SIRPA Protein

SIRPA Protein

Overview

Signal regulatory protein alpha (SIRPA), encoded by the SIRPA gene located on chromosome 20q13.13, is a transmembrane glycoprotein belonging to the immunoglobulin superfamily. SIRPA (also known as MFR, SHPS-1, and P84) functions as a key regulator of innate immune signaling and cell-cell interactions. The protein comprises an extracellular domain with three immunoglobulin-like domains, a transmembrane domain, and a cytoplasmic tail containing immunoreceptor tyrosine-based inhibitory motifs (ITIMs). SIRPA is expressed primarily on myeloid cells, including microglia, macrophages, and dendritic cells, making it particularly relevant to neuroinflammatory processes in the central nervous system.

Function and Biology

SIRPA functions as a "don't eat me" signal through its interaction with CD47, a membrane protein widely expressed on cell surfaces including neurons. This interaction prevents phagocytosis and promotes cell survival. When SIRPA engages CD47, the protein recruits Src homology 2 domain-containing protein tyrosine phosphatase (SHP-2) to its cytoplasmic ITIMs, triggering inhibitory signaling cascades that suppress microglial activation and phagocytic capacity. Beyond the CD47 interaction, SIRPA also binds thrombospondin-1 (TSP-1) and other ligands that modulate immune responses. The protein plays essential roles in regulating microglial surveillance behavior, controlling immune tolerance, and maintaining appropriate neuroinflammatory homeostasis within the brain parenchyma.

Role in Neurodegeneration

SIRPA Protein

Overview

Signal regulatory protein alpha (SIRPA), encoded by the SIRPA gene located on chromosome 20q13.13, is a transmembrane glycoprotein belonging to the immunoglobulin superfamily. SIRPA (also known as MFR, SHPS-1, and P84) functions as a key regulator of innate immune signaling and cell-cell interactions. The protein comprises an extracellular domain with three immunoglobulin-like domains, a transmembrane domain, and a cytoplasmic tail containing immunoreceptor tyrosine-based inhibitory motifs (ITIMs). SIRPA is expressed primarily on myeloid cells, including microglia, macrophages, and dendritic cells, making it particularly relevant to neuroinflammatory processes in the central nervous system.

Function and Biology

SIRPA functions as a "don't eat me" signal through its interaction with CD47, a membrane protein widely expressed on cell surfaces including neurons. This interaction prevents phagocytosis and promotes cell survival. When SIRPA engages CD47, the protein recruits Src homology 2 domain-containing protein tyrosine phosphatase (SHP-2) to its cytoplasmic ITIMs, triggering inhibitory signaling cascades that suppress microglial activation and phagocytic capacity. Beyond the CD47 interaction, SIRPA also binds thrombospondin-1 (TSP-1) and other ligands that modulate immune responses. The protein plays essential roles in regulating microglial surveillance behavior, controlling immune tolerance, and maintaining appropriate neuroinflammatory homeostasis within the brain parenchyma.

Role in Neurodegeneration

SIRPA's involvement in neurodegeneration centers on its regulation of microglial-mediated neuronal clearance and inflammatory responses. In Alzheimer's disease, impaired SIRPA-CD47 signaling may contribute to excessive microglial phagocytosis of viable neurons and synapses, a phenomenon termed "over-pruning." Studies demonstrate that aging and neuroinflammatory conditions can reduce SIRPA expression on microglia, compromising the protective CD47 signal and rendering neurons more susceptible to microglial attack. In Parkinson's disease, dysregulated SIRPA signaling correlates with enhanced microglial activation in response to α-synuclein pathology. The protein has also been implicated in amyotrophic lateral sclerosis (ALS), where altered SIRPA expression on myeloid cells may contribute to the neuroinflammatory cascade driving motor neuron degeneration. Additionally, accumulating evidence suggests that SIRPA dysfunction exacerbates neurodegeneration in Huntington's disease through enhanced striatal inflammation and impaired microglial regulation.

Molecular Mechanisms

SIRPA exerts neuroprotective effects through several interconnected mechanisms. The SIRPA-CD47-SHP-2 axis generates inhibitory signals that suppress the pro-inflammatory transcription factors NF-κB and STAT3, thereby reducing production of tumor necrosis factor-alpha (TNF-α), interleukin-1beta (IL-1β), and other cytotoxic mediators. SIRPA activation also enhances microglial expression of anti-inflammatory and tissue-remodeling factors, including interleukin-10 (IL-10) and transforming growth factor-beta (TGF-β). The protein regulates microglial metabolic reprogramming, shifting cells from pro-inflammatory glycolytic metabolism toward anti-inflammatory oxidative phosphorylation. Furthermore, SIRPA modulates synaptic pruning dynamics by controlling the complement cascade and the fractalkine-CX3CR1 chemokine axis, both critical determinants of microglial-synaptic interactions. Loss or reduced SIRPA signaling permits excessive complement deposition and synaptic engulfment, contributing to cognitive decline and neuronal loss.

Clinical and Research Significance

SIRPA represents an emerging therapeutic target for neurodegeneration. Strategies to enhance SIRPA-CD47 signaling—such as blocking antibodies against CD47 competitors, CD47-expressing exosomes, or SIRPA-agonist compounds—show promise in preclinical models. Clinical interest centers on SIRPA polymorphisms associated with neurodegenerative disease risk and progression rates. Biomarker studies investigate soluble SIRPA levels in cerebrospinal fluid and plasma as indicators of microglial dysfunction in neurodegenerative conditions.

Related Entities

• CD47: Primary SIRPA ligand mediating "don't eat me" signals
• SHP-2: Phosphatase recruited by SIRPA ITIMs
• Microglia: Primary SIRPA-expressing cells in the brain
• Thrombospondin-1: Alternative SIRPA ligand
• Complement system: Interacting pathway in synaptic pruning
• CX3CL1/Fractalkine: Related chemokine axis regulating microglial function