The Potential of Minocycline for Neuroprotection in Human Neurologic Disease

Minocycline is a member of the tetracycline class of molecules with broad-spectrum antibiotic activity. The unique properties of minocycline result in increased tissue distribution when compared with the other tetracyclines. Of particular interest is the ability of minocycline to diffuse into the central nervous system at clinically effective levels. Aside from its antimicrobial properties, minocycline has been found to have beneficial effects on inflammation, microglial activation, matrix metalloproteinases, nitric oxide production, and apoptotic cell death. Concordantly, minocycline has been found to have neuroprotective effects in animal models of a number of diseases including stroke, multiple sclerosis, and Parkinson disease. The proven safety of minocycline over decades of use as an antibiotic suggests that it may have potential for development into an effective treatment of multiple neurologic conditions in humans.

Minocycline worsens hypoxic-ischemic brain injury in a neonatal mouse model

Hypoxic-ischemic encephalopathy (HIE) is a leading cause of mortality and morbidity during the perinatal period, and currently no therapeutic drug is available. Minocycline, an antibiotic, has recently been shown to have neuroprotective effects distinct from its antimicrobial effect in several neurological disorders including ischemic brain injury. We examined the effect of minocycline on neonatal hypoxic-ischemic brain injury by using histologic scoring in both mouse and rat models. Mouse (C57Bl/6) and rat (SD) pups were exposed to a unilateral hypoxic-ischemic insult at 8 and 7 days of age, respectively. Minocycline hydrochloride was administered according to protocols that were reported to provide neuroprotection in adult or neonatal rats. Seven days after the insult, we examined brain injury in Nissl stained sections. Although minocycline ameliorated brain injury in the developing rat, it increased injury in the developing mouse. This detrimental effect in the mouse was consistent across different regions (cortex, striatum, and thalamus), with both single and multiple injection protocols and with both moderate and high-dose treatment (P < 0.05). The mechanism of the contrasting effects in mouse and rat is not clear and remains to be elucidated. Minocycline has been used as an antibiotic in the clinical setting for decades; therefore, it may be considered for use in infants with hypoxic-ischemic brain damage, based on prior reports of neuroprotection in the rat. However, it is important to examine this drug carefully before clinical use in human infants, taking our data in the mouse model into consideration.

Minocycline protects PC12 cells from ischemic-like injury and inhibits 5-lipoxygenase activation

Minocycline protects animals against cerebral ischemia by inhibiting inflammation. To determine whether minocycline protects PC12 cells from in vitro ischemic-like injury and affects pro-inflammatory 5-lipoxygenase activation, the cell viability and 5-lipoxygenase translocation to nuclear membrane were observed after oxygen-glucose deprivation (OGD). We found that OGD reduced cell viability, which was attenuated by minocycline and 5-lipoxygenase inhibitor caffeic acid. 5-Lipoxygenase protein was detected in PC12 cells by immunohistochemical and Western blot analyses. OGD induced 5-lipoxygenase translocation to nuclear membranes, which was abolished by minocycline and caffeic acid. Thus, minocycline can protect PC12 cells from in vitro ischemic-like injury, and this effect may be partly related to the inhibition of 5-lipoxygenase activation.

Minocycline reduces the lipopolysaccharide-induced inflammatory reaction, peroxynitrite-mediated nitration of proteins, disruption of the blood-brain barrier, and damage in the nigral dopaminergic system

We have evaluated the potential neuroprotectant activity of minocycline in an animal model of Parkinson's disease induced by intranigral injection of lipopolysaccharide. Minocycline treatment was very effective in protecting number of nigral dopaminergic neurons and loss of reactive astrocytes at 7 days postlesion. Evaluation of microglia revealed that minocycline treatment highly prevented the lipopolysaccharide-induced activation of reactive microglia as visualized by OX-42 and OX-6 immunohistochemistry. Short-term RT-PCR analysis demonstrated that minocycline partially prevented the lipopolysaccharide-induced increases of mRNA levels for interleukin-1alpha and tumor necrosis factor-alpha. In addition, lipopolysaccharide highly induced protein nitration as seen by 3-nitrotyrosine immunoreactivity in the ventral mesencephalon. Minocycline treatment strongly diminished the extent of 3-nitrotyrosine immunoreactivity. We also found a direct correlation between location of IgG immunoreactivity-a marker of blood-brain barrier disruption-and neurodegenerative processes including death of nigral dopaminergic cells and reactive astrocytes. There was also a precise spatial correlation between disruption of blood-brain barrier and 3-nitrotyrosine immunoreactivity. We discuss potential involvement of lipopolysaccharide-induced formation of peroxynitrites and cytokines in the pathological events in substantia nigra in response to inflammation. If inflammation is proved to be involved in the ethiopathology of Parkinson's disease, our data support the use of minocycline in parkinsonian patients.

