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The excitation cascade of Limulus ventral photoreceptors: guanylate cyclase as the link between InsP3-mediated Ca2+ release and the opening of cGMP-gated channels.

Garger AV, Richard EA, Lisman JE - BMC Neurosci (2004)

Bottom Line: Similarly, GtetP reduced the excitation caused by intracellular injection of Ca2+.In contrast, this GC inhibitor did not affect the excitation produced by injection of a cGMP analog.We conclude that GC is downstream of InsP3-induced Ca2+ release and is the final enzymatic step of the excitation cascade.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Biology and Center for Complex Systems, Brandeis University, Waltham, MA 02454-9110, USA. alexander_garger@nymc.edu

ABSTRACT

Background: Early stages in the excitation cascade of Limulus photoreceptors are mediated by activation of Gq by rhodopsin, generation of inositol-1,4,5-trisphosphate by phospholipase-C and the release of Ca2+. At the end of the cascade, cGMP-gated channels open and generate the depolarizing receptor potential. A major unresolved issue is the intermediate process by which Ca2+ elevation leads to channel opening.

Results: To explore the role of guanylate cyclase (GC) as a potential intermediate, we used the GC inhibitor guanosine 5'-tetraphosphate (GtetP). Its specificity in vivo was supported by its ability to reduce the depolarization produced by the phosphodiesterase inhibitor IBMX. To determine if GC acts subsequent to InsP3 production in the cascade, we examined the effect of intracellular injection of GtetP on the excitation caused by InsP3 injection. This form of excitation and the response to light were both greatly reduced by GtetP, and they recovered in parallel. Similarly, GtetP reduced the excitation caused by intracellular injection of Ca2+. In contrast, this GC inhibitor did not affect the excitation produced by injection of a cGMP analog.

Conclusion: We conclude that GC is downstream of InsP3-induced Ca2+ release and is the final enzymatic step of the excitation cascade. This is the first invertebrate rhabdomeric photoreceptor for which transduction can be traced from rhodopsin photoisomerization to ion channel opening.

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A model for Limulus excitation. The cascade is initiated by the isomerization of rhodopsin to metarhodopsin by light. Metarhodopsin catalyzes exchange of GTP for GDP on multiple G proteins (Gq). Gq-GTP binds and activates phospholipase C (PLC). This complex cleaves phosphatidyl inositol-4,5-bisphosphate (PIP2) producing InsP3. InsP3 opens Ca2+ ion channels in the endoplasmic reticulum (ER) leading to the release of Ca2+ into the cytosol. Ca2+ release activates GC. A rise in cGMP opens cyclic nucleotide-gated ion channels (CNCG) in the plasma membrane.
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Figure 5: A model for Limulus excitation. The cascade is initiated by the isomerization of rhodopsin to metarhodopsin by light. Metarhodopsin catalyzes exchange of GTP for GDP on multiple G proteins (Gq). Gq-GTP binds and activates phospholipase C (PLC). This complex cleaves phosphatidyl inositol-4,5-bisphosphate (PIP2) producing InsP3. InsP3 opens Ca2+ ion channels in the endoplasmic reticulum (ER) leading to the release of Ca2+ into the cytosol. Ca2+ release activates GC. A rise in cGMP opens cyclic nucleotide-gated ion channels (CNCG) in the plasma membrane.

Mentions: In Limulus, excitation is initiated by conversion of rhodopsin to metarhodopsin by light (Fig. 5). The active state of metarhodopsin activates a G protein as evidenced by the fact that G protein inhibitors decrease the light-response, whereas G protein activators mimic the light response [31,35,36]. Metarhodopsin is inactivated in less than 150 ms [37]; while active about 10 G proteins are turned on [38]. The G protein involved has been identified as Gq in Limulus [39,40]. In the next stage of the cascade, PLC is activated by Gq, resulting in the hydrolysis of phosphatidyl inositol-4,5-bisphosphate to produce InsP3 and diacyglycerol. PLC antagonists such as neomycin, spermine, and U-73122 decrease the response to light [41,42] (see also [15]). Diacyglycerol may be important for excitation in Drosophila [10]; however in Limulus this is not likely to be the case [43]. InsP3 has been shown to meet all the requirements for acting as an intracellular second messenger necessary for excitation in Limulus: endogenous synthesis, increased concentration in response to light, and excitation through exogenous application [6,7].


