Dysfunction of the blood-brain barrier and neurovascular unit in amyotrophic lateral sclerosis - From advanced human models to treatments.

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder, characterized by the degeneration of motor neurons, resulting in progressive and eventually complete loss of motor function. The phenotypical heterogeneity of ALS poses a challenge to early diagnosis and treatment, with currently available therapeutics only capable of relieving symptoms and slowing progression. While ALS is classified as a motor neuron disease (MND), research has identified significant impairment to the blood-brain barrier (BBB) and neurovascular unit (NVU) of ALS patients. The BBB along with supporting cells of the NVU selectively control the entry of substances into the central nervous system (CNS) and maintain brain homeostasis, with its many cellular components demonstrating impaired function, specifically relating to inflammation, oxidative stress and TDP-43 proteinopathy. The advent of more advanced in vitro techniques as well as use of patient-derived BBB cells has presented a promising alternative to recreate the in vivo structure of the BBB/NVU in a highly controlled platform. Such approaches have ranged from simple 2-dimensional (2D) cell culture to intricate 3-dimensional (3D) co-culture systems, as well as microfluidic systems and organoids, with a potential to investigate ALS associated BBB/NVU dysfunction and drug discovery. This review provides a comprehensive assessment of known BBB/NVU dysfunction in ALS and methods of investigating this using in vitro and in vivo platforms with a focus on inflammation, oxidative stress and pathological TDP-43 expression. This review also presents a future perspective to overcoming barriers to treatment of ALS using new patient-derived BBB/NVU model systems.