Molecular mechanism of immunoglobulin V-region diversification regulated by transcription and RNA metabolism in antigen-driven B cells.

The immune system produces specific antibodies (Ab) against any antigens (Ag) of exogenous and endogenous origins with a diverse repertoire of V-region specificities. The primary V-region repertoire is created by the rearrangement of immunoglobulin (Ig) V-region, D- and J-segments with the insertion of N- and P-sequences during early B cell differentiation. Recent studies revealed that secondary diversification of the IgV-region generated in the peripheral lymphoid organs plays a critical role in the generation of effective Ab production for protection from various pathogens. Naïve B cells that react with Ags initiate proliferation and differentiation in the follicular region and create the germinal centres (GCs), where activation-induced cytidine deaminase (AID)-dependent IgV-region somatic hypermutation (SHM) and class-switch recombination generate high-affinity and class-switched mature Ag-specific B cells. Our studies have discovered a 210-kDa nuclear protein, named GC-associated nuclear protein (GANP) that is up-regulated in GC B cells during the T cell-dependent (TD) immune responses. By studying mice with mutant forms of the ganp gene, we demonstrated that GANP is essential for the generation of high-affinity B cells against TD-Ag by affecting SHM at the IgV-regions. GANP is associated with AID in the cytoplasm and the GANP/AID complex is recruited to the nucleus, specifically, the chromatin, and targeted selectively to the IgV-region gene in B cells. GANP augments the access of AID towards IgV-regions in B cells. Here, we review the role of GANP in acquired immunity through the detailed analysis of the molecular mechanism generating SHM specifically at IgV-regions in B cells.

Thymus and autoimmunity. Transplantation of the thymus from cyclosporin A-treated mice causes organ-specific autoimmune disease in athymic nude mice.

Organ-specific autoimmune diseases such as gastritis, oophoritis, thyroiditis, or insulitis developed in athymic nu/nu mice after engraftment of the thymus from euthymic nu/+ mice treated with cyclosporin A (CsA), a potent immuno-suppressant. The development of autoimmune disease in the nu/nu mice was prevented by inoculation of thymocyte suspensions prepared from normal nu/+ mice, but not by thymocyte suspensions from CsA-treated nu/+ mice. Cotransplantation of normal nu/+ mouse thymus with CsA-treated thymus also suppressed the development of autoimmune disease. Inoculation of spleen cell suspensions prepared from normal adult nu/+ mice prevented autoimmune disease, but inoculation of those from newborn nu/+ mice did not. Thus, CsA appears to interfere selectively with the thymic production of certain suppressor T cells controlling self-reactive (autoimmune) T cells, allowing the latter to expand and cause autoimmune disease.

Thymus and autoimmunity: capacity of the normal thymus to produce pathogenic self-reactive T cells and conditions required for their induction of autoimmune disease.

BALB/c athymic nu/nu mice spontaneously developed organ-specific (gastritis, thyroiditis, oophoritis, or orchitis) and systemic (arteritis, glomerulonephritis, and polyarthritis) autoimmune diseases when transplanted with neonatal BALB/c thymuses. Transplantation of thymuses from adult BALB/c mice was far less effective in inducing histologically evident organ-specific autoimmune disease in nu/nu mice. Autoimmune disease developed, however, when adult thymuses were irradiated at a T cell-depleting dose before transplantation. Engrafting newborn thymuses into BALB/c mice T cell depleted by thymectomy, irradiation, and bone marrow transplantation produced similar organ-specific autoimmune disease as well, but thymus engrafting into T cell-nondepleted BALB/c mice (i.e., mice thymectomized as adults, but not irradiated) did not, despite the fact that transplanted thymuses grew well in both groups of mice. The mice with organ-specific autoimmune disease produced autoantibodies specific for the respective organ components, such as gastric parietal cells, thyroglobulins, oocytes, or sperm. The thymus-transplanted nu/nu mice also had hypergammaglobulinemia and developed anti-DNA autoantibodies, rheumatoid factors, and immune complexes in the circulation. These results indicate that: (a) the thymus of a murine strain that does not develop spontaneous autoimmune disease can produce pathogenic self-reactive T cells that mediate organ-specific and/or systemic autoimmune diseases; and (b) such self-reactive T cells, especially those mediating organ-specific autoimmune disease, spontaneously expand and cause autoimmune disease when released to the T cell-deficient or -eliminated periphery.

