Bulk degradation of dendritic cargos requires Rab7-dependent transport in Rab7-positive/LAMP1-negative endosomes to somatic lysosomes.

Abstract

Regulation of protein homeostasis (\"proteostasis\") is necessary for maintaining healthy cells. Disturbances in proteostasis lead to aggregates, cellular stress and can result in toxicity. There is thus great interest in when and where proteins are degraded in cells. Neurons are very large as well as very long-lived, creating unusually high needs for effective regulation of protein turnover in time and space. We previously discovered that the dendritic membrane proteins Nsg1 and Nsg2 are short-lived with half-lives of less than two hours. Their short half-lives enabled us to ask whether these proteins are degraded by local degradative pathways in dendrites. We discovered a striking spatial gradient of late endosomes/lysosomes in dendrites, with late endosomes (Rab7-positive/LAMP1-negative/cathepsinB-negative) found in distal portion of dendrites, and degradative lysosomes (LAMP1-positive/cathepsinB-positive) being overwhelmingly found in the soma and in the proximal portion of dendrites. Surprisingly, the majority of dendritic Rab7-positive late endosomes do not contain LAMP1, unlike Rab7-positive late endosomes in fibroblasts. Secondly, Rab7 activity is required to mobilize these distal pre-degradative dendritic late endosomes for transport to the soma and degradation. We conclude that the vast majority of dendritic LAMP1-positive endosomes are not degradative lysosomes and that bulk degradation of dendritic cargos, such as Nsg1, Nsg2, and DNER, requires Rab7-dependent transport in late endosomes to somatic lysosomes.