Peripheral Inflammation Control by Sensory Neuron-Derived Arc
June Bryan de la Peña, Paulino Barragan-Iglesias, Tzu-Fang Lou, Nikesh Kunder, Sarah Loerch et al.
(see pages 7712–7726)
In addition to gathering information about potential bodily damage, pain-sensing neurons (nociceptors) can also influence it by releasing signaling factors in the periphery. But only a handful of such factors have been identified, and little is known about how they are produced and delivered. Now, de la Peña et al. show that afferent neurons from mouse dorsal root ganglia (DRGs) respond to peripheral inflammation with local translation and release of Arc, an immediate early gene that plays an important role in synaptic plasticity and memory.
Following stimulation of cultured DRG neurons with the proinflammatory molecules nerve growth factor (NGF) and interleukin 6 (IL-6), ribosome profiling showed that translation of >200 proteins increased, including, intriguingly, Arc. Immunohistochemical experiments confirmed that Arc protein levels increased in DRG neurons by 15-fold following treatment with NGF/IL-6.
To visualize Arc expression in vivo, the researchers used a fluorescently labeled reporter strain (EGFP-Arc), which confirmed that Arc was expressed throughout the mouse DRGs and in fibers that innervate the skin. Immunohistochemistry confirmed that Arc was rapidly expressed in the skin of the hindpaw following inflammation. A transcription inhibitor had little effect, indicating that Arc was translated from an existing pool of mRNA. Following transection of the sciatic nerve, which eradicated nerve fibers from the skin, treatment with NGF/IL-6 did not induce Arc translation, pointing to afferent nerves as the source of Arc in skin.
How Arc is released from DRG nerve endings was unclear, but when the researchers expressed Arc in a nociceptor cell line, they saw an approximately fourfold increase in the presence of extracellular vesicles (EVs), which contained Arc and Arc mRNA. The EVs also contained mRNA for other proteins, presumably being transported from nociceptors to other cells.
Thermal imaging showed that paw temperature increased following injection of the robust inflammatory complete Freund's adjuvant by ∼1.4°C in wild-type mice, but the increase was nearly twice that in mice lacking Arc. Injection of Arc-containing EVs rescued the temperature modulation effect. Arc similarly mitigated blood flow in the paw, indicating that vasodilation was responsible for the thermal increase and was modulated by Arc.
The work reveals a new role for Arc as a peripheral signaling molecule that is rapidly translated in skin nerve endings following inflammation that may act as a regulator of intercellular mRNA transport contributing to the control of inflammatory processes.
Pain nerve fibers in the mouse glabrous skin express EGFP-tagged Arc and CGRP.
Autism-Related Mutations' Effects on Synaptogenesis
Chen Tian, Jeremiah D. Paskus, Erin Fingleton, Katherine W. Roche, and Bruce E. Herring
(see pages 7768–7778)
Among the several hundred genetic mutations now found to contribute to autism spectrum disorder (ASD), many are associated with molecular pathways that contribute to synaptic function. Beyond that, little is known about how mutations give rise to ASD or intellectual disability. Now Tian, Paskus et al. delve into how mutations affect the function of Trio, a rhodopsin guanine nucleotide exchange factor recently found to harbor ASD-related mutations. One mutation, D1368V, makes the protein hyperfunctional, leading to a massive increase in glutamatergic synapse formation and in currents through both NMDA- and AMPA-type glutamate receptors. But how Trio contributes to synaptogenesis was so far unknown.
Another mutation in the 8th spectrin repeat domain of Trio, N1080I, was identified in a nonverbal individual profoundly affected by ASD. The authors first wanted to determine the functional consequences of this N1080I mutation, so they transfected organotypic slice cultures from rat hippocampus with the mutant protein and made electrophysiological recordings from the CA1 pyramidal neurons while stimulating Schaffer collaterals. Overexpression of Trio-9 usually causes a doubling of AMPA-mediated evoked EPSC (eEPSC) amplitude, but the N1080I mutant-transfected neurons showed no such increase. NMDA-type receptor-mediated currents were not affected by the N1080I mutation. Next, the authors generated Trio-9 containing both mutations; the addition of N1080I completely blocked the amplifying effects of the D1368V mutation on spine formation and glutamatergic currents. Together, the findings suggest that the N1080I mutation interferes with a protein–protein interaction that is required for D1368V Trio to boost synaptogenesis.
That protein interaction was found to be with neuroligin 1 (NLGN1), which is known to drive synaptogenesis; their association was reduced by half when Trio contained both mutations. In CA1 neurons transfected with NLGN1, spine density increased by 40% and was similarly increased when endogenous Trio and its paralog Kalirin were replaced with transgenic Trio-9, indicating that Trio supports NLGN1-mediated synaptogenesis. Introduction of N1080I Trio, however, blocked the increase entirely. Electrophysiological experiments showed that NLGN1 augmented glutamatergic currents, but expression of N1080I Trio prevented that increase in NMDA eEPSCs. AMPA currents in contrast were still twice as large as eEPSCs from control neurons, which the authors conclude is a result of increased AMPA receptor expression at existing synapses. Finally, NLGN1 expression was required for D1368V to increase spine formation.
Footnotes
This Week in The Journal was written by Stephani Sutherland, Ph.D.
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