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Phylogeny and regulation of four lipocalin genes clustered in the chicken genome: evidence of a functional diversification after gene duplication
Producción CientíficaA novel lipocalin gene is here reported that represents the fourth member of a cluster we have identified in the chicken genome. This
cluster also includes Chondrogenesis-Associated Lipocalins h and g (CALh, CALg) and Extracellular Fatty Acid Binding Protein (Ex-
FABP). The new gene codes for a 22-kDa secreted protein with three cysteine residues and a series of sequence features well conserved in the
lipocalin family. All the genes in the cluster are structurally similar presenting comparable exon/intron boundary positions and exon sizes. A
phylogenetic analysis indicates the monophyletic grouping of these genes, and their relationship with the lipocalins a-1-microglobulin
(A1mg), complement factor 8g chain (C8GC), prostaglandin D synthase (PGDS), and neutrophil-gelatinase-associated lipocalin (NGAL).
The new cluster gene appears to be the ortholog of the mammalian C8GC and was thus named Ggal-C8GC. This orthology also suggests that
this lipocalin was present in the ancestor common to reptiles and mammals.
In addition to other expressing tissues, Ex-FABP, CALh and CALg genes are highly transcribed in chondrocytes at late stages of
chondrogenesis during endochondral bone formation and/or upon inflammatory stimulation. Here, we show that they are also
transcriptionally induced when chondrocytes are subjected to various biological events as cell quiescence, cell shape transition, and hormonal
stimulation. By contrast, Ggal-C8GC transcripts are only barely detectable in chondrocytes, but are more abundant in liver, kidney, brain,
heart, skeletal muscle and particularly in skin. Moreover, no expression induction was observed neither during chondrocyte differentiation,
nor upon any of the stimulations mentioned above. This indicates that the Ggal-C8GC gene was co-opted for a novel function after the
duplication events that gave rise to the cluster.
The peculiar coordinated regulation of Ex-FABP, CALh and CALg, and the apparent divergent role of Ggal-C8GC suggest that these
gene duplications may have been maintained during evolution by a sub-functionalization mechanism where some common function(s) are
shared by several members of the cluster and some other specialized function(s) are unique to other members