Mitochondrial Dynamics and Hypothalamic Regulation of Metabolism

Abstract

Mitochondria are cellular organelles that play an important role in bioenergetic processes. In the central nervous system, high energy–demanding neurons are critically dependent on mitochondria to fulfill their appropriate functions. The hypothalamus is a key brain area for maintaining glucose and energy homeostasis via the ability of hypothalamic neurons to sense, integrate, and respond to numerous metabolic signals. Mitochondrial function has emerged as an important component in the regulation of hypothalamic neurons controlling glucose and energy homeostasis. Although the underlying mechanisms are not fully understood, emerging evidence indicates that mitochondrial dysfunction in hypothalamic neurons may contribute to the development of various metabolic diseases, including obesity and type 2 diabetes mellitus (T2DM). In this review, we summarize recent studies demonstrating the link between mitochondria and hypothalamic neural control of glucose and energy homeostasis. Finally, this review provides an insight to understand how mitochondria in hypothalamic neurons may contribute to the development of metabolic disorders, such as T2DM and obesity.

The prevalence of metabolic disorders, including obesity and type 2 diabetes mellitus (T2DM), continues to rise at alarming rates around the world, including the United States (1). Insufficient understanding of the pathophysiological mechanisms that promote these disorders has resulted in a substantial financial burden on health care.

The central nervous system (CNS) plays a pivotal role in the regulation of whole-body energy and glucose metabolism (2). In particular, the hypothalamus, an important coordinator of the endocrine and the autonomic nervous system, has been considered a key brain area in regulating metabolism through the ability of neurons to sense, integrate, and respond to numerous metabolic signals, such as hormones, including leptin, ghrelin and insulin, and nutrients, including glucose, amino acids, and fatty acids (3, 4). Several neuronal populations in the hypothalamus have been identified for their property to sense and respond to changes in circulating metabolic signals. However, the intracellular mechanisms underlying their ability to sense, respond, and regulate metabolism remain elusive.

Mitochondria, cytosolic organelles, are considered the powerhouse of the cell because they generate energy for cellular processes. Neurons rely on mitochondrial oxidative phosphorylation to meet their high energy requirements (5). Mitochondria are highly dynamic organelles. Besides being able to move within the cell according to its metabolic needs, they are able to undergo active morphological changes. These changes are defined as mitochondrial dynamics and are in response to various metabolic stimuli that will influence several mitochondrial properties, including mitochondrial bioenergetics and quality control (6, 7).

Recent studies have demonstrated that mitochondrial dynamics in hypothalamic neurons are critical events in mediating neural activity in response to changes in metabolic states (8, 9). This suggests that mitochondrial dysfunction might be involved in the development of metabolic disorders, such as T2DM and obesity. In this review, we discuss the importance of mitochondrial function and dynamics, particularly in the hypothalamus; explain their role in the neuronal regulation of glucose and energy homeostasis; and provide some perspectives on their role in the development of metabolic disorders.

Hypothalamic Regulation of Glucose and Energy Homeostasis

The CNS has been shown to be an important regulator of whole-body energy and glucose homeostasis. Specifically, within the CNS, the hypothalamus plays a key role in maintaining homeostasis by sensing and integrating signals such as hormones and nutrients, as well as responding to these signals by modulating the activity of peripheral organs. This precise communication between the CNS and the peripheral organs occurs because of the ability of the hypothalamus to modulate the endocrine and the autonomic nervous system. Several hypothalamic nuclei have been identified and studied for their roles in metabolism regulation, including the nuclei of the arcuate (ARC), ventromedial (VMH), paraventricular, and dorsomedial hypothalamus and the lateral hypothalamic area (10).

The ARC plays a fundamental role in sensing whole-body energy status (11). Within the ARC, the melanocortin system has been shown to play a key role in the regulation of energy and glucose metabolism. The ARC melanocortin system consists of two neuronal populations: the orexigenic Agouti-related protein (AgRP)–expressing and neuropeptide Y (NPY)–expressing neurons and the anorexigenic pro-opiomelanocortin (POMC) neurons. In addition, these two neuronal populations also play an antagonistic role in modulating glucose metabolism.

