Up-regulation of activated macrophages in response to degeneration in the taste system: Effects of dietary sodium restriction

Maintenance of taste receptor cell presynaptic sites requires gustatory nerve fibers

The turnover and re-establishment of peripheral taste synapses is vital to maintain connectivity between the primary taste receptor cells and the gustatory neurons which relay taste information from the tongue to the brain. Despite the importance of neuron-taste cell reconnection, mechanisms governing synapse assembly and the specificity of synaptic connections is largely unknown. Here we use the expression of presynaptic proteins, CALHM1 and Bassoon, to probe whether nerve fiber connectivity is an initiating factor for the recruitment of presynaptic machinery in different populations of taste cells. Under homeostatic conditions, the vast majority (>90%) of presynaptic sites are directly adjacent to nerve fibers. In the days immediately following gustatory nerve transection and complete denervation, Bassoon and CALHM1 puncta are markedly reduced. This suggests that nerve fiber innervation is crucial for the recruitment and maintenance of presynaptic sites. In support of this, we find that expression of Bassoon and Calhm1 mRNA transcripts are significantly reduced after denervation. During nerve fiber regeneration into the taste bud, presynaptic sites begin to replenish, but are not as frequently connected to nerve fibers as intact controls (∼50% compared to >90%). This suggests that gustatory neuron proximity, rather than direct contact, likely drives taste receptor cells to express and aggregate presynaptic proteins at the cell membrane. Together, these data support the idea that trophic factors secreted by gustatory nerve fibers prompt taste receptor cells to produce presynaptic specializations at the cell membrane, which in turn may guide neurons to form mature synapses. These findings provide new insights into the mechanisms driving synaptogenesis and synaptic plasticity within the rapidly changing taste bud environment.

Interleukin (IL)-1 Receptor Signaling Is Required for Complete Taste Bud Regeneration and the Recovery of Neural Taste Responses following Axotomy

Experimental or traumatic nerve injury causes the degeneration of associated taste buds. Unlike most sensory systems, the sectioned nerve and associated taste buds can then regenerate, restoring neural responses to tastants. It was previously unknown whether injury-induced immune factors mediate this process. The proinflammatory cytokines, interleukin (IL)-1α and IL-1β, and their requisite receptor are strongly expressed by anterior taste buds innervated by the chorda tympani nerve. We tested taste bud regeneration and functional recovery in mice lacking the IL-1 receptor. After axotomy, the chorda tympani nerve regenerated but was initially unresponsive to tastants in both WT and Il1r KO mice. In the absence of Il1r signaling, however, neural taste responses remained minimal even >8 weeks after injury in both male and female mice, whereas normal taste function recovered by 3 weeks in WT mice. Failed recovery was because of a 57.8% decrease in regenerated taste buds in Il1r KO compared with WT axotomized mice. Il1a gene expression was chronically dysregulated, and the subset of regenerated taste buds were reinnervated more slowly and never reached full volume as progenitor cell proliferation lagged in KO mice. Il1r signaling is thus required for complete taste bud regeneration and the recovery of normal taste transmission, likely by impairing taste progenitor cell proliferation. This is the first identification of a cytokine response that promotes taste recovery. The remarkable plasticity of the taste system makes it ideal for identifying injury-induced mechanisms mediating successful regeneration and recovery.SIGNIFICANCE STATEMENT Taste plays a critical role in nutrition and quality of life. The adult taste system is highly plastic and able to regenerate following the disappearance of most taste buds after experimental nerve injury. Several growth factors needed for taste bud regeneration have been identified, but we demonstrate the first cytokine pathway required for the recovery of taste function. In the absence of IL-1 cytokine signaling, taste bud regeneration is incomplete, preventing the transmission of taste activity to the brain. These results open a new direction in revealing injury-specific mechanisms that could be harnessed to promote the recovery of taste perception after trauma or disease.

Immune responses in the injured olfactory and gustatory systems: a role in olfactory receptor neuron and taste bud regeneration?

Sensory cells that specialize in transducing olfactory and gustatory stimuli are renewed throughout life and can regenerate after injury unlike their counterparts in the mammalian retina and auditory epithelium. This uncommon capacity for regeneration offers an opportunity to understand mechanisms that promote the recovery of sensory function after taste and smell loss. Immune responses appear to influence degeneration and later regeneration of olfactory sensory neurons and taste receptor cells. Here we review surgical, chemical, and inflammatory injury models and evidence that immune responses promote or deter chemosensory cell regeneration. Macrophage and neutrophil responses to chemosensory receptor injury have been the most widely studied without consensus on their net effects on regeneration. We discuss possible technical and biological reasons for the discrepancy, such as the difference between peripheral and central structures, and suggest directions for progress in understanding immune regulation of chemosensory regeneration. Our mechanistic understanding of immune-chemosensory cell interactions must be expanded before therapies can be developed for recovering the sensation of taste and smell after head injury from traumatic nerve damage and infection. Chemosensory loss leads to decreased quality of life, depression, nutritional challenges, and exposure to environmental dangers highlighting the need for further studies in this area.

Regression of Lingual Lymphatic Vessels in Sodium-restricted Mice

Lymphatic vessel networks can expand and regress, with consequences for interstitial fluid drainage and nutrient supply to tissues, inflammation, and tumor spread. A diet high in sodium stimulates hyperplasia of cutaneous lymphatic capillaries. We hypothesized that dietary sodium restriction would have the opposite effect, shrinking lymphatic capillaries in the tongue. Lingual lymphatic capillary density and size was significantly reduced in mice fed a low-sodium diet (0.03%) for 3 weeks compared with control-fed mice. Blood vessel density was unchanged. Despite lymphatic capillary shrinkage, lingual edema was not observed. The effect on lymphatic capillaries was reversible, as lymphatic density and size in the tongue were restored by 3 weeks on a control diet. Lymphatic hyperplasia induced by a high-sodium diet is dependent on infiltrating macrophages. However, lingual CD68+ macrophage density was unchanged by sodium deficiency, indicating that distinct mechanisms may mediate lymphatic regression. Further studies are needed to test whether dietary sodium restriction is an effective, non-invasive co-therapy for oral cancer.

Inflammatory stimuli acutely modulate peripheral taste function

Inflammation-mediated changes in taste perception can affect health outcomes in patients, but little is known about the underlying mechanisms. In the present work, we hypothesized that proinflammatory cytokines directly modulate Na(+) transport in taste buds. To test this, we measured acute changes in Na(+) flux in polarized fungiform taste buds loaded with a Na(+) indicator dye. IL-1β elicited an amiloride-sensitive increase in Na(+) transport in taste buds. In contrast, TNF-α dramatically and reversibly decreased Na(+) flux in polarized taste buds via amiloride-sensitive and amiloride-insensitive Na(+) transport systems. The speed and partial amiloride sensitivity of these changes in Na(+) flux indicate that IL-1β and TNF-α modulate epithelial Na(+) channel (ENaC) function. A portion of the TNF-mediated decrease in Na(+) flux is also blocked by the TRPV1 antagonist capsazepine, although TNF-α further reduced Na(+) transport independently of both amiloride and capsazepine. We also assessed taste function in vivo in a model of infection and inflammation that elevates these and additional cytokines. In rats administered systemic lipopolysaccharide (LPS), CT responses to Na(+) were significantly elevated between 1 and 2 h after LPS treatment. Low, normally preferred concentrations of NaCl and sodium acetate elicited high response magnitudes. Consistent with this outcome, codelivery of IL-1β and TNF-α enhanced Na(+) flux in polarized taste buds. These results demonstrate that inflammation elicits swift changes in Na(+) taste function, which may limit salt consumption during illness.

Shrinkage of ipsilateral taste buds and hyperplasia of contralateral taste buds following chorda tympani nerve transection

