TAM Receptor Signaling

TAM Receptor Signaling

Overview

TAM receptor signaling refers to signaling through the receptor tyrosine kinases TYRO3, AXL, and MERTK (also written as MER) and their canonical ligands GAS6 (growth arrest-specific 6) and PROS1 (protein S). This signaling system represents a critical regulator of innate immunity, phagocytosis, and tissue homeostasis in the nervous system. In the brain, TAM signaling helps regulate microglial phagocytosis, inflammatory tone, glial homeostasis, and neural stem cell survival and differentiation [@insight2023][@tam2015].

TAM Receptor Family

TYRO3

TYRO3 was the first TAM receptor discovered and is expressed predominantly in the nervous system and reproductive tissues. In the brain, TYRO3 is expressed on [neurons](/entities/neurons), [astrocytes](/entities/astrocytes), and oligodendrocyte precursor cells. TYRO3 signaling promotes cell survival and regulates synaptic function. Mutations in TYRO3 are associated with rare developmental disorders [@tyro2020][@tyro2019].

AXL

AXL (also known as TYRO3) is the most widely studied TAM receptor in the context of disease. It is highly expressed on [microglia](/cell-types/microglia-neuroinflammation) and macrophages, where it mediates phagocytosis of apoptotic cells and cellular debris. AXL is often upregulated in response to injury and inflammation, making it a biomarker for immune activation. Soluble AXL (sAXL) can be detected in cerebrospinal fluid and serves as a disease biomarker [@axl2021][@soluble2020].

MERTK

TAM Receptor Signaling

Overview

TAM receptor signaling refers to signaling through the receptor tyrosine kinases TYRO3, AXL, and MERTK (also written as MER) and their canonical ligands GAS6 (growth arrest-specific 6) and PROS1 (protein S). This signaling system represents a critical regulator of innate immunity, phagocytosis, and tissue homeostasis in the nervous system. In the brain, TAM signaling helps regulate microglial phagocytosis, inflammatory tone, glial homeostasis, and neural stem cell survival and differentiation [@insight2023][@tam2015].

TAM Receptor Family

TYRO3

TYRO3 was the first TAM receptor discovered and is expressed predominantly in the nervous system and reproductive tissues. In the brain, TYRO3 is expressed on [neurons](/entities/neurons), [astrocytes](/entities/astrocytes), and oligodendrocyte precursor cells. TYRO3 signaling promotes cell survival and regulates synaptic function. Mutations in TYRO3 are associated with rare developmental disorders [@tyro2020][@tyro2019].

AXL

AXL (also known as TYRO3) is the most widely studied TAM receptor in the context of disease. It is highly expressed on [microglia](/cell-types/microglia-neuroinflammation) and macrophages, where it mediates phagocytosis of apoptotic cells and cellular debris. AXL is often upregulated in response to injury and inflammation, making it a biomarker for immune activation. Soluble AXL (sAXL) can be detected in cerebrospinal fluid and serves as a disease biomarker [@axl2021][@soluble2020].

MERTK

MERTK is the central phagocytic receptor for apoptotic cell clearance in the brain. Unlike AXL, which is inducible, MERTK is constitutively expressed on microglia and is essential for efficient phagocytosis of apoptotic neurons and debris. Mutations in MERTK cause retinal degeneration and phagocytic defects in mice and humans. MERTK polymorphisms are associated with increased risk for neurodegenerative diseases [@mertk2020][@mertk2017].

Ligands

GAS6

Growth arrest-specific 6 (GAS6) is the founding TAM ligand, originally identified as a gene upregulated during cellular senescence. GAS6 is expressed in neurons, astrocytes, and endothelial cells. It functions as a soluble bridge molecule, binding to phosphatidylserine on apoptotic cells via its Gla domain and simultaneously engaging TAM receptors on phagocytes. GAS6 has multiple domains including an LG domain that mediates receptor binding [@gastam2015][@structure2020].

PROS1

Protein S (PROS1) is best known as a cofactor for anticoagulation, but it also functions as a TAM ligand. Like GAS6, PROS1 binds to phosphatidylserine on apoptotic cells and engages TAM receptors. The relative contributions of GAS6 and PROS1 vary by tissue and context. In the brain, both ligands can support microglial phagocytosis [@protein2017][@pros2018].

Signaling Mechanisms

Receptor Activation

TAM receptors are activated through two main mechanisms: ligand-dependent autophosphorylation and ligand-independent transactivation. Binding of GAS6 or PROS1 induces receptor dimerization and autophosphorylation of tyrosine residues in the cytoplasmic domain. This creates docking sites for downstream signaling proteins containing SH2 domains [@tam2013][@tam2019].

