Nerve growth factor in pituitary development and pituitary tumors

A bioinformatics-based approach and expression assay for identification of dysregulated genes in pituitary adenoma

Non-functioning pituitary adenomas (NFPAs) are a group of pituitary neuroendocrine tumors that are associated with morbidity. The exact pathophysiological process leading to this pathology is not known. Nerve growth factor (NGF) is a neurotropic factor that might be involved in this process. We used bioinformatics tools to analyze expression of genes in NFPA samples. Our analyses led to identification of NGF-related genes, namely ARC, ID1, and SH3GL3 - as well as one long non-coding RNA (lncRNA) called myocardial infarction associated transcript (MIAT). Then, we assessed their expression in NFPAs and their adjacent non-cancerous samples. While expression levels of SH3GL3 and MIAT were different between NFPA samples and control samples, expressions of ARC and ID1 were not meaningfully different between these two groups of specimens. SH3GL3 was over-expressed in NFPA samples compared with control samples (expression ratio (95% CI)= 8.22 (1.51-44.6), P value= 0.03). Similarly, expression of MIAT was higher in NFPAs compared with controls (expression ratio (95% CI)= 7.7 (1.7-33.6), P value= 0.009). Taken together, we validated the bioinformatics results regarding the expression of SH3GL3 and MIAT. This study provides a deeper understanding of the involvement of these genes in the pituitary tumorigenesis.

Cellular interactions in the pituitary stem cell niche

Stem cells in the anterior pituitary gland can give rise to all resident endocrine cells and are integral components for the appropriate development and subsequent maintenance of the organ. Located in discreet niches within the gland, stem cells are involved in bi-directional signalling with their surrounding neighbours, interactions which underpin pituitary gland homeostasis and response to organ challenge or physiological demand. In this review we highlight core signalling pathways that steer pituitary progenitors towards specific endocrine fate decisions throughout development. We further elaborate on those which are conserved in the stem cell niche postnatally, including WNT, YAP/TAZ and Notch signalling. Furthermore, we have collated a directory of single cell RNA sequencing studies carried out on pituitaries across multiple organisms, which have the potential to provide a vast database to study stem cell niche components in an unbiased manner. Reviewing published data, we highlight that stem cells are one of the main signalling hubs within the anterior pituitary. In future, coupling single cell sequencing approaches with genetic manipulation tools in vivo, will enable elucidation of how previously understudied signalling pathways function within the anterior pituitary stem cell niche.

Time-Resolved Integrative Optical Imaging of Diffusion during Spreading Depression

An improved version of the integrative optical imaging method has been developed that substantially increases the time resolution of diffusion measurements. We present a theory for time-resolved integrative optical imaging that incorporates a time-dependent effective diffusion coefficient in homogeneous anisotropic media and a time-dependent nonspecific linear clearance. The method was applied to measure the very fast changes in extracellular diffusion that occur during spreading depression in rat hippocampal slices. We were able to achieve a time resolution of approximately 1 s, an improvement of at least 10 times compared to the standard methods for extracellular diffusion measurement. We have found that diffusion of a small fluorescent extracellular marker (MW 3000) completely stopped during the maximum direct current shift associated with the spreading depression wave, then gradually resumed over several minutes afterward. The effect of spreading depression on extracellular space is much larger than previously estimated by other methods with lower time resolution.

Atypical pituitary adenoma with neurocytic transformation

Here, we report an example of an atypical prolactin-producing pituitary adenoma showing clear morphologic and immunohistochemical evidence of neurocytic transformation. Its features support the concept that neoplastic neuroendocrine cells, in this case adenohypophyseal cells, are capable of neuronal differentiation and broaden the morphologic spectrum of such rare tumors. Our findings have implications with respect to the nosology of neuronal tumors of the adenohypophysis.

Cellular and molecular specificity of pituitary gland physiology

The anterior pituitary gland has the ability to respond to complex signals derived from central and peripheral systems. Perception of these signals and their integration are mediated by cell interactions and cross-talk of multiple signaling transduction pathways and transcriptional regulatory networks that cooperate for hormone secretion, cell plasticity, and ultimately specific pituitary responses that are essential for an appropriate physiological response. We discuss the physiopathological and molecular mechanisms related to this integrative regulatory system of the anterior pituitary gland and how it contributes to modulate the gland functions and impacts on body homeostasis.

