Immune-Mediated Peripheral Neuropathies
These neuropathies are rare but potentially catastrophic autoimmune disorders that are clinically heterogeneous with known or suspected infectious etiologies. Included among these disabling disorders is acute inflammatory demyelinating polyradiculopathy (AIDP), a common North American and European variant of Guillain-Barré Syndrome (GBS). GBS first gained public attention briefly in 1976 following vaccination for the swine flu. Affected individuals initially experienced varying degrees of weakness or tingling sensations distally in the legs. For many patients, these symptoms spread to the arms and upper body increasing in intensity until muscle control was lost and total paralysis occurred. For these cases, GBS is life-threatening. The length of illness is unpredictable and often months of hospital care are required. Fortunately, most patients eventually recover from GBS. However, nearly a third of GBS patients continue to experience residual weakness after three years. Relapse occurs in approximately 3% of affected individuals.
In the US, thousands of GBS cases are reported annually with an overall yearly social and economic burden exceeding an estimated $1.8 billion. Subsequent to the virtual eradication of poliomyelitis, GBS is now considered the leading cause of acute flaccid paralysis in Western countries. GBS occurs with an incidence rate of 0.2-4.0 cases per 100,000, a frequency that is alarmingly similar to that reported for multiple sclerosis.
Why GBS affects some individuals and not others remains unclear. In some patients, symptoms of a respiratory or gastrointestinal viral infection precede, by a few days or weeks, the onset of GBS. Rarely, surgery or vaccinations will trigger the syndrome. The cause of AIDP/GBS remains unknown. Enhanced infiltration of inflammatory cells into peripheral nerves of some GBS patients, however, is strongly suggestive of a cellular immune-mediated pathogenic process. To date, the cellular immunity directed against specific constituents of the peripheral nerve myelin sheath is considered causal in AIDP.
The treatment of GBS is currently palliative and utilizes non-specific immune-modulating therapies. We believe that the advancement of care for patients affected by GBS and related diseases awaits the development of novel selective immune-modulating April 6, 2012 for expediting aggressive treatment options.
Peer-reviewed published findings from this laboratory have addressed the application of statins for the treatment of AIDP/GBS. Statins are a group of potent 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors that are clinically approved for cholesterol reduction. Increasing clinical and experimental evidence, however, strongly support anti-inflammatory effects of statins. We found that a short-term course of parenterally administered high-dose lovastatin markedly inhibits the development and progression of EAN without eliciting hepatotoxic or myotoxic complications. Independent of cholesterol reduction, lovastatin-treatment was found to protect against EAN-induced peripheral nerve conduction deficits and morphologic nerve injury by a mechanism that limits the proliferation and transendothelial migration of autoreactive leukocytes. We postulate that a short-term application of high-dose statins may prove clinically useful in the management of inflammatory demyelinating peripheral nerve diseases, including some clinical sub-groups of Guillain-Barré syndrome.
Our studies addressing humoral immune-mediated peripheral nerve disorders have focused on MGUS neuropathy, a hematological condition in which a single clone of B cells abnormally proliferate and produce an excess of immunoglobulin in the absence of apparent neoplastic disease. MGUS is a disease of the aged and is found in 3% of adults older than 70 years of age. The reported incidence of peripheral neuropathy in MGUS patients varies between 6%-31%, depending on the isotype of the patients’ paraprotein. Altered humoral, but not cellular, immune responses are strongly suspected as causal in some MGUS neuropathies. Although the development of neuropathy in some patients may be unrelated to their paraproteinemia, the occurrence of MGUS in affected patients is significantly higher than in the normal populations. Antibodies that react with peripheral nerve antigens have been strongly associated with some neuropathy syndromes, but in many cases the causal relationship of a monoclonal antibody to the associated neurological disease remains to be established.
Our peer-reviewed published studies have advanced this field by successfully identifying novel anti-neural polyclonal antibodies present in sera from patients with IgG MGUS neuropathy. These antibodies were distinct from the patients’ monoclonal IgG antibody, a novel finding that uniquely distinguishes IgG MGUS neuropathy from IgM MGUS neuropathy. Two discrete immunoreactive antigens were identified by sequence analysis: a high molecular weight neurofilament protein (NFH) and L-periaxin, an intracellular protein expressed in mature myelin. Antibodies to intracellular antigens are associated with a variety of neurological diseases, including autoimmune peripheral neuropathies. Using intraneural injection methodology, we found that sera containing polyclonal antibodies to NFH significantly diminished evoked motor compound muscle action potential (CMAP) amplitudes with morphologic evidence of axon damage. Intraneural injection of sera containing polyclonal antibodies to L-periaxin, on the other hand, produced significant sensory nerve conduction deficits with motor sparing and morphologic evidence of demyelination. We speculate that anti-neural polyclonal antibodies in sera of IgG MGUS neuropathy patients may have a greater pathologic significance than previously anticipated.
Studies addressing clinical strategies for the treatment of individuals with GBS or related inflammatory demyelinating peripheral neuropathies remain poorly represented. Long-term goals for these applied studies remain focused on the development of improved therapeutic strategies for the treatment and management of these potentially devastating disorders of the peripheral nerves.
Continued collaborations with our clinician colleagues have resulted in the successful procurement of additional extramural funds from the Department of Veterans Affairs to conduct a randomized blinded prospective clinical trial designed to rigorously identify the type and determine the significance of exercise on recovery of peripheral nerve function in type 2 diabetic Veterans with neuropathy.
