Clinical manifestations of impaired GnRH neuron development and function

Gonadotropin-releasing hormone (GnRH) and olfactory neurons migrate together in embryologic development, and disruption of this process causes idiopathic hypogonadotropic hypogonadism (IHH) with anosmia (Kallmann syndrome (KS)). Patients with IHH/KS generally manifest irreversible pubertal delay and subsequent infertility due to deficient pituitary gonadotropins or GnRH. The molecular basis of IHH/KS includes genes that: (1) regulate GnRH and olfactory neuron migration; (2) control the synthesis or secretion of GnRH; (3) disrupt GnRH action upon pituitary gonadotropes, or (4) interfere with pituitary gonadotropin synthesis or secretion. KS patients may also have midline facial defects indicating the diverse developmental functions of genes involved. Most causative genes cause either normosmic IHH or KS except FGFR1, which may cause either phenotype. Recently, several balanced chromosomal translocations have been identified in IHH/KS patients, which could lead to the identification of new disease-producing genes. Although there are two cases reported who have digenic disease, this awaits confirmation in future larger studies. The challenge will be to determine the importance of these genes in the 10-15% of couples with normal puberty who have infertility.     

Taste Perception in Kallmann Syndrome,A Model of Congenital Anosmia

ObjectiveTo investigate taste, a component of flavor perception, using electrogustometry (EG) in patients with congenital anosmia associated with Kallmann syndrome (KS).MethodsFour patients with KS and 4 control subjects participated in this study. During the first phase of the investigation, the study subjects were administered the University of Pennsylvania Smell Identification Test. During the second phase of the study, EG testing of 2 regions on the anterior tongue tip was performed through an electrode.ResultsPatients with KS, as expected, scored in the anosmic range on the University of Pennsylvania Smell Identification Test, whereas the control group had a normal sense of smell. The difference in the olfaction scores was significant between the 2 study groups (P < 0.015). The result of taste assessment of patients with KS and control subjects with use of EG was not significantly different between the 2 study groups (P = 0.874).ConclusionThe current study demonstrates that patients with KS have a normal sense of taste, as determined by EG. This finding is consistent with the fact that the deficit in KS is purely olfactory. Because flavor perception is not a common complaint in patients with this condition, it may be postulated that persons with KS compensate for the absent sense of smell. Further studies need to be undertaken to explore how patients with KS compensate for the olfactory dysfunction, information that should contribute to the understanding of the interplay of the various components of flavor perception. (Endocr Pract. 2007;13:716-720)     

2013 William Allan Award: My Multifactorial Journey

Gonadotropin-releasing hormone (GnRH) neurons originate outside the CNS in the olfactory placode and migrate into the CNS, where they become integral components of the hypothalamic-pituitary-gonadal (HPG) axis. Disruption of this migration results in Kallmann syndrome (KS), which is characterized by anosmia and pubertal failure due to hypogonadotropic hypogonadism. Using candidate-gene screening, autozygosity mapping, and whole-exome sequencing in a cohort of 30 individuals with KS, we searched for genes newly associated with KS. We identified homozygous loss-of-function mutations in FEZF1 in two independent consanguineous families each with two affected siblings. The FEZF1 product is known to enable axons of olfactory receptor neurons (ORNs) to penetrate the CNS basal lamina in mice. Because a subset of axons in these tracks is the migratory pathway for GnRH neurons, in FEZF1 deficiency, GnRH neurons also fail to enter the brain. These results indicate that FEZF1 is required for establishment of the central component of the HPG axis in humans.     

Revealing the mystery of persistent smell loss in Long COVID patients

COVID-19 is hopefully approaching its end in many countries as herd immunity develops and weaker strains of SARS-CoV-2 dominate. However, a new concern occurs over the long-term effects of COVID-19, collectively called “Long COVID”, as some symptoms of the nervous system last even after patients recover from COVID-19. This review focuses on studies of anosmia, i.e., impairment of smell, which is the most common sensory defect during the disease course and is caused by olfactory dysfunctions. It remains mysterious how the olfactory functions are affected since the virus can''t invade olfactory receptor neurons. We describe several leading hypotheses about the mystery in hope to provide insights into the pathophysiology and treatment strategies for anosmia.     

