The L1 Cell Adhesion Molecule Constrains Dendritic Spine Density through Ankyrin Binding in Pyramidal Neurons of the Mouse Cerebral Cortex

Abstract

A novel function for L1 cell adhesion molecule and its interaction with Ankyrin, an actin-spectrin adaptor protein, was identified in constraining dendritic spine density on pyramidal neurons in the mouse neocortex. In an L1-null mouse mutant increased spine density was observed on apical but not basal dendrites of pyramidal neurons in diverse cortical areas (prefrontal cortex layer 2/3, motor cortex layer 5, visual cortex layer 4).The Ankyrin binding motif (FIGQY) in L1’s cytoplasmic domain was critical for spine formation, as demonstrated by increased spine density in the prefrontal cortex of a mouse mutant (L1YH) harboring a tyrosine to histidine mutation in this motif, which disrupts L1-Ankyrin association. This mutation is a known variant in the human L1 syndrome. In both mutants mature mushroom spines rather than immature spines were predominant. L1 was detected in spines and dendrites of wild-type prefrontal cortical neurons by immmunostaining. L1 coimmunoprecipitated with Ankyrin B (220 kDa) from cortical lysates of wild-type but not L1YH mice. Spine pruning assays in cortical neuron cultures from wild-type and L1YH mutant mice showed that the L1-Ankyrin interaction mediated spine retraction in response to the class 3 Semaphorins, Sema3F and to a lesser extent Sema3B. These ligands also induce spine pruning through other L1 family adhesion molecules, NrCAM and Close Homolog of L1 (CHL1), respectively. This study provides insight into the molecular mechanism of spine regulation and underscore the potential for this adhesion molecule to regulate cognitive and other L1-related functions that are abnormal in the L1 syndrome.

Ankyrin B Promotes Developmental Spine Regulation in the Mouse Prefrontal Cortex

“…NrCAM engaged AnkB-220 at a conserved FIGQY motif in the NrCAM cytoplasmic domain. This motif was required for spine elimination induced by Sema3F, as mutation of FIGQY to FIGQH disrupted AnkB-220 binding to NrCAM and blocked spine retraction in cortical neurons, similar to a previous study in L1 knock-in mice with a substitution in the FIGQY motif (FIGQH) (Murphy et al 2023a). As shown in Figure 4F, NrCAM binds Npn2 through extracellular domain interactions which increase Npn2/PlexA3 affinity (Mohan et al 2019a), activating the intrinsic Rap-GAP activity of PlexA3 (Wang et al 2013).…”

“…NrCAM F/F : RCE) (Mohan et al 2019b), as well as in Sema3F-null, Npn2-null, and PlexA3-null mice (Tran et al 2009;Mohan et al 2019b). This elevated spine phenotype on apical dendrites is consistent with AnkB-mediated spine elimination through the Sema3F holoreceptor complex and was also observed in the PFC of CHL1-null and L1null mutant mice (Murphy et al 2023a). Since CHL1 shares a homologous Ankyrin binding site (FIGAY) and binds Npn2, Sema3B-induced retraction of a distinct spine subpopulation likely also depends on AnkB.…”

“…Neuronglial related CAM (NrCAM), Npn2, and PlexA3 constitute a receptor complex for Sema3F (Demyanenko et al 2014;Mohan et al 2019a), while Close Homolog of L1 (CHL1), Npn2, and PlexA4 form a receptor for Sema3B (Mohan et al 2019b). Genetic knockout of NrCAM, CHL1 or L1 (Murphy et al 2023a) in mice increased spine density on apical dendrites of cortical pyramidal neurons. Sema3F and Sema3B are secreted by neurons in an activity-dependent manner, consistent with the notion that less active or immature spines may be pruned by Sema3s released from active synaptic neighbors to refine cortical circuits in development (Wang et al 2017;Mohan et al 2019a).…”

“…The copyright holder for this preprint (which this version posted July 11, 2023. ; https://doi.org/10.1101/2023.07.11.548527 doi: bioRxiv preprint All L1-CAMs bind the actin-spectrin adaptor protein Ankyrin B (AnkB) at a conserved cytoplasmic domain motif (FIGQY) (Bennett et al 2009). Mice with a point mutation in the L1 Ankyrin binding site (FIGQH) display increased spine density in the prefrontal cortex (Murphy et al 2023a) suggesting that AnkB may be required for spine pruning in vivo. This L1 mutation is a human pathological mutation associated with intellectual disability (Hortsch et al 2014).…”