PTP1B mediates the inhibitory effect of MFGE8 on insulin signaling through the ß5 integrin.

Integrins are cell adhesion receptors that dimerize to mediate cell-cell interactions and regulate processes, including proliferation, inflammation, and tissue repair. The role of integrins in regulating insulin signaling is incompletely understood. We have previously shown that binding of the integrin ligand milk fat globule epidermal growth factor like 8 (MFGE8) to the αvß5 integrin promotes termination of insulin receptor signaling in mice. Upon ligation of MFGE8, integrin ß5 complexes with the insulin receptor beta (IRß) in skeletal muscle, resulting in dephosphorylation of IRß and reduction of insulin-stimulated glucose uptake. Here, we investigate the mechanism by which the interaction between ß5 and IRß impacts IRß phosphorylation status. We show in in vitro and in vivo in skeletal muscle in mice that antibody-mediated blockade of the ß5 integrin inhibits and recombinant MFGE8 promotes PTP1B binding to and dephosphorylation of IRß resulting in increased or reduced insulin-stimulated glucose uptake, respectively. The ß5-PTP1B complex is recruited by MFGE8 to IRß leading to termination of canonical insulin signaling. ß5 blockade enhances insulin-stimulated glucose uptake in wildtype but not Ptp1b KO mice indicating that PTP1B functions downstream of MFGE8 in modulating insulin receptor signaling. Furthermore, in a human cohort, we report serum MFGE8 levels correlate with indices of insulin resistance. These data provide mechanistic insights into the role of MFGE8 and ß5 in regulating insulin signaling.

Effects of Pharmacotherapy for the Treatment of Obesity in an Urban, Safety-Net Population.

OBJECTIVE: To evaluate the effect of common weight loss pharmacotherapies among low-income, racially diverse adult patients at an urban safety-net weight management clinic. METHODS: Our retrospective review from 2015 to 2019 examined patients who took either GLP-1 analog (GL) or phentermine/topiramate (PT) for ≥90 days and patients who exclusively pursued non-pharmacologic treatment for comparison. Changes in weight, blood pressure, and hemoglobin A1c at 1-year follow-up were reported. RESULTS: We analyzed 22 GL and 26 PT patients and included 40 patients who pursued only lifestyle modifications (LM). All three groups achieved significant weight loss at one year: GL -3.69 (interquartile range (IQR): -11.0, -1.77) kg (p=0.0004), PT -7.01 (IQR: -13.4, -1.45) kg (p<0.001), and LM -3.01 (IQR: -6.81, 1.13) kg (p=0.005). There was no significant difference in the median weight loss (p=0.11) between the three groups. We observed no significant changes in systolic blood pressure but saw a significant change of -0.75 in hemoglobin A1c (IQR: -1.35, -0.25) (p=0.01) among patients with diabetes in the GL group. CONCLUSIONS: Our real-world applications of GLP-1 and phentermine/topiramate suggest that both are effective weight loss medication regimens in low-socioeconomic status patients.

MFGE8 inhibits insulin signaling through PTP1B.

The role of integrins in regulating insulin signaling is incompletely understood. We have previously shown that binding of the integrin ligand milk fat globule epidermal growth factor like 8 (MFGE8) to the αvß5 integrin promotes termination of insulin receptor signaling in mice. Upon ligation of MFGE8, ß5 complexes with the insulin receptor beta (IRß) in skeletal muscle resulting in dephosphorylation of IRß and reduction of insulin-stimulated glucose uptake. Here we investigate the mechanism by which the interaction between ß5 and IRß impacts IRß phosphorylation status. We show that ß5 blockade inhibits and MFGE8 promotes PTP1B binding to and dephosphorylation of IRß resulting in reduced or increased insulin-stimulated myotube glucose uptake respectively. The ß5-PTP1B complex is recruited by MFGE8 to IRß leading to termination of canonical insulin signaling. ß5 blockade enhances insulin-stimulated glucose uptake in wild type but not Ptp1b KO mice indicating that PTP1B functions downstream of MFGE8 in modulating insulin receptor signaling. Furthermore, in a human cohort, we report serum MFGE8 levels correlate with indices of insulin resistance. These data provide mechanistic insights into the role of MFGE8 and ß5 in regulating insulin signaling.

Autoantibodies to Perilipin-1 Define a Subset of Acquired Generalized Lipodystrophy.

