Physiology of lens and Cataractogenesis

Physiology of lens and Cataractogenesis

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  PHYSIOLOGY OF LENSAND CATARACTOGENESIS DR. SRISTI THAKUR LEI, NAMS 1ST YEAR RESIDENT
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  LAYOUT Physiological processes concerned withthe functioning of lens • Biochemical composition of lens • Metabolism of lens • Lens transparency • Antioxidation mechanism Cataractogenesis • Introduction • Congenital cataract And Developmental cataract • Acquired cataract
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  INTRODUCTION • Transparent, avascular,biconvex, crystalline structure • located between the iris and the vitreous in the patellar fossa • aid in focussing light rays on retina. • Suspended from the ciliary body by zonular fibres. • formation of the human lens begins very early in embryogenesis i.e at 3rd week of gestation.
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  BIOCHEMICAL COMPOSITION OFLENS 64% water 35% wet wtproteins Others : lipids, amino acids, carbohydrates, inorganic ions, ascorbic acid, glutathione 1%
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  1. LENS PROTEINS(35% WET WT) Alfa A Alfa B 1/3r d 55% Gamma crystallin s
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  SOLUBLE PROTEINS Alpha crystallins •31.7% • Highest molecular wt 600 kilodaltons (kDa) • 2 subunits: A(thiol group) and B • Polymer (50 monomer) • prevent the complete denaturation and insolubilization of the other crystallins. Beta crystallins • 53.4% • 3 polypeptide chains • Mol wt 23kDa • Relatively high thiol content • Coded by 7 gene • beta H (beta high molecular weight) and • beta L (beta low molecular weight) fractions. Gamma crystallins • 15% • Monomers • Lowest molecular weight 20 kDa or less • High in nucleus • Coded by 4 gene
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  INSOLUBLE PROTEINS • canbe further separated into 2 fractions - soluble and insoluble in 8 molar urea • Urea-soluble fraction: • Protein vimentin • Beaded filaments - made from the proteins phakinin and filensin • Urea-insoluble fraction: • Major intrinsic protein (MIP)
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  Albuminoid : • Chiefinsoluble protein • Molecular weight – 3,70,000 • 12.5% of total protein • partly digested by urea
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  • As thelens ages - soluble alpha crystallins are gradually converted into insoluble albuminoids. • Lens proteins are organ specific • Individual can become sensitised to one’s own lens proteins (phacoanaphylactic uveitis).
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  • Increase ofwater insoluble proteins with age • lens protein aggregate form very large particles become water insoluble scatter light increase opacity of lens • Brunescent cataract – increase in amount of water-insoluble protein
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  Functions of lensproteins • Refractive function • Contributes to lens transparency • Helps in change of shape during cell differentiation • Involved in the assembly & disassembly of the lens cytoskeleton • Provide the lens with stress-resistant & oxidative properties
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  2. Amino acids •2 groups of amino acids : 1. Proteogenic - alanine, leucine, glutamic acid, aspartic acid glycine, valine, phenylalanine, tyrosine, serine, isoleucine, lysine, histidine, methionine, proline, threonine, and arginine 2. Non proteogenic - taurine, ornithine butyric acid
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  • Consist allthe amino acids present in any tissue except tryptophan, cysteine and hydroxy proline • Concentration higher in lens than aqueous or vitreous humour • Actively transported into the lens
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  3. Carbohydrates Free carbohydrates Derivativesof sugar • Glucose • Fructose • Glycogen • Sorbitol • Inositol • Ascorbic acid • Gluconic acid • Glucosamine
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  • FRUCTOSE FORMGLUCOSE • GLYCOGEN – IN TRACES..CONC VARIES WITH AGE. • .LOCALISED IN NUCLEUS AND APPEARS TO REPLACE GAMMA CRYSTALLIN NORMALY PRESENT THERE TO INCREASE REFRACTIVE INDEX • IONOSITOL- INVOLE IN METABOLISM OF PHOSPHOLIPID
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  4. Lens lipid •Cholesterol (50-60%) • Phospholipids – sphingomyelin, cephalin, isolecithin, lipoproteins • Glycosphingolipids • Functions- • Principal constituents of lens cell membrane • Also associated with lens epithelial cell division
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  Chemical composition oflens Aqueous humour Lens Water 99% 66% of wet wt. Sodium 144 20 mM Potassium 4.5 125mM Chloride 110 18mM Glucose Lactic acid 6 7.4 1 mM 14 mM Glutathione Inositol 0 0.1 12mM 5.9mM Amino acids Proteins 5 0.04% 25 mM 33% of wet wt.