Autoimmune disease as a consequence of developmental abnormality of a T cell subpopulation.

Neonatal thymectomy (NTx), especially around day 3 after birth, causes various organ-specific autoimmune diseases in mice. This report shows that: (a) T cells expressing the interleukin 2 receptor alpha chains (CD25) ontogenically begin to appear in the normal periphery immediately after day 3, rapidly increasing within 2 wk to nearly adult levels (approximately 10% of CD3+ cells, especially of CD4+ cells); (b) NTx on day 3 eliminates CD25+ T cells from the periphery for several days; inoculation immediately after NTx of CD25+ splenic T cells from syngeneic non-Tx adult mice prevents autoimmune development, whereas inoculation of CD25- T cells even at a larger dose does not; and furthermore, (c) similar autoimmune diseases can be produced in adult athymic nu/nu mice by inoculating either spleen cell suspensions from 3-d-old euthymic nu/+ mice or CD25+ cell-depleted spleen cell suspensions from older, even 1-yr-old, nu/+ mice. The CD25- populations from neonates or adults are also similar in the profile of cytokine formation. These results, taken together, indicate that one aspect of peripheral self-tolerance is maintained by CD25+ T cells that sustain potentially pathogenic self-reactive T cells in a CD25- dormant state; the thymic production of the former is developmentally programmed to begin on day 3 after birth in mice. Thus, NTx on day 3 can, at least transiently, eliminate/reduce the autoimmune-preventive CD25+ T cells, thereby leading to activation of the self-reactive T cells that have been produced before NTx.

Immunologic self-tolerance maintained by CD25(+)CD4(+) regulatory T cells constitutively expressing cytotoxic T lymphocyte-associated antigen 4.

This report shows that cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) plays a key role in T cell-mediated dominant immunologic self-tolerance. In vivo blockade of CTLA-4 for a limited period in normal mice leads to spontaneous development of chronic organ-specific autoimmune diseases, which are immunopathologically similar to human counterparts. In normal naive mice, CTLA-4 is constitutively expressed on CD25(+)CD4(+) T cells, which constitute 5-10% of peripheral CD4(+) T cells. When the CD25(+)CD4(+) T cells are stimulated via the T cell receptor in vitro, they potently suppress antigen-specific and polyclonal activation and proliferation of other T cells, including CTLA-4-deficient T cells, and blockade of CTLA-4 abrogates the suppression. CD28-deficient CD25(+)CD4(+) T cells can also suppress normal T cells, indicating that CD28 is dispensable for activation of the regulatory T cells. Thus, the CD25(+)CD4(+) regulatory T cell population engaged in dominant self-tolerance may require CTLA-4 but not CD28 as a costimulatory molecule for its functional activation. Furthermore, interference with this role of CTLA-4 suffices to elicit autoimmune disease in otherwise normal animals, presumably through affecting CD25(+)CD4(+) T cell-mediated control of self-reactive T cells. This unique function of CTLA-4 could be exploited to potentiate T cell-mediated immunoregulation, and thereby to induce immunologic tolerance or to control autoimmunity.

B cell growth factors and B cell differentiation factor from human T hybridomas. Two distinct kinds of B cell growth factor and their synergism in B cell proliferation.

