ADAM22 (A Disintegrin And Metalloproteinase domain 22) is a member of the ADAM family of transmembrane proteins that plays a critical role in synaptic function and neurological disease. Unlike many ADAM family members, ADAM22 is catalytically inactive, functioning primarily as a cell adhesion molecule rather than a protease. This unique characteristic makes it a crucial mediator of synaptic architecture and neuronal signaling through its role as the primary receptor for the leucine-rich glioma inactivated 1 (LGI1) protein[@fukata2006].
The gene encoding ADAM22 is located on chromosome 7q21.2 and is expressed predominantly in the central nervous system, particularly in brain regions essential for cognitive function and motor control. The protein localizes to postsynaptic membranes, where it forms trans-synaptic complexes with presynaptic proteins to regulate neurotransmitter release and synaptic plasticity. Mutations in ADAM22 have been implicated in genetic epilepsy, Alzheimer's disease, and autism spectrum disorder, highlighting its importance in maintaining normal neurological function[@schulte2016].
Research over the past two decades has established ADAM22 as a critical node in synaptic signaling networks, with effects on AMPA receptor trafficking, neuronal excitability, and myelination. This comprehensive review examines the molecular biology of ADAM22, its interactions with LGI1 and other proteins, and its involvement in neurodegenerative and neurodevelopmental disorders.
ADAM22 (A Disintegrin And Metalloproteinase domain 22) is a member of the ADAM family of transmembrane proteins that plays a critical role in synaptic function and neurological disease. Unlike many ADAM family members, ADAM22 is catalytically inactive, functioning primarily as a cell adhesion molecule rather than a protease. This unique characteristic makes it a crucial mediator of synaptic architecture and neuronal signaling through its role as the primary receptor for the leucine-rich glioma inactivated 1 (LGI1) protein[@fukata2006].
The gene encoding ADAM22 is located on chromosome 7q21.2 and is expressed predominantly in the central nervous system, particularly in brain regions essential for cognitive function and motor control. The protein localizes to postsynaptic membranes, where it forms trans-synaptic complexes with presynaptic proteins to regulate neurotransmitter release and synaptic plasticity. Mutations in ADAM22 have been implicated in genetic epilepsy, Alzheimer's disease, and autism spectrum disorder, highlighting its importance in maintaining normal neurological function[@schulte2016].
Research over the past two decades has established ADAM22 as a critical node in synaptic signaling networks, with effects on AMPA receptor trafficking, neuronal excitability, and myelination. This comprehensive review examines the molecular biology of ADAM22, its interactions with LGI1 and other proteins, and its involvement in neurodegenerative and neurodevelopmental disorders.
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<tr><th colspan="2" style="background:#e8f4f8; text-align:center; font-size:1.1em;">ADAM Metallopeptidase Domain 22</th></tr>
<tr><td><strong>Gene Symbol</strong></td><td>ADAM22</td></tr>
<tr><td><strong>Full Name</strong></td><td>ADAM metallopeptidase domain 22</td></tr>
<tr><td><strong>Chromosome</strong></td><td>7q21.2</td></tr>
<tr><td><strong>NCBI Gene ID</strong></td><td>[5365](https://www.ncbi.nlm.nih.gov/gene/5365)</td></tr>
<tr><td><strong>OMIM</strong></td><td>605283</td></tr>
<tr><td><strong>Ensembl ID</strong></td><td>ENSG00000010704</td></tr>
<tr><td><strong>UniProt ID</strong></td><td>[Q9P121](https://www.uniprot.org/uniprot/Q9P121)</td></tr>
<tr><td><strong>Associated Diseases</strong></td><td>Epilepsy, Alzheimer's Disease, Autism Spectrum Disorder</td></tr>
</table>
</div>
The ADAM22 gene spans approximately 25 kilobases and consists of 24 exons encoding a protein of 834 amino acids. The genomic structure follows the conserved pattern of ADAM family genes, with a prodomain, metalloproteinase domain, disintegrin domain, cysteine-rich region, EGF-like domain, transmembrane domain, and cytoplasmic tail. However, critical mutations in the zinc-binding motif (HEXGHNLG) of the metalloproteinase domain render ADAM22 catalytically inactive[@sagane2008].
The ADAM22 protein contains several distinct domains that mediate its functions:
The cytoplasmic tail of ADAM22 is particularly important, terminating in the sequence EVKV, which binds to PDZ domain-containing proteins including MAGUK family members. This interaction is essential for clustering ADAM22 at synaptic membranes and coupling it to intracellular signaling pathways[@owu2013].
