Phospho-dependent ubiquitination and degradation of PAR-1 regulates synaptic morphology and tau-mediated Aβ toxicity in Drosophila

Alzheimer's disease pathophysiology in the Retina

The retina is an emerging CNS target for potential noninvasive diagnosis and tracking of Alzheimer's disease (AD). Studies have identified the pathological hallmarks of AD, including amyloid β-protein (Aβ) deposits and abnormal tau protein isoforms, in the retinas of AD patients and animal models. Moreover, structural and functional vascular abnormalities such as reduced blood flow, vascular Aβ deposition, and blood-retinal barrier damage, along with inflammation and neurodegeneration, have been described in retinas of patients with mild cognitive impairment and AD dementia. Histological, biochemical, and clinical studies have demonstrated that the nature and severity of AD pathologies in the retina and brain correspond. Proteomics analysis revealed a similar pattern of dysregulated proteins and biological pathways in the retina and brain of AD patients, with enhanced inflammatory and neurodegenerative processes, impaired oxidative-phosphorylation, and mitochondrial dysfunction. Notably, investigational imaging technologies can now detect AD-specific amyloid deposits, as well as vasculopathy and neurodegeneration in the retina of living AD patients, suggesting alterations at different disease stages and links to brain pathology. Current and exploratory ophthalmic imaging modalities, such as optical coherence tomography (OCT), OCT-angiography, confocal scanning laser ophthalmoscopy, and hyperspectral imaging, may offer promise in the clinical assessment of AD. However, further research is needed to deepen our understanding of AD's impact on the retina and its progression. To advance this field, future studies require replication in larger and diverse cohorts with confirmed AD biomarkers and standardized retinal imaging techniques. This will validate potential retinal biomarkers for AD, aiding in early screening and monitoring.

Cortical tension promotes Kibra degradation via Par-1

The Hippo pathway is an evolutionarily conserved regulator of tissue growth. Multiple Hippo signaling components are regulated via proteolytic degradation. However, how these degradation mechanisms are themselves modulated remains unexplored. Kibra is a key upstream pathway activator that promotes its own ubiquitin-mediated degradation upon assembling a Hippo signaling complex. Here, we demonstrate that Hippo complex-dependent Kibra degradation is modulated by cortical tension. Using classical genetic, osmotic, and pharmacological manipulations of myosin activity and cortical tension, we show that increasing cortical tension leads to Kibra degradation, whereas decreasing cortical tension increases Kibra abundance. Our study also implicates Par-1 in regulating Kib abundance downstream of cortical tension. We demonstrate that Par-1 promotes ubiquitin-mediated Kib degradation in a Hippo complex-dependent manner and is required for tension-induced Kib degradation. Collectively, our results reveal a previously unknown molecular mechanism by which cortical tension affects Hippo signaling and provide novel insights into the role of mechanical forces in growth control.

Multi-layered transcriptomic analysis reveals a pivotal role of FMR1 and other developmental genes in Alzheimer's disease-associated brain ceRNA network

Alzheimer's disease (AD) is an increasingly neurodegenerative disorder that causes progressive cognitive decline and memory impairment. Despite extensive research, the underlying causes of late-onset AD (LOAD) are still in progress. This study aimed to establish a network of competing regulatory interactions involving circular RNAs (circRNAs), microRNAs (miRNAs), RNA-binding proteins (RBPs), and messenger RNAs (mRNAs) connected to LOAD. A systematic analysis of publicly available expression data was conducted to identify integrated differentially expressed genes (DEGs) from the hippocampus of LOAD patients. Subsequently, gene co-expression analysis identified modules comprising highly expressed DEGs that act cooperatively. The competition between co-expressed DEGs and miRNAs/RBPs and the simultaneous interactions between circRNA and miRNA/RBP revealed a complex ceRNA network responsible for post-transcriptional regulation in LOAD. Hippocampal expression data for miRNAs, circRNAs, and RBPs were used to filter relevant relationships for AD. An integrated topological score was used to identify the highly connected hub gene, from which a brain core ceRNA subnetwork was generated. The Fragile X Messenger Ribonucleoprotein 1 (FMR1) coding for the RBP FMRP emerged as the prominent driver gene in this subnetwork. FMRP has been previously related to AD but not in a ceRNA network context. Also, the substantial number of neurodevelopmental genes in the ceRNA subnetwork and their related biological pathways strengthen that AD shares common pathological mechanisms with developmental conditions. Our results enhance the current knowledge about the convergent ceRNA regulatory pathways underlying AD and provide potential targets for identifying early biomarkers and developing novel therapeutic interventions.

A Novel Interaction between MFN2/Marf and MARK4/PAR-1 Is Implicated in Synaptic Defects and Mitochondrial Dysfunction

As cellular energy powerhouses, mitochondria undergo constant fission and fusion to maintain functional homeostasis. The conserved dynamin-like GTPase, Mitofusin2 (MFN2)/mitochondrial assembly regulatory factor (Marf), plays a role in mitochondrial fusion, mutations of which are implicated in age-related human diseases, including several neurodegenerative disorders. However, the regulation of MFN2/Marf-mediated mitochondrial fusion, as well as the pathologic mechanism of neurodegeneration, is not clearly understood. Here, we identified a novel interaction between MFN2/Marf and microtubule affinity-regulating kinase 4 (MARK4)/PAR-1. In the Drosophila larval neuromuscular junction, muscle-specific overexpression of MFN2/Marf decreased the number of synaptic boutons, and the loss of MARK4/PAR-1 alleviated the synaptic defects of MFN2/Marf overexpression. Downregulation of MARK4/PAR-1 rescued the mitochondrial hyperfusion phenotype caused by MFN2/Marf overexpression in the Drosophila muscles as well as in the cultured cells. In addition, knockdown of MARK4/PAR-1 rescued the respiratory dysfunction of mitochondria induced by MFN2/Marf overexpression in mammalian cells. Together, our results indicate that the interaction between MFN2/Marf and MARK4/PAR-1 is fine-tuned to maintain synaptic integrity and mitochondrial homeostasis, and its dysregulation may be implicated in neurologic pathogenesis.

Role of proline-rich tyrosine kinase 2 (Pyk2) in the pathogenesis of Alzheimer's disease

Alzheimer's disease (AD), a progressive neurodegenerative disorder, is the most common form of dementia in the elderly. Two major pathological hallmarks have been identified for AD: extracellular amyloid plaques and intracellular neurofibrillary tangles (NFT). Recently, proline-rich tyrosine kinase 2 (Pyk2), which belongs to the focal adhesion kinase (FAK) non-receptor tyrosine kinase family, was recognized to contribute significantly towards the pathogenesis of AD. Pyk2 can influence the formation of amyloid plaques as well as NFTs. The kinase can directly phosphorylate tau, which is a significant component of NFTs and enhances tau pathology. Several competitive inhibitors have been developed for Pyk2, tested in several cancer models, as Pyk2 is known to be overexpressed under those conditions. The current review article discusses the possible mechanistic pathways by which Pyk2 can influence the pathogenesis of AD. Besides, it describes various inhibitors for Pyk2 and their potential role as therapeutics for AD in the future.

