<table class="infobox infobox-gene">
<tr>
<th class="infobox-header" colspan="2">SIGLEC9 — Sialic Acid Binding Ig-Like Lectin 9</th>
</tr>
<tr>
<td class="label">Feature</td>
<td>Value</td>
</tr>
<tr>
<td class="label">Gene Symbol</td>
<td>SIGLEC9</td>
</tr>
<tr>
<td class="label">Chromosomal Location</td>
<td>19q13.4</td>
</tr>
<tr>
<td class="label">NCBI Gene ID</td>
<td>27180</td>
</tr>
<tr>
<td class="label">UniProt ID</td>
<td>Q9Y736</td>
</tr>
<tr>
<td class="label">Genomic Coordinates</td>
<td>chr19:50,761,328-50,778,310 (GRCh38)</td>
</tr>
<tr>
<td class="label">Exon Count</td>
<td>11</td>
</tr>
<tr>
<td class="label">Protein Length</td>
<td>482 amino acids</td>
</tr>
<tr>
<td class="label">Molecular Weight</td>
<td>~55 kDa</td>
</tr>
<tr>
<td class="label">Expression</td>
<td>Neutrophils, monocytes, NK cells, B cells</td>
</tr>
<tr>
<td class="label">Associated Diseases</td>
<td><a href="/wiki/als" style="color:#ef9a9a">Als</a>, <a href="/wiki/alzheimer" style="color:#ef9a9a">Alzheimer</a>, <a href="/wiki/dementia" style="color:#ef9a9a">Dementia</a>, <a href="/wiki/glioblastoma" style="color:#ef9a9a">Glioblastoma</a>, <a href="/wiki/ms" style="color:#ef9a9a">Ms</a></td>
</tr>
<tr>
<td class="label">KG Connections</td>
<td><a href="/atlas" style="color:#4fc3f7">20 edges</a></td>
</tr>
</table>
<table class="infobox infobox-gene">
<tr>
<th class="infobox-header" colspan="2">SIGLEC9 — Sialic Acid Binding Ig-Like Lectin 9</th>
</tr>
<tr>
<td class="label">Feature</td>
<td>Value</td>
</tr>
<tr>
<td class="label">Gene Symbol</td>
<td>SIGLEC9</td>
</tr>
<tr>
<td class="label">Chromosomal Location</td>
<td>19q13.4</td>
</tr>
<tr>
<td class="label">NCBI Gene ID</td>
<td>27180</td>
</tr>
<tr>
<td class="label">UniProt ID</td>
<td>Q9Y736</td>
</tr>
<tr>
<td class="label">Genomic Coordinates</td>
<td>chr19:50,761,328-50,778,310 (GRCh38)</td>
</tr>
<tr>
<td class="label">Exon Count</td>
<td>11</td>
</tr>
<tr>
<td class="label">Protein Length</td>
<td>482 amino acids</td>
</tr>
<tr>
<td class="label">Molecular Weight</td>
<td>~55 kDa</td>
</tr>
<tr>
<td class="label">Expression</td>
<td>Neutrophils, monocytes, NK cells, B cells</td>
</tr>
<tr>
<td class="label">Associated Diseases</td>
<td><a href="/wiki/als" style="color:#ef9a9a">Als</a>, <a href="/wiki/alzheimer" style="color:#ef9a9a">Alzheimer</a>, <a href="/wiki/dementia" style="color:#ef9a9a">Dementia</a>, <a href="/wiki/glioblastoma" style="color:#ef9a9a">Glioblastoma</a>, <a href="/wiki/ms" style="color:#ef9a9a">Ms</a></td>
</tr>
<tr>
<td class="label">KG Connections</td>
<td><a href="/atlas" style="color:#4fc3f7">20 edges</a></td>
</tr>
</table>
SIGLEC9 (Sialic Acid Binding Ig-Like Lectin 9) is a member of the CD33-related Siglec family of inhibitory lectins. It is encoded by the SIGLEC9 gene located on chromosome 19q13.4 and is expressed predominantly on neutrophils, monocytes, and to a lesser extent on natural killer (NK) cells and B cells [1][2]. SIGLEC9 functions as an inhibitory receptor that regulates immune cell activation through recognition of sialylated glycans on host cells and pathogens. This lectin plays critical roles in immune homeostasis, pathogen recognition, and inflammatory responses, with emerging evidence suggesting involvement in neuroinflammation and neurodegenerative diseases. [@zhang2021]
The SIGLEC9 protein is also known as CD329 and shares significant sequence homology with SIGLEC10, with which it is physically linked on chromosome 19q13.4. Both genes likely arose from a gene duplication event and have overlapping but distinct functions. [@crocker2020]
SIGLEC9 exhibits the characteristic Siglec domain organization:
SIGLEC9 exhibits preferential binding to specific sialylated structures:
High Affinity Targets:
SIGLEC9 transmits inhibitory signals through its ITIM motifs:
Activation Sequence:
ITIM Sequence:
Neutrophil Functions:
SIGLEC9 recognizes sialylated structures on various pathogens:
Bacterial Recognition:
Tumor Cell Recognition:
SIGLEC9 involvement in Alzheimer's disease centers on neuroinflammation:
Microglial Regulation:
SIGLEC9 plays multiple roles in Parkinson's disease:
Microglial Activation:
Evidence for SIGLEC9 in ALS includes:
Neuroinflammation:
SIGLEC9 in multiple sclerosis:
Autoimmunity:
Common SIGLEC9 variants include:
Specific haplotypes have been associated with:
Agonists:
SIGLEC9 measurement may help:
SIGLEC9 is an inhibitory sialic acid-binding lectin primarily expressed on neutrophils and monocytes that plays essential roles in immune regulation and inflammation. Through its ITIM-mediated signaling, SIGLEC9 suppresses immune cell activation and helps maintain immune homeostasis. Its expression on microglia and capacity to modulate neuroinflammation link it to neurodegenerative diseases including Alzheimer's disease, Parkinson's disease, and ALS. The recognition of sialylated molecules on pathological protein aggregates and the genetic associations with neurodegenerative disease risk suggest that SIGLEC9 may represent both a therapeutic target and a biomarker for these conditions. Further research into SIGLEC9 biology will clarify its contributions to neurodegeneration and potentially lead to novel treatment strategies.
