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Endocytosis in the inner segment of rod photoreceptors: analysis of Xenopus laevis retinas using horseradish peroxidase.
Hollyfield JG
,
Rayborn ME
.
Abstract
To evaluate endocytosis in rod photoreceptor inner segments, isolated retinas from Xenopus laevis were maintained in vitro in the presence of the tracer, horseradish peroxidase (HRP). HRP-positive vesicles, 110 +/- 82 nm in diam. (mean +/- S.D.), appeared in the inner segment cytoplasm within 2 min of incubation. Within 8-15 min, 20-30% of the vesicles in the ellipsoid were HRP-positive, reaching approx. 50% by 30-60 min and maintaining this level through 4 hr of culture. In retinas incubated 30 min with HRP and then cultured for an additional period without tracer, the HRP-labeled vesicles were reduced in number by 50- and 75% in the 1- and 2-hr chase incubations, respectively. In retinas cultured for 8 min or longer, multivesicular bodies in the inner segments also contained HRP reaction product, suggesting that the HRP-labeled endocytotic vesicles enter multivesicular bodies. Retinas cultured with HRP for greater than or equal to 1 hr contained significantly more vesicles (per unit area of ellipsoid cytoplasm) as compared with unincubated retinas, retinas incubated from 1 hr in normal media or retinas incubated 30 min with HRP followed by normal chase medium for greater than or equal to 30 min. Taken together, these data indicate that the rod photoreceptor inner segment is capable of extensive endocytotic activity, a process which may function to recover components of the inner segment plasma membrane and the interphotoreceptor matrix.
Fxo. 1. Electron micrographs showing details of the ellipsoid region of rod photoreceptors in the region
of the connecting cilium from retinas that have been incubated for 8 rain (A), 15 rain (B) and 30 rain (C,
D) in the presence of horseradish peroxidase. Several vesicles are present in the cytoplasm surrounding
the cilium. Solid arrows point to some of the vesicles that contain HRP reaction product, whereas open
arrows point to several of the vesicles that are unlabeled. Extracellular fibrillar strands (double
arrowheads) were routinely seen near the cilium and associated with the periciliary ridge complex. This
extracellular marker was useful in determining the proximity of the periciliary ridge complex in sections
that did not contain the cilium or a clear profile of the groove or ridge associated with this site. All
micrographs are printed at the same magnification. Bar in lower right of (A) represents 1"0 pro.
Fro. 2. Electron micrographs through a portion of the ellipsoid in rod photoreceptors from retinas
that have been incubated for 30 rain (A), I hr {E), and 2 hr (B-D) in the presence of horseradish
peroxidase. ~:~ote the vesicles in the perimitochondrial cytoplasm. In (A) a vesicle filled with HRP
reaction product appears to be in continuity with the plasma membrane at the base of one of the grooves
of the periciliary ridge complex, suggesting a stage of endocytosis at this site just prior to detachment
from the base of the groove (open arrowhead). In (D), ~n enlargement shows a continuity between the
membrane of an unlabeled vesicle with the plasma membrane which is densely decorated with HRP
(small arrow) suggesting a stage in the fusion of unlabeled vesicle with the plasma membrane. In (E) note
the long cytoplasmic continuity from the base of the cilium through the mitochondrial mass. Numerous
profiles of vesicles are scattered along this mitochondria-free channel. Arrow designations are the same
as described in legend of Fig. 1. All mierographs are printed at the same magnification. Bar in (B)
represents 1-0 #m.
FIo. 3. Five consecutive serial sections through the connecting cilium and subjacent cytoplasm in a
rod photoreceptor from a retina incubated for 30 rain in the presence of horseradish peroxidase are
presented in (A-E). Asterisks denote two of tile ridges of the periciliary ridge complex sectioned at an
oblique angle in (A, B). The connecting cilium is first shown ill continuity with the inner segment of the
plasma membrane in (D). Both the cilium and basal body are evident in (E). Numerous vesicles are
present in the subjacent cytoplasm surrounding the cilium. In (F) the positions of several vesicles have
been reconstructed from the serial sections shown in (A-E}, and are superimposed in the appropriate
spatial position over a photograph of the section shown in (E). The dark spheres represent those vesicles
which contained HRP reaction product whereas the lighter spheres represent those vesicles in which no
extracellular tracer was evident. All of the vesicles reconstruct as near-perfect spheres, except for one
tubular structure with a diameter similar to the spherical vesicles. All micrographs are printed at same
magnification. Bar in (E) represents 1-0 pro. See text for further explanation.
