HMGB1 Signaling in Neurodegeneration

HMGB1 Signaling in Neurodegeneration

Introduction

High mobility group box 1 (HMGB1) is a ubiquitous nuclear protein that functions as a damage-associated molecular pattern (DAMP) when released extracellularly, playing a critical role in neuroinflammation and neurodegenerative diseases. This page provides comprehensive coverage of HMGB1 signaling in Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and stroke. [@yanai2022]

Overview

HMGB1 is a 215-amino acid, 30 kDa protein that binds to DNA and participates in transcriptional regulation, DNA repair, and chromatin remodeling. Under pathological conditions, HMGB1 can be actively secreted by immune cells or passively released from damaged [neurons](/entities/neurons) and glia, where it acts as a potent pro-inflammatory cytokine. Extracellular HMGB1 binds to pattern recognition receptors (TLR2, [TLR4](/entities/tlr4), TLR9) and the receptor for advanced glycation end products (RAGE), triggering downstream inflammatory signaling cascades that contribute to chronic neuroinflammation, blood-brain barrier (BBB) disruption, and neuronal death in neurodegenerative diseases. [@andersson2022]

HMGB1 Structure and Function

Nuclear Functions

HMGB1 Signaling in Neurodegeneration

Introduction

High mobility group box 1 (HMGB1) is a ubiquitous nuclear protein that functions as a damage-associated molecular pattern (DAMP) when released extracellularly, playing a critical role in neuroinflammation and neurodegenerative diseases. This page provides comprehensive coverage of HMGB1 signaling in Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and stroke. [@yanai2022]

Overview

HMGB1 is a 215-amino acid, 30 kDa protein that binds to DNA and participates in transcriptional regulation, DNA repair, and chromatin remodeling. Under pathological conditions, HMGB1 can be actively secreted by immune cells or passively released from damaged [neurons](/entities/neurons) and glia, where it acts as a potent pro-inflammatory cytokine. Extracellular HMGB1 binds to pattern recognition receptors (TLR2, [TLR4](/entities/tlr4), TLR9) and the receptor for advanced glycation end products (RAGE), triggering downstream inflammatory signaling cascades that contribute to chronic neuroinflammation, blood-brain barrier (BBB) disruption, and neuronal death in neurodegenerative diseases. [@andersson2022]

HMGB1 Structure and Function

Nuclear Functions

HMGB1 contains two DNA-binding HMG boxes (A-box and B-box) and a acidic C-terminal tail. In the nucleus, HMGB1 facilitates DNA bending and unwinding, promotes access to transcription factors ([NF-κB](/entities/nf-kb), p53, steroid hormone receptors), and participates in V(D)J recombination and DNA repair. The protein is highly expressed in neurons, [astrocytes](/entities/astrocytes), and [microglia](/cell-types/microglia-neuroinflammation) throughout the brain. [@matsumoto2023]

Extracellular Release

HMGB1 release occurs through two primary mechanisms: [@fang2022]

The redox state of HMGB1 determines its biological activity: reduced HMGB1 promotes chemotaxis via [RAGE](/entities/rage-receptor), disulfide HMGB1 signals through TLR4, and oxidized HMGB1 is immunologically silent. [@paudel2023]

Receptors and Signaling Pathways

RAGE (Receptor for Advanced Glycation End Products)

RAGE is a pattern recognition receptor belonging to the immunoglobulin superfamily. HMGB1-RAGE signaling triggers: [@zhao2021]

• NF-κB activation via MyD88-dependent pathway
• MAP kinase (p38, JNK, ERK) activation
• Rac1 and Cdc42-mediated cytoskeletal reorganization
• Pro-inflammatory cytokine production (IL-1β, IL-6, TNF-α)
• Matrix metalloproteinase (MMP-9) upregulation
• Neuronal [apoptosis](/entities/apoptosis) and synaptic dysfunction

Toll-Like Receptors

TLR4

• NF-κB and AP-1 activation
• Type I interferon (IFN-β) production via TRIF pathway
• Inflammasome priming
• Microglial activation and proliferation

TLR2 and TLR9

Downstream Signaling Cascades

Role in Neurodegenerative Diseases

Alzheimer's Disease

In Alzheimer's disease (AD), HMGB1 is implicated in multiple pathogenic mechanisms: [@musumeci2014]

• Amyloid-β interaction: HMGB1 binds to amyloid-β plaques and enhances [Aβ](/proteins/amyloid-beta) fibril formation while reducing clearance. The HMGB1-Aβ complex activates TLR4 and RAGE on microglia, amplifying neuroinflammation.
• [Tau](/proteins/tau) pathology: HMGB1 promotes tau phosphorylation via RAGE-mediated MAPK activation and contributes to tau spread between neurons.
• Synaptic dysfunction: Extracellular HMGB1 impairs [long-term potentiation](/mechanisms/long-term-potentiation) (LTP) and promotes synaptic loss through RAGE-dependent mechanisms.
• Blood-brain barrier disruption: HMGB1-RAGE signaling increases BBB permeability by upregulating MMP-9 and downregulating tight junction proteins (claudin-5, occludin).
• Metaflammation: HMGB1 contributes to metabolic inflammation in AD through [NLRP3](/entities/nlrp3-inflammasome) inflammasome activation.

