The crosstalk between the Notch, Wnt, and SHH signaling pathways in regulating the proliferation and regeneration of sensory progenitor cells in the mouse cochlea

Notch signaling in neurogenesis

The Notch signaling pathway plays a crucial role in neurogenesis by regulating cell fate specification. However, its complexity poses challenges in uncovering the mechanisms underlying these decisions. This Review explores the intricacies of the Notch pathway, including its diverse activation mechanisms and the influence of post-translational modifications of Notch receptors and ligands on pathway outcomes. We discuss how Notch signaling regulates embryonic neurogenesis via interactions with proneural genes and with other signaling pathways. We also examine the role of Notch in adult neurogenesis, and the therapeutic potential of leveraging Notch signaling to reprogram glia in the adult brain. Lastly, we highlight emerging technologies that measure Notch dynamics and discuss remaining knowledge gaps. Together, these insights underscore the multifaceted role of Notch signaling and outline key directions for future research.

Personalized Stem Cell-Based Regeneration in Spinal Cord Injury Care

Spinal cord injury (SCI) remains a major clinical challenge, with limited therapeutic options for restoring lost neurological function. While efforts to mitigate secondary damage have improved early-phase management, achieving sustained neurorepair and functional recovery remains elusive. Advances in stem cell engineering and regenerative medicine have opened new avenues for targeted interventions, particularly through the transplantation of neural stem/progenitor cells (NSPCs), induced pluripotent stem cells (iPSCs), and mesenchymal stem cells (MSCs). However, patient-specific factors such as cellular senescence, genetic and epigenetic variability, injury microenvironment, and comorbidities influence the efficacy of stem cell therapies by affecting graft survival and differentiation. Overcoming these challenges necessitates cutting-edge technologies, including single-cell transcriptomics, CRISPR-mediated hypoimmunogenic engineering, and biomaterial-based delivery platforms, which enable personalized and precision-driven SCI repair. Leveraging these advancements may help stem cell therapies overcome translational barriers and establish clinically viable regenerative solutions. This review explores the intersection of patient-specific variability, bioengineering innovations, and transcriptomic-guided precision medicine to define the next frontier in SCI therapy.

Shh agonist enhances maturation in homotypic Lgr5-positive inner ear organoids

Background: The regeneration of functional hair cells (HCs) remains a critical challenge in addressing sensorineural hearing loss. This study aimed to investigate the molecular and functional mechanisms driving stereocilia maturation within inner ear organoids (IEO) derived from homogenic Lgr5-positive progenitor cells (LPCs) and to compare outcomes with traditional heterotypic cultures. Methods: Mouse cochlear LPCs were isolated via magnetic-activated cell sorting (MACS) to establish homotypic cultures, ensuring purity and eliminating the heterotypic influences present in traditional manual isolation (MI) methods. Differentiation into HCs was induced through Wnt and Notch signaling modulation. Transcriptomic profiling using bulk and single-cell RNA sequencing (scRNA-seq) identified gene expression changes linked to stereocilia development. A Sonic Hedgehog (Shh) agonist was applied to enhance structural maturation of HCs. Functional assessment included electron microscopy, FM1-43 uptake assays, and microelectrode array recordings in assembloids of IEO with primary spiral ganglion neurons (SGN) co-cultures. Results: While homotypic LPC-derived IEOs successfully differentiated into HC-like cells, initial morphological assessment revealed immature stereocilia structures. Bulk RNA-seq analysis highlighted a downregulation of morphogenesis-related genes in these organoids. The application of a Shh agonist, acting as a key morphogen, promoted stereocilia development, as evidenced by enhanced ultrastructural features and increased expression of cuticular plate-associated genes (Pls1, Lmo7 and Lrba). Single-cell RNA sequencing (scRNA-seq) further identified distinct cell clusters, which exhibited robust expression of stereocilia-related genes (Espn, Lhfpl5, Loxhd1 and Tmc1), indicative of advanced HC maturation. Electrophysiological assessments of IEO-SGN assembloids using microelectrode arrays confirmed functional mechanoelectrical transduction between cells. Conclusion: This integrated approach elucidates critical pathways and cellular dynamics underpinning stereocilia maturation and functional HC development in EIOs. These findings provide new insights into the molecular regulation of HC maturation and support the utility of Shh-modulated IEOs as a promising platform for inner ear regeneration and therapeutic development for inner ear regenerative therapies.