The morphological changes that occur in the taste buds after denervation are not well understood in rats, especially in the contralateral tongue epithelium. In this study, we investigated the time course of morphological changes in the taste buds following unilateral nerve transection. The role of the trigeminal component of the lingual nerve in maintaining the structural integrity of the taste buds was also examined. Twenty-four Sprague-Dawley rats were randomly divided into three groups: control, unilateral chorda tympani nerve transection and unilateral chorda tympani nerve transection + lingual nerve transection. Rats were allowed up to 42 days of recovery before being euthanized. The taste buds were visualized using a cytokeratin 8 antibody. Taste bud counts, volumes and taste receptor cell numbers were quantified and compared among groups. No significant difference was detected between the chorda tympani nerve transection and chorda tympani nerve transection + lingual nerve transection groups. Taste bud counts, volumes and taste receptor cell numbers on the ipsilateral side all decreased significantly compared with control. On the contralateral side, the number of taste buds remained unchanged over time, but they were larger, and taste receptor cells were more numerous postoperatively. There was no evidence for a role of the trigeminal branch of the lingual nerve in maintaining the structural integrity of the anterior taste buds.

Role of neurotrophin in the taste system following gustatory nerve injury

Taste system is a perfect system to study degeneration and regeneration after nerve injury because the taste system is highly plastic and the regeneration is robust. Besides, degeneration and regeneration can be easily measured since taste buds arise in discrete locations, and nerves that innervate them can be accurately quantified. Neurotrophins are a family of proteins that regulate neural survival, function, and plasticity after nerve injury. Recent studies have shown that neurotrophins play an important role in the developmental and mature taste system, indicating neurtrophin might also regulate taste system following gustatory nerve injury. This review will summarize how taste system degenerates and regenerates after gustatory nerve cut and conclude potential roles of neurotrophin in regulating the process.

Ingestion of bacterial lipopolysaccharide inhibits peripheral taste responses to sucrose in mice

A fundamental role of the taste system is to discriminate between nutritive and toxic foods. However, it is unknown whether bacterial pathogens that might contaminate food and water modulate the transmission of taste input to the brain. We hypothesized that exogenous, bacterially-derived lipopolysaccharide (LPS), modulates neural responses to taste stimuli. Neurophysiological responses from the chorda tympani nerve, which innervates taste cells on the anterior tongue, were unchanged by acute exposure to LPS. Instead, neural responses to sucrose were selectively inhibited in mice that drank LPS during a single overnight period. Decreased sucrose sensitivity appeared 7days after LPS ingestion, in parallel with decreased lingual expression of Tas1r2 and Tas1r3 transcripts, which are translated to T1R2+T1R3 subunits forming the sweet taste receptor. Tas1r2 and Tas1r3 mRNA expression levels and neural responses to sucrose were restored by 14 days after LPS consumption. Ingestion of LPS, rather than contact with taste receptor cells, appears to be necessary to suppress sucrose responses. Furthermore, mice lacking the Toll-like receptor (TLR) 4 for LPS were resistant to neurophysiological changes following LPS consumption. These findings demonstrate that ingestion of LPS during a single period specifically and transiently inhibits neural responses to sucrose. We suggest that LPS drinking initiates TLR4-dependent hormonal signals that downregulate sweet taste receptor genes in taste buds. Delayed inhibition of sweet taste signaling may influence food selection and the complex interplay between gastrointestinal bacteria and obesity.

Impact of chorda tympani nerve injury on cell survival, axon maintenance, and morphology of the chorda tympani nerve terminal field in the nucleus of the solitary tract

Chorda tympani nerve transection (CTX) has been useful to study the relationship between nerve and taste buds in fungiform papillae. This work demonstrated that the morphological integrity of taste buds depends on their innervation. Considerable research focused on the effects of CTX on peripheral gustatory structures, but much less research has focused on the central effects. Here, we explored how CTX affects ganglion cell survival, maintenance of injured peripheral axons, and the chorda tympani nerve terminal field organization in the nucleus of the solitary tract (NTS). After CTX in adult rats, the chorda tympani nerve was labeled with biotinylated dextran amine at 3, 7, 14, 30, and 60 days post-CTX to allow visualization of the terminal field associated with peripheral processes. There was a significant and persistent reduction of the labeled chorda tympani nerve terminal field volume and density in the NTS following CTX. Compared with controls, the volume of the labeled terminal field was not altered at 3 or 7 days post-CTX; however, it was significantly reduced by 44% and by 63% at 30 and 60 days post-CTX, respectively. Changes in the density of labeled terminal field in the NTS paralleled the terminal field volume results. The dramatic decrease in labeled terminal field size post-CTX cannot be explained by a loss of geniculate ganglion neurons or degeneration of central axons. Instead, the function and/or maintenance of the peripheral axonal process appear to be affected. These new results have implications for long-term functional and behavioral alterations.

