Alzheimer’s disease (AD) is a neurodegenerative disease pathologically characterized by the progressive buildup of two types of proteinaceous deposits in the brain: amyloid β (Aβ) and tau tangles. Whereas therapeutic strategies targeting Aβ have largely dominated, they have shown only modest clinical benefits; consequently, the early pathogenic cascade of AD remains to be studied intensively [1]. Recent advances have linked AD pathology to disruptions in calcium signaling, involving multiple cellular pathways including mitochondrial function and the autophagy-lysosome system [2]. However, selectively targeting calcium homeostasis remains a substantial challenge. In this context, Griffioen’s team has proposed a self-reinforcing amplification between the cytoplasmic calcium concentration, [Ca2+]cyto, and AD pathology, such that decreasing elevated [Ca2+]cyto might offer neuroprotective benefits [3]. Their subsequent research, published in Science in May 2024, introduces ReS19-T, a small-molecule that stabilizes septin filaments, thereby specifically restoring calcium homeostasis under pathological conditions [4].
As reported in Science by Princen et al., the researchers developed a cell-based screening assay that mimics tau- and Ca2+-induced toxicity in human neuroblastoma cells, which they subsequently used to identify ReS19-T. This compound regulates intracellular calcium ion concentrations exclusively in the pathological state. Comprehensive techniques such as mammalian cell three-hybrid screening, flow-induced dispersion analysis, and RNA interference silencing confirmed that SEPT6 family members, particularly SEPT6, are high-affinity targets of ReS19-T. SEPT6, a key regulator of store-operated calcium entry (SOCE), is a cytoskeletal component that assembles into cellular microfilaments together with SEPT2 and SEPT7 [5]. REM127, the most potent compound in the ReS19-T series, was found to selectively block tau-dependent pathological forms of SOCE in a concentration-dependent manner, thus suggesting a new avenue for therapeutic intervention (Figure 1).
Ligand Gaussian Accelerated Molecular Dynamics (LiGaMD) analysis indicated that REM127 has the strongest affinity for the SEPT 6/7 dimer. Moreover, AlphaFold 2.0 simulations [6] have revealed that REM127 aligns along the mouse SEPT6/7 interface, contacting multiple amino acids in SEPT6 and SEPT7, thus affecting septal filament assembly and restoring the structural stability disrupted by pathological tau proteins. The therapeutic effects of ReS19-T compounds, including decreasing Aβ plaque deposition and abnormal tau protein accumulation, were supported by immunohistochemistry, electroencephalography, water maze tests, and other experiments in AD transgenic mouse models with various pathological manifestations and patient-derived induced pluripotent stem cell neuron models.
Intriguingly, studies by Lobato-Márquez et al., using cryo-electron tomography coupled with in vitro septin cage reconstitution techniques, have explored the SEP2/6/7 hexamer’s role in maintaining immune function [7]. That work has prompted questions regarding whether ReS19-T might affect immune function and the immune response. Moreover, the experimental results of Princen et al. indicated that this compound does not affect diffuse amyloid plaques, which correlate with astrocytes in patients with AD [4]. However, the formation of dense core amyloid plaques, which correlate with microglia [8], was alleviated by REM127. Further research is necessary to understand why and how ReS19-T influences dense core amyloid plaques exclusively. The findings from Princen et al. suggest that aberrant calcium entry is an integral component of tau pathology and indicate that the inhibitory effects of ReS19-T on Aβ pathology (Aβ plaques and Aβ42 amount) are at least partly due to normalizing tau-induced calcium dysregulation. However, given that tau is hyperphosphorylated in many APP models (including those used by Princen et al.), questions persist regarding whether ReS19-T might be effective in APP models if tau is knocked out or in an amyloid-only model. SOCE has two main components; beyond the Stim1 protein highlighted by Princen et al., Orai, which shares a related role in the maintenance of calcium ion homeostasis, is also of interest. Orai2 is expressed in the human brain at a slightly lower level than Stim1, and Orai 1-mediated SOCE drives the production of inflammatory cytokines and the proliferation of microglial cells [9]. Whether calcium ions enter cells via Orai and whether ReS-19T acts on Orai, and is required for its selectivity, remain to be clarified. The SOCE pathway is mediated by Stim1’s activation of Orai1 and TRPC1, and Orai1 dysfunction has been shown to lead to profound changes in brain function [10]. The TRPC-specific inhibitor SKF-96365 is highly selective, and its relationship with AD remains to be investigated [11]. Future studies might use SKF-96365 to validate the therapeutic effect of Res19-T. In conclusion, the study by Princen et al. underscores the potential of calcium-lowering interventions in AD therapy by exploring the actions of ReS19-T and its target, SEPT6. These findings provide crucial insights into the relationship between calcium homeostasis and the self-amplifying cycle of AD pathology, thus suggesting that ReS19-T might serve as an alternative to Aβ-targeted therapies. Although the results are promising, translating these findings to human patients will require extensive testing to confirm treatment efficacy and safety. Further investigations are also required to understand how these compounds might affect other cellular functions mediated by the septin cytoskeleton. Nonetheless, this research represents a substantial step toward developing more effective treatments for AD, by focusing on cellular mechanisms beyond the traditional targets of Aβ and tau pathologies.
Schematic depicting the roles of ReS19-T and Septin2/6/7 in calcium homeostasis in AD.
ReS19-T stabilizes the structure of septin2/6/7, thereby selectively blocking the tau-activated pathological disfunction of SOCE and restoring calcium homeostasis. These findings suggest an emerging target for therapeutic intervention in AD.