Thyroid Hormone Deiodinases: Dynamic Switches in Developmental Transitions

Abstract

Thyroid hormones exert pleiotropic, essential actions in mammalian, including human, development. These actions depend on provision of thyroid hormones in the circulation but also to a remarkable extent on deiodinase enzymes in target tissues that amplify or deplete the local concentration of the primary active form of the hormone T3 (3,5,3′-triiodothyronine), the high affinity ligand for thyroid hormone receptors. Genetic analyses in mice have revealed key roles for activating (DIO2) and inactivating (DIO3) deiodinases in cell differentiation fates and tissue maturation, ultimately promoting neonatal viability, growth, fertility, brain development, and behavior, as well as metabolic, endocrine, and sensory functions. An emerging paradigm is how the opposing activities of DIO2 and DIO3 are coordinated, providing a dynamic switch that controls the developmental timing of a tissue response, often during neonatal and maturational transitions. A second paradigm is how cell to cell communication within a tissue determines the response to T3. Deiodinases in specific cell types, often strategically located near to blood vessels that convey thyroid hormones into the tissue, can regulate neighboring cell types, suggesting a paracrine-like layer of control of T3 action. We discuss deiodinases as switches for developmental transitions and their potential to influence tissue dysfunction in human thyroid disorders.

Thyroid hormones regulate multiple functions in growth and development. Adequate provision of thyroid hormones in the circulation is a prerequisite for these actions as has long been known from the serious impairments that may arise in congenital thyroid disorders, including both hypothyroid and hyperthyroid conditions (1, 2). However, it is increasingly evident that deiodinase enzymes within a given target tissue or cell type regulate the local content of 3,5,3′-triiodothyronine (T3), the primary biologically active form of thyroid hormone, and thereby confer tissue-intrinsic control over the responses. This intrinsic control is dependent on the complement of activating and inactivating deiodinases present in the tissue.

The type 1, 2, and 3 deiodinases (DIO1, DIO2, and DIO3, respectively) are selenoenzymes with related features including an N-terminal transmembrane domain and a catalytic domain incorporating the rare amino acid selenocysteine that is essential for enzymatic function (3, 4) (Fig. 1A). The deiodinases remove an iodine residue from the outer or inner ring of thyroid hormones (Fig. 1B). In development, both the activating DIO2 and the inactivating DIO3 enzymes are critical. DIO2 generates T3 by outer ring deiodination of the more abundant precursor thyroxine (T4), whereas DIO3 depletes T3 sources by inner ring deiodination of both T3 and T4. Owing to their opposing activities and distinct expression patterns, DIO2 and DIO3 are uniquely positioned to control development. Genetic mouse models have been instrumental in demonstrating the biological functions of deiodinases, which add a prereceptor layer of complexity to the control of T3 action (5).

Figure 1.

Main characteristics of deiodinases (DIOs). A, Protein domains of deiodinases with conservation between enzyme domains indicated, type of activity, and selected main sites of expression. B, Structure of thyroid hormones and main biochemical conversions catalyzed by the type 2 deiodinase (DIO2) and type 3 deiodinase (DIO3) enzymes (bold arrows).

Here, we review the developmental roles of deiodinases, based mainly on rodent studies and consider implications for human disorders. We highlight 2 emerging mechanistic concepts: (i) how the coordinated expression of DIO3 and DIO2 provides a dynamic developmental switch, as suggested by early observations 40 years ago in the rat by Kaplan and Yaskoski (6); and (ii) how deiodinases in specific cell types regulate responses in other nearby cell types, indicating paracrine-like control of cellular responses to T3 within a tissue. The insights gained into the developmental roles of deiodinases suggest the potential utility of these enzymes as future therapeutic targets to counter tissue dysfunction in disease.

Deiodinase Switches in Neonatal and Maturational Transitions

Coordinated, often reciprocal, profiles of DIO3 and DIO2 activities underlie the development of many tissues, providing a dynamic, double control over the T3 content within tissues (Fig. 2A). DIO3 activity generally peaks at fetal and neonatal stages and is also present at a high level in the placenta and uterus during pregnancy (7-9), suggesting that it constrains T3 action at sensitive early stages. DIO2 activity often rises later, as DIO3 activity declines, suggesting a means by which T3 content can be enhanced as tissues mature. The composite profiles of both enzymes indicate a dynamic switch from constraint to amplification of the T3 signal during developmental transitions in many organs.