Functional role for interleukin-1 in the injured peripheral taste system

The peripheral taste system presents an excellent model for studying the consequences of neural injury, for the damaged nerve and sensory cells and the neighboring, intact neural cells. Sectioning a primary afferent nerve, the chorda tympani (CT), rapidly recruits neutrophils to both sides of the tongue. The bilateral neutrophil response induces transient functional deficits in the intact CT. Normal function is subsequently restored as macrophages respond to injury. We hypothesized that macrophages produce the proinflammatory cytokine interleukin (IL)-1, which contributes to the maintenance of normal taste function after nearby injury. We demonstrate that IL-1β protein levels are significantly increased on the injured side of the tongue at day 2 after injury. Dietary sodium deficiency, a manipulation that prevents macrophage recruitment, inhibits the elevation in IL-1β. IL-1β was expressed in several cell populations, including taste receptor cells and infiltrating neutrophils and macrophages. To test whether IL-1 modulates taste function after injury, we blocked signaling with an IL-1 receptor antagonist (IL-1 RA) and recorded taste responses from the intact CT. This treatment inhibited the bilateral macrophage response to injury and impaired taste responses in the intact CT. Cytokine actions in the taste system are largely unstudied. These results demonstrate that IL-1 has a beneficial effect on taste function after nearby injury, in contrast to its detrimental role in the injured central nervous system.

Aging profoundly delays functional recovery from gustatory nerve injury

The peripheral taste system remains plastic during adulthood. Sectioning the chorda tympani (CT) nerve, which sends sensory information from the anterior tongue to the central nervous system, causes degeneration of distal fibers and target taste buds. However, taste function is restored after about 40 days in young adult rodents. We tested whether aging impacts the reappearance of neural responses after unilateral CT nerve injury. Taste bud regeneration was minimal at day 50-65 after denervation, and most aged animals died before functional recovery could be assessed. A subset (n=3/5) of old rats exhibited normal CT responses at day 85 postsectioning, suggesting the potential for efficient recovery. The aged taste system is fairly resilient to sensory receptor loss and major functional changes in normal aging. However, injury to the taste system reveals a surprising vulnerability in old rodents. The gustatory system provides an excellent model to study mechanisms underlying delayed recovery from peripheral nerve injury. Strategies to accelerate recovery and restore normal function will be of interest, as the elderly population continues to grow.

Interferon-gamma, macrophages, and virus spread after HSV-1 injection

PURPOSE

After uniocular anterior chamber (AC) injection of HSV-1, the anterior segment of BALB/c mice becomes inflamed and infected; however, virus does not spread from the anterior segment to cause retinitis in the injected eye. The purpose of these studies was to determine whether interferon (IFN-)-γ and Mac-1(+) cells play a role in preventing direct anterior-to-posterior spread of HSV-1 in the injected eye.

METHODS

One AC of adult female BALB/c mice was injected with HSV-1 (KOS). The location of IFN-α, IFN-β, and IFN-γ in the injected eye was determined by immunofluorescence, and mRNA expression was quantified by qPCR. Injected eyes of IFN-γ knockout or clodronate-treated macrophage-depleted mice were examined to determine whether the absence of IFN-γ or Mac-1(+) macrophages affected the sites or timing of virus spread.

RESULTS

IFN-α, IFN-β, and IFN-γ were observed in the anterior segment of injected eyes through 72 hours and mRNA levels of IFN-β and IFN-γ were increased in virus-infected eyes 48 to 120 hours after infection. However, the absence of IFN-γ or macrophages did not affect either the sites or the timing of HSV-1 infection in injected eyes.

CONCLUSIONS

Protection of the retina of the injected eye does not depend on a single cell type or cytokine. In addition, in the eye, as in other sites of the body, there are redundancies in the innate response to virus infection.

