Prosurvival and proapoptotic intracellular signaling in rat spiral ganglion neurons in vivo after the loss of hair cells

Abstract

Neurons depend on afferent input for survival. Rats were given daily kanamycin injections from P8 to P16 to destroy hair cells, the sole afferent input to spiral ganglion neurons (SGNs). Most SGNs die over an approximately 14-week period after deafferentation. During this period, the SGN population is heterogeneous. At any given time, some SGNs exhibit apoptotic markers--TUNEL and cytochrome c loss--whereas others appear nonapoptotic. We asked whether differences among SGNs in intracellular signaling relevant to apoptotic regulation could account for this heterogeneity. cAMP response element binding protein (CREB) phosphorylation, which reflects neurotrophic signaling, is reduced in many SGNs at P16, P23, and P32, when SGNs begin to die. In particular, nearly all apoptotic SGNs exhibit reduced phospho-CREB, implying that apoptosis is due to insufficient neurotrophic support. However, >32% of SGNs maintain high phospho-CREB levels, implying access to neurotrophic support. By P60, when approximately 50% of the SGNs have died, phospho-CREB levels in surviving neurons are not reduced, and SGN death is no longer correlated with reduced phospho-CREB. Activity in the proapoptotic Jun N-terminal kinase (JNK)-Jun signaling pathway is elevated in SGNs during the cell death period. This too is heterogeneous: <42% of the SGNs exhibited high phospho-Jun levels, but nearly all SGNs undergoing apoptosis exhibited elevated phospho-Jun. Thus, heterogeneity among SGNs in prosurvival and proapoptotic signaling is correlated with apoptosis. SGN death following deafferentation has an early phase in which apoptosis is correlated with reduced phospho-CREB and a later phase in which it is not. Proapoptotic JNK-Jun signaling is tightly correlated with SGN apoptosis.

Kanamycin‐Furosemide Ototoxicity in the Mouse Cochlea: A 3‐Dimensional Analysis

“…Otolaryngology-Head and Neck Surgery 150 (4) The loss of SGNs, although not as extensive as the IHC loss, was still apparent. Our results agree with a study by Alam et al, 17 who injected rats with kanamycin alone for 8 days to observe the effect on SGNs. They reported a decrease in SGN density 20 days after the first injection.…”

Inner ear development: building a spiral ganglion and an organ of Corti out of unspecified ectoderm

“…However, it is remarkable that simple delayed loss of Atoh1 or loss of several other transcription factors can cause rapid demise of hair cells leading to the nearly flat epithelium, also observed in many human cases of long term hair cell loss. Equally remarkable is that in most of these mice with genetically induced hair cell loss there is substantial and long lasting innervation of parts of the OC (Fritzsch et al, 2005b, Pan et al, 2011, Xiang et al, 2003), which resembles more with human hearing loss conditions compared to hair cell loss induced by other means (Alam et al, 2007). …”

Factors influencing neurotrophic effects of electrical stimulation in the deafened developing auditory system

“…Thus, the data were fitted to a two-time constant model, shown by the 2 exponential functions (dashed lines) and a combined function (solid line), which suggests an early rapid phase of SG cell degeneration over about the first 2 months of deafness and a slower phase thereafter. An important recent study from investigators at the University of Iowa (Alam and Green, 2005;Alam et al, 2007) demonstrated that there are also 2 phases in the degeneration of SG neurons after ototoxic drug induced deafness in rats, an early phase in which apoptosis is correlated with reduced neurotrophic signaling (reduced CREB phosphorylation) and a later phase (after ~postnatal day 60) when activity in the proapoptotic JNK-Jun signaling pathway is tightly correlated with apoptosis of SG neurons. We hypothesize that the two phases of SG cell degeneration observed in our neonatally deafened cats are correlated with the same mechanisms that underlie the two phases of apoptosis in rats, and further, that these mechanisms may be conserved across species and may be relevant to the human cochlea as well.…”