A ZFP42/MARK2 regulatory network reduces the damage of retinal ganglion cells in glaucoma: a study based on GEO dataset and in vitro experiments

Glaucoma is a common disorder in which the death of retinal ganglion cells (RGCs) results in a progressive loss of sight, even blindness. This study was performed to reveal the key molecular mechanism of RGC damage in glaucoma based on the Gene Expression Omnibus database. Glaucoma-related microarray datasets were identified, followed by collection of differentially expressed genes (DEGs) with the key genes discovered by weighted gene co-expression network analysis. Through LASSO regression analysis, candidate genes involved in the pathogenesis of glaucoma were identified with their accuracy evaluated by receiver operating characteristic curve analysis. The glaucoma-specific transcriptional regulatory network was constructed to determine the key transcription factor regulatory axis. Then, in vitro cell models were established using H₂O₂ for further verifying the regulatory role of identified ZFP42/MARK2 axis in RGC damage in glaucoma. Differential analysis of GSE27276, GSE45570, and GSE101727 microarray datasets yielded 165 DEGs, and 22 key genes were identified following. Then, 9 candidate genes involved in the pathogenesis of glaucoma was collected and the key ZFP42/MARK2 regulatory axis was found. In vitro cell experiments further confirmed that ZFP42 and MARK2 were down-regulated in RGCs treated with H₂O₂. In addition, overexpression of ZFP42 increased the expression of MARK2 to increase RGC cell viability, and reduce cell apoptosis and ROS levels, while silencing MARK2 could reverse the protective effect of ZFP42. We confirmed that ZFP42 reduced the damage of RGCs in glaucoma by up-regulating the expression of MARK2.

Ssu72 is a T-cell receptor-responsive modifier that is indispensable for regulatory T cells

The homeostatic balance between effector T cells and regulatory T cells (Tregs) is crucial for adaptive immunity; however, epigenetic programs that inhibit phosphorylation to regulate Treg development, peripheral expression, and suppressive activity are elusive. Here, we found that the Ssu72 phosphatase is activated by various T-cell receptor signaling pathways, including the T-cell receptor and IL-2R pathways, and localizes at the cell membrane. Deletion of Ssu72 in T cells disrupts CD4+ T-cell differentiation into Tregs in the periphery via the production of high levels of the effector cytokines IL-2 and IFNγ, which induce CD4+ T-cell activation and differentiation into effector cell lineages. We also found a close correlation between downregulation of Ssu72 and severe defects in mucosal tolerance in patients. Interestingly, Ssu72 forms a complex with PLCγ1, which is an essential effector molecule for T-cell receptor signaling as well as Treg development and function. Ssu72 deficiency impairs PLCγ1 downstream signaling and results in failure of Foxp3 induction. Thus, our studies show that the Ssu72-mediated cytokine response coordinates the differentiation and function of Treg cells in the periphery.

Tau Accumulation via Reduced Autophagy Mediates GGGGCC Repeat Expansion-Induced Neurodegeneration in Drosophila Model of ALS

Expansions of trinucleotide or hexanucleotide repeats lead to several neurodegenerative disorders, including Huntington disease [caused by expanded CAG repeats (CAGr) in the HTT gene], and amyotrophic lateral sclerosis [ALS, possibly caused by expanded GGGGCC repeats (G4C2r) in the C9ORF72 gene], of which the molecular mechanisms remain unclear. Here, we demonstrated that lowering the Drosophila homologue of tau protein (dtau) significantly rescued in vivo neurodegeneration, motor performance impairments, and the shortened life-span in Drosophila expressing expanded CAGr or expanded G4C2r. Expression of human tau (htau4R) restored the disease-related phenotypes that had been mitigated by the loss of dtau, suggesting an evolutionarily-conserved role of tau in neurodegeneration. We further revealed that G4C2r expression increased tau accumulation by inhibiting autophagosome-lysosome fusion, possibly due to lowering the level of BAG3, a regulator of autophagy and tau. Taken together, our results reveal a novel mechanism by which expanded G4C2r causes neurodegeneration via an evolutionarily-conserved mechanism. Our findings provide novel autophagy-related mechanistic insights into C9ORF72-ALS and possible entry points to disease treatment.

The Accumulation of Tau in Postsynaptic Structures: A Common Feature in Multiple Neurodegenerative Diseases?

Increasingly, research suggests that neurodegenerative diseases and dementias are caused not by unique, solitary cellular mechanisms, but by multiple contributory mechanisms manifesting as heterogeneous clinical presentations. However, diverse neurodegenerative diseases also share common pathological hallmarks and cellular mechanisms. One such mechanism involves the redistribution of the microtubule associated protein tau from the axon into the somatodendritic compartment of neurons, followed by the mislocalization of tau into dendritic spines, resulting in postsynaptic functional deficits. Here we review various signaling pathways that trigger the redistribution of tau to the cell body and dendritic tree, and its mislocalization to dendritic spines. The convergence of multiple pathways in different disease models onto this final common pathway suggests that it may be an attractive pathway to target for developing new treatments for neurodegenerative diseases.

Actin-microtubule crosslinker Pod-1 tunes PAR-1 signaling to control synaptic development and tau-mediated synaptic toxicity

Partitioning-defective 1 (PAR-1), a conserved cell polarity regulator, plays an important role in synaptic development, and its mutation affects the formation of synaptic boutons and localization of postsynaptic density protein Discs large (Dlg) at the neuromuscular junction (NMJ) in Drosophila. Drosophila PAR-1 and its human homolog, Microtubule affinity-regulating kinases (MARK), are also known to be implicated in Alzheimer's disease (AD) by controlling tau-mediated Aβ toxicity. However, the molecular mechanisms of PAR-1 function remain incompletely understood. Here we identified Pod-1, an actin-microtubule crosslinker, which functionally and physically interacts with PAR-1 in Drosophila. Pod-1 prominently co-localizes with PAR-1 in the postsynaptic region and regulates PAR-1 activity at the NMJ. Synaptic defects, including the reduction of boutons and delocalization of Dlg caused by PAR-1 overexpression, were rescued by Pod-1 knockdown. Conversely, the reduction of synaptic boutons in PAR-1 overexpressed NMJ was synergistically enhanced by the overexpression of Pod-1. Furthermore, Pod-1 increases the PAR-1 dependent S262 phosphorylation of tau, which is known to contribute to tau-mediated Aβ toxicity. In line with the change of tau phosphorylation, Pod-1 knockdown rescued tau-mediated synaptic toxicity at the NMJ. Our results suggest that Pod-1 may act as a modulator of PAR-1 in synaptic development and tau-mediated toxicity.

Role of Post-translational Modifications in Alzheimer's Disease

The global burden of Alzheimer's disease (AD) is growing. Valiant efforts to develop clinical candidates for treatment have continuously met with failure. Currently available palliative treatments are temporary and there is a constant need to search for reliable disease pathways, biomarkers and drug targets for developing diagnostic and therapeutic tools to address the unmet medical needs of AD. Challenges in drug-discovery efforts raise further questions about the strategies of current conventional diagnosis; drug design; and understanding of disease pathways, biomarkers and targets. In this context, post-translational modifications (PTMs) regulate protein trafficking, function and degradation, and their in-depth study plays a significant role in the identification of novel biomarkers and drug targets. Aberrant PTMs of disease-relevant proteins could trigger pathological pathways, leading to disease progression. Advancements in proteomics enable the generation of patterns or signatures of such modifications, and thus, provide a versatile platform to develop biomarkers based on PTMs. In addition, understanding and targeting the aberrant PTMs of various proteins provide viable avenues for addressing AD drug-discovery challenges. This review highlights numerous PTMs of proteins relevant to AD and provides an overview of their adverse effects on the protein structure, function and aggregation propensity that contribute to the disease pathology. A critical discussion offers suggestions of methods to develop PTM signatures and interfere with aberrant PTMs to develop viable diagnostic and therapeutic interventions in AD.