The immunoreceptor tyrosine-based inhibition motif (ITIM) signaling pathway in SIGLEC9 involves a precisely orchestrated cascade of molecular events. Upon engagement of the V-type domain with sialylated ligands, SIGLEC9 molecules cluster on the cell surface, creating a high local concentration of ITIM-bearing receptors. Src family kinases, particularly Lyn and Fyn, recognize the consensus ITIM sequence (I/V/L/S)YXXL/V and phosphorylate the tyrosine residues within the motif. The phosphorylated ITIMs then serve as docking sites for SH2 domain-containing phosphatases, predominantly SHP-1 (PTPN6) and SHP-2 (PTPN11). These phosphatases possess high affinity for the phosphorylated ITIM sequences and are recruited rapidly to the receptor complex. Once recruited, SHP-1 and SHP-2 dephosphorylate key signaling molecules downstream of activating receptors, effectively dampening the immune response. The specificity of this inhibition depends on the tissue-specific expression of phosphatases and the particular signaling pathways active in the cell type [@lall2023].
SIGLEC9 exhibits remarkable specificity for sialylated glycans, which are terminal modifications on glycoproteins and glycolipids. The V-type domain contains a conserved binding pocket that recognizes the carboxyl group of sialic acid and the underlying glycan structure. The specificity for different sialylated structures is determined by a small set of amino acid residues in the binding pocket. SIGLEC9 preferentially binds to α2,3-linked sialic acids found on certain leukocyte populations and some pathogen surfaces. The affinity for sialyl-Lewis X (sLeX), a tetrasaccharide expressed on activated leukosomes and some tumor cells, is particularly high. This differential binding enables SIGLEC9 to distinguish between self and non-self patterns of sialylation. The sialic acid modifications themselves can vary, with N-acetylneuraminic acid (Neu5Ac) being the most common form in humans, while N-glycolylneuraminic acid (Neu5Gc) is not synthesized in humans but can be incorporated from dietary sources [@angata2022].
The Siglec family in humans comprises 14 members divided into two evolutionary groups: the CD33-related SIGLECs (SIGLEC3, 5, 6, 7, 8, 9, 10, 11, 14, 16) and the conserved subgroup (SIGLEC1, 2, 4). SIGLEC9 and SIGLEC10 are closely related, located in tandem on chromosome 19q13.4, and likely arose from a gene duplication event during primate evolution. Both proteins share similar domain architecture and binding preferences but have distinct expression patterns. SIGLEC9 is predominantly expressed on neutrophils, while SIGLEC10 is more restricted to B cells and eosinophils. This functional divergence reflects different biological roles in immune regulation. The Siglec family shows remarkable species-specific expansions, with mice having a different repertoire of Siglecs compared to humans. This evolutionary diversification suggests that Siglecs have adapted to species-specific immune challenges and host-pathogen interactions [@angata2022].
While SIGLEC9 is primarily known for its expression on peripheral immune cells, emerging evidence suggests that microglia, the resident immune cells of the brain, express SIGLEC family members including SIGLEC9. Microglial SIGLEC9 likely plays similar roles in these cells as it does in peripheral monocytes, providing inhibitory signals that help maintain the balance between protective immune surveillance and pathological inflammation. Under normal conditions, SIGLEC9 may help keep microglial activation in check, preventing excessive inflammatory responses that could damage neurons. In neurodegenerative diseases, this inhibitory function may be overwhelmed or dysregulated, contributing to chronic neuroinflammation. The interaction of SIGLEC9 with sialylated molecules on amyloid-β plaques and α-synuclein aggregates suggests a role in modulating microglial clearance of these pathological proteins [@bov2024].