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  Glucose metabolism • Mainsource of energy • Lens requires continuous supply of energy for • Active transport of ions and amino acids • Maintenance of lens dehydration and transparency • Synthesis of protein and glutathione • Most of the energy produced is utilized in the epithelium which is the major site of all active transport processes
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  Glucose metabolism inlens
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  • Most ofglucose transported into lens phosphorylated to glucose 6-phosphate by enzyme hexokinase • Rxn 70-100 times sloer than that of other enzymes involved in lens glycolysis-so rate limited • G6p enters 1 of 2 metabolic pathways : anerobic glycolysis or HMP shunt • Most active-anerobic – provides most of high energy phosphate bond required for lens metabolism • ADP to ATP occurs at 2 steps along the pathway from glucose metabolism to lactate • less efficient as only 2 atps produced for each glucose molecule utilized • Aerobic produce 36 molecules atp from each molecule in glucose in citric acid cycle(0xidative metabolism) • Low o2 tension In lens , only 3%of lens glucose passes through the krebs citric acid cycle to produce atp—approx. 25% lens ATP
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  Pathways of glucosemetabolism S.N o Pathway Main Intermediates End products % Of O2 ATP gain 1. Glycolytic G6P, fructose 1,6 - diphosphate, PA Lactic acid 80 2 2. Kreb’s cycle TCA CO2, H20 3 36 3. HMP shunt Pentoses CO2, NADPH 10 - 4. Sorbitol Sorbitol, fructose Lactic acid 5 2
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  Clinical correlation  Sugarcataract- 1. True diabetic cataract - accumulation of sorbitol within the lens and accompanying changes in hydration -increased non-enzymatic glycosylation (glycation) of lens proteins, or greater oxidative stress from alternations in lens metabolism. 1. Galactosemic cataract - accumulation of galactose and galactitol within lens.
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  Sugar cataract Diabetic cataract; Snowflakeor snowstorm cataract Galactosemic cataract; oil drop cataract
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  GLUTATHIONE A tripeptide :glutamate, cysteine and glycine • 3.5-5.5 mmol/g wet weight • More in the epithelial layer • Half life-1 to 2 days Functions: • Maintenance of protein thiols in the reduced state • Protection of protein involved in cation transport & permeability • Protection against oxidative damage • Removal of xenobiotics
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  Antioxidant mechanism Reactive oxygenspecies from cell metabolism and photochemical reactions Superoxide anions, hydroxyl free radicals, hydroperoxyl radicals, singlet oxygen, H2O2 a) Cause damage in the bases of DNA b) Polymerize & cross link protein. c) Peroxidize membrane lipids which forms cross links between membrane proteins & lipids causing opacity of lens.
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  Clinical correlation • Reducedglutathione levels in lens epithelial cells or whole lenses causes cell damage and cataract formation • Higher oxygen levels near the crystalline lens induce nuclear cataract • With vitreous syneresis or after vitrectomy, the lens has much greater exposure to oxygen levels from the choroid - induces nuclear sclerosis.
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   Protein metabolism Protein synthesis ◦ Synthesized from free amino acids which are actively transported from aqueous ◦ Requires ATP and the appropriate RNA template ◦ Occurs throughout life (crystallins & MIP)  Protein breakdown Endopeptidases Exopeptidases Protein Peptides A.A Ca++ & Mg
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  Transport of waterand ions
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  Maintenance of lenswater and cation balance • Transparency highly dependent on the structural and macro molecules components of the lens; perturbation of cellular hydration can lead to opacification • Lens capsule freely permeable to water, ions and proteins with molecular weight upto 70 kda • Epithelial cells and fibers possess a number of different channels, pumps, and transporters that enable transepithelial movement to and from the extracellular milieu
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  • Cation balancebetween inside and outside of the lens is due to - Permeability properties of the lens cell membrane - Activity of the sodium pump ( reside within the cell membrane of the lens epithelium and each lens fiber) • Sodium pump functions by pumping sodium ion out while taking potassium ions in • Inhibition of NA+ K+ ATPase leads to loss of cation balance and elevated water content in the lens.
