adam17

Overview

Pathway Diagram

graph TD
    ADAM17["ADAM17"]
    style ADAM17 fill:#1a237e,stroke:#4fc3f7,stroke-width:3px
    ADAM17 -->|"activates"| TNF_alpha([TNF-alpha])
    ADAM17 -->|"activates"| EGFR_ligands([EGFR ligands])
    ADAM17 -->|"regulates"| epithelial_tissue_development(epithelial tissue development)
    ADAM17 -->|"regulates"| p75NTR["p75NTR"]
    ADAM17 -->|"regulates"| tPA["tPA"]
    ADAM17 -->|"regulates"| fibrin["fibrin"]
    ADAM17 -->|"regulates"| nerve_conduction_velocity(nerve_conduction_velocity)
    ADAM17 -->|"regulates"| macrophage_recruitment["macrophage_recruitment"]
    ADAM17 -->|"regulates"| cognitive_function(cognitive_function)
    ADAM17 -->|"regulates"| endothelium_dependent_vasodila["endothelium-dependent_vasodilation"]
    PDK1([PDK1]) -->|"regulates"| ADAM17
    ERK1_2([ERK1/2]) -->|"regulates"| ADAM17
    RSK2([RSK2]) -->|"regulates"| ADAM17
    iRhom2([iRhom2]) -->|"regulates"| ADAM17
    APOPTOSIS([APOPTOSIS]) -->|"expressed in"| ADAM17
    NEURODEGENERATION([NEURODEGENERATION]) -->|"implicated in"| ADAM17

ADAM17 (A Disintegrin And Metalloproteinase 17), also known as TACE (TNF-alpha Converting Enzyme), is a transmembrane zinc-dependent metalloproteinase that functions as the primary alpha-secretase for amyloid precursor protein (APP) processing. Located on chromosome 2p25.1, ADAM17 is a critical regulator of non-amyloidogenic APP processing, making it a key therapeutic target for Alzheimer’s Disease (AD)[1][2].

ADAM17 is one of the most extensively studied ADAM family members due to its pivotal role in releasing the extracellular domains of numerous membrane proteins—a process termed “ectodomain shedding.” This activity regulates signaling molecules including TNF-alpha, Notch, EGFR ligands, and APP, positioning ADAM17 at the intersection of inflammation, development, and neurodegeneration[“3”][4].

Gene and Protein Structure

Gene Location and Organization

The ADAM17 gene spans approximately 20 kb on chromosome 2p25.1 and consists of 22 exons encoding an 824-amino acid protein. The gene structure includes:

• Exon organization: 22 exons distributed across the coding sequence

• Promoter region: Contains response elements for various transcription factors including NF-κB and AP-1

• Alternative splicing: Multiple transcript variants produce different isoforms with tissue-specific expression patterns[5]

Protein Domain Architecture

The ADAM17 protein contains the characteristic ADAM family architecture:

The catalytic domain contains the conserved HEXGHXXGXXH zinc-binding motif essential for proteolytic activity. The disintegrin and cysteine-rich domains together form a “binding pocket” that determines substrate specificity[1][6].

Biological Functions

Ectodomain Shedding

ADAM17 is the prototypical “sheddase,” releasing soluble ectodomains from over 100 membrane-bound substrates[3]:

1. Cytokines and growth factors:

   • TNF-α: ADAM17 was first identified as the enzyme converting pro-TNF-α to soluble TNF-α

   • TGF-α (transforming growth factor alpha)

   • Amphiregulin

   • Epiregulin

   • Heparin-binding EGF-like growth factor (HB-EGF)

2. Signaling receptors:

   • Notch: Essential for Notch cleavage and activation

   • IL-6R (interleukin-6 receptor)

   • EGFR (epidermal growth factor receptor) ligands

3. Adhesion molecules:

   • L-selectin

   • VCAM-1

   • ICAM-1

4. APP and amyloid-related proteins:

   • Amyloid precursor protein: Primary alpha-secretase activity

   • APLP1 and APLP2 (APP-like proteins)[7][8]

Alpha-Secretase Activity and APP Processing

ADAM17 is the major physiological alpha-secretase in human neurons, responsible for cleaving APP within the Aβ sequence to generate soluble APPα (sAPPα) and prevent amyloid-beta formation[9][10][11]:

APP (cell membrane)
    ↓ ADAM17 (α-secretase)
sAPPα + C83 (CTF)

Alternative: BACE1 (β-secretase) → sAPPβ + C99 → Aβ

This non-amyloidogenic pathway competes with the amyloidogenic β-secretase (BACE1) pathway, making ADAM17 activation a promising therapeutic strategy to reduce Aβ production[12].

Notch Signaling

ADAM17-mediated Notch cleavage is essential for Notch receptor activation during development and adulthood. The enzyme processes Notch at the S2 cleavage site, enabling subsequent S3/S4 cleavage by γ-secretase to release the Notch intracellular domain (NICD) that translocates to the nucleus[13].

