EDITORIAL
Neuromuscular junction as Achilles’ heel Yet another autoantibody?
Daniel B. Drachman, MD Every contraction of skeletal muscle entails the release Henry J. Kaminski, MD of acetylcholine (ACh) from the motor nerve terminal, its passage across a submicroscopic space, and combination with the postsynaptic ACh receptors Correspondence to (AChRs), which in turn triggers the events of muscle Dr. Drachman: contraction. This tiny movement of ACh across a [email protected] 70-nM gap not only is highly complex but is vulner® able to a wide variety of attacks or errors that may Neurology 2014;82:1942–1943 cause weakness or paralysis. Some examples include bacteria (8 different botulinum toxins, tetanus toxin), plants (curare), snakes (cobratoxin, bungarotoxin), black widow spiders (latrotoxin), frogs (histrionicotoxin), and, of course, humans (anti-cholinesterase war gases sarin, tabun). Myasthenia gravis (MG), the iconic autoimmune disorder of the neuromuscular junction (NMJ), is manifested by weakness and fatigue of skeletal muscles. MG results from a deficit of available AChRs at NMJs due to an antibody-mediated autoimmune attack. By far the most common antibodies in MG are those directed against AChRs, and they are detectable in approximately 85% of patients by a standard radioimmunoassay. These antibodies can deplete AChRs by one or more of 3 mechanisms1: (1) cross-linking AChRs, which induces accelerated uptake and degradation by muscle cells; (2) complement binding, which damages the AChRs and NMJs; and (3) blocking the actual ACh binding sites, thereby preventing transmission. The severity of MG depends on how efficiently a given patient’s repertoire of AChR antibodies can reduce the available AChRs.2 What of the 15% to 20% of patients with generalized MG who do not have AChR antibodies detectable by radioimmunoassay? Early studies showed that these patients do indeed have antibodies with suspicious properties suggesting that they might cause MG3,4: in most cases, the antibodies bound to skeletal muscle fibers in vitro. On passive transfer to mice, they significantly3 reduced the numbers of junctional AChRs and the amplitudes of miniature endplate potentials (p , 0.001). Unlike AChR antibodies, they did not cause either accelerated degradation or blockade of the
AChRs. These findings suggested that the antibodies either (1) were directed against some component of the NMJ different from the AChR but closely associated with it, or (2) had low affinity binding (and therefore might not show up in a fluid-based antibody assay).5 Both of these possibilities have now been confirmed. Antibodies against other components of the NMJ, i.e., MuSK (muscle-specific protein kinase)6,7 and LRP48 (low-density lipoprotein-related receptor 4), have now been identified. Antibodies to MuSK are present in approximately 40% of AChR antibody–negative patients and produce a myasthenic syndrome that closely resembles classic AChR-antibody MG. They are believed to interfere with clustering of AChRs at NMJs. More recently, antibodies to LRP4 have been identified in 3% to 10% (or 50%?)8 of antibody-negative sera9 and are being tested for their myasthenogenic potential. The identification of new pathogenic antibodies not only is of biological interest but also influences the clinical phenotype of the MG and appears to affect the patient’s responsiveness to therapeutic agents. In the article by Gasperi et al.10 in this issue of Neurology®, antibodies to another key component of the NMJ, agrin, have been found, raising the possibility that they too may have a role in the pathogenesis of MG in some patients. Role of agrin. Proper function of the NMJ requires that the release of ACh by motor nerves occur precisely at the sites of AChRs (figure). It is the job of the motor nerve to induce and maintain clustering of AChRs in exact apposition to the nerve terminals. Motor nerves secrete a special form of agrin (“z1”), which binds to its receptor molecule, Lrp4, and together they activate MuSK. Activated MuSK serves a complex function that results in the clustering of AChRs and postsynaptic structures of the NMJ. Because of the role of agrin in this mechanism, it seemed reasonable that antibodies to agrin could produce a myasthenic syndrome. How can the pathogenicity of such antibodies be evaluated? Criteria for defining a pathogenic role of autoantibodies include the following11: (1) presence
See page 1976 From the Department of Neurology (D.B.D.), Johns Hopkins School of Medicine, Baltimore, MD; and George Washington University (H.J.K.), Washington, DC. Go to Neurology.org for full disclosures. Funding information and disclosures deemed relevant by the authors, if any, are provided at the end of the editorial. 1942
© 2014 American Academy of Neurology
Figure
Now it will be important to determine whether anti-agrin antibodies can affect the Achilles’ heel at the NMJ and cause yet another myasthenic syndrome.
Role of agrin
STUDY FUNDING No targeted funding reported.
DISCLOSURE The authors report no disclosures relevant to the manuscript. Go to Neurology.org for full disclosures.
