Lipids in Regulated Exocytosis: What are They Doing?

t-SNARE Transmembrane Domain Clustering Modulates Lipid Organization and Membrane Curvature

The t-SNARE complex plays a central role in neuronal fusion. Its components, syntaxin-1 and SNAP25, are largely present in individual clusters and partially colocalize at the presumptive fusion site. How these protein clusters modify local lipid composition and membrane morphology is largely unknown. In this work, using coarse-grained molecular dynamics, the transmembrane domains (TMDs) of t-SNARE complexes are shown to form aggregates leading to formation of lipid nanodomains, which are enriched in cholesterol, phosphatidylinositol 4,5-bisphosphate, and gangliosidic lipids. These nano-domains induce membrane curvature that would promote a closer contact between vesicle and plasma membrane.

Phospholipase D and Its Essential Role in Cancer

The role of phospholipase D (PLD) in cancer development and management has been a major area of interest for researchers. The purpose of this mini-review is to explore PLD and its distinct role during chemotherapy including anti-apoptotic function. PLD is an enzyme that belongs to the phospholipase super family and is found in a broad range of organisms such as viruses, yeast, bacteria, animals, and plants. The function and activity of PLD are widely dependent on and regulated by neurotransmitters, hormones, small monomeric GTPases, and lipids. A growing body of research has shown that PLD activity is significantly increased in cancer tissues and cells, indicating that it plays a critical role in signal transduction, cell proliferation, and anti-apoptotic processes. In addition, recent studies show that PLD is a downstream transcriptional target of proteins that contribute to inflammation and carcinogenesis such as Sp1, NFκB, TCF4, ATF-2, NFATc2, and EWS-Fli. Thus, compounds that inhibit expression or activity of PLD in cells can be potentially useful in reducing inflammation and sensitizing resistant cancers during chemotherapy.

Phosphatidic acid-producing enzymes regulating the synaptic vesicle cycle: Role for PLD?

In cortical and hippocampal neurons of the mammalian brain, the synaptic vesicle cycle is a series of steps that tightly regulate exo- and endocytosis of vesicles. Many proteins contribute to this regulation, but lipids have recently emerged as critical regulators as well. Of all the many lipid signaling molecules, phosphatidic acid is important to the physical processes of membrane fusion. Therefore, the lipid-metabolizing enzymes that produce phosphatidic acid are vital to the regulation of the cycle. Our lab is particularly interested in the potential regulatory mechanisms and neuronal roles of two phosphatidic acid-producing enzymes: diacylglycerol kinase theta (DGKθ) and phospholipase D (PLD). We recently discovered a regulatory role of DGKθ on evoked endocytosis (Goldschmidt et al., 2016). In addition to this enzyme, studies implicate PLD1 in neurotransmission, although its precise role is of some debate. Altogether, the production of phosphatidic acid by these enzymes offer an interesting and novel pathway for the regulation of the synaptic vesicle cycle.

v-SNARE function in chromaffin cells

Vesicle fusion is elementary for intracellular trafficking and release of signal molecules, thus providing the basis for diverse forms of intercellular communication like hormonal regulation or synaptic transmission. A detailed characterization of the mechanisms underlying exocytosis is key to understand how the nervous system integrates information and generates appropriate responses to stimuli. The machinery for vesicular release employs common molecular players in different model systems including neuronal and neuroendocrine cells, in particular members of the SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptors) protein family, Sec1/Munc18-like proteins, and other accessory factors. To achieve temporal precision and speed, excitable cells utilize specialized regulatory proteins like synaptotagmin and complexin, whose interplay putatively synchronizes vesicle fusion and enhances stimulus-secretion coupling. In this review, we aim to highlight recent progress and emerging views on the molecular mechanisms, by which constitutively forming SNAREpins are organized in functional, tightly regulated units for synchronized release. Specifically, we will focus on the role of vesicle associated membrane proteins, also referred to as vesicular SNAREs, in fusion and rapid cargo discharge. We will further discuss the functions of SNARE regulators during exocytosis and focus on chromaffin cell as a model system of choice that allows for detailed structure-function analyses and direct measurements of vesicle fusion under precise control of intracellular [Ca]i.

