Connexin43 in astroglial gap junctions. Regulation by age, neurotransmitters, GH and IGF-I

Abstract

Connexins form gap junctions that are aqueous pores allowing low-molecular (<1000 daltons) compounds to pass from cell to cell. In astroglial cells, connexin43 (cx43) makes up gap junctions, which in turn mediate intercellular communication within the astroglial network. Growth hormone (GH) and insulin-like growth factor I (IGF-I) have been shown to affect, for example, levels of neurotransmitters in the brain, myelination and even memory and learning. As glial cells express receptors for GH and IGF-I, we hypothesized that these hormones could affect astrocytic gap junctions and cx43. The objective of this thesis was to study the expression of cx43 in the rat brain, the effects of GH and IGF-I on cx43 expression in vivo and its regulation by neurotransmitters (5-HT and glutamate), GH, and IGF-I and their effects on gap junctional coupling (GJC) in vitro. First, the postnatal and adult expression of cx43 mRNA and protein was studied in seven brain regions. The data show a brain region-specific postnatal increase in cx43 expression with a different regulation for mRNA and protein abundance. In addition, mixed neuronal-astroglial cell cultures from four of the seven brain regions were studied in terms of cx43 mRNA and protein, which was related to functional GJC studies. Furthermore, cx43 and GJC were partially related to the extent of astroglial calcium wave propagation. Glutamate and serotonin each differentially affected GJC in astroglial primary cultures from different brain regions.The effects of bGH and rhIGF-I on cx43 expression and GJC were studied in vivo and in vitro. In vivo, hypophysectomized adult female rats, with low circulating levels of GH and IGF-I, were substituted with bGH (1 mg/kg) and rhIGF-I (0.85 mg/kg) and in vitro, bGH and rhIGF-I were added to primary astroglial cell cultures derived from the cerebral cortices of newborn rats. Systemic bGH but not rhIGF-I, increased the expression of cx43 mRNA and protein in the cerebral cortex and hypothalamus of the brain. However, in vitro, rhIGF-I increased GJC and cx43 abundance. Moreover, at a high concentration of rhIGF-I, where astroglial proliferation increased, the increase in GJC and cx43 disappeared. The astroglial cells secreted IGBFPs in vitro, and a high abundance of these was associated with a low response to rhIGF-I. As GH is known to induce IGF-I synthesis in the brain, these results indicate that the effect of GH on cx43 expression in vivo, may be mediated locally by IGF-I and interactions with IGBFPs.In conclusion, the short-term response to neurotransmitters is heterogeneous with respect to brain region in vitro as is the long-term response after treatment with GH in vivo. Altogether, the results show that cx43 and astroglial gap junctions are under dynamic regulation during maturation and that cx43 and GJC can be regulated by neurotransmitters and hormones. This may be of significance under normal physiological conditions by enhancing the spatial buffering capacity of astrocytes needed to sustain extended neurotransmission, as well as after insults to the nervous system.