Agrin released from the nerve binds to LRP4, and through a complex with MuSK induces clustering of AChRs, which is further stabilized by rapsyn. Dok7 is required for proper activation of MuSK by nerve-derived agrin. See text re autoantibodies that have been demonstrated to produce myasthenia gravis and those that have yet to achieve requirements for confirmation as pathogenic antibodies. ACh 5 acetylcholine; AChR 5 acetylcholine receptor; Dok7 5 docking protein 7; LRP4 5 low-density lipoprotein-related receptor 4; MuSK 5 muscle-specific protein kinase.
of the antibody in patients with the disease; (2) antibody binding to the target antigen; (3) reproduction of the disease features by passive transfer; (4) production of a model disease by immunization with antigen; and (5) amelioration of the disease by antibody reduction. In the case of anti-agrin antibodies, criteria 1 and 2 have been satisfied thus far. Anti-agrin antibodies were present in approximately 10% of patients in this series and in 15% of patients with MG in a prior reported series of 161 patients.12 All 5 anti-agrin patients in this series had antibodies to other NMJ components, but nearly half of those in the series by Cossins et al.12 had only antibodies to agrin. The antibodies bound appropriately to agrin in an ELISA assay and to agrin expressed by cultured HEK293 cells.
REFERENCES 1. Drachman DB, Adams RN, Stanley EF, Pestronk A. Mechanisms of acetylcholine receptor loss in myasthenia gravis. J Neurol Neurosurg Psychiatry 1980;43:601–610. 2. Drachman DB, Adams RN, Josifek LF, Self SG. Functional activities of anti-AChR autoantibodies and clinical severity of myasthenia gravis. N Engl J Med 1982;307: 769–775. 3. Drachman DB, DeSilva S, Ramsay D, Pestronk A. Humoral pathogenesis of myasthenia gravis. Ann NY Acad Sci 1987;505:90–104. 4. Mossman S, Vincent A, Newsom-Davis J. Myasthenia gravis without acetylcholine-receptor antibody: a distinct disease entity. Lancet 1986;1:116–119. 5. Vincent A, Waters P, Leite I, et al. Antibodies identified by cell-based assays in myasthenia gravis and associated diseases. Ann NY Acad Sci 2012;1275:92–98. 6. Hoch W, McConville J, Helms S, et al. Autoantibodies to the receptor tyrosine kinase MuSK in patients with myasthenia gravis without acetylcholine receptor antibodies. Nat Med 2001;7:365–368. 7. Shigemoto K, Kubo S, Maruyama N, et al. Induction of myasthenia by immunization against muscle specific kinase. J Clin Invest 2006;116:1016–1024. 8. Pevzner A, Schoser B, Peters K, et al. Anti-LRP4 autoantibodies in AChR- and MuSK-antibody-negative myasthenia gravis. J Neurol 2012;259:427–435. 9. Zhang B, Tzartos JS, Belimezi M, et al. Autoantibodies to low density lipoprotein related protein 4 in patients with double seronegative myasthenia gravis. Arch Neurol 2012; 69:445–451. 10. Gasperi C, Melms A, Schoser B, et al. Anti-agrin autoantibodies in myasthenia gravis. Neurology 2014;82:1976–1983. 11. Drachman DB. How to recognize an antibody-mediated autoimmune disease: criteria. In: Waksman B, editor. Immunologic Mechanisms in Neurologic and Psychiatric Disease. New York: Raven Press; 1990:183–186. 12. Cossins J, Belaya K, Zoltowska K, et al. The search for new antigenic targets in myasthenia gravis. Ann NY Acad Sci 2012;1275:123–128.
Neurology 82
June 3, 2014
1943
Neuromuscular junction as Achilles' heel: Yet another autoantibody? Daniel B. Drachman and Henry J. Kaminski Neurology 2014;82;1942-1943 Published Online before print May 2, 2014 DOI 10.1212/WNL.0000000000000486 This information is current as of May 2, 2014 Updated Information & Services
including high resolution figures, can be found at: http://www.neurology.org/content/82/22/1942.full.html
References
This article cites 11 articles, 2 of which you can access for free at: http://www.neurology.org/content/82/22/1942.full.html##ref-list-1
Citations
This article has been cited by 1 HighWire-hosted articles: http://www.neurology.org/content/82/22/1942.full.html##otherarticles
Permissions & Licensing
Information about reproducing this article in parts (figures,tables) or in its entirety can be found online at: http://www.neurology.org/misc/about.xhtml#permissions
Reprints
Information about ordering reprints can be found online: http://www.neurology.org/misc/addir.xhtml#reprintsus
Neurology ® is the official journal of the American Academy of Neurology. Published continuously since 1951, it is now a weekly with 48 issues per year. Copyright © 2014 American Academy of Neurology. All rights reserved. Print ISSN: 0028-3878. Online ISSN: 1526-632X.