Acquired lipodystrophy is often characterized as an idiopathic subtype of lipodystrophy. Despite suspicion of an immune-mediated pathology, biomarkers such as autoantibodies are generally lacking. Here, we used an unbiased proteome-wide screening approach to identify autoantibodies to the adipocyte-specific lipid droplet protein perilipin 1 (PLIN1) in a murine model of autoimmune polyendocrine syndrome type 1 (APS1). We then tested for PLIN1 autoantibodies in human subjects with acquired lipodystrophy with two independent severe breaks in immune tolerance (including APS1) along with control subjects using a specific radioligand binding assay and indirect immunofluorescence on fat tissue. We identified autoantibodies to PLIN1 in these two cases, including the first reported case of APS1 with acquired lipodystrophy and a second patient who acquired lipodystrophy as an immune-related adverse event following cancer immunotherapy. Lastly, we also found PLIN1 autoantibodies to be specifically enriched in a subset of patients with acquired generalized lipodystrophy (17 of 46 [37%]), particularly those with panniculitis and other features of autoimmunity. These data lend additional support to new literature that suggests that PLIN1 autoantibodies represent a marker of acquired autoimmune lipodystrophies and further link them to a break in immune tolerance.

A gene-diet interaction controlling relative intake of dietary carbohydrates and fats.

OBJECTIVE: Preference for dietary fat vs. carbohydrate varies markedly across free-living individuals. It is recognized that food choice is under genetic and physiological regulation, and that the central melanocortin system is involved. However, how genetic and dietary factors interact to regulate relative macronutrient intake is not well understood. METHODS: We investigated how the choice for food rich in carbohydrate vs. fat is influenced by dietary cholesterol availability and agouti-related protein (AGRP), the orexigenic component of the central melanocortin system. We assessed how macronutrient intake and different metabolic parameters correlate with plasma AGRP in a cohort of obese humans. We also examined how both dietary cholesterol levels and inhibiting de novo cholesterol synthesis affect carbohydrate and fat intake in mice, and how dietary cholesterol deficiency during the postnatal period impacts macronutrient intake patterns in adulthood. RESULTS: In obese human subjects, plasma levels of AGRP correlated inversely with consumption of carbohydrates over fats. Moreover, AgRP-deficient mice preferred to consume more calories from carbohydrates than fats, more so when each diet lacked cholesterol. Intriguingly, inhibiting cholesterol biosynthesis (simvastatin) promoted carbohydrate intake at the expense of fat without altering total caloric consumption, an effect that was remarkably absent in AgRP-deficient mice. Finally, feeding lactating C57BL/6 dams and pups a cholesterol-free diet prior to weaning led the offspring to prefer fats over carbohydrates as adults, indicating that altered cholesterol metabolism early in life programs adaptive changes to macronutrient intake. CONCLUSIONS: Together, our study illustrates a specific gene-diet interaction in modulating food choice.

Probing Insulin Sensitivity with Metabolically Competent Human Stem Cell-Derived White Adipose Tissue Microphysiological Systems.

Impaired white adipose tissue (WAT) function has been recognized as a critical early event in obesity-driven disorders, but high buoyancy, fragility, and heterogeneity of primary adipocytes have largely prevented their use in drug discovery efforts highlighting the need for human stem cell-based approaches. Here, human stem cells are utilized to derive metabolically functional 3D adipose tissue (iADIPO) in a microphysiological system (MPS). Surprisingly, previously reported WAT differentiation approaches create insulin resistant WAT ill-suited for type-2 diabetes mellitus drug discovery. Using three independent insulin sensitivity assays, i.e., glucose and fatty acid uptake and suppression of lipolysis, as the functional readouts new differentiation conditions yielding hormonally responsive iADIPO are derived. Through concomitant optimization of an iADIPO-MPS, it is abled to obtain WAT with more unilocular and significantly larger (≈40%) lipid droplets compared to iADIPO in 2D culture, increased insulin responsiveness of glucose uptake (≈2-3 fold), fatty acid uptake (≈3-6 fold), and ≈40% suppressing of stimulated lipolysis giving a dynamic range that is competent to current in vivo and ex vivo models, allowing to identify both insulin sensitizers and desensitizers.

The East Asian gut microbiome is distinct from colocalized White subjects and connected to metabolic health.

