The HHQK domain of beta-amyloid provides a structural basis for the immunopathology of Alzheimer's disease.

The beta-amyloid peptide 1-42 (Abeta1-42), a major component of neuritic and core plaques found in Alzheimer's disease, activates microglia to kill neurons. Selective modifications of amino acids near the N terminus of Abeta showed that residues 13-16, the HHQK domain, bind to microglial cells. This same cluster of basic amino acids is also known as a domain with high binding affinity for heparan sulfate. Both Abeta binding to microglia and Abeta induction of microglial killing of neurons were sensitive to heparitinase cleavage and to competition with heparan sulfate, suggesting membrane-associated heparan sulfate mediated plaque-microglia interactions through the HHQK domain. Importantly, small peptides containing HHQK inhibited Abeta1-42 cell binding as well as plaque induction of neurotoxicity in human microglia. In vivo experiments confirmed that the HHQK peptide reduces rat brain inflammation elicited after infusion of Abeta peptides or implantation of native plaque fragments. Strategies which exploit HHQK-like agents may offer a specific therapy to block plaque-induced microgliosis and, in this way, slow the neuronal loss and dementia of Alzheimer's disease.

Specific domains of beta-amyloid from Alzheimer plaque elicit neuron killing in human microglia.

Alzheimer's disease (AD) is found to have striking brain inflammation characterized by clusters of reactive microglia that surround senile plaques. A recent study has shown that microglia placed in contact with isolated plaque fragments release neurotoxins. To explore further this process of immunoactivation in AD, we fractionated plaque proteins and tested for the ability to stimulate microglia. Three plaque-derived fractions, each containing full-length native A beta 1-40 or A beta 1-42 peptides, elicited neurotoxin release from microglia. Screening of various synthetic peptides (A beta 1-16, A beta 1-28, A beta 12-28, A beta 25-35, A beta 17-43, A beta 1-40, and A beta 1-42) confirmed that microglia killed neurons only after exposure to nanomolar concentrations of human A beta 1-40 or human A beta 1-42, whereas the rodent A beta 1-40 (5Arg-->Gly, 10Tyr-->Phe 13His-->Arg) was not active. These findings suggested that specific portions of human A beta were necessary for microglia-plaque interactions. When coupled to microspheres, N-terminal portions of human A beta (A beta 1-16, A beta 1-28, A beta 12-28) provided anchoring sites for microglial adherence whereas C-terminal regions did not. Although itself not toxic, the 10-16 domain of human A beta was necessary for both microglial binding and activation. Peptide blockade of microglia-plaque interactions that occur in AD might prevent the immune-driven injury to neurons.

Senile plaques stimulate microglia to release a neurotoxin found in Alzheimer brain.

Senile plaques found in the brains of patients with Alzheimer's disease (AD) are surrounded by clusters of reactive microglia. Isolated human microglia placed in contact with plaques in vitro are activated to release a factor which is toxic to neurons. This same neurotoxin is found in AD brain tissue and causes damage to pyramidal neurons in vivo when infused into rat hippocampus. Highest concentrations of the neurotoxin are in those brain structures most burdened by reactive microglia, suggesting that plaque-activated cells contribute to the neuronal damage and impaired cognition seen in patients with Alzheimer's dementia.

Phosphorylation of v-mos Ser 47 by the mitotic form of p34cdc2.

P85gag-mos is hyperphosphorylated during mitosis in normal rat kidney (NRK) cells transformed by Moloney murine sarcoma virus ts110. We now report that P85gag-mos is phosphorylated in vitro by the mitotic form of the cdc2 kinase (p34cdc2, known as M-phase kinase) derived from virus-transformed cells. The major site of P85gag-mos phosphorylation by the M-phase kinase in vitro lies within the amino-terminal portion of the viral mos protein sequence spanning residues 45-53, as determined by tryptic peptide mapping. A synthetic peptide corresponding to amino acids 37-55 of v-mos was specifically phosphorylated by the M-phase kinase, whereas v-mos peptides either lacking Ser 47 or substituted with Ala at residue 47 were not phosphorylated. Protein sequencing analyses established that the M-phase kinase specifically phosphorylates Ser 47. Tryptic phosphopeptide mapping of the in vivo-phosphorylated gag-mos protein from mitotic cells indicated that the 45-53 v-mos region was also phosphorylated within mitotic cells. These findings demonstrate that the M-phase kinase phosphorylates the viral mos protein at Ser 47. These results were unexpected in view of earlier reports regarding cdc2 kinase activation/stabilization by the c-mos kinase in maturing oocytes.

