Expression of huntingtin-associated protein 1 in adult mouse dorsal root ganglia and its neurochemical characterization in reference to sensory neuron subpopulations

Corneal Sensory Nerve Injury Disrupts Lacrimal Gland Function by Altering Circadian Rhythms in Mice

Purpose

To investigate the impact of corneal sensory nerve injury on lacrimal gland function, focusing on mechanisms involving the superior salivatory nucleus (SSN), circadian rhythm disruption, immune microenvironment alterations, and the potential for neural regeneration.

Methods

A murine model of corneal sensory nerve injury was used to assess lacrimal gland function, with tear secretion measured using the phenol red thread test. Transcriptomic analysis of lacrimal glands examined circadian rhythm and immune-related gene expression. Basic fibroblast growth factor (bFGF) was used to promote corneal nerve regeneration, and its effects on tear secretion and nerve repair were evaluated.

Results

Corneal nerve injury resulted in a 35% reduction in tear secretion and significantly impaired SSN activity, as evidenced by a 31% decrease in c-FOS-positive neurons in choline acetyltransferase (ChAT)-expressing neurons. Transcriptomic analysis revealed significant downregulation of immune-related pathways, including Toll-like receptor (TLR), NOD-like receptor (NLR), and T-cell receptor signaling. Circadian rhythm gene expression exhibited phase shifts, with a 2.13-hour delay in peak expression and a substantial change in the number and types of rhythmic genes, which were enriched in different signaling pathways. The bFGF treatment restored tear secretion by 22% and promoted nerve regeneration, although nerve fiber density remained 74% lower than that of controls.

Conclusions

Corneal sensory nerve injury disrupts both central and peripheral circadian clock functions in the lacrimal gland, leading to reduced tear secretion and immune dysregulation. These findings highlight the novel role of circadian rhythms and neural-immune interactions in lacrimal gland dysfunction. Neural regeneration strategies, such as bFGF, offer therapeutic potential for dry eye syndrome, providing new directions for clinical intervention.

Neuroanatomical distribution of endogenous huntingtin and its immunohistochemical relationships with STB/HAP1 in the adult mouse brain and spinal cord

Huntingtin-associated protein 1 (HAP1) is an essential constituent of the stigmoid body (STB) and is known as a neuroprotective interactor with causal agents for several neurodegenerative disorders, including huntingtin (HTT) in Huntington's disease. Previous in vitro studies showed that compared to normal HTT, STB/HAP1 exhibited a higher binding affinity for mutant HTT. The detailed in vivo relationships of STB/HAP1 with endogenous HTT, however, have not been clarified yet. This study examined the distribution of endogenous HTT and its relationships with STB/HAP1 in the adult mouse brain and spinal cord using light/fluorescence microscopy. Our results show that HTT immunoreactivity is highly distributed in the striatum, medial septal nucleus (MS), nucleus of the horizontal limb/ vertical limb of the diagonal band of Broca (HDB, VDB), substantia innominata basal part (SIB), pedunculopontine tegmental nucleus (PPTg), laterodorsal tegmental nucleus (LDTg), autonomic preganglionic neurons, and brainstem/spinal motoneurons. More than 90 % of HTT-immunoreactive (ir) neurons contain STB/HAP1 immunoreactivity in MS, VBD/HDB, SIB, PPTg, LDTg, and autonomic preganglionic nuclei. HTT-ir neurons in the striatal and motor nuclei, however, do not exhibit HAP1 immunoreactivity. These suggest that due to the absence of STB/HAP1-protectivity, HTT-ir striatal/motor neurons are more vulnerable to neurodegeneration than other HAP1-expressing HTT neurons. Our current findings might provide a framework for elucidating the pathophysiological functions of endogenous HTT and HAP1 in the central nervous system.

