Transformation, migration and outcome of residual bodies in the seminiferous tubules of the rat testis.

Experiments were performed to study the transformation, migration and outcome of residual bodies (RBs) in the seminiferous tubules of the rat testes. One part of the testes from adult Sprague-Dawley rats was used to generate paraffin sections to observe RBs and RB precursors through specific staining, and the other part of the testes was used to generate ultrathin sections to observe RBs under a transmission electron microscope. Deep blue particles of different sizes were observed in some seminiferous tubules through specific staining for RBs and RB precursors. These particles first appeared in the seminiferous tubules at stage I of the spermatogenic cycle, and after spermiation, the particles travelled rapidly towards the deeper region of the seminiferous epithelium and soon appeared close to the basement membrane of the seminiferous tubule. All of the particles in the tubules disappeared at stage IX. Using transmission electron microscopy, components of different electron densities were observed in the RBs on the surface of the seminiferous epithelium, all of which gradually formed in the cytoplasm of spermatozoon in later stages of spermiogenesis. After the spermatozoa were released, the RBs in the epithelium travelled quickly to the edge of the tube and were gradually transformed into lipid inclusions. These lipid inclusions ultimately became lipidlike particles. The lipidlike particles were discharged into the interstitial tissue. RBs initiate their own digestive process before their formation during spermiation in the rat testes. After spermiation, the RBs transform into lipid inclusions and finally into lipidlike particles. These lipidlike particles can be eliminated from the seminiferous tubules.

Expression of ß-catenin in rheumatoid arthritis fibroblast-like synoviocytes.

OBJECTIVE: ß-Catenin is the key mediator of the Wnt signal and also a component of E-cadherin complexes at the intercellular adhering junction, which mediates cell-cell adhesion. We hypothesized that ß-catenin might be involved in the long-lasting changed phenotype of rheumatoid arthritis fibroblast-like synoviocytes (RA-FLS) and could play a role in the pathogenesis of RA. In this study we investigated the expression of ß-catenin in RA-FLS. METHODS: Synovial tissues were obtained during joint replacement surgery or arthroscopy from six patients with RA, six patients with osteoarthritis (OA), and six patients with joint trauma (Trauma group). Immunohistochemical analysis of ß-catenin was performed in the synovial tissues from the three groups. Synovial tissues from three patients in each group were selected at random for FLS isolation. Expression of ß-catenin in FLS from the three groups was evaluated at the protein level by western blotting and at the mRNA level by reverse transcription polymerase chain reaction (RT-PCR). RESULTS: Immunohistochemistry revealed that the expression of ß-catenin in synovial lining cells of the RA samples was significantly greater than that of the OA or trauma samples (p < 0.01). Western blotting and RT-PCR showed that ß-catenin expression was elevated in RA-FLS compared with that in OA-FLS or Trauma-FLS (p < 0.05) at the protein level but no difference was found at the mRNA level. CONCLUSIONS: Expression of ß-catenin is elevated in RA-FLS, not only in vitro but also in vivo. The increase is due to activation of Wnt/ß-catenin signalling. Wnt/ß-catenin signalling is activated in RA-FLS, and contributes to the stable activation of RA-FLS.

Massive deletion in AZFb/b+c and azoospermia with Sertoli cell only and/or maturation arrest.

Recent studies have revealed that AZF deletion in Y chromosome is the most common known molecular genetic cause of spermatogenetic failure leading to male infertility. Characteristics of AZFa, AZFb and AZFc deletions and their association with spermatogenic impairment have been reported in a large number of populations. However, the distributions of those larger deletions resulted from P5/proximal-P1, P5/distal-P1 and P4/distal-P1 recombinations are still unclear as the literature on their frequencies is limited, and the contribution of these deletions to spermatogenetic failure remain to be confirmed by population studies. In this study, we investigated such massive deletions in 387 idiopathic azoospermic, 269 oligozoospermic patients and 315 men with normal spermatogenesis using 21 AZFb/c specific sequence-tagged sites. As a result, nine uninterrupted massive deletions were observed exclusively in men with azoospermia caused by either Sertoli cell only (SCO) or maturation arrest (MA). Prevalence of the deletion was 2.33%, in which five deletions arose from non-allelic homologous recombination between the palindromes P5 and P1 and two between P4 and P1. In other two deletions, novel proximal breakpoints in the interval region between P4 and P3 were observed. Our findings strongly support that the massive deletions in the AZFb or AZFb+c regions are important genetic causes of SCO and/or MA resulting in azoospermia and, besides the P5/proximal-P1, P5/distal-P1 and P4/distal-P1 deletions there may be other massive deletions in these regions resulting in severe spermatogenic impairment.

