ProNGF induces p75-mediated death of oligodendrocytes following spinal cord injury

Abstract

The neurotrophin receptor p75 is induced by various injuries to the nervous system, but its role after injury has remained unclear. Here, we report that p75 is required for the death of oligodendrocytes following spinal cord injury, and its action is mediated mainly by proNGF. Oligodendrocytes undergoing apoptosis expressed p75, and the absence of p75 resulted in a decrease in the number of apoptotic oligodendrocytes and increased survival of oligodendrocytes. ProNGF is likely responsible for activating p75 in vivo, since the proNGF from the injured spinal cord induced apoptosis among p75(+/+), but not among p75(-/-), oligodendrocytes in culture, and its action was blocked by proNGF-specific antibody. Together, these data suggest that the role of proNGF is to eliminate damaged cells by activating the apoptotic machinery of p75 after injury.

Figure 1. Spinal Cord Injury Leads to a Loss of Oligodendrocytes Contusion injury in rats leads to a loss of oligodendrocytes

Estimated total number of oligodendrocytes within the confines of the rat dorsal columns at 7 and 13 mm rostral and 7 and 13 mm caudal to a contusion lesion at spinal level T10 (25 mm, NYU device). Counts were made using a stereology program (see Experimental Procedures). An approximate 50% reduction in numbers was seen after 8 and 21 days with significant reductions continuing to 42 days (p < 0.05). Similar results were obtained when measuring the density (number per mm³).

Figure 2. Spinal Cord Injury-Specific Induction of p75 among Oligodendrocytes

(A) Injury-specific induction of p75 in rats after contusion injury. A 5 mm spinal cord block was taken from rostral (R), caudal (C), as well as from the injury epicenter (L) at the indicated times after initial injury and processed for anti-p75 Western. The data are from a representative Western blot, and the same pattern was observed from five independent sets of samples. The lysates from 293 cells transfected with vector (GFP) or rat-p75 cDNA (HA-p75) were used as a negative or positive control, respectively. The same blot was reprobed for actin as a negative control. (B) Oligodendrocytes express p75 after spinal cord injury. Colocalization of CC1 and p75 in the dorsal column region 13 mm rostral to the injury epicenter and 8 days after the lesion. The arrows point to CC1⁺/p75⁺ cells. Scale bar, 12.5 μm.

Figure 3. p75⁺ Oligodendrocytes Are Positive for Active Caspase 3

(A) Colocalization of p75⁺ and active caspase 3 in the rostral region at 8 days after spinal cord injury. The arrows point to cells that are positive for both. (B) Colocalization of CC1⁺ and active caspase 3 in the rostral region at 8 days after spinal cord injury. The arrows point to cells that are positive for both. Scale bar, 12.5 μm.

Figure 4. Mouse Oligodendrocytes Fail to Die in Culture in the Absence of p75

(A) A representative picture of MBP⁺ cells counted for data in Figure 4B. After 6 days in culture, cells were treated with purified mature NGF at 100 ng/ml. At the amounts of time indicated in Figure 4B, cells were fixed, stained for TUNEL, then subsequently for MBP. The arrows point to TUNEL⁺ cells in the field. Scale bar, 20 μm. (B) Quantification of TUNEL⁺ MBP⁺ cells following purified NGF treatment at 100 ng/ml. The quantification data are from six independent experiments, with 80-120 cells counted in each experiment, for a total of 480-720 cells counted.

Figure 5. Increased Survival and Decreased Apoptosis of Oligodendrocytes after Spinal Cord Injury in p75^-/- Mice

(A) Left: induction of p75 among oligodendrocytes in mice after hemisection. Scale bar, 12.5 μm. Right: representative pictures of cells stained for cleaved caspase 3 and CC1 at 5 days postinjury. The arrows point to cells that are stained for both antibodies. Note the decrease in the number of CC1⁺/cleaved caspase 3⁺ cells in p75^-/- mice. Scale bar, 20 μm. (B) The loss of oligodendrocytes is attenuated in the dorsal funiculus of p75^-/- mice after dorsal hemisection (p < 0.05). The number is presented as the density of oligodendrocytes per mm³. Counts were made using a stereology program. (C) Reduction in the number of oligodendrocytes expressing active caspase 3 in p75^-/- mice after dorsal hemisection (p < 0.01). The number represents average cell counts in the dorsal columns per coronal section of the spinal cord.

