Molecular Mechanisms of Transcription, 
Regulation, and Development 
of the Neuroendocrine System 
Michael G. Rosenfeld, M.D. — Investigator 
Dr. Rosenfeld is also Professor of Medicine in the Eukaryotic Regulatory Biology Program at the University 
of California, San Diego, School of Medicine. He received his undergraduate degree from the Johns Hopkins 
University and his medical degree from the University of Rochester. His internship and medical residency 
were completed at Barnes Hospital, St. Louis. Before accepting his current position, he received 
postdoctoral training at NIH. Dr. Rosenfeld also holds an adjunct position at the Salk Institute. 
OVER the past year our central research focus 
has been the determination of molecular 
mechanisms that induce specific neuroendocrine 
phenotypes and the further definition of signal 
transduction pathways that lead to regulated pat- 
terns of gene expression. 
The neuroendocrine system coordinates the 
complex pattern of regulation necessary to 
achieve the precise temporal, spatial, and homeo- 
static patterns of gene expression required by 
complex organisms. Development of the central 
nervous system and endocrine organs involves 
precise patterns of responses to morphogens and 
other regulatory signals that ultimately establish 
the intricate patterns of neural and endocrine 
phenotypes. The cloning and analysis of genes 
encoding receptors and cell-specific transcrip- 
tion factors have permitted an initial definition of 
developmental and regulatory strategies. 
The anterior pituitary gland has provided a suit- 
able model to investigate the molecular basis for 
generating specific cell phenotypes in an organ. 
The rat genes for growth hormone and prolactin 
(hormone that stimulates and sustains lactation) 
exhibit precisely restricted expression in the 
cells of origin, somatotrophs and lactotrophs, re- 
spectively. We found that prolactin gene expres- 
sion is dictated by a distal enhancer and a proxi- 
mal region, each containing at least four critical 
cell-specific elements. These two domains, each 
capable of targeting tissue-specific gene expres- 
sion, act synergistically to generate high levels of 
prolactin gene expression in transgenic mice. 
Similarly, grov^h hormone gene expression in- 
volves the action of related cell-specific cis- 
active elements. A 33-kDa cell-specific transcrip- 
tion factor, referred to as Pit- 1 , was characterized 
and its encoding cDNA defined. 
Bacterially expressed Pit-1 specifically and 
with high affinity binds to prolactin and growth 
hormone promoters. Additional elements and 
factors are required to achieve the full physiologi- 
cal levels and restricted patterns of expression of 
the prolactin and growth hormone genes. For ex- 
ample, the estrogen receptor synergistically acts 
with Pit- 1 in activation of the rat prolactin gene's 
distal enhancer. 
Pit-1 is a member of a family of regulators that 
contain a POU domain consisting of a variant ho- 
meodomain and a second, 76-amino acid se- 
quence of striking homology. We have found that 
the 76-amino acid POU-specific domain func- 
tions in high-affinity DNA binding, in conferring 
site-specificity, and in protein-protein interac- 
tions critical for transcriptional activation by 
Pit-1. The major transcription-activating domain 
of Pit-1 is a 70-amino acid, N'-terminal, serine, 
threonine-rich sequence, distinct from recog- 
nized motifs. A detailed structure-function analy- 
sis has suggested that the determinants of DNA 
binding by the Pit-1 POU-domain protein are dis- 
tinct from those of the classical homeodomain 
proteins, and that Pit- 1 DNA binding is regulated 
by phosphorylation of a specific, conserved resi- 
due in the POU homeodomain. 
A genetic approach was utilized to determine 
the functional role of Pit-1 during organogenesis. 
Based on the demonstration of disrupted patterns 
of the Pit-1 gene in genetic dwarf mice, it has 
been established that this POU-domain protein 
acts as a developmental regulator to determine 
patterns of commitment, progression, and prolif- 
eration of three specific cell types in the anterior 
pituitary gland. In the case of lactotrophs and so- 
matotrophs, proliferation is restricted in the 
Pit- 1 -defective animal cell type, while the thyro- 
troph provides an initial example of an estab- 
lished cell phenotype that disappears at the time 
the initial Pit-1 protein is expressed. These data 
indicate that one role of a developmental tran- 
scriptional regulator can be survival of an estab- 
lished cell type. 
A strategy was devised to isolate new members 
of the POU-domain gene family. Ten new 
members were identified in neural tissues. Two, 
referred to as Brn-1 and Brn-2, exhibit virtually 
identical patterns of expression in the central 
nervous system, though Brn-1 is clearly ex- 
pressed in the medullary zone of the kidney 
while Brn-2 is not. A third member, Brn-3, is pre- 
dominantly expressed in the peripheral nervous 
system. Tst l transcripts are present in mamma- 
lian brain cells and in testis. Subsequently other 
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