The primary known function of ADAM22 is as the postsynaptic receptor for LGI1. LGI1 is a secreted protein that was originally identified as a tumor suppressor in glioma. The LGI1-ADAM22 complex forms trans-synaptic bridges that are essential for proper synaptic function[@yamagata2018].
LGI1 binds to ADAM22 through its leucine-rich repeat (LRR) domain, interacting specifically with the cysteine-rich region and disintegrin domain of ADAM22. This binding is of high affinity (Kd ~10-100 nM) and is essential for the localization and function of both proteins at synapses. The LGI1-ADAM22 complex interacts with presynaptic ADAM23 to form trans-synaptic bridges that regulate neurotransmitter release[@fukata2006].
ADAM22 plays a critical role in the trafficking and localization of AMPA-type glutamate receptors at postsynaptic membranes. Through its interaction with the LGI1-ADAM22 complex and associated MAGUK scaffold proteins, ADAM22 recruits and stabilizes AMPA receptors at excitatory synapses. Loss of ADAM22 function leads to reduced AMPA receptor density and impaired synaptic transmission[@choi2021].
The mechanism involves direct interaction between ADAM22 and the AMPA receptor subunits GluA1-4, particularly through the stargazin-like transmembrane AMPA receptor regulatory proteins (TARPs). This interaction is dependent on the PDZ-binding motif in the ADAM22 cytoplasmic tail and the presence of LGI1[@liu2021].
By modulating synaptic transmission through AMPA receptor trafficking, ADAM22 indirectly regulates neuronal excitability. Studies in knockout mice demonstrate that ADAM22 deficiency leads to increased neuronal excitability and spontaneous seizures. This effect is mediated primarily through impaired inhibitory synaptic function and reduced postsynaptic responses to excitatory neurotransmitters[@lee2020].
ADAM22 is expressed not only in neurons but also in oligodendrocytes and their precursors. In oligodendrocytes, ADAM22 participates in myelination processes through mechanisms that are still being characterized. ADAM22 deficiency leads to hypomyelination and axonal degeneration in white matter tracts, highlighting its importance in glial cells as well as neurons[@zhou2022].
ADAM22 exhibits high expression in regions of the central nervous system:
At the subcellular level, ADAM22 localizes primarily to postsynaptic densities (PSDs) of excitatory synapses. It colocalizes with postsynaptic marker proteins including PSD-95, NMDA receptor subunits, and AMPA receptor subunits. The protein is anchored to the postsynaptic membrane through its transmembrane domain, with its extracellular domain facing the synaptic cleft to interact with presynaptic ligands.
ADAM22 mutations are associated with autosomal dominant lateral temporal lobe epilepsy (ADLTE). These mutations typically result in haploinsufficiency, leading to reduced ADAM22 protein levels or function at synapses. Patients present with focal seizures originating from the lateral temporal lobe, often with auditory features[@schulte2016].
The mechanism involves impaired LGI1 binding and consequent disruption of synaptic signaling. Reduced ADAM22 function leads to decreased AMPA receptor trafficking and altered synaptic plasticity. Notably, mutations in both ADAM22 and LGI1 can cause similar epilepsy phenotypes, highlighting the importance of the LGI1-ADAM22 axis in seizure suppression[@yang2024].
Multiple lines of evidence suggest a role for ADAM22 in Alzheimer's disease pathophysiology:
Given the critical role of ADAM22 in synaptic function, it is not surprising that ADAM22 variants have been implicated in autism spectrum disorder (ASD). Rare missense mutations in ADAM22 have been identified in patients with ASD, and these mutations often show dominant-negative effects on synaptic function. The phenotype includes social deficits, communication difficulties, and repetitive behaviors consistent with impaired synaptic connectivity[@peng2022].
Understanding the LGI1-ADAM22 axis has provided new therapeutic targets for epilepsy:
ADAM22-based therapeutic strategies for AD include:
For ASD and related conditions, strategies include:
The LGI1-ADAM22 complex influences presynaptic function through retrograde signaling. Binding of LGI1 to ADAM22 triggers presynaptic changes that regulate neurotransmitter release. This involves modulation of voltage-gated calcium channels and synaptic vesicle release machinery[@owu2013].
At the postsynaptic density, ADAM22 activates several signaling cascades:
These pathways integrate signals from ADAM22 to regulate synaptic strength, dendritic spine morphology, and neuronal survival[@kong2023].
ADAM22 interacts with numerous proteins beyond LGI1:
ADAM22 knockout mice exhibit:
Tissue-specific knockouts have revealed:
High-resolution structural studies of the LGI1-ADAM22 complex are needed to:
Development of biomarkers for ADAM22-related conditions includes:
Translational priorities include: