Serum concentration of 20K human growth hormone (20K hGH) measured by a specific enzyme-linked immunosorbent assay. Study Group of 20K hGH

Several GH isoforms have been identified in pituitary and serum, the most abundant of which is the 22K human GH (hGH) isoform. The 20K hGH isoform is produced by alternative splicing of GH messenger ribonucleic acid and comprises approximately 10% of all GH in the pituitary. The physiological role of 20K hGH remains to be determined, partly because of the lack of a simple and specific assay. We have established sensitive enzyme-linked immunosorbent assays (ELISAs) specific to 20K and 22K hGH. To determine whether regulation of 20K hGH secretion is the same as that for 22K hGH, we measured serum concentrations of both species of hGH in normal subjects and patients with a variety of endocrine disorders. The serum levels of 20K hGH after overnight fasting was 118 +/- 178 pg/mL (n = 282) in normal women, significantly higher than that in normal men (64 +/- 170 pg/mL; n = 226). However, there was no difference in the proportion of 20K hGH to 20K plus 22K hGH between men (6.3 +/- 2.6%, mean +/- SD; n = 176) and women (6.3 +/- 2.1%; n = 263). No correlation was detected between the ratio of 20K hGH and age, body height, body weight, or body fat mass in normal subjects. The proportion of 20K hGH was significantly (P < 0.001) higher in patients with active acromegaly (9.2 +/- 2.2%; n = 33) and patients with anorexia nervosa (9.0 +/- 1.9; n = 8), both of which are characterized by chronic elevation of circulating GH levels. The proportion of 20K hGH in successfully treated acromegalic patients did not differ from that in normal subjects, suggesting that GH-producing pituitary tumors secrete a higher proportion of 20K hGH, or that a chronic excess of 22K hGH alters the MCR of 20K hGH. The values in patients with adult GH deficiency, hyperthyroidism, primary hypothyroidism, or GH-independent short stature did not differ from those in normal subjects. The 20K ratio did not change after acute GH provocative tests, such as the insulin tolerance test and the GHRH test. These results suggest that secretion of 20K hGH from the pituitary is under the same control as that of 22K hGH. This new assay may provide a tool for understanding the physiological or pathophysiological role of the 20K hGH isoform.

Redundant and distinct functions for dynamin-1 and dynamin-2 isoforms

A role for dynamin in clathrin-mediated endocytosis is now well established. However, mammals express three closely related, tissue-specific dynamin isoforms, each with multiple splice variants. Thus, an important question is whether these isoforms and splice variants function in vesicle formation from distinct intracellular organelles. There are conflicting data as to a role for dynamin-2 in vesicle budding from the TGN. To resolve this issue, we compared the effects of overexpression of dominant-negative mutants of dynamin-1 (the neuronal isoform) and dynamin-2 (the ubiquitously expressed isoform) on endocytic and biosynthetic membrane trafficking in HeLa cells and polarized MDCK cells. Both dyn1(K44A) and dyn2(K44A) were potent inhibitors of receptor-mediated endocytosis; however neither mutant directly affected other membrane trafficking events, including transport mediated by four distinct classes of vesicles budding from the TGN. Dyn2(K44A) more potently inhibited receptor-mediated endocytosis than dyn1(K44A) in HeLa cells and at the basolateral surface of MDCK cells. In contrast, dyn1(K44A) more potently inhibited endocytosis at the apical surface of MDCK cells. The two dynamin isoforms have redundant functions in endocytic vesicle formation, but can be targeted to and function differentially at subdomains of the plasma membrane.

Visualization of melanosome dynamics within wild-type and dilute melanocytes suggests a paradigm for myosin V function In vivo

Unlike wild-type mouse melanocytes, where melanosomes are concentrated in dendrites and dendritic tips, melanosomes in dilute (myosin Va-) melanocytes are concentrated in the cell center. Here we sought to define the role that myosin Va plays in melanosome transport and distribution. Actin filaments that comprise a cortical shell running the length of the dendrite were found to exhibit a random orientation, suggesting that myosin Va could drive the outward spreading of melanosomes by catalyzing random walks. In contrast to this mechanism, time lapse video microscopy revealed that melanosomes undergo rapid ( approximately 1.5 microm/s) microtubule-dependent movements to the periphery and back again. This bidirectional traffic occurs in both wild-type and dilute melanocytes, but it is more obvious in dilute melanocytes because the only melanosomes in their periphery are those undergoing this movement. While providing an efficient means to transport melanosomes to the periphery, this component does not by itself result in their net accumulation there. These observations, together with previous studies showing extensive colocalization of myosin Va and melanosomes in the actin-rich periphery, suggest a mechanism in which a myosin Va-dependent interaction of melanosomes with F-actin in the periphery prevents these organelles from returning on microtubules to the cell center, causing their distal accumulation. This "capture" model is supported by the demonstration that (a) expression of the myosin Va tail domain within wild-type cells creates a dilute-like phenotype via a process involving initial colocalization of tail domains with melanosomes in the periphery, followed by an approximately 120-min, microtubule-based redistribution of melanosomes to the cell center; (b) microtubule-dependent melanosome movement appears to be damped by myosin Va; (c) intermittent, microtubule-independent, approximately 0.14 microm/s melanosome movements are seen only in wild-type melanocytes; and (d) these movements do not drive obvious spreading of melanosomes over 90 min. We conclude that long-range, bidirectional, microtubule-dependent melanosome movements, coupled with actomyosin Va-dependent capture of melanosomes in the periphery, is the predominant mechanism responsible for the centrifugal transport and peripheral accumulation of melanosomes in mouse melanocytes. This mechanism represents an alternative to straightforward transport models when interpreting other myosin V mutant phenotypes.

Tropomyosin-containing actin cables direct the Myo2p-dependent polarized delivery of secretory vesicles in budding yeast

The actin cytoskeleton in budding yeast consists of cortical patches and cables, both of which polarize toward regions of cell growth. Tropomyosin localizes specifically to actin cables and not cortical patches. Upon shifting cells with conditionally defective tropomyosin to restrictive temperatures, actin cables disappear within 1 min and both the unconventional class V myosin Myo2p and the secretory vesicle-associated Rab GTPase Sec4p depolarize rapidly. Bud growth ceases and the mother cell grows isotropically. When returned to permissive temperatures, tropomyosin-containing cables reform within 1 min in polarized arrays. Cable reassembly permits rapid enrichment of Myo2p at the focus of nascent cables as well as the Myo2p- dependent recruitment of Sec4p and the exocyst protein Sec8p, and the initiation of bud emergence. With the loss of actin cables, cortical patches slowly assume an isotropic distribution within the cell and will repolarize only after restoration of cables. Therefore, actin cables respond to polarity cues independently of the overall distribution of cortical patches and are able to directly target the Myo2p-dependent delivery of secretory vesicles and polarization of growth.

