Figure 2 . A summary of the modifications to TJ, adhesion junctions and efflux transporter expression at ALS CNS barriers. The abundance of TJ and adhesion proteins are generally reduced, leading to a compromised junctional integrity and increase in paracellular permeability. The basement membrane thickness is reported to either increase as a reparative mechanism or decrease with reduced collagen and laminin in ALS, with discrepancies reported in human tissues, while being consistently decreased in animal models. The expression of P-gp and BCRP are generally increased in ALS, which is expected to result in reduced CNS exposure of drugs that are substrates for these efflux transporters. ZO: zonula occludens, P-gp: P-glycoprotein, BCRP: breast cancer resistance protein, JAM: junctional adhesion molecules.
Summary and future directions
Human and rodent studies have consistently demonstrated ultrastructural abnormality, increased paracellular permeability and elevated P-gp (and sometimes BCRP) expression and activity at the CNS barriers in ALS. Based on the reports of reduced TJ function, it would be expected that general CNS permeability to drugs would be elevated in ALS, however, this may be counteracted especially if the drugs are substrates of P-gp and/or BCRP. Furthermore, if there is indeed a thickening of the cerebral or spinal microvasculature basement membrane, this could result in lower brain and spinal cord access of drugs as has been reported in a mouse model of AD with thickened cerebrovascular basement membrane (Mehta, Short & Nicolazzo, 2013). Therefore, it is clear that a more detailed functional analysis of transport processes of many drugs which are trafficked via different mechanisms (i.e. paracellular, transcellular, substrates of influx and efflux transporters) is required so as to predict how CNS access of drugs indeed alters in ALS.
With disease progression, the expression and activity of P-gp and BCRP generally increases, which may lead to suboptimal drug delivery, while this yet to be confirmed in people with ALS, for example, by using PET imaging. If this is validated in humans, specialised approaches can be trialled to improve the CNS access of riluzole (and other CNS-acting drugs) to improve therapeutic outcomes. This can be via pharmacological manipulation of P-gp expression and activity or by transiently disrupting the CNS barriers using emerging technology such as MR-guided focused ultrasound (Abrahao et al., 2019).
Our current understanding of the CNS barriers in ALS is still limited. The majority of studies have only investigated P-gp and BCRP expression. Proteomic studies can be performed using microvessels isolated from post-mortem ALS human brain/spinal cords or from transgenic ALS mice, to generate a more detailed status of the ALS BBB and BSCB. A better appreciation of the status of BBB/BSCB influx transporters in ALS can assist in the design of new chemical entities that can specifically target these influx transporters to enhance CNS exposure of otherwise impermeable drugs. These studies will also highlight, based on their affinity to transporters, which drugs not intended to reach the CNS have increased CNS access in ALS, informing which drugs may require dosage adjustment so as to avoid excessive CNS exposure. Ultimately, this will guide optimum dosing in individuals for all medications consumed by people with ALS to maximise effectiveness (when CNS access is required) and minimise CNS toxicity (when CNS access is not desirable), overall enhancing optimum use of medicines in individuals with ALS.