Bio

Professional Education


  • Doctor of Philosophy, Simon Fraser University (2015)

Stanford Advisors


Publications

All Publications


  • Imaging organelle transport in primary hippocampal neurons treated with amyloid-β oligomers. Methods in cell biology Gan, K. J., Silverman, M. A. 2016; 131: 425–51

    Abstract

    We describe a strategy for fluorescent imaging of organelle transport in primary hippocampal neurons treated with amyloid-β (Aβ) peptides that cause Alzheimer's disease (AD). This method enables careful, rigorous analyses of axonal transport defects, which are implicated in AD and other neurodegenerative diseases. Moreover, we present and emphasize guidelines for investigating Aβ-induced mechanisms of axonal transport disruption in the absence of nonspecific, irreversible cellular toxicity. This approach should be accessible to most laboratories equipped with cell culture facilities and a standard fluorescent microscope and may be adapted to other cell types.

    View details for DOI 10.1016/bs.mcb.2015.06.012

    View details for PubMedID 26794527

  • Atlas stumbled: Kinesin light chain-1 variant E triggers a vicious cycle of axonal transport disruption and amyloid-ß generation in Alzheimer's disease. BioEssays Gan, K. J., Morihara, T., Silverman, M. A. 2015; 37 (2): 131-141

    Abstract

    Substantial evidence implicates fast axonal transport (FAT) defects in neurodegeneration. In Alzheimer's disease (AD), it is controversial whether transport defects cause or arise from amyloid-β (Aβ)-induced toxicity. Using a novel, unbiased genetic screen, Morihara et al. identified kinesin light chain-1 splice variant E (KLC1vE) as a modifier of Aβ accumulation. Here, we propose three mechanisms to explain this causal role. First, KLC1vE reduces APP transport, leading to Aβ accumulation. Second, reduced transport of APP by KLC1vE triggers an ER stress response that activates the amyloidogenic pathway. Third, KLC1vE impairs transport of other KLC1 cargos that regulate amyloidogenesis, promoting Aβ retention within the secretory pathway. Collectively, KLC1vE perpetuates a vicious cycle of Aβ generation, kinase dysregulation, and global FAT impairment that inevitably leads to cellular toxicity. These concepts implicate alternative splicing of KLC1 in AD and suggest that the reciprocal influence of transport mechanisms on disease states contributes to neurodegeneration.

    View details for DOI 10.1002/bies.201400131

    View details for PubMedID 25394182

  • Dendritic and axonal mechanisms of Ca2+ elevation impair BDNF transport in Aβ oligomer-treated hippocampal neurons. Molecular biology of the cell Gan, K. J., Silverman, M. A. 2015

    Abstract

    Disruption of fast axonal transport (FAT) and intracellular Ca(2+) dysregulation are early pathological events in Alzheimer's disease (AD). Amyloid-β oligomers (AβOs), a causative agent of AD, impair transport of BDNF independent of tau by non-excitotoxic activation of calcineurin (CaN). Ca(2+)-dependent mechanisms that regulate the onset, severity, and spatiotemporal progression of BDNF transport defects from dendritic and axonal AβO binding sites are unknown. Here, we show that BDNF transport defects in dendrites and axons are induced simultaneously but exhibit different rates of decline. The spatiotemporal progression of FAT impairment correlates with Ca(2+) elevation and CaN activation first in dendrites and subsequently in axons. Although many axonal pathologies have been described in AD, studies have primarily focused only on the dendritic effects of AβOs despite compelling reports of presynaptic AβOs in AD models and patients. Indeed, we observe that dendritic CaN activation converges on Ca(2+) influx through axonal voltage-gated Ca(2+) channels to impair FAT. Finally, FAT defects are prevented by dantrolene, a clinical compound that reduces Ca(2+) release from the ER. This work establishes a novel role for Ca(2+) dysregulation in BDNF transport disruption and tau-independent Aβ toxicity in early AD.

