Targeting SARM1 improves autophagic stress-induced axonal neuropathy.

ABBREVIATIONS: AAV: adeno-associated virus; ATF3: activating transcription factor 3; ATG7: autophagy related 7; AVIL: advillin; cADPR: cyclic ADP ribose; CALC: calcitonin/calcitonin-related polypeptide; CMT: Charcot-Marie-Tooth disease; cKO: conditional knockout; DEG: differentially expressed gene; DRG: dorsal root ganglion; FE-SEM: field emission scanning electron microscopy; IF: immunofluorescence; NCV: nerve conduction velocity; PVALB: parvalbumin; RAG: regeneration-associated gene; ROS: reactive oxygen species; SARM1: sterile alpha and HEAT/Armadillo motif containing 1; SYN1: synapsin I.

Heat Shock Protein 90 is Required for cAMP-Induced Differentiation in Rat Primary Schwann Cells.

Schwann cells (SCs) play an important role in producing myelin for rapid neurotransmission in the peripheral nervous system. Activation of the differentiation and myelination processes in SCs requires the expression of a series of transcriptional factors including Sox10, Oct6/Pou3f1, and Egr2/Krox20. However, functional interactions among several transcription factors are poorly defined and the important components of the regulatory network are still unknown. Until now, available evidence suggests that SCs require cAMP signaling to initiate the myelination program. Heat shock protein 90 (Hsp90) is known as a chaperone required to stabilize ErbB2 receptor. In recent years, it was reported that cAMP transactivated the ErbB2/ErbB3 signaling in SCs. However, the relationship between Hsp90 and cAMP-induced differentiation in SCs is undefined. Here we investigated the role of Hsp90 during cAMP-induced differentiation of SCs using Hsp90 inhibitor, geldanamycin and Hsp90 siRNA transfection. Our results showed that dibutyryl-cAMP (db-cAMP) treatment upregulated Hsp90 expression and led to nuclear translocation of Gab1/ERK, the downstream signaling pathway of the ErbB2 signaling mechanism in myelination. The expression of myelin-related genes and nuclear translocation of Gab1/ERK following db-cAMP treatment was inhibited by geldanamycin pretreatment and Hsp90 knockdown. These findings suggest that Hsp90 might play a role in cAMP-induced differentiation via stabilization of ErbB2 and nuclear translocation of Gab1/ERK in SCs.

Cooperative interaction of hepatocyte growth factor and neuregulin regulates Schwann cell migration and proliferation through Grb2-associated binder-2 in peripheral nerve repair.

The sequential reactive changes in Schwann cell phenotypes in transected peripheral nerves, including dedifferentiation, proliferation and migration, are essential for nerve repair. Even though the injury-induced migratory and proliferative behaviors of Schwann cells resemble epithelial and mesenchymal transition (EMT) in tumors, the molecular mechanisms underlying this phenotypic change of Schwann cells are still unclear. Here we show that the reactive Schwann cells exhibit migratory features dependent on the expression of a scaffolding oncoprotein Grb2-associated binder-2 (Gab2), which was transcriptionally induced by neuregulin 1-ErbB2 signaling following nerve injury. Injury-induced Gab2 expression was dependent on c-Jun, a transcription factor critical to a Schwann cell reprograming into a repair-type cell. Interestingly, the injury-induced activation (tyrosine phosphorylation) of Gab2 in Schwann cells was regulated by an EMT signal, the hepatocyte growth factor-c-Met signaling, but not by neuregulin 1. Gab2 knockout mice exhibited a deficit in nerve repair after nerve transection due to limited Schwann cell migration. Furthermore, Gab2 was required for the proliferation of Schwann cells following nerve injury and in vitro, and was over-expressed in human Schwann cell-derived tumors. In contrast, the tyrosine phosphorylation of Gab1 after nerve injury was principally regulated by the neuregulin 1-ErbB2 signaling and was indispensable for remyelination after crush injury, but not for the proliferation and migration of Schwann cells. Our findings indicate that Gab1 and Gab2 in Schwann cells are nonredundant and play a crucial role in peripheral nerve repair.

