Koon-Sea Hui

A genomic screen for modifiers of tauopathy identifies puromycin-sensitive aminopeptidase as an inhibitor of tau-induced neurodegeneration.

Neurofibrillary tangles (NFT) containing tau are a hallmark of neurodegenerative diseases, including Alzheimers disease (AD). NFT burden correlates with cognitive decline and neurodegeneration in AD. However, little is known about mechanisms that protect against tau-induced neurodegeneration. We used a cross species functional genomic approach to analyze gene expression in multiple brain regions in mouse, in parallel with validation in Drosophila, to identify tau modifiers, including the highly conserved protein puromycin-sensitive aminopeptidase (PSA/Npepps). PSA protected against tau-induced neurodegeneration in vivo, whereas PSA loss of function exacerbated neurodegeneration. We further show that human PSA directly proteolyzes tau in vitro. These data highlight the utility of using both evolutionarily distant species for genetic screening and functional assessment to identify modifiers of neurodegeneration. Further investigation is warranted in defining the role of PSA and other genes identified here as potential therapeutic targets in tauopathy.

The breakdown of the individual neurofilament proteins by cathepsin D

In a continuing study of proteolysis of CNS proteins by CNS enzymes, neurofilament proteins (210 K, 155 K, 70 K) and desmin were separated, and the breakdown of individual proteins by purified brain cathepsin D was measured and compared to breakdown by plasma thrombin. With both cathepsin D and thrombin, the rate of breakdown of the 70 K protein was the highest, followed by the 155 K, and that of the 210 K was the lowest. With each substrate cathepsin D breakdown was the highest at pH 3; small but significant breakdown could be seen at pH 6. The pattern of intermediate breakdown products depended on pH, with greater amounts of fragments detected at higher pH, and the patterns with the two enzymes were different. We showed that differences exist in cleavage sites and breakdown rates of the neurofilament proteins. The capacity of the cathepsin D present in the tissue to hydrolyze these substrates was high, even at pH close to neutral, and was greatly in excess of that needed for physiological neurofilament turnover.

A Novel Neuron-specific Aminopeptidase in Rat Brain Synaptosomes ITS IDENTIFICATION, PURIFICATION, AND CHARACTERIZATION

A specific aminopeptidase localized exclusively in neurons of the central nervous system was identified with an automated continuous-flow aminopeptidase analyzer developed recently in this laboratory. The enzyme was purified from rat brain 4933-fold to homogeneity with 9.3% recovery by ammonium sulfate fractionation, followed by column chromatography successively on phenyl-Sepharose, Sephadex G-200, and twice on Mono Q FPLC. The purified single-chain enzyme was estimated to be 110 kDa in molecular mass. It has a pI of 5.25 and a pH optimum of 7.0. Only Mg(II) restores the activity of the apoenzyme. The neutral aminopeptidase hydrolyzes β-naphthylamides of amino acids with aliphatic, polar uncharged, positively charged, or aromatic side chains. It has a K m of 95 μm and a k cat of 7.8 s−1 on methionine-enkephalin, releasing only the N-terminal tyrosine. The thiol-dependent metallo-enzyme is most sensitive to amastatin inhibition with a K i of 0.04 μm, and is the aminopeptidase most sensitive to puromycin. Its properties are different from those of the ubiquitous puromycin-sensitive aminopeptidase obtained from the same enzyme preparation. The blocked N terminus, substrate and inhibitor specificity, hydrolytic coefficiency, metal effects, pI, molecular weight, and catalytic site show that this enzyme is distinct from all other known aminopeptidases. Its enrichment in the synaptosomes suggests that this first reported neuron-specific peptidase plays a role in neurotransmission and synaptic differentiation.