Reduced growth, abnormal kidney structure, and type 2 (AT2) angiotensin receptor-mediated blood pressure regulation in mice lacking both AT1A and AT1B receptors for angiotensin II

The classically recognized functions of the renin-angiotensin system are mediated by type 1 (AT1) angiotensin receptors. Whereas man possesses a single AT1 receptor, there are two AT1 receptor isoforms in rodents (AT1A and AT1B) that are products of separate genes (Agtr1a and Agtr1b). We have generated mice lacking AT1B (Agtr1b -/-) and both AT1A and AT1B receptors (Agtr1a -/-Agtr1b -/-). Agtr1b -/- mice are healthy, without an abnormal phenotype. In contrast, Agtr1a -/-Agtr1b -/- mice have diminished growth, vascular thickening within the kidney, and atrophy of the inner renal medulla. This phenotype is virtually identical to that seen in angiotensinogen-deficient (Agt-/-) and angiotensin-converting enzyme-deficient (Ace -/-) mice that are unable to synthesize angiotensin II. Agtr1a -/-Agtr1b -/- mice have no systemic pressor response to infusions of angiotensin II, but they respond normally to another vasoconstrictor, epinephrine. Blood pressure is reduced substantially in the Agtr1a -/- Agtr1b -/- mice and following administration of an angiotensin converting enzyme inhibitor, their blood pressure increases paradoxically. We suggest that this is a result of interruption of AT2-receptor signaling. In summary, our studies suggest that both AT1 receptors promote somatic growth and maintenance of normal kidney structure. The absence of either of the AT1 receptor isoforms alone can be compensated in varying degrees by the other isoform. These studies reaffirm and extend the importance of AT1 receptors to mediate physiological functions of the renin-angiotensin system.

Gene structure, expression pattern, and biological activity of mouse killer cell activating receptor-associated protein (KARAP)/DAP-12

Natural killer cell and T cell subsets express at their cell surface a repertoire of receptors for MHC class I molecules, the natural killer cell receptors (NKRs). NKRs are characterized by the existence of inhibitory and activating isoforms, which are encoded by highly homologous but separate genes present in the same locus. Inhibitory isoforms express an intracytoplasmic immunoreceptor tyrosine-based inhibition motif, whereas activating isoforms lack any immunoreceptor tyrosine-based inhibition motif but harbor a charged amino acid residue in their transmembrane domain. We previously characterized KARAP (killer cell activating receptor-associated protein), a novel disulfide-linked tyrosine-phosphorylated dimer that selectively associates with the activating NKR isoforms. We report here the identification of the mouse KARAP gene, its localization on chromosome 7 and its genomic organization in five exons. Point mutation and transfection studies revealed that KARAP is a novel signaling transmembrane subunit whose transduction function depends on the integrity of an intracytoplasmic immunoreceptor tyrosine-based activation motif. In contrast to previous members of the immunoreceptor tyrosine-based activation motif polypeptide family, KARAP is ubiquitously expressed on hematopoietic and nonhematopoietic cells, suggesting its association with a broad range of activating receptors in a variety of tissues.

Gs protein-coupled serotonin receptors: receptor isoforms and functional differences

Three distinct mammalian Gs coupled serotonin receptor genes have been identified, 5-HT4, 5-ht6, and 5-HT7, which produce at least seven different functional receptors through alternative splicing. One of the chief questions facing workers in this area mirrors that confronting the serotonin receptor field as a whole: why so many subtypes? The answer to this question is made more elusive at present by two further considerations. First, there may well be additional Gs coupled receptor subtypes yet to be described. Secondly, although the various isoforms of 5-HT4 and 5-HT7 have been shown to be functional in in vitro assays, it remains to be shown that all isoforms have biological significance. This paper will summarize some of the differences at the molecular and cellular level that are becoming apparent among the 5-HT4, 5-ht6 and 5-HT7 receptor subtypes and their various isoforms. As an example, it will focus on the 5-HT7 system, and describe recent developments in ascribing particular functions to differences due to alternative splicing.

Transactivation-defective c-MycS retains the ability to regulate proliferation and apoptosis

Transcriptional activation by c-Myc through specific E box elements is thought to be essential for its biological role. However, c-MycS is unable to activate transcription through these elements and yet retains the ability to stimulate proliferation, induce anchorage-independent growth, and induce apoptosis. In addition, c-MycS retains the ability to repress transcription of several specific promoters. Furthermore, c-MycS can rescue the c-myc null phenotype in fibroblasts with homozygous deletion of c-myc. Taken together, our data argue against the paradigm that all of the biological functions of c-Myc are mediated by transcriptional activation of specific target genes through E box elements.

A big gene linked to small eyes encodes multiple Mitf isoforms: many promoters make light work

Among more than 80 different loci related to mouse coat color, microphthalmia-associated transcription factor (Mitf) encoded at the mouse microphthalmia locus is one of the most exciting molecules that regulates the development and survival of many cell types, including melanocyte, retinal pigment epithelium (RPE), and mast cells. Mitf and its human homolog MITF consist of at least three isoforms, referred to as Mitf-A/MITF-A, the heart-type Mitf-H/MITF-H, and the melanocyte lineage-specific Mitf-M/MITF-M, respectively. These isoforms differ in the amino-terminal domains but share a transactivation domain and a basic helix-loop-helix and leucine-zipper structure that is required for DNA binding and dimerization. MITF-M is exclusively expressed in melanocytes and melanoma cells, but not in other cell types, including RPE cells. In contrast, MITF-A mRNA is widely expressed in many cell types. These three isoform mRNAs are possibly generated by differential usage of the gene promoters and by alternative splicing. We predict that the entire MITF gene spans about 200 kb of DNA. Like MITF-M, MITF-A is able to activate the two melanogenesis gene promoters, tyrosinase and tyrosinase-related protein 1. These results suggest that melanogenesis may be regulated by different MITF isoforms in melanocyte and RPE. Possible implications of the multiplicity in Mitf/MITF isoforms are discussed.

Fibronectin and collagen I matrixes promote calcification of vascular cells in vitro, whereas collagen IV matrix is inhibitory

Vascular calcification is a frequent component of atherosclerosis, yet the pathological mechanisms that regulate its formation are poorly understood. Calcification of the vessel wall may represent a process by which cells that normally exhibit a smooth muscle phenotype differentiate into cells that exhibit an osteoblast-like phenotype. One of the determinants of cellular phenotype is extracellular matrix; thus, we undertook the current study to evaluate the influence of extracellular matrix on calcification of vascular cells in vitro. Cell lines derived from bovine aortic media were divided into 1 of 3 groups: those that did not mineralize, those that mineralized slowly, or those that mineralized rapidly. When slowly mineralizing cells were plated onto matrix produced by rapidly mineralizing cells, the time required for mineralization decreased from 33+/-3.0 days to 7.8+/-1.3 days. Matrix produced by rapidly mineralizing cells was found to contain 3 times the amount of collagen I and fibronectin but 70% less collagen IV than nonmineralizing clones. When slowly mineralizing cells were cultured on purified collagen I or fibronectin, mineralized nodule formation, calcium incorporation, von Kossa staining, and alkaline phosphatase activity increased. In contrast, culturing slowly mineralizing cells on purified collagen IV inhibited these mineralization parameters. Furthermore, blocking antibodies to alpha5 integrins significantly inhibited the fibronectin-mediated increases in alkaline phosphatase activity, indicating that integrin-based signaling may be involved. These data suggest that matrix composition can regulate development of arterial calcification and that a subpopulation of vascular cells preferentially produces positively regulating matrix components.

