Mark C. Willingham

Immunohistochemical localization in normal tissues of different epitopes in the multidrug transport protein P170: evidence for localization in brain capillaries and crossreactivity of one antibody with a muscle protein.

Using peroxidase immunohistochemistry, we examined the distribution of P170, a multidrug transport protein, in normal tissues by use of two different monoclonal antibodies (MAb). MAb MRK16 is a MAb that has been shown to react with an epitope in P170 located on the external face of the plasma membrane of multidrug-resistant human cells. MAb C219 has been shown to react with P170 in many mammalian species, and detects an epitope located on the cytoplasmic face of the plasma membrane. Using MRK16, we have previously described the localization of P170 on the bile canalicular face of hepatocytes, the apical surface of proximal tubular cells in kidney, and the surface epithelium in the lower GI tract in normal human tissues. In this work, we report that MRK16 also detects P170 in the capillaries of some human brain samples. A similar pattern was found using MAb C219 in rat tissues. in addition, MAb C219 showed intense localization in selected skeletal muscle fibers and all cardiac muscle fibers in rat and human tissues. ATPase cytochemistry showed that these reactive skeletal muscle fibers were of the type I (slow-twitch) class. Other additional sites of C219 reactivity in rat tissues were found in pancreatic acini, seminal vesicle, and testis. Electrophoretic gel immunoblotting showed two protein bands reactive with MAb C219. In liver, MAb C219 reacted with a approximately 170 KD band. In skeletal and cardiac muscle, MAb C219 reacted with a approximately 200 KD band which migrated in the same position as myosin. This band also reacted with an antibody to skeletal muscle myosin. This result suggests that C219 may crossreact with the heavy chain of muscle myosin in cardiac and skeletal muscle. Because MAb C219 reacts with proteins other than P170, it should be used with caution in studies of multidrug resistance.

Major DNA Fragmentation Is a Late Event in Apoptosis

Apoptosis, the terminal morphological and biochemical events of programmed cell death, is characterized by specific changes in cell surface and nuclear morphology. In addition, DNA fragmentation in an internucleosomal pattern is detectable in mass cultures of apoptotic cells. However, DNA fragmentation and nuclear morphological changes may not necessarily be associated events. In this study, we examined OVCAR-3 and KB human carcinoma cells using time-lapse video phase-contrast microscopy to characterize the surface and nuclear morphological features of apoptosis in response to treatment with either taxol or ricin. The surface morphological features of apoptosis were the same in both cell types and with both drugs. Using an in situ nick-translation histochemical assay, these single cells were also examined for DNA strand breaks during apoptosis. Surface morphological changes demonstrated discrete stages of cell rounding, surface blebbing, followed by cessation of movement and the extension of thin surface microspikes, followed much later by surface blistering and cell lysis. Nuclear features examined by DAPI cytochemistry demonstrated apoptotic nuclear condensation very early in this sequence, usually at the time of initial surface blebbing. The nick-translation assay, however, demonstrated DNA strand breaks at a much later time, only after the formation of separated apoptotic bodies or after final cell lysis. This study points out the differences between surface and nuclear morphological changes in apoptosis, and the large temporal separation between nuclear morphological changes and major DNA fragmentation detectable by this in situ technique. This result suggests caution in using in situ nick-translation as a direct correlate of internucleosomal DNA fragmentation in apoptosis.

Clathrin-coated vesicles: Isolation, dissociation and factor-dependent reassociation of clathrin baskets