Somatostatin-immunoreactive neurons of the rat gut during the development

Somatostatin (SST) is a peptide expressed in the peripheral and central nervous systems, as well as in endocrine and immune cells. The aim of the current study is to determine the percentage of SST immunoreactive (IR) neurons and their colocalization with choline acetyltransferase (ChAT), neuronal nitric oxide synthase (nNOS), neuropeptide Y (NPY), and glial fibrillary acidic protein (GFAP) in the myenteric plexus (MP) and submucous plexus (SP) of the small intestine (SI) and large intestine (LI) of rats across different age groups from newborn to senescence using immunohistochemistry. In the MP of the SI and LI, the percentage of SST-IR neurons significantly increased during early postnatal development from 12 ± 2.4 (SI) and 13 ± 3.0 (LI) in newborn rats to 23 ± 1.5 (SI) and 18 ± 1.6 (LI) in 20-day-old animals, remaining stable until 60 days of age. The proportion of SST-IR cells then decreased in aged 2-year-old animals to 14 ± 2.0 (SI) and 10 ± 2.6 (LI). In the SP, the percentage of SST-IR neurons significantly rose from 22 ± 3.2 (SI) and 23 ± 1.7 (LI) in newborn rats to 42 ± 4.0 in 20-day-old animals (SI) and 32 ± 4.9 in 30-day-old animals (LI), before declining in aged 2-year-old animals to 21 ± 2.6 (SI) and 28 ± 7.4 (LI). Between birth and 60 days of age, 97-98% of SST-IR neurons in the MP and SP colocalized with ChAT in both plexuses of the SI and LI. The percentage of SST/ChAT neurons decreased in old rats to 85 ± 5.0 (SI) and 90 ± 3.8 (LI) in the MP and 89 ± 3.2 (SI) and 89 ± 1.6 (LI) in the SP. Conversely, in young rats, only a few SST-IR neurons colocalized with nNOS, but this percentage significantly increased in 2-year-old rats. The percentage of SST/NPY-IR neurons exhibited considerable variation throughout postnatal development, with no significant differences across different age groups in both the MP and SP of both intestines. No colocalization of SST with GFAP was observed in any of the studied animals. In conclusion, the expression of SST in enteric neurons increases in young rats and decreases in senescence, accompanied by changes in SST colocalization with ChAT and nNOS.

Research advances in huntingtin-associated protein 1 and its application prospects in diseases

Huntingtin-associated protein 1 (HAP1) was the first protein discovered to interact with huntingtin. Besides brain, HAP1 is also expressed in the spinal cord, dorsal root ganglion, endocrine, and digestive systems. HAP1 has diverse functions involving in vesicular transport, receptor recycling, gene transcription, and signal transduction. HAP1 is strongly linked to several neurological diseases, including Huntington's disease, Alzheimer's disease, epilepsy, ischemic stroke, and depression. In addition, HAP1 has been proved to participate in cancers and diabetes mellitus. This article provides an overview of HAP1 regarding the tissue distribution, cell localization, functions, and offers fresh perspectives to investigate its role in diseases.

Pain in Huntington's disease and its potential mechanisms

Pain is common and frequent in many neurodegenerative diseases, although it has not received much attention. In Huntington's disease (HD), pain is often ignored and under-researched because attention is more focused on motor and cognitive decline than psychiatric symptoms. In HD progression, pain symptoms are complex and involved in multiple etiologies, particularly mental issues such as apathy, anxiety and irritability. Because of psychiatric issues, HD patients rarely complain of pain, although their bodies show severe pain symptoms, ultimately resulting in insufficient awareness and lack of research. In HD, few studies have focused on pain and pain-related features. A detailed and systemic pain history is crucial to assess and explore pain pathophysiology in HD. This review provides an overview concentrating on pain-related factors in HD, including neuropathology, frequency, features, affecting factors and mechanisms. More attention and studies are still needed in this interesting field in the future.

Huntington-associated protein 1 inhibition contributes to neuropathic pain by suppressing Cav1.2 activity and attenuating inflammation

ABSTRACT

Although pain dysfunction is increasingly observed in Huntington disease, the underlying mechanisms still unknown. As a crucial Huntington-associated protein, Huntington-associated protein 1 (HAP1) is enriched in normal spinal dorsal horn and dorsal root ganglia (DRG) which are regarded as "primary sensory center," indicating its potential functions in pain process. Here, we discovered that HAP1 level was greatly increased in the dorsal horn and DRG under acute and chronic pain conditions. Lack of HAP1 obviously suppressed mechanical allodynia and hyperalgesia in spared nerve injury (SNI)-induced and chronic constriction injury-induced pain. Its deficiency also greatly inhibited the excitability of nociceptive neurons. Interestingly, we found that suppressing HAP1 level diminished the membrane expression of the L-type calcium channel (Cav1.2), which can regulate Ca 2+ influx and then influence brain-derived neurotrophic factor (BDNF) synthesis and release. Furthermore, SNI-induced activation of astrocytes and microglia notably decreased in HAP1-deficient mice. These results indicate that HAP1 deficiency might attenuate pain responses. Collectively, our results suggest that HAP1 in dorsal horn and DRG neurons regulates Cav1.2 surface expression, which in turn reduces neuronal excitability, BDNF secretion, and inflammatory responses and ultimately influences neuropathic pain progression.