Neuromuscular junction as Achilles’ heel Yet another autoantibody?
Daniel B. Drachman, MD Every contraction of skeletal muscle entails the release Henry J. Kaminski, MD of acetylcholine (ACh) from the motor nerve terminal, its passage across a submicroscopic space, and combination with the postsynaptic ACh receptors Correspondence to (AChRs), which in turn triggers the events of muscle Dr. Drachman: contraction. This tiny movement of ACh across a [email protected] 70-nM gap not only is highly complex but is vulner® able to a wide variety of attacks or errors that may Neurology 2014;82:1942–1943 cause weakness or paralysis. Some examples include bacteria (8 different botulinum toxins, tetanus toxin), plants (curare), snakes (cobratoxin, bungarotoxin), black widow spiders (latrotoxin), frogs (histrionicotoxin), and, of course, humans (anti-cholinesterase war gases sarin, tabun). Myasthenia gravis (MG), the iconic autoimmune disorder of the neuromuscular junction (NMJ), is manifested by weakness and fatigue of skeletal muscles. MG results from a deficit of available AChRs at NMJs due to an antibody-mediated autoimmune attack. By far the most common antibodies in MG are those directed against AChRs, and they are detectable in approximately 85% of patients by a standard radioimmunoassay. These antibodies can deplete AChRs by one or more of 3 mechanisms1: (1) cross-linking AChRs, which induces accelerated uptake and degradation by muscle cells; (2) complement binding, which damages the AChRs and NMJs; and (3) blocking the actual ACh binding sites, thereby preventing transmission. The severity of MG depends on how efficiently a given patient’s repertoire of AChR antibodies can reduce the available AChRs.2 What of the 15% to 20% of patients with generalized MG who do not have AChR antibodies detectable by radioimmunoassay? Early studies showed that these patients do indeed have antibodies with suspicious properties suggesting that they might cause MG3,4: in most cases, the antibodies bound to skeletal muscle fibers in vitro. On passive transfer to mice, they significantly3 reduced the numbers of junctional AChRs and the amplitudes of miniature endplate potentials (p , 0.001). Unlike AChR antibodies, they did not cause either accelerated degradation or blockade of the
AChRs. These findings suggested that the antibodies either (1) were directed against some component of the NMJ different from the AChR but closely associated with it, or (2) had low affinity binding (and therefore might not show up in a fluid-based antibody assay).5 Both of these possibilities have now been confirmed. Antibodies against other components of the NMJ, i.e., MuSK (muscle-specific protein kinase)6,7 and LRP48 (low-density lipoprotein-related receptor 4), have now been identified. Antibodies to MuSK are present in approximately 40% of AChR antibody–negative patients and produce a myasthenic syndrome that closely resembles classic AChR-antibody MG. They are believed to interfere with clustering of AChRs at NMJs. More recently, antibodies to LRP4 have been identified in 3% to 10% (or 50%?)8 of antibody-negative sera9 and are being tested for their myasthenogenic potential. The identification of new pathogenic antibodies not only is of biological interest but also influences the clinical phenotype of the MG and appears to affect the patient’s responsiveness to therapeutic agents. In the article by Gasperi et al.10 in this issue of Neurology®, antibodies to another key component of the NMJ, agrin, have been found, raising the possibility that they too may have a role in the pathogenesis of MG in some patients. Role of agrin. Proper function of the NMJ requires that the release of ACh by motor nerves occur precisely at the sites of AChRs (figure). It is the job of the motor nerve to induce and maintain clustering of AChRs in exact apposition to the nerve terminals. Motor nerves secrete a special form of agrin (“z1”), which binds to its receptor molecule, Lrp4, and together they activate MuSK. Activated MuSK serves a complex function that results in the clustering of AChRs and postsynaptic structures of the NMJ. Because of the role of agrin in this mechanism, it seemed reasonable that antibodies to agrin could produce a myasthenic syndrome. How can the pathogenicity of such antibodies be evaluated? Criteria for defining a pathogenic role of autoantibodies include the following11: (1) presence
See page 1976 From the Department of Neurology (D.B.D.), Johns Hopkins School of Medicine, Baltimore, MD; and George Washington University (H.J.K.), Washington, DC. Go to Neurology.org for full disclosures. Funding information and disclosures deemed relevant by the authors, if any, are provided at the end of the editorial. 1942
© 2014 American Academy of Neurology
Figure
Now it will be important to determine whether anti-agrin antibodies can affect the Achilles’ heel at the NMJ and cause yet another myasthenic syndrome.
Role of agrin
STUDY FUNDING No targeted funding reported.
DISCLOSURE The authors report no disclosures relevant to the manuscript. Go to Neurology.org for full disclosures.