Na+-dependent phosphate cotransporters: the NaPi protein families

In vertebrates, the level of inorganic phosphate (Pi) is tightly balanced both inside the cell and in the whole organism. A number of different Na+-dependent Pi cotransport systems involved in Pi homeostasis have been identified and characterized at the molecular level in the past 7 years. The transporters constitute three different protein families denoted NaPi-I, NaPi-II and NaPi-III. NaPi-I from the rabbit was the first member of this family to be cloned. However, it still resists efforts to unravel its physiological role and a clear-cut functional identity: is it a Cl-channel, a Na+/Pi cotransporter, a regulator, or does it perform a combination of these functions? These questions provide a slight taste of the problems associated with orphan genes derived from sequencing projects. The members of the NaPi-II protein family are crucially involved in tightly controlled renal Pi excretion and, as recently discovered, intestinal Pi absorption. The expression and the cellular distribution of NaPi-II in the proximal tubular epithelium are affected by hormonal and metabolic factors known to influence extracellular fluid Pi homeostasis. Recently, the expression of NaPi-II has been demonstrated in osteoclasts and brain;however, the physiological roles of NaPi-II in these tissues remain to be established. The members of the third protein family, NaPi-III, have been identified on the basis of their function as viral receptors. The widespread expression of this family suggests that NaPi-III is involved in supplying the basic cellular metabolic needs for Pi.

[Cadherins, the development and regeneration of the neuromuscular axis]

Various cell adhesion molecules of the cadherin family characterize the neuromuscular system. During development, cadherins N and M are sequentially expressed by myogenic cells during the two waves of myoblast fusion. Two other cadherins, called 6 and 11, are also expressed during the embryonic musculature development. In adult muscle, cadherins N and M, whose expression is suppressed by muscle activity, persist only at the neuromuscular junction and are reexpressed at the surface of denervated fibers. Cadherins N, M and E are also expressed in adult peripheral nerves. Their differential localization at Schmidt-Lanterman clefts, Ranvier nodes and neuromuscular junctions suggest that these molecules contribute to the stabilization of specialized intercellular contacts. In conclusion, a combination of cadherins, the expression of which is spatially and temporally regulated, participates in the differentiation and maintenance of the organization of the various cellular and tissular components of the neuromuscular system.

Signaling mechanisms underlying muscarinic receptor-mediated increase in contraction rate in cultured heart cells

We have investigated the mechanisms by which stimulation of cardiac muscarinic receptors result in paradoxical stimulatory effects on cardiac function, using cultured neonatal rat ventricular myocytes as a model system. Application of low concentrations of carbachol (CCh) (EC50 = 35 nM) produced an atropine-sensitive decrease in spontaneous contraction rate, while, in cells pretreated with pertussis toxin, higher concentrations of CCh (EC50 = 26 microM) elicited an atropine-sensitive increase in contraction rate. Oxotremorine, an m2 muscarinic acetylcholine receptor (mAChR) agonist, mimicked the negative but not the positive chronotropic response to CCh. Reverse transcription followed by polymerase chain reaction carried out on mRNA obtained from single cells indicated that ventricular myocytes express mRNA for the m1, m2, and, possibly, m4 mAChRs. The presence of m1 and m2 mAChR protein on the surface membranes of the cultured ventricular myocytes was confirmed by immunofluorescence. The CCh-induced positive chronotropic response was significantly inhibited by fluorescein-tagged antisense oligonucleotides directed against the m1, but not the m2 and m4, mAChR subtypes. The response was also inhibited by antisense oligonucleotides against Gqalpha protein. Finally, inhibition of CCh-induced phosphoinositide hydrolysis with 500 microM neomycin or 5 microM U73122 completely abolished the CCh-induced positive chronotropic response. These results are consistent with the stimulatory effects of mAChR activation on the rate of contractions in cultured ventricular myocytes being mediated through the m1 mAChR coupled through Gq to phospholipase C-induced phosphoinositide hydrolysis.

Complexity of trypanosomatid endocytosis pathways revealed by Rab4 and Rab5 isoforms in Trypanosoma brucei

Small G proteins of the Rab family are responsible for vesicle fusion and control flux during intracellular transport. Rab5 is important in endosome maturation and Rab4 in recycling of endocytic material. Three Rab5 isoforms identified so far in mammals and three in the yeast genome suggest that conservation of multiple Rab5 isoforms is required for sophisticated regulation of endocytosis. Trypanosoma brucei homologues of Rab5 and Rab4 (TbRab5A and TbRab4) have been identified. Here we report cloning of a second Rab5 homologue, TbRab5Bp. The TbRAB5A and -5B genes are not linked in the genome, and phylogenetic reconstruction indicates that multiple Rab5 isoforms in yeast, mammals, and trypanosomes evolved independently. Northern blots demonstrate that TbRab5A, -5B, and TbRab4 messages are expressed in bloodstream form (BSF) and procyclic forms of the parasite even though endocytosis is not very active in the latter form. mRNA levels of TbRab5A and -4 are constitutive. Multiple-sized TbRab5B messages at very low abundance are detected, with greater expression in BSF. Also, the TbRab5B mRNA has a large 3'-untranslated region suggestive of potentially complex regulation, and therefore TbRab5Bp may be an important regulator of differential endocytosis levels between BSF and procyclic stage parasites. Affinity purified antibodies raised to C-terminal peptide sequences of all three TbRab proteins recognized small vesicular cytoplasmic structures, which for TbRab5Ap and -5Bp are predominantly near the flagellar pocket. TbRab5Bp colocalizes with invariant surface glycoprotein 100 (ISG100), a protein entering the endocytotic pathway in BSF parasites, whereas in procyclic cells populations of vesicles stained with both TbRab5Ap and -5Bp substantially overlap; TbRab5 proteins are therefore components of the endocytotic pathway. TbRab4p localizes to vesicular structures throughout the cytoplasm, with some overlap with TbRab5Bp, but the majority occupying a different compartment to the TbRab5s. Therefore the trypanosome endosomal system has been functionally dissected for the first time; these reagents provide a unique opportunity for manipulation of the protozoan endosomal system to further our understanding of drug uptake mechanisms and virulence.

Characterization of a murine type II sodium-phosphate cotransporter expressed in mammalian small intestine

An isoform of the mammalian renal type II Na/Pi-cotransporter is described. Homology of this isoform to described mammalian and nonmammalian type II cotransporters is between 57 and 75%. Based on major diversities at the C terminus, the new isoform is designed as type IIb Na/Pi-cotransporter. Na/Pi-cotransport mediated by the type IIb cotransporter was studied in oocytes of Xenopus laevis. The results indicate that type IIb Na/Pi-cotransport is electrogenic and in contrast to the renal type II isoform of opposite pH dependence. Expression of type IIb mRNA was detected in various tissues, including small intestine. The type IIb protein was detected as a 108-kDa protein by Western blots using isolated small intestinal brush border membranes and by immunohistochemistry was localized at the luminal membrane of mouse enterocytes. Expression of the type IIb protein in the brush borders of enterocytes and transport characteristics suggest that the described type IIb Na/Pi-cotransporter represents a candidate for small intestinal apical Na/Pi-cotransport.