Optimal delivery of minocycline to the brain: implication for human studies of acute neuroprotection

Minocycline is currently under development as a neuroprotective agent in many different brain diseases. In more than a dozen experimental investigations in various models of brain injury, high doses of minocycline have been administered intraperitoneally. This report details new concerns with this route of administration and makes a case for intravenous dosing in experimental animals, particularly for acute neuroprotection, to optimize delivery to the brain and facilitate translation to human studies.

Minocycline up-regulates Bcl-2 and protects against cell death in mitochondria

Robust neuroprotective effects have been shown for minocycline. Whether it also protects nonneuronal cells or tissues is unknown. More importantly, the mechanisms of minocycline protection appear multifaceted and remain to be clarified. Here we show that minocycline can protect kidney epithelial cells in vitro and protect the kidneys from ischemic injury in vivo. We further show that Bcl-2 is a key molecular determinant of minocycline protection. Minocycline protected kidney epithelial cells against apoptosis induced by hypoxia, azide, cisplatin, and staurosporine. The protection occurred at mitochondria, involving the suppression of Bax accumulation, outer membrane damage, and cytochrome c release. Minocycline induced Bcl-2, which accumulated in mitochondria and interacted with death-promoting molecules including Bax, Bak, and Bid. Down-regulation of Bcl-2 by specific antisense oligonucleotides abolished the cytoprotective effects of minocycline. Thus, minocycline can protect neuronal as well as nonneuronal cells and tissues. One mechanism for minocycline protection involves the induction of Bcl-2, an antiapoptotic protein.

Efficient three-drug cocktail for disease induced by mutant superoxide dismutase

There is currently no effective pharmacological treatment for amyotrophic lateral sclerosis (ALS). Because evidence suggests that multiple pathways may contribute to ALS pathogenesis, we tested in a mouse model of ALS (SOD1(G37R) mice) a combination approach consisting of three drugs for distinct targets in the complex pathway to neuronal death: minocycline, an antimicrobial agent that inhibits microglial activation, riluzole, a glutamate antagonist, and nimodipine, a voltage-gated calcium channel blocker. The efficacy of this three-drug cocktail was remarkable when administered in the diet from late presymptomatic stage (8-9 months). It delayed the onset of disease, slowed the loss of muscle strength, and increased the average longevity of SOD1(G37R) mice by 6 weeks. The protective effect of the treatment was corroborated by the reduced immunodetection signals for markers of gliosis and neurodegeneration in the spinal cord of SOD1(G37R) mice. These results indicate that such three-drug combination may represent an effective strategy for ALS treatment.

MAPKs: new targets for neurodegeneration

Neurodegenerative diseases remain a huge unmet pharmaceutical need. For some diseases such as Parkinson's disease, there are currently only palliative therapies, and for others such as Alzheimer's disease there are no proven therapies on the market that have any significant impact on disease progression. Recent work has suggested that cell death may play a key role in a number of neurodegenerative diseases, and halting this aberrant cell death may halt disease progression. Kinases identified in cell death pathways may be attractive targets for neurodegenerative diseases. In this review, the authors will focus on three families of related mitogen-activated protein kinases (MAPKs), namely, the extracellular signal-regulated protein kinases (ERKs), the c-Jun N-terminal kinases (JNKs) and the p38 MAPKs. The evidence for activation of each of these pathways in disease states and in models of neurodegenerative disorders will be examined. Effects of inhibitors, where available, will be discussed, and potential problems and side effects of kinase inhibitors as therapeutics will be addressed.

Additive neuroprotective effects of minocycline with creatine in a mouse model of ALS

The known neuroprotective effects of minocycline and creatine in animal models of amyotrophic lateral sclerosis (ALS) led us to examine whether the combination of these agents would result in increased neuroprotection. As previously reported, we confirmed in ALS mice that either minocycline or creatine treatment results in improvement in motor performance and extended survival. We report that combination of minocycline and creatine resulted in additive neuroprotection, suggesting this to be a novel potential strategy for the treatment of ALS. To our knowledge, this is the first report demonstrating additive neuroprotection of a combinatorial approach in a mouse model of ALS. Adding relevancy to our findings, minocycline and creatine, are relatively safe, cross the blood-brain barrier, and are currently available for human evaluation.

