Development and regional expression of chicken neuroleukin (glucose‐6‐phosphate isomerase) messenger RNA

Abstract

Neuroleukin (NLK) is a protein identical with the glycolytic enzyme glucose-6-phosphate isomerase (GPI) that has been reported to support the survival of a subpopulation of neurons in embryonic dorsal root ganglia and spinal cord neurons in culture. In this report we have studied the developmental expression of NLK mRNA in the chick embryo in order to evaluate its possible role as a neurotrophic factor. The chicken gene encoding NLK was isolated by cross-hybridization to a mouse NLK cDNA clone. A DNA fragment from the chicken NLK gene with a 90% nucleotide sequence homology to mouse NLK cDNA encoding amino acids 310-355 was then used as a hybridization probe in a series of RNA-blots. In the entire embryo NLK mRNA was found already at embryonic day 3.5 (E3.5) and the level of expression was significantly decreased between E3.5 and hatching. Roughly similar levels of NLK mRNA were found in all tissues of the E8 embryo analyzed with the exception of the brain, which contained only low levels. When the developmental expression was analyzed in different tissues separately, NLK mRNA expression was found to decrease during development in the heart and bursa of Fabricius, whereas the level of mRNA in the brain showed a large increase shortly after hatching. The spinal cord and the pectoral and femoral muscles all showed high levels of NLK mRNA throughout development. In the adult chick, the highest levels of NLK mRNA were found in the muscle, brain, and kidney, where the NLK mRNA was estimated to account for approximately 0.1% of the total mRNA in these tissues. A widespread expression of NLK mRNA was observed in the adult brain with approximately similar levels in all brain regions tested. Similar results were also obtained when NLK mRNA expression was analyzed in adult rats. Our results show that developmental expression of the NLK gene is independently regulated in different tissues. The widespread and abundant expression of both the avian and rodent NLK gene is in accordance with its newly discovered identity as a glycolytic enzyme. Consequently, the developmental and adult pattern of NLK mRNA expression does not favour a specific trophic role for this protein in accordance with other known neurotrophic factors.