Cathepsin gene expression profile in rat acute pneumococcal otitis media

Acute otitis media, often caused by infection with Streptococcus pneumoniae, is characterized by inflammation of the middle ear mucosa. A prominent feature of the host response to bacterial infection of the middle ear mucosa is an influx of inflammatory cells that contributes to the local pool of inflammatory mediators by releasing additional inflammatory chemicals, which in turn cause further tissue injury. The objective was to identify candidate effector and signaling molecules involved in acute otitis media pathogenesis caused by S pneumoniae infection.

Male Sprague-Dawley rats were randomly assigned to 1 of 5 groups, including 1 control group without treatment, 2 placebo groups (12 and 48 hours) and 2 infected groups (12 and 48 hours). The rat middle ear was bilaterally inoculated with either 25 microL of tryptic soy broth (TSB group) or 25 microL of TSB containing approximately 1.24 x 10(9) cfu/mL of S pneumonias type 6A (SP group). Rats were killed at 12 and 48 hours after inoculation and the middle ear mucosa was collected. Total RNA was extracted and pooled from each group for gene expression assays.

Gene expression profiles for rat middle ear mucosa at 12 and 48 hours after S pneumoniae or placebo inoculation were constructed using microarray technology (Clontech Atlas Rat 1.2 Array, 1176 cDNAs). Genes of interest were further validated by real-time polymerize chain reaction.

Middle ear mucosa expression of a gene cluster encoding the lysosomal cysteine proteases, cathepsins B (Ctsb), L (Ctsl), and K (Ctsk), was modified after S pneumoniae challenge. Specifically, at 12 hours, Ctsk and Ctsl messenger RNA that was abundantly expressed in the normal middle ear mucosa was decreased, whereas Ctsb transcript was induced. The changes in Ctsb and Ctsk gene expression were sustained at 48 hours.

The constitutive expression of Ctsk and Ctsl messenger RNA in normal middle ear mucosa supports a function in the maintenance of middle ear mucosa homeostasis, and their downregulation as an early event in acute otitis media may reflect a disruption in that function. The induction of Ctsb messenger RNA in the infected middle ear mucosa suggests a role in early tissue injury; thus, Ctsb may represent a potential target for molecular diagnostics and/or rational intervention during the development of acute otitis media.