Advances toward structure-based drug discovery for inflammasome targets.

Inflammasome proteins play an important role in many diseases of high unmet need, making them attractive drug targets. However, drug discovery for inflammasome proteins has been challenging in part due to the difficulty in solving high-resolution structures using cryo-EM or crystallography. Recent advances in the structural biology of NLRP3 and NLRP1 have provided the first set of data that proves a promise for structure-based drug design for this important family of targets.

A human tissue-based functional assay platform to evaluate the immune function impact of small molecule inhibitors that target the immune system.

While the immune system is essential for the maintenance of the homeostasis, health and survival of humans, aberrant immune responses can lead to chronic inflammatory and autoimmune disorders. Pharmacological modulation of drug targets in the immune system to ameliorate disease also carry a risk of immunosuppression that could lead to adverse outcomes. Therefore, it is important to understand the 'immune fingerprint' of novel therapeutics as they relate to current and, clinically used immunological therapies to better understand their potential therapeutic benefit as well as immunosuppressive ability that might lead to adverse events such as infection risks and cancer. Since the mechanistic investigation of pharmacological modulators in a drug discovery setting is largely compound- and mechanism-centric but not comprehensive in terms of immune system impact, we developed a human tissue based functional assay platform to evaluate the impact of pharmacological modulators on a range of innate and adaptive immune functions. Here, we demonstrate that it is possible to generate a qualitative and quantitative immune system impact of pharmacological modulators, which might help better understand and predict the benefit-risk profiles of these compounds in the treatment of immune disorders.

Inhibition of αvß5 Integrin Attenuates Vascular Permeability and Protects against Renal Ischemia-Reperfusion Injury.

Ischemia-reperfusion injury (IRI) is a leading cause of AKI. This common clinical complication lacks effective therapies and can lead to the development of CKD. The αvß5 integrin may have an important role in acute injury, including septic shock and acute lung injury. To examine its function in AKI, we utilized a specific function-blocking antibody to inhibit αvß5 in a rat model of renal IRI. Pretreatment with this anti-αvß5 antibody significantly reduced serum creatinine levels, diminished renal damage detected by histopathologic evaluation, and decreased levels of injury biomarkers. Notably, therapeutic treatment with the αvß5 antibody 8 hours after IRI also provided protection from injury. Global gene expression profiling of post-ischemic kidneys showed that αvß5 inhibition affected established injury markers and induced pathway alterations previously shown to be protective. Intravital imaging of post-ischemic kidneys revealed reduced vascular leak with αvß5 antibody treatment. Immunostaining for αvß5 in the kidney detected evident expression in perivascular cells, with negligible expression in the endothelium. Studies in a three-dimensional microfluidics system identified a pericyte-dependent role for αvß5 in modulating vascular leak. Additional studies showed αvß5 functions in the adhesion and migration of kidney pericytes in vitro Initial studies monitoring renal blood flow after IRI did not find significant effects with αvß5 inhibition; however, future studies should explore the contribution of vasomotor effects. These studies identify a role for αvß5 in modulating injury-induced renal vascular leak, possibly through effects on pericyte adhesion and migration, and reveal αvß5 inhibition as a promising therapeutic strategy for AKI.

Inhibition of spleen tyrosine kinase attenuates IgE-mediated airway contraction and mediator release in human precision cut lung slices.

BACKGROUND AND PURPOSE: Asthma presents as a heterogeneous syndrome characterized by airway obstruction, inflammation and hyper-reactivity (AHR). Spleen tyrosine kinase (Syk) mediates allergen-induced mast cell degranulation, a central component of allergen-induced inflammation and AHR. However, the role of Syk in IgE-mediated constriction of human small airways remains unknown. In this study, we addressed whether selective inhibition of Syk attenuates IgE-mediated constriction and mast cell mediator release in human small airways. EXPERIMENTAL APPROACH: Human precision cut lung slices (hPCLS) ex vivo derived from non-asthmatic donors were incubated overnight with human IgE, dexamethasone, montelukast, antihistamines or a selective Syk inhibitor (SYKi). High-affinity IgE receptor (FcεRI) activation by anti-IgE cross-linking was performed, and constriction and mediator release measured. Airway constriction was normalized to that induced by maximal carbachol stimulation. Syk expression (determined by qPCR and immunoblot) was also evaluated in human primary airway smooth muscle (HASM) cells to determine whether Syk directly modulates HASM function. KEY RESULTS: While dexamethasone had little effect on FcεR-mediated contraction, montelukast or antihistamines partially attenuated the response. SYKi abolished anti-IgE-mediated contraction and suppressed the release of mast cell or basophil mediators from the IgE-treated hPCLS. In contrast, SYKi had little effect on the non-allergic contraction induced by carbachol. Syk mRNA and protein were undetectable in HASM cells. CONCLUSIONS AND IMPLICATIONS: A selective Syk inhibitor, but not corticosteroids, abolished FcεR-mediated contraction in human small airways ex vivo. The mechanism involved FcεRI receptor activation on mast cells or basophils that degranulate causing airway constriction, rather than direct actions on HASM.

