. Immunofluorescence for mTOR localization to LAMP2⁺ lysosomes [27].
cAMP/PKA(1) BASAL (low cAMP) (2) ELEVATED (uniform rise) (3) OSCILLATORY (cAMP transients) (4) COMPARTMENTALIZED (microdomain)BASAL: low [cAMP], PKA holoenzyme inactive (R₂C₂). ELEVATED: AC activated, high cAMP, PKA dissociates → catalytic subunits free. OSCILLATORY: cAMP pulses (period ~2–10 min) from interplay of AC and PDEs. COMPARTMENTALIZED: localized cAMP domains near plasma membrane vs. perinuclear; AKAPs tether PKA to specific subcellular sites.FRET-based cAMP sensors (Epac1-camps, ICUE); PKA activity FRET sensors (AKAR). Compartmentalization revealed by local PDE inhibition and targeted sensors [28,29].
NFAT (Ca²⁺ → calcineurin → NFAT)(1) OFF (hyper-phosphorylated, cytoplasmic) (2) ON (dephosphorylated, nuclear)OFF: NFAT heavily phosphorylated by CK1/GSK3, sequestered in cytoplasm. ON: Ca²⁺ rises → calcineurin activated → dephosphorylates NFAT → rapid nuclear translocation. NFAT shows sustained nuclear localization as long as Ca²⁺ stays elevated (unlike fast nucleocytoplasmic shuttling of NF-κB).NFAT-GFP translocation; phospho-NFAT Western; NFAT-driven luciferase. NFAT isoforms (NFATc1–c4) differ in nuclear retention dynamics [30].

Detailed Commentary on Selected Pathways

NF-κB: Digital activation + temporal encoding

At the single-cell level, NF-κB activation by TNF-α is a digital (all-or-none) event — individual cells either activate or not, with the fraction of responding cells scaling with TNF dose. The responding cells encode dose in analogue parameters: the amplitude of the first nuclear peak, the time to first peak, the total number of oscillations, and the damping rate [1,2]. The NF-κB system thus exhibits multiple discrete attractors (OFF, single-pulse, oscillatory, sustained) that can be discriminated by live-cell time-lapse microscopy of p65-GFP [3].

ERK/MAPK: Duration, amplitude, and frequency codes

ERK exhibits at least four distinguishable macrostates: OFF, transient ON, sustained ON, and oscillatory ON. The duration of ERK activity is a classic decoding mechanism — transient ERK in PC12 cells drives proliferation, while sustained ERK drives neuronal differentiation [4,5]. The MAPK cascade contains positive-feedback motifs that create bistability (switch-like OFF→ON transitions) and delayed negative feedback from ERK to RAF that generates oscillations with a period of ~15–20 min [6,7].

p53: Frequency-modulated pulses

p53 responds to DNA damage with fixed-amplitude digital pulses, whose frequency (number of pulses per unit time) encodes the amount of DNA damage [8,9]. This is an unusually clear example of frequency-modulated (FM) signaling — the “state” is best described not merely as ON/OFF but by the pulse count per cell (0, 1, 2, 3… pulses). Under severe, irreparable damage, p53 transitions to a sustained high state that triggers apoptosis [10].

Ca²⁺: A multi-attractor system

Ca²⁺ is perhaps the most macroscopically rich second messenger, supporting spiking (single transients), bursting (clusters of spikes), waves (spatial propagation) and sustained plateaus [20,21]. The frequency of Ca²⁺ oscillations (spikes/min) is the best-characterized FM signaling code in biology — different frequencies activate different downstream effectors via frequency-dependent activation of CaM kinase II, NFAT, and NF-κB [20].

GPCRs: Beyond binary ON/OFF — biased signaling

GPCRs do not occupy a single “ON” state but rather an ensemble of active conformations. Different agonists can stabilize different subsets of these conformations, producing biased signaling: the receptor in a G-protein–preferring conformation vs. a β-arrestin–preferring conformation [22–24]. This goes beyond a simple ON/OFF classification into distinct agonist-specific active states (a form of multistability).

Integrins: Three-state conformational machine

Integrins are mechanistically well-characterized as a three-state device: bent-closed (low-affinity, OFF), extended-closed (intermediate), and extended-open (high-affinity, ON) [25,26]. The transition from bent to extended can be driven by “inside-out” signals (talin binding to β-tail) while ligand binding stabilizes the open headpiece for “outside-in” signaling.

