Netrins and Receptors

Background

Netrins are a family of extracellular matrix proteins that play crucial roles in guiding the growth of axons during neural development and in various cellular processes. They were first identified in the context of neuronal guidance, where they help steer neurons toward their target cells. Netrins contain a conserved structure and mediate their effects primarily through interactions with specific receptors on cell surfaces.

Key Features of Netrins

• Structure: Netrins are characterized by their structural motifs, which include several conserved domains. They can exist in soluble forms in the extracellular environment or be associated with the cell membrane.
• Types: The most studied netrins are netrin-1, netrin-2, and netrin-3. Each has distinct roles in the nervous system and other tissues.
• Functions: Netrins guide axons toward their targets by either attracting or repelling them, depending on the receptors they bind to. They are also involved in processes like cell adhesion and the formation of synapses.

Netrin Receptors

Netrins exert their effects through specific receptors, primarily:

1. DCC (Deleted in Colorectal Cancer)

• Structure: DCC is a transmembrane protein consisting of multiple immunoglobulin (Ig) domains and fibronectin type III repeats.
• Function: DCC is primarily a receptor for Netrin-1. Its activation is critical for mediating attractive guidance cues during neuronal development. DCC facilitates cytoskeletal remodeling and promotes growth cone advancement towards Netrin-1 sources.

2. UNC5 Receptors

• Family Members: Includes UNC5A, UNC5B, UNC5C, and UNC5D.
• Structure: Like DCC, UNC5 receptors have extracellular domains that allow Netrin binding, but they possess a different signaling mechanism.
• Function: UNC5 receptors are involved in repulsive signaling. When activated by Netrin-1, they often trigger pathways that inhibit cell growth or promote apoptosis, particularly in specific neuronal populations.

3. Neogenin

• Role: Neogenin is another receptor activated by Netrins, playing a critical role in the development of the nervous system, tissue repair, and cell migration.
• Function: It can mediate both attractive and repulsive effects, depending on the context of other signaling pathways.

Mechanisms of Action

1. Attractive Guidance (DCC)

• When Netrin-1 binds to DCC, it activates intracellular signaling pathways.
• Signaling Pathways: This often involves the activation of Src family kinases, which can lead to changes in cytoskeleton dynamics and promote growth cone attraction.
• Cytoskeletal Rearrangement: DCC activation leads to the reorganization of actin filaments, facilitating growth cone advance.

2. Repulsive Guidance (UNC5)

• UNC5 receptors, when bound to Netrin-1, can initiate signaling cascades that lead to growth cone collapse or repulsion.
• Intracellular Signaling: This may involve Rho GTPases, which regulate cytoskeletal dynamics and drive the growth cone away from the Netrin source.

Netrins and their receptors are vital for the proper wiring of the nervous system. Understanding their mechanisms can provide insights into developmental biology and potential therapeutic targets for neurological disorders.

Fig.1 Netrins and their receptors. (Larrieu-Lahargue F, et al., 2012)

Netrins and Receptors-Related Molecules

The following are the various genes and proteins associated with axon guidance and neuronal development in the context of netrin and receptors:

Genes and Proteins	Details
DCC (Deleted in Colorectal Cancer)	DCC is a key receptor for Netrins, particularly Netrin-1. It mediates attractive responses to Netrins, guiding axons toward their targets during neural development. It is associated with neural guidance and cell migration, and DCC plays a role in neurodevelopment and tumor progression.
DSCAM (Down syndrome cell adhesion molecule)	DSCAM is involved in neural development and axon guidance. It plays a role in mediating cell-cell adhesion and recognition processes. It plays an important role in CNS development and is involved in neuronal interactions and connections.
LRRC4B and LRRC4C (Leucine-rich repeat containing 4B and 4C)	LRRC4B and LRRC4C are proteins that have been implicated in axon guidance and neural circuit formation. They may interact with Netrins and other guidance cues to regulate axonal pathfinding. They play a role in synapse formation and function and are involved in neurotransmission.
NTN1 (Netrin-1), NTN3 (Netrin-3), Netrin-4	Netrin-1, Netrin-3, and Netrin-4 are members of the Netrin family of proteins. They function as guidance cues for axon guidance and are involved in various aspects of neural development. They are important extracellular matrix proteins, guiding neuronal growth and localization roles, involved in the formation of neural networks and intercellular interactions.
UNC5A, UNC5B, UNC5C, and UNC5D	UNC5 receptors are a family of receptors for Netrins. They are involved in mediating repulsive responses to Netrins, steering axons away from inappropriate targets. These are proteins associated with the Netrin receptor and are involved in apoptosis, neural development, and axon guidance.