Key Downstream Pathways

• PI3K/Akt pathway — Promotes cell survival and cytoskeletal reorganization for phagocytosis
• MAPK/ERK pathway — Regulates cell proliferation and differentiation
• [NF-κB](/entities/nf-kb) pathway — Modulates inflammatory gene expression
• STAT pathway — Regulates immune cell activation and cytokine production
• Rho GTPases — Control actin dynamics for phagocytic engulfment

Negative Regulation

TAM signaling is subject to negative regulation through multiple mechanisms. Protein tyrosine phosphatases (PTPs) dephosphorylate activated receptors. Suppressors of cytokine signaling (SOCS) proteins inhibit signaling through ubiquitination and degradation. Soluble receptor domains (sTYRO3, sAXL, sMERTK) can act as decoys [@negative2016][@socs2019].

Role in Neuroinflammation

Microglial Phagocytosis

The primary function of TAM receptors in the brain is to mediate microglial phagocytosis. MERTK is essential for the engulfment of apoptotic neurons, while AXL contributes to debris clearance. Impaired TAM signaling leads to accumulation of apoptotic cells and cellular debris, which can trigger chronic inflammation. This is particularly relevant in neurodegenerative diseases where inefficient clearance contributes to pathology [@tam2015a][@phagocytic2020].

Inflammatory Tone Regulation

TAM signaling acts as a brake on excessive innate immune activation. GAS6/TAM signaling negatively regulates inflammatory induction of cytokines including GM-CSF, TNF-α, and IL-1β. This anti-inflammatory function is mediated in part through inhibition of NF-κB signaling and promotion of anti-inflammatory cytokine production. Dysregulation of this brake contributes to chronic19][@antiinflammatory2019].

T neuroinflammation [^REM2 Interaction

TAM receptors cooperate with [TREM2](/proteins/trem2) (triggering receptor expressed on myeloid cells 2) in regulating microglial function. Both pathways are involved in phagocytosis and inflammatory responses. TREM2 variants are major risk factors for Alzheimer's disease, and there is evidence for functional interactions between these two microglial receptor systems [@trem2019][@microglial2020].

Role in Neurodegenerative Diseases

Alzheimer's Disease

In AD, TAM receptor expression is altered in response to amyloid pathology. AXL is upregulated in AD brain and in mouse models of amyloid deposition. GAS6 levels are reduced in AD cerebrospinal fluid, potentially reflecting increased consumption or reduced production. MERTK expression on microglia is reduced in AD, which may contribute to impaired phagocytosis of amyloid plaques and apoptotic cells. Targeting TAM signaling has been proposed as a therapeutic strategy [@tam2017][@gas2019].

Multiple Sclerosis

In MS and its animal model EAE, TAM signaling modulates disease severity. GAS6 deficiency worsens clinical scores, while exogenous GAS6 ameliorates disease. The mechanism involves regulation of microglial activation and phagocytic clearance of myelin debris. TAM receptors may also affect remyelination by regulating oligodendrocyte precursor cell function [@tam2018][@gas2019a].

ALS

In ALS, TAM receptors are expressed in microglia and astrocytes. Changes in TAM signaling may contribute to the neuroinflammatory environment that surrounds motor neurons. MERTK polymorphisms have been associated with ALS risk in some populations. The role of TAM signaling in ALS is an area of active investigation [@tam2020][@mertk2015].

Parkinson's Disease

PD involves progressive loss of dopaminergic neurons in the substantia nigra. Microglial TAM signaling may affect the inflammatory environment surrounding vulnerable neurons. AXL has been detected in microglia in PD brain, and changes in soluble AXL have been reported. The contribution of TAM signaling to PD pathogenesis remains to be fully elucidated [@axl2020][@microglial2020a].

Therapeutic Implications

Targeting TAM Signaling

• TAM agonists — Recombinant GAS6 or PROS1 to enhance phagocytosis
• Small molecule activators — Compounds that activate TAM receptors directly
• Gene therapy — Viral vectors to express GAS6 or activate TAM pathways

Biomarker Potential

• Soluble AXL — Marker of microglial activation in CSF and blood
• GAS6 levels — Potential biomarker for disease progression
• TAM expression — Imaging targets for microglial burden

Mermaid Diagram: TAM Receptor Signaling

TAM Receptor Signaling in Development

Neural Development

TAM receptors play crucial roles in nervous system development:

• Neurogenesis: Regulation of neural stem cell proliferation[@insight2023]
• Neuronal migration: Axon guidance and cell positioning
• Synapse formation: Synaptic development and maintenance
• Myelination: Oligodendrocyte differentiation and function