Human T hybridomas secreting B cell growth factors (BCGF) and B cell differentiation factor (BCDF) have been established. Hybrid clones 77-A, 94-C, and 98-F secreted BCGF that induced proliferation of anti-IgM-stimulated normal B cells. The culture supernatant from 77-A cells could also maintain continuous proliferation of colony-forming B cells, but the factor from 94-C could not. The addition of the supernatant from 94-C cells to that from 77-A cells, however, synergistically augmented the proliferation of colony-forming B cells, demonstrating the existence of two distinct kinds of BCGF and the synergism between them. These supernatants, however, showed no interleukin 2 (IL-2) or BCDF activity. A hybrid clone, 90-E, secreted BCDF. The culture supernatant induced Ig production in Cowan I-stimulated normal B cells or in a transformed B cell line, CESS. However, the supernatant had no BCGF or IL-2 activity. Anti-Ig-stimulated B cells, but not IL-2-dependent T cells, absorbed BCGF activity and CESS cells absorbed BCDF activity but not BCGF activity in the culture supernatants from T hybridomas. Taken collectively, the results demonstrated that IL-2, BCGF, and BCDF were different molecules and acceptors specific for the each molecule are present on the each target cell.

Activation mediated by RP105 but not CD40 makes normal B cells susceptible to anti-IgM-induced apoptosis: a role for Fc receptor coligation.

Signals through the B cell antigen receptor lead to a variety of cellular events such as activation, anergy, and apoptosis. B cells select these outcomes to establish and maintain self-tolerance, and to mount adequate antibody responses. However, it is not fully understood how one and the same signal causes such different consequences. In the present study, we have studied the effect of activation signals on the outcome of responses to antigen receptor ligation. Two distinct growth-promoting signals were used to activate B cells. Ligation of either RP105, a newly discovered B cell surface molecule, or the CD40 molecule, drove B cells to proliferate. Resultant blastic cells were then exposed to anti-immunoglobulin M (IgM). Blast cells that had been stimulated with anti-RP105 ceased growing and underwent apoptosis after cross-linking of surface IgM. Coligation of the Fc gamma receptor IIB with surface IgM augmented, rather than aborted, this response. In contrast to RP105-activated B cells, blast cells that had been activated by CD40 ligation were unaltered by anti-IgM. On the other hand, CD40-activated B cells became extremely susceptible to Fas-mediated apoptosis, whereas RP105-activated B cells were much less sensitive. Anti-IgM-induced apoptosis in RP105 blasts was independent of Fas, because it was demonstrable with Fas-deficient MRL-lpr/lpr mice. These results demonstrate that the nature of an initial activation signal has a great influence on the fate of activated B cells after (re)engagement of the antigen receptor. RP105, as well as CD40, may be important in this life/death decision.

Antiapoptotic function of 17AA(+)WT1 (Wilms' tumor gene) isoforms on the intrinsic apoptosis pathway.

The WT1 gene is overexpressed in human primary leukemia and a wide variety of solid cancers. The WT1 gene is alternatively spliced at two sites, yielding four isoforms: 17AA(+)KTS(+), 17AA(+)KTS(-), 17AA(-)KTS(+), and 17AA(-)KTS(-). Here, we showed that 17AA(+)WT1-specific siRNA induced apoptosis in three WT1-expressing leukemia cell lines (K562, HL-60, and Kasumi-1), but not in WT1-non-expressing lymphoma cell line (Daudi). 17AA(+)WT1-specific siRNA activated caspase-3 and -9 in the intrinsic apoptosis pathway but not caspase-8 in the extrinsic one. On the other hand, 17AA(-)WT1-specific siRNA did not induce apoptosis in the three WT1-expressing cell lines. The apoptosis was associated with activation of proapoptotic Bax, which was activated upstream of the mitochondria. Constitutive expression of 17AA(+)WT1 isoforms inhibited apoptosis of K562 leukemia cells induced by apoptosis-inducing agents, etoposide and doxorubicin, through the protection of mitochondrial membrane damages, and DNA-binding zinc-finger region of 17AA(+)WT1 isoform was essential for the antiapoptotic functions. We further studied the gene(s) whose expression was altered by the expression of 17AA(+)WT1 isoforms and showed that the expression of proapoptotic Bak was decreased by the expression of 17AA(+)KTS(-)WT1 isoform. Taken together, these results indicated that 17AA(+)WT1 isoforms played antiapoptotic roles at some points upstream of the mitochondria in the intrinsic apoptosis pathway.