Molecular Signaling Pathways in Wound-Induced Hair-Follicle Neogenesis

Wound-induced hair-follicle neogenesis (WIHN) is the phenomenon of regenerating new hair follicles from wounds in mammals. The WIHN involves both developmental and adult wound-healing processes. Moreover, the WIHN is regulated by a variety of factors, particularly multiple molecular signaling pathways produced in several types of cells. Here, the role of multiple signaling in different types of cells in WIHN is comprehensively described. Furthermore, the lack of dermal γδ T cells in the human scalp has hindered the clinical application of WIHN, but the development of drugs such as Wnt signaling activators is increasing the effectiveness of WIHN in humans. Overall, understanding the underlying mechanisms that regulate WIHN may help treat skin diseases, including alopecia.

The Notch ligand Jagged1 plays a dual role in cochlear hair cell regeneration

Hair cells (HCs) within the inner ear cochlea are specialized mechanoreceptors required for hearing. Cochlear HCs are not regenerated in mammals, and their loss is a leading cause of deafness in humans. Cochlear supporting cells (SCs) in newborn mice have the capacity to regenerate HCs, but persistent Notch signaling, presumably activated by SC-specific Notch ligand Jagged1 (JAG1), prevents SCs from converting into HCs. Here, employing an organoid platform, we show that while JAG1 participates in HC-fate repression, JAG1's primary function is to preserve the "progenitor-like characteristics" of cochlear SCs. Transcriptomic and mechanistic studies reveal that JAG1/Notch signaling maintains the expression of progenitor and metabolic genes in cochlear SCs and sustains pro-growth pathways, including PI3K-Akt-mTOR signaling, a function that is mediated by Notch1 and Notch2. Finally, we show that JAG1/Notch signaling stimulation with JAG1-Fc peptide enhances the HC-forming capacity of cochlear SCs undergoing maturation in cochlear explants and in vivo .

Immunomodulatory Tissue Factors in the Gallbladder Walls of Pediatric Patients with Chronic Calculous Cholecystitis

BACKGROUND

The rising rates of gallstones and cholecystectomy in pediatric populations underscore the increasing concern regarding chronic cholecystitis. However, the morphopathogenesis of pediatric calculous cholecystitis is still not well understood. This study aimed to determine the expression and distribution of immunomodulatory factors interleukin-12 (IL-12), interleukin-13 (IL-13), interleukin-1β (IL-1β), sonic hedgehog protein (SHH), nuclear factor NF-kappa-B p65 subunit (NFkBp65), and heat shock protein 60 (HSP60) in the gallbladder walls of pediatric patients with chronic calculous cholecystitis.

METHODS

In total, 11 gallbladder samples were collected from pediatric patients with calculous cholecystitis during cholecystectomy, while 5 healthy gallbladder samples served as controls. IL-12, IL-13, IL-1β, SHH, NFkBp65, and HSP60 were detected by immunohistochemistry. The number of positive structures in gallbladder wall epithelium, vasculature, and inflammatory infiltrate was assessed semi-quantitatively by microscopy. A Mann-Whitney U test and Spearman's rank-order correlation coefficient were calculated.

RESULTS

Statistically significant differences were observed between patient and control samples in the expression of IL-1β, SHH, and NFkBp65 in the epithelium, as well as in the expression of IL-12, SHH, and HSP60 in the blood vessels. The expression of IL-1β was stronger in the epithelium of controls, while other markers were more prominent in patient samples.

CONCLUSIONS

An increased number of NFkBp65, IL-12, and HSP60 positive cells in patient gallbladder tissue suggests a significant role of these tissue factors in driving immune modulation and sustaining the inflammation in pediatric chronic calculous cholecystitis. The noticeable expression of SHH in patient gallbladder tissue indicates its part in tissue regeneration and repair processes, as well as in modulating inflammation and vascular responses in calculous cholecystitis. The significant positive correlations between the factors studied highlight the importance of their coordinated interaction and intricate crosstalk in the morphopathogenesis of calculous cholecystitis.