An intriguing aspect of the biology of the ARC melanocortin cells is that they have opposing activation states; when NPY/AgRP neurons are active, POMC cells are quiescent and vice versa. From the perspective of their functional relevance (hunger vs satiety), this finding is not unexpected. Nevertheless, the intracellular mechanisms that underlie this reciprocal activation remain elusive. Under conditions of negative energy balance, in which glucose levels are decreased and levels of circulating ghrelin are increased, NPY/AgRP neurons are activated and POMC neurons are silent, which leads to the development of hunger sensations and increased lipid metabolism in peripheral tissues. During negative energy balance, ghrelin-induced β-oxidation of fatty acids provides a continuous fuel supply to NPY/AgRP neurons, which enables maintenance of the activated state (12). These ghrelin-mediated effects are driven by AMP-activated protein kinase (AMPK) signaling, a pathway that decreases levels of malonyl coenzyme A (CoA) via inactivation of acetyl CoA carboxylase and leads to enhanced activity of carnitine O-palmitoyltransferase 1, muscle isoform (CPT1-M) (12, 13). The activity of AMPK is subject to regulation by the AMP/ATP ratio in the cytoplasm. During negative energy balance, AMPK regulates mitochondrial biogenesis in the peripheral tissues, such as skeletal muscle (14), and its levels are elevated in the hypothalamus (15, 16). AMPK signaling also induces mitochondrial uptake of long-chain fatty acyl CoAs via CPT1-M (12), leading to the generation of ATP via β-oxidation of fatty acids (17). Hypothalamic CPT1-M is a crucial regulator of feeding (18), which indicates that an intracellular lipid-metabolizing pathway is present in the hypothalamus.

Following food intake, circulating levels of glucose and leptin increase and POMC neurons are activated, which promotes the cessation of feeding. Glucose sensing is critical for POMC neuronal activation (19); glucose uptake and metabolism in these neurons have an important role in regulating systemic glucose homeostasis (20). Glucose-dependent POMC activity is suggested to be promoted, in part, by the inactivation of ATP-sensitive potassium channels, which results in depolarization of the cells (19, 21, 22). In addition, leptin promotes the opening of nonspecific cation channels that depolarize POMC.

Leptin also promotes signal transducer and activator of transcription 3 phosphorylation via Janus kinase 2 and subsequent expression of POMC (23). However, under conditions of elevated levels of glucose and leptin, an increase in the activity of POMC neurons is also associated with a concomitant increase in levels of reactive oxygen species (ROS) (12, 24). Intracerebroventricular injection of a ROS scavenging cocktail promoted hunger (12), and increases in levels of hypothalamic ROS seem to mediate POMC neuronal activation and feelings of satiety (25). In diet-induced obese (DIO) mice, impairment of POMC neuronal firing is reversed by intracerebroventricular ROS injection and suppression of peroxisome proliferator–activated receptor γ signaling (25).

Taken together, these data suggest that fundamentally different metabolic conditions and changes in fuel availability may mediate the promotion of hunger and satiety. Given that sensations of hunger and satiety can occur hours apart, it is possible that intracellular mechanisms and, more specifically, fuel availability–driven mitochondrial mechanisms may be critical to these processes and that hunger and satiety ensure the required switches in fuel utilization that support the appropriate activity of orexigenic and anorexigenic neurons. This also suggests that fuel availability–mediated intracellular (mitochondrial) mechanisms may be responsible for the melanocortin neuronal responses under different diet compositions, including high-fat diet (HFD)–induced obesity, a diet known to induce a condition called leptin resistance.

The VMH has been identified as a crucial brain region to monitor and regulate energy and glucose homeostasis (26–29). The VMH contains glucose-sensing neurons that respond to either high or low glucose levels. In addition, a subgroup of the VMH neurons also expresses insulin receptors, and recent studies have shown that CNS insulin signaling in patients with T2DM is altered (30). Many studies have also pointed to the VMH as being a critical site for counterregulatory responses following insulin-triggered hypoglycemia, which includes elevated secretion of glucagon to restore glucose levels (31–33). More recent studies also demonstrated that the VMH plays an important role in regulating insulin secretion in response to hyperglycemia (29, 34, 35).

