Developmental synaptic regulator,TWEAK/Fn14 signaling,is a determinant of synaptic function in models of stroke and neurodegeneration

Identifying molecular mediators of neural circuit development and/or function that contribute to circuit dysfunction when aberrantly reengaged in neurological disorders is of high importance. The role of the TWEAK/Fn14 pathway, which was recently reported to be a microglial/neuronal axis mediating synaptic refinement in experience-dependent visual development, has not been explored in synaptic function within the mature central nervous system. By combining electrophysiological and phosphoproteomic approaches, we show that TWEAK acutely dampens basal synaptic transmission and plasticity through neuronal Fn14 and impacts the phosphorylation state of pre- and postsynaptic proteins in adult mouse hippocampal slices. Importantly, this is relevant in two models featuring synaptic deficits. Blocking TWEAK/Fn14 signaling augments synaptic function in hippocampal slices from amyloid-beta–overexpressing mice. After stroke, genetic or pharmacological inhibition of TWEAK/Fn14 signaling augments basal synaptic transmission and normalizes plasticity. Our data support a glial/neuronal axis that critically modifies synaptic physiology and pathophysiology in different contexts in the mature brain and may be a therapeutic target for improving neurophysiological outcomes.

Neural circuit patterning, refinement, and plasticity are enabled by the dynamic strengthening, weakening, and pruning of chemical synapses in response to circuit activity. However, synapse loss and reduced plasticity are early hallmarks of chronic neurological disorders such as autism, schizophrenia and Alzheimer’s disease (AD) (1–3). It is therefore hypothesized that the underlying molecular mechanisms of pruning, although normally balanced in health, are dysregulated in disease. Particularly interesting is the notion that the mechanisms responsible for the reduction in functional synapses in disease reflect the aberrant reactivation of pathways important for synapse elimination in development. For example, in an AD model, synapse elimination was shown to be mediated by the complement pathway in the hippocampus (HC), reflecting aberrant reactivation of complement-dependent synapse elimination that occurs in the dorsal lateral geniculate nucleus (dLGN) of the thalamus during visual development (4). In such a paradigm, the reactivation of developmental mechanisms enables pathways that can act universally across different ages, circuits, and brain regions. Thus, the mechanisms underlying normal circuit development and their potential reactivation as key contributors to neurological diseases are areas of deep interest.In addition to chronic neurological disorders, circuitry changes also occur in acute ischemic stroke, the second leading cause of death worldwide and a cause of debilitating long-term disability. Interruptions in blood flow that deprive neurons of oxygen and nutrients result in significant cell death, followed by deficits in neurophysiological activity that are associated with poor motor recovery (5). Remarkably, the adult brain can undergo some degree of spontaneous poststroke recovery, apparently by engaging neuroplasticity mechanisms including remapping, synaptogenesis, and synaptic strengthening (5, 6). Despite these adaptations, over half of ischemic stroke patients fail to recover completely and continue to experience persistent long-term disability (7). The underlying signaling pathways that regulate synaptic physiology after stroke are an active topic of investigation.TNF-like weak inducer of apoptosis (TWEAK) protein, originally discovered as a cytokine produced by macrophages (8), signals through its injury-inducible transmembrane receptor, FGF-inducible molecule-14 (Fn14) (9). Consequently, the function of TWEAK/Fn14 signaling was elucidated as a driver of tissue remodeling in contexts of injury and disease in a variety of organ systems (10). Recently, findings have suggested a role for the TWEAK/Fn14 pathway in the central nervous system (CNS). Namely, several compelling observations indicate that TWEAK signaling through Fn14 might be a key molecular modulator of synaptic function in contexts of neurological challenge. TWEAK and Fn14 are up-regulated in the CNS in AD (11, 12, 13 and SI Appendix, Fig. S6A) and after ischemic stroke in humans and mice (14–16). Importantly, TWEAK/Fn14 signaling was also recently shown to be a pathway necessary for synapse maturation during experience-dependent visual development. Light-induced up-regulation of Fn14 in thalamocortical excitatory neurons and corresponding up-regulation of TWEAK in microglia mediate the elimination of weak synapses and strengthening of remaining synapses in the dLGN (17, 18). Indeed, the communication between neurons and supporting microglia has emerged as a key mechanism regulating neuronal circuitry, with microglia deploying their ramified processes to continuously survey and refine synapses in response to neural activity. Interestingly, TWEAK expression has also been shown to be microglia-enriched in the mouse cortex (19), suggesting that it may play a role in multiple brain regions. Thus, like the complement pathway, the TWEAK/Fn14 pathway could be an important regulator of synapse biology in visual development which is re-engaged and acts generally in different ages and brain regions to contribute to pathology.The involvement of TWEAK/Fn14 signaling in synapse physiology or pathophysiology outside of the developing visual system is unknown. We considered it to be a strong candidate modifier of synaptic function in adults given that Fn14 is up-regulated and required for synaptic refinement in experience-dependent visual development, and TWEAK and Fn14 are up-regulated in contexts of neurological injury/disease, suggesting that the TWEAK/Fn14 system is tuned to periods of substantial change in neuronal activity levels or environment (e.g., eye opening, ischemic stroke). We employed HC slices to test the hypothesis that the TWEAK/Fn14 pathway regulates synaptic function in adult mice and in different disease contexts and delineate its mechanism of action. Herein, we reveal that TWEAK, through neuronal Fn14, mediates acute dampening of basal synaptic transmission and synaptic plasticity in hippocampal slices from mature mice. Furthermore, we demonstrate that TWEAK/Fn14 signaling broadly impacts the phosphorylation state of critical synaptic proteins, suggesting a general role in synapse modulation. Finally, we show that pathway deficiency or pharmacological inhibition of TWEAK/Fn14 signaling augments synaptic transmission and plasticity in amyloid-beta (Aβ)–overexpressing mice and post ischemic stroke animals, two model systems featuring synaptic functional deficits. Thus, our results support that TWEAK/Fn14 constitutes a synaptic regulatory pathway with therapeutic potential for CNS disorders in the adult brain.