Multiple Signaling Pathways Govern Calcium Homeostasis in Photoreceptor Inner Segments

Fig. 1. A Generic rod photoreceptor. The outer segment (OS) hosts the phototransduction apparatus. The inner segment (IS) downstream from the OS is formed by three anatomically distinct domains: (1) ellipsoid, which contains most of cell’s mitochondria; (2) the cell body, which contains the cell nucleus, nuclear envelope formed by the endoplasmic reticulum (ER) cisternae; and (3) the synaptic terminal, packed with synaptic vesicles and cisternae of smooth ER. B Dissociated salamander rod and C salamander cone photoreceptor. Ca2+ sequestration and release from the mitochondria occur via Ca2+ uniporter channels and Na+/Ca2+ transporters, respectively. PMCA plasma membrane Ca adenosine triphosphatase (ATPase), VGCC voltage-gated channel. Scale bars 5 m.

dedicated to transducing photon energy into graded changes in the photoreceptor membrane potential. In contrast, photoreceptor regions downstream from the OS are responsible for life support and survival and for synaptic transmission of light-evoked changes in membrane potential to the rest of the visual system. The inner segment (IS) itself is comprised of a number of anatomically discrete domains (the ellipsoid, the myoid, the cell body, and the synaptic terminal), each of which contains distinct sets of intracellular organelles and ion transporters (Fig. 1).

The IS and the OS are connected by incessant large-scale movement of proteins and lipids guided by specialized motors and Ca2+ buffers through a thin nonmotile cilium (Besharse et al., 2003; Giessl et al., 2006) as well as by cyclical light-dependent translocation of at least three phototransduction proteins between the OS and the IS (McGinnis et al., 2002; Sokolov et al., 2002; Strissel et al., 2005). While photoreceptors do not possess a bona fide action potential-conducting axon (their individual neurite may better be thought of as an enlargement of the initial segment of the axon of a conventional neuron), each neurite ends in a specialized synaptic terminal (pedicle in cones, spherule

Fig. 2. A Simultaneous [Ca2+]i measurement from the cell body and outer segment of a salamander rod. The ryanodine receptor agonist caffeine transiently elevated [Ca2+]i in the inner, but not the outer, segment. B Rod light responses to 200-ms, 567-nm flash (upper panel −3.5 log; bottom panel −1.0 log quanta) before and during caffeine exposure. No significant effect of caffeine was observed on either the transient hyperpolarization of the rod or the rod “tail” for brighter flashes; slightly slower rise times were observed for caffeine exposures during dim flashes. IS inner segment, OS outer segment.

in rods; Lasansky, 1973; Haverkamp et al., 2000). Terminals in mammalian species such as mouse and rat possess a single large mitochondrion, whereas those of amphibian and reptilian photoreceptors appear to lack mitochondria (Lasansky, 1973; Choi et al., 2005). The main point of our chapter is to review how the highly compartmentalized anatomy of rod and cone ISs is associated with region-specific regulation of intracellular calcium concentration [Ca2+] and to relate those differences to homeostatic Ca2+ mechanisms particular to each photoreceptor region. In that regard, the OS and IS express different sets of plasma membrane, intracellular store transporters, and ion channels, which in turn impart differential voltage sensitivity, Ca2+ affinities, and transport and modulation properties to each segment (Krizaj and Copenhagen, 2002). An example is illustrated in Fig. 2, which shows that stimulation of ryanodine receptors evokes large-scale Ca2+ release from internal stores in the rod IS but has no effect on [Ca2+]i homeostasis in the OS or the light response of rod photoreceptors.

A great deal has been learned from molecular, physiological, and genetic studies of Ca2+ regulation in the OS (Lamb and Pugh, 2006). These studies established that the OS possesses a single Ca2+ entry pathway (the cyclic guanosine monophosphate [cGMP]- dependent [cyclic nucleotide-gated, CNG] channel) and one Ca2+ clearance pathway, the Na,K+/Ca2+ exchanger (NKCX) (Korenbrot and Rebrik, 2002; Palczewski et al., 2004; Paillart et al., 2006). While in some retinas OSs are also labeled by antibodies raised against ryanodine, inositol triphosphate (IP3), and sarcoplasmic-endoplasmic reticulum Ca2+ ATPase (SERCA) channels and transporters (Wang et al., 1999; Krizaj, 2005b; Shoshan-Barmatz et al., 2005), a functional role for specific Ca2+ store transporters in OS Ca2+ regulation still needs to be defined. In contrast to the OS, the IS is characterized by a multitude of Ca2+ influx, clearance, and storage mechanisms, including powerful intracellular organelles such as the endoplasmic reticulum (ER) and mitochondria.

OVERVIEW OF CA2+ REGULATION IN THE INNER SEGMENT

In sensory neurons, Ca2+ acts globally (across tens of micrometers) as well as locally (within “microdomains”) to coordinate, integrate, and tune sensory transduction, gene expression, and neurotransmission through a wide array of signaling systems with differing Ca2+ affinities (e.g., Roberts, 1994; Neher, 1995; Zufall and Leinders-Zufall, 2000; Nakatani et al., 2002a and 2002b; Berridge et al., 2003). Vertebrate rod and cone photoreceptors exemplify this strategy; most, if not all, key biochemical processes in these cells are modulated by spatial, temporal, and amplitude aspects of changes in [Ca2+]i microdomains (reviewed in Fain et al., 2001; Krizaj and Copenhagen, 2002; Heidelberger et al., 2005). [Ca2+]IS is determined by the interplay between activation of Ca2+ entry and clearance mechanisms in the plasma membrane, ER, and mitochondria that keeps [Ca2+]i in darkness high (at 300–600nM) and low (30–50nM) in the light (Krizaj et al., 2003; Szikra and Krizaj, 2006).

Figure 3 illustrates the pathways for Ca2+ entry into the IS cytosol: (1) plasma membrane voltage-activated Ca2+ channels (Corey et al., 1984); (2) store-operated transient receptor potential canonical (TRPC) or receptor-operated transient receptor potential vanilloid (TRPV)-like channels (Zimov and Yazulla, 2004; Szikra et al., 2006); (3) CNG channels (Rieke and Schwartz, 1994); (4) ryanodine and IP3 receptor-operated release channels (Peng et al., 1991; Krizaj et al., 2004); and (5) Ca2+ release from the mitochondria (Krizaj et al., 2003; Szikra and Krizaj 2006).

Fig. 3. Schema of Ca regulation in the inner segment (IS). The endoplasmic reticulum (ER) Ca store represents a central hub for intracellular Ca homeostasis, communicating with both plasma membrane and mitochondria. Black arrows calcium fluxes, gray arrows activation pathways. CNGC cyclic guanosine monophosphate (cGMP)-gated channel, GPCR G proteincoupled receptor, IP3R, triphosphate (IP3) receptor, NCX mitochondrial Na/Ca exchanger, PLC phospholipase C, PMCA plasma membrane Ca ATPase, RyR ryanodine receptor, SERCA, sarco- plasmic-endoplasmic reticulum Ca adenosine triphosphatase (ATPase), SOC store-operated Ca channel, VOCC L-type voltage-operated Ca channel, CICR, Calcium-Induced Calcium release; DAG, diacyl glycerol; Gprot, G protein.