Endogenous GSK‐3/Shaggy Regulates Bidirectional Axonal Transport of the Amyloid Precursor Protein

Abstract

Neurons rely on microtubule (MT) motor proteins such as kinesin-1 and dynein to transport essential cargos between the cell body and axon terminus. Defective axonal transport causes abnormal axonal cargo accumulations and is connected to neurodegenerative diseases, including Alzheimer's disease (AD). Glycogen synthase kinase 3 (GSK-3) has been proposed to be a central player in AD and to regulate axonal transport by the MT motor protein kinesin-1. Using genetic, biochemical and biophysical approaches in Drosophila melanogaster, we find that endogenous GSK-3 is a required negative regulator of both kinesin-1-mediated and dynein-mediated axonal transport of the amyloid precursor protein (APP), a key contributor to AD pathology. GSK-3 also regulates transport of an unrelated cargo, embryonic lipid droplets. By measuring the forces motors generate in vivo, we find that GSK-3 regulates transport by altering the activity of kinesin-1 motors but not their binding to the cargo. These findings reveal a new relationship between GSK-3 and APP, and demonstrate that endogenous GSK-3 is an essential in vivo regulator of bidirectional APP transport in axons and lipid droplets in embryos. Furthermore, they point to a new regulatory mechanism in which GSK-3 controls the number of active motors that are moving a cargo.

Lipid Droplets Purified from Drosophila Embryos as an Endogenous Handle for Precise Motor Transport Measurements

“…However, lipid droplets from Drosophila facilitate the study of motor regulation, since the droplets carry both-polarity microtubule motors and have the added advantages of their extensively characterized transport in vivo (1,3,32-34), a published proteome (31), and the tractable genetics of the fly. Further, we have recently shown that lipid droplets can be used as a model to study neuronal transport, as some of the regulators of axonal cargoes regulate lipid droplets similarly (16).…”

“…Several recent studies have demonstrated the ability to make precision measurements on individual endogenous cargoes in living cells (3,(12)(13)(14)(15), and such measurements have been used to study the coordination of opposite polarity motors (1) and motor regulation (16). However, in vivo measurements suffer from three shortcomings that reduce the ability to dissect the details of motor dynamics and function with high precision.…”

Regulation of Axonal Transport by Protein Kinases

“…rAT, retrograde axonal transport; aAT, anterograde axonal transport; +, enhanced; À, inhibited; NE, no effect. [27], and lipid droplets in larval segmental nerves [27]. There are conflicting views over whether GSK3 also inhibits the axonal transport of synaptic vesicles in these nerves [25,27].…”

“…[27], and lipid droplets in larval segmental nerves [27]. There are conflicting views over whether GSK3 also inhibits the axonal transport of synaptic vesicles in these nerves [25,27]. Stimulation of GSK3b by pathological proteins, such as presenilin 1 and amyloid beta (Ab) (see section on Relevance to central nervous system function and disease) also inhibits bidirectional axonal transport in mammalian neurons [29,30] (Table 1).…”

Axonal Transport Defects in Alzheimer’s Disease

“…As mentioned previously, recent studies found that endogenous GSK-3 is a required negative regulator of both bidirectional APP transport in axons using genetic, biochemical, and biophysical approaches in drosophila melanogaster. It was noted that by measuring the forces that motors generate in vivo, GSK-3 regulated transport by altering the activity of kinesin-1 motors but not their binding to the cargoes [28]. Additionally, due to BACE1 initiated Aβ production, enhanced β-secretase cleavage reduced the anterograde AT of APP, while inhibited β-cleavage stimulated APP anterograde AT [29].…”