Receptor Mediated and Fluid Phase Uptake

Slide 1 · Receptor-Mediated and

Fluid-Phase Uptake

Cellular housekeeping • Endocytosis, exocytosis, transcytosis and cytoskeletal context

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning outcomes

By the end of the session, explain how cells:

Internalize selected ligands by caveolae- and clathrin-associated routes.

Sort cargo and receptors within acidified endosomal compartments.

Balance endocytosis with secretion, recycling, and transcytosis.

Use cytoskeletal systems for cell structure, movement, and transport.

Slide 3 · Three directions of membrane traffic

Endocytosis

Material enters the cell

Surface membrane forms internal vesicles

Exocytosis

Vesicles fuse with the surface

Secreted proteins leave the cell

Transcytosis

Vesicles traverse a cell

Transfer across an epithelial barrier

Slide 4 · The basic traffic problem

Select cargo

Form vesicle

Sort internalized material

Reuse or dispose

Entry, intracellular sorting, and membrane return are linked processes.

Cargo molecules and their receptors do not necessarily share the same final destination.

Slide 5 · Which route is used?

Caveolae

Small receptor-bound molecules, including some vitamins

Caveolin-associated membrane domains

Clathrin-coated pits

Examples: LDL and transferrin receptor cargo

Clathrin-associated internalization

Fluid-phase capture

Extracellular fluid is co-internalized

Can accompany coated-vesicle uptake

Slide 6 · Caveolae: Membrane invaginations

Extracellular space

Cytosol

Caveola

Caveolin-rich membrane

Vesicle after scission

Identity

“Little caves” at the plasma membrane.

Associated with caveolin rather than a clathrin coat.

Support uptake of bound molecules and extracellular fluid.

Slide 7 · The lipid environment of caveolae

Caveolae share properties with membrane rafts.

Membrane domains are enriched in cholesterol and glycosphingolipids.

The lipid setting contributes to localization of specific membrane-associated proteins.

Caveolin is the major structural protein identified in tissue analysis.

Slide 8 · Caveolin and membrane organization

Extracellular space

Cytosol

Caveola

Caveolin-rich membrane

Vesicle after scission

Structural concept

Caveolin organizes the caveolar membrane domain.

The domain can internalize associated ligands.

These are schematic relationships, not a structural micrograph.

Slide 9 · Proteins associated with caveolae

Several classes.

Glycosylphosphatidylinositol (GPI)-linked molecules.

CAMP-binding proteins and Src-family kinases.

Folate receptors involved in uptake of some small molecules.

Receptors and integrins can be internalized in caveolar pathways.

Slide 10 · Potocytosis: “cellular sipping”

Caveolar domain

Bound molecule + fluid

Internalization

Intracellular delivery

Potocytosis specifically names internalization of caveolae with associated extracellular fluid.

Slide 11 · Folate as a caveolar cargo example

Vesicle cargo

Transport to next compartment

Trace the example

Folate receptor is associated with caveolae.

Receptor-associated uptake can deliver folate across the plasma membrane.

The source treats this as an example, not an exclusive uptake mechanism.

Slide 12 · Caveolae extend beyond nutrient uptake

They also participate in cell-surface regulation.

Internalization of receptors can influence transmembrane signaling.

Integrin internalization links caveolae to cellular adhesion.

Membrane transport and surface signaling are mechanistically connected.

Slide 13 · One caveolar trafficking sequence

Selected surface cargo

Caveolar invagination

Dynamin-dependent release

Cargo redistribution

Dynamin activity is required for the vesicle-release step described for caveolae.

Slide 14 · Clathrin-coated pits

Extracellular space

Cytosol

Receptor-bound cargo

Clathrin coat

Architecture

Clathrin assembles as an intracellular basket-like lattice.

The coat helps drive membrane invagination.

Coated vesicles carry receptor-associated macromolecules.

Slide 15 · Two classic receptor-cargo pairs

LDL pathway

LDL receptor binds LDL-associated cargo.

Internalization supports cholesterol delivery.

Transferrin pathway

Transferrin receptor binds transferrin cargo.

Internalization supports iron delivery.

Slide 16 · Formation of a coated vesicle

Ligand binds receptor

Receptors localize to pit

Clathrin drives budding

Vesicle separates

Receptor-mediated uptake concentrates bound macromolecules in specialized membrane regions.

