Activation of Transient Receptor Potential Vanilloid 4 Impairs the Dendritic Arborization of Newborn Neurons in the Hippocampal Dentate Gyrus through the AMPK and Akt Signaling Pathways

TRPV4 Blockage Inhibits the Neurogenesis in the Adult Hippocampal Dentate Gyrus Following Pilocarpine‑Induced Status Epilepticus

Aberrant neurogenesis in the adult hippocampal dentate gyrus (DG) contributes to synapse remodeling during temporal lobe epilepsy (TLE). Transient receptor potential vanilloid 4 (TRPV4) is involved in the pathogenesis of TLE. Activation of TRPV4 can modulate neurogenesis in the adult hippocampal DG. The present study examined whether TRPV4 is responsible for the aberrant neurogenesis in the adult hippocampal DG during TLE. Herein, administration of a TRPV4-specific antagonist, HC-067047, attenuated the enhanced neural stem cell proliferation in the adult hippocampal DG in mice following pilocarpine‑induced status epilepticus (PISE). HC-067047 reduced the heightened hippocampal protein levels of cyclin-dependent kinase (CDK) 2, CDK6, cyclin E1, cyclin A2, and phosphorylated retinoblastoma (p-Rb) observed following PISE. Meanwhile, HC-067047 inhibited the extracellular signal-regulated kinase 1/2 (ERK1/2) and p38 mitogen-activated protein kinase (p38 MAPK) pathways that were enhanced and responsible for the increased proliferation of stem cells and higher levels of CDKs, cyclins, and p-Rb protein. HC-067047 reduced the 28-day-old BrdU+ cells but increased the ratio of 28-day-old BrdU+ cells to 1-day-old BrdU+ cells, indicating that TRPV4 blockage reduced the number but increased the survival rate of newborn cells following PISE. Finally, HC-067047 increased the Akt signaling that was inhibited and responsible for the decreased survival rate of newborn cells following PISE. It is concluded that TRPV4 blockage inhibits stem cell proliferation in the hippocampal DG following PISE, likely through inhibiting ERK1/2 and p38 MAPK signaling to decrease cell cycle-related protein expression, and increases newborn cell survival rate likely through increasing phosphoinositide 3 kinase-Akt signaling.

Advancements in Targeting Ion Channels for the Treatment of Neurodegenerative Diseases

Ion channels are integral membrane proteins embedded in biological membranes, and they comprise specific proteins that control the flow of ion transporters in and out of cells, playing crucial roles in the biological functions of different cells. They maintain the homeostasis of water and ion metabolism by facilitating ion transport and participate in the physiological processes of neurons and glial cells by regulating signaling pathways. Neurodegenerative diseases are a group of disorders characterized by the progressive loss of neurons in the central nervous system (CNS) or peripheral nervous system (PNS). Despite significant progress in understanding the pathophysiological processes of various neurological diseases in recent years, effective treatments for mitigating the damage caused by these diseases remain inadequate. Increasing evidence suggests that ion channels are closely associated with neuroinflammation; oxidative stress; and the characteristic proteins in neurodegenerative diseases, including Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), amyotrophic lateral sclerosis (ALS), and multiple sclerosis (MS). Therefore, studying the pathogenic mechanisms closely related to ion channels in neurodegenerative diseases can help identify more effective therapeutic targets for treating neurodegenerative diseases. Here, we discuss the progress of research on ion channels in different neurodegenerative diseases and emphasize the feasibility and potential of treating such diseases from the perspective of ion channels.

