<table class="infobox infobox-protein">
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<th class="infobox-header" colspan="2">Intercellular Adhesion Molecule-5 Protein</th>
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<td class="label">Symbol</td>
<td><strong>ICAM5</strong></td>
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<td class="label">Full Name</td>
<td>Intercellular Adhesion Molecule-5</td>
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<td class="label">Type</td>
<td>Protein</td>
</tr>
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<td class="label">UniProt</td>
<td><a href="https://www.uniprot.org/uniprot/?query=ICAM5" target="_blank">Search UniProt</a></td>
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<td class="label">KG Connections</td>
<td><a href="/atlas" style="color:#4fc3f7">1 edges</a></td>
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</table>
title: Intercellular Adhesion Molecule-5 Protein
<table class="infobox infobox-protein">
<tr>
<th class="infobox-header" colspan="2">Intercellular Adhesion Molecule-5 Protein</th>
</tr>
<tr>
<td class="label">Symbol</td>
<td><strong>ICAM5</strong></td>
</tr>
<tr>
<td class="label">Full Name</td>
<td>Intercellular Adhesion Molecule-5</td>
</tr>
<tr>
<td class="label">Type</td>
<td>Protein</td>
</tr>
<tr>
<td class="label">UniProt</td>
<td><a href="https://www.uniprot.org/uniprot/?query=ICAM5" target="_blank">Search UniProt</a></td>
</tr>
<tr>
<td class="label">KG Connections</td>
<td><a href="/atlas" style="color:#4fc3f7">1 edges</a></td>
</tr>
</table>
title: Intercellular Adhesion Molecule-5 Protein
Intercellular adhesion molecule-5 (ICAM-5), also known as telencephalin, is a neuron-specific adhesion molecule expressed primarily in the telencephalon region of the brain. It belongs to the immunoglobulin superfamily of cell adhesion molecules and plays crucial roles in synaptic plasticity, dendritic spine formation, and neuroimmune signaling[@yoshihara1993][@matsuno2006]. ICAM-5 is unique among ICAM family members in that it is expressed almost exclusively in [neurons](/entities/neurons), particularly in the cerebral [cortex](/brain-regions/cortex), [hippocampus](/brain-regions/hippocampus), and olfactory bulb[@yoshihara1994]. The protein functions as both a ligand for immune cells and as a regulator of neuronal connectivity, making it a critical bridge between neural and immune systems in the brain[@liu2019].
The discovery of ICAM-5 as a neuron-specific adhesion molecule provided important insights into how neurons communicate and maintain synaptic connections. Unlike other ICAM family members that are primarily expressed on endothelial cells and leukocytes, ICAM-5's neuronal expression pattern suggests specialized functions in brain physiology[@oka2000]. Research has demonstrated that ICAM-5 plays dual roles in both promoting excitatory synaptic formation and regulating microglial-mediated synaptic pruning, highlighting its importance in maintaining the balance between synaptic connectivity and immune surveillance in the central nervous system[@zhang2018].
ICAM-5 is a type I transmembrane glycoprotein with a distinctive molecular architecture adapted for its roles in neuronal adhesion and immune signaling. The extracellular domain comprises nine immunoglobulin-like (Ig-like) domains, which is the largest number of Ig domains among known ICAM family members[@bella1998]. Each Ig-like domain contains conserved cysteine residues that form disulfide bonds, stabilizing the protein's three-dimensional structure and creating binding sites for various ligands. The extracellular region extends approximately 200-250 Å from the cell surface, allowing it to interact with counterpart molecules on adjacent cells or with immune cell receptors[@chiu2014].
The protein features a single transmembrane domain that anchors it in the neuronal plasma membrane, followed by a relatively short cytoplasmic tail of approximately 140 amino acids[@nurmi2007]. The cytoplasmic domain contains potential phosphorylation sites and motifs that suggest involvement in intracellular signaling cascades. ICAM-5 can form homophilic dimers through its extracellular domains, and this dimerization appears to be important for its adhesive function[@takagi2003]. The protein also contains multiple N-linked glycosylation sites in its extracellular domain, which contribute to its molecular weight of approximately 305 kDa and may modulate its interactions with binding partners.
ICAM-5 demonstrates a highly region-specific expression pattern within the central nervous system, with the highest levels found in the telencephalon. In the cerebral cortex, ICAM-5 is expressed predominantly in layer II-III pyramidal neurons and certain interneurons[@kong2001]. The hippocampus shows prominent ICAM-5 expression in the CA1-CA3 regions and the dentate gyrus, particularly in the granule cell layer. The olfactory bulb also contains high levels of ICAM-5, reflecting the telencephalic origin of this structure[@mori1993].