Figure 2.

Type 3 deiodinase (DIO3) to type 2 deiodinase (DIO2) switches in developmental transitions. A, Opposing activities of DIO3 and DIO2 provide dynamically balanced control of the 3,5,3′-triiodothyronine (T3) signal in a tissue. B and C, DIO3 to DIO2 switches in cerebellar development in mice and humans, and relationship to systemic thyroxine (T4) and T3 levels. (Cerebellar differentiation and the increase in circulating T4 and T3 occur earlier in human than mouse development.) The mouse profile parallels that originally shown for rat (6). The human profile is based on fetal data reported by Kester et al (19). Circulating hormone profiles are adapted from reported data for mice (11) and humans (124, 125). D, DIO3-DIO2 switches are customized in each tissue and often encompass terminal differentiation events and onset of organ function. All deiodinase activity profiles in panels B, C, and D are approximate and based on publications cited in the text.

DIO3 to DIO2 switches often encompass the maturation of tissue function during transitions from fetal to neonatal life or during later transitions that allow autonomous juvenile and adult life. In the brain, a switch accompanies differentiation and synaptogenesis as circuits are established, for example, in the cerebellum as locomotor control matures (Fig. 2B) (6, 10). In sensory systems, switches in the cochlea and retina encompass the onset of auditory and visual function, respectively (11-13) (Fig. 2D). Switches underlie gonadal and reproductive maturation (14, 15) as well as the neonatal activation of brown adipose tissue and facultative thermogenesis (16, 17). A switch in the hypothalamus accompanies the maturation of the hypothalamic-pituitary control of thyroid gland function (6, 18).

Although human data are limited, evidence supports conserved trends with DIO3 preceding DIO2 activity in fetal brain regions (19) such as the cerebellum (Fig. 2C). Transcriptomic data from human tissues across developmental stages also indicate a progressive switch from DIO3 to DIO2 in the cerebrum, cerebellum, testis, kidney, and heart (20). Another way to assess the roles of deiodinases in human development is to study the differentiation of organoids derived from induced pluripotent stem cells in culture. For example, human retinal organoids display a DIO3 to DIO2 switch at the RNA level (21) resembling the enzyme profiles in rodent retina (12, 22) (see Fig. 2D). In mice, many systems mature at the postnatal or juvenile stages, whereas in humans equivalent events often occur at the fetal or perinatal stages. Thus, deiodinase switches may be expected earlier, in utero, and during the fetal to neonatal transition in human development.

Deiodinase switches are customized in each tissue and can precede or lag the systemic rise in circulating T4 and T3 levels, thereby allowing partly autonomous control over development (see Fig. 2). For example, DIO2 activity rises in the rodent retina in concert with the rise of the systemic hormone level. However, in the cochlea or brown adipose tissue (see Fig. 2D), DIO2 rises before the systemic hormone rises, suggesting an earlier action uncoupled to an extent from circulating hormone levels. Conversely, in the testis, a sustained DIO3 profile suggests prolonged constraint of responses despite the systemic rise of T4 and T3.

At adult stages, the activities of DIO3 and DIO2 display lower, more restricted profiles and are involved in responses to endocrine and environmental challenges as well as responses to injury, infection, tumorigenesis, or other disease states as reviewed elsewhere (23-25).

Developmental Functions of Deiodinases

We illustrate here examples of the versatile developmental roles of the DIO2 and DIO3 enzymes. The DIO1 enzyme lacks known developmental functions although subtle roles cannot be excluded. DIO1 has dual activating and inactivating activities and influences the disposal of iodothyronines and iodine economy but lacks an obvious role in the maintenance of systemic euthyroidism (26, 27). DIO1 deiodinates the outer ring of T4 to generate T3 but also deiodinates the inner ring, especially of sulfated and glucuronidated iodothyronine derivatives, and its preferred substrate is reverse T3 (rT3), a largely inactive metabolite.

The DIO2 and DIO3 enzymes play an important role in many tissues that are not reviewed in detail here, including bone (28, 29), muscle (30, 31), skin (14, 32), heart (33), metabolic systems (34, 35), and innate immune cells (23) (Table 1). Although not the focus of this article, many of these actions may involve a developmental component.

Table 1.

