Simona Suppo & Falconieri Marita
DEAFNESS and APOPTOSIS
Epidemiological studies have shown that hearing impairment affects about 10% of the adult population and more than 40% of people over 60 years old. In over 80% of cases, it seems to be caused directly or indirectly by degeneration and death of cochlear neurosensory cells, i.e. the auditory hair cells (HCs) and/or their associated spiral ganglion neurons (SGNs). In mammals, HCs and SGNs develop during the embryonic period and are expected to survive for the whole life of the individual. Underlying the irreversibility of hearing loss in mammals is the incapacity of inner ear to replace lost HCs or SGNs. The development of therapeutic techniques designed to replace them is a challenge.
The vulnerability of the cochlea mainly resides in its low amount of neurosensory cells. Therefore, even a slight damage to neurosensory cells of the cochlea can be responsible for compromised hearing. The causes of sensory loss within the inner ear are diverse, but by far the largest number of cases are related to the lack of essential growth factors, or to damage caused by ototoxic drugs or by noise. But the basic molecular mechanisms involved in the control of survival and regeneration of cochlear neurosensory cells remain still unclear.
At least two modes of cell death can be distinguished: apoptosis and necrosis. Although apoptosis and necrosis can be well distinguished with respect to both morphological and biochemical characteristics, the limit between these two modes of cell death may be very tight and even absent, in view of the intermediate form of cell death falling along an apoptotis-necrosis continuum.
INNER EAR DISORDERS AND POTENTIAL THERAPEUTIC PATHWAYS
During the past several years, progress in understanding intracellular events that mediate aspects of damage to the neurosensory cells of cochlea have been made and concluded that the death of HCs and SGNs occurs via an apoptotic process, whether caused by ototoxic drugs such as aminoglycosides or cisplatin, sound trauma, or by trophic factor withdrawal.
The aminoglycosides are antibiotic molecules whose primary effect is irreversible inhibition of bacterial protein synthesis. In the cochlea, aminoglycosides destroy the sensory and supporting cells of the organ of Corti in a systematic manner beginning with the HCs of the lower turn and progressing toward the apex, defining a pathological pattern of early high-frequency hearing loss. Moreover, as a result of auditory HC loss, in a second stage, their associated SGNs can degenerate, due to the loss of trophic factor delivery.
Based on the morphological characteristics of nuclear fragmentation accompanying HC death, a lot of studies have shown that the deletion of affected HCs exposed aminoglycosides occurs through the process of apoptosis. Since then, caspase activation has been described in damaged vestibular sensory HCs. Several reports indicate that the main deleterious event aminoglycoside-induced HC damage is ROS production, which would be induced by an iron-aminogly-aminoglycoside complex. Cells protect themselves against metabolic-derived oxidative stress via an antioxidant defense, which utilizes free radical scavengers and other enzymes that maintain the appropriate redox state of cellular proteins. Then, in oxidative stress-induced apoptosis of cochlear sensory cells, the leading strategy was to find molecules with efficacy of radical scavengers and iron chelators which prevent ROS production or limit their harmfulness, acting as potent endogenous antioxidants.
Another mechanism by which aminoglycosides may induce sensory loss implies NMDA receptors located on the terminals of the SGNs. Evidence has been obtained indicating that aminoglycosides can mimic the modulatory actions of polyamines on the NMDA receptor, causing excitotoxicity on SGNs and death of HCs.
HC damage induced by aminoglycosides can also be prevented by blocking directly the intracellular signalling cascade associated with apoptosis. HCs damaged by aminoglycosides activate the JNK pathway leading to apoptosis. As ROS can activate JNK pathway, it is possible that aminoglycoside-induced ROS production may lead to the activation of the JNK signaling pathway in HCs.
Cisplatin is a widely used chemotherapeutic agent gene- generally recognized as a DNA-damaging drug. However, rally its use is limited by serious side effects including ototoxicity [199, 200]. Ototoxic effects are manifested as a high frequency hearing loss that progresses towards the low frequencies. Studies on SGNs as well as auditory HCs suggested that in cochlear tissue, cisplatin increases the production of ROS and causes a decline in antioxidant enzymes which trigger activation of the apoptosis cascade. Several antioxidant molecules including methio- methionine, diethyldithiocarbamate, sodium nine thiosulfate and L-n-acetylcysteine, were shown to reduce cisplatin ototoxicity. However, high concentrations of these compounds are needed to obtain efficacy, and as their mechanism of action is based on a direct binding of cisplatin compounds, a decrease in cisplatin plasma concentration may occur. Furthermore, treatment with M40403, a superoxide dismutase mimetic, does not protect cochlear HCs from cisplatin toxicity. Neurotrophins have also been shown to ameliorate survival of SGNs after cisplatin-induced injury in vitro and in vivo, but with no effect on HCs death, limiting their therapeutic use. Studies on dorsal root ganglion neurons have shown that in postmitotic cells, cisplatin-induced neuronal cell death involves an attempt to re-enter the cell cycle, an event which may generate DNA damage and p53 activation and can lead to the activation of the apoptotic program. Finally, caspase inhibitors have been shown to protect both HCs and SGNs in vitro after cisplatin damage.
Intense sound stimulation results in various structural changes leading to functional auditory impairment. Then, the acute hearing losses after acoustic trauma are due both to HCs injuries and the dendrite breaking. This intense noise can also lead to a secondary neuronal cell death due to the loss of trophic support.
Although downstream intracellular pathways underlying HC death after noise trauma have been more but not completely investigated, the very early mechanisms constituting the initial stress signal are not clear. One event, and not the least, is a potent and sustained elevation of Ca2+ in HCs and the surrounding support cells, perhaps through HC transducer channels. Consequently, a series of Ca2+-dependent processes may be activated including the activation of numerous enzymes. The propagation of this Ca2+ wave could provide a simple but critical mechanism to signal the occurrence of HC damage across the auditory sensory epithelium. Thus, calcium ions play a crucial role in hearing processes and one could then expect that this well-regulated mechanism of cell to cell talking have to play a physiological role under non-traumatic conditions.
CONCLUSIONS AND PERSPECTIVES
In conclusion, it seems likely that the new developments will gradually further clarify the molecular players in cochlear cell death, indicating other possible avenues of therapeutic manipulations. Optimal otoprotection might require administration of a pharmacological cocktail directed against several pathogenic events associated with apoptosis, but also necrosis.
(Lallemend et al.- “Molecular Pathways Involved in Apoptotic Cell Death in the Injured Cochlea: Cues to Novel Therapeutic Strategies” - Current Pharmaceutical Design, 2005, 11, 2257-2275 - Review)