Notch & Hedgehog: Binary switches

Both Notch and Hedgehog pathways approximate binary fate switches. In Notch, the key event is proteolytic liberation of NICD — it is either present or absent in the nucleus, with no intermediate amplitude modulation (though the system can exhibit bistable dynamics via downstream feedback) [12,13]. In Hedgehog, the switch operates at the level of Gli processing: OFF = GliR (repressor form), ON = GliA (full-length activator form) [14,15].

References

  1. Tay S, Hughey JJ, Lee TK, Lipniacki T, Quake SR, Covert MW. Single-cell NF-κB dynamics reveal digital activation and analogue information processing. Nature. 2010;466(7303):267–271. DOI: 10.1038/nature09145
  2. Zhang Q, Gupta S, Schipper DL, Kowalczyk GJ, Mancini AE, Budinger GRS, et al. NF-κB Dynamics Discriminate between TNF Doses in Single Cells. Cell Systems. 2017;5(6):638-645.e5. DOI: 10.1016/j.cels.2017.10.011
  3. Longo DM, Selimkhanov J, Kearns JD, Hasty J, Hoffmann A, Tsimring LS. Dual Delayed Feedback Provides Sensitivity and Robustness to the NF-κB Signaling Module. PLoS Computational Biology. 2013;9(6):e1003112. DOI: 10.1371/journal.pcbi.1003112
  4. Sturm OE, Orton R, Grindlay J, Birtwistle M, Vyshemirsky V, Gilbert D, et al. The Mammalian MAPK/ERK Pathway Exhibits Properties of a Negative Feedback Amplifier. Science Signaling. 2010;3(153):ra90. DOI: 10.1126/scisignal.2001212
  5. Arkun Y, Yasemi M. Dynamics and control of the ERK signaling pathway: Sensitivity, bistability, and oscillations. PLOS ONE. 2018;13(4):e0195513. DOI: 10.1371/journal.pone.0195513
  6. Sepulchre J-A, Ventura AC. Intrinsic Feedbacks in MAPK Signaling Cascades Lead to Bistability and Oscillations. Acta Biotheoretica. 2013;61(1):59–78. DOI: 10.1007/s10441-013-9177-5
  7. Fritsche-Guenther R, Witzel F, Sieber A, Herr R, Schmidt N, Braun S, et al. Strong negative feedback from Erk to Raf confers robustness to MAPK signalling. Molecular Systems Biology. 2011;7(1):489. DOI: 10.1038/msb.2011.27
  8. Kalo A, Shav-Tal Y. Acting on impulse: dissecting the dynamics of the NFAT transcriptional response. Genome Biology. 2013;14(1):102. DOI: 10.1186/gb-2013-14-1-102
  9. Purvis JE, Karhohs KW, Mock C, Batchelor E, Loewer A, Lahav G. p53 Dynamics Control Cell Fate. Science. 2012;336(6087):1440–1444. DOI: 10.1126/science.1218351
  10. Batchelor E, Mock CS, Bhan I, Loewer A, Lahav G. Recurrent initiation: A mechanism for triggering p53 pulses in response to DNA damage. Molecular Cell. 2008;30(3):277–289. DOI: 10.1016/j.molcel.2008.03.016
  11. Cadigan KM. Wnt/β-Catenin Signaling: Turning the Switch. Developmental Cell. 2008;14(3):322–323. DOI: 10.1016/j.devcel.2008.02.006
  12. Vujovic F, Hunter N, Farahani RM. Notch pathway: a bistable inducer of biological noise? Cell Communication and Signaling. 2019;17(1):133. DOI: 10.1186/s12964-019-0453-0
  13. Kopan R, Ilagan MXG. Notch Signaling: The Core Pathway and Its Posttranslational Regulation. Developmental Cell. 2009;16(5):619–632. DOI: 10.1016/j.devcel.2009.03.010
  14. Wang B, Fallon JF, Beachy PA. Hedgehog-Regulated Processing of Gli3 Produces an Anterior/Posterior Repressor Gradient in the Developing Vertebrate Limb. Cell. 2000;100(4):423–434. DOI: 10.1016/s0092-8674(00)80678-9
  15. Niewiadomski P, Kong JH, Ahrends R, Ma Y, Humke EW, Khan S, et al. Gli Protein Activity Is Controlled by Multisite Phosphorylation in Vertebrate Hedgehog Signaling. Cell Reports. 2014;6(1):168–181. DOI: 10.1016/j.celrep.2013.12.003
  16. Hansen CG, Moroishi T, Guan K-L. YAP and TAZ: a nexus for Hippo signaling and beyond. Trends in Cell Biology. 2015;25(9):499–513. DOI: 10.1016/j.tcb.2015.05.002