Puromycin-Sensitive Aminopeptidase (PSA/NPEPPS) Impedes Development of Neuropathology in hPSA/TAUP301L Double Transgenic Mice

Accumulation of neurotoxic hyperphosphorylated TAU protein is a major pathological hallmark of Alzheimer disease and other neurodegenerative dementias collectively called tauopathies. Puromycin-sensitive aminopeptidase (PSA/NPEPPS) is a novel modifier of TAU-induced neurodegeneration with neuroprotective effects via direct proteolysis of TAU protein. Here, to examine the effects of PSA/NPEPPS overexpression in vivo in the mammalian system, we generated and crossed BAC-PSA/NPEPPS transgenic mice with the TAU(P301L) mouse model of neurodegeneration. PSA/NPEPPS activity in the brain and peripheral tissues of human PSA/NPEPPS (hPSA) mice was elevated by ∼2-3-fold with no noticeable deleterious physiological effects. Double-transgenic animals for hPSA and TAU(P301L) transgenes demonstrated a distinct trend for delayed paralysis and showed significantly improved motor neuron counts, no gliosis and markedly reduced levels of total and hyperphosphorylated TAU in the spinal cord, brain stem, cortex, hippocampus and cerebellum of adult and aged animals when compared with TAU(P301L) mice. Furthermore, endogenous TAU protein abundance in human neuroblastoma SH-SY5Y cells was significantly reduced or augmented by overexpression or knockdown of PSA/NPEPPS, respectively. This study demonstrated that without showing neurotoxic effects, elevation of PSA/NPEPPS activity in vivo effectively blocks accumulation of soluble hyperphosphorylated TAU protein and slows down the disease progression in the mammalian system. Our data suggest that increasing PSA/NPEPPS activity may be a feasible therapeutic approach to eliminate accumulation of unwanted toxic substrates such as TAU.

Brain-Specific Aminopeptidase: From Enkephalinase to Protector Against Neurodegeneration

The major breakthrough discovery of enkephalins as endogenous opiates led our attempts to determine their inactivation mechanisms. Because the NH2-terminal tyrosine is absolutely necessary for the neuropeptides to exert analgesic effects, and aminopeptidase activities are extraordinarily high in the brain, a specific “amino-enkephalinase” should exist. Several aminopeptidases were identified in the central nervous system during the search. In fact, our laboratory found two novel neuron-specific aminopeptidases: NAP and NAP-2. NAP is the only functionally active brain-specific enzyme known. Its synaptic location coupled with its limited substrate specificity could constitute a “functional” specificity and contribute to enkephalin-specific functions. In addition, NAP was found to be essential for neuron growth, differentiation, and death. Thus, aminopeptidases are likely important for mental health and neurological diseases. Recently, puromycin-sensitive aminopeptidase (PSA) was identified as a modifier of tau-induced neurodegeneration. Because the enzymatic similarity between PSA and NAP, we believe that the depletion of NAP in Alzheimer’s disease (AD) brains plays a causal role in the development of AD pathology. Therefore, use of the puromycin-sensitive neuron-aminopeptidase NAP could provide neuroprotective mechanisms in AD and similar neurodegenerative diseases.

Activation and Inhibition of Cerebral Prolidase

Purification of prolidase from calf brain (acetone and [NH4]2SO4 fractionation) separated this enzyme from proteases, leucine aminopeptidase, master dipeptidase, and Gly‐Gly dipeptidase. Prolidase was tested with peptidase and protease inhibitors, used at higher levels (35 times or more) than their ID50 for peptidases and proteases. Bacitracin, leupeptin, chymostatin, and antipain had no effect; pepstatin slightly increased activity, and only bestatin was inhibitory. Antibiotics that affect protein synthesis did not inhibit prolidase. Peptides with proline at the NH2 end activated prolidase, whereas those with proline at the carboxyl end inhibited it. Di, tri, and tetra‐Pro peptides increased prolidase activity. Thyrotropin‐releasing hormone had no effect on prolidase; its analog Pro‐His‐Pro‐NH2 gave high activation and decreased the Km from 20 mm to 1.54 mm. Pro‐peptide inhibitors and activators were not themselves split by prolidase. The results indicate influences of specific peptides, for both inhibition and activation, on prolidase activity.