RIBEYE(B)-domain binds to lipid components of synaptic vesicles in an NAD(H)-dependent, redox-sensitive manner

Synaptic ribbons are needed for fast and continuous exocytosis in ribbon synapses. RIBEYE is a main protein component of synaptic ribbons and is necessary to build the synaptic ribbon. RIBEYE consists of a unique A-domain and a carboxyterminal B-domain, which binds NAD(H). Within the presynaptic terminal, the synaptic ribbons are in physical contact with large numbers of synaptic vesicle (SV)s. How this physical contact between ribbons and synaptic vesicles is established at a molecular level is not well understood. In the present study, we demonstrate that the RIBEYE(B)-domain can directly interact with lipid components of SVs using two different sedimentation assays with liposomes of defined chemical composition. Similar binding results were obtained with a SV-containing membrane fraction. The binding of liposomes to RIBEYE(B) depends upon the presence of a small amount of lysophospholipids present in the liposomes. Interestingly, binding of liposomes to RIBEYE(B) depends on NAD(H) in a redox-sensitive manner. The binding is enhanced by NADH, the reduced form, and is inhibited by NAD⁺, the oxidized form. Lipid-mediated attachment of vesicles is probably part of a multi-step process that also involves additional, protein-dependent processes.

Comparative Characterization of Phosphatidic Acid Sensors and Their Localization during Frustrated Phagocytosis

Phosphatidic acid (PA) is the simplest phospholipid naturally existing in living organisms, but it constitutes only a minor fraction of total cell lipids. PA has attracted considerable attention because it is a phospholipid precursor, a lipid second messenger, and a modulator of membrane shape, and it has thus been proposed to play key cellular functions. The dynamics of PA in cells and in subcellular compartments, however, remains an open question. The recent generation of fluorescent probes for PA, by fusing GFP to PA-binding domains, has provided direct evidence for PA dynamics in different intracellular compartments. Here, three PA sensors were characterized in vitro, and their preferences for different PA species in particular lipidic environments were compared. In addition, the localization of PA in macrophages during frustrated phagocytosis was examined using these PA sensors and was combined with a lipidomic analysis of PA in intracellular compartments. The results indicate that the PA sensors display some preferences for specific PA species, depending on the lipid environment, and the localization study in macrophages revealed the complexity of intracellular PA dynamics.

The Multifaceted Role of SNARE Proteins in Membrane Fusion

Membrane fusion is a key process in all living organisms that contributes to a variety of biological processes including viral infection, cell fertilization, as well as intracellular transport, and neurotransmitter release. In particular, the various membrane-enclosed compartments in eukaryotic cells need to exchange their contents and communicate across membranes. Efficient and controllable fusion of biological membranes is known to be driven by cooperative action of SNARE proteins, which constitute the central components of the eukaryotic fusion machinery responsible for fusion of synaptic vesicles with the plasma membrane. During exocytosis, vesicle-associated v-SNARE (synaptobrevin) and target cell-associated t-SNAREs (syntaxin and SNAP-25) assemble into a core trans-SNARE complex. This complex plays a versatile role at various stages of exocytosis ranging from the priming to fusion pore formation and expansion, finally resulting in the release or exchange of the vesicle content. This review summarizes current knowledge on the intricate molecular mechanisms underlying exocytosis triggered and catalyzed by SNARE proteins. Particular attention is given to the function of the peptidic SNARE membrane anchors and the role of SNARE-lipid interactions in fusion. Moreover, the regulatory mechanisms by synaptic auxiliary proteins in SNARE-driven membrane fusion are briefly outlined.

Phosphatidic Acid-Mediated Signaling Regulates Microneme Secretion in Toxoplasma

The obligate intracellular lifestyle of apicomplexan parasites necessitates an invasive phase underpinned by timely and spatially controlled secretion of apical organelles termed micronemes. In Toxoplasma gondii, extracellular potassium levels and other stimuli trigger a signaling cascade culminating in phosphoinositide-phospholipase C (PLC) activation, which generates the second messengers diacylglycerol (DAG) and IP3 and ultimately results in microneme secretion. Here we show that a delicate balance between DAG and its downstream product, phosphatidic acid (PA), is essential for controlling microneme release. Governing this balance is the apicomplexan-specific DAG-kinase-1, which interconverts PA and DAG, and whose depletion impairs egress and causes parasite death. Additionally, we identify an acylated pleckstrin-homology (PH) domain-containing protein (APH) on the microneme surface that senses PA during microneme secretion and is necessary for microneme exocytosis. As APH is conserved in Apicomplexa, these findings highlight a potentially widely used mechanism in which key lipid mediators regulate microneme exocytosis.

