The Therapeutic Potential of Neuronal K-Cl Co-Transporter KCC2 in Huntington's Disease and Its Comorbidities

Restoring Compromised Cl- in D2 Neurons of a Huntington's Disease Mouse Model Rescues Motor Disability

Huntington's disease (HD) is a progressive neurodegenerative disorder with no cure, characterized by significant neurodegeneration of striatal GABAergic medium spiny neurons (MSNs). Early stages of the disease are characterized by the loss of dopamine 2 receptor-expressing MSNs (D2 MSNs) followed by degeneration of dopamine 1 receptor-expressing MSNs (D1 MSNs), leading to aberrant basal ganglia signaling. While the early degeneration of D2 MSNs and impaired GABAergic transmission are well-documented, potassium chloride cotransporter 2 (KCC2), a key regulator of intracellular chloride (Cl-), and therefore GABAergic signaling, has not been characterized in D1 and D2 MSNs in HD. We aimed to investigate whether Cl- regulation was differentially altered in D1 and D2 MSNs and may contribute to the early degeneration of D2 MSNs in male and female symptomatic R6/2 mice. We used electrophysiology to record the reversal potential for GABAA receptors (E GABA), a read-out for the efficacy of Cl- regulation, in striatal D1 and D2 MSNs and their corresponding output structures. During the early symptomatic phase (P55-P65), Cl- impairments were observed in D2 MSNs in R6/2 mice, with no change in D1 MSNs. Cl- regulation was also dysfunctional in the globus pallidus externa, resulting in GABA-mediated excitation. When we overexpressed KCC2 in D2 MSNs using AAV-mediated delivery, we delayed the onset of motor impairments in R6/2 mice. We demonstrate that Cl- homeostasis is differentially altered in D1 and D2 MSNs and may contribute to the enhanced susceptibility of D2 MSNs during HD progression.

SNARE protein SNAP25 regulates the chloride-transporter KCC2 in neurons

Inhibitory synaptic neurotransmission mediated by GABA requires a low concentration of chloride ions (Cl-) in neurons, which is established and maintained by the potassium-chloride co-transporter 2 (KCC2). While KCC2-interacting proteins are known to regulate KCC2 protein level and function, specific KCC2-interacting partners are still being identified and characterized. We asked whether SNAP25, an integral component of the SNARE-complex and a novel KCC2 interactor, regulates KCC2 protein and function in mice. We demonstrated that SNAP25 interacts with KCC2, and that this interaction is regulated by protein kinase C (PKC)-mediated phosphorylation. We also discovered that SNAP25 knockdown decreases total KCC2 in cortical neurons, and reduces the strength of synaptic inhibition, as demonstrated through a depolarization of the reversal potential for GABA (EGABA), indicating reduced KCC2 function. Our biochemical and electrophysiological data combined demonstrate that SNAP25 regulates KCC2 membrane expression and function, and in doing so, regulates inhibitory synaptic transmission.

Neuronal K+-Cl- cotransporter KCC2 as a promising drug target for epilepsy treatment

Epilepsy is a prevalent neurological disorder characterized by unprovoked seizures. γ-Aminobutyric acid (GABA) serves as the primary fast inhibitory neurotransmitter in the brain, and GABA binding to the GABAA receptor (GABAAR) regulates Cl- and bicarbonate (HCO3-) influx or efflux through the channel pore, leading to GABAergic inhibition or excitation, respectively. The neuron-specific K+-Cl- cotransporter 2 (KCC2) is essential for maintaining a low intracellular Cl- concentration, ensuring GABAAR-mediated inhibition. Impaired KCC2 function results in GABAergic excitation associated with epileptic activity. Loss-of-function mutations and altered expression of KCC2 lead to elevated [Cl-]i and compromised synaptic inhibition, contributing to epilepsy pathogenesis in human patients. KCC2 antagonism studies demonstrate the necessity of limiting neuronal hyperexcitability within the brain, as reduced KCC2 functioning leads to seizure activity. Strategies focusing on direct (enhancing KCC2 activation) and indirect KCC2 modulation (altering KCC2 phosphorylation and transcription) have proven effective in attenuating seizure severity and exhibiting anti-convulsant properties. These findings highlight KCC2 as a promising therapeutic target for treating epilepsy. Recent advances in understanding KCC2 regulatory mechanisms, particularly via signaling pathways such as WNK, PKC, BDNF, and its receptor TrkB, have led to the discovery of novel small molecules that modulate KCC2. Inhibiting WNK kinase or utilizing newly discovered KCC2 agonists has demonstrated KCC2 activation and seizure attenuation in animal models. This review discusses the role of KCC2 in epilepsy and evaluates its potential as a drug target for epilepsy treatment by exploring various strategies to regulate KCC2 activity.