Further studies on the glycosylated gag gene products of Rauscher murine leukemia virus: identification of an N-terminal 45,000-dalton cleavage product.

A glycosylated 45,000-Mr protein containing Rauscher murine leukemia virus p15 and p12 antigenic sites and tryptic peptides was identified in Rauscher murine leukemia virus-infected cells. This glycoprotein, termed gP45gag, was also shown to contain a single tryptic peptide also present in gPr80gag and its unglycosylated apoprotein precursor Pr75gag, but lacking in Pr65gag or Pr40gag. The presence of this peptide only in viral precursor proteins containing the so-called leader (L) sequence strongly suggests that gPr45gag is an N-terminal fragment of larger glycosylated gag polyproteins, composed of L sequences in addition to p15 and p12. The kinetics of appearance of radiolabeled gPr45gag and its disappearance during chase-incubation is suggestive of a precursor-like role for this intermediate gene product. An observed 27,000-Mr glycosylated polypeptide, termed gP27gag and containing p15 but not p12, p30, or p10 antigenic determinants, is a candidate cleavage product derived from gPr45gag. These observations suggest that gPr45gag and its putative cleavage product gP27gag represent an authentic pathway for intracellular processing of glycosylated core proteins.

The structural relatedness of the virus core proteins of Rauscher and Moloney murine leukaemia virus.

The virus core proteins p30, p15, pp12 and p10 of Rauscher (R-MuLV) and Moloney murine leukaemia virus (Mo-MuLV) were purified. Two-dimensional peptide maps of 3H-leucine-containing tryptic peptides as well as elution profiles from ion-exchange chromatography of tryptic peptides derived from 3H-tyrosine-labelled R-MuLV core proteins and 14C-tyrosine-labelled Mo-MuLV core proteins were compared. The results show that the p30 and p10 proteins are very similar but that p15 and pp12 exhibit significant differences.

Characterization of 40,000- and 25,000-dalton intermediate precursors to Rauscher murine leukemia virus gag gene products.

Under steady-state labeling conditions, Rauscher murine leukemia virus-infected NIH Swiss mouse cells contain at least three major polyproteins derived from the viral gag gene. They have molecular weights of 65,000, 40,000, and 25,000. They have been termed pPr65gag, Pr40gag, and pPr25gag. pPr65gag has been shown by a number of laboratories to be composed of all four core proteins (p15, pp12, p30, and p10). In this paper, Pr40gag was found to contain p30 and p10 antigenic determinants and peptide sequences, whereas pPr25gag was found to contain p15 and pp12. Pr40gag and pPr25gag are rapidly labeled precursor proteins that were detectable early in pulse-chase experiments. Both precursors disappeared during the later stages of the chase period concurrent with the appearance of the mature viral core proteins. pPr65gag and pPr25gag were found to be phosphorylated, pPr25 having a higher specific activity of 32P than pPr65. In spite of this, peptide mapping studies, as well as the identification of the phosphorylated amino acid residues of pPr65, and pPr25, and pp12, indicated that the same sites are phosphorylated regardless of whether the precursors or the mature pp12 are examined.

Tryptic peptide analysis of gag and gag-pol gene products of Rauscher murine leukemia virus.