Slide 17 · Dynamin: The vesicle scission step

Extracellular space

Cytosol

Dynamin collar

Scission occurs at the narrow neck

Mechanistic point

Dynamin is a GTPase.

It functions as a molecular scissor at vesicle release.

Required for the caveolar and clathrin routes described.

Slide 18 · Coated vesicles do not stay coated

Plasma membrane

Coated vesicle

Clathrin uncoating

Early endosome fusion

Loss of the coat occurs rapidly after release, before endosomal fusion.

Slide 19 · Early endosome: First sorting station

Early endosome

Acidic compartment

Cargo separates from receptor

Endosomal role

Receives vesicles after uncoating.

Maintains an acidic intraluminal environment.

Provides conditions for ligand-receptor separation.

Slide 20 · Why acidic endosomes matter

Low pH changes cargo-receptor association.

LDL and transferrin receptors release their transported cargo.

Cholesterol and iron are then transported into the cytosol.

The receptor can follow a separate sorting route after ligand release.

Slide 21 · Progressive endosomal maturation

Early endosome

Late endosome

Lysosomal fusion

Cargo processing

The text describes maturation through a late endosome to eventual lysosomal fusion.

Slide 22 · Lysosomes in the endocytic pathway

Early endosome

Acidic compartment

Cargo separates from receptor

Terminal destination

Late endosomes ultimately fuse with lysosomes.

Some internalized proteins and receptors are degraded.

Recycling is a distinct alternative to lysosomal loss.

Slide 23 · Receptor fate is a sorting decision

Ligand separates

Receptor sorted

Surface recycling / reuse

OR lysosomal degradation

The two fates have different consequences for cell-surface receptor abundance.

Slide 24 · Recycling restores the surface pool

Cell surface

Endosome

Recycling

Surface reuse

Examples

LDL receptors can be reused.

Transferrin receptors can be reused.

Ligand release can precede return to the plasma membrane.

Slide 25 · Degradation is a regulatory choice

Not every receptor is returned to the surface.

Internalized receptors may be directed to lysosomes.

Receptor degradation reduces the number of available surface receptors.

The text uses epidermal growth factor receptor (EGFR) as an example.

Slide 26 · EGFR internalization limits signaling

Surface EGFR

Ligand-associated uptake

Lysosomal degradation

Receptor downregulation

The important consequence is attenuation of receptor-mediated signaling.

Slide 27 · Recycling versus downregulation

Return and reuse

Removal and degradation

After cargo release, receptor is directed back to plasma membrane.

Cell preserves a reusable receptor pool.

Examples in the text: LDL and transferrin receptors.

Endocytic routing can direct receptor toward lysosomal destruction.

Surface receptor abundance declines.

EGFR exemplifies signaling control by downregulation.

Slide 28 · LDL receptor pathway: Key itinerary

LDL

LDL receptor

Endosome

Internalization delivers cholesterol; Defective LDL transport is linked to FH

Follow the cargo

LDL receptor binds extracellular cargo.

Clathrin-associated internalization delivers it to endosomes.

Endosomal release permits cellular cholesterol delivery.

Slide 29 · Transferrin receptor: Same overall logic

Transferrin-bound receptor

Coated-pit uptake

Acidic endosomal release

Iron delivery + receptor reuse

The textbook specifically pairs transferrin uptake with iron delivery.

Slide 30 · Clinicopathologic bridge: Familial hypercholesterolemia

When LDL receptor-mediated transport is defective:

LDL transport into cells is impaired.

Familial hypercholesterolemia is identified in tissue analysis as a disease association.

The source points to Chapter 5 for a fuller description; Detailed genetics are outside this unit.

Slide 31 · Fluid-phase pinocytosis

Endocytosis does not carry only receptor-bound cargo.

As a coated membrane pit closes, it traps extracellular fluid.

This accompanying fluid uptake is termed fluid-phase pinocytosis.

Receptor-associated cargo selection and nonspecific fluid capture can occur together.

Slide 32 · One vesicle, two types of contents

Vesicle cargo

Transport to next compartment

Contents

Selected receptor-bound ligand.

An aliquot of surrounding extracellular fluid.

Both can enter the vesicular trafficking pathway.

Slide 33 · Pinocytosis and phagocytosis

Pinocytosis

“Cellular drinking”: Fluid/small vesicle uptake.

Includes the fluid captured with coated vesicles.