East Asians (EAs) experience worse metabolic health outcomes compared to other ethnic groups at lower body mass indices; however, the potential role of the gut microbiota in contributing to these health disparities remains unknown. We conducted a multi-omic study of 46 lean and obese East Asian and White participants living in the San Francisco Bay Area, revealing marked differences between ethnic groups in bacterial richness and community structure. White individuals were enriched for the mucin-degrading Akkermansia muciniphila. East Asian subjects had increased levels of multiple bacterial phyla, fermentative pathways detected by metagenomics, and the short-chain fatty acid end-products acetate, propionate, and isobutyrate. Differences in the gut microbiota between the East Asian and White subjects could not be explained by dietary intake, were more pronounced in lean individuals, and were associated with current geographical location. Microbiome transplantations into germ-free mice demonstrated stable diet- and host genotype-independent differences between the gut microbiotas of East Asian and White individuals that differentially impact host body composition. Taken together, our findings add to the growing body of literature describing microbiome variations between ethnicities and provide a starting point for defining the mechanisms through which the microbiome may shape disparate health outcomes in East Asians.

The community of microbes living in the human gut varies based on where a person lives, in part because of differences in diets but also due to factors still incompletely understood. In turn, this 'microbiome' may have wide-ranging effects on health and diseases such as obesity and diabetes. Many scientists want to understand how differences in the microbiome emerge between people, and whether this may explain why certain diseases are more common in specific populations. Self-identified race or ethnicity can be a useful tool in that effort, as it can serve as a proxy for cultural habits (such as diets) or genetic information. In the United States, self-identified East Asian Americans often have worse 'metabolic health' (e.g. levels of sugar or certain fat molecules in the blood) at a lower weight than those identifying as White. Ang, Alba, Upadhyay et al. investigated whether this health disparity was linked to variation in the gut microbiome. Samples were collected from 46 lean and obese individuals living in the San Francisco Bay Area who identified as White or East Asian. The analyses showed that while the gut microbiome of White participants changed in association with obesity, the microbiomes of East Asian participants were distinct from their White counterparts even at normal weight, with features mirroring what was seen in White individuals in the context of obesity. Although these differences were connected to people's current address, they were not attributable to dietary differences. Ang, Alba, Upadhyay et al. then transplanted the microbiome of the participants into genetically identical mice with microbe-free guts. The differences between the gut microbiomes of White and East Asian participants persisted in recipient animals. When fed the same diet, the mice also gained different amounts of weight depending on the ethnic identity of the microbial donor. These results show that self-identified ethnicity may be an important variable to consider in microbiome studies, alongside other factors such as geography. Ultimately, this research may help to design better, more personalized treatments for an array of conditions.

CD81 Controls Beige Fat Progenitor Cell Growth and Energy Balance via FAK Signaling.

Adipose tissues dynamically remodel their cellular composition in response to external cues by stimulating beige adipocyte biogenesis; however, the developmental origin and pathways regulating this process remain insufficiently understood owing to adipose tissue heterogeneity. Here, we employed single-cell RNA-seq and identified a unique subset of adipocyte progenitor cells (APCs) that possessed the cell-intrinsic plasticity to give rise to beige fat. This beige APC population is proliferative and marked by cell-surface proteins, including PDGFRα, Sca1, and CD81. Notably, CD81 is not only a beige APC marker but also required for de novo beige fat biogenesis following cold exposure. CD81 forms a complex with αV/ß1 and αV/ß5 integrins and mediates the activation of integrin-FAK signaling in response to irisin. Importantly, CD81 loss causes diet-induced obesity, insulin resistance, and adipose tissue inflammation. These results suggest that CD81 functions as a key sensor of external inputs and controls beige APC proliferation and whole-body energy homeostasis.

Genetic basis for the cooperative bioactivation of plant lignans by Eggerthella lenta and other human gut bacteria.

Plant-derived lignans, consumed daily by most individuals, are thought to protect against cancer and other diseases1; however, their bioactivity requires gut bacterial conversion to enterolignans2. Here, we dissect a four-species bacterial consortium sufficient for all five reactions in this pathway. A single enzyme (benzyl ether reductase, encoded by the gene ber) was sufficient for the first two biotransformations, variable between strains of Eggerthella lenta, critical for enterolignan production in gnotobiotic mice and unique to Coriobacteriia. Transcriptional profiling (RNA sequencing) independently identified ber and genomic loci upregulated by each of the remaining substrates. Despite their low abundance in gut microbiomes and restricted phylogenetic range, all of the identified genes were detectable in the distal gut microbiomes of most individuals living in northern California. Together, these results emphasize the importance of considering strain-level variations and bacterial co-occurrence to gain a mechanistic understanding of the bioactivation of plant secondary metabolites by the human gut microbiome.

