2.1 Organelles, Cytoskeleton & Membrane Transport

[!NOTE] MRCP Part 1 Blueprint Focus: The cellular and molecular biology syllabus tests the mechanistic basis of systemic disease and targeted pharmacotherapy. Candidates are frequently tested on organelle pathology (e.g., lysosomal and peroxisomal storage defects), intermediate filament immunohistochemistry, cell cycle checkpoint kinetics (p53 and Rb tumor suppressor pathways), intrinsic vs. extrinsic apoptosis, and second messenger cascades.

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Organelle Function, Trafficking & Pathological Correlates

Eukaryotic cells rely on compartmentalized organelles to separate biosynthetic, metabolic, and degradative processes. Structural or enzymatic failure within these compartments produces characteristic clinical syndromes.

Endoplasmic Reticulum and Golgi Apparatus

• Rough Endoplasmic Reticulum (RER): Site of synthesis of membrane-bound, lysosomal, and secretory proteins. Proteins enter the RER lumen via signal peptides. Molecular chaperones (BiP, calreticulin, calnexin) ensure correct tertiary folding.
• Unfolded Protein Response (UPR): Accumulation of misfolded proteins triggers endoplasmic reticulum stress, activating sensor proteins (PERK, IRE1, ATF6) that inhibit general translation while upregulating chaperone synthesis. Prolonged ER stress induces apoptosis via CHOP-mediated Bim activation.
• Smooth Endoplasmic Reticulum (SER): Devoid of ribosomes; executes lipid and steroid hormone synthesis (abundant in adrenal cortex and gonads), glycogen catabolism, and cytochrome P450-mediated drug detoxification (in hepatocytes). The sarcoplasmic reticulum of myocytes acts as a dedicated intracellular calcium store.
• Golgi Apparatus: Coordinates post-translational modifications, sorting, and packaging. Executes N-linked oligosaccharide modification, O-linked glycosylation, and sulfation.
• Mannose-6-Phosphate (M6P) Tagging: Acid hydrolases destined for lysosomes receive an M6P tag in the cis-Golgi via phosphotransferase (N-acetylglucosaminyl-1-phosphotransferase). In I-cell disease (mucolipidosis type II), deficiency of this phosphotransferase leaves lysosomal enzymes untagged; they are erroneously secreted into extracellular fluid. Lysosomes become engorged with undegraded substrates, producing coarse facial features, skeletal dysostosis multiplex, severe psychomotor retardation, and early childhood mortality.

Peroxisomes

Peroxisomes contain oxidative enzymes (catalase, urate oxidase) that catabolise very long chain fatty acids (VLCFAs, >C22) via beta-oxidation, branched-chain fatty acids (phytanic acid) via alpha-oxidation, and synthesize plasmalogens (essential myelin phospholipids).

• Zellweger Syndrome: Autosomal recessive mutation in PEX genes encoding peroxin proteins essential for peroxisome assembly. Patients present in the neonatal period with severe hypotonia ("floppy infant"), craniofacial dysmorphism (high forehead, wide fontanelles), hepatomegaly, and death within the first year.
• Refsum Disease: Autosomal recessive deficiency of phytanoyl-CoA hydroxylase, blocking alpha-oxidation of phytanic acid. Clinical triad: cerebellar ataxia, retinitis pigmentosa, and peripheral sensorimotor neuropathy, accompanied by ichthyosis and sensorineural hearing loss. Treatment requires dietary restriction of dairy products and ruminant meats.
• X-Linked Adrenoleukodystrophy (X-ALD): Inactivating mutation in the ABCD1 peroxisomal transporter gene, preventing VLCFA entry into peroxisomes. VLCFAs accumulate in the cerebral white matter and adrenal cortex, causing progressive neurocognitive decline, spastic quadriparesis, and primary adrenal insufficiency (Addisonian crisis) in young boys.

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Cytoskeletal Architecture & Diagnostic Immunohistochemistry

The eukaryotic cytoskeleton comprises microfilaments, intermediate filaments, and microtubules, maintaining spatial organization, cell motility, and intracellular transport.

Immunohistochemical Utility of Intermediate Filaments

When pathologists evaluate an undifferentiated metastatic carcinoma, sarcoma, or lymphoma, intermediate filament immunohistochemistry defines the tissue of origin:

• Cytokeratin: Epithelial cells; positive in carcinomas.
• Vimentin: Mesenchymal cells; positive in sarcomas, renal cell carcinoma, and endometrial carcinoma.
• Desmin: Skeletal, smooth, and cardiac muscle; positive in rhabdomyosarcomas and leiomyosarcomas.
• Glial Fibrillary Acidic Protein (GFAP): Astrocytes, oligodendrocytes, ependymal cells; positive in astrocytomas and glioblastoma multiforme.
• Neurofilaments: Neuronal axons and dendrites; positive in neuroblastoma, pheochromocytoma, and ganglioneuroma.

Microtubule Dynamics and Molecular Motor Proteins

Microtubules are polar polymers with dynamic plus ends (growing toward the cell periphery) and minus ends (anchored at the centrosome/microtubule-organizing center).