Germinal Center B-Cell-Associated Nuclear Protein (GANP) Involved in RNA Metabolism for B Cell Maturation.

Germinal center B-cell-associated nuclear protein (GANP) is upregulated in germinal center B cells against T-cell-dependent antigens in mice and humans. In mice, GANP depletion in B cells impairs antibody affinity maturation. Conversely, its transgenic overexpression augments the generation of high-affinity antigen-specific B cells. GANP associates with AID in the cytoplasm, shepherds AID into the nucleus, and augments its access to the rearranged immunoglobulin (Ig) variable (V) region of the genome in B cells, thereby precipitating the somatic hypermutation of V region genes. GANP is also upregulated in human CD4(+) T cells and is associated with APOBEC3G (A3G). GANP interacts with A3G and escorts it to the virion cores to potentiate its antiretroviral activity by inactivating HIV-1 genomic cDNA. Thus, GANP is characterized as a cofactor associated with AID/APOBEC cytidine deaminase family molecules in generating diversity of the IgV region of the genome and genetic alterations of exogenously introduced viral targets. GANP, encoded by human chromosome 21, as well as its mouse equivalent on chromosome 10, contains a region homologous to Saccharomyces Sac3 that was characterized as a component of the transcription/export 2 (TREX-2) complex and was predicted to be involved in RNA export and metabolism in mammalian cells. The metabolism of RNA during its maturation, from the transcription site at the chromosome within the nucleus to the cytoplasmic translation apparatus, needs to be elaborated with regard to acquired and innate immunity. In this review, we summarize the current knowledge on GANP as a component of TREX-2 in mammalian cells.

Four subclones with distinct immunoglobulin light chain phenotypes. (kappa+lambda+, kappa+, lambda+ and kappa-lambda-) from acute leukemia.

A human acute lymphoblastic leukemia (ALL) cell line, BALM-9, was established from the peripheral blood specimen of a immunoglobin (lg) phenotype, the established BALM-9 cell line expressed both kappa and lambda light (L) chains simultaneously in a range of 30-80%. Two-color flow cytometric analysis demonstrated that there was a distinct population of kappa lambda double positive cells as well as kappa single, lambda single, and double negative populations. Therefore, subclones were obtained from each population by limiting dilution and were designated BALM-9KL (kappa+lambda+), BALM-9K (kappa+lambda-), BALM 9N (kappa-lambda-). Western blotting confirmed the results of the immunofluorescence test at the protein level. In BALM-9N, L chains were absent even in the cytoplasm as demonstrated by Western blotting. Evidence that the subclones have the same ancestry was provided both by cytogenetic analysis and by Southern blotting, which revealed the 14q32 chromosomal rearrangement as a common abnormality and the same IgH gene arrangement among the subclones. The existence of a kappa lambda positive B cell population suggests a transient stage of normal B cell maturation. These subclones might represent such a stage and thus provide a useful means of analyzing the mechanism of this double light chain expression.

A novel ALL-L3 cell line, BALM-16, lacking expression of immunoglobulin chains derived from a patient with hypercalcemia.

