Cystic Fibrosis Gene Normal Structure and Function

Slide 1 · Cystic Fibrosis Gene:

Normal Structure and Function

CFTR domains · regulated gating · tissue-specific epithelial transport

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning objectives

Identify the CFTR gene locus and five major protein domains.

Explain phosphorylation- and ATP-dependent CFTR gating.

Compare CFTR roles in airway, intestine, and sweat duct.

Relate bicarbonate transport to secretion and mucin behavior.

Slide 3 · CFTR connects a gene to epithelial fluid physiology

CFTR gene

CFTR protein

Ion transport

Surface fluid

A single gene encodes an apical epithelial anion channel.

Channel conductance and transport regulation determine surface-fluid conditions.

The physiological effect depends on the epithelial tissue.

Slide 4 · The CFTR locus is on chromosome 7

The cystic fibrosis transmembrane conductance regulator gene is at 7q31.2.

Its protein product regulates epithelial chloride and bicarbonate transport.

Chromosomal location identifies the gene; It does not explain channel gating.

Chromosome 7

Long arm (q)

CFTR locus: 7q31.2

Slide 5 · One CFTR gene produces a multidomain protein

CFTR encodes a polypeptide containing 1480 amino acids.

Distinct protein domains carry out pore formation and regulation.

The major architecture can be grouped into five functional domains.

Epithelial lipid bilayer

TMD1 (6 helices)

TMD2 (6 helices)

NBD1

NBD2

R domain

Cytoplasm

Slide 6 · The five-domain CFTR architecture

Two transmembrane domains form the channel-bearing portion.

Two nucleotide-binding domains respond to ATP.

One regulatory (R) domain contains kinase phosphorylation sites.

Epithelial lipid bilayer

TMD1 (6 helices)

TMD2 (6 helices)

NBD1

NBD2

R domain

Cytoplasm

Slide 7 · Transmembrane domains traverse the lipid bilayer

CFTR has two transmembrane domains, TMD1 and TMD2.

Each domain contains six α-helices.

Together, these membrane-embedded structures help form the anion pore.

Epithelial lipid bilayer

TMD1 (6 helices)

TMD2 (6 helices)

NBD1

NBD2

R domain

Cytoplasm

Slide 8 · Twelve membrane-spanning helices organize the pore

Six helices belong to the first transmembrane domain.

Six helices belong to the second transmembrane domain.

The paired domain architecture provides the pathway for chloride passage.

Epithelial lipid bilayer

TMD1 (6 helices)

TMD2 (6 helices)

NBD1

NBD2

R domain

Cytoplasm

Slide 9 · The pore provides an anion-conduction pathway

The transmembrane domains form the channel through which chloride passes.

A permeation pathway is distinct from the signals that govern pore opening.

Transport requires a channel at the appropriate epithelial membrane.

Epithelial lipid bilayer

TMD1 (6 helices)

TMD2 (6 helices)

NBD1

NBD2

R domain

Cl⁻

Cytoplasm

Slide 10 · NBD1 and NBD2 are cytoplasmic domains

CFTR contains two cytoplasmic nucleotide-binding domains.

These domains interact with adenosine triphosphate (ATP).

ATP-dependent domain activity contributes to opening and closing of the pore.

Epithelial lipid bilayer

TMD1 (6 helices)

TMD2 (6 helices)

NBD1

NBD2

R domain

Cytoplasm

Slide 11 · ATP binding is essential to CFTR gating

ATP availability

NBD binding

Channel gating

Anion flow

ATP binding occurs at the nucleotide-binding domains.

This activity is necessary for normal channel opening and closing.

ATP is therefore a gating requirement, not merely a metabolic background factor.

Slide 12 · ATP hydrolysis participates in pore cycling

ATP hydrolysis occurs at the NBDs.

Binding and hydrolysis together support channel opening/closing.

The kinetic cycle is regulated further by phosphorylation of the R domain.

Agonist → cAMP → PKA

R-domain phosphorylation

ATP binding / hydrolysis at NBDs

Channel opening and closing

Slide 13 · The regulatory domain controls activation

The R domain is distinct from the TMDs and NBDs.

It contains phosphorylation sites for protein kinase A (PKA).

R-domain phosphorylation enhances ATP-dependent channel activity.

