Rescue of auditory hair cells from aminoglycoside toxicity by Clostridium difficile toxin B, an inhibitor of the small GTPases Rho/Rac/Cdc42

The hair cells (HCs) are the most vulnerable elements in the cochlea and damage to them is the most common cause of sensorineural hearing loss. Understanding the intracellular events that lead to the death of HCs is a key to developing protective strategies. Recently, it has been shown that the c-Jun-N-terminal kinase (JNK) pathway is activated in HCs in response to aminoglycosides (J. Neurosci. 20 (2000) 43). We have studied the upstream events leading to JNK activation in aminoglycoside toxicity in vitro. The small GTPases Rac and Cdc42 are well known upstream activators of JNK in other cell types. Clostridium difficile toxin B monoglucosylates all members of the Rho GTPase subfamily (Rho, Rac and Cdc42 isoforms) and inhibits GTP binding by steric interference (Nature 341 (1989) 209). Organ of Corti explants from p5 rat basal turns were maintained in tissue culture and treated with C. difficile toxin B for 12 h. They were then treated with toxin B plus gentamicin for 72 h. Significantly less HC death was observed compared to with gentamicin alone. Toxin B alone had no effect on HCs at the highest concentration used. Using antibodies against phospho-c-Jun, we observed background immunoreactivity in control explants, strong staining of outer hair cell nuclei in gentamicin treated explants, and weaker immunostaining in explants treated with gentamicin and C. difficile toxin B. We conclude that Rho family small GTPases play a role in aminoglycoside toxicity signaling as upstream activators of the JNK signaling pathway

Inflammatory signals increase Fas ligand expression by inner ear cells

There is considerable evidence that hearing and vestibular function can be influenced by immune processes. The inner ear has evolved mechanisms, such as the blood-labyrinthine barrier that limit immune responses and autoimmune processes to reduce the potential for damage to cochlear cells. Recently, expression of Fas ligand (FasL) in some non-lymphoid tissue, as in the anterior chamber of the eye, has been hypothesized to play a role in protection of sensitive organs from activated T-cells. We show that under resting conditions, cochlear cells express little or no FasL. However, after exposure to interferon-gamma in vitro, FasL is induced in many neonatal cochlear cells. In addition, we show that FasL is upregulated in adult cochlear cells after induction of a sterile labyrinthitis in vivo. The induction of FasL by inflammation may serve to limit cochlear immune responses, and to protect sensorineural tissue from immune and autoimmune damage

Inflammatory signals increase Fas ligand expression by inner ear cells

There is considerable evidence that hearing and vestibular function can be influenced by immune processes. The inner ear has evolved mechanisms, such as the blood-labyrinthine barrier that limit immune responses and autoimmune processes to reduce the potential for damage to cochlear cells. Recently, expression of Fas ligand (FasL) in some non-lymphoid tissue, as in the anterior chamber of the eye, has been hypothesized to play a role in protection of sensitive organs from activated T-cells. We show that under resting conditions, cochlear cells express little or no FasL. However, after exposure to interferon-gamma in vitro, FasL is induced in many neonatal cochlear cells. In addition, we show that FasL is upregulated in adult cochlear cells after induction of a sterile labyrinthitis in vivo. The induction of FasL by inflammation may serve to limit cochlear immune responses, and to protect sensorineural tissue from immune and autoimmune damage