Thursday, April 4, 2019
Effects of Paraneoplastic LEMS on Neuromuscular Junction
Effects of Paraneoplastic LEMS on Neuromuscular sumQing Zhi TanEffects of ParaneoplasticLambert-Eaton Myasthenic Syndrome on the Neuromuscular JunctionIntroductionThe Lambert-Eaton myasthenic syndrome (LEMS) is an autoimmune disease. It is characterised by muscle weakness of the proximal lowerlimbs as a result of auto-antibodies produced against the voltage-gated calcium transmit (VGCC) prime on the presynaptic membrane of the neuromuscular junction(1). Most of the time, LEMS is classified as a paraneoplastic syndrome as it is very much associated with squirtcer (small electric cell lung cancer in particular) (2, 3).One of the earliest descriptions of this disease was reported by Anderson and his colleagues in the year 1953, describing a 47-year-old patient with bronchial carcinoma presenting with progressive muscle weakness(4). Upon examining this patient, Anderson wroteHis (the patients) muscle reason was generally weaker than his muscle bulk would suggest, especially the mu scles of the shoulder girdle, hip girdle, trunk, and anterior compartments of the legs.(4)Since then, much research has been done to find kayoed more about this disease. It is now known that the auto-antibodies are primarily directed against the P/Q-type VGCCs at the neuromuscular junction (1-3, 5, 6). This report will discuss the effects of LEMS on the neuromuscular junction.Voltage-Gated Calcium ChannelsIt would be serviceable to first understand the organise of the voltage-gated calcium channels (VGCC). In general, VGCCs are made up of 5 subunits (1, 2, , , ) and can be divided, according to the electrophysiological and pharmacological properties of their 1 subunit, into 5 different subtypes (L, N, P/Q, R and T-type)(2, 7). The structure of a typical VGCC is shown in Figure 1 below.As mentioned before, the LEMS auto-antibodies are mainly directed against the P/Q-type VGCCs (1-3, 5, 6). The 1 subunit of the P/Q-type VGCCs is known as 1A(8). This 1A subunit forms the channel wh ich allows the give-up the ghostment of Ca2+ ions into the synaptic knob(8), and can be further broken down into 4 identical domains (I, II, III, IV), each possessing 6 transmembrane segments (S1-S6)(2, 8) (see Figure 2).What Happens at The Neuromuscular Junction?Now that we piddle understood the basic structure of the VGCCs, let us move on to its authority and how it is affected by the pathogenesis of LEMS. The neuromuscular junction is made up of the pre-synaptic membrane, the synaptic cleave and the post-synaptic membrane (5, 9, 10). P/Q-type VGCCs present on the surface of the pre-synaptic membrane play a vital role in allowing synaptic transmissions to cross the neuromuscular junction (7, 9).In studies conducted by multiple different researchers, anti-P/Q-type VGCC auto-antibodies were found in more than 85% of LEMS patients (3, 6, 11). Japanese researchers Masaharu Takamori and his colleagues even went a step further to determine the special(prenominal) immunodominant si tes within the P/Q-type VGCC in which the auto-antibodies would bind. In their study, they found that majority of their test subjects had auto-antibodies directed against domains II and IV of the 1A subunit in the P/Q-type VGCC(12)(highlighted in red in Figure 2).Depolarisation of the pre-synaptic membrane occurs when stirred by an action authorisation initiated by a nerve impulse(13). This membrane depolarisation causes the VGCCs to open, allowing the local inflow of calcium ions (Ca2+)into the synaptic knob(13). The increase in Ca2+ concentration will stimulate the exocytosis of vesicles containing the neurotransmitter Acetylcholine (ACh) into the synaptic cleft(13). ACh then goes on to bind to the nicotinic acetylcholine receptors found on the motor end plate which will trigger an action potential and subsequently causing muscle contraction.(10, 13)In patients with LEMS, the LEMS auto-antibodies would bind to the VGCC present on the pre-synaptic membrane of the neuromuscular j unction(5). This causes the VGCC to set down its ability to function as an ion channel, thus inhibiting the influx of Ca2+ into the synaptic knob during membrane depolarisation(5). Since the exocytosis of synaptic vesicles are dependent on Ca2+ (14), it is believed that a decrease in Ca2+ influx would result in a simplification in synaptic vesicle exocytosis(5, 13, 15).A recent study conducted in 2015 confirmed the phenomenon in which the LEMS immunoglobulin G directly causes a reduction in the exocytosis of synaptic vesicles (16). They did this by using fluorescence imaging techniques to observe the exocytosis of synaptic vesicles in rat neurons incubated with LEMS IgG (16). Pooled IgG from healthy individuals were used in comparison and as a control (16).Hence, it is evident that the muscle weakness seen in LEMS patients is a direct consequence of reduced exocytosis of ACh-containing synaptic vesicles.In summary, the Lambert-Eaton myasthenic syndrome is an autoimmune disease tha t causes muscle weakness in patients. The reduction in muscle power is due to insufficient ACh release at the neuromuscular junction. In 85% of these patients, this is caused by auto-antibodies inactivating the P/Q-type VGCCs on the presynaptic membrane.Going Beyond VGCCsSo, what happens in the remaining 10-15% of LEMS patients that do not have anti-P/Q-type VGCC antibodies? Many scientists have asked the same question and investigations have been carried out to find alternative mechanisms in the pathogenesis of LEMS in seronegative patients. (The term seronegative is used to tell patients with undetectable anti-P/Q-type antibodies.)In the absence of anti-P/Q-type VGCC antibodies, it is almost impossible that LEMS would have the same pathogenesis in seronegative patients. Yet, it is found that in that location are no significant differences in the electrophysiological and clinical characteristics between the seronegative and seropositive patients(17, 18).One possible explanation for this phenomenon is that on that point might be auto-antibodies directed against a different molecule involved in the synaptic transmission hence causing seronegative patients to have the same clinical and electrophysical features albeit having no anti-P/Q-type VGCC antibodies.To date, many auto-antibodies with different target molecules have been found in LEMS patients. One of the earliest to be discovered is an auto-antibody to synaptotagmin, which is a protein involved in synaptic vesicle exocytosis (2, 5, 13, 15, 19).Furthermore, antibodies to the M1-type presynaptic muscarinic ACh receptor (M1-mAChR) have as well been discovered(20). The M1-mAChR is a G-protein coupled receptor that regulates ACh release at the neuromuscular junction(5, 20). We will not be going into the specifics of these antibodies in this report as the mechanisms involved are for the most part complicated.ConclusionEven though LEMS is a rare autoimmune disease, the prevalence of LEMS as a paraneoplasti c syndrome in patients with small cell lung cancer (SCLC) is extremely high. More than half of LEMS patients have SCLC and more often than not, the diagnosis of LEMS precedes the diagnosis of SCLC(21). Therefore, LEMS could potentially play a crucial role in allowing certain cancers to be detected at an earlier stage.Hence, understanding the pathophysiology of this disease would not only prove useful in discovering better treatments for LEMS patients but also aid the early detection of cancer.References1.Weiss N, Koschak A. Pathologies of Calcium Channels. Weiss N, Koschak A, editors Berlin, Heidelberg Springer Berlin Heidelberg Imprint Springer 2014.2.Takamori M. 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