The Ubiquitin System in Alzheimer's Disease

Alzheimer's disease (AD) is the most common form of dementia, most prevalent in the elderly population and has a significant impact on individuals and their family as well as the health care system and the economy. While the number of patients affected by various forms of dementia including AD is on the increase, there is currently no cure. Although genome-wide association studies have identified genetic markers for familial AD, the molecular mechanisms underlying the initiation and development of both familial and sporadic AD remain poorly understood. Most neurodegenerative diseases and in particular those associated with dementia have been defined as proteinopathies due to the presence of intra- and/or extracellular protein aggregates in the brain of affected individuals. Although loss of proteostasis in AD has been known for decades, it is only in recent years that we have come to appreciate the role of ubiquitin-dependent mechanisms in brain homeostasis and in brain diseases. Ubiquitin is a highly versatile post-translational modification which regulates many aspects of protein fate and function, including protein degradation by the Ubiquitin-Proteasome System (UPS), autophagy-mediated removal of damaged organelles and proteins, lysosomal turnover of membrane proteins and of extracellular molecules brought inside the cell through endocytosis. Amyloid-β (Aβ) fragments as well as hyperphosphorylation of Tau are hallmarks of AD, and these are found in extracellular plaques and intracellular fibrils in the brain of individuals with AD, respectively. Yet, whether it is the oligomeric or the soluble species of Aβ and Tau that mediate toxicity is still unclear. These proteins impact on mitochondrial energy metabolism, inflammation, as well as a number of housekeeping processes including protein degradation through the UPS and autophagy. In this chapter, we will discuss the role of ubiquitin in neuronal homeostasis as well as in AD; summarise crosstalks between the enzymes that regulate protein ubiquitination and the toxic proteins Tau and Aβ; highlight emerging molecular mechanisms in AD as well as future strategies which aim to exploit the ubiquitin system as a source for next-generation therapeutics.

Alzheimer's Disease Associated Genes Ankyrin and Tau Cause Shortened Lifespan and Memory Loss in Drosophila

Alzheimer's disease (AD) is the most common form of dementia and is characterized by intracellular neurofibrillary tangles of hyperphosphorylated Tau, including the 0N4R isoform and accumulation of extracellular amyloid beta (Aβ) plaques. However, less than 5% of AD cases are familial, with many additional risk factors contributing to AD including aging, lifestyle, the environment and epigenetics. Recent epigenome-wide association studies (EWAS) of AD have identified a number of loci that are differentially methylated in the AD cortex. Indeed, hypermethylation and reduced expression of the Ankyrin 1 (ANK1) gene in AD has been reported in the cortex in numerous different post-mortem brain cohorts. Little is known about the normal function of ANK1 in the healthy brain, nor the role it may play in AD. We have generated Drosophila models to allow us to functionally characterize Drosophila Ank2, the ortholog of human ANK1 and to determine its interaction with human Tau and Aβ. We show expression of human Tau 0N4R or the oligomerizing Aβ 42 amino acid peptide caused shortened lifespan, degeneration, disrupted movement, memory loss, and decreased excitability of memory neurons with co-expression tending to make the pathology worse. We find that Drosophila with reduced neuronal Ank2 expression have shortened lifespan, reduced locomotion, reduced memory and reduced neuronal excitability similar to flies overexpressing either human Tau 0N4R or Aβ42. Therefore, we show that the mis-expression of Ank2 can drive disease relevant processes and phenocopy some features of AD. Therefore, we propose targeting human ANK1 may have therapeutic potential. This represents the first study to characterize an AD-relevant gene nominated from EWAS.

Transcriptome profiling data reveals ubiquitin-specific peptidase 9 knockdown effects

Ubiquitin specific peptidase 9 (USP9) is a deubiquitinase encoded by a sex-linked gene with a Y-chromosomal form (USP9Y) and an X-chromosomal form (USP9X) that escapes X-inactivation. Since USP9 is a key regulatory gene with sex-linked expression in the human brain, the gene may be of interest for researchers studying molecular gender differences and ubiquitin signaling in the brain.

To assess the downstream effects of knocking down USP9X and USP9Y on a transcriptome-wide scale, we have conducted microarray profiling experiments using the human DU145 prostate cancer cell culture model, after confirming the robust expression of both USP9X and USP9Y in this model. By designing shRNA constructs for the specific knockdown of USP9X and the joint knockdown of USP9X and USP9Y, we have compared gene expression changes in both knockdowns to control conditions to infer potential shared and X- or Y-form specific alterations. Here, we provide details of the corresponding microarray profiling data, which has been deposited in the Gene Expression Omnibus database (GEO series accession number GSE79376). A biological interpretation of the data in the context of a potential involvement of USP9 in Alzheimer's disease has previously been presented in Köglsberger et al. (2016). To facilitate the re-use and re-analysis of the data for other applications, e.g. the study of ubiquitin signaling and protein turnover control, and the regulation of molecular gender differences in the human brain and brain-related disorders, we provide a more in-depth discussion of the data properties, specifications and possible use cases.

MiR-219 represses expression of dFMR1 in Drosophila melanogaster

AIMS

Fragile X mental retardation protein (FMRP) plays a vital role in mRNA trafficking and translation inhibition to regulate the synthesis of local proteins in neuronal axons and dendritic terminals. However, there are no reports on microRNA (miRNA)-mediated regulation of FMRP levels in Drosophila. Here, we aimed to identify miRNAs regulating FMRP levels in Drosophila.

MAIN METHODS

Using online software, we predicted and selected 11 miRNAs potentially acting on the Drosophila fragile X mental retardation 1 (dFMR1) transcript. These candidates were screened for modulation of dFMR1 transcript levels at the cellular level using a dual luciferase reporter system. In addition, we constructed a transgenic Drosophila model overexpressing miR-219 in the nervous system and quantified dFMRP by western blotting. The neuromuscular junction phenotype in the model was studied by immunofluorescence staining.

KEY FINDINGS

Among the 11 miRNAs screened, miR-219 and miR-960 reduced luciferase gene activity by binding to the 3'-UTR of the dFMR1 transcript. Mutation of the miR-219 or miR-960 binding sites on the transcript resulted in complete or partial elimination of the miRNA-induced repression. Western blots revealed that dFMRP expression was decreased in the miR-219 overexpression model (Elav>miR-219). Drosophila larvae overexpressing miR-219 showed morphological abnormalities at the neuromuscular junction (increased synaptic boutons and synaptic branches). This finding is consistent with some phenotypes observed in dfmr1 mutants.

SIGNIFICANCE

Our results suggest that miR-219 regulates dFMR1 expression in Drosophila and is involved in fragile X syndrome pathogenesis. Collectively, these findings expand the current understanding of miRNA-mediated regulation of target molecule-related functions.