These genes and proteins collectively contribute to the intricate processes of axon guidance, neuronal development, and the establishment of neural circuits. Their interactions with Netrins and other guidance cues are essential for the precise wiring of the nervous system during embryogenesis and beyond.

Fig.2 Netrin-1 receptor signaling and known functions. (Ranganathan P, et al., 2014)

Role of Netrins and Receptors in Neural Development and Axon Guidance

Netrins and their receptors play a pivotal role in neural development and axon guidance, ensuring the proper wiring of the nervous system. Here's an overview of their significant contributions to these processes:

• Axon Guidance: Netrins are key players in guiding axon growth during neural development. They can attract or repel axons, directing them towards their appropriate targets. This guidance is crucial for establishing precise neural connections necessary for the functioning of the nervous system.
• Receptor Interaction: Netrins interact with specific receptors on the surface of growth cones, the dynamic structures at the tips of extending axons. The main receptors for Netrins include DCC (Deleted in Colorectal Cancer) and UNC5 family receptors. This interaction triggers intracellular signaling cascades that regulate axon growth cone motility and steering.
• Chemoattractant and Chemorepellent Properties: Netrins can act as chemoattractants, guiding axons towards their target cells or regions with high Netrin concentration. Conversely, they can also serve as chemorepellents, steering axons away from areas where Netrin levels are low or absent. This dual functionality allows for precise axon pathfinding.
• Neural Circuit Formation: Proper neural circuit formation relies on the accurate guidance of axons to their appropriate targets. Netrins and their receptors help establish and refine neural circuits by ensuring that axons navigate to their specific destinations and form synaptic connections with the correct target cells.
• Synaptic Plasticity: Netrins are involved in synaptic plasticity, the ability of synapses to strengthen or weaken over time in response to activity. By influencing axon guidance and synaptogenesis, Netrins contribute to the dynamic changes in neural circuits that underlie learning, memory, and adaptive behaviors.
• Neurodevelopmental Disorders: Dysregulation of Netrin signaling pathways has been implicated in various neurodevelopmental disorders, such as autism spectrum disorders, schizophrenia, and intellectual disabilities. Understanding the role of Netrins and their receptors in neural development is crucial for unraveling the mechanisms underlying these conditions.

In summary, Netrins and their receptors are essential components of the intricate processes involved in neural development and axon guidance. Their ability to attract, repel, and guide axons to their targets is fundamental for the establishment of functional neural circuits and proper brain function. Studying the mechanisms by which Netrins and receptors operate provides valuable insights into neurodevelopment and the potential for therapeutic interventions in neurological disorders.

Potential Implications of Hedgehog Signaling Dysregulation in Neurodevelopmental Disorders

Netrins are a family of extracellular proteins involved in guiding cell migration and axon pathfinding during development. They interact with a variety of receptors, notably the Deleted in Colorectal Cancer (DCC) and the UNC5 family, to influence neuronal connectivity and survival. Their roles extend beyond development, implicating them in various neurodegenerative diseases, neurodevelopmental defects, and other disorders.

Netrins and Neurodegenerative Diseases

• Alzheimer's Disease: Netrin-1 has been shown to have protective roles for neurons. Its levels are often reduced in Alzheimer's, correlating with neurodegeneration. Netrin-1 can inhibit the apoptosis of neurons, suggesting its potential as a therapeutic target.
• Parkinson's Disease: Altered expression of netrins and their receptors has been implicated in dopaminergic neuron survival. Netrin-1 may help protect these neurons in models of Parkinson's, potentially by modulating inflammation and promoting cell survival.
• Amyotrophic Lateral Sclerosis (ALS): Studies suggest that netrin signaling pathways may influence motor neuron fate. Disruptions in these pathways could contribute to the disease pathology.

Netrins and Neurodevelopmental Defects

• Spinal Cord Development: Netrins are crucial in spinal cord development, guiding neuronal projections. Deficiencies in netrins can result in axon misrouting and defects in spinal cord connectivity, potentially leading to conditions like spina bifida.
• Autism Spectrum Disorders (ASD): Genetic studies have linked alterations in netrin signaling to ASD. Aberrant synapse formation influenced by netrins may contribute to the behavioral and cognitive symptoms of these disorders.

Other Disorders

• Schizophrenia: Research indicates netrins may play roles in synaptic function and plasticity, with potential links to schizophrenia. Disruptions in netrin signaling could affect neural circuitry associated with the disorder.
• Cancer: Beyond the nervous system, netrins have roles in tumor progression and metastasis, often exhibiting pro- or anti-tumorigenic effects depending on context. The interaction between netrins and their receptors can influence cell migration and invasion.

