Small heat shock protein 22 preserves neuronal function in a murine model of tauopathy

Proteinopathies is a family of diseases associated with the pathological aggregation of protein. There are a multitude of proteinopathies, like tauopathy and synucleinopathy that contribute to neurodegenerative diseases like Alzheimer’s disease (AD), Creutzfeldt-Jakob, Pick’s disease, Frontotemporal dementia, and Parkinson’s disease (PD). Currently, one focus of research in the field is mitigating aggregation-prone proteins contributing to disease state. One method of targeting aggregation is the use of chaperones. Chaperones are molecular machinery that help maintain homeostasis in the cells, through various roles and mechanisms, one of those methods is to regulate protein aggregation. Chaperones can achieve this by affecting protein-protein interactions, protein degradation, protein clearance, protein refolding, protein disaggregation, and even force protein aggregation to insolubility in cases where there is less pathology in an insoluble protein state, as seen before with tau. Tau is a microtubule-associated protein responsible for offering stability to microtubules while maintaining their dynamic nature. Tau when unbound to the microtubule, either through stress induced phosphorylation, physical trauma, or non-native tau interaction, causes tau to pathologically aggregate and cause cytotoxicity, this disease progression is known as a tauopathy. In the first part of the study, we highlight the diversity and function of small heat shock proteins (sHsp) and highlight their connections to proteinopathies, including tauopathies. sHsp are a family of ten molecular chaperones that are adenine triphosphate (ATP) independent, some are tissue specific and others are ubiquitously expressed. Overall, most sHsps have holdase activity, allowing them to interact with aggregation-prone proteins and prevent further pathology. sHsp are not limited to just holdase activity, they have also shown other functions in the cell, like stress granule formation and proteostasis in general, as seen with HSPB1 and HSPB8. We also learned sHsp members can regulate their activity through self-interactions and cross sHsp member interactions. What does remain a gap in knowledge is how sHsp members are able to affect pathways and regulators dysregulated by proteinopathies and reduce toxicity. This leads us to the second part of the study where we investigated how HSPB8, also known as Hsp22, when overexpressed can protect a tauopathic brain. We tested this by overexpressing Hsp22 (wtHsp22), a phosphomimic mutant (mtHsp22), and GFP in the hippocampus and frontal cortex of rTg4510 tau transgenic mice. We showed through the radial arm water maze (RAWM), mice overexpressing mtHsp22 maintain spatial reversal learning and memory. Using long term potentiation (LTP), we demonstrated Hsp22 overexpression increased synaptic plasticity in rTg4510 mice. Tau levels and associated pathogenic parkers were then tested through immunohistochemistry and immunoblotting and there were no significant differences between overexpressed Hsp22 and control mice. This was interesting since phenotypic deficits that were rescued in RAWM and LTP have been related to tau accumulation, leading us to believe Hsp22 overexpression could be protecting the brain from other tau-mediated deficits. To examine this, we decided to look at overall neuronal count, which revealed rTg4510 mice overexpressing Hsp22 had a higher maintenance of neurons. To understand how Hsp22 was protecting and preventing toxicity in the brain, we decided to look at the proteome of the rTg4510 using mass spectrometry (MS). This revealed Hsp22 overexpression was able to regulate key pathways and regulators involved with neuronal viability, synaptogenesis, and cellular metabolism that were dysregulated by the rTg4510. Overall, this part of the study demonstrated Hsp22 can protect and rescue tauopathy induced toxicity independently of tau. However, what remains a gap in knowledge is how Hsp22 is modulating these pathways and regulators in the rTg4510, and how Hsp22 holdase activity contributes to such rescue. To address this gap more research needs to be done on tau and related models to understand the relationship between tau dysregulation and Hsp22 rescue. This leads us to the final part of the study where we look at tau and different tauopathy models and how they regulate the proteome and transcriptome when compared to wildtype mice. For this we used datasets curated by Ingenuity Pathway Analysis (IPA) and compared tauopathy models, rTg4510 (P301L 4R0N strain) and TPR50 (P301S 4R2N strain), which revealed overlapping dysregulation of the same key pathways and regulators involved primarily in neuronal viability and synaptogenesis. We also showed there were differences in expression levels of regulators involved in protein stability, neuronal death, and cognitive regulation, demonstrating isoforms and modifications like mutations or post-translational modifications can have distinct effects on protein networks and pathology. However, this biology needs to be further studied to have better therapeutic targets and develop better disease models. Overall, this work demonstrates sHsp members, specifically Hsp22, can be great targets for restoring the deficits found in the neurodegenerative brain, which revealed a novel way of alleviating stress and cytotoxicity induced by tauopathy and possibly other proteinopathies