Cloning and expressions of three mammalian homologues of Drosophila slit suggest possible roles for Slit in the formation and maintenance of the nervous system

In Drosophila embryogenesis, the slit gene has been shown to play a critical role in CNS midline formation. However, no slit homologues have been reported in vertebrates. Here, we have identified mammalian homologues of the slit gene (human Slit-1, Slit-2, Slit-3, and rat Slit-1). Each Slit gene encodes a putative secreted protein, which contains conserved protein-protein interaction domains including leucine-rich repeats (LRR) and epidermal growth factor (EGF)-like motifs, like that of the Drosophila protein. Northern blot analysis revealed that the human Slit-1, -2, and -3 mRNAs are exclusively expressed in the brain, spinal cord, and thyroid, respectively. In situ hybridization studies indicated that the rat Slit-1 mRNA is specifically expressed in the neurons of fetal and adult forebrains. Our data suggest that Slit genes form an evolutionary conserved group in vertebrates and invertebrates, and that the mammalian Slit proteins may participate in the formation and maintenance of the nervous and endocrine systems by protein-protein interactions.

The laminins: role in intestinal morphogenesis and differentiation

Dynamic and reciprocal heterotypic cell interactions are crucial for intestinal morphogenesis and differentiation. This paper emphasizes the role of basement membrane molecules and in particular of laminins as potent mediators in this intercellular cross talk. Changes in the expression or localization of laminin isoforms or of integrins during development and cell migration strengthen the concept that heterogeneity in cell-matrix interactions could mediate distinct cell responses. A combination of genetic or biochemical approaches associated with in vitro models allows us to study the potential role of each laminin isoform in basement membrane assembly, cell migration, or cell differentiation.

Spatially restricted expression of pipe in the Drosophila egg chamber defines embryonic dorsal-ventral polarity

Expression of pipe in the somatic tissue of the Drosophila ovary is required for the formation of embryonic dorsal-ventral polarity. pipe, which encodes an enzyme similar to the glycosaminoglycan-modifying enzyme heparan sulfate 2-O-sulfotransferase, is expressed in a spatially restricted domain of follicle cells on the ventral side of the egg chamber. Mutations that affect follicular polarity correspondingly alter the spatial pattern of pipe expression. Directed expression of pipe in otherwise pipe mutant females restores embryonic lateral and ventral pattern elements and can orient the dorsal-ventral axis of the embryo. Thus, the localized expression of pipe and the spatially restricted modification of carbohydrate chains play pivotal roles in the mechanisms that establish embryonic pattern and integrate follicular and embryonic polarity.

Transport of ER vesicles on actin filaments in neurons by myosin V

Axoplasmic organelles in the giant axon of the squid have been shown to move on both actin filaments and microtubules and to switch between actin filaments and microtubules during fast axonal transport. The objectives of this investigation were to identify the specific classes of axoplasmic organelles that move on actin filaments and the myosin motors involved. We developed a procedure to isolate endoplasmic reticulum (ER) from extruded axoplasm and to reconstitute its movement in vitro. The isolated ER vesicles moved on exogenous actin filaments adsorbed to coverslips in an ATP-dependent manner without the addition of soluble factors. Therefore myosin was tightly bound and not extracted during isolation. These vesicles were identified as smooth ER by use of an antibody to an ER-resident protein, ERcalcistorin/protein disulfide isomerase (EcaSt/PDI). Furthermore, an antibody to squid myosin V was used in immunogold EM studies to show that myosin V localized to these vesicles. The antibody was generated to a squid brain myosin (p196) that was classified as myosin V based on comparisons of amino acid sequences of tryptic peptides of this myosin with those of other known members of the myosin V family. Dual labeling with the squid myosin V antibody and a kinesin heavy chain antibody showed that the two motors colocalized on the same vesicles. Finally, antibody inhibition experiments were performed with two myosin V-specific antibodies to show that myosin V motor activity is required for transport of vesicles on actin filaments in axoplasm. One antibody was made to a peptide in the globular tail domain and the other to the globular head fragment of myosin V. Both antibodies inhibited vesicle transport on actin filaments by greater than 90% compared to controls. These studies provide the first direct evidence that ER vesicles are transported on actin filaments by myosin V. These data confirm the role of actin filaments in fast axonal transport and provide support for the dual filament model of vesicle transport.

AF-2-dependent potentiation of CCAAT enhancer binding protein beta-mediated transcriptional activation by glucocorticoid receptor

We report glucocorticoid-dependent induction of transcription from the herpes simplex virus thymidine kinase gene promoter proximal regulatory region in the absence of glucocorticoid response elements and independent of the ability of glucocorticoid receptor (GR) to bind DNA. Examination of the thymidine kinase promoter localized glucocorticoid responsiveness to a binding site for CCAAT enhancer-binding proteins (C/EBPs). Further analysis indicated that GR specifically potentiated the induction of transcription by C/EBP beta, but not C/EBP alpha or delta, and that full induction could be obtained by the ligand-binding domain (LBD) of GR alone. C/EBP beta, but not C/EBP alpha or delta, reciprocally potentiated transcriptional activation by DNA-bound GR LBD. However, C/EBP beta was unable to increase activation by a GR LBD with a short C-terminal truncation, indicating that the functional interaction between the two factors was dependent upon the GR AF-2. Surprisingly, despite the specificity in functional effects, all three C/EBPs bound indistinguishably to GR in GST pull-down and immunoprecipitation assays. Indeed, several nuclear receptors, including the estrogen (ER alpha), progesterone, retinoic acid (RAR), and androgen receptors, displayed a similar potential to bind C/EBPs. Previous reports have demonstrated that ER alpha and RARs repress transcriptional activation by C/EBP beta in ways that were dependent on their related AF-2 functions. Therefore, the GR AF-2 may encode functional features that distinguish the transcriptional regulatory potential of GR from that of ER and RAR. Finally, C/EBP binding mapped to the GR DNA-binding domain, which was not required for functional interaction with C/EBP beta. Thus, the potentiation of C/EBP beta-mediated transcription by GR would appear to require the presence of an intermediary factor.

The plasma membrane calcium pump, its role and regulation: new complexities and possibilities

Significant progress has been achieved in elucidating the role of the plasma membrane Ca2(+)-ATPase in cellular Ca2+ homeostasis and physiology since the enzyme was first purified and physiology since the enzyme was first purified and cloned a number of years ago. The simple notion that the PM Ca2(+)-ATPase controls resting levels of [Ca2+]CYT has been challenged by the complexity arising from the finding of four major isoforms and splice variants of the Ca2+ pump, and the finding that these are differentially localized in various organs and subcellular regions. Furthermore, the isoforms exhibit differential sensitivities to Ca2+, calmodulin, ATP, and kinase-mediated phosphorylation. The latter pathways of regulation can give rise to activation or inhibition of the Ca2+ pump activity, depending on the kinase and the particular Ca2+ pump isoform. Significant progress is being made in elucidating subtle and more profound roles of the PM Ca2(+)-ATPase in the control of cellular function. Further understanding of these roles awaits new studies in both transfected cells and intact organelles, a process that will be greatly aided by the development of new and selective Ca2+ pump inhibitors.