Netrins and their receptors are crucial for both the development and maintenance of neural systems. Their dysregulation is associated with a range of neurodegenerative and neurodevelopmental disorders, making them promising targets for therapy. Further research is needed to clarify their exact roles and develop potential interventions that could modulate netrin signaling to improve outcomes in these conditions.

Fig.3 Signaling pathways associated with the ligand Netrin-1 and receptor DCC. (Zhang L, et al., 2021)

Case Study

Case 1: Jarjour AA, Manitt C, Moore SW, Thompson KM, Yuh SJ, Kennedy TE. Netrin-1 is a chemorepellent for oligodendrocyte precursor cells in the embryonic spinal cord. J Neurosci. 2003;23(9):3735-3744.

Netrin-1, which is produced by floor plate cells, plays a crucial role in guiding axon extension concerning the ventral midline of the developing spinal cord. Oligodendrocyte precursor (OP) cells are initially located near the ventral midline and move away from the floor plate during development. The study demonstrates that OP cells, identified by their expression of the platelet-derived growth factor α receptor, possess the netrin receptors DCC and UNC5H1 but do not produce netrin-1 themselves.

Through experiments using a microchemotaxis assay, it was shown that OP cells exhibit a repulsive response to a netrin-1 gradient in vitro. Moreover, the application of netrin-1 to OP cells in vitro leads to the retraction of OP cell processes. In vivo studies reveal that in the absence of netrin-1 or functional Deleted in Colorectal Cancer (DCC), fewer OP cells migrate from the ventral to the dorsal regions of the embryonic spinal cord, supporting the notion that netrin-1 acts as a repellent cue in this context.

While DCC and UNC-5 homologs are known to function as proapoptotic dependence receptors, triggering cell death in the absence of netrin-1, this study found no evidence of increased OP cell death either in vivo or in vitro when netrin-1 or DCC was absent. These results suggest that netrin-1 serves as a repellent signal for migrating OP cells within the embryonic spinal cord, shedding light on the intricate mechanisms involved in cell migration and guidance during neural development.

Fig.1 Netrin-1 is a chemorepellent for oligodendrocyte precursor cells in vitro.

Case 2: Díaz MM, Tsenkina Y, Arizanovska D, Mehlen P, Liebl DJ. DCC/netrin-1 regulates cell death in oligodendrocytes after brain injury. Cell Death Differ. 2023;30(2):397-406.

The hallmark pathological features of brain trauma include axonal degeneration and demyelination, primarily due to the vulnerability of myelin-producing oligodendrocytes (OLs) to injury-induced death signals. To elucidate the mechanisms behind this OL loss, a study examined a unique class of "death receptors" known as dependence receptors (DepRs). These receptors trigger pro-death signals in the absence of their ligands, yet their function following injury remains poorly understood. The research focused on determining the involvement of the deleted colorectal cancer (DCC) dependence receptor in OL loss post-brain injury.

The study revealed that administering the netrin-1 ligand of DCC effectively prevented OL cell death. Additionally, it was observed that DCC expression increased while netrin-1 levels decreased in the tissues surrounding the injury shortly after the trauma. Furthermore, by genetically inhibiting the pro-death activity through DCCD1290N mutant mice, enhanced OL survival, improved myelin integrity, and better motor function were observed. These results highlight a novel role for the netrin-1/DCC pathway in regulating OL loss in the injured brain, shedding light on potential therapeutic targets for managing traumatic brain injuries.

Fig.2 Schematic model of Netrin-1/DCC receptor function in the injured brain.

Related References

• Moore SW, Tessier-Lavigne M, Kennedy TE. Netrins and their receptors. Adv Exp Med Biol. 2007;621:17-31.
• Yamagishi S, Bando Y, Sato K. Involvement of netrins and their receptors in neuronal migration in the cerebral cortex. Front Cell Dev Biol. 2021;8:590009.
• Larrieu-Lahargue F, Thomas KR, Li DY. Netrin ligands and receptors: lessons from neurons to the endothelium. Trends Cardiovasc Med. 2012;22(2):44-47.
• Ranganathan P, Mohamed R, Jayakumar C, Ramesh G. Guidance cue netrin-1 and the regulation of inflammation in acute and chronic kidney disease. Mediators Inflamm. 2014;2014:525891.
• Zhang L, Qi Z, Li J, et al. Roles and mechanisms of axon-guidance molecules in Alzheimer's disease. Mol Neurobiol. 2021;58(7):3290-3307.