Elastase 3B mutation links to familial pancreatitis with diabetes and pancreatic adenocarcinoma.

While improvements in genetic analysis have greatly enhanced our understanding of the mechanisms behind pancreatitis, it continues to afflict many families for whom the hereditary factors remain unknown. Recent evaluation of a patient with a strong family history of pancreatitis sparked us to reexamine a large kindred originally reported over 50 years ago with an autosomal dominant inheritance pattern of chronic pancreatitis, diabetes and pancreatic adenocarcinoma. Whole exome sequencing analysis identified a rare missense mutation in the gene encoding pancreas-specific protease Elastase 3B (CELA3B) that cosegregates with disease. Studies of the mutant protein in vitro, in cell lines and in CRISPR-Cas9 engineered mice indicate that this mutation causes translational upregulation of CELA3B, which upon secretion and activation by trypsin leads to uncontrolled proteolysis and recurrent pancreatitis. Although lesions in several other pancreatitic proteases have been previously linked to hereditary pancreatitis, this is the first known instance of a mutation in CELA3B and a defect in translational control contributing to this disease.

Sweet cognition: The differential effects of glucose consumption on attentional food bias in individuals of lean and obese status.

In general, glucose consumption improves cognitive performance; however, it is unknown whether glucose specifically alters attentional food bias, and how this process may vary by BMI status. We hypothesized that glucose consumption would increase attentional food bias among individuals of obese BMI status more so than among individuals of lean BMI status. Participants (N = 35) completed the n-back, a working memory task modified to assess attentional food bias (ATT-Food), under fasting and glucose challenge conditions. We computed pre-post changes in ATT-Food, blood glucose and insulin (∆BG & ∆BI), and perceived task-stress (∆stress). After the second cognitive test and blood draw, participants ate lunch and completed a "taste test" of highly palatable foods, and we recorded food consumption. Pre-post changes in ATT-Food were greater among participants of obese (relative to lean) BMI status (F(1,33) = 5.108, p = .031). Greater ∆ATT-Food was significantly associated with greater ∆BG (r = .462, p = .007) and reduced ∆stress (r =-.422, p = .011), and marginally associated with greater taste-test eating (r =.325, p = .057), but was not associated with ∆BI. Our findings suggest that individuals of obese BMI status may exhibit "sweet cognition," as indexed by greater attentional food bias following glucose ingestion, relative to individuals of lean BMI status. Among individuals of obese BMI status, sweet cognition may arise from difficulty broadening attention toward non-food cues after consuming a high glucose load, thereby potentially perpetuating sugar consumption. If confirmed by further research, measures of sweet cognition may help identify individuals with a phenotype of risk for obesity and greater sugar consumption, who may benefit from tailored interventions.

Obesity and Fat Metabolism in Human Immunodeficiency Virus-Infected Individuals: Immunopathogenic Mechanisms and Clinical Implications.

Metabolic complications relating to complex effects of viral and immune-mediated mechanisms are now a focus of clinical care among persons living with human immunodeficiency virus (PLHIV), and obesity is emerging as a critical problem. To address knowledge gaps, the US National Institutes of Health sponsored a symposium in May 2018 entitled "Obesity and Fat Metabolism in HIV-infected Individuals." Mechanisms relating to adipose dysfunction and fibrosis, immune function, inflammation, and gastrointestinal integrity were highlighted as contributors to obesity among PLHIV. Fibrotic subcutaneous adipose tissue is metabolically dysfunctional and loses its capacity to expand, leading to fat redistribution, including visceral obesity and ectopic fat accumulation, promoting insulin resistance. Viral proteins, including viral protein R and negative regulatory factor, have effects on adipogenic pathways and cellular metabolism in resident macrophages and T cells. HIV also affects immune cell trafficking into the adipose compartments, with effects on adipogenesis, lipolysis, and ectopic fat accumulation. Key cellular metabolic functions are likely to be affected in PLHIV by gut-derived cytokines and altered microbiota. There are limited strategies to reduce obesity specifically in PLHIV. Enhancing our understanding of critical pathogenic mechanisms will enable the development of novel therapeutics that may normalize adipose tissue function and distribution, reduce inflammation, and improve insulin sensitivity in PLHIV.

