Disruption of the expression of the proprotein convertase PC7 reduces BDNF production and affects learning and memory in mice

PC7 belongs to the proprotein convertase family, whose members are implicated in the cleavage of secretory precursors. The in vivo function of PC7 is unknown. Herein, we find that the precursor proBDNF is processed into mature BDNF in COS-1 cells coexpressing proBDNF with either PC7 or Furin. Conversely, the processing of proBDNF into BDNF is markedly reduced in the absence of either Furin or PC7 in mouse primary hepatocytes. In vivo we observe that BDNF and PC7 mRNAs are colocalized in mouse hippocampus and amygdala and that mature BDNF protein levels are reduced in these brain areas in PC7 KO mice but not in the hippocampus of PC1/3 KO mice. Various behavioral tests reveal that in PC7 KO mice spatial memory is intact and plasticity of responding is mildly abnormal. Episodic and emotional memories are severely impaired, but both are rescued with the tyrosine receptor kinase B agonist 7,8-dihydroxyflavone. Altogether, these results support an in vivo role for PC7 in the regulation of certain types of cognitive performance, in part via proBDNF processing. Because polymorphic variants of human PC7 are being characterized, it will be important in future studies to determine their effects on additional physiological and behavioral processes.Nine secretory proprotein convertases (PCs) play major roles in regulating multiple cellular and extracellular processes both in health and disease states (reviewed in refs. 1 and 2). The convertases PC1/3, PC2, Furin, PC4, PC5/6, PACE4, and PC7 cleave their substrates after single or pairs of basic amino acids, SKI-1/S1P processes protein precursors after nonbasic residues, and PCSK9 has no known substrates other than itself (1). Studies that have analyzed tissue expression, levels, regulation, ontogeny, phenotypes of model animals lacking one or more PCs, and human/mouse natural mutations are starting to provide clues as to the specific roles of these enzymes in cells and whole-animal physiological and pathological processes.The type I membrane-bound PC7 is the most ancient member of the mammalian basic amino acid-specific PC family, and it exhibits the closest homology to yeast kexin (3). Human PC7 is synthesized as an N-glycosylated zymogen (proPC7) that, like most other PCs, undergoes autocatalytic cleavage in the endoplasmic reticulum at RAKR₁₄₁↓SV. Mature PC7 can reach the cell surface by an unconventional route from the endoplasmic reticulum (4), but it also accumulates in the trans Golgi network (TGN) and can cycle between the cell surface and TGN via endosomes, in part by virtue of a Pro-Leu-Cys₇₂₆ motif in its cytosolic tail (5). The tail also contains two cysteine residues, Cys₆₉₉ and Cys₇₀₄, which are palmitoylated (3, 4, 6), that may assist in this process. Confocal and electron microscopy studies have revealed that PC7 localizes to vesicles located immediately beneath the plasma membrane (4, 7). No soluble shed forms of PC7 have been detected. Enzymatic activity assays using only the soluble luminal/extracellular domain of PC7 (sol.PC7) and fluorogenic substrates have indicated that this Ca²⁺-dependent enzyme exhibits a neutral pH optimum and a cleavage specificity similar to that of Furin, cleaving within the general motif (R/K)-2X_n-R↓ where n = 0–2 (8, 9).Only the membrane-bound PC7 induces the processing of proepidermal growth factor into a ∼115-kDa transmembrane form (10). PC7 is abundant in neurons (11) but is also expressed in microglia (12). It has been shown that PC7 exerts an important function in MHC class I-mediated antigen presentation (7), befitting its high expression within the immune system (3). Finally, PC7 is unique because it is able to shed the human transferrin receptor 1 (TfR1) into a soluble form by cleavage at KTECER₁₀₀↓LA within endosomes (13).The physiological importance of the PCs is illustrated by the early death or major phenotypes observed in mice lacking one or more convertase (1, 14). Although generated several years ago, deletion of PC7 is the only PC KO mouse for which no overt phenotype(s) has been described (15). In contrast, PC7 knockdown in Xenopus is embryonic lethal. These embryos lack eyes and brain and exhibit abnormal anterior neural development (16). Unless this knockdown is due to an off-target effect, amphibian PC7 seems to fulfill essential neuronal functions that, in mammals, may be either nonessential or redundantly assumed by other PCs. Notably, the regulation of the PC7 gene (PCSK7) has not been examined (3, 17).In the present work we describe behavioral alterations in mice lacking PC7. Our results show that PC7 KO mice have lower levels of BDNF in the hippocampus and amygdala than WT mice and exhibit learning and memory impairments. Reduced BDNF levels are likely responsible for some of these deficits, because they are rescued by an agonist to the BDNF receptor tyrosine receptor kinase B (TrkB). Therefore, PC7 seems to play unique roles in the CNS, at least in part, through regulating levels of BDNF.