Heat Shock Proteins (HSPs)

What Are HSPs?

Classification - The Seven Functional Categories

Key HSP Families

HSP70 Family

• The most extensively studied chaperone family
• Binds short hydrophobic amino acid sequences on nascent polypeptides as they emerge from the ribosome, preventing premature aggregation while proper secondary structures form
• Uses ATP/ADP cycling: the ADP-chaperone complex binds unfolded protein; ADP is then replaced by ATP, which causes release of properly folded segments; the cycle repeats until folding is complete
• Plays a key role in anti-apoptotic signaling - overexpression shown to limit apoptosis via an increased Bcl-2/Bax ratio in renal models - Brenner and Rector's The Kidney
• Mediates immune signaling via the cell surface receptor CD91 and CD40

HSP90 Family

• Does not fold most proteins de novo; instead, it holds specific "client proteins" (transcription factors, kinases, signaling molecules) in a partially folded, activation-ready state
• Critical clients include: steroid hormone receptors (glucocorticoid receptor, mineralocorticoid receptor), oncogenic kinases, and eNOS
• Glucocorticoid receptor (GR) example: in the cytoplasm, GR is held in a high-affinity ligand-binding conformation by an HSP90-HSP70 complex; ligand binding displaces HSP90, allowing receptor-ligand translocation to the nucleus - Medical Physiology; Harper's Biochemistry
• HSP90-eNOS coupling: cytosolic HSP90 couples with eNOS to regulate NO production and intravascular tone - particularly important in renal ischemia-reperfusion injury - Brenner and Rector's The Kidney
• Cancer target: HSP90 is a major anticancer drug target because many oncoproteins (HER2, BCR-ABL, AKT, EGFR) are HSP90 clients; inhibiting HSP90 destabilizes multiple oncoproteins simultaneously - Harper's Biochemistry

HSP60 / Chaperonins

• Act later in the folding process, often working downstream of HSP70
• Form a distinctive donut-shaped oligomer (800 kDa) with a hydrophobic central cavity
• The misfolded protein enters the cavity, unfolds completely, then refolds in this protected, isolated microenvironment - avoiding aggregation with other proteins
• Bacterial equivalent: GroEL/GroES (extensively studied as the model chaperonin)
• Eukaryotic equivalent: HSP60 in mitochondria; CCT/TRiC in the cytosol
• Harper's Biochemistry

HSP40 (DNAJ Family)

• Act as co-chaperones that stimulate the ATPase activity of HSP70
• Help deliver substrate proteins to HSP70

Small HSPs (sHSPs, e.g., HSP25/HSP27)

• Act as "holdases" - they bind unfolded proteins and prevent aggregation but do not actively refold
• HSP25/HSP27 specifically protects the actin cytoskeleton from disruption during stress
• Brenner and Rector's The Kidney

HSP47

• A collagen-specific chaperone in the ER
• Binds the triple-helix collagen molecule, stabilizing it and preventing premature aggregation of trimers within the cell before secretion
• Histology - Ross & Pawlina

gp96 (GRP94)

• ER-resident chaperone
• Major HSP involved in antigen presentation via CD91 - released during necrotic (but not apoptotic) cell death, carrying antigenic peptides to APCs
• Mulholland & Greenfield's Surgery

Molecular Mechanism: The HSP Chaperone Cycle

Stressor → protein unfolding/misfolding
         ↓
HSP70 binds exposed hydrophobic regions (ADP state = high affinity)
         ↓
Nucleotide exchange: ADP → ATP (low affinity → substrate release)
         ↓
Properly folded? → Released for function
Improperly folded? → Re-enter chaperone cycle OR
                  → HSP60 for further refolding OR
                  → Ubiquitin-Proteasome System (UPS) for degradation

Immunological Roles

1. Antigen chaperoning: HSPs (especially HSP70, HSP90, gp96) capture peptide antigens from all cellular compartments and chaperone them to the cell surface or to APCs. CD91 is the primary receptor for HSP-peptide complexes on APCs, delivering antigens to the MHC class I pathway for CD8+ T cell activation. - Mulholland & Greenfield's Surgery
2. Danger signaling (DAMPs): Released from necrotic (but not apoptotic) cells, extracellular HSPs act as damage-associated molecular patterns (DAMPs), activating innate immune responses via TLR2, TLR4, CD91, CD40, and other receptors.
3. Tumor immunity: Tumor-derived gp96-peptide complexes elicit specific anti-tumor T cell responses; this has been explored in cancer vaccines.
4. Autoimmunity: Cross-reactivity between microbial HSPs and self-proteins may contribute to autoimmune disease:
   • In rheumatoid arthritis (RA), T cells from synovial fluid proliferate briskly to recombinant HSP65 (mycobacterial). γδ T cell clones recognizing mycobacterial HSP65 can cross-react with cartilage proteoglycan epitopes. Synovial fluid mononuclear cells activated by HSP60 inhibit proteoglycan production by cartilage explants. - Firestein & Kelley's Rheumatology