Changes in Lean Mass, Absolute and Relative Muscle Strength, and Physical Performance After Gastric Bypass Surgery.

Context: Bariatric surgery results in reduced muscle mass as weight is lost, but postoperative changes in muscle strength and performance are incompletely understood. Objective: To examine changes in body composition, strength, physical activity, and physical performance following Roux-en-Y gastric bypass (RYGB). Design, Participants, Outcomes: In a prospective cohort of 47 adults (37 women, 10 men) aged 45 ± 12 years (mean ± SD) with body mass index (BMI) 44 ± 8 kg/m2, we measured body composition by dual-energy X-ray absorptiometry, handgrip strength, physical activity, and physical performance (chair stand time, gait speed, 400-m walk time) before and 6 and 12 months after RYGB. Relative strength was calculated as absolute handgrip strength/BMI and as absolute strength/appendicular lean mass (ALM). Results: Participants experienced substantial 12-month decreases in weight (-37 ± 10 kg or 30% ± 7%), fat mass (-48% ± 12%), and total lean mass (-13% ± 6%). Mean absolute strength declined by 9% ± 17% (P < 0.01). In contrast, relative strength increased by 32% ± 25% (strength/BMI) and 9% ± 20% (strength/ALM) (P < 0.01 for both). There were clinically significant postoperative improvements in all physical performance measures, including mean improvement in gait speed of >0.1 m/s (P < 0.01) and decrease in 400-m walk time of nearly a full minute. Conclusions: In the setting of dramatic weight loss, lean mass and absolute grip strength declined after RYGB. However, relative muscle strength and physical function improved meaningfully and are thus noteworthy positive outcomes of gastric bypass.

Subcutaneous Fat Fibrosis Links Obesity to Insulin Resistance in Chinese Americans.

Objective: Type 2 diabetes presents at a lower body mass index (BMI) in Chinese individuals than in white individuals. We sought to determine the role of subcutaneous adipose tissue (SCAT)-intrinsic factors, vs BMI or adiposity per se, in the vulnerability of Chinese individuals to obesity-associated impairment of insulin sensitivity. Research Design and Methods: Thirty-two Chinese and 30 white men and women from a cohort in the San Francisco Bay Area underwent anthropometric measurements, body composition (dual-energy X-ray absorptiometry) analyses, and measurement of fasting plasma glucose and insulin. Forty-eight also provided abdominal SCAT samples for transcriptional and biochemical analyses of tissue fibrosis. Results: BMI correlated with total body fat in white (r = 0.74, P < 0.001) but not Chinese individuals, whereas BMI correlated with visceral adipose tissue (VAT) accrual in both ethnicities (r = 0.88 and 0.81, respectively; P < 0.01). Insulin resistance (homeostatic model assessment of insulin resistance) worsened with VAT mass, but not total body fat, in Chinese subjects (r = 0.63, P < 0.01), whereas it worsened with both in white individuals. By contrast, SCAT mRNA levels of genes encoding profibrotic proteins rose remarkably along with both BMI and VAT mass in Chinese but not white subjects. Similarly, SCAT levels of hydroxyproline, an indicator of tissue collagen content that correlated with increasing VAT mass, were higher in Chinese vs white subjects, particularly in the setting of relative insulin resistance. Conclusions: Our findings dissociate BMI from adiposity in Chinese individuals and instead highlight SCAT fibrosis as a process linked to visceral adiposity and insulin resistance in this group.

Repression of Adipose Tissue Fibrosis through a PRDM16-GTF2IRD1 Complex Improves Systemic Glucose Homeostasis.

