Control of outflow resistance by soluble adenylyl cyclase

Abstract

Abstract Glaucoma is a leading cause of blindness in the United States affecting as many as 2.2 million Americans. All current glaucoma treatment strategies aim to reduce intraocular pressure, even in patients with normal tension glaucoma. Typically, this is accomplished by reducing the rate of aqueous flow by limiting aqueous production or enhancing drainage using drugs and surgery. Whereas these strategies are effective in diminishing vision loss, some patients continue to lose vision and many discontinue use of their medications because of undesirable side effects. Drugs known to be effective in altering conventional outflow have for the most part been abandoned from modern clinical practice due to undesirable side effects. Identification of new drugs that could enhance conventional outflow, would offer additional options in the treatment of glaucoma and ocular hypertension. To this end, our laboratory has recently uncovered a novel pathway for regulation of conventional outflow by the ciliary body. This pathway is dependent on soluble adenylyl cyclase, an enzyme that catalyzes the generation of cyclic adenosine 3',5' monophosphate (cAMP) in response to bicarbonate.

FIG. 1.

Two sources of cyclic adenosine 3′,5′ monophosphate (cAMP) in the mammalian cells. Membrane-based adenylyl cyclases (transmembrane adenylyl cyclase [tmAC]) are activated by G proteins and forskolin. Soluble adenylyl cyclase (sAC) is not a membrane protein, is found in a variety of intracellular locations, and is activated by bicarbonate but not forskolin.

FIG. 2.

sAC is expressed in the ciliary body (CB), but not drainage tissues. (A) Mouse CB contains mRNA for sAC as evidenced by RT-PCR (A) using primer sets designed to span multiple exons covering 2 different regions of sAC. Bands matched those obtained from testis (T) and were absent in water controls (neg). Immunofluorescence staining for sAC (B) demonstrates that it is highly expressed in the CB, where it colocalizes with the nonpigmented epithelium (NPE)-specific protein Bestrophin-2 (Best2) (C). (D) Detailed examination of a merged fluorescence and differential interference contrast image demonstrates that sAC is highly expressed in the CB, where it is concentrated in the NPE cells and stroma, but is not observed in pigment epithelium cells. Whereas sAC is also expressed in corneal endothelia, epithelia, and retina, no specific sAC staining was noted in drainage tissues. (This figure was originally published in The Journal of Biological Chemistry. See reference © the American Society for Biochemistry and Molecular Biology.) Color images available online at www.liebertpub.com/jop

FIG. 3.

Potential mechanism of outflow resistance control by sAC in the CB. Outflow resistance could be regulated by controlling the level of cAMP in the AH. cAMP in the aqueous humor could affect outflow by conversion to adenosine by phosphodiesterases, interaction with an as yet unknown cAMP receptor, or uptake using an anion transporter such as the organic anion transporter 1 (OAT-1) triggering structural changes that could accommodate better drainage of AH through the trabecular meshwork (TM) and Schlemm's canal (SC).