Drosophila INDY and Mammalian INDY: Major Differences in Transport Mechanism and Structural Features despite Mostly Similar Biological Functions

Abstract

INDY (I’m Not Dead Yet) is a plasma membrane transporter for citrate, first identified in Drosophila. Partial deficiency of INDY extends lifespan in this organism in a manner similar to that of caloric restriction. The mammalian counterpart (NaCT/SLC13A5) also transports citrate. In mice, it is the total, not partial, absence of the transporter that leads to a metabolic phenotype similar to that caloric restriction; however, there is evidence for subtle neurological dysfunction. Loss-of-function mutations in SLC13A5 (solute carrier gene family 13, member A5) occur in humans, causing a recessive disease, with severe clinical symptoms manifested by neonatal seizures and marked disruption in neurological development. Though both Drosophila INDY and mammalian INDY transport citrate, the translocation mechanism differs, the former being a dicarboxylate exchanger for the influx of citrate2− in exchange for other dicarboxylates, and the latter being a Na+-coupled uniporter for citrate2−. Their structures also differ as evident from only ~35% identity in amino acid sequence and from theoretically modeled 3D structures. The varied biological consequences of INDY deficiency across species, with the beneficial effects predominating in lower organisms and detrimental effects overwhelming in higher organisms, are probably reflective of species-specific differences in tissue expression and also in relative contribution of extracellular citrate to metabolic pathways in different tissues

Mapping the Metabolic Niche of Citrate Metabolism and SLC13A5

“…Drosophila INDY turned out to be a transporter with a preference for citrate over dicarboxylates and is the mammalian ortholog of NaCT [ 30 ]. Loss of function of NaCT activity leads to severe neurological dysfunction in humans, but to a less severe phenotype in mice, thus suggesting species-specific differences ( Figure 1 A) [ 31 , 32 ]. Indeed, human SLC13A5 expression is highest in the liver followed by the brain and testes.…”

Molecular phenotypes segregate missense mutations in SLC13A5 Epilepsy

“…Such potentiators have been highly successful in the treatment of gating mutations of CFTR (e.g., G551D); they act by non-specifically opening the channel to enable ion fluxes. However, potentiators have not been described so far for any disease-causing mutations of transporters that utilize an elevator-type mechanism, such as NaCT (Jaramillo-Martinez et al, 2021a; Jaramillo-Martinez et al, 2021b; Jaramillo-Martinez et al, 2021c; Mulligan et al, 2016; Sauer et al, 2021a; Sauer et al, 2021b).…”

SLC13A5 Deficiency Disorder: From Genetics to Gene Therapy

“…While Slc13a5 KO mice model salient aspects of SDD that can be utilized to develop an AAV9 SLC13A5 gene replacement therapy, they do not show signs of overt behavioral or cognitive impairment seen in patients with SDD [ 3 ]. These differences between mice and humans may be indicative of underlying biochemical (e.g., different K M ) or physiological differences in NaCT function between species [ 37 , 38 ]. This difference may limit the applicability of the Slc13a5 KO mouse model and there is need for further development of SDD animal models that recapitulate additional clinical symptoms.…”

“…Rodent NaCT has a much greater affinity for citrate than the human and non-human primate counterparts. Additionally, lithium inhibits NaCT in rodents, but stimulates NaCT in primates, suggesting species-specific biochemical differences in the transporter [ 37 ]. The development of gene replacement therapy for SDD should therefore place emphasis on non-human primate toxicological investigations and include neurological assessments before advancing to clinical trials.…”