Glial Development

• Astrocyte maturation: Support of astrocytic differentiation
• Oligodendrocyte precursor cell fate: lineage determination
• Microglial colonization: Brain immune cell development

TAM in Specific Neurodegenerative Diseases

Amyotrophic Lateral Sclerosis

• Motor neuron environment: Microglial TAM signaling
• Astrocytic involvement: Support cell dysfunction
• Therapeutic potential: Modulating neuroinflammation
• Genetic associations: MERTK variants in ALS[@tam2015]

Frontotemporal Dementia

• Temporal lobe involvement: Region-specific effects
• Microglial activation: Inflammatory pathways
• TDP-43 relationship: Proteinopathy interactions
• Therapeutic targeting: Modulating TAM pathways

Huntington's Disease

• Striatal degeneration: Affected brain region impacts
• Microglial proliferation: Reactive microgliosis
• Therapeutic approaches: TAM modulation strategies
• Genetic models: Understanding disease mechanisms

Multiple System Atrophy

• Oligodendrocyte pathology: Myelin degeneration
• Microglial involvement: Neuroinflammation
• α-Synuclein interaction: Protein aggregation
• Therapeutic targets: Multiple pathways

Therapeutic Development

Agonist Approaches

• Recombinant GAS6: Protein therapeutics
• Small molecule activators: Direct receptor activation
• Protein S mimetics: PROS1-based approaches
• Gene therapy: AAV-mediated expression

Antagonist Approaches

• Receptor tyrosine kinase inhibitors: Blocking activation
• Soluble receptor decoys: sAXL, sMERTK
• Neutralizing antibodies: Ligand blockade
• Signal transduction inhibitors: Downstream pathways

Biomarker Development

• Soluble AXL: Diagnostic and prognostic marker
• GAS6 levels: Disease progression indicator
• Cerebrospinal fluid markers: CNS involvement
• Imaging targets: PET ligand development

Research Methods

Molecular Techniques

• CRISPR/Cas9: Genetic manipulation
• RNA sequencing: Transcriptomic analysis
• Proteomics: Protein interaction networks
• Single-cell analysis: Cellular heterogeneity

Animal Models

• Knockout mice: TAM receptor deficiency
• Transgenic models: Disease-associated mutations
• Humanized models: Human TAM expression
• Conditional knockouts: Tissue-specific deletion

Clinical Studies

• Biomarker validation: Clinical utility assessment
• Therapeutic trials: Drug development
• Genetic associations: Risk factor identification
• Natural history: Disease progression studies

TAM Receptor Structure

Extracellular Domain

• Receptor dimerization: Ligand binding interface
• Ig-like domains: Protein interactions
• Growth factor repeats: Specificity determinants

Kinase Domain

• Activation loop: Autophosphorylation sites
• SH2 docking: Signal transduction
• Catalytic activity: Tyrosine kinase function

Cytoplasmic Signaling

• Phosphorylation sites: Downstream signaling
• Adaptor protein binding: Signal complexity
• Negative regulation: Feedback mechanisms

Interaction Networks

TREM2 Interaction

TREM2 and TAM cooperate in microglia:

• Phagocytic pathways: Complementary functions
• Inflammatory signaling: Cross-talk mechanisms
• Disease relevance: AD risk and progression
• Therapeutic potential: Combined targeting

Cytokine Networks

• Interferon responses: Type II interferon interactions
• IL-10 signaling: Anti-inflammatory pathways
• TNF modulation: Inflammatory regulation
• Chemokine production: Immune cell recruitment

Cell Adhesion

• Integrin interactions: Cell-matrix relationships
• Ephrin signaling: Developmental pathways
• Cadherin systems: Cell-cell adhesion

Future Directions

Basic Science Questions

• Receptor activation: Precise molecular mechanisms
• Cell-type specificity: Tissue-specific functions
• Temporal dynamics: Developmental vs. adult roles
• Species differences: Human vs. mouse biology

Clinical Translation

• Target validation: Clinical utility confirmation
• Biomarker development: Patient stratification
• Therapeutic window: Safety and efficacy
• Combination approaches: Multi-target strategies

Emerging Technologies

• Single-cell proteomics: Cellular resolution
• Spatial transcriptomics: Tissue architecture
• Organoid models: 3D culture systems
• Gene editing: Therapeutic applications

TAM Receptor Signaling in Other CNS Disorders

Stroke and Brain Injury

• Ischemic stroke: TAM in post-stroke recovery
• Traumatic brain injury: Neuroinflammation modulation
• Hemorrhagic injury: Vascular damage responses
• Rehabilitation: Recovery mechanisms