The ability of the VMH to regulate peripheral organs strongly depends on its connection with the autonomic nervous system. The VMH efferents directly project to sympathetic preganglionic cells in the spinal cord (36, 37). By projecting onto other hypothalamic neurons (including the ARC POMC neurons), the VMH can also indirectly influence sympathetic outflow to regulate glucose homeostasis (38). On the other hand, no direct projections were found from the VMH to the parasympathetic preganglionic neurons in the dorsal motor nucleus of the vagus (37) to the pancreas (39). However, blockade of parasympathetic input to pancreatic α-cells prevented hypoglycemia-induced and neuroglycopenia-induced increases in glucagon (40, 41). The intracellular mechanisms underlying the ability of VMH neurons to sense and respond to changes in peripheral glucose levels in the control of glucose homeostasis remain elusive. Recent evidence suggests that mitochondria play a role in these mechanisms. Specifically, mitochondrial dynamics have recently been shown to be tightly associated with the ability of these neurons to respond to the alteration in glucose levels, influencing neuronal activation and thus systemic glucose metabolism (42).

Mitochondrial Dynamics

Mitochondria not only have a fundamental role in regulating ATP production, the main energy source to maintain cell survival and cellular metabolic homeostasis (43, 44), but also are involved in pivotal cellular processes, including calcium homeostasis, redox signaling, autophagy, and immune responses (43). Because of their functions, mitochondria are highly dynamic organelles. These dynamic processes include directed mitochondrial movement along the cytoskeleton, interaction with the endoplasmic reticulum (ER) (45), highly dynamic structures regulating cellular processes and signaling pathways (46), and mitochondrial morphological changes due to fusion and fission that control mitochondrial shape, size, and number within the cell (47). These morphological changes, defined as mitochondrial dynamics, are in support of the energetic needs of the cell (48).

Mitochondrial dynamics are highly regulated processes. Mitochondrial fusion is controlled by two outer membrane-anchored dynamin family member proteins named mitofusin 1 and mitofusin 2 (MFN2) and by a single inner-membrane dynamin family member called optic atrophy 1 [for review, see Youle and van der Bliek (49) and Kasahara and Scorrano (50)]. Mitochondrial fission is controlled by mitochondrial fission 1, mitochondrial fission factor, and dynamin-related protein 1 (DRP1). Mitochondrial fission 1 and mitochondrial fission factor are localized on the outer mitochondrial membrane, whereas DRP1 is localized mainly in the cytosol. DRP1 recruitment to the mitochondria and its oligomerization drive mitochondrial constriction and fragmentation. DRP1 translocation from the cytosol to the mitochondrial fission sites is mediated via its dephosphorylation at serine 637 by calcineurin [for review, see Kasahara and Scorrano (50)]. On the other hand, phosphorylation at serine 616 has been associated with increased mitochondrial fragmentation (51) and thus its activation. The mechanism by which initial constriction of mitochondria occurs remains elusive, but the subsequent constriction and scission processes are known to be mediated by DRP1 (52, 53).

It also has been suggested that mitochondrial dynamics are important processes for controlling mitochondrial quality. For instance, mitochondria can undergo fusion processes to exchange content between healthy mitochondria as a complementary function. Unlike fusion, fission process allows the removal of damaged mitochondria through mitophagy, which is a form of autophagy that selectively degrades damaged mitochondria (7, 47, 54, 55). Besides its role in mitochondrial quality control, continuous mitochondrial dynamics occur in response to energy demands of the cell and to rapidly adapt to changes in the cellular environment (56). Thus, mitochondrial dynamics are an essential cellular and physiological process for mitochondria and cellular homeostasis.

Aberrant mitochondrial dynamics in tissues have been associated with several neurodegenerative diseases, cardiovascular disorders, and cancer (57–59). Furthermore, alterations in mitochondrial dynamics have been observed in different in vitro and in vivo models of metabolic disorders such as T2DM (6, 60–62). In addition, alterations in mitochondria-ER interaction, which plays an important role in cellular function in response to metabolic status (46), have been shown to play a crucial role in the development of obesity and leptin resistance in peripheral tissues (63).