Increased bleeding tendency and decreased susceptibility to thromboembolism in mice lacking the prostaglandin E receptor subtype EP(3).

BACKGROUND: Among the prostanoids, thromboxane (TX) A(2) is a potent stimulator of platelets, whereas prostaglandin (PG) I(2) inhibits their activation. The roles of PGE(2) in the regulation of platelet function have not been established, however, and the contribution of PGE(2) in hemostasis and thromboembolism is poorly understood. The present study was intended to clarify these roles of PGE(2) by using mice lacking the PGE(2) receptor subtype 3 (EP(3)(-/-) mice). METHODS AND RESULTS: Expression of mRNAs for EP(3) in murine platelets was confirmed by quantitative reverse transcription-polymerase chain reaction. PGE(2) and AE-248, a selective EP(3) agonist, showed concentration-dependent potentiation of platelet aggregation induced by U46619, a TXA(2) receptor agonist, although PGE(2) alone could not induce aggregation. PGE(2) and AE-248 increased cytosolic calcium ion concentration ([Ca(2+)](i)), and AE-248 inhibited the forskolin-induced increase in cytosolic cAMP concentration ([cAMP](i)), suggesting G(i) coupling of EP(3). The potentiating effects of PGE(2) and AE-248 on platelet aggregation along with their effects on [Ca(2+)](i) and [cAMP](i) were absent in EP(3)(-/-) mice. In vivo, the bleeding time was significantly prolonged in EP(3)(-/-) mice. Moreover, when mice were challenged intravenously with arachidonic acid, mortality and thrombus formation in the lung were significantly reduced in EP(3)(-/-) mice. CONCLUSIONS: - PGE(2) potentiated platelet aggregation induced by U46619 via EP(3) by increasing [Ca(2+)](i), decreasing [cAMP](i), or both. This potentiating action of PGE(2) via EP(3) is essential in mediating both physiological and pathological effects of PGE(2) in vivo.

Roles of prostaglandin I(2) and thromboxane A(2) in cardiac ischemia-reperfusion injury: a study using mice lacking their respective receptors.

BACKGROUND: Prostaglandin (PG) I(2) and thromboxane (TX) A(2), the most common prostanoids in the cardiovascular system, are produced abundantly during cardiac ischemia/reperfusion (I/R); their roles in I/R injury, however, remain undetermined. We intended to clarify these roles of PGI(2) and TXA(2) using mice lacking the PGI(2) receptor, IP(-/-) mice, or the TXA(2) receptor, TP(-/-) mice. METHODS AND RESULTS: The left anterior descending coronary artery was occluded for 1 hour and then reperfused for 24 hours. The size of myocardial infarct in IP(-/-) mice was significantly larger than that in wild-type mice, although the size of the area at risk was similar between the 2 groups of mice. In contrast, there was no such difference between TP(-/-) and wild-type mice. To further determine whether PGI(2) and TXA(2) act directly on the cardiac tissue or indirectly through their action on blood constituents, we perfused excised heart according to the Langendorff technique. The isolated heart was then subjected to global ischemia followed by reperfusion. In IP(-/-) mice, developed tension and coronary flow rate during reperfusion were significantly lower and release of creatine kinase was significantly higher than those in wild-type mice. There were no such differences, however, between TP(-/-) and wild-type mice. CONCLUSIONS: PGI(2), which was produced endogenously during cardiac I/R, exerts a protective effect on cardiomyocytes independent of its effects on platelets and neutrophils. In contrast, TXA(2) has little role in the cardiac I/R injury.

An engineered site for protein kinase C flanking the SV40 large T-antigen NLS confers phorbol ester-inducible nuclear import.

Nuclear import of simian virus SV40 large tumour antigen (T-ag) is enhanced by the protein kinase CK2 (CK2) site flanking the nuclear localisation sequence (NLS). We report here that replacement of this site with a consensus site for protein kinase C (PK-C) can alter the regulation of T-ag nuclear import and render it inducible by phorbol ester. Measurement of nuclear import kinetics using fluorescently labelled proteins and confocal laser scanning microscopy show that the introduced PK-C site is functional in enhancing T-ag nuclear import compared to a protein lacking the CK2 site. Treatment with the PK-C activator phorbol 12-myristate 13-acetate (PMA) further increases the level of maximal nuclear accumulation and the initial nuclear import rate. This engineered PMA-responsive NLS may have application in targeting of molecules of interest to the nucleus in response to agents stimulating PK-C.

