A properly functioning nervous system is necessary for life. If the propagation of nerve signals to parts of the human body is hindered or blocked altogether, that part of the body is unable to follow the commands given by the brain in a reasonable reaction time. In addition to physical damage that cuts the nerves, such effects can also happen by many types of neural diseases that target the myelin sheath. Rapid nerve conduction velocity is reliant of the insulative nature of myelin. Therefore, damage to the myelin sheath that reveals parts of the axons to the extracellular side can significantly slow down the nerve signals or prevent them from reaching their targets altogether. Repeated enough along a specific pathway, this can result in, for instance, slow reaction times in limbs.
Diseases that target the myelin sheath structure without damaging the axon it- self are referred to as demyelinating diseases[77]. Generally, the most dangerous of these diseases are those targeting the central nervous system due to its reduced regen- erative capabilities, but some diseases of the peripheral nervous system may also be extremely dangerous. Not all neural diseases are caused directly by demyelination, but many are involved in reducing nerve conduction velocity in some way. This can also be achieved by deformation of the myelin sheath. If the myelin sheath is not folded properly, resulting in bulges or loose contacts, the insulation around the axon is weakened. This is often due to dysfunction of the proteins involved in the myelin contraction process. Therefore, myelin proteins are also potential targets for myelin-related diseases. Other main types of diseases found in the nervous system are neurodegenerative diseases, where the nerve cells disintegrate over time for some reason[78]. Neurodegenerative diseases are often incurable. They are usually caused by genetic mutations, protein misfolding or incorrectly activated programmed cell death, mainly targeting the central nervous system.
The most common demyelinating disease in the central nervous system is mul- tiple sclerosis (MS), targeting the myelin sheaths of the nerve cells in the brain and the spinal cord[8]. The demyelination process is visualized in Figure 3.4. The ef- fects of MS vary greatly between patients, making it difficult to diagnose without performing biopsy. Generally, the symptoms include muscle weakness, coordina-
Nerve fiber Myelin
Healthy nerve Myelin begins to disintegrate Disrupted axon function
Figure 3.4 Illustration of the progress of demyelination in the MS disease. Damage to the myelin sheath reveals the nerve fiber and weakens nerve signal transduction. Eventually signals are unable to be transmitted when the axon is damaged enough. Figure modified from Ref. [79].
tion trouble, double vision or blindness. The main mechanisms initiating MS are thought to be the failure of the immune system or the inability to produce myeli- nating cells in the CNS. The common factor between MS patients is the destruction of the myelin sheath and the loss of oligodendrocytes[80]. However, the root cause still remains unknown[81]. The first symptoms are typically observed as early as between ages 20 and 40 and it remains the leading cause for neural non-traumatic disabilities in young adults[82]. No known cures for MS yet exist, but patients are being treated both with physical treatment and different types of medication.
One example of a neurodegenerative disease in the CNS is the Alzheimer’s disease (AD)[83]. It is a slowly progressing disease mainly targeting people over the age of 65. The main symptoms revolve around memory loss, beginning from occasionally forgetting small details, evolving to greater difficulties in remembering words, places and names, and finally resulting in loss of thinking and speaking abilities. The main cause behind AD is relatively unknown, but several theories exist revolving around CNS protein abnormalities initiating cell death in the neurons of the brain[84, 85].
No cures or definitive ways to prevent AD currently exist, but intellectual activities and a healthy diet have been shown to reduce the risk of the onset of the Alzheimer’s disease[86, 87].
In the peripheral nervous system there are two major demyelinating diseases. The Guillain–Barré syndrome (GBS) is a rapidly progressing neurological disorder where the peripheral nervous system is attacked by the immune system[88]. The myelin sheath around the axon, and in some cases the axon itself, can be damaged as a result, resulting in reduced nerve conduction velocity[89]. The Schwann cells of the PNS are able to rapidly repair the myelin damage, but by creating shorted internodes, that leaves the newly created myelin vulnerable for further damage. The symptoms typically include muscle weakness in the hands and the feet and can intensify signif-
icantly in a period of a few weeks to both sides of the body. In the most severe cases of GBS, the nerves of the respiratory system are damaged leading to extreme difficul- ties in breathing. There are no actual treatments for GBS, but the majority of people who experience the disease recover fully after intensive supportive care during the most dangerous period of the disease.
One of the most common inherited diseases of the nervous system is the Charcot- Marie-Tooth neuropathy (CMT)[90]. It is primarily a demyelinating disease of the peripheral nervous system which onsets when mutations in the proteins facilitating myelin sheath formation cause misfolding of the myelin sheath [91]. This results in reduced nerve conduction velocity in both motor and sensory nerves presenting itself as a loss of functional muscle tissue, touch sensitivity and slower reflexes in separate parts of the body. CMT usually presents itself at quite a young age, mainly varying from childhood to early adulthood[92]. The symptoms are usually visible on the patient, usually initially observed in the foot, where it is characterized with a loss of muscle, high arch and curved toes. At later stages the disease may spread to other peripheral parts of the body causing neuropathic pain and severely hindering day-to-day life. As with many other nervous diseases, there is currently no existing drug treatments available, and the main suggested therapy is exercise which main- tains muscle strength in the afflicted regions[93].
CMT has been quite well studied. The root causes behind the disease have been identified as mutations in the genes (e.g.pmp22, mpz, pmp2) encoding the main pro- teins of the myelin sheath[10, 75, 94, 95]. The types of the Charcot-Marie-Tooth disease are named differently depending on the gene from which it originates and whether it targets the myelin sheath or the axons (e.g. CMT1A-F, CMT2, CMT3).
CMT1 variants target the myelin sheath proteins and recent studies have associated certain mutations in thepmp2-gene with the CMT1A-variant of the disease, typi- cally associated with the protein PMP22[7]. Thepmp2-gene encodes the peripheral myelin protein P2, which has been a main point of focus in this Thesis. The point mutants related to CMT1A are further discussed in Section 7.2 of this Thesis along with the research published in Paper II.
Most of the CMT variants cause demyelination in the myelin sheath, but some can also cause damage to the axon itself. The regenerative capabilities of Schwann cells help the myelin sheath rapidly reconstruct around the axon, but the mutations in the myelin proteins reduce the effectiveness of the protein-lipid interplay, essen-
tially resulting in a looser structure. This in turn results in a deformed myelin struc- ture, or in some cases, a so-called onion bulb structure caused by repeating episodes of demyelination and remyelination, eventually causing thinning of the total func- tional myelin sheath. This in turn leads to a loss of functional nerve fibers in parts of the PNS[96].
4 SCRAMBLASES AND LIPID FLIP–FLOP
The second part of this Thesis focuses on another type of signaling where dysfunc- tioning proteins are also at the root of several serious diseases. Cellular signaling is often dependent on a certain lipid composition on the outer side of a cellular bi- layer. This lipid composition is regulated by specific types of proteins known as flippases and scramblases. Their function is to facilitate lipid translocation across the membrane interior, which is intrinsically a rare event. In addition to cellular signaling, rapid lipid translocation is necessary for other cellular activities as well.
Failure to transport lipids from one side of the bilayer to the other, or transport of the wrong types of lipids, usually has severe consequences for the cell. In Paper IV of this Thesis we present extensive discussion on the roles lipids have in cellular sig- naling. The author contributed to the publication by studying the scramblase prop- erties of rhodopsin and the mechanisms with which it transports lipids across the bilayer. In this chapter, we discuss transbilayer lipid motion, the roles of scramblase proteins and the potential diseases that occur if this process is somehow hindered.