Levels of Par-1 kinase determine the localization of Bruchpilot at the Drosophila neuromuscular junction synapses

Functional synaptic networks are compromised in many neurodevelopmental and neurodegenerative diseases. While the mechanisms of axonal transport and localization of synaptic vesicles and mitochondria are relatively well studied, little is known about the mechanisms that regulate the localization of proteins that localize to active zones. Recent finding suggests that mechanisms involved in transporting proteins destined to active zones are distinct from those that transport synaptic vesicles or mitochondria. Here we report that localization of BRP-an essential active zone scaffolding protein in Drosophila, depends on the precise balance of neuronal Par-1 kinase. Disruption of Par-1 levels leads to excess accumulation of BRP in axons at the expense of BRP at active zones. Temporal analyses demonstrate that accumulation of BRP within axons precedes the loss of synaptic function and its depletion from the active zones. Mechanistically, we find that Par-1 co-localizes with BRP and is present in the same molecular complex, raising the possibility of a novel mechanism for selective localization of BRP-like active zone scaffolding proteins. Taken together, these data suggest an intriguing possibility that mislocalization of active zone proteins like BRP might be one of the earliest signs of synapse perturbation and perhaps, synaptic networks that precede many neurological disorders.

Expression profiling of ubiquitin-related genes in LKB1 mutant lung adenocarcinoma

Liver kinase B1 (LKB1) is a tumor suppressor, and there is a very high proportion of LKB1 mutation in lung adenocarcinoma. The function of LKB1 is closely related to that of ubiquitin related genes. Our objective is to analyze the changes in ubiquitin-related genes in LKB1 mutant lung adenocarcinoma. We searched The Cancer Genome Atlas (TCGA) and obtained gene expression profiles from 230 lung adenocarcinoma patients, which were then analyzed using R software. Kaplan–Meier curves and Cox proportional hazards regression were applied to estimate survival. Real-time reverse transcription PCR was used to verify gene expression. Gene function was explored by gene set enrichment analysis. There were significantly expressed differences in the ubiquitin-related gene SH3RF1 between the LKB1 mutant and wild-type lung adenocarcinoma patients (p = 9.78013E-05). Patients with LKB1 mutation and high expression of SH3RF1 had a better prognosis than the low expression group (HR 0.356, 95% CI 0.136–0.929, p = 0.035). SH3RF1 can influence cell cycle, apoptosis, DNA replication and the p53 signaling pathway. SH3RF1 might have great clinical value act as a diagnostic biomarker and indicator to evaluate the prognosis of LKB1 mutant lung adenocarcinoma patients. This gene also can become a new treatment target for LKB1 mutant lung adenocarcinoma patients.

Altered ER-mitochondria contact impacts mitochondria calcium homeostasis and contributes to neurodegeneration in vivo in disease models

Calcium (Ca²⁺) homeostasis is essential for neuronal function and survival. Altered Ca²⁺ homeostasis has been consistently observed in neurological diseases. How Ca²⁺ homeostasis is achieved in various cellular compartments of disease-relevant cell types is not well understood. Here we show in Drosophila Parkinson's disease (PD) models that Ca²⁺ transport from the endoplasmic reticulum (ER) to mitochondria through the ER-mitochondria contact site (ERMCS) critically regulates mitochondrial Ca²⁺ (mito-Ca²⁺) homeostasis in dopaminergic (DA) neurons, and that the PD-associated PINK1 protein modulates this process. In PINK1 mutant DA neurons, the ERMCS is strengthened and mito-Ca²⁺ level is elevated, resulting in mitochondrial enlargement and neuronal death. Miro, a well-characterized component of the mitochondrial trafficking machinery, mediates the effects of PINK1 on mito-Ca²⁺ and mitochondrial morphology, apparently in a transport-independent manner. Miro overexpression mimics PINK1 loss-of-function effect, whereas inhibition of Miro or components of the ERMCS, or pharmacological modulation of ERMCS function, rescued PINK1 mutant phenotypes. Mito-Ca²⁺ homeostasis is also altered in the LRRK2-G2019S model of PD and the PAR-1/MARK model of neurodegeneration, and genetic or pharmacological restoration of mito-Ca²⁺ level is beneficial in these models. Our results highlight the importance of mito-Ca²⁺ homeostasis maintained by Miro and the ERMCS to mitochondrial physiology and neuronal integrity. Targeting this mito-Ca²⁺ homeostasis pathway holds promise for a therapeutic strategy for neurodegenerative diseases.

Microtubule affinity-regulating kinases are potential druggable targets for Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder that affects normal functions of the brain. Currently, AD is one of the leading causes of death in developed countries and the only one of the top ten diseases without a means to prevent, cure, or significantly slow down its progression. Therefore, newer therapeutic concepts are urgently needed to improve survival and the quality of life of AD patients. Microtubule affinity-regulating kinases (MARKs) regulate tau-microtubule binding and play a crucial role in neurons. However, their role in hyperphosphorylation of tau makes them potential druggable target for AD therapy. Despite the relevance of MARKs in AD pathogenesis, only a few small molecules are known to have anti-MARK activity and not much has been done to progress these compounds into therapeutic candidates. But given the diverse role of MARKs, the specificity of novel inhibitors is imperative for their successful translation from bench to bedside. In this regard, a recent co-crystal structure of MARK4 in association with a pyrazolopyrimidine-based inhibitor offers a potential scaffold for the development of more specific MARK inhibitors. In this manuscript, we review the biological role of MARKs in health and disease, and draw attention to the largely unexplored area of MARK inhibitors for AD.

Shedding Light on Alzheimer's β-Amyloidosis: Photosensitized Methylene Blue Inhibits Self-Assembly of β-Amyloid Peptides and Disintegrates Their Aggregates

Abnormal aggregation of β-amyloid (Aβ) peptides is a major hallmark of Alzheimer’s disease (AD). In spite of numerous attempts to prevent the β-amyloidosis, no effective drugs for treating AD have been developed to date. Among many candidate chemicals, methylene blue (MB) has proved its therapeutic potential for AD in a number of in vitro and in vivo studies; but the result of recent clinical trials performed with MB and its derivative was negative. Here, with the aid of multiple photochemical analyses, we first report that photoexcited MB molecules can block Aβ₄₂ aggregation in vitro. Furthermore, our in vivo study using Drosophila AD model demonstrates that photoexcited MB is highly effective in suppressing synaptic toxicity, resulting in a reduced damage to the neuromuscular junction (NMJ), an enhanced locomotion, and decreased vacuole in the brain. The hindrance effect is attributed to Aβ₄₂ oxidation by singlet oxygen (¹O₂) generated from photoexcited MB. Finally, we show that photoexcited MB possess a capability to disaggregate the pre-existing Aβ₄₂ aggregates and reduce Aβ-induced cytotoxicity. Our work suggests that light illumination can provide an opportunity to boost the efficacies of MB toward photodynamic therapy of AD in future.