PKM2 controls cochlear development through lactate-dependent transcriptional regulation

Understanding the role of metabolic processes during inner ear development is essential for identifying targets for hair cell (HC) regeneration, as metabolic choices play a crucial role in cell proliferation and differentiation. Among the metabolic processes, growing evidence shows that glucose metabolism is closely related to organ development. However, the role of glucose metabolism in mammalian inner ear development and HC regeneration remains unclear. In this study, we found that glycolytic metabolism is highly active during mouse and human cochlear prosensory epithelium expansion. Using mouse cochlear organoids, we revealed that glycolytic activity in cochlear nonsensory epithelial cells was predominantly dominated by pyruvate kinase M2 (PKM2). Deletion of PKM2 induced a metabolic switch from glycolysis to oxidative phosphorylation, impairing cochlear organoid formation. Furthermore, conditional loss of PKM2 in cochlear progenitors hindered sensory epithelium morphogenesis, as demonstrated in PKM2 knockout mice. Mechanistically, pyruvate is generated by PKM2 catalysis and then converted into lactate, which then lactylates histone H3, regulating the transcription of key genes for cochlear development. Specifically, accumulated lactate causes histone H3 lactylation at lysine 9 (H3K9la), upregulating the expression of Sox family transcription factors through epigenetic modification. Moreover, overexpression of PKM2 in supporting cells (SCs) triggered metabolism reprogramming and enhanced HC generation in cultured mouse and human cochlear explants. Our findings uncover a molecular mechanism of sensory epithelium formation driven by glycolysis-lactate flow and suggest unique approaches for mammalian HC regeneration.

Activation of Wnt/β-catenin signaling to increase B lymphoma Moloney murine leukemia virus insertion region 1 by lithium chloride attenuates the toxicity of cisplatin in the HEI-OC1 auditory cells

Cisplatin is widely used in anti-tumor therapy, but the ototoxicity caused by high-dose cisplatin often limits its efficacy, and the specific mechanism of cisplatin-induced cochlear damage is still not perfect. The Wnt/β-catenin signaling pathway is closely related to aging, embryonic development, and apoptosis. Meanwhile, B lymphoma Moloney murine leukemia virus insertion region 1 (BMI1) plays a certain role in the evolution and development of the inner ear and the occurrence and development of inner ear-related diseases. Our study intends to explore the role and specific mechanism of the Wnt/β-catenin signaling pathway and BMI1 in improving cisplatin ototoxicity. The appropriate experimental concentrations for each drug were selected by CCK-8 cell proliferation assay and Western Blot to detect apoptosis. The lentivirus transfection of HEI-OC1 cochlear hair cells was used to overexpress BMI1. Western Blot, qPCR, and immunofluorescence detected the activation of each component of BMI1 and Wnt/β-catenin signaling pathway in each experimental model. Wnt/β-catenin signaling pathway and BMI1 are jointly involved in cisplatin-induced cell injury. Low lithium chloride (LiCl) concentrations activated the Wnt/β-catenin pathway, increased BMI1 expression, and reduced cisplatin-induced hair cell injury. In contrast, overexpression of BMI1 inhibited the Wnt/β-catenin pathway and reduced hair cell injury. Meanwhile, the increased cisplatin-induced damage to hair cells by inhibiting BMI1 could not be rescued by LiCl. In conclusion, LiCl can ameliorate cisplatin ototoxicity by elevating BMI1 expression through activation of the Wnt/β-catenin pathway. Overexpression of BMI1 inhibits the Wnt/β-catenin pathway and reduces cisplatin-induced hair cell damage.

Implication of GPRASP2 in the Proliferation and Hair Cell-Forming of Cochlear Supporting Cells

G protein-coupled receptor-associated sorting protein 2 (GPRASP2) has been identified as the causative gene for X-linked recessive syndromic hearing loss (SHL) in our previous study. However, the role of GPRASP2 in auditory function remains unclear. The present study demonstrated that Gprasp2 overexpression in mouse organoids promoted the proliferation of supporting cells (SCs), which was mainly mediated by the Hedgehog signalling pathway. Meanwhile, GPRASP2 promoted hair cell (HC) formation from SCs via β-catenin signalling. In addition, GPRASP2 deficiency resulted in increased lysosomal degradation of SMO protein, leading to decreased expression of β-catenin and the Hedgehog pathway transcription factor GLI1. In neomycin-treated mouse cochlear explant, the smoothened agonist (SAG) recured the HC loss and further facilitated AAV-ie-Gprasp2 to promote the proliferation of SCs and formation of HCs. Our results suggested that GPRASP2 could be a potential candidate for gene therapy in the regeneration of HCs.

