p62/SQSTM1 (Sequestosome-1)

Introduction

P62 Sqstm1 (Sequestosome 1) is an important component in the neurobiology of neurodegenerative diseases. This page provides detailed information about its structure, function, and role in disease processes.

Overview

p62, also known as Sequestosome-1 (SQSTM1), is a multifunctional scaffold protein that serves as the classical selective autophagy receptor in mammalian cells. It plays a central role at the intersection of the ubiquitin-proteasome system, selective autophagy, and multiple cell signaling pathways including NF-κB, mTOR, and Nrf2 oxidative stress response. p62 is encoded by the SQSTM1 gene on chromosome 5q35.^{1Moscat & Diaz-Meco, p62 at the Crossroads of Autophagy, Apoptosis, and Cancer, Cell 20092009 · DOI 10.1016/j.cell.2009.05.023Open reference 2SQSTM1 Mutations in Familial and Sporadic ALS, Archives of Neurology 20112011 · DOI 10.1001/archneurol.2011.250Open reference}

In neurodegenerative diseases, p62 has emerged as a pivotal player due to its dual role: (1) as a cargo receptor that tags and delivers ubiquitinated protein aggregates for autophagic degradation, and (2) as a component of the protein inclusions that characterize virtually all major neurodegenerative conditions. p62-positive inclusions are found in the neurofibrillary tangles of Alzheimer’s disease, Lewy bodies of Parkinson’s disease, and ubiquitinated inclusions of ALS/FTD. Mutations in SQSTM1 are directly linked to familial ALS and frontotemporal dementia, establishing p62 as both a pathological marker and a causal factor in neurodegeneration.^{2SQSTM1 Mutations in Familial and Sporadic ALS, Archives of Neurology 20112011 · DOI 10.1001/archneurol.2011.250Open reference 3Homeostatic Levels of p62 Control Cytoplasmic Inclusion Body Formation, J Cell Biol 20072007 · DOI 10.1083/jcb.200702050Open reference}

Structure and Domains

p62 is a 440-amino acid protein with a modular domain architecture that enables its diverse functions: ^{4p62 Filaments Capture and Present Ubiquitinated Cargos for Autophagy, EMBO J 20182018 · DOI 10.15252/embj.201798تار8Open reference}

Key Functional Domains

1. PB1 (Phox and Bem1) domain (residues 1–102): Mediates oligomerization and interactions with atypical protein kinases C (aPKCs) and other PB1-containing proteins. Self-oligomerization via PB1 is essential for forming p62 bodies — phase-separated condensates that concentrate cargo for autophagy

2. ZZ-type zinc finger (residues 128–163): Binds RIP1 kinase, mediating NF-κB signaling activation

3. TRAF6-binding domain (TBS, residues 225–250): Interacts with TRAF6, an E3 ubiquitin ligase critical for NF-κB activation and neuroinflammatory signaling

4. LIR (LC3-interacting region) (residues 335–341): The critical motif for autophagy receptor function — binds LC3/GABARAP proteins on the autophagosome membrane, tethering ubiquitinated cargo to the autophagic machinery

5. KIR (Keap1-interacting region) (residues 349–361): Binds Keap1, releasing Nrf2 for nuclear translocation and activation of antioxidant and cytoprotective gene expression

6. UBA (ubiquitin-associated) domain (residues 386–440): Binds K48- and K63-linked polyubiquitin chains on cargo proteins destined for degradation^{3Homeostatic Levels of p62 Control Cytoplasmic Inclusion Body Formation, J Cell Biol 20072007 · DOI 10.1083/jcb.200702050Open reference}

Structural Organization for Cargo Recognition

The spatial arrangement of the LIR and UBA domains allows p62 to simultaneously bind ubiquitinated substrates (via UBA) and autophagosome membranes (via LIR), functioning as a molecular bridge between cargo and the autophagic machinery. Self-oligomerization via the PB1 domain creates large p62 condensates that concentrate cargo, enhancing the efficiency of selective autophagy.^{4p62 Filaments Capture and Present Ubiquitinated Cargos for Autophagy, EMBO J 20182018 · DOI 10.15252/embj.201798تار8Open reference 5Sánchez-Martín & Bhatt, SQSTM1/p62: A Potential Target for Neurodegenerative Disease, ACS Chemical Neuroscience 20192019 · DOI 10.1021/acschemneuro.8b00516Open reference}