Abstract The nature of the protein coat on clathrin-coated vesicles and the interactions responsible for the structure and reformation of clathrin baskets have been investigated. Coated vesicles were isolated from bovine brain using a rapid, one-day modification of the method of Pearse (1975). The vesicles are composed predominantly of clathrin (175,000) with smaller amounts of 110,000 and 55,000 molecular weight polypeptides. Clathrin was released in a solubilized form from these vesicles by treatment with 0.5 M Tris(hydroxymethyl)methyl ammonium chloride (Tris-Cl) or other protonated amines at neutral pH. It was not released on treatment with thiols, thiol reagents, Triton X-100 or sodium chloride, leading us to suggest that specific amino-carboxylate salt linkages are necessary for maintenance of the basket structure. When viewed by electron microscopy, the solubilized proteins in Tris-Cl are present in the form of filamentous aggregates and no basket structures are observed. Gel filtration of the extract in Tris-Cl resolves a clathrin-containing fraction (I) from one consisting predominantly of the 110,000 molecular weight polypeptide (II). We were able to reconstitute basket structures from the unfractionated Tris-Cl extract by dialyzing it against the vesicle isolation buffer, a solution of moderate ionic strength (Γ/2 = 0.11). Neither of the resolved fractions (I or II) alone yielded baskets on dialysis, but reconstitution was successful when both I and II were combined and dialyzed. The activity in II, which appears to be a basket-assembly factor, was heat-labile and could not be replaced by bovine serum albumin or brain calcium-dependent modulator protein. If a solution of low ionic strength (Γ/2 = 0.01) containing calcium was used as the dialysate, the clathrin fraction (I) alone was capable of reforming baskets. Thus the clathrin coat is a labile structure that can be solubilized by nondenaturing treatments, and baskets can be reformed from the extracted material.

The receptosome: An intermediate organelle of receptor-mediated endocytosis in cultured fibroblasts

Receptor-mediated endocytosis of specific ligands is mediated through clustering of receptor-ligand complexes in coated pits on the cell surface. Following this clustering event, the ligand is internalized into a noncoated intracellular vesicle, the receptosome, which selectively avoids fusion with lysosomes and moves toward the Golgi region of the cell by saltatory motion. Using alpha 2-macroglobulin as the ligand and electron microscopic cytochemical methods, we have shown the unusual appearance of this previously undescribed organelle and have followed the labeled ligand in these vesicles in the cytoplasm. To accomplish this, cells were incubated with immunolabeled alpha 2-macroglobulin at 4C under conditions where ligand-receptor complexes cluster into coated pits on the cell surface. Formation of the receptosome occurs between 2 and 5 min after raising the temperature of cells to 37C. These labeled receptosomes were seen to associate with many small vesicular elements in the cytoplasm, and were often found near the Golgi or GERL region after 15 min. Between 15 and 30 min a significant transfer of labeled ligand occurred from the receptosomal population to a population of small uniform lysosomes. By 60 min, all of the label was contained in these small lysosomes. Immunocytochemical studies showed that the receptosomes were not associated with clathrin, actin, myosin or tubulin. This unique, short-lived, specialized organelle selectively delivers the products of receptor-mediated endocytosis to intracellular sites.

Cytochemical Methods for the Detection of Apoptosis

Detection of apoptotic cell death in cells and tissues has become of paramount importance in many fields of modern biology, including studies of embryonic development, degenerative disease, and cancer biology. In addition to methods that employ biochemical analysis of large populations of cells, cytochemical methods have recently been extensively used both in individual cells and in tissues. Most of these methods exploit properties of dying cells that are more or less specific for the apoptotic process. However, considerable confusion exists over the interpretation of some of these methods and their usefulness in all settings. This review attempts to summarize the more recent advances in cytochemical detection of apoptosis and emphasizes some of the pitfalls that confuse the interpretation of results of these methods.

Collection of insulin, EGF and α2-Macroglobulin in the same patches on the surface of cultured fibroblasts and common internalization

We have used video intensification microscopy to observe fluorescent derivatives of insulin, epidermal growth factor and alpha2-macroglobulin added to Swiss 3T3-4 cells. At 4 degrees C, each of these polypeptides binds diffusely to specific receptors on the cell surface. When the cells are warmed to 23 or 37 degrees C, the bound insulin epidermal growth factor or alpha2-macroglobulin rapidly forms patches on the cell surface and is internalized. Using fluorescein-labeled alpha2-macroglobulin and rhodamine-labeled derivatives of insulin and epidermal growth factor, we show that all three polypeptides are internalized within the same vesicles by a common pathway. The mechanism for the internalization of these molecules is discussed.

Localization of the src gene product of the Harvey strain of MSV to plasma membrane of transformed cells by electron microscopic immunocytochemistry

Using electron microscopic immunocytochemistry, we have investigated the intracellular location of the src protein (p21) in cells transformed by the Harvey strain of Murine Sarcoma Virus (Ha-MSV). Antibodies to p21 were derived from tumor-bearing rats inoculated with Ha-NRK cells. The distribution of p21 in intracellular sites in MDCK dog cells transformed by Ha-MSV was examined and quantified using a recently developed immunocytochemical technique. More than 95% of p21 was localized to the inner surface of the plasma membrane in these Ha-MSV-transformed cells; p21 was not exposed on the outside surface of the plasma membrane. A similar location was observed by immunofluorescence in other Ha-MSV-transformed cell lines, including cells derived from rat, mouse and mink. This finding, and the previous demonstration that p60src of avian sarcoma virus is concentrated on the inner surface of the plasma membrane, suggests that the plasma membrane is a major site of action for transforming proteins.

