ARNT2
ACTS AS A DIMERIZATION PARTNER FOR SIM1 DURING DEVELOPMENT OF THE HYPOTHALAMUS
NEUROENDOCRINE LINEAGES
This study is the result of a collaboration between
Jacques
L. Michaud1, Charles De Rossi2, Noah R. May3,
Chen-Ming Fan3, and Bernadette C. Holdener2. 1Service
de Génétique Médicale, Hôpital Sainte-Justine, Montréal,
Québec; 2Department of Biochemistry and Cell Biology and the
Institute for Cell and Developmental Biology, State University of New York at
Stony Brook, Stony Brook, NY 11794-5215; and 3Department of Embryology,
Carnegie Institution of Washington, Baltimore, Md.
The transcription factor SIM1 is a critical regulator of neuronal differentiation.
In mice, several neuronal lineages located within three nuclei of the hypothalamus,
the paraventricular (PVN), the anterior periventricular (aPV) and the supraoptic
(SON) nuclei fail to develop in the absence of Sim1 gene function. These
nuclei are major components of the hypothalamic-pituitary axis. Several neuroendocrine
fail to develop in the hypothalamus of Sim1 mutant mice.
SIM1 is a member of an emerging family of transcription
factors characterized by the presence of bHLH (basic helix-loop-helix) and PAS
(for Per, ARNT and Sim, the first proteins shown to contain this motif) domains.
Close relatives of SIM1 include CLOCK, which participates in the control of
circadian rhythms, hypoxia-inducible factor-1a
(HIF-1a) and endothelial
PAS domain protein 1 (EPAS1) which regulate the response to hypoxia and the
arylhydrocarbon receptor (AHR) which mediates the effects of dioxin. These bHLH-PAS
factors form a group of proteins that heterodimerize with members of another
group of bHLH-PAS proteins, for which there are only three representatives yet
described: ARNT, ARNT2 and BMAL1. Arnt2 is completely removed by the c112K
deletion. Using co-immunoprecipitation assays, expression studies and mutant
analysis, we provide evidence that ARNT2 is the in vivo dimerization
partner of SIM1 during development of the PVN/SON.
FIG. 1: Hypothalamus
neuroendocrine lineage development
FIG. 2:
Arnt2 and Sim1 are Co-expressed in the Developing
PVN and SON
FIG. 3:
The PVN/SON Fails to Develop in Arnt2 Deficient Mice
FIG. 4:
Brn2 and Sim1 are not Expressed in the PVN/SON of
Arnt2-Deficient
Fetuses at E18.5
FIG. 5:
All Magnocellular Neurons and a Subset of Parvocellular Neurons
are
Missing from the Hypothalamus of Arnt2-Deficient
Fetuses
Conclusions
FIG. 1: Hypothalamus
neuroendocrine lineage development
- Figure adapted from Kandel et al., Principles of Neuroscience, Third
Ed. (1991) and Michaud et al., Genes & Dev. 12:3264-3275 (1998)
Note, the brain on the left is really a human not rodent brain.
- The hypothalamus maintains homeostasis by modulating the secretion of pituitary
hormones (note this is a human brain not a rodent brain)
- The paraventricular (PVN), anterior periventricular (aPV, not shown), and
supraoptic nucleus (SON) are integration centers within the hypothalamus and
are major components of the hypothalamic-pituitary axis
- Magnocellular neurons within the PVN and SON project their axons to the
posterior pituitary where they secrete vasopressin (Vaso) and oxytocin (OT)
- Parvocellular neurons within the PVN and aPV project axons to the median
eminence of the pituitary where they secrete corticotropin-releasing hormone
(Crh) and thyrotropin-releasing hormone (Trh); Somatostatin (SS) is produced
mainly by cells in the aPV
- In mice, loss of Sim1 results in failure of the entire magnocellular
neurosecretory system and at least three parvocellular lineages to develop
- Sim1 is a member of an emerging family of transcription factors characterized
by the presence of a bHLH (basic-helix-loop-helix) and PAS (for founding members
Per, ARNT, and Sim) domains
- In the following figures, we provide evidence that Arnt2 is the in-vivo
dimerization partner of Sim1 during development of the PVN/SON
FIG. 2:
Arnt2 and Sim1 are Co-expressed in the Developing PVN and SON
- Arnt2 is ubiquitously
expressed in the E13.5 brain, with higher levels in a region of the PVN that
overlaps with Sim1 expression
- Arnt2 expression coincides with Sim1 expression at E18.5 in
the PVN and its derivative, the SON
FIG. 3:
The PVN/SON Fails to Develop in Arnt2 Deficient Mice
- The PVN and SON are hypocellular in Arnt2-/- newborn mice
- Expansion of the third ventricle is also apparent in Arnt2 deficient
mice
FIG. 4:
Brn2 and Sim1 are not Expressed in the PVN/SON of Arnt2-Deficient
Fetuses at E18.5
- Brn2 expression is not maintained in the PVN and SON of E18.5 Arnt2-/-
fetuses
- Sim1 expression is not maintained in the PVN and SON of E18.5 Arnt2-/-
fetuses; interestingly, ectopic domains of Sim1 expression can be seen
lateral to the PVN
FIG. 5:
All Magnocellular Neurons and a Subset of Parvocellular Neurons are
Missing from the Hypothalamus of Arnt2-Deficient
Fetuses
- Magnocellular neurons marked by the production of Vasopressin (Vaso) or
Oxytocin (OT, not shown) are absent in the developing PVN and SON of Arnt2-/-
fetuses
- Parvocellular neurons in the PVN, marked by expression of Corticotropin-releasing
hormone (Crh) and Thyrotropin-releasing hormone (Trh), are missing in Arnt2-/-
fetuses
- Parvocellular neurons in the aPV, marked by production of Somatostatin (SS),
are absent in Arnt2-/- fetuses
Conclusions
- Sim1 and Arnt2
have overlapping expression domains within the PVN and SON
- Loss of Arnt2 results in a phenotype similar to Sim1-/- mice
- Arnt2 is required for proper formation of all magnocellular neurons
within the PVN and SON and a subset of parvocellular neurons within the PVN
and aPV
