An intracellular domain with a novel sequence regulates cell surface expression and synaptic clustering of leucine-rich repeat transmembrane proteins in hippocampal neurons

Large Donor CRISPR for Whole-Coding Sequence Replacement of Cell Adhesion Molecule LRRTM2

The cell adhesion molecule leucine-rich repeat transmembrane neuronal protein 2 (LRRTM2) is crucial for synapse development and function. However, our understanding of its endogenous trafficking has been limited due to difficulties in manipulating its coding sequence (CDS) using standard genome editing techniques. Instead, we replaced the entire LRRTM2 CDS by adapting a two-guide CRISPR knock-in method, enabling complete control of LRRTM2. In primary rat hippocampal cultures dissociated from embryos of both sexes, N-terminally tagged, endogenous LRRTM2 was found in 80% of synapses, and synaptic LRRTM2 content correlated with PSD-95 and AMPAR levels. LRRTM2 was also enriched with AMPARs outside synapses, demonstrating the sensitivity of this method to detect relevant new biology. Finally, we leveraged total genomic control to increase the synaptic levels of LRRTM2 via simultaneous mutation of its C-terminal domain, which did not correspondingly increase AMPAR enrichment. The coding region of thousands of genes span lengths suitable for whole-CDS replacement, suggesting this simple approach will enable straightforward structure-function analysis in neurons.

Role of Neural Circuits in Cognitive Impairment

Cognitive impairment refers to abnormalities in learning, memory and cognitive judgment, mainly manifested as symptoms such as decreased memory, impaired orientation and reduced computational ability. As the fundamental unit of information processing in the brain, neural circuits have recently attracted great attention due to their functions in regulating pain, emotion and behavior. Furthermore, a growing number of studies have suggested that neural circuits play an important role in cognitive impairment. Neural circuits can affect perception, attention and decision-making, they can also regulate language skill, thinking and memory. Pathological conditions crucially affecting the integrity and preservation of neural circuits and their connectivity will heavily impact cognitive abilities. Nowadays, technological developments have led to many novel methods for studying neural circuits, such as brain imaging, optogenetic techniques, and chemical genetics approaches. Therefore, neural circuits show great promise as a potential target in mitigating cognitive impairment. In this review we discuss the pathogenesis of cognitive impairment and the regulation and detection of neural circuits, thus highlighting the role of neural circuits in cognitive impairment. Hence, therapeutic agents against cognitive impairment may be developed that target neural circuits important in cognition.

LRRTM2 controls presynapse nano-organization and AMPA receptor sub-positioning through Neurexin-binding interface

Synapses are organized into nanocolumns that control synaptic transmission efficacy through precise alignment of postsynaptic neurotransmitter receptors and presynaptic release sites. Recent evidence show that Leucine-Rich Repeat Transmembrane protein LRRTM2, highly enriched and confined at synapses, interacts with Neurexins through its C-terminal cap, but the role of this binding interface has not been explored in synapse formation and function. Here, we develop a conditional knock-out mouse model (cKO) to address the molecular mechanisms of LRRTM2 regulation, and its role in synapse organization and function. We show that LRRTM2 cKO specifically impairs excitatory synapse formation and function in mice. Surface expression, synaptic clustering, and membrane dynamics of LRRTM2 are tightly controlled by selective motifs in the C-terminal domain. Conversely, the N-terminal domain controls presynapse nano-organization and postsynapse AMPAR sub-positioning and stabilization through the recently identified Neurexin-binding interface. Thus, we identify LRRTM2 as a central organizer of pre- and post- excitatory synapse nanostructure through interaction with presynaptic Neurexins.

Structure, function, and pathology of Neurexin-3

Neurexin-3 is primarily localized in the presynaptic membrane and forms complexes with various ligands located in the postsynaptic membrane. Neurexin-3 has important roles in synapse development and synapse functions. Neurexin-3 mediates excitatory presynaptic differentiation by interacting with leucine-rich-repeat transmembrane neuronal proteins. Meanwhile, neurexin-3 modulates the expression of presynaptic α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors and γ-aminobutyric acid A receptors by interacting with neuroligins at excitatory and inhibitory synapses. Numerous studies have documented the potential contribution of neurexin-3 to neurodegenerative and neuropsychiatric disorders, such as Alzheimer's disease, addiction behaviors, and other diseases, which raises hopes that understanding the mechanisms of neurexin-3 may hold the key to developing new strategies for related illnesses. This review comprehensively covers the literature to provide current knowledge of the structure, function, and clinical role of neurexin-3.

Role of regulatory C-terminal motifs in synaptic confinement of LRRTM2

Leucine Rich Repeat Transmembrane proteins (LRRTMs) are neuronal cell adhesion molecules involved in synapse development and plasticity. LRRTM2 is the most synaptogenic isoform of the family, and its expression is strongly restricted to excitatory synapses in mature neurons. However, the mechanisms by which LRRTM2 is trafficked and stabilized at synapses remain unknown. Here, we examine the role of LRRTM2 intracellular domain on its membrane expression and stabilization at excitatory synapses, using a knock-down strategy combined to single molecule tracking and super-resolution dSTORM microscopy. We show that LRRTM2 operates an important shift in mobility after synaptogenesis in hippocampal neurons. Knock-down of LRRTM2 during synapse formation reduced excitatory synapse density in mature neurons. Deletion of LRRTM2 C-terminal domain abolished the compartmentalization of LRRTM2 in dendrites and disrupted its synaptic enrichment. Furtheremore, we show that LRRTM2 diffusion is increased in the absence of its intracellular domain, and that the protein is more dispersed at synapses. Surprisingly, LRRTM2 confinement at synapses was strongly dependent on a YxxC motif in the C-terminal domain, but was independent of the PDZ-like binding motif ECEV. Finally, the nanoscale organization of LRRTM2 at excitatory synapses depended on its C-terminal domain, with involvement of both the PDZ-binding and YxxC motifs. Altogether, these results demonstrate that LRRTM2 trafficking and enrichment at excitatory synapses are dependent on its intracellular domain.