Fie. 4. Myoid region of a rod photoreceptor from a retina incubated for 30 rain in the presence of
horseradish peroxidase. In (A), arrowheads denote numerous vesicles labeled with HRP l~aetion product
scattered throughout the cytoplasm surrounding the Golgi and rough-surfaced endoplasmie reticulum.
The open arrowhead points to a large multivesicular body that is heavily labeled with HRP. Note the
very intense labeling of the plasma membrane. In (B), two multivesicular bodies are indicated with
asterisks ; one heavily labeled with HRP reaction product and the other unlabeled. Bar in (A) represents
1.0/~m, in (B), 0"5/~m.
FIQ. 5. Time course of appearance of labeled vesicles in the ellipsoid cytoplasm of rod photoreceptors
from Xenopus laevis retinas incubated for the indicated intervals in the presence of HRP. Each point
represents the mean±s.n, of vesicle counts from 7-10 rods from each of three retinas analyzed at each
recovery time. Recovery times during the first hour were made at 2; 4; 8; 15; and 30 rain of incubation.
At the shortest recovery times approx. 10-20% of the vesicles contain HRP reaction product. The
proportion of labeled vesicles reaches 20-30 % by 8-15 min in culture. By 30 rain-1 hr, approx. 50% of
the vesicles contain HRP. This proportion is maintained through 4 h of culture. See text for further
explanation.
Fio. 6. Comparison of the proportion of HRP-labeled (shaded bars) and unlabeled vesicles (open bars)
in the ellipsoid from retinas incubated in the continuous presence of HRP in the incubation medium (A),
or from retinas cultured for 30 rain in medium with HRP followed by removal to chase media for
additional intervals (B). Note the relatively constant proportions and larger numbers of HRP-labeled
vesicles in (A), independent of the period of time culture. In (B) the proportion of labeled and unlabeled
vesicles decreases following removal to chase media. Lines above each mean value presented represent
I S.D. Time intervals presented in the abscissa refer to the total period of time in culture. See text for
further explanation.
FIo. 7. Histogram showing the number of HRP-labeled vesicles in the myoid of rod photoreceptors
from which the ellipsoid data presented in Fig. 6 were taken. (A) shows the number of labeled vesicles
following incubation in the presence of HRP for the indicated times, (B) presents the number of HRPlabeled
vesicles present after 30 rain in the presence of HRP and for additional intervals in chase
medium. Note the decrease in the number of HRP-labeled vesicles observed during the chase incubation
periods. Time intervals indicated in the abscissa refer to the total period of time in culture.
Fro. 8. Histogram showing the proportion of HRP-labeled (shaded bars) and unlabeled vesicles (open
bars) in the ellipsoid (upper panel) and tbe number of labeled vesicles in the myoid (lower panel) from
rod photoreceptors in retinas which had been cultured in the presence of HRP applied according to the
following conditions. Condition 1 : retinas incubated for 1 hr in the presence of HRP. Condition 2 : retinas
incubated for 3 hr in the presence of HRP. Condition 3: retinas.incubated for 2 hr in normal media and
were then transferred to media containing HRP and cultured for an additional 1 hr, Note the similarity
in the total numbers of vesicles observed under each condition as well as the similarities in the proportion
of ItRP-labeled and unlabeled vesicles. See text for further explanation.
Fzo. 9. Histograms showing the proportions of HRP-labeled (shaded bars) and unlabeled (open bars)
multivesicular bodies observed at different times of culture in rod photor~,ceptors incubated throughout
the culture period in the presence of HRP (A), or in the presence of HRP for 30 min followed by an
additional time in chaue media (B). Time in culture presented on the abscissa indicates the total time in
culture. Note that in all reeo¢-ery times, both labeled and unlabeled multivesicular bodies are present.
The proportion of labeled and unlabeled multi~,esicular bodies are similar through 3 hr of culture but
increase dramatically in the 4-hr recoveries. Larger numbers of multive~icular bodies are observed when
HRP is continuously present after 4 hr than is observed at this recovery time in the pulse-chase
incubations. See text for further explanation.
