Abstract
Neurons are particularly vulnerable to perturbations in endo-lysosomal transport, as several neurological disorders are caused by a primary deficit in this pathway. In this report, we used positional cloning to show that the spontaneously occurring neurological mutation teetering (tn) is a single nucleotide substitution in hepatocyte growth factor-regulated tyrosine kinase substrate (Hgs/Hrs), a component of the endosomal sorting complex required for transport (ESCRT). The tn mice exhibit hypokenesis, muscle weakness, reduced muscle size and early perinatal lethality by 5-weeks of age. Although HGS has been suggested to be essential for the sorting of ubiquitinated membrane proteins to the lysosome, there were no alterations in receptor tyrosine kinase levels in the central nervous system, and only a modest decrease in tropomyosin receptor kinase B (TrkB) in the sciatic nerves of the tn mice. Instead, loss of HGS resulted in structural alterations at the neuromuscular junction (NMJ), including swellings and ultra-terminal sprouting at motor axon terminals and an increase in the number of endosomes and multivesicular bodies. These structural changes were accompanied by a reduction in spontaneous and evoked release of acetylcholine, indicating a deficit in neurotransmitter release at the NMJ. These deficits in synaptic transmission were associated with elevated levels of ubiquitinated proteins in the synaptosome fraction. In addition to the deficits in neuronal function, mutation of Hgs resulted in both hypermyelinated and dysmyelinated axons in the tn mice, which supports a growing body of evidence that ESCRTs are required for proper myelination of peripheral nerves. Our results indicate that HGS has multiple roles in the nervous system and demonstrate a previously unanticipated requirement for ESCRTs in the maintenance of synaptic transmission.
Highlights
In neurons, endosomal transport and sorting of internalized cargo affects the abundance of plasma membrane proteins and regulates a diverse group of cellular processes such as signal transduction and synaptic vesicle recycling [1,2,3]
Many factors involved in endosomal sorting in mammalian cells have been identified, and mutations in these components are associated with a variety of neurological disorders
While the function of endosomal sorting components has been intensely studied in immortalized cell lines, it is not known what role these factors play in endosomal sorting in the nervous system
Summary
Endosomal transport and sorting of internalized cargo affects the abundance of plasma membrane proteins and regulates a diverse group of cellular processes such as signal transduction and synaptic vesicle recycling [1,2,3]. Four distinct ESCRT complexes, ESCRT-0, -I, -II and—III, act sequentially to contribute to the sorting of membrane proteins, determining if endocytosed receptors are recycled back to the cell surface or are further sorted into intraluminal vesicles (ILVs) within specialized endosomes called multivesicular bodies (MVBs) for their eventual degradation by lysosomes [6]. The ESCRT-0 complex, composed of HGS and signal transducing adaptor molecule 1 (STAM1) [11,12,13], is essential for the initial recognition of ubiquitinated cargo that will be sorted at the endosomal membrane and degraded upon endo-lysosomal fusion [14,15]. Despite the insights into HGS function gained from the studies described above, the precise role of HGS in endosomal trafficking has not been determined in the mammalian nervous system
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