Biological activity of the growth factor-induced cytokine N51: structure-function analysis using N51/Interleukin-8 chimeric molecules

Fortilin binds Ca2+ and blocks Ca2+-dependent apoptosis in vivo

Fortilin, a 172-amino-acid polypeptide present both in the cytosol and nucleus, possesses potent anti-apoptotic activity. Although fortilin is known to bind Ca2+, the biochemistry and biological significance of such an interaction remains unknown. In the present study we report that fortilin must bind Ca2+ in order to protect cells against Ca2+-dependent apoptosis. Using a standard Ca2+-overlay assay, we first validated that full-length fortilin binds Ca2+ and showed that the N-terminus (amino acids 1-72) is required for its Ca2+-binding. We then used flow dialysis and CD spectropolarimetry assays to demonstrate that fortilin binds Ca2+ with a dissociation constant (Kd) of approx. 10 mM and that the binding of fortilin to Ca2+ induces a significant change in the secondary structure of fortilin. In order to evaluate the impact of the binding of fortilin to Ca2+ in vivo, we measured intracellular Ca2+ levels upon thapsigargin challenge and found that the lack of fortilin in the cell results in the exaggerated elevation of intracellular Ca2+ in the cell. We then tested various point mutants of fortilin for their Ca2+ binding and identified fortilin(E58A/E60A) to be a double-point mutant of fortilin lacking the ability of Ca2+-binding. We then found that wild-type fortilin, but not fortilin(E58A/E60A), protected cells against thapsigargin-induced apoptosis, suggesting that the binding of fortilin to Ca2+ is required for fortilin to protect cells against Ca2+-dependent apoptosis. Together, these results suggest that fortilin is an intracellular Ca2+ scavenger, protecting cells against Ca2+-dependent apoptosis by binding and sequestering Ca2+ from the downstream Ca2+-dependent apoptotic pathways.

Neutrophil infiltration, glial reaction, and neurological disease in transgenic mice expressing the chemokine N51/KC in oligodendrocytes

Chemokines (pro-inflammatory chemoattractant cytokines) are expressed in pathological conditions of the central nervous system (CNS). Previous studies suggested that the CNS is relatively resistant to leukocyte diapedesis after chemokine injection, leaving their functional role unresolved. The CNS function of N51/KC, a neutrophil-selective chemokine, was addressed by expressing N51/KC under control of the myelin basic protein (MBP) promoter in transgenic (tg) mice (MBP-N51/KC mice). CNS-specific N51/KC expression produced remarkable neutrophil infiltration into perivascular, meningeal, and parenchymal sites, demonstrating that this chemokine exerts the multiple functions in vivo required to recruit leukocytes into the CNS. MBP-N5 1/KC mice represent an incisive model for the molecular dissection of neutrophil entry into the CNS. Unexpectedly, MBP-N51/KC mice developed a neurological syndrome of pronounced postural instability and rigidity at high frequency beginning at 40 days of age, well after peak chemokine expression. 68/182 mice in one tg fine were found dead before one year of age, with prominent neurological symptoms premortem in 26 (38%). Florid microglial activation and blood-brain barrier disruption without dysmyelination were the major neuropathological alterations. Late-onset neurological symptoms in MBP-N51/KC mice may indicate unanticipated consequences of CNS chemokine expression.

Site-directed mutagenesis of monocyte chemoattractant protein-1 identifies two regions of the polypeptide essential for biological activity

Monocyte chemoattractant protein-1 (MCP-1) mediates monocyte migration into tissues in inflammatory diseases and atherosclerosis. We have investigated structure-activity relationships for human MCP-1. Mutations were introduced based upon differences between MCP-1 and the structurally related but functionally distinct molecule interleukin-8 (IL-8). Mutant proteins produced using the baculovirus/insect cell expression system were purified and their ability to stimulate monocyte chemotaxis and elevation of intracellular calcium in THP-1 monocytic leukaemia cells was measured. Two regions in MCP-1 were identified as important for its biological activity. One region consists of the sequence Thr-Cys-Cys-Tyr (amino acids 10-13). Point mutations of Thr-10 to Arg and Tyr-13 to Ile greatly lowered MCP-1 activity. The second functionally important region is formed by Ser-34 and Lys-35. Insertion of a Pro between these two residues, or their substitution by the sequence Gly-Pro-His, caused nearly complete loss of MCP-1 activity. Competition binding experiments showed that the mutations that affected activity also lowered the ability to compete with wild-type MCP-1 for receptors on THP-1 cells. Point mutations at positions 8, 15, 30, 37, 38 and 68 had little effect on MCP-1 activity. The important regions that we have identified in MCP-1 correspond with previously identified functionally important regions of IL-8, suggesting that the two molecules bind to their respective receptors by similar contacts.

Expression of the chemokine N51/KC in the thymus and epidermis of transgenic mice results in marked infiltration of a single class of inflammatory cells

Transgenic mice expressing the chemokine N51/KC in thymus, skin, and tongue showed a marked infiltration of a single class of inflammatory cells (neutrophils) in the sites of transgene expression. In the thymus, neutrophils were most numerous in the cortex and juxta-medullary regions, often forming aggregates or clusters. A similar, but less intense, neutrophilic infiltrate occurred in close proximity to the epidermal basal layer of the tongue and skin. No morphologic evidence of injury was observed in the thymus, skin, or tongue of these transgenic mice, indicating that N51/KC expression induces recruitment but not inflammatory activation of neutrophils. The lack of activation in the thymus resulted in a large senescent neutrophilic population that was phagocytosed by thymic macrophages and epithelial-reticular cells. These results indicate that N51/KC is a neutrophil chemoattractant in vivo and establish these transgenic mice as effective models to study the phenomena of recruitment and clearance of neutrophils, events that are critical for the initiation and resolution of the inflammatory response.