Neuronal guidance genes in health and diseases

Abstract

Neurons migrate from their birthplaces to the destinations, and extending axons navigate to their synaptic targets by sensing various extracellular cues in spatiotemporally controlled manners. These evolutionally conserved guidance cues and their receptors regulate multiple aspects of neural development to establish the highly complex nervous system by mediating both short- and long-range cell-cell communications. Neuronal guidance genes (encoding cues, receptors, or downstream signal transducers) are critical not only for development of the nervous system but also for synaptic maintenance, remodeling, and function in the adult brain. One emerging theme is the combinatorial and complementary functions of relatively limited classes of neuronal guidance genes in multiple processes, including neuronal migration, axonal guidance, synaptogenesis, and circuit formation. Importantly, neuronal guidance genes also regulate cell migration and cell-cell communications outside the nervous system. We are just beginning to understand how cells integrate multiple guidance and adhesion signaling inputs to determine overall cellular/subcellular behavior and how aberrant guidance signaling in various cell types contributes to diverse human diseases, ranging from developmental, neuropsychiatric, and neurodegenerative disorders to cancer metastasis. We review classic studies and recent advances in understanding signaling mechanisms of the guidance genes as well as their roles in human diseases. Furthermore, we discuss the remaining challenges and therapeutic potentials of modulating neuronal guidance pathways in neural repair.

Keywords: angiogenesis; axon guidance; cancer metastasis; cell-cell communications; neural circuit formation; neural mapping; neuronal migration; organogenesis; synaptogenesis.

Figure 1.

Neuronal guidance cues and receptors. A schematic illustration of guidance cues and receptors based on domain structures annotated by EMBL-SMART. CC: conserved cytoplasmic; CT: C-terminal cysteine knot; CUB: found in C1r, C1s, uEGF, and BMP; DD: death domain; FN3: fibronectin type 3; F V/VIII: factors V and VIII (coagulation factors V and VIII); GAP: GTPase-activating protein; Ig: immunoglobulin-like; IPT: immunoglobulin-plexin-transcription; Lam: laminin-type; LBD: ligand-binding domain; LRR: leucine-rich repeat; MAM: present in meprin, A5, receptor protein tyrosine phosphatase mu; PSI: plexin-semaphorin-integrin; SAM: sterile alpha motif; TSP: thrombospondin type 1; UPA: UNC5-PIDD-ankirin; ZU5: present in ZO-1 and UNC5. The LRR domain structure of FLRT3 is modified based on Seiradake et al. (2014). Each LRR domain of SLIT2 is composed of multiple LRR assemblies. EFNBs and EPH receptors C-terminally bear PDZ domain (the scaffold domain shared by Postsynaptic density-95)-binding motifs. Reticulon-4 (RTN4)/NOGO interacts with RTN4 receptors (RTN4Rs). Note that not included in the diagram are non-canonical guidance cues and the recent finding of interaction between RTN4R and brain-specific angiogenesis inhibitors (BAIs) (Wang et al., 2021).

Figure 2.

Mechanisms regulating presentation of neuronal guidance cues and expression/function of their receptors. (A) Axons sense guidance cues (e.g., NTN1), which are presented on axonal membranes. NTN1, DCC, non-DCC NTN receptors and NTN-binding proteins are depicted on the right. NTN1 has multiple receptor-binding sites (Meijers et al., 2020). (B) A NTN1 (blue)-expressing axon presents NTN1 to a growing axon (pink) (left panel). Alternatively, a non-NTN1-expressing cell captures NTN1, presents NTN1 to a growing axon and direct its growth (right panel). (C) A chemotropism-based model (left) versus an adhesive guidance model (right) for axon guidance at the midline. In the right panel, radial glial progenitors at the ventricular zone (VZ) present NTN1 to growing commissural axons (pink), using DCC distributed on their long radial processes (blue lines). For clarity, radial glia and their processes are illustrated on the left half of the spinal cord. D: dorsal; FP: floorplate; V: ventral. (D) Local-translation and endocytic recycling-based control of axonal sensitivity to guidance cues. Internalized, cue-bound receptors can continue to emanate signals from signaling endosomes, which may be different from those from the plasma membrane-localized receptors. In addition, the efficiency of intracellular signaling cascades is also modulatable, affecting the overall cellular responses.

Figure 3.

