To Eat or Not to Eat: Targeting autophagy to enhance memory immune responses
Keywords: RUBCN, autophagy, memory B cell

Memory B cells depend on autophagy for their survival, but the protein Rubicon is thought to hinder this process.
Dr. Chao-Yuan Tsai and the researchers of Osaka University have discovered a shorter isoform of Rubicon called
RUBCN100, which enhances autophagy in B cells. Mice that lacked the longer isoform, RUBCN130, produced morememory B cells in a way that relied on autophagy. These findings provide further insight into the role of Rubicon inautophagy.
Autophagy is a mechanism that degrades and recycles damaged components or organelles in cells, which plays
an important role in various systems. In the immune system, autophagy is involved in B cell self-renewal and thesurvival of memory and plasma B cells. However, the effects of enhancing autophagy in B cells are not wellunderstood.
The protein Rubicon (RUBCN) inhibits autophagy and impacts various conditions like fatty liver, kidney fibrosis,
brain α-synuclein accumulation, systemic fat atrophy, osteoporosis, and myocardial ischemia/reperfusion injury.
However, RUBCN deficiency can also lead to metabolic syndrome and decreased Sertoli cell function in mice.
RUBCN also regulates noncanonical autophagic processes like LC-3–associated phagocytosis (LAP), LC-3–associated endocytosis (LANDO), or micropinocytosis (LAM). Loss of RUBCN impairs LAP/LANDO and enhances
fungal infection and neurodegeneration, but enhances antitumor "T cell" responses and germinal center (GC) B(BGC)–cell differentiation in mouse models. However, the exact mechanisms of RUBCN's control over autophagy
and other functions are still not fully understood.
Here, the research group has identified and characterized a RUBCN isoform called RUBCN100. RUBCN100 istranslated from alternative start codons of the same messenger RNA that encodes RUBCN130. Autophagy is enhanced
in B cells deficient in RUBCN130 (Figure 1) or expressing RUBCN100.

Figure 1. Autophagy is enhanced in the RUBCN130-deficient B cells. License: CC BY-NC 4.0

In mice, selective deficiency of RUBCN130 in B cells enhances the generation of memory B cells in an autophagydependent manner (Figure 2). The increase in B cell memory is further increased by enhanced recall responses in B
cell-specific RUBCN130 knockout (RUBCN B-KO) mice. Importantly, elevated canonical autophagy alters the fateof BGC cell differentiation without affecting GC formation or cell survival in RUBCN B-KO mice.

Figure 2. Mice lacking RUBCN130 specifically in B cells produce a greater number of memory B cells in an autophagydependent manner. CD19-Cre+/-: wild-type; ATG7 B-KO: B cell-specific ATG7 KO (autophagy-deficient);
ATG7/RUBCN B-DKO; double KO (autophagy-deficient). License: CC BY-NC 4.0

The presence or absence of the RUN domain in RUBCN isoforms may explain their distinct functions inregulating autophagy (Figure 3). RUBCN100-VPS34 complexes enhance VPS34 activity, promoting autophagosomeformation. On the other hand, RUBCN130-VPS34 complexes suppress autophagy by inhibiting VPS34 activity andautophagosome-lysosome fusion. Additionally, RUBCN100 restrains mTORC1 in early endosomes, delaying its
activation. Put simply, RUBCN100 promotes autophagy, while RUBCN130 has a suppressive role.

Figure 3. A model for the functional interaction between RUBCN100 and RUBCN130 in regulating autophagy. License:
CC BY-NC 4.0

In summary, the group found a RUBCN isoform called RUBCN100 and showed that the interaction between thetwo RUBCN isoforms is important for regulating autophagy. The balance of these isoforms is crucial for maintaining
cellular balance and controlling autophagy and mTORC1 activity. A deficiency in RUBCN130 and enhancing
autophagy with drugs can increase memory B cell generation. The discovery of RUBCN100 not only resolves
conflicting findings about RUBCN's functions but also sheds light on manipulating B cell memory.

Conceptual Figure: A RUBCN100 stick cues the "Autophagy" ball to pocket the "B" ball into the “Mem” pocket.
(GC: germinal center; B: B cells; PB: plasma cells; Mem: memory B cells)
License: CC BY-NC-ND 4.0

Journal: Science Signaling (Sep. 19, 2023 online)
Title: Opposing roles of RUBCN isoforms in autophagy and memory B cell generation
Authors: Chao-Yuan Tsai*, Shuhei Sakakibara, Yu-Diao Kuan, Hiroko Omori, Maruwa Ali El Hussien, DaisukeOkuzaki, Shiou-Ling Lu, Takeshi Noda, Keisuke Tabata, Shuhei Nakamura, Tamotsu Yoshimori, Hitoshi Kikutani.
(*corresponding)https://www.science.org/doi/10.1126/scisignal.ade3599