Functions in the Nervous System

Selective Autophagy Receptor

p62 is the prototypical receptor for aggrephagy — the selective autophagic degradation of protein aggregates. In neurons, which are post-mitotic and cannot dilute toxic aggregates through cell division, this function is critically important. p62 recognizes and binds: ^{6PINK1/Parkin-Mediated Mitophagy Is Dependent on VDAC1 and p62/SQSTM1, Nature Cell Biology 20102010 · DOI 10.1038/ncb2012Open reference}

• Ubiquitinated misfolded proteins (via UBA domain)

• Tau(/proteins/tau) aggregates in Alzheimer’s disease

• alpha-synuclein aggregates in Parkinson’s disease

• TDP-43 and FUS aggregates in ALS/FTD

• Huntingtin polyglutamine aggregates in Huntington’s disease^{5Sánchez-Martín & Bhatt, SQSTM1/p62: A Potential Target for Neurodegenerative Disease, ACS Chemical Neuroscience 20192019 · DOI 10.1021/acschemneuro.8b00516Open reference}

Mitophagy

p62 participates in PINK1/Parkin-dependent mitophagy — the selective degradation of damaged mitochondria. When PINK1 accumulates on depolarized mitochondria and recruits Parkin, the E3 ubiquitin ligase activity of Parkin ubiquitinates outer mitochondrial membrane proteins. p62 recognizes these ubiquitinated mitochondrial substrates and facilitates their engulfment by autophagosomes. Impaired mitophagy due to p62 dysfunction contributes to mitochondrial dysfunction in neurodegeneration.^{2SQSTM1 Mutations in Familial and Sporadic ALS, Archives of Neurology 20112011 · DOI 10.1001/archneurol.2011.250Open reference}0 ^{2SQSTM1 Mutations in Familial and Sporadic ALS, Archives of Neurology 20112011 · DOI 10.1001/archneurol.2011.250Open reference}1

NF-κB Signaling

Through its ZZ and TBS domains, p62 activates the NF-κB inflammatory pathway. In microglia and potentially harmful (via [neuroinflammation) — makes p62 a complex therapeutic target.^{2SQSTM1 Mutations in Familial and Sporadic ALS, Archives of Neurology 20112011 · DOI 10.1001/archneurol.2011.250Open reference}2 ^{2SQSTM1 Mutations in Familial and Sporadic ALS, Archives of Neurology 20112011 · DOI 10.1001/archneurol.2011.250Open reference}3

Nrf2-Keap1 Oxidative Stress Response

When p62 accumulates (e.g., due to autophagy impairment), it sequesters Keap1 via its KIR domain, releasing Nrf2 to translocate to the nucleus and activate antioxidant response element (ARE)-driven genes. This p62-Keap1-Nrf2 axis provides a feedback loop linking autophagy status to the oxidative stress response — a mechanism particularly relevant in neurons under proteotoxic stress.^{2SQSTM1 Mutations in Familial and Sporadic ALS, Archives of Neurology 20112011 · DOI 10.1001/archneurol.2011.250Open reference}4 ^{2SQSTM1 Mutations in Familial and Sporadic ALS, Archives of Neurology 20112011 · DOI 10.1001/archneurol.2011.250Open reference}5

Role in Neurodegenerative Diseases

Alzheimer’s Disease

In Alzheimer’s disease, p62 expression is reduced in affected brain regions, particularly the frontal cortex and hippocampus, beginning early in the disease process. This reduction correlates with impaired autophagic clearance of both tau](/proteins/tau) tangles and Amyloid-Beta aggregates. p62 co-localizes with neurofibrillary tangles and is believed to participate in tau](/proteins/tau) degradation via selective autophagy. p62 also modulates APP processing: loss of p62 function shifts APP processing toward the amyloidogenic pathway, increasing Aβ production.^{2SQSTM1 Mutations in Familial and Sporadic ALS, Archives of Neurology 20112011 · DOI 10.1001/archneurol.2011.250Open reference}6 ^{2SQSTM1 Mutations in Familial and Sporadic ALS, Archives of Neurology 20112011 · DOI 10.1001/archneurol.2011.250Open reference}7

CSF p62 concentrations are significantly elevated in AD patients compared to controls, suggesting that neuronal release of p62 (from dying neurons or exosomal secretion) may serve as a biomarker of autophagic dysfunction.^{2SQSTM1 Mutations in Familial and Sporadic ALS, Archives of Neurology 20112011 · DOI 10.1001/archneurol.2011.250Open reference}8 ^{2SQSTM1 Mutations in Familial and Sporadic ALS, Archives of Neurology 20112011 · DOI 10.1001/archneurol.2011.250Open reference}9

