Expression of kinesin heavy chain isoforms in retinal pigment epithelial cells

Sumoylation in axons triggers retrograde transport of the RNA-binding protein La

A surprisingly large population of mRNAs has been shown to localize to sensory axons, but few RNA-binding proteins have been detected in these axons. These axonal mRNAs include several potential binding targets for the La RNA chaperone protein. La is transported into axonal processes in both culture and peripheral nerve. Interestingly, La is posttranslationally modified in sensory neurons by sumoylation. In axons, small ubiquitin-like modifying polypeptides (SUMO)-La interacts with dynein, whereas native La interacts with kinesin. Lysine 41 is required for sumoylation, and sumoylation-incompetent La(K41R) shows only anterograde transport, whereas WT La shows both anterograde and retrograde transport in axons. Thus, sumoylation of La determines the directionality of its transport within the axonal compartment, with SUMO-La likely recycling to the cell body.

Actin-dependent motility of melanosomes from fish retinal pigment epithelial (RPE) cells investigated using in vitro motility assays

Melanosomes (pigment granules) within retinal pigment epithelial (RPE) cells of fish and amphibians undergo massive migrations in response to light conditions to control light flux to the retina. Previous research has shown that melanosome motility within apical projections of dissociated fish RPE cells requires an intact actin cytoskeleton, but the mechanisms and motors involved in melanosome transport in RPE have not been identified. Two in vitro motility assays, the Nitella assay and the sliding filament assay, were used to characterize actin-dependent motor activity of RPE melanosomes. Melanosomes applied to dissected filets of the Characean alga, Nitella, moved along actin cables at a mean rate of 2 microm/min, similar to the rate of melanosome motility in dissociated, cultured RPE cells. Path lengths of motile melanosomes ranged from 9 to 37 microm. Melanosome motility in the sliding filament assay was much more variable, ranging from 0.4-33 microm/min; 70% of velocities ranged from 1-15 microm/min. Latex beads coated with skeletal muscle myosin II and added to Nitella filets moved in the same direction as RPE melanosomes, indicating that the motility is barbed-end directed. Immunoblotting using antibodies against myosin VIIa and rab27a revealed that both proteins are enriched on melanosome membranes, suggesting that they could play a role in melanosome transport within apical projections of fish RPE.

Bidirectional pigment granule migration in isolated retinal pigment epithelial cells requires actin but not microtubules

In the teleost retinal pigment epithelium (RPE), melanin pigment granules disperse into long apical projections in the light and reaggregate into the cell body in the dark. To investigate the cytoskeletal mechanisms responsible for these movements, we have examined the effects of cytoskeletal inhibitors on pigment granule transport in cultured, dissociated RPE cells using time-lapse video microscopy. The kinetics of pigment granule transport during normal aggregation and dispersion are quite distinct: during aggregation, all pigment granules undergo simultaneous, nonsaltatory centripetal movement (mean velocity 3.6 microm/min); during dispersion, individual granules undergo independent, bidirectional saltatations (mean velocities 3.7 microm/min centrifugal; 1.1 microm/min centripetal). Nocodazole disruption of microtubules within the RPE apical projections had little effect on the kinetics of pigment granule movement, and essentially no effect on extent of pigment granule aggregation or dispersion, or on maintenance of the fully aggregated or fully dispersed states. In contrast, cytochalasin D (CD) treatment blocked net aggregation and dispersion of pigment granules, and compromised maintenance of the fully aggregated and dispersed states. These observations suggest that the actin cytoskeleton plays an important role in both centripetal and centrifugal transport of pigment granules in teleost RPE cells.

Photoreceptor localization of the KIF3A and KIF3B subunits of the heterotrimeric microtubule motor kinesin II in vertebrate retina

The heterotrimeric microtubule motor kinesin II has been shown to be required for morphogenesis and maintenance of both motile flagella and immotile sensory cilia. Recently, we showed that the KIF3A subunit of kinesin II is concentrated in the inner segment and connecting cilium of fish photoreceptors. Here we report the gene structure of human KIF3A (HsKIF3A) and describe its localization in human and monkey retina. We also describe the localization of both KIF3A and KIF3B kinesin II subunits in Xenopus retina. Using a portion of HsKIF3A we had amplified from adult human retinal cDNA, we found by a GenBank database search that an identical sequence had already been obtained by the Human Genome Center at Lawrence Berkeley National Laboratories in a direct sequencing analysis of 680 kb of human chromosome 5q31. By comparing the genomic sequence of HsKIF3A to the open reading frame (ORF) of the highly homologous mouse Kif3A, we determined that the HsKIF3A gene has 17 exons and an ORF of approximately 2.1 kb, predicting a protein of 80.3 kDa. Antibodies against sea urchin KRP85, a KIF3A homologue, bound to a single band of approximately 85 kDa in immunoblots of total retina protein from human, monkey and Xenopus. In these same samples, a single band of approximately 95 kDa is recognized by antibodies against Xklp3, a Xenopus KIF3B homologue. In sections of Xenopus retina, both antibodies strongly labelled photoreceptor inner segments and the outer limiting membrane. Both antibodies also labelled photoreceptor axonemes. The axonemal localization of kinesin II subunits suggests that kinesin II may play a role in transport of materials from the photoreceptor cell body to the outer segment.

