The danger control concept in kidney disease: mesangial cells.

Kidney remodeling is a response to intrinsic or extrinsic triggers of kidney injury. Injury initiates a set of universal response programs that were positively selected through evolution to control potentially life-threatening dangers and to regain homeostasis, including tissue repair. These danger control programs are (i) clotting, to control the risk of bleeding; (ii) inflammation, to 
control the risk of infection; (iii) epithelial repair; (iv) mesenchymal repair; and (v) scar resolution or minimization. In this review we focus on the role of mesangial cells in glomerular disorders and how their behaviors follow these danger control programs. We review the role of mesangial cells in glomerular coagulation and fibrinolysis, as well as their role in triggering glomerular inflammation and mesangioproliferative disorders. Furthermore, we discuss how the mesangium self-repairs, how podocyte injury triggers a "mesenchymal healing"-kind of response that leads to glomerular fibrosis and sclerosis. Thus, we can better appreciate the contribution of mesangial cells to glomerular pathology when we understand their behavior as an attempt to support the evolutionally conserved universal danger control programs. However, these mechanisms often result in maladaptive processes that destroy the complex glomerular ultrastructure rather than help to regain tissue homeostasis.