The Intraflagellar Transport Protein IFT20 Recruits ATG16L1 to Early Endosomes to Promote Autophagosome Formation in T Cells.

Francesca Finetti,Fabrizia Zevolini,Cosima T Baldari,Anna Onnis,Monica Gesualdo,Valentina Cianfanelli,Jlenia Brunetti,Chiara Cassioli,Francesco Cecconi

doi:10.3389/fcell.2021.634003

Abstract

Lymphocyte homeostasis, activation and differentiation crucially rely on basal autophagy. The fine-tuning of this process depends on autophagy-related (ATG) proteins and their interaction with the trafficking machinery that orchestrates the membrane rearrangements leading to autophagosome biogenesis. The underlying mechanisms are as yet not fully understood. The intraflagellar transport (IFT) system, known for its role in cargo transport along the axonemal microtubules of the primary cilium, has emerged as a regulator of autophagy in ciliated cells. Growing evidence indicates that ciliogenesis proteins participate in cilia-independent processes, including autophagy, in the non-ciliated T cell. Here we investigate the mechanism by which IFT20, an integral component of the IFT system, regulates basal T cell autophagy. We show that IFT20 interacts with the core autophagy protein ATG16L1 and that its CC domain is essential for its pro-autophagic activity. We demonstrate that IFT20 is required for the association of ATG16L1 with the Golgi complex and early endosomes, both of which have been identified as membrane sources for phagophore elongation. This involves the ability of IFT20 to interact with proteins that are resident at these subcellular localizations, namely the golgin GMAP210 at the Golgi apparatus and Rab5 at early endosomes. GMAP210 depletion, while leading to a dispersion of ATG16L1 from the Golgi, did not affect basal autophagy. Conversely, IFT20 was found to recruit ATG16L1 to early endosomes tagged for autophagosome formation by the BECLIN 1/VPS34/Rab5 complex, which resulted in the local accumulation of LC3. Hence IFT20 participates in autophagosome biogenesis under basal conditions by regulating the localization of ATG16L1 at early endosomes to promote autophagosome biogenesis. These data identify IFT20 as a new regulator of an early step of basal autophagy in T cells.

Highlights

Autophagy is a degradative process that subserves the dual function of eliminating damaged macromolecules and organelles while providing endogenous energy sources and building blocks to maintain cellular homeostasis (Dikic and Elazar, 2018)
The evidence that T cells share common regulators of vesicular trafficking with ciliated cells (Cassioli and Baldari, 2019) and that IFT20 deficiency is associated to impaired autophagic flux in T cells (Finetti et al, 2020) raises the question of whether IFT20 may interact with ATG16L1 in nonciliated T cells to promote autophagy
Since the IFT20-dependent recruitment of ATG16L1 to the Golgi apparatus does not appear to contribute to its proautophagic function in T cells (Figures 3G,H), we focused on early endosomes

Summary

Introduction

Autophagy is a degradative process that subserves the dual function of eliminating damaged macromolecules and organelles while providing endogenous energy sources and building blocks to maintain cellular homeostasis (Dikic and Elazar, 2018). T cell activation and proliferation are fine-tuned by selective autophagy (Zaffagnini and Martens, 2016). In this context, the autophagy cargo is initially switched from mitochondria to macromolecules including inhibitors of T cell receptor (TCR) signaling (Hubbard et al, 2010; Valdor et al, 2014) and cell cycle (Jia et al, 2015). Autophagy impacts on helper and cytotoxic T cell effectors, with Th1, Th2, and Treg cells relying on autophagy for survival (Kovacs et al, 2012; Kabat et al, 2016; Wei et al, 2016) and cytotoxic T lymphocytes (CTL) for their function and for memory maintenance (Puleston et al, 2014; Xu et al, 2014)

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