Improved Detection of Apoptotic Cells in Archival Paraffin Sections: Immunohistochemistry Using Antibodies to Cleaved Caspase 3

Apoptosis has gained central importance in the study of many biological processes, including neoplasia, neurodegenerative diseases, and development. One of the limitations of many studies is the difficulty of specifically identifying individual apoptotic cells. Of the many specific methods developed to detect apoptotic cells, most are not applicable to histological sections of archival paraffin-embedded tissues. Recently, advances in the understanding of the molecular events in apoptosis have led to the realization that caspase activation is by far the most specific indicator of this cell suicide mechanism. Several publications have reported the development of antibodies directed at neoepitopes that are generated in various substrates through the action of caspases. One of these is that present on activated caspase 3, a ubiquitously distributed caspase that is a main effector caspase of the apoptotic cascade within cells. This study demonstrates the utility of using a recently commercially available antibody to cleaved caspase 3 in archival paraffin sections, suggesting that this may be a highly specific and sensitive method generally applicable to many studies of archival material.

Hepatic Niemann-Pick C1–like 1 regulates biliary cholesterol concentration and is a target of ezetimibe

Niemann-Pick C1-like 1 (NPC1L1) is required for cholesterol absorption. Intestinal NPC1L1 appears to be a target of ezetimibe, a cholesterol absorption inhibitor that effectively lowers plasma LDL-cholesterol in humans. However, human liver also expresses NPC1L1. Hepatic function of NPC1L1 was previously unknown, but we recently discovered that NPC1L1 localizes to the canalicular membrane of primate hepatocytes and that NPC1L1 facilitates cholesterol uptake in hepatoma cells. Based upon these findings, we hypothesized that hepatic NPC1L1 allows the retention of biliary cholesterol by hepatocytes and that ezetimibe disrupts hepatic function of NPC1L1. To test this hypothesis, transgenic mice expressing human NPC1L1 in hepatocytes (L1-Tg mice) were created. Hepatic overexpression of NPC1L1 resulted in a 10- to 20-fold decrease in biliary cholesterol concentration, but not phospholipid and bile acid concentrations. This decrease was associated with a 30%-60% increase in plasma cholesterol, mainly because of the accumulation of apoE-rich HDL. Biliary and plasma cholesterol concentrations in these animals were virtually returned to normal with ezetimibe treatment. These findings suggest that in humans, ezetimibe may reduce plasma cholesterol by inhibiting NPC1L1 function in both intestine and liver, and hepatic NPC1L1 may have evolved to protect the body from excessive biliary loss of cholesterol.

Inhibition of VSV binding and infectivity by phosphatidylserine: Is phosphatidylserine a VSV-binding site?

Recently we described a saturable, high-affinity binding site for vesicular stomatitis virus (VSV) on the surface of Vero cells that appears to mediate viral infectivity. To isolate this binding site, we have extracted Vero cells with the detergent, octyl-beta-D-glucopyranoside. The dialyzed detergent extract specifically inhibits the saturable, high-affinity binding of 35S-methionine-labeled VSV to Vero cells. The inhibitory activity is resistant to protease, neuraminidase and heating to 100 degrees C. It is soluble in chloroform-methanol and inactivated by phospholipase C, suggesting that it is a phospholipid. Of various purified lipids tested, only phosphatidylserine was capable of totally inhibiting the high-affinity binding of VSV. The half-maximal inhibitory concentration for phosphatidylserine was 1 microM. Phosphatidylserine also inhibited VSV plaque formation by 80%-90%; Herpes simplex virus plaque formation was unaffected. Centrifugation and electron microscopy studies have shown that phosphatidylserine-containing liposomes bind to VSV. The finding that phosphatidylserine directly binds to VSV and inhibits VSV attachment and infectivity suggests that plasma membrane phosphatidylserine could function as a binding site or portion of a binding site for VSV.