A human acute lymphoblastic leukemia (ALL) cell line, BALM-16, was established from the peripheral blood specimen of a patient with B cell ALL L3 type (ALL-L3) in relapse. As with the original leukemia cells, the established line was negative for both cell surface and cytoplasmic immunoglobulin (Ig) chains. Absence of Ig expression was confirmed by Western blotting. Southern blot analysis demonstrated homozygous deletion of the C kappa gene, germ line configuration of the C lambda and rearrangement of IgJH genes. Cytogenetic analysis of both leukemic bone marrow and BALM-16 cells showed the t(8;22)(q24;q11) abnormality which is specifically associated with ALL-L3 and Burkitt lymphoma. The patient's serum showed hypercalcemia, prompting further investigation of the established cell lines which showed parathyroid hormone-related peptide (PTHrP) mRNA detected by reverse-transcriptase polymerase chain reaction. However, PTHrP production was not detected in the culture supernatant. The established cell line, BALM-16, could provide a useful material for analyzing the lack of Ig expression and of clarifying the pathogenesis of this type of B cell malignancy.

Amla (Emblica officinalis Gaertn.) extracts reduce oxidative stress in streptozotocin-induced diabetic rats.

The antioxidant properties of amla extracts and their effects on the oxidative stress in streptozotocin-induced diabetes were examined in rats. Amla in the form of either the commercial enzymatic extract SunAmla (Taiyo Kagaku Co. Ltd., Yokkaichi, Japan) (20 or 40 mg/kg of body weight/day) or a polyphenol-rich fraction of ethyl acetate extract (10 or 20 mg/kg of body weight/day) was given orally for 20 days to the streptozotocin-induced diabetic rats. Amla extracts showed strong free radical scavenging activity. Amla also showed strong inhibition of the production of advanced glycosylated end products. The oral administration of amla extracts to the diabetic rats slightly improved body weight gain and also significantly alleviated various oxidative stress indices of the serum of the diabetic rats. The elevated serum levels of 5-hydroxymethylfurfural, which is a glycosylated protein that is an indicator of oxidative stress, were significantly reduced dose-dependently in the diabetic rats fed amla. Similarly, the serum level of creatinine, yet another oxidative stress parameter, was also reduced. Furthermore, thiobarbituric acid-reactive substances levels were significantly reduced with amla, indicating a reduction in lipid peroxidation. In addition, the decreased albumin levels in the diabetic rats were significantly improved with amla. Amla also significantly improved the serum adiponectin levels. These results form the scientific basis supporting the efficacy of amla for relieving the oxidative stress and improving glucose metabolism in diabetes.

Induction of 19S IgM secretion in a murine pre-B cell line, 70Z/3, by cell hybridization with non-secreting myeloma cells.

Somatic cell hybrids were prepared between the TK-deficient variant of murine pre-B cell line, 70Z/3, cells and the HGPRT-deficient variant of non-secreting myeloma cells. Several hybrid clones which secreted IgM but did not express surface IgM were isolated. LPS stimulation did not induce the expression of surface IgM. SDS-PAGE analysis indicated that the IgM secreted by one of the hybrid clones was a 19S pentamer and that the size of its mu-chains was the same as that of mu-chains from MOPC 104E myeloma IgM. Two-dimensional gel electrophoresis of biosynthetically labeled immunoglobulin showed that the same pattern was obtained with kappa-chains from two hybrid clones and from the LPS-induced 70Z/3 cells. The result showed that cell hybridization could induce L-chain synthesis in the pre-B cell line. Anti-idiotypic antibody against the secreted IgM was prepared and it was shown that the surface IgM expressed on all LPS-stimulated 70Z/3 cells bore the same idiotype. Those results indicated that the specificity commitment has already occurred in 70Z/3 cells.

Causes and mechanism of autoimmune disease: cyclosporin A as a probe for the investigation.