Characterization of a novel C-kinesin (KIFC3) abundantly expressed in vertebrate retina and RPE

Many forms of intracellular transport are mediated by microtubule-dependent motors of the kinesin superfamily (KIFs). To identify kinesins expressed in human retina and RPE, we used degenerate primer RT-PCR to amplify a approximately 440 bp kinesin motor domain fragment from human retinal and RPE messenger RNAs. Four distinct kinesins were detected: one C-kinesin (HsKIFC3); one kinesin from the unc104/KIF1 family [HsKIF1A]; and the ubiquitous and neuronal forms of conventional kinesin heavy chain [HsuKHC and HsnKHC]. The C-kinesin HsKIFC3 comprised 33.3% of the retinal clones and was 60% identical to FKIF2, the most abundant kinesin detected in a previous screen of fish retina and 95% identical to a fragment of MmKifC3 recently amplified from mouse brain. Elsewhere we have reported the sequence of HsKIFC3 and shown that it maps to the same locus on chromosome 16q13-q21 as Bardet-Biedl syndrome Type II, a hereditary retinal degeneration. We describe here the kinesin PCR screen of human retina and RPE and examine the tissue and subcellular distribution of KIFC3 in both fish and human retina using an antibody raised against a peptide conserved between FKIF2 and HsKIFC3. This peptide antibody identified a single approximately 80 kDa band in Western blots of fish and human retina and RPE. In both fish and human retina this antibody strongly labeled photoreceptor terminals in the outer plexiform layer, suggesting that FKIF2/KIFC3 may play some role in the photoreceptor synapse.

Identification and classification of 16 new kinesin superfamily (KIF) proteins in mouse genome

KIF (kinesin superfamily) proteins are microtubule-dependent molecular motors that play important roles in intracellular transport and cell division. The extent to which KIFs are involved in various transporting phenomena, as well as their regulation mechanism, are unknown. The identification of 16 new KIFs in this report doubles the existing number of KIFs known in the mouse. Conserved nucleotide sequences in the motor domain were amplified by PCR using cDNAs of mouse nervous tissue, kidney, and small intestine as templates. The new KIFs were studied with respect to their expression patterns in different tissues, chromosomal location, and molecular evolution. Our results suggest that (i) there is no apparent tendency among related subclasses of KIFs of cosegregation in chromosomal mapping, and (ii) according to their tissue distribution patterns, KIFs can be divided into two classes-i.e., ubiquitous and specific tissue-dominant. Further characterization of KIFs may elucidate unknown fundamental phenomena underlying intracellular transport. Finally, we propose a straightforward nomenclature system for the members of the mouse kinesin superfamily.

Calcium-independent regulation of pigment granule aggregation and dispersion in teleost retinal pigment epithelial cells

In the eyes of teleosts and amphibians, melanin pigment granules of the retinal pigment epithelium (RPE) migrate in response to changes in light conditions. In the light, pigment granules disperse into the cells' long apical projections, thereby shielding the rod photoreceptor outer segments and reducing their extent of bleach. In darkness, pigment granules aggregate towards the base of the RPE cells. In vitro, RPE pigment granule aggregation can be induced by application of nonderivatized cAMP, and pigment granule dispersion can be induced by cAMP washout. In previous studies based on RPE-retina co-cultures, extracellular calcium was found to influence pigment granule migration. To examine the role of calcium in regulation of RPE pigment granule migration in the absence of retinal influences, we have used isolated RPE sheets and dissociated, cultured RPE cells. Under these conditions depletion of extracellular or intracellular calcium ([Ca2+]o, [Ca2+]i) had no effect on RPE pigment granule aggregation or dispersion. Using the intracellular calcium dye fura-2 and a new dye, fura-pe3, to monitor calcium dynamics in isolated RPE cells, we found that [Ca2+]i did not change from basal levels when pigment granule aggregation was triggered by cAMP, or dispersion was triggered by cAMP washout. Also, no change in [Ca2+]i was detected when dispersion was triggered by cAMP washout in the presence of 10 microM dopamine, a treatment previously shown to enhance dispersion. In addition, elevation of [Ca2+]i by addition of ionomycin neither triggered pigment movements, nor interfered with pigment granule motility elicited by cAMP addition or washout. Since other studies have indicated that actin plays a role in both pigment granule dispersion and aggregation in RPE, our findings suggest that RPE pigment granule migration depends on an actin-based motility system that is not directly regulated by calcium.