The fatigue-inducing effects of cancer and its therapy are characterized by decreased physical activity in the absence of any motivational deficit.

Although cancer and its therapy are well known to be associated with fatigue, the exact nature of cancer-related fatigue remains ill-defined. We previously reported that fatigue-like behavior induced independently by tumor growth and by the chemotherapeutic agent cisplatin is characterized by reduced voluntary wheel running and an intact motivation to expand effort for food rewards. The present set of experiments was initiated to characterize the functional consequences of fatigue induced by chemoradiotherapy in tumor-bearing mice and relate them to changes in the expression of genes coding for inflammation, mitochondria dynamics and metabolism. Two syngeneic murine models of cancer were selected for this purpose, a model of human papilloma virus-related head and neck cancer and a model of lung cancer. In both models, tumor-bearing mice were submitted to chemoradiotherapy to limit tumor progression. Two dimensions of fatigue were assessed, the physical dimension by changes in physical activity in mice trained to run in wheels and the motivational dimension by changes in the performance of mice trained to nose poke to obtain a food reward in a progressive ratio schedule of food reinforcement. Chemoradiotherapy reliably decreased wheel running activity but had no effect on performance in the progressive ratio in both murine models of cancer. These effects were the same for the two murine models of cancer and did not differ according to sex. Livers and brains were collected at the end of the experiments for qRT-PCR analysis of expression of genes coding for inflammation, mitochondria dynamics, and metabolism. The observed changes were mainly apparent in the liver and typical of activation of type I interferon and NF-κB-dependent signaling, with alterations in mitochondrial dynamics and a shift toward glycolysis. Although the importance of these alterations for the pathophysiology of cancer-related fatigue remains to be explored, the present findings indicate that fatigue brought on by cancer therapy in tumor-bearing mice is more physical than motivational.

Cisplatin decreases voluntary wheel-running activity but does not impair food-motivated behavior in mice.

Cisplatin is a chemotherapeutic agent that is still commonly used to treat solid tumors. However, it has several toxic side effects due in large part to the mitochondrial damage that it induces. As this mitochondrial damage is likely to result in a decrease in the amount of metabolic energy that is available for behavioral activities, it is not surprising that fatigue develops in cancer patients treated with cisplatin. The present preclinical study was initiated to determine whether the detrimental effects of cisplatin were more pronounced on physical effort requiring a lot of energy versus effort that not only requires less energy but also procures energy in the form of food. For this purpose, mice were either trained to run in a wheel or to work for food in various schedules of food reinforcement before being treated with cisplatin. The experiments were carried out only in male mice as we had already reported that sex differences in cisplatin-induced neurotoxicities are minimal. Cisplatin was administered daily for one cycle of five days, or two cycles separated by a five-day rest. As observed in previous experiments, cisplatin drastically reduced voluntary wheel running. In contrast, when cisplatin was administered to food-restricted mice trained to work for a food reward in a progressive ratio schedule or in a fixed-interval schedule, it tended to increase the number of responses emitted to obtain the food rewards. This increase was not associated with any change in the temporal distribution of responses during the interval between two reinforcements in mice submitted to the fixed interval schedule of food reinforcement. When cisplatin was administered to food-restricted mice trained in an effort-based decision-making task in which they had to choose between working for a grain pellet with little effort and working for a preferred chocolate pellet with more effort, it decreased the total number of responses emitted to obtain food rewards. However, this effect was much less marked than the decrease in wheel running induced by cisplatin. The decrease in the effort invested in the procurement of food rewards was not associated with any change in the relative distribution of effort between low reward and high reward during the time course of the test session. These findings show that cisplatin decreases energy-consuming activities but not energy-procuring activities unless they require a choice between options differing in their cost-benefit ratio. Furthermore, they indicate that the physical dimension of fatigue is more likely to develop in cisplatin-treated individuals than the motivational dimension of fatigue.

The GDF15-GFRAL axis mediates chemotherapy-induced fatigue in mice.

