SPOP loss of function protects against tauopathy

Unraveling Molecular Targets for Neurodegenerative Diseases Through Caenorhabditis elegans Models

Neurodegenerative diseases (NDDs), including Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), Huntington's disease (HD), and prion disease, represent a group of age-related disorders that pose a growing and formidable challenge to global health. Despite decades of extensive research that has uncovered key genetic factors and biochemical pathways, the precise molecular mechanisms underlying these diseases and effective therapeutic strategies remain elusive. Caenorhabditis elegans (C. elegans) has emerged as a powerful model organism for studying NDDs due to its unique biological features such as genetic tractability, conserved molecular pathways, and ease of high-throughput screening. This model provides an exceptional platform for identifying molecular targets associated with NDDs and developing novel therapeutic interventions. This review highlights the critical role of C. elegans in elucidating the complex molecular mechanisms of human NDDs, with a particular focus on recent advancements and its indispensable contributions to the discovery of molecular targets and therapeutic strategies for these NDDs.

Exposure to Manganese Induces Autophagy-Lysosomal Pathway Dysfunction-Mediated Tauopathy by Activating the cGAS-STING Pathway in the Brain

Manganese (Mn) exposure leads to pathological accumulation of Tau-associated neurodegenerative disease and has become a major public health concern. However, the precise mechanism underlying this effect remains unclear. Here, the mechanism by which Mn induces dysfunction of autophagy-lysosomal pathway-mediated tauopathy by activating the cGAS-STING pathway was explored both in vitro and in vivo. Mn exposure induced tauopathy in microglia and in mice while activating the cGAS-STING pathway, inducing type I interferon production, and impairing the degradation function of the autophagy-lysosomal pathway. Importantly, inactivation of the cGAS-STING pathway rescued the degradation activity of the autophagy-lysosomal pathway, while tauopathy was markedly attenuated, as shown in both cGAS-knockout and STING-knockout BV2 microglia and in mice. Moreover, the autophagy inhibitor 3-methyladenine (3-MA) restored the impaired degradation activity of the autophagy-lysosomal pathway by inactivating the cGAS-STING pathway, thereby clearing Tau aggregation. Taken together, these results indicate that Mn exposure induces tauopathy by impairing the function of the autophagy-lysosomal pathway through the activation of the cGAS-STING pathway. Thus, this study identifies a novel mechanism by which Mn exposure induces Tau aggregation, which in turn triggers potential neurotoxicity, providing a foundation for future drug target research.

TMEM106B C-terminal fragments aggregate and drive neurodegenerative proteinopathy in transgenic Caenorhabditis elegans

INTRODUCTION

Genetic variation in the lysosomal and transmembrane protein 106B (TMEM106B) modifies risk for several neurodegenerative disorders, especially frontotemporal lobar degeneration (FTLD). The C-terminal (CT) domain of TMEM106B occurs as fibrillar protein deposits in the brains of dementia patients.

METHODS

To determine the TMEM CT aggregation propensity and neurodegenerative potential, we generated transgenic Caenorhabditis elegans expressing the human TMEM CT fragment aggregating in FTLD cases.

RESULTS

Pan-neuronal expression of human TMEM CT in C. elegans causes severe neuronal dysfunction driving neurodegeneration.  Cytosolic aggregation of TMEM CT proteins accompanied by behavioral dysfunction and neurodegeneration. Loss of pgrn-1 did not modify TMEM CT phenotypes suggesting TMEM CT aggregation occurs downstream of PGRN loss of function. The mechanistic drivers of TMEM106B proteinopathy appear distinct from known modifiers of tauopathy.

DISCUSSION

Our data demonstrate that TMEM CT aggregation can kill neurons. TMEM106B transgenic C.elegans provide a useful model for characterizing TMEM106B proteinopathy-mediated neurodegeneration in FTLD.

HIGHLIGHTS

Pan-neuronal expression of human TMEM106B C-terminal fragments (TMEM CT) in C. elegans neurons drives a suite of disease-related phenotypes useful for modeling the molecular and cellular features of TMEM106B neuropathology. TMEM CT expression results in extensive TMEM aggregation and accumulation of highly detergent insoluble protein species. TMEM CT expression causes moderate to severe neuronal dysfunction dependent on TMEM CT abundance as measured by stereotypical behavioral readouts. TMEM CT expression drives significant neurodegenerative changes. Dendra2 tagged TMEM exhibits similar properties to untagged TMEM allowing ready visualization of the protein. TMEM CT aggregates accumulate adjacent to but not within lysosomes. PGRN loss of function does not impact TMEM CT toxicity. Modifiers of tau and TDP-43 proteinopathies have little impact on TMEM CT-related neurodegenerative phenotypes.