Creating intracellular structural domains: spatial segregation of actin and tropomyosin isoforms in neurons

Actin microfilaments play a direct role in a variety of cell processes. Distinct populations of microfilaments are associated with different cellular compartments, such as growth cones, filipodia, stress fibers, and lamellipodia. It is becoming clear that these different populations are often composed of different isoforms of the two core microfilament components, actin and tropomyosin. This is particularly true in neurons, where actin and tropomyosin isoforms are segregated into different intracellular compartments which correspond to functionally distinct regions of the neuron. Developmental regulation of this isoform sorting suggests a specific role for some isoforms in growth and for others in stabilization of neuronal structure. This provides a mechanism by which a neuron can create and independently regulate intracellular domains composed of microfilaments with different functional properties.

Endothelin-1 induces production of the neutrophil chemotactic factor interleukin-8 by human brain-derived endothelial cells

Increased levels of endothelin-1 (Et-1), a potent vasoconstrictor, have been correlated with hypertension and neuronal damage in ischemic/reperfusion injury. The presence of polymorphonuclear cells (PMNs) in the brain has been shown to be directly responsible for this observed pathology. To address the question of whether Et-1 plays a role in this process, human brain-derived endothelial cells (CNS-ECs) were cultured with Et-1. The results demonstrate that Et-1 induces production of the neutrophil chemoattractant interleukin-8 (IL-8) twofold to threefold after 72 hours; mRNA was maximal after 1 hour of stimulation. Conditioned culture medium derived from Et-1-stimulated CNS-ECs induced a chemotactic response in the PMN migration assay. The inflammatory cytokines tumor necrosis factor-alpha (TNF) and IL-1beta functioned additively with Et-1 in increasing IL-8 production. In contrast, transforming growth factor-beta (TGF-beta), but not IL-10, completely abolished the effect of Et-1 on IL-8 production. However, Et-1 did not modulate intercellular adhesion molecule-1 (ICAM-1) expression. These data demonstrate that Et-1 may be a risk factor in ischemic/reperfusion injury by inducing increased levels of the neutrophil chemoattractant IL-8.

A conventional myosin motor drives neurite outgrowth

Neuritic outgrowth is a striking example of directed motility, powered through the actions of molecular motors. Members of the myosin superfamily of actin-associated motors have been implicated in this complex process. Although conventional myosin II is known to be present in neurons, where it is localized at the leading edge of growth cones and in the cell cortex close to the plasma membrane, its functional involvement in growth cone motility has remained unproven. Here, we show that antisense oligodeoxyribonucleotides, complementary to a specific isoform of conventional myosin (myosin IIB), attenuate filopodial extension whereas sense and scrambled control oligodeoxyribonucleotides have no effect. Attenuation is shown to be reversible, neurite outgrowth being restored after cessation of the antisense regimen. Myosin IIB mRNA was present during active neurite extension, but levels were minimal in phenotypically rounded cells before neurite outgrowth and message levels decreased during antisense treatment. By contrast, the myosin IIA isoform is shown to be expressed constitutively both before and during neurite outgrowth and throughout exposure to myosin IIB antisense oligodeoxyribonucleotides. These results provide direct evidence that a conventional two-headed myosin is required for growth cone motility and is responsible, at least in part, for driving neuritic process outgrowth.

Laminin isoforms and epithelial development

Several different approaches suggest that basement-membrane assembly is important for epithelial development. Basement membranes contain isoforms of collagen IV, proteoglycans, and noncollagenous glycoproteins such as the laminins and nidogens. The expression and role of laminins for epithelial morphogenesis is reviewed. Laminins are large heterotrimeric proteins composed of alpha, beta, and gamma chains. Many major epithelial laminins and their receptors have been identified recently, and the extracellular protein-protein interactions that drive basement-membrane assembly are beginning to be understood. Three laminin alpha-chains are typically made by epithelial, alpha 1, alpha 3, and alpha 5. Three major epithelial heterotrimers can at present be distinguished--laminin-1 (alpha 1 beta 1 gamma 1), laminin-5 (alpha 3 beta 3 gamma 2), and laminin-10 (alpha 5 beta 1 gamma 1)--but other heterotrimers may exist in epithelia. Laminins containing either alpha 1 or alpha 3 chains are largely limited to epithelia, whereas the alpha 5 is also found in endothelial and muscle basement membranes, particularly in the adult. Some epithelial cell types express several laminin alpha-chains, so it is relevant to test how the different laminins affect epithelial cells. Laminins interact with integrin type of receptors on the cell surface, but binding to other proteins has also recently been demonstrated. Two important recent discoveries are the identification of dystroglycan as a major laminin receptor in muscle and epithelia, and nidogen as a high-affinity laminin-binding protein important for basement-membrane assembly. Antibody perturbation experiments suggest that these protein-protein interactions are important for epithelial morphogenesis.

GABAA-receptor subtypes in developing brain. Actors or spectators?

Distinct GABAA-receptor subtypes, differing in subunit composition, physiology, and pharmacology, are expressed in fetal, neonatal, and adult brain. Their developmental schedule, evidenced by the differential maturation of the GABAA-receptor subunits alpha 1, alpha 2, and alpha 5, is similar in rodents and primates, indicating that the regulation of receptor subtypes is conserved across species. "Adult" GABAA-receptors, characterized by the alpha 1-subunit immunoreactivity, are largely absent from fetal brain. They appear, however, before the onset of functional inhibitory connections, suggesting that GABAA-receptors may play an active role in the formation of GABAergic synapses. In neocortex, the maturation of GABAA-receptor subtypes is governed by an intrinsic program, leading to an area- and lamina-specific distribution as early as E20 in rats. In primary somatosensory and visual areas, this pattern is influenced postnatally by the ingrowing thalamocortical projection, a process that can be prevented experimentally by lesioning the thalamus at birth. Altogether, the expression of GABAA-receptor subtypes in developing brain reflects the changing functional needs of neurons during differentiation, the formation of inhibitory circuits, and the emergence of functionally distinct brain compartments.

Differentially expressed GABAA-receptor subunits result in structurally and functionally receptor assemblies following excitatory afferent synaptic transmission

Cerebellar granule cells isolated from postnatal day 7 rat pups are ideal for studying epigenetic events associated with the regulation of neuronal gene expression. These cultures contain from 90 to 95% glutamatergic granule cells and express mRNAs encoding a variety of ionotropic and metabotropic glutamate receptors as well as virtually all of the GABAA-receptor subunit mRNAs to different extents. A unique feature of this culture system is that the neurons undergo time-dependent maturation changes in vitro that mimic many of the characteristics of these receptors occurring in vivo. Granule cell cultures in vitro require depolarizing concentrations of KCl for long-term growth and survival. Both N-methyl-D-aspartate (NMDA) and GABA have been reported to exert trophic actions on these cells replacing the requirement for maintaining the cultures in high KCl. Cerebellar granule cells maintained under different conditions in vitro can be induced to alter their patterns of maturation, as indicated by the different temporal changes in gene expression of receptor subunit mRNAs and proteins. The focus of the current studies is the effect of NMDA afferent synaptic signaling on the changes in mRNA content and functional properties of GABAA receptors and how this may relate to comparable changes shown to occur in vivo.