Enhancement of angiogenic effect of co-transfection human NGF and VEGF genes in rat bone marrow mesenchymal stem cells

The current study explored the feasibility and efficacy of co-transfection of the human nerve growth factor (NGF) and vascular endothelial growth factor 165 (VEGF165) genes in rat bone marrow mesenchymal stem cells (BMSCs). The obtained hNGF and vascular endothelial growth factor (VEGF) cDNAs were cloned into the pEGFP-C1 expression vector to construct the recombinant vectors. Co-transfection in rat BMSCs was performed and the expressions of both genes were detected by RT-PCR, Western blot, and enzyme-linked immunospecific assay. The biological activity of recombinant NGF and VEGF proteins was confirmed using the Chick Chorioallantoic Membrane (CAM) assay. NGF and VEGF genes could be expressed successfully in rat BMSCs. The recombinant NGF and VEGF from the rat BMSCs showed a more significant synergetic biological activity compared with single recombinant NGF or VEGF. These findings demonstrate that the co-transfection of hNGF+VEGF genes can enhance the angiogenic effect in vivo.

Modulation of the tyrosine kinase receptor Ret/glial cell-derived neurotrophic factor (GDNF) signaling: a new player in reproduction induced anterior pituitary plasticity?

During gestation, parturition, and lactation, the endocrine axis of the dam must continually adapt to ensure the continual and healthy development of offspring. The anterior pituitary gland, which serves as the endocrine interface between the brain and periphery, undergoes adaptations that contribute to regulation of the reproductive axis. Growth factors and their receptors are potential candidates for intrapituitary and paracrine factors to participate in the functional and anatomical plasticity of the gland. We examined the involvement of the growth factor glial cell-derived neurotrophic factor (GDNF) and its receptor tyrosine kinase rearranged during transfection (Ret) in the physiological functional and anatomical plasticity of the anterior pituitary gland. We found that variations in both expression and subcellular localization of Ret during gestation and lactation are temporally correlated with changes in pituitary gland function. We showed that Ret/GDNF signaling could endorse two different functional roles depending on the physiological status. At the end of lactation and after weaning, Ret was colocalized with markers of apoptosis. We found that Ret could therefore act as a physiological dependence receptor capable of inducing apoptosis in the absence of GDNF. In addition, we identified the follicullostellate cell as a probable source for intrapituitary GDNF and proposed GDNF as a potential physiological modulator of endocrine cell function. During all stages studied, we showed that acute application of GDNF to pituitary slices was able to modulate both positively and negatively intracellular calcium activity. Altogether our results implicate Ret/GDNF as a potent pleiotropic factor able to influence pituitary physiology during a period of high plasticity.

High-level expression of recombinant human nerve growth factor beta in milk of nontransgenic rabbits

The technology for the large-scale production of therapeutic recombinant proteins remains a challenge in the biopharmaceutical industry. In this study, we reported a nontransgenic approach to producing a large quantity of human nerve growth factor beta (hNGF-beta) in rabbit milk by employing a recombinant adenoviral expression system. After directly instilling hNGF-beta recombinant adenoviruses into rabbit mammary glands, a polypeptide with a molecular weight of 13.2 kDa was detected in rabbit milk. The maximal expression level of hNGF-beta reached 346 mug/ml. The biological activity of recombinant hNGF-beta was confirmed using PC12 cells and cultures of dorsal root ganglion neurons from chicken embryos. Our data suggest that instilling recombinant adenovirus directly into the mammary gland of mammals is an efficient approach to producing a large quantity of hNGF-beta.

Advances in the treatment of prolactinomas

Prolactinomas account for approximately 40% of all pituitary adenomas and are an important cause of hypogonadism and infertility. The ultimate goal of therapy for prolactinomas is restoration or achievement of eugonadism through the normalization of hyperprolactinemia and control of tumor mass. Medical therapy with dopamine agonists is highly effective in the majority of cases and represents the mainstay of therapy. Recent data indicating successful withdrawal of these agents in a subset of patients challenge the previously held concept that medical therapy is a lifelong requirement. Complicated situations, such as those encountered in resistance to dopamine agonists, pregnancy, and giant or malignant prolactinomas, may require multimodal therapy involving surgery, radiotherapy, or both. Progress in elucidating the mechanisms underlying the pathogenesis of prolactinomas may enable future development of novel molecular therapies for treatment-resistant cases. This review provides a critical analysis of the efficacy and safety of the various modes of therapy available for the treatment of patients with prolactinomas with an emphasis on challenging situations, a discussion of the data regarding withdrawal of medical therapy, and a foreshadowing of novel approaches to therapy that may become available in the future.