Negative charge at the protein kinase CK2 site enhances recognition of the SV40 large T-antigen NLS by importin: effect of conformation.

SV40 large tumor-antigen (T-ag) nuclear import is enhanced by the protein kinase CK2 (CK2) site (Ser111Ser112) flanking the nuclear localization sequence (NLS). Here we use site-directed mutagenesis to examine the influence of negative charge and conformation at the site on T-ag nuclear import and recognition by the NLS-binding importin subunits. Negative charge through aspartic acid in place of Ser111 simulated CK2 phosphorylation in enhancing nuclear accumulation to levels well above those of proteins lacking a functional CK2 site. This was shown to be through enhancement of T-ag NLS recognition by importin using an ELISA-based assay. Asp112-substituted mutants containing proline at positions 109, 110 (wild-type position) or 111 were compared to assess the role of conformation at the CK2 site. Maximal nuclear import of the protein with Pro109 was lower than that of the Pro110 derivative, with the Pro111 variant even lower, these differences also being attributable to effects on importin binding. All results indicate a correlation of the initial nuclear import rate with the importin binding affinity, demonstrating that NLS recognition by importin is a key rate-determining step in nuclear import.

Characterization of prostanoid receptors mediating contraction of the gastric fundus and ileum: studies using mice deficient in prostanoid receptors.

Receptors mediating prostanoid-induced contractions of longitudinal sections of gastric fundus and ileum were characterized by using tissues obtained from mice deficient in each type and subtype of prostanoid receptors. The fundus and ileum from mice deficient in either EP(3) (EP(3)(-/-) mice), EP(1) (EP(1)(-/-) mice) and FP (FP(-/-) mice) all showed decreased contraction to PGE(2) compared to the tissues from wild-type mice, whereas contraction of the fundus slightly increased in EP(4)(-/-) mice. 17-phenyl-PGE(2) also showed decreased contraction of the fundus from EP(3)(-/-), EP(1)(-/-) and FP(-/-) mice. Sulprostone showed decreased contraction of the fundus from EP(3)(-/-) and FP(-/-) mice, and decreased contraction of the ileum to this compound was seen in tissues from EP(3)(-/-), EP(1)(-/-) and FP(-/-) mice. In DP(-/-) mice, sulprostone showed increased contraction. DI-004 and AE-248 caused the small but concentration-dependent contraction of both tissues, and these contractions were abolished in tissues obtained from EP(1)(-/-) and EP(3)(-/-) mice, respectively, but not affected in other mice. Contractions of both fundus and ileum to PGF(2)alpha was absent at lower concentrations (10(-9) to 10(-7) M), and suppressed at higher concentrations (10(-6) to 10(-5) M) of the agonist in the FP(-/-) mice. Suppression of the contractions at the higher PGF(2)alpha concentrations was also seen in the fundus from EP(3)(-/-), EP(1)(-/-) and TP(-/-) mice and in the ileum from EP(3)(-/-) and TP(-/-) mice. Contraction of the fundus to PGD(2) was significantly enhanced in DP(-/-) mice, and contractions of the fundus and ileum to this PG decreased in FP(-/-) and EP(3)(-/-) mice. Contractions of both tissues to I-BOP was absent at 10(-9) to 10(-7) M and much suppressed at higher concentrations in TP(-/-) mice. Slight suppression to this agonist was also observed in the tissues from EP(3)(-/-) mice. PGI(2) induced small relaxation of both tissues from wild-type mice. These relaxation reactions were much potentiated in EP(3)(-/-) mice. On the other hand, significant contraction to PGI(2) was observed in both tissues obtained from IP(-/-) mice. These results show that contractions of the fundus and ileum induced by each prostanoid agonist are mediated by actions of this agonist on multiple types of prostanoid receptors and in some cases modified by its action on relaxant receptors.

Differential effects of Ca2+ channel blockers on Ca2+ overload induced by lysophosphatidylcholine in cardiomyocytes.