Comparison of the effects of anosmia induced by either peripheral lesion or bulbectomy upon the feeding pattern of the rat (author's transl)]

In order to support the contention that the feeding pattern seen after olfactory bulb removal is due to a sensory loss, the feeding pattern of rats was studied after a peripheral chemical lesion of the olfactory mucosa. A conditioned smell aversion procedure was used to assess the occurence and duration of anosmia after the topical application of zinc sulfate to the olfactory mucosa. It was found that the sensory deficit induced by the peripheral lesion lasted from four to six days. The occurrence of the disrupted feeding pattern in the peripherally lesioned rats coincided in time with the short period of anosmia. Thus, the disruption of the feeding pattern after bulbectomy and after lesions of the central olfactory pathways is clearly the result of anosmia and not of the loss of other non-sensory functions of the olfactory bulbs.     

Assessment of Olfactory Nerve by SPECT-MRI Image with Nasal Thallium-201 Administration in Patients with Olfactory Impairments in Comparison to Healthy Volunteers

Purpose

The aim of this study was to assess whether migration of thallium-201 (²⁰¹Tl) to the olfactory bulb were reduced in patients with olfactory impairments in comparison to healthy volunteers after nasal administration of ²⁰¹Tl.

Procedures

10 healthy volunteers and 21 patients enrolled in the study (19 males and 12 females; 26–71 years old). The causes of olfactory dysfunction in the patients were head trauma (n = 7), upper respiratory tract infection (n = 7), and chronic rhinosinusitis (n = 7). ²⁰¹TlCl was administered unilaterally to the olfactory cleft, and SPECT-CT was conducted 24 h later. Separate MRI images were merged with the SPECT images. ²⁰¹Tl olfactory migration was also correlated with the volume of the olfactory bulb determined from MRI images, as well as with odor recognition thresholds measured by using T&T olfactometry.

Results

Nasal ²⁰¹Tl migration to the olfactory bulb was significantly lower in the olfactory-impaired patients than in healthy volunteers. The migration of ²⁰¹Tl to the olfactory bulb was significantly correlated with odor recognition thresholds obtained with T&T olfactometry and correlated with the volume of the olfactory bulb determined from MRI images when all subjects were included.

Conclusions

Assessment of the ²⁰¹Tl migration to the olfactory bulb was the new method for the evaluation of the olfactory nerve connectivity in patients with impaired olfaction.     

GnRH deficiency: new insights from genetics

The acquisition of a sexually dimorphic phenotype is a critical event in mammalian development. Hypogonadotropic hypogonadism (HH) results from impaired secretion of GnRH. The patients display with delayed puberty, micropenis and cryptorchidism in the male reflecting gonadotropin insufficiency, and amenorrhea in the female. Kallmann's syndrome (KS) is defined by the association of HH and anosmia or hyposmia (absent smelling sense). Segregation analysis in familial cases has demonstrated diverse inheritance patterns, suggesting the existence of several genes regulating GnRH secretion. The X-linked form of the disease was associated with a genetic defect in the KALI gene located on the Xp22.3 region. KAL1 gene encodes an extracellular matrix glycoprotein anosmin-1, which facilitates neuronal growth and migration. Abnormalities in the migratory processes of the GnRH neurons with the olfactory neurons explain the association of HH with anosmia. Recently, mutations in the FGF recepteur 1 (FGFR1) gene were found in KS with autosomal dominant mode of inheritance. The role of FGFR1 in the function of reproduction requires further investigation. Besides HH with anosmia, there are isolated HH (IHH). No human GnRH mutations have been reported although hypogonadal mice due to a GnRH gene deletion exist. In patients with idiopathic HH and without anosmia an increasing number of GnRH receptor (GnRHR) mutations have been described which represent about 50% of familial cases. The clinical features are highly variable and there is a good relationship between genotype and phenotype. A complete loss of function is associated with the most severe phenotype with resistance to pulsatile GnRH treatment, absence of puberty and cryptorchidism in the male. In contrast, milder loss of function mutations causes incomplete failure of pubertal development. The preponderant role of GnRH in the secretion of LH by the gonadotrophs explains the difference of the phenotype between male and female with partial GnRH resistance. Affected females can have spontaneous telarche and normal breast development while affected males exhibit no pubertal development but normal testis volume, a feature described as "fertile-eunuch". High-dose pulsatile GnRH has been used to induce ovulation. Another gene, called GPR54, responsible for idiopathic HH has been recently described by segregation analysis in two different consanguineous families. The GPR54 gene is an orphan receptor, and its putative ligand is the product of the KISS-1 gene, called metastine. Their roles in the function of reproduction are still unknown.     

Kallmann De Morsier syndrome: FGF-signaling insufficiency?