The excitation cascade of Limulus ventral photoreceptors: guanylate cyclase as the link between InsP3-mediated Ca2+ release and the opening of cGMP-gated channels.

Garger AV, Richard EA, Lisman JE - BMC Neurosci (2004)

A model for Limulus excitation. The cascade is initiated by the isomerization of rhodopsin to metarhodopsin by light. Metarhodopsin catalyzes exchange of GTP for GDP on multiple G proteins (Gq). Gq-GTP binds and activates phospholipase C (PLC). This complex cleaves phosphatidyl inositol-4,5-bisphosphate (PIP2) producing InsP3. InsP3 opens Ca2+ ion channels in the endoplasmic reticulum (ER) leading to the release of Ca2+ into the cytosol. Ca2+ release activates GC. A rise in cGMP opens cyclic nucleotide-gated ion channels (CNCG) in the plasma membrane.
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Related In: Results  -  Collection

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Figure 5: A model for Limulus excitation. The cascade is initiated by the isomerization of rhodopsin to metarhodopsin by light. Metarhodopsin catalyzes exchange of GTP for GDP on multiple G proteins (Gq). Gq-GTP binds and activates phospholipase C (PLC). This complex cleaves phosphatidyl inositol-4,5-bisphosphate (PIP2) producing InsP3. InsP3 opens Ca2+ ion channels in the endoplasmic reticulum (ER) leading to the release of Ca2+ into the cytosol. Ca2+ release activates GC. A rise in cGMP opens cyclic nucleotide-gated ion channels (CNCG) in the plasma membrane.
Mentions: In Limulus, excitation is initiated by conversion of rhodopsin to metarhodopsin by light (Fig. 5). The active state of metarhodopsin activates a G protein as evidenced by the fact that G protein inhibitors decrease the light-response, whereas G protein activators mimic the light response [31,35,36]. Metarhodopsin is inactivated in less than 150 ms [37]; while active about 10 G proteins are turned on [38]. The G protein involved has been identified as Gq in Limulus [39,40]. In the next stage of the cascade, PLC is activated by Gq, resulting in the hydrolysis of phosphatidyl inositol-4,5-bisphosphate to produce InsP3 and diacyglycerol. PLC antagonists such as neomycin, spermine, and U-73122 decrease the response to light [41,42] (see also [15]). Diacyglycerol may be important for excitation in Drosophila [10]; however in Limulus this is not likely to be the case [43]. InsP3 has been shown to meet all the requirements for acting as an intracellular second messenger necessary for excitation in Limulus: endogenous synthesis, increased concentration in response to light, and excitation through exogenous application [6,7].

Bottom Line: Similarly, GtetP reduced the excitation caused by intracellular injection of Ca2+.In contrast, this GC inhibitor did not affect the excitation produced by injection of a cGMP analog.We conclude that GC is downstream of InsP3-induced Ca2+ release and is the final enzymatic step of the excitation cascade.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Biology and Center for Complex Systems, Brandeis University, Waltham, MA 02454-9110, USA. alexander_garger@nymc.edu

ABSTRACT

Background: Early stages in the excitation cascade of Limulus photoreceptors are mediated by activation of Gq by rhodopsin, generation of inositol-1,4,5-trisphosphate by phospholipase-C and the release of Ca2+. At the end of the cascade, cGMP-gated channels open and generate the depolarizing receptor potential. A major unresolved issue is the intermediate process by which Ca2+ elevation leads to channel opening.

Results: To explore the role of guanylate cyclase (GC) as a potential intermediate, we used the GC inhibitor guanosine 5'-tetraphosphate (GtetP). Its specificity in vivo was supported by its ability to reduce the depolarization produced by the phosphodiesterase inhibitor IBMX. To determine if GC acts subsequent to InsP3 production in the cascade, we examined the effect of intracellular injection of GtetP on the excitation caused by InsP3 injection. This form of excitation and the response to light were both greatly reduced by GtetP, and they recovered in parallel. Similarly, GtetP reduced the excitation caused by intracellular injection of Ca2+. In contrast, this GC inhibitor did not affect the excitation produced by injection of a cGMP analog.

Conclusion: We conclude that GC is downstream of InsP3-induced Ca2+ release and is the final enzymatic step of the excitation cascade. This is the first invertebrate rhabdomeric photoreceptor for which transduction can be traced from rhodopsin photoisomerization to ion channel opening.

Show MeSH
Related in: MedlinePlus