WT1 interacts with the splicing factor U2AF65 in an isoform-dependent manner and can be incorporated into spliceosomes

WT1 is essential for normal kidney development, and genetic alterations are associated with Wilms' tumor, Denys Drash (DDS), and Frasier syndromes. Although generally considered a transcription factor this study has revealed that WT1 interacts with an essential splicing factor, U2AF65, and associates with the splicing machinery. WT1 is alternatively spliced and isoforms that include three amino acids, KTS, show stronger interaction with U2AF65 in vitro and better colocalization with splicing factors in vivo. Interestingly a mutation associated with DDS enhanced both -KTS WT1 binding to U2AF65 and splicing-factor colocalization. These data illustrate the functional importance of WT1 isoforms and suggest that WT1 plays a role in pre-mRNA splicing.

Laminin-like proteins are differentially regulated during cerebellar development and stimulate granule cell neurite outgrowth in vitro

The basement membrane glycoprotein laminin-1 is a potent stimulator of neurite outgrowth. Although a variety of laminin isoforms have been described in recent years, the role of alternative laminin isoforms in neural development remains largely uncharacterized. We found that a polyclonal antibody raised against the alpha1, beta1, and gamma1 chains of laminin-1 and a monoclonal antibody raised against the alpha2 chain of laminin-2 detect immunoreactive material in neuronal cell bodies in the developing mouse cerebellum. In addition, laminin-1-like immunoreactivity was found in cell types throughout the cerebellum, but laminin-alpha2-like immunoreactivity was restricted to the Purkinje cells. Purified laminin-1 and laminin-2 stimulated neurite outgrowth in primary cultures of mouse cerebellar granule neurons to a similar extent, whereas the synthetic peptides tested appeared to be active only for cell adhesion and not for stimulation of neurite outgrowth. The E8 proteolytic fragment of laminin-1 contained full neurite outgrowth activity. The identity of laminins expressed in granule neurons was also examined by Western blotting; laminin-like complexes were associated with the cell and appeared to have novel compositions. These results suggest that laminin-like complexes play important roles in cerebellar development.

Functional characterization of T7 and T8 of human apolipoprotein (a)

Lipoprotein (a) [Lp(a)], a risk factor for coronary artery disease, is a LDL-like particle with apolipoprotein (a) [apo(a)] covalently linked to apolipoprotein B (apoB). Apo(a) has many repeats of kringle 4-like domain, classified as type 1 through type 10 (T1-T10). Deletion analysis was performed to define the functional modules of human apo(a). We found that T7 has an affinity for cell surfaces and is required for Lp(a) formation. Cell surface binding was inhibited by L-proline, KI = 4.7 +/- 3.6 mM (n=3). We also found that T8 has an affinity for subendothelial extracellular matrix (ECM). ECM binding was inhibited modestly by L-proline (KI = 6.1 +/- 1.9 mM, n=3), and more effectively by L-lysine (KI = 2.7 +/- 1.0 mM, n=3) and its analogue, 6-aminohexanoic acid (KI = 0.35 +/- 0.13 mM, n=3). These data point to T7 and T8 as important functional modules of apo(a).

Multiple isoforms of thyroid hormone receptor: an analysis of their relative contribution in mediating thyroid hormone action

Thyroid hormone is essential for normal development and maintaining metabolic homeostasis. In mediating the thyroid hormone action, the thyroid hormone receptor (TR) plays a key role. Almost one decade ago, the cloning of TR was achieved, revealing the existence of at least two genes, TR alpha and TR beta, which encode TR. From these genes several TR isoforms can be generated by alternative splicing. They are designated as TR alpha 1, TR alpha 2 (inactive form), TR beta 1 and TR beta 2. Since the discovery of these TR isoforms, many studies have attempted to demonstrate their relative contribution to mediate thyroid hormone in various tissues. The distinct tissue distribution and the ontogenic expression of the TR isoforms, and the fact that TR gene abnormalities associated with the syndrome of resistance to thyroid hormone (RTH) have been found only in the TR beta gene, indicate that products of TR alpha and TR beta have distinct roles. However, no direct evidence of the distinct roles of the TR isoforms has been shown. Gene knockouts of either TR isoform would provide important information to understanding their specific roles. In this review, the history of the TR isoform discovery and studies attempting to demonstrate the specific roles of TR isoforms are summarized, and recent reports dealing with knockouts of TR isoforms are comprehensively presented.

Characterization of the isoform-specific differences in the gating of neuronal and muscle sodium channels

Human heart (hH1), human skeletal muscle (hSkM1), and rat brain (rIIA) Na channels were expressed in cultured cells and the activation and inactivation of the whole-cell Na currents measured using the patch clamp technique. hH1 Na channels were found to activate and inactivate at more hyperpolarized voltages than hSkM1 and rIIA. The conductance versus voltage and steady state inactivation relationships have midpoints of -48 and -92 mV (hH1), -28 and -72 mV (hSkM1), and -22 and -61 mV (rIIA). At depolarized voltages, where Na channels predominately inactivate from the open state, the inactivation of hH1 is 2-fold slower than that of hSkM1 and rIIA. The recovery from fast inactivation of all three isoforms is well described by a single rapid component with time constants at -100 mV of 44 ms (hH1), 4.7 ms (hSkM1), and 7.6 ms (rIIA). After accounting for differences in voltage dependence, the kinetics of activation, inactivation, and recovery of hH1 were found to be generally slower than those of hSkM1 and rIIA. Modeling of Na channel gating at hyperpolarized voltages where the channel does not open suggests that the slow rate of recovery from inactivation of hH1 accounts for most of the differences in the steady-state inactivation of these Na channels.

Potential for pharmacology of ryanodine receptor/calcium release channels

Calcium release channels, known also as ryanodine receptors (RyRs), play an important role in Ca2+ signaling in muscle and nonmuscle cells by releasing Ca2+ from intracellular stores. Mammalian tissues express three different RyR isoforms comprising four 560-kDa (RyR polypeptide) and four 12-kDa (FK506 binding protein) subunits. The large protein complexes conduct monovalent and divalent cations and are capable of multiple interactions with other molecules. The latter include small diffusible endogenous effector molecules including Ca2+, Mg2+, adenine nucleotides, sufhydryl modifying reagents (glutathione, NO, and NO adducts) and lipid intermediates, and proteins such as protein kinases and phosphatases, calmodulin, immunophilins (FK506 binding proteins), and in skeletal muscle the dihydropyridine receptor. Because of their role in regulating intracellular Ca2+ levels and their multiple ligand interactions, RyRs constitute an important, potentially rich pharmacological target for controlling cellular functions. Exogenous effectors found to affect RyR function include ryanoids, toxins, xanthines, anthraquinones, phenol derivatives, adenosine and purinergic agonists and antagonists, NO donors, oxidizing reagents, dantrolene, local anesthetics, and polycationic reagents.

