Novel regulation of mucosal innate defense by AMPK in Otitis Media

Mucin, a major protein component in mucus, plays a critical role in mucosal innate defense by providing a physical barrier and trapping pathogens for mucociliary clearance.

If uncontrolled, excessive mucin production overwhelms mucociliary clearance and causes conductive hearing loss in otitis media (OM) and mucus obstruction in lung infections. Therefore, mucin production must be tightly regulated. However, the molecular mechanisms underlying the tight regulation of mucin remain largely unknown.

Otitis media (OM) is the most common childhood bacterial infection and the leading cause of conductive hearing loss. It remains a major health problem and a substantial socioeconomic burden. S. pneumoniae, Sp, represents a major gram-positive bacterial pathogen for OM. Currently available Sp vaccines have a limited impact on OM.

Moreover, inappropriate antibiotic use increased antibiotic-resistance.

There is an urgent need for developing innovative non-antibiotic therapeutic agent for suppressing mucus overproduction.

Our long-term goal is to elucidate the molecular mechanisms underlying OM pathogenesis and identify novel therapeutic targets.

In contrast to the relatively well-known toll-like receptor (TLR)-dependent mechanisms by which Sp and pneumolysin (PLY ? a key virulence factor produced by virtually all clinical Sp isolates) induce host mucosal immune response, the TLR-independent mechanisms including the key regulators remain largely unclear.

Adenosine 5?-monophosphate-activated protein kinase ?1 (AMPK?1) has emerged as a master regulator of host energy homeostasis.

Its role in infectious diseases, in particular in the host mucosal innate defense response, e.g. mucus production, remains largely unclear.

Our encouraging preliminary data suggest that Sp and PLY may up-regulate mucin MUC5AC and MUC5B via activation of AMPK?1 in a TLR2/4-independent manner in the middle ear and airway epithelial cells in vitro and in the mouse models of both acute and chronic OM.

Interestingly, Sp and PLY may activate AMPK?1 by inducing novel non-traditional (protein degradation- independent) ubiquitination of AMPK?1 likely via downregulating a key deubiquitinase CYLD.

Together, these exciting preliminary data have thus provided a solid foundation for us to hypothesize that [1] AMPK?1 acts as a key regulator for Sp-induced up-regulation of MUC5AC and MUC5B via TLR-independent signaling; [2] Activation of AMPK?1 by interplay between polyubiquitination and phosphorylation plays a critical role in Sp- induced up-regulation of MUC5AC and MUC5B (hypothesis).

To test our hypothesis, we will pursue two specific aims to determine (Aim 1) the role of AMPK?1 in OM pathogenesis in both AOM and COM; and (Aim 2) how Sp activates AMPK?1.

These studies will significantly advance our understanding of the key regulators including AMPK in TLR-independent host mucosal innate defense in bacterial infections and lead to the identification of novel therapeutic targets for controlling mucus overproduction.

Our AMPK signaling studies may also help understand molecular mechanisms of other AMPK-related diseases (Significance and Impact).