Phenotypes of global and tissue-specific deiodinase deficiencies in mice

Gene	Phenotypes	References
Dio2	HPT axis
	Pituitary resistance to thyroid hormone (elevated TSH and T4)	(36-38)
	CNS/behavior
	Reduced brain T3 content and altered gene expression	(50, 51)
	Increased anxiety-like behavior	(52, 53)
	Sensory function
	Deafness	(64)
	Other phenotypes
	Impaired BAT thermogenesis	(17, 126)
	Increased bone fracture susceptibility	(29)
	Skeletal muscle regeneration defect	(127)
	Increased susceptibility to obesity	(128)
	Altered programming of liver lipid metabolism	(114, 115)
	Defective macrophage function	(129)
Dio3	General phenotypes
	perinatal lethality (> 30%, genetic background dependent)	(18)
	growth retardation (20%-30% body wt reduction)	(18)
	Impaired fertility in both sexes	(15, 18)
	HPT axis
	Neonatal thyrotoxicosis	(18)
	Adult central hypothyroidism	(18, 130)
	(Small thyroid gland, low T4 and T3, unchanged TSH)	(10, 130)
	CNS/behavior
	Developmental and adult brain hyperthyroidism	(54, 55)
	Cerebellar maldevelopment and motor deficits	(10)
	Hyperactivity	(57)
	Reduced anxiety- and depression-like behaviors	(57)
	Increased aggression and poor maternal behavior	(58)
	Sensory function
	Deafness	(63)
	Color blindness (achromatopsia)	(13)
	Olfactory impairment	(58)
	Other phenotypes
	Reduced adiposity and altered energy balance	(59)
	Male hypogonadism and reduced testis size	(15)
	Muscle regeneration/stem cell defects	(131)
	Altered pancreatic and insulin homeostasis	(35)
	Keratinocyte developmental defects	(32)
	Susceptibility to cardiomyopathy	(33)
	Impaired bacterial clearance, neutrophil function defect	(132, 133)
Dio2/Dio3	Exacerbated deafness	(22)
	Partial or full rescue of:
	Testis size	(15)
	Retinal cone survival	(22)
	HPT axis parameters, brain thyroid status	(22, 41)
Dio1	Elevated rT3, decreased iodide urinary excretion	(27)
Dio1/Dio2	Exacerbated impairment of rT3 clearance Abnormal brain gene expression	(39, 134)

Abbreviations: BAT, brown adipose tissue; CNS, central nervous system; Dio1, type 1 deiodinase gene; Dio2, type 2 deiodinase gene; Dio3, type 3 deiodinase gene; HPT, hypothalamic-pituitary-thyroid; rT3, reverse 3,3,5′-triiodothyronine; T3, 3,3,5′-triiodothyronine; T4, thyroxine; TSH, thyrotropin.

Hypothalamic-Pituitary-Thyroid Axis

During development, the thyroid gland, under the control of the hypothalamus and pituitary, secretes calibrated, progressively increasing amounts of T4 and T3 into the circulation, as appropriate for stimulating the maturation of many organs. Also, during development, the hypothalamic-pituitary-thyroid (HPT) axis acquires its own set-point sensitivity for negative feedback control by systemic levels of T4 and T3. The HPT axis may be viewed as a master system under control by deiodination, since other tissues are influenced by the resultant output of circulating T4 and T3.

DIO2 and DIO3 are critical for the development of the function and set point of the HPT axis. Normally, the thyrotropin (TSH) released by the pituitary stimulates the thyroid gland to produce appropriate amounts of T4 and T3. T4 and T3 in turn control their own production via negative feedback that suppresses TSH production in the pituitary. In Dio2^–/– mice, the HPT axis fails to mature correctly and displays central resistance to feedback control. Dio2^–/– mice display elevated levels of both T4 and TSH, suggesting insensitivity of the pituitary to T4-mediated suppression (36). The elevation of T4 as early as postnatal day 8 is largely recapitulated in mice with pituitary-specific Dio2 deficiency (37), which also show a blunted rise of TSH in response to hypothyroidism (38). Thus, DIO2-mediated deiodination of T4 in the pituitary and possibly hypothalamus contributes to establishing feedback control of the axis. The elevation of T4 and TSH in Dio2^–/– mice is slightly exacerbated in Dio2;Dio1 double-knockout mice (39), suggesting that DIO1 cooperates with DIO2 in the HPT axis. Conversely, the elevation of rT3 in Dio1^–/– mice is exacerbated in Dio2;Dio1 double knockouts, indicating that DIO2 contributes to the clearance of rT3 in Dio1^–/– mice (39).

