Biology subjects

Tan, S. M.

Publications and source records attributed to Tan, S. M..

7 recordsLinked to original sources

bioRxiv2026

Cholesterol transport cycle of human ABCA2

Cholesterol is a key component of cellular membranes and is critical for brain function, particularly axon myelination. Among the 48 human ATP-binding cassette (ABC) transporters, ABCA2 exhibits the highest expression in the brain and is involved in cholesterol metabolism, primarily in oligodendrocytes. Notably, ABCA2 has been associated with myelin sheath integrity and maintenance, as well as Alzheimer's disease. Here, we report cryo-EM structures of human ABCA2 that reveal critical endogenous lipid-binding sites unique to ABCA2. Our five distinct conformations include a previously uncharacterized intermediate between the closed and apo states of ABCA subfamily transporters. Most importantly, we elucidated the cholesterol transport mechanism of ABCA2, which involves novel interdependent rotations of the exocytoplasmic domains (ECDs) and regulatory domains (RDs). Our structural findings provide a new perspective on ABCA transporter function and highlight the role of ABCA2 in facilitating efficient cholesterol recycling and transport in the brain.

molecular biology↗

bioRxiv2025

Characterizing the transcriptomic response to interferon and infection in European Domestic Ferret respiratory tissues using long-read RNA sequencing

The European domestic ferret (Mustela putorius furo) is considered the gold-standard small animal model for studying human and avian influenza virus infections. However, experimental characterization of the transcriptomic response to interferon (IFN) stimulation and/or influenza virus infection has been limited, particularly in defining the induction of interferon-stimulated genes (ISGs), with most being computationally predicted. In this study, we present a comprehensive transcriptome-wide assessment of the ferret transcriptome following IFN- treatment of a ferret lung (FRL) cell line, as well as in nasal turbinates from influenza A virus (IAV)-infected ferrets using long-read RNA sequencing. We have identified a panel of ferret genes orthologous to human ISGs that are upregulated both in response to IFN- stimulation in vitro and IAV infection in vivo. We have also identified novel interferon stimulated genes and transcripts. Furthermore, we observed elongation of the poly(A) tails of genes in the ribosome and Coronavirus Disease-19 pathways in response to IFN- treatment in vitro, suggesting a relationship between poly(A) elongation and the antiviral responses of the host. These results illuminate the dynamics of the transcriptional innate immune response of the domestic ferret and provide an important resource for better utilizing ferrets as a small animal model to study influenza virus infections.

immunology↗

bioRxiv2023

Overcoming Resolution Attenuation During Tilted Cryo-EM Data Collection

Structural biology efforts using cryogenic electron microscopy are frequently stifled by specimens adopting "preferred orientations" on grids, leading to anisotropic map resolution and impeding structure determination. Tilting the specimen stage during data collection is a generalizable solution but has historically led to substantial resolution attenuation. Here, we develop updated data collection and image processing workflows and demonstrate, using multiple specimens, that resolution attenuation is negligible or significantly reduced across tilt angles. Reconstructions with and without the stage tilted as high as 60{degrees} are virtually indistinguishable. These strategies allowed the reconstruction to 3 [A] resolution of a bacterial RNA polymerase with preferred orientation. Furthermore, we present a quantitative framework that allows cryo-EM practitioners to define an optimal tilt angle for dataset acquisition. These data reinforce the utility of employing stage tilt for data collection and provide quantitative metrics to obtain isotropic maps.

biophysics↗

bioRxiv2021

Optogenetic inhibition of the dorsal hippocampus CA3 region during early-stage cocaine-memory reconsolidation disrupts subsequent context-induced cocaine seeking in rats

The dorsal hippocampus (DH) is key to the long-term maintenance of cocaine memories following retrieval-induced memory destabilization; even though, it is not the site of protein synthesis-dependent memory reconsolidation. Here, we took advantage of the temporal and spatial specificity of an optogenetic manipulation to examine the role of the cornu ammonis 3 subregion of the DH (dCA3) in early-stage cocaine-memory reconsolidation. Male Sprague-Dawley rats expressing eNpHR3.0 in the DH were trained to self-administer cocaine in a distinct context and underwent extinction training in a different context. Rats then received a 15-min memory-reactivation session, to destabilize cocaine memories and trigger reconsolidation, or remained in their home cages (no-reactivation controls). Optogenetic inhibition of the dCA3 for 1 h immediately, but not 1 h, after memory reactivation resulted in cocaine-memory impairment as indicated by reduction in drug-seeking behavior selectively in the cocaine-paired context 3 d later, at test, relative to responding in no-inhibition, no-reactivation, and no-eNpHR3.0 controls. Cocaine-memory impairment was associated with reduced c-Fos expression, an index of neuronal activation, in the dCA3 stratum lucidum (SL) and stratum pyramidale (SP) at test. Based on these observations and extant literature, we postulate that recurrent circuits in the SP are activated during early-stage memory reconsolidation to maintain labile cocaine memories prior to protein synthesis-dependent restabilization in another brain region, such as the basolateral amygdala. Furthermore, SL and SP interneurons may enhance memory reconsolidation by limiting synaptic noise in the SP and also contribute to recall as elements of the updated cocaine engram or retrieval links.

neuroscience↗

bioRxiv2021

Construction of a bioluminescence-based assay for bitter taste receptors (TAS2Rs)