Upregulation of hypothalamic TRPV4 via S100a4/AMPKα signaling pathway promotes the development of diet-induced obesity

Obesity is associated with abnormal regulation of energy metabolism in the hypothalamus. Transient receptor potential vanilloid 4 (TRPV4) is involved in regulating osmotic pressure, temperature and mechanical force transmission, but little is known about its role in obesity. Herein, the present study aimed to elucidate the effect of hypothalamic TRPV4 on high-fat diet-induced obesity (DIO) and evaluate its potential for regulating energy metabolism. Here we show that hypothalamic TRPV4 content is increased in DIO rats. Central administration of adeno-associated virus expressing TRPV4 in these animals remarkably increased body weight and fat mass by activating the S100a4/AMPKα signaling pathway, thereby promoting positive energy metabolism. Overexpressed hypothalamic TRPV4 impaired glucose tolerance, while promoting the accumulation of fat in liver cells, resulting in hepatic steatosis. In addition, the upregulation of hypothalamic TRPV4 reduces high-fat induced central inflammation. This study provides evidence that hypothalamic TRPV4 plays a significant role in regulating homeostasis. Hypothalamic TRPV4 emerges as a target for therapeutic intervention against obesity.

TRPV4-mediated Ca2+ deregulation causes mitochondrial dysfunction via the AKT/α-synuclein pathway in dopaminergic neurons

Mutations in the gene encoding the transient receptor potential vanilloid member 4 (TRPV4), a Ca2+ permeable nonselective cation channel, cause TRPV4-related disorders. TRPV4 is widely expressed in the brain; however, the pathogenesis underlying TRPV4-mediated Ca2+ deregulation in neurodevelopment remains unresolved and an effective therapeutic strategy remains to be established. These issues were addressed by isolating mutant dental pulp stem cells from a tooth donated by a child diagnosed with metatropic dysplasia with neurodevelopmental comorbidities caused by a gain-of-function TRPV4 mutation, c.1855C > T (p.L619F). The mutation was repaired using CRISPR/Cas9 to generate corrected isogenic stem cells. These stem cells were differentiated into dopaminergic neurons and the pharmacological effects of folic acid were examined. In mutant neurons, constitutively elevated cytosolic Ca2+ augmented AKT-mediated α-synuclein (α-syn) induction, resulting in mitochondrial Ca2+ accumulation and dysfunction. The TRPV4 antagonist, AKT inhibitor, or α-syn knockdown, normalizes the mitochondrial Ca2+ levels in mutant neurons, suggesting the importance of mutant TRPV4/Ca2+/AKT-induced α-syn in mitochondrial Ca2+ accumulation. Folic acid was effective in normalizing mitochondrial Ca2+ levels via the transcriptional repression of α-syn and improving mitochondrial reactive oxygen species levels, adenosine triphosphate synthesis, and neurite outgrowth of mutant neurons. This study provides new insights into the neuropathological mechanisms underlying TRPV4-related disorders and related therapeutic strategies.

mTOR (Mammalian Target of Rapamycin): Hitting the Bull's Eye for Enhancing Neurogenesis After Cerebral Ischemia?

Ischemic stroke remains a leading cause of morbidity and disability around the world. The sequelae of serious neurological damage are irreversible due to body's own limited repair capacity. However, endogenous neurogenesis induced by cerebral ischemia plays a critical role in the repair and regeneration of impaired neural cells after ischemic brain injury. mTOR (mammalian target of rapamycin) kinase has been suggested to regulate neural stem cells ability to self-renew and differentiate into proliferative daughter cells, thus leading to improved cell growth, proliferation, and survival. In this review, we summarized the current evidence to support that mTOR signaling pathways may enhance neurogenesis, angiogenesis, and synaptic plasticity following cerebral ischemia, which could highlight the potential of mTOR to be a viable therapeutic target for the treatment of ischemic brain injury.

