HIV and Cocaine exposure promote Tau phosphorylation through RSK-1 in a GSK3β-independent manner.

HIV and cocaine are known to disrupt neuronal signaling and contribute to neurocognitive dysfunction, yet the underlying molecular mechanisms are not clear. In this study, we delineate the underlying molecular mechanism by which HIV and/or cocaine enhance Tau phosphorylation (p-Tau S396), a marker of Tau-mediated neuropathies. Furthermore, we elucidate how these two independent neuropathogenic factors, cocaine and HIV, exploit distinct yet convergent signaling pathways to drive this pathological event. We demonstrate that HIV robustly activates and upregulates RSK1, which functions upstream of AKT and promotes Tau phosphorylation through an AKT-independent mechanism while simultaneously inactivating GSK3β via serine-9 phosphorylation (p-GSK3β S9). However, cocaine not only activates RSK1 but also strongly stimulates AKT1, resulting in sustained GSK3β inhibition and persistent Tau phosphorylation. Notably, Tau phosphorylation persists even under conditions of GSK3β inactivation in both HIV and cocaine exposure, revealing a previously unrecognized GSK3β-independent mechanism of Tau modification. Collectively, these findings identify RSK1 as the primary mediator of Tau phosphorylation upon HIV and/or cocaine exposure, and uncover a novel RSK1-driven, GSK3β-independent pathway contributing to Tauopathy. Through a combination of immunofluorescence, immunoblotting, genetic knockout, and overexpression approaches, we establish RSK1 as a central signaling hub linking the AKT-GSK3β pathway to Tau phosphorylation. We demonstrate that RSK1 operates as a critical upstream regulator of AKT and GSK3β signaling, playing dual roles, both activating AKT and suppressing GSK3β, thereby uncovering a novel layer of pathways that regulates Tau phosphorylation. The reproducibility of these main signaling pathways across SH-SY5Y neurons, mixed cell 3D spheroids, and human brain organoids underscores the robustness and biological relevance of this mechanism. Collectively, these findings reveal mechanistic convergence of HIV and cocaine on RSK1-dependent signaling and provide critical insight into how diverse neuropathic / neuropathological factors remodel neuronal signaling to drive Tau-associated dysfunction. These findings provide novel mechanistic insight into the molecular underpinnings of neuro-HIV and substance abuse associated Tauopathy. By identifying RSK1 as a master regulator and demonstrating that Tau phosphorylation can bypass GSK3β inhibition, our study advances understanding of signaling complexity and highlights new opportunities for therapeutic intervention. Targeting RSK1 may represent a promising strategy to mitigate Tau pathology, induced due to insoluble aggregates of phosphorylated Tau, a common factor promoting cognitive decline not only in individuals with Alzheimer's disease but also in those exposed to cocaine or/and infected with HIV.