Minireview: intraislet regulation of insulin secretion in humans

The higher organization of β-cells into spheroid structures termed islets of Langerhans is critical for the proper regulation of insulin secretion. Thus, rodent β-cells form a functional syncytium that integrates and propagates information encoded by secretagogues, producing a "gain-of-function" in hormone release through the generation of coordinated cell-cell activity. By contrast, human islets possess divergent topology, and this may have repercussions for the cell-cell communication pathways that mediate the population dynamics underlying the intraislet regulation of insulin secretion. This is pertinent for type 2 diabetes mellitus pathogenesis, and its study in rodent models, because environmental and genetic factors may converge on these processes in a species-specific manner to precipitate the defective insulin secretion associated with glucose intolerance. The aim of the present minireview is therefore to discuss the structural and functional underpinnings that influence insulin secretion from human islets, and the possibility that dyscoordination between individual β-cells may play an important role in some forms of type 2 diabetes mellitus.

Lipotoxicity disrupts incretin-regulated human β cell connectivity

Pancreatic β cell dysfunction is pathognomonic of type 2 diabetes mellitus (T2DM) and is driven by environmental and genetic factors. β cell responses to glucose and to incretins such as glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are altered in the disease state. While rodent β cells act as a coordinated syncytium to drive insulin release, this property is unexplored in human islets. In situ imaging approaches were therefore used to monitor in real time the islet dynamics underlying hormone release. We found that GLP-1 and GIP recruit a highly coordinated subnetwork of β cells that are targeted by lipotoxicity to suppress insulin secretion. Donor BMI was negatively correlated with subpopulation responses to GLP-1, suggesting that this action of incretin contributes to functional β cell mass in vivo. Conversely, exposure of mice to a high-fat diet unveiled a role for incretin in maintaining coordinated islet activity, supporting the existence of species-specific strategies to maintain normoglycemia. These findings demonstrate that β cell connectedness is an inherent property of human islets that is likely to influence incretin-potentiated insulin secretion and may be perturbed by diabetogenic insults to disrupt glucose homeostasis in humans.

Navigating pituitary structure and function - defining a roadmap for hormone secretion

The human pituitary gland may be the size of a pea, but it punches well above its weight due to secretion of hormones that regulate essential homeostatic processes ranging from growth to stress. Recent advances in imaging and genetics have allowed large-scale interrogation of pituitary structure and function, and in doing so, have revealed that output relies on its organisation into highly plastic networks.

Cells co-expressing luteinising hormone and thyroid-stimulating hormone are present in the ovine pituitary pars distalis but not the pars tuberalis: implications for the control of endogenous circannual rhythms of prolactin

BACKGROUND/AIMS

A mammalian circannual pacemaker responsible for regulating the seasonal pattern of prolactin has been recently described in sheep. This pacemaker resides within the pars tuberalis, an area of the pituitary gland that densely expresses melatonin receptors. However, the chemical identity of the cell type which acts as the pacemaker remains elusive. Mathematical-modelling approaches have established that this cell must be responsive to the static melatonin signal as well as prolactin negative feedback. Considering that in sheep the gonadotroph is the only cell in the pars tuberalis which expresses the prolactin receptor, and that in other photoperiodic species the thyrotroph is the only cell expressing the melatonin receptor in this tissue, a cell type which expresses both proteins would fulfil the theoretical criteria of a circannual pacemaker.

METHODS

Pituitary glands were obtained from female sheep under short days (breeding season) and long days (non-breeding season) and double immunofluorescent staining was conducted to determine the prevalence of bi-hormonal cells in the pars distalis and pars tuberalis using specific antibodies to luteinising hormone-β and thyroid-stimulating hormone-β.

RESULTS

The results reveal that whilst such a bihormonal cell is clearly present in the pars distalis and constitute 4% of the gonadotroph population in this region, the same cell type is completely absent from the pars tuberalis even though LH gonadotrophs are abundantly expressed.

