Download T32-Iris and Lens Biochemistry 02 Sep 09

Scientific Basis of Vision
Biochemistry- Iris and Lens
Sep 02, 2009
Shiva Swamynathan
Department of Ophthalmology
University of Pittsburgh School of Medicine
List the components of the Uveal Tract and their
relative location
Iris (Anterior)
Ciliary Body (Middle)
Choroid (Posterior)
Where is the uveal tract attached to the sclera?
Uveal tract is attached to the sclera at three points:
1. The sclera spur,
2. The exit points of vortex veins and
3. The optic nerve.
What is Iris?
A highly pigmented tissue that functions as a diaphragm
between the anterior and the posterior chambers of the
eye, regulating the amount of light reaching the retina.
What are the two chambers of the anterior compartment?
The iris divides the anterior compartment, the space separating the
cornea and the lens, into 2 chambers:
1. the larger anterior chamber (between the cornea and the iris), and
2. the smaller posterior chamber (between the iris and the lens).
What are the boundaries of the anterior and
posterior chambers?
Anterior Chamber- bounded by cornea, anterior iris and
anterior lens surface.
Posterior Chamber- bounded by ciliary body, lens and
posterior iris. Note: this is not the vitreous cavity.
What is the basis for eye color?
Eye color comes from the iris (stroma in particular).
When there is abundant melanin in the stroma and
epithelial cells, the iris is brown; less melanin results in
lighter shades of brown or yellow. Blue or green irides
partly are the result of lack of melanin in the stroma
combined with melanin in the epithelium and some
other ill-defined structural change.
What are the different tissues in the iris?
The Anterior endothelium consists of an incomplete layer of
endothelial (mesothelial) cells on the anterior surface of the iris.
The Irideal Stroma accounts for most of the mass of the iris and is
composed of connective tissue, blood vessels and nerve fibers.
Scattered within the stroma are numerous pigmented cells, usually
containing melanin. The posterior boundary of the iris stroma,
peripheral to the sphincter muscle, is demarcated by another sheet
of smooth muscle, the dilator muscle.
The Irideal Epithelium is composed of two layers of densely
pigmented cells.
What are Sphincter and dilator muscles?
Sphincter muscle. The iris sphincter muscle encircles the pupil, and
constricts the pupil in bright light or during accommodation.
Dimensions: about 0.75 mm wide by 0.15 mm thick.
Sphincter is controlled by parasympathetic nerve fibers.
Dilator muscle. The dilator muscle fibers, arranged radially, extends
from the base of the iris to the sphincter muscle. The fibers of the
dilator muscle are derived from, and remain in continuity with, the
cuboidal pigmented cell bodies in the iris pigment epithelium.
Dilator muscle is controlled mainly by sympathetic nerve fibers,
although some parasympathetic fibers are associated with it.
What is miosis?
Constriction of the pupil (Narrow pupil)
What is mydriasis?
Relaxation of the pupil (Dilated pupil)
What is the effect of miotic agents and mydriatic
agents on sphincter and dilator muscles?
Miotic agents - Cholinergic agonists stimulate the
sphincter and adrenergic blockers block the dilator
Mydriatic agents – Cholinergic blockers block the
sphincter and adrenergic agonists stimulate the dilator.
What is the source of blood supply to the iris?
The iris is supplied from the major arterial circle in the
ciliary body.
The blood vessels of the iris run in a radial direction.
The anterior border layer contains very few vessels.
Iris blood vessels appear sheathed and have a
characteristic loose appearance.
How is the iris innervated?
The iris is supplied by the long and short ciliary nerves. The ciliary
nerves pierce the sclera around the entrance of the optic nerve,
and run forward in the perichoroidal space.
After reaching the iris the ciliary nerves form a plexus around its
attached margin. Nerve fibers ending in the anterior surface of
the iris, Sphincter and Dilator are derived from these.
The fibers derived through the motor root of the ciliary ganglion
from the oculomotor nerve supply the Sphincter, while those
derived from the sympathetic supply the Dilator.
What is the ciliary body?
A structure connected to the iris and sclera, which
produces aqueous fluid that fills the anterior
compartment, thus maintaining the eye pressure.
The ciliary muscle helps in focusing by facilitating
the lens accommodation.
What is the main pharmacological target in treating
glaucoma? Why?
Ciliary body is the main target for treating glaucoma.
It is rich in adrenergic, cholinergic and prostaglandins
receptors, and their respective signal transduction
pathways.
What germ-layer is the source of iris and ciliary
body smooth muscle?
Neuroectoderm
(Unlike the smooth muscle elsewhere in the
body, which is derived from the mesoderm)
What are the main functions of the iris and ciliary
body smooth muscle?
•
•
•
•
Contraction/relaxation,
Receptor characterization,
Second messenger formation and regulation
Arachidonic acid release and eicosanoid biosynthesis
What are the broad classes of proteins expressed
in the ciliary body?
20% - Protein synthesis, folding, secretion and degradation
12% - Energy supply and biosynthesis
6% - Cytoskeletal structure and contractility
7% - Cellular signaling and cell cycle regulation
2% - Nerve cell related
37% - Unknown
What is the rate of aqueous humor production in humans?
