T Li

Massachusetts Eye and Ear Infirmary, Boston, Massachusetts, United States

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Publications (9)42.44 Total impact

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    ABSTRACT: The X-linked RP3 gene codes for the ciliary protein RPGR and accounts for over 10% of inherited retinal degenerations. The critical RPGR-ORF15 splice variant contains a highly repetitive purine-rich linker region that renders it unstable and difficult to adapt for gene therapy. To test the hypothesis that the precise length of the linker region is not critical for function, we evaluated whether AAV-mediated replacement gene therapy with a human ORF15 variant containing in-frame shortening of the linker region could reconstitute RPGR function in vivo. We delivered human RPGR-ORF15 replacement genes with deletion of most (314-codons, 'short form') or 1/3 (126-codons, 'long form') of the linker region to Rpgr null mice. Human RPGR-ORF15 expression was detected post-treatment with both forms of ORF15 transgenes. However, only the long form correctly localized to the connecting cilia and led to significant functional and morphological rescue of rods and cones. Thus the highly repetitive region of RPGR is functionally important but that moderate shortening of its length, which confers the advantage of added stability, preserves its function. These findings provide a theoretical basis for optimizing replacement gene design in clinical trials for X-linked RP3.Gene Therapy accepted article preview online, 08 September 2015. doi:10.1038/gt.2015.93.
    Full-text · Article · Sep 2015 · Gene therapy
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    ABSTRACT: The actin-binding LIM domain protein (abLIM) is the mammalian homologue of UNC-115, a protein mediating axon guidance in C. elegans. AbLIM is widely expressed with three isoforms differing from one another by the length of their amino termini. Experiments utilizing dominant-negative mutants in the chick retina suggested a role for abLIM in axon path finding in retinal ganglion cells (RGCs). To investigate which variant is involved in the regulation of mammalian RGC axon guidance, we analyzed their expression profile in mice. The longest variant, abLIM-L, is highly enriched in the ganglion cell layer. AbLIM-L is up-regulated postnatally which temporally overlaps with the period of RGC axon remodeling. In contrast, the abLIM-M and abLIM-S variants are widespread and remain relatively constant through development. By selective gene targeting, we ablated abLIM-L to explore its functional significance in vivo. AbLIM-L mutant mice exhibit no apparent morphological or functional defects in photoreceptors and inner retinal neurons. Retinofugal projections and synaptic maturation also appear normal. These data suggest that abLIM-M is likely the isoform performing the essential function related to axon guidance.
    No preview · Article · Feb 2003 · Neuroscience
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    ABSTRACT: To test the hypothesis that a lack of Tubby-like protein 1 (TULP1) function causes aberrant transport of nascent rhodopsin and to examine the functional relationship between the homologous proteins TULP1 and Tubby by studying mice carrying combined mutations. Subcellular localization of TULP1 and rhodopsin in photoreceptors was determined by immunofluorescence and by postembedding immunoelectron microscopy. Mice carrying different tulp1/tubby allele combinations were examined by histology, electroretinograms (ERGs), and immunofluorescence microscopy. TULP1 is distributed throughout the photoreceptor cytoplasm but is excluded from the outer segments and the nuclei. In the tulp1-/- mice, ectopic accumulation of rhodopsin occurs at an early age. Both the vesicular profiles in the interphotoreceptor space and the inner segment plasma membranes are immunoreactive for rhodopsin. Mice doubly homozygous for null mutations in the tulp1 and tubby genes initially develop photoreceptors and express a battery of photoreceptor markers at age 14 days. Thereafter their photoreceptors undergo a fulminant degeneration that reaches completion by postnatal day 17. The disease phenotype in the double homozygote is much more severe than either single homozygote. Double heterozygotes are phenotypically normal. A lack of TULP1 function results in misrouting of nascent rhodopsin. TULP1 may be a component of the cellular machinery that targets nascent rhodopsin to the outer segments. Comparison of disease phenotypes in the single and double mutants suggests that TULP1 and Tubby are not functionally interchangeable in photoreceptors nor do they form an obligate functional complex.
