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

Abstract

Precise wiring of neural circuits is essential for brain connectivity and function. During development, axons respond to diverse cues present in the extracellular matrix or at the surface of other cells to navigate to specific targets, where they establish precise connections with post-synaptic partners. Cell adhesion molecules (CAMs) represent a large group of structurally diverse proteins well known to mediate adhesion for neural circuit assembly. Through their adhesive properties, CAMs act as major regulators of axon navigation, fasciculation, and synapse formation. While the adhesive functions of CAMs have been known for decades, more recent studies have unraveled essential, non-adhesive functions as well. CAMs notably act as guidance cues and modulate guidance signaling pathways for axon pathfinding, initiate contact-mediated repulsion for spatial organization of axonal arbors, and refine neuronal projections during circuit maturation. In this review, we summarize the classical adhesive functions of CAMs in axonal development and further discuss the increasing number of other non-adhesive functions CAMs play in neural circuit assembly.

Keywords: axon targeting; contact; growth cone; pathfinding; signaling; synaptic specificity.

FIGURE 1

Main families of CAMs expressed in the nervous system. CAMs have historically been classified into families based on the structural composition of their extracellular domain. Integrins form obligate heterodimers composed of α and β integrin subunits that cluster at the plasma membrane to mediate adhesion to the extracellular matrix. CAMs of the Immunoglobulin Superfamily (IgSF) are characterized by the presence of one or more Ig-like domains that can be followed by Fibronectin type III domain (Fn3) repeats. IgSF CAMs mediate adhesion by engaging in homophilic or heterophilic interactions. Most of them include a transmembrane and intracellular domains, but some like Contactins are GPI-anchored. CAMs of the Cadherin Superfamily mostly engage in homophilic interactions and are characterized by the presence of one or more calcium-binding cadherin repeats. The Leucine-rich repeat (LRR) Superfamily includes adhesion molecules that are characterized by the presence or LRRs and can include Ig or Fn3 domains in their extracellular domain. LRR CAMs are often found at synapses and engage in both homophilic or heterophilic trans-interactions. Neurexins and Neuroligins engage in heterophilic interactions in trans at nascent synapses to promote synaptic differentiation and stabilization. Teneurins are type II single-pass transmembrane proteins that interact homophilically or engage in trans-interactions with Latrophilins, a class of adhesion G-protein coupled receptors (not shown).

FIGURE 2

Adhesive functions of CAMs in circuit wiring. (A) Integrins mediate adhesion to the ECM for growth cone advance. Binding of integrins to ECM ligands leads to the clustering of integrins and the recruitment of adaptor proteins linking integrins to the actin cytoskeleton. The point contacts hence formed promote growth cone advance by stabilizing filopodial protrusions and restraining the retrograde flow of actin at the growth cone periphery. (B) Integrins mediate adhesion to the ECM for target selection. In the visual system, integrin α8β1 is selectively expressed by retinal ganglion cells projecting ipsilaterally. Its ligand, the ECM glycoprotein Nephronectin, is restricted to a sublamina at the target. Interaction between integrin α8β1 and Nephronectin is necessary for the laminar targeting of ipsilateral axons to the rostral stratum opticum (SO). Deleting integrin α8β1 or Nephronectin causes a dramatic loss of ipsilateral projections while contralateral projections remain unaffected in the stratum griseum superficial (SGS). Adapted from Su et al. (2021). (C,D) Protocadherin-17 (Pcdh17) mediates trans-axonal interactions for proper tract formation. (C) Pcdh17 accumulates at homotypic contacts between growth cones and axons from amygdala neurons, where it recruits the WAVE complex, Lamellipodin, and Ena/VASP proteins that remodel the actin cytoskeleton and promote membrane protrusion. Pcdh17-mediated adhesion enhances growth cone motility and enable growth cone advance along homotypic axons. (D) Pcdh17 is required for the extension of amygdala axons through the stria terminalis toward the hypothalamus. Adapted from Hayashi et al. (2014). (E) A CAM code specifies laminar targeting and synaptic specificity in the retina. Sdk1, Sdk2, Dscam, DscamL, and Cntns (not all a represented here) are expressed in non-overlapping subsets of bipolar (green), amacrine (orange), and retinal ganglion cells (purple) and engage in homophilic trans-interactions to direct synapse formation between matching partners in specific laminae (S1–S5) of the inner plexiform layer (IPL). Classical cadherins (Cdh) also contribute to the molecular code specifying connections. INL, inner nuclear layer; GCL, ganglion cell layer. Adapted from Sanes and Zipursky (2020).

FIGURE 3

Non-adhesive functions of CAMs in circuit assembly. (A) CAMs regulate transcription for axon growth by activating intracellular signaling pathways from the plasma membrane (1), acting directly in the nucleus after proteolytic cleavage (2), or regulating the transport of molecules with transcriptional activity (3). NCAM, for instance, interacts with FGFR to activate the MAPK pathway and in turn, transcription (1). Alternatively, proteolytic processing of NCAM releases a fragment that is trafficked through endosomes and the endoplasmic reticulum (ER), released in the cytoplasm, and finally translocated into the nucleus where it regulates transcription (2). NF-protocadherin (NF-Pcdh) directly interacts with TAF1, a component of the basal transcription factor complex TFIID, suggesting that NF-Pcdh might regulate axon elongation through TAF1-mediated transcriptional control. (B) CAMs instruct axon repulsion by modulating signaling pathways. Both L1CAM and Cntn2 form a complex with Nrp1, the receptor to the repulsive guidance cue Sema3A, at the plasma membrane. Cntn2 modulates axon response to Sema3A by regulating the endocytosis of the Nrp1/L1CAM/Sema3A complex. After internalization, L1CAM and Nrp1 become segregated by Cntn2 into two distinct trafficking pathways. Nrp1 is routed to endocytic compartments where its increased association with PlexinA4 signals for collapse. Adapted from Law et al. (2008) and Dang et al. (2012). (C) CAMs instruct repulsion by acting as guidance cues. In the hippocampus, reciprocal repulsions mediated by Tenm3 and Lphn2 ensure proper target selection. Axons originating from proximal CA1 (pCA1) neurons (green) express Tenm3 and project to the distal subiculum (dSub) after being repelled by Lphn2 (pink) present in the proximal subiculum (pSub). Conversely, distal CA1 (dCA1) axons expressing Lphn2 (pink) are repelled by Tenm3 in dSub (green) and project to pSub. Adapted from Pederick et al. (2021). (D) Clustered Pcdhs regulate self-avoidance. Pcdh genes are organized into three adjacent clusters that include several variable exons. Each variable exon codes for an extracellular and transmembrane domains and is preceded by a promoter randomly activated in individual neurons to drive transcription. Stochastic promoter choice leads to the production of different Pcdh isoforms from each of the three clusters in a cell-specific manner, thereby generating a unique combination of Pcdh α, β, and γ expression in each neuron. Sister branches from the same terminal arbor express the same code of Pcdhs at their surface and repel each other after Pcdhs interact homophilically in trans. (E) CAMs ensure tiling of terminal arbors. In the Drosophila visual system, DSCAM2, Turtle (Tutl), and Flamingo (Fmi) together with Gogo engage in homophilic trans-interactions to activate repulsion, thereby control the proper spacing of L1-L5, R8, and R7 terminal arbors, respectively, in the medulla. Adapted from Spead and Poulain (2021).