MARK2 inhibits the growth of HeLa cells through AMPK and reverses epithelial-mesenchymal transition

Microtubule affinity-regulating kinases (MARKs; MARK1, MARK2, MARK3 and MARK4) act directly downstream of LKB1, the multitasking tumor-suppressor kinase, and thereby mediate its biological effects. Current understanding of the function of MARKs is greatly restricted to regulation of cell polarity. However, whether or how MARKs contribute to cellular growth control remains largely unknown. In the present study, we utilized an inducible lentiviral expression system that allows rapid MARK expression in LKB1-deficient HeLa cells, and characterized additional functions of MARKs: overexpression of MARK2 in HeLa cells resulted in a decrease in cell growth, inhibition of colony formation and arrest in G1 cell cycle phase, with AMPK as the putative downstream effector upregulating the expression of p21 and p16. MARK2 was found to play a role in F-actin reorganization and to contribute to reversal of epithelial‑mesenchymal transition (EMT) as exemplified in the case of HeLa cells that exhibited phenotypic changes, reduced cell migration and invasion. Our findings unveil the coordinated regulation of cell growth and EMT mediated by MARK2, and also provide new insights into the mechanisms underlying the anti-metastatic activity of MARK2.

CDK5-Mediated Phosphorylation-Dependent Ubiquitination and Degradation of E3 Ubiquitin Ligases GP78 Accelerates Neuronal Death in Parkinson's Disease

The molecular mechanisms responsible for the loss of dopaminergic neurons in Parkinson's disease (PD) remain obscure. Loss of function of E3 ubiquitin ligases is associated with mitochondria dysfunction, dysfunction of protein degradation, and α-synuclein aggregation, which are major contributors to neurodegeneration in PD. Recent research has thus focused on E3 ubiquitin ligase glycoprotein 78 (GP78); however, the role of GP78 in PD pathogenesis remains unclear. Notably, cyclin-dependent kinase 5 (CDK5) controls multiple cellular events in postmitotic neurons, and CDK5 activity has been implicated in the pathogenesis of PD. Thus, we addressed the relationship between CDK5 and GP78 in MPTP-based PD models. We found that GP78 expression is decreased in MPTP-based cellular and animal PD models, and CDK5 directly phosphorylated GP78 at Ser516, which promoted the ubiquitination and degradation of GP78. Importantly, overexpression of GP78 or interference of GP78 Ser516 phosphorylation protected neurons against MPP⁺-induced cell death. Thus, our research reveals that the CDK5-GP78 pathway is involved in the pathogenesis of PD and could be a novel candidate drug target for the treatment of PD.

Phenotypic differences between Drosophila Alzheimer's disease models expressing human Aβ42 in the developing eye and brain

Drosophila melanogaster expressing amyloid-β42 (Aβ42) transgenes have been used as models to study Alzheimer's disease. Various Aβ42 transgenes with different structures induce different phenotypes, which make it difficult to compare data among studies which use different transgenic lines. In this study, we compared the phenotypes of four frequently used Aβ42 transgenic lines, UAS-Aβ42^2X , UAS-Aβ42^BL33770 , UAS-Aβ42^11C39 , and UAS-Aβ42^H29.3 . Among the four transgenic lines, only UAS-Aβ42^2X has two copies of the upstream activation sequence-amyloid-β42 (UAS-Aβ42) transgene, while remaining three have one copy. UAS-Aβ42^BL33770 has the 3' untranslated region of Drosophila α-tubulin, while the others have that of SV40. UAS-Aβ42^11C39 and UAS-Aβ42^H29.3 have the rat pre-proenkephalin signal peptide, while UAS-Aβ42^2X and UAS-Aβ42^BL33770 have that of the fly argos protein. When the transgenes were expressed ectopically in the developing eyes of the flies, UAS-Aβ42^2X transgene resulted in a strongly reduced and rough eye phenotype, while UAS-Aβ42^BL33770 only showed a strong rough eye phenotype; UAS-Aβ42^H29.3 and UAS-Aβ42^11C39 had mild rough eyes. The levels of cell death and reactive oxygen species (ROS) in the eye imaginal discs were consistently the highest in UAS-Aβ42^2X , followed by UAS-Aβ42^BL33770 , UAS-Aβ42^11C39 , and UAS-Aβ42^H29.3 . Surprisingly, the reduction in survival during the development of these lines did not correlate with cell death or ROS levels. The flies which expressed UAS-Aβ42^11C39 or UAS-Aβ42^H29.3 experienced greatly reduced survival rates, although low levels of ROS or cell death were detected. Collectively, our results demonstrated that different Drosophila AD models show different phenotypic severity, and suggested that different transgenes may have different modes of cytotoxicity. Abbreviations: Aβ42: amyloid-β42; AD: Alzheimer's disease; UAS: upstream activation sequence.

Active zone proteins are transported via distinct mechanisms regulated by Par-1 kinase

Disruption of synapses underlies a plethora of neurodevelopmental and neurodegenerative disease. Presynaptic specialization called the active zone plays a critical role in the communication with postsynaptic neuron. While the role of many proteins at the active zones in synaptic communication is relatively well studied, very little is known about how these proteins are transported to the synapses. For example, are there distinct mechanisms for the transport of active zone components or are they all transported in the same transport vesicle? Is active zone protein transport regulated? In this report we show that overexpression of Par-1/MARK kinase, a protein whose misregulation has been implicated in Autism spectrum disorders (ASDs) and neurodegenerative disorders, lead to a specific block in the transport of an active zone protein component- Bruchpilot at Drosophila neuromuscular junctions. Consistent with a block in axonal transport, we find a decrease in number of active zones and reduced neurotransmission in flies overexpressing Par-1 kinase. Interestingly, we find that Par-1 acts independently of Tau-one of the most well studied substrates of Par-1, revealing a presynaptic function for Par-1 that is independent of Tau. Thus, our study strongly suggests that there are distinct mechanisms that transport components of active zones and that they are tightly regulated.

Synapses consist of pre- and postsynaptic partners. Proper function of active zones, a presynaptic component of synapse, is essential for efficacious neuronal communication. Disruption of neuronal communication is an early sign of both neurodevelopmental as well as neurodegenerative diseases. Since proteins that reside in active zones are used so frequently during the neuronal communication, they must be constantly replenished to maintain active zones. Axonal transport of these proteins plays an important role in replenishing these vital components necessary for the health of active zones. However, the mechanisms that transport components of active zones are not well understood. Our data suggest that there are distinct mechanisms that transport various active zone cargoes and this process is likely regulated by kinases. Further, our data show that disruption in the transport of one such active zone components causes reduced neuronal communication emphasizing the importance of the process of axonal transport of active zone protein(s) for neuronal communication. Understanding the processes that govern the axonal transport of active zone components will help dissect the initial stages of pathogenesis in both neurodevelopmental and neurodegenerative diseases.

Gender-Specific Expression of Ubiquitin-Specific Peptidase 9 Modulates Tau Expression and Phosphorylation: Possible Implications for Tauopathies

Public transcriptomic studies have shown that several genes display pronounced gender differences in their expression in the human brain, which may influence the manifestations and risk for neuronal disorders. Here, we apply a transcriptome-wide analysis to discover genes with gender-specific expression and significant alterations in public postmortem brain tissue from Alzheimer’s disease (AD) patients compared to controls. We identify the sex-linked ubiquitin-specific peptidase 9 (USP9) as an outstanding candidate gene with highly significant expression differences between the genders and male-specific underexpression in AD. Since previous studies have shown that USP9 can modulate the phosphorylation of the AD-associated protein MAPT, we investigate functional associations between USP9 and MAPT in further detail. After observing a high positive correlation between the expression of USP9 and MAPT in the public transcriptomics data, we show that USP9 knockdown results in significantly decreased MAPT expression in a DU145 cell culture model and a concentration-dependent decrease for the MAPT orthologs mapta and maptb in a zebrafish model. From the analysis of microarray and qRT-PCR experiments for the knockdown in DU145 cells and prior knowledge from the literature, we derive a data-congruent model for a USP9-dependent regulatory mechanism modulating MAPT expression via BACH1 and SMAD4. Overall, the analyses suggest USP9 may contribute to molecular gender differences observed in tauopathies and provide a new target for intervention strategies to modulate MAPT expression.