Neutrophil responses to injury or inflammation impair peripheral gustatory function

The adult peripheral taste system is capable of extensive functional plasticity after injury. Sectioning the chorda tympani (CT), a primary sensory afferent nerve, elicits transient changes in the uninjured, contralateral population of taste receptor cells. Remarkably, the deficits are specific to the sodium transduction pathway. Normal function is quickly restored in the intact nerve, in parallel with an influx of macrophages to both the denervated and uninjured sides of the tongue. However, changing the dietary environment by restricting sodium blocks the macrophage response and prolongs functional alterations. Since the functional deficits occur before macrophages are present in the peripheral taste system, we hypothesized that neutrophils play a role in modulating neural responses in the intact CT. First, the dynamics of the neutrophil response to nerve injury were analyzed in control-fed and sodium-deficient rats. Nerve sectioning briefly increased the number of neutrophils on both the denervated and uninjured sides of the tongue. The low-sodium diet amplified and extended the bilateral neutrophil response to injury, in parallel with the persistent changes in sodium taste function. To test the impact of neutrophils on taste function, we depleted these cells prior to nerve sectioning and recorded neural responses from the intact CT. This treatment restored normal sodium responses in the uninjured nerve. Moreover, recruiting neutrophils to the tongue induced deficits in sodium taste function in both CT nerves. Neutrophils play a critical role in ongoing inflammatory responses in the oral cavity, and may induce changes in taste perception. We also suggest that balanced neutrophil and macrophage responses enable normal neural responses after neural injury.

Attenuation of peripheral salt taste responses and local immune function contralateral to gustatory nerve injury: effects of aldosterone

Dietary sodium restriction coupled with axotomy of the rat chorda tympani nerve (CTX) results in selectively attenuated taste responses to sodium salts in the contralateral, intact chorda tympani nerve. Converging evidence indicates that sodium deficiency also diminishes the activated macrophage response to injury on both the sectioned and contralateral, intact sides of the tongue. Because a sodium-restricted diet causes a robust increase in circulating aldosterone, we tested the hypothesis that changes in neurophysiological and immune responses contralateral to the CTX could be mimicked by aldosterone administration instead of the low-sodium diet. Taste responses in rats with CTX and supplemental aldosterone for 4-6 days were similar to rats with CTX and dietary sodium restriction. Responses to sodium salts were as much as 50% lower compared with sham-operated and vehicle-supplemented rats. The group-related functional differences were eliminated with lingual application of amiloride, suggesting that a major transduction pathway affected was through epithelial sodium channels. Consistent with the functional results, few macrophages were observed on either side of the tongue in rats with CTX and aldosterone. In contrast, macrophages were elevated on both sides of the tongue in rats with CTX and the vehicle. These results show that sodium deficiency or administration of aldosterone suppresses the immune response to neural injury, resulting in attenuation of peripheral gustatory function. They also show a potential key link among downstream consequences of sodium imbalance, taste function, and immune activity.

Fungiform taste bud degeneration in C57BL/6J mice following chorda-lingual nerve transection

Taste buds are dependent on innervation for normal morphology and function. Fungiform taste bud degeneration after chorda tympani nerve injury has been well documented in rats, hamsters, and gerbils. The current study examines fungiform taste bud distribution and structure in adult C57BL/6J mice from both intact taste systems and after unilateral chorda-lingual nerve transection. Fungiform taste buds were visualized and measured with the aid of cytokeratin 8. In control mice, taste buds were smaller and more abundant on the anterior tip (<1 mm) of the tongue. By 5 days after nerve transection taste buds were smaller and fewer on the side of the tongue ipsilateral to the transection and continued to decrease in both size and number until 15 days posttransection. Degenerating fungiform taste buds were smaller due to a loss of taste bud cells rather than changes in taste bud morphology. While almost all taste buds disappeared in more posterior fungiform papillae by 15 days posttransection, the anterior tip of the tongue retained nearly half of its taste buds compared to intact mice. Surviving taste buds could not be explained by an apparent innervation from the remaining intact nerves. Contralateral effects of nerve transection were also observed; taste buds were larger due to an increase in the number of taste bud cells. These data are the first to characterize adult mouse fungiform taste buds and subsequent degeneration after unilateral nerve transection. They provide the basis for more mechanistic studies in which genetically engineered mice can be used.