Precision Prediction of Alzheimer's Disease: Integrating Mitochondrial Energy Metabolism and Immunological Insights

Alzheimer's disease (AD), a prevalent neurodegenerative disorder, is characterized by mitochondrial dysfunction and immune dysregulation. This study is aimed at developing a risk prediction model for AD by integrating multi-omics data and exploring the interplay between mitochondrial energy metabolism-related genes (MEMRGs) and immune cell dynamics. We integrated four GEO datasets (GSE132903, GSE29378, GSE33000, GSE5281) for differential gene expression analysis, functional enrichment, and weighted gene co-expression network analysis (WGCNA). We identified two key gene modules (turquoise and magenta) significantly correlated with AD. Subsequently, we constructed a risk prediction model incorporating five MEMRGs (MRPL15, RBP4, ABCA1, MPV17, and MRPL37) and clinical factors using LASSO regression. The model demonstrated robust predictive performance (AUC > 0.815) in both internal and external validation (GSE44770) cohorts. Downregulation of MRPL15, RBP4, MPV17, and MRPL37 in AD brain regions (validated using AlzData and qRT-PCR) suggests impaired mitochondrial function. Conversely, ABCA1 upregulation may represent a compensatory response. Furthermore, significant differences in immune cell proportions, particularly gamma delta T cells (p = 0.002) and activated CD4 memory T cells (p = 0.027), were found between AD and non-demented samples. We observed significant correlations between MEMRG expression and specific immune cell fractions, indicating a potential link between mitochondrial dysfunction and immune dysregulation in AD. Our study provides a reliable risk prediction model for AD and highlights the crucial roles of MEMRGs and immune responses in disease pathogenesis, offering potential targets for therapeutic interventions.

Endoplasmic reticulum unfolded protein response transcriptional targets of XBP-1s mediate rescue from tauopathy

Pathological tau disrupts protein homeostasis (proteostasis) within neurons in Alzheimer's disease (AD) and related disorders. We previously showed constitutive activation of the endoplasmic reticulum unfolded protein response (UPRER) transcription factor XBP-1s rescues tauopathy-related proteostatic disruption in a tau transgenic Caenorhabditis elegans (C. elegans) model of human tauopathy. XBP-1s promotes clearance of pathological tau, and loss of function of the ATF-6 branch of the UPRER prevents XBP-1s rescue of tauopathy in C. elegans. We conducted transcriptomic analysis of tau transgenic and xbp-1s transgenic C. elegans and found 116 putative target genes significantly upregulated by constitutively active XBP-1s. Among these were five candidate XBP-1s target genes with human orthologs and a previously known association with ATF6 (csp-1, dnj-28, hsp-4, ckb-2, and lipl-3). We examined the functional involvement of these targets in XBP-1s-mediated tauopathy suppression and found loss of function in any one of these genes completely disrupts XBP-1s suppression of tauopathy. Further, we demonstrate upregulation of HSP-4, C. elegans BiP, partially rescues tauopathy independent of other changes in the transcriptional network. Understanding how the UPRER modulates pathological tau accumulation will inform neurodegenerative disease mechanisms and direct further study in mammalian systems with the long-term goal of identifying therapeutic targets in human tauopathies.

TMEM106B C-terminal fragments aggregate and drive neurodegenerative proteinopathy

