We have demonstrated a novel, progesterone-dependent regulation of NRG1 mRNA and protein in astrocytes in vitro
. Conversely, we observed no induction of NRG1 mRNA and protein in cortical neurons following treatment with either progesterone or 17β-estradiol. These findings indicate that a progesterone-dependent regulatory system exists in the CNS. It is known that the actions of gonadal steroids are highly tissue-specific and depend on coregulator proteins that act as either positive or negative regulators of steroid hormone-receptor signaling (Khan and Nawaz, 2003
; Hall and McDonnell, 2005
). Furthermore, it has been demonstrated previously that progesterone regulates the concentration of NRG1 in epithelial cells (Balana et al., 1999
) and that NRG1 regulates expression of the progesterone receptor in a reciprocal fashion (Labriola et al., 2003
). In this manner, progesterone and NRG1 act together in a feed-forward mechanism that potentiates both progesterone sensitivity and increases the expression of NRG1. These data, together with our findings, indicate that a progesterone–NRG1 regulatory system might also be present in CNS astrocytes. It remains to be determined whether treatment of cultured astrocytes with NRG1 regulates progesterone-receptor expression in a manner similar to epithethial cells.
A growing body of evidence has demonstrated a key role for growth factors and neurotrophic factors such as NRG1 as algogenic mediators in chronic pain states. Astrocytes have been postulated to have a key role in the production and secretion of these growth factors, and have been demonstrated to increase their expression of several growth factors after they are activated in response to CNS injury (Ridet et al., 1997
). In this manner, increased local production of NRG1 by astrocytes might simultaneously serve beneficial and detrimental functions in chronic pain states. It is known that neuregulins have major trophic (Oka et al., 2000
; Chen et al., 2003
) and promyelination functions (Cannella et al., 1998
; Michailov et al., 2004
; Lemke, 2006
) which might protect the damaged CNS while simultaneously sensitizing nociceptive systems.
PSD-95 is a key component in the architecture of excitatory synapses. PSD-95 acts a scaffolding protein that brings together a variety of receptors, signaling molecules and kinases via a special protein-interaction PDZ domain (Hata and Takai, 1999
). In particular, PSD-95 has been shown to interact with several subunits of the N
-methyl-D-aspartate (NMDA) receptor (Kornau et al., 1995
) and might have a key role in clustering these receptors at the post-synaptic membrane to allow efficient crosstalk with downstream signaling molecules. Several studies have shown that PSD-95, which is normally located diffusely in cultured neurons, localizes into distinct puncta in the presence of a synaptogenic signal. A study by Ullian et al. (2001)
has shown that medium from astrocytes in culture contains factors that induce PSD-95 localization into puncta as an element of synapse formation in neurons in response to ACM (Pfrieger and Barres, 1997
; Ullian et al., 2004
). Attempts to isolate individual factor(s) in ACM that promote synapse formation led to the identification of thrombospondin-1 and thrombospondin-2 (Christopherson et al., 2005
). Our finding that NRG1-β1 also induces PSD-95 to localize into puncta indicates that this astrocyte-derived factor might also have a role in the astrocyte-mediated modulation of synapses. Injection of NRG1-β1 into the hippocampus regulates synaptic activity transiently, as measured by electrophysiology in vivo
(Roysommuti et al., 2003
). Further studies investigating the role of NRG1 as a synaptic modulator should uncover the molecular events that mediate this change in synaptic activity in neurons.
In conclusion, we have demonstrated that progesterone, but not 17β-estradiol, regulates the expression of NRG1 mRNA and protein in astrocytes in culture. Conversely, no significant regulation of NRG1 was observed in cultured neurons treated with either progesterone or 17β-estradiol. Finally, we have shown that NRG1-β1 and ACM cause PSD-95 to localize in puncta to a similar extent. These data highlight the importance of steroid-hormone regulation of astrocyte factors that might regulate synaptic function in neurons, and highlight the complex control of glial–neuronal signaling networks.