Kallmann syndrome (KAL) associates hypogonadotropic hypogonadism and anosmia, i.e. a deficiency of the sense of smell. Anosmia is related to the absence or the hypoplasia of the olfactory bulbs. Hypogonadism is due to GnRH deficiency, and is likely to result from the failed embryonic migration of GnRH-synthesizing neurons. These cells normally migrate from the olfactory epithelium to the forebrain along the olfactory nerve pathway. Kallmann syndrome is genetically heterogeneous. The gene responsible for the X-chromosome linked form of the disease, KAL-1, has been identified in 1991. KAL1 encodes a ~95 kDa glycoprotein of unknown function, which is present locally in various extracellular matrices during the period of organogenesis. The recent finding that FGFR1 mutations are involved in an autosomal dominant form of Kallmann syndrome (KAL-2), combined to the analysis of mutant mouse embryos that no longer express Fgfr1 in the telencephalon, suggests that the disease results from a deficiency in FGF-signaling at the earliest stage of olfactory bulb morphogenesis. We propose that the role of the KAL1 gene product, the extracellular matrix protein anosmin-1, is to enhance FGF-signaling, and suggest that the gender difference in anosmin-1 dosage (because KAL1 partially escapes X-inactivation) explains the higher prevalence of the disease in males.     

Le syndrome de kallmann de morsier aspect génétique

Kallmann syndrome (KS) is a rare, heterogeneous disorder consisting of congenital hypogonadotropic hypogonadism, associated with anosmia (or hyposmia) and other clinical manifestations such as mirror movements, and renal, urological and neurosensory disorders. The presence of anosmia with micropenis in boys is suggestive of the diagnostic of KS. In KS, the GnRH neurons do not migrate correctly from the olfactory placode to the hypothalamus during development and olfactory bulbs also fail to form, leading to anosmia. Mutations in KAL1 which encodes Anosmin-1, are responsible for the X-linked form of KS. Anosmin-1 is normally expressed in the brain, facial mesenchyme, mesonephros and metanephros. It is required to promote migration of GnRH neurons into the hypothalamus. It also allows migration of olfactory neurons from the olfactory bulbs to the hypothalamus. The loss of function mutations in FGFR1 “fibroblast growth factor” were identified in 2003 as a cause of autosomal forms of this disease. An additional autosomal cause of Kallmann syndrome was recently identified by a mutation in the prokineticin receptor-2 gene (PROKR2) (KAL-3) and its ligand prokineticin 2 (PROK2) (KAL-4). Mutations in these genes induce various degrees of olfactory and reproductive dysfunction, but not the other symptoms seen in KAL-1 and KAL-2 forms of KS. Neuropilin2, which has an important role in migration of GnRH neurons, is a recent candidate gene for KS. The authors describe the genetic features and recent findings of KS, necessary to understand this disease.     

Assessment of the Olfactory Function in Italian Patients with Type 3 von Willebrand Disease Caused by a Homozygous 253 Kb Deletion Involving VWF and TMEM16B/ANO2

Type 3 Von Willebrand disease is an autosomal recessive disease caused by the virtual absence of the von Willebrand factor (VWF). A rare 253 kb gene deletion on chromosome 12, identified only in Italian and German families, involves both the VWF gene and the N-terminus of the neighbouring TMEM16B/ANO2 gene, a member of the family named transmembrane 16 (TMEM16) or anoctamin (ANO). TMEM16B is a calcium-activated chloride channel expressed in the olfactory epithelium. As a patient homozygous for the 253 kb deletion has been reported to have an olfactory impairment possibly related to the partial deletion of TMEM16B, we assessed the olfactory function in other patients using the University of Pennsylvania Smell Identification Test (UPSIT). The average UPSIT score of 4 homozygous patients was significantly lower than that of 5 healthy subjects with similar sex, age and education. However, 4 other members of the same family, 3 heterozygous for the deletion and 1 wild type, had a slightly reduced olfactory function indicating that socio-cultural or other factors were likely to be responsible for the observed difference. These results show that the ability to identify odorants of the homozygous patients for the deletion was not significantly different from that of the other members of the family, showing that the 253 kb deletion does not affect the olfactory performance. As other genes may compensate for the lack of TMEM16B, we identified some predicted functional partners from in silico studies of the protein-protein network of TMEM16B. Calculation of diversity for the corresponding genes for individuals of the 1000 Genomes Project showed that TMEM16B has the highest level of diversity among all genes of the network, indicating that TMEM16B may not be under purifying selection and suggesting that other genes in the network could compensate for its function for olfactory ability.     

