Cul3-KLHL20 Ubiquitin Ligase Governs the Turnover of ULK1 and VPS34 Complexes to Control Autophagy Termination, 2016, Liu et al

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Cul3-KLHL20 Ubiquitin Ligase Governs the Turnover of ULK1 and VPS34 Complexes to Control Autophagy Termination

Liu, Chin-Chih; Lin, Yu-Ching; Chen, Yu-Hsuan; Chen, Chun-Ming; Pang, Liang-Yu; Chen, Hsuan-An; Wu, Pei-Rung; Lin, Mei-Yao; Jiang, Si-Tse; Tsai, Ting-Fen; Chen, Ruey-Hwa

Abstract
Autophagy, a cellular self-eating mechanism, is important for maintaining cell survival and tissue homeostasis in various stressed conditions. Although the molecular mechanism of autophagy induction has been well studied, how cells terminate autophagy process remains elusive.

Here, we show that ULK1, a serine/threonine kinase critical for autophagy initiation, is a substrate of the Cul3-KLHL20 ubiquitin ligase. Upon autophagy induction, ULK1 autophosphorylation facilitates its recruitment to KLHL20 for ubiquitination and proteolysis. This autophagy-stimulated, KLHL20-dependent ULK1 degradation restrains the amplitude and duration of autophagy.

Additionally, KLHL20 governs the degradation of ATG13, VPS34, Beclin-1, and ATG14 in prolonged starvation through a direct or indirect mechanism. Impairment of KLHL20-mediated regulation of autophagy dynamics potentiates starvation-induced cell death and aggravates diabetes-associated muscle atrophy.

Our study identifies a key role of KLHL20 in autophagy termination by controlling autophagy-dependent turnover of ULK1 and VPS34 complex subunits and reveals the pathophysiological functions of this autophagy termination mechanism.

Link (Molecular Cell)
https://doi.org/10.1016/j.molcel.2015.11.001

 

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This is one of the papers cited in the DecodeMe candidate genes and links the ubiquitin-proteasome system (UPS) and KLHL20 to autophagy termination

From the discussion

In this study, we uncover a key role of ubiquitin-proteasome system in autophagy termination and identify the critical function of KLHL20-based ubiquitin ligase in this process. Upon autophagy induction, KLHL20 orchestrates the degradation of multiple components of the ULK1 and VPS34 complexes, including ULK1, ATG13, VPS34, Beclin-1, and ATG14, through a direct or indirect mechanism. Thus, although the catalytic activities of ULK1 and VPS34 are rapidly induced during the early stage of autophagy, the abundance of these two complexes is gradually declined. This destruction of multiple proteins that act in the early steps of autophagy would comprise a timely and effective negative feedback mechanism to restrain autophagy activity, thereby shaping the dynamics of autophagy. In line with this notion, KLHL20 depletion significantly increases the amplitude and duration of autophagy response. Thus, our study identifies KLHL20 as a master regulator of autophagy termination and uncovers an intriguing crosstalk between ubiquitin-proteasome system and autophagy-lysosome system.

The identification of a role of KLHL20 in coordinating the feedback regulation of multiple autophagy factors offers a unique opportunity to explore the physiological and pathological importance of autophagy termination.

At an organism level, skeletal muscle-specific ablation of KLHL20 enhances experimental diabetes-associated muscle atrophy, which is reversed by blockage of autophagic degradation. Although we cannot completely rule out an involvement of autophagy-independent effect of KLHL20 in the muscle phenotype, our study supports that KLHL20-mediated autophagy termination acts as an adaptive mechanism to prevent excessive muscle loss under this disease state. Furthermore, these findings are consistent with a muscle-wasting effect induced by excessive autophagy, even though basal autophagy activity is needed to preserve muscle structure and function by removing damaged or dysfunctional cell components.

 

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A couple more snippets

Under stressed conditions, ULK1 catalytic activity is induced through AMPK-mediated phosphorylation and/or loss of mTOR-mediated repression. The activated ULK1 phosphorylates downstream targets to trigger autophagy cascade.

We show that ULK1 autophosphorylation at S1042/T1046 potentiates its interaction with KLHL20, leading to ULK1 degradation

Conversely, our study has provided substantial evidence for the destabilization effect of ULK1 autophosphorylation at S1042/T1046. This finding also indicates a tight coupling of ULK1 catalytic activity to its degradation, an ideal mode for negative feedback regulation. Indeed, ablation of this ULK1 autophosphorylation leads to persistent autophagic activity.

We noticed that, unlike the evenly distributed Cul3, KLHL20 in starved cells was enriched in punctate structures. Intriguingly, these KLHL20-positive puncta were frequently situated at the edge of ATG16 puncta, but not at lysosomes/autolysosomes. Furthermore, a partial co-localization of Beclin-1 with KLHL20 in ATG16-positive phagophore structures was also frequently observed

 

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A brief summary (my interpretation , possibly over simplistic but hopefully not too off the mark…)
- ULK1 is important to autophagy
- KLHL20 tags ULK1 (and some other things) so they are degraded, so helping keep a brake on the process
- In mouse models without KLHL20 this doesn’t happen
- So you get a ULK1 feedback loop
- And increased autophagy
- Chloroquine reduced the autophagy in mice

A few things I found interesting here
- We have a feedback loop here, ULK1 triggers it’s own destruction… normally
- And a link, crosstalk, between two maintenance systems, the ubiquitin proteasome system and the autophagy lysosome system
- There seems to be some localised effects, with enrichment of KLHL20 in specific areas

And from DecodeME, the allele associated with an increased risk of ME/CFS is associated with decreasing KLHL20 expression, so presumably associated with a less effective/strong brake on this process and feedback loop.