In vivo Validation of Bimolecular Fluorescence Complementation (BiFC) to Investigate Aggregate Formation in Amyotrophic Lateral Sclerosis (ALS)

Functional analysis of protein interactions using coupled bi-fluorescence complementation/GFP nanobody techniques

Transcription factors (TFs) form homo- or hetero-dimeric DNA binding complexes along with associated co-regulators that can have transcriptional repressor or activator functions. Defining the specific composition of the complexes is therefore key to understanding their biological role. Here, we utilized bimolecular fluorescence complementation (BiFC) to visualize the formation of defined TF dimers and associated co-regulators derived from the activator protein-1 (AP-1) and myocyte enhancer factor 2 (MEF2) families. Firstly, BiFC signals were observed in cells co-expressing TFs tagged with complimentary combinations of the split fluorescent protein, demonstrating the engineered formation of defined dimer complexes. Next, we applied this approach and determined that defined AP-1 dimers localized at discrete sub-nuclear locations. Subsequently, a combination of BiFC coupled with GFP binding peptide (GBP)-nanotrap allowed observation of protein-protein interactions between a co-regulator, HDAC4, and defined BiFC-MEF2 engineered dimers. To determine transactivation properties of defined TF dimers in a cellular system, the Gal4-DNA binding domain fused to GBP was utilized to assess the transcriptional properties of the BiFC-TF dimers using a generically applicable Gal4/UAS luciferase reporter gene assay system. Here, we report efficacy of a BiFC/GBP-nanobody approach that allows engineering, visualization, and functional analysis of defined TF dimers.

RNA-binding properties orchestrate TDP-43 homeostasis through condensate formation in vivo

Insoluble cytoplasmic aggregate formation of the RNA-binding protein TDP-43 is a major hallmark of neurodegenerative diseases including Amyotrophic Lateral Sclerosis. TDP-43 localizes predominantly in the nucleus, arranging itself into dynamic condensates through liquid-liquid phase separation (LLPS). Mutations and post-translational modifications can alter the condensation properties of TDP-43, contributing to the transition of liquid-like biomolecular condensates into solid-like aggregates. However, to date it has been a challenge to study the dynamics of this process in vivo. We demonstrate through live imaging that human TDP-43 undergoes nuclear condensation in spinal motor neurons in a living animal. RNA-binding deficiencies as well as post-translational modifications can lead to aberrant condensation and altered TDP-43 compartmentalization. Single-molecule tracking revealed an altered mobility profile for RNA-binding deficient TDP-43. Overall, these results provide a critically needed in vivo characterization of TDP-43 condensation, demonstrate phase separation as an important regulatory mechanism of TDP-43 accessibility, and identify a molecular mechanism of how functional TDP-43 can be regulated.

Cyclin F can alter the turnover of TDP-43

Previously, we demonstrated that the SCFcyclin F complex directly mediates the poly-ubiquitylation of TDP-43, raising the question of whether cyclin F can be used to enhance the turnover of TDP-43. A hurdle to the use of cyclin F, however, is that the overexpression of cyclin F can lead to the initiation of cell death pathways. Accordingly, the aim of this study was to identify and evaluate a less toxic variant of cyclin F. To do so, we first confirmed and validated our previous findings that cyclin F binds to TDP-43 in an atypical manner. Additionally, we demonstrated that mutating the canonical substrate region in cyclin F (to generate cyclin FMRL/AAA) led to reduced binding affinity to known canonical substrates without impacting the interaction between cyclin F and TDP-43. Notably, both wild-type and cyclin FMRL/AAA effectively reduced the abundance of TDP-43 in cultured cells whilst cyclin FMRL/AAA also demonstrated reduced cell death compared to the wild-type control. The decrease in toxicity also led to a reduction in morphological defects in zebrafish embryos. These results suggest that cyclin F can be modified to enhance its targeting of TDP-43, which in turn reduces the toxicity associated with the overexpression of cyclin F. This study provides greater insights into the interaction that occurs between cyclin F and TDP-43 in cells and in vivo.

Zebrafish as a Model Organism for Studying Pathologic Mechanisms of Neurodegenerative Diseases and other Neural Disorders

Zebrafish are widely considered an excellent vertebrate model for studying the pathogenesis of human diseases because of their transparency of embryonic development, easy breeding, high similarity with human genes, and easy gene manipulation. Previous studies have shown that zebrafish as a model organism provides an ideal operating platform for clarifying the pathological and molecular mechanisms of neurodegenerative diseases and related human diseases. This review mainly summarizes the achievements and prospects of zebrafish used as model organisms in the research of neurodegenerative diseases and other human diseases related to the nervous system in recent years. In the future study of human disease mechanisms, the application of the zebrafish model will continue to provide a valuable operating platform and technical support for investigating and finding better prevention and treatment of these diseases, which has broad application prospects and practical significance. Zebrafish models used in neurodegenerative diseases and other diseases related to the nervous system.

Expression of Interleukin-2 receptor subunit gamma (IL-2Rγ) and its binding with IL-2 induced activation of CD4 T lymphocytes in flounder (Paralichthys olivaceus)

Interleukin-2 receptor (IL-2R), as the specific ligand of interleukin-2 (IL-2), binds to IL-2 and transmits signals and then can induce the proliferation of T lymphocytes in mammals. In this paper, the subunit of IL-2R in flounder (Paralichthys olivaceus), interleukin-2 receptor subunit gamma (IL-2Rγ) was cloned, and polyclonal antibodies (Abs) against its extracellular region were produced, then the expression of flounder IL-2Rγ (fIL-2Rγ) at transcriptional and cellular levels were characterized. Moreover, the interaction of flounder IL-2 (fIL-2) with fIL-2Rγ was investigated, and the variations on CD4+/IL-2Rγ+ cells in flounder after treatment with recombinant IL-2 (rIL-2), anti-IL-2Rγ Abs were detected, respectively. The results showed that fIL-2Rγ protein had a typical fibronectin type III (FN3) domain. The Abs could specifically recognize native fIL-2Rγ molecules at 39.9 kDa. FIL-2Rγ was localized on both T and B lymphocytes, and the percentages of CD4+/IL-2Rγ+ and IgM+/IL-2Rγ+ lymphocytes were high in spleen. In addition, pBiFC-VN173-IL-2Rγ plasmids could bind to pBiFC-VC155-IL-2 plasmids. The percentage of CD4+/IL-2Rγ+ lymphocytes was significantly decreased after blocking with anti-IL-2Rγ Abs both in vivo and in vitro. In the meantime, four T cell markers genes and six IL-2-IL-2R pathway genes were down-regulated in anti-IL-2Rγ Abs group. These results first demonstrated that fIL-2Rγ molecules were expressed on both T and B lymphocytes in flounder, and the bond between fIL-2Rγ and fIL-2 activated the CD4 T lymphocytes. This study gave a new sight into the exploration of IL-2R function on T lymphocytes proliferation in fish.