Figure 6. Spinal Cord Injury-Specific Induction of ProNGF

(A) High-molecular weight NGF is predominantly induced after spinal cord injury. 30 μg of extracts were analyzed at each time point in Western analyses for NGF (Chemicon), BDNF (Promega), and NT3 (Promega). The extracts were prepared from rostral 5 mm (rats) or 3 mm (mice) blocks from the lesion center, but the pattern of NGF expression did not vary whether the sample was taken caudally or from the lesion center. The arrow indicates the position of recombinant BDNF and NT3, which were used at 250 ng as a control for antibody specificity. The star points to purified NGF being weakly recognized by anti-BDNF antibody. (B) The high-molecular weight NGF is proNGF. 350 μg of lysates was immunoprecipitated with proNGF antibody, and probed with anti-mature-NGF antibody (Chemicon). The bands at 26-32 KDa disappear almost completely following depletion with proNGF antibody, indicating that these bands are proNGF (compare lanes dep and undep). Although not shown, mature NGF is present in undepleted rat samples in longer exposure. (C) Specificity of proNGF and proBDNF antibodies. Western blot of 50 ng of mature NGF (Harlan Bioproducts for Science), 50 ng of mature BDNF (Promega), 50 μg of lysate from 293 cells stably expressing cleavage resistant proNGF, and 250 ng of supernatant from 293 cells infected with cleavage-resistant adenoviral proBDNF. A set of pro and mature NGF was probed with anti-pro/mature NGF antibody (Cedarlane) and proNGF antibody, and a set of pro and mature BDNF was probed with anti-mature-BDNF (Santa Cruz) and anti-proBDNF antibody. (D) ProNGF antibody depletes proNGF, but not mature NGF. Recombinant proNGF (lanes 1 and 2) or recombinant pro and mature NGF (lanes 3 and 4) were subjected to immunodepletion using proNGF antibody. Note that proNGF antibody depletes only proNGF, but not mature NGF (compare lanes 3 and 4).

Figure 7. ProNGF Present in Injured Mouse Spinal Cord Extracts Is Active in p75-Mediated Apoptosis of Oligodendrocytes in Culture

(A) p75 is required for apoptosis mediated by the injured spinal cord extracts. Extracts from sham-operated or injured mouse spinal cord extracts were added to either p75^+/+ orp75^-/- mouse oligodendrocytes. After 24 hr, cells were processed for TUNEL/MBP stain as described. For quantification of apoptotic cells, 150-200 cells were counted in each of two to four independent experiments to yield the total cell count of 300-800. (B) ProNGF in the injured spinal cord extracts behaves similarly to recombinant, cleavage-resistant proNGF in its affinity to induce apoptosis of oligodendrocytes. The concentration of proNGF in the extracts was estimated when added to 1 ml media. After 24 hr, cells were processed for TUNEL/MBP staining as described. For quantification of apoptotic cells, 250-350 cells were counted in each of three independent experiments to yield the total cell count of 750-1050. Abbreviations: rec pNGF, recombinant, cleavage-resistant proNGF; SCI, spinal cord injury extracts; rec mNGF, recombinant mature NGF. (C) ProNGF present in the spinal cord injury extracts is responsible for apoptosis of oligodendrocytes. Top: the data are represented in terms of the fold increase in the proportion of TUNEL⁺ MBP⁺ cells compared to that with the samples treated with individual antibody. The death mediated by the spinal cord injury extract is significantly attenuated with Chem-icon anti-NGF and proNGF antibodies, but not by preimmune serum or proBDNF antibody. The cells treated with purified NGF (Harlan Bioproducts for Science) were used as a positive control. For quantification of apoptotic cells, 250-350 cells were counted in each of three independent experiments to yield the total cell count of 750-1050. Bottom: purified NGF from Harlan contains proNGF. (D) Functional specificity of proNGF antibody. Rat oligodendrocytes were treated with 50 ng/ml of purified, recombinant mature NGF or 0.5 ng/ml of purified, recombinant, cleavage-resistant proNGF in the presence of either preimmune serum or anti-proNGF antibody. After 24 hr, cells were processed for TUNEL/MBP staining as described. For quantification of apoptotic cells, 200-300 cells were counted in each of three independent experiments to yield the total cell count of 600-900. Note that both proNGF and mature NGF can induce apoptosis, but proNGF is at least 50-fold more active than mature NGF.