Increased Bone Mineral Density in Male Patients With Idiopathic Hypogonadotropic Hypogonadism Who Undergo Sex Hormone Therapy: Findings From Cross-Sectional and Longitudinal Studies

ObjectiveThis retrospective observational study assessed the long-term impact of pulsatile gonadotropin-releasing hormone, combined gonadotropin, or testosterone replacement therapy on total hip, femoral, and lumbar bone mineral density (BMD) and Z-scores in adult men with idiopathic hypogonadotropic hypogonadism (IHH).MethodsIn the cross-sectional study, 69 patients were allocated to untreated (n = 42) and treated (n = 27) groups. The untreated group included IHH patients without hormone therapy history, while the treated group included age- and body mass index-matched patients who had received hormone therapy for at least 5 years. The longitudinal study included 53 IHH patients, and their hip and lumbar BMDs were measured several times during hormone therapy. We then evaluated the changes in their BMD.ResultsOur cross-sectional study showed that the treated group had a significantly higher BMD and Z-score for total hip, femoral neck, and lumbar spine (P < 0.001 for all) than the untreated group, and the average bone mass even reached the age-matched normal range. The prevalence of low BMD was 80.95% and 11.11% in untreated and treated groups, respectively. In the longitudinal study (N = 53), the total hip, femoral neck, and lumbar spine BMD gradually increased during treatment. The lumbar spine showed a greater increment in BMD compared with the total hip and femoral neck (P < 0.05).ConclusionSex hormone therapy improved hip and lumbar spine BMD and Z-scores in patients with IHH. The lumbar spine showed a greater improvement in BMD compared with the total hip and femoral neck.     

Kallmann syndrome: mutations in the genes encoding prokineticin-2 and prokineticin receptor-2

Kallmann syndrome combines anosmia, related to defective olfactory bulb morphogenesis, and hypogonadism due to gonadotropin-releasing hormone deficiency. Loss-of-function mutations in KAL1 and FGFR1 underlie the X chromosome-linked form and an autosomal dominant form of the disease, respectively. Mutations in these genes, however, only account for approximately 20% of all Kallmann syndrome cases. In a cohort of 192 patients we took a candidate gene strategy and identified ten and four different point mutations in the genes encoding the G protein-coupled prokineticin receptor-2 (PROKR2) and one of its ligands, prokineticin-2 (PROK2), respectively. The mutations in PROK2 were detected in the heterozygous state, whereas PROKR2 mutations were found in the heterozygous, homozygous, or compound heterozygous state. In addition, one of the patients heterozygous for a PROKR2 mutation was also carrying a missense mutation in KAL1, thus indicating a possible digenic inheritance of the disease in this individual. These findings reveal that insufficient prokineticin-signaling through PROKR2 leads to abnormal development of the olfactory system and reproductive axis in man. They also shed new light on the complex genetic transmission of Kallmann syndrome.     

Metamorphic remodeling of the primary olfactory projection in Xenopus: Developmental independence of projections from olfactory neuron subclasses

In adult Xenopus, the nasal cavity is divided into separate middle (MC) and principal (PC) cavities; the former is used to smell water-borne odorants, the latter air-borne odorants. Recent work has shown that olfactory neurons of each cavity express a distinct subclass of odorant receptors. Moreover, MC and PC axons project to distinct regions of the olfactory bulb. To examine the developmental basis for this specificity in the olfactory projection, we extirpated the developing MC from early metamorphic (stage 54–57) tadpoles and raised the animals through metamorphosis. In most lesioned animals, the MC partly regenerated. Compared with the unlesioned side, reduction of the region of the glomerular layer of the olfactory bulb receiving MC afferents ranged from 70% to 95%. PC afferents did not occupy regions of the olfactory bulb deprived of MC afferents. These results support a model in which intrinsic cues in the olfactory bulb control the projection pattern attained by ingrowing olfactory axons. © 1997 John Wiley & Sons, Inc. J Neurobiol 32: 213–222, 1997.     

Olfactory function in patients with hypogonadotropic hypogonadism: an all-or-none phenomenon?

