[Pathological changes in human brain biopsies from patients with idiopathic normal pressure hydrocephalus]

Aquaporin-4 in glymphatic system, and its implication for central nervous system disorders

The clearance function is essential for maintaining brain tissue homeostasis, and the glymphatic system is the main pathway for removing brain interstitial solutes. Aquaporin-4 (AQP4) is the most abundantly expressed aquaporin in the central nervous system (CNS) and is an integral component of the glymphatic system. In recent years, many studies have shown that AQP4 affects the morbidity and recovery process of CNS disorders through the glymphatic system, and AQP4 shows notable variability in CNS disorders and is part of the pathogenesis of these diseases. Therefore, there has been considerable interest in AQP4 as a potential and promising target for regulating and improving neurological impairment. This review aims to summarize the pathophysiological role that AQP4 plays in several CNS disorders by affecting the clearance function of the glymphatic system. The findings can contribute to a better understanding of the self-regulatory functions in CNS disorders that AQP4 were involved in and provide new therapeutic alternatives for incurable debilitating neurodegenerative disorders of CNS in the future.

Glymphatic MRI techniques in sleep and neurodegenerative diseases

PURPOSE OF REVIEW

The purpose of this review article is to summarize the current in-vivo imaging techniques for the evaluation of the glymphatic function and discuss the factors influencing the glymphatic function and research directions in the future.

RECENT FINDINGS

The glymphatic system allows the clearance of metabolic waste from the central nervous system (CNS). The glymphatic pathway has been investigated using intrathecal or intravenous injection of a gadolinium-based contrast agent (GBCA) on MRI, so-called glymphatic MRI. The glymphatic MRI indirectly visualizes the dynamic CSF flow and evaluated the glymphatic function in the animal and human models. Several clinical and preclinical studies using glymphatic MRI have confirmed that the glymphatic function is impaired in neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, and idiopathic normal pressure hydrocephalus. Furthermore, physiologic process such as sleep facilitates the glymphatic clearance, thus clearing accumulation of protein deposition, such as amyloid or tau, potentially delaying the progression of neurodegenerative diseases.

SUMMARY

The glymphatic system plays a crucial role in clearing metabolic wastes in the brain. Glymphatic MR imaging using GBCA administration serves as a functional imaging tool to measure the glymphatic function and investigate various pathophysiologies of neurodegenerative diseases.

Aquaporin-4 and Cognitive Disorders

Aquaporin-4 (AQP4) is the most abundantly expressed aquaporin in the central nervous system (CNS) and is an integral part of the glymphatic system that cannot be ignored. The CNS has the glymphatic system instead of the conventional lymphatic system. The glymphatic system plays an essential role in the pathophysiological processes of many cognitive disorders. AQP4 shows noteworthy changes in various cognitive disorders and is part of the pathogenesis of these diseases. For this reason, AQP4 has attracted attention as a potential and promising target for regulating and even reversing cognitive dysfunction. This review will summarize the role of AQP4 in the pathophysiological processes of several cognitive disorders as reported in recent studies.

Glymphatic System Impairment in Alzheimer's Disease and Idiopathic Normal Pressure Hydrocephalus

Approximately 10% of dementia patients have idiopathic normal pressure hydrocephalus (iNPH), an expansion of the cerebrospinal fluid (CSF)-filled brain ventricles. iNPH and Alzheimer's disease (AD) both exhibit sleep disturbances, build-up of brain metabolic wastes and amyloid-β (Aβ) plaques, perivascular reactive astrogliosis, and mislocalization of astrocyte aquaporin-4 (AQP4). The glia-lymphatic (glymphatic) system facilitates brain fluid clearance and waste removal during sleep via glia-supported perivascular channels. Human studies have implicated impaired glymphatic function in both AD and iNPH. Continued investigation into the role of glymphatic system biology in AD and iNPH models could lead to new strategies to improve brain health by restoring homeostatic brain metabolism and CSF dynamics.