Abstract
ABSTRACTThe mouse Gpi1 gene encodes the glycolytic enzyme glucose phosphate isomerase. Homozygous Gpi1−/− null mouse embryos die but a previous study showed that some homozygous Gpi1−/− null cells survived when combined with wild-type cells in fetal chimaeras. One adult female Gpi1−/−↔Gpi1c/c chimaera with functional Gpi1−/− null oocytes was also identified in a preliminary study. The aims were to characterise the survival of Gpi1−/− null cells in adult Gpi1−/−↔Gpi1c/c chimaeras and determine if Gpi1−/− null germ cells are functional. Analysis of adult Gpi1−/−↔Gpi1c/c chimaeras with pigment and a reiterated transgenic lineage marker showed that low numbers of homozygous Gpi1−/− null cells could survive in many tissues of adult chimaeras, including oocytes. Breeding experiments confirmed that Gpi1−/− null oocytes in one female Gpi1−/−↔Gpi1c/c chimaera were functional and provided preliminary evidence that one male putative Gpi1−/−↔Gpi1c/c chimaera produced functional spermatozoa from homozygous Gpi1−/− null germ cells. Although the male chimaera was almost certainly Gpi1−/−↔Gpi1c/c, this part of the study is considered preliminary because only blood was typed for GPI. Gpi1−/− null germ cells should survive in a chimaeric testis if they are supported by wild-type Sertoli cells. It is also feasible that spermatozoa could bypass a block at GPI, but not blocks at some later steps in glycolysis, by using fructose, rather than glucose, as the substrate for glycolysis. Although chimaera analysis proved inefficient for studying the fate of Gpi1−/− null germ cells, it successfully identified functional Gpi1−/− null oocytes and revealed that some Gpi1−/− null cells could survive in many adult tissues.
Highlights
The dimeric glycolytic enzyme, glucose phosphate isomerase (GPI; E.C. 5.3.1.9), known as glucose-6-phosphate isomerase, phosphoglucose isomerase or phosphohexose isomerase, catalyses the conversion of glucose-6-phosphate to fructose-6-phosphate at the second step in glycolysis and the reverse reaction during gluconeogenesis in some cell types
The aims of the current study were (i) to characterise the extent of survival of homozygous Gpi1−/− null cells in adult mouse chimaeras, (ii) to extend the previous preliminary study to evaluate whether female Gpi1−/−↔Gpi1c/c chimaeras can produce offspring from GPI-null oocytes and (iii) to determine whether male Gpi1−/−↔Gpi1c/c chimaeras can sire offspring from GPI-null spermatozoa derived from homozygous Gpi1−/− null germ cells
Chimaera analysis proved to be an inefficient approach to study the fate of Gpi1−/− null germ cells as only two of ten Gpi1−/−↔Gpi1c/c chimaeras produced offspring from the genetically pigmented, Gpi1−/− cell population
Summary
The dimeric glycolytic enzyme, glucose phosphate isomerase (GPI; E.C. 5.3.1.9), known as glucose-6-phosphate isomerase, phosphoglucose isomerase or phosphohexose isomerase, catalyses the conversion of glucose-6-phosphate to fructose-6-phosphate at the second step in glycolysis and the reverse reaction during gluconeogenesis in some cell types. Evidence has accumulated that several different non-enzymatic, paracrine and autocrine functions are mediated by one or more forms of secreted, extracellular monomeric GPI, which bind to cell membrane receptors (reviewed by Henderson and Martin, 2014; Jeffery, 1999; Kim and Dang, 2005). These proteins have more restricted tissue distributions than the ubiquitous dimeric GPI enzyme and may be truncated forms of the GPI monomer with different quaternary structures (Baumann and Brand, 1988; Mizrachi, 1989; Repiso et al, 2008). The term ‘protein moonlighting’ has been coined to describe proteins, such as GPI, that can perform multiple functions (Jeffery, 1999) and databases of these proteins are available (see Henderson and Martin, 2014)
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