Changes in puromycin-sensitive aminopeptidases in postmortem schizophrenic brain regions

We studied the distribution of puromycin-sensitive aminopeptidase (PSA) in well-defined human brain ares by Western immunoblot in an attempt to examine its possible role in schizophrenia. The schizophrenic brains were from suicide victims (n = 13) of either sex, with an age range of 30-60 yr (average 45). The controls were mostly victims of myocardial infarction (n = 12), of either sex and between 32 and 56 yr old (average 44). The brain regions were obtained within 48 h after death. After ultracentrifugation the PSA was quantified by Western blot analysis using a PSA antiserum. The distribution of the two most abundant antigens, MW 100 kDa (PSA-100) and 170 kDA (PSA-170), were compared. PSA-100 had peptidase activity, PSA-170 did not. PSA-100 was found in all of the region studied. In the control brain areas prefrontal cingulate and frontal cortices, thalamus, hippocampus, hypothalamus and outer globus pallidus contained significantly more PSA-100 than the corresponding areas from schizophrenic brain. PSA-170 was mostly found only in areas of schizophrenic brains. In three control brains, in one area of each, it could be detected, but the level in each of these regions was less than 30% of that in the corresponding schizophrenic area. PSA-170 was found in all the schizophrenic brains, in 20 of the 35 regions we studied, with parahippocampal cortex the highest (134 ng/g wet tissue) and frontal inferior cortex the lowest (9.3 ng/g wet tissue). It was not detectable in cerebral or cerebellar white matter. Our data show that the amounts and distribution of PSA-170, a protein of unknown function, is restricted mostly to schizophrenic brain areas. The difference is not due either to neuroleptic treatment of the patient or to the postmortem proteolysis of the brain samples.

Enkephalin-containing polypeptides are potent inhibitors of enkephalin degradation

Enkephalin-containing polypeptides derived from pro-enkephalin A, pro-enkephalin B, or pro-opiomelanocortin were inhibitors of enkephalin degradation by aminoenkephalinases purified from cytosol or membranes. Of the peptides, Argo-Met-enkephalin was the most potent inhibitor for the aminoenkephalinases, with an IC50 of about 0.6 microM, it was more effective than bestatin (IC50 = 0.8-1.0 microM). This inhibition was partly due to substrate competition. Argo-Met-enkephalin was hydrolyzed by aminoenkephalinases to form Arg, Tyr, and Gly-Gly-Phe-Met in a substrate-inhibited manner. The hexapeptide also inhibited the breakdown of Arg- and Tyr-beta-naphthylamide by the membrane aminoenkephalinase. Since Argo-Met-enkephalin did not inhibit leucine aminopeptidase, it was a more selective inhibitor than bestatin of Met-enkephalin breakdown by aminopeptidases. Argo-Met-enkephalin inhibited enkephalin breakdown by synaptosomal plasma membranes but not by brain slices. Our data suggest that in addition to their possible role as opioids, the enkephalin-containing polypeptides may be regulators of enkephalin levels.

Prolidase activity in rat brain; developmental, regional and subcellular distribution.

We determined the regional and subcellular distribution of prolidase in rat brain and its changes with development. The most rapid changes in enzyme activity occurred perinatally, with a maximum level of activity 2 days before birth and a minimum 1 day after birth. Of the 7 regions examined, cerebellum had the highest enzyme level, followed by medulla. The lowest levels were found in the hypothalamus in the adult and in the midbrain in the young. Polidase was mainly soluble; over 55% was recovered in the S2fraction, and the rest was released from the particulate fractions by hypotonic shock. Brains of male rats contained a slightly higher level of the enzyme.

Analysis of copper in brain by the mass-spectrometric integrated-ioncurrent procedure

Through use of the high-resolution double-focusing mass spectrometer, copper has been identified in various regions of the mouse, rat, guinea pig, rabbit, and human brain. The procedure depends on converting the copper (in ashed tissue) to its chloride salt, followed by derivatization with tetraphenylporphyrin (TPP) to yield a TPP chelate. After chromatographic separation, this chelate is assessed in the mass spectrometer by the integrated-ion-current procedure. Deuterated metal TPP chelates and the rare stable isotope65Cu were used as internal standards. Whole brain values obtained were as follows: mouse, 6.67±0.16 (mean±SEM) μg/g wet weight of tissue; rat, 1.06±0.05; guinea pig, 5.40±0.63; and rabbit, 7.52±0.76. In the rat, the cerebellum contained the highest concentration (1.25 μg/g), and the striatum the lowest (0.70 μg/g). In the human brain, the cortex (gray) and the striatum were relatively the highest copper-containing regions, with the cerebellum (white) being the lowest.