Phenotypic responses of differentiated asthmatic human airway epithelial cultures to rhinovirus.

OBJECTIVES: Human airway epithelial cells are the principal target of human rhinovirus (HRV), a common cold pathogen that triggers the majority of asthma exacerbations. The objectives of this study were 1) to evaluate an in vitro air liquid interface cultured human airway epithelial cell model for HRV infection, and 2) to identify gene expression patterns associated with asthma intrinsically and/or after HRV infection using this model. METHODS: Air-liquid interface (ALI) human airway epithelial cell cultures were prepared from 6 asthmatic and 6 non-asthmatic donors. The effects of rhinovirus RV-A16 on ALI cultures were compared. Genome-wide gene expression changes in ALI cultures following HRV infection at 24 hours post exposure were further analyzed using RNA-seq technology. Cellular gene expression and cytokine/chemokine secretion were further evaluated by qPCR and a Luminex-based protein assay, respectively. MAIN RESULTS: ALI cultures were readily infected by HRV. RNA-seq analysis of HRV infected ALI cultures identified sets of genes associated with asthma specific viral responses. These genes are related to inflammatory pathways, epithelial structure and remodeling and cilium assembly and function, including those described previously (e.g. CCL5, CXCL10 and CX3CL1, MUC5AC, CDHR3), and novel ones that were identified for the first time in this study (e.g. CCRL1). CONCLUSIONS: ALI-cultured human airway epithelial cells challenged with HRV are a useful translational model for the study of HRV-induced responses in airway epithelial cells, given that gene expression profile using this model largely recapitulates some important patterns of gene responses in patients during clinical HRV infection. Furthermore, our data emphasize that both abnormal airway epithelial structure and inflammatory signaling are two important asthma signatures, which can be further exacerbated by HRV infection.

Immunoglobulin E signal inhibition during allergen ingestion leads to reversal of established food allergy and induction of regulatory T cells.

Immunoglobulin E (IgE) antibodies are known for triggering immediate hypersensitivity reactions such as food anaphylaxis. In this study, we tested whether they might additionally function to amplify nascent antibody and T helper 2 (Th2) cell-mediated responses to ingested proteins and whether blocking IgE would modify sensitization. By using mice harboring a disinhibited form of the IL-4 receptor, we developed an adjuvant-free model of peanut allergy. Mast cells and IgE were required for induction of antibody and Th2-cell-mediated responses to peanut ingestion and they impaired regulatory T (Treg) cell induction. Mast-cell-targeted genetic deletion of the FcεRI signaling kinase Syk or Syk blockade also prevented peanut sensitization. In mice with established allergy, Syk blockade facilitated desensitization and induction of Treg cells, which suppressed allergy when transferred to naive recipients. Our study suggests a key role for IgE in driving Th2 cell and IgE responses while suppressing Treg cells in food allergy.

Inhibition of spleen tyrosine kinase attenuates allergen-mediated airway constriction.

Spleen tyrosine kinase (SYK) is a key activator of signaling pathways downstream of multiple surface receptors implicated in asthma. SYK function has been extensively studied in mast cells downstream of the high-affinity IgE receptor, FcεR1. Preclinical studies have demonstrated a role for SYK in models of allergic inflammation, but a role in airway constriction has not been demonstrated. Here, we have used a potent and selective pharmacological inhibitor of SYK to determine the role of SYK in allergen-mediated inflammation and airway constriction in preclinical models. Attenuation of allergic airway responses was evaluated in a rat passive anaphylaxis model and rat and sheep inhaled allergen challenge models, as well as an ex vivo model of allergen-mediated airway constriction in rats and cynomolgus monkeys. Pharmacological inhibition of SYK dose-dependently blocked IgE-mediated tracheal plasma extravasation in rats. In a rat ovalbumin-sensitized airway challenge model, oral dosing with an SYK inhibitor led to a dose-dependent reduction in lung inflammatory cells. Ex vivo analysis of allergen-induced airway constriction in ovalbumin-sensitized brown Norway rats showed a complete attenuation with treatment of a SYK inhibitor, as well as a complete block of allergen-induced serotonin release. Similarly, allergen-mediated airway constriction was attenuated in ex vivo studies from nonhuman primate lungs. Intravenous administration of an SYK inhibitor attenuated both early- and late-phase allergen-induced increases in airway resistance in an Ascaris-sensitive sheep allergen challenge model. These data support a key role for SYK signaling in mediating allergic airway responses.

