Cell-Type Specific β2AR Signaling Results in a Reduction in the Basal pERK Level of Primary Human Trabecular Meshwork Cells

Brian Hudson, Melanie Kelly. Dalhousie University, Halifax, NS, Canada.

In the eye intraocular pressure (IOP) is maintained by the balance of aqueous humour production from the ciliary body and outflow predominantly through the trabecular meshwork (TM). Since elevated IOP is the primary risk factor for glaucoma, a blinding eye disease, understanding the function of the ciliary body and TM is critical in the development of novel glaucoma therapeutics. One target for the reduction of IOP in both the ciliary body and TM are β-adrenergic receptors (β-ARs). Due to their ability to decrease aqueous secretion by the ciliary body and lower IOP, β-AR antagonists are first line treatments for glaucoma. Although not as well understood, activation of β-ARs in the TM also results in a reduction in IOP. Therefore, the present study examined β-AR signaling in primary human TM cells.

Treatment of TM cells with the non-selective β-AR agonist isoproterenol (ISO) produced a 13% decrease (p<0.001) in the basal phospho-ERK (pERK) level, which was blocked by a β2AR inverse agonist ICI 118551 (p<0.001). Since, β2AR activation is traditionally associated with an increase in pERK, the mechanism of this TM specific pERK decrease was examined. The response was insensitive to PTx, indicating a non-Gi pathway. However, long term pre-treatment with cholera toxin to desensitize Gs resulted in a reversal of the response, yielding instead a 12% increase in pERK. Treatment with forskolin yielded a 20% (p<0.05) decrease pERK, suggesting the involvement of adenylyl cyclase and cAMP. The PKA inhibitor H-89, like cholera toxin, reversed the ISO pERK response. Taken together, these findings suggest that in TM cells β2AR-Gs signaling via adenylyl cyclase, cAMP and PKA produces a decrease in pERK, while blocking this pathway unmasks the underlying traditional β2AR mediated increase in pERK. The importance of this unique β2AR signaling pathway in TM function, aqueous humor outflow and IOP regulation remains to be elucidated..