Genetic variation in the lysosomal and transmembrane protein 106B (TMEM106B) modifies risk for a diverse range of neurodegenerative disorders, especially frontotemporal lobar degeneration (FTLD) with progranulin (PGRN) haplo-insufficiency, although the molecular mechanisms involved are not yet understood. Through advances in cryo-electron microscopy (cryo-EM), homotypic aggregates of the C-Terminal domain of TMEM106B (TMEM CT) were discovered as a previously unidentified cytosolic proteinopathy in the brains of FTLD, Alzheimer's disease, progressive supranuclear palsy (PSP), and dementia with Lewy bodies (DLB) patients. While it remains unknown what role TMEM CT aggregation plays in neuronal loss, its presence across a range of aging related dementia disorders indicates involvement in multi-proteinopathy driven neurodegeneration. To determine the TMEM CT aggregation propensity and neurodegenerative potential, we characterized a novel transgenic C. elegans model expressing the human TMEM CT fragment constituting the fibrillar core seen in FTLD cases. We found that pan-neuronal expression of human TMEM CT in C. elegans causes neuronal dysfunction as evidenced by behavioral analysis. Cytosolic aggregation of TMEM CT proteins accompanied the behavioral dysfunction driving neurodegeneration, as illustrated by loss of GABAergic neurons. To investigate the molecular mechanisms driving TMEM106B proteinopathy, we explored the impact of PGRN loss on the neurodegenerative effect of TMEM CT expression. To this end, we generated TMEM CT expressing C. elegans with loss of pgrn-1, the C. elegans ortholog of human PGRN. Neither full nor partial loss of pgrn-1 altered the motor phenotype of our TMEM CT model suggesting TMEM CT aggregation occurs downstream of PGRN loss of function. We also tested the ability of genetic suppressors of tauopathy to rescue TMEM CT pathology. We found that genetic knockout of spop-1, sut-2, and sut-6 resulted in weak to no rescue of proteinopathy phenotypes, indicating that the mechanistic drivers of TMEM106B proteinopathy may be distinct from tauopathy. Taken together, our data demonstrate that TMEM CT aggregation can kill neurons. Further, expression of TMEM CT in C. elegans neurons provides a useful model for the functional characterization of TMEM106B proteinopathy in neurodegenerative disease.

Transgenic Dendra2::tau expression allows in vivo monitoring of tau proteostasis in Caenorhabditis elegans

Protein homeostasis is perturbed in aging-related neurodegenerative diseases called tauopathies, which are pathologically characterized by aggregation of the microtubule-associated protein tau (encoded by the human MAPT gene). Transgenic Caenorhabditis elegans serve as a powerful model organism to study tauopathy disease mechanisms, but moderating transgenic expression level has proven problematic. To study neuronal tau proteostasis, we generated a suite of transgenic strains expressing low, medium or high levels of Dendra2::tau fusion proteins by comparing integrated multicopy transgene arrays with single-copy safe-harbor locus strains generated by recombinase-mediated cassette exchange. Multicopy Dendra2::tau strains exhibited expression level-dependent neuronal dysfunction that was modifiable by known genetic suppressors or an enhancer of tauopathy. Single-copy Dendra2::tau strains lacked distinguishable phenotypes on their own but enabled detection of enhancer-driven neuronal dysfunction. We used multicopy Dendra2::tau strains in optical pulse-chase experiments measuring tau turnover in vivo and found that Dendra2::tau turned over faster than the relatively stable Dendra2. Furthermore, Dendra2::tau turnover was dependent on the protein expression level and independent of co-expression with human TDP-43 (officially known as TARDBP), an aggregating protein interacting with pathological tau. We present Dendra2::tau transgenic C. elegans as a novel tool for investigating molecular mechanisms of tau proteostasis.

Sut-6/NIPP1 modulates tau toxicity

Neurodegenerative diseases exhibiting the pathological accumulation of tau such as Alzheimer's disease and related disorders still have no disease-modifying treatments and the molecular mechanisms of neurodegeneration remain unclear. To discover additional suppressor of tauopathy (sut) genes that mediate or modulate the toxicity of pathological tau, we performed a classical genetic screen using a tau transgenic Caenorhabditis elegans model. From this screen, we identified the suppressing mutation W292X in sut-6, the C. elegans homolog of human NIPP1, which truncates the C-terminal RNA-binding domain. Using CRISPR-based genome editing approaches, we generated null and additional C-terminally truncated alleles in sut-6 and found that loss of sut-6 or sut-6(W292X) suppresses tau-induced behavioral locomotor deficits, tau protein accumulation and neuron loss. The sut-6(W292X) mutation showed stronger and semi-dominant suppression of tau toxicity while sut-6 deletion acted recessively. Neuronal overexpression of SUT-6 protein did not significantly alter tau toxicity, but neuronal overexpression of SUT-6 W292X mutant protein reduced tau-mediated deficits. Epistasis studies showed tauopathy suppression by sut-6 occurs independent of other known nuclear speckle-localized suppressors of tau such as sut-2, aly-1/aly-3 and spop-1. In summary, we have shown that sut-6/NIPP1 modulates tau toxicity and found a dominant mutation in the RNA-binding domain of sut-6 which strongly suppresses tau toxicity. This suggests that altering RNA-related functions of SUT-6/NIPP1 instead of complete loss of SUT-6/NIPP1 will provide the strongest suppression of tau.