The role of SIGLEC9 in neuroinflammatory signaling extends beyond direct immune cell modulation. In the Alzheimer's disease brain, chronic neuroinflammation drives disease progression through multiple pathways. Microglial activation leads to production of pro-inflammatory cytokines including TNF-α, IL-1β, and IL-6, which can exacerbate tau pathology and neuronal loss. SIGLEC9, through its inhibitory signaling, may help limit this pathological inflammation. Genetic variants in SIGLEC9 that alter its function could modify an individual's susceptibility to neuroinflammation-driven neurodegeneration. Studies have identified polymorphisms in the SIGLEC9 gene region that are associated with altered Alzheimer's disease risk, suggesting that this immune regulatory gene may contribute to disease susceptibility. The expression levels of SIGLEC9 in the brain may also serve as a biomarker for neuroinflammatory status in neurodegenerative diseases [@yang2023].
In Parkinson's disease, SIGLEC9 plays multiple roles in the neuroimmune interface. The recognition of sialylated forms of α-synuclein by SIGLEC9 on microglia may modulate the clearance of Lewy bodies, the pathological protein aggregates characteristic of PD. Impaired SIGLEC9 function could reduce the efficiency of this clearance mechanism, allowing pathological proteins to accumulate. Additionally, SIGLEC9 regulates peripheral immune responses that may influence CNS pathology through multiple mechanisms. Gut inflammation, which is increasingly recognized as a contributing factor in PD, may involve SIGLEC9 dysregulation given the importance of this receptor in intestinal immune responses. The bidirectional communication between the gut and brain through the gut-brain axis provides multiple points where SIGLEC9 could influence PD pathogenesis [@liu2024].
SIGLEC9 serves as an important pattern recognition receptor for pathogens that display sialylated surface structures. Several bacterial species have evolved to express sialylated molecules that engage SIGLECs, potentially modulating the host immune response. Haemophilus influenzae, Neisseria meningitidis, and Streptococcus pneumoniae all express sialylated surface glycans that can bind to SIGLEC9. This interaction may provide these pathogens with a mechanism to suppress host immune responses by engaging inhibitory receptors. Conversely, the host may have evolved to use SIGLECs as part of the first line of defense against sialylated pathogens. The balance between pathogen exploitation of SIGLEC signaling and host utilization of SIGLEC for immune defense represents a dynamic evolutionary arms race [@frickel2023].
The role of SIGLEC9 in cancer immunology has received increasing attention in recent years. Many tumors overexpress sialylated structures including sLeX and sLeA, which can engage SIGLEC9 on natural killer cells and other immune effectors. This engagement may suppress anti-tumor immune responses, allowing tumors to evade immune surveillance. SIGLEC9 on NK cells can recognize tumor cells expressing high levels of sialylated glycans, leading to ITIM-mediated inhibition of NK cell cytotoxicity. This represents a novel immune checkpoint mechanism that tumors may exploit. Therapeutic strategies targeting this pathway include blocking antibodies that prevent SIGLEC9 engagement with tumor-associated glycans, potentially restoring NK cell-mediated anti-tumor immunity. The development of glycan-based biomarkers that predict SIGLEC9 engagement may enable patient stratification for immunotherapy approaches [@hsu2023].
SIGLEC9 contributes to immune homeostasis and may play a role in preventing autoimmunity. The receptor helps distinguish self from non-self based on patterns of sialylation, with loss of self-sialylation potentially triggering autoimmune responses. In rheumatic diseases, alterations in SIGLEC9 expression or function have been reported, suggesting a role in disease pathogenesis. The balance between activating and inhibitory Siglecs determines the threshold for immune activation, and dysregulation of this balance can lead to either excessive inflammation or impaired immune surveillance. Understanding SIGLEC9 function in autoimmunity may lead to novel therapeutic approaches that restore immune balance.
The development of SIGLEC9-targeted therapeutics follows several strategies depending on the desired outcome. Agonists that activate SIGLEC9 inhibitory signaling could be useful in treating inflammatory and autoimmune conditions. These could be synthetic small molecules, antibody-based agonists, or sialylated ligand mimetics that engage the receptor without triggering pathogen exploitation. Conversely, antagonists that block SIGLEC9 function could enhance anti-tumor immunity by removing an inhibitory checkpoint. Chimeric antigen receptor (CAR) T cells engineered to lack SIGLEC9 expression may show enhanced anti-tumor activity. The development of SIGLEC9-modulating therapeutics requires careful consideration of the desired outcome and potential off-target effects [@padler2024].
SIGLEC9 has potential as a biomarker for multiple conditions. Soluble SIGLEC9 levels in cerebrospinal fluid may reflect neuroinflammatory status in neurodegenerative diseases. Peripheral expression of SIGLEC9 on neutrophils and monocytes could serve as a biomarker for systemic inflammation. Genetic variants in SIGLEC9 may predict disease risk or progression, enabling personalized medicine approaches. The development of robust assays for SIGLEC9 measurement in clinical samples is an active area of research. Combination biomarkers incorporating SIGLEC9 with other immune parameters may provide more comprehensive assessments of disease status.
The following diagram shows the key molecular relationships involving SIGLEC9 — Sialic Acid Binding Ig-Like Lectin 9 discovered through SciDEX knowledge graph analysis: