Role of Guanylate Cyclase Activating Proteins in photoreceptor cells of the retina in health and disease

Abstract

In the last two decades, it has been done a thoroughly research about the role of Guanylate Cyclase Activating Proteins (GCAPs) in photoreceptor cells of the retina as activity regulators of Retinal Guanylate Cyclase (RetGC), which allow to restore cGMP levels to darkness ones when intracellular Ca2+ falls.

However, little is known about: a) ¿What determines GCAPs distribution within the cell?, b) ¿Which other functions GCAP proteins, GCAP1 and GCAP2, carry out at other cellular compartments different from the sensory one? and c) ¿How they cause cell death when they are mutated? In this study we want address these questions.

1. First of all, we own a mouse model that expresses a GCAP2 mutated form unable to bind Ca2+ (bEF-GCAP2). Other mutations described for GCAP1 and present in some autosomic dominant Cone Rod Dystrophies (adCORD), prevent Ca2+ binding to some of its EF-hand domains which produces the constitutive activation of RetGC, and consequently, high cGMP levels that result in toxicity for the cell. However, we observe that our model causes the death by other mechanism, as RetGC is not activated by GCAP2, because GCAP2 is retained in the inner segment and does not translocate to the sensory compartment. We want to identify interactions that GCAP2 establish differentially in this compartment and could be retaining it. We find out 14-3-3 family of proteins by mass-spectrometry and liquid chromatography. Furthermore, bEF-GCAP2 is abnormally phosphorylated in vivo and GCAP2 phosphorylation promotes its binding to 14-3-3 binding. We demonstrate that GCAP2 phosphorylation in residue serine 201 is the cause of its retention in the inner segment, avoiding its translocation to the outer segment, and when we mutate serine 201 into a glycine, this retention is reverted in vivo. Finally, we propose that GCAP2 phosphorylation and its binding to 14-3-3 is what retains GCAP2 in the inner segment, and this happens in a balance way during dark/light day cycles. When this system overloads will cause retinal degeneration by the formation of aggregates. We believe that mutations in GCAP2 or light conditions promoting GCAP2 accumulation in its Ca2+-free form in the inner segment of the cell, bring to cell death by GCAP2 conformational instability. Most important, we propose that this will also apply for genetic scenarios mimicking the effects to constant light exposure, the so called “equivalent-light” scenarios.

2. Secondly, as a result of the identification of GCAP2 interaction to RIBEYE (Venkatesan et al. 2010) the major component of synaptic ribbons in the photoreceptor cell terminal, we developed an ultrastructural study of the role that GCAP2 may play in this compartment. Through confocal and electronic microscopy we have demonstrated the presence of GCAP1 and GCAP2 in rod synaptic ribbons. However, GCAP1 and GCAP2 are not necessary during synaptic ribbons assembling and basic maintenance. As GCAP2 overexpression in the wildtype background (which means a higher GCAP2:GCAP1 ratio) promotes ribbons disassembling, we propose that GCAP2 may play a role mediating the morphological changes that take place in the synaptic ribbons in response to variations in [Ca2+].

En las dos últimas décadas se ha investigado a fondo el papel que juegan las Proteínas Activadoras de Guanilato Ciclasa (GCAPs) en las células fotorreceptor de la retina como proteínas encargadas de regular la actividad de la Guanilato Ciclasa (GC).

Sin embargo se sabe poco acerca de: a) ¿Qué determina la distribución de GCAPs en la célula?, b) ¿Qué otras funciones ejercen GCAP1 y GCAP2 en otros compartimentos celulares distintos al segmento sensorial? y c) ¿Cómo dan lugar a muerte celular cuando están mutadas? En este estudio hemos querido encarar estas preguntas.

1. En primer lugar, poseemos un modelo de ratón que expresa una forma mutante de GCAP2 que no une Ca2+ (bEF-GCAP2). A diferencia de otras mutaciones descritas para GCAP1, en que se ha observado que la muerte celular es producida por niveles tóxicos de cGMP, observamos que nuestro modelo produce la muerte celular por otro mecanismo en que GCAP2 se acumula en el segmento interno. Identificamos abundantemente las distintas isoformas de 14-3-3 como interactores diferenciales de bEF-GCAP2, que a su vez está anormalmente fosforilada in vivo. Tras una serie de experimentos para caracterizar esta interacción, proponemos que la fosforilación de GCAP2 y su unión a 14-3-3 retienen a GCAP2 en el segmento interno, y si este mecanismo se sobrecarga por a) mutaciones en GCAP2, b) condiciones de luz que promuevan la acumulación de GCAP2 en su forma libre de Ca2+ en el segmento interno o c) condiciones genéticas que mimeticen los efectos de exposición a luz prolongada, tendría lugar la degeneración de la retina por la formación de agregados debido a la inestabilidad conformacional de GCAP2.

2. En segundo lugar, tras la identificación de la interacción de GCAP2 con RIBEYE (Venkatesan et al. 2010), el componente mayoritario de las cintillas sinápticas de fotorreceptores, realizamos un estudio ultrastructural del papel que puede estar jugando GCAP2 en este compartimento mediante microscopia electrónica y confocal, demostrando la presencia de GCAP1 y GCAP2 en las cintillas sinápticas de bastones. GCAP1 y GCAP2 son prescindibles en el ensamblaje y mantenimiento básico de las cintillas sinápticas, pero la sobreexpresión de GCAP2 en el fenotipo salvaje, que incrementa el ratio GCAP2:GCAP1, promueve el desensamblaje de las cintillas. Proponemos que GCAP2 podría jugar un papel mediando cambios morfológicos en las cintillas sinápticas promovidas por cambios en [Ca2+].