Functional comparison of two human monocyte chemotactic protein-2 isoforms, role of the amino-terminal pyroglutamic acid and processing by CD26/dipeptidyl peptidase IV

Human Monocyte Chemotactic Protein (MCP)-2 has originally been isolated from stimulated osteosarcoma cells as a chemokine coproduced with MCP-1 and MCP-3. Here, a 5'-end extended MCP-2 cDNA was cloned from a human testis cDNA library. It encoded a 76 residue MCP-2 protein, but differed from the reported bone marrow-derived MCP-2 cDNA sequence in codon 46, which coded for a Lys instead of a Gln. This MCP-2Lys46 variant, caused by a single nucleotide polymorphism (SNP), was biologically compared with MCP-2Gln46. The coding regions were subcloned into the bacterial expression vector pHEN1, and after transformation of Escherichia coli, the two MCP-2 protein variants were recovered from the periplasm. The recombinant proteins were purified to homogeneity by heparin-Sepharose affinity chromatography and reversed-phase HPLC. Edman degradation revealed a Gln residue at the NH2 terminus instead of a pGlu. To evaluate the influence of the cyclization, this Gln was chemically converted into pGlu in both MCP-2 variants. The conversion was confirmed by electrospray mass spectrometry. rMCP-2Gln46 and rMCP-2Lys46 and the NH2-terminal cyclic counterparts were tested on monocytic cells in calcium mobilization and chemotaxis assays. No significant difference in biological activity was observed between the rMCP-2Gln46 and rMCP-2Lys46 isoforms. However, for both MCP-2 variants the NH2-terminal pyroglutamate was shown to be essential for chemotaxis, but not for calcium mobilization. NH2-terminal truncation of rMCP-2Lys46 by the serine protease CD26/dipeptidyl peptidase IV (CD26/DPP IV) resulted in the cleavage of the NH2-terminal Gln-Pro dipeptide, whereas synthetic MCP-2 with an amino-terminal pGlu remained unaffected. CD26/DPP IV-clipped rMCP-2Lys46(3-76) was almost completely inactive in both chemotaxis and signaling assays. These observations indicate that the NH2-terminal pGlu in MCP-2 is necessary for chemotactic activity but also that it protects the protein against degradation by CD26/DPP IV.

Molecular cloning, water channel activity and tissue specific expression of two isoforms of radish vacuolar aquaporin

A major membrane intrinsic protein (VM23) in vacuoles of radish (Raphanus) tap root was investigated. The cDNAs for two isoforms of VM23, gamma- and delta-VM23, encode polypeptides of 253 and 248 amino acids, respectively. gamma- and delta-VM23 correspond to the gamma- and delta-TIP (tonoplast intrinsic protein) of Arabidopsis. The deduced amino acid sequences of the two VM23 isoforms were 60% identical. The amino-terminal sequence of gamma-VM23 showed agreement with the direct sequence of the purified VM23, suggesting that gamma-VM23 is the most abundant molecule among the VM23 isoforms. When mRNAs of gamma- and delta-VM23 were injected into Xenopus oocytes, the osmotic water permeability of oocytes increased 6-fold (60 to 200 microns s-1) of the control oocytes. The transcripts of both isoforms were detected in a high level in growing hypocotyls and young leaves, but delta-VM23 was not detected in seedling roots. Light illumination enhanced the transcription of two genes of VM23 in cotyledons and roots but suppressed their expression in hypocotyls the growth of which was inhibited by light. These findings suggest that the expression of VM23 is tightly related to cell elongation.

Modulation of MAP kinase signaling and growth characteristics by the overexpression of protein kinase C in NIH3T3 cells

This study was performed to examine effects of the overexpression of protein kinase C (PKC) isoforms (i.e., beta I, beta II, gamma, delta, eta, and zeta) on mitogen-activated protein (MAP) kinase (Erk-1 and -2) signaling and growth characteristics of NIH3T3 cells. Phorbol ester (PMA) activated endogenous and ectopically expressed PKC alpha, beta I, beta II, gamma, delta, epsilon, and eta. Overexpression of the examined PKC isoforms enhanced PMA-induced MAP kinase activation. Potentiation of MAP kinase activation was also observed upon stimulation of cells with platelet-derived growth factor (PDGF) although there was no indication for the activation PKC isoforms by PDGF. Inhibition of PKC blocked PMA- but not PDGF-induced MAP kinase activation. Thus, potentiation of PDGF-induced MAP kinase activation appears to be independent to PKC activity, while PMA-induced MAP kinase activation requires PKC activity. The ability of PKC isoforms to potentiate MAP kinase activation is not related to the growth characteristics of cells because individual PKC isoforms differentially regulated maximum density and proliferation of cells.

Deficits in memory tasks of mice with CREB mutations depend on gene dosage

Studies in Aplysia, Drosophila, and mice have shown that the transcription factor CREB is involved in formation and retention of long-term memory. To analyze the impact of differential CREB levels on learning and memory, we varied the gene dosage of CREB in two strains of mutant mice: (1) CREBalphadelta mice, in which the alpha and delta isoforms are disrupted, but a third isoform beta is strongly up-regulated; (2) CREBcomp, a compound strain with one alphadelta allele and one CREBnull allele in which all CREB isoforms are disrupted. To minimize genetic background effects, CREB mutations were backcrossed into a C57BL/6 and a FVB/N strain, respectively, and studies were performed in F1 hybrids from these lines. CREBcomp but not CREBalphadelta F1 hybrids were impaired in water maze learning and fear conditioning, demonstrating a CREB gene dosage effect. However, analysis of the platform searching strategies in the water maze task suggested that CREBcomp mutants are impaired in behavioral flexibility rather than in spatial memory. In contrast to previous experiments using CREBalphadelta mice with different genetic background, the F1 hybrid CREBalphadelta and CREBcomp mice did not show deficits in a social transmission of food preference task nor in dentate gyrus and CA1 LTP as recorded from slice preparations. These data indicate that the hybrid vigor typical for F1 hybrids may compensate for a reduction in CREB levels in some tests. On the other hand, the persistence of clear behavioral deficits as shown by the F1 hybrid CREBcomp mice in water maze and fear conditioning indicates a robust and repeatable phenomenon that will permit further functional analysis of CREB.

Suppression of neoplastic transformation and regulation of cytoskeleton by tropomyosins

Down regulation of Tropomyosins (TMs) is a consistent biochemical change observed in many transformed cells. Our previous work has demonstrated that Tropomyosin-1 is an antioncogene and it is a class II tumor suppressor. Using ras-transformed murine fibroblasts (DT cells), we have examined the effects of co-expression of two isoforms of TM on cell morphology, cytoskeleton and tumorigenecity. Enhanced expression of TM1, a suppressor of transformation, along with TM2 which is not a tumor suppressor results in the formation of well-organized microfilaments, a morphology that resembles normal fibroblasts, and suppression of tumorigenecity. Tumor formation in vivo was compatible with the persistence of high-level of TM2, but not TM1. Homodimers of TM1 and TM2 were observed in these cells. Thus, restoration of expression of TM1 and TM2 protein in ras-transformed cells suppresses the transformed phenotype with dramatic re-organization of microfilaments. These data show that TM2 cooperates with TM1 in the reorganization of microfilaments, while TM1 is a suppressor of the transformed phenotype.