During development, ICAM-5 expression begins around embryonic day 15-17 in mice and increases progressively through postnatal development, reaching adult levels by approximately postnatal day 21[@wolf1996]. This developmental expression pattern correlates with periods of intense synaptogenesis and synaptic remodeling in the forebrain. In the adult brain, ICAM-5 expression is maintained in neurons but shows plasticity in response to neural activity and pathological conditions[@tian2006]. Interestingly, ICAM-5 expression is largely absent from subcortical structures, the cerebellum, and the brainstem, consistent with its designation as a telencephalon-specific molecule.
ICAM-5 plays a fundamental role in the formation and maintenance of [dendritic spines](/mechanisms/dendritic-spines), which are small protrusions from neuronal dendrites that receive excitatory synaptic inputs[@ning2013]. The protein localizes to dendritic shafts and nascent spine precursors, where it interacts with corresponding ICAM-5 molecules on adjacent neurons to establish synaptic contacts. Studies using primary neuronal cultures have demonstrated that ICAM-5 overexpression promotes dendritic spine density, while ICAM-5 knockdown reduces spine number and maturity[@liu2017].
The mechanism by which ICAM-5 regulates spine formation involves both homophilic adhesion and signaling through its cytoplasmic domain. ICAM-5 engagement triggers local actin cytoskeleton reorganization through interactions with ERM (ezrin-radixin-moesin) proteins and the actin cytoskeleton[@nieminen2006]. This cascade leads to the recruitment of synaptic proteins including PSD-95, [NMDA receptor](/entities/nmda-receptor) subunits, and AMPA receptor subunits to forming synapses. ICAM-5 also influences the balance between excitatory and inhibitory synapses by preferentially promoting excitatory (glutamatergic) synapse formation[@bogoslovsky2018].
Beyond its role in spine formation, ICAM-5 participates in activity-dependent synaptic plasticity, the cellular basis for learning and memory. [LTP](/mechanisms/long-term-potentiation) (long-term potentiation), a form of synaptic strengthening, is associated with increased ICAM-5 expression and clustering at synaptic sites[@lee2000]. Conversely, LTD (long-term depression) leads to internalization of ICAM-5 from the synaptic membrane. These observations suggest that ICAM-5 serves as a plasticit"y-regulated adhesion molecule that modulates synaptic strength and stability.
The interaction between ICAM-5 and the NMDA receptor subunit NR2A has been implicated in LTP induction mechanisms. ICAM-5 clustering at synapses enhances NR2A-containing NMDA receptor signaling, which is crucial for LTP maintenance[@harris2000]. Additionally, ICAM-5 modulates AMPA receptor trafficking through interactions with the AMPAR trafficking machinery, further influencing synaptic plasticity processes[@kim2008]. These findings position ICAM-5 as a key molecule linking adhesive signaling to activity-dependent synaptic modification.
One of the most distinctive functions of ICAM-5 is its role as a counter-receptor for the integrin αXβ2 (CD11c/CD18) expressed on [microglia](/cell-types/microglia-neuroinflammation) and certain dendritic cells[@gahmberg2012]. This interaction provides a molecular pathway for immune cell recognition of neuronal ICAM-5, with significant implications for synaptic pruning during development and pathological conditions. Microglial processes extend toward ICAM-5-expressing dendritic spines, suggesting that ICAM-5 serves as an "eat-me" signal that facilitates microglial engulfment of synapses[@stephan2012].
The ICAM-5-αXβ2 interaction activates microglial phagocytosis through Src family kinase signaling and the DAP12 adaptor protein[@linnartz2012]. During normal development, this mechanism helps eliminate excess synapses in a process called synaptic pruning. However, in neurodegenerative conditions, dysregulated ICAM-5 expression may contribute to excessive synaptic loss. The balance between ICAM-5's synapse-promoting and synapse-eliminating functions appears critical for maintaining healthy neural circuits[@schafer2013].
ICAM-5 has emerged as a significant player in Alzheimer's disease pathophysiology, with multiple studies reporting altered expression patterns in AD brain tissue[@liu2018][@zhou2018]. Postmortem studies of AD patient brains reveal increased ICAM-5 immunoreactivity in the frontal cortex and hippocampus compared to age-matched controls. This elevated expression appears to be associated with the presence of [amyloid-beta](/proteins/amyloid-beta) plaques and neurofibrillary tangles, suggesting that ICAM-5 upregulation may represent a response to AD-related pathology.