Notch signaling regulates:

• Neuronal differentiation

• Synaptic plasticity

• Learning and memory

• Neurogenesis

Dysregulated Notch processing contributes to neurodegenerative processes[14].

Regulation of Synaptic Function

ADAM17 influences synaptic function through multiple mechanisms:

• Synaptic adhesion: Shedding of synaptic adhesion molecules regulates synapse formation and maintenance

• NMDA receptor modulation: Affects NMDA receptor subunit composition and function

• Dendritic spine morphology: Controls spine density and morphology through cleavage of synaptic proteins[15]

Expression Pattern

Brain Expression

ADAM17 is widely expressed throughout the brain:

• Neurons: High expression in pyramidal neurons of the cortex and hippocampus

• Astrocytes: Moderate expression in astrocytes throughout the brain

• Microglia: Low baseline expression, upregulated in neuroinflammation

• Oligodendrocytes: Expression in developing and mature oligodendrocytes

Regional distribution:

• Cerebral cortex (layers II-VI)

• Hippocampus (CA1, CA3, dentate gyrus)

• Cerebellum (Purkinje cells and granule cells)

• Basal ganglia

• Brainstem nuclei

Peripheral Expression

High expression in:

• Heart (cardiomyocytes)

• Liver (hepatocytes)

• Kidney (epithelial cells)

• Lung (alveolar epithelium)

• Immune cells (monocytes, T cells)

Disease Associations

Alzheimer’s Disease

ADAM17 plays a central role in AD pathophysiology through its alpha-secretase activity[16][17][18]:

Amyloid Processing

1. Competition with BACE1: ADAM17 and BACE1 compete for APP as a substrate. Enhancing ADAM17 activity shifts APP processing away from the amyloidogenic pathway.

2. sAPPα functions: The sAPPα fragment generated by ADAM17 cleavage has neuroprotective properties:

   • Promotes neuronal survival

   • Enhances synaptic plasticity

   • Exhibits neuroprotective effects against excitotoxicity

   • May support neurogenesis

3. ADAM17 dysfunction in AD:

   • Reduced ADAM17 activity in AD brain[19]

   • Altered subcellular localization

   • Impaired regulation by cellular signaling

Neuroinflammation

ADAM17 regulates neuroinflammation through TNF-α shedding[20]:

• Elevated TNF-α in AD brain contributes to neuroinflammation

• ADAM17-mediated shedding releases soluble TNF-α that activates microglia

• Chronic neuroinflammation accelerates neurodegeneration

Synaptic Dysfunction

ADAM17 deficiency contributes to synaptic deficits in AD:

• Impaired LTP (long-term potentiation)

• Reduced dendritic spine density

• Altered NMDA receptor function

• Memory deficits in animal models[21]

Genetic Associations

Polymorphisms in the ADAM17 gene have been associated with AD risk:

• Certain ADAM17 variants may influence age of onset

• Expression quantitative trait loci (eQTLs) in AD brain tissue

• Interaction with other AD risk genes[22][23]

Parkinson’s Disease

ADAM17 involvement in PD includes:

1. Neuroinflammation: TNF-α shedding contributes to dopaminergic neuron degeneration

2. Alpha-synuclein processing: Potential effects on alpha-synuclein aggregation

3. Microglial activation: ADAM17-mediated TNF-α release activates microglia

4. Mitochondrial function: ADAM17 may affect mitochondrial quality control

Inflammatory Diseases

Beyond neurodegeneration, ADAM17 is implicated in:

• Rheumatoid arthritis: Elevated TNF-α shedding drives inflammation

• Psoriasis: ADAM17 processes growth factors involved in skin pathology

• Inflammatory bowel disease: TNF-α and growth factor shedding

• Cardiovascular disease: Regulates inflammation in atherosclerosis

Cancer

ADAM17 is frequently overexpressed in cancers:

• Promotes tumor growth through growth factor shedding

• Enhances invasion and metastasis

• Associated with poor prognosis

• Target for cancer therapy development

Therapeutic Targeting

ADAM17 Activation Strategies

Enhancing ADAM17 activity represents a promising AD therapeutic approach[24][25]:

Pharmacological Activators

1. Small molecule activators: Compounds that directly activate ADAM17 catalytic activity

   • Currently in preclinical development

   • Must achieve brain penetration

2. Protein kinase C (PKC) activators: Phorbol esters and other PKC agonists stimulate ADAM17 activity

   • 12-O-tetradecanoylphorbol-13-acetate (TPA) -Bryostatin analogs

3. Phosphatidylinositol-3-kinase (PI3K) modulators: Enhance ADAM17 surface expression and activity

Natural Compounds

Several natural compounds have been shown to modulate ADAM17:

• Flavonoids: Quercetin, epigallocatechin gallate (EGCG)

• Polyphenols: Resveratrol

• Curcuminoids: Curcumin

These compounds may offer neuroprotection through multiple mechanisms including ADAM17 activation.

