Past exam of the biochemistry course of the University of Cambridge / 2025 / Paper 4 / Section I

phosphoinositides provide changing membrane identities that recruit, activate and release the endocytic machinery. The strongest evidence combines specific lipid-binding measurements, acute lipid manipulation, genetic rescue and time-resolved imaging. An altered uptake endpoint alone cannot identify whether the defect is pit initiation, membrane deformation, membrane fission, uncoating or later endosome traffic.

phosphoinositides are phosphorylated derivatives of phosphatidylinositol. Their headgroups differ in phosphate position, so chemically similar membrane anchors carry distinct recognition signals. At the plasma membrane, phosphatidylinositol 4,5-bisphosphate supports recruitment and activation of multiple components of clathrin-mediated endocytosis, including the AP-2 complex, epsin and dynamin. Binding can involve both electrostatic attraction and stereospecific recognition; showing binding to a highly charged artificial surface does not establish physiological selectivity. Compare equal-charge membranes with different headgroups, vary the target lipid at constant total charge, and verify that a binding-site mutation preserves protein folding.

A decisive experiment is to remove phosphatidylinositol 4,5-bisphosphate rapidly after the biological cell has developed normally. Chemically induced recruitment of a lipid phosphatase to the plasma membrane caused loss of endocytic coats and impaired uptake, giving a causal link on a timescale much shorter than a constitutive knockout.^[ref] A catalytically inactive phosphatase, recruitment without the active enzyme and monitoring of lipid depletion separate enzyme action from recruitment artifacts. Because the phosphatase converts one phosphoinositide into another, the perturbation changes both substrate and product; it is not equivalent to selectively deleting one molecular interaction. Recovery of lipid and coat assembly, or an independent depletion method, strengthens specificity.

That experiment supports a requirement for phosphatidylinositol 4,5-bisphosphate, but does not by itself assign its action to one protein. Multiple coat and actin regulators bind the same lipid. To isolate a particular recruitment step, compare normal and lipid-binding-defective AP-2 complexes at matched abundance, test cargo binding independently, and assay membrane association and internalization separately. In a reconstitution, purified adaptors, clathrin, defined lipid vesicles and a cargo-bearing membrane can distinguish direct lipid-dependent assembly from secondary changes in cell physiology. The interpretation must also account for membrane tension, which can change the requirement for actin-generated force.

Removal of the same lipid is needed later in the pathway. synaptojanin is a phosphoinositide phosphatase whose loss in mouse neurons caused abnormal phosphoinositide metabolism, accumulation of coated intermediates and defects in synaptic vesicle recycling.^[ref] This establishes that productive traffic requires turnover as well as production of the lipid. A reasonable mechanism is that dephosphorylation weakens membrane binding of adaptors after budding, permitting coat release and component reuse. Constitutive loss can also affect development and several trafficking steps, so the morphological phenotype should be combined with acute interventions and rescue by normal versus catalytically defective synaptojanin. Neither an increased number of coated profiles nor reduced synaptic transmission alone uniquely identifies defective uncoating.

Different phosphoinositides can act sequentially. Experiments implicating phosphatidylinositol 3,4-bisphosphate and PI3K-C2α in late coated-pit maturation linked local lipid production to recruitment of SNX9, a membrane-remodeling factor.^[ref] The mechanistic point is a timed recruitment signal, rather than a permanently uniform membrane composition. Compare kinase depletion with rescue by a normal or catalytic mutant, measure pit lifetimes and morphology, and verify that cargo remains available at the surface. A fluorescent clathrin punctum can disappear by internalization, disassembly or movement out of the observation plane; an independent scission or uptake assay is necessary.

Live imaging provides temporal information that bulk lipid extraction lacks. Coincidence-detecting sensors were developed to report phosphoinositides specifically at clathrin-associated structures and follow their conversion during traffic.^[ref] A probe's fluorescence reports its binding equilibrium, geometry and accessibility, rather than an absolute local lipid count. Overexpressed probes can buffer the lipid being measured; some domains bind soluble headgroups as well. Use low expression, lipid-binding-defective probes, known enzymatic perturbations and an independent reporter. Sensor arrival after a coat marker is evidence about accessible lipid at that time, not necessarily the instant of synthesis.

Downstream, phosphatidylinositol 3-phosphate helps recruit effectors to early endosomes, while phosphatidylinositol 3,5-bisphosphate is associated with later endolysosomal function. These roles should be separated experimentally from initial uptake. Pulse-label a surface cargo, remove extracellular label, and follow its delivery, recycling and degradation. If uptake is normal but degradation fails, the result supports a sorting or maturation defect rather than failed entry. General inhibition of all phosphoinositide kinases is especially difficult to interpret because it perturbs several compartments simultaneously.

A useful synthesis is a regulated cycle: local lipid synthesis makes an assembly site competent; coat and curvature factors recognize that site; membrane deformation and scission proceed; lipid conversion changes the set of favored partners; uncoating and endosomal sorting follow. This is a model assembled from distinct experiments, not proof that every endocytic event follows one rigid lipid sequence. The evidence supports both spatial identity and temporal switching: a lipid can be required for assembling an endocytic structure, while its subsequent removal is required for that structure to complete its task.

