N-Glycosylation is required for secretion of the precursor to brain-derived neurotrophic factor (proBDNF) carrying sulfated LacdiNAc structures

Abstract

Brain-derived neurotrophic factor (BDNF) is generated by proteolytic cleavage of a prodomain from the proBDNF precursor either intracellularly by furin-like proteases or extracellularly by plasmin or matrix metalloproteinases. ProBDNF carries a single N-glycosylation sequon (Asn-127) that remains virtually unstudied despite being located in a highly conserved region proximal to the proteolytic site. To study the proBDNF structure and function, here we expressed the protein and its nonglycosylated N127Q mutant in HEK293F cells. We found that mutation of the Asn-127 prevents intracellular maturation and secretion, an effect reproduced in WT proBDNF by tunicamycin-induced inhibition of N-glycosylation. Absence of the N-glycan did not affect the kinetics of proBDNF cleavage by furin in vitro, indicating that effects other than a direct furin-proBDNF interaction may regulate proBDNF maturation. Using an optimized LC-MS/MS workflow, we demonstrate that secreted proBDNF is fully glycosylated and carries rare N-glycans terminated by GalNAcβ1-4GlcNAcβ1-R (LacdiNAc) extensively modified by terminal sulfation. We and others noted that this type of glycosylation is protein-specific, extends to proBDNF expressed in PC12 cells, and implies the presence of interacting partners that recognize this glycan epitope. The findings of our study reveal that proBDNF carries an unusual type of N-glycans important for its processing and secretion. Our results open new opportunities for functional studies of these protein glycoforms in different cells and tissues.

Keywords: LacdiNAc; N121Q; brain-derived neurotrophic factor (BDNF); glycoprotein; glycosylation; mass spectrometry (MS); maturation; neurotrophin; sulfation.

Scheme 1.

A, schematic illustration of proBDNF with zoom into the region containing the N-glycosylation and proteolytic cleavage sites involved in the generation of mature BDNF; B, alignment of the sequences demonstrates a high degree of conservation among species (source www.uniprot.org) (please note that the JBC is not responsible for the long-term archiving and maintenance of this site or any other third party hosted site) (multiple alignments were performed with Align tool of the Clustal Omega 1.2.4 engine). Letters in red represent amino acids that differ from the human sequence. Underlined text highlights the NX(S/T) N-glycosylation sequon.

Figure 1.

ProBDNF stably expressed in HEK293F cells is glycosylated. N-Glycosylation of proBDNF and the prodomain is documented by mass shift following deglycosylation with PNGase F: A, detection of proBDNF and BDNF by an antibody recognizing the BDNF region; B, detection of proBDNF and the prodomain by an antibody recognizing the prodomain region. The gel images were spliced as indicated by space to exclude samples not related to the study.

Figure 2.

Extracted ion chromatograms (XIC) of (A) nonglycosylated, NYLDAANMSMR and (B) deglycosylated, NYLDAADMSMR glycopeptide of proBDNF secreted by HEK293F cells confirm site occupancy higher than 99%. C, tandem mass spectrum verifies deglycosylation of the proBDNF peptide under [¹⁸O]water.

Figure 3.

Expression of wildtype (WT) proBDNF and its N121Q mutant (NQ) in stably transfected HEK293F cells. Protein expression was determined by Western blotting using an antibody, recognizing both BDNF and proBDNF, in the conditioned media (A) and cell lysates (B). Arrows point to (pro)BDNF forms: glycosylated proBDNF (black arrow), nonglycosylated proBDNF (white arrow), and mature BDNF (gray arrow). M indicates MagicMark molecular weight marker. Expression of mRNA was determined in WT and N121Q mutant by RT-PCR (C). ProBDNF/BDNF expression in cell lysates (D) or conditioned media (E) of HEK293F cells expressing N121Q mutant treated with proteasome inhibitor MG132 (MG), lysosomal inhibitor chloroquine (CQ), or chemical chaperone 4-phenylbutyric acid (PBA). The black arrow points to the position of nonglycosylated proBDNF.

Figure 4.

Kinetics of cleavage of glycosylated and nonglycosylated proBDNF by furin. Mass difference between glycosylated and nonglycosylated proBDNF enabled densitometric quantification of each proBDNF form: A, representative blot of the cleavage kinetic at the indicated time intervals; B, densitometric analysis of proBDNF intensities at the indicated time intervals. Results are expressed as percent of the proBDNF concentration (mean ± S.D., n = 3) at the beginning of the reaction. Note that proBDNF without C-terminal Myc-FLAG tag was used in the reaction as described under “Experimental procedures.”

Figure 5.

Mass spectra of proBDNF under the following conditions. A, precursor profile of the tryptic glycopeptide NYLDAANM(ox)SM(ox)R of proBDNF expressed in HEK293F cells; B, HCD fragmentation spectrum of the major glycoform (m/z 1132.42) at low collision energy (10% NCE) showing that characteristic LacdiNAc oxonium ion HexNAc-HexNAc (m/z 407.17) and its fucosylated form (m/z 553.22) with complementary y-ions (m/z 1373, 1381, 1454, and 1555) dominate the composition.

Figure 6.

Extracted ion chromatograms of four of the most abundant glycopeptides of proBDNF expressed in PC12 cells. A, C, and D, LacdiNAc containing structures; B, LacNAc containing structure. The arrow points to the peaks of glycopeptides aligned with slight retention time shifts as expected based on the hydrophilicity of the attached glycan. Numbers represent intensity of the extracted peaks (×10⁵).

Figure 7.

Impact of modulators of glycosylation on the BDNF/proBDNF ratio in HEK293F cells. Western blotting using antibodies to prodomain and mature BDNF regions was analyzed by densitometry for the following: A, intracellular BDNF to proBDNF ratio; B, secreted BDNF to proBDNF ratio; C, intracellular prodomain to proBDNF ratio; and D, secreted prodomain to proBDNF ratio. The representative Western blotting is labeled as the graphs: CTRL, control; CHLOR, 50 mm sodium chlorate; KIF, 1 μg/ml kifunensine; TNMC, 5 μg/ml of tunicamycin. Results are expressed as scatter plot (mean ± S.D., n = 3). *, p < 0.05; **, p < 0.01; ***, p < 0.001 compared with CTRL, one-way analysis of variance with Bonferroni adjustment.