    Preview · Article · Sep 2001 · Investigative Ophthalmology & Visual Science
  • C Lu · Y.W. Peng · J Shang · B.S. Pawlyk · F Yu · T Li
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    ABSTRACT: The retinal degeneration B (rdgB) gene in Drosophila is essential for photoreceptor function and survival. The rdgB mutant fly exhibits an abnormal electroretinogram and a light-dependent photoreceptor degeneration. The function of RdgB is not fully understood, but the presence of a phosphatidylinositol transfer protein domain suggests a possible role in phosphatidylinositol metabolism and signaling. Two mammalian homologs, M-RdgB1 and M-RdgB2, are known. While M-RdgB1 is widely expressed, M-RdgB2 is found primarily in the retina and the dentate gyrus. Functional conservation between the Drosophila and mammalian RdgBs was demonstrated by the ability of both M-RdgBs to rescue the photoreceptor phenotype in rdgB mutant flies through transgenic expression. To investigate the role of M-RdgB2 in the mammalian retina, we disrupted the m-rdgB2 gene in mice by gene targeting. The homozygous knockout mice are fertile and apparently healthy. By light microscopy, immunocytochemistry and electroretinograms, mice up to 18 months of age showed normal photoreceptor function and survival. The inner retinal neurons were also examined by immunolabeling with a number of cell-specific markers and no apparent defects were found in the major cell populations. We conclude that M-rdgB2 is not essential for phototransduction and photoreceptor survival. Thus, m-rdgB2 is not a candidate gene for human retinal degenerations. Whether M-rdgB2 has a role in visual processing in the inner retina, or whether it is required for hippocampal function, remains to be determined.
    No preview · Article · Feb 2001 · Neuroscience
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    SA Hagstrom · M Duyao · MA North · TS Li
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    ABSTRACT: The Tulp1 gene is a member of the tubby gene family with unknown function. Mutations in the human TULP1 gene cause autosomal recessive retinitis pigmentosa. To understand the pathogenic mechanism associated with TULP1 mutations and to explore the physiologic function of this protein, we examined tissue distribution of the Tulp1 protein in normal mice and the photoreceptor disease phenotype in Tulp1-ablated mice. Tissue distribution of the Tulp1 protein in normal mice was examined by immunoblotting and immunocytochemistry. The disease phenotype in tulp1-/- mice was studied by light and electron microscopy, electroretinography (ERG), and immunocytochemistry. These results were compared with another mouse model of retinal degeneration carrying a rhodopsin mutation. Tulp1 is found exclusively in photoreceptors, localizing predominantly in the inner segments. It is a soluble protein with an apparent molecular weight of approximately 70 kDa. Photoreceptor degeneration developed in tulp1-/- mice, with early involvement of both rods and cones. At the early stage of degeneration, rod and cone opsins, but not peripherin/RDS, exhibited prominent ectopic localization. Electron microscopy revealed massive accumulation of extracellular vesicles surrounding the distal inner segments. The function of Tulp1 is required to maintain viability of rod and cone photoreceptors. Extracellular vesicular accumulation is not a common phenomenon associated with photoreceptor degeneration but appears to be a distinct ultrastructural feature shared by a small group of retinal disease models. The defect in tulp1-/- mice may be consistent with a loss of polarized transport of nascent opsin to the outer segments.
    Preview · Article · Dec 1999 · Investigative Ophthalmology & Visual Science
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    CW Lu · T.S. Vihtelic · D.R. Hyde · TS Li
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    ABSTRACT: Mutations in the Drosophila retinal degeneration B (rdgB) gene cause a rapid loss of the electrophysiological light response and subsequent light-enhanced photoreceptor degeneration. The rdgB gene encodes a protein with an N-terminal phosphatidylinositol transfer protein domain, a large C-terminal segment, and several hydrophobic regions thought to multiply span the subrhabdomeric cisternal membrane. A mammalian rdgB homolog (m-rdgB1) was previously identified and shown to exhibit widespread tissue distribution and functionally rescue the Drosophila rdgB mutant phenotypes. We describe a second mammalian rdgB homolog (m-rdgB2) that possesses 46% amino acid identity to Drosophila RdgB and 56% identity to M-RdgB1. M-RdgB2 possesses a neuronal-specific expression pattern, with high levels in the retina and the dentate gyrus mossy fibers and dendritic field. Using M-RdgB2-specific antibodies and subcellular fractionation, we demonstrate that M-RdgB2 is not an integral membrane protein but is stably associated with a particulate fraction through protein-protein interactions. Although transgenic expression of M-RdgB2 in rdgB2 null mutant flies suppressed the retinal degeneration, it failed to fully restore the electrophysiological light response. Because transgenic expression of M-RdgB2 does not restore the wild-type phenotype to rdgB2 mutant flies to the same extent as M-RdgB1, functional differences likely exist between the two M-RdgB homologs.