Dio3 ^–/– mice exhibit complex alterations in the HPT axis, indicating major functions for DIO3 consistent with its profile in the hypothalamus (see Fig. 2D) (6, 18). In Dio3^–/– mice, the thyroid gland is small, the HPT axis does not respond to hypothyroidism, the pituitary response to thyrotropin-releasing hormone (TRH) is blunted, and the thyroid gland response to TSH is impaired (40). In neonatal Dio3^–/– mice, serum T3 is elevated and T4 is decreased but this pattern shifts progressively to a pattern of central hypothyroidism in adults, with moderately reduced T3 and T4 and minimally altered TSH (18).

The proposal that coordination of the opposing actions of DIO3 and DIO2 provides balanced control is supported by deletion of Dio2 from Dio3^–/– mice, which substantially restores thyroid gland size and serum levels of T4, T3, and TSH at postnatal and adult ages (22). Conversely, the elevated T4 and TSH levels in Dio2^–/– mice or double Dio1;Dio2 knockouts, indicative of central resistance to T4, are partly ameliorated in double Dio2;Dio3 knockouts (22) or triple Dio1;Dio2;Dio3 knockouts (41). Future studies may elucidate the critical but complex roles of DIO3 and DIO2 in the HPT axis in the context of specialized cell types both in the hypothalamus and pituitary.

Brain

Thyroid hormones modulate a range of functions in brain development (42-45), many of which may be modified by DIO3 and DIO2 within the brain. In rodents, DIO3 activity declines and DIO2 activity rises in brain regions including the cerebrum and cerebellum during the second postnatal week (6, 14) (see Fig. 2B). This deiodinase switch allows brain tissues to amplify T3 levels during a period that encompasses events such as oligodendrocyte differentiation, myelination, and synaptogenesis and correlates with heightened sensitivity of T3-responsive genes (46, 47).

Although neurons are among the main cell types that respond to T3, a remarkable finding was that DIO2 is located predominantly in glial cells in regions such as the cerebral cortex (48, 49), supporting a model whereby separation of T3-producing and T3-responding cell types provides a paracrine-like form of control (Fig. 3A and 3C). Other forms of cellular segregation, for example, in the cochlea and testis, support paracrine-like control of T3 transfer between cell types within tissues (see later sections).

Figure 3.

Conceptual diagrams of paracrine-like control of 3,5,3′-triiodothyronine (T3) action by type 2 deiodinase (DIO2) and type 3 deiodinase (DIO3) within a tissue. A, DIO2+ cell types close to capillaries take up thyroxine (T4) from the circulation, convert T4 to T3, then transfer T3 to neighboring, T3-responsive cell types. Examples are found in the cochlea and cerebrum. B, DIO3+ support cell types deplete T4 and T3, thereby constraining the T3 signal available for other response cell types. An example is found in the testis. Transplasma membrane transport of T4 and T3 by specific transporter proteins is an integral part of the models shown in panels A and B. C, Images of DIO2+ support cells (pale blue) in proximity to blood vessels (red): specifically DIO2+ supporting cells in the cochlea and DIO2+ astrocytes in the cerebral cortex (images are the authors’ own work using a Dio2cre knockin; manuscript in preparation). TR, thyroid hormone receptor in the nucleus of target cells.

The proposal that the requisite T3 content in brain tissues (a euthyroid state) is maintained in part by DIO2 predicts that DIO2 deficiency would cause pronounced neurological deficits comparable to those in hypothyroidism. However, although 2-week-old Dio2^–/– mice exhibit low brain T3 content, some neurological functions, as assessed by motor and behavioral tests, are only modestly affected compared to the impact of severe hypothyroidism (50). In addition, changes in T3-dependent gene expression in the cerebrum are milder in Dio2^–/– mice than in hypothyroid mice (50, 51), suggesting that compensatory mechanisms partly protect T3-dependent responses in Dio2^–/– mice. However, Dio2^–/– mice do exhibit increased anxiety and fear memory (52). Astrocyte-specific Dio2 deficiency (53) also results in depression-like behavior and hypothyroid-like changes in gene expression, including neuronal gene expression (53), supporting the proposal that glial-neuronal transfer modifies T3 responses in the brain.