About 2 ml/min.
What are the components of the aqueous humor?
Growth factors
Enzymes such as carbonic anhydrase, lysozyme, diamine
oxidase, dopamine hydroxylase, phospholipase A2,
plasminogen activator, cAMP, prostaglandin,
catecholamines, steroid hormones and hyaluronic acid.
How does aqueous humor enter the posterior
chamber? Actively or passively?
By both means.
Actively, by means of energy dependent secretion
including carbonic anhydrase-II activity.
Passively, by diffusion and ultrafiltration. Active
secretion accounts for a major amount of aqueous
humor.
Describe the aqueous fluid outflow path
The aqueous fluid produced by the
ciliary body flows between the iris and
lens, through the pupil to the drainage
angle at the junction of the iris and
the cornea. Aqueous fluid exits the
eye through the trabecular meshwork
and Schlemm’s canal, interscleral
channels and episcleral vein.
Aqueous humor is rich in macromolecules. True/False.
False.
Aqueous humor is practically free of proteins and other
macromolecules, allowing for optical clarity. For example, it
contains about 1/500 of the albumin present in the plasma.
What are Eicosanoids?
Eicosanoids are signaling molecules with hormonal activity, made
by oxygenation of twenty-carbon essential fatty acids. They exert
complex control over inflammation or immunity, and serve as
messengers in the central nervous system. They include
compounds such as prostaglandins, prostacyclins, thromboxanes,
and leukotrienes.
Eicosanoids derive from either omega-3 (ω-3) or omega-6 (ω-6)
EFAs. The ω-6 eicosanoids are generally pro-inflammatory; ω-3's
are much less so. Anti-inflammatory drugs such as aspirin and
other NSAIDs act by downregulating eicosanoid synthesis.
Which of the following statements about prostaglandin synthesis is
true?
A. In response to cytokines, neurotransmitters or pharmacologic
treatments, phospholipase-A2 is activated, releasing arachidonic
acid from membrane phospholipids.
B. Free arachidonic acid is converted by cyclooxygenase-I (Cox1) or
Cox2, to prostaglandin H2 endoperoxide intermediates.
C. Free arachidonic acid can also be metabolized through 5’lipoxygenases and cytochrome P-450 pathways to generate
leukotrienes and epoxides, respectively.
D. Phospholipase A2 can be inhibited by corticosteroids; Cox1 by
non-steroidal anti-inflammatory drugs (NSAIDS), and the 5lipoxygenase pathway by nordihydroguaiaretic acid (NDGA).
E. All of the above.
Answer- E
True or False?
A. Prostaglandin analogs enhance outflow rather than formation,
of aqueous humor.
B. b-blockers, CA-inhibitors and a2 agonists decrease the
formation of aqueous humor.
C. NSAIDs bind irreversibly to cyclooxygenases blocking
biosynthesis of prostaglandins from arachidonic acid.
D. COX1 is widely and constitutively expressed.
E. COX2 is expressed at low levels in normal physiologic conditions
and upregulated in response to pro-inflammatory signals.
F. COX2 inhibitors (Vioxx, Celebrex, Bextra, etc) increase the risks
of cardiovascular toxicity and complications.
G. Prostaglandin receptors are G-protein coupled 7transmembrane domain containing membrane proteins
H. All of the above are true
What is the effect of Carbonic Anhydrase-II
inhibitors on aqueous flow?
CA-II inhibitors reduce the rate of entry of sodium
and bicarbonate into the aqueous, thus reducing the
aqueous humor formation.
What are the functions of the blood-aqueous and
blood-retinal barriers?
They control the composition and amounts of all
material entering and leaving the eye. Only exception is
the material that leaves the eye through Schlemm’s
canal. When these barriers are defective, blood
constituents mix with ocular fluids, causing plasmoid
aqueous, retinal exudates or retinal edema.
What is the lens? What keeps it in place?
The lens is a transparent soft encapsulated biconvex structure
composed of crystallins.
The lens is suspended by thin zonules that are attached to the
ciliary body.
What is the size of the lens at birth and in adults?
• The average anterior-posterior human lens thickness at birth
is 3.5 - 4 mm and 4.5 - 5 mm after 65 years of age.
• The diameter of the human lens at birth is 6.0 - 6.5 mm and
9.0 - 9.5 mm after 65 years of age.
• The anterior surface of the lens has a greater radius of
curvature than the posterior surface.
True/False
1. The lens is avascular and nourished by diffusion from the aqueous
and vitreous.
2. The lens capsule is thicker at the anterior, compared to the posterior
of the lens.
3. The radius of curvature of the anterior surface averages 10 mm, and
it is subject to marked changes during accommodation.
4. Lens epithelial cells are metabolically active and regulate the water
and ion balance of the entire lens.
5. Elimination of cellular organelles is necessary to reduce scatter of
light passing through the lens.
6. The lens capsule allows free diffusion of water, ions, and small
molecules, while acting as a barrier to large proteins such as serum
albumin.