CONCLUSIONS

Based on these findings, together with existing data, we are able to propose an alternative model where the gonadotroph itself is controlled indirectly by neighbouring melatonin responsive cells, allowing it to act as a pacemaker.

Anterior pituitary cell networks

Both endocrine and non-endocrine cells of the pituitary gland are organized into structural and functional networks which are formed during embryonic development but which may be modified throughout life. Structural mapping of the various endocrine cell types has highlighted the existence of distinct network motifs and relationships with the vasculature which may relate to temporal differences in their output. Functional characterization of the network activity of growth hormone and prolactin cells has revealed a role for cell organization in gene regulation, the plasticity of pituitary hormone output and remarkably the ability to memorize altered demand. As such, the description of these endocrine cell networks alters the concept of the pituitary from a gland which simply responds to external regulation to that of an oscillator which may memorize information and constantly adapt its coordinated networks' responses to the flow of hypothalamic inputs.

A tridimensional view of pituitary development and function

Recent advances in tridimensional (3D) tissue imaging have considerably enriched our view of the pituitary gland and its development. Whereas traditional histology of the pituitary anterior lobe portrayed this tissue as a patchwork of cells, 3D imaging revealed that cells of each lineage form extensive and structured homotypic networks. In the adult gland these networks contribute to the robustness and coordination of the cell response to secretagogs. In addition, the network organization adapts to changes in endocrine environment, as revealed by the sexually dimorphic growth hormone (GH) cell network. Further work is required to establish better the molecular basis for homotypic and heterotypic interactions in the pituitary as well as the implications of these interactions for pituitary function and dysfunction in humans.

Photoperiodic modulation of the suppressive actions of prolactin and dopamine on the pituitary gonadotropin responses to gonadotropin-releasing hormone in sheep

In a variety of species, the LH-secretory response to gonadotropin-releasing hormone (GnRH) is completely suppressed by the combined actions of prolactin (PRL) and dopamine (DA). In sheep, this effect is only observed under long days (nonbreeding season [NBS]). To investigate the level at which these mechanisms operate, we assessed the effects of PRL and bromocriptine (Br), a DA agonist, on the gonadotropin-secretory and mRNA responses to GnRH in pituitary cell cultures throughout the ovine annual reproductive cycle. As expected, the LH-secretory response to GnRH was only abolished during the NBS following combined PRL and Br application. Conversely, the LHB subunit response to GnRH was reduced during both the BS and NBS by the combined treatment and Br alone. Similar results were obtained in pars distalis-only cultures, indicating that the effects are pars tuberalis (PT)- independent. Further signaling studies revealed that PRL and Br alter the LH response to GnRH via convergence at the level of PLC and PKC. Results for FSH generally reflected those for LH, except during the BS where removal of the PT allowed PRL and Br to suppress the FSH-secretory response to GnRH. These data show that suppression of the LH-secretory response to GnRH by PRL and DA is accompanied by changes in mRNA synthesis, and that the photoperiodic modulation of this inhibition operates primarily at the level of LH release through alterations in PKC and PLC. Furthermore, the suppressive effects of PRL and DA on the secretion of FSH are photoperiodically regulated in a PT-dependent manner.

Pituitary endocrine cell networks - 10 years and beyond

The secretion of peptide hormone by the pituitary gland is obligatory for controlling a wide-range of downstream physiological processes. To achieve this, endocrine cells must respond co-ordinately to hypothalamic input to release defined pulses of hormone into the bloodstream. The organ context is clearly essential for proper hormone release since enzymatically dispersed cells mount attenuated responses to secretagogue. Yet, scant attention has been paid to whether endocrine cell populations are organized at the tissue level within the pituitary. Recently, using transgenic animals allied to sophisticated in situ imaging techniques, we have shown that endocrine cells are homotypically organized into three-dimensional networks. In addition, using multicellular calcium imaging combined with online/real-time measures of hormone secretion, we have demonstrated that these networks play a crucial role in hormone release by integrating hypothalamic signals at the population level. As such, it is anticipated that perturbation of endocrine network function may underlie hormone deficits associated with pituitary dysfunction.