Minocycline Slows Disease Progression in a Mouse Model of Amyotrophic Lateral Sclerosis

There is currently no effective pharmacological treatment for amyotrophic lateral sclerosis (ALS). Because recent evidence suggests that secondary inflammation and caspase activation may contribute to neurodegeneration in ALS, we tested the effects of minocycline, a second-generation tetracycline with anti-inflammatory properties, in mice expressing a mutant superoxide dismutase (SOD1(G37R)) linked to human ALS. Administration of minocycline into the diet, beginning at late presymptomatic stage (7 or 9 months of age), delayed the onset of motor neuron degeneration, muscle strength decline, and it increased the longevity of SOD1(G37R) mice by approximately 5 weeks for approximately 70% of tested mice. Moreover, less activation of microglia was detected at early symptomatic stage (46 weeks) and at the end stage of disease in the spinal cord of SOD1(G37R) mice treated with minocycline. These results indicate that minocycline, which is clinically well tolerated, may represent a novel and effective drug for treatment of ALS.

Minocycline markedly protects the neonatal brain against hypoxic-ischemic injury

Hypoxic-ischemic brain injury in the perinatal period is a major cause of morbidity and mortality. Presently, there are no proven effective therapies with which to safeguard the human neonatal brain against this type of injury. Minocycline, a semisynthetic tetracycline, has been shown to be neuroprotective in certain adult ischemic injury/stroke and neurodegenerative disease models. However, minocycline's neuroprotective effects have not been assessed after insults to the neonatal brain. We now report that minocycline administered either immediately before or immediately after a hypoxic-ischemic insult substantially blocks tissue damage in a rodent model of neonatal hypoxic-ischemic brain injury. Minocycline treatment prevents the formation of activated caspase-3, a known effector of apoptosis, as well as the appearance of a calpain cleaved substrate, a marker of excitotoxic/necrotic cell death. To our knowledge, this is the first report of a systemic treatment that can be administered after a hypoxic-ischemic insult, which provides robust, nearly complete neuroprotection to the developing brain. Our data suggest that minocycline or a related neuroprotective tetracycline may be a candidate to consider in human clinical trials to protect the developing brain against hypoxic-ischemic-induced damage.

Minocycline inhibits cytochrome c release and delays progression of amyotrophic lateral sclerosis in mice

Minocycline mediates neuroprotection in experimental models of neurodegeneration. It inhibits the activity of caspase-1, caspase-3, inducible form of nitric oxide synthetase (iNOS) and p38 mitogen-activated protein kinase (MAPK). Although minocycline does not directly inhibit these enzymes, the effects may result from interference with upstream mechanisms resulting in their secondary activation. Because the above-mentioned factors are important in amyotrophic lateral sclerosis (ALS), we tested minocycline in mice with ALS. Here we report that minocycline delays disease onset and extends survival in ALS mice. Given the broad efficacy of minocycline, understanding its mechanisms of action is of great importance. We find that minocycline inhibits mitochondrial permeability-transition-mediated cytochrome c release. Minocycline-mediated inhibition of cytochrome c release is demonstrated in vivo, in cells, and in isolated mitochondria. Understanding the mechanism of action of minocycline will assist in the development and testing of more powerful and effective analogues. Because of the safety record of minocycline, and its ability to penetrate the blood-brain barrier, this drug may be a novel therapy for ALS.

Neuronal survival and cell death signaling pathways

Neuronal viability is maintained through a complex interacting network of signaling pathways that can be perturbed in response to a multitude of cellular stresses. A shift in the balance of signaling pathways after stress or in response to pathology can have drastic consequences for the function or the fate of a neuron. There is significant evidence that acutely injured and degenerating neurons may die by an active mechanism of cell death. This process involves the activation of discrete signaling pathways that ultimately compromise mitochondrial structure, energy metabolism and nuclear integrity. In this review we examine recent evidence pertaining to the presence and activation of anti- and pro-cell death regulatory pathways in nervous system injury and degeneration.

Caspases in Huntington's disease

Huntington's disease (HD) is an autosomal dominant condition, resulting from a mutation in huntingtin (htt). Htt is a novel protein, and its normal function is at present not well understood. Nuclear translocation of mutant htt in vitro up-regulates expression of the cell death gene caspase-1. We have demonstrated in a transgenic HD mouse model that caspase-1 and caspase-3 are transcriptionally up-regulated and activated. Underscoring the relevancy of these findings, recent results suggest that caspase-1 is activated in brains of humans with HD. Caspase activation results in the proteolytic cleavage of key cellular targets, including htt, leading to cell dysfunction. Caspase activation leading to cell dysfunction and death correlates with disease progression. In HD-transgenic mice, caspase inhibition resulted in a delayed onset of symptoms, a slowed progression, and prolonged survival. Caspase inhibition is a therapeutic strategy that merits evaluation in humans with HD.