Erythroid spectrin is the predominant component of the twodimensional protein network called the membrane skeleton, underlying the lipid bilayer of red cells (for recent reviews, seeRefs. 1-3). Formation of the membrane skeleton involves multiple protein-protein interactions among integral membrane proteins. Interactions of spectrin with other membrane proteins such as ankyrin, protein 4.1, and adducin provide a linkage of spectrin either to the plasma membrane or among spectrin tetramers. Many hereditary anemia mutations affect interactions of these integral membrane proteins, resulting in increased fragility and shortened lifespan of erythrocytes. In hereditary elliptocytosis and pyropoikilocytosis, the mutations have been localized in the ␣-and ␤-subunits of spectrin (reviewed in Refs. 4 and 5). Many of these proteins, including spectrin, which were first identified in red cells, have isoforms expressed in nonerythroid cells, but the structure and regulatory processes of the nonerythroid membrane skeleton are less well understood (reviewed in Refs. 1-3, 6, and 7). Functional differences between the membranes of erythroid and nonerythroid cells argue against the simple erythrocyte model of the membrane skeleton. Major differences between the erythroid model and other cells include differences in the expression of spectrin (8 -11) and ankyrin isoforms (12-15) (reviewed in Ref.16), interactions of spectrin and ankyrin with additional proteins (17-21), localization of spectrin in the cytoplasm as well as in the plasma membrane (10,11,22), and the potential for dramatic rearrangements of spectrin's cellular location (23, 24) (reviewed in Refs. 2 and 7).Several studies have demonstrated that both erythroid and nonerythroid spectrins are expressed in brain tissue (8 -11, 25). Neuronal compartmentalization of brain spectrin isoforms into axons and presynaptic terminals (nonerythroid spectrin) and into cell bodies and dendrites (erythroid spectrin) (10, 25) suggests that brain spectrin isoforms may perform related but distinct functions in neuronal cells. It has been suggested that nonerythroid spectrin performs a more general, constitutive role, while erythroid spectrin takes part in more specialized activities of differentiated cells (26). The ␣-subunit of erythroid spectrin, ␣I (27), 1 and the ␣-subunit of nonerythroid spectrin, ␣II (28, 29), each contains a unique SH3 2 domain. Distinct protein interactions are likely to involve these domains, and they may be important for specific distribution and specialized roles of brain spectrin isoforms.

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Amyloid (A ) immunoreactivity in neurons was examined in brains of 32 control subjects, 31 people with Down syndrome, and 36 patients with sporadic Alzheimer's disease to determine if intraneuronal A immunoreactivity is an early manifestation of Alzheimer-type pathology leading to Wbrillar plaque formation and/or neuroWbrillary degeneration. The appearance of A immunoreactivity in neurons in infants and stable neuron-type speciWc A immunoreactivity in a majority of brain structures during late childhood, adulthood, and normal aging does not support this hypothesis. The absence or detection of only traces of reaction with antibodies against 4-13 aa and 8-17 aa of A in neurons indicated that intraneuronal A was mainly a product of -and -secretases (A 17-40/42 ). The presence of N-terminally truncated A 17-40 and A 17-42 in the control brains was conWrmed by Western blotting and the identity of A 17-40 was conWrmed by mass spectrometry. The prevalence of products of -and -secretases in neurons and -and -secretases in plaques argues against major contribution of A -immunopositive material detected in neuronal soma to amyloid deposit in plaques. The strongest intraneuronal A 17-42 immunoreactivity was observed in structures with low susceptibility to Wbrillar A deposition, neuroWbrillary degeneration, and neuronal loss compared to areas more vulnerable to Alzheimertype pathology. These observations indicate that the intraneuronal A immunoreactivity detected in this study is not a predictor of brain amyloidosis or neuroWbrillary degeneration. [389][390][391][392][393][394][395][396][397][398][399][400][401][402] 123 immunoreactivity in structures free from neuronal pathology during essentially the entire life span suggests that intraneuronal amino-terminally truncated A represents a product of normal neuronal metabolism.

The scrapie agent protein (Sp33-37) is a degradation-resistant protein that aggregates into fibrils and amyloid plaques. This protein is derived from a normal cellular protein (Cp33-37). Understanding the mechanism responsible for the conversion of Cp33-37 to Sp33-37 may explain scrapie agent replication. Cp33-37 was extracted from normal hamster brain and purified 2700-fold by an immunoaffinity method. Both Cp33-37 purified from normal hamster brain and Sp33-37 purified from scrapie-affected hamster brain had apparent masses of 33-37 kilodaltons and displayed microheterogeneity characteristic of glycoproteins. Cp33-37 was completely digested by proteinase K under conditions that resulted in conversion of Sp33-37 to the protease-resistant fragment PrP27-30. Cp33-37 did not cause scrapie when inoculated intracerebrally into hamsters. Fractions containing purified Sp33-37 had average titers of greater than 10(11) LD50 of the scrapie agent/mg of protein; these titers were not diminished by proteinase K. These results indicate that altered sensitivity to proteolysis in vitro reflects an intrinsic difference between Sp33-37 and Cp33-37.

Alzheimer amyloid-beta is deposited in the neuropil and in brain blood vessels in transgenic Tg2576 mice that overexpress human amyloid-beta precursor protein (AbetaPP) containing the Swedish mutation (AbetaPP-Swe). Because the AbetaPP transgene in Tg2576 mice is placed behind the PrP promoter, all amyloid-beta, including vascular amyloid, is considered to be of neuronal origin. We studied the expression of the transgenic AbetaPP in smooth muscle cells cultured from brain blood vessels from Tg2576 mice. We found that brain vascular smooth muscle cells overexpressed human AbetaPP-Swe approximately 4 times the physiological levels of mouse AbetaPP. The cultured cells secreted abundant Abeta1-40 and Abeta1-42 and formed intracellular Abeta-immunoreactive granules. The percentage of cells containing intracellular Abeta and the amount of intracellular Abeta were significantly higher in cultures obtained from 14-month-old than from 4-month-old mice, as tested on first or second passages. During cell senescence in culture, intracellular accumulation of Abeta and C-terminal fragments of AbetaPP increased in cells derived from both 4- and 14-month-old mice. Vascular muscle cells from Tg2576 mice appear to be a valuable model of the intracellular accumulation of Abeta. We suggest that vascular muscle cells may be involved in the production of cerebrovascular amyloid in Tg2576 mice.