Mitochondrial Dynamics in Hypothalamic Neurons

Mitochondrial dynamics in ARC neurons

Previous studies have shown that neuronal activation of the NPY/AgRP neurons, such as during fasting or after ghrelin administration, is associated with an increase in mitochondria density and a decrease in mitochondria size (Table 1) (12, 64). Thus, these data suggest that changes in mitochondrial dynamics and specifically the induction of mitochondrial fission may play a role in NPY/AgRP neuronal activation during negative energy balance. In agreement with this, a recent study has revealed that in DIO mice, silent NPY/AgRP neurons show increased size and elongated mitochondria (65), suggesting that changes in mitochondrial dynamics and specifically the induction of mitochondrial fusion may play a role in the inactivation of these neurons (Table 2).

Table 1.

Neuron Type–Specific Alteration of Mitochondrial Processes in Various Metabolic States

Neurons	Energy Status	Mitochondrial Processes	Reference
AgRP	Fasting	Fission	(65)
	Ghrelin	Fission	(12)
	HFD	Fusion	(65)
POMC	Fed	Fusion (decreased DRP1 levels)	(66, 67)
	HFD	Loss of mitochondria-ER contacts, increased free intracellular Ca2+	(66, 68)
SF1	Glucose	Fission (mediated by UCP2)	(42)

Table 2.

Mitochondrial Processes in Transgenic Animal Models in Various Metabolic States

Genetic Model	Energy Status	Mitochondrial Processes (Compared to Control)	Observed Outcomes	Reference
Mfn1KOAgRP	ND	More rounded shape	No change in metabolic phenotypes	(65)
	HFD	Increased mitochondria number and coverage	Decreased firing rate	(65)
			Decreased food intake and body weight in female
Mfn2KOAgRP	ND	Increased mitochondria size without changes in shape	Decreased body weight in female	(65)
	HFD	Increased mitochondria number and coverage	Decreased firing rate	(65)
			Decreased body weight
			Improved glucose tolerance
Mfn1KOPOMC	ND	Reduced mitochondria size	Abnormal glucose homeostasis by impaired pancreatic insulin release	(66, 69)
		Increased ROS levels
Mfn2KOPOMC	HFD	Loss of mitochondria-ER contacts	Obesity and leptin resistance	(66)
Opa1KOPOMC	ND	Reduced mitochondrial fusion	No change in metabolic phenotypes before onset of obesity	(69)
Drp1KOPOMC	ND	Increased mitochondria size and rounded shape	Improvement of glucose metabolism and leptin sensitivity	(67)
		Increased ROS levels
Ucp2KO	Ghrelin	Loss of UCP2-dependent increased ROS production	Impaired ghrelin effect	(12)
	Glucose	No change in glucose-induced mitochondrial fission in SF1 neurons	Decreased glucose tolerance, reduced insulin sensitivity	(70)
Ucp2KOKISF1	Glucose	DRP1-mediated mitochondrial fission in SF1 neurons	Increased glucose tolerance	(42)
			Enhanced insulin sensitivity
Ucp2KISF1	Glucose	DRP1-mediated mitochondrial fission in SF1 neurons	Increased glucose tolerance	(42)
			Enhanced insulin sensitivity

Abbreviation: ND, normal diet.

When Mfn1 and Mfn2 were deleted in NPY/AgRP neurons, neuronal firing frequency was impaired in DIO mice (65). The impairment of AgRP neuronal activation was reversed by increasing intracellular ATP levels, suggesting that the decrease in firing rate is likely due to changes in intracellular ATP levels. Collectively, these data indicate that in the orexigenic AgRP neurons, Mfns-mediated mitochondrial fusion plays a role in regulating neural activity thorough perturbation of ATP levels, which, in turn, will affect energy and glucose homeostasis (Fig. 1).

Figure 1.

Mitochondria dynamics in ARC melanocortin neurons. Schematic illustration showing changes in mitochondrial dynamics in anorexigenic POMC and orexigenic AgRP neurons of the ARC nucleus. Negative energy balance induces mitochondrial fission in orexigenic AgRP neurons. During positive energy balance, such as HFD feeding, mitochondria density and coverage decrease, indicating mitochondrial fusion. In contrast to AgRP neurons, the activation of anorexigenic POMC neurons under satiety condition leads to mitochondrial fusion, which is associated with elevated ROS levels. In POMC neurons, HFD feeding results in decreased mitochondria-ER interaction and increased free Ca²⁺. 3V, third ventricle.