Diabetes mellitus (diabetes) is the most common cause of blindness, end-stage renal disease, and non-traumatic lower extremity limb amputations in adults. Whether insulin-dependent (type 1) or non-insulin dependent (type 2), diabetes is defined as a metabolic, rather than an immune-mediated, disorder complicated by long-term microvascular, neurologic, and macrovascular complications.
Type 2 diabetes has reached epidemic proportions in the United States, affecting an estimated 24 million Americans, or greater than 8% of the population. Obesity, secondary to physical inactivity, is cited as a major factor in the increasing rates of type 2 diabetes. Greater than half of all patients with diabetes develop polyneuropathy, a debilitating progressive deterioration of peripheral and autonomic nerves. Vascular dysfunction, leading to reduced nerve blood flow and reduced endoneurial oxygen tension, is considered a primary contributor to diabetic polyneuropathy. Currently, diabetic polyneuropathy is untreatable in humans. Innovative treatment strategies designed to address pre-existing nerve injury in the aged diabetic patient are critically needed.
Therapeutic interventions that improve blood flow to peripheral nerves, such as exercise, is expected to slow or perhaps reverse the progression of peripheral nerve disease in the aged diabetic patient. Supervised moderate intensity exercise is an established safe and integral approach to the management of patients with diabetes. In a pilot study of five type 2 diabetic Veterans with length-dependent sensory polyneuropathy, our collaborators found that moderate intensity exercise produces measurable, and in some cases significant (p<0.03), improvement over baseline in motor and sensory nerve conduction electrodiagnostic primary outcome studies. We postulate that recovery of peripheral nerve function occurs by neuroplastic remodeling (growth of new nerve fibers) secondary to exercise-improved tissue oxygenation.
The applied goal of this clinical study is to quantitatively document the effects of different types of exercise on recovery of peripheral nerve function in the type 2 diabetic Veteran with neuropathy. The definitive benefits of exercise on recovery of peripheral nerve function in diabetic patients have not been established. We believe that this on-going single-site clinical study will provide the critical evidence required to objectively and critically define the importance of exercise in rehabilitation of the neuron-compromised diabetic Veteran.
Lessons learned from our immune-mediated program are being successfully applied to test novel therapeutic interventions for the management glaucomatous neuropathy. Our intent is to determine whether statins may serve as a novel therapeutic strategy for the management of primary open angle glaucoma (POAG), a leading cause of blindness in the United States and worldwide.
POAG is a slowly progressive optic neuropathy with characteristic optic nerve degeneration and progressive visual field loss. The prevalence of POAG in the US population 40 years and older is approximately 1.86% or nearly 2 million individuals. Over 60 million cases of blindness can be attributed to glaucoma worldwide. The personal, social, and medical burden of this disease is experienced globally and remains a significant concern. Studies designed to treat POAG are extremely important and are currently under represented.
Elevated intraocular pressure (IOP) is considered a primary risk factor for the initiation and progression of glaucomatous neuropathy. As such, current therapeutic options for the glaucomatous patient are limited to the management of IOP. Adequate IOP control in many patients, however, is insufficient to prevent further progressive loss of vision. Thus, the development of more selective therapeutic strategies that address the molecular defects responsible for optic nerve injury are critically needed to advance the care of the glaucomatous patient.
Statins are now being aggressively evaluated as therapeutic agents for the management of a wide variety of disorders, including those affecting vision. Long-term statin therapy in patients with cardiovascular problems has been associated with a lower incidence and slowed progression of primary open angle glaucoma. Systemic administration of simvastatin lowers intraocular pressure (IOP) in humans, possibly by facilitating outflow of aqueous humor (AH). Our own preliminary studies support a protective role of simvastatin during experimental ischemic-reperfusion retinal injury. The mechanism(s) by which statins increase AH outflow, reduces IOP, and protect against retinal injury remains unclear.
Disruption of Rho G-protein mediated signaling events may be of central importance in statin-dependent regulation of AH outflow. Inhibition of Rho with C3 exoenzyme, or expression of a dominant-negative mutant RhoA GTPase, in human eye anterior segments has been shown to increase AH outflow. The interaction of RhoA with Rho kinase effectors (ROCK1 (ROKb, p160ROCK) or ROCK2 (ROKa, Rho Kinase)) is believed to regulate actin polymerization, making it a putative pharmacologic target for the regulation of AH outflow.
Given the importance of the signaling pathways governed by Rho proteins and their role in regulating AH outflow, we are currently investigating the consequence of disrupting Rho isoprenylation on human trabecular meshwork (TM) cell function. We recently reported that inhibition of HMG-CoA reductase with lovastatin elicits a marked and selective increase in RhoA and RhoB mRNA and G-protein content in human TM cells, in part, by limiting geranylgeranyl isoprenylation of these G-proteins. We propose that post-translational geranylgeranylation facilitates RhoA and RhoB protein expression and processing in TM cells. Increased accumulation of unprenylated inactive (GDP-bound) forms of RhoA and RhoB may disrupt Rho-dependent regulation of TM cell cytoskeletal organization.
A thorough understanding of how statins influence AH outflow and affect retinal function is critically needed before the development of more specific and effective therapeutic options for the glaucomatous patient can be realized.