Existence of long-lasting experience-dependent plasticity in endocrine cell networks

Experience-dependent plasticity of cell and tissue function is critical for survival by allowing organisms to dynamically adjust physiological processes in response to changing or harsh environmental conditions. Despite the conferred evolutionary advantage, it remains unknown whether emergent experience-dependent properties are present in cell populations organized as networks within endocrine tissues involved in regulating body-wide homeostasis. Here we show, using lactation to repeatedly activate a specific endocrine cell network in situ in the mammalian pituitary, that templates of prior demand are permanently stored through stimulus-evoked alterations to the extent and strength of cell–cell connectivity. Strikingly, following repeat stimulation, evolved population behaviour leads to improved tissue output. As such, long-lasting experience-dependent plasticity is an important feature of endocrine cell networks and underlies functional adaptation of hormone release.

Experience-dependent plasticity and functional adaptation are thought to be restricted to the central nervous and immune systems. This study shows that long-lasting experience-dependent plasticity is a key feature of endocrine cell networks, allowing improved tissue function and hormone output following repeat demand.

Coordination of calcium signals by pituitary endocrine cells in situ

The pulsatile secretion of hormones from the mammalian pituitary gland drives a wide range of homeostatic responses by dynamically altering the functional set-point of effector tissues. To accomplish this, endocrine cell populations residing within the intact pituitary display large-scale changes in coordinated calcium-spiking activity in response to various hypothalamic and peripheral inputs. Although the pituitary gland is structurally compartmentalized into specific and intermingled endocrine cell networks, providing a clear morphological basis for such coordinated activity, the mechanisms which facilitate the timely propagation of information between cells in situ remain largely unexplored. Therefore, the aim of the current review is to highlight the range of signalling modalities known to be employed by endocrine cells to coordinate intracellular calcium rises, and discuss how these mechanisms are integrated at the population level to orchestrate cell function and tissue output.

Influence of estrogens on GH-cell network dynamics in females: a live in situ imaging approach

The secretion of endocrine hormones from pituitary cells finely regulates a multitude of homeostatic processes. To dynamically adapt to changing physiological status and environmental stimuli, the pituitary gland must undergo marked structural and functional plasticity. Endocrine cell plasticity is thought to primarily rely on variations in cell proliferation and size. However, cell motility, a process commonly observed in a variety of tissues during development, may represent an additional mechanism to promote plasticity within the adult pituitary gland. To investigate this, we used multiphoton time-lapse imaging methods, GH-enhanced green fluorescent protein transgenic mice and sexual dimorphism of the GH axis as a model of divergent tissue demand. Using these methods to acutely (12 h) track cell dynamics, we report that ovariectomy induces a dramatic and dynamic increase in cell motility, which is associated with gross GH-cell network remodeling. These changes can be prevented by estradiol supplementation and are associated with enhanced network connectivity as evidenced by increased coordinated GH-cell activity during multicellular calcium recordings. Furthermore, cell motility appears to be sex-specific, because reciprocal alterations are not detected in males after castration. Therefore, GH-cell motility appears to play an important role in the structural and functional pituitary plasticity, which is evoked in response to changing estradiol concentrations in the female.

Endocrine cells and blood vessels work in tandem to generate hormone pulses

Hormones are dynamically collected by fenestrated capillaries to generate pulses, which are then decoded by target tissues to mount a biological response. To generate hormone pulses, endocrine systems have evolved mechanisms to tightly regulate blood perfusion and oxygenation, coordinate endocrine cell responses to secretory stimuli, and regulate hormone uptake from the perivascular space into the bloodstream. Based on recent findings, we review here the mechanisms that exist in endocrine systems to regulate blood flow, and facilitate coordinated cell activity and output under both normal physiological and pathological conditions in the pituitary gland and pancreas.