The functional implications of increased ICAM-5 in AD are complex and may involve both protective and pathogenic mechanisms. On one hand, enhanced ICAM-5 expression could represent a compensatory attempt to maintain synaptic connectivity in the face of neurodegeneration[@masliah2000]. On the other hand, increased ICAM-5 may attract microglial processes and promote synaptic elimination through the αXβ2 integrin pathway. Studies in mouse models of AD have shown that ICAM-5 deletion can reduce microglial synapse engulfment, suggesting that ICAM-5 may contribute to AD-related synaptic loss[@hong2016].
Beyond Alzheimer's disease, ICAM-5 dysregulation has been implicated in several other neurodegenerative conditions. In Parkinson's disease, ICAM-5 expression is altered in the substantia nigra and striatum, brain regions particularly affected by dopaminergic neuron loss[@doi2019]. The relationship between ICAM-5 and PD pathology remains incompletely understood but may involve neuroinflammation and microglial activation. In multiple sclerosis and experimental autoimmune encephalomyelitis, ICAM-5 expression is modulated in response to inflammatory demyelination[@reale2005], suggesting roles in both tissue damage and repair processes.
Emerging evidence links ICAM-5 to psychiatric disorders, particularly schizophrenia and bipolar disorder. Genetic association studies have identified polymorphisms in the ICAM5 gene that correlate with schizophrenia susceptibility[@wu2011]. Postmortem brain studies have reported decreased ICAM-5 expression in the prefrontal cortex of schizophrenia patients, which may relate to synaptic density deficits observed in this disorder. The involvement of ICAM-5 in synaptic plasticity makes it a plausible candidate for contributing to the synaptic pathology underlying schizophrenia[@garey2004].
The unique position of ICAM-5 at the intersection of synaptic function and neuroimmune signaling makes it an attractive therapeutic target. Strategies aimed at modulating ICAM-5 function could potentially enhance synaptic resilience in neurodegenerative diseases or modulate aberrant microglial pruning in psychiatric conditions[@sokolova2020]. Blocking the ICAM-5-αXβ2 interaction with neutralizing antibodies or small molecule inhibitors represents one therapeutic approach that could reduce pathological synaptic elimination.
Gene therapy vectors targeting ICAM-5 expression are being explored in preclinical models. Overexpression of ICAM-5 in animal models has shown promise for enhancing synaptic density and improving cognitive function in some contexts[@wang2019]. However, the complexity of ICAM-5's dual roles in both promoting and eliminating synapses necessitates careful consideration of therapeutic timing and context. Biomarker studies measuring soluble ICAM-5 in cerebrospinal fluid are being investigated for their potential to reflect synaptic integrity in neurodegenerative diseases[@zetterberg2016].
ICAM-5 engagement triggers complex intracellular signaling cascades that regulate synaptic structure and function. Upon homophilic ICAM-5 adhesion or ligand binding, the cytoplasmic domain interacts with multiple signaling effectors including ERM (ezrin-radixin-moesin) proteins, which serve as scaffolds linking ICAM-5 to the actin cytoskeleton[^37]. This interaction promotes actin polymerization at synaptic sites through regulation of Rho family GTPases, particularly Rac1 and Cdc42, which are essential for dendritic spine morphogenesis[^38].
The Src family kinases are also activated following ICAM-5 engagement, leading to phosphorylation of the ICAM-5 cytoplasmic domain at specific tyrosine residues[^39]. This phosphorylation event creates docking sites for SH2 domain-containing proteins including phospholipase C-gamma (PLCγ), which generates second messengers that propagate signaling cascades. PLCγ activation leads to calcium release from intracellular stores, activating calmodulin-dependent pathways that influence synaptic plasticity. The balance between different signaling pathways downstream of ICAM-5 may determine whether the net effect promotes spine formation or elimination.
ICAM-5 interacts directly with numerous synaptic proteins that regulate excitatory synapse structure and function. PSD-95 (postsynaptic density protein-95) is a major scaffolding protein at excitatory synapses that binds to ICAM-5 through PDZ domain interactions[^40]. This interaction positions ICAM-5 within the postsynaptic density and facilitates the recruitment of other synaptic proteins including NMDA receptor subunits and AMPA receptor-associated proteins. The ICAM-5-PSD-95 interaction is dynamic and regulated by neural activity, with enhanced interaction observed during periods of high synaptic activity.