The effects of Ca2+ channel blockers (verapamil, diltiazem, nicardipine, bepridil and flunarizine) on Ca2+ overload induced by lysophosphatidylcholine were examined in rat isolated cardiomyocytes. Addition of lysophosphatidylcholine (15 microM) produced Ca2+ overload as evidenced by a marked increase in the concentration of intracellular Ca2+ and hypercontracture of cells. Verapamil, flunarizine and bepridil concentration dependently inhibited the lysophosphatidylcholine-induced Ca2+ overload, whereas diltiazem and nicardipine did not. Lysophosphatidylcholine increased the release of creatine kinase, which was significantly attenuated by verapamil, flunarizine or bepridil (5 microM for each), but not by diltiazem or nicardipine (20 microM for each). Verapamil, flunarizine, bepridil (which attenuated the lysophosphatidylcholine-induced Ca2+ overload) and nicardipine (which did not) inhibited the veratridine-induced increase in the concentration of intracellular Na+ (indicated by the increase in fluorescence ratio of Na(+)-binding benzofuran isophthalate) and cell contracture, whereas diltiazem did not. These results suggest that verapamil, bepridil and flunarizine attenuate the Ca2+ overload induced by lysophosphatidylcholine, and that the Ca2+ channel blocking action of these drugs does not contribute substantially to this effect. The Na+ channel inhibition together with high lipophilicity of these drugs may be important for the attenuation of the lysophosphatidylcholine-induced Ca2+ overload.

Both D-cis- and L-cis-diltiazem attenuate hydrogen peroxide-induced derangements in rat hearts.

The effects of D-cis- and L-cis-diltiazem on the hydrogen peroxide (H2O2)-induced derangements of mechanical function and energy metabolism, and accumulation of intracellular Na+ were studied in isolated rat hearts. The intracellular concentration of Na+ ([Na+]i) in the myocardium was measured using a nuclear magnetic resonance technique. H2O2 (600 microM) increased the left ventricular end-diastolic pressure, decreased the tissue level of ATP, and increased the release of lactate dehydrogenase from the myocardium. These alterations induced by H2O2 were significantly attenuated by D-cis-diltiazem (15 microM) or L-cis-diltiazem (15 microM). H2O2 (1 mM) produced a marked increase in the myocardial [Na+]i, which was effectively inhibited by tetrodotoxin (3 microM), D-cis-diltiazem (15 microM) or L-cis-diltiazem (15 microM). These results suggest that both D-cis- and L-cis-diltiazem protect the myocardium against the H2O2-induced derangements in the isolated, perfused rat heart. The protective action of D-cis- and L-cis-diltiazem may be due to their ability to inhibit the H2O2-induced increase in [Na+]i, at least in part.

Lysophosphatidylcholine induces Ca2+-independent cellular injury attenuated by d-propranolol in rat cardiomyocytes.

In isolated rat cardiomyocytes, exogenous lysophosphatidylcholine (LPC) (15 microM) increased the intracellular Ca2+ concentration (Ca2+]i) from 72 +/- 5 to 3042 +/- 431 nM accompanied by cell injury as indicated by the hypercontracture of the cells and the increase in creatine phosphokinase (CPK) release. In order to understand whether the cell injury induced by LPC was a consequence of the elevation of [Ca2+]i, the effect of LPC was examined in the Ca2+-free solution containing EGTA. Under the Ca2+ -free conditions, LPC did not increase [Ca2+]i, whereas it still inflicted injury on the cells in terms of cell-shape change and CPK release to the same degree as that under the Ca2+-present condition. Addition of ryanodine (10 microM) failed to prevent the changes in cell-shape and CPK release induced by LPC under both Ca2+-free and Ca2+-present conditions. Preincubation of the myocytes with d-propranolol (50 microM) inhibited the LPC-induced changes in cell-shape and CPK release under both Ca2+ -free and Ca2+ -present conditions (p < 0.05). Our study provides clear evidence that the cellular injury induced by LPC could be independent of the increase in [Ca2+]i, and the Ca2+-independent cellular injury induced by LPC could be attenuated by d-propranolol, although the mechanism remains unknown.

A new approach to the development of anti-ischemic drugs: protective drugs against cell injury induced by lysophosphatidylcholine.

Recent studies have revealed that lysophosphatidylcholine (LPC) produces mechanical and metabolic derangements in perfused working rat hearts and Ca2+-overload in isolated cardiac myocytes. Thus, LPC possesses an ischemia-like effect on the heart. Therefore, a drug that possesses an anti-LPC action would protect or improve ischemia/reperfusion damage. We examined the effects of various anti-ischemic drugs on the Ca2+ overload induced by LPC. Our data suggest that a drug with high lipophilicity possesses a protective effect on cell injury induced by LPC, probably because of preservation of membrane integrity.