HSPs in Disease

Therapeutic Relevance

• HSP90 inhibitors (e.g., geldanamycin, 17-AAG, ganetespib): disrupt the client-HSP90 interaction, simultaneously destabilizing multiple oncoproteins - under investigation for various cancers. The HSP90 multi-functionality in cancer review (Albakova, 2024) provides a recent update (PMID: 39148727).
• HSP70 inhibitors: also being investigated as anticancer targets, as HSP70 is overexpressed in many tumors and promotes cell survival.
• HSP induction (preconditioning): prior heat stress or pharmacologic HSP induction before ischemia is protective in cardiac and renal ischemia-reperfusion models.
• HSP-based vaccines: tumor-derived HSP-peptide complexes (e.g., HSPPC-96/Oncophage) have been tested in melanoma and renal cell carcinoma trials.

Key Summary Points

• HSPs are evolutionary conserved stress-response proteins induced by heat, ischemia, oxidants, toxins, and other stressors
• Their core function is molecular chaperoning: assisting protein folding, preventing aggregation, and targeting irreparably damaged proteins for degradation
• They operate as part of the broader proteostasis network alongside the UPS and autophagy-lysosome pathway
• Major families: HSP70 (folding), HSP90 (client regulation), HSP60 (chaperonin), sHSPs (holdases), HSP47 (collagen)
• Beyond folding, they have major roles in immunity, signal transduction, apoptosis regulation, and cancer biology
• They are active targets for both cancer therapy (inhibition) and cytoprotective strategies (induction)

The Kallikrein-Kinin System (KKS)

Components of the KKS

1. Kininogens - the precursor proteins (substrates)
2. Kallikreins - the serine proteases that cleave kininogens
3. Kinins - the active effector peptides (bradykinin, kallidin)
4. Kinin receptors - B1R and B2R
5. Kininases - enzymes that inactivate kinins

1. Kininogens

2. Kallikreins

Plasma Kallikrein

• Circulates as prekallikrein (synthesized in the liver)
• Activated by Factor XIIa (Hageman factor) on negatively charged surfaces - this links the KKS directly to the intrinsic coagulation cascade
• Activated plasma kallikrein reciprocally activates more Factor XII, creating a positive feedback loop
• Primary substrate: HMW kininogen → releases bradykinin (9-amino acid peptide)
• Also activates neutrophils and cleaves complement components

Tissue Kallikrein (KLK1)

• 15 tissue kallikreins have been identified; in humans only KLK1 participates in local kinin production
• Genes clustered on chromosome 19q13.3-13.4
• Synthesized as a zymogen (prekallikrein) with a signal peptide and activation sequence
• Found abundantly in kidney (connecting tubules), pancreas, salivary glands, prostate
• Primary substrate: LMW kininogen → releases kallidin (Lys-bradykinin, 10 amino acids)

3. The Two KKS Subsystems

4. Kinin Structures

5. Kinin Receptors

B2 Receptor (B2R) - "Constitutive"

• Widely expressed in most normal tissues under basal conditions
• Mediates all physiological effects of kinins normally
• Binds intact bradykinin and kallidin
• Rapidly desensitized (downregulated) after agonist stimulation via a Ser/Thr-rich C-terminal cluster
• Signaling: couples via Gq → PLC → ↑intracellular Ca²⁺

B1 Receptor (B1R) - "Inducible"

• Normally absent or expressed at very low levels
• Upregulated during inflammation - induced by tissue injury, cytokines (IL-1β, TNF-α), endotoxins, and growth factors
• Ligands: des-Arg metabolites of bradykinin (des-Arg⁹-bradykinin) and kallidin (des-Arg¹⁰-kallidin)
• B1R response is not desensitized - sustaining prolonged inflammatory signaling
• Carboxypeptidase M (which generates B1R ligands) co-localizes with B1R on cell surfaces, forming an efficient signaling complex

Shared Signaling Pathways - Goodman & Gilman's

• Both B1R and B2R → Gq → PLC → ↑Ca²⁺ → diverse responses depending on cell type
• On endothelial cells: B2R activation → Ca²⁺-calmodulin → eNOS → NO → cGMP → smooth muscle relaxation (vasodilation)
• On smooth muscle directly: B1R or B2R → Gq → ↑Ca²⁺ → contraction
• Both activate NF-κB (proinflammatory) via Gα and βγ subunits
• Both activate PLA₂ → arachidonic acid → prostaglandins and EETs
• During inflammation: B2R → prolonged eNOS/NO via MEK1/2-JNK; B1R → iNOS → high-output NO