Hypogonadotropic hypogonadism (HH) refers to an endocrine defectof hypothalamic origin resulting in gonadal hypoplasia and frequentlyassociated with anosmia or severely impaired olfactory function(Kallmann's syndrome). This apparently results from a disruptionin the migration of neurons from the olfactory placode to thebulb and hypothalamus early in development, and so providesa unique opportunity to investigate olfactory function in humansubjects with congenitally incomplete peripheral systems. Olfactoryperformance in 37 HH patients and 37 age-matched controls wascompared using a modified version of the Munich Olfaction Test.This test is based on the sniff-bottle method and includes testsof (i) odor quality discrimination, (ii) intensity discrimination,(iii) detection thresholds, and (iv) recognition, hedonic evaluationand identification ability. The patients could be divided intotwo distinct groups differing significantly on all four subtestsand showing no overlap in performance: 20 anosmics, conformingto Kallmann's syndrome, and 17 apparent normosmics whose performancewas slightly poorer, but not significantly different to thatof the controls. The unexpected failure to find a continuumof olfactory dysfunction now raises the question whether HHwith or without anosmia represents two syndromes with distinctetiologies, or rather reflects the ability of the olfactorysystem to function well despite morphological impairment.     

Wilson's Storm Petrels Oceanites oceanicus Recognise the Olfactory Signature of Their Mate

Chemical signals in birds have rarely been considered as recognition cues. Nevertheless, recent experiments showed that several petrel species are able to recognize their nest by smell, and in at least one species even their mate. But the use of smell may be different across the petrel species and olfactory nest recognition appears to be dependent on species’ breeding biology. To increase our knowledge of individual olfactory recognition in petrels and the relationships between breeding biology and use of smell, we tested Wilson’s storm petrels Oceanites oceanicus in Antarctica. In previous experiments, these birds failed to home if rendered anosmic, but the method employed to obtain anosmia (potentially stressing birds) and the fact that they breed in 24‐h daylight suggest that they might use visual, rather than olfactory, cues to recognize their nest. Our birds were tested in T‐maze experiments where nest odours or partner odours were presented. Wilson’s storm petrels preferred odours of their own nest and mate. Results on olfactory nest recognition confirm and complete previous results, viz. anosmic Wilson’s storm petrels do not home. Storm petrels olfactory mate recognition suggests that this ability may be widespread in burrowing petrels and implements olfactory nest recognition.     

Computer modeling of mechanisms of the information processing in the olfactory bulb. II. Mechanisms of identification and short-term storage in the olfactory bulb: results of the computer experimentation

The results of experimentation with the computer model of the olfactory bulb are presented. The architecture and scenario of the work of the model were described previously. The dynamic character of the identification process and the mechanism of memorizing short-term of smell stimuli are described. During the identification, a self-adjustment of the olfactory bulb to incoming signals occurs. The self-modification of mitral and tufted cell synapses enhances responses of the cells; upon subsequent presentation of the stimulus, the olfactory bulb responds with a higher activity. The modeling confirmed the validity of the assumption that the functions of mitral and tufted cells are to identify the components of a complex smell and the image of the smell as the whole.     

Brain Development after Neonatal Intermittent Hyperoxia-Hypoxia in the Rat Studied by Longitudinal MRI and Immunohistochemistry

Background

Neonatal intermittent hyperoxia-hypoxia (IHH) is involved in the pathogenesis of retinopathy of prematurity. Whether similar oxygen fluctuations will create pathological changes in the grey and white matter of the brain is unknown.

Methods

From birth until postnatal day 14 (P14), two litters (total n = 22) were reared in IHH: hyperoxia (50% O₂) interrupted by three consecutive two-minute episodes of hypoxia (12% O₂) every sixth hour. Controls (n = 8) were reared in room-air (20.9% O₂). Longitudinal MRI (Diffusion Tensor Imaging and T₂-mapping) was performed on P14 and P28 and retinal and brain tissue were examined for histopathological changes. Long-term neurodevelopment was assessed on P20 and P27.

Results

Mean, radial and axial diffusivity were higher in white matter of IHH versus controls at P14 (p < 0.04), while fractional anisotropy (FA) was lower in the hippocampal fimbria and tended to be lower in corpus callosum (p = 0.08) and external capsule (p = 0.05). White matter diffusivity in IHH was similar to controls at P28. Higher cortical vessel density (p = 0.005) was observed at P14. Cortical and thalamic T₂-relaxation time and mean diffusivity were higher in the IHH group at P14 (p ≤ 0.03), and albumin leakage was present at P28. Rats in the IHH group ran for a longer time on a Rotarod than the control group (p ≤ 0.005). Pups with lower bodyweight had more severe MRI alterations and albumin leakage.

Conclusion

IHH led to subtle reversible changes in brain white matter diffusivity, grey matter water content and vascular density. However, alterations in blood-brain barrier permeability may point to long-term effects. The changes seen after IHH exposure were more severe in animals with lower bodyweight and future studies should aim at exploring possible interactions between IHH and growth restriction.