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Aging is a major risk factor to develop neurodegenerative diseases and is associated with decreased buffering capacity of the proteostasis network. We investigated the significance of the unfolded protein response (UPR), a major signaling pathway activated to cope with endoplasmic reticulum (ER) stress, in the functional deterioration of the mammalian brain during aging. We report that genetic disruption of the ER stress sensor IRE1 accelerated age-related cognitive decline. In mouse models, overexpressing an active form of the UPR transcription factor XBP1 restored synaptic and cognitive function, in addition to reducing cell senescence. Proteomic profiling of hippocampal tissue showed that XBP1 expression significantly restore changes associated with aging, including factors involved in synaptic function and pathways linked to neurodegenerative diseases. The genes modified by XBP1 in the aged hippocampus where also altered. Collectively, our results demonstrate that strategies to manipulate the UPR in mammals may help sustain healthy brain aging.

The coordinated movement of many organisms relies on efficient nerve–muscle communication at the neuromuscular junction (NMJ), a peripheral synapse composed of a presynaptic motor axon terminal, a postsynaptic muscle specialization, and non‐myelinating terminal Schwann cells. NMJ dysfunctions are caused by traumatic spinal cord or peripheral nerve injuries as well as by severe motor pathologies. Compared to the central nervous system, the peripheral nervous system displays remarkable regenerating abilities; however, this capacity is limited by the denervation time frame and depends on the establishment of permissive regenerative niches. At the injury site, detailed information is available regarding the cells, molecules, and mechanisms involved in nerve regeneration and repair. However, a regenerative niche at the final functional step of peripheral motor innervation, i.e. at the mature neuromuscular synapse, has not been deciphered. In this review, we integrate classic and recent evidence describing the cells and molecules that could orchestrate a dynamic ecosystem to accomplish successful NMJ regeneration. We propose that such a regenerative niche must ensure at least two fundamental steps for successful NMJ regeneration: the proper arrival of incoming regenerating axons to denervated postsynaptic muscle domains, and the resilience of those postsynaptic domains, in morphological and functional terms. We here describe and combine the main cellular and molecular responses involved in each of these steps as potential targets to help successful NMJ regeneration.

The frog neuromuscular junction (NMJ) has been extensively used as a model system to dissect the mechanisms involved in synapse formation, maturation, maintenance, regeneration, and function. Early NMJ synaptogenesis relies on a combination of cell-autonomous and interdependent pre/postsynaptic communication processes. Due to their transparency, comparatively easy manipulation, and remarkable regenerative abilities, frog tadpoles constitute an excellent model to study NMJ formation and regeneration. Here, we aimed to contribute new aspects on the characterization of the ontogeny of NMJ formation in Xenopus embryos and to explore the morphological changes occurring at the NMJ after spinal cord injury. Following analyses of X. tropicalis tadpoles during development we found that the early pathfinding of rostral motor axons is likely helped by previously formed postsynaptic specializations, whereas NMJ formation in recently differentiated ventral muscles in caudal segments seems to rely on presynaptic inputs. After spinal cord injury of X. laevis tadpoles our results suggest that rostral motor axon projections help caudal NMJ re-innervation before spinal cord connectivity is repaired.

Brain ageing is the main risk factor to develop dementia and neurodegenerative diseases, associated with a decay in the buffering capacity of the proteostasis network. We investigated the significance of the unfolded protein response (UPR), a major signaling pathway to cope with ER stress, to the functional deterioration of the brain during aging. Genetic disruption of the ER stress sensor IRE1α accelerated cognitive and motor decline during ageing. Exogenous bolstering of the UPR by overexpressing an active form of the UPR transcription factor XBP1 restored synaptic and cognitive function, in addition to reducing cell senescence. Proteomic profiling of hippocampal tissue indicated that XBP1s expression attenuated age-related alterations to synaptic function and pathways linked to neurodegenerative diseases. Overall, our results demonstrate that strategies to manipulate the UPR in mammals may sustain healthy brain ageing.