Dysregulation of AMPK-mTOR signaling leads to comorbid anxiety in Dip2a KO mice

Autism is often comorbid with other psychiatric disorders. We have previously shown that Dip2a knockout (KO) induces autism-like behaviors in mice. However, the role of Dip2a in other psychiatric disorders remains unclear. In this paper, we revealed that Dip2a KO mice had comorbid anxiety. Dip2a KO led to a reduction in the dendritic length of cortical and hippocampal excitatory neurons. Molecular mechanism studies suggested that AMPK was overactivated and suppressed the mTOR cascade, contributing to defects in dendritic morphology. Deletion of Dip2a in adult-born hippocampal neurons (Dip2a conditional knockout (cKO)) increased susceptibility to anxiety upon acute stress exposure. Application of (2R,6R)-hydroxynorketamine (HNK), an inhibitor of mTOR, rescued anxiety-like behaviors in Dip2a KO and Dip2a cKO mice. In addition, 6 weeks of high-fat diet intake alleviated AMPK-mTOR signaling and attenuated the severity of anxiety in both Dip2a KO mice and Dip2a cKO mice. Taken together, these results reveal an unrecognized function of DIP2A in anxiety pathophysiology via regulation of AMPK-mTOR signaling.

TRPV4 inhibitor HC067047 produces antidepressant-like effect in LPS-induced depression mouse model

Inflammation is a crucial component that contributes to the pathogenesis of major depressive disorder. It has been revealed that the nonselective cation channel transient receptor potential vanilloid 4 (TRPV4) profoundly affects a variety of physiological processes, including inflammation. However, its roles and mechanisms in LPS-induced depression are still unclear. Here, for the first time, we found that there was a significant increase in TRPV4 in the hippocampus in a depression mouse model induced by LPS. TRPV4 inhibitor HC067047 or knockdown the hippocampal TRPV4 with TRPV4 shRNA could effectively rescue the aberrant behaviors. Furthermore, TRPV4 inhibitor HC067047 reduced the activation of astrocyte and microglia, decreased expression of CaMKII-NLRP3 inflammasome and increased the expression of neurogenesis marker DCX in the hippocampus. In addition, enhanced neuroinflammation in the serum was also reversed by TRPV4 inhibitor HC067047. Thus, we consider that TRPV4 has an important role in contributing to the depression-like behavior following LPS-induced systemic inflammation.

Amelioration of Scopolamine-induced Learning and Memory Impairment by the TRPV4 Inhibitor HC067047 in ICR Mice

Alzheimer's disease (AD) is one of the most common causes of neurodegenerative diseases in the elderly. Cholinergic dysfunction is one of the pathological hallmarks of AD and leads to learning and memory impairment. Transient receptor potential vanilloid 4(TRPV4), a nonselective cation channel, is involved in learning and memory functions. HC067047, a TRPV4 specific inhibitor, has been reported to protect neurons against cerebral ischemic injury and amyloid-β -(Aβ) 40-induced hippocampal cell death. However, whether HC067047 could improve scopolamine (SCP)-induced cognitive dysfunction in mice is still unknown. The aims of this study were to verify whether HC067047 could ameliorate the SCP-induced learning and memory impairments in mice and to elucidate its underlying mechanisms of action. In this study, we examined the neuroprotective effect of the HC067047 against cognitive dysfunction induced by SCP (5 mg/kg, i.p.), a muscarinic receptor antagonist. The results showed that administration of HC067047(10 mg/kg, i.p.) significantly ameliorated SCP-induced cognitive dysfunction as assessed by the novel place recognition test (NPRT) and novel object recognition test (NORT). In the Y-maze test, HC067047 significantly enhanced the time spent in the novel arm in SCP mice. To further investigate the molecular mechanisms underlying the neuroprotective effect of HC067047, expression of several proteins involved in apoptosis was examined. The results demonstrated that HC067047 treatment decreased the protein levels of proapoptotic proteins such as Bax and caspase-3 in the hippocampus of SCP mice. In addition, HC067047 enhanced expression of the neurogenesis marker DCX and improved levels of the mature neuronal marker NeuN in SCP mice. These findings suggest the neuroprotective potential of the TRPV4 inhibitor HC067047 for the management of dementia with learning and memory loss.

