K. Mandal

Magnetocaloric effect in reactively-milled LaFe11.57Si1.43Hy intermetallic compounds

Hydrides of LaFe11.57Si1.43 intermetallic compound have been prepared by high-energy ball milling in the presence of hydrogen gas, a process known as reactive milling. The Curie temperature of the samples was tuned within the temperature range of 199 K to 346 K by changing the hydrogen content from 0 to 2.3 at. % without compromising much of the magnitude of the magnetocaloric effect. Arrott plots and large hysteresis in the magnetization vs magnetic field curves confirm that the first-order itinerant-electron metamagnetic transition is the reason for large entropy change in the parent as well as in the hydride samples. The present study indicates that reactive milling can be an effective method for incorporating interstitial hydrogen within these compounds in order to raise their TC to room temperature.

Paramagnetism in single-phase Sn1−xCoxO2 dilute magnetic semiconductors

Transition metal (TM) Co doped SnO2 dilute magnetic semiconductor (DMS) in both nano- and bulk state are prepared by solvothermal and mechanosyhtesis route, respectively. Contraction in unit cell volume of tetragonal rutile SnO2 after Co doping and redshift in energy band gap compared to that of undoped SnO2 ensures the incorporation of smaller Co2+ ions replacing larger host cations Sn4+. Vibrating sample magnetometer measurements show that paramagnetism is the intrinsic magnetic property in single-phase Sn1−xCoxO whereas non-DMS related ferromagnetism is associated only with the corresponding nanostructures. Paramagnetism is also confirmed by thermal dependence of magnetization M (T) and magnetic susceptibility (χ) measurement in between 80≤T≤350 K.

Exchange bias in Co-Cr2O3 nanocomposites

The possibility of using exchange bias in a ferromagnetic-antiferromagnetic system to overcome the effect of superparamagnetism in small ferromagnetic nanoparticles is explored. We have prepared Co-Cr2O3 nanocomposite powders using a chemical method and shown that the effect of superparamagnetism in cobalt nanoparticles could be overcome using exchange bias between Co and Cr2O3. The superparamagnetic blocking temperature of 3 nm cobalt particles has been increased to above room temperature. The choice of Cr2O3 is vital as its TN is higher compared to other antiferromagnetic materials used for this purpose such as CoO. The field cooled and zero field cooled hysteresis measurements of the samples confirm the existence of exchange bias interaction in this system.

Contributions to giant magnetoimpedance from different domain regions of Co68.15Fe4.35Si12.5B15 amorphous wire

The frequency dependence of magnetoimpedance (MI) of amorphous wire Co68.15Fe4.35Si12.5B15 has been studied in the presence of various tensile stresses up to 285MPa to understand the contributions to giant magnetoimpedance from different domain regions. The external stress affects MI of these materials to a large extent as magnetocrystalline anisotropy is absent and magnetoelastic anisotropy is the main source of anisotropy in these materials. At a particular frequency, a stress-induced anisotropy changes the domain structure, magnetization dynamics, and hence MI. Single peak MI characteristics obtained at lower frequencies (approximately a few kilohertz) are changed to a double-peak behavior with the application of a tensile stress or increase in frequency. Asymmetry between the two peaks has also been developed only by sending a dc of 5mA. Hysteresis loop measurement of amorphous wire at various frequencies and tensile stresses helps us understand the MI results.