The advent of highly selective inhibitors of cyclooxygenase--a review

Cyclooxygenase (COX) exists in two isoforms, COX-1 and COX-2, COX-1 is present and is constitutively expressed in most cells and tissues, whereas COX-2 is felt to principally mediate inflammation. However, this distinction appears to be challenged by recent observations. This review addresses the roles of COX-1 and COX-2 isoforms in physiologic and pathophysiologic states and reviews potential therapeutic roles for selective COX inhibitors.

The diversity of GABAA receptors. Pharmacological and electrophysiological properties of GABAA channel subtypes

The amino acid gamma-aminobutyric-acid (GABA) prevails in the CNS as an inhibitory neurotransmitter that mediates most of its effects through fast GABA-gated Cl(-)-channels (GABAAR). Molecular biology uncovered the complex subunit architecture of this receptor channel, in which a pentameric assembly derived from five of at least 17 mammalian subunits, grouped in the six classes alpha, beta, gamma, delta, sigma and epsilon, permits a vast number of putative receptor isoforms. The subunit composition of a particular receptor determines the specific effects of allosterical modulators of the GABAARs like benzodiazepines (BZs), barbiturates, steroids, some convulsants, polyvalent cations, and ethanol. To understand the physiology and diversity of GABAARs, the native isoforms have to be identified by their localization in the brain and by their pharmacology. In heterologous expression systems, channels require the presence of alpha, beta, and gamma subunits in order to mimic the full repertoire of native receptor responses to drugs, with the BZ pharmacology being determined by the particular alpha and gamma subunit variants. Little is known about the functional properties of the beta, delta, and epsilon subunit classes and only a few receptor subtype-specific substances like loreclezole and furosemide are known that enable the identification of defined receptor subtypes. We will summarize the pharmacology of putative receptor isoforms and emphasize the characteristics of functional channels. Knowledge of the complex pharmacology of GABAARs might eventually enable site-directed drug design to further our understanding of GABA-related disorders and of the complex interaction of excitatory and inhibitory mechanisms in neuronal processing.

An alternative first exon in the distal end of the erythroid ankyrin gene leads to production of a small isoform containing an NH2-terminal membrane anchor

Mouse erythroid ankyrin is encoded by the Ank1 gene on Chromosome 8. The best studied isoform is 210 kDa and contains three large functional domains. We have recently reported a small Ank1 isoform (relative mobility 25 kDa) that localizes to the M and Z lines in skeletal muscle. Analyses of cDNA and genomic clones show that three transcripts of 3.5, 2.0, and 1.6 kb code for this protein. The different transcript sizes are due to their 3'-untranslated regions. They are encoded by a new first exon located in intron 39 of the Ank1 gene and three previously described Ank1 exons (40, 41, and 42). The 5'-flanking region contains a putative muscle-specific promoter. The sequence of the first 72 amino acids is novel and is predicted to form a transmembrane helix at the NH2-terminus. Functional testing of the putative transmembrane segment indicates that it acts as a membrane anchor, suggesting that the new Ank1 isoform may play an important role in organizing the contractile apparatus within the cell.

Steroid receptor isoforms: exception or rule?

Since the first steroid receptor was cloned, it was quickly identified as one of many such receptors constituting a gene superfamily which has grown to include not only steroid receptors but also receptors for thyroid hormone, retinoic acid, 1,25-dihydroxyvitamin D3 as well as a number of less traditional ligands, including farnesoids and fatty acids. Interestingly, these receptors are far outnumbered by the 'orphan' receptors for which ligands are still being sought. The original cloning of nuclear receptors, although sometimes identifying more than one receptor form, led to the general premise that each ligand has its cognate receptor through which signal is transduced to the transcriptional machinery. Regulation of this process was found to occur at the level of receptor expression, ligand availability, and more recently, through post-translational modifications of the receptor and interaction of a variety of coactivators/corepressors with the receptor protein. The continuing identification of more than a single form for many of the receptors directed the attention of a number of investigators toward defining possible roles for these 'extras'. This review examines the different forms of nuclear receptor gene family members and how they may provide an additional level of regulation.

Soluble Fas/Apo-1 splicing variants and apoptosis

In addition to the full length mRNA activated human peripheral blood mononuclear cells (PBMC) and T cell tumor lines express several alternatively spliced Fas variants. At least five of these code for soluble Fas (CD95) molecules. In vitro studies suggest that these soluble Fas isoforms inhibit apoptosis induced by agonistic antibodies and, more importantly, by the natural Fas ligand in Fas-bearing sensitive cells. Interestingly, this functional property can be assigned to the first 49 aminoacids of the mature protein, the only region shared by the soluble Fas molecules.

Functional heterogeneity of CD44 molecules in ovarian cancer cell lines

We have previously shown that CD44 partly mediates ovarian cancer cell attachment to peritoneal mesothelium through recognition of mesothelial-associated hyaluronate. CD44 is a major receptor for hyaluronate and exists as a standard 90-180-kDa form (CD44H), as well as several higher molecular mass variant forms produced by alternative splicing. To determine whether functional differences exist between CD44H and its variants we have investigated the relationship between CD44 isoform expression and mesothelial adhesion in 12 ovarian cancer cell lines. Eight lines were CD44 positive (range, 83-94%) and demonstrated significant binding to mesothelium and hyaluronate, whereas two lines showed reduced CD44 levels (3-13%) and demonstrated decreased binding. Interestingly, two other lines (OVC-3 and SW626) expressed CD44 in the majority of cells (>93%) and yet bound weakly to mesothelium. Mean linear fluorescence intensity of CD44 expressed by OVC-3 and SW626 cells was approximately one-half that of strongly binding cell lines, suggesting that the ability to adhere may be partly related to CD44 surface density. However, immunoprecipitation and immunoblot analyses revealed that standard CD44H represented only 23-31% of total CD44 in weakly binding cells, with the majority of species being comprised of CD44 variants. Indirect immunofluorescence of OVC-3 and SW626 cells confirmed the presence of CD44 variants containing exons v3, v6, and v9. In contrast, CD44H represented the majority (75-86%) of total CD44 expressed by strongly binding cell lines such as CAOV-3 and UPN36T. Transfection of CD44H cDNA into weakly binding OVC-3 cells restored significant mesothelial binding which was partly blocked by anti-CD44 antibody. These data suggest that the expression of CD44 is necessary but not sufficient for mediating attachment of ovarian cancer cells to mesothelium. Although CD44 variants may constitute the major CD44 species in certain ovarian cancer cell lines, it appears that these CD44 species are not always capable of mediating significant binding to mesothelium or hyaluronate. Rather, an adequate level of CD44H is the critical determinant of binding in this system. The role of CD44 variants in the process of ovarian cancer metastasis will require further investigation.