Akt3-mTOR regulates hippocampal neurogenesis in adult mouse

Akt signaling has been associated with adult neurogenesis in the hippocampal dentate gyrus (DG). We reported cognitive dysfunction in Akt3 knockout (Akt3-KO) mice with the down-regulation of mTOR activation. However, little is known about the effects of Akt3 signaling on hippocampal neurogenesis. Herein, we show that progenitor cells, neuroblasts, and mature newborn neurons in hippocampal DG expressed Akt3 protein. The Akt3 phosphorylation in hippocampal DG was increased after voluntary wheel running for 7 days in wild-type mice (running WT mice), but not in Akt3-KO mice (running Akt3-KO mice). Subsequently, we observed that the proliferation of progenitor cells was suppressed in Akt3-KO mice and the mTOR inhibitor rapamycin-treated mice, whereas enhanced in running WT mice rather than running Akt3-KO mice. Neurite growth of neuroblasts was impaired in Akt3-KO mice and rapamycin-treated mice. In contrast, neither differentiation of progenitor cells nor migrating of newly generated neurons was altered in Akt3-KO mice or running WT mice. The levels of p70S6K and 4EBP1 phosphorylation were declined in Akt3-KO mice and elevated in running WT mice depending on mTOR activation. Furthermore, telomerase activity, telomere length, and expression of telomerase reverse transcriptase (TERT) were decreased in Akt3-KO mice but increased in running WT mice rather than running Akt3-KO mice, which required the mTOR activation. The study provides in vivo evidence that Akt3-mTOR signaling plays an important role in the proliferation of progenitor cells and neurite growth through positive regulated TERT expression and activation of p70S6K and 4EBP1.

The TRPV4 Agonist GSK1016790A Regulates the Membrane Expression of TRPV4 Channels

TRPV4 is a non-selective cation channel that tunes the function of different tissues including the vascular endothelium, lung, chondrocytes, and neurons. GSK1016790A is the selective and potent agonist of TRPV4 and a pharmacological tool that is used to study the TRPV4 physiological function in vitro and in vivo. It remains unknown how the sensitivity of TRPV4 to this agonist is regulated. The spatial and temporal dynamics of receptors are the major determinants of cellular responses to stimuli. Membrane translocation has been shown to control the response of several members of the transient receptor potential (TRP) family of ion channels to different stimuli. Here, we show that TRPV4 stimulation with GSK1016790A caused an increase in [Ca²⁺]_i that is stable for a few minutes. Single molecule analysis of TRPV4 channels showed that the density of TRPV4 at the plasma membrane is controlled through two modes of membrane trafficking, complete, and partial vesicular fusion. Further, we show that the density of TRPV4 at the plasma membrane decreased within 20 min, as they translocate to the recycling endosomes and that the surface density is dependent on the release of calcium from the intracellular stores and is controlled via a PI3K, PKC, and RhoA signaling pathway.

TRPV4 Inhibition Improved Myelination and Reduced Glia Reactivity and Inflammation in a Cuprizone-Induced Mouse Model of Demyelination

The inhibition of demyelination and the promotion of remyelination are both considerable challenges in the therapeutic process for many central nervous system (CNS) diseases. Increasing evidence has demonstrated that neuroglial activation and neuroinflammation are responsible for myelin sheath damage during demyelinating disorders. It has been revealed that the nonselective cation channel transient receptor potential vanilloid 4 (TRPV4) profoundly affects a variety of physiological processes, including inflammation. However, its roles and mechanisms in demyelination have remained unclear. Here, for the first time, we found that there was a significant increase in TRPV4 in the corpus callosum in a demyelinated mouse model induced by cuprizone (CPZ). RN-1734, a TRPV4-antagonist, clearly alleviated demyelination and inhibited glial activation and the production of tumor necrosis factor α (TNF-α) and interleukin 1β (IL-1β) without altering the number of olig2-positive cells. In vitro, RN-1734 treatment clearly inhibited the influx of calcium and decreased the levels of IL-1β and TNF-α in lipopolysaccharide (LPS)-activated microglial cells by suppressing NF-κB P65 phosphorylation. Apoptosis of oligodendrocyte induced by LPS-activated microglia was also alleviated by RN-1734. The results suggest that activation of TRPV4 in microglia is involved in oligodendrocyte apoptosis through the activation of the NF-κB signaling pathway, thus revealing a new mechanism of CNS demyelination.