Aberrant Axonal Arborization of PDF Neurons Induced by Aβ42-Mediated JNK Activation Underlies Sleep Disturbance in an Alzheimer's Model

Impaired sleep patterns are common symptoms of Alzheimer's disease (AD). Cellular mechanisms underlying sleep disturbance in AD remain largely unknown. Here, using a Drosophila Aβ42 AD model, we show that Aβ42 markedly decreases sleep in a large population, which is accompanied with postdevelopmental axonal arborization of wake-promoting pigment-dispersing factor (PDF) neurons. The arborization is mediated in part via JNK activation and can be reversed by decreasing JNK signaling activity. Axonal arborization and impaired sleep are correlated in Aβ42 and JNK kinase hemipterous mutant flies. Image reconstruction revealed that these aberrant fibers preferentially project to pars intercerebralis (PI), a fly brain region analogous to the mammalian hypothalamus. Moreover, PDF signaling in PI neurons was found to modulate sleep/wake activities, suggesting that excessive release of PDF by these aberrant fibers may lead to the impaired sleep in Aβ42 flies. Finally, inhibition of JNK activation in Aβ42 flies restores nighttime sleep loss, decreases Aβ42 accumulation, and attenuates neurodegeneration. These data provide a new mechanism by which sleep disturbance could be induced by Aβ42 burden, a key initiator of a complex pathogenic cascade in AD.

Attenuation of synaptic toxicity and MARK4/PAR1-mediated Tau phosphorylation by methylene blue for Alzheimer's disease treatment

Alzheimer's disease (AD) is a neurodegenerative disease characterized by genotypic and phenotypic heterogeneity. Critical components of the two AD pathological pathways, Aβ-amyloidosis and Tauopathy, have been considered as therapeutic targets. Among them, much effort is focused on aberrant Tau phosphorylation and targeting Tau-phosphorylating kinases. Methylene blue (MB), a phenothiazine dye that crosses the blood-brain barrier, has been shown to hit multiple molecular targets involved in AD and have beneficial effects in clinical studies. Here we present evidence that microtubule affinity-regulating kinase (MARK4) is a novel target of MB. MB partially rescued the synaptic toxicity in Drosophila larva overexpressing PAR1 (MARK analog). In 293T culture, MB decreased MARK4-mediated Tau phosphorylation in a dose dependent manner. Further studies revealed a two-fold mechanism by MB including down-regulation of MARK4 protein level through ubiquitin-proteasome pathway and inhibition of MARK4 kinase activity in vitro. This study highlights the importance of MARK4 as a viable target for Tauopathy and provides fresh insight into the complex mechanism used by MB to treat AD.

The novel SH3 domain protein Dlish/CG10933 mediates fat signaling in Drosophila by binding and regulating Dachs

Much of the Hippo and planar cell polarity (PCP) signaling mediated by the Drosophila protocadherin Fat depends on its ability to change the subcellular localization, levels and activity of the unconventional myosin Dachs. To better understand this process, we have performed a structure-function analysis of Dachs, and used this to identify a novel and important mediator of Fat and Dachs activities, a Dachs-binding SH3 protein we have named Dlish. We found that Dlish is regulated by Fat and Dachs, that Dlish also binds Fat and the Dachs regulator Approximated, and that Dlish is required for Dachs localization, levels and activity in both wild type and fat mutant tissue. Our evidence supports dual roles for Dlish. Dlish tethers Dachs to the subapical cell cortex, an effect partly mediated by the palmitoyltransferase Approximated under the control of Fat. Conversely, Dlish promotes the Fat-mediated degradation of Dachs.

DOI:http://dx.doi.org/10.7554/eLife.16624.001

Tau phosphorylation at Alzheimer's disease-related Ser356 contributes to tau stabilization when PAR-1/MARK activity is elevated

Abnormal phosphorylation of the microtubule-associated protein tau is observed in many neurodegenerative diseases, including Alzheimer's disease (AD). AD-related phosphorylation of two tau residues, Ser262 and Ser356, by PAR-1/MARK stabilizes tau in the initial phase of mismetabolism, leading to subsequent phosphorylation events, accumulation, and toxicity. However, the relative contribution of phosphorylation at each of these sites to tau stabilization has not yet been elucidated. In a Drosophila model of human tau toxicity, we found that tau was phosphorylated at Ser262, but not at Ser356, and that blocking Ser262 phosphorylation decreased total tau levels. By contrast, when PAR-1 was co-overexpressed with tau, tau was hyperphosphorylated at both Ser262 and Ser356. Under these conditions, the protein levels of tau were significantly elevated, and prevention of tau phosphorylation at both residues was necessary to completely suppress this elevation. These results suggest that tau phosphorylation at Ser262 plays the predominant role in tau stabilization when PAR-1/MARK activity is normal, whereas Ser356 phosphorylation begins to contribute to this process when PAR-1/MARK activity is abnormally elevated, as in diseased brains.

Stabilization of Microtubule-Unbound Tau via Tau Phosphorylation at Ser262/356 by Par-1/MARK Contributes to Augmentation of AD-Related Phosphorylation and Aβ42-Induced Tau Toxicity

Abnormal accumulation of the microtubule-interacting protein tau is associated with neurodegenerative diseases including Alzheimer’s disease (AD). β-amyloid (Aβ) lies upstream of abnormal tau behavior, including detachment from microtubules, phosphorylation at several disease-specific sites, and self-aggregation into toxic tau species in AD brains. To prevent the cascade of events leading to neurodegeneration in AD, it is essential to elucidate the mechanisms underlying the initial events of tau mismetabolism. Currently, however, these mechanisms remain unclear. In this study, using transgenic Drosophila co-expressing human tau and Aβ, we found that tau phosphorylation at AD-related Ser262/356 stabilized microtubule-unbound tau in the early phase of tau mismetabolism, leading to neurodegeneration. Aβ increased the level of tau detached from microtubules, independent of the phosphorylation status at GSK3-targeted SP/TP sites. Such mislocalized tau proteins, especially the less phosphorylated species, were stabilized by phosphorylation at Ser262/356 via PAR-1/MARK. Levels of Ser262 phosphorylation were increased by Aβ42, and blocking this stabilization of tau suppressed Aβ42-mediated augmentation of tau toxicity and an increase in the levels of tau phosphorylation at the SP/TP site Thr231, suggesting that this process may be involved in AD pathogenesis. In contrast to PAR-1/MARK, blocking tau phosphorylation at SP/TP sites by knockdown of Sgg/GSK3 did not reduce tau levels, suppress tau mislocalization to the cytosol, or diminish Aβ-mediated augmentation of tau toxicity. These results suggest that stabilization of microtubule-unbound tau by phosphorylation at Ser262/356 via the PAR-1/MARK may act in the initial steps of tau mismetabolism in AD pathogenesis, and that such tau species may represent a potential therapeutic target for AD.