Functional importance of blood flow dynamics and partial oxygen pressure in the anterior pituitary

The pulsatile release of hormone is obligatory for the control of a range of important body homeostatic functions. To generate these pulses, endocrine organs have developed finely regulated mechanisms to modulate blood flow both to meet the metabolic demand associated with intense endocrine cell activity and to ensure the temporally precise uptake of secreted hormone into the bloodstream. With a particular focus on the pituitary gland as a model system, we review here the importance of the interplay between blood flow regulation and oxygen tensions in the functioning of endocrine systems, and the known regulatory signals involved in the modification of flow patterns under both normal physiological and pathological conditions.

Investigating and modelling pituitary endocrine network function

Endocrine cells in the mammalian pituitary are arranged into three-dimensional homotypic networks that wire the gland and act to optimise hormone output by allowing the transmission of information between cell ensembles in a temporally precise manner. Despite this, the structure-function relationships that allow cells belonging to these networks to display coordinated activity remain relatively uncharacterised. This review discusses the recent technological advances that have allowed endocrine cell network structure and function to be probed and the mathematical models that can be used to analyse and present the resulting data. In particular, we focus on the mechanisms that allow endocrine cells to dynamically function as a population to drive hormone release as well as the experimental and theoretical methods that are used to track and model information flow through the network.

Role of prolactin in the gonadotroph responsiveness to gonadotrophin-releasing hormone during the equine annual reproductive cycle

A combined suppressive effect of prolactin (PRL) and dopamine on the secretion of luteinising hormone (LH) at the level of the pituitary gland has been identified in sheep, a short-day breeder. However, little is known about the role of PRL in the intra-pituitary regulation of the gonadotrophic axis in long-day breeders. In the present study, we investigated the effects of PRL on LH and follicle-stimulating hormone (FSH) secretion during the equine annual reproductive cycle. Horse pituitaries were obtained during the breeding season (BS) and nonbreeding season (NBS). Cells were dispersed, plated to monolayer cultures and assigned to one of the following specific treatments: (i) medium (Control); (ii) rat PRL (rPRL); (iii) thyrotrophin-releasing hormone (TRH); (iv) bromocriptine (Br); and (v) Br + rPRL. Gonadotrophin-releasing hormone (GnRH) dose-dependently stimulated LH release during the BS and NBS. During the BS, neither rPRL nor TRH affected the LH response to GnRH, but Br significantly (P < 0.01) enhanced both basal and GnRH-stimulated LH release through a mechanism that did not involve alterations in the concentrations of PRL. However, rPRL prevented the Br-induced increase in basal and GnRH-stimulated LH output, and suppressed LH below basal values (P < 0.05). Conversely, during the NBS, no significant effects of treatments were observed. Interestingly, at this time of year, the incidence of pituitary gap junctions within the pars distalis decreased by 50% (P < 0.01). By contrast to the effects on LH, no treatment effects were detected on the FSH response to GnRH, which was only apparent during the NBS. These results reveal no direct effects of PRL but an interaction between PRL and dopamine in the inhibitory regulation of LH, but not FSH, release at the level of the pituitary in the horse, and a modulatory role of season/photoperiod associated with alterations in folliculostellate cell-derived gap junctions.