Inhibition of Na+ channel or Na+/H+ exchanger attenuates the hydrogen peroxide-induced derangements in isolated perfused rat heart.

The effect of tetrodotoxin, a specific inhibitor of the Na+ channel, and 5-(N,N-dimethyl)-amiloride, a specific inhibitor of the Na+/H+ exchanger, on the mechanical and metabolic derangements induced by hydrogen peroxide (H2O2) was studied in the isolated perfused rat heart. The isolated rat heart was perfused aerobically at a constant flow rate and driven electrically. H2O2 (600 microM) decreased the left ventricular developed pressure and increased the left ventricular end-diastolic pressure (i.e. mechanical dysfunction), decreased the tissue levels of adenosine triphosphate and adenosine diphosphate (i.e. metabolic derangement), and increased the tissue level of malondialdehyde (i.e. lipid peroxidation). These mechanical and metabolic derangements induced by H2O2 were significantly attenuated by tetrodotoxin (3 microM) or 5-(N,N-dimethyl)-amiloride (15 microM). Neither tetrodotoxin nor 5-(N,N-dimethyl)-amiloride modified the tissue malondialdehyde level, which was increased by H2O2. In the normal (H2O2-untreated) heart, neither tetrodotoxin nor 5-(N,N-dimethyl)-amiloride affected the mechanical function and energy metabolism. These results suggested that inhibition of the Na+ channel or Na+/H+ exchanger was effective in attenuating the H2O2-induced mechanical dysfunction and metabolic derangements in the isolated perfused rat heart.

[Analysis of patients of psoriasis coexisting with chylothorax].

Six patients diagnosed as Psoriasis with complication of chylothorax (3 of chylothorax, 3 of both chylothorax and chyloperitoneum, age from 21 to 50, male 4, female 2) were reported. All patients have a history of taking a chinese medicine named "complex Wulong powder" for treating psoriasis. All patients have not the history of trauma, operation, and the history of living in epidemic focus of filariasis. Their X-ray exam and CT exam of chest did not show lung lesion but pleural effusion. Chylothorax coexisting with psoriasis was not found in literature. This result suggests that using complex WuLong powder might be the cause of chylothorax. The mechanism was unknown.

[Primary pericardial mesothelioma: a clinicopathological analysis of 7 cases].

Primary pericardial mesotheliona is a clinical rarity. 7 cases were reported in this paper. There were no typical clinical symptoms and signs. It could not be diagnosed simply with chest film, ECG or echocardiagram and was usually misdiagnosed as constritive pericarditis, tuberculous pericarditis etc. In order to make correct diagnosis, some investigative diagnose methods such as pathological examination of pericardial fluid and pericardial biopsy, Gallium-67 scintigraphy, Ber-EP4 antibody and immunohistochemical procedures should be employed.

[Inhibitory effects of balsam pear on the mutagenic activity of cyclophosphamide in vivo].

Inhibitory effects of balsam pear on the mutagenic activity of cyclophosphamide were studied in the cells of bone marrow of mouse in vivo. It was found that balsam pear juice itself showed no effect on the incidence of sister chromatid exchanges (SCE) and of micronuclei (MN) on the cells of bone marrow. But balsam pear juice reduced the incidence of SCE (from 29.1 times/cell to 13.27-28.38 times/cell) and MN (from 60.0% to 27.0%-50.0%) induced by cyclophosphamide respectively. The inhibitory effects showed a dose-dependent relation, which was statistically significant.

[Anti-virus effect of aralosides].

The result showed that the anti-virus effect of aralosides on infections with poliovirus II, ECHO delta virus, adenovirus II, herpes simplex virus I, coxsackie B3 virus and coxsackie A16 virus was remarkable. Aralosides could inhibit the development of cytopathic effect (CPE) and protect cultural cells from being infected with the above viruses.

The protein kinase CK2 site (Ser111/112) enhances recognition of the simian virus 40 large T-antigen nuclear localization sequence by importin.