Nerve growth factor affects Ca2+ currents via the p75 receptor to enhance prolactin mRNA levels in GH3 rat pituitary cells

In clonal pituitary GH(3) cells, spontaneous action potentials drive the opening of Ca(v)1 (L-type) channels, leading to Ca(2+) transients that are coupled to prolactin gene transcription. Nerve growth factor (NGF) has been shown to stimulate prolactin synthesis by GH(3) cells, but the underlying mechanisms are unknown. Here we studied whether NGF influences prolactin gene expression and Ca(2+) currents. By using RT-PCR, NGF (50 ng ml(-1)) was found to augment prolactin mRNA levels by approximately 80% when applied to GH(3) cells for 3 days. A parallel change in the prolactin content was detected by Western blotting. Both NGF-induced responses were mimicked by an agonist (Bay K 8644) and prevented by a blocker (nimodipine) of L-type channels. In whole-cell patch-clamp experiments, NGF enhanced the L-type Ca(2+) current by approximately 2-fold within 60 min. This effect reversed quickly upon growth factor withdrawal, but was maintained for days in the continued presence of NGF. In addition, chronic treatment (>or= 24 h) with NGF amplified the T-type current, which flows through Ca(v)3 channels and is thought to support pacemaking activity. Thus, NGF probably increases the amount of Ca(2+) that enters per action potential and may also induce a late increase in spike frequency. MC192, a specific antibody for the p75 neurotrophin receptor, but not tyrosine kinase inhibitors (K252a and lavendustin A), blocked the effects of NGF on Ca(2+) currents. Overall, the results indicate that NGF activates the p75 receptor to cause a prolonged increase in Ca(2+) influx through L-type channels, which in turn up-regulates the prolactin mRNA.

Triiodothyronine expands the lactotroph and maintains the lactosomatotroph population, whereas thyrotrophin-releasing hormone augments thyrotroph abundance in aggregate cell cultures of postnatal rat pituitary gland

In the present study, we used a three-dimensional pituitary cell culture system from early postnatal rats to examine the in vitro developmental potential of triiodothyronine (T3) and thyrotrophin-releasing hormone (TRH). Cell types were identified at the hormone mRNA level by single-cell reverse transcription-polymerase chain reaction and any change in abundance was further examined by immunofluorescence staining of the corresponding hormone protein. In aggregates from 14-day-old rats, long-term (12-16 days) treatment with T3 (0.5 nM) increased the abundance of cells expressing prolactin mRNA (PRLmRNA cells) by 2.5-fold and lowered that of nonhormonal cells and thyroid-stimulating hormone beta (TSHbeta)mRNA cells. The abundance of growth hormone (GH)mRNA cells decreased during culture compared to that in the freshly dispersed pituitary gland and T3 did not significantly affect this cell population. Cells coexpressing PRL mRNA and GH mRNA virtually disappeared during culture but reappeared in the presence of T3. T3 increased the abundance of PRL-immunoreactive (ir) cells in aggregates from 14-day-old rats, as well as in aggregates from newborn and 1-week-old rats. As estimated by bromodeoxyuridine (BrdU) labelling, a 3-day treatment with T3 enhanced the number of PRL-ir cells that had incorporated BrdU, but did not yet expand the total population of PRL-ir cells. Long-term treatment with TRH (100 nM) did not affect the proportion of PRLmRNA or GHmRNA cells, but consistently increased the proportional number of TSHbeta(mRNA) and TSHbeta-ir cells. The present data confirm the findings obtained in recent in vivo loss of function genetic studies suggesting that T3 plays a prominent role in postnatal expansion of the lactotroph population and that TRH is important for thyrotroph development. The data suggest that the effect of T3 is brought about by a direct action on the pituitary gland through a cell proliferation mechanism. T3 also appears to support the lactosomatotroph population. In view of the established theory that lactotrophs develop from GH-expressing progenitor cells and that this is a post mitotic event, we propose that T3 is mitogenic for GHmRNA cells that lack GH-ir material and that transdifferentiate into PRL-ir cells, but that a pathway of PRL cell development from mitotic nonhormonal cell progenitors may also be involved.

Pituitary tumors: prognostic indicators

Many factors influence the proliferation of pituitary adenomas: angiogenesis, apoptosis, growth factors, oncogenes, tumor suppressor genes, and hormone receptors. These elements can be demonstrated by immunohistochemistry and/or molecular pathology but no single factor can be used for determination of biological behavior resp. prognosis. Pituitary adenomas can be enclosed or invasive and may be very large or may be microadenomas, but the most important point for prognosis is the total resection in the first or second surgery or the reaction on treatments by drugs. Especially for residual tumor tissue proliferation, markers are important because they may indicate the growth rate and the aggressiveness of the tumor. Radiation therapy is indicated in many of these recurrent tumors and can improve the prognosis.

