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1.  Submembrane Assembly and Renewal of Rod Photoreceptor cGMP-Gated Channel: Insight into the Actin-Dependent Process of Outer Segment Morphogenesis 
The Journal of Neuroscience  2014;34(24):8164-8174.
The photoreceptor outer segment (OS) is comprised of two compartments: plasma membrane (PM) and disk membranes. It is unknown how the PM renewal is coordinated with that of the disk membranes. Here we visualized the localization and trafficking process of rod cyclic nucleotide-gated channel α-subunit (CNGA1), a PM component essential for phototransduction. The localization was visualized by fusing CNGA1 to a fluorescent protein Dendra2 and expressing in Xenopus laevis rod photoreceptors. Dendra2 allowed us to label CNGA1 in a spatiotemporal manner and therefore discriminate between old and newly trafficked CNGA1-Dendra2 in the OS PM. Newly synthesized CNGA1 was preferentially trafficked to the basal region of the lateral OS PM where newly formed and matured disks are also added. Unique trafficking pattern and diffusion barrier excluded CNGA1 from the PM domains, which are the proposed site of disk membrane maturation. Such distinct compartmentalization allows the confinement of cyclic nucleotide-gated channel in the PM, while preventing the disk membrane incorporation. Cytochalasin D and latrunculin A treatments, which are known to disrupt F-actin-dependent disk membrane morphogenesis, prevented the entrance of newly synthesized CNGA1 to the OS PM, but did not prevent the entrance of rhodopsin and peripherin/rds to the membrane evaginations believed to be disk membrane precursors. Uptake of rhodopsin and peripherin/rds coincided with the overgrowth of the evaginations at the base of the OS. Thus F-actin is essential for the trafficking of CNGA1 to the ciliary PM, and coordinates the formations of disk membrane rim region and OS PM.
PMCID: PMC4051972  PMID: 24920621
CNGA1; cyclic nucleotide-gated channel; morphogenesis; photoreceptor; retina; rhodopsin
2.  An Unconventional Secretory Pathway Mediates the Cilia Targeting of Peripherin/rds 
The Journal of Neuroscience  2014;34(3):992-1006.
It is unclear how unconventional secretion interplays with conventional secretion for the normal maintenance and renewal of membrane structures. The photoreceptor sensory cilium is recognized for fast membrane renewal, for which rhodopsin and peripherin/rds (P/rds) play critical roles. Here, we provide evidence that P/rds is targeted to the cilia by an unconventional secretion pathway. When expressed in ciliated hTERT-RPE1 human cell line, P/rd is localized to cilia. Cilium trafficking of P/rds was sustained even when the Golgi functions, including trans-Golgi-mediated conventional secretion, were inhibited by the small molecules brefeldin A, 30N12, and monensin. The unconventional cilia targeting of P/rds is dependent on COPII-mediated exit from the ER, but appears to be independent of GRASP55-mediated secretion. The regions in the C-terminal tail of P/rds are essential for this unconventional trafficking. In the absence of the region required for cilia targeting, P/rds was prohibited from entering the secretory pathways and was retained in the Golgi apparatus. A region essential for this Golgi retention was also found in the C-terminal tail of P/rds and supported the cilia targeting of P/rds mediated by unconventional secretion. In ciliated cells, including bovine and Xenopus laevis rod photoreceptors, P/rds was robustly sensitive to endoglycosidase H, which is consistent with its bypassing the medial Golgi and traversing the unconventional secretory pathway. Because rhodopsin is known to traffic through conventional secretion, this study of P/rds suggests that both conventional secretion and unconventional secretion need to cooperate for the renewal of the photoreceptor sensory cilium.
PMCID: PMC3891973  PMID: 24431457
cilia; peripherin/rds; photoreceptor; retina; trafficking; unconventional secretion
3.  Signals Governing the Trafficking and Mistrafficking of a Ciliary GPCR, Rhodopsin 
The Journal of Neuroscience  2013;33(34):13621-13638.
Rhodopsin is a cilia-specific GPCR essential for vision. Rhodopsin mislocalization is associated with blinding diseases called retinal ciliopathies. The mechanism by which rhodopsin mislocalizes in rod photoreceptor neurons is not well understood. Therefore, we investigated the roles of trafficking signals in rhodopsin mislocalization. Rhodopsin and its truncation mutants were fused to a photoconvertible fluorescent protein, Dendra2, and expressed in Xenopus laevis rod photoreceptors. Photoconversion of Dendra2 causes a color change from green to red, enabling visualization of the dynamic events associated with rhodopsin trafficking and renewal. We found that rhodopsin mislocalization is a facilitated process for which a signal located within 322–326 aa (CCGKN) is essential. An additional signal within 327–336 aa further facilitated the mislocalization. This collective mistrafficking signal confers toxicity to rhodopsin and causes mislocalization when the VXPX cilia-targeting motif is absent. We also determined that the VXPX motif neutralizes this mistrafficking signal, enhances ciliary targeting at least 10-fold, and accelerates trafficking of post-Golgi vesicular structures. In the absence of the VXPX motif, mislocalized rhodopsin is actively cleared through secretion of vesicles into the extracellular milieu. Therefore, this study unveiled the multiple roles of trafficking signals in rhodopsin localization and renewal.
