Microglia are crucial for immune responses in the brain. Although their origin from the yolk sac has been recognized for some time, their precise precursors and the transcription program that is used are not known. We found that mouse microglia were derived from primitive c-kit(+) erythromyeloid precursors that were detected in the yolk sac as early as 8 d post conception. These precursors developed into CD45(+) c-kit(lo) CX(3)CR1(-) immature (A1) cells and matured into CD45(+) c-kit(-) CX(3)CR1(+) (A2) cells, as evidenced by the downregulation of CD31 and concomitant upregulation of F4/80 and macrophage colony stimulating factor receptor (MCSF-R). Proliferating A2 cells became microglia and invaded the developing brain using specific matrix metalloproteinases. Notably, microgliogenesis was not only dependent on the transcription factor Pu.1 (also known as Sfpi), but also required Irf8, which was vital for the development of the A2 population, whereas Myb, Id2, Batf3 and Klf4 were not required. Our data provide cellular and molecular insights into the origin and development of microglia.

Research on matrix metalloproteinases (MMPs) and in particular on gelatinase B, alias MMP-9, has grown exponentially in the decade 2003-2012. Structural details about flexibility of MMP-9 monomers, together with glycosylation, oligomerization, heterogeneity and instability of the wildtype enzyme explain why crystallography experiments have not yet been successful for the intact enzyme. MMP-9 may be viewed as a multidomain enzyme in which the hemopexin, the O-glycosylated and the catalytic domains yield support for attachment, articulation and catalysis, respectively. The stepwise proteolytic activation of the inactive zymogen into a catalytically active form becomes gradually better understood. Priming of activation by MMP-3 may be executed by meprins that destabilize the interaction of the aminoterminus with the third fibronectin repeat. Alternatively, autocatalytic activation may occur in the presence of molecules that tightly bind to the catalytic site and that push the cystein residue in the prodomain away from the catalytic zinc ion. Thanks to the development of degradomics technologies, substrate repertoires of MMP-9 have been defined, but it remains a challenge to determine and prove which substrates are biologically relevant. The substrate repertoire has been enlarged from extracellular to membrane-bound and efficient intracellular substrates, such as crystallins, tubulins and actins. Biological studies of MMP-9 have tuned the field from being primarily cancer-oriented towards vascular and inflammatory research. In tumor biology, it has been increasingly appreciated that MMP-9 from inflammatory cells, particularly neutrophils, co-determines prognosis and outcome. Aside from the catalytic functions executed by aminoterminal domains of MMP-9, the carboxyterminal hemopexin (PEX) domain of gelatinase B exerts non-catalytic anti-apoptotic signaling effects. The recognition that gelatinase B is induced by many pro-inflammatory cytokines, whereas its inhibitors are increased by anti-inflammatory cytokines, has generated interest to target MMP-9 in acute lethal conditions, such as bacterial meningitis, sepsis and endotoxin shock, and in acute exacerbations of chronic diseases. Previously described transcriptional regulation of MMP-9 is complemented by epigenetic checkpoints, including histone modifications and microRNAs. Because activation of proMMP-9 may be executed by other MMPs, the therapeutic dogma that MMP inhibitors need to be highly selective may be keyed down for the treatment of life-threatening conditions. When inflammation and MMP-9 fulfill beneficial functions to clear damaging protein complexes, such as in systemic autoimmune diseases, therapeutic MMP inhibition has to be avoided. In Mmp9 gene knockout mice, specific spontaneous phenotypes emerged with effects on the skeletal, reproductive and nervous systems. These findings not only have clinical correlates in bone growth and fertility, but also stimulate research on the roles of MMPs and MMP-9 in endocrinology, immunology and the neurosciences. Mm...

The matrix metalloproteinases (MMPs) form an enzyme family of which gelatinase B (MMP-9) represents the largest and most complex member. We focus here on the biochemical properties, regulation, and functions of gelatinase B. The tight regulation of gelatinase B activity is highly complex and is established at five different levels. The transcription of the gelatinase B-gene depends on various cis-elements in its gene promotor and is induced or repressed by a large variety of soluble factors, including cytokines, growth factors, and hormones and by cellular contacts acting through specific signaling pathways. The specific regulation of its secretion occurs in cells storing gelatinase B in granules. After secretion, progelatinase B must be activated through an activation network. The enzyme activity is further regulated by inhibition and by other mechanisms, such as fine-tuning and stabilization by glycosylation. The ability of gelatinase B to degrade components of the extracellular matrix and to regulate the activity of a number of soluble proteins confers an important role in various physiological and pathological processes. These include reproduction, growth, development, inflammation, and vascular and proliferative diseases.