Upregulation of intracellular adhesion molecule (ICAM)-1 and vascular cell adhesion molecule (VCAM)-1 after unilateral nerve injury in the peripheral taste system

In the peripheral taste system, activated macrophages are recruited to both sides of the tongue after unilateral sectioning of the chorda tympani nerve (CT). Neural degeneration elicits macrophage entry in other systems by upregulating vascular adhesion molecules. We hypothesized that CT sectioning leads to a bilateral increase in intracellular adhesion molecule (ICAM)-1 and vascular cell adhesion molecule (VCAM)-1 expression on lingual vessels. To test this hypothesis, rats were euthanized at time points from 6 hr to 7 days post-sectioning. Frozen sections of tongue were processed for immunohistochemical staining for ICAM-1 and VCAM-1. Tongue homogenates from additional rats were analyzed with ELISA. ICAM-1 expression increases first on the denervated side of the tongue at 24 hr post-section and then on the uninjured side at 48 hr post-section. ICAM-1 remains elevated through Day 7 post-sectioning on both sides of the tongue. Dietary sodium restriction, which prevents the macrophage response to nerve sectioning, had no effect on ICAM-1 levels. VCAM-1+ vessels are increased on the denervated side of the tongue at 24-48 hr post-section in control-fed rats. However, dietary sodium restriction prevents the increase. These results indicate that vascular adhesion molecules are differentially regulated by CT sectioning. We suggest that macrophage entry, migration, and modulation of taste function are downstream of dynamic expression of adhesion molecules.

Upregulation of the chemokine monocyte chemoattractant protein-1 following unilateral nerve injury in the peripheral taste system

Macrophages are recruited to both sides of the tongue following unilateral chorda tympani (CT) nerve injury. The mechanisms responsible for recruiting these macrophages to the peripheral taste system are unknown. Neural degeneration in other systems leads to the upregulation of small molecules that function as chemoattractant cytokines, or chemokines. The chemokines monocyte chemoattractant protein (MCP)-1 and macrophage inflammatory protein (MIP)-1alpha are important regulators of macrophage recruitment to sites of infection and injury. We hypothesized that CT nerve sectioning leads to a bilateral upregulation of MCP-1 and MIP-1alpha. We examined lingual protein levels of MCP-1 and MIP-1alpha by enzyme-linked immunosorbent assays (ELISA)s at several time points after unilateral CT section in rats. MCP-1 was significantly upregulated on the intact side of the tongue at 12 h after sectioning, and on the injured side at 24-48 h post-injury. However, MIP-1alpha expression did not significantly change following CT nerve sectioning. These data indicate that chemokines are differentially regulated following neural injury, and that MCP-1 may contribute to the bilateral macrophage response to neural injury. Furthermore, the increase in MCP-1 occurs even in uninjured, distant sites, and may be upstream from the deficits in neural responses from the contralateral CT after sectioning.

Lipopolysaccharide-induced up-regulation of activated macrophages in the degenerating taste system

Unilateral chorda tympani (CT) nerve section and maintenance on a sodium-restricted diet leads to a rapid decrease in neurophysiological taste responses to sodium in the contralateral, intact CT nerve. Up-regulation of immune function with lipopolysaccharide (LPS; 100 microg i.p.) induces a recovery of normal sodium taste responses, suggesting that the sodium-deficient diet is immunosuppressive. In fact, there is a bilateral increase in the number of lingual, activated macrophages in control-fed rats receiving CT nerve section that does not occur in sodium-deficient rats after sectioning. In the current study, we hypothesized that the LPS-induced recovery of normal taste function in sodium-deficient rats is based on an increase in the activated macrophage response to denervation. Rats receiving a unilateral CT nerve section, a sodium-restricted diet, and/or an injection of LPS (100 microg; i.p.) were overdosed with pentobarbital at day 2 postsectioning, and tongues were rapidly dissected and frozen. Cryosections were then immunohistochemically stained to determine the percentage of ED1 staining for activated macrophages or the number of alphabeta or gammadelta T cells. Activated macrophage levels were significantly increased in sodium-restricted rats that received LPS following unilateral CT nerve section, supporting our hypothesis. These novel findings suggest that LPS overcomes the immunosuppression induced by the sodium-restricted diet and also indicate that the immune system plays a role in regulating taste function after neural injury.