SEZ6L2

SEZ6L2 is a neuronal type-1 transmembrane protein that functions as both a cargo receptor and a synaptic scaffolding molecule to regulate endosomal trafficking, neurite outgrowth, and excitatory synaptic connectivity. SEZ6L2 binds cathepsin D and mediates its mannose-6-phosphate-independent sorting to endosomes; cathepsin D in turn cleaves SEZ6L2 to release a soluble N-terminal fragment that promotes neurite outgrowth, establishing a proteolytic feedback loop that couples lysosomal enzyme trafficking to neuronal differentiation (PMID:26698217). SEZ6L2 also scaffolds AMPA receptor subunit GluR1 to adducin, regulating adducin phosphorylation and excitatory synaptic signaling, and Sez6l2 knockout mice display impaired motor coordination, reduced sociability, and decreased dendritic spine length with diminished postsynaptic protein expression (PMID:29032200, PMID:41502956). Autoimmunity against the SEZ6L2 extracellular domain elicits CD4+ T cell-mediated cerebellar pathology and ataxia-like motor deficits in mice (PMID:41291765).

1. 2015 High

   Resolved how cathepsin D reaches endosomes independently of mannose-6-phosphate receptors: SEZ6L2 directly binds cathepsin D and mediates its endosomal sorting, and cathepsin D reciprocally cleaves SEZ6L2 to generate a soluble fragment that drives neurite outgrowth, revealing a proteolytic feedback loop coupling lysosomal enzyme trafficking to neuronal differentiation.

   Evidence Co-binding assays, siRNA knockdown in neuroblastoma cells, subcellular fractionation, HeLa complementation, and neurite outgrowth assays with cleavage fragments

   PMID:26698217

   Open questions at the time

   • Structural basis of the SEZ6L2–cathepsin D interaction is undefined
   • Whether this alternative sorting pathway operates in all neuronal subtypes in vivo is untested
   • Relative contribution of SEZ6L2 versus other alternative cathepsin D receptors is unknown
2. 2017 Medium

   Identified a scaffolding function at excitatory synapses: SEZ6L2 bridges AMPA receptor subunit GluR1 to adducin, regulating adducin phosphorylation and linking glutamate receptor signaling to cytoskeletal remodeling.

   Evidence Proteomics-based interactome, co-immunoprecipitation, siRNA knockdown, and overexpression with phosphorylation readouts in neuronal cells

   PMID:29032200

   Open questions at the time

   • Direct binding domains on SEZ6L2 for GluR1 and adducin have not been mapped
   • Functional impact on AMPA receptor surface expression or synaptic transmission not measured electrophysiologically
   • Findings from a single lab without independent replication
3. 2020 Medium

   Extended SEZ6L2 biology beyond neurons: SEZ6L2 knockdown in colorectal cancer cells triggers caspase-dependent apoptosis via the cytochrome C pathway, and SEZ6L2 surface upregulation on drug-resistant lung cancer cells contributes to chemoresistance, indicating a pro-survival role exploited by tumors.

   Evidence shRNA knockdown with apoptosis assays and xenografts (colorectal); antibody blocking of surface SEZ6L2 on drug-resistant lung cancer spheroids

   PMID:32105413 PMID:33202873

   Open questions at the time

   • Mechanism linking SEZ6L2 to mitochondrial apoptosis pathway components is not defined
   • Whether the cathepsin D transport or scaffolding functions underlie the cancer phenotypes is unknown
   • Antibody blocking specificity validated only by flow cytometry
4. 2022 Medium

   Defined transcriptional regulation of SEZ6L2: USF1 directly binds the SEZ6L2 promoter to activate its transcription in breast cancer cells.

   Evidence Promoter binding assay (ChIP/luciferase), knockdown and overexpression with proliferation and invasion readouts

   PMID:35523305

   Open questions at the time

   • Whether USF1 regulates SEZ6L2 in neurons is untested
   • Other transcription factors contributing to SEZ6L2 expression are uncharacterized
   • Single lab finding without independent confirmation
5. 2025 Medium

   Established post-transcriptional control of SEZ6L2: IGF2BP2 stabilizes SEZ6L2 mRNA through m6A methylation, providing a mechanism for SEZ6L2 upregulation in cisplatin-resistant NSCLC.

   Evidence RNA immunoprecipitation, RNA decay assay, dual-luciferase reporter, functional rescue, and xenograft models

   PMID:40801328

   Open questions at the time

   • Specific m6A sites on SEZ6L2 mRNA are not mapped at nucleotide resolution
   • Relevance of IGF2BP2-mediated stabilization in neuronal contexts is unknown
   • Single lab study
6. 2025 Medium

   Demonstrated that autoimmunity against SEZ6L2 is pathogenic: immunization of mice with SEZ6L2 generates anti-SEZ6L2 IgG and drives CD4+ T cell infiltration into the cerebellum, producing ataxia-like motor deficits.

   Evidence Mouse active immunization model with cerebellar IgG deposition, T-cell epitope mapping, and behavioral assays

   PMID:41291765

   Open questions at the time

   • Whether antibody-mediated or T cell-mediated mechanisms are primary drivers of pathology is not resolved
   • Transfer experiments with purified patient IgG have not been performed
   • Relevance to human autoimmune cerebellar ataxia requires clinical correlation

• The in vivo requirement of SEZ6L2 for synaptic connectivity and behavior has been characterized in a knockout mouse showing reduced dendritic spine length and postsynaptic protein loss with motor, social, and sensory deficits, but the molecular pathway linking SEZ6L2 loss to these synaptic phenotypes — whether through impaired cathepsin D trafficking, disrupted GluR1-adducin scaffolding, or both — remains unresolved.
• Electrophysiological characterization of synaptic function in Sez6l2 KO mice has not been reported
• Whether cathepsin D missorting contributes to the synaptic and behavioral phenotypes is untested
• No conditional or cell-type-specific knockout has been generated to distinguish cell-autonomous roles