The chemokine CCL2 activates p38 mitogen-activated protein kinase pathway in cultured rat hippocampal cells

Abstract

Emerging evidence indicates that chemokines can regulate both the physiology and biochemistry of CNS neurons and glia. In the current study, Western blot analysis showed that in rat hippocampal neuronal/glial cultures the signal transduction pathway activated by CCL2, a chemokine expressed in the normal brain and at elevated levels during neuroinflammation, involves a G-protein coupled receptor, p38 MAPK as well as its immediate upstream kinase MKK3/6, and the downstream transcription factor CREB. ERK 1/2 and the transcription factors STAT1 and STAT3 do not play a prominent role. CCL2 also altered Ca(2+) influx and synaptic network activity in the hippocampal neurons. These results suggest an important role for p38 MAPK and CREB in hippocampal actions of CCL2.

Figure 1

Effect of CCL2 on the level of phosphorylated MAPK in hippocampal cultures. (A) Concentration-dependent effect of CCL2 (10 min exposure) on the level of phosphorylated p38 MAPK (p-p38 MAPK). (B) Time dependent effect of CCL2 (25 nM) on the level of p-p38 MAPK. (C) Time dependent effect of CCL2 (25 nM) on the level of phosphorylated ERK1/2 (p-ERK1/2). For all studies, representative immunoblots and a graph of mean values are shown. CCL2 significantly increased the level of p-p38 MAPK in the hippocampal cultures but not the level of p-ERK1/2. Methods were similar for all studies. Hippocampal cultures were serum-starved overnight and treated at 13 DIV with CCL2 (0–25 nM) for the times specified. Whole cell lysates were prepared and 20 μg of total protein per lane was separated on the SDS-PAGE and immunoblotted with the antibody specific for p-p38 MAPK or p-ERK1/2. The blot was stripped and reprobed with the antibody specific for β-actin. The density of each band was quantified and normalized against the corresponding density of β-actin in the same lane. All data were then normalized to the normalized value (i.e., p38 MAPK/b-actin) for control conditions (0 CCL2). Densitometry data are presented as the mean ± S.E.M. *, P < 0.05 vs control conditions. n = the number of independent samples tested. Samples were derived from at least 3 different culture sets.

Figure 2

Effect of CCL2 on the level of p-MKK3/6 in hippocampal cultures. (A) Concentration-dependent effect of CCL2 (10 min exposure) on the level of p-MKK3/6. (B) Time-dependent effect of CCL2 (25 nM) on the level of p-MKK3/6. (C) Effect of PTX (100 ng/ml) on the CCL2-induced increases in the levels of p-MKK3/6. Representative immunoblots and a graph of mean values are shown for all studies. The methods were similar to the methods described in Figure 1 for p38 MAPK. Cultures were pretreated with PTX overnight. In C, data from CCL2-treated cultures were normalized to data from untreated control cultures; data from CCL2- and PTX-treated cultures was normalized to data from cultures treated with PTX but no CCL2.

Figure 3

Effect of CCL2 on the levels of phosphorylated CREB, STAT1 and STAT3 in hippocampal cultures. (A) Concentration-dependent effect of CCL2 (10 min exposure) on the level of p-CREB. (B, E, F) Time-dependent effect of CCL2 (25 nM) on the level of p-CREB (B), p-STAT3 (E) and p-STAT1 (F). CCL2 significantly increased the levels of p-CREB in a time-dependent manner. No significant effect on levels of p-STAT1 and p-STAT3 was observed. (C,D) Effects of PTX (100 ng/ml) and SB203580 (2 μM) on the CCL2-induced increase in the levels of p-CREB. Both inhibition of Gi/Go by PTX and inhibition of p38 MAPK by SB203580 blocked the CCL2-induced increase in the level of p-CREB, consistent with the involvement of a Gi/Go-coupled receptor and p38 MAPK in the actions of CCL2. Cultures were pretreated with either PTX overnight or SB203580 for 30 min, and the methods were similar to the methods described in the Figure 1. In C, data from CCL2-treated cultures were normalized to data from untreated control cultures; data from CCL2-and PTX-treated cultures was normalized to data from cultures treated with PTX but no CCL2. In D, data from CCL2-treated cultures were normalized to data from untreated control cultures; data from CCL2- and SB203580-treated cultures were normalized to data from cultures treated with SB203580 but no CCL2. For all studies, representative immunoblots and a graph of mean values are shown.

Figure 4

Hippocampal neurons express functional CCR2. (A) Immunocytochemical staining of rat hippocampal cultures using the antibody specific against CCR2B (1:100 dilution). Prominent immunostaining was observed in the neurons (e.g., black arrow) but not in the underlying glial cell layer (e.g., white arrow). Neurons were identified by morphological criteria established by immunostaining with cell type specific antibodies. The phase contrast images (left panel) show all cells in the microscopic field. The Hoffman optics images (right panel) show the immunostained cells in the field. (B) Specific immunostaining for CCR2 was blocked by preincubation of the antibody with the antigenic peptide used to raise the CCR2 antibody. (C–F) Effect of CCL2 on Ca²⁺ signals generated by spontaneous network synaptic activity in hippocampal neurons. (C) Gray scale digitized image showing a representative field of fura-2 loaded hippocampal neurons. (D) Mean (SEM) values for average Ca²⁺ levels under control conditions and after addition of CCL2 (25 nM) to the bath saline. Recordings were made in the presence (antagonists) and absence (no antagonists) of TTX plus glutamate and GABA receptor antagonists. Numbers in bars indicate the number of cells measured. (E, F) Representative recordings of intracellular Ca²⁺ levels before and after (solid bars) bath exchange. Dashed lines below the traces indicate when the bath was exchanged during the recording. In F, the recording was stopped during the bath exchange. In E, the bath contained TTX plus receptor antagonists to block synaptic network activity. In F, TTX and receptor antagonists were not used. The vehicle control bath exchange was used to identify potential changes in Ca²⁺ signals due to mechanical stimuli caused by the bath exchange.