  17. Dupont S, Morsut L, Aragona M, Enzo E, Giulitti S, Cordenonsi M, et al. Role of YAP/TAZ in mechanotransduction. Nature. 2011;474(7350):179–183. DOI: 10.1038/nature10137
  18. Schmierer B, Hill CS. TGFβ–SMAD signal transduction: molecular specificity and functional flexibility. Nature Reviews Molecular Cell Biology. 2007;8(12):970–982. DOI: 10.1038/nrm2297
  19. Swameye I, Muller TG, Timmer J, Sandra O, Klingmüller U. Identification of nucleocytoplasmic cycling as a remote sensor in cellular signaling by databased modeling. Proceedings of the National Academy of Sciences. 2003;100(3):1028–1033. DOI: 10.1073/pnas.0237333100
  20. Berridge MJ, Bootman MD, Roderick HL. Calcium signalling: dynamics, homeostasis and remodelling. Nature Reviews Molecular Cell Biology. 2003;4(7):517–529. DOI: 10.1038/nrm1155
  21. Shuttleworth TJ, Thompson JL. Ca²⁺ entry modulates oscillation frequency by triggering Ca²⁺ release. Biochemical Journal. 1996;313(3):815–819. DOI: 10.1042/bj3130815
  22. Perez DM, Karnik SS. Multiple Signaling States of G-Protein-Coupled Receptors. Pharmacological Reviews. 2005;57(2):147–161. DOI: 10.1124/pr.57.2.2
  23. Wootten D, Christopoulos A, Marti-Solano M, Babu MM, Sexton PM. Mechanisms of signalling and biased agonism at G protein-coupled receptors. Nature Reviews Molecular Cell Biology. 2018;19(10):638–653. DOI: 10.1038/s41580-018-0049-3
  24. Violin JD, Lefkowitz RJ. β-Arrestin-biased ligands at seven-transmembrane receptors. Trends in Pharmacological Sciences. 2007;28(8):416–422. DOI: 10.1016/j.tips.2007.06.006
  25. Kim C, Ye F, Ginsberg MH. Regulation of Integrin Activation. Annual Review of Cell and Developmental Biology. 2011;27(1):321–345. DOI: 10.1146/annurev-cellbio-100109-104104
  26. Springer TA, Wang J. The three-dimensional structure of integrins and their ligands, and conformational regulation of cell adhesion. Advances in Protein Chemistry. 2004;68:29–63. DOI: 10.1016/S0065-3233(04)68002-3
  27. Saxton RA, Sabatini DM. mTOR Signaling in Growth, Metabolism, and Disease. Cell. 2017;168(6):960–976. DOI: 10.1016/j.cell.2017.02.004
  28. Zaccolo M, Pozzan T. Discrete microdomains with high concentration of cAMP in stimulated rat neonatal cardiac myocytes. Science. 2002;295(5560):1711–1715. DOI: 10.1126/science.1069982
  29. Baillie GS. Compartmentalized signalling: spatial regulation of cAMP by the action of compartmentalized phosphodiesterases. FEBS Journal. 2009;276(7):1790–1799. DOI: 10.1111/j.1742-4658.2009.06926.x
  30. Yissachar N, Sharar Fischler T, Cohen AA, Reich-Zeliger S, Russ D, Shifrut E, et al. Dynamic Response Diversity of NFAT Isoforms in Individual Living Cells. Molecular Cell. 2013;49(2):322–330. DOI: 10.1016/j.molcel.2012.11.003

Key interpretive notes

  • Bistability (two stable steady states, OFF and ON, with hysteresis) is a property of many of these systems — notably ERK/MAPK, Wnt/β-catenin, Notch, and cell-cycle transitions — enabled by positive or double-negative feedback loops [Ferrell 2002, ref 11 in table].
  • Oscillations are not a single “state” but rather a dynamical regime that can coexist with ON/OFF states. NF-κB, p53, ERK, Ca²⁺, and JAK-STAT all exhibit oscillatory attractors under some conditions.
  • Digital vs. analogue encoding: NF-κB shows digital (binary, all-or-none) activation at the single-cell level but analogue (continuous) encoding in peak amplitude and pulse number [1]. ERK shows both graded responses (at low stimulus) and switch-like, bistable responses (at threshold) [5,6].
  • Attractor landscapes are stimulus-dependent: These pathways are not hard-wired to a single macrostate — they explore different regimes depending on ligand identity, concentration, duration, and cellular context.