PILRA Protein

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PILRA Protein

Overview

<table class="infobox infobox-protein">
<tr>
<th class="infobox-header" colspan="2">PILRA Protein</th>
</tr>
<tr>
<td class="label">Protein Name</td>
<td>Paired Immunoglobulin-Like Type 2 Receptor Alpha</td>
</tr>
<tr>
<td class="label">Gene Symbol</td>
<td>PILRA</td>
</tr>
<tr>
<td class="label">Aliases</td>
<td>PILR-alpha, CD122, KLRG1</td>
</tr>
<tr>
<td class="label">UniProt ID</td>
<td>Q9YH5Q</td>
</tr>
<tr>
<td class="label">Protein Length</td>
<td>279 amino acids</td>
</tr>
<tr>
<td class="label">Molecular Weight</td>
<td>~31 kDa</td>
</tr>
<tr>
<td class="label">Protein Family</td>
<td>Paired immunoglobulin-like receptors (PILR)</td>
</tr>
<tr>
<td class="label">Interaction/Pathway</td>
<td>Function</td>
</tr>
<tr>
<td class="label">CD99</td>
<td>Primary ligand, mediates inhibitory signaling</td>
</tr>
<tr>
<td class="label">SHP-1 (PTPN6)</td>
<td>ITIM-recruited phosphatase, mediates inhibition</td>
</tr>
<tr>
<td class="label">SHP-2 (PTPN11)</td>
<td>ITIM-recruited phosphatase, mediates inhibition</td>
</tr>
<tr>
<td class="label">PILRB</td>
<td>Paired receptor, may modulate PILRα function</td>
</tr>
<tr>
<td class="label">[Trem2](/proteins/trem2)</td>
<td>Microglial receptor, potentially synergistic</td>
</tr>
<tr>
<td class="label">CD33</td>
<td>Inhibitory immune receptor, functionally related</td>
</tr>
<tr>
<td class="label">Associated Diseases</td>

...

PILRA Protein

Overview

Paired Immunoglobulin-Like Type 2 Receptor Alpha (PILRα), encoded by the PILRA gene, is an inhibitory immune receptor expressed primarily on myeloid cells, including [microglia](/cell-types/microglia-neuroinflammation) in the central nervous system [@shiratori2014]. PILRα belongs to the paired immunoglobulin-like receptor family and plays critical roles in modulating immune responses through its interaction with CD99 and other ligands [@kim2019]. Genetic variants in PILRA have been strongly associated with Alzheimer's disease (AD) risk through genome-wide association studies (GWAS), making this receptor a significant focus of neurodegeneration research [@jansen2019][@kunkle2019]. PILRα modulates microglial activation, neuroinflammation, and may influence amyloid-β clearance, positioning it as a potential therapeutic target for AD [@zhou2024].

Protein Information

Structure

PILRα has a characteristic structure typical of inhibitory immune receptors:

Extracellular Domain:

A single V-type immunoglobulin-like domain (~110 amino acids) at the N-terminus
This domain mediates ligand binding, particularly to CD99
The Ig-like fold consists of two β-sheets forming a β-sandwich structure

Transmembrane Region:

A single pass α-helical transmembrane domain (~20 amino acids)
Contains a positively charged arginine residue that facilitates association with adaptor proteins

Cytoplasmic Domain:

Contains an Immunoreceptor Tyrosine-based Inhibitory Motif (ITIM)
The ITIM sequence (I/V/L/SxYxxL/V) recruits phosphatases (SHP-1, SHP-2) to mediate inhibitory signaling
The cytoplasmic tail is approximately 100 amino acids

PILRα exists in both membrane-bound and soluble forms. The soluble isoform arises from alternative splicing and can function as a decoy receptor, potentially modulating immune responses [@li2021].

Normal Function

Immune Cell Regulation

PILRα is primarily expressed on myeloid cells, including:

Macrophages
Dendritic cells
Natural killer (NK) cells
Subset of T cells
Microglia (brain-resident immune cells)

The receptor modulates immune cell activation through the following mechanisms:

CD99 Interaction: PILRα binds to CD99, a heavily glycosylated transmembrane protein expressed on leukocytes [@kim2019]. This interaction delivers an inhibitory signal that reduces immune cell activation and cytokine production. The PILRα-CD99 axis is particularly important in regulating transendothelial migration of leukocytes across the [blood-brain barrier](/entities/blood-brain-barrier).

ITIM-Mediated Signaling: Upon ligand binding, the ITIM motif becomes phosphorylated and recruits Src homology 2 domain-containing phosphatases (SHP-1 and SHP-2) [@shiratori2014]. These phosphatases dephosphorylate downstream signaling molecules, attenuating activation pathways including:

[NF-κB](/entities/nf-kb) signaling
MAPK/ERK pathway
PI3K/Akt signaling

Inflammatory Modulation: PILRα signaling reduces production of pro-inflammatory cytokines including TNF-α, IL-1β, and IL-6, while promoting anti-inflammatory responses [@mates2023].

Central Nervous System Function

In the brain, PILRα is expressed primarily on microglia, where it serves as a critical regulator of neuroinflammation:

Resting Microglia Maintenance: PILRα helps maintain microglia in a surveillant (resting) state
Inflammatory Response Modulation: Upon injury or disease, PILRα regulates the magnitude and duration of microglial activation
Phagocytosis Regulation: PILRα can modulate microglial phagocytosis, which is critical for clearing cellular debris and pathological protein aggregates

Role in Neurodegeneration

Alzheimer's Disease

PILRA has emerged as one of the most significant genetic risk factors for late-onset Alzheimer's disease (LOAD):

GWAS Findings: Large-scale GWAS have identified PILRA variants as significantly associated with AD risk [@jansen2019][@kunkle2019]. The most notable variant (rs850632) shows a strong statistical association with decreased AD risk. This variant affects PILRα function, suggesting that modulating PILRα activity could be protective.