6. Kinin Inactivation (Kininases)

7. The KKS Cascade Diagram

8. Physiological Effects of Kinins

Cardiovascular

• Vasodilation and hypotension: bradykinin infusion causes vasodilation via B2R on endothelial cells → NO + prostacyclin + hyperpolarizing EETs
• Animals deficient in tissue kallikrein show impaired flow-dependent vasodilation and endothelial dysfunction
• Cardioprotective: bradykinin contributes to ischemic preconditioning; stimulates tPA release from vascular endothelium (fibrinolysis)
• Many beneficial effects of ACE inhibitors are attributed to bradykinin accumulation (antiproliferative, anti-hypertrophic, improved insulin sensitivity)
• Goodman & Gilman's

Renal (Renal KKS)

• Kallikrein is produced in high amounts by connecting tubule epithelium - closely associated anatomically with afferent arterioles of the JGA
• Functions: regulates RBF, GFR, renin release, natriuresis, and diuresis
• Kinins influence tubuloglomerular feedback via prostaglandin production
• Modulates other vasoactive systems: renin-angiotensin, eicosanoids, catecholamines, NO, vasopressin, endothelin
• Urinary kallikrein is a marker of renal KKS activity; decreased in hypertension
• Brenner and Rector's The Kidney

Inflammation and Pain

• Kinins are potent mediators of inflammation: cause vasodilation, increase vascular permeability, edema
• Pain: bradykinin is one of the most potent endogenous pain-producing substances - activates sensory nociceptors (B1R and B2R)
• Stimulate prostaglandin synthesis in many tissues

Other Effects

• Smooth muscle contraction (airways, gut)
• Stimulates electrolyte and water transport across epithelia
• Cellular growth regulation

9. KKS and Other Systems - Intersections

10. Clinical Relevance and Pharmacology

Hereditary Angioedema (HAE)

• Caused by C1-inhibitor (C1-INH) deficiency; C1-INH normally inhibits plasma kallikrein and Factor XIIa
• Without C1-INH, uncontrolled kallikrein activation → massive bradykinin generation → angioedema
• Treatments targeting the KKS:
  • Icatibant (Firazyr): selective competitive B2R antagonist; decapeptide; self-administered subcutaneously - Katzung
  • C1-INH concentrates (Berinert, Cinryze): replace the missing inhibitor
  • Ecallantide: recombinant plasma kallikrein inhibitor; more potent and selective than C1-INH
  • Lanadelumab: monoclonal antibody inhibiting plasma kallikrein; HAE prophylaxis
  • Donidalorsen: modified antisense oligonucleotide reducing plasma prekallikrein production

ACE Inhibitor Cough and Angioedema

• Bradykinin accumulation due to blocked kininase II → dry cough in ~10-15% of patients
• Rare but serious: bradykinin-mediated angioedema (does NOT respond to antihistamines/epinephrine - requires icatibant or C1-INH)

Inflammation and Pain Targets

• B2R antagonists (e.g., HOE-140): research tools; investigated for inflammatory pain
• Oral nonpeptide B2R antagonists (FR173657, NPC 18884): inhibit bradykinin-induced bronchoconstriction and nociception in animal models
• B1R antagonists (SSR240612): anti-inflammatory and analgesic in animal models
• Aprotinin: broad kallikrein inhibitor (used historically in cardiac surgery to reduce bleeding)
• Aspirin/NSAIDs: block prostaglandin-mediated kinin effects nonspecifically

Hypertension

• Urinary kallikrein is decreased in hypertension; kininogen-deficient rat strains show increased salt/angiotensin pressor sensitivity
• KKS appears minor in normal BP regulation but more important in hypertensive states

Cardiovascular Disease

• Selective B2R agonists under investigation for hypertension, myocardial hypertrophy

Summary

Prekallikrein (plasma, liver-derived)
       ↓ Factor XIIa (contact activation)
Plasma Kallikrein
       ↓ cleaves HMW kininogen
    Bradykinin (B2R → vasodilation, ↑permeability, pain)
       ↓ Kininase I (Carboxypeptidase N/M)
  des-Arg⁹-bradykinin (B1R → inflammation)
       ↓ Kininase II (ACE / Neprilysin)
    Inactive peptides

Tissue Kallikrein (KLK1, local synthesis)
       ↓ cleaves LMW kininogen
    Kallidin (B2R → same as bradykinin + local effects)
       ↓ Aminopeptidase
    Bradykinin (interconversion)

The Lyon Hypothesis (X-Chromosome Inactivation / Lyonization)

Note: You likely meant the Lyon hypothesis (not "Layon") - named after English geneticist Mary Frances Lyon (1925-2014), who proposed it in her landmark 1961 paper in Nature: "Gene Action in the X-chromosome of the Mouse."