In Dio3^–/– mice, the absence of DIO3 together with a transient neonatal excess of circulating T3 results in a thyrotoxic state in the brain that persists into adulthood (54). Abnormalities include increased thickness of the neocortex (55) and premature differentiation of the cerebellum with accelerated regression of the external germinal layer and locomotor defects (10). Dio3^–/– mice exhibit increases in T3-dependent expression both of glial and neuronal genes (54-56), in accordance with a complex cellular role of DIO3 and possible control over glial-neuronal transfer of T3.

Dio3 ^–/– mice exhibit other neuroendocrine and behavioral phenotypes. Oxytocin and arginine-vasopressin functions are altered in neonatal and adult Dio3^–/– mice and are associated in both sexes with aggressive behavior, hyperactivity, reduced anxiety-like and depression-like behaviors, and lack of maternal behavior in females (57, 58). These phenotypes are likely developmental in origin, as adult-onset DIO3 deficiency recapitulates only a mild form of hyperactivity (56). Concerning energy balance, Dio3^–/– mice are abnormally lean and have alterations in serum leptin and reduced Pomc and increased Agrp expression in the hypothalamus (59), both factors being part of the leptin-melanocortin system that regulates food intake and energy expenditure. Dio3^–/– mice also show lengthening of the nocturnal phase of physical activity, suggesting that DIO3 influences hypothalamic and circadian regulation of energy balance and activity (59).

The proposed control by a balance between DIO3 and DIO2 activities is supported by measurements of T3 content in brain tissues. Dio3^–/– and Dio2^–/– mice exhibit increased and decreased T3 content, respectively, whereas this content is normalized in mice with triple deiodinase deficiency (41).

Cochlea

Thyroid hormones promote auditory development and have a major site of action in the cochlea (60-62). Terminal differentiation of the cochlea depends not only on systemic thyroid hormone levels, but also on the local control of T3 content by deiodinases. In the cochlea, early DIO3 activity is followed by a dramatic rise of DIO2 expression preceding the onset of hearing (see Fig. 2D) (11), and deletion of either enzyme results in deafness (63, 64). Dio3^–/– and Dio2^–/– mice exhibit opposite cochlear phenotypes of accelerated and delayed differentiation, respectively, illustrating how DIO3 and DIO2 activities safeguard the appropriate time course of development.

Dio2 ^–/– mice display retarded remodeling of the sensory epithelium, which includes the mechanosensory hair cells, and deformity of the tectorial membrane (64), similar to defects in hypothyroid (65-68) or Thrb (thyroid hormone receptor β)-deficient mice (60, 69). In contrast, Dio3^–/– mice exhibit premature remodeling of the sensory epithelium (70) in accord with a protective role for DIO3 at early stages. Excessive T3 given during a transient neonatal phase in wild-type mice also accelerates cochlear differentiation and causes permanent deafness (71).

The vascular network that carries circulating T4 and T3 into the cochlea enters through the central modiolus, then extends laterally through the spiral ligament and stria vascularis. This network is compartmentalized from the sensory tissues in the cochlear duct (72) and implies the need for a cellular route of hormonal transfer from the circulation to internal target tissues. A proposed route involves uptake of circulating T4 from capillaries, conversion of T4 into T3 by DIO2 in intermediary support cell types, then transfer of T3 to internal target cell types (73). The Dio2 gene is expressed in vascularized support tissues in the modiolus and lateral wall rather than internal response tissues (11) and Dio2-positive supporting cells have been identified that extend projections to nearby blood vessels in the cochlea (Fig. 3A and 3C). In contrast, Dio3 expression partly overlaps with internal T3-response tissues (70) suggesting that the T3 content is limited by DIO3 activity in the more immediate vicinity of target cells.

The evidence supports a model whereby the opposing activities of DIO3 and DIO2 modulate the rate of cochlear development and the onset of hearing. However, combined deletion of Dio3 and Dio2 does not restore normal development but instead exacerbates the deafness, suggesting that in the cochlea, DIO3 and DIO2 control complex events perhaps at distinct time points, and possibly in some independent locations, ultimately resulting in an additive phenotype in Dio3;Dio2 double deficiency (22).

Retina

The functions of T3 in retinal development are still being elucidated but include a key role in the cone photoreceptor diversity that allows color vision. Most mammals have cone types for sensitivity to short (S, “blue”) and medium-long (M, “green”) wavelengths of light (74) but in mice lacking thyroid hormone receptor TRβ2 (encoded by the Thrb gene), all cones differentiate as S type cones (75), resulting in a form of color blindness. Related defects occur in hypothyroid rodents (76-79). Human THRB mutations also impair cone functions (80-82) and impair differentiation of medium-long cone-like cells in retinal organoids in culture (21).