Increased serum concentration of nerve growth factor in patients with microprolactinoma

Nerve growth factor (NGF) is known to play a role as a circulating neurokine, integrating signals from the neuro-immuno-endocrine system. The ability of NGF to activate the pituitary-adrenocortical axis, together with the increase of its serum concentration in pregnancy and lactation, supports the hypothesis that NGF is secreted by the pituitary gland and plays a role as modulator of endocrine functions. Evidence obtained both in vitro and in vivo in experimental animal models suggests that lactotroph cells secrete both prolactin (PRL) and NGF. Furthermore, in previous studies we demonstrate that cell lines derived from dopamine (DA)-sensitive human prolactinomas express high levels of NGF messenger RNA and protein. On these basis, we studied serum NGF concentrations in female patients with microprolactinoma (n = 4) and in control women (n = 5). PRL and NGF were measured at the diagnosis, during the thyrotropin releasing hormone (TRH) test and after the therapy with DA D2 receptor agonist cabergoline (0.25 mg, twice a week). Results obtained suggested that hyperprolactinemia (70.3+/-8.4 ng/ml) paralleled markedly higher NGF levels (58.4+/-18.7 pg/ml) compared to controls (PRL 8.7+/-3.2 ng/ml, NGF 8.4+/-1.3 pg/ml). Serum concentrations of NGF and PRL during the TRH test were closely associated (r = 0.943, p < 0.01). Cabergoline therapy normalized PRL (7.9+/-3.6 ng/ml) and induced a significant decrease of NGF levels (12.5+/-4.9 pg/ml). In conclusions, data reported here indicated that, in human microprolactinomas, NGF is released in the bloodstream paralleling PRL-secretion and it is modulated by a neurotransmitter-regulated mechanism, since the normalization of PRL elicited by the DA D2 receptor agonist cabergoline induced a significant decrease of serum NGF as well.

Molecular defects in the pathogenesis of pituitary tumours

The majority of pituitary adenomas are trophically stable and change relatively little in size over many years. A comparatively small proportion behave more aggressively and come to clinical attention through inappropriate hormone secretion or adverse effects on surrounding structures. True malignant behaviour with metastatic spread is very atypical. Pituitary adenomas that come to surgery are predominantly monoclonal in origin and roughly half are aneuploid, indicating either ongoing genetic instability or transition through a period of genetic instability at some time during their development. Few are associated with the classical mechanisms of tumour formation but it is generally believed that the majority harbour quantitative if not qualitative differences in molecular composition compared to the normal pituitary. Despite their prevalence and the ready availability of biopsy material, at the present time, the precise molecular pathogenesis of the majority of pituitary adenomas remains unclear. This review summarizes current thinking.

Sex-related variations in serum nerve growth factor concentration in humans

A role of nerve growth factor (NGF) in the neuro-endocrine-immune interactions has been recently suggested by the presence of NGF and its receptors in cells of the immune and endocrine systems. The improvement in the comprehension of the role played by NGF in humans is linked to the availability of a sensitive and reliable method to quantify NGF concentrations in body fluids and tissues. As a consequence of different methods used, normal levels of human serum NGF reported in the literature show wide differences. The present results indicate that ELISA appears very sensitive (detection limit 1.4pg/ml) and allows the discrimination of subtle variations of serum NGF concentrations. ELISA performed in serum obtained from men indicated that NGF concentration was 40.8+/-10.8pg/ml, whereas women showed significantly lower levels that were influenced by the menstrual cycle. In particular, the mean value of this neurotrophin during the follicular phase was 8.2+/-1.4pg/ml; the luteal phase, in turn, showed levels up to 14.4+/-2.9pg/ml. The difference of serum NGF concentrations between the follicular and luteal phase in each woman was statistically significant. Differences in NGF concentrations between men and women (in both phases of the menstrual cycles) were also statistically significant. In conclusion, a possible role of sex steroids as modulators of NGF secretion in humans is strongly supported by the present paper. However, mechanisms underlying this phenomenon are still unknown. The evidence indicating physiological sex hormone-related variations in NGF levels would be of interest in view of the possible use of circulating NGF modifications as a laboratory biomarker in different diseases.

