Description

Repetitive increases in the cytoplasmic calcium concentration (Ca²⁺ oscillation) control a wide variety of biological events (Dupont et al. 2011). In fertilization, a sperm-bearing factor, phospholipase C zeta 1 (PLCz1), triggers Ca²⁺ oscillation and resumes cell division in eggs. Citrate synthase (CS) is localized to the mitochondrial matrix, where it catalyzes a reaction between acetyl-coenzyme A (CoA) and oxaloacetate to form citric acid (Surpin and Chory 1997) (Figure 1A). The extra-mitochondrial form of CS (eCS) is encoded by a separate gene in mice and is expressed by alternative splicing from the CS gene in humans (Kang et al. 2020). eCS functions as a secondary factor triggering Ca²⁺ oscillation, which is transferred from the sperm to the eggs (Kang et al. 2020). More specifically, eCS has been found to trigger an initial Ca²⁺ spike using a PLCz1-independent mechanism. However, the role of eCS-triggered Ca²⁺ oscillation in broad cell functions is unknown.

Presently, we studied the neuronal involvement of eCS and its neuronal expression using eCs-deficient (KO) male mice carrying the LacZ reporter gene inserted into the eCs-null allele. KO mice were born healthy, but their body size was noticeably small. Therefore, KO mice were weighed daily after birth and compared with wild-type (WT) mice (Figure 1B). From the first to the second week, the body weight was comparable between KO and WT mice. However, KO mice continuously weighed less than WT mice during the third week (6.7 ± 0.5 g vs. 11.2 ± 1.1 g; p < 0.0001), fourth week (9.1 ± 0.4 g vs. 14.0 ± 2.5 g; p < 0.0001), fifth week (15.6 ± 0.6 g vs. 19.5 ± 0.6 g; p < 0.0001), and sixth week (18.3 ± 0.6 g vs. 20.4 ± 0.7 g; p < 0.0001). No difference was noted from the seventh week onward. Moreover, eCS expression was detected in a narrow layer of the cerebellar cortex, probably the Purkinje layer (Figure 1C). From this result, we assumed that eCs and eCS-expressed neuronal cells could regulate the rapid increase in body weight during childhood.

Growth retardation is linked to low concentrations of growth hormone (GH) in humans and mice. GH, also known as somatotropin, is a peptide hormone that mainly functions in growth, cell replication, and cell regeneration (Velloso 2008). GH stimulates the production of the insulin-like growth factor-1 (IGF-1) (also called growth-promoting hormone) to regulate overall body growth (Yakar et al. 2002). Moreover, GH release conclusively depends on changes in intracellular Ca²⁺ concentration, indicating a critical role of intracellular Ca²⁺ as a mediator of GH function (Cuttler et al. 1992). Upon the development of the central nervous system, IGF-1 is abundantly expressed in neurons, especially in Purkinje cells, to promote cell survival in the cerebellum (Torres-Aleman et al. 1994; Chrysis et al. 2001), suggesting a possible contribution of eCS to neuronal survival.

Otherwise, spontaneous Ca²⁺ oscillation is induced in astrocytes both in vitro and in vivo (Zhou et al. 2020). Ca²⁺ oscillation also controls the fate determination of cultured neural stem cells (Glaser et al. 2020). From our results, we concluded that eCS expressed in the Purkinje cell layer could trigger neuronal signaling via Ca²⁺ oscillation, subsequently enhancing the rapid growth observed during childhood.

Citrate has been shown to be a regulator of various biological processes, such as insulin secretion (Iacobazzi and Infantino 2014). Citrates act as suppressors of problematic events, such as in the reduction of oxidative stress and inflammation (Iacobazzi and Infantino 2014) and in the protection against traumatic brain injury (Kilbaugh et al. 2015). However, while it is known that citrates are specifically synthesized and released from astrocytes (Westergaard et al. 2017), presently, knowledge of the role of citrates is insufficient. Our results largely contribute to the understanding of eCS-mediated Ca²⁺ oscillation in the brain.