DIO3 and DIO2 activity profiles switch during retinal differentiation in rodents (12, 13, 22) (Fig. 2D). DIO3 activity has a protective role as Dio3^–/– mice lose cones by T3-induced apoptosis resulting in achromatopsia (13), or color blindness and loss of visual function in daylight conditions. There is a slight loss of rods, the photoreceptors that mediate vision in dim conditions, suggesting that DIO3 also limits T3 availability for rods and possibly other retinal cell types (13). The cone loss is rescued by deletion of TRβ2, demonstrating that cone death results from T3 acting on a specific receptor (13).

The DIO2 peak at juvenile stages suggests that DIO2 amplifies T3 content during retinal maturation. Retinal functions for DIO2 identified to date are subtle. Dio2^–/– mice do not display overt defects in cone diversity although rod function is modestly impaired (22). It has been reported that Dio2 expression is under circadian control and modestly influences cone opsin expression in a limited portion of central retina (83). It is possible that more obvious retinal phenotypes in Dio2^–/– mice are masked by compensatory mechanisms. A major role for the DIO2 enzyme is unmasked by deletion of Dio2 in Dio3^–/– mice, which reverses the cone cell death and restores both M- and S-cone function (22). This combined control of retinal development by DIO3 and DIO2 activities may involve both tissue-intrinsic functions and complex, indirect changes in circulating T4 and T3 levels. The Dio2 gene is expressed in Müller glial cells, which provide support functions for retinal neurons and might modify the T3 supply for photoreceptors (22), raising the possibility of paracrine-like T3 actions in the retina.

Gonads

Hyperthyroidism in rodents causes male hypogonadism associated with low numbers of Sertoli cells, whereas hypothyroidism causes an opposite, hyperplastic outcome in the testis (84). It has been proposed that T3 delineates the time window during which Sertoli cell precursors proliferate. In the testis, DIO3 has activity spanning the fetal and postnatal stages, whereas DIO2 has a modest postnatal peak, suggesting that differentiation depends on sustained protection by DIO3.

Dio3 ^–/– mice display dramatic testicular defects (15). The adult testes are 25% of normal size with prematurely arrested Sertoli cell proliferation, and these changes are associated with delayed puberty and reduced spermatogenesis. The defects are partially rescued by deletion of the T3 receptor TRα1 or MCT8 plasma membrane transporter (15), demonstrating that the phenotype reflects aberrant T3 signaling. The hypogonadism arises at neonatal stages and is associated with altered gene expression in the testis (85), reduced testosterone levels, and increased expression of gonadotropins follicle-stimulating hormone and luteinizing hormone by the pituitary (15). Spermatogonia-specific inactivation of DIO3 also results in reduced testis size and changes in testicular markers (85), demonstrating that defective clearance of T3 by spermatogonia contributes to the Sertoli cell defects and hypogonadism. The results suggest a variation on the theme of paracrine-like control in which DIO3 activity limits the T3 available for transfer from spermatogonia to Sertoli cells and possibly Leydig or other cell types (15) (Fig. 3B).

The testicular abnormalities in Dio3^–/– mice are partially rescued in double-Dio2/Dio3 mutants (15), supporting the proposal that the balance between DIO3 and DIO2 activities determines the appropriate T3 content for the correct differentiation of the tissue.

Deiodinases, an Integral Component of Tissue-Specific Developmental Control

The evidence discussed in the preceding sections indicates that the developmental actions of thyroid hormones in different tissues depend on an intricate combination of endocrine and paracrine-like mechanisms. A corollary of the proposed paracrine-like control by deiodinases within a tissue is a requirement for transfer of T4 and T3 between cell types (see Fig. 3A and 3B). Deiodinases are likely to cooperate closely with specific plasma membrane transporters, such as MCT8, that mediate the cellular uptake and cell-to-cell transfer of T4 and T3 (44, 86-88). For example, Mct8-deficient mice exhibit complex brain dysfunction and behavioral outcomes that may be consistent both with hypothyroid and hyperthyroid states in different brain regions as a result of defects in cell-specific transport of T3 and T4 (89). The potential for cooperation between deiodinases and different plasma membrane transporters in specific cell types presumably confers precision in determining the local availability of ligand for the nuclear receptors that elicit a given cellular transcriptional response.

