Histochemical study of enzymes of optic nerve of monkey and rat

Bioenergetics shapes cellular death pathways in Leber's hereditary optic neuropathy: a model of mitochondrial neurodegeneration

Leber's hereditary optic neuropathy (LHON) was the first maternally inherited disease to be associated with point mutations in mitochondrial DNA and is now considered the most prevalent mitochondrial disorder. The pathology is characterized by selective loss of ganglion cells in the retina leading to central vision loss and optic atrophy, prevalently in young males. The pathogenic mtDNA point mutations for LHON affect complex I with the double effect of lowering the ATP synthesis driven by complex I substrates and increasing oxidative stress chronically. In this review, we first consider the biochemical changes associated with the proton-translocating NADH-quinone oxidoreductase of mitochondria in cybrid cells carrying the most common LHON mutations. However, the LHON cybrid bioenergetic dysfunction is essentially compensated under normal conditions, i.e. in glucose medium, but is unrevealed by stressful conditions such as growing cybrids in glucose free/galactose medium, which forces cells to rely only on respiratory chain for ATP synthesis. In fact, the second part of this review deals with the investigation of LHON cybrid death pathway in galactose medium. The parallel marked changes in antioxidant enzymes, during the time-course of galactose experiments, also reveal a relevant role played by oxidative stress. The LHON cybrid model sheds light on the complex interplay amongst the different levels of biochemical consequences deriving from complex I mutations in determining neurodegeneration in LHON, and suggests an unsuspected role of bioenergetics in shaping cell death pathways.

Histochemical localisation of mitochondrial enzyme activity in human optic nerve and retina

AIMS

To demonstrate the quantitative distribution of mitochondrial enzymes within the human optic nerve and retina in relation to the pathogenesis of ophthalmic disease.

METHODS

Enucleations were performed at the time of multiple organ donation and the optic nerve and peripapillary retina immediately excised en bloc and frozen. Reactivities of the mitochondrial enzymes cytochrome c oxidase and succinate dehydrogenase were demonstrated in serial cryostat sections using specific histochemical assays.

RESULTS

In the optic nerve the unmyelinated prelaminar and laminar regions were rich in both cytochrome c oxidase and succinate dehydrogenase. Myelination of fibres as they exited the lamina cribrosa was associated with an abrupt reduction in enzyme activity. Within the retina, high levels of enzyme activity were found localised within the retinal ganglion cells and nerve fibre layer, the outer plexiform layer, inner segments of photoreceptors, and the retinal pigment epithelium.

CONCLUSIONS

Mitochondrial enzyme activity is preserved in human optic nerve and retina retrieved at the time of multiple organ donation. The distribution of enzyme activity within the eye has implications for the understanding of the pattern of ophthalmic involvement seen in mitochondrial diseases and the site of ganglion cell dysfunction in those patients with optic nerve involvement.

Mastocytosis-induced nyctalopia

Systemic mastocytosis is characterized by an increased number of mast cells in multiple organs particularly skin. A 55-year-old man with mastocytosis presented with nyctalopia caused by malabsorption of vitamin A. Diagnosis was made by documenting a low vitamin A level and an ERG that showed rod-cone deficiency with rods affected more than cones. Vitamin A therapy led to return of good visual function. To our knowledge, this is the first reported case of mastocytosis induced nyctalopia. Vitamin A deficiency should be considered as a potential cause of visual loss in patients with sudden onset of night blindness.

Optic disk cupping and electrocardiographic abnormalities in an American pedigree with Leber's hereditary optic neuropathy

We examined the clinical characteristics of affected and unaffected members of an American black family with the 11778 mitochondrial DNA mutation associated with Leber's hereditary optic neuropathy. Thirty-six individuals from four generations were included. All maternally related subjects were shown to be essentially homoplasmic for the 11778 mitochondrial DNA mutation in blood. Paternally related subjects did not carry this mutation. Patients affected with optic neuropathy had optic nerve head cupping. Loss of unmyelinated axons from the prelaminar optic nerve may be responsible for cupping in these patients. Electrocardiographic analysis of subjects carrying the 11778 mitochondrial DNA mutation disclosed statistically significant (P = .02) prolongation of the corrected OT interval as compared to control subjects. While the clinical significance of this magnitude of corrected QT prolongation is unknown, it may represent a systemic manifestation of the 11778 mutation. No electrocardiographic evidence of preexcitation syndromes was seen.

Pattern of myelination and distribution of neuroglial cells along the developing optic system of the rat and rabbit

In order to gather information about the factors influencing myelination and neuroglial cell formation in the central nervous system, the development of myelin and neuroglia was investigated along the length of the optic nerve and tract of rats and rabbits. The results of this study revealed complex developmental patterns for both of these processes. Furthermore, the pattern of myelination for the rat is different from that of the rabbit. Myelination of the rat optic pathway exhibits a general rostro-caudal gradient extending from the eye to the lateral geniculate nucleus, whereas a more homogeneous pattern is present in the rabbit. Myelination along the nerves of both species shows many sharp peaks and troughs, especially in the region of the optic canal. Formation of myelin begins at several different sites along the nerve, with the first fibers appearing on both sides of the optic canal. Another small site of myelination develops just in front of the optic chiasm. Myelination of the entire length of an individual axon is not homogeneous but instead it begins at several discrete sites. The number of glial cells along the nerve and tract also fluctuates sharply but their pattern does not precisely parallel that of myelination. A better correlation is obtained by comparing the distribution of axons being myelinated with that of oligodendroglial cells. The complex patterns and the species differences observed in this study indicate that myelination and neuroglial cell development must be regulated by both neuronal and local factors. The data presented here together with other available information suggest that the vascularization of the nerve and the direct effects of hormones upon glial cells are possible factors that contribute to the local variability.