Investigating the role of the imprinted Grb10 gene in the regulation of maternal nutrient transfer.

Imprinted genes are a subset of loci, positioned on autosomes and the X-chromosome, which are expressed monoallelically in a parent-of-origin specific manner. The influence of such genes on the regulation of embryonic growth and postnatal energy homeostasis is well established. The parental conflict hypothesis predicts that, in utero, paternally-expressed genes will promote maternal resource acquisition and thus growth, whereas maternally-expressed genes will oppose this action, restricting resource investment in a single brood in the interests of the lifetime reproductive success of the mother. Grb10 is an imprinted gene which encodes the cytoplasmic adaptor protein Growth factor receptor bound protein 10. In the majority of tissues, Grb10 is expressed from the maternally-derived chromosome. Consistent with conflict theory, transgenic mice inheriting a disrupted Grb10 allele through the maternal line (Grb10Δ2-4m/+) exhibit embryonic overgrowth, although the mechanisms and signalling pathways responsible for this effect are unclear. Grb10Δ2-4m/+ mice also demonstrate enhanced insulin signalling and improved whole body glucose clearance, consistent with the established role of imprinted genes in the regulation of postnatal metabolism. An integrated LacZ gene-trap in the Grb10Δ2-4 allele failed to fully recapitulate endogenous Grb10 expression, notably within the central nervous system. To address this issue, a second transgenic mouse line, Grb10KO, was created. This allele produced strong LacZ reporter expression in the central nervous system, but only when transmitted through the paternal line (Grb10KO+/p), establishing Grb10 as the only known imprinted gene with a reciprocal imprinting profile between the central nervous system and peripheral tissues. Grb10KO+/p mice exhibit a social dominance phenotype, suggesting distinct roles for maternally- and paternally-expressed Grb10, consistent with their respective sites of expression. The current study characterised the Grb10KO allele at the genetic level, and in doing so, revealed a phenotypic difference between Grb10KOm/p and Grb10Δ2-4m/p mice for which a possible explanation was provided. Importantly, with this knowledge, the current study elucidated the genetic and molecular basis for inconsistencies in reporter expression between the two transgenic lines, identifying a novel tissue-specific enhancer element at the locus. In addition to the central nervous system, this enhancer appeared to be active in the mammary epithelium, identifying a novel site of Grb10 expression, which was pregnancy-dependent and specifically from the maternally-inherited chromosome. Characterisation of the functional significance of expression in this tissue revealed that maternally-expressed Grb10 mediates a supply/demand system between lactating mother and suckling pup, acting as a supply promoter and demand suppressor. This role is inconsistent with conflict theory, but suggests the maintenance of the Grb10 imprint in the mammary epithelium might be associated with improved coadaptiveness between mother and offspring. Intriguingly, in utero, Grb10 is both a demand and supply suppressor. When considered together, these findings suggest a wider role for maternally-expressed Grb10 in the homeostatic control of growth and achievement of optimal fitness.