Sculpting Dendritic Spines during Initiation and Maintenance of Neuropathic Pain

Abstract

Accumulating evidence has established a firm role for synaptic plasticity in the pathogenesis of neuropathic pain. Recent advances have highlighted the importance of dendritic spine remodeling in driving synaptic plasticity within the CNS. Identifying the molecular players underlying neuropathic pain induced structural and functional maladaptation is therefore critical to understanding its pathophysiology. This process of dynamic reorganization happens in unique phases that have diverse pathologic underpinnings in the initiation and maintenance of neuropathic pain. Recent evidence suggests that pharmacological targeting of specific proteins during distinct phases of neuropathic pain development produces enhanced antinociception. These findings outline a potential new paradigm for targeted treatment and the development of novel therapies for neuropathic pain. We present a concise review of the role of dendritic spines in neuropathic pain and outline the potential for modulation of spine dynamics by targeting two proteins, srGAP3 and Rac1, critically involved in the regulation of the actin cytoskeleton.

Figure 1.

Structural and functional dendritic spine adaptation. Dendritic spines are dynamic structures that change their shape and membrane composition following synchronous and coordinated presynaptic signaling. (1) Sustained postsynaptic depolarization causes removal of the magnesium blockade from N-methyl-D-aspartate (NMDA) receptors, which allows increased intracellular calcium concentrations. (2) Increased intracellular calcium induces phosphorylation (P) of CaMKII, which increases its activity and drives postsynaptic events leading to synaptic potentiation. (3) One of the postsynaptic events triggered by CaMKII activation is the insertion of additional α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic (AMPA) receptors into the head of the dendritic spine in the region of the PSD. (4) High levels of postsynaptic activity trigger polymerization of soluble G-actin into filamentous F-actin, which form branching networks with the actin regulating protein 2/3 (Arp2/3) serving as a nucleation site. Highly branched actin networks are found in mature dendritic spines. (5) The thinner neck found in mushroom-shaped spines restricts the diffusion of calcium into the dendritic shaft and allows for locally elevated levels to be maintained. Three spatial dimensions, head diameter (D1), spine length (D2), and neck width (D3) are primarily used to classify spines into defined categories. Spine with large heads tend to have a greater volume, larger PSD, more glutamatergic receptors, and greater signaling efficacy.

Figure 2.

Structural changes to dendritic spines in neuropathic pain. Sensory neurons receive nociceptive input at excitatory dendritic spines located at the distal dendritic branches. Spatial and temporal summation occurs in the soma, and output is transmitted along the axon as an action potential. Dendritic spines are broadly classified into four categories: stubby, mushroom, filopodial, and thin spines. Mushroom spines are mature, stable, and have highly branched actin networks. Filopodial spines are like thin spines, which generally act as a means of sensing new synaptic connections. Thin spines are transient and on sustained synaptic activity can mature into mushroom-shaped spines. Stubby spines are also a mature category with a short but wide neck region that has limited capacity for restriction of calcium diffusion. The spine life cycle begins with immature filopodial spines and progresses to mature mushroom spines, which correlates with the level of synaptic activity. Neuropathic pain results in common changes to the dendritic spine profile of sensory neurons in the dorsal horn of the spinal cord characterized by increased density of mature/mushroom spines, increased neuronal excitability, and a redistribution of spines toward the soma. These changes can be ameliorated by inhibition of Rac1 using intrathecal administration of NSC23766 and blockade of srGAP3 using intrathecal siRNA injection.

Figure 3.

Rac1 signaling cycle in dendritic spines. The spatial and temporal activity of Rac1 is tightly controlled through a network of signaling interactions and post-translational modifications. (1) The primary regulators of Rac1 are the GEFs, which activate Rac1 by exchanging GDP for GTP, and the GAPs, which inactivate Rac1 by accelerating the hydrolysis of GTP. The compound NSC23766 specifically inhibits Rac1 activity by blocking the GEFs Tiam1 and Trio. (2) In its inactive state, Rho GDIs act to sequester Rac1 in the nucleus and protect it from proteolytic degradation. (3) The enzyme GeranylGeranyl Transferase 1 (GGT1) post-translationally modifies Rac1 through the addition of a geranylgeranyl lipid group. (4) Following prenylation, Rac1 is ready to be activated. (5) Upon activation of an upstream effector, such as receptor tyrosine kinase or mGluRs, the prenylated Rac1 loses the Rho GDI and has GDP exchanged for GTP, resulting in activation. (6) Activated Rac1 then binds to WAVE1 at the membrane and causes activation of Arp2/3, which can then bind to already established F-actin. (7) Binding of Arp2/3 increases the number of branched actin filaments and drives the dendritic spine into a more stable configuration. Rho GEF, Rho guanine nucleotide exchange factor; Rho GAP, Rho GTPase activating protein; Rho GDI, Rho guanine nucleotide dissociation inhibitor.

Figure 4.

Functional domains of srGAP3 drive diverse interactions. The structure of srGAP3 is highly conserved with other srGAP proteins and contains three unique domains. At the N-terminal region is an IF-BAR domain that is composed of α-helical coils that bind the membrane as homodimers through recognition of specific membrane phospholipids, such as PIP2 and PIP3. In the central region, there is a GAP domain, which is specific for and inhibits the activity of Rac1. Toward the C-terminal tail of the protein is an SH3 domain that acts as a scaffolding adaptor for recognition of several different interactors, such as WAVE1, lamellopodin, and its canonical receptor Robo1. These regions allow srGAP3 to mediate a variety of different intracellular functions through interactions with a diverse set of interaction partners. The numbers depict the specific amino acid residues where each domain begins and ends in the human srGAP3 protein.