Type 3 Deiodinase and Cell-Fate Decisions

The expression of DIO3 in immature tissues often correlates with developmental windows when progenitor and precursor cell types commit to specific fates: proliferation, cell death, or terminal differentiation into mature cell types. A number of studies have now indicated that DIO3 specifically controls several cell-fate decisions. We highlight here examples of DIO3-dependent cell-fate decisions.

The expression of DIO3 in the immature retina presumably forms a sink that minimizes exposure of cone photoreceptors to T3 thereby preventing T3-induced apoptosis (13) (Fig. 4A). Although some T3 is required for cones to diversify into the M- and S-types required for color vision, these precursors sit on a knife edge between normal differentiation and apoptosis if the T3 signal is excessive. DIO3 crucially constrains the T3 signal to promote the former over latter outcome.

Figure 4.

Control of cell differentiation fates by type 3 deiodinase (DIO3). A, DIO3 critically limits the 3,5,3′-triiodothyronine (T3) signal to allow normal differentiation vs T3-induced cell death. Examples show differentiation of cone photoreceptors in the retina and myoblasts during muscle regeneration. B, For Sertoli cell precursors in the testis, DIO3 limits the T3 signal to allow an appropriate period of proliferation vs premature arrest of proliferation, thereby generating a full complement of Sertoli cells.

A similar paradigm is observed for muscle satellite cells, which differentiate into the myoblasts that ultimately form muscle fibers (see Fig. 4A) (90). During muscle regeneration in injury models, satellite stem cells are prone to T3-induced apoptosis so that DIO3 activity is critical in safeguarding normal differentiation as these cells form myoblasts (91).

In the testis, DIO3 activity is thought to control whether Sertoli precursor cells continue to proliferate or exit from the cell cycle and enter terminal differentiation (Fig. 4B). In Dio3^–/– mice, unconstrained T3 signals lead to premature arrest of proliferation, resulting in a reduced pool of Sertoli cell precursors and consequently fewer mature Sertoli cells, leading to testicular hypoplasia (15, 85). Sertoli cell sensitivity is controlled at least in part by DIO3 in adjacent spermatogonia, as discussed earlier.

Together these findings suggest that the timing, duration, and level of T3 exposure as determined in part by DIO3 are critical factors in determining cell-fate decisions. We suggest that further studies will reveal additional controls by DIO3 over specific cell lineages in a wider range of tissues. Better definition of DIO3 and DIO2 functions at this cellular level awaits improved methods for identifying the cell types that express each enzyme. This has been difficult because deiodinases are expressed transiently and often at low levels in restricted cell populations.

Deiodinases and Human Development

Inactivating mutations of human DIO2 and DIO3 are currently unknown but evidence from mouse models predicts that such mutations might be associated with a range of syndromic features (see Table 1). We discuss here other known forms of genetic changes that may influence DIO2 and DIO3 function.

Type 2 Deiodinase

Approximately 30% of human populations carry a single-nucleotide variation that substitutes alanine for threonine at position 92 of the DIO2 protein (92). This T92A variant modestly reduces T3 generation from T4 in cultured cells or when introduced into mice (93). Studies in mice carrying the human variant suggest that T92A alters proteostasis associated with binding of chaperones that traffic and recycle the protein between the Golgi and endoplasmic reticulum (93). In hypothyroid patients who also carry T92A, thyroid status and neurological outcomes improve when treated with a combination of T4 and T3, instead of T4 alone (94), suggesting impaired T4 to T3 conversion in T92A carriers. This hypomorphic variant has been associated with impaired response of the HPT axis to TRH (95) and an increased incidence of metabolic and neurological disorders (96-98).

Type 3 Deiodinase

DIO3 is an imprinted gene in the imprinted DLK1 to DIO3 cluster on chromosome 14 in humans (chromosome 12 in mice) and may contribute to abnormalities in human uniparental disomy of chromosome 14 (UPD14) (99). DIO3 is preferentially expressed from the paternally inherited allele (100-102). Inheritance of 2 copies of maternal or paternal DIO3 in UPD14 patients results in a double dosage of the underexpressed maternal or overexpressed paternal allele, respectively. Reduced DIO3 activity is expected with maternal UPD14 (Temple syndrome), characterized by neonatal failure to thrive, growth retardation, reduced head circumference, hydrocephalus, mild mental impairment, and early puberty (103-105). Conversely, DIO3 overexpression is expected with paternal UPD14 (Kagami-Ogata syndrome), characterized by cranial dysmorphism, neurological impairment, and rib cage abnormalities (105). Aberrant dosage of other genes in the DLK1 to DIO3 cluster presumably contributes to the pathophysiology of both UPD14 syndromes.