• Kinesin: ATP-dependent motor protein moving anterograde (minus-to-plus, centrosome to cell periphery; transports vesicles and organelles toward the synaptic terminal).
• Dynein: ATP-dependent motor protein moving retrograde (plus-to-minus, cell periphery to centrosome; transports endosomes, neurotropic viruses such as Rabies and HSV, and retrograde signals).
• Primary Ciliary Dyskinesia (Kartagener Syndrome): Autosomal recessive defect in ciliary dynein arms, immobilizing respiratory cilia and sperm flagella. Classic triad: bronchiectasis, chronic sinusitis, and situs inversus (Kartagener syndrome = PCD with situs inversus due to absent embryonic nodal ciliary rotation), plus male subfertility.
• Pharmacological Modulators: Colchicine binds tubulin dimers to inhibit polymerisation (blocking neutrophil extravasation in acute gout). Vinca alkaloids (vincristine, vinblastine) arrest cells in metaphase by disrupting spindle assembly. Taxanes (paclitaxel, docetaxel) hyperstabilize microtubules, preventing depolymerisation and blocking anaphase completion.

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Membrane Transport Systems & Channelopathies

Biological membranes exhibit selective permeability maintained by specialized transport proteins.

Membrane Transport Systems
├── Passive Transport
│   ├── Simple Diffusion (lipophilic molecules: O2, CO2, steroids)
│   └── Facilitated Diffusion (GLUT transporters, resting ion channels)
└── Active Transport
    ├── Primary Active Transport (direct ATP hydrolysis: Na+/K+ ATPase, SERCA, H+/K+ ATPase)
    └── Secondary Active Transport (uses electrochemical gradient created by primary transport)
        ├── Symport / Cotransport (SGLT1/2, Na+-K+-2Cl- cotransporter / NKCC2)
        └── Antiport / Exchanger (Na+/Ca2+ exchanger / NCX, Na+/H+ exchanger / NHE1)

Primary vs. Secondary Active Transport

• Na+/K+ ATPase: Primary active, electrogenic antiporter. Hydrolyses one ATP molecule to pump 3 Na+ out of the cell and 2 K+ into the cell against steep concentration gradients. Phosphorylated state (E2) binds cardiac glycosides (digoxin), which competitively block extracellular K+ binding, raising intracellular Na+. High intracellular Na+ reduces the driving force for the Na+/Ca2+ exchanger (NCX), elevating cytosolic Ca2+ to increase myocardial contractility.
• Sodium-Glucose Cotransporter 2 (SGLT2): Secondary active symporter located in the S1 segment of the renal proximal convoluted tubule. Couples the downhill reabsorption of Na+ to the uphill reabsorption of glucose. Selectively inhibited by gliflozins (dapagliflozin, empagliflozin), promoting glycosuria, osmotic diuresis, blood pressure reduction, and nephroprotection.

ABC Transporters & CFTR Pathophysiology

The ATP-Binding Cassette (ABC) transporter superfamily utilizes ATP hydrolysis to translocate substrates across lipid bilayers.

• Multidrug Resistance Protein 1 (MDR1 / P-glycoprotein): Overexpressed on malignant cells; actively extrudes hydrophobic chemotherapeutic agents (doxorubicin, etoposide, vincristine) from the cytoplasm, producing chemoresistance.
• Cystic Fibrosis Transmembrane Conductance Regulator (CFTR): A unique ABC transporter that functions as an ATP-gated, cAMP-regulated chloride channel across epithelial apical membranes. CFTR also tonically inhibits the epithelial sodium channel (ENaC) in respiratory and intestinal epithelia, while activating ENaC in sweat glands.

[!IMPORTANT] The F508del Mutation: Deletion of 3 base pairs (CTT) results in loss of a single phenylalanine residue at position 508. This causes protein misfolding in the rough endoplasmic reticulum. Chaperones detect the abnormal conformation, targeting the channel for polyubiquitination and degradation by the 26S proteasome. As a result, virtually zero functional CFTR reaches the plasma membrane.

Aquaporins & Ion Channelopathies

• Aquaporin-2 (AQP2): Expressed in the apical membrane of principal cells in the renal medullary collecting duct. Under unstimulated conditions, AQP2 is stored in intracellular endosomes. Vasopressin binds basolateral V2 receptors, elevating cAMP via Gs, which triggers protein kinase A (PKA) to phosphorylate AQP2 and mobilize it to the apical plasma membrane, concentrating urine. Inactivating mutations in AVPR2 (X-linked) or AQP2 (autosomal) cause congenital nephrogenic diabetes insipidus.
• Ryanodine Receptor 1 (RYR1): Calcium-release channel on the sarcoplasmic reticulum membrane of skeletal myocytes. Autosomal dominant gain-of-function mutations cause malignant hyperthermia. Exposure to volatile halogenated anaesthetics (halothane, isoflurane) or depolarising muscle relaxants (suxamethonium) triggers uncontrolled sarcoplasmic reticulum Ca2+ release, causing sustained muscular rigidity, hypermetabolism, hyperthermia, rhabdomyolysis, and life-threatening lactic/respiratory acidosis. Treated immediately with dantrolene, which blocks the RYR1 channel.

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