Organ-specific autoimmune disease can be elicited in rodents by manipulating the thymus/T cells. For example, elimination of a particular T-cell subset causes organ-specific autoimmune diseases, such as thyroiditis and gastritis, in otherwise normal mice. Environmental agents can cause similar autoimmune diseases by affecting the thymus/T cells. Cyclosporin A (CsA), a potent immunosuppressive drug, is an example. When a particular strain of newborn mice are daily administered with CsA for a limited period, they spontaneously develop organ-specific autoimmune disease, such as gastritis with anti-parietal cell autoantibodies, later in life. CsA abrogates the production of CD4+T cells and CD8+T cells in the thymus. Consequently, these T cells are substantially depleted from the peripheral lymphoid organs, especially when the drug is administered from the day of birth. The autoimmune disease is prevented when CsA-treated newborn mice are inoculated with splenic T cells from normal syngeneic adult mice. On the other hand, removal of the thymus immediately after neonatal CsA treatment produces autoimmune disease with a higher incidence and in a wider spectrum of organs, i.e., thyroiditis, sialoadenitis of the salivary gland, gastritis, insulitis of the endocrine pancreas, adrenalitis, oophoritis, or orchitis. Each autoimmune disease is accompanied by the development of circulating autoantibodies specific for the corresponding organ-specific antigens. These findings taken together indicate that CsA causes autoimmune disease not by affecting the target self-antigens, but by interfering with a thymus/T cell-dependent control mechanism on the production/expansion of pathogenic self-reactive T cells. Various other environmental insults (such as ionizing radiation or virus) can also cause similar autoimmune diseases, presumably by a similar mechanism.

The gene structure and promoter analysis of mouse lymphocyte signal transduction molecule alpha 4 that is related to the yeast TAP42 involved in a rapamycin-sensitive pathway.

The mouse alpha 4 phosphoprotein encoding a component associated with the B cell antigen receptor (BCR)-mediated signal transduction is suggested to be involved in a unique rapamycin-sensitive pathway. We studied the structure and the molecular mechanism of the expression of alpha 4 gene by isolating two phage clones, named #10 and #23, covering entire exons of the mouse alpha 4 gene. The alpha 4 gene is located within about 25 kb and composed of six exons. To analyze the regulation of alpha 4 gene expression, we determined the nucleotide sequence toward 2 kb upstream of the translation start site of the alpha 4 gene. The 5'-flanking region does not contain a typical TATA box or the initiation consensus sequence, but it contains a CCAAT box, E-boxes, and several DNA binding motifs such as c-Myc, c-Myb, and c-Ets. Transcription of the alpha 4 gene starts at four different sites, determined by primer extension analysis, that were surrounded by Y-rich sequences. We further characterized the functional promoter of the alpha 4 gene at the region between -263 and the transcription start site of alpha 4 gene by luciferase assay system and suggested that the 5' upstream region of alpha 4 gene contains the silencer element of MT repetitive sequence.

Structure, expression, and chromosomal localization of the human gene encoding a germinal center-associated nuclear protein (GANP) that associates with MCM3 involved in the initiation of DNA replication.

A 210kDa protein named GANP is upregulated in germinal center (GC)-B cells in the spleen of antigen-immunized mouse. We studied a human ganp gene (hganp) encoding a putative polypeptide of 1980 amino acids. The carboxyl-terminal 721-amino-acid sequence of hGANP is identical to Map80, that is presumably generated by alternative splicing of hganp/Map80 gene. The genomic segment carrying hganp and Map80 genes was isolated, and the chromosomal location was determined on 21q22.3. Northern blot analysis with RNAs from various organs demonstrated a single band of 7kb hganp mRNA, which suggests a preferential transcription of hganp gene from the hganp/Map80 locus. The hGANP expression was upregulated in GCs of the tonsil, as demonstrated by in-situ RNA hybridization and immunohistochemical analyses. The hGANP, with the domain (Map-box) capable of binding to MCM3 in B cells, might be involved in regulation of cell-cycle progression and DNA replication of GC-B cells.

Alpha4 protein as a common regulator of type 2A-related serine/threonine protein phosphatases.