Epilepsy

• Seizure-induced changes: TAM expression alterations
• Inflammatory responses: Glial involvement
• Therapeutic targeting: Anti-epileptic potential
• Status epilepticus: Long-term consequences

Mood Disorders

• Depression: TAM signaling abnormalities
• Anxiety: Inflammatory mechanisms
• Bipolar disorder: TAM in mood regulation
• Therapeutic implications: Novel treatments

Addiction

• Dopaminergic systems: Mesolimbic pathways
• Microglial involvement: Reward circuitry
• Relapse mechanisms: Neuroinflammation
• Treatment targets: TAM modulation

Advanced Research Techniques

Single-Cell Analysis

• Single-cell RNA-seq: Cellular heterogeneity
• ATAC-seq: Chromatin accessibility
• Spatial transcriptomics: Tissue architecture
• Proteomics: Protein levels

Structural Biology

• Cryo-EM: Receptor structures
• X-ray crystallography: Ligand binding
• Molecular docking: Drug design
• Mutagenesis studies: Functional domains

Live Imaging

• Intravital microscopy: Brain imaging
• Two-photon imaging: Neuronal activity
• Light sheet fluorescence: Large volumes
• Super-resolution: Nanoscale imaging

TAM Signaling in Pain

Neuropathic Pain

• Microglial TAM: Pain modulation
• Inflammatory pain: Cytokine involvement
• Peripheral sensitization: Nerve injury
• Therapeutic targeting: Pain management

Inflammatory Pain

• TAM in inflammation: Pro- and anti-inflammatory
• Cytokine interactions: Complex signaling
• Peripheral mechanisms: Non-neuronal cells
• Analgesic approaches: Novel targets

TAM and Neurogenesis

Adult Neurogenesis

• Neural stem cells: TAM expression
• Proliferation: Growth factor effects
• Differentiation: Lineage commitment
• Therapeutic potential: Regeneration

Developmental Neurogenesis

• Embryonic development: Temporal patterns
• Radial glia: Stem cell niches
• Migration: Neuronal positioning
• Circuit formation: Integration

Comparative TAM Biology

Evolution of TAM Receptors

• Vertebrate origins: Evolutionary conservation
• Gene duplications: Receptor family expansion
• Species differences: Functional variations
• Phylogenetic analysis: Family relationships

TAM in Non-Mammalian Systems

• Zebrafish models: Developmental studies
• Drosophila: Homolog function
• C. elegans: Conserved pathways
• Xenopus: Developmental biology

Clinical Trial Design

Biomarker Selection

• Soluble AXL: Patient stratification
• GAS6 levels: Treatment response
• Imaging markers: Neuroinflammation
• Genetic markers: Pharmacogenomics

Endpoint Selection

• Clinical measures: Cognitive and functional
• Biomarker endpoints: Surrogate markers
• Patient-reported outcomes: Quality of life
• Composite endpoints: Multiple measures

Trial Populations

• Disease staging: Early vs. advanced
• Genetic subtyping: Mutation carriers
• Comorbidities: Exclusion criteria
• Prior treatment: Washout periods

Future Therapeutic Directions

Combination Therapies

• TAM + TREM2: Dual targeting
• TAM + anti-amyloid: Disease modification
• TAM + anti-inflammatory: Comprehensive
• TAM + neurotrophic: Regeneration

Personalized Medicine

• Genetic testing: Risk stratification
• Biomarker-driven: Patient selection
• Response prediction: Precision medicine
• Dose optimization: Individualized treatment

Novel Delivery Methods

• Brain penetration: CNS delivery strategies
• AAV vectors: Gene therapy
• Nanoparticles: Targeted delivery
• Cell-penetrating peptides: Intracellular delivery

Summary

TAM receptor signaling (TYRO3, AXL, MERTK) and their ligands (GAS6, PROS1) form a critical regulatory system for neuroinflammation and phagocytosis in the brain. These receptors are essential for microglial clearance of apoptotic cells and debris, and they modulate inflammatory tone through negative regulation of cytokine production. Dysregulated TAM signaling contributes to the pathogenesis of Alzheimer's disease, multiple sclerosis, ALS, and Parkinson's disease. Therapeutic targeting of TAM pathways offers potential for modulating neuroinflammation and enhancing debris clearance in neurodegenerative conditions.

