A simplified mass-transfer model for visual pigments in amphibian retinal-cone outer segments

Molecular basis for photoreceptor outer segment architecture

To serve vision, vertebrate rod and cone photoreceptors must detect photons, convert the light stimuli into cellular signals, and then convey the encoded information to downstream neurons. Rods and cones are sensory neurons that each rely on specialized ciliary organelles to detect light. These organelles, called outer segments, possess elaborate architectures that include many hundreds of light-sensitive membranous disks arrayed one atop another in precise register. These stacked disks capture light and initiate the chain of molecular and cellular events that underlie normal vision. Outer segment organization is challenged by an inherently dynamic nature; these organelles are subject to a renewal process that replaces a significant fraction of their disks (up to ∼10%) on a daily basis. In addition, a broad range of environmental and genetic insults can disrupt outer segment morphology to impair photoreceptor function and viability. In this chapter, we survey the major progress that has been made for understanding the molecular basis of outer segment architecture. We also discuss key aspects of organelle lipid and protein composition, and highlight distributions, interactions, and potential structural functions of key OS-resident molecules, including: kinesin-2, actin, RP1, prominin-1, protocadherin 21, peripherin-2/rds, rom-1, glutamic acid-rich proteins, and rhodopsin. Finally, we identify key knowledge gaps and challenges that remain for understanding how normal outer segment architecture is established and maintained.

Cone outer segments: a biophysical model of membrane dynamics, shape retention, and lamella formation

An hypothesis is developed to explain how the unique, right circular conical geometry of cone outer segments (COSs) in Xenopus laevis and other lower vertebrates is maintained during the cycle of axial shortening by apical phagocytosis and axial elongation via the addition of new basal lamellae. Extension of a new basal evagination (BE) applies radial (lateral) traction to membrane and cytoplasmic domains, achieving two coupled effects. 1), The bilayer domain is locally stretched/dilated, creating an entropic driving force that draws membrane components into the BE from the COS's distributed bilayer phase, i.e., plasmalemma and older lamellae (membrane recycling). Membrane proteins, e.g., opsins, are carried passively in this advective, bilayer-driven process. 2), With BE stretching, hydrostatic pressure within the BE cytoplasm is reduced slightly with respect to that of the axonemal cytoplasmic reservoir, allowing cytoplasmic flow into the BE. Attendant lowering of the reservoir's hydrostatic pressure facilitates the subsequent transfer of cytoplasm from lamellar domains to the reservoir (cytoplasmic recycling). The geometry of the BE reflects the membrane/cytoplasm ratio needed for its construction, and essentially specifies the ratio of components recycled from older lamellae. Length and taper angle of the COS reflect the ratio of recycled/new components constructing a new BE. The model also integrates the trajectories and dynamics of lamella open margin lattice components. Although not fully evaluated, the initial model has been assessed against the relevant literature, and three experimental predictions are derived.