Phagocytosis

“Cellular eating”: Ingestion of larger material.

Textbook context: Macrophages and neutrophils ingest microbes/debris.

Slide 34 · Endocytosis moves substantial volume

10–20%

Of a cell’s own volume taken up from extracellular space per hour (textbook estimate)

This is a routine, not exceptional, flux.

Vesicle trafficking must therefore be sustained.

Membrane turnover cannot proceed without recycling.

Slide 35 · Plasma membrane turns over rapidly

1–2%

Of plasma membrane internalized each minute (textbook estimate)

Unchecked uptake would deplete the cell surface.

Internalized membrane must be returned.

Endocytosis and exocytosis are tightly coupled.

Slide 36 · Why membrane recycling is essential

Endocytosis

Exocytosis

Membrane in

Membrane out

Coupled cycling stabilizes surface area

Conservation

Endocytosis removes plasma membrane.

Exocytosis restores membrane to the surface.

Balance avoids large changes in membrane area.

Slide 37 · The membrane conservation cycle

Surface invagination

Vesicular uptake

Intracellular routing

Return/fusion at surface

Multiple rounds of uptake require re-use of membrane after internalization.

Slide 38 · Secretory proteins: Preparation for export

Rough ER

Golgi

Vesicle

Protein synthesis

Packaging

Surface fusion

Biosynthetic route

Secreted proteins originate in rough endoplasmic reticulum (RER).

Golgi apparatus processes and packages cargo.

Proteins accumulate in secretory vesicles.

Slide 39 · Exocytosis transfers vesicle contents outward

Secretory vesicle

Approaches plasma membrane

Membrane fusion

Cargo released extracellularly

Vesicle membrane also becomes part of the plasma membrane during fusion.

Slide 40 · Two examples of secreted proteins

Peptide hormones

Cytokines

Insulin is an example named in the text.

Protein is synthesized and packaged before release.

The vesicle expels its contents after plasma membrane fusion.

Cytokines are another example named in the text.

Exocytosis supports export of secreted signaling proteins.

The cargo exits without crossing the bilayer as an isolated solute.

Slide 41 · Transcytosis: Across the cell

Epithelial cell

Apical

Basolateral

Transport direction

A vesicle moves between apical and basolateral compartments.

Transfer can move intact macromolecules across epithelial barriers.

Transcytosis differs from simple delivery into the cytosol.

Slide 42 · Why epithelial polarity matters

An epithelium has distinct membrane domains.

Apical and basolateral compartments occupy different sides of the barrier.

Transcytosis links these compartments by intracellular vesicular movement.

The payload can remain intact during barrier transfer.

Slide 43 · Intact proteins cross an epithelial barrier

Apical uptake

Vesicular transfer

Basolateral delivery

Intact protein on opposite side

The textbook provides maternal milk antibodies as an example of intact-protein transfer.

Slide 44 · Transcytosis can also support bulk transport

The text identifies two functional roles.

Movement of intact proteins across an epithelial interface.

Rapid movement of large solute volumes.

The process is distinguished by passage across the cell rather than retention.

Slide 45 · Compare the main transport pathways

Caveolae

Caveolin-associated

Folate as an example

Clathrin-mediated uptake

Clathrin-coated pits

LDL/transferrin examples

Exocytosis

Secretory vesicle fusion

Insulin/cytokines

Transcytosis

Apical ↔ basolateral

Maternal antibody example

Slide 46 · Clinical application: LDL uptake

Apply the mechanism

A cell has defective LDL receptor-mediated transport. Which trafficking step is most directly compromised?

Specify the surface recognition and internalization stages.

Explain the likely consequence for LDL-associated cargo delivery.

Slide 47 · Explanation: Defective LDL transport

Reasoning and mechanism

The affected step is receptor-mediated internalization of LDL-associated cargo.

The textbook connects defects in this process with familial hypercholesterolemia.

The present unit does not specify individual gene variants or lipid concentrations.

Slide 48 · Mechanism check: EGFR

Apply the mechanism

Why can routing an internalized growth factor receptor to the lysosome attenuate cellular signaling?

Distinguish the fate of the ligand from the fate of its receptor.

Predict the change in available receptor abundance at the plasma membrane.

Slide 49 · Explanation: Receptor downregulation

Reasoning and mechanism

Lysosomal degradation reduces the receptor population available for subsequent signaling.