In humans, a family of 25 bitter taste receptors (TAS2Rs) mediates bitter taste perception. A common approach to characterize bitter causative agents involves expressing TAS2Rs and the appropriate signal transducers in heterologous cell systems, and monitoring changes in the intracellular free calcium levels upon ligand exposure using a fluorescence-based modality, which typically suffers from a low signal window, and is susceptible to interference by autofluorescence, therefore prohibiting its application to screening of plant or food extracts, which are likely to contain autofluorescent compounds. Here, we report the development and validation of a bioluminescence-based intracellular calcium release assay for TAS2Rs that has a better assay performance than a fluorescence-based assay. Furthermore, the bioluminescence-based assay enables the evaluation of TAS2R agonists within an autofluorescent matrix, highlighting its potential utility in the assessment of the bitterness-inducing properties of plant or food fractions by the food industry. Additionally, improvement to the bioluminescence-based assay for some TAS2Rs was achieved by altering their N-terminal signal sequences, leading to signal window enhancement. Altogether, the bioluminescence-based TAS2R assay can be used to perform functional studies of TAS2Rs, evaluate TAS2R-modulating properties of autofluorescent samples, and facilitate the discovery of compounds that can function as promising bitter taste modulators.

biochemistry↗

bioRxiv2020

CB1 receptor signaling modulates amygdalar plasticity during context-cocaine memory reconsolidation to promote subsequent cocaine seeking

Contextual drug-associated memories precipitate craving and relapse in cocaine users. Such associative memories can be weakened through interference with memory reconsolidation, a process by which memories are maintained following memory retrieval-induced destabilization. We hypothesized that cocaine-memory reconsolidation requires cannabinoid type 1 receptor (CB1R) signaling based on the fundamental role of the endocannabinoid system in synaptic plasticity and emotional memory processing. Using an instrumental rat model of cocaine relapse, we evaluated whether systemic CB1R antagonism (AM251; 3 mg/kg, I.P.) during memory reconsolidation alters (a) subsequent drug context-induced cocaine-seeking behavior, as well as (b) cellular adaptations and (c) excitatory synaptic physiology in the basolateral amygdala (BLA). Systemic CB1R antagonism - during, but not after, cocaine-memory reconsolidation - reduced drug context-induced cocaine-seeking behavior three days, but not three weeks, later. CB1R antagonism also inhibited memory retrieval-associated increases in BLA zinc finger 268 (zif268) and activity regulated cytoskeletal-associated protein (Arc) immediate-early gene expression and changes in BLA -amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) and N-methyl-D-aspartate receptor (NMDAR) subunit phosphorylation that likely contribute to increased receptor membrane trafficking and synaptic plasticity during memory reconsolidation. Furthermore, CB1R antagonism increased memory reconsolidation-associated spontaneous excitatory post-synaptic current frequency in BLA principal neurons during memory reconsolidation. Together, these findings suggest that CB1R signaling modulates cellular and synaptic mechanisms in the BLA during cocaine-memory reconsolidation, thereby facilitating cocaine-memory maintenance. These findings identify the CB1R as a potential therapeutic target for relapse prevention. SIGNIFICANCE STATEMENTDrug relapse can be triggered by the retrieval of context-drug memories upon re-exposure to a drug-associated environment. Context-drug associative memories become destabilized upon retrieval and must be reconsolidated into long-term memory stores in order to persist. Hence, targeted interference with memory reconsolidation can weaken maladaptive context-drug memories and reduce the propensity for drug relapse. Our findings indicate that cannabinoid type 1 receptor (CB1R) signaling is critical for context-cocaine memory reconsolidation and subsequent drug context-induced reinstatement of cocaine-seeking behavior. Furthermore, cocaine-memory reconsolidation is associated with CB1R-dependent immediate-early gene expression and changes in excitatory synaptic proteins and physiology in the basolateral amygdala. Together, our findings provide initial support for CB1R as a potential therapeutic target for relapse prevention.

neuroscience↗

bioRxiv2019

Resistant starch does not protect against nephropathy or alter intestinal permeability in an STZ-induced diabetic mouse model

Alterations in gut homeostasis may contribute to the progression of diabetic nephropathy. There has been recent attention on the renoprotective effects of metabolite-sensing receptors in chronic renal injury, including the G-protein-coupled-receptor (GPR)109a, which ligates the short chain fatty acid butyrate. However, the role of GPR109a in the development of diabetic nephropathy, a milieu of diminished microbiome-derived metabolites, has not yet been determined. This study aimed to assess the effects of insufficient GPR109a signalling via genetic deletion of GPR109a on the development of renal injury in diabetic nephropathy. Gpr109a-/- mice or their wildtype littermates (Gpr109a+/+) were rendered diabetic with streptozotocin (STZ). Mice received a control diet or an isocaloric high fiber diet (12.5% resistant starch) for 24 weeks and gastrointestinal permeability and renal injury were determined. Diabetes was associated with increased albuminuria, glomerulosclerosis and inflammation. In comparison, Gpr109a-/- mice with diabetes did not show an altered renal phenotype. Resistant starch supplementation did not afford protection from renal injury in diabetic nephropathy. Whilst diabetes was associated with alterations in intestinal morphology, intestinal permeability assessed in vivo using the FITC-dextran test was unaltered. GPR109a deletion did not worsen gastrointestinal permeability. Further, 12.5% resistant starch supplementation, at physiological concentrations, had no effect on intestinal permeability or morphology. These studies indicate that GPR109a does not play a critical role in intestinal homeostasis in a model of type 1 diabetes or in the development of diabetic nephropathy.

molecular biology↗