Human BDNF Isoforms are Differentially Expressed in Cocaine Addicts and are Sorted to the Regulated Secretory Pathway Independent of the Met66 Substitution

Differential BDNF gene (BDNF) promoter use leads to protein isoforms differing by 8 or 15 N-terminal residues (BDNF1 and BDNF2) whose regulation and function are not completely understood versus the well-known 247-aa BDNF short form. To describe how BDNF isoform levels were regulated by chronic drug use, we measured BDNF isoform-specific mRNA levels in different human brain regions from cocaine addicts relative to age, race, and gender-matched controls. The cocaine group had threefold higher levels of exon 4-specific (BDNF Short) mRNAs in cerebellum versus controls (P < 0.01). In cortex, exon 4 and exon 1-specific BDNF mRNA levels (BDNF1) were significantly reduced in the cocaine group relative to controls (40%, P < 0.01). We also tested the hypothesis that the signal peptides of isoforms BDNF1 and BDNF2 confer different functional properties and determined if the functional Val66Met polymorphism influenced these functions. In contrast to transfected AtT-20 cells producing BDNF Short, regulated secretion of BDNF1 or BDNF2 was not affected by the Met66 substitution. Hippocampal neurons producing BDNF1 or BDNF2 on either the Val66 or Met66 background were similarly distributed in dendrites and had similar colocalization patterns with the secretory granule marker Sec II. This pattern differed from neurons producing BDNF Short Met66, which had impaired trafficking. Together, these findings support a mechanism by which variant BDNF proteins can overcome the functional defect of the Met66 substitution and suggest how functional differences in BDNF may impact brain responses in disease.