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J. Biol. Chem., Vol. 280, Issue 17, 16784-16789, April 29, 2005

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A Direct Signaling Role for Phosphatidylinositol 4,5-Bisphosphate (PIP₂) in the Visual Excitation Process of Microvillar Receptors^*

Maria del Pilar Gomez and Enrico Nasi

From the Department of Physiology and Biophysics, Boston University School of Medicine, Boston, Massachusetts 02118 and the Marine Biological Laboratory, Woods Hole, Massachusetts 02543

In microvillar photoreceptors the pivotal role of phospholipase C in light transduction is undisputed, but previous attempts to account for the photoresponse solely in terms of downstream products of phosphatidylinositol 4,5-bisphosphate (PIP₂) hydrolysis have proved wanting. In other systems PIP₂ has been shown to possess signaling functions of its own, rather than simply serving as a precursor molecule. Because illumination of microvillar photoreceptors cells leads to PIP₂ break-down, a potential role for this phospholipid in phototransduction would be to help maintain some element(s) of the transduction cascade in the inactive state. We tested the effect of intracellular dialysis of PIP₂ on voltage-clamped molluscan photoreceptors and found a marked reduction in the amplitude of the photocurrent; by contrast, depolarization-activated calcium and potassium currents were unaffected, thus supporting the notion of a specific effect on light signaling. In the dark, PIP₂ caused a gradual outward shift of the holding current; this change was due to a decrease in membrane conductance and may reflect the suppression of basal openings of the light-sensitive conductance. The consequences of depleting PIP₂ were examined in patches of light-sensitive microvillar membrane screened for the exclusive presence of light-activated ion channels. After excision, superfusion with anti-PIP₂ antibodies induced the appearance of single-channel currents. Replenishment of PIP₂ by exogenous application reverted the effect. These data support the notion that PIP₂, in addition to being the source of inositol trisphosphate and diacylglycerol, two messengers of visual excitation, may also participate in a direct fashion in the control of the light-sensitive conductance.

Received for publication, December 23, 2004 , and in revised form, February 28, 2005.

^* This work was supported by National Institutes of Health Grant EY07559. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

To whom correspondence should be addressed: Dept. of Physiology and Biophysics, Boston University School of Medicine, 715 Albany St., Boston, MA 02118. Tel.: 617-638-4072; Fax: 617-638-4273; E-mail: mpgomez{at}bu.edu.

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