Gene expression profiling following NRF2 and KEAP1 siRNA knockdown in human lung fibroblasts identifies CCL11/Eotaxin-1 as a novel NRF2 regulated gene.

BACKGROUND: Oxidative Stress contributes to the pathogenesis of many diseases. The NRF2/KEAP1 axis is a key transcriptional regulator of the anti-oxidant response in cells. Nrf2 knockout mice have implicated this pathway in regulating inflammatory airway diseases such as asthma and COPD. To better understand the role the NRF2 pathway has on respiratory disease we have taken a novel approach to define NRF2 dependent gene expression in a relevant lung system. METHODS: Normal human lung fibroblasts were transfected with siRNA specific for NRF2 or KEAP1. Gene expression changes were measured at 30 and 48 hours using a custom Affymetrix Gene array. Changes in Eotaxin-1 gene expression and protein secretion were further measured under various inflammatory conditions with siRNAs and pharmacological tools. RESULTS: An anti-correlated gene set (inversely regulated by NRF2 and KEAP1 RNAi) that reflects specific NRF2 regulated genes was identified. Gene annotations show that NRF2-mediated oxidative stress response is the most significantly regulated pathway, followed by heme metabolism, metabolism of xenobiotics by Cytochrome P450 and O-glycan biosynthesis. Unexpectedly the key eosinophil chemokine Eotaxin-1/CCL11 was found to be up-regulated when NRF2 was inhibited and down-regulated when KEAP1 was inhibited. This transcriptional regulation leads to modulation of Eotaxin-1 secretion from human lung fibroblasts under basal and inflammatory conditions, and is specific to Eotaxin-1 as NRF2 or KEAP1 knockdown had no effect on the secretion of a set of other chemokines and cytokines. Furthermore, the known NRF2 small molecule activators CDDO and Sulphoraphane can also dose dependently inhibit Eotaxin-1 release from human lung fibroblasts. CONCLUSIONS: These data uncover a previously unknown role for NRF2 in regulating Eotaxin-1 expression and further the mechanistic understanding of this pathway in modulating inflammatory lung disease.

Systems analysis of eleven rodent disease models reveals an inflammatome signature and key drivers.

Common inflammatome gene signatures as well as disease-specific signatures were identified by analyzing 12 expression profiling data sets derived from 9 different tissues isolated from 11 rodent inflammatory disease models. The inflammatome signature significantly overlaps with known drug targets and co-expressed gene modules linked to metabolic disorders and cancer. A large proportion of genes in this signature are tightly connected in tissue-specific Bayesian networks (BNs) built from multiple independent mouse and human cohorts. Both the inflammatome signature and the corresponding consensus BNs are highly enriched for immune response-related genes supported as causal for adiposity, adipokine, diabetes, aortic lesion, bone, muscle, and cholesterol traits, suggesting the causal nature of the inflammatome for a variety of diseases. Integration of this inflammatome signature with the BNs uncovered 151 key drivers that appeared to be more biologically important than the non-drivers in terms of their impact on disease phenotypes. The identification of this inflammatome signature, its network architecture, and key drivers not only highlights the shared etiology but also pinpoints potential targets for intervention of various common diseases.

Discovery of MK-7246, a selective CRTH2 antagonist for the treatment of respiratory diseases.

In this manuscript we wish to report the discovery of MK-7246 (4), a potent and selective CRTH2 (DP2) antagonist. SAR studies leading to MK-7246 along with two synthetic sequences enabling the preparation of this novel class of CRTH2 antagonist are reported. Finally, the pharmacokinetic and metabolic profile of MK-7246 is disclosed.

Pharmacological characterization of MK-7246, a potent and selective CRTH2 (chemoattractant receptor-homologous molecule expressed on T-helper type 2 cells) antagonist.