Historical Background

The Core Postulates of the Lyon Hypothesis

The Process Illustrated

Why Is Dosage Compensation Needed?

Timing of X Inactivation

• XCI begins at the earliest stages of embryogenesis - XIST RNA begins accumulating on one X chromosome around the 8-cell stage in human preimplantation embryos
• By approximately the 16-cell stage, inactivation is established and random
• Once set, the inactive X identity is mitotically stable and transmitted to all progeny cells
• In the germ line (oocytes), the inactive X is reactivated - so meiosis occurs with two active X chromosomes, ensuring that each egg carries one active X - Emery's; Basic Medical Biochemistry

Species difference: In humans and most placental mammals, inactivation is random (maternal or paternal X inactivated with equal probability). In marsupials, the paternally derived X chromosome is consistently inactivated (imprinted XCI). - Emery's

Molecular Mechanism of X Inactivation

Steps:

1. Counting: The cell "counts" the number of X chromosomes and inactivates all but one
2. Choice: A stochastic process determines which X chromosome will be inactivated
3. Initiation: The XIST gene on the future inactive X (Xi) begins to be expressed
4. Spreading: XIST produces a long non-coding RNA (lncRNA) that "coats" the Xi in cis, spreading a methylation/silencing signal along the entire chromosome in both directions from the XIC
5. Maintenance: Multiple epigenetic mechanisms lock in the silenced state:

Epigenetic Features Distinguishing Active X (Xa) vs. Inactive X (Xi): - Thompson & Thompson

The Barr Body

Genes That Escape X Inactivation

• Pseudoautosomal regions (PAR1 and PAR2): At the tips of the X chromosome short and long arms; these regions are essentially identical on X and Y, undergo obligate recombination during male meiosis, and are not inactivated
• Distal Xp: Up to 50% of genes on distal Xp escape inactivation
• Xq: Only a few percent of genes escape inactivation

Females as Genetic Mosaics

• Cells where the maternal X is active (Xm active, Xp inactive)
• Cells where the paternal X is active (Xp active, Xm inactive)

Skewed X-Inactivation

• If one X carries a lethal or severely deleterious mutation, cells with that X active will be eliminated by selection - the surviving cells preferentially have the normal X active
• Structural abnormalities: An abnormal X chromosome (translocations, deletions) is usually preferentially inactivated - only cells where the normal X is active survive
• Extreme skewing (>90:10) can allow a female carrier of an X-linked recessive disorder to become symptomatic (manifesting carrier)
• Emery's; Thompson & Thompson

Clinical Implications

1. X-Linked Recessive Disorders in Carrier Females

• Carrier females are usually unaffected because cells with the mutant X active are compensated by cells with the normal X active
• Extreme skewed inactivation toward the mutant X → manifesting carriers (e.g., mild hemophilia A, mild Duchenne muscular dystrophy, red-green color blindness in some female carriers)

2. X-Linked Dominant Disorders

• Males with only one X are often more severely affected or lethal (e.g., incontinentia pigmenti, Rett syndrome - males often do not survive)
• Females are more mildly affected due to mosaicism

3. Skewed XCI in Disease Detection

• Methylation-sensitive probes detecting XIST methylation patterns allow carrier detection for some X-linked diseases (e.g., Wiskott-Aldrich syndrome, X-linked severe combined immunodeficiency) - Emery's

4. ACE Inhibitors and XCI

• The XIST-based DNA methylation pattern has been exploited in clonality assays - useful for determining whether tumors or hematopoietic disorders (e.g., polycythemia vera, CML) are clonal expansions

5. Turner Syndrome (45,X)

• Only one X present → no Barr body, no XCI needed
• Clinical features (short stature, gonadal dysgenesis, lymphedema) arise from haploinsufficiency of genes that normally escape XCI and require two active copies (especially genes on Xp)

Key Summary

Zygote (XmXp) - both X chromosomes active
        ↓ ~8-16 cell stage
Random choice: Xm or Xp inactivated in each cell
        ↓
XIST lncRNA expressed from Xi → coats Xi → spreads silencing
        ↓
Epigenetic marks lock in Xi: hypermethylation, macroH2A, late replication, heterochromatin
        ↓
Clonally maintained in all daughter cells → mosaic female
        ↓
~15% of X genes escape; PAR genes always escape