Phosphatidic acid and neurotransmission

Lipids play a vital role in the health and functioning of neurons and interest in the physiological role of neuronal lipids is certainly increasing. One neuronal function in which neuronal lipids appears to play key roles in neurotransmission. Our understanding of the role of lipids in the synaptic vesicle cycle and neurotransmitter release is becoming increasingly more important. Much of the initial research in this area has highlighted the major roles played by the phosphoinositides (PtdIns), diacylglycerol (DAG), and phosphatidic acid (PtdOH). Of these, PtdOH has not received as much attention as the other lipids although its role and metabolism appears to be extremely important. This lipid has been shown to play a role in modulating both exocytosis and endocytosis although its precise role in either process is not well defined. The currently evidence suggest this lipid likely participates in key processes by altering membrane architecture necessary for membrane fusion, mediating the penetration of membrane proteins, serving as a precursor for other important SV cycling lipids, or activating essential enzymes. In this review, we address the sources of PtdOH, the enzymes involved in its production, the regulation of these enzymes, and its potential roles in neurotransmission in the central nervous system.

A central role for phosphatidic acid as a lipid mediator of regulated exocytosis in apicomplexa

Lipids are commonly known for the structural roles they play, however, the specific contribution of different lipid classes to wide-ranging signalling pathways is progressively being unravelled. Signalling lipids and their associated effector proteins are emerging as significant contributors to a vast array of effector functions within cells, including essential processes such as membrane fusion and vesicle exocytosis. Many phospholipids have signalling capacity, however, this review will focus on phosphatidic acid (PA) and the enzymes implicated in its production from diacylglycerol (DAG) and phosphatidylcholine (PC): DGK and PLD respectively. PA is a negatively charged, cone-shaped lipid identified as a key mediator in specific membrane fusion and vesicle exocytosis events in a variety of mammalian cells, and has recently been implicated in specialised secretory organelle exocytosis in apicomplexan parasites. This review summarises the recent work implicating a role for PA regulation in exocytosis in various cell types. We will discuss how these signalling events are linked to pathogenesis in the phylum Apicomplexa.

Relation between plasma and brain lipids

PURPOSE OF REVIEW

This article evaluates recent experimental and human evidence regarding the involvement of lipids, lipoproteins, and apolipoproteins in neurodegenerative diseases, and reviews the current literature of the effects of cholesterol-lowering treatment on cognition.

RECENT FINDINGS

Plasma levels of traditional lipids and lipoproteins are not consistently associated with risk of dementia even though low plasma levels of apolipoprotein E, through unknown mechanisms, robustly predict future dementia. Experimental evidence suggests neuroprotective roles of several brain and cerebrospinal fluid apolipoproteins. Whether plasma levels of apolipoprotein E, or any other apolipoprotein with possible central nervous system and/or blood-brain barrier functions (apolipoproteins J, A-I, A-II, A-IV, D, C-I, and C-III) may become accessible biomarker components that improve risk prediction for dementia together with genetic risk variants and cardiovascular risk factors remains to be determined.

SUMMARY

Apolipoproteins with well established functions in peripheral lipid metabolism may play important roles for brain vascular health and Alzheimer's disease pathophysiology. Experimental work on lipids, lipoproteins, and apolipoproteins in the central nervous system together with robust prospective human studies will help to substantiate the drug target potential of these lipid components.

Lipids implicated in the journey of a secretory granule: from biogenesis to fusion

The regulated secretory pathway begins with the formation of secretory granules by budding from the Golgi apparatus and ends by their fusion with the plasma membrane leading to the release of their content into the extracellular space, generally following a rise in cytosolic calcium. Generation of these membrane-bound transport carriers can be classified into three steps: (i) cargo sorting that segregates the cargo from resident proteins of the Golgi apparatus, (ii) membrane budding that encloses the cargo and depends on the creation of appropriate membrane curvature, and (iii) membrane fission events allowing the nascent carrier to separate from the donor membrane. These secretory vesicles then mature as they are actively transported along microtubules toward the cortical actin network at the cell periphery. The final stage known as regulated exocytosis involves the docking and the priming of the mature granules, necessary for merging of vesicular and plasma membranes, and the subsequent partial or total release of the secretory vesicle content. Here, we review the latest evidence detailing the functional roles played by lipids during secretory granule biogenesis, recruitment, and exocytosis steps. In this review, we highlight evidence supporting the notion that lipids play important functions in secretory vesicle biogenesis, maturation, recruitment, and membrane fusion steps. These effects include regulating various protein distribution and activity, but also directly modulating membrane topology. The challenges ahead to understand the pleiotropic functions of lipids in a secretory granule's journey are also discussed. This article is part of a mini review series on Chromaffin cells (ISCCB Meeting, 2015).

