Evidence for negative control of growth by adenosine in the mammalian embryo: Induction of Hmx/+ mutant limb outgrowth by adenosine deaminase

Abstract

We investigated the growth-regulatory actions of adenosine and adenosine deaminase (ADA) during embryonic limb development in the mouse. Polydactylous outgrowth, an expression of the Hemimelia-extra toe (Hm x) mutant phenotype, was experimentally regulated in hind-limb buds explanted into a serum-free in vitro system at stage 18 of gestation. Its expression was promoted by exposure to 0.1 or 0.2 IU/ml exogenous ADA and suppressed by co-exposure to 10 nM (—)-N 6-(R-phenylisopropy1)-adenosine (N 6-PIA). Evidence that N 6-PIA acted as a high-affinity agonist against the external adenosine receptor was provided by experiments in which 100 μM caffeine, a known antagonist, competitively blocked its effect. The endogenous adenosine content was analyzed by reversed-phase high-performance liquid chromatography with fluorometric detection following its conversion to the 1,N 6ethenoadenosine derivative. At stage 18. the adenosine levels were 0.5 pmol/μg DNA in whole embryos and 0.08 pmol/pg DNA in hindlimb buds. At the same stage, partially purified extracts of the embryonal plasma enriched fraction contained high levels of ADA activity (0.04-0.06 IU/embryo, or 0.7-1.0 IU/mg protein). In contrast, blood cells contained 0.0001 IU/embryo (or 0.01 IU/mg protein). This enzyme occurred as a single kinetic form with a molecular weight of 4500047000 daltons and an apparent K m of 36-38 μM. Its presence in the embryonal plasma argues against an endocrine mechanism of adenosine secretion in favor of autocrine (self-regulatory) or paracrine (proximate-regulatory) mechanisms. Taken together, our results suggest that the in vitro outgrowth of the prospective polydactylous region is induced upon escape from the local growth-inhibitory influence of extracellular adenosine. It is concluded that growth during early limb development is under systemic as well as local control.