Highlights d Tmem106b knockout leads to LAMP1-positive vacuoles at the axon initial segment d Vacuolization is mostly confined to motoneurons d Vacuoles develop due to impaired axonal trafficking of LAMP1-positive organelles d Degradation of autophagic cargo is impaired due to TMEM106B deficiency
Axon injury in the peripheral nervous system (PNS) induces a regeneration-associated gene (RAG) response. Atf3 (activating transcription factor 3) is such a RAG and ATF3's transcriptional activity might induce ‘effector’ RAGs (e.g. small proline rich protein 1a (Sprr1a), Galanin (Gal), growth-associated protein 43 (Gap43)) facilitating peripheral axon regeneration. We provide a first analysis of Atf3 mouse mutants in peripheral nerve regeneration. In Atf3 mutant mice, facial nerve regeneration and neurite outgrowth of adult ATF3-deficient primary dorsal root ganglia neurons was decreased. Using genome-wide transcriptomics, we identified a neuropeptide-encoding RAG cluster (vasoactive intestinal peptide (Vip), Ngf, Grp, Gal, Pacap) regulated by ATF3. Exogenous administration of neuropeptides enhanced neurite growth of Atf3 mutant mice suggesting that these molecules might be effector RAGs of ATF3's pro-regenerative function. In addition to the induction of growth-promoting molecules, we present data that ATF3 suppresses growth-inhibiting molecules such as chemokine (C-C motif) ligand 2. In summary, we show a pro-regenerative ATF3 function during PNS nerve regeneration involving transcriptional activation of a neuropeptide-encoding RAG cluster. ATF3 is a general injury-inducible factor, therefore ATF3-mediated mechanisms identified herein might apply to other cell and injury types.
Traumatic injuries to human peripheral nerves are frequently associated with damage to nerve surrounding tissues including muscles and blood vessels. Currently, most rodent models of peripheral nerve injuries (e.g., facial or sciatic nerve) employ surgical nerve transection with scissors or scalpels. However, such an isolated surgical nerve injury only mildly damages neighboring tissues and weakly activates an immune response. In order to provide a rodent nerve injury model accounting for such nerve-associated tissue damage and immune cell activation, we developed a drop tower-based facial nerve trauma model in mice. We compare nerve regeneration in this novel peripheral nerve trauma model with the established surgical nerve injury along several parameters. These include gene expression, histological and functional facial motoneuron (FMN) regeneration, facial nerve degeneration, immune cell activation and muscle damage. Regeneration-associated genes (RAGs; e.g., Atf3) were strongly induced in FMNs subjected to traumatic and surgical injury. Regeneration of FMNs and functional recovery of whisker movement were faster in traumatic versus complete surgical injury, thus cutting down experimentation time. Wallerian degeneration of distal nerve stumps was readily observed in this novel trauma injury model. Importantly, drop tower-inflicted facial nerve injury resulted in muscle damage, activation of muscle satellite cell markers (PAX7) and pronounced infiltration of immune cells to the injury site only in this model but not upon surgical nerve transection. Thus, we provide a novel rodent PNS trauma model that can be easily adopted to other PNS nerves such as the sciatic nerve. Since this nerve trauma model replicates multiple tissue damage frequently encountered in clinical routine, it will be well suited to identify molecular and cellular mechanisms of PNS nerve repair in wild-type and genetically modified rodents.
MRI (magnetic resonance imaging) is an indispensable tool in the diagnosis of centrals nervous system (CNS) disorders such as spinal cord injury and multiple sclerosis (MS). In contrast, diagnosis of peripheral nerve injuries largely depends on clinical and electrophysiological parameters. Thus, currently MRI is not regularly used which in part is due to small nerve calibers and isointensity with surrounding tissue such as muscles. In this study we performed translational MRI research in mice to establish a novel MRI protocol visualizing intact and injured peripheral nerves in a non-invasive manner without contrast agents. With this protocol we were able to image even very small nerves and nerve branches such as the mouse facial nerve (diameter 100–300 μm) at highest spatial resolution. Analysis was performed in the same animal in a longitudinal study spanning 3 weeks after injury. Nerve injury caused hyperintense signal in T 2 -weighted images and an increase in nerve size of the proximal and distal nerve stumps were observed. Further hyperintense signal was observed in a bulb-like structure in the lesion site, which correlated histologically with the production of fibrotic tissue and immune cell infiltration. The longitudinal MR representation of the facial nerve lesions correlated well with physiological recovery of nerve function by quantifying whisker movement. In summary, we provide a novel protocol in rodents allowing for non-invasive, non-contrast agent enhanced, high-resolution MR imaging of small peripheral nerves longitudinally over several weeks. This protocol might further help to establish MRI as an important diagnostic and post-surgery follow-up tool to monitor peripheral nerve injuries in humans.
traumatic injury of peripheral nerves typically also damages nerve surrounding tissue including muscles. Hence, molecular and cellular interactions of neighboring damaged tissues might be decisive for successful axonal regeneration of injured nerves. So far, the contribution of muscles and muscle-derived molecules to peripheral nerve regeneration has only poorly been studied. Herein, we conditionally ablated SRF (serum response factor), an important myofiber transcription factor, in skeletal muscles of mice. Subsequently, the impact of this myofiber-restricted SRF deletion on peripheral nerve regeneration, i.e. facial nerve injury was analyzed. Quantification of facial nerve regeneration by retrograde tracer transport, inspection of neuromuscular junctions (nMJs) and recovery of whisker movement revealed reduced axonal regeneration upon muscle specific Srf deletion. in contrast, responses in brainstem facial motor neuron cell bodies such as regeneration-associated gene (RAG) induction of Atf3, synaptic stripping and neuroinflammation were not overly affected by SRF deficiency. Mechanistically, SRF in myofibers appears to stimulate nerve regeneration through regulation of muscular satellite cell (Sc) proliferation. in summary, our data suggest a role of muscle cells and SRf expression within muscles for regeneration of injured peripheral nerves.
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