Annexin A2-dependent actin bundling promotes secretory granule docking to the plasma membrane and exocytosis

Annexin A2, a calcium-, actin-, and lipid-binding protein involved in exocytosis, mediates the formation of lipid microdomains required for the structural and spatial organization of fusion sites at the plasma membrane. To understand how annexin A2 promotes this membrane remodeling, the involvement of cortical actin filaments in lipid domain organization was investigated. 3D electron tomography showed that cortical actin bundled by annexin A2 connected docked secretory granules to the plasma membrane and contributed to the formation of GM1-enriched lipid microdomains at the exocytotic sites in chromaffin cells. When an annexin A2 mutant with impaired actin filament-bundling activity was expressed, the formation of plasma membrane lipid microdomains and the number of exocytotic events were decreased and the fusion kinetics were slower, whereas the pharmacological activation of the intrinsic actin-bundling activity of endogenous annexin A2 had the opposite effects. Thus, annexin A2-induced actin bundling is apparently essential for generating active exocytotic sites.

Function Suggests Nano-Structure: Quantitative Structural Support for SNARE-Mediated Pore Formation

Granule secretory content is released in either basal or calcium-activated complete exocytosis mode. A vital element in these processes is the establishment of a fusion pore between the granule membrane and the plasma membrane, initiated by the formation of a circular rosette docking arrangement of SNARE protein complexes. The controversially disputed number of SNARE complexes needed for granule priming leading to the formation of the fusion pore, is granule-size dependent and varies between secretion modes. Resorting to a statistical mechanics approach that views SNARE complexes and Ca(2+) ions as interacting particles, we have developed a relationship that links secretion rate to SNARE rosette size, Ca(2+) concentration and Ca(2+) ion cooperativity. Data are presented and discussed which suggest this SNARE-dependent generalization of existing narrow-range biophysical models that correlate secretion rate with Ca(2+) concentration and maximal Ca(2+) ion cooperativity. Evidence from dozens of examples in the literature advocate for this relation, which holds through the entire biological range. The coalescence of so many areas of diverse research methodologies has greatly augmented our understanding of so many different sequences of granule life cycle. Accordingly, these new tools may become valuable in a variety of electrophysiological experiments.

Platelet membrane variations and their effects on δ-granule secretion kinetics and aggregation spreading among different species

Platelet exocytosis is regulated partially by the granular/cellular membrane lipids and proteins. Some platelets contain a membrane-bound tube, called an open canalicular system (OCS), which assists in granular release events and increases the membrane surface area for greater spreading. The OCS is not found in all species, and variations in membrane composition can cause changes in platelet secretion. Since platelet studies use various animal models, it is important to understand how platelets differ in both their composition and granular release to draw conclusions among various models. The relative phospholipid composition of the platelets with (mouse, rabbit) and without (cow) an OCS was quantified using UPLC-MS/MS. Cholesterol and protein composition was measured using an Amplex Red Assay and BCA Assay. TEM and dark field platelet images were gathered and analyzed with Image J. Granular release was monitored with single cell carbon fiber microelectrode amperometry. Cow platelets contained greater amounts of cholesterol and sphingomyelin. In addition, they yield greater serotonin release and longer δ granule secretion times. Finally, they showed greater spreading area with a greater range of spread. Platelets containing an OCS had more similarities in their membrane composition and secretion kinetics compared to cow platelets. However, cow platelets showed greater fusion pore stability which could be due to extra sphingomyelin and cholesterol, the primary components of lipid rafts. In addition, their greater stability may lead to many granules assisting in spreading. This study highlights fundamental membrane differences and their effects on platelet secretion.

