Impairment of agonist-induced M2 muscarinic receptor activation by autoantibodies from chagasic patients with cardiovascular dysautonomia

Paraneoplastic Neurologic Syndromes

OBJECTIVE

Progress is ongoing in understanding paraneoplastic neurologic disorders, with new syndromes and antibodies being described and more detailed evidence available to guide workup for diagnosis and treatment to improve outcomes. Many excellent reviews have summarized the molecular features of different antibodies, but this article emphasizes the clinical features of each syndrome that may help guide initial diagnosis and treatment, which often should occur before an antibody or cancer is found to confirm the diagnosis.

LATEST DEVELOPMENTS

Recent findings include updated diagnostic criteria with validated sensitivity and specificity, discovery of novel antibodies, and clinical findings that increase the likelihood of an underlying paraneoplastic disorder. Suggestive syndromes that have been recently identified include faciobrachial dystonic seizures and pilomotor auras in anti-leucine-rich glioma inactivated protein 1 encephalitis, extreme delta brush on EEG in N-methyl-d-aspartate (NMDA)-receptor encephalitis, déjà vu aura in anti-glutamic acid decarboxylase 65 (GAD65) encephalitis, and sleep disturbances in several disorders. In addition, there is confirmed utility of brain positron emission tomography (PET) and CSF markers, including carcinoembryonic antigen and oligoclonal bands, as well as improved tests for the presence of leptomeningeal cancer cells in CSF. Associations of cancer immunotherapies with paraneoplastic syndromes and herpes simplex virus encephalitis (and COVID-19) with NMDA-receptor encephalitis have been described.

ESSENTIAL POINTS

All neurologists should be aware of advances regarding paraneoplastic neurologic syndromes, as patients can present with a wide variety of neurologic symptoms and earlier diagnosis and treatment can improve outcomes.

Implications of Antimuscarinic Autoantibodies in Postural Tachycardia Syndrome

Functional autoantibodies directed to the M2 muscarinic acetylcholine receptor (M2R) could affect the heart rate directly by altering cardiac M2R activity and/or indirectly by changing vagal-mediated cardiac M2R activity. We measured M2R autoantibody activity in sera from 10 subjects with postural tachycardia syndrome (POTS) and 5 healthy control subjects using a cell-based bioassay. Half of the POTS subjects demonstrated presence of elevated M2R autoantibody activity, while no significant M2R autoantibody activity was found in the healthy subjects. Serum-derived immunoglobulin G (IgG) from antibody-positive POTS patients induced a dose-dependent activation of M2R, which was blocked by the muscarinic antagonist atropine. Moreover, antibody-positive POTS IgG decreased the responsiveness to oxotremorine, an orthosteric muscarinic agonist, indicating an indirect inhibitory effect. These data suggest that M2R autoantibodies may contribute to the pathophysiology of POTS by increasing the normal vagal withdrawal during upright posture through its negative allosteric modulation of M2R activity. M2 muscarinic receptor-activating autoantibodies are present in a subgroup of patients with POTS and act as a negative allosteric modulator of the orthosteric ligand response.

Autoantibodies as Endogenous Modulators of GPCR Signaling

Endogenous self-reactive autoantibodies (AAs) recognize a range of G-protein-coupled receptors (GPCRs). They are frequently associated with cardiovascular, neurological, and autoimmune disorders, and in some cases directly impact disease progression. Many GPCR AAs modulate receptor signaling, but molecular details of their modulatory activity are not well understood. Technological advances have provided insight into GPCR biology, which now facilitates deeper understanding of GPCR AA function at the molecular level. Most GPCR AAs are allosteric modulators and exhibit a broad range of pharmacological properties, altering both receptor signaling and trafficking. Understanding GPCR AAs is not only important for defining how these unusual GPCR modulators function in disease, but also provides insight into the potential use and limitations of using therapeutic antibodies to modulate GPCR signaling.