HMGB1 functional plasticity in neurodegenerative diseases and its translational implications.

High mobility group box 1 (HMGB1) is a chromatin-associated protein that can be redistributed and released during cellular stress, when it acts as an extracellular alarmin within the broader family of damage-associated molecular patterns (DAMPs). Its functions depend on subcellular localization, redox and post-translational modification state, cellular source, mode of release, and molecular partners. This critical narrative review examines how these variables shape HMGB1 biology in Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), Huntington's disease (HD), spinocerebellar ataxias (SCAs), frontotemporal dementia (FTD), and related disorders, and considers their implications for biomarker and therapeutic development. Intracellular HMGB1 contributes to chromatin organization, DNA repair, mitochondrial genome maintenance, and proteostasis. After redistribution and release, extracellular HMGB1 signals through receptors including the receptor for advanced glycation end products (RAGE) and Toll-like receptor 4 (TLR4), interacts with disease-associated proteins, and links proteostatic stress with glial activation, neuronal dysfunction, and blood-brain barrier impairment. Across disease models, two recurrent patterns emerge: loss or sequestration of intracellular HMGB1 in models of polyglutamine disorders, including HD and SCAs, and extracellular HMGB1 signaling as a progression-associated inflammatory amplifier in AD, PD, and ALS models. Central-peripheral inflammatory communication and cell-specific HMGB1 release further shape these responses. Clinical studies have associated total HMGB1 or anti-HMGB1 autoantibodies with clinical features, but current assays do not resolve redox form, cellular source, or biological activity. Most therapeutic studies remain preclinical, while available human intervention studies involve drugs with multiple molecular actions. Clinical translation requires redox-resolved and standardized assays, longitudinal human sampling, compartment- and cell-specific causal studies, and interventions with documented target engagement, central nervous system exposure, and long-term safety. HMGB1 is best understood as a context-dependent mediator rather than a uniformly pathogenic factor or a single treatment target.