Phospholipase C and D regulation of Src, calcium release and membrane fusion during Xenopus laevis development

This review emphasizes how lipids regulate membrane fusion and the proteins involved in three developmental stages: oocyte maturation to the fertilizable egg, fertilization and during first cleavage. Decades of work show that phosphatidic acid (PA) releases intracellular calcium, and recent work shows that the lipid can activate Src tyrosine kinase or phospholipase C during Xenopus fertilization. Numerous reports are summarized to show three levels of increase in lipid second messengers inositol 1,4,5-trisphosphate and sn 1,2-diacylglycerol (DAG) during the three different developmental stages. In addition, possible roles for PA, ceramide, lysophosphatidylcholine, plasmalogens, phosphatidylinositol 4-phosphate, phosphatidylinositol 5-phosphate, phosphatidylinositol 4,5-bisphosphate, membrane microdomains (rafts) and phosphatidylinositol 3,4,5-trisphosphate in regulation of membrane fusion (acrosome reaction, sperm-egg fusion, cortical granule exocytosis), inositol 1,4,5-trisphosphate receptors, and calcium release are discussed. The role of six lipases involved in generating putative lipid second messengers during fertilization is also discussed: phospholipase D, autotaxin, lipin1, sphingomyelinase, phospholipase C, and phospholipase A2. More specifically, proteins involved in developmental events and their regulation through lipid binding to SH3, SH4, PH, PX, or C2 protein domains is emphasized. New models are presented for PA activation of Src (through SH3, SH4 and a unique domain), that this may be why the SH2 domain of PLCγ is not required for Xenopus fertilization, PA activation of phospholipase C, a role for PA during the calcium wave after fertilization, and that calcium/calmodulin may be responsible for the loss of Src from rafts after fertilization. Also discussed is that the large DAG increase during fertilization derives from phospholipase D production of PA and lipin dephosphorylation to DAG.

The phospholipase D superfamily as therapeutic targets

The phospholipase D (PLD) lipid-signaling enzyme superfamily has long been studied for its roles in cell communication and a wide range of cell biological processes. With the advent of loss-of-function genetic mouse models that have revealed that PLD1 and PLD2 ablation is overtly tolerable, small-molecule PLD1/2 inhibitors that do not cause unacceptable clinical toxicity, a PLD2 polymorphism that has been linked to altered physiology, and growing delineation of processes that are subtly altered in mice lacking PLD1/2 activity, the stage is being set for assessment of PLD1/2 inhibition for therapeutic purposes. Based on findings to date, PLD1/2 inhibition may be of more utility in acute rather than chronic settings, although this generalization will depend on the specific risks and benefits in each disease setting.

Plant mitochondrial dynamics and the role of membrane lipids

Mitochondria are highly dynamic organelles that are continuously shaped by the antagonistic fission and fusion processes. The major machineries of mitochondrial fission and fusion, as well as mechanisms that regulate the function of key players in these processes have been analyzed in different experimental systems. In plants however, the mitochondrial fusion machinery is still largely unknown, and the regulatory mechanisms of the fission machinery are just beginning to be elucidated. This review focuses on the molecular mechanisms underlying plant mitochondrial dynamics and regulation of some of the key factors, especially the roles of membrane lipids such as cardiolipin.

Role of mitochondrial lipids in guiding fission and fusion

Clinically important links have been established between mitochondrial function and cardiac physiology and disease in the context of signaling mechanisms, energy production, and muscle cell development. The proteins and processes that drive mitochondrial fusion and fission are now known to have emergent functions in intracellular calcium homeostasis, apoptosis, vascular smooth muscle cell proliferation, myofibril organization, and Notch-driven cell differentiation, all key issues in cardiac disease. Moreover, decreasing fission may confer protection against ischemic heart disease, particularly in the setting of obesity, diabetes, and heart failure. The importance of lipids in controlling mitochondrial fission and fusion is increasingly becoming appreciated. Roles for the bulk and signaling lipids cardiolipin, phosphatidylethanolamine, phosphatidic acid, diacylglycerol, and lysophosphatidic acid and the enzymes that synthesize or metabolize them in the control of mitochondrial shape and function are reviewed here. A number of diseases have been linked to loss-of-function alleles for a subset of the enzymes, emphasizing the importance of the lipid environment in this context.