Agrin released from the nerve binds to LRP4, and through a complex with MuSK induces clustering of AChRs, which is further stabilized by rapsyn. Dok7 is required for proper activation of MuSK by nerve-derived agrin. See text re autoantibodies that have been demonstrated to produce myasthenia gravis and those that have yet to achieve requirements for confirmation as pathogenic antibodies. ACh 5 acetylcholine; AChR 5 acetylcholine receptor; Dok7 5 docking protein 7; LRP4 5 low-density lipoprotein-related receptor 4; MuSK 5 muscle-specific protein kinase.
of the antibody in patients with the disease; (2) antibody binding to the target antigen; (3) reproduction of the disease features by passive transfer; (4) production of a model disease by immunization with antigen; and (5) amelioration of the disease by antibody reduction. In the case of anti-agrin antibodies, criteria 1 and 2 have been satisfied thus far. Anti-agrin antibodies were present in approximately 10% of patients in this series and in 15% of patients with MG in a prior reported series of 161 patients.12 All 5 anti-agrin patients in this series had antibodies to other NMJ components, but nearly half of those in the series by Cossins et al.12 had only antibodies to agrin. The antibodies bound appropriately to agrin in an ELISA assay and to agrin expressed by cultured HEK293 cells.
REFERENCES 1. Drachman DB, Adams RN, Stanley EF, Pestronk A. Mechanisms of acetylcholine receptor loss in myasthenia gravis. J Neurol Neurosurg Psychiatry 1980;43:601–610. 2. Drachman DB, Adams RN, Josifek LF, Self SG. Functional activities of anti-AChR autoantibodies and clinical severity of myasthenia gravis. N Engl J Med 1982;307: 769–775. 3. Drachman DB, DeSilva S, Ramsay D, Pestronk A. Humoral pathogenesis of myasthenia gravis. Ann NY Acad Sci 1987;505:90–104. 4. Mossman S, Vincent A, Newsom-Davis J. Myasthenia gravis without acetylcholine-receptor antibody: a distinct disease entity. Lancet 1986;1:116–119. 5. Vincent A, Waters P, Leite I, et al. Antibodies identified by cell-based assays in myasthenia gravis and associated diseases. Ann NY Acad Sci 2012;1275:92–98. 6. Hoch W, McConville J, Helms S, et al. Autoantibodies to the receptor tyrosine kinase MuSK in patients with myasthenia gravis without acetylcholine receptor antibodies. Nat Med 2001;7:365–368. 7. Shigemoto K, Kubo S, Maruyama N, et al. Induction of myasthenia by immunization against muscle specific kinase. J Clin Invest 2006;116:1016–1024. 8. Pevzner A, Schoser B, Peters K, et al. Anti-LRP4 autoantibodies in AChR- and MuSK-antibody-negative myasthenia gravis. J Neurol 2012;259:427–435. 9. Zhang B, Tzartos JS, Belimezi M, et al. Autoantibodies to low density lipoprotein related protein 4 in patients with double seronegative myasthenia gravis. Arch Neurol 2012; 69:445–451. 10. Gasperi C, Melms A, Schoser B, et al. Anti-agrin autoantibodies in myasthenia gravis. Neurology 2014;82:1976–1983. 11. Drachman DB. How to recognize an antibody-mediated autoimmune disease: criteria. In: Waksman B, editor. Immunologic Mechanisms in Neurologic and Psychiatric Disease. New York: Raven Press; 1990:183–186. 12. Cossins J, Belaya K, Zoltowska K, et al. The search for new antigenic targets in myasthenia gravis. Ann NY Acad Sci 2012;1275:123–128.
Neurology 82
June 3, 2014
1943
Neuromuscular junction as Achilles' heel: Yet another autoantibody? Daniel B. Drachman and Henry J. Kaminski Neurology 2014;82;1942-1943 Published Online before print May 2, 2014 DOI 10.1212/WNL.0000000000000486 This information is current as of May 2, 2014 Updated Information & Services
including high resolution figures, can be found at: http://www.neurology.org/content/82/22/1942.full.html
References
This article cites 11 articles, 2 of which you can access for free at: http://www.neurology.org/content/82/22/1942.full.html##ref-list-1
Citations
This article has been cited by 1 HighWire-hosted articles: http://www.neurology.org/content/82/22/1942.full.html##otherarticles
Permissions & Licensing
Information about reproducing this article in parts (figures,tables) or in its entirety can be found online at: http://www.neurology.org/misc/about.xhtml#permissions
Reprints
Information about ordering reprints can be found online: http://www.neurology.org/misc/addir.xhtml#reprintsus
Neurology ® is the official journal of the American Academy of Neurology. Published continuously since 1951, it is now a weekly with 48 issues per year. Copyright © 2014 American Academy of Neurology. All rights reserved. Print ISSN: 0028-3878. Online ISSN: 1526-632X.