In addition to its role in mitochondrial fusion, MFN2 has been shown to play a role in tethering ER to mitochondria, a juxtaposition required for mitochondria calcium uptake (71). Interestingly, HFD feeding has been associated with an alteration in mitochondria-ER interaction in POMC neurons (66), and diet-resistant mice display increased mitochondria-ER interaction in POMC neurons compared with obese controls (72). Consistent with this, POMC-specific deletion of Mfn2 induced reduced mitochondria-ER interaction, increased hypothalamic ER stress, and impaired POMC activity, resulting in obesity (66). These results suggest that MFN2-mediated mitochondria-ER interaction in POMC neurons is an essential event for POMC function in regulating energy homeostasis. Similarly, mice with selective POMC deletion of Opa1, another mitochondrial fusion protein, display an obese phenotype (69). On the other hand, deletion of Mfn1 in POMC neurons has been shown to alter glucose sensing, leading to impaired glucose homeostasis with reduced insulin release from pancreatic β-cells (69). Altogether, these data suggest that mitochondrial fusion is required for POMC neuronal functions in the regulation of energy and glucose homeostasis (Fig. 1).

In agreement with a role for mitochondrial fusion in supporting POMC neuronal function, activated DRP1, a major regulator of mitochondrial fission, was found to be expressed in silent POMC neurons (67). Consistent with this, inducible and selective ablation of Drp1 in POMC neurons increased POMC neuronal activity in response to glucose and leptin, as well as improved glucose metabolism (67), suggesting that DRP1-mediated mitochondrial fission regulates POMC glucose and leptin sensitivity. Collectively, these data suggest that mitochondrial dynamics and mitochondria-ER interaction are essential regulators of POMC neurons in modulating energy and glucose homeostasis (Fig. 1).

Mitochondrial dynamics on SF1 neurons

The VMH plays an important role in glucose sensing and hypoglycemia-related adaptations of the autonomic nervous system. Glucose-sensing neurons in this region are either activated (glucose-excited neurons) or inhibited (glucose-inhibited neurons) by elevated glucose levels (73, 74). The glucose-excited and glucose-inhibited neurons play a key role in the regulation of pancreatic insulin release in response to hyperglycemic conditions and in counterregulatory responses induced by hypoglycemia (29, 75), respectively. However, how intracellular signaling modalities within these VMH neurons bring about physiological and pathological responses to the changing glucose environment remains elusive.

Recent studies have shown that DRP1 in VMH neurons plays a pivotal role in glucose sensing and thus in the regulation of systemic glucose metabolism (Tables 1 and 2) (42, 70). Specifically, a study from our laboratory has shown that systemic glucose administration induced decreased mitochondrial size and increased mitochondrial density without affecting the total mitochondrial area in the cytosol of VMH neurons, suggesting that glucose promotes mitochondrial fission. In support of this, glucose administration was found to significantly increase activated DRP1, thus confirming glucose-induced mitochondrial fission in VMH neurons (42). Furthermore, DRP1 activation was found to be mediated by a mitochondrial protein, uncoupling protein 2 (UCP2), as glucose-induced mitochondrial fission in VMH neurons was not observed in glucose-impaired Ucp2KO mice, and selective reexpression of UCP2 in neurons of Ucp2KO mice reversed these effects (42). Thus, all together these data indicate that mitochondrial fission driven by UCP2 plays a role in VMH glucose sensing and in peripheral regulation of glucose metabolism (Fig. 2).

Figure 2.

Mitochondrial dynamics in VMH glucose-excited neurons. Schematic illustration showing increased glucose levels induce mitochondrial fission and increased neuronal activation in glucose-excited neurons of the VMH. 3V, third ventricle.