This differs from recycling, which returns receptors to the surface.

EGFR is the source’s example of degradation-mediated downregulation.

Slide 50 · Mechanism check: Membrane mass

Apply the mechanism

What would happen to plasma membrane area if endocytosis continued without exocytotic return?

Use the source’s rapid uptake rates to justify the direction of change.

Identify the balancing process that prevents the problem.

Slide 51 · Explanation: Membrane homeostasis

Reasoning and mechanism

The plasma membrane would be rapidly depleted unless internalized membrane is returned.

The source estimates 1–2% membrane internalization each minute.

Coupled exocytosis and recycling stabilize the surface area.

Slide 52 · Endocytic pathways: Five decisions

Ask these questions whenever tracing vesicle traffic.

What cargo is selected, and by which surface receptor or membrane domain?

How does budding occur, and what mediates vesicle scission?

Where does the vesicle deliver its contents?

Does the receptor recycle or undergo lysosomal downregulation?

Slide 53 · Cytoskeleton: Linked Robbins unit

The index groups cytoskeleton with this topic.

The cytoskeleton governs shape, polarity, organelle organization, and cell migration.

Its major structural systems are actin microfilaments, intermediate filaments, and microtubules.

The more dynamic systems reorganize as cellular needs change.

Slide 54 · Functions of an intracellular scaffold

Not a static framework.

Establishes cellular shape and mechanical organization.

Contributes to polarity and intracellular organelle positioning.

Supports migration and motor-dependent intracellular movements.

Rapid assembly and disassembly are important to many functions.

Slide 55 · Three classes of cytoskeletal polymers

Actin microfilaments

5–9 nm diameter

Actin polymers

Dynamic shape/movement

Intermediate filaments

10 nm diameter

Protein-family dependent

Tensile strength

Microtubules

~25 nm diameter

α/β-tubulin dimers

Tracks and cilia/mitosis

Slide 56 · Actin: G-actin to F-actin

F-actin: Intertwined polarized filaments

− end

+ end

Molecular architecture

Globular G-actin monomers polymerize noncovalently.

F-actin strands intertwine as polarized double helices.

Actin is a major cytosolic protein.

Slide 57 · Polarized actin filaments

Polymer assembly is directional.

Actin filaments possess plus and minus ends.

New subunits are typically added at the plus end.

Removal is commonly associated with the minus end.

The persistent filament can thus exhibit turnover despite structural continuity.

Slide 58 · Actin treadmilling

Monomer available

Plus-end addition

Filament turnover

Minus-end loss

Actin nucleation and regulatory proteins control assembly, bundling, and branching.

Slide 59 · Actin and myosin

Contractile force requires a motor protein.

Myosin uses ATP hydrolysis to move actin filaments relative to one another.

Skeletal and cardiac muscle arrange actin-myosin structures in visible bands.

Myosin-dependent activity also supports cellular transport and movement.

Slide 60 · Actin-related functions

In muscle

Highly organized contractile arrays

Myosin generates relative filament sliding

Elsewhere in cells

Vesicular transport and movement

Epithelial barrier regulation and migration

Slide 61 · Intermediate filaments: Mechanical resilience

Intermediate filament: Rope-like bundle

Tensile support

Junctional anchorage

Core property

Rope-like polymers of diverse proteins.

About 10 nm in diameter.

Generally less dynamically reorganized than actin and microtubules.

Slide 62 · Intermediate filaments support epithelia

Structural integrity depends on anchorage.

Intermediate filaments connect to desmosomes and hemidesmosomes.

This network permits tissues to tolerate mechanical stress.

Cytoskeletal organization is therefore integrated with cell-cell and matrix adhesion.

Slide 63 · Examples of intermediate-filament proteins

Epithelial / mesenchymal

Cytokeratins: Epithelial lineages

Vimentin: Mesenchymal cells

Nuclear lamins: Nuclear lamina

Muscle / neural / glial

Desmin: Muscle scaffold

Neurofilaments: Neurons

GFAP: Glial cells

Slide 64 · Pathology relevance: Lineage identification

Tissue-specific expression patterns are diagnostically useful.

Individual intermediate filaments are expressed in characteristic cell populations.

These patterns can inform assignment of origin in poorly differentiated tumors.

The text lists cytokeratin, vimentin, desmin, neurofilament, and GFAP lineages as examples.