Defective Wnt3 expression by testicular Sertoli cells compromise male fertility

Testicular Sertoli cells make a niche for the division and differentiation of germ cells. Sertoli cells respond to increased follicle-stimulating hormone (FSH) and testosterone (T) levels at the onset of puberty by producing paracrine factors which affect germ cells and trigger robust onset of spermatogenesis. Such paracrine support to germ cells is absent during infancy, despite Sertoli cells being exposed to high FSH and T within the infant testis. This situation is similar to certain cases of male idiopathic infertility where post-pubertal Sertoli cells fail to support germ cell division and differentiation in spite of endogenous or exogenous hormonal support. Defective Sertoli cells in such individuals may fail to express the full complement of their paracrine repertoire. Identification and supplementation with such factors may overcome Sertoli cells deficiencies and help trigger quantitatively and qualitatively normal differentiation of germ cells. To this end, we compared the transcriptome of FSH- and T-treated infant and pubertal monkey Sertoli cells by DNA microarray. Expression of Wnt3, a morphogen of the Wnt/β-catenin pathway, was higher in pubertal Sertoli cells relative to infant Sertoli cells. Transgenic mice were generated by us in which Wnt3 expression was curtailed specifically in post-pubertal Sertoli cells by shRNA. Subfertility and oligozoospermia were noticed in such animals with low Wnt3 expression in post-pubertal Sertoli cells along with diminished expression of Connexin43, a gap-junctional molecule essential for germ cell development. We report that the FSH- and T-targetedf Wnt3 governs Sertoli cell-mediated regulation of spermatogenesis and hence is crucial for fertility.

An integrated transcriptomics-guided genome-wide promoter analysis and next-generation proteomics approach to mine factor(s) regulating cellular differentiation

Abstract Differential next-generation-omics approaches aid in the visualization of biological processes and pave the way for divulging important events and/or interactions leading to a functional output at cellular or systems level. To this end, we undertook an integrated Nextgen transcriptomics and proteomics approach to divulge differential gene expression of infant and pubertal rat Sertoli cells (Sc).Unlike, pubertal Sc, infant Sc are immature and fail to support spermatogenesis. We found exclusive association of 14 and 19 transcription factor binding sites to infantile and pubertal states of Sc, respectively, using differential transcriptomics-guided genome-wide computational analysis of relevant promoters employing 220 Positional Weight Matrices from the TRANSFAC database. Proteomic SWATH-MS analysis provided extensive quantification of nuclear and cytoplasmic protein fractions revealing 1,670 proteins differentially located between the nucleus and cytoplasm of infant Sc and 890 proteins differentially located within those of pubertal Sc. Based on our multi-omics approach, the transcription factor YY1 was identified as one of the lead candidates regulating differentiation of Sc.YY1 was found to have abundant binding sites on promoters of genes upregulated during puberty. To determine its significance, we generated transgenic rats with Sc specific knockdown of YY1 that led to compromised spermatogenesis.

Sertoli cell specific knockdown of RAR-related orphan receptor (ROR) alpha at puberty reduces sperm count in rats

Globally, there is an alarming decline in sperm count. Very often hormonal supplementation fails to restore normal sperm count. Sertoli cells (Sc) present within seminiferous tubules provide appropriate niche and factors required for the differentiation of germ cells (Gc) into mature sperm (spermatogenesis). Functionally compromised Sc may be one of the reasons for failure of hormones to facilitate normal spermatogenesis. Although role of secretory proteins and signaling molecules of Sc has been studied well, role of transcription factors regulating sperm count has not been addressed appropriately. Retinoic acid receptor-related orphan receptor (ROR)-alpha is one of such transcription factors reported in testis but its role in testicular function is not yet known. In a separate study, we found abundant ROR-alpha binding sites on promoter regions of several genes upregulated in pubertal rat Sc as compared to infant Sc. Immunostaining studies also revealed presence of ROR alpha in nucleus of pubertal Sc. We generated a transgenic knockdown rat model expressing shRNA targeted to ROR-alpha under Sc specific promoter, which is transcriptionally active only at and after puberty. ROR-alpha knockdown animals were found to have abnormal association of Sc and Gc, including Gc sloughing and restricted release of sperm. The knockdown animals displayed compromised spermatogenesis leading to significant reduction in sperm count. This is the first report describing the Sc specific role of ROR-alpha in maintaining quantitatively normal sperm output. Identification of various such molecules can generate avenues to limit or reverse an alarmingly declining sperm count witnessed globally in men.