Epithelial lipid bilayer

TMD1 (6 helices)

TMD2 (6 helices)

NBD1

NBD2

R domain

Cytoplasm

Slide 14 · CFTR architecture separates three tasks

Domain

Structural identity

Main function

TMD1 + TMD2

Two × six α-helices

Anion pore

NBD1 + NBD2

Cytoplasmic domains

ATP-dependent gating

R domain

PKA sites

Regulatory phosphorylation

Slide 15 · Biosynthesis begins before membrane transport

Gene → mRNA

ER translation

Golgi processing

Apical CFTR

The CFTR gene is transcribed into mRNA.

Translation and folding occur in the endoplasmic reticulum.

Processing and glycosylation precede delivery of mature CFTR to its membrane location.

Slide 16 · CFTR is synthesized and folded in the ER

Translation converts CFTR mRNA into the polypeptide.

Protein folding occurs in the endoplasmic reticulum.

Proper biosynthetic processing is relevant to how the channel reaches the cell surface.

Nucleus

CFTR mRNA

Endoplasmic reticulum

Translation / folding

Golgi

Processing / glycosylation

Apical membrane

CFTR channel

Slide 17 · Golgi processing precedes mature surface CFTR

The biosynthetic pathway includes Golgi processing.

CFTR undergoes glycosylation along the maturation route.

The final functional destination is the epithelial cell membrane.

Nucleus

CFTR mRNA

Endoplasmic reticulum

Translation / folding

Golgi

Processing / glycosylation

Apical membrane

CFTR channel

Slide 18 · Apical localization enables directional transport

CFTR operates at the luminal (apical) epithelial surface.

The luminal compartment differs between an airway and a sweat duct.

The same protein can consequently participate in secretion or reabsorption.

APICAL MEMBRANE

Lumen: External to epithelial cell

Epithelial cytoplasm

Directed anion transport

Slide 19 · Channel activation begins with a signaling agonist

Agonist

↑ cAMP

PKA activation

R-domain signal

An extracellular agonist initiates an epithelial signaling response.

The response increases intracellular cyclic AMP (cAMP).

This signaling step precedes activation of protein kinase A.

Slide 20 · CAMP links a receptor signal to the channel

Cyclic AMP is the intracellular signal described for CFTR activation.

Increased cAMP activates protein kinase A.

The cAMP step translates cellular stimulation into channel regulation.

Agonist signal

↑ cAMP

PKA activated

R-domain target

Slide 21 · Protein kinase A targets the R domain

Activated PKA phosphorylates R-domain sites.

Phosphorylation enhances channel opening in the presence of ATP.

Thus, a protein kinase links signaling to the pore.

↑ cAMP

PKA activated

R phosphorylation

CFTR activation

Slide 22 · Phosphorylation and ATP regulate the same pore

R-domain phosphorylation promotes channel activation.

ATP binding/hydrolysis at NBDs supports the opening–closing cycle.

Neither mechanism should be confused with the ion pathway in the TMDs.

Agonist → cAMP → PKA

R-domain phosphorylation

ATP binding / hydrolysis at NBDs

Channel opening and closing

Slide 23 · The full gating sequence is a coupled process

Agonist/cAMP

PKA + R domain

ATP + NBDs

Pore gating

Agonist → cAMP → PKA occurs upstream.

PKA phosphorylates the regulatory domain.

ATP-dependent NBD behavior then supports pore opening and closing.

Slide 24 · Activation differs from permeation

Question

Gating

Permeation

What changes?

Open/closed state

Ion movement

Key domains

R domain + NBDs

TMD1 + TMD2

Main inputs

PKA phosphorylation, ATP

Electrochemical conditions

Slide 25 · CFTR conducts chloride across the epithelium

CFTR was initially characterized as a chloride-conductance channel.

The direction of net transport is tissue-dependent.

Chloride movement contributes to luminal electrolyte and water balance.

AIRWAY LUMEN

Epithelial cytoplasm

CFTR

ENaC

Cl⁻ secretion

Transport direction is tissue-specific

Slide 26 · CFTR also transports bicarbonate

The CFTR pore is not completely selective for chloride.

Bicarbonate (HCO₃⁻) can also pass through the channel.

Bicarbonate transport changes the acid–base character of secretions.

SECRETORY LUMEN

Epithelial cytoplasm

CFTR

SLC26

HCO₃⁻ secretion

Transport direction is tissue-specific

Slide 27 · CFTR has functions beyond its own pore

CFTR regulates other ion channels and cellular processes.

Effects involving chloride, potassium, and gap-junction channels.

Other regulated processes include ATP transport and mucus secretion.

CFTR

Other Cl⁻ channels

Kir6.1 K⁺ channels

ENaC / gap junctions

Slide 28 · The ENaC interaction is physiologically important

ENaC is the epithelial sodium channel at the apical surface.