    View details for DOI 10.1091/mbc.E14-12-1612

    View details for PubMedID 25609087

  • Amyloid-beta oligomers induce tau-independent disruption of BDNF axonal transport via calcineurin activation in cultured hippocampal neurons MOLECULAR BIOLOGY OF THE CELL Ramser, E. M., Gan, K. J., Decker, H., Fan, E. Y., Suzuki, M. M., Ferreira, S. T., Silverman, M. A. 2013; 24 (16): 2494-2505

    Abstract

    Disruption of fast axonal transport (FAT) is an early pathological event in Alzheimer's disease (AD). Soluble amyloid-β oligomers (AβOs), increasingly recognized as proximal neurotoxins in AD, impair organelle transport in cultured neurons and transgenic mouse models. AβOs also stimulate hyperphosphorylation of the axonal microtubule-associated protein, tau. However, the role of tau in FAT disruption is controversial. Here we show that AβOs reduce vesicular transport of brain-derived neurotrophic factor (BDNF) in hippocampal neurons from both wild-type and tau-knockout mice, indicating that tau is not required for transport disruption. FAT inhibition is not accompanied by microtubule destabilization or neuronal death. Significantly, inhibition of calcineurin (CaN), a calcium-dependent phosphatase implicated in AD pathogenesis, rescues BDNF transport. Moreover, inhibition of protein phosphatase 1 and glycogen synthase kinase 3β, downstream targets of CaN, prevents BDNF transport defects induced by AβOs. We further show that AβOs induce CaN activation through nonexcitotoxic calcium signaling. Results implicate CaN in FAT regulation and demonstrate that tau is not required for AβO-induced BDNF transport disruption.

    View details for DOI 10.1091/mbc.E12-12-0858

    View details for Web of Science ID 000324492900002

    View details for PubMedID 23783030

  • Thyroid hormone accelerates opsin expression during early photoreceptor differentiation and induces opsin switching in differentiated TRa-expressing cones of the salmonid retina. Developmental dynamics Gan, K. J., Novales Flamarique, I. 2010; 239 (10): 2700-2713

    Abstract

    Thyroid hormone and its receptors (TRs) regulate photoreceptor differentiation and visual pigment protein (opsin) expression in the retinas of several vertebrates, including rodents and fish. In some of these animals, opsin expression can arise through switches within differentiated cone photoreceptors. In salmonid fishes, single cones express ultraviolet (SWS1) opsin during embryonic development and switch to blue (SWS2) opsin as the fishes grow. It is unknown whether thyroid hormone regulates opsin expression during early cone differentiation and acts through TRs to induce opsin switches in differentiated cones of the salmonid retina. Using in situ hybridization, we characterized the spatiotemporal dynamics of opsin expression and switching in embryos treated with exogenous thyroid hormone or propylthiouracil (PTU), a pharmacological inhibitor of thyroid hormone synthesis. We combined immunohistochemistry with in situ hybridization to map TRα expression with respect to cones undergoing the opsin switch in older juvenile fish. Thyroid hormone accelerated opsin expression in differentiating cones and induced the opsin switch in differentiated single cones, whereas PTU repressed the opsin switch. TRα was not detected in differentiating photoreceptors as opsin expression initiated, but was later expressed in differentiated single cones before the onset of the opsin switch. TRα expression exhibited a dynamic dorsoventral distribution that paralleled the progression of the opsin switch. Together, our results show that thyroid hormone is required for opsin switching in the retina of salmonid fishes and suggest that TRα may be involved in regulating this phenomenon.