Cerebrolysin enhances the expression of the synaptogenic protein LRRTM4 in the hippocampus and improves learning and memory in senescent rats

Aging reduces the efficiency of the organs and systems, including the cognitive functions. Brain aging is related to a decrease in the vascularity, neurogenesis, and synaptic plasticity. Cerebrolysin, a peptide and amino acid preparation, has been shown to improve the cognitive performance in animal models of Alzheimer's disease. Similarly, the leucine-rich repeat transmembrane 4 protein exhibits a strong synaptogenic activity in the hippocampal synapses. The aim of this study was to evaluate the effect of the cerebrolysin treatment on the learning and memory abilities, sensorimotor functions, and the leucine-rich repeat transmembrane 4 protein expression in the brain of 15-month-old rats. Cerebrolysin (1076 mg/kg) or vehicle was administered to Wistar rats intraperitoneally for 4 weeks. After the treatments, learning and memory were tested using the Barnes maze test, and the acoustic startle response, and its pre-pulse inhibition and habituation were measured. Finally, the leucine-rich repeat transmembrane 4 expression was measured in the brainstem, striatum, and hippocampus using a Western-blot assay. The 15-month-old vehicle-treated rats showed impairments in the habituation of the acoustic startle response and in learning and memory when compared to 3-month-old rats. These impairments were attenuated by the subchronic cerebrolysin treatment. The leucine-rich repeat transmembrane 4 protein expression was lower in the old vehicle-treated rats than in the young rats; the cerebrolysin treatment attenuated that decrease in the old rats. The leucine-rich repeat transmembrane 4 protein was not expressed in striatum or brainstem. These results suggest that the subchronic cerebrolysin treatment enhances the learning and memory abilities in aging by increasing the expression of the leucine-rich repeat transmembrane 4 protein in the hippocampus.

Trafficking mechanisms of synaptogenic cell adhesion molecules

Nearly every aspect of neuronal function, from wiring to information processing, critically depends on the highly polarized architecture of neurons. Establishing and maintaining the distinct molecular composition of axonal and dendritic compartments requires precise control over the trafficking of the proteins that make up these cellular domains. Synaptic cell adhesion molecules (CAMs), membrane proteins with a critical role in the formation, differentiation and plasticity of synapses, require targeting to the correct pre- or postsynaptic compartment for proper functioning of neural circuits. However, the mechanisms that control the polarized trafficking, synaptic targeting, and synaptic abundance of CAMs are poorly understood. Here, we summarize current knowledge about the sequential trafficking events along the secretory pathway that control the polarized surface distribution of synaptic CAMs, and discuss how their synaptic targeting and abundance is additionally influenced by post-secretory determinants. The identification of trafficking-impairing mutations in CAMs associated with various neurodevelopmental disorders underscores the importance of correct protein trafficking for normal brain function.

Role of LRRTMs in synapse development and plasticity

Leucine-rich-repeat transmembrane neuronal proteins (LRRTMs) are a family of four synapse organizing proteins critical for the development and function of excitatory synapses. The genes encoding LRRTMs and their binding partners, neurexins and HSPGs, are strongly associated with multiple psychiatric disorders. Here, we review the literature covering their structural features, expression patterns in the developing and adult brains, evolutionary origins, and discovery as synaptogenic proteins. We also discuss their role in the development and plasticity of excitatory synapses as well as their disease associations.

LRRTM3 Regulates Excitatory Synapse Development through Alternative Splicing and Neurexin Binding

The four members of the LRRTM family (LRRTM1-4) are postsynaptic adhesion molecules essential for excitatory synapse development. They have also been implicated in neuropsychiatric diseases. Here, we focus on LRRTM3, showing that two distinct LRRTM3 variants generated by alternative splicing regulate LRRTM3 interaction with PSD-95, but not its excitatory synapse-promoting activity. Overexpression of either LRRTM3 variant increased excitatory synapse density in dentate gyrus (DG) granule neurons, whereas LRRTM3 knockdown decreased it. LRRTM3 also controlled activity-regulated AMPA receptor surface expression in an alternative splicing-dependent manner. Furthermore, Lrrtm3-knockout mice displayed specific alterations in excitatory synapse density, excitatory synaptic transmission and excitability in DG granule neurons but not in CA1 pyramidal neurons. Lastly, LRRTM3 required only specific splice variants of presynaptic neurexins for their synaptogenic activity. Collectively, our data highlight alternative splicing and differential presynaptic ligand utilization in the regulation of LRRTMs, revealing key regulatory mechanisms for excitatory synapse development.