RT Journal Article
SR Electronic
T1 The Structural Basis for SARM1 Inhibition, and Activation Under Energetic Stress
JF bioRxiv
FD Cold Spring Harbor Laboratory
SP 2020.08.05.238287
DO 10.1101/2020.08.05.238287
A1 Michael Sporny
A1 Julia Guez-Haddad
A1 Tami Khazma
A1 Avraham Yaron
A1 Moshe Dessau
A1 Carsten Mim
A1 Michail N. Isupov
A1 Ran Zalk
A1 Michael Hons
A1 Yarden Opatowsky
YR 2020
UL http://biorxiv.org/content/early/2020/08/05/2020.08.05.238287.abstract
AB SARM1 is a central executor of axonal degeneration (1). Mechanistically, SARM1 contains NADase activity, which, in response to nerve injury, depletes the key cellular metabolite, NAD+ (2–5). Interestingly, SARM1 knockout mouse models do not present any apparent physiological impairment. Yet, the lack of SARM1 protects against various neuropathies (6, 7), thereby highlighting SARM1 as a likely safe and effective drug target (8). However, the absence of a SARM1 structure, in its active or inhibited form, makes it impossible to understand the molecular basis of SARM1 inhibition, and its activation under stress conditions. In this study we present two cryo-EM maps of SARM1 (at 2.6 Å and 2.9 Å resolution). We show that the inhibited SARM1 homo-octamer assumes a packed conformation with well-ordered inner and peripheral rings. Here the catalytic TIR domains are held apart from each other and are unable to form dimers, which is a prerequisite for NADase activity. More importantly, after screening several cellular metabolites we discovered that the inactive conformation is stabilized by the binding of SARM1’s own substrate: NAD+. The NAD+ inhibitory allosteric site is located away from the NAD+ catalytic site of the TIR domain. Site-directed mutagenesis of the allosteric site leads to constitutive active SARM1. Based on our data we propose that a reduction of cellular NAD+ concentrations (an early indication of disease-associated and age-related neurodegeneration (9)) disassemble SARM1’s peripheral ring, which allows NADase activity. This leads to an energetic catastrophe and eventually cell death. The discovery of the allosteric inhibitory site opens the door for the development of effective drugs that will prevent SARM1 activation, rather than compete for binding to the NADase catalytic site.Brief description It is not known how NAD+ depletion brings about neurodegeneration. Here, we show that the intrinsic NADase activity of SARM1 is allosterically inhibited by physiological concentrations of NAD+. NAD+ stabilizes a compact, auto-inhibited conformation of the SARM1 octamer. Once NAD+ levels are depleted, the allosteric inhibition is released, enabling SARM1’s NADase activity, which eventually leads to energetic catastrophe and cell death.Competing Interest StatementThe authors have declared no competing interest.