Abstract
<jats:p>Astrocytes are among the earliest cells to exhibit dysfunction in Alzheimer's disease (AD), developing profound calcium signalling deficits before amyloid plaques have formed, yet the underlying mechanisms remain unknown. Lysosomal dysfunction is a hallmark of AD, but whether it initiates this early functional impairment or arises as a consequence of established pathology remains unresolved. Here, we find that astrocytic cytosolic calcium activity is suppressed prior to amyloid plaque deposition and is accompanied by reduced lysosomal acidification in vivo. Using a lysosome-targeted calcium indicator selectively expressed in astrocytes, we directly visualise lysosomal calcium dynamics in vivo and reveal a profound early loss of lysosomal calcium release, identifying lysosomal failure as an initiating event in astrocyte dysfunction in AD. Reduced expression of the lysosomal calcium channel TRPML1 provides the mechanistic basis for this deficit. Astrocyte-specific restoration of TRPML1 expression rescues lysosomal homeostasis and cytosolic calcium signalling, and prevents astrocyte reactivity and morphological hypertrophy. Strikingly, early TRPML1 restoration prevents both the initial calcium hypoactivity observed before plaque formation and the later hyperactivity that characterises post-plaque disease, demonstrating that lysosomal calcium homeostasis stabilises astrocyte function across the disease trajectory. TRPML1 restoration also reduces amyloid plaque burden, indicating that astrocytic lysosomal competence directly shapes disease pathology. These findings identify lysosomal calcium failure as an early organelle-level mechanism linking amyloid stress to astrocyte dysfunction in AD, and position TRPML1-mediated lysosomal calcium signalling as a tractable target for limiting disease progression.</jats:p>