Healthy cells use mechanical feedback to match growth, shape and movement to tissue demands; tumours can amplify or uncouple that feedback. Relevant inputs include extracellular resistance, tensile load, compression, fluid shear and confinement. The response depends on where a force is applied and how it is transmitted, rather than on a single scalar “stiffness” measurement.

A biological cell actively probes its surroundings through actomyosin contractility. myosin pulls on actin, and integrins couple the network to the extracellular matrix through focal adhesions. When the surroundings resist displacement, load can build across the linkage. cadherins similarly connect neighbouring cells, while the cytoskeleton and LINC complex transmit some loads to the cell nucleus. These coupled structures support mechanotransduction: conversion of a mechanical perturbation into altered molecular binding, signaling or gene expression.

One physical mechanism is force-dependent exposure of binding sites. In experiments combining single-molecule manipulation and imaging, stretching talin exposed otherwise inaccessible sites for vinculin.^[ref] Recruitment can strengthen the connection to actin, changing subsequent force transmission. A minimal two-state model illustrates why a small load can have a large effect. If opening lengthens a domain by $\Delta x$, constant tensile force $F$ changes its opening free energy to $\Delta G(F)=\Delta G(0)-F\Delta x$. Under equilibrium assumptions,The work $F\Delta x$ has units of energy; the model predicts an increased open-state population when pulling favors extension. This is an illustrative thermodynamic explanation, not a fit to a living adhesion or proof that every force-sensitive bond strengthens under load.

Mechanical context can alter cell fate. Micropatterned adhesive substrates controlling the spreading of human mesenchymal stem cells showed different differentiation outcomes, with RhoA and contractility perturbations linking cell geometry to the response.^[ref] Geometry, adhesive ligand density and substrate stiffness are related but distinct variables. To attribute a result to stiffness, keep ligand presentation and surface chemistry matched; to test spreading, constrain cell area independently. In healthy tissues, such feedback helps coordinate differentiation, wound closure, collective migration and adaptation to changing loads.

Nuclear transcriptional regulators integrate these inputs. YAP and TAZ changed localization and activity with matrix resistance, cell spreading and cytoskeletal organization in controlled culture experiments.^[ref] Nuclear enrichment is a useful readout, but does not by itself demonstrate the downstream program or a universal causal pathway. Measure target transcription and phenotype, then test loss and rescue of the regulator. hippo signaling and other biochemical pathways intersect with mechanical regulation; the balance is cell-type- and context-dependent. Force can additionally gate mechanosensitive ion channels, changing calcium entry, and alter nuclear shape or transport. Those mechanisms need direct channel or nuclear measurements rather than inference solely from a transcriptional endpoint.

The normal feedback is bidirectional. Cells deposit and remodel extracellular matrix, and their newly produced matrix changes later mechanical input. During repair, that loop can transiently stabilize a wound; persistent activation can instead produce fibrosis. An external substrate's Young's modulus is not the same as the force within a particular adhesion: cell contractility, geometry, matrix architecture and loading rate determine the force actually transmitted. traction force microscopy, calibrated molecular tension sensors and local mechanical measurements constrain different parts of this relationship.

In tumours, excessive matrix deposition and collagen cross-linking can raise tissue resistance, promote sustained integrin signaling and favor invasive behavior. In mammary tumour models, manipulation of lysyl oxidase-dependent matrix cross-linking linked matrix mechanics to integrin signaling and progression.^[ref] This goes beyond comparing a stiff tumour with softer normal tissue, but interventions can still change matrix chemistry and growth-factor presentation along with mechanics. Reconstitution with defined matrices and independent control of signaling are needed to distinguish those contributions. Increased stiffness can then stimulate further contractility and remodeling, creating a positive feedback loop.

Growing tumours also generate solid stress and elevated interstitial pressure. Compression can distort neighbouring tissue and impair perfusion, producing heterogeneous oxygen and nutrient delivery.^[ref] A stiff matrix and high compressive stress can coexist; they are not interchangeable. Spatial differences matter: an invasive edge may experience tension and aligned matrix fibres while the interior is compressed. Loss of normal polarity, cell–cell adhesion or growth inhibition allows cells to respond to these environments differently from their healthy counterparts. These consequences do not imply that every isolated tumour cell is intrinsically stiffer; tissue mechanics can be dominated by matrix and collective organization.

Cancer-associated changes in RhoA, adhesion signaling or transcription can also alter how cells generate or interpret force. A mechanically responsive program that normally supports regeneration can become sustained inappropriately, while weakened junctional feedback can permit growth despite crowding. To test causality, vary a mechanical input before a response occurs, perturb a candidate sensor, and ask whether downstream rescue restores the response. Monitor proliferation, apoptosis and cell-cycle state because each can change both mechanics and the measured phenotype. Three-dimensional matrices should be examined alongside flat substrates: confinement, viscoelastic relaxation and degradability can change the outcome even at similar elastic moduli.

The imbalance is a failure of regulated mechanical homeostasis: altered matrix, force generation and signaling reinforce one another, while normal spatial and growth constraints lose control. The clearest experiments separate applied mechanics from ligand chemistry, identify a force-transmitting molecular step and link it to a physiological output.