PMCID: PMC3755712  PMID: 23966685
4.  The Mechanosensory Structure of the Hair Cell Requires Clarin-1, a Protein Encoded by Usher Syndrome III Causative Gene 
Mutation in the clarin-1 gene results in loss of hearing and vision in humans (Usher syndrome III), but the role of clarin-1 in the sensory hair cells is unknown. Clarin-1 is predicted to be a four transmembrane domain protein similar to members of the tetraspanin family. Mice carrying null mutation in the clarin-1 (Clrn1−/−) gene show loss of hair cell function and a possible defect in ribbon synapse. We investigated the role of clarin-1 using various in vitro and in vivo approaches. We show by immunohistochemistry and patch-clamp recordings of Ca2+ currents and membrane capacitance from IHCs that clarin-1 is not essential for formation or function of ribbon synapse. However, reduced cochlear microphonic potentials, FM1-43 loading and transduction currents pointed to diminished cochlear hair bundle function in Clrn1−/− mice. Electron microscopy of cochlear hair cells revealed loss of some tall stereocilia and gaps in the v-shaped bundle, although tip-links and staircase arrangement of stereocilia were not primarily affected by Clrn1−/− mutation. Human clarin-1 protein expressed in transfected mouse cochlear hair cells localized to the bundle; however, the pathogenic variant, p.N48K, failed to localize to the bundle. The mouse model generated to study the in vivo consequence of p. N48K in clarin-1 (Clrn1N48K) supports our in vitro and Clrn1−/− mouse data and the conclusion that CLRN1 is an essential hair bundle protein. Further, the ear phenotype in the Clrn1N48K mouse suggests that it is a valuable model for ear disease in CLRN1N48K, the most prevalent Usher III mutation in North America.
PMCID: PMC3422646  PMID: 22787034
USH3; Clarin-1; hair bundle; stereocilia
5.  Increased expression of cholesterol 24S-hydroxylase results in disruption of glial glutamate transporter EAAT2 association with lipid rafts: a potential role in Alzheimer’s disease 
Journal of neurochemistry  2010;113(4):978-989.
The glial glutamate transporter EAAT2 is the major mediator of glutamate clearance that terminates glutamate-mediated neurotransmission. Loss of EAAT2 and associated glutamate uptake function has been reported in the brains of patients with Alzheimer’s disease (AD). We previously reported that EAAT2 is associated with lipid raft microdomains of the plasma membrane. In the present study, we demonstrated that association of EAAT2 with lipid rafts is disrupted in AD brains. This abnormality is not a consequence of neuron degeneration, oxidative stress, or amyloid beta toxicity. In AD brains, cholesterol 24S-hydroxylase (CYP46), a key enzyme in maintenance of cholesterol homeostasis in the brain, is markedly increased in astrocytes but decreased in neurons. We demonstrated that increased expression of CYP46 in primary astrocytes results in a reduction of membrane cholesterol levels and leads to the dissociation of EAAT2 from lipid rafts and the loss of EAAT2 and associated glutamate uptake function. These results suggest that a disturbance of cholesterol metabolism may contribute to loss of EAAT2 in AD.
PMCID: PMC3010752  PMID: 20193040
glutamate transporter EAAT2; lipid raft microdomain; Alzheimer’s disease; cholesterol 24S-hydroxylase; excitotoxicity
6.  Messenger RNA Oxidation Occurs Early in Disease Pathogenesis and Promotes Motor Neuron Degeneration in ALS 
PLoS ONE  2008;3(8):e2849.
Accumulating evidence indicates that RNA oxidation is involved in a wide variety of neurological diseases and may be associated with neuronal deterioration during the process of neurodegeneration. However, previous studies were done in postmortem tissues or cultured neurons. Here, we used transgenic mice to demonstrate the role of RNA oxidation in the process of neurodegeneration.
Methodology/Principal Findings
We demonstrated that messenger RNA (mRNA) oxidation is a common feature in amyotrophic lateral sclerosis (ALS) patients as well as in many different transgenic mice expressing familial ALS-linked mutant copper-zinc superoxide dismutase (SOD1). In mutant SOD1 mice, increased mRNA oxidation primarily occurs in the motor neurons and oligodendrocytes of the spinal cord at an early, pre-symptomatic stage. Identification of oxidized mRNA species revealed that some species are more vulnerable to oxidative damage, and importantly, many oxidized mRNA species have been implicated in the pathogenesis of ALS. Oxidative modification of mRNA causes reduced protein expression. Reduced mRNA oxidation by vitamin E restores protein expression and partially protects motor neurons.
These findings suggest that mRNA oxidation is an early event associated with motor neuron deterioration in ALS, and may be also a common early event preceding neuron degeneration in other neurological diseases.
PMCID: PMC2481395  PMID: 18682740

Results 1-6 (6)