The chemoattractant receptor-homologous molecule expressed on T-helper type 2 cells (CRTH2) is a G protein-coupled receptor that has been reported to modulate inflammatory responses in various rodent models of asthma, allergic rhinitis and atopic dermatitis. In this study, we describe the biological and pharmacological properties of {(7R)-7-[[(4-fluorophenyl)sulfonyl](methyl)amino]-6,7,8,9-tetrahydropyrido[1,2-a]indol-10-yl}acetic acid (MK-7246), a novel synthetic CRTH2 antagonist. We show that MK-7246 1) has high affinity for the human, monkey, dog, rat, and mouse CRTH2, 2) interacts with CRTH2 in a reversible manner, 3) exhibits high selectivity over all prostanoid receptors as well as 157 other receptors and enzymes, 4) acts as a full antagonist on recombinant and endogenously expressed CRTH2, 5) demonstrates good oral bioavailability and metabolic stability in various animal species, 6) yields ex vivo blockade of CRTH2 on eosinophils in monkeys and sheep, and 7) significantly blocks antigen-induced late-phase bronchoconstriction and airway hyper-responsiveness in sheep. MK-7246 represents a potent and selective tool to further investigate the in vivo function of CRTH2.

Identification of distinct plasma biomarker signatures in patients with rapid and slow declining forms of COPD.

Chronic obstructive pulmonary disease (COPD) is a prevalent pulmonary disease characterized by a progressive decline in lung function. The identification of biomarkers capable of predicting the rate of lung function decline or capable of giving an early read on drug efficacy in clinical trials would be very useful. The aim of this study was to identify plasma biomarkers capable of accurately distinguishing patients with COPD from healthy controls. Eighty-nine plasma markers in 40 COPD patients and 20 healthy smoker controls were analyzed. The COPD patients were divided into two subgroups, rapid and slow decliners based on their rate of lung function decline measured over 15 years. Univariate analysis revealed that 25 plasma markers were statistically different between rapid decliners and controls, 4 markers were different between slow decliners and controls, and 10 markers were different between rapid and slow decliners (p < 0.05). Multivariate analysis led to the identification of groups of plasma markers capable of distinguishing rapid decliners from controls (signature 1), slow decliners from controls (signature 2) and rapid from slow decliners (signature 3) with over 90% classification accuracy. Importantly, signature 1 was shown to be longitudinally stable using plasma samples taken a year later from a subset of patients. This study describes a novel set of plasma markers differentiating slow from rapid decline of lung function in COPD. If validated in distinct and larger cohorts, the signatures identified will have important implications in both disease diagnosis, as well as the clinical evaluation of new therapies.

Loss of PTEN attenuates the development of pathological hypertrophy and heart failure in response to biomechanical stress.

AIMS: The maladaptive response to biomechanical stress is a fundamental response in heart disease. Loss of the 3'-lipid phosphatase, phosphatase and tensin homolog deleted on chromosome ten (PTEN), is associated with increased phosphorylation of Akt/protein kinase B and glycogen synthase kinase-beta. We hypothesize that these key changes will halt the development of pathological hypertrophy and the progression to heart failure in response to pressure overload. METHODS AND RESULTS: In mice, muscle-specific knockout of PTEN, mckCRE-PTEN(flox/flox) (PTEN KO), resulted in basal hypertrophy and mild reduction in left ventricular (LV) systolic function. Male mice were subjected to aortic banding (AB) or sham operation. In contrast to mckCRE-PTEN(+/+) control mice, pressure overload in PTEN KO mice resulted in reduced pathological hypertrophy, less interstitial fibrosis, and reduced apoptosis with a marked preservation of LV function. Western blot analysis of mitogen-activated protein kinase (MAPK) signalling showed equivalent phosphorylation of extracellular signal-regulated kinase (ERK)1 and ERK2 with markedly reduced phosphorylation of jun N-terminal kinase (JNK)1 and JNK2, and p38 in PTEN KO mice subjected to AB. Loss of PTEN was associated with increased expression of the proangiogenic factors, vascular endothelial growth factor-A and angiopoietin-2, with preservation of the myocardial capillary density in response to pressure overload. Moreover, banded PTEN KO mice maintained the expression of several key metabolic genes that are known to be dysregulated in heart failure. In contrast, a subpressor dose of the G protein-coupled receptor (GPCR) agonist angiotensin II (Ang II) leads to increased pathological hypertrophy and MAPK activation in PTEN KO mice. CONCLUSION: Loss of PTEN prevents the development of maladaptive ventricular remodelling with preservation of angiogenesis and metabolic gene expression in response to pressure overload but not in response to the GPCR agonist, Ang II. Inhibition of PTEN signalling in the heart may represent a novel approach to slow the progression of heart failure in response to pathological biomechanical stress.