    Full-text · Article · Oct 1999 · The Journal of Neuroscience : The Official Journal of the Society for Neuroscience
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    ABSTRACT: The missense mutation Lys-296-->Glu (K296E) in the rhodopsin gene produces an opsin with no chromophore binding site and therefore is not activated by light. Nevertheless, the mutant opsin constitutively activates transducin in vitro and causes photoreceptor degeneration in vivo, possibly by continuously activating the phototransduction cascade, analogous to constant exposure to environmental light. We studied the K296E mutation in eight lines of transgenic mice. Each line developed photoreceptor degeneration with the rate of degeneration increasing monotonically as the ratio of mutant:wild-type opsin mRNA increased. At no time in the course of degeneration was there endogenous light adaptation in the retina as measured by the electroretinogram. The mutant opsin was found to be invariably phosphorylated and stably bound to arrestin. Light-independent activation of transducin was demonstrated only after the removal of arrestin and dephosphorylation of K296E opsin. Thus, K296E opsin in vivo does not activate the phototransduction cascade because it is shut off by photoreceptor inactivation mechanisms. Our data show that the K296E mutation does not cause photoreceptor degeneration by continuous activation of phototransduction.
    Preview · Article · May 1995 · Proceedings of the National Academy of Sciences
  • T P Dryja · T Li
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    ABSTRACT: Hereditary degenerations and dysfunctions of the retina are an extremely heterogeneous group of diseases. This summary deals with recent advances in the molecular genetics of a subset of those disorders, namely, those encompassed under the diagnosis 'retinitis pigmentosa'. Over 20 loci where mutations cause retinitis pigmentosa have been mapped; the review focuses on the seven retinitis pigmentosa loci that have been identified.
    No preview · Article · Feb 1995 · Human Molecular Genetics
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    ABSTRACT: The ability of replication-deficient adenovirus to mediate gene transfer to retinal cells was evaluated. A replication-deficient adenoviral vector, AdCMV beta A.ntlacZ, which contains the bacterial beta-galactosidase (lacZ) reporter gene, was injected into the subretinal space of normal, rd, and rds strains of mice at various ages. The efficiency and duration of transgene expression were assessed by histochemical examination and transmission electron microscopy. AdCMV beta A.ntlacZ was effective in mediating gene transfer to the retinal pigment epithelial cells, rod and cone photoreceptor cells, and cells in the inner nuclear layer of the retina for periods of up to 1 month. Gene transfer to retinal pigment epithelial cells occurred at much lower viral titers than was required for gene transfer to photoreceptor cells. The extent to which photoreceptor cells could be transduced varied with the age of the animals and the conditions of the photoreceptor cells: greater numbers of photoreceptor cells were transduced in 5- to 7-day-old pups and in mice at the initial stages of photoreceptor degeneration than in normal adult mice. No evidence of gross pathogenic effects or viremia in recipient mice was observed. Replication-deficient adenovirus mediates transfer and expression of a foreign gene in retinal pigment epithelial and photoreceptor cells. Gene transfer to photoreceptor cells is enhanced in developing retinas or at the predegenerate stage of photoreceptors in genetically programmed retinal degeneration.
    Preview · Article · May 1994 · Investigative Ophthalmology & Visual Science

Publication Stats

677 Citations
42.44 Total Impact Points

Institutions

  • 1994-2015
    • Massachusetts Eye and Ear Infirmary
      • Berman-Gund Laboratory for the Study of Retinal Degenerations
      Boston, Massachusetts, United States
  • 1999-2003
    • Harvard Medical School
      Boston, Massachusetts, United States