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  Pump-leak theory
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  • NA FLOWSIN THROUGH BACK OF LENS WITH CONCENTRATION GRADIENT AND ACTIVELY EXCHANGED FOR K BY EPITHELIUM • K CONCENTRATED IN ANTERIOR LENS WHEREAS NA IN POSTERIOR
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  • Active transportmechanism are lost if the capsule and attached epithelium are removed from the lens but not if the capsule alone is removed by enzymatic degradation(collagenase) • Membrane transport process establishes the ion gradient across cell membranes and generates extra cellular currents around the outside of lens.
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  • Unequal distributionof electrolytes across the lens cell membranes results in an electrical potential difference between inside & outside of lens. • Inside of lens is electronegative about -70mv, there is -23mv potential difference between anterior & posterior surfaces of lens. • Normal potential difference of about 70mv is readily altered by changes in pump activity or membrane permeability.
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  Calcium homeostasis •
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  • Critical tolens • large transmembrane calcium gradient is maintained by calcium pump(ca+ atpase) • Lens cell membrane impermeable to calcium • Increased levels of calcium –depressed glucose metabolism , formation of high molecular weight protein aggregates, and activation of destructive proteases
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  Transport functions inthe lens •Simple diffusion •Facilitated diffusion Glucose •Dependent upon the sodium gradientAmino acid •specialized carrier-mediated transport system Choline, myo inositol, ascorbic acid •Simple diffusionWaste products
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  LENS TRANSPARENCY Lens transparencydepends on: A . Anatomical factors 1. Single layered lens epithelial cells. 2. Tight arrangement of fibers with little extracellular space. 3. Loss of cellular organelles and nucleus preventing scattering of light. 4. Permeability of lens capsule. 5. Pattern of distribution of the protein within the cells. 6. Avascularity of lens 41
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  B . Physiologicalfactor: 1. Water and electrolyte balance of lens fiber maintaining relative state of dehydration. 2. Presence of alpha crystallin protein preventing aggregation of lens protein. 3. Low oxygen tension around the lens. 4. Auto-oxidative mechanism 42
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  Accomodation
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  • Mechanism bywhich the eye changes focus from distant to near images, produced by a changes in lens shape resulting from the action of the ciliary muscle on the zonular fibers. • lens substance progressively loses its ability to change shape with age. • After 40, rigidity of lens nucleus clinically reduces accommodation because sclerotic nucleus can’t bulge anteriorly & change it’s anterior curvature.
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  • Accommodation ismediated by the parasympathetic fibers of cranial nerve III (oculomotor). • Parasympathomimetic drugs (pilocarpine) induce accommodation, • Parasympatholytic medications (atropine) block accommodation. • Drugs that relax the ciliary muscle are called cycloplegics.
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  Theory of accomodation
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  PRESBYOPIA • Loss ofaccommodation due to aging • Usually after 40 yrs • According to theory of von helmholtz, as the crystalline Lens ages, it becomes firmer and more sclerotic and resists deformation when the ciliary muscles contracts • Hence, it can not bulge enough anteriorly to increase the lens curvature and dioptric power to focus at near.
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  Changes in aginglens Accommodation changes: • Amplitude of accommodation : amount of change in the eye’s refractive power that is produced by accommodation; diminishes with age and may be affected by some medications and diseases 10yrs=12-16d, 40yrs=4-8 d , 60yrs= <2 d 1. Decreased capsular elasticity 2. Increase in stiffness of lens substance 3. Radius of curvature of anterior capsule decreases lens rounder 4. Distance between anterior surface of lens & cornea decreases 5. Internal apical region of the ciliary body moves forward & inward
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  Changes in aginglens Morphological changes: •  In weight and thickness of the lens • Epithelial cells become thinner with flat nuclei , develop electron dense bodies and vacuoles which increases density of surface & cytoskeleton . proliferative capacity decreases. • Lens fibers- loss of plasma membrane proteins & cytoskeletal proteins • Capsule becomes thick and loses its elasticity. •  Disulfide bond with  sulfhydryl group of lens proteins → conversion of soluble lens proteins into insoluble proteins → lens opacification
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  Changes in aginglens • Biophysical changes: • Colorless/pale yellow → darker yellow to brown or black. •  In lens transparency(aggregation of lens protein) • Increase in light scattering • Fluorescence property increases with the age • Absorption of UV & visible light increases with age.
