Immunoglobulin G regulates α-MSH and α-MSH-like Enterobacterial ClpB protein anorexigenic signaling and their levels in the brain

Enterobacterial caseinolytic protease B (ClpB) plays a key role in the development of activity-based anorexia (ABA) in mice due to its mimetism with α-melanocyte-stimulating hormone (α-MSH), an anorexigenic peptide. ClpB immunization stimulates α-MSH cross-reactive IgG influencing ABA outcome depending on IgG binding properties. As such, low affinity IgG blocks melanocortin type 4 receptor (MC4R) activation by ClpB and α-MSH, protecting against ABA. In contrast, higher affinity IgG was associated with increased plasma ClpB and α-MSH, and ABA aggravation. However, it remained unknown if such IgG may influence MC4R signaling and if ClpB and IgG may reach the brain, questions that were addressed in this study. We report here that higher affinity IgG efficiently protects α-MSH from degradation in plasma and enhances MC4R activation by both α-MSH and ClpB. We also reveal that ClpB is present in the brain with higher levels in the hypothalamus. Brain ClpB levels were further increased in mice developing higher affinity IgG who had also increased hypothalamic levels of both ClpB-binding IgG and α-MSH. Opposite effects were observed in ABA mice developing low affinity IgG characterized by the preferential IgG binding to the α-MSH-like epitope of ClpB. We also show that ClpB binds to MC1R, MC3R, MC4R and MC5R. Thus, these results reveal that both ClpB protein and ClpB-binding IgG can reach the brain depending on IgG properties modulating ClpB and α-MSH anorexigenic signaling. These data further clarify the putative role of Enterobacterial ClpB and of ClpB- and α-MSH-binding IgG in the pathophysiology of anorexia nervosa.