Interrelationship between poly (ADP-Rib) synthesis, intracellular NAD levels, and muscle or cartilage differentiation from mesodermal cells of embryonic chick limb.

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Abstract
Mesodermal cells of embryonic chick limbs have the capacity to differentiate into either muscle or cartilage. Previous reports from this laboratory show a correlation between pyridine nucleotide levels and this differentiation, and thus suggest that fluctuations in the cellular NAD levels play a role in the control of muscle versus cartilage development. Poly(ADP-Rib) is chromatin-associated and forms from the polymerization of NAD with the excision of nicotinamide. The studies reported here show that: (A) the rate of net synthesis of poly(ADP-Rib) is correlated with the differentiation of chondrogenic cells from stage 24 limb mesodermal cells grown in vitro; (B) inhibition of chondrogenic expression caused by exposure to nicotinamide or BrdUrd is correlated with maintenance of basal levels of poly(ADP-Rib) synthesis, and this inhibition is dependent on the concentration of nicotinamide or BrdUrd exogenously supplied; (C) potentiation of chondrogenic expression caused by exposure of limb mesodermal cells in vitro to 3-acetylpyridine is correlated with stimulation of the rate of poly(ADP-Rib) synthesis if corrected for the specific activity of the ATP pool or compared to untreated cultures on a per cell basis; (D) isolated chromatin from mesodermal cells has the enzymatic machinery for synthesizing poly(ADP-Rib); (E) this machinery is inhibited by nicotinamide, thymidine, and 3-acetylpyridine; and (F) newly synthesized poly(ADP-Rib) is either associated with a discrete fraction of chromatin or is completely extracted from chromatin by the high column salts, which result in an aggregation with eventual elution at the exclusion volume position of the agarose column. Taken together, these observations provide a possible explanation for how fluctuations in cellular NAD levels can communicate with or be "sensed" by genomic related machinery and eventually result in differtial phenotypic expression.