The catalytic activity of the C subunit of serine/threonine phosphatase 2A is regulated by the association with A (PR65) and B subunits. It has been reported that the alpha4 protein, a yeast homolog of the Tap42 protein, binds the C subunit of serine/threonine phosphatase 2A and protein phosphatase 2A-related protein phosphatases such as protein phosphatase 4 and protein phosphatase 6. In the present study, we showed that alpha4 binds these three phosphatases and the association of alpha4 reduces the activities of these phosphatases in vitro. In contrast, PR65 binds to the C subunit of serine/threonine phosphatase 2A but not to protein phosphatase 4 and protein phosphatase 6. These results suggest that the alpha4 protein is a common regulator of the C subunit of serine/threonine phosphatase 2A and protein phosphatase 2A-related protein phosphatases.

Immunologic self tolerance maintained by T-cell-mediated control of self-reactive T cells: implications for autoimmunity and tumor immunity.

T-cell-mediated dominant control of self-reactive T cells is one mechanism for maintaining immunologic self tolerance. It also hampers the generation of immunity to autologous tumor cells. Abrogation of the control can evoke potent tumor immunity as well as autoimmunity in normal animals. This common regulatory mechanism for autoimmunity and tumor immunity can be exploited to devise a novel immunotherapy against cancer.

A case of von Recklinghausen's disease with bilateral pheochromocytoma-malignant peripheral nerve sheath tumors of the adrenal and gastrointestinal autonomic nerve tumors.

A 48-year-old man with neurofibromatosis type 1 presented with chest pain, paroxysmal hypertension, tachycardia, and progressive respiratory insufficiency. Clinical investigation displayed calcified tumors in the anterior mediastinum and pararenal region. Histological examination at autopsy revealed composite tumors consisting of pheochromocytoma and malignant peripheral nerve sheath tumor (MPNST) at two sites: the left adrenal gland and the region surrounding the inferior vena cava, probably corresponding to the right adrenal gland. The MPNST component showed a varied histological appearance, including hyalinized bands with polygonal cells, a cartilaginous and myxoid stroma, a hemangiopericytomatous architecture, and a fibrosarcomatous structure, which suggested osteosarcoma, chondrosarcoma, angiosarcoma, and fibrosarcoma, respectively. In addition, based on the ultrastructural findings, the gastrointestinal tract was involved with mesenchymal tumors showing neurogenic differentiation. These lesions suggest the divergent cellular differentiation of neural crest-derived cells to mesenchymal elements as well as neuroectodermal neoplasms.

Immunohistochemical localization of the Na-K-Cl co-transporter (NKCC1) in the gerbil inner ear.

We mapped the cellular and subcellular distribution of the Na-K-Cl co-transporter (NKCC) in the adult gerbil inner ear by immunostaining with a monoclonal antibody (MAb T4) generated against human colon NKCC. Heavy immunolabeling was seen in the basolateral plasma membrane of marginal cells in the stria vascularis and dark cells in the vestibular system. Subpopulations of fibrocytes in the cochlear spiral ligament and limbus and underlying the vestibular neurosensory epithelium also stained with moderate to strong intensity, apparently along their entire plasmalemma. Because MAb T4 recognizes both the basolateral secretory (NKCC1) and the apical absorptive (NKCC2) isoforms of the co-transporter, we employed reverse transcription and the polymerase chain reaction (RT-PCR) to explore isoform diversity in inner ear tissues. Using NKCC1 and NKCC2 isoform-specific PCR primers based on mouse and human sequences, only transcripts for NKCC1 were detected in the gerbil inner ear. The presence of abundant NKCC1 in the basolateral plasmalemma of strial marginal and vestibular dark cells confirms conclusions drawn from pharmacological and physiological data. The co-expression of NKCC1 and Na,K-ATPase in highly specialized subpopulations of cochlear and vestibular fibrocytes provides further evidence for their role in recycling K+ leaked or effluxed through hair cells into perilymph back to endolymph, as postulated in current models of inner ear ion homeostasis.