It mediates uptake of sodium from luminal fluid.

The relationship between CFTR and ENaC differs by tissue.

CFTR

ENaC

In airway: Restraint

Na⁺ enters cell through ENaC

ENaC direction = lumen → cell

Slide 29 · ENaC normally absorbs luminal sodium

Sodium moves from the epithelial lumen into cells through ENaC.

This uptake changes luminal electrolyte balance.

Sodium transport affects the movement of water across epithelium.

AIRWAY LUMEN

Epithelial cytoplasm

CFTR

ENaC

Na⁺ uptake

Transport direction is tissue-specific

Slide 30 · In the airway, CFTR restrains ENaC activity

Normal CFTR-mediated inhibition of airway ENaC.

This restrains sodium uptake from the airway surface liquid.

Combined with anion secretion, it helps preserve the surface-fluid layer.

CFTR

ENaC

In airway: Restraint

Na⁺ enters cell through ENaC

ENaC direction = lumen → cell

Slide 31 · Ion movement governs luminal water volume

Epithelial ion flux alters osmotic driving forces for water movement.

Anion secretion helps support luminal fluid.

Excess sodium reabsorption can favor passive withdrawal of water from the lumen.

AIRWAY LUMEN

Epithelial cytoplasm

CFTR

ENaC

Cl⁻ secretion

Na⁺ uptake restrained

Surface hydration maintained

Transport direction is tissue-specific

Slide 32 · CFTR physiology is explicitly tissue-specific

Tissue

Major normal CFTR role

ENaC relation

Sweat duct

Luminal Cl⁻ reabsorption

Supports Na⁺ reabsorption

Airway

Luminal Cl⁻ secretion

Restraint of Na⁺ uptake

Intestine

Luminal Cl⁻ secretion

Regulation of luminal fluid

Slide 33 · Sweat ducts reclaim chloride from their lumen

Ductal CFTR mediates reabsorption of luminal chloride.

This contrasts with chloride secretion in airways.

The effect is a reduction in salt remaining in final sweat.

SWEAT DUCT LUMEN

Epithelial cytoplasm

CFTR

ENaC

Cl⁻ uptake

Transport direction is tissue-specific

Slide 34 · Sweat-duct sodium reabsorption accompanies chloride

CFTR activity augments ENaC-mediated sodium reabsorption in sweat ducts.

Na⁺ and Cl⁻ are removed from the duct lumen.

This supports relatively low salt content of normal sweat.

SWEAT DUCT LUMEN

Epithelial cytoplasm

CFTR

ENaC

Cl⁻ uptake

Na⁺ uptake

Transport direction is tissue-specific

Slide 35 · Normal ductal transport reduces final sweat salt

Sweat enters duct

Cl⁻ reabsorption

Na⁺ reabsorption

Less NaCl in sweat

Sweat duct epithelium reabsorbs salt as fluid passes through the duct.

CFTR and ENaC support this salt-reabsorptive function.

The concentration of sodium chloride in secreted sweat therefore falls.

Slide 36 · Loss of sweat-duct CFTR impairs salt reclamation

CFTR dysfunction reduces ductal chloride reabsorption.

Robbins also describes reduced ENaC activity in the sweat duct.

More sodium chloride remains in final sweat.

SWEAT DUCT LUMEN

Epithelial cytoplasm

CFTR (impaired)

ENaC

↓ Cl⁻ uptake

↓ Na⁺ uptake

Reduced CFTR-dependent ion transport

Slide 37 · The sweat phenotype follows an absorption defect

Step

Normal sweat duct

CFTR dysfunction

CFTR Cl⁻ uptake

Effective

Reduced

ENaC Na⁺ uptake

Augmented

Reduced

Final sweat salt

Lower

Higher

Slide 38 · Airway CFTR supports luminal chloride secretion

The airway lumen is outside the apical epithelial surface.

CFTR transfers chloride toward airway surface liquid.

This supplies anions to the luminal fluid compartment.

AIRWAY LUMEN

Epithelial cytoplasm

CFTR

ENaC

Cl⁻ secretion

Transport direction is tissue-specific

Slide 39 · Normal airway transport restrains sodium uptake

At the airway surface, CFTR inhibits ENaC activity.

Less luminal sodium is absorbed when the channel network is intact.

This effect complements CFTR-mediated chloride secretion.

AIRWAY LUMEN

Epithelial cytoplasm

CFTR

ENaC

Cl⁻ secretion

Na⁺ uptake restrained

Transport direction is tissue-specific

Slide 40 · Hydration sustains the airway surface layer

Surface liquid overlays respiratory epithelial cells.

