Browsing by Author "Tan WJ"

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    Deciphering the roles of subcellular distribution and interactions involving the MEF2 binding region, the ankyrin repeat binding motif and the catalytic site of HDAC4 in Drosophila neuronal morphogenesis
    (BioMed Central Ltd, 2024-12) Tan WJ; Hawley HR; Wilson SJ; Fitzsimons HL
    BACKGROUND: Dysregulation of nucleocytoplasmic shuttling of histone deacetylase 4 (HDAC4) is associated with several neurodevelopmental and neurodegenerative disorders. Consequently, understanding the roles of nuclear and cytoplasmic HDAC4 along with the mechanisms that regulate nuclear entry and exit is an area of concerted effort. Efficient nuclear entry is dependent on binding of the transcription factor MEF2, as mutations in the MEF2 binding region result in cytoplasmic accumulation of HDAC4. It is well established that nuclear exit and cytoplasmic retention are dependent on 14-3-3-binding, and mutations that affect binding are widely used to induce nuclear accumulation of HDAC4. While regulation of HDAC4 shuttling is clearly important, there is a gap in understanding of how the nuclear and cytoplasmic distribution of HDAC4 impacts its function. Furthermore, it is unclear whether other features of the protein including the catalytic site, the MEF2-binding region and/or the ankyrin repeat binding motif influence the distribution and/or activity of HDAC4 in neurons. Since HDAC4 functions are conserved in Drosophila, and increased nuclear accumulation of HDAC4 also results in impaired neurodevelopment, we used Drosophila as a genetic model for investigation of HDAC4 function. RESULTS: Here we have generated a series of mutants for functional dissection of HDAC4 via in-depth examination of the resulting subcellular distribution and nuclear aggregation, and correlate these with developmental phenotypes resulting from their expression in well-established models of neuronal morphogenesis of the Drosophila mushroom body and eye. We found that in the mushroom body, forced sequestration of HDAC4 in the nucleus or the cytoplasm resulted in defects in axon morphogenesis. The actions of HDAC4 that resulted in impaired development were dependent on the MEF2 binding region, modulated by the ankyrin repeat binding motif, and largely independent of an intact catalytic site. In contrast, disruption to eye development was largely independent of MEF2 binding but mutation of the catalytic site significantly reduced the phenotype, indicating that HDAC4 acts in a neuronal-subtype-specific manner. CONCLUSIONS: We found that the impairments to mushroom body and eye development resulting from nuclear accumulation of HDAC4 were exacerbated by mutation of the ankyrin repeat binding motif, whereas there was a differing requirement for the MEF2 binding site and an intact catalytic site. It will be of importance to determine the binding partners of HDAC4 in nuclear aggregates and in the cytoplasm of these tissues to further understand its mechanisms of action.
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    Increased Abundance of Nuclear HDAC4 Impairs Neuronal Development and Long-Term Memory
    (Frontiers Media, 30/03/2021) Fitzsimons H; Main P; Wheeler D; Tan WJ
    Dysregulation of the histone deacetylase HDAC4 is associated with both neurodevelopmental and neurodegenerative disorders, and a feature common to many of these disorders is impaired cognitive function. HDAC4 shuttles between the nucleus and cytoplasm in both vertebrates and invertebrates and alterations in the amounts of nuclear and/or cytoplasmic HDAC4 have been implicated in these diseases. In Drosophila, HDAC4 also plays a critical role in the regulation of memory, however, the mechanisms through which it acts are unknown. Nuclear and cytoplasmically-restricted HDAC4 mutants were expressed in the Drosophila brain to investigate a mechanistic link between HDAC4 subcellular distribution, transcriptional changes and neuronal dysfunction. Deficits in mushroom body morphogenesis, eye development and long-term memory correlated with increased abundance of nuclear HDAC4 but were associated with minimal transcriptional changes. Although HDAC4 sequesters MEF2 into punctate foci within neuronal nuclei, no alteration in MEF2 activity was observed on overexpression of HDAC4, and knockdown of MEF2 had no impact on long-term memory, indicating that HDAC4 is likely not acting through MEF2. In support of this, mutation of the MEF2 binding site within HDAC4 also had no impact on nuclear HDAC4-induced impairments in long-term memory or eye development. In contrast, the defects in mushroom body morphogenesis were ameliorated by mutation of the MEF2 binding site, as well as by co-expression of MEF2 RNAi, thus nuclear HDAC4 acts through MEF2 to disrupt mushroom body development. These data provide insight into the mechanisms through which dysregulation of HDAC4 subcellular distribution impairs neurological function and provides new avenues for further investigation.
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    Loss of Drosophila Coq8 results in impaired survival, locomotor deficits and photoreceptor degeneration
    (BioMed Central, 9/02/2022) Hura A; Hannah H; Tan WJ; Penny R; Jessie J; Fitzsimons H
    Coenzyme Q8A encodes the homologue of yeast coq8, an ATPase that is required for the biosynthesis of Coenzyme Q10, an essential component of the electron transport chain. Mutations in COQ8A in humans result in CoQ10 deficiency, the clinical features of which include early-onset cerebellar ataxia, seizures and intellectual disability. The rapid advancement of massively parallel sequencing has resulted in the identification of more than 40 new mutations in COQ8A and functional studies are required to confirm causality and to further research into determining the specific mechanisms through which the mutations result in loss of function. To that end, a Drosophila model of Coq8 deficiency was developed and characterized to determine its appropriateness as a model system to further explore the role of Coq8 in the brain, and for functional characterisation of Coq8 mutations. Pan-neuronal RNAi knockdown of Coq8 was largely lethal, with female escapers displaying severe locomotor deficits. Knockdown of Coq8 in the eye resulted in degeneration of photoreceptors, progressive necrosis and increased generation of reactive oxygen species. Reintroduction of wild-type Coq8 restored normal function, however expression of human wild-type COQ8A exacerbated the eye phenotype, suggesting it was acting as a dominant-negative. This model is therefore informative for investigating the function of Drosophila Coq8, however human COQ8A mutations cannot be assessed as hCOQ8A does not rescue Coq8 deficiency.