Type 1 Deiodinase

Two families have been identified that carry heterozygous DIO1 mutations (106). The mutations change amino acids 94 and 201 and decrease enzymatic activity in vitro largely due to lower substrate affinity. As in Dio1^+/– mice (or more obviously in Dio1^–/– mice) (27), the human carriers exhibit impaired metabolism of iodothyronines with elevated rT3 and higher rT3/T3 ratios presumably due to reduced clearance of rT3 by DIO1. As reported to date, the patients display otherwise mild symptoms.

Other Genes

As selenoproteins, all 3 deiodinases require specialized factors, including SECISBP2, for incorporation of the rare amino acid selenocysteine during protein translation (107). Human mutations that interfere with this incorporation impair deiodinases and other selenoproteins and are associated with abnormal metabolism of thyroid hormones (108, 109), hearing loss, azoospermia, skin photosensitivity, as well as growth and metabolic impairment (110, 111). Inactivation of SECISBP2 in humans or mice results in elevated T4 and rT3 and decreased T3, indicative of impaired deiodination (112, 113). Many of the human symptoms, for example, in the HPT axis, hearing and growth, potentially indicate the involvement of all 3 deiodinases in multiple human tissues.

Future Challenges

The expression of deiodinases in most, if not all, tissues in mammalian development suggests that additional functions remain to be discovered, and that these functions may profoundly influence tissue maturation. The overall phenotype of Dio3^–/– mice (see Table 1) is the most severe of any deiodinase deficiency, suggesting that new DIO3 functions remain to be elucidated, including key life-sustaining functions at perinatal stages. Although DIO2 tends to be associated with later maturation, a novel early role was reported in perinatal programming of liver function in mice (114). Hepatocyte-specific Dio2 deficiency was associated with altered lipid metabolism, rendering the liver resistant to high-fat diet– or alcohol-induced steatosis (114, 115).

Apart from overt mutations in deiodinase genes, further studies may investigate whether persistent environmental pollutants that act as endocrine-disrupting chemicals might disturb deiodinase activities. Environmental chemicals may produce relatively modest changes in circulating T4 and T3 but more severe end-tissue defects (116, 117), for example, in brain function (118) or hearing (119), suggesting possible interference with tissue determinants of thyroid hormone action including deiodinases.

Finally, knowledge of the developmental roles of deiodinases in determining cell death or survival may offer opportunities for exploiting these enzymes as therapeutic targets in a range of pathological conditions. For example, deletion of Dio2 recovers cone photoreceptor survival in Dio3^–/– mice (22) but, remarkably, also improves cone survival in other forms of retinal degeneration caused by defects unrelated to T3 signaling (120). The possibility that suppressing T3 signaling in the diseased retina helps photoreceptor survival has been supported by interventions in the eye in mouse models of Leber congenital amaurosis by overexpression of DIO3 via adenoviral vectors or by application of iopanoic acid, an inhibitor of deiodinase activity (121). There is much scope to explore targeting of deiodinases in different tissues as a way to ameliorate disease outcomes. Accessible tissues, such as the eye, may be amenable to direct intervention, bypassing some of the concerns that arise with systemic treatments. The use of novel inhibitors of deiodinase activity (122, 123) or gene therapy may allow therapeutic approaches based on local interference with T3 signaling.

Glossary

Abbreviations

Dio1

type 1 deiodinase gene

Dio2

type 2 deiodinase gene

Dio3

type 3 deiodinase gene

DIO1

type 1 deiodinase enzyme

DIO2

type 2 deiodinase enzyme

DIO3

type 3 deiodinase enzyme

HPT axis

hypothalamic-pituitary-thyroid axis;

M opsin

medium-long wavelength opsin;

rT3

reverse 3,3′,5′-triiodothyronine;

S opsin

short wavelength opsin;

T3

3,5,3′-triiodothyronine;

T4

thyroxine;

THRB

thyroid hormone receptor β;

TRH

thyrotropin-releasing hormone;

TSH

thyrotropin;

UPD14

uniparental disomy of chromosome 14

Additional Information

Disclosures: The authors have nothing to disclose.

Data Availability

Data sharing is not applicable to this article because no data sets were generated or analyzed during the present study.