Wnt Signaling Pathway in Tumor Biology

Relapse and metastasis are the major challenges that stand in the way of cancer healing and survival, mainly attributed to cancer stem cells (CSCs). Their capabilities of self-renewal and tumorigenic potential leads to treatment resistance development. CSCs function through signaling pathways such as the Wnt/β-catenin cascade. While commonly involved in embryogenesis and adult tissues homeostasis, the dysregulation of the Wnt pathway has direct correlations with tumorigenesis, metastasis, and drug resistance. The development of therapies that target CSCs and bulk tumors is both crucial and urgent. However, the extensive crosstalk present between Wnt and other signaling networks (Hedgehog and Notch) complicates the development of efficient long-term therapies with minimal side-effects on normal tissues. Despite the obstacles, the emergence of Wnt inhibitors and subsequent modulation of the signaling pathways would provide dynamic therapeutic approaches to impairing CSCs and reversing resistance mechanisms.

Unraveling the Mechanisms of Vestibular Neuron Formation from Human Induced Pluripotent Stem Cells

The development of in vitro models that accurately recapitulate the complex cellular and molecular interactions of the inner ear is crucial for understanding inner ear development, function, and disease. In this study, we utilized a customized microfluidic platform to generate human induced pluripotent stem cell (hiPSC)-derived three-dimensional otic sensory neurons (OSNs). hiPSC-derived otic neuronal progenitors (ONPs) were cultured in hydrogel-embedded microfluidic channels over a 40-day period. Careful modulation of Wnt and Shh signaling pathways was used to influence dorsoventral patterning and direct differentiation toward a vestibular neuron lineage. After validating the microfluidic platform, OSN spheroid transcription factor and protein expression were assessed using real-time quantitative polymerase chain reaction (RT-qPCR), immunocytochemistry, and flow cytometry. The results demonstrated the successful differentiation of hiPSCs into ONPs and subsequent divergent differentiation into vestibular neuronal lineages, as evidenced by the expression of characteristic markers. Overall, our microfluidic platform provides a physiologically relevant environment for the culture and differentiation of hiPSCs, offering a valuable tool for studying inner ear development, disease and drug screening, and regenerative medicine applications.

Pharmacological modulation of Sonic Hedgehog signaling pathways in Angiogenesis: A mechanistic perspective

The Sonic hedgehog (SHh) signaling pathway is an imperative operating network that helps in regulates the critical events during the development processes like multicellular embryo growth and patterning. Disruptions in SHh pathway regulation can have severe consequences, including congenital disabilities, stem cell renewal, tissue regeneration, and cancer/tumor growth. Activation of the SHh signal occurs when SHh binds to the receptor complex of Patch (Ptc)-mediated Smoothened (Smo) (Ptc-smo), initiating downstream signaling. This review explores how pharmacological modulation of the SHh pathway affects angiogenesis through canonical and non-canonical pathways. The canonical pathway for angiogenesis involves the activation of angiogenic cytokines such as fibroblast growth factor (FGF), vascular endothelial growth factor (VEGF), placental growth factor (PGF), hepatocyte growth factor (HGF), platelet-derived growth factor (PDGF), stromal cell-derived factor 1α, transforming growth factor-β1 (TGF-β1), and angiopoietins (Ang-1 and Ang-2), which facilitate the process of angiogenesis. The Non-canonical pathway includes indirect activation of certain pathways like iNOS/Netrin-1/PKC, RhoA/Rock, ERK/MAPK, PI3K/Akt, Wnt/β-catenin, Notch signaling pathway, and so on. This review will provide a better grasp of the mechanistic approach of SHh in mediating angiogenesis, which can aid in the suppression of certain cancer and tumor growths.

The protective role of Wnt3a in peroxynitrite-induced damage of cochlear hair cells in vitro

OBJECTIVE

To investigate the effect of peroxynitrite on the cultured cochlear hair cells of C57BL/6 P3 mice in vitro as well as the role of Wnt3a, as an activator of the canonical Wnt signaling pathway, underlying the action of such an oxidative stress.

METHODS

The in vitro primary cultured cochlear hair cells were subjected to l00 μM peroxynitrite and l00 μM peroxynitrite +25 ng/mL Wnt3a for 24 h, the cell survival and morphological changes were examined by immunofluorescence and transmission electron microscopy.

RESULTS

The number of surviving hair cells was significantly reduced in the 100 μM peroxynitrite group, while it was significantly higher in the Wnt3a + peroxynitrite treated group compared with the peroxynitrite treated group. The transmission electron microscopy showed that exposure to peroxynitrite induced a dramatic decrease in the number of mitochondria and severely disrupted mitochondrial ultrastructure, while Wnt3a clearly diminished the disruption of mitochondrial structure and preserved a higher number of mitochondria.