The neuromuscular junction (NMJ) is the peripheral synapse that controls the coordinated movement of many organisms. The NMJ is also an archetypical model to study synaptic morphology and function. As the NMJ is the primary target of neuromuscular diseases and traumatic injuries, the establishment of suitable models to study the contribution of specific postsynaptic muscle-derived proteins on NMJ maintenance and regeneration is a permanent need. Considering the unique experimental advantages of the levator auris longus (LAL) muscle, here we present a method allowing for efficient electroporation-mediated gene transfer and subsequent detailed studies of the morphology and function of the NMJ and muscle fibers. Also, we have standardized efficient facial nerve injury protocols to analyze LAL muscle NMJ degeneration and regeneration. Our results show that the expression of a control fluorescent protein does not alter either the muscle structural organization, the apposition of the pre- and post-synaptic domains, or the functional neurotransmission parameters of the LAL muscle NMJs; in turn, the overexpression of MuSK, a major regulator of postsynaptic assembly, induces the formation of ectopic acetylcholine receptor clusters. Our NMJ denervation experiments showed complete reinnervation of LAL muscle NMJs four weeks after facial nerve injury. Together, these experimental strategies in the LAL muscle constitute effective methods to combine protein expression with accurate analyses at the levels of structure, function, and regeneration of the NMJ. Fondo Nacional de Desarrollo Cientifico y Tecnologico (FONDECYT) 1170614 Fondo Nacional de Desarrollo Cientifico y Tecnologico (FONDECYT) 1130321 Fondo Nacional de Desarrollo Cientifico y Tecnologico (FONDECYT) 3190255 Fondo Nacional de Desarrollo Cientifico y Tecnologico (FONDECYT) 3170464 Ministerio de Economia, Industria y Competitividad, Gobierno de Espana/FEDER BFU201678934-P

Aging is the major risk factor for the development of dementia and neurodegenerative disorders, and the aging brain manifests severe deficits in buffering capacity by the proteostasis network. Accordingly, we investigated the significance of the unfolded protein response (UPR), a major signaling pathway that copes with endoplasmic reticulum (ER) stress, to normal mammalian brain aging. Genetic disruption of ER stress sensor IRE1α accelerated cognitive and motor dysfunction during aging. Exogenous bolstering of the UPR by overexpressing an active form of the transcription factor XBP1 restored synaptic and cognitive function in addition to reducing cell senescence. Remarkably, proteomic profiling of hippocampal tissue indicated that XBP1s expression corrected age-related alterations in synaptic function. Collectively, our data demonstrate that strategies to manipulate the UPR sustain healthy brain aging.One Sentence SummaryThe IRE1/XBP1 pathway dictates when and how brain function declines during aging.

The coordinated movement of organisms relies on efficient nerve-muscle communication at the neuromuscular junction. After peripheral nerve injury or neurodegeneration, motor neurons and Schwann cells increase the expression of the p75NTRpan-neurotrophin receptor. Even though p75NTRtargeting has emerged as a promising therapeutic strategy to delay peripheral neuronal damage progression, the effects of long-term p75NTRinhibition at the mature neuromuscular junction have not been elucidated. We performed quantitative neuroanathomical analyses of the neuromuscular junction in p75NTRnull mice by laser confocal and electron microscopy, which were complemented with electromyography, locomotor tests, and pharmacological intervention studies. Mature neuromuscular synapses of p75NTRnull mice show impaired postsynaptic organization and ultrastructural complexity, which correlate with altered synaptic function at the levels of nerve activity-induced muscle responses, muscle fiber structure, force production, and locomotor performance. Our results on primary myotubes and denervated muscles indicate that muscle-derived p75NTRdoes not play a major role on postsynaptic organization. In turn, motor axon terminals of p75NTRnull mice display a strong reduction in the number of synaptic vesicles and active zones. According to the observed pre and postsynaptic defects, pharmacological acetylcholinesterase inhibition rescued nerve-dependent muscle response and force production in p75NTRnull mice. Our findings revealing that p75NTRis required to organize mature neuromuscular junctions contribute to a comprehensive view of the possible effects caused by therapeutic attempts to target p75NTR.