Alzheimer’s disease (AD) is the most common cause of dementia resulting from progressive neuron loss. Two proteins, β-amyloid (Aβ) and tau, accumulate in AD brains and are involved in AD pathogenesis. In healthy neurons, tau binds to microtubules to regulate its stability; in AD brains, however, tau is detached from microtubules and phosphorylated at multiple sites. Such abnormal tau behavior, which is likely to be triggered by Aβ, results in generation of pathological tau species that mediate neuron loss. However, the detailed mechanisms underlying this event remain incompletely understood. Using transgenic flies expressing human tau and Aβ as a model system, we found that tau phosphorylation at specific AD-related sites stabilized microtubule-unbound tau in the early phase of tau mismetabolism to generate toxic tau species. Moreover, this process is critical for Aβ to promote subsequent tau phosphorylation and neurodegeneration. Our results reveal a critical step in the initiation of tau mismetabolism, and this process may represent a potential therapeutic target for AD.

Deletion of endogenous Tau proteins is not detrimental in Drosophila

Human Tau (hTau) is a highly soluble and natively unfolded protein that binds to microtubules within neurons. Its dysfunction and aggregation into insoluble paired helical filaments is involved in the pathogenesis of Alzheimer’s disease (AD), constituting, together with accumulated β-amyloid (Aβ) peptides, a hallmark of the disease. Deciphering both the loss-of-function and toxic gain-of-function of hTau proteins is crucial to further understand the mechanisms leading to neurodegeneration in AD. As the fruit fly Drosophila melanogaster expresses Tau proteins (dTau) that are homologous to hTau, we aimed to better comprehend dTau functions by generating a specific tau knock-out (KO) fly line using homologous recombination. We observed that the specific removal of endogenous dTau proteins did not lead to overt, macroscopic phenotypes in flies. Indeed, survival, climbing ability and neuronal function were unchanged in tau KO flies. In addition, we did not find any overt positive or negative effect of dTau removal on human Aβ-induced toxicity. Altogether, our results indicate that the absence of dTau proteins has no major functional impact on flies, and suggests that our tau KO strain is a relevant model to further investigate the role of dTau proteins in vivo, thereby giving additional insights into hTau functions.

Cellular and molecular modifier pathways in tauopathies: the big picture from screening invertebrate models

Abnormal tau accumulations were observed and documented in post-mortem brains of patients affected by Alzheimer's disease (AD) long before the identification of mutations in the Microtubule-associated protein tau (MAPT) gene, encoding the tau protein, in a different neurodegenerative disease called Frontotemporal dementia and Parkinsonism linked to chromosome 17 (FTDP-17). The discovery of mutations in the MAPT gene associated with FTDP-17 highlighted that dysfunctions in tau alone are sufficient to cause neurodegeneration. Invertebrate models have been diligently utilized in investigating tauopathies, contributing to the understanding of cellular and molecular pathways involved in disease etiology. An important discovery came with the demonstration that over-expression of human tau in Drosophila leads to premature mortality and neuronal dysfunction including neurodegeneration, recapitulating some key neuropathological features of the human disease. The simplicity of handling invertebrate models combined with the availability of a diverse range of experimental resources make these models, in particular Drosophila a powerful invertebrate screening tool. Consequently, several large-scale screens have been performed using Drosophila, to identify modifiers of tau toxicity. The screens have revealed not only common cellular and molecular pathways, but in some instances the same modifier has been independently identified in two or more screens suggesting a possible role for these modifiers in regulating tau toxicity. The purpose of this review is to discuss the genetic modifier screens on tauopathies performed in Drosophila and C. elegans models, and to highlight the common cellular and molecular pathways that have emerged from these studies. Here, we summarize results of tau toxicity screens providing mechanistic insights into pathological alterations in tauopathies. Key pathways or modifiers that have been identified are associated with a broad range of processes including, but not limited to, phosphorylation, cytoskeleton organization, axonal transport, regulation of cellular proteostasis, transcription, RNA metabolism, cell cycle regulation, and apoptosis. We discuss the utility and application of invertebrate models in elucidating the cellular and molecular functions of novel and uncharacterized disease modifiers identified in large-scale screens as well as for investigating the function of genes identified as risk factors in genome-wide association studies from human patients in the post-genomic era. In this review, we combined and summarized several large-scale modifier screens performed in invertebrate models to identify modifiers of tau toxicity. A summary of the screens show that diverse cellular processes are implicated in the modification of tau toxicity. Kinases and phosphatases are the most predominant class of modifiers followed by components required for cellular proteostasis and axonal transport and cytoskeleton elements.

La FAM fatale: USP9X in development and disease

Deubiquitylating enzymes (DUBs), act downstream of ubiquitylation. As such, these post-post-translational modifiers function as the final arbitrators of a protein substrate’s ubiquitylation status, thus regulating its fate. In most instances, DUBs moderate the absolute level of a substrate, its locality or activity, rather than being an “all-or-none” phenomenon. Yet, disruption of this quantitative regulation can produce dramatic qualitative differences. The ubiquitin-specific protease 9X (USP9X/FAM) is a substrate-specific DUB, which displays an extraordinarily high level of sequence conservation from Drosophila to mammals. It is primarily the recent revelations of USP9X’s pivotal role in human cancers, both as oncogene or tumour suppressor, in developmental disorders including intellectual disability, epilepsy, autism and developmental delay that has led to a subsequent re-examination of its molecular and cellular functions. Results from experimental animal models have implicated USP9X in neurodegeneration, including Parkinson’s and Alzheimer’s disease, as well as autoimmune diseases. In this review, we describe the current and accumulated knowledge on the molecular, cellular and developmental aspects of USP9X function within the context of the biological consequences during normal development and disease.

Activation of mTOR: a culprit of Alzheimer's disease?

Alzheimer's disease (AD) is characterized by cognitive impairment in clinical presentation, and by β-amyloid (Aβ) production and the hyper-phosphorylation of tau in basic research. More highlights demonstrate that the activation of the mammalian target of rapamycin (mTOR) enhances Aβ generation and deposition by modulating amyloid precursor protein (APP) metabolism and upregulating β- and γ-secretases. mTOR, an inhibitor of autophagy, decreases Aβ clearance by scissoring autophagy function. mTOR regulates Aβ generation or Aβ clearance by regulating several key signaling pathways, including phosphoinositide 3-kinase (PI3-K)/protein kinase B (Akt), glycogen synthase kinase 3 [GSK-3], AMP-activated protein kinase (AMPK), and insulin/insulin-like growth factor 1 (IGF-1). The activation of mTOR is also a contributor to aberrant hyperphosphorylated tau. Rapamycin, the inhibitor of mTOR, may mitigate cognitive impairment and inhibit the pathologies associated with amyloid plaques and neurofibrillary tangles by promoting autophagy. Furthermore, the upstream and downstream components of mTOR signaling are involved in the pathogenesis and progression of AD. Hence, inhibiting the activation of mTOR may be an important therapeutic target for AD.