Oestrogen regulates neurofilament expression in a subset of anterior pituitary cells of the adult female rat

It is the prevailing view that the neurohypophysis derives from neural crest while the pituitary's anterior lobe is of ectodermal origin. However, it has been recently suggested that anterior pituitary cells could have in part neuro-ectodermal origin, and thus should express specific neuronal markers. This issue was examined previously with conflicting results. The present study attempts to clarify the question of whether or not neuronal markers are expressed in the adenohypophysis. Using quantitative immunofluorescence, we have positively identified a subset of anterior pituitary cells, which express immunoreactivity for neuronal markers, including 68 kDa neurofilament (NF68). Interestingly, we noticed that the expression of NF68 is sexually dimorphic (i.e. neurofilament-positive cells are more abundant in sexually mature female rats). In addition, NF68 expression in female rats increases during ontogenic development and reaches a plateau level after puberty. Thereafter, it displays plastic changes along the oestrous cycle, with the maximum of neurofilament expression at oestrus and the minimum at proestrus. NF68 immunoreactivity was examined after ovariectomy, oestradiol replacement and treatment with an specific oestrogen receptor antagonist. Bilateral ovariectomy induced a significant reduction in the number of NF68-positive cells. This effect was completely prevented by treatment of ovariectomized rats with oestradiol. When intact female rats were treated with the anti-oestrogen tamoxifen, a drastic decrease in NF68 expression in anterior pituitary cells was observed. Furthermore, oestradiol administration in castrated male rats increased NF68 immunoreactivity. Double-immunolabelling experiments provided evidence that pituitary cells expressing neuronal traits correspond to subsets of lactotrophs, somatotrophs, thyrotrophs and gonadotrophs. It remains to be established if NF68 induction in the pituitary is due to direct and/or indirect effects of oestrogens. Also, the possible functional role of this subset of NF68-positive anterior pituitary cells in the female rat remains to be examined.

Dopamine as a prolactin (PRL) inhibitor

Dopamine is a small and relatively simple molecule that fulfills diverse functions. Within the brain, it acts as a classical neurotransmitter whose attenuation or overactivity can result in disorders such as Parkinson's disease and schizophrenia. Major advances in the cloning and characterization of biosynthetic enzymes, transporters, and receptors have increased our knowledge regarding the metabolism, release, reuptake, and mechanism of action of dopamine. Dopamine reaches the pituitary via hypophysial portal blood from several hypothalamic nerve tracts that are regulated by PRL itself, estrogens, and several neuropeptides and neurotransmitters. Dopamine binds to type-2 dopamine receptors that are functionally linked to membrane channels and G proteins and suppresses the high intrinsic secretory activity of the pituitary lactotrophs. In addition to inhibiting PRL release by controlling calcium fluxes, dopamine activates several interacting intracellular signaling pathways and suppresses PRL gene expression and lactotroph proliferation. Thus, PRL homeostasis should be viewed in the context of a fine balance between the action of dopamine as an inhibitor and the many hypothalamic, systemic, and local factors acting as stimulators, none of which has yet emerged as a primary PRL releasing factor. The generation of transgenic animals with overexpressed or mutated genes expanded our understanding of dopamine-PRL interactions and the physiological consequences of their perturbations. PRL release in humans, which differs in many respects from that in laboratory animals, is affected by several drugs used in clinical practice. Hyperprolactinemia is a major neuroendocrine-related cause of reproductive disturbances in both men and women. The treatment of hyperprolactinemia has greatly benefited from the generation of progressively more effective and selective dopaminergic drugs.

Prolactin: structure, function, and regulation of secretion

Prolactin is a protein hormone of the anterior pituitary gland that was originally named for its ability to promote lactation in response to the suckling stimulus of hungry young mammals. We now know that prolactin is not as simple as originally described. Indeed, chemically, prolactin appears in a multiplicity of posttranslational forms ranging from size variants to chemical modifications such as phosphorylation or glycosylation. It is not only synthesized in the pituitary gland, as originally described, but also within the central nervous system, the immune system, the uterus and its associated tissues of conception, and even the mammary gland itself. Moreover, its biological actions are not limited solely to reproduction because it has been shown to control a variety of behaviors and even play a role in homeostasis. Prolactin-releasing stimuli not only include the nursing stimulus, but light, audition, olfaction, and stress can serve a stimulatory role. Finally, although it is well known that dopamine of hypothalamic origin provides inhibitory control over the secretion of prolactin, other factors within the brain, pituitary gland, and peripheral organs have been shown to inhibit or stimulate prolactin secretion as well. It is the purpose of this review to provide a comprehensive survey of our current understanding of prolactin's function and its regulation and to expose some of the controversies still existing.

