Effects of intravitreal insulin and insulin signaling cascade inhibitors on emmetropization in the chick.

PURPOSE: Intravitreal insulin has been shown to be a powerful stimulator of myopia in chickens, in particular if the retinal image is degraded or defocused. In most tissues, the insulin receptor activates two main signaling pathways: a) the mitogen-activated protein kinase (MAPK) cascade (e.g., mitogen-activated protein kinasem kinase [MEK] and extracellular regulated kinase [ERK]) and b) the phosphatidylinositol 3-kinase (PI3K)/protein kinase B (Akt) pathway. In the current study, insulin was injected, and these pathways were separately inhibited to determine which is activated when the retinal image is defocused by spectacle lenses. METHODS: Chicks were treated with either +7 D, -7 D, or no lenses. They were intravitreally injected with insulin, the MEK inhibitor U0126, the PI3K inhibitor Ly294002, or a combination of insulin and one of the inhibitors. Refractions and ocular dimension were measured at the beginning and after four days of treatment. The retinal proteins of the chicks were measured with western blots after 2 h and four days of treatment. Incubation occurred with anti-Akt1, anti-Erk1/2, anti-phospho-Akt(Thr308), and anti-phospho-Erk1/2((Thr202/Tyr204)) antibodies, and the ratio between the relative intensity of the phospho-form and the total-form was calculated. RESULTS: Chicks wearing positive lenses and injected with saline and with PI3K inhibitor compensated for the imposed defocus and became hyperopic. Insulin injections and insulin plus PI3K inhibitor injections prevented lens-induced hyperopia, whereas the MEK inhibitor alone and insulin plus MEK inhibitor had no effect. Obviously, the MEK inhibitor suppressed the effect of insulin on eye growth in the plus lens-treated animals. Chicks treated with negative lenses and injected with insulin, or with insulin plus MEK inhibitor, overcompensated for the imposed defocus. This effect of insulin was not detected in eyes injected with PI3K inhibitor plus insulin, suggesting that the PI3K inhibitor suppressed the effects of insulin in minus lens-treated animals. Insulin increased the ratio of phospho-Akt/total-Akt in animals with normal visual exposure but even more so in chicks wearing plus or minus lenses. The increase was blocked by simultaneous PI3K inhibitor injections in control eyes but not in lens-treated eyes. Insulin also increased the ratio of phospho-ERK/total-ERK in animals with normal visual exposure and in animals wearing positive lenses, compared to U0126- and Ly294002-injected eyes. In contrast, no significant activation of the MEK/ERK pathway was observed in the negative lens-treated animals. CONCLUSIONS: Intravitreal insulin promoted axial eye growth and stimulated both signaling pathways. The PI3K/Akt pathway was activated in control and plus and minus lens-treated eyes, but the MEK/ERK pathway was activated only with positive lenses or no lenses. With negative lenses, insulin did not stimulate the MEK/ERK signaling cascade. Independent of the pathway stimulated after insulin binding, the effect on insulin was always the same: an increase in eye growth.

Lactate dehydrogenase isoenzyme pattern in the eye muscles. Deviation in myopia.

The lactate dehydrogenase isoenzyme patterns in 46 extraocular eye muscle samples removed at surgery for squint were determined by acrylamide-gel electrophoresis and reduction of NAD coupled with formazan reaction. Muscle type subunits predominated in the isoenzymes of the medial and lateral rectus muscles of emmetropic and hypermetropic eyes, whereas heart type subunits predominated in those of myopic eyes.

Might the refractive state in oculocutaneous albino patients be a clue for distinguishing between tyrosinase-positive and tyrosinase-negative forms of oculocutaneous albinism?

In oculocutaneous albinism (OCA), one can distinguish between a tyrosinase-negative form (no residual activity of the enzyme tyrosinase) and a tyrosinase-positive form (with detectable residual enzymatic activity) and their respective subtypes. In infancy and early childhood the clinical discrimination between tyrosinase-positive OCA and tyrosinase-negative OCA can be very difficult. To date, only the hair-bulb L-dopa incubation test has been helpful in discriminating between the tyrosinase (ty)-negative and ty-positive forms of OCA. In 68 patients with albinism of the eye, 24 had an oculocutaneous form of albinism. We determined the ty relationship either by the L-dopa incubation test (younger patients) or by clinical appearance (older patients). We determined the full cycloplegic refraction in all patients and looked for a possible correlation of the refraction with the ty relationship. Our data suggest that an OCA patient with a hyperopic refractive error of > 4.0 D might have a ty-positive form of OCA. A patient with a moderate to high degree of myopia is more likely to have a ty-negative form of OCA. Determination of ty relationship is more reliable in patients with high degrees of hyperopia and, therefore, in patients with a ty-positive form of OCA. Statistical evaluation of the data was not possible in a sensible way due to the small number of patients involved in the present study.