The use of strictly standardized mean difference for hit selection in primary RNA interference high-throughput screening experiments.

RNA interference (RNAi) high-throughput screening (HTS) has been hailed as the 2nd genomics wave following the 1st genomics wave of gene expression microarrays and single-nucleotide polymorphism discovery platforms. Following an RNAi HTS, the authors are interested in identifying short interfering RNA (siRNA) hits with large inhibition/activation effects. For hit selection, the z-score method and its variants are commonly used in primary RNAi HTS experiments. Recently, strictly standardized mean difference (SSMD) has been proposed to measure the siRNA effect represented by the magnitude of difference between an siRNA and a negative reference group. The links between SSMD and d+-probability offer a clear interpretation of siRNA effects from a probability perspective. Hence, SSMD can be used as a ranking metric for hit selection. In this article, the authors investigated both the SSMD-based testing process and the use of SSMD as a ranking metric for hit selection in 2 primary siRNA HTS experiments. The analysis results showed that, as a ranking metric, SSMD was more stable and reliable than percentage inhibition and led to more robust hit selection results. Using the SSMD -based testing method, the false-negative rate can more readily be obtained. More important, the use of the SSMD-based method can result in a reduction in both the false-negative and false-positive rates. The applications presented in this article demonstrate that the SSMD method addresses scientific questions and fills scientific needs better than both percentage inhibition and the commonly used z-score method for hit selection.

Mice with a targeted mutation of patched2 are viable but develop alopecia and epidermal hyperplasia.

Hedgehog (Hh) signaling plays pivotal roles in tissue patterning and development in Drosophila melanogaster and vertebrates. The Patched1 (Ptc1) gene, encoding the Hh receptor, is mutated in nevoid basal cell carcinoma syndrome, a human genetic disorder associated with developmental abnormalities and increased incidences of basal cell carcinoma (BCC) and medulloblastoma (MB). Ptc1 mutations also occur in sporadic forms of BCC and MB. Mutational studies with mice have verified that Ptc1 is a tumor suppressor. We previously identified a second mammalian Patched gene, Ptc2, and demonstrated its distinct expression pattern during embryogenesis, suggesting a unique role in development. Most notably, Ptc2 is expressed in an overlapping pattern with Shh in the epidermal compartment of developing hair follicles and is highly expressed in the developing limb bud, cerebellum, and testis. Here, we describe the generation and phenotypic analysis of Ptc2(tm1/tm1) mice. Our molecular analysis suggests that Ptc2(tm1) likely represents a hypomorphic allele. Despite the dynamic expression of Ptc2 during embryogenesis, Ptc2(tm1/tm1) mice are viable, fertile, and apparently normal. Interestingly, adult Ptc2(tm1/tm1) male animals develop skin lesions consisting of alopecia, ulceration, and epidermal hyperplasia. While functional compensation by Ptc1 might account for the lack of a strong mutant phenotype in Ptc2-deficient mice, our results suggest that normal Ptc2 function is required for adult skin homeostasis.

Structure-function relationships in the neuropeptide S receptor: molecular consequences of the asthma-associated mutation N107I.

Neuropeptide S (NPS) and its receptor (NPSR) are thought to have a role in asthma pathogenesis; a number of single nucleotide polymorphisms within NPSR have been shown to be associated with an increased prevalance of asthma. One such single nucleotide polymorphism leads to the missense mutation N107I, which results in an increase in the potency of NPS for NPSR. To gain insight into structure-function relationships within NPS and NPSR, we first carried out a limited structural characterization of NPS and subjected the peptide to extensive mutagenesis studies. Our results show that the NH(2)-terminal third of NPS, in particular residues Phe-2, Arg-3, Asn-4, and Val-6, are necessary and sufficient for activation of NPSR. Furthermore, part of a nascent helix within the peptide, spanning residues 5 through 13, acts as a regulatory region that inhibits receptor activation. Notably, this inhibition is absent in the asthma-linked N107I variant of NPSR, suggesting that residue 107 interacts with the aforementioned regulatory region of NPS. Whereas this interaction may be at the root of the increase in potency associated with the N107I variant, we show here that the mutation also causes an increase in cell-surface expression of the mutant receptor, leading to a concomitant increase in the maximal efficacy (E(max)) of NPS. Our results identify the key residues of NPS involved in NPSR activation and suggest a molecular basis for the functional effects of the N107I mutation and for its putative pathophysiological link with asthma.