Slide 65 · Microtubules: Tubulin-built cylinders

Hollow tube assembled from α/β-tubulin dimers

Minus end (centrosome)

Plus end (dynamic)

Construction

α- and β-tubulin dimers assemble noncovalently.

The resulting structure is a hollow tube roughly 25 nm wide.

Microtubules are polarized and dynamically remodel.

Slide 66 · Microtubule polarity and dynamics

The centrosome organizes many microtubules.

The minus end is typically anchored at a microtubule organizing center.

The plus end can elongate or retract through tubulin dimer exchange.

Dynamic behavior permits rapid reorganization of intracellular architecture.

Slide 67 · Motor proteins on microtubules

Kinesin

Dynein

Typically outward

Typically inward

Transport systems

ATP-dependent motors travel along microtubules.

Kinesin generally mediates anterograde transport.

Dynein generally mediates retrograde transport; Exceptions exist.

Slide 68 · Major functions of microtubules

A dynamic structural scaffold with multiple outputs.

Serves as tracks for intracellular vesicles and organelles.

Enables sister-chromatid segregation during mitosis.

Forms the core of primary cilia.

Also supports motile cilia and sperm flagella.

Slide 69 · Microtubule-based cilia

Microtubule-based surface projections

Primary cilium: Sensory / regulatory

Core idea

Cilia and flagella depend on microtubular structures.

Primary cilia are generally nonmotile and regulatory.

Motile cilia and flagella support movement.

Slide 70 · Primary cilia and disease association

The textbook links primary cilia to cellular regulation.

Primary cilia contribute to proliferation and differentiation signaling.

Mutations in primary-cilia complex proteins are associated with some polycystic kidney disease forms.

The mechanism is presented here as a chapter-level connection, not a full renal disease lecture.

Slide 71 · Examples of motile microtubular structures

Location helps connect structure with function.

Bronchial epithelial cells possess motile cilia.

The sperm flagellum is another microtubule-based motile structure.

These functions are distinct from the typically nonmotile primary cilium.

Slide 72 · Cytoskeletal systems: Distinguishing features

Actin

5–9 nm

Polar filament network

Myosin interaction

Contraction and movement

Intermediate filaments

10 nm

Rope-like strength

Lineage-specific proteins

Mechanical integrity

Microtubules

25 nm

Tubulin-based hollow tube

Kinesin/dynein tracks

Mitosis, cilia, trafficking

Slide 73 · Cytoskeleton application

Apply the mechanism

A poorly differentiated tumor is positive for a characteristic intermediate-filament marker. What can that finding contribute?

Explain what the marker can indicate about cell lineage.

Identify why a single structural protein is not automatically a complete diagnosis.

Slide 74 · Explanation: Intermediate-filament markers

Reasoning and mechanism

The result can support assignment of a cellular or tissue lineage.

Intermediate-filament expression follows characteristic tissue patterns.

This supports diagnosis but does not replace overall morphologic and clinicopathologic assessment.

Slide 75 · A connected cellular transport system

Membrane invagination

Vesicle release/sorting

Motor-linked transport

Recycling or secretion

Microtubule motors and actomyosin participate in intracellular transport.

The membrane and cytoskeleton are integrated, although every step has its own molecular machinery.

Slide 76 · Take-home concepts

Six statements to retain.

Caveolin-rich caveolae and clathrin-coated pits are distinct uptake architectures.

Dynamin is needed for vesicle release in both routes described.

Acidic endosomes permit cargo release and receptor sorting.

Recycling, degradation, and exocytosis determine surface receptor and membrane balance.

Slide 77 · Take-home concepts (continued)

Cross-barrier transport and mechanical infrastructure.

Transcytosis transfers vesicular contents across epithelial compartments.

Actin, intermediate filaments, and microtubules have distinct dimensions and functions.

Intermediate-filament patterns can inform tumor cell origin.

Microtubules support transport, mitosis, and ciliary structures.

Slide 78 · Primary source and scope

Primary reference

Robbins, Cotran & Kumar: Pathologic Basis of Disease. 11th ed. (2026). Chapter 1, “The Cell as a Unit of Health and Disease,” pp. 10–11.

Source boundaries: lecture content is derived from the supplied Robbins Markdown extraction. Textbook figures were not reproduced; mechanisms and structures are shown with new, editable schematics. The original chapter PDF was not supplied for page-image verification.