Circulating Follistatin-like 1 associates with inflammation, cognitive decline, and amyloid pathology in Alzheimer’s disease and promotes Aβ-induced neuronal ferroptosis via PI3K/Akt signaling

Alzheimer’s disease (AD) is characterized by progressive cognitive decline, neuroinflammation, and neuronal loss. However, the molecular factors linking amyloid pathology to regulated neuronal death and clinically accessible biomarkers remain incompletely understood. Follistatin-like 1 (FSTL1) is a secreted glycoprotein implicated in inflammatory signaling and cell survival; however, its role in AD pathogenesis remains unclear. This study investigated the clinical relevance and mechanistic function of FSTL1 in AD. A total of 143 participants were enrolled, including 93 patients with AD and 50 healthy controls. Serum FSTL1 levels, inflammatory cytokines, cognitive performance, and cerebrospinal fluid (CSF) biomarkers were analyzed. In vitro, Aβ₁₋₄₂-treated HT22 hippocampal neurons were used to examine the functional role of FSTL1 and its downstream signaling mechanisms. Serum FSTL1 levels were significantly elevated in patients with AD and increased progressively with disease severity. Elevated FSTL1 concentrations correlated positively with pro-inflammatory cytokines (IL-1β, IL-6, and TNF-α) and CSF biomarkers of amyloid and tau pathology (Aβ₁₋₄₂, Aβ₁₋₄₀, p-tau181, and total tau), while showing a negative correlation with cognitive performance assessed by the Montreal Cognitive Assessment. Receiver operating characteristic (ROC) analysis demonstrated high diagnostic accuracy of serum FSTL1 for distinguishing AD patients from healthy controls. Mechanistically, Aβ₁₋₄₂ exposure upregulated FSTL1 expression in HT22 neurons in a concentration-dependent manner. Silencing FSTL1 activated the PI3K/Akt signaling, reduced neuronal apoptosis, attenuated oxidative stress and reactive oxygen species (ROS) production, and suppressed ferroptosis, as evidenced by decreased intracellular Fe2⁺ accumulation, downregulation of acyl-CoA synthetase long-chain family member 4 (ACSL4), and upregulation of solute carrier family 7 member 11 (SLC7A11) and glutathione peroxidase 4 (GPX4). Pharmacological inhibition of PI3K/Akt signaling partially abolished these neuroprotective effects. These findings identify FSTL1 as a clinically relevant biomarker and a mechanistic mediator linking amyloid toxicity to neuronal ferroptosis via the PI3K/Akt signaling pathway, highlighting its diagnostic value and therapeutic potential in AD.