PI(4,5)P₂-binding effector proteins for vesicle exocytosis

PI(4,5)P₂participates directly in priming and possibly in fusion steps of Ca²⁺-triggered vesicle exocytosis. High concentration nanodomains of PI(4,5)P₂reside on the plasma membrane of neuroendocrine cells. A subset of vesicles that co-localize with PI(4,5)P₂ domains appear to undergo preferential exocytosis in stimulated cells. PI(4,5)P₂directly regulates vesicle exocytosis by recruiting and activating PI(4,5)P₂-binding proteins that regulate SNARE protein function including CAPS, Munc13-1/2, synaptotagmin-1, and other C2 domain-containing proteins. These PI(4,5)P₂effector proteins are coincidence detectors that engage in multiple interactions at vesicle exocytic sites. The SNARE protein syntaxin-1 also binds to PI(4,5)P₂, which promotes clustering, but an activating role for PI(4,5)P₂in syntaxin-1 function remains to be fully characterized. Similar principles underlie polarized constitutive vesicle fusion mediated in part by the PI(4,5)P₂-binding subunits of the exocyst complex (Sec3, Exo70). Overall, focal vesicle exocytosis occurs at sites landmarked by PI(4,5)P2, which serves to recruit and/or activate multifunctional PI(4,5)P₂-binding proteins. This article is part of a Special Issue entitled Phosphoinositides.

Regulation of mitochondrial morphology by lipids

Although great progress has been made in identifying key protein factors that regulate mitochondrial morphology through mediating fission and fusion, signaling lipids are increasingly being recognized as important in the process as well. We review here roles that have been proposed for the signaling and bulk lipids cardiolipin, phosphatidic acid, lysophosphatidic acid, diacylglycerol, and phosphatidylethanolamine and the enzymes that generate or catabolize them in the regulation of mitochondrial morphology in yeast and mammals. Mutations in some of these enzymes are causal in a number of disease settings, highlighting the significance of controlling the lipid environment in this setting.

Biosynthesis and roles of phospholipids in mitochondrial fusion, division and mitophagy

Mitochondria move, fuse and divide in cells. The dynamic behavior of mitochondria is central to the control of their structure and function. Three conserved mitochondrial dynamin-related GTPases (i.e., mitofusin, Opa1 and Drp1 in mammals and Fzo1, Mgm1 and Dnm1 in yeast) mediate mitochondrial fusion and division. In addition to dynamins, recent studies demonstrated that phospholipids in mitochondria also play key roles in mitochondrial dynamics by interacting with dynamin GTPases and by directly changing the biophysical properties of the mitochondrial membranes. Changes in phospholipid composition also promote mitophagy, which is a selective mitochondrial degradation process that is mechanistically coupled to mitochondrial division. In this review, we will discuss the biogenesis and function of mitochondrial phospholipids.

Exocytosis and endocytosis in neuroendocrine cells: inseparable membranes!

Although much has been learned concerning the mechanisms of secretory vesicle formation and fusion at donor and acceptor membrane compartments, relatively little attention has been paid toward understanding how cells maintain a homeostatic membrane balance through vesicular trafficking. In neurons and neuroendocrine cells, release of neurotransmitters, neuropeptides, and hormones occurs through calcium-regulated exocytosis at the plasma membrane. To allow recycling of secretory vesicle components and to preserve organelles integrity, cells must initiate and regulate compensatory membrane uptake. This review relates the fate of secretory granule membranes after full fusion exocytosis in neuroendocrine cells. In particular, we focus on the potential role of lipids in preserving and sorting secretory granule membranes after exocytosis and we discuss the potential mechanisms of membrane retrieval.

Incidence of hypothyroidism after 131I therapy for hyperthyroidism. Relation to pretherapy serum levels of T3, T4 and thyroid antibodies

A correlation is reported between serum levels of triiodothyronine (S-T3) and thyroxine (S-T4) before treatment, as well as levels of thyroid antibodies before treatment, and the development of hypothyroidism following 131I therapy in 86 patients with hyperthyroidism. Patients with marked elevation of S-T3 or S-T4 had demonstrable antibodies to thyroid cytoplasmic antigen more often than those with normal or moderately elevated levels, and patients with markedly elevated levels of S-T3 also had a higher incidence of hypothyroidism after treatment. Patients with nodular thyroid glands and with markedly elevated levels of S-T3 required a larger number of 131I doses before no signs of hyperthyroidism persisted in comparison to those with moderately elevated levels.