OP: Corneal Endothelial Pump

What is the % of H2O in the corneal stroma?

78%

What two molecules are moved transcellularly? What about paracellularly?

Trans: H2O + Cl-

Para: Na+ + H2O

What divides the apical and basolateral domains of the cell? What does this division allow for?

Tight junctions; paracellular transport

What are the 6 enzymes of the basolateral domain. Name what each transports, the type of transporter, and what this domain faces overall.

• NKA (sodium potassium atpase)

  • 3 Na+ out and 2 K+ in (antiporter)

• NBC (sodium bicarbonate co-transporter)

  • 1 Na+ and 1 HCO3- in (symporter)

• NHE (sodium hydrogen exchanger)

  • 1 Na+ in and 1 H+ out (antiporter)

• AE (anion exchanger)

  • 1 HCO3- out and 1 Cl- in (antiporter)

• NKCC (sodium potassium chloride co-transporter)

  • 1 Na+ and K+ and 2 Cl- in (symporter)

• MCT (monocarboxylate transporters)

  • 1 H+ and lactate (-) in (symporter)

How does the gradient of lactate change from the basolateral to the apical domain of the corneal endothelium?

Decreases

What 3 enzymes are responsible for paracellular transport of water? What reaction does each do? Furthermore, where is each enzyme located?

• Carbonic Anhydrase II: inside endothelial cell

  • Converts H2CO3 → CO2 + H2O

• Carbonic Anhydrase IV: on apical domain

  • Converts CO2 + H2O → HCO3- + H+ (negative charge for water paracellular transport)

• CACC: apical domain

  • 1 Cl- out (activated with Ca+2)

• CFTCR

  • 1 Cl- and HCO3- out

• MCT

  • 1 H+ and lactate (-) out

What are the 4 enzymes of the apical domain of the endothelium? What does each transport and name the transportation method each utilizes. Lastly, what structure does the apical domain face?

• CACC (Ca+2 activated chloride channel): uniporter

  • 1 Cl- out (requires Ca+2 activation)

• CFTCR (cystic fibrosis transmembrane conductance regulator): symporter

  • 1 Cl- and HCO3- out

• MCT (monocarboxylate transporters): symporter

  • 1 H+ and lactate (-) out

• Anterior chamber/lumen

What does corneal hydration exist as? How does it function with the endothelium in normal conditions? Lastly, what is it required for?

• Corneal hydration is a carefully balanced equilibrium

• Under normal conditions the water is continuously drawn into the stroma through the endothelium and continuously pumped out

• Required for transparency

What is the graphical relationship between hydration and thickness? What is this due to?

• Linear relationship between hydration and thickness

  • Increase in water = increase in corneal thickness

  • Due to stroma water content

What will result if the leakage of water is greater than pump function of the endothelium? What about when the leakage of water is less than pump function of the endothelium?

• Corneal edema

• Corneal deturgescence or removal of water

What are the 2 structures that act as barriers to water in the cornea? What are the 2 junctional molecules that mediate this barrier function? Lastly, which is more permeable, why, and how do the leakiness of the two structures compare?

• Epithelium and endothelium are barriers to water

• Barrier function is mediated by junctional molecules

  • Tight junctions

  • Claudin proteins

• The endothelium is more permeable than the epithelium

  • D/t differential expression of claudin protein types

  • Leakiness of endo > epi

What is the epithelial permeability mediated by, which junctional protein varies in distinct epithelia, and which junctions are found on superficial epithelial and endothelial cells?

• Epithelial permeability is mediated by tight junctions

• Tight junctional protein (claudin) expression varies in distinct epithelia

• Tight junctions found in superficial epithelial and endothelial cells

What are claudins? What is their function and how does their differential expression effect the epithelium?

• Claudins: transmembrane tight junctional proteins

• Form small pores large enough for ion and water movement but small enough to prohibit diffusion of proteins or other larger molecules

• Differential claudin expression likely determines the leakiness of an epithelium

How does injury/cell volume alterations affect endothelial ion transport? Name the 2 ways, how each is triggered, and what each leads to. Lastly, what is notably not necessary for this effect?

• Injury and cell volume alterations can lead to increased endothelium ion transport

• 2 ways transport is stimulated

  • Activating CaCC via increasing intracellular Ca+2

  • Activation of purinergic receptors

    • Triggered by secreted ATP

    • Activation leads to increased intracellular cAMP

    • cAMP increase leads to activation of CFTCR

  • No neural stimulation required

What are the water channels present in the conj/cornea/endothelium of cornea?

Aquaporins

Where are aquaporins highly expressed, what domain are they present in, what 2 things do they allow for, and lastly what is their function in the pump mechanism?

• Highly expressed in corneal endothelium

• Apical and basolateral protein

• Allows bidirectional water flow between aqueous and stroma

• Acts to move water transcellularly

• Function unclear in pump mechanism

What does CHED stand for, when does it occur, how frequent is it roughly, what is the presentation, what is the cause, and how does it disrupt function?

• Congenital hereditary endothelial dystrophy (CHED)

  • Can first manifest at birth or later in life

  • 3/100k newborns

  • Thickening of stroma d/t increased water content

  • Mutations in ion transport encoding gene

  • Disrupts function of a putative NBC co-transporter

How does hydration of the cornea change in newborns vs adults?

Decreases (infants born with some corneal edema)

How does endothelial cell density change with age? What does it start at and where does it end? How much density is required for normal function and if you drop below this range, what does this cause?

• Endothelial density decreases with age

  • 4k cells at birth vs 2.5k in adults

• Need at least 400-750 to have normal function

• Further reduction = leaking > pump function

What is the reduction in endothelial cells d/t? Why?

• Reduction in density is d/t cell loss over time

• Very low mitotic rate does not replenish lost cells but instead simply gets bigger and amorphous

Does age dependent loss of endothelial cells prevent endothelial function normally? Why or why not? Is there any limit and what occurs if this limit is exceeded?

• Age dependent loss of endothelial cells normally doesn’t prevent endothelial function

• Activity and number of NKA pumps can increase to compensate for cell loss

• Compensation has limitations

  • Combined with pathological loss of endothelial cells and/or function can lead to edema

What are the 5 factors contributing to endothelial cell loss? How does this effect pump function if combined with aging?

• CLS

• Crosslinking therapy

• Refractive/Cataract surgery

• Trauma

• Corneal transplants

Combined with aging can result in dramatic reduction in pump fxn.