Nerve growth factor and retinoic acid inhibit proliferation and invasion in thyroid tumor cells

NGF has anti-proliferative and anti-invasive effects in neuroendocrine tumors. In the present work we examined the effects of NGF and retinoic acid on cell proliferation and invasion in thyroid carcinoma cells. We found that NGF and retinoic acid do not affect cell proliferation on their own but in combination they produce a strong inhibition. We also found that retinoic acid regulates the matrix metalloproteinase 2 activity and invasion. In contrast, NGF inhibited invasion and reverted the effect of retinoic acid. This effect of NGF is likely mediated by an increase in adhesion to laminin and collagen IV and the inhibition of cell migration. NGF also induced the expression of the p75 NGF receptor. In conclusion, NGF and retinoic acid in combination inhibit proliferation and invasion of thyroid papillary carcinoma cells. These data open the possibility of a potential combined therapy for thyroid papillary carcinomas.

Expression of Growth Factors in Normal and Neoplastic Pituitary Tissues

Many growth factors are expressed in normal pituitary cells and pituitary tumors. They are involved in gene expression for pituitary hormones and in cell proliferation. Some appear to be important for prognosis or treatment. Strong overexpression of some growth factors may indicate a more rapid growth. The significance of the different growth factors for pituitary function and pathology is discussed.

Progenitor cells in the embryonic anterior pituitary abruptly and concurrently depress mitotic rate before progressing to terminal differentiation

The control of progenitor cell proliferation in concert with terminal differentiation during embryonic development is poorly understood. The present paper examines this issue in the different cell lineages of the fetal mouse pituitary. Mouse fetuses were pulse-exposed to 3H-thymidine (3H-T) on a single day between embryonic day (E) 10 and E16 (prior to the onset of hormone phenotype expression) and the 3H-T labeling index of each cell type determined 3 or 4 days later (E13-19), when hormone phenotypes were detectable. In the pars tuberalis primordium, TSHbeta appeared from E13. Of these cells 75.5% were labeled when 3H-T had been administered on E10. Label decreased to 40.8% when it had been incorporated on E11 and was negligible (4.2%) when it had been taken up on E12. In the pars distalis, ACTH appeared on E13, TSHbeta, and PRL on E14, LHbeta/FSHbeta on E15 and GH on E16. When examined on E16, all these cell types were labeled for 50-60% if 3H-T had been injected on E12, but this number dropped to about 15% when 3H-T had been given on E13. Only 5-10% of the hormonal cells had taken up label when E14, 15, and 16 were the days of 3H-T administration. The decline in overall labeling index (LI) within both parts of the pituitary was significantly smaller than that in the hormone expressing cells. It is concluded that an outspoken decline in proliferation of the cells destined to become hormone-expressing cell types occurs one to several days before these hormones come to expression. In the pars distalis, this decline occurs at a common time point i.e. between E12 and E13 for each cell type. Pars tuberalis and pars distalis TSHbeta cells show distinct 3H-T labeling profiles, suggesting distinct cell lineage sources for each.

Thyroid hormone is essential for pituitary somatotropes and lactotropes

Mice homozygous for a disruption in the alpha-subunit essential for TSH, LH, and FSH activity (alphaGsu-/-) exhibit hypothyroidism and hypogonadism similar to that observed in TSH receptor-deficient hypothyroid mice (hyt) and GnRH-deficient hypogonadal mutants (hpg). Although the five major hormone-producing cells of the anterior pituitary are present in alphaGsu-/- mice, the relative proportions of each cell type are altered dramatically. Thyrotropes exhibit hypertrophy and hyperplasia, and somatotropes and lactotropes are underrepresented. The size and number of gonadotropes in alphaGsu mutants are not remarkable in contrast to the hypertrophy characteristic of gonadectomized animals. The reduction in lactotropes is more severe in alphaGsu mutants (13-fold relative to wild-type) than in hyt or hpg mutants (4.5- and 1.5-fold, respectively). In addition, T4 replacement therapy of alphaGsu mutants restores lactotropes to near-normal levels, illustrating the importance of T4, but not alpha-subunit, for lactotrope proliferation and function. T4 replacement is permissive for gonadotrope hypertrophy in alphaGsu mutants, consistent with the role for T4 in the function of gonadotropes. This study reveals the importance of thyroid hormone in developing the appropriate proportions of anterior pituitary cell types.