Branching Morphogenesis
The mammary gland develops postnatally and undergoes regression and amplification under the control of endocrine hormones, estrogen, progesterone, growth hormone, and prolactin. Like most other endocrine factors, these hormones do not bind to heparan sulfate but rather act on the mammary stroma to induce the local expression of soluble growth factors, many of which bind to heparan sulfate. Reducing the extent of sulfation specifically in the mammary epithelia using the Cre-loxP system had no effect on the viability or expected Mendelian ratio of offspring, but the pups failed to survive due to insufficient lactation by the mothers. Normal primary ductal proliferation and branching occurred, but a striking deficiency in lobuloalveolar development was manifest. Western blotting revealed a lack of Cyclin D1 expression that accounts for the reduced proliferation. This was accompanied by reduced phosphorylation of AKT/PKB, which resulted in precocious involution of the gland. In other studies we found that deletion of all sulfation or the inhibition of heparan sulfate polymerization prevented earlier stages in branching morphogenesis. Studies are underway to determine the primary growth factor signaling deficiencies responsible for these defects. The studies complement the branching system in the kidney under study in Sanjay Nigam? laboratory.