Gene Therapy Approaches

• Viral vector-mediated ADAM17 overexpression in brain

• CRISPR activation of endogenous ADAM17 expression

• siRNA-mediated reduction of ADAM17 in specific contexts

ADAM17 Inhibitors

While activation is desired for AD, inhibition may be beneficial in other conditions:

• Inflammatory disorders: TNF-α blocking agents (already in clinical use)

• Cancer: Targeting tumor growth and metastasis

• Skin diseases: Psoriasis treatment

However, broad ADAM17 inhibition causes side effects due to disruption of multiple signaling pathways.

Clinical Considerations

1. Blood-brain barrier penetration: Critical challenge for CNS-targeted drugs

2. Selectivity: Avoiding inhibition of other ADAMs (especially ADAM10, also an alpha-secretase)

3. Substrate specificity: Different substrates may require different modulatory approaches

4. Timing of intervention: Optimal treatment may require early intervention

Signaling Pathways and Regulation

Post-Translational Regulation

ADAM17 activity is tightly regulated:

1. Prodomain removal: The prodomain maintains latency; removal is required for activity

   • Furin-like convertases process ADAM17 in the Golgi

   • Autocatalytic cleavage also occurs

2. Phosphorylation: Kinases regulate ADAM17:

   • PKC phosphorylation increases activity

   • MAPK pathways can modulate shedding

3. Trafficking: Cell surface delivery is regulated:

   • TGN to plasma membrane transport

   • Recycling through endosomes

Transcriptional Regulation

ADAM17 expression is modulated by:

• NF-κB: Pro-inflammatory signaling increases ADAM17 transcription

• AP-1: Immediate-early gene regulation

• ** glucocorticoids**: May reduce ADAM17 expression

• Cellular stress: Various stressors alter expression

Interaction Partners

ADAM17 interacts with numerous proteins:

Animal Models

Knockout Mice

ADAM17 global knockout:

• Perinatal lethality (died within 24-48 hours)

• Defects in heart (lack of ventricular septa)

• Skin abnormalities (impaired epidermal growth)

• Immune system defects (lack mature T cells)

• Unable to process TNF-α[26]

Phenotypic limitations: Severe phenotypes limit use for neurodegeneration studies.

Conditional Knockouts

Neuron-specific ADAM17 knockout:

• Viable with neurological phenotypes

• Impaired synaptic plasticity

• Memory deficits

• Reduced sAPPα production

Astrocyte-specific ADAM17 knockout:

• Altered neuroinflammation responses

• Modified astrocyte-neuron communication

Transgenic Models

ADAM17 overexpression:

• Increased sAPPα production

• Improved synaptic function

• Reduced Aβ pathology in some models

• Enhanced cognitive performance

Heterozygous Models

ADAM17+/- mice:

• Partial loss of function

• Show intermediate phenotypes

• Useful for studying haploinsufficiency

Research Directions

Current Priorities

1. Brain-penetrant ADAM17 activators: Develop small molecules that cross the BBB

2. Selective modulators: Distinguish ADAM17 from ADAM10 activity

3. Biomarkers: Identify biomarkers for ADAM17-targeted therapy response

4. Combination therapy: ADAM17 activators with BACE inhibitors or immunotherapies

Emerging Approaches

• Allosteric modulators: Target regulatory domains rather than catalytic site

• Protein-protein interaction inhibitors: Block ADAM17-inhibitor interactions

• Antibody-based activators: Engineered antibodies that enhance activity

• Gene editing: CRISPR approaches to enhance ADAM17 expression

Cross-Links

• Alzheimer’s Disease - Primary disease context

• APP Gene - Primary substrate

• BACE1 Gene - Competing amyloidogenic enzyme

• TNF Gene - Major substrate

• Alpha-Secretase - Functional role

• Amyloid-beta - Product of amyloidogenic pathway

• Notch Signaling Pathway - Related pathway

• ADAM10 Gene - Related alpha-secretase

Summary

ADAM17 (TACE) is a critical metalloproteinase with multifaceted roles in normal physiology and disease. Its function as the primary neuronal alpha-secretase makes it a key therapeutic target for Alzheimer’s disease, where enhancing its activity could shift APP processing away from amyloidogenic BACE1-mediated cleavage toward non-amyloidogenic sAPPα production. Beyond APP processing, ADAM17 regulates neuroinflammation through TNF-α shedding, Notch signaling, and synaptic function, all processes relevant to neurodegeneration. While ADAM17 activation represents a promising therapeutic strategy, challenges remain in developing brain-penetrant, selective activators that can be brought to clinical use.