The Semaphorin family of guidance cues and their Plexin receptors play critical roles in neuronal development, synaptic plasticity, and axonal regeneration. These molecules are increasingly recognized as important players in neurodegenerative disease pathogenesis, with dysregulated semaphorin/plexin signaling contributing to synaptic loss, impaired regeneration, and neuroinflammation in Alzheimer's disease (AD), Parkinson's disease (PD), and amyotrophic lateral sclerosis (ALS). This pathway page explores the molecular mechanisms by which semaphorins and plexins influence neurodegeneration and evaluates therapeutic strategies targeting this signaling system. [1]
Semaphorins are a large family of secreted and membrane-bound proteins originally characterized for their role in axonal guidance during development. They are classified into eight classes (1-7, plus viral semaphorins) based on their structural features and species distribution. In mammals, the family includes approximately 20 semaphorins divided into classes 3-7, with class 3 being the only secreted vertebrate semaphorins and classes 4-7 being membrane-associated [1]. [2]
The semaphorin family includes key members such as: [3]
Plexins are the primary receptors for semaphorins, with four main subfamilies: Plexin-A1-A4, Plexin-B1-B3, Plexin-C1, and Plexin-D1. Each plexin subfamily exhibits distinct ligand specificity and expression patterns [2]. [4]
Plexin-A1 is the primary receptor for Sema3A and signals through interactions with Neuropilin co-receptors. It regulates axonal repulsion, dendritic morphogenesis, and synaptic plasticity. Plexin-A1 is widely expressed in the brain, including the hippocampus and cerebral cortex. [5]
Plexin-A2 shares significant homology with Plexin-A1 and often cooperates in semaphorin signaling. It plays roles in neural crest cell migration and hippocampal circuit formation. [6]
Plexin-A3 is highly expressed in developing neurons and regulates axonal guidance. It has been implicated in cortical neuron connectivity and is important for proper hippocampal function. [7]
Plexin-A4 mediates Sema3A and Sema3F signaling in sensory neurons and contributes to pain pathways. In the CNS, it regulates synaptic function and plasticity. [8]
Plexin-B1 binds Sema4D and Sema4C, activating Rho GTPase signaling cascades. It is expressed in oligodendrocytes and regulates myelination. Plexin-B1 has been implicated in synaptic function and dendritic spine morphology. [9]
Plexin-B2 binds Sema4D and Sema4G, playing roles in immune cell trafficking and neural development. It is involved in astrocyte function and has been linked to neuroinflammatory processes. [10]
Plexin-C1 binds Sema3E and viral semaphorins, with roles in immune regulation and endothelial cell function. Plexin-D1 is primarily involved in vascular development but also has CNS expression. [11]
Neuropilins (NRP1 and NRP2) serve as critical co-receptors for class 3 semaphorins, dramatically increasing their binding affinity and signaling specificity. NRP1 primarily partners with Plexin-A1/A2 to mediate Sema3A signaling, while NRP2 pairs with Plexin-A3/A4 for Sema3F responses [3]. [12]
Neuropilins are transmembrane proteins with extracellular domains that bind semaphorins and growth factors (including VEGF). Their cytoplasmic domains lack intrinsic kinase activity but contain a PDZ-binding motif that recruits downstream signaling partners.
In the adult brain, neuropilins continue to be expressed in neurons and glia, where they regulate synaptic plasticity, adult neurogenesis, and glial function. NRP1 is highly expressed in hippocampal neurons, particularly in the CA1 region and dentate gyrus, areas critical for learning and memory.
Semaphorin/plexin signaling activates multiple downstream pathways that regulate cytoskeletal dynamics, cell adhesion, and gene expression.
The primary downstream effectors of plexin signaling are Rho GTPases, including Rac1, RhoA, and Cdc42. Plexin cytoplasmic domains interact with various Rho GTPase GAPs (GTPase-activating proteins) and GEFs (guanine nucleotide exchange factors) to regulate actin cytoskeleton dynamics [4].
Rac1: Activated by plexin signaling to promote actin polymerization and membrane protrusion. In neurons, Rac1 regulates dendritic spine formation and synaptic plasticity.
RhoA: Often inhibited by semaphorin signaling, leading to growth cone collapse. RhoA activation can promote actomyosin contractility and growth cone retraction.
Cdc42: Regulates filopodia formation and axonal outgrowth. Plexin-mediated Cdc42 signaling influences dendritic branching and spine morphology.
