mu-Crystallin, thyroid hormone-binding protein, is expressed abundantly in the murine inner root sheath cells

Emerging role of T3-binding protein μ-crystallin (CRYM) in health and disease

Thyroid hormones are essential metabolic and developmental regulators that exert a huge variety of effects in different organs. Triiodothyronine (T3) and thyroxine (T4) are synthesized in the thyroid gland and constitute unique iodine-containing hormones that are constantly regulated by a homeostatic feedback mechanism. T3/T4 activity in cells is mainly determined by specific transporters, cytosolic binding proteins, deiodinases (DIOs), and nuclear receptors. Modulation of intracellular T3/T4 level contributes to the maintenance of this regulatory feedback. μ-Crystallin (CRYM) is an important intracellular high-affinity T3-binding protein that buffers the amount of T3 freely available in the cytosol, thereby controlling its action. In this review, we focus on the molecular and pathological properties of CRYM in thyroid hormone signaling, with emphasis on its critical role in malignancies.

Biomaterial bridges enable regeneration and re-entry of corticospinal tract axons into the caudal spinal cord after SCI: Association with recovery of forelimb function

Severed axon tracts fail to exhibit robust or spontaneous regeneration after spinal cord injury (SCI). Regeneration failure reflects a combination of factors, including the growth state of neuronal cell bodies and the regeneration-inhibitory environment of the central nervous system. However, while spared circuitry can be retrained, target reinnervation depends on longitudinally directed regeneration of transected axons. This study describes a biodegradable implant using poly(lactide-co-glycolide) (PLG) bridges as a carrier scaffold to support regeneration after injury. In order to detect regeneration of descending neuronal tracts into the bridge, and beyond into intact caudal parenchyma, we developed a mouse cervical implantation model and employed Crym:GFP transgenic mice. Characterization of Crym:GFP mice revealed that descending tracts, including the corticospinal tract, were labeled by green fluorescent protein (GFP), while ascending sensory neurons and fibers were not. Robust co-localization between GFP and neurofilament-200 (NF-200) as well as GFP and GAP-43 was observed at both the rostral and caudal bridge/tissue interface. No evidence of similar regeneration was observed in mice that received gelfoam at the lesion site as controls. Minimal co-localization between GFP reporter labeling and macrophage markers was observed. Taken together, these data suggest that axons originating from descending fiber tracts regenerated, entered into the PLG bridge at the rostral margin, continued through the bridge site, and exited to re-enter host tissue at the caudal edge of the intact bridge. Finally, regeneration through implanted bridges was associated with a reduction in ipsilateral forelimb errors on a horizontal ladder task.

Lysine metabolism in mammalian brain: an update on the importance of recent discoveries

The lysine catabolism pathway differs in adult mammalian brain from that in extracerebral tissues. The saccharopine pathway is the predominant lysine degradative pathway in extracerebral tissues, whereas the pipecolate pathway predominates in adult brain. The two pathways converge at the level of ∆(1)-piperideine-6-carboxylate (P6C), which is in equilibrium with its open-chain aldehyde form, namely, α-aminoadipate δ-semialdehyde (AAS). A unique feature of the pipecolate pathway is the formation of the cyclic ketimine intermediate ∆(1)-piperideine-2-carboxylate (P2C) and its reduced metabolite L-pipecolate. A cerebral ketimine reductase (KR) has recently been identified that catalyzes the reduction of P2C to L-pipecolate. The discovery that this KR, which is capable of reducing not only P2C but also other cyclic imines, is identical to a previously well-described thyroid hormone-binding protein [μ-crystallin (CRYM)], may hold the key to understanding the biological relevance of the pipecolate pathway and its importance in the brain. The finding that the KR activity of CRYM is strongly inhibited by the thyroid hormone 3,5,3'-triiodothyronine (T3) has far-reaching biomedical and clinical implications. The inter-relationship between tryptophan and lysine catabolic pathways is discussed in the context of shared degradative enzymes and also potential regulation by thyroid hormones. This review traces the discoveries of enzymes involved in lysine metabolism in mammalian brain. However, there still remain unanswered questions as regards the importance of the pipecolate pathway in normal or diseased brain, including the nature of the first step in the pathway and the relationship of the pipecolate pathway to the tryptophan degradation pathway.

Imine reductases: a comparison of glutamate dehydrogenase to ketimine reductases in the brain

A key intermediate in the glutamate dehydrogenase (GDH)-catalyzed reaction is an imine. Mechanistically, therefore, GDH exhibits similarities to the ketimine reductases. In the current review, we briefly discuss (a) the metabolic importance of the GDH reaction in liver and brain, (b) the mechanistic similarities between GDH and the ketimine reductases, (c) the metabolic importance of the brain ketimine reductases, and (d) the neurochemical consequences of defective ketimine reductases. Our review contains many historical references to the early work on amino acid metabolism. This work tends to be overlooked nowadays, but is crucial for a contemporary understanding of the central importance of ketimines in nitrogen and intermediary metabolism. The ketimine reductases are important enzymes linking nitrogen flow among several key amino acids, yet have been little studied. The cerebral importance of the ketimine reductases is an area of biomedical research that deserves far more attention.

Mammalian forebrain ketimine reductase identified as μ-crystallin; potential regulation by thyroid hormones

Ketimine reductase (E.C. 1.5.1.25) was purified to apparent homogeneity from lamb forebrain by means of a rapid multi-step chromatography protocol. The purified enzyme was identified by MS/MS (mass spectrometry) as μ-crystallin. The identity was confirmed by heterologously expressing human μ-crystallin in Escherichia coli and subsequent chromatographic purification of the protein. The purified human μ-crystallin was confirmed to have ketimine reductase activity with a maximum specific activity similar to that of native ovine ketimine reductase, and was found to catalyse a sequential reaction. The enzyme substrates are putative neuromodulator/transmitters. The thyroid hormone 3,5,3'-l-triiodothyronine (T3) was found to be a strong reversible competitive inhibitor, and may have a novel role in regulating their concentrations. μ-Crystallin is also involved in intracellular T3 storage and transport. This research is the first to demonstrate an enzyme function for μ-crystallin. This newly demonstrated enzymatic activity identifies a new role for thyroid hormones in regulating mammalian amino acid metabolism, and a possible reciprocal role of enzyme activity regulating bioavailability of intracellular T3.

Genomic analysis of the function of the transcription factor gata3 during development of the mammalian inner ear

We have studied the function of the zinc finger transcription factor gata3 in auditory system development by analysing temporal profiles of gene expression during differentiation of conditionally immortal cell lines derived to model specific auditory cell types and developmental stages. We tested and applied a novel probabilistic method called the gamma Model for Oligonucleotide Signals to analyse hybridization signals from Affymetrix oligonucleotide arrays. Expression levels estimated by this method correlated closely (p<0.0001) across a 10-fold range with those measured by quantitative RT-PCR for a sample of 61 different genes. In an unbiased list of 26 genes whose temporal profiles clustered most closely with that of gata3 in all cell lines, 10 were linked to Insulin-like Growth Factor signalling, including the serine/threonine kinase Akt/PKB. Knock-down of gata3 in vitro was associated with a decrease in expression of genes linked to IGF-signalling, including IGF1, IGF2 and several IGF-binding proteins. It also led to a small decrease in protein levels of the serine-threonine kinase Akt2/PKBbeta, a dramatic increase in Akt1/PKBalpha protein and relocation of Akt1/PKBalpha from the nucleus to the cytoplasm. The cyclin-dependent kinase inhibitor p27(kip1), a known target of PKB/Akt, simultaneously decreased. In heterozygous gata3 null mice the expression of gata3 correlated with high levels of activated Akt/PKB. This functional relationship could explain the diverse function of gata3 during development, the hearing loss associated with gata3 heterozygous null mice and the broader symptoms of human patients with Hearing-Deafness-Renal anomaly syndrome.

Identification of mu-crystallin as an androgen-regulated gene in human prostate cancer

BACKGROUND

Androgen receptor (AR) signaling is implicated in prostate cancer progression. Therefore, identification of AR downstream genes is potentially important for selection of novel markers and therapy targets in prostate cancer.

METHODS

Expression of a thyroid hormone T3-binding protein mu-crystallin (CRYM) mRNA and protein in cell lines was evaluated by real-time PCR and Western blot, respectively. CRYM expression in vivo was analyzed in patients' samples by immunohistochemistry. The effects of androgen and T3 on proliferation of MDA PCa 2b cells were assessed by (3)H-thymidine uptake assay.

RESULTS

CRYM expression was detected in AR-positive LNCaP and MDA PCa 2b cells. In MDA PCA 2b cells, CRYM was regulated by androgens. Androgen-induced CRYM expression was diminished by antiandrogens or AR siRNA. Inhibition of transcription by alpha-amanitin caused a reduction in CRYM mRNA. The lack of CRYM expression was noted in LAPC-4 cells and in AR-negative prostate cancer cell lines PC-3 and DU-145. CRYM protein was increased in cancer tissue and decreased in samples from patients after hormonal therapy. In samples from patients with therapy-refractory cancer CRYM was not detectable. We also found that androgens and T3 have additive effects on stimulation of MDA PCa 2b cells proliferation.

CONCLUSION

CRYM is a novel androgen-regulated gene whose expression is elevated in prostate cancer but down-regulated in castration therapy-resistant tumors.

mu-crystallin, a NADPH-dependent T(3)-binding protein in cytosol

Thyroid hormone action is initiated through nuclear thyroid hormone receptors (TRs). Before the discovery of these nuclear receptors, possible major binding sites for thyroid hormones were thought to be cytosolic owing to high thyroid hormone-binding activity in crude cytosolic fractions. Several cytosolic thyroid hormone-binding proteins have been identified, including reduced nicotinamide adenine dinucleotide phosphate (NADPH)-dependent 3,5,3'-triiodo-L-thyronine (T(3))-binding protein, also known as mu-crystallin, which was initially cloned as the ortholog of bacterial ornithine cyclodeaminase. The expression of mu-crystallin is developmentally regulated and cell-type specific. Recently, patients with nonsyndromic deafness were reported to have point mutations in the mu-crystallin gene. Cytosolic thyroid hormone-binding proteins, especially mu-crystallin, have roles in adaptation to environmental alterations by thyroid hormone, which might have a role in hearing function.

The Near-Naked Hairless (Hr) Mutation Disrupts Hair Formation but Is Not Due to a Mutation in the Hairless Coding Region

Near-naked hairless (Hr(N)) is a semi-dominant, spontaneous mutation that was suggested by allelism testing to be allelic with mouse Hairless (Hr). Hr(N) mice differ from other Hr mutants in that hair loss appears as the postnatal coat begins to emerge, rather than as an inability to regrow hair after the first catagen and that the mutation displays semi-dominant inheritance. We sequenced the Hr cDNA in Hr(N)/Hr(N) mice and characterized the pathological and molecular phenotypes to identify the basis for hair loss in this model. Hr(N)/Hr(N) mice exhibit dystrophic hairs that are unable to emerge consistently from the hair follicle, whereas Hr(N)/+ mice display a sparse coat of hair and a milder degree of follicular dystrophy than their homozygous littermates. DNA microarray analysis of cutaneous gene expression demonstrates that numerous genes are downregulated in Hr(N)/Hr(N) mice, primarily genes important for hair structure. By contrast, Hr expression is significantly increased. Sequencing the Hr-coding region, intron-exon boundaries, 5'- and 3'-untranslated region, and immediate upstream region did not reveal the underlying mutation. Therefore, Hr(N) does not appear to be an allele of Hr but may result from a mutation in a closely linked gene or from a regulatory mutation in Hr.

Transcriptome and phenotypic analysis reveals Gata3-dependent signalling pathways in murine hair follicles

The transcription factor Gata3 is crucially involved in epidermis and hair follicle differentiation. Yet, little is known about how Gata3 co-ordinates stem cell lineage determination in skin, what pathways are involved and how Gata3 differentially regulates distinct cell populations within the hair follicle. Here, we describe a conditional Gata3-/- mouse (K14-Gata3-/-) in which Gata3 is specifically deleted in epidermis and hair follicles. K14-Gata3-/- mice show aberrant postnatal growth and development, delayed hair growth and maintenance, abnormal hair follicle organization and irregular pigmentation. After the first hair cycle, the germinative layer surrounding the dermal papilla was not restored; instead, proliferation was pronounced in basal epidermal cells. Transcriptome analysis of laser-dissected K14-Gata3-/- hair follicles revealed mitosis, epithelial differentiation and the Notch, Wnt and BMP signaling pathways to be significantly overrepresented. Elucidation of these pathways at the RNA and protein levels and physiologic endpoints suggests that Gata3 integrates diverse signaling networks to regulate the balance between hair follicle and epidermal cell fates.

Major shifts in genomic activity accompany progression through different stages of the hair cycle

Hair follicles display a unique pattern of cyclic growth and regression involving cell proliferation, differentiation and migration. The molecular details of these processes are largely unexplored. Global expression analyses on the basis of about 20,000 genes for each morphologically distinguishable stage of the hair cycle revealed unexpected complexities of and major temporal shifts in transcriptional programs involving about 13% of all genes. In particular, hundreds of genes characterise the pattern of genomic activity during regression and resting phases; selected genes can be used to monitor hair growth in mice. We demonstrate that temporal expression patterns predict gene expression domains within the hair follicle. Expression of insulin-like growth factor binding proteins and anti-angiogenic factors is associated with the regression phase of the hair cycle.

Follicle-stimulating hormone suppresses cytosolic 3,5,3'-triiodothyronine-binding protein messenger ribonucleic acid expression in rat granulosa cells

FSH plays crucial roles in differentiation of granulosa cells and development of follicles. Considering the broad scope of FSH effects, a large number of genes are likely responsive to the hormone. However, only a limited number of genes have been identified as FSH-regulated genes, particularly during the preantral stage. In an attempt to better define genes involved in follicular development, we examined primary granulosa cell cultures, an undifferentiated rat ovarian granulosa cell line and rat ovaries, using differential display, quantitative RT-PCR, Northern blot analysis, and in situ hybridization. We report, for the first time, that nicotinamide adenine dinucleotide phosphate-dependent cytosolic T(3)-binding protein mRNA is expressed in the ovary, particularly in the granulosa cell layer of preantral and early antral follicles, but not in large preovulatory follicles. Its expression markedly declines in response to FSH, which is dependent on the period of the exposure. This FSH-responsive down-regulation is dependent on granulosa cell differentiation and follicular development. FSH down-regulates the mRNA via the adenylyl cyclase/cAMP pathway, and the down-regulation requires de novo synthesis of a regulatory protein(s). The cytosolic T(3)-binding protein may play a significant role in the regulation of steroidogenesis and follicular development in the mammalian ovary.