Influence of mitochondrial protein synthesis inhibition on deafferentation?induced ultrastructural changes in nucleus magnocellularis of developing chicks

Abstract

Following cochlea removal in developing chicks, about 30% of the neurons in the ipsilateral second-order auditory nucleus, nucleus magnocellularis, undergo cell death. Administration of chloramphenicol, a mitochondrial protein synthesis inhibitor, results in a pronounced increase in deafferentation-induced cell death. In this study, we examined whether the chloramphenicol enhancement of deafferentation-induced cell death reveals the same ultrastructural characteristics that are seen in degenerating nucleus magnocellularis neurons after cochlea removal alone. Unilateral cochlea removal was performed on anaesthetized posthatch chicks. One group of animals was simultaneously treated with chloramphenicol. Six, twelve, or twenty-four hours following cochlea removal, n. magnocellularis neurons were studied by routine transmission electron microscopy. Particular attention was paid to the integrity of the polyribosomes and rough endoplasmic reticulum. Two ultrastructurally different types of neuronal degeneration were observed in the deafferented nucleus magnocellularis neurons: an early onset electron-lucent type that always involved ribosomal dissociation and a late-onset electron-dense type displaying nuclear pyknosis and severely damaged mitochondria. The percentage of nucleus magnocellularis neurons displaying ribosomal disintegration following cochlea removal was found to be markedly increased after chloramphenicol treatment. This finding suggests that mitochondrial function is important for the maintenance of a functional protein synthesis apparatus following deafferentation.

Histochemical and Fluorescent Analyses of Mitochondrial Integrity in Chick Auditory Neurons following Deafferentation

“…I n nucleus magnocellularis auditory neurons, mitochondria are clearly involved in the cellular response to deafferentation. Previous studies revealed significant morphological changes in neuronal mitochondria, including rapid proliferation across the entire deafferented nucleus (Hyde and Durham, 1994), ultrastructural vacuolization within the subset undergoing ribosomal degradation and endoplasmic reticulum (ER) fragmentation (Born and Rubel, 1985;Rubel et al, 1991;Hyde and Durham, 1994;Hartlage-Rubsamen and Rubel, 1996), and loss of mitochondrial DNA integrity in the subset destined to die (Karnes et al, 2009). Here, we demonstrate disturbances in mitochondrial functional status in deafferented neurons as a corollary to the previously described alterations of mitochondrial structure.…”

“…Calcium influx triggers rapid proliferation of mitochondria within NM neurons (Hyde and Durham, 1994). Mitochondria unable to buffer the rise in calcium undergo vacuolization (Hyde and Durham, 1994;Hartlage-Rubsamen and Rubel, 1996), whether in response to persistent elevated calcium levels (Brookes et al, 2004) or from the loss of ER-mitochondrial junctional integrity (Pinton et al, 2008). ER-mitochondrial junctional impairment releases lipids and calcium into the cellular cytoplasm, which perpetuates ROS generation (Nicholas and Hyson, 2006;Nicholls, 2008), shifts metabolism toward the TCA cycle (Duchen, 2004;Gunter et al, 2004), and increases SDH levels.…”

“…Surprisingly, TUNEL labeling accumulated in neuronal cytoplasm, implicating fragmentation of mitochondrial DNA in the deafferentation-induced death process. Mitochondria are intimately involved in the neuronal response to deafferentation in NM (Durham and Rubel, 1985;Hyde and Durham, 1990;Garden et al, 1994;Hyde and Durham, 1994;Hartlage-Rubsamen and Rubel, 1996).…”

Life and Death in Otolaryngology

“…71 When investigated further, it was found that inhibition of mitochondrial but not cytoplasmic protein synthesis specifically potentiates cell death in NM after afferent deprivation. 69,70,74 These observations indicate a protective role for mitochondria in determining cell fate after deafferentation.…”

“…64,67,68 During this process, these neurons undergo characteristic metabolic and morphologic changes similar to apoptosis. 69,70 These changes include (but are not limited to) an increase in mitochondrial density, an increase in in-tracellular calcium, decreased RNA and protein synthesis, and changes in oxidative metabolism. 67,[71][72][73] Once again, it appears that mitochondria are of central importance in determining cell survival in this model.…”