Loss of angiotensin-converting enzyme-2 leads to the late development of angiotensin II-dependent glomerulosclerosis.

Angiotensin-converting enzyme-2 (ACE2), a membrane-bound carboxymonopeptidase highly expressed in the kidney, functions as a negative regulator of the renin-angiotensin system. Here we report early accumulation of fibrillar collagen in the glomerular mesangium of male ACE2 mutant (ACE2-/y) mice followed by development of glomerulosclerosis by 12 months of age whereas female ACE2 mutant (ACE2-/-) mice were relatively protected. Progressive kidney injury was associated with increased deposition of collagen I, collagen III and fibronectin in the glomeruli and increased urinary albumin excretion compared to age-matched control mice. These structural and functional changes in the glomeruli of male ACE2 mutant mice were prevented by treatment with the angiotensin II type-1 receptor antagonist irbesartan. Loss of ACE2 was associated with a marked increase in renal lipid peroxidation product formation and activation of mitogen-activated protein kinase and extracellular signal-regulated kinases 1 and 2 in glomeruli, events that are also prevented by angiotensin II type-1 receptor blockade. We conclude that deletion of the ACE2 gene leads to the development of angiotensin II-dependent glomerular injury in male mice. These findings have important implications for our understanding of ACE2, the renin-angiotensin system, and gender in renal injury, with ACE2 likely to be an important therapeutic target in kidney disease.

Angiotensin-converting enzyme 2 protects from severe acute lung failure.

Acute respiratory distress syndrome (ARDS), the most severe form of acute lung injury, is a devastating clinical syndrome with a high mortality rate (30-60%) (refs 1-3). Predisposing factors for ARDS are diverse and include sepsis, aspiration, pneumonias and infections with the severe acute respiratory syndrome (SARS) coronavirus. At present, there are no effective drugs for improving the clinical outcome of ARDS. Angiotensin-converting enzyme (ACE) and ACE2 are homologues with different key functions in the renin-angiotensin system. ACE cleaves angiotensin I to generate angiotensin II, whereas ACE2 inactivates angiotensin II and is a negative regulator of the system. ACE2 has also recently been identified as a potential SARS virus receptor and is expressed in lungs. Here we report that ACE2 and the angiotensin II type 2 receptor (AT2) protect mice from severe acute lung injury induced by acid aspiration or sepsis. However, other components of the renin-angiotensin system, including ACE, angiotensin II and the angiotensin II type 1a receptor (AT1a), promote disease pathogenesis, induce lung oedemas and impair lung function. We show that mice deficient for Ace show markedly improved disease, and also that recombinant ACE2 can protect mice from severe acute lung injury. Our data identify a critical function for ACE2 in acute lung injury, pointing to a possible therapy for a syndrome affecting millions of people worldwide every year.

Mutant meiotic chromosome core components in mice can cause apparent sexual dimorphic endpoints at prophase or X-Y defective male-specific sterility.

Genetic modifications causing germ cell death during meiotic prophase in the mouse frequently have sexually dimorphic phenotypes where oocytes reach more advanced stages than spermatocytes. To determine to what extent these dimorphisms are due to differences in male versus female meiotic prophase development, we compared meiotic chromosome events in the two sexes in both wild-type and mutant mice. We report the abundance and time course of appearance of structural and recombination-related proteins of fetal oocyte nuclei. Oocytes at successive days post coitus show rapid, synchronous meiotic prophase development compared with the continuous spermatocyte development in adult testis. Consequently, a genetic defect requiring 2-3 days from the onset of prophase to reach arrest registers pachytene as the developmental endpoint in oocytes. Pachytene spermatocytes, on the other hand, which normally accumulate during days 4-10 after the onset of prophase, will be rare, giving the appearance of an earlier endpoint than in oocytes. We conclude that these different logistics create apparent sexually dimorphic endpoints. For more pronounced sexual dimorphisms, we examined meiotic prophase of mice with genetic modifications of meiotic chromosome core components that cause male but not female sterility. The correlations between male sterility and alterations in the organization of the sex chromosome cores and X-Y chromatin may indicate that impaired signals from the XY domain (XY chromosome cores, chromatin, dense body and sex body) may interfere with the progression of the spermatocyte through prophase. Oocytes, in the absence of the X-Y pair, do not suffer such defects.