Ultrastructural localization of glycerolipid synthesis in rod cells of the isolated frog retina

Abstract

The incorporation of two glycerolipid precursors, 3H-glycerol and 3H-choline, into rod cells of the isolated frog retina has been studied using quantitative electron microscope autoradiography. The results indicate that the endoplasmic reticulum (ER) is the major site of early incorporation of these precursors suggesting that the ER is the primary site of lipid synthesis. Of the different types of ER present in rod cells, the rough ER (RER) and nuclear envelope predominate in this activity. The organized region of smooth ER (SER) in the subellipsoid region does not appear to be of major quantitative importance, although SER closely intermingled with RER in the myoid region may be involved to some extent. We also compared the pattern of labelling observed at various incubation times in 3H-glycerol and 3H-choline with that observed with 3H-leucine. Differences were observed between the pattern of lipid and protein labelling, particularly in the labelling of the Golgi apparatus, mitochondria, plasma membrane, presynaptic terminals and outer segments. This suggests that lipids and proteins may differ in some aspects of the routes and mechanisms by which they are transported from their sites of synthesis to the membrane delimited compartments for which they are destined.

Smooth endoplasmic reticulum and other agranular reticulum in frog retinal photoreceptors

“…We do not see continuities of this SER with mitochondria in the ellipsoid region, such as those reported for the axonal SER in certain neurons (Spa~ek & Lieberman, 1980), and in the companion paper (Mercurio & Holtzman, 1982) evidence is presented which indicates that the subellipsoid SER is not a major site of glycerolipid synthesis in the rod cell, one of the ways it might have been thought to contribute to the maintenance of the mitochondria or the outer segment.…”

“…The mode(s) of origin of key structures, notably synaptic vesicles, remains in dispute, and, while there is an abundant and exciting literature on axonal transport, the nature and organization of the cellular elements involved in this transport have yet to be identified unambiguously (Schwartz, 1979;Grafstein & Forman, 1980;Rambourg & Droz, 1980). The present paper, and its companion (Mercurio & Holtzman, 1982), report our efforts to analyse central features of one prominent class of neuronal structures, the agranular (that is, ribosome-lacking) sacs and tubules, distinct from the Golgi apparatus, that are found in all regions of the neuron. Structures of this sort are often indiscriminately referred to as smooth endoplasmic reticulum (SER), but this is usually done without any evidence that they are actually part of the endoplasmic reticulum (ER).…”

“…We were especially interested in determining which of the elements of the agranular reticulum are part of the ER and in determining the organization of these elements. The companion paper (Mercurio & Holtzman, 1982) concerns the localization of glycerolipid synthesis, a role often attributed to the SER. Preliminary reports of some of the findings covered in this paper have been published (Mercurio & Holtzman, 1978, 1979.…”

Photoreceptor renewal: A role for peripherin/rds

“…In a cell culture system the normal ROS structure (organized stacked flattened membrane disks) can, however, be maintained in a culture system supplemented with permissive sugars: galactose and lactose (Stiemke and Hollyfield, 1994). Early studies in which retinas were treated with tunicamycin Ulshafer et al, 1986), an inhibitor of N-linked glycosylation, documented newly synthesized membrane material to form whorl-like or tubulovesicular structures rather than normal ROS disks (Mercurio and Holtzman, 1982). These vesicular structures are newly assembled opsin-containing membranes, which have been prepared for disk morphogenesis but are incapable of forming normal closed disks .…”

Involvement of the Golgi apparatus in sorting of materials to opposite ends of frog rod retinal photoreceptors

“…However, a related point to keep in mind is that not all sorting of membrane materials in neurons need be via the Golgi apparatus. Our autoradiographic studies and investigations with monensin suggest that presynaptic terminals of rod photoreceptors can very rapidly acquire some newly made proteins by a Golgi-independent route (Mercurio and Holtzman, 1982b;Matheke and Holtzman, 1984). The axonal endoplasmic reticulum, which is continuous from the rod cell body to the terminal, and which, although largely smooth, includes some ribosome-studded patches even in the axons and terminals, could be a key element in this route (Mercurio and Holtzman, 1982a).…”