Multiple mechanisms of rhabdom shedding in the lateral eye of Limulus polyphemus

Abstract

Rhabdom shedding in horseshoe crab lateral eye photoreceptors was studied with anti-opsin and anti-arrestin immunocytochemistry. Two, possibly three, distinct shedding mechanisms were revealed in animals maintained in natural lighting. Transient rhabdom shedding, triggered by dawn, is a brief, synchronous event that removes up to 10% of the rhabdom membrane. Whorls of rhabdomeral membrane break into vesicles and form compact multivesicular bodies. These debris particles are immunoreactive for opsin and are of a relatively uniform size, averaging approximately 2 microm(2) in area. Transient shedding requires that input from circadian efferent fibers to the retina precedes the light trigger, and cutting the optic nerve blocks efferent input and transient shedding. Light-driven rhabdom shedding is a progressive process. Rhabdomeral membrane is removed by coated vesicles that accumulate into loosely packed multivesicular bodies. These debris particles label with antibodies directed against opsin, arrestin, and adaptin, and they have a large distribution of sizes, averaging almost 6 microm(2) in area and ranging up to 25 microm(2) or more. The amount of rhabdomeral membrane removed by light-driven shedding has seasonal variation and depends on latitude. Light-driven shedding does not require circadian efferent input. A possible third shedding mechanism, light-independent shedding, is observed when transient shedding is blocked either by 48 hours of darkness or by cutting the optic nerve. Small particles, averaging 1.8 microm(2) in area, exhibiting opsin but not arrestin immunoreactivity can then be found in the cytoplasm surrounding the rhabdom. The nature of light-independent shedding is not yet clear.

Evolution of arthropod visual systems: Development of the eyes and central visual pathways in the horseshoe crabLimulus polyphemusLinnaeus, 1758 (Chelicerata, Xiphosura)

“…Furthermore, clusters of vesicles in the retinular cells are strongly labeled. The opsin in these vesicles is presumably internalized in membranes during transient and light-driven rhabdom shedding (Sacunas et al, 2002).…”

“…Opsin is the protein moiety of the visual pigment rhodopsin, which is responsible for absorbing light and initiating the phototransduction cascade (Smith et al, 1993). Lp opsin localizes to the rhabdom of photoreceptors and is associated with cytosplasmic vesicles containing internalized rhabdomeral membranes (Battelle et al, 2001;Sacunas et al, 2002). Lp myosin III is present throughout the photoreceptors, including the region occupied by the photosensitive rhabdom, the cytosol of their cell bodies (Battelle et al, 1998(Battelle et al, , 2001, and their axons and terminals (Edwards et al, 1990;and Battelle, unpublished observations).…”

“…Evidence from studies with adult animals indicates the protein is specific to photoreceptors. Visual arrestin, another photoreceptor specific protein, is involved in the termination of the photoresponse and may function in rhodopsin endocytosis (Sacunas et al, 2002). It is concentrated in the rhabdom of adult L. polyphemus lateral eyes and is also present in axons of the lateral optic nerve (Battelle et al, 2000(Battelle et al, , 2001.…”

“…Aspects that have been explored in the past include adult ommatidial morphology of the lateral eyes (review Fahrenbach, 1975), organization of the retinal projections into the brain as well as efferent projections Barlow, 1980, 1982;Batra and Chamberlain, 1985;, and distribution of the visual pathway neurotransmitters (reviewed in Battelle et al, 1999) octopamine (Evans et al, 1983), serotonin , FMRFamide and substance P (Chamberlain and Engbretson, 1982;Lewandoswski et al, 1989), histamine , and acetylcholine (Hornstein et al, 1994). Additionally, circadian modulation of the photoresponse (e.g., Battelle et al, 1998Battelle et al, , 2000Battelle et al, , 2001Sacunas et al, 2002;Waren and Chamberlain, 2002;Ankrom and Chamberlain, 2002;Dalal et al, 2003;Pieprzyk et al, 2003;Runyon et al, 2004;reviews, Barlow, 2001;Battelle, 2002), and both computational and behavioral aspects of vision (reviewed in Barlow et al, 2001) are active areas of current research.…”

Opsin1-2, Gqα and arrestin levels at Limulus rhabdoms are controlled by diurnal light and a circadian clock

“…TRS is triggered by the dim light of dawn and is characterized by a rapid, synchronous shedding of opsin-containing rhabdomeral membranes. TRS does not occur if clock input to the LE is blocked (Chamberlain and Barlow, 1979;Chamberlain and Barlow, 1984;Sacunas et al, 2002). A separate process of rhabdom shedding called light-driven shedding (LDS) is independent of clock input and becomes apparent as light intensity increases during the day (Sacunas et al, 2002).…”