Semaphorin/plexin signaling modulates PI3K/Akt pathway activity, influencing cell survival, metabolism, and protein synthesis. Sema3A signaling through Neuropilin-1/Plexin-A1 can activate Akt, which has complex effects on neuronal viability depending on context [5].
The MAPK/ERK pathway is activated downstream of plexin receptors, regulating gene expression, cell differentiation, and synaptic plasticity. Sema3A signaling can modulate ERK activity in neurons, influencing long-term potentiation and memory formation.
Plexin receptors interact with MERM proteins to regulate cytoskeletal organization and cell morphology. Merlin (NF2) is particularly important as it links plexin signaling to Hippo pathway regulation of cell proliferation and tissue homeostasis.
Semaphorin/plexin signaling critically regulates synaptic plasticity, the cellular basis of learning and memory. Sema3A is highly expressed in the hippocampus where it modulates long-term potentiation (LTP) and long-term depression (LTD) [6]. In AD, elevated Sema3A levels contribute to synaptic dysfunction through several mechanisms:
Sema3A inhibits neurite outgrowth and axonal regeneration, which becomes particularly problematic in AD where synaptic loss is a hallmark. Amyloid-beta (Aβ) oligomers upregulate Sema3A expression in neurons and astrocytes, creating a regenerative failure state [7].
Aβ-induced Sema3A elevation:
Semaphorin signaling intersects with tau pathology through multiple pathways. Tau phosphorylation affects microtubule dynamics and axonal transport, which may influence semaphorin-mediated guidance. Conversely, Sema3A signaling can modulate tau kinases (including GSK-3β), potentially affecting tau pathology progression [8].
Targeting semaphorin/plexin signaling in AD presents several therapeutic opportunities:
| Strategy | Approach | Status |
|---|---|---|
| Sema3A neutralization | Anti-Sema3A antibodies | Preclinical |
| Plexin-A1 modulation | Small molecule agonists | Research |
| Neuropilin-1 inhibition | Peptide antagonists | Research |
| Downstream pathway targeting | Rho GTPase modulators | Preclinical |
During development, semaphorins guide dopaminergic neurons from the substantia nigra to the striatum. In adult PD, semaphorin signaling influences dopaminergic neuron survival and axonal maintenance [9].
Sema3A and Sema3F are expressed in the substantia nigra and striatum where they:
One hallmark of PD is the progressive loss of dopaminergic axons in the striatum. While semaphorins normally inhibit axonal regeneration, understanding this pathway may reveal ways to overcome it. Sema3A repels regenerating axons, but targeted inhibition could promote recovery [10].
α-Synuclein aggregation, the pathological hallmark of PD, interacts with semaphorin signaling. Evidence suggests:
Plexin-B receptors on microglia and astrocytes respond to semaphorin signals during neuroinflammation. Sema4D and Sema4A activate inflammatory responses through Plexin-B1/B2, potentially exacerbating dopaminergic neuron loss in PD [11].
Semaphorins play critical roles in motor neuron development, including axonal pathfinding from the spinal cord to target muscles. In ALS, semaphorin signaling contributes to:
Elevated Sema3A levels have been observed in ALS models and patient tissues. This increase contributes to:
Plexin-A receptors mediate repulsive axon guidance, which becomes problematic in ALS when semaphorin expression is dysregulated. The balance between attractive and repulsive cues is disrupted, leading to:
Plexin-B1 signaling regulates oligodendrocyte differentiation and myelination. In ALS, white matter pathology involves oligodendrocyte dysfunction, and semaphorin signaling may contribute to myelin abnormalities [12].
Several approaches are being developed to block excessive semaphorin signaling:
Anti-Sema3A antibodies: Neutralize Sema3A to promote neurite outgrowth and synaptic recovery. Under investigation in AD and ALS models.
Receptor blockers: Peptide antagonists that block semaphorin binding to plexins or neuropilins.
Soluble receptors: Engineered Plexin-A/Fc fusion proteins that sequester semaphorins.
Rho GTPase modulators: Targeting downstream effectors to overcome semaphorin-mediated inhibition of axonal growth.
GSK-3β inhibitors: Addressing the intersection between semaphorin and tau pathology.
cAMP enhancers: Raising cAMP levels can convert semaphorin responses from repulsive to attractive.
Viral vector delivery of:
Given the complexity of semaphorin/plexin signaling, combination approaches may be most effective:
This pathway intersects with several other neurodegenerative disease mechanisms: