Extra-mitochondrial citrate synthase initiates calcium oscillation and suppresses age-dependent sperm dysfunction

Abstract

Men and women become infertile with age, but the mechanism of declining male fertility, more specifically, the decrease in in sperm quality, is not well known. Citrate synthase (CS) is a core enzyme of the mitochondrial tricarboxylic acid (TCA) cycle, which directly controls cellular function. Extra-mitochondrial CS (eCS) is produced and abundant in the sperm head; however, its role in male fertility is unknown. We investigated the role of eCS in male fertility by producing eCs-deficient (eCs-KO) mice. The initiation of the first spike of Ca 2+ oscillation was substantially delayed in egg fused with eCs-KO sperm, despite normal expression of sperm factor phospholipase C zeta 1. The eCs-KO male mice were initially fertile, but the fertility dropped with age. Metabolomic analysis of aged sperm revealed that the loss of eCS enhances TCA cycle in the mitochondria with age, presumably leading to depletion of extra-mitochondrial citrate. The data suggest that eCS suppresses age-dependent male infertility, providing insights into the decline of male fertility with age.

Emerging Role of TCA Cycle-Related Enzymes in Human Diseases

“…In contrast, cytosolic sperm extracts have been reported to trigger an increase in Ca 2+ concentration in newt eggs [ 80 ]; the responsible factor for this was identified as a CS [ 80 ]. A single gene produces variants corresponding to CS, and eCS lacking mitochondrial-targeting sequences (MTS) in various plant and animal species [ 33 ].…”

The soluble sperm factor that activates the egg: PLCzeta and beyond

“…Furthermore, PAWP is supposed to cause Ca 2+ oscillations during ICSI, but we now know that PLCζ accounts for these Ca 2+ oscillations. Another study has suggested that extramitochondrial citrate synthase is the second sperm factor in mammals (Kang et al 2020). However, the phenotype of extramitochondrial citrate synthase KO sperm at fertilization is apparently the same as PLCζ KO sperm, with delayed Ca 2+ oscillations (Kang et al 2020).…”

“…Another study has suggested that extramitochondrial citrate synthase is the second sperm factor in mammals (Kang et al 2020). However, the phenotype of extramitochondrial citrate synthase KO sperm at fertilization is apparently the same as PLCζ KO sperm, with delayed Ca 2+ oscillations (Kang et al 2020). This result is difficult to rationalize because these citrate synthase KO sperm still contained PLCζ and the initial Ca 2+ oscillations should not be delayed.…”

Citrate Synthase and OGDH as Potential Biomarkers of Atherosclerosis under Chronic Stress

“…Citrate synthase, also known as citrate condensation enzyme, is an important component of energy metabolism. It catalyzes the reaction of acetyl-CoA and oxaloacetate, releasing coenzyme A and citric acid, which participate in the TCA cycle as rate-limiting enzymes and help regulate the glyoxylate cycle [ 30 ]. This enzyme is present in almost all cells that can undertake oxidative metabolism.…”

“…Through epidemiological investigations and laboratory studies, it has been found that stress load has become one of the most important factors in the development of AS in modern society [15]. It is extremely important to fully explore the [30]. This enzyme is present in almost all cells that can undertake oxidative metabolism.…”