Gonadotropin-releasing hormone stimulates prolactin release from lactotrophs in photoperiodic species through a gonadotropin-independent mechanism

Previous studies have provided evidence for a paracrine interaction between pituitary gonadotrophs and lactotrophs. Here, we show that GnRH is able to stimulate prolactin (PRL) release in ovine primary pituitary cultures. This effect was observed during the breeding season (BS), but not during the nonbreeding season (NBS), and was abolished by the application of bromocriptine, a specific dopamine agonist. Interestingly, GnRH gained the ability to stimulate PRL release in NBS cultures following treatment with bromocriptine. In contrast, thyrotropin-releasing hormone, a potent secretagogue of PRL, stimulated PRL release during both the BS and NBS and significantly enhanced the PRL response to GnRH during the BS. These results provide evidence for a photoperiodically modulated functional interaction between the GnRH/gonadotropic and prolactin axes in the pituitary gland of a short day breeder. Moreover, the stimulation of PRL release by GnRH was shown not to be mediated by the gonadotropins, since immunocytochemical, Western blotting, and PCR studies failed to detect pituitary LH or FSH receptor protein and mRNA expressions. Similarly, no gonadotropin receptor expression was observed in the pituitary gland of the horse, a long day breeder. In contrast, S100 protein, a marker of folliculostellate cells, which are known to participate in paracrine mechanisms within this tissue, was detected throughout the pituitaries of both these seasonal breeders. Therefore, an alternative gonadotroph secretory product, a direct effect of GnRH on the lactotroph, or another cell type, such as the folliculostellate cell, may be involved in the PRL response to GnRH in these species.

Methylocystis hirsuta sp. nov., a novel methanotroph isolated from a groundwater aquifer

Strain CSC1T, a Gram-negative, aerobic, methane-oxidizing bacterium, was isolated from an uncontaminated aquifer nearly 20 years ago. Based on 16S rRNA gene sequence similarity, this strain was identified as a member of the Alphaproteobacteria, most closely related to an uncultured member of the Methylocystaceae as well as two cultured organisms, Methylocystis sp. L32 and Methylocystis sp. SC2. This strain differed from extant species in cell shape, size, expression of soluble methane monooxygenase and its unique spiny surface layers, composed of polysaccharide. DNA–DNA hybridization results showed only 3.8 % relatedness with Methylocystis echinoides NCIMB 13100 and 41.1 % relatedness with Methylocystis rosea SV97T. Based on these genotypic and physiological differences, this isolate is proposed as a member of a novel species of the genus Methylocystis, Methylocystis hirsuta sp. nov. (type strain CSC1T =ATCC BAA-1344T =DSM 18500T).

Primary central nervous system lymphoma: a single-centre experience of 55 unselected cases

AIMS

Current treatment for primary central nervous system lymphoma (PCNSL) involves high-dose methotrexate (HDMTX) with or without radiotherapy. Many published studies describing this approach include a highly selected group of patients. We report a single-centre experience of unselected cases of PCNSL.

MATERIALS AND METHODS

We retrospectively reviewed the case notes of 55 consecutive patients diagnosed with biopsy-proven PCNSL between 1995 and 2003 at Addenbrooke's Hospital Cambridge, UK. We describe the treatment and outcome, including survival, treatment-related toxicity and long-term functional disability.

RESULTS

At diagnosis, 45% of patients were considered unfit to receive treatment with HDMTX, owing to poor performance status or comorbidity. These patients had a median survival of 46 days and may not have been included in other published studies. The remaining patients were treated with a chemotherapy regimen, which included HDMTX. Patients who received at least one cycle of a chemotherapy containing HDMTX had a median survival of 31 months. Forty per cent did not complete planned chemotherapy owing to toxicity, disease progression or death. The median survival of patients treated with HDMTX aged 60 years compared with patients aged under 60 years was 26 months vs 41 months (P = 0.07), respectively. Younger patients treated with HDMTX, who achieved complete remission with chemotherapy, had a median survival of 56 months. We identified a high incidence of functional disability among survivors, resulting from a combination of the tumour itself, the neurosurgical procedure required for diagnosis and the late neurotoxicity of combined chemoradiotherapy.

CONCLUSION

The treatment of PCNSL is associated with significant early and late toxicity. Further attempts to improve treatment should address mechanisms to reduce this toxicity. In particular, the benefit of radiotherapy in patients who achieve complete remission with HDMTX will remain uncertain until it is addressed in a multicentre, randomised trial.