A serine protease in Alzheimer's disease cells cleaves a 16K-peptide with flanking residues upstream to beta-amyloid-N-terminus as natural substrate

Pharmacological Stimulation of Phagocytosis Enhances Amyloid Plaque Clearance; Evidence from a Transgenic Mouse Model of ATTR Neuropathy

Hereditary ATTR V30M amyloidosis is a lethal autosomal dominant sensorimotor and autonomic neuropathy caused by deposition of aberrant transthyretin (TTR). Immunohistochemical examination of sural nerve biopsies in patients with amyloidotic neuropathy show co-aggregation of TTR with several proteins; including apolipoprotein E, serum amyloid P and components of the complement cascade. Complement activation and macrophages are increasingly recognized to play a crucial role in amyloidogenesis at the tissue bed level. In the current study we test the effect of two C5a receptor agonists and a C5a receptor antagonist (PMX53) on disease phenotype in ATTR V30M mice. Our results indicate that amyloid deposition was significantly reduced following treatment with the C5a receptor agonists, while treatment with the antagonist resulted in a significant increase of amyloid load. Administration of the C5a receptor agonists triggered increased recruitment of phagocytic cells resulting in clearance of amyloid deposits.

ST101 induces a novel 17 kDa APP cleavage that precludes Aβ generation in vivo

OBJECTIVE

Inhibiting Aβ generation is a prime therapeutic goal for preventing or treating Alzheimer disease. Here we sought to identify any disease-modifying properties of an azaindolizinone derivative, spiro[imidazo[1,2-a]pyridine-3,2-idan]-2(3H)-one (ST101 or ZSET1446).

METHODS

The effects of ST101 were studied in 3xTg-AD mice and young cynomolgus monkeys using a combination of biochemical and histological analyses.

RESULTS

Here we describe that ST101 induces cleavage of APP protein at a novel site, generating a 17 kDa C-terminal fragment. This 17 kDa APP cleavage product does not appear to be a substrate for either α- or β-secretase, and thus bypasses generation of Aβ. ST101 is orally active, efficacious at low doses, improves memory function, and robustly reduces brain Aβ in transgenic mice and nonhuman primates.

INTERPRETATION

Using rodent and nonhuman primate models, we show that ST101 represents a novel class of small molecules that reduce central nervous system levels of Aβ by inducing an alternate pathway of APP cleavage.

A novel carboxypeptidase B that processes native beta-amyloid precursor protein is present in human hippocampus

The processing of beta-amyloid precursor protein (APP) and generation of beta-amyloid (Abeta) are associated with the pathophysiology of Alzheimer's disease (AD). As the proteases responsible for the process in the human brain have yet to be clarified, we have searched for activities capable of cleaving native brain APP in the human hippocampus. A 40-kDa protein with proteolytic activity that degrades native brain APP in vitro was purified and characterized; molecular analysis identified it as a novel protease belonging to the carboxypeptidase B (CPB) family. PC12 cells overexpressing the cDNA encoding this protease generate a major 12-kDa beta-amyloid-bearing peptide in cytosol, a peptide which has also been detected in a cell-free system using purified brain APP as substrate. Although the protease is homologous to plasma CPB synthesized in liver, it has specific domains such as C-terminal 14 amino acid residues. Western analysis, cDNA-cloning process and Northern analysis suggested a brain-specific expression of this protease. An immunohistochemical study showed that the protease is expressed in various neuronal perikarya, including those of pyramidal neurons of the hippocampus and ependymal-choroid plexus cells, and in a portion of the microglia of normal brains. In brains of patients with sporadic AD, there is decreased neuronal expression of the protease, and clusters of microglia with protease immunoreactivity associated with its extracellular deposition are detected. These findings suggest that brain CPB has a physiological function in APP processing and may have significance in AD pathophysiology.

A human brain proteolytic activity capable of cleaving natural beta-amyloid precursor protein is affected by its substrate glycoconjugates

Human brain proteins were partially purified by using arginine-Sepharose 4B affinity chromatography, which traps proteins having an affinity to certain groups of arginine residue, such as serine proteases and zymogens. Bound proteins were analyzed for binding and cleavage related to the brain beta-amyloid precursor protein (APP). They were then further separated and isolated using a preparative gel system having a liquid-phase collection apparatus, using a non-denaturing gel system. Each fractionated protein was also analyzed for the above activity using natural APP. Among these, we found several fractions that bind preferentially to APP treated with chondroitinase ABC but not to intact APP, and that also generate particular beta-amyloid containing C-terminal peptides of APP via proteolysis. Our results suggest that sulfated glycoconjugates attached to APP play a role in the substrate specificity of APP for proteases, and also that the nature of natural APP processing mechanisms in vivo is very complex.

Enhanced aggregation of beta-amyloid-containing peptides by extracellular matrix and their degradation by the 68 kDa serine protease prepared from human brain

To explore whether extracellular matrix components in human brain affect the deposition and aggregation of beta-amyloid containing peptides, human brain samples from patients with sporadic Alzheimer's disease and normal aged were analyzed by Western blot analysis. All major beta-amyloid-containing peptides contained epitope(s) which is recognized by anti heparan sulfate antibody. Incubation of brain beta-amyloid-containing peptides with human collagen type IV in neutral pH efficiently generated a high molecular weight aggregated band, approximately 5-fold that of the control sample. We have previously found a serine protease which is capable of cleaving an oligopeptide at the N-terminus of beta-amyloid. In this study, the protease, which also contains heparan sulfate glycoconjugates, degraded the above brain peptides as natural substrates, although with different efficiency. These findings suggest that extra-cellular matrix components affect the processing and aggregation of beta-amyloid-containing peptides in human brain.

Human brain beta-secretase contains heparan sulfate glycoconjugates

A polyclonal antibody against the 68 kDa beta-secretase was established, which recognizes a single 68 kDa band in brain homogenate of Alzheimer's disease patients and normal aged. Western analysis revealed that the protease is an acidic glycoprotein with negative charge on its glycoconjugate(s). Sensitiveness to heparitinase and glycopeptidase A indicates that the protease contains asparagine-linked oligosaccharide with heparan sulfate moieties. Specific detection of the 68 kDa band in the analysis using anti-heparan sulfate antibody, and its time-course-dependent degradation, also confirm the above results. It seems that, like human blood coagulation factors IXa and XIa, the glycoconjugate(s) attached to the protease interfere with substrate specificity, stability and topological restriction of proteolysis in brain extracellular matrix, where diffuse plaque formation is taking place.