ALS and Frontotemporal Dementia

Mutations in SQSTM1 are directly causative of familial ALS and FTD. Over 25 pathogenic variants have been identified, most affecting the UBA domain and impairing ubiquitin binding. Key mutations include: ^{2SQSTM1 Mutations in Familial and Sporadic ALS, Archives of Neurology 20112011 · DOI 10.1001/archneurol.2011.250Open reference}0

• P392L — the most common mutation, originally identified in Paget disease of bone and subsequently linked to ALS

• L341V, P348L — affect the LIR domain, disrupting LC3 binding

• K238E, D337E — impair NF-κB signaling or autophagy flux

p62-positive, TDP-43-positive inclusions are the hallmark pathology of sporadic ALS and C9orf72-linked ALS/FTD, indicating that p62-mediated aggregate clearance is overwhelmed in these conditions.^{2SQSTM1 Mutations in Familial and Sporadic ALS, Archives of Neurology 20112011 · DOI 10.1001/archneurol.2011.250Open reference}1

Parkinson’s Disease and Lewy Body Dementia

p62 is a consistent component of Lewy bodies, where it co-localizes with ubiquitinated alpha-synuclein. In models of Parkinson’s disease, overexpression of p62 enhances alpha-synuclein clearance, while p62 knockdown accelerates aggregate accumulation and neuronal death. The connection between p62 and PINK1/Parkin-mediated mitophagy further links p62 to PD pathogenesis.^{2SQSTM1 Mutations in Familial and Sporadic ALS, Archives of Neurology 20112011 · DOI 10.1001/archneurol.2011.250Open reference}2

Huntington’s Disease

p62 recognizes polyglutamine-expanded huntingtin aggregates for autophagic clearance. In Huntington’s disease models, enhancing p62 expression or activity promotes aggregate clearance and improves neuronal survival.^{2SQSTM1 Mutations in Familial and Sporadic ALS, Archives of Neurology 20112011 · DOI 10.1001/archneurol.2011.250Open reference}3

Childhood-Onset Neurodegeneration

Biallelic loss-of-function mutations in SQSTM1 cause a severe childhood-onset neurodegenerative syndrome characterized by progressive cerebellar ataxia, dystonia, and supranuclear gaze palsy, demonstrating that complete loss of p62 function is incompatible with normal neuronal maintenance.^{2SQSTM1 Mutations in Familial and Sporadic ALS, Archives of Neurology 20112011 · DOI 10.1001/archneurol.2011.250Open reference}4

Therapeutic Implications

p62 as a Drug Target

Several strategies are being explored to modulate p62 function therapeutically:

• Enhancing p62 expression: Small molecules and gene therapy approaches to increase p62 levels and boost selective autophagy

• Stabilizing p62-LC3 interaction: Compounds that enhance the LIR-LC3 binding to improve autophagy flux

• p62-based degraders: Using p62’s UBA domain as a targeting module in chimeric degrader molecules (autophagy-targeting chimeras, AUTACs)

• Nrf2 activation: Leveraging the p62-Keap1-Nrf2 axis for antioxidant neuroprotection^{2SQSTM1 Mutations in Familial and Sporadic ALS, Archives of Neurology 20112011 · DOI 10.1001/archneurol.2011.250Open reference}5

Biomarker Applications

CSF and blood levels of p62 are being investigated as biomarkers for:

• Autophagic dysfunction in Alzheimer’s disease and ALS

• Disease progression and treatment response in clinical trials targeting the autophagy-lysosomal pathway

• Genetic risk assessment in ALS/FTD families^{2SQSTM1 Mutations in Familial and Sporadic ALS, Archives of Neurology 20112011 · DOI 10.1001/archneurol.2011.250Open reference}6

See Also

• Parkin

• PINK1

• TDP-43 (TAR DNA-Binding Protein 43)

External Links

• UniProt — SQSTM1 Human

• OMIM — SQSTM1

• NCBI Gene — SQSTM1

Brain Atlas Resources

• Allen Human Brain Atlas: p62/SQSTM1 expression search

• Allen Mouse Brain Atlas: p62/SQSTM1 search

• Allen Cell Type Atlas: Transcriptomic cell type reference

• BrainSpan Developmental Transcriptome: p62/SQSTM1 developmental expression

Background

The study of P62 Sqstm1 (Sequestosome 1) has evolved significantly over the past decades. Research in this area has revealed important insights into the underlying mechanisms of neurodegeneration and continues to drive therapeutic development.

Historical context and key discoveries in this field have shaped our current understanding and will continue to guide future research directions.