The mechanism by which phosphorylation regulates nuclear localization sequence (NLS)-dependent nuclear protein import is largely unclear. Whereas nuclear accumulation of SV40 large tumor antigen (T-ag) fusion proteins is completely dependent on the T-ag NLS (amino acids 126-132), the rate of nuclear import is increased 50-fold by amino acid residues 111-125 and in particular a site for the protein kinase CK2 (CK2) at serine 111/112. Because the first step of nuclear protein import involves the binding of the NLS by an NLS-receptor complex such as the importin 58/97 heterodimer, we established a novel enzyme-linked immunosorbent assay to test whether NLS recognition is influenced by amino acids amino-terminal to the NLS and the CK2 site. We found that recognition of the T-ag NLS by importin 58/97 was enhanced 10-fold in the presence of amino acid residues 111-125 and strongly dependent on importin 97. A T-ag fusion protein in which the spacer between the CK2 site and the NLS was decreased showed 30% reduced binding by importin 58/97. Maximal nuclear accumulation of this protein was reduced by more than 50%, indicating the physiological importance of the correctly positioned CK2 site. Phosphorylation by CK2 increased the T-ag NLS binding affinity for importin 58/97 by a further 40%. We conclude that flanking sequences and in particular phosphorylation at the CK2 site are mechanistically important in NLS recognition and represent the basis of their enhancement of T-ag nuclear import. This study thus represents the first elucidation of the mechanistic basis of the regulation of nuclear protein import through phosphorylation within a phosphorylation-regulated NLS.

SV40 large tumor antigen nuclear import is regulated by the double-stranded DNA-dependent protein kinase site (serine 120) flanking the nuclear localization sequence.

Nuclear localization sequence (NLS)-dependent nuclear import of SV40 large tumor antigen (T-Ag) fusion proteins is regulated by phosphorylation sites for casein kinase II (CKII) and the cyclin-dependent kinase Cdc2 amino-terminal to the NLS (amino acids 126-132). Between the T-Ag CKII and Cdc2 sites is a site (Ser120) for the double-stranded DNA-dependent protein kinase (dsDNA-PK), which we show here for the first time to play a role in regulating T-Ag nuclear import. We replaced Ser120 by aspartic acid or alanine using site-directed mutagenesis and assessed the effects on nuclear transport kinetics both in vivo (microinjected cells) and in vitro (mechanically perforated cells) in HTC rat hepatoma cells. Maximal nuclear accumulation of the Asp120 and Ala120 protein derivatives was approximately 40% and 70% reduced in vivo, respectively, compared with that of the wild type protein, and similarly reduced in vitro, although to a lesser extent. This implies that the dsDNA-PK site regulates the maximal level of nuclear accumulation, normally functioning to enhance T-Ag nuclear transport; the higher accumulation of the Asp120 protein compared with the Ala120 protein indicates that negative charge at the dsDNA-PK site is mechanistically important in regulating nuclear import. The Asp120 protein accumulated in the nucleus at a faster rate than the wild type protein, implying that phosphorylation at Ser120 may also regulate the nuclear import rate. CKII phosphorylation of the Asp120 protein in cytosol or by purified CKII was approximately 30% higher than that of the Ser120 and Ala120 proteins, while negative charge at the CKII site increased dsDNA-PK phosphorylation of Ser120 by approximately 80% compared with wild type, implying physical and functional interactions between the two phosphorylation sites. Quantitation of NLS recognition by the importin 58/97 subunits using an enzyme-linked immunosorbent assay indicated that while the Ala120 protein derivative had a binding affinity very similar to that of wild type, the Asp120 derivative showed 40% higher affinity. In vitro CKII phosphorylation increased importin binding by about 30% in all cases. These results imply that negative charge at the dsDNA-PK site may enhance nuclear import through increasing both NLS recognition by importin subunits, and phosphorylation at the CKII site, which itself also facilitates NLS recognition by importin 58/97.

Nuclear targeting signal recognition: a key control point in nuclear transport?

Recent progress indicates that there are multiple pathways of nucleocytoplasmic transport which involve specific targeting sequences, such as nuclear localization sequences (NLSs), and cytosolic receptor molecules of the importin/karyopherin superfamily which recognise and dock the NLS-containing proteins at the nuclear pore. This first step of nuclear import/export is of central importance, with the affinity of the importin-targeting sequence interaction a critical parameter in determining transport efficiency. Different importins possess distinct NLS-binding specificities, which allows the system to be modulated through differential expression of the importins themselves, as well as through competition between different importins for the same protein, and between different proteins for the same importin. The targeting sequence-importin interaction can also be influenced directly by phosphorylation increasing the affinity of the interaction with importins or by targeting sequence masking through phosphorylation or specific protein binding. Targeting sequence recognition thus appears to represent a key control point in the regulation of nuclear transport. BioEssays 22:532-544, 2000.