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  Changes in aginglens Biochemical changes: • Changes in the cellular junctions and alteration on cation permeability → constant K+ level,↑Na (40meq/L)→ ↑optical density • Decrease in MIP reduces cell to cell communication. • ↑ Cholesterol:phospholipid ratio→↓ in membrane fluidity • Membrane potential  es(due to change in free Ca levels)from –50mv (at age of 20 yrs) to –20mv (at the age of 80 yrs) • Glutathione and ascorbate levels ↓ • Superoxide dismutase and catalase activity ↓
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  Changes in aginglens Changes in crystallins: • Loss of α – crystallins from soluble proteins of lens nucleus and b crystalline become more polydisperse • Loss of γ – crystallins and increase in disulphide bond • ↑ Insolubility  accumulation of HMW aggregates
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  CATARACTOGENESIS
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  Cataract means opacificationof crystalline lens and its capsule • d/t Precipitation, denaturation, coagulation or agglutination of soluble protein Risk factors: • Heredity • Exposure to radiation • Dietary factors
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  • Severe diarrhoea •Diabetes • Renal failure • Hypertension & diuretics • Myopia • Misc – smoking, alcohol, glaucoma, steroids
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  ETIOPATHOGENESIS
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  • Cataract-caused bydegeneration and opacification of lens fibres • Lens fibres already formed • Formation of aberrant lens fibre and deposition of other material in there place • Disorganisation of lens fibre and abnormalities in lens protein • Loss of transpararency
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  • Any factorsphysical or chemical which disturb the critical intra and extra cellular equilibrium of water and electrolytes and deranges the colloid system within fibres—bring opacification
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  • Aberrant lensfibre germinal epithelium loses its ability to form normal fibres –there is posterior subcapsular cortical cataract • Fibrous metaplasia of fibres - complicated cataract • Epithelial cell necrosis - focal opacification of lens epithelium as glaucomfleken in acute ACG
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  ETIOPATHOGENESIS • Biochemically 3factors evident in cataract formation 1. Hydration – soft cataract 2. Denaturation of lens protein 3. Usual degenerative change (hard caratact)
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  1. Hydration • Frequentlyactual droplet of fluids gather under capsule forming lacunae between fibres • Entire tissue swells • Intumesence • Become opaque • To some extent process reversible and opacities thus formed may clear up as in juvenile insulin dependent diabetic patient whose lens become clearer after control of hyperglycemia
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  • Hydration dueto osmotic changes within lens or to change in semipermeability of capsule impaired • Inactive insoluble protein and anti oxidative mechanism less effective • Process dramatic in traumatic cataract when capsule ruptures and lens fibres swells and bulge out into anterior chamber
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  2. Denaturation oflens protein • Protein denatured with increase in insoluble protein • Dense opacity produced • Irreversible opacity don’t clear up • Mostly in young lens and cortex of adult lens where metabolism active (Rare in old and inactive fibres) • Hard-slow sclerosis
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  3. Aging • Semipermeabilityof capsule impaired • Inactive insoluble protein increase and antioxidative mechanism less effective • Normal lens-sulphydryl containing reduced glutathione and vit c—both decrease with age and in cataract • Deficiency, either of aa(tryptophan or vit b2(riboflavin) or by administration of toxic substances naphthalene, lactose, galactose, selenite, thallium etc
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  • Dinitrophenol (forslimming) and paradichlorobenzene(in insecticide) - lens opacity in posterior cortex • Cyanate from cigarrete smoke and from urea in renal failure and dehydration—carbamylation and protein denaturation (as do sugars by glycation in diabetes) • Hypocalcemia - same result - alter ionic balance - cataract of parathyroid tetany • Cataractous changes - use of stronger anticholinesterase group of miotics and after prolongrd systemic use of corticosteroid
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  AGE RELATED CATARACTS •• Most commonest • • B/L and asymmetrical • • Three main types 1.Nuclear cataracts 2.Cortical cataracts 3.Posterior subcapsular cataracts 70
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  NUCLEAR CATARACT • Mostcommon type, >60% • In asian population, cortical cataract predominates • associated with the oxidative damage to the proteins and lipids, leading to hardening of the lens nucleus and increased light Scattering • Hardening increases refractive index myopic shift second sight 71