Adipose tissue fibrosis is a hallmark of malfunction that is linked to insulin resistance and type 2 diabetes; however, what regulates this process remains unclear. Here we show that the PRDM16 transcriptional complex, a dominant activator of brown/beige adipocyte development, potently represses adipose tissue fibrosis in an uncoupling protein 1 (UCP1)-independent manner. By purifying the PRDM16 complex, we identified GTF2IRD1, a member of the TFII-I family of DNA-binding proteins, as a cold-inducible transcription factor that mediates the repressive action of the PRDM16 complex on fibrosis. Adipocyte-selective expression of GTF2IRD1 represses adipose tissue fibrosis and improves systemic glucose homeostasis independent of body-weight loss, while deleting GTF2IRD1 promotes fibrosis in a cell-autonomous manner. GTF2IRD1 represses the transcription of transforming growth factor ß-dependent pro-fibrosis genes by recruiting PRDM16 and EHMT1 onto their promoter/enhancer regions. These results suggest a mechanism by which repression of obesity-associated adipose tissue fibrosis through the PRDM16 complex leads to an improvement in systemic glucose homeostasis.

Developmental staging of male murine embryonic gonad by SAGE analysis.

Despite the identification of key genes such as Sry integral to embryonic gonadal development, the genomic classification and identification of chromosomal activation of this process is still poorly understood. To better understand the genetic regulation of gonadal development, we performed Serial Analysis of Gene Expression (SAGE) to profile the genes and novel transcripts, and an average of 152,000 tags from male embryonic gonads at E10.5 (embryonic day 10.5), E11.5, E12.5, E13.5, E15.5 and E17.5 were analyzed. A total of 275,583 non-singleton tags that do not map to any annotated sequence were identified in the six gonad libraries, and 47,255 tags were mapped to 24,975 annotated sequences, among which 987 sequences were uncharacterized. Utilizing an unsupervised pattern identification technique, we established molecular staging of male gonadal development. Rather than providing a static descriptive analysis, we developed algorithms to cluster the SAGE data and assign SAGE tags to a corresponding chromosomal position; these data are displayed in chromosome graphic format. A prominent increase in global genomic activity from E10.5 to E17.5 was observed. Important chromosomal regions related to the developmental processes were identified and validated based on established mouse models with developmental disorders. These regions may represent markers for early diagnosis for disorders of male gonad development as well as potential treatment targets.

Application of transcriptional and biological network analyses in mouse germ-cell transcriptomes.

Serial analysis of gene expression (SAGE) provides a global analysis platform for profiling mRNA populations present in cells of interest without the constraint of gene selection and the ambiguous nature of data obtained. However, most of the reports on SAGE and germ cell development are limited to descriptive analyses. Here, we report a series of bioinformatic analyses using recently published SAGE data on the transcriptome of mouse type A spermatogonia (Spga), pachytene spermatocytes (Spcy), and round spermatids (Sptd). Tags with a total count of > or =20 in three SAGE libraries were examined. Our aim was to identify and discover potential transcriptional regulators and pathways involved at different stages of spermatogenesis. Unsupervised hierarchical clustering based on tag expression and Gene Ontology analysis were applied to identify genes and biological processes overrepresented at a particular stage of development. The 5' cis-regulatory elements were examined for common regulators in different functional clusters. Potential biological networks were also constructed to reveal the link between the gene candidates. Biological pathways related to the three germ cell stages were constructed. A number of known transcription regulators in spermatogenesis, including NF-kappaB, SP1, AP-1, and EGR, were identified. Novel promoter elements such as the E box in Spga-specific genes, GATA in Spcy-specific genes, and GKLF in Sptd-specific genes were also observed. Taken together, our approach is reliable and provides a foundation for the generation of novel biological hypotheses for studying spermatogenesis.

Transcriptome analyses of male germ cells with serial analysis of gene expression (SAGE).

Serial analysis of gene expression (SAGE) provides an alternative with additional advantages to microarrays for studying gene expression during spermatogenesis. The digitized transcriptome provided by SAGE of purified mouse germ cells identified 27,504 species of transcripts expressed in type A spermatogonia, pachytene spermatocytes, and round spermatids. Over 2700 of these transcripts were novel. Computational analyses allowed the identification of clusters of co-regulated genes, cell-specific promoter modules, cell-specific biological processes, as well as "preferential" biological networks in different cell types. These analyses provided potential drug targets for interference of specific pathways at different stages of spermatogenesis. Analyses of the transcriptomes revealed the prominent role of cytochrome c oxidase in germ cells and suggest a novel role for this enzyme in cytochrome c-mediated apoptosis in spermatogonia. A number of genes were shown to undergo differential splicing during spermatogenesis giving rise to cell-specific splice variants.