Calcium Homeostasis, ROS, and UCP2 in the Hypothalamus

It is well known that mitochondria not only play an important role in generating ATP but also contribute to critical cellular signaling pathways, including Ca²⁺ and ROS signaling (76). For example, as mitochondrial dynamics and mitochondria-ER interactions have been found to be altered during DIO in hypothalamic neurons (66), a recent study has shown that DIO is associated with impaired intracellular Ca²⁺ handling, including mitochondrial Ca²⁺ uptake, in POMC neurons (68). These changes in POMC intracellular Ca²⁺ handling properties were associated with membrane hyperpolarization and a marked decrease in excitability of POMC neurons. By experimentally altering POMC intracellular Ca²⁺ concentrations, the electrophysiological properties observed in DIO were reproduced, thus suggesting that high-fat feeding-induced deterioration of Ca²⁺ homeostasis in POMC neurons during obesity development may contribute to the dysregulation of metabolism by affecting neuronal functions.

Mitochondria are also a major source of oxidative stress caused by ROS, which are generated as byproducts of cellular process (77). Oxidative stress has been implicated in the pathogenesis of several diseases, including cardiovascular diseases, neurodegenerative diseases, cancer, and metabolic diseases (78). In addition, mitochondrial ROS have emerged as important signaling molecules that play a key role in hypothalamic glucose sensing (79).

It has been reported that mitochondrial ROS production is negatively regulated by mitochondrial UCP2 (80), a mitochondrial inner membrane protein, which is highly expressed in the hypothalamus (81). UCP2 function is still unclear. As an uncoupler, it mediates the reentry of protons into the mitochondrial matrix, a process known as proton leak. However, most recently, it has been suggested that UCP2 may play a role as a metabolite transporter (82). It has been demonstrated that UCP2 is involved in impairment glucose-induced ATP production and associated with obesity-induced loss of glucose sensing in POMC neurons (20). Moreover, it has been suggested that UCP2 may negatively affect POMC neuronal activation by buffering ROS in DIO, as ROS levels have been shown to be an important regulator of POMC activity (25). In contrast to POMC neurons, UCP2-dependent mitochondrial function is an important positive regulator of hypothalamic AgRP neurons in response to ghrelin and associated feeding behavior (12). Hypothalamic activation of AMPK mediates mitochondrial UCP2 (12) activation, which, by limiting the generation of ROS (83, 84) that are associated with mitochondrial β-oxidation of fatty acids (12, 85, 86), enables NPY/AgRP neurons to be active at a time of negative energy balance. AMPK-mediated UCP2 levels and activity lead to an increase in the number of mitochondria in NPY/AgRP neurons during fasting and in response to ghrelin (12, 64), which seems to play a pivotal role in regulating AgRP neuronal activation. Activation of AMPK also has been shown to increase the expression of UCP2 in other tissues, such as liver and endothelial cells (87).

In addition to the ARC POMC and AgRP neurons, more recently, it has been shown that in the VMH, glucose-induced DRP1-mediated mitochondrial fission and neuronal activation are dependent on UCP2 (42).

Altogether, these data suggested that UCP2-mediated mitochondrial function and dynamics play a crucial role in the regulation of hypothalamic neural activity for energy metabolism and glucose homeostasis.

Conclusions and Future Directions

Mitochondrial dynamics play crucial roles in remodeling of mitochondrial structure and regulating their cellular function in response to the changing metabolic milieu. Mitochondrial dysfunction can increase the risk of many health problems, including metabolic diseases such as T2DM and obesity. The data reviewed here imply that mitochondrial dynamics in most hypothalamic neurons are involved in regulating energy metabolism and glucose homeostasis, although the precise mechanisms underlying mitochondrial dynamics–induced neural adaptations remain unclear.

Glossary

Abbreviations:

AgRP

Agouti-related protein

AMPK

AMP-activated protein kinase

ARC

arcuate hypothalamus

CNS

central nervous system

CoA

coenzyme A

CPT1-M

carnitine O-palmitoyltransferase 1, muscle isoform

DIO

diet-induced obese

DRP1

dynamin-related protein 1

ER

endoplasmic reticulum

HFD

high-fat diet

MFN2

mitofusin 2

NPY

neuropeptide Y

POMC

pro-opiomelanocortin

ROS

reactive oxygen species

T2DM

type 2 diabetes mellitus

UCP2

uncoupling protein 2

VMH

ventromedial hypothalamus