Cell adhesion molecule L1 interacts with the chromo shadow domain of heterochromatin protein 1 isoforms α, β, and ɣ via its intracellular domain

Abstract

Cell adhesion molecule L1 regulates multiple cell functions and L1 deficiency is linked to several neural diseases. Proteolytic processing generates functionally decisive L1 fragments, which are imported into the nucleus. By computational analysis, we found at L1's C‐terminal end the chromo shadow domain‐binding motif PxVxL, which directs the binding of nuclear proteins to the heterochromatin protein 1 (HP1) isoforms α, β, and ɣ. By enzyme‐linked immunosorbent assay, we show that the intracellular L1 domain binds to all HP1 isoforms. These interactions involve the HP1 chromo shadow domain and are mediated via the sequence 1158KDET1161 in the intracellular domain of murine L1, but not by L1's C‐terminal PxVxL motif. Immunoprecipitation using nuclear extracts from the brain and from cultured cerebellar and cortical neurons indicates that HP1 isoforms interact with a yet unknown nuclear L1 fragment of approximately 55 kDa (L1‐55), which carries ubiquitin residues. Proximity ligation indicates a close association between L1‐55 and the HP1 isoforms in neuronal nuclei. This association is reduced after the treatment of neurons with inhibitors of metalloproteases, β‐site of amyloid precursor protein cleaving enzyme (BACE1), or ɣ‐secretase, suggesting that cleavage of full‐length L1 by these proteases generates L1‐55. Reduction of HP1α, ‐β, or ‐ɣ expression by siRNA decreases L1‐dependent neurite outgrowth from cultured cortical neurons and decreases the L1‐dependent migration of L1‐transfected HEK293 cells in a scratch assay. These findings indicate that the interaction of the novel fragment L1‐55 with HP1 isoforms in nuclei affects L1‐dependent functions, such as neurite outgrowth and neuronal migration.

The Cell Adhesion Molecule L1 Interacts with Methyl CpG Binding Protein 2 via Its Intracellular Domain

“…Recently, we identified a ~55 kDa L1 fragment (L1−55) interacting with heterochromatin protein 1 isoforms α, β, and γ via the KDET motif, and we proposed that the generation of this fragment involves the metalloprotease ADAM-10 and/or -17, BACE1, and γ-secretase [ 39 ]. Since it was very likely that HP1-binding L1−55 is identical to the ~55 kDa L1 fragment that binds to MeCP2, we analyzed whether the generation of the MeCP2-binding ~55 kDa L1 fragment also involves ADAM-10 and/or -17, BACE1, and γ-secretase.…”

“…Of note, similar numbers of L1/MeCP2-positive puncta were found in non-stimulated and antibody 557-stimulated cortical neurons ( Figure 7 c), indicating that the interaction between L1 and MeCP2 is not enhanced by the stimulation of cortical neurons with antibody 557. Interestingly, the interaction between L1 and HP1, which is also mediated by the KDET motif [ 39 ], was not affected by the peptide in the absence of antibody 557, whereas the antibody 557-induced L1/HP1 interaction was inhibited by the peptide ( Figure 7 d).…”

“…It is noteworthy in this respect that MeCP2 interacts with HP1γ and that the MeCP2/HP1γ interaction contributes to the reorganization of heterochromatin during myogenesis [ 58 , 59 ]. Interestingly, HP1α, -β, and -γ, like MeCP2, interact with L1−55 [ 39 ] and regulate L1 functions, such as cell migration and neurite outgrowth. Histone H1.4, another nuclear L1 binding partner [ 23 ], competes with MeCP2 for common chromatin binding sites and decreases the binding of HP1γ to DNA [ 60 ].…”

To Stick or Not to Stick: The Multiple Roles of Cell Adhesion Molecules in Neural Circuit Assembly

“…The sumoylated L1CAM fragment hence generated traffics to endosomes and the cytoplasm before translocating in a sumoylation-dependent manner to the nucleus where it interacts with multiple nuclear proteins ( Girbes Minguez et al, 2020 ). Interaction with heterochromatin protein 1 is notably required for L1CAM-mediated neurite outgrowth in cultured cortical neurons ( Kleene et al, 2022 ). Conversely, the intracellular domains generated after cleavage of DSCAM and DSCAML1 by γ-secretase alter the transcription of genes regulating circuit formation and inhibit axon growth when overexpressed in cortical neurons ( Sachse et al, 2019 ).…”

Shining Light on the Dark Side of the Genome

“…Interactions between HP1a and SU(VAR)3-9 propagate heterochromatin along the chromosome, generating domains of silent chromatin [28][29][30][31][32]. The CSDs of HP1 dimers form a platform for interaction with chromatin proteins that possess the pentapeptide PxVxL (x = any amino acid) or a variant of that sequence [33][34][35][36][37] (Figure 1b). CSD interaction partners include the histone demethylase dKDM4A [38], which co-localizes with HP1a within centric heterochromatin and plays a role in repair of heterochromatic DNA damage [35,39,40].…”