Immunohistochemical Distribution and Neurochemical Characterization of Huntingtin-Associated Protein 1 Immunoreactive Neurons in the Adult Mouse Lingual Ganglia

Huntingtin-associated protein 1 (HAP1) is a determinant marker for the stigmoid body (STB), a neurocytoplasmic physiological inclusion. STB/HAP1 enriched areas in the brain/spinal cord are usually protected from neurodegenerative diseases, whereas the regions with tiny amounts or no STB/HAP1 are affected. In addition to the brain/spinal cord, HAP1 is highly expressed in the myenteric/submucosal plexuses of the enteric nervous system in the gastrointestinal tract. The tongue is attached to the pharynx by the hyoid bone as an extension of the gastrointestinal system. To date, the immunohistochemical distribution and neurochemical characterization of HAP1 have not been elucidated in the lingual ganglia. Using immunohistochemistry and light microscopy, our current study demonstrates the expression and immunohistochemical phenotype of HAP1 in the lingual ganglia of adult mice. We showed that HAP1 was profoundly distributed in the intralingual ganglion (ILG) and the ganglia near the root of the tongue (which we coined as "lingual root ganglion"; LRG). Neurons in ILG and LRG exhibited high coexpression of HAP1 with NOS or ChAT. Furthermore, most HAP1-immunoreactive neurons contained SP, CGRP, and VIP immunoreactivity in both ILG and LRG. The current results might serve as an essential base for future studies to elucidate the pathological/physiological functions of HAP1 in the lingual ganglia.

Neurochemical phenotypes of huntingtin-associated protein 1 in reference to secretomotor and vasodilator neurons in the submucosal plexuses of rodent small intestine

Huntingtin-associated protein 1(HAP1) is an immunohistochemical marker of the stigmoid body (STB). Brain and spinal cord regions with lack of STB/HAP1 immunoreactivity are always neurodegenerative targets, whereas STB/HAP1 abundant regions are usually spared from neurodegeneration. In addition to the brain and spinal cord, HAP1 is abundantly expressed in the excitatory and inhibitory motor neurons in myenteric plexuses of the enteric nervous system (ENS). However, the detailed expression of HAP1 and its neurochemical characterization in submucosal plexuses of ENS are still unknown. In this study, we aimed to clarify the expression and neurochemical characterization of HAP1 in the submucosal plexuses of the small intestine in adult mice and rats. HAP1 was highly expressed in the submucosal plexuses of both rodents. The percentage of HAP1-immunoreactive submucosal neurons was not significantly varied between the intestinal segments of these rodents. Double immunofluorescence results revealed that almost all the cholinergic secretomotor neurons containing ChAT/ CGRP/ somatostatin/ calretinin, non-cholinergic secretomotor neurons containing VIP/NOS/TH/calretinin, and vasodilator neurons containing VIP/calretinin expressed HAP1. Our current study is the first to clarify that STB/HAP1 is expressed in secretomotor and vasodilator neurons of submucosal plexuses, suggesting that STB/HAP1 might modulate or protect the secretomotor and vasodilator functions of submucosal neurons in ENS.

Mapping of STB/HAP1 Immunoreactivity in the Mouse Brainstem and its Relationships with Choline Acetyltransferase, with Special Emphasis on Cranial Nerve Motor and Preganglionic Autonomic Nuclei