    View details for DOI 10.1002/dvdy.22392

    View details for PubMedID 20730870

  • Thyroid Hormone Induces a Time-Dependent Opsin Switch in the Retina of Salmonid Fishes INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE Cheng, C. L., Gan, K. J., Flamarique, I. N. 2009; 50 (6): 3024-3032

    Abstract

    To determine the role of thyroid hormone in inducing the UV (SWS1)-to-blue (SWS2) opsin switch in the retina of two salmonid fishes, the coho salmon (Oncorhynchus kisutch) and the rainbow trout (O. mykiss).Fish were treated with thyroid hormone (T(4)) or the vehicle solution (0.1 M NaOH, control), exogenously or by intraocular injection, at different life history stages. Microspectrophotometry and in situ hybridization with riboprobes against the SWS1 and SWS2 opsins were used to reveal the dynamics of opsin expression in treated and control animals. To assess whether thyroid hormone induced differentiation of retinal progenitor cells into cones, treated and control fish were injected intraocularly with bromodeoxyuridine (BrdU) and the number of proliferating cells in the outer nuclear layer (ONL) determined. These observations were accompanied by histologic counts of cone densities.Thyroid hormone induced a reversible UV-to-blue opsin switch in differentiated single cones of juvenile salmonids (alevin and parr stages), but failed to exert any effect in the retina of older fish (smolt stage). The switch progressed from the ventral to the dorsal retina in clockwise fashion. Thyroid hormone did not induce cone density changes or alterations in the number of BrdU-labeled cells, which were the same in control and treated animals.Thyroid hormone induces a UV (SWS1)-to-blue (SWS2) opsin switch in the retina of young salmonid fishes that is identical with that occurring during natural development. The switch occurs in differentiated photoreceptors, is reversible (maintained by thyroid hormone exposure), and can be induced only before its natural onset. Thyroid hormone did not cause changes in the number of proliferating cells in the ONL. These results conform to the dynamics of thyroid hormone-induced opsin expression in the mouse and are consistent with the opsin plasticity found in differentiated photoreceptors of the fruit fly, Drosophila melanogaster. This work establishes a role for thyroid hormone in triggering opsin switches in the vertebrate retina.

    View details for DOI 10.1167/iovs.08-2713

    View details for Web of Science ID 000266403800067

    View details for PubMedID 19218617

  • The ultraviolet opsin is the first opsin expressed during retinal development of salmonid fishes INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE Cheng, C. L., Gan, K. J., Flamarique, I. N. 2007; 48 (2): 866-873

    Abstract

    To determine the spatial and temporal progression of opsin appearance during retinal development of salmonid fishes (genus Oncorhynchus and Salmo).Reverse transcription-polymerase chain reaction (RT-PCR) and in situ hybridization with riboprobes against the five classes of opsins present in salmonids (UV, blue, green, red, and rhodopsin) were used to establish the sequence of opsin appearance and the localization of opsins to specific morphologic photoreceptor types.Both detection methods revealed that UV opsin mRNA was expressed first and was followed closely by red opsin mRNA. In situ hybridization results indicated the following opsin sequence: UV, red, rhodopsin, green, and blue. The UV opsin riboprobe labeled single cones, whereas the red and green riboprobes labeled opposite members of double cones. The blue riboprobe started labeling single center cones approximately 1 month after initial UV riboprobe labeling, confirming a switch in opsin expression of these cones from UV to blue. All probes first labeled a small patch of cells in the centrotemporal retina, and expression then expanded primarily toward the temporal and dorsal retina, with the exception of the blue opsin which expanded ventrally at first.The sequence of cone opsin appearance in salmonid fishes is similar to that in mammals, in which a violet-blue (SWS1) opsin is expressed first followed by a red (M/LWS) opsin. This sequence is different from that in zebrafish, goldfish, and chick, in which red and green opsins are expressed first. As in mammals, rhodopsin expression in salmonid fishes arises after the first cone opsin. The findings show similarity in the sequence of opsin expression between a group of lower vertebrates, the salmonid fishes, and mammals.

    View details for DOI 10.1167/iovs.06-0442

    View details for Web of Science ID 000243729300052

    View details for PubMedID 17251489