Cancer-related fatigue is defined as a distressing persistent subjective sense of physical, emotional, and/or cognitive tiredness or exhaustion related to cancer or cancer treatment that is not proportional to recent activity and that interferes with usual functioning. This form of fatigue is highly prevalent during cancer treatment and in some patients, it can persist for years after treatment has ended. An understanding of the mechanisms that drive cancer-related fatigue is still lacking, which hampers the identification of effective treatment options. Various chemotherapeutic agents including cisplatin are known to induce mitochondrial dysfunction and this effect is known to mediate chemotherapy-induced peripheral neuropathy and cognitive dysfunction. Mitochondrial dysfunction results in the release of mitokines that act locally and at distance to promote metabolic and behavioral adjustments to this form of cellular stress. One of these mitokines, growth differentiation factor 15 (GDF15) and its receptor, glial cell line-derived neurotrophic factor family receptor α-like (GFRAL), have received special attention in oncology as activation of GFRAL mediates the anorexic response that is responsible for cancer anorexia. The present study was initiated to determine whether GDF15 and GFRAL are involved in cisplatin-induced fatigue. We first tested the ability of cisplatin to increase circulating GDF15 in mice before assessing whether GDF15 can induce behavioral fatigue measured by decreased wheel running in healthy mice and increase behavioral fatigue induced by cisplatin. Mice administered a long acting form of GDF15, mGDF15-fc, decreased their voluntary wheel running activity. When the same treatment was administered to mice receiving cisplatin, it increased the amplitude and duration of cisplatin-induced decrease in wheel running. To determine whether endogenous GDF15 mediates the behavioral fatigue induced by cisplatin, we then administered a neutralizing monoclonal antibody to GFRAL to mice injected with cisplatin. The GFRAL neutralizing antibody mostly prevented cisplatin-induced decrease in wheel running and accelerated recovery. Taken together these findings demonstrate for the first time the role of the GDF15/GFRAL axis in cisplatin-induced behaviors and indicate that this axis could be a promising therapeutic target for the treatment of cancer-related fatigue.

Consumption of dietary fat causes loss of olfactory sensory neurons and associated circuitry that is not mitigated by voluntary exercise in mice.

Excess nutrition causes loss of olfactory sensory neurons (OSNs) and reduces odour discrimination and odour perception in mice. To separate diet-induced obesity from the consumption of dietary fat, we designed pair-feeding experiments whereby mice were maintained on isocaloric diets for 5 months, which prevented increased fat storage. To test our hypothesis that adiposity was not a prerequisite for loss of OSNs and bulbar projections, we used male and female mice with an odorant receptor-linked genetic reporter (M72tauLacZ; Olfr160) to visualize neural circuitry changes resulting from elevated fat in the diet. Simultaneously we monitored glucose clearance (diagnostic for prediabetes), body fat deposition, ingestive behaviours, select inflammatory markers and energy metabolism. Axonal projections to defined olfactory glomeruli were visualized in whole-mount brains, and the number of OSNs was manually counted across whole olfactory epithelia. After being pair fed a moderately high-fat (MHF) diet, mice of both sexes had body weight, adipose deposits, energy expenditure, respiratory exchange ratios and locomotor activity that were unchanged from control-fed mice. Despite this, they were still found to lose OSNs and associated bulbar projections. Even with unchanged adipocyte storage, pair-fed animals had an elevation in TNF cytokines and an intermediate ability for glucose clearance. Albeit improving health metrics, access to voluntary running while consuming an ad libitum fatty diet still precipitated a loss of OSNs and associated axonal projections for male mice. Our results support that long-term macronutrient imbalance can drive anatomical loss in the olfactory system regardless of total energy expenditure. KEY POINTS: Obesity can disrupt the structure and function of organ systems, including the olfactory system that is important for food selection and satiety. We designed dietary treatments in mice such that mice received fat, but the total calories provided were the same as in control diets so that they would not gain weight or increase adipose tissue. Mice that were not obese but consumed isocaloric fatty diets still lost olfactory neuronal circuits, had fewer numbers of olfactory neurons, had an elevation in inflammatory signals and had an intermediate ability to clear glucose (prediabetes). Mice were allowed access to running wheels while consuming fatty diets, yet still lost olfactory structures. We conclude that a long-term imbalance in nutrition that favours fat in the diet disrupts the olfactory system of mice in the absence of obesity.

Long-term obesogenic diet and targeted deletion of potassium channel Kv 1.3 have differing effects on voluntary exercise in mice.

Voluntary exercise is frequently employed as an intervention for obesity. The voltage-gated potassium channel Kv 1.3 is also receiving attention as a therapeutic target for obesity, in addition to potential therapeutic capabilities for neuroinflammatory diseases. To investigate the combinatorial effects of these two therapies, we have compared the metabolic status and voluntary exercise behavior of both wild-type mice and a transgenic line of mice that are genetic knockouts for Kv 1.3 when provided with a running wheel and maintained on diets of differing fat content and caloric density. We tracked the metabolic parameters and wheel running behavior while maintaining the mice on their assigned treatment for 6 months. Wild-type mice maintained on the fatty diet gain a significant amount of bodyweight and adipose tissue and display significantly impaired glucose tolerance, though all these effects were partially reduced with provision of a running wheel. Similar to previous studies, the Kv 1.3-null mice were resistant to obesity, increased adiposity, and impaired glucose tolerance. Both wild-type and Kv 1.3-null mice maintained on the fatty diet displayed increased wheel running activity compared to control-fed mice, which was caused primarily by a significant increase in the amount of time spent running as opposed to an increase in running velocity. Interestingly, the patterns of running behavior differed between wild-type and Kv 1.3-null mice. Kv 1.3-null mice spent significantly less time running during the light phase and displayed a decrease in running 1-2 h before the onset of the light phase, seemingly in anticipation of the dark-to-light phase transition. These studies indicate that voluntary exercise combats metabolic maladies and running behavior is modified by both consumption of an obesogenic diet and deletion of the Kv 1.3 channel.