Ion transport governs the amount of fluid in that layer.

Adequate fluid supports movement of the mucus layer.

Mucus / airway surface liquid

Respiratory epithelial surface

Hydrated layer permits mucociliary movement

Slide 41 · Normal mucus transport requires mucociliary action

Mucociliary clearance moves mucus along the airway surface.

A hydrated surface layer permits this transport.

The significance of CFTR is therefore functional, not only biochemical.

Mucus / airway surface liquid

Respiratory epithelial surface

Hydrated layer permits mucociliary movement

Slide 42 · CFTR dysfunction lowers airway chloride secretion

Impaired CFTR reduces transfer of chloride into the lumen.

Reduced anion secretion contributes to surface-fluid depletion.

This differs from the reduction in chloride reabsorption in sweat ducts.

AIRWAY LUMEN

Epithelial cytoplasm

CFTR (impaired)

ENaC

↓ Cl⁻ secretion

Reduced CFTR-dependent ion transport

Slide 43 · ENaC disinhibition increases airway sodium uptake

Loss of normal CFTR restraint on airway ENaC.

Sodium uptake from the lumen increases.

Water is then more readily reabsorbed from the airway surface layer.

AIRWAY LUMEN

Epithelial cytoplasm

CFTR (impaired)

ENaC

↑ Na⁺ uptake

↑ H₂O reabsorption

Reduced CFTR-dependent ion transport

Slide 44 · The airway defect primarily reduces fluid volume

The airway surface liquid becomes depleted in volume.

An isotonic but low-volume surface layer.

The principal change is not increased salt concentration of airway surface fluid.

Mucus / airway surface liquid

Below: Lower-volume layer when CFTR is impaired

Respiratory epithelial surface

Volume decreases despite little change in salt concentration

Slide 45 · Dehydrated airway mucus becomes concentrated

Water loss concentrates secreted mucins.

Mucus becomes viscid and less readily cleared.

The dense material can obstruct conducting airways.

Mucus / airway surface liquid

Respiratory epithelial surface

Concentrated mucus; Clearance impaired

Slide 46 · Defective clearance explains downstream risk

↓ Cl⁻ secretion

↑ Na⁺/water uptake

Low-volume fluid

Poor mucus clearance

Poor mucociliary transport permits mucus retention.

Retained secretions facilitate recurrent pulmonary infection.

These consequences arise downstream of altered epithelial ion transport.

Slide 47 · Intestinal CFTR similarly supports secretion

Intestinal epithelium is another site of luminal chloride secretion.

The luminal water balance depends partly on epithelial ion distribution.

Reduced normal CFTR activity favors dehydrated, obstructive secretions.

INTESTINAL LUMEN

Epithelial cytoplasm

CFTR

ENaC

Cl⁻ secretion

Na⁺ uptake restrained

Surface hydration maintained

Transport direction is tissue-specific

Slide 48 · The same mutation can have opposite ion phenotypes

Compartment

Normal CFTR direction

Effect of CFTR loss

Sweat duct

Cl⁻ lumen → cell

NaCl retention in sweat

Airway

Cl⁻ cell → lumen

Reduced surface-fluid volume

Intestine

Cl⁻ cell → lumen

Impaired hydrated secretion

Slide 49 · Bicarbonate is a second major CFTR substrate

The channel directly carries bicarbonate ions.

Its contribution is not adequately captured by chloride flux alone.

Bicarbonate transport helps support normal alkaline secretions.

SECRETORY LUMEN

Epithelial cytoplasm

CFTR

SLC26

HCO₃⁻ secretion

Cl⁻

Transport direction is tissue-specific

Slide 50 · CFTR coordinates with SLC26 exchangers

SLC26-family anion exchangers are co-expressed at apical surfaces.

Reciprocal interactions with CFTR.

This regulatory relationship contributes to epithelial bicarbonate transport.

CFTR

HCO₃⁻ permeation

SLC26 exchangers

Apical secretion

Slide 51 · Direct and indirect bicarbonate actions coexist

CFTR pore

CFTR–SLC26

HCO₃⁻ transport

Alkaline fluid

CFTR itself conducts HCO₃⁻.

CFTR also regulates apical SLC26-family anion exchangers.

Both modes contribute to bicarbonate handling at epithelial surfaces.

Slide 52 · Normal bicarbonate secretion produces alkaline fluid

Bicarbonate supports the luminal pH of epithelial secretions.