Huntingtin-associated protein 1 (HAP1) is a core component of stigmoid body (STB) and is known as a neuroprotective interactor with causal agents for various neurodegenerative diseases. Brain regions rich in STB/HAP1 immunoreactivity are usually spared from cell death, whereas brain regions with negligible STB/HAP1 immunoreactivity are the major neurodegenerative targets. Recently, we have shown that STB/HAP1 is abundantly expressed in the spinal preganglionic sympathetic/parasympathetic neurons but absent in the motoneurons of spinal cord, indicating that spinal motoneurons are more vulnerable to neurodegenerative diseases. In light of STB/HAP1 neuroprotective effects, it is also essential to clarify the distribution of STB/HAP1 in another major neurodegenerative target, the brainstem. Here, we examined the expression and detailed immunohistochemical distribution of STB/HAP1 and its relationships with choline acetyltransferase (ChAT) in the midbrain, pons, and medulla oblongata of adult mice. Abundant STB/HAP1 immunoreactive neurons were disseminated in the periaqueductal gray, Edinger-Westphal nucleus, raphe nuclei, locus coeruleus, pedunculopontine tegmental nucleus, superior/inferior salivatory nucleus, and dorsal motor nucleus of vagus. Double-label immunohistochemistry of HAP1 with ChAT (or with urocortin-1 for Edinger-Westphal nucleus centrally projecting population) confirmed that STB/HAP1 was highly present in parasympathetic preganglionic neurons but utterly absent in cranial nerve motor nuclei throughout the brainstem. These results suggest that due to deficient putative STB/HAP1-protectivity, cranial nerve motor nuclei might be more vulnerable to certain neurodegenerative stresses than STB/HAP1-expressing brainstem nuclei, including preganglionic parasympathetic nuclei. Our current results also lay a basic foundation for future studies that seek to clarify the physiological/pathological roles of STB/HAP1 in the brainstem.

Uncovering a Genetic Polymorphism Located in Huntingtin Associated Protein 1 in Modulation of Central Pain Sensitization Signaling Pathways

Fibromyalgia syndrome (FMS) is characterized by widespread pain and increased sensitivity to nociceptive stimulus or tenderness. While familial aggregation could suggest a potential hereditary component in FMS development, isolation of genetic determinants has proven difficult due to the multi-factorial nature and complexity of the syndrome. Central sensitization is thought to be one of the key mechanisms leading to FMS in a subset of patients. Enhanced central pain signaling can be measured using the Nociceptive Flexion Reflex (NFR) or RIII threshold. We performed a genome-wide association study (GWAS) using an array to genotype 258,756 human genetic polymorphisms in 225 FMS patients and 77 healthy volunteers and searched for genetic variants associated with a lowered NFR threshold. We have identified a potential association between a single nucleotide polymorphism resulting in a common non-synonymous coding mutation in the Huntingtin associated protein 1 (HAP1) gene (rs4796604, MAF = 0.5) and the NFR threshold (p = 4.78E−06). The Hap1 protein is involved in trafficking and is particularly enriched in neurons. Our results suggest a possible involvement of the neuronal trafficking protein HAP1 in modulating pain signaling pathways and thus participate in the establishment of the NFR threshold.

Electroacupuncture Regulates TRPV1 through PAR2/PKC Pathway to Alleviate Visceral Hypersensitivity in FD Rats

Visceral hypersensitivity (VH) is the predominant pathogenesis of functional dyspepsia (FD). Duodenal hypersensitivity along with nausea further reduces the comfort level in gastric balloon dilatation and inhibits gastric receptive relaxation. The potential mechanism behind electroacupuncture- (EA-) mediated alleviation of VH has not been elucidated. In an FD rat model with tail clamping stress, iodine acetamide (IA) induced VH. The rats were treated with EA with or without PAR2 antagonist FSLLRY-NH2, and the body weight, gastric sensitivity, compliance, and gastrointestinal motility were determined. Mast cells and activated degranulation were stained with toluidine blue (TB) staining and visualized under a transmission electron microscope (TEM). Immunofluorescence was used to detect the expression of PAR2, PKC, and TRPV1 in the duodenum and dorsal root ganglion (DRG) and that of CGRP, SP in DRG, and c-fos in the spinal cord. EA alone and EA + antagonist enhanced the gastrointestinal motility but diminished the expression of TRPV1, CGRP, SP, and c-fos-downstream of PAR2/PKC pathway and alleviated VH in FD rats. However, there was no obvious superposition effect between the antagonists and EA + antagonists. The effect of EA alone was better than that of antagonists and EA + antagonists 2 alone. EA-induced amelioration of VH in FD rats was mediated by TRPV1 regulation through PAR2/PKC pathway. This protective mechanism involved several pathways and included several targets.

