Close Homolog of L1 Regulates Dendritic Spine Density in the Mouse Cerebral Cortex Through Semaphorin 3B

Abstract

Dendritic spines in the developing mammalian neocortex are initially overproduced and then eliminated during adolescence to achieve appropriate levels of excitation in mature networks. We show here that the L1 family cell adhesion molecule Close Homolog of L1 (CHL1) and secreted repellent ligand Semaphorin 3B (Sema3B) function together to induce dendritic spine pruning in developing cortical pyramidal neurons. Loss of CHL1 in null mutant mice in both genders resulted in increased spine density and a greater proportion of immature spines on apical dendrites in the prefrontal and visual cortex. Electron microscopy showed that excitatory spine synapses with postsynaptic densities were increased in the CHL1-null cortex, and electrophysiological recording in prefrontal slices from mutant mice revealed deficiencies in excitatory synaptic transmission. Mechanistically, Sema3B protein induced elimination of spines on apical dendrites of cortical neurons cultured from wild-type but not CHL1-null embryos. Sema3B was secreted by the cortical neuron cultures, and its levels increased when cells were treated with the GABA antagonist gabazine. In vivo CHL1 was coexpressed with Sema3B in pyramidal neuron subpopulations and formed a complex with Sema3B receptor subunits Neuropilin-2 and PlexinA4. CHL1 and NrCAM, a closely related L1 adhesion molecule, localized primarily to distinct spines and promoted spine elimination to Sema3B or Sema3F, respectively. These results support a new concept in which selective spine elimination is achieved through different secreted semaphorins and L1 family adhesion molecules to sculpt functional neural circuits during postnatal maturation. Significance StatementDendritic spines in the mammalian neocortex are initially overproduced and then pruned in adolescent life through unclear mechanisms to sculpt maturing cortical circuits. Here, we show that spine and excitatory synapse density of pyramidal neurons in the developing neocortex is regulated by the L1 adhesion molecule, Close Homolog of L1 (CHL1). CHL1 mediated spine pruning in response to the secreted repellent ligand Semaphorin 3B and associated with receptor subunits Neuropilin-2 and PlexinA4. CHL1 and related L1 adhesion molecule NrCAM localized to distinct spines, and promoted spine elimination to Semaphorin 3B and -3F, respectively. These results support a new concept in which selective elimination of individual spines and nascent synapses can be achieved through the action of distinct secreted semaphorins and L1 adhesion molecules.

Molecular Mechanisms of L1 and NCAM Adhesion Molecules in Synaptic Pruning, Plasticity, and Stabilization

“…L1-CAMs are established promoters of neural adhesion, migration, and process outgrowth ( Maness and Schachner, 2007 ). Among these, NrCAM and CHL1 were found to mediate adolescent spine pruning in mouse genetic models ( Demyanenko et al, 2014 ; Mohan et al, 2019a , b ). Variations in the NrCAM gene have been implicated in autism ( Hutcheson et al, 2004 ; Bonora et al, 2005 ; Sakurai et al, 2006 ; Marui et al, 2009 ; Pinto et al, 2010 ; Voineagu et al, 2011 ) and schizophrenia ( Kim et al, 2009 ; Ayalew et al, 2012 ; Zhang et al, 2015 ), while mutations in the CHL1 (CALL) gene are linked to intellectual disability ( Angeloni et al, 1999 ; Frints et al, 2003 ; Cuoco et al, 2011 ), schizophrenia ( Sakurai et al, 2002 ; Chen et al, 2005 ; Chu and Liu, 2010 ), and autism ( Salyakina et al, 2011 ).…”

Patient iPSC-astrocytes show transcriptional and functional dysregulation in schizophrenia

“…Notably, we demonstrated strongly altered astrocyte expression of neural cell adhesion molecule L1-like protein (CHL1) between affected and unaffected males and females, and even between unaffected and healthy males. CHL1 regulates neuronal survival and growth, and functions together with Semaphorin 3B to induce dendritic spine pruning in developing pyramidal neurons 37 . CHL1 has been linked to SCZ in several studies and its expression is regulated by clozapine 38 .…”

A Novel Mechanism Underlying Activity-Dependent Pruning in Postnatal Prefrontal Cortex

“…L1 cell adhesion molecules are also known to interact with class 3 semaphorins (Sema3; Wright et al, 2007;Mohan et al, 2019a), a family of secreted guidance cues that induce repulsion of growing neural processes (Liu and Strittmatter, 2001) and can promote dendritic retraction. In a recent study, Mohan et al (2019b) examined how Close Homolog of L1 (CHL1) affects dendritic spine dynamics in cortical regions during adolescence, and whether dendritic remodeling at this time is mediated by CHL1-Sema3 interactions.…”

“…Although it has been speculated that cell adhesion receptors and diffusible molecules that guide connectivity during early development are involved in synaptic remodeling in adolescent PFC, few of the precise molecular players that drive adolescent pruning have been identified. Here, Mohan et al (2019b) identify interactions between CHL1 and Sema3B as a novel mechanism by which immature or unneeded synapses may be selectively pruned during postnatal remodeling of PFC circuitry. Their findings indicate that CHL1/Sema3B-mediated pruning is likely to occur in an activity-dependent manner, but how spines are selected for elimination by this mechanism, and also the circumstances under which Sema3B is secreted in response to activity require further examination.…”

“…Although Mohan et al (2019b) found that Sema3B was secreted in response to a widespread, experimentally induced increase in neural activity in culture, it will be important to determine in what situations Sema3s are secreted in intact circuits and in response to experience. One possibility is that release of Sema3B by recently potentiated synapses drives elimination of less active neighboring spines expressing CHL1 receptor complexes.…”