Robbins characterizes normal secretions as alkaline.

PH is therefore an output of epithelial transport, not merely a laboratory value.

Apical epithelial anion transport

HCO₃⁻-rich fluid

Alkaline secretion

Luminal pH reflects bicarbonate availability

Slide 53 · Loss of bicarbonate transport acidifies secretions

Certain CFTR defects can markedly impair HCO₃⁻ transport.

Some of these defects preserve substantial chloride transport.

The resulting secretion may become abnormally acidic.

Apical epithelial anion transport

Reduced HCO₃⁻ transport

Lower luminal pH

Acidification can alter mucins and duct patency

Slide 54 · Lower luminal pH changes mucin behavior

↓ HCO₃⁻

↓ Luminal pH

Mucin precipitation

Duct plugging

Reduced bicarbonate lowers luminal pH.

Acidic secretions to increased mucin precipitation.

Precipitated mucin can contribute to obstruction of ducts.

Slide 55 · Acidic secretions can favor bacterial binding

HCO₃⁻ defect

Acidic secretion

↑ Bacterial–mucin binding

Host-surface effects

Acidic fluid can increase bacterial binding to mucins.

This effect is distinct from the low-volume mucus mechanism.

CFTR transport changes both fluid amount and luminal chemistry.

Slide 56 · Pancreatic relevance highlights bicarbonate function

Pancreatic duct secretions depend on bicarbonate transport.

Marked bicarbonate-conductance defects to pancreatic insufficiency.

The clinical implication follows from abnormal fluid chemistry and ductal plugging.

SECRETORY LUMEN

Epithelial cytoplasm

CFTR

SLC26

HCO₃⁻ secretion

Transport direction is tissue-specific

Slide 57 · CFTR effects occur at multiple biological scales

Gene

Protein domains

Cell transport

Tissue function

Gene location specifies the encoded protein.

Domains mediate gating and anion permeation.

Epithelial polarity and partner channels determine tissue phenotype.

Slide 58 · Contrast three CFTR-dependent physiological tasks

Level

Key determinant

Normal output

Molecular

R domain + NBDs

Regulated opening

Sweat duct

CFTR + ENaC uptake

Reduced final NaCl

Airway

Cl⁻ secretion + ENaC restraint

Hydrated surface liquid

Secretory epithelia

CFTR + SLC26

HCO₃⁻-rich fluid

Slide 59 · Interpretation exercise: Sweat and airway

A defective channel yields salty sweat and dehydrated airway mucus.

Is the direction of normal chloride transport the same in both tissues?

Identify the normal direction before considering the disease consequence.

Where does Cl⁻ normally move?

Reason from the normal physiological mechanism.

Slide 60 · Explanation: Different transport directions

Sweat duct CFTR reabsorbs Cl⁻ from the lumen.

Airway CFTR secretes Cl⁻ into the lumen.

Both consequences follow from loss of the same protein in different epithelia.

Sweat duct

Airway

Lumen

Lumen

Cl⁻ uptake

Cl⁻ secretion

Cell

Cell

Slide 61 · Interpretation exercise: Anion conductance

A hypothetical CFTR protein reaches the apical membrane.

It has an intact pore but cannot be phosphorylated at its R domain.

Which step of normal function is directly compromised?

What is most directly affected?

Reason from the normal physiological mechanism.

Slide 62 · Explanation: Pore structure is not enough

TMDs can form the ion pathway without guaranteeing activation.

R-domain phosphorylation normally supports channel opening.

ATP-dependent NBD function then couples to gating of the pore.

Agonist → cAMP → PKA

R-domain phosphorylation

ATP binding / hydrolysis at NBDs

Channel opening and closing

Slide 63 · Five principles to retain

CFTR is a chromosome 7q31.2–encoded, 1480-residue anion channel.

Its 2 TMDs, 2 NBDs, and R domain perform distinct tasks.

CAMP/PKA and ATP regulate chloride and bicarbonate conductance.

Tissue-specific direction of ion transport explains contrasting effects.

7q31.2 → 1480-aa CFTR

2 TMDs + 2 NBDs + R

CAMP/PKA + ATP gating

Cl⁻ and HCO₃⁻ transport

Slide 64 · Primary source and scope

Robbins, Cotran & Kumar. Pathologic Basis of Disease. 11th ed. (2026).

Chapter 10, pp. 424–426: “Cystic Fibrosis Gene: Normal Structure and Function.”

Conceptual diagrams are original native PowerPoint schematics; no textbook figure was reproduced.