See Also

• [Microglia](/cell-types/microglia) — Brain immune cells
• [Neuroinflammation](/mechanisms/neuroinflammation) — Inflammatory processes
• [Alzheimer's Disease](/diseases/alzheimers-disease) — AD pathophysiology
• [Phagocytosis](/mechanisms/phagocytosis-neurodegeneration) — Cellular clearance
• [BDNF Therapy](/therapeutics/bdnf-therapy) — Neurotrophic factor

External Links

• [PubMed](https://pubmed.ncbi.nlm.nih.gov/)
• [TAM System Review](https://doi.org/10.1038/s41583-019-0150-4)

Recent Research Updates (2024-2026)

• Li C et al. (2025 Dec) [Efferocytosis in Health and Disease.](https://pubmed.ncbi.nlm.nih.gov/41403915/). MedComm (2020)*
• Ravichandran KA et al. (2025 Dec 1) [Enhancing Tyro3 signaling ameliorates IL-1β production through STAT1 in Alzheimer's disease models.](https://pubmed.ncbi.nlm.nih.gov/41206011/). J Leukoc Biol*
• Fabiano MP et al. (2025 Aug 26) [Plasma extracellular vesicle surface-located GAS6/PROS1 and CD39/CD73 attenuate inflammation.](https://pubmed.ncbi.nlm.nih.gov/40751911/). Cell Rep*
• Yuan K et al. (2025 Aug) [Deacetylase SIRT2 Inhibition Promotes Microglial M2 Polarization Through Axl/PI3K/AKT to Alleviate White Matter Injury After Subarachnoid Hemorrhage.](https://pubmed.ncbi.nlm.nih.gov/39103659/). Transl Stroke Res*
• Carrera Silva EA et al. (2025 Jan) [New potential ligand-receptor axis involved in tissue repair as therapeutic targets in progressive multiple sclerosis.](https://pubmed.ncbi.nlm.nih.gov/39892997/). J Pharmacol Exp Ther*

TAM Signaling in Neurodegenerative Disease: Detailed Analysis

Molecular Mechanisms in AD

Amyloid-GAS6 Interaction

• GAS6 binding: Soluble AXL induction
• Phagocytic enhancement: Amyloid clearance
• Cytokine modulation: Anti-inflammatory effects
• Therapeutic implications: Receptor activation

Microglial TAM Expression

• AXL upregulation: Response to pathology
• MERTK reduction: Impaired phagocytosis
• Age-related changes: Glial senescence
• Therapeutic targeting: Expression modulation

Molecular Mechanisms in PD

Alpha-Synuclein and TAM

• GAS6 modulation: Ligand level changes
• Phagocytic dysfunction: Clearance impairment
• Neuroinflammation: Chronic activation
• Therapeutic approaches: Pathway activation

Molecular Mechanisms in ALS

Motor Neuron Environment

• Astrocytic TAM: Support cell signaling
• Microglial activation: Inflammatory phenotype
• Mutant SOD1 effects: Disease-specific changes
• Therapeutic potential: Multi-target approach

TAM as Therapeutic Target

Agonist Development

Recombinant Proteins

• GAS6 production: Purification challenges
• PROS1 therapeutics: Alternative ligand
• Fusion proteins: Enhanced activity
• Clinical status: Current trials

Small Molecule Agonists

• High-throughput screening: Identification
• Lead optimization: Drug development
• Preclinical models: Efficacy testing
• Clinical translation: Future directions

Biomarker-Driven Therapy

Patient Selection

• soluble AXL levels: Biomarker stratification
• GAS6 measurement: Disease monitoring
• Genetic testing: Risk assessment
• Imaging markers: Target engagement

Treatment Response

• Biomarker changes: On-target effects
• Clinical endpoints: Efficacy measures
• Safety monitoring: Adverse events
• Dose optimization: Individualized treatment

TAM in Psychiatric Conditions

Depression

• Inflammatory hypothesis: Cytokine involvement
• TAM signaling: Anti-inflammatory effects
• Treatment implications: Novel mechanisms
• Research directions: Clinical studies

Anxiety

• Neuroimmune interactions: Stress response
• GAS6 effects: Anxiolytic potential
• Microglial modulation: Target engagement
• Therapeutic approaches: Drug development

Schizophrenia

• Neurodevelopmental hypothesis: Early events
• Microglial involvement: Developmental pruning
• TAM functions: Brain development
• Treatment targeting: Disease modification

TAM Research: Future Directions

Basic Science Priorities

• Receptor structure: Crystallography studies
• Signal transduction: Pathway elucidation
• Cell-type specificity: Cellular resolution
• Temporal dynamics: Time course studies

Clinical Development

• Trial design: Patient enrichment
• Endpoint selection: Validated measures
• Combination approaches: Multi-target
• Personalized medicine: Biomarker-driven

Technology Development

• Imaging ligands: Target visualization
• Biomarker platforms: Diagnostic assays
• Gene therapy: Vector development
• Cell therapy: Stem cell approaches

References