CONCLUSION

These results indicated that peroxynitrite could cause oxidative damage to the cochlear hair cells, and low concentrations of Wnt3a has a protective effect against oxidative damage.

LEVEL OF EVIDENCE

Level 2.

The ndrg2 Gene Regulates Hair Cell Morphogenesis and Auditory Function during Zebrafish Development

Damages of sensory hair cells (HCs) are mainly responsible for sensorineural hearing loss, however, its pathological mechanism is not yet fully understood due to the fact that many potential deafness genes remain unidentified. N-myc downstream-regulated gene 2 (ndrg2) is commonly regarded as a tumor suppressor and a cell stress-responsive gene extensively involved in cell proliferation, differentiation, apoptosis and invasion, while its roles in zebrafish HC morphogenesis and hearing remains unclear. Results of this study suggested that ndrg2 was highly expressed in the HCs of the otic vesicle and neuromasts via in situ hybridization and single-cell RNA sequencing. Ndrg2 loss-of-function larvae showed decreased crista HCs, shortened cilia, and reduced neuromasts and functional HCs, which could be rescued by the microinjection of ndrg2 mRNA. Moreover, ndrg2 deficiency induced attenuated startle response behaviors to sound vibration stimuli. Mechanistically, there were no detectable HC apoptosis and supporting cell changes in the ndrg2 mutants, and HCs were capable of recovering by blocking the Notch signaling pathway, suggesting that ndrg2 was implicated in HC differentiation mediated by Notch. Overall, our study demonstrates that ndrg2 plays crucial roles in HC development and auditory sensory function utilizing the zebrafish model, which provides new insights into the identification of potential deafness genes and regulation mechanism of HC development.

Regeneration of Hair Cells from Endogenous Otic Progenitors in the Adult Mammalian Cochlea: Understanding Its Origins and Future Directions

Sensorineural hearing loss is caused by damage to sensory hair cells and/or spiral ganglion neurons. In non-mammalian species, hair cell regeneration after damage is observed, even in adulthood. Although the neonatal mammalian cochlea carries regenerative potential, the adult cochlea cannot regenerate lost hair cells. The survival of supporting cells with regenerative potential after cochlear trauma in adults is promising for promoting hair cell regeneration through therapeutic approaches. Targeting these cells by manipulating key signaling pathways that control mammalian cochlear development and non-mammalian hair cell regeneration could lead to regeneration of hair cells in the mammalian cochlea. This review discusses the pathways involved in the development of the cochlea and the impact that trauma has on the regenerative capacity of the endogenous progenitor cells. Furthermore, it discusses the effects of manipulating key signaling pathways targeting supporting cells with progenitor potential to promote hair cell regeneration and translates these findings to the human situation. To improve hearing recovery after hearing loss in adults, we propose a combined approach targeting (1) the endogenous progenitor cells by manipulating signaling pathways (Wnt, Notch, Shh, FGF and BMP/TGFβ signaling pathways), (2) by manipulating epigenetic control, and (3) by applying neurotrophic treatments to promote reinnervation.

1'-O-methyl-averantin isolated from the endolichenic fungus Jackrogersella sp. EL001672 suppresses colorectal cancer stemness via sonic Hedgehog and Notch signaling

Endolichenic fungi are host organisms that live on lichens and produce a wide variety of secondary metabolites. Colorectal cancer stem cells are capable of self-renewal and differentiation into cancer cells, which makes cancers difficult to eradicate. New alternative therapeutics are needed to inhibit the growth of tumor stem cells. This study examined the ability of an extract of Jackrogersella sp. EL001672 (derived from the lichen Cetraria sp.) and the isolated compound 1'-O-methyl-averantin to inhibit development of cancer stemness. The endolichenic fungus Jackrogersella sp. EL001672 (KACC 83021BP), derived from Cetraria sp., was grown in culture medium. The culture broth was extracted with acetone to obtain a crude extract. Column chromatography and reverse-phase HPLC were used to isolate an active compound. The anticancer activity of the extract and the isolated compound was evaluated by qRT-PCR and western blotting, and in cell viability, spheroid formation, and reporter assays. The acetone extract of EL001672 did not affect cell viability. However, 1'-O-methyl-averantin showed cytotoxic effects against cancer cell lines at 50 μg/mL and 25 μg/mL. Both the crude extract and 1'-O-methyl-averantin suppressed spheroid formation in CRC cell lines, and downregulated expression of stemness markers ALDH1, CD44, CD133, Lgr-5, Msi-1, and EphB1. To further characterize the mechanism underlying anti-stemness activity, we examined sonic Hedgehog and Notch signaling. The results showed that the crude extract and the 1'-O-methyl-averantin inhibited Gli1, Gli2, SMO, Bmi-1, Notch-1, Hes-1, and the CSL complex. Consequently, an acetone extract and 1'-O-methyl-averantin isolated from EL001672 suppresses colorectal cancer stemness by regulating the sonic Hedgehog and Notch signaling pathways.