7. Adult lens is surrounded by a single celled epithelial layer.
8. Mature fibers support active transcription.
9. Central epithelial cells are mitotically active.
What is the lens capsule? What is it made of?
The lens capsule is a 10-20 µm thick elastic basement
membrane that completely envelopes the lens.
It consists of hyaline material containing type IV collagen,
small amounts of other collagens, glycosaminoglycans,
laminin, fibronectin, and heparan sulfate proteoglycan.
What is the lens epithelium?
The lens epithelium is a single-celled layer of large
cuboidal epithelial cells, beneath the anterior lens capsule.
The cells of the lens epithelium continue to divide with
age; thus, the lens continuously grows till death. The lens
epithelial cells differentiate into fiber cells at the equator.
The posterior capsule does not have any epithelial cells
associated with it.
What is the structure of the lens cortex and nucleus?
The adult lens can be divided into the cortex or the outer
fibers laid down after the age of about 20 years, and the
internal nucleus, consisting of cells produced from
embryogenesis through adolescence. New lens cells are
added to the margin of the lens throughout life from the lens
epithelium. The cells at the lens nucleus do not undergo
turnover or replacement and are among the oldest cells in the
body of an adult.
What is the structure of the lens zonule?
The lens zonule suspends the lens and holds it in place. The
zonular fibers extend from both the anterior and posterior
capsule of the lens to join the span fibers of the ciliary body
and insert on the pars plana and retina.
What are lens sutures?
As the differentiating fibers become fully elongated, they
meet with fibers that have elongated from the other end,
making the lens sutures.
Name the major lens-specific integral membrane protein.
Aquaporin-0 or Major Intrinsic Protein (Aqp0 or MIP).
What are the abundant water-soluble proteins in the lens
called?
Crystallins.
Alert- There is a wrong statement about the crystallins in section 2 of
BCSC. In page 275, it states that crystallins constitute 90-95% of total
lens protein. It is just not true. It should be as high as 90-95% of watersoluble proteins, and not total proteins.
What do lens fiber cell membranes contain?
Tight packing and low fluidity in the mature lens fiber
cell membranes is achieved by
• High amount of saturated fatty acids,
• High cholesterol:phospholipid ratio, and a
• High concentration of sphingomyelin
• Lipids contribute about 1% of total lens mass
Name two requirements for proteins to be recruited as
crystallins.
• Should be water-soluble, without precipitating at high
concentrations
• Should be stable and stay active for a long time
What are the two general groups of crystallins?
• Ubiquitous Crystallins (Present in all vertebrates)
e.g.,a-Crystallin and b/g-crystallins
• Taxon-specific crystallins (Different in different species)
e.g., e-crystallin
What is the native molecular mass of a-crystallin?
Larger than 600kDa; can be as high as 1500kDa.
aA- and aB-crystallins are both around 20kDa monomers.
a-crystallin is a super-aggregate of these monomers.
How is spherical aberration avoided by the lens?
Glass bead in water
Decreasing gradient of refractive index from
the centre to the periphery of the lens
resulting from continual lens development
minimizes the spherical aberration
Fish lens in water
True/False
 b- and g-crystallins are structurally related.
 True
 aB-crystallin is a widely and constitutively expressed member of the small heat
shock proteins family and is inducible by heat and other forms of stress.
 True
 g-crystallin tends to be concentrated in the nuclear region of the lens, as it is
abundantly expressed early in development.
 True
 Most taxon-specific crystallins are oxidoreductases which bind pyridine nucleotides.
Reduced nucleotides absorb UV light, protecting the retina from oxidative damage.
 True
 In the normal lens, concentrations of sodium are low (~10mMol/L) and potassium,
high (~120mMol/L) relative to aqueous humor, which contains high sodium
(~150mMol/L) and low potassium (~5mMol/L).
 True
 There is a smooth gradient of refractive index in the lens, with the oldest cells in the
center having the lowest refractive index and the newest cells in the periphery having
the highest refractive index.
 False
 aA-crystallin has chaperone-like activity, which is absent in the aB-crystallin.
False
What happens when lens proteins aggregate?
Results in cataract- scatter of light, loss of transparency.
Thus, chaperone activity of a-crystallin is crucial for longterm maintenance of lens transparence.
List two metabolic causes of cataract.
Sugar cataract caused due to diabetes mellitus
Sugar cataract caused due to defective galactose metabolism.
What maintains the ionic balance in the lens?
• A sodium-potassium ATPase pump, an intrinsic membrane protein
that hydrolyzes ATP, to transport Na+ out and K+ in to the lens.
• Na+-K+-pumps are found primarily in the anterior surface of the
lens, in the epithelium and outer, immature fiber cells.
How do lens cells communicate with each other?
Through gap junctions consisting of connexin 43 in the epithelial
cells and Cx-46 and -50 in the fiber cells.
MIP (Aqp0) also helps in intercellular communication.
What is the primary source of energy in the lens?
Anaerobic glycoslysis is the primary source of energy in the anterior
lens. Pentose phosphate pathway is used in oxidative stress
conditions, to replenish NADPH.
Questions/Comments?
Room 1025, EEI
Phone: 412-802-6437
[email protected]
Genes involved in iris development