Preproglucagon neurons in the hindbrain have IL-6 receptor-α and show Ca²⁺ influx in response to IL-6

Abstract

Neuronal circuits in the hypothalamus and hindbrain are of importance for control of food intake, energy expenditure, and fat mass. We have recently shown that treatment with exendin-4 (Ex-4), an analog of the proglucagon-derived molecule glucagon-like peptide 1 (GLP-1), markedly increases mRNA expression of the cytokine interleukin-6 (IL-6) in the hypothalamus and hindbrain and that this increase partly mediates the suppression of food intake and body weight by Ex-4. Endogenous GLP-1 in the central nervous system (CNS) is produced by preproglucagon (PPG) neurons of the nucleus of the solitary tract (NTS) in the hindbrain. These neurons project to various parts of the brain, including the hypothalamus. Outside the brain, IL-6 stimulates GLP-1 secretion from the gut and pancreas. In this study, we aim to investigate whether IL-6 can affect GLP-1-producing PPG neurons in the nucleus of the solitary tract (NTS) in mouse hindbrain via the ligand binding part of the IL-6 receptor, IL-6 receptor-α (IL-6Rα). Using immunohistochemistry, we found that IL-6Rα was localized on PPG neurons of the NTS. Recordings of these neurons in GCaMP3/GLP-1 reporter mice showed that IL-6 enhances cytosolic Ca(2+) concentration in neurons capable of expressing PPG. We also show that the Ca(2+) increase originates from the extracellular space. Furthermore, we found that IL-6Rα was localized on cells in the caudal hindbrain expressing immunoreactive NeuN (a neuronal marker) or CNP:ase (an oligodendrocyte marker). In summary, IL-6Rα is present on PPG neurons in the NTS, and IL-6 can stimulate these cells by increasing influx of Ca(2+) to the cytosol from the extracellular space.

Serotonergic modulation of the activity of GLP-1 producing neurons in the nucleus of the solitary tract in mouse

“…1 nM leptin increased intracellular Ca 2+ in all imaged PPG neurons (n = 17 cells), whereas only a subset of NTS PPG neurons exhibited activation upon superfusion with 200 nM CCK-8 (five out of 14 imaged PPG neurons). These data along with previously published immunohistochemical verification [37] validate our model and demonstrate that it is well-suited to investigate responses to other neurotransmitters.

Figure 3 5-HT evoked Ca 2+ transients in PPG neurons are mainly observed in dendrites . (A) Intracellular Ca 2+ changes in PPG neurons can be monitored using the genetically encoded Ca 2+ indicator, GCaMP3.

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“…GCaMP3 was detected in the caudal part of the NTS (Figure 3B) and in the dorsomedial part of the intermediate reticular nucleus (data not shown). GCaMP3-expressing neurons have been shown previously to express GLP-1 in this transgenic mouse model [37]. To demonstrate the ability to record changes in intracellular Ca 2+ from PPG neurons in vitro , coronal brainstem slices were superfused with 100 μM glutamate for 1 min.…”

Glucagon-Like Peptide-1-, but not Growth and Differentiation Factor 15-, Receptor Activation Increases the Number of Interleukin-6-Expressing Cells in the External Lateral Parabrachial Nucleus

“…Previous studies in IL-6 -/mice with phenotypic mature-onset obesity have shown that IL-6 inhibits obesity via effects in CNS [24]. This assumption is supported by the fact that intracerebroventricular injections of IL-6, but not peripheral injections, cause weight-reducing effects by means of increased energy expenditure [25,26] presumably via phosphorylation of STAT3 [14]. Furthermore, it is likely that pathologically high IL-6 release contributes to cachexia development [1,27,28], since blocking IL-6 synthesis in several mouse cancer models attenuates the progression of the wasting syndrome [29,30].…”

“…The clinical implications of stimulation and inhibition of the IL-6 receptor alpha (IL-6Rα) is a topic of ongoing studies [13]. IL-6Rα is located outside the CNS, but also in many CNS regions, including many of the fundamental energy-regulatory hypothalamic and hindbrain nuclei [14][15][16][17][18][19][20]. Much is known about the exact localization of IL-6Rα thanks to the availability of well validated antibodies against IL-6Rα itself [15][16][17] and against markers of IL-6Rα activation such as pSTAT3 [21].…”

The Cardiovascular Biology of Glucagon-like Peptide-1

“…Evidence for the importance of a cytokine-PGDP axis in the rat and mouse brain derives from observations that centrally administered exendin-4 increased the expression of IL-6 and IL-1b in the hypothalamus (and IL-6 in the hindbrain), whereas pharmacological disruption or genetic attenuation of either IL-6 or IL-1b signaling reduced the delayed anorectic response to central or peripheral exendin-4 (Shirazi et al, 2013). IL-6 may also be important for the synthesis, secretion, and anorectic actions of GLP-1 in the brain, as almost 40% of the GLP-1+ neurons within the nucleus of the solitary tract express the IL-6 receptor a, and these PGDP+ neurons respond to exogenous IL-6 with an increase in cytosolic Ca 2+ (Anesten et al, 2016).…”