Genome-wide DNA methylation analysis of middle-aged and elderly monozygotic twins with age-related hearing loss in Qingdao, China

OBJECTIVE

To explore the differences in DNA methylation associated with age-related hearing loss in a study of 57 twin pairs from China.

DESIGN

Monozygotic twins were identified through the Qingdao Twin Registration system. The median age of participants was >50 years. Their hearing thresholds were measured using a multilevel pure-tone audiometry assessment. The pure-tone audiometry was calculated at low frequencies (0.5, 1.0, and 2.0 kHz), speech frequencies (0.5, 1.0, 2.0, and 4.0kHz), and high frequencies (4.0 and 8 kHz). The CpG sites were tested using a linear mixed-effects model, and the function of the cis-regulatory regions and ontological enrichments were predicted using the online Genomic Regions Enrichment of Annotations Tool. The differentially methylated regions were identified using a comb-p python library approach.

RESULTS

In each of the PTA categories (low-, speech-, high-frequency), age-related hearing loss was detected in 25.9%, 19.3%, and 52.8% of participants. In the low-, speech- and high-frequency categories we identified 18, 42, and 12 individual CpG sites and 6, 11, and 6 differentially methylated regions. The CpG site located near DUSP4 had the strongest association with low- and speech-frequency, while the strongest association with high-frequency was near C21orf58. We identified associations of ALG10 with high-frequency hearing, C3 and LCK with low- and speech-frequency hearing, and GBX2 with low-frequency hearing. Top pathways that may be related to hearing, such as the Notch signaling pathway, were also identified.

CONCLUSION

Our study is the first of its kind to identify these genes and their associated with DNA methylation may play essential roles in the hearing process. The results of our epigenome-wide association study on twins clarify the complex mechanisms underlying age-related hearing loss.

Understanding the Notch Signaling Pathway in Acute Myeloid Leukemia Stem Cells: From Hematopoiesis to Neoplasia

The Notch signaling pathway is fundamental to early fetal development, but its role in acute myeloid leukemia is still unclear. It is important to elucidate the function that contains Notch, not only in acute myeloid leukemia, but in leukemic stem cells (LSCs). LSCs seem to be the principal cause of patient relapse. This population is in a quiescent state. Signaling pathways that govern this process must be understood to increase the chemosensitivity of this compartment. In this review, we focus on the conserved Notch signaling pathway, and its repercussions in hematopoiesis and hematological neoplasia. We found in the literature both visions regarding Notch activity in acute myeloid leukemia. On one hand, the activation of Notch leads to cell proliferation, on the other hand, the activation of Notch leads to cell cycle arrest. This dilemma requires further experiments to be answered, in order to understand the role of Notch not only in acute myeloid leukemia, but especially in LSCs.

Primary cilia and ciliary signaling pathways in aging and age-related brain disorders

Brain disorders are characterized by the progressive loss of structure and function of the brain as a consequence of progressive degeneration and/or death of nerve cells. Aging is a major risk factor for brain disorders such as Alzheimer’s disease (AD), Parkinson’s disease (PD), amyotrophic lateral sclerosis (ALS), and stroke. Various cellular and molecular events have been shown to play a role in the progress of neurodegenerative diseases. Emerging studies suggest that primary cilia could be a key regulator in brain diseases. The primary cilium is a singular cellular organelle expressed on the surface of many cell types, such as astrocytes and neurons in the mature brain. Primary cilia detect extracellular cues, such as Sonic Hedgehog (SHH) protein, and transduce these signals into cells to regulate various signaling pathways. Abnormalities in ciliary length and frequency (ratio of ciliated cells) have been implicated in various human diseases, including brain disorders. This review summarizes current findings and thoughts on the role of primary cilia and ciliary signaling pathways in aging and age-related brain disorders.