The F-box protein Slmb restricts the activity of aPKC to polarize epithelial cells

The Par-3/Par-6/aPKC complex is the primary determinant of apical polarity in epithelia across animal species, but how the activity of this complex is restricted to allow polarization of the basolateral domain is less well understood. In Drosophila, several multiprotein modules antagonize the Par complex through a variety of means. Here we identify a new mechanism involving regulated protein degradation. Strong mutations in supernumerary limbs (slmb), which encodes the substrate adaptor of an SCF-class E3 ubiquitin ligase, cause dramatic loss of polarity in imaginal discs accompanied by tumorous proliferation defects. Slmb function is required to restrain apical aPKC activity in a manner that is independent of endolysosomal trafficking and parallel to the Scribble module of junctional scaffolding proteins. The involvement of the Slmb E3 ligase in epithelial polarity, specifically limiting Par complex activity to distinguish the basolateral domain, points to parallels with polarization of the C. elegans zygote.

Slmb antagonises the aPKC/Par-6 complex to control oocyte and epithelial polarity

The Drosophila anterior-posterior axis is specified when the posterior follicle cells signal to polarise the oocyte, leading to the anterior/lateral localisation of the Par-6/aPKC complex and the posterior recruitment of Par-1, which induces a microtubule reorganisation that localises bicoid and oskar mRNAs. Here we show that oocyte polarity requires Slmb, the substrate specificity subunit of the SCF E3 ubiquitin ligase that targets proteins for degradation. The Par-6/aPKC complex is ectopically localised to the posterior of slmb mutant oocytes, and Par-1 and oskar mRNA are mislocalised. Slmb appears to play a related role in epithelial follicle cells, as large slmb mutant clones disrupt epithelial organisation, whereas small clones show an expansion of the apical domain, with increased accumulation of apical polarity factors at the apical cortex. The levels of aPKC and Par-6 are significantly increased in slmb mutants, whereas Baz is slightly reduced. Thus, Slmb may induce the polarisation of the anterior-posterior axis of the oocyte by targeting the Par-6/aPKC complex for degradation at the oocyte posterior. Consistent with this, overexpression of the aPKC antagonist Lgl strongly rescues the polarity defects of slmb mutant germline clones. The role of Slmb in oocyte polarity raises an intriguing parallel with C. elegans axis formation, in which PAR-2 excludes the anterior PAR complex from the posterior cortex to induce polarity, but its function can be substituted by overexpressing Lgl.

miR-124 controls Drosophila behavior and is required for neural development

MicroRNA-124 (miR-124) is an evolutionarily conserved, small, noncoding RNA molecule that participates in the central nervous system (CNS) developmental control of gene expression. In the current study, we found that Drosophila embryos lacking the mir-124 gene did not exhibit detectable defects in axon growth or CNS development. On the other hand, adult mutants showed severe problems in locomotion, flight, and female fertility. Furthermore, the deficits that we observed in the adult stage could be marginally rescued with elav-GAL4 driven expression of miR-124, making elav-GAL4 an excellently simulated driver to induce entopic over-expression of miR-124. Further developmental assessment in the third instar larval neuromuscular junction (NMJ) and dendritic arborization (DA) neurons was performed with miR-124 knock outs, flies over-expressing miR-124, and rescue models. Typically, the absence and over-abundance of a molecular signal exerts opposite effects on development or phenotype. However, we determined that both miR-124 knock-outs and over-expressing flies displayed reduced NMJ 6/7 bouton number and branch length. Similarly, reduced ddaE branching numbers were observed between the two mutant lines. As to ddaF, we found that branching number was not influenced by mir-124 knock out, but was statistically reduced by miR-124 over-expression. While we were not able to determine any causal relationship between behavioral defects and dysplasia of NMJs or DA neurons, there were some accompanying relationships among behavioral phenotypes, NMJ abnormalities, and ddaE/ddaF dendritic branching which were all controlled by miR-124.

Synaptic abnormalities in a Drosophila model of Alzheimer's disease

Alzheimer's disease (AD) is an age-related neurodegenerative disease characterized by memory loss and decreased synaptic function. Advances in transgenic animal models of AD have facilitated our understanding of this disorder, and have aided in the development, speed and efficiency of testing potential therapeutics. Recently, we have described the characterization of a novel model of AD in the fruit fly, Drosophila melanogaster, where we expressed the human AD-associated proteins APP and BACE in the central nervous system of the fly. Here we describe synaptic defects in the larval neuromuscular junction (NMJ) in this model. Our results indicate that expression of human APP and BACE at the larval NMJ leads to defective larval locomotion behavior, decreased presynaptic connections, altered mitochondrial localization in presynaptic motor neurons and decreased postsynaptic protein levels. Treating larvae expressing APP and BACE with the γ-secretase inhibitor L-685,458 suppresses the behavioral defects as well as the pre- and postsynaptic defects. We suggest that this model will be useful to assess and model the synaptic dysfunction normally associated with AD, and will also serve as a powerful in vivo tool for rapid testing of potential therapeutics for AD.

The synaptic maintenance problem: membrane recycling, Ca2+ homeostasis and late onset degeneration

Most neurons are born with the potential to live for the entire lifespan of the organism. In addition, neurons are highly polarized cells with often long axons, extensively branched dendritic trees and many synaptic contacts. Longevity together with morphological complexity results in a formidable challenge to maintain synapses healthy and functional. This challenge is often evoked to explain adult-onset degeneration in numerous neurodegenerative disorders that result from otherwise divergent causes. However, comparably little is known about the basic cell biological mechanisms that keep normal synapses alive and functional in the first place. How the basic maintenance mechanisms are related to slow adult-onset degeneration in different diseasesis largely unclear. In this review we focus on two basic and interconnected cell biological mechanisms that are required for synaptic maintenance: endomembrane recycling and calcium (Ca²⁺) homeostasis. We propose that subtle defects in these homeostatic processes can lead to late onset synaptic degeneration. Moreover, the same basic mechanisms are hijacked, impaired or overstimulated in numerous neurodegenerative disorders. Understanding the pathogenesis of these disorders requires an understanding of both the initial cause of the disease and the on-going changes in basic maintenance mechanisms. Here we discuss the mechanisms that keep synapses functional over long periods of time with the emphasis on their role in slow adult-onset neurodegeneration.

The importance of tau phosphorylation for neurodegenerative diseases

Fibrillar deposits of highly phosphorylated tau are a key pathological feature of several neurodegenerative tauopathies including Alzheimer's disease (AD) and some frontotemporal dementias. Increasing evidence suggests that the presence of these end-stage neurofibrillary lesions do not cause neuronal loss, but rather that alterations to soluble tau proteins induce neurodegeneration. In particular, aberrant tau phosphorylation is acknowledged to be a key disease process, influencing tau structure, distribution, and function in neurons. Although typically described as a cytosolic protein that associates with microtubules and regulates axonal transport, several additional functions of tau have recently been demonstrated, including roles in DNA stabilization, and synaptic function. Most recently, studies examining the trans-synaptic spread of tau pathology in disease models have suggested a potential role for extracellular tau in cell signaling pathways intrinsic to neurodegeneration. Here we review the evidence showing that tau phosphorylation plays a key role in neurodegenerative tauopathies. We also comment on the tractability of altering phosphorylation-dependent tau functions for therapeutic intervention in AD and related disorders.