Parkinson's Disease

In Parkinson's disease (PD), HMGB1 plays a significant role in dopaminergic neuron degeneration: [@lai2017]

• [α-Synuclein](/proteins/alpha-synuclein) interaction: HMGB1 binds to α-synuclein and facilitates its aggregation and cell-to-cell transmission. The HMGB1-α-synuclein complex is more neurotoxic than either molecule alone.
• Microglial activation: HMGB1 released from damaged neurons activates microglia via TLR4 and RAGE, creating a vicious cycle of neuroinflammation.
• Blood-brain barrier permeability: HMGB1 contributes to BBB disruption in PD, facilitating peripheral immune cell infiltration.
• Mitochondrial dysfunction: HMGB1-TLR4 signaling impairs mitochondrial biogenesis and promotes mitochondrial fragmentation in dopaminergic neurons.

Amyotrophic Lateral Sclerosis

In ALS, HMGB1 is involved in motor neuron degeneration:

• Excitotoxicity: HMGB1 enhances glutamate-induced excitotoxicity through RAGE-mediated mechanisms.
• Glial activation: HMGB1 promotes astrocyte and microglia activation, creating a toxic microenvironment for motor neurons.
• RNA metabolism: HMGB1 interacts with [TDP-43](/mechanisms/tdp-43-proteinopathy) protein (aggregated in most ALS cases), potentially affecting RNA processing.

Stroke and Brain Injury

In ischemic stroke and traumatic brain injury (TBI):

• Early phase: HMGB1 is released within hours of stroke onset, contributing to excitotoxicity and infarct expansion.
• BBB disruption: HMGB1-mediated MMP-9 upregulation degrades tight junction proteins.
• Inflammatory cascade: HMGB1 amplifies post-stroke neuroinflammation through TLR4 and RAGE.

Therapeutic Targeting

HMGB1 Inhibitors

| Agent | Mechanism | Stage | Reference |
|-------|-----------|-------|-----------|
| Anti-HMGB1 monoclonal antibodies | Neutralize extracellular HMGB1 | Preclinical | [Zhao et al., 2017](https://pubmed.ncbi.nlm.nih.gov/29126423/) |
| HMGB1 Box A | RAGE antagonist, blocks HMGB1 signaling | Preclinical | [Schiraldi et al., 2009](https://pubmed.ncbi.nlm.nih.gov/19544420/) |
| Glycyrrhizin | HMGB1 binding, inhibitor | Preclinical | [Musumeci et al., 2014](https://pubmed.ncbi.nlm.nih.gov/24722772/) |
| Sodium butyrate | HMGB1 acetylation inhibition | Preclinical | [Lai et al., 2017](https://pubmed.ncbi.nlm.nih.gov/28526670/) |

Downstream Pathway Inhibitors

• RAGE inhibitors: FPS-ZM1, PF-04494700 (in clinical trials for diabetic neuropathy)
• TLR4 antagonists: Eritoran, TAK-242
• NF-κB inhibitors: Pyridine dithiocarbamate, BAY 11-7082
• MMP-9 inhibitors: SB-3CT

Repurposed Drugs

• Statins: Downregulate HMGB1 release and TLR4 expression
• Minocycline: Inhibits HMGB1-mediated microglial activation
• Edaravone: Antioxidant, reduces HMGB1 release after stroke
• Curcumin: Modulates HMGB1 signaling pathways

Cross-Talk with Other Pathways

HMGB1 signaling intersects with several other neurodegeneration-related pathways:

• [NF-κB signaling](/mechanisms/nfkb-signaling-neurodegeneration): HMGB1 is both a target and activator of NF-κB, creating a positive feedback loop for neuroinflammation.
• [NLRP3 inflammasome](/mechanisms/nlrp3-inflammasome): HMGB1 provides signal 1 (priming) for NLRP3 inflammasome activation.
• [TREM2 signaling](/mechanisms/trem2-signaling): HMGB1 and [TREM2](/proteins/trem2) compete for TLR4 binding, modulating microglial responses.
• [cGAS-STING pathway](/mechanisms/cgas-sting-neurodegeneration): Both pathways contribute to type I interferon responses in neurodegeneration.
• [Microglial polarization](/mechanisms/microglial-polarization): HMGB1 promotes pro-inflammatory (M1) microglial polarization.
• [Blood-brain barrier](/entities/blood-brain-barrier): HMGB1-RAGE signaling disrupts BBB integrity.

Summary

HMGB1 represents a critical nexus between tissue damage, neuroinflammation, and neurodegeneration. Its release following neuronal injury triggers widespread inflammatory cascades through RAGE and TLR receptors, contributing to chronic neuroinflammation, blood-brain barrier disruption, and progressive neuronal loss in AD, PD, ALS, and stroke. Targeting HMGB1 signaling offers a promising therapeutic strategy, though delivery to the CNS remains a significant challenge. Understanding the redox-dependent activities of HMGB1 and its interactions with other DAMPs may enable more precise therapeutic interventions.

See Also

• [NF-κB Signaling in Neurodegeneration](/mechanisms/nfkb-signaling-neurodegeneration)
• [NLRP3 Inflammasome Pathway](/mechanisms/nlrp3-inflammasome)
• [TREM2 Signaling in Neurodegeneration](/mechanisms/trem2-signaling)
• [Neuroinflammation Pathway](/mechanisms/neuroinflammation-pathway)
• [Alzheimer's Disease](/diseases/alzheimers-disease)
• [Parkinson's Disease](/diseases/parkinsons-disease)
• [Amyotrophic Lateral Sclerosis](/diseases/amyotrophic-lateral-sclerosis)