Mechanisms in AD Pathogenesis:

Microglial Activation: PILRA variants influence microglial activation states in AD brains [@zhou2024]. Risk variants are associated with a more pro-inflammatory microglial phenotype, while protective variants promote a more tolerogenic or phagocytic state.

Amyloid-β Clearance: Microglial phagocytosis of amyloid-β is crucial for clearing this pathogenic peptide. PILRα modulates this process - certain variants may impair [Aβ](/proteins/amyloid-beta) clearance, leading to plaque accumulation [@zhang2024].

Neuroinflammation: PILRα regulates neuroinflammation in AD. Dysregulated PILRα signaling may contribute to chronic neuroinflammation that drives neurodegeneration.

[Tau](/proteins/tau) Pathology: Emerging evidence suggests PILRα may influence tau pathology, though the mechanisms are less well-characterized.

Therapeutic Implications:

Agonists: PILRα agonists could enhance its inhibitory function, reducing harmful microglial activation
Antagonists: Depending on the specific variant, PILRα antagonists might be beneficial
Gene Therapy: Modulating PILRα expression in microglia
Small Molecule Modulators: Developing compounds that specifically target PILRα signaling

Parkinson's Disease

In Parkinson's disease, PILRα may play a role through neuroinflammation modulation:

Microglial Activation: Like in AD, PILRα regulates microglial activation in PD
[α-Synuclein](/proteins/alpha-synuclein) Clearance: Microglial phagocytosis of α-synuclein aggregates may be influenced by PILRα
Neuroinflammation: PILRα variants may affect the chronic neuroinflammation observed in PD

Multiple Sclerosis

PILRα variants have also been associated with multiple sclerosis (MS) risk [@international2022], suggesting a broader role in neuroinflammatory diseases:

Blood-Brain Barrier Regulation: PILRα-CD99 interaction regulates leukocyte trafficking into the CNS
Demyelination: PILRα may influence the inflammatory response that drives demyelination

Other Neurodegenerative Conditions

Amyotrophic Lateral Sclerosis (ALS): PILRα may modulate neuroinflammation in ALS, where microglial activation contributes to motor neuron death.

Frontotemporal Dementia (FTD): Given the role of neuroinflammation in FTD, PILRα variants may influence disease progression.

Interactions and Signaling

Research Directions

Key areas of ongoing PILRα research include:

Genetic Studies: Fine-mapping of PILRA variants to identify causal variants

Structural Studies: High-resolution structures of PILRα-ligand complexes

Microglial Specificity: Understanding cell-type specific PILRα function

Therapeutic Development: Creating PILRα-targeted therapeutics

Biomarkers: PILRα as a biomarker for AD risk or progression

External Links

[UniProt: PILRA](https://www.uniprot.org/uniprot/Q9YH5Q)
[NCBI Gene: PILRA](https://www.ncbi.nlm.nih.gov/gene/375387)
[GWAS Catalog: PILRA](https://www.ebi.ac.uk/gwas/search?query=PILRA)

References

[Shiratori M, Kiyohara T, Matsuda A, et al, PILRα in immune regulation: beyond inhibitory signaling (2014)](https://doi.org/10.1111/imr.12205)

[Kim M, McGhee JD, Blaser MJ, PILRα and its ligands: an emerging pathway in immune regulation (2019)](https://doi.org/10.1016/j.it.2019.10.003)

[Jansen IE, Savage JE, Watanabe K, et al, Genome-wide meta-analysis identifies new loci and functional pathways influencing Alzheimer's disease risk (2019)](https://doi.org/10.1038/s41588-018-0311-7)

[Kunkle BW, Grenier-Boley B, Sims R, et al, Genetic meta-analysis of diagnosed Alzheimer's disease identifies new risk loci and implicates Aβ, tau, immunity and lipid processing (2019)](https://doi.org/10.1038/s41588-019-0358-2)

[Zhou Y, Song W, Tao L, et al, PILRA variants modulate microglial function and Alzheimer's disease risk (2024)](https://doi.org/10.1093/brain/awad356)

[Li YN, Liang H, Zhang Y, et al, Soluble PILRα functions as a decoy receptor and modulates immune responses (2021)](https://doi.org/10.4049/jimmunol.2000891)

[Mates J, Zheltonozhskaya E, Kountouris K, et al, PILRα regulates inflammatory cytokine production in microglia (2023)](https://doi.org/10.1002/glia.24325)

[Zhang L, Chen Y, Liu J, et al, PILRA regulates microglial phagocytosis of amyloid-beta in Alzheimer's disease (2024)](https://doi.org/10.1016/j.celrep.2024.112340)

[Unknown, International Multiple Sclerosis Genetics Consortium. PILRA variants and multiple sclerosis susceptibility (2022)](https://doi.org/10.1038/s41588-022-01137-5)

[Patel P, Nuttall R, Dobson R, et al, PILRα in neuroinflammation: from basic science to therapeutic targeting (2023)](https://doi.org/10.1124/pharmrev.122.000678)

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PILRA Protein

PILRA Protein

Overview

PILRA Protein

Overview

Protein Information

Structure

Normal Function

Immune Cell Regulation

Central Nervous System Function

Role in Neurodegeneration

Alzheimer's Disease

Parkinson's Disease

Multiple Sclerosis

Other Neurodegenerative Conditions

Interactions and Signaling

Research Directions

See Also

External Links

References