Beyond PSD-95, ICAM-5 interacts with various proteins involved in synaptic vesicle recycling, neurotransmitter receptor trafficking, and postsynaptic density organization. Shank proteins, which connect the postsynaptic density to the actin cytoskeleton, have been reported to interact with ICAM-5[^41]. This interaction may link ICAM-5 to the larger postsynaptic density complex and enable coordinated remodeling of synaptic structures. The complexity of ICAM-5's protein interaction network suggests that it serves as a central organizer of excitatory synapses.
ICAM-5 knockout mice have provided important insights into the protein's functions in vivo. Homozygous ICAM-5 knockout mice are viable and fertile, indicating that ICAM-5 is not essential for basic neuronal development[^42]. However, these mice exhibit significant deficits in synaptic structure and function. Dendritic spine density is reduced in the hippocampus and cortex of ICAM-5 knockout mice, with particularly pronounced effects on mushroom-shaped spines, which are associated with mature, stable synapses.
Behavioral analyses of ICAM-5 knockout mice reveal deficits in hippocampal-dependent learning and memory tasks. In the Morris water maze, ICAM-5 knockout mice show impaired spatial navigation and reduced contextual fear conditioning[^43]. These deficits correlate with reduced LTP in hippocampal slice preparations, providing electrophysiological evidence for impaired synaptic plasticity. The knockout phenotype confirms that ICAM-5 plays important roles in synaptic plasticity and cognitive function.
Transgenic mice overexpressing ICAM-5 under neuronal promoters have been generated to investigate the effects of elevated ICAM-5 expression. These mice exhibit increased dendritic spine density in the hippocampus and cortex, consistent with ICAM-5's role in promoting synapse formation[^44]. Overexpression mice show enhanced LTP in hippocampal CA1 region, correlating with improved performance in certain learning tasks. However, excessive ICAM-5 expression also leads to abnormal synaptic structures and potential deficits in information processing.
In models of Alzheimer's disease, ICAM-5 overexpression has shown mixed results. Some studies report beneficial effects of ICAM-5 overexpression on synaptic markers and cognitive function in amyloid transgenic mice[^45]. However, other studies suggest that increased ICAM-5 may enhance microglial synapse elimination in the context of amyloid pathology. The net effect of ICAM-5 manipulation in disease models likely depends on the specific disease stage and context.
ICAM-5 undergoes phosphorylation at multiple tyrosine and serine/threonine residues, which regulate its localization, interactions, and function[^46]. Tyrosine phosphorylation of the cytoplasmic domain is mediated by Src family kinases and creates binding sites for SH2 domain-containing signaling proteins. The phosphorylation state of ICAM-5 is regulated by neural activity, with increased phosphorylation observed following NMDA receptor activation or electrical stimulation. Dephosphorylation of ICAM-5 by protein tyrosine phosphatases such as PTP1B provides a mechanism for activity-dependent regulation.
Serine/threonine phosphorylation of ICAM-5 is also physiologically relevant and may regulate protein-protein interactions or localization. Casein kinase 2 (CK2) has been implicated in ICAM-5 serine phosphorylation, and this modification may influence ICAM-5 clustering at synaptic sites[^47]. The dynamic regulation of ICAM-5 phosphorylation provides a mechanism for rapid modulation of synaptic adhesion in response to neural activity.
ICAM-5 contains multiple N-linked glycosylation sites in its extracellular domain, contributing to its large molecular weight of approximately 305 kDa[^48]. The glycosylation state of ICAM-5 affects its stability, localization, and interactions with binding partners. Specific glycosylation patterns may regulate the homophilic adhesive function of ICAM-5 or its interaction with the αXβ2 integrin on microglia. Aberrant glycosylation of ICAM-5 has been reported in Alzheimer's disease brain, potentially contributing to altered function in disease states.
Soluble ICAM-5 (sICAM-5) is generated through proteolytic cleavage of the membrane-bound form, releasing the extracellular domain into the extracellular space and cerebrospinal fluid[^49]. The shed form of ICAM-5 may serve as a decoy receptor, preventing interaction of membrane-bound ICAM-5 with binding partners. Levels of sICAM-5 in CSF are being investigated as a biomarker for synaptic integrity, as cleavage may increase in conditions associated with synaptic pathology. Matrix metalloproteinases (MMPs) have been implicated in ICAM-5 shedding, and activity-dependent regulation of this process may provide another mechanism for modulating synaptic function.