Immunohistochemical expression and neurochemical phenotypes of huntingtin-associated protein 1 in the myenteric plexus of mouse gastrointestinal tract

Huntingtin-associated protein 1 (HAP1) is a neural huntingtin interactor and being considered as a core molecule of stigmoid body (STB). Brain/spinal cord regions with abundant STB/HAP1 expression are usually spared from neurodegeneration in stress/disease conditions, whereas the regions with little STB/HAP1 expression are always neurodegenerative targets. The enteric nervous system (ENS) can act as a potential portal for pathogenesis of neurodegenerative disorders. However, ENS is also a neurodegenerative target in these disorders. To date, the expression of HAP1 and its neurochemical characterization have never been examined there. In the current study, we determined the expression of HAP1 in the ENS of adult mice and characterized the morphological relationships of HAP1-immunoreactive (ir) cells with the markers of motor neurons, sensory neurons, and interneurons in the myenteric plexus using Western blotting and light/fluorescence microscopy. HAP1-immunoreaction was present in both myenteric and submucosal plexuses of ENS. Most of the HAP1-ir neurons exhibited STB in their cytoplasm. In myenteric plexus, a large number of calretinin, calbindin, NOS, VIP, ChAT, SP, somatostatin, and TH-ir neurons showed HAP1-immunoreactivity. In contrast, most of the CGRP-ir neurons were devoid of HAP1-immunoreactivity. Our current study is the first to clarify that HAP1 is highly expressed in excitatory motor neurons, inhibitory motor neurons, and interneurons but almost absent in sensory neurons in myenteric plexus. These suggest that STB/HAP1-ir neurons are mostly Dogiel type I neurons. Due to lack of putative STB/HAP1 protectivity, the sensory neurons (Dogiel type II) might be more vulnerable to neurodegeneration than STB/HAP1-expressing motoneurons/interneurons (Dogiel type I) in myenteric plexus.

Androgen Affects the Inhibitory Avoidance Memory by Primarily Acting on Androgen Receptor in the Brain in Adolescent Male Rats

Adolescence is the critical postnatal stage for the action of androgen in multiple brain regions. Androgens can regulate the learning/memory functions in the brain. It is known that the inhibitory avoidance test can evaluate emotional memory and is believed to be dependent largely on the amygdala and hippocampus. However, the effects of androgen on inhibitory avoidance memory have never been reported in adolescent male rats. In the present study, the effects of androgen on inhibitory avoidance memory and on androgen receptor (AR)-immunoreactivity in the amygdala and hippocampus were studied using behavioral analysis, Western blotting and immunohistochemistry in sham-operated, orchiectomized, orchiectomized + testosterone or orchiectomized + dihydrotestosterone-administered male adolescent rats. Orchiectomized rats showed significantly reduced time spent in the illuminated box after 30 min (test 1) or 24 h (test 2) of electrical foot-shock (training) and reduced AR-immunoreactivity in amygdala/hippocampal cornu Ammonis (CA1) in comparison to those in sham-operated rats. Treatment of orchiectomized rats with either non-aromatizable dihydrotestosterone or aromatizable testosterone were successfully reinstated these effects. Application of flutamide (AR-antagonist) in intact adolescent rats exhibited identical changes to those in orchiectomized rats. These suggest that androgens enhance the inhibitory avoidance memory plausibly by binding with AR in the amygdala and hippocampus.

Immunohistochemical relationships of huntingtin-associated protein 1 with enteroendocrine cells in the pyloric mucosa of the rat stomach

Huntingtin-associated protein 1 (HAP1) is a neuronal cytoplasmic protein that is predominantly expressed in the brain and spinal cord. In addition to the central nervous system, HAP1 is also expressed in the peripheral organs including endocrine system. Different types of enteroendocrine cells (EEC) are present in the digestive organs. To date, the characterization of HAP1-immunoreactive (ir) cells remains unreported there. In the present study, the expression of HAP1 in pyloric stomach in adult male rats and its relationships with different chemical markers for EEC [gastrin, marker of gastrin (G) cells; somatostatin, marker of delta (D) cells; 5-HT, marker of enterochromaffin (EC) cells; histamine, marker of enterochromaffin-like (ECL) cells] were examined employing single- or double-labelled immunohistochemistry and with light-, fluorescence- or electron-microscopy. HAP1-ir cells were abundantly expressed in the glandular mucosa but were very few or none in the surface epithelium. Double-labelled immunofluorescence staining for HAP1 and markers for EECs showed that almost all the G-cells expressed HAP1. In contrast, HAP1 was completely lacking in D-cells, EC-cells or ECL-cells. Our current study is the first to clarify that HAP1 is selectively expressed in G-cells in rat pyloric stomach, which probably reflects HAP1's involvement in regulation of the secretion of gastrin.