Therapeutic potential for KCC2-targeted neurological diseases

Patients with neurological diseases, such as schizophrenia, tend to show low K+-Cl- co-transporter 2 (KCC2) levels in the brain. The cause of these diseases has been associated with stress and neuroinflammation. However, since the pathogenesis of these diseases is not yet fully investigated, drug therapy is still limited to symptomatic therapy. Targeting KCC2, which is mainly expressed in the brain, seems to be an appropriate approach in the treatment of these diseases. In this review, we aimed to discuss about stress and inflammation, KCC2 and Gamma-aminobutyric acid (GABA) function, diseases which decrease the KCC2 levels in the brain, factors that regulate KCC2 activity, and the possibility to overcome neuronal dysfunction targeting KCC2. We also aimed to discuss the relationships between neurological diseases and LPS caused by Porphyromonas gingivalis (P. g), which is a type of oral bacterium. Clinical trials on oxytocin, sirtuin 1 (SIRT1) activator, and transient receptor potential cation channel subfamily V Member 1 activator have been conducted to develop effective treatment methods. We believe that KCC2 modulators that regulate mitochondria, such as oxytocin, glycogen synthase kinase 3β (GSK3β), and SIRT1, can be potential targets for neurological diseases.

Rett and Rett-related disorders: Common mechanisms for shared symptoms?

Rett syndrome is a neurodevelopmental disorder caused by loss-of-function mutations in the methyl-CpG binding protein-2 (MeCP2) gene that is characterized by epilepsy, intellectual disability, autistic features, speech deficits, and sleep and breathing abnormalities. Neurologically, patients with all three disorders display microcephaly, aberrant dendritic morphology, reduced spine density, and an imbalance of excitatory/inhibitory signaling. Loss-of-function mutations in the cyclin-dependent kinase-like 5 (CDKL5) and FOXG1 genes also cause similar behavioral and neurobiological defects and were referred to as congenital or variant Rett syndrome. The relatively recent realization that CDKL5 deficiency disorder (CDD), FOXG1 syndrome, and Rett syndrome are distinct neurodevelopmental disorders with some distinctive features have resulted in separate focus being placed on each disorder with the assumption that distinct molecular mechanisms underlie their pathogenesis. However, given that many of the core symptoms and neurological features are shared, it is likely that the disorders share some critical molecular underpinnings. This review discusses the possibility that deregulation of common molecules in neurons and astrocytes plays a central role in key behavioral and neurological abnormalities in all three disorders. These include KCC2, a chloride transporter, vGlut1, a vesicular glutamate transporter, GluD1, an orphan-glutamate receptor subunit, and PSD-95, a postsynaptic scaffolding protein. We propose that reduced expression or activity of KCC2, vGlut1, PSD-95, and AKT, along with increased expression of GluD1, is involved in the excitatory/inhibitory that represents a key aspect in all three disorders. In addition, astrocyte-derived brain-derived neurotrophic factor (BDNF), insulin-like growth factor 1 (IGF-1), and inflammatory cytokines likely affect the expression and functioning of these molecules resulting in disease-associated abnormalities.

Inhibitory Synaptic Influences on Developmental Motor Disorders

During development, GABA and glycine play major trophic and synaptic roles in the establishment of the neuromotor system. In this review, we summarise the formation, function and maturation of GABAergic and glycinergic synapses within neuromotor circuits during development. We take special care to discuss the differences in limb and respiratory neuromotor control. We then investigate the influences that GABAergic and glycinergic neurotransmission has on two major developmental neuromotor disorders: Rett syndrome and spastic cerebral palsy. We present these two syndromes in order to contrast the approaches to disease mechanism and therapy. While both conditions have motor dysfunctions at their core, one condition Rett syndrome, despite having myriad symptoms, has scientists focused on the breathing abnormalities and their alleviation-to great clinical advances. By contrast, cerebral palsy remains a scientific quagmire or poor definitions, no widely adopted model and a lack of therapeutic focus. We conclude that the sheer abundance of diversity of inhibitory neurotransmitter targets should provide hope for intractable conditions, particularly those that exhibit broad spectra of dysfunction-such as spastic cerebral palsy and Rett syndrome.

KCC2 overexpressed exosomes meditated spinal cord injury recovery in mice

Exosomes show great potential in treating diseases of the central nervous system including spinal cord injury (SCI), still better engineered exosomes have more advantages. In this study, we purified exosomes from K⁺-Cl^-co-transporter (KCC2) overexpressed bone marrow mesenchymal stem cells (Exo_KCC2), to investigate the effect of Exo_KCC2on neural differentiationin vitroand the repairing function of Exo_KCC2in SCI micein vivo. Compared to bone marrow mesenchymal stem cells (BMSC)-derived exosomes (Exo), Exo_KCC2could better promote neural stem cell differentiated into neurons, ameliorate the function recovery of SCI mice, and accelerate the neural regeneration at the lesion site. Altogether, engineered Exo_KCC2may prove to be an advantageous strategy for SCI treatment.