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  • lens normallyexists in an extremely hypoxic Environment. • Patients treated with long-term hyperbaric oxygen therapy develop a myopic shift nuclear cataracts • Post vitrectomy and age related degeneration of Vitreous also plays significan role in nuclear cataracts 72
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  CORTICAL CATARACT • Firstappear at age of onset of presbyopia • Mature fibres on surface of cells are affected • Most common site is inferonasal quadrant • Starts at periphery and takes years to obscure vision Risk factors • Exposure to sunlight • Thinner lens • Dm 73
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  MECHANISMS • Disruption ofpumps • Physical or chemical damage to cell plasma proteins • Damage to ca homeostasis • Glutathione loss 74
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  POSTERIOR SUBCAPSULAR CATARACT • Caused bycluster of swollen cells at posterior pole of lens just below capsule • Opacity in optical axis, disabling Risk factors • Steroid intake • Exposure to radiation • Trauma 75
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  SUGAR CATARACT • Associatedwith galactosaemia and daibetes mellitus • Galactosaemic cataract: seen in galactosaemia (a.Classical galactosaemia due to galactose-1-p04 uridyltransferase deficiency, b. Due to deficiency of galactokinase); oil drop cataract • True diabetic cataract: snowflake or snowstorm cataract 76
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  SUGAR CATARACT CONTD.. PATHOGENESIS: 1.SORBITOL, ALDOSE REDUCTASE AND OSMOTIC HYPOTHESIS: 77
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  2.Theory of non-enzymaticglycosylation: most important theory of etiology of diabetic cataract till date Increase glucose non-enzymatic glycosylation Of lens proteins conformational change cataract 78
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  RADIATION CATARACT • Dueto damage to the germinative zones of lens epithelium Infrared(heat) cataract: • posterior subcapsular cataract • seen in workers of glass industries so called glass blower’s and glass worker’s cataract Non ionizing radiations-uv rays: • UV-B not UV-A responsible for cortical cataract • Mechanism in humans not clear • May be due to excess formation of free radicals • Mainly cortical cataracts are formed 79
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  STEROID INDUCED CATARACT •• Children more susceptible than adults • • Mechanism- increase glucose levels -Inhibition of na-k-atpase pump -Loss of ATP -Increased cation permeability • • Common is posterior subcapsular opacities due to aberrant differentiation and Migration of epithelial cells. 82
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  ELECTRICAL INJURY • •Cause protein coagulation and cataract Formation • • More likely when the transmission of current Involves the patient's head • • Initially, lens vacuoles appear in the anterior midperiphery of the lens, followed by linear Opacities in the anterior subcapsular cortex 83
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  CATARACT DUE TOUVEITIS /INTRAOCULAR SURGERY / RETINITIS PIGMENTOSA • In chronic uveitis - due to uncontrolled and sustained inflammation and prolonged use of steroids • Following vitreo-retinal surgery -breakdown of blood vitreous barrier - posterior subcapsular cataracts • In RP– breakdown of the blood vitreous barrier by Destruction of pigment epithelium. 84
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  DRUG-INDUCED CATARACT 85 STATINS AMIODARONE• PHENOTHIAZI NES:
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  TRAUMATIC CATARACT • ROSETTECATARACT 86
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  CATARACT IN CHALCOSIS 87 SUNFLOWERCATARACT AS IN WILSON DISEASE
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  MYOTONIC DYSTROPHIC CATARACT •POLYCHROMATIC CRYSTALS IN CHRISTMAS TREE PATTERN 88
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  Present at birth Involvementof the fetal nucleus Diameter less than 5.75mm Pediatric: Congenital Cataract 89
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  TOXIC AGENTS Corticosteroids Discoid,PSCC Anticholinesterases Ant subcapsular Chlorpromazine Yellow/ brown ant. Capsular/ stellate/ ant. Polar Busulfan Pscc Chloroquine White/flaky PSCC Amiodarone Ant. Subcapsular Cigarette smoking Nuclear Iron Brown discoloration Gold Golden ant. Capsular deposits
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  Reference • American academyof ophthalmology [lens and cataract ] ,2014- 2015 • Parson’s disease of the eye, 20th edition • Anthony j bron, ramesh c tripathi, brenda j tripathi, wolff’s anatomy of the eye and orbit, 8th edition. • Jack j kanski, brad bowling, clinical ophthalmology, 8th edition • Internet resources: www.Oculist.Com