Networks of netrin and Slit signaling pathways. (A) Ligand-receptor relationships of NTN, FLRTs and SLIT pathways. Also depicted are the interactions of PLXNA1 with SLIT-C, SEMA3B and NRP2. The interactions of DSCAM with SLIT-N or with ROBO1 in vertebrates remain to be verified (shown by red dotted lines). (B) The intracellular signaling cascades triggered by NTN1-DCC and SLIT-ROBO1 pathways that share several key signaling components. The signaling networks are presented based on biochemical interactions. (C) Axon fasciculation upon exposure to SLIT2 (25 pmol/L) in primary cultures of mouse dorsal spinal cord neurons (time-lapse differential interference contrast [DIC] imaging in live cells) and cortical neurons (DIC images of fixed cells) and in floorplate-containing spinal cord explant culture (immunostained with anti-L1CAM, a post-crossing commissural axon marker) (see Kinoshita-Kawada et al., 2019). In these cultures, axon fasciculation rapidly proceeds upon SLIT2 stimulation. In time-lapse imaging (left panels), the same neurons were stimulated sequentially with a control, then with SLIT2 (two axon bundles formed are marked with arrowheads). Note that dissociated cortical neurons also exhibited cell-adhesive responses (right lower panels). Scale bars: 10 μm.

Figure 4.

Networks of semaphorin and ephrin signaling pathways. Semaphorin and ephrin pathways are substantially heterogeneous, although they share several common signaling cascades (see text for details). (A–D) Semaphorin pathways. (E–G) Ephrin pathways. Bidirectional and reverse signaling pathways are also depicted.

Figure 5.

Neuronal migration into the substantia nigra and the formation of representative neural maps. (A) A schematic illustration of two routes of neuronal migration guided by NTN1. Migration of DCC-expressing dopamine (DA) neurons (green cells) into the SNc is guided by NTN1 presented along VZ radial glial fibers (purple lines), whereas migration of DSCAM-expressing GABAergic neurons (red cells) into the SNr is directed by NTN1 axonally delivered from the forebrain striatum (purple). GABAergic neurons further restrict tangentially migrating DA neurons to the SNc. (B) The retinocollicular (or retinotectal in lower vertebrates) projection. Two countergradients (EFNA and EPHA) contribute to the establishment of the retinotopy on the superior colliculus (SC) along the rostral-caudal (RC) axis by triggering EFNA-EPHA forward and reverse signaling. Temporal retinal axons project to the rostral SC, whereas nasal axons project to the caudal SC, thereby establishing a coarse retinocollicular map. A mature continuous map is formed after refinements. (C, D) The projection map formation of axons of olfactory sensory neurons (OSNs) on the olfactory epithelium (OE) to the glomerulus in the olfactory bulb (OB). During early phases of development, SEMA3A-NRP1 signaling regulates pre-target axonal sorting within axon bundles along the RC axis (C), whereas SEMA3F-NRP2 signaling controls the sequential arrival of OSN axons at the OB along the dorsal-ventral (DV) axis (D), thereby generating a coarse map. Note that, along the RC axis, odorant receptor (OR)-dependent spontaneous cAMP signaling specifies expression levels of SEMA3A and NRP1 in OSNs and their axon projection sites in the OB. (E) OR-directed, structured patterns of spontaneous neural activity fine-tune olfactory axon glomerular projections by inducing specific sets of axon-sorting molecules, to form a discrete map. (F) The hippocampal networks are generated by complementary gradients of TEN3 and LPHN2 in the hippocampal CA1 and subiculum (Sub): TEN3-TEN3 homophilic attraction and TEN3-LPHN2 reciprocal repulsion drives axon targeting. The TEN-FLRT-LPHN transsynaptic supercomplex then confers the synaptic specificity. d: distal; p: proximal.

Figure 6.

Neuronal guidance genes in regulating angiogenesis and cancer metastasis. (A) Models proposed for functions of guidance cues in angiogenesis. In model 1, in response to SLIT, ROBO4 stablizes the vasculature by suppressing ARF6 activity and thus VEGFA-induced angiogenesis and hyperpermeability. In model 2, SLIT activates angiogenic processes via ROBO1/2. In model 3, upon ROBO4-UNC5 binding, UNC5 suppresses VEGFA-induced angiogenesis and hyperpermeability. (B) Roles of PLXND1 in mechanosensation and in SEMA3E-induced repulsion in endothelial cells. PLXND1 forms mechanosensory or SEMA3E receptor complexes by differentially interacting with NRP1 and VEGFR2. (C) Intratumoral SLIT-ROBO1-MYO9B signaling suppresses cancer metastasis. (D) SLIT acts as a chemoattractant for tumor cells, when endothelial cells express SLIT at higher levels than tumor cells. SLIT-ROBO1 signaling may thus promote cancer metastasis.