[3H]tyrosine-labeled viral precursor polyproteins and known mature viral proteins derived from the Rauscher murine leukemia virus gag and pol genes were examined by two-dimensional tryptic peptide mapping. Pr200gag-pol was found to contain peptide sequences of the viral core proteins p30, p15, p12, and p10, as well as peptide sequences found in the cell-associated reverse transcriptase. Intermediate reverse transcriptase precursor Pr125pol lacked peptide sequences of the four-core proteins but contained reverse transcriptase-specific tryptic peptides plus two additional tyrosine-containing tryptic peptides not related to gag or pol gene products. Methionine-containing tryptic peptide analysis also suggested the presence of additional protein material in Pr125pol (Kopchick et al., Proc. Natl. Acad. Sci. U.S.A. 75:2016-2020, 1978). Pr200gag-pol, although containing both viral core and reverse transcriptase-assoicated methionine and tyrosine tryptic peptides, also contained additional tryptic peptides. Thes are of two classes: (i) tryptic peptides associated with the Pr125pol but not Pr80pol and (ii) tryptic peptides not found in Pr125pol or in any known viral protein. One interpretation of these results is that Pr200gag-pol contains additional gene products aside from the gag and pol genes. Pr80gag and Pr65gag peptide maps were also examined and found to have sequences of all four core proteins. Pr65gag was found to contain two p30 tyrosine tryptic peptides that were absent in Pr80gag, suggesting that Pr80gag may not be the precursor to Pr65gag. Pr80gag, as expected from its larger size, also contained tryptic peptides not found in Pr65gag. Two of these additional Pr80gag tryptic peptides were found in Pr80pol as well but not in any of the viral core proteins, suggesting that Pr80gag and Pr80pol may have overlapping peptide sequences. Consistent with this finding is the conclusion that Pr80gag terminates within the pol gene. A model that describes the relationship of these recent findings to viral gene products is presented.

Common precursor for Rauscher leukemia virus gp69/71, p15(E), and p12(E).

Rauscher murine leukemia virus glycoprotein gp69/71 and non-glycosylated p15(E) are synthesized by way of a 90,000-dalton precursor glycoprotein, termed Pr2a+b. Peptide mapping experiments showed that Pr2a+b contains all the tyrosine-containing tryptic peptides of gp69/71. Two additional tyrosine-containing tryptic peptides in Pr2a+b that are not detected in gp69/71 are found in p15(E). Thus, gp69/71 and p15(E) peptide sequences account for all the tyrosine tryptic peptides of Pr2a+b. The gene order of the two proteins was determined by pulse-labeling infected cells in the presence and absence of pactamycin at concentrations of the inhibitor that prevent initiation of translation, but not elongation. The gene order was found to be: (2)HN-gp69/71-p15(E)-COOH. A newly identified major viral protein, termed p12(E), migrates in sodium dodecyl sulfate-polyacrylamide gels in the "p12" region. It is related to p15(E) as determined by tryptic mapping experiments. p15(E) and p12(E) are not phosphorylated, and both can be separated from phosphoprotein p12 by guanidine hydrochloride-agarose chromatography. p12(E) and p15(E) elute in the void volume fraction, whereas phosphoprotein p12 elutes between p15 and p10. The two p12 proteins can also be separated from each other by two-dimensional gel electrophoresis involving isoelectric focusing in the first dimension and sodium dodecyl sulfate-gel electrophoresis in the second dimension.

A fucose-deficient glycoprotein precursor to Rauscher leukemia virus gp69/71.

Rauscher leukemia virus glycoprotein gp69/71 is synthesized in virus-infected cells by way of a 90,000 dalton glycoprotein precursor, termed Pr2a+b. This precursor could be labeled with radioactive glucosamine and methionine but not with fucose; whereas gp69/71 could be detected by labeling with radioactive glucosamine, fucose, or a mixture of amino acids but seemed to be deficient in methionine relative to Pr2a+b. Pr2a+b and gp69/71, were specifically precipitated by an antiserum prepared against phosphocellulose purified Rauscher gp69/71. Other virus-specific precursors, in addition to Pr2a+b, could be precipitated by antiserum prepared against detergent disrupted virus. Neither Pr2a+b nor gp69/71 was precipitated from cell extracts by antisera to Rauscher p30. Tryptic maps of Pr2a+b and gp69/71 showed that these glycoproteins share many tryptic peptides. Pulse-chase experiments with 14C-labeled amino acids indicated that gp69/71 was not radio-labeled during the pulse-labeling period but slowly appeared during the chase incubations. Pr2a+b, however, was rapidly labeled and tended to disappear during long chases. Furthermore, two nonglycosylated viral proteins, termed p15E and p12E, are structurally related to Pr2a+b. Viral p15E and p12E contained the same methionine-containing tryptic peptide fraction as Pr2a+b as determined by ion-exchange chromatography. These results provide evidence that Pr2a+b is a precursor to gp69/71 and establish a structural and possible precursor-product relationship between Pr2a+b, p15E, and p12E.