The Scaffolding Protein, Grb2-associated Binder-1, in Skeletal Muscles and Terminal Schwann Cells Regulates Postnatal Neuromuscular Synapse Maturation.

The vertebrate neuromuscular junction (NMJ) is considered as a "tripartite synapse" consisting of a motor axon terminal, a muscle endplate, and terminal Schwann cells that envelope the motor axon terminal. The neuregulin 1 (NRG1)-ErbB2 signaling pathway plays an important role in the development of the NMJ. We previously showed that Grb2-associated binder 1 (Gab1), a scaffolding mediator of receptor tyrosine kinase signaling, is required for NRG1-induced peripheral nerve myelination. Here, we determined the role of Gab1 in the development of the NMJ using muscle-specific conditional Gab1 knockout mice. The mutant mice showed delayed postnatal maturation of the NMJ. Furthermore, the selective loss of the gab1 gene in terminal Schwann cells produced delayed synaptic elimination with abnormal morphology of the motor endplate, suggesting that Gab1 in both muscles and terminal Schwann cells is required for proper NMJ development. Gab1 in terminal Schwann cells appeared to regulate the number and process elongation of terminal Schwann cells during synaptic elimination. However, Gab2 knockout mice did not show any defects in the development of the NMJ. Considering the role of Gab1 in postnatal peripheral nerve myelination, our findings suggest that Gab1 is a pleiotropic and important component of NRG1 signals during postnatal development of the peripheral neuromuscular system.

Laminin 211 inhibits protein kinase A in Schwann cells to modulate neuregulin 1 type III-driven myelination.

Myelin is required for proper nervous system function. Schwann cells in developing nerves depend on extrinsic signals from the axon and from the extracellular matrix to first sort and ensheathe a single axon and then myelinate it. Neuregulin 1 type III (Nrg1III) and laminin α2ß1γ1 (Lm211) are the key axonal and matrix signals, respectively, but how their signaling is integrated and if each molecule controls both axonal sorting and myelination is unclear. Here, we use a series of epistasis experiments to show that Lm211 modulates neuregulin signaling to ensure the correct timing and amount of myelination. Lm211 can inhibit Nrg1III by limiting protein kinase A (PKA) activation, which is required to initiate myelination. We provide evidence that excessive PKA activation amplifies promyelinating signals downstream of neuregulin, including direct activation of the neuregulin receptor ErbB2 and its effector Grb2-Associated Binder-1 (Gab1), thereby elevating the expression of the key transcription factors Oct6 and early growth response protein 2 (Egr2). The inhibitory effect of Lm211 is seen only in fibers of small caliber. These data may explain why hereditary neuropathies associated with decreased laminin function are characterized by focally thick and redundant myelin.

Transient lysosomal activation is essential for p75 nerve growth factor receptor expression in myelinated Schwann cells during Wallerian degeneration.

Myelinated Schwann cells in the peripheral nervous system express the p75 nerve growth factor receptor (p75NGFR) as a consequence of Schwann cell dedifferentiation during Wallerian degeneration. p75NGFR has been implicated in the remyelination of regenerating nerves. Although many studies have shown various mechanisms underlying Schwann cell dedifferentiation, the molecular mechanism contributing to the re-expression of p75NGFR in differentiated Schwann cells is largely unknown. In the present study, we found that lysosomes were transiently activated in Schwann cells after nerve injury and that the inhibition of lysosomal activation by chloroquine or lysosomal acidification inhibitors prevented p75NGFR expression at the mRNA transcriptional level in an ex vivo Wallerian degeneration model. Lysosomal acidification inhibitors suppressed demyelination, but not axonal degeneration, thereby suggesting that demyelination mediated by lysosomes may be an important signal for inducing p75NGFR expression. Tumor necrosis factor-α (TNF-α) has been suggested to be involved in regulating p75NGFR expression in Schwann cells. In this study, we found that removing TNF-α in vivo did not significantly suppress the induction of both lysosomes and p75NGFR. Thus, these findings suggest that lysosomal activation is tightly correlated with the induction of p75NGFR in demyelinating Schwann cells during Wallerian degeneration.