expression occurs on E 1 7-E 1 8 in the central caudal 
stem cells. 
High levels of cell-specific expression of the rat 
prolactin gene are dictated by two separate regions 
— a distal enhancer (—1830 to —1530) and a proxi- 
mal region; mutation of even a single cis-active ele- 
ment in either gene can reduce gene expression by 
90-98%. Structurally similar elements within the 
initial 180 bp of the growth hormone promoter that 
act in a position- and orientation-dependent fashion 
are required for cell-specific expression of heterolo- 
gous genes in transgenic mice. The consensus bind- 
ing site for this critical cis-active element bound a 
tissue-specific POU domain transcription factor, ac- 
tivating both a 60-amino acid divergent homeo- 
domain and a 76- to 78-amino acid region of 
homology with several other transcriptional or de- 
velopmental factors (Oct-1 and Oct-2, and unc-86) 
that may specify cell lineage relationships. Dr. 
Rosenfeld and his colleagues have shown that the 
POU-specific domain of Pit-1 activates DNA, is re- 
quired for high-affinity site-specific binding, and is 
important in cooperative, DNA-dependent dimer 
binding on native sites. Consistent with Pit-1 regu- 
lating the prolactin and growth hormone gene acti- 
vation, the onset of expression of Pit-1 protein on 
El 5.5 correlates closely with that of the prolactin 
and growth hormone genes. However, the failure of 
the dwarf animals to develop thyrotrophs, which 
appear more than a day before detectable Pit- 1 gene 
expression, suggests that Pit-1 may be involved in 
thyrotroph survival or maintenance, and indeed 
thyrotrophs appear normally on El 3 in Snell dwarfs. 
Mouse genetics has provided direct evidence for 
the developmental role of the tissue-specific tran- 
scription factors. A variety of developmental mu- 
tants have been identified by their dwarf pheno- 
types. Among these are the Snell, Jackson, and Ames 
dwarf mutants that exhibit a virtually identical phe- 
notype: they contain no lactotrophs, somatotrophs, 
or thyrotrophs and have markedly hypoplastic ante- 
rior pituitary glands. Jackson and Snell dwarfs are 
allelic on chromosome 16, and it has been demon- 
strated that both dwarf phenotypes result from mu- 
tations of the Pit-1 gene. The Jackson mutation is a 
rearrangement, whereas the Snell phenotype is a 
transversion mutation in the Pit-1 POU homeodo- 
main that alters a crucial residue in the DNA recogni- 
tion helix, conserved among all homeodomain pro- 
teins. The mutation impairs the ability of Pit-1 to 
bind to its DNA recognition elements. The expres- 
sion of thyrotroph embryonic factor (TEE) , a basic- 
leucine repeat transcription factor, and not that of 
Pit-1, correlates spatially and temporally with the 
onset of thyroid-stimulating hormone-;5 (TSH|S) ex- 
pression in the rostral tip and can activate the TSH/? 
promoter expression in heterologous cell types. 
These data indicate that Pit-1 is necessary for the 
specification of phenotype of three cell types 
within the anterior pituitary and directly link a tran- 
scription factor to commitment and progression 
events in mammalian organogenesis. The hypoplas- 
tic nature of the dwarf pituitary indicates that cell 
proliferation is a critical component of the develop- 
mental program specified by Pit- 1 . However, a com- 
plex combinatorial code is responsible for the ini- 
tial activation of the Pit-1 gene. In a collaborative 
project with Dr. John Parks, a point mutation (a 
transfer) that alters a single residue in the POU- 
specific domain of human Pit- 1 results in short stat- 
ure with absent growth hormone and prolactin gene 
expression but a normal-sized gland. This implies 
distinct requirements for activation of distal target 
genes and the Pit-1 -dependent genes that are re- 
quired for cell proliferation. 
Pit- 1 positively autoregulates the expression of its 
promoter as a consequence of binding to two Pit- 1 
elements. These data are consistent with a positive, 
attenuated autoregulatory loop that appears to func- 
tion as a molecular memory in maintaining Pit-1 
gene expression. 
Combinatorial positive and negative events ulti- 
mately lead to cell-specific expression of the growth 
hormone and prolactin genes in distinct cell types. 
Based on a synergistic interaction with Pit- 1 activat- 
ing the prolactin gene distal enhancer, the estrogen 
receptor appears to be another critical regulator. 
Pit-1 actions are modulated in the mature pitu- 
itary gland. Pit-1 is a phosphoprotein regulated by 
cAMP and phorbol esters, with phosphorylation at 
two specific residues (Ser*'^, Thr^^°). One func- 
tional consequence of this post-translational modifi- 
cation is to alter affinity for cognate Pit- 1 DNA recog- 
nition element. The direction of the altered affinity 
is site dependent, with marked inhibition of binding 
on growth hormone sites but actually increased 
binding to the Pit-1 autoregulatory site. 
A Large Family of POU Domain Proteins 
in Mammalian Brain Development 
Dr. Rosenfeld and his colleagues subsequently 
cloned multiple additional mammalian members of 
the POU domain gene family and demonstrated that 
all of the known POU domain genes are expressed 
during neural development and exhibit precisely re- 
stricted temporal and spatial patterns of gene ex- 
pression. In this regard, the POU domain family of 
transcription factors resembles the developmental 
patterns of the hierarchy of regulatory genes that are 
sequentially activated during Drosophila develop- 
NEUROSCIENCE 427 
