379 LACTATION INDUCTION IN PREPUBERTAL BULLS AND HEIFERS AS A TOOL FOR PREDICTING MAMMARY SPECIFIC TRANSGENE EXPRESSION IN CATTLE

Abstract

The bovine's long generation interval results in a lapse, from the time of birth, of two to three years before mammary-specific transgenes can be assessed in genetically engineered animals. This experiment was conducted in an attempt to reduce that waiting period by up to two years. Lactation was induced in prepubertal bull and heifer calves, 3 to 8 mo of age, as a means of predicting transgene behavior during subsequent normal lactations in the heifers and daughters of bulls. Transgenic animals tested were either founder animals, produced by somatic cell nuclear transfer, or G1 offspring of founder bulls. The transgene consists of a lactation specific sequence encoding lysostaphin, an antimicrobial protein targeted against Staphylococcus aureus, a mastitis-causing pathogen. Estrogen, progesterone, and dexamethasone were administered as previously described (Ball et al. 2000 J. Dairy Sci. 83, 2459) to nine heifers (transgenics = 5) ranging in weight from 90 to 165 kg. Eight bull calves (transgenics = 7) weighing from 81 to 178 kg received additional estrogen and progesterone injections as well as reserpine prior to dexamethasone treatment. Animals were hand-milked twice daily for 4 to 7 days. All nine heifers responded to the milk induction scheme, yielding between 19 mL and 4.5 L. Milk volume from the three responding males (100 ¼L to 2.5 mL) was significantly less than that harvested from females (P = 0.025). Only bull calves over 150 kg had a positive response. Transgenic females produce less milk then non-transgenics (313 ± 494 vs. 2276 ± 552 mL, respectively; P = 0.033). Most importantly, there was no detectable difference between the concentration of lysostaphin in milk from induction (8.1 ± 2.7 ¼g/mL) and natural lactations (3.5 ± 2.6 ¼g/mL) in the four transgenic heifers tested (P = 0.229). The result was the same when lysostaphin was analyzed as a percentage of total protein (P = 0.427). Induction of a G1 heifer and a bull calf from the same founder bull produced similar lysostaphin concentrations in their milk (5.6 ± 0.9 and 5.2 ± 0.5 ¼g/mL; respectively). ²-lactoglobulin concentration was also similar during induced and natural lactation (P = 0.165) for all animals studied. However, total protein was greater in induced milk samples compared to natural lactation samples (28.4 ± 1.7 vs. 21.2 ± 1.7 mg/mL; P = 0.007) as was lactoferrin (707 ± 51 vs. 213 ± 51 ¼g/mL; P < 0.001). Conversely, compared to that in induced milk samples, lactose was more concentrated in the natural lactation samples (34.6 ± 2.5 vs. 46.0 ± 2.1 g/L). In this study transgene expression was detected in milk from induced lactations and its concentration in those samples was generally predictive of product concentration in the natural lactation milk. The induction protocol was effective in male (>150 kg) and female calves.