Elevated Anxiety and Impaired Attention in Super-Smeller, Kv1.3 Knockout Mice.

It has long been recognized that olfaction and emotion are linked. While chemosensory research using both human and rodent models have indicated a change in emotion can contribute to olfactory dysfunction, there are few studies addressing the contribution of olfaction to a modulation in emotion. In mice, olfactory deficits have been linked with heightened anxiety levels, suggesting that there could be an inverse relationship between olfaction and anxiety. Furthermore, increased anxiety is often co-morbid with psychiatric conditions such as attention disorders. Our study aimed to investigate the roles of olfaction in modulating anxiety. Voltage-gated potassium ion channel Kv1.3 knockout mice (Kv1.3-/-), which have heightened olfaction, and wild-type (WT) mice were examined for anxiety-like behaviors using marble burying (MB), light-dark box (LDB) and elevated-plus maze (EPM) tests. Because Kv1.3-/- mice have increased locomotor activity, inattentive and hyperactive behaviors were quantified for both genotypes. Kv1.3-/- mice showed increased anxiety levels compared to their WT counterparts and administration of methylphenidate (MPH) via oral gavage alleviated their increased anxiety. Object-based attention testing indicated young and older Kv1.3-/- mice had attention deficits and treatment with MPH also ameliorated this condition. Locomotor testing through use of a metabolic chamber indicated that Kv1.3-/- mice were not significantly hyperactive and MPH treatment failed to modify this activity. Our data suggest that heightened olfaction does not necessarily lead to decreased anxiety levels, and that Kv1.3-/- mice may have behaviors associated with inattentiveness.

Mitochondrial Ultrastructure and Glucose Signaling Pathways Attributed to the Kv1.3 Ion Channel.

Gene-targeted deletion of the potassium channel Kv1.3 (Kv1.3(-∕-)) results in "Super-smeller" mice with a sensory phenotype that includes an increased olfactory ability linked to changes in olfactory circuitry, increased abundance of olfactory cilia, and increased expression of odorant receptors and the G-protein, Golf. Kv1.3(-∕-) mice also have a metabolic phenotype including lower body weight and decreased adiposity, increased total energy expenditure (TEE), increased locomotor activity, and resistance to both diet- and genetic-induced obesity. We explored two cellular aspects to elucidate the mechanism by which loss of Kv1.3 channel in the olfactory bulb (OB) may enhance glucose utilization and metabolic rate. First, using in situ hybridization we find that Kv1.3 and the insulin-dependent glucose transporter type 4 (GLUT4) are co-localized to the mitral cell layer of the OB. Disruption of Kv1.3 conduction via construction of a pore mutation (W386F Kv1.3) was sufficient to independently translocate GLUT4 to the plasma membrane in HEK 293 cells. Because olfactory sensory perception and the maintenance of action potential (AP) firing frequency by mitral cells of the OB is highly energy demanding and Kv1.3 is also expressed in mitochondria, we next explored the structure of this organelle in mitral cells. We challenged wildtype (WT) and Kv1.3(-∕-) male mice with a moderately high-fat diet (MHF, 31.8 % kcal fat) for 4 months and then examined OB ultrastructure using transmission electron microscopy. In WT mice, mitochondria were significantly enlarged following diet-induced obesity (DIO) and there were fewer mitochondria, likely due to mitophagy. Interestingly, mitochondria were significantly smaller in Kv1.3(-∕-) mice compared with that of WT mice. Similar to their metabolic resistance to DIO, the Kv1.3(-∕-) mice had unchanged mitochondria in terms of cross sectional area and abundance following a challenge with modified diet. We are very interested to understand how targeted disruption of the Kv1.3 channel in the OB can modify TEE. Our study demonstrates that Kv1.3 regulates mitochondrial structure and alters glucose utilization; two important metabolic changes that could drive whole system changes in metabolism initiated at the OB.