Matrix metalloprotease-9 inhibition improves amyloid beta-mediated cognitive impairment and neurotoxicity in mice

Plasma MMP-9/TIMP-1 ratio serves as a novel potential biomarker in Alzheimer's disease

This study aimed to explore the diagnostic potential of plasma MMP-9, TIMP-1 and MMP-9/TIMP-1 ratio for Alzheimer's disease (AD). This retrospective study was performed in a cohort consisting of patients with AD (AD group) and cognitive normal subjects (HC group). Cerebrospinal fluid (CSF) classic biomarkers including Aβ42, Aβ40, total tau (t-tau), and phosphorylated tau (p-tau) levels, and plasma MMP-9 and TIMP-1 levels were measured by commercially available ELISA kits, respectively. The differential diagnostic potential of plasma MMP-9, TIMP-1 and MMP-9/TIMP-1 ratio was evaluated using the receiver operating characteristic curve analysis. It was observed that significantly elevated levels of plasma MMP-9 and MMP-9/TIMP-1 ratio in patients with AD than HC. Both MMP-9 and MMP-9/TIMP-1 ratios were negatively correlated with CSF Aβ42/Aβ40 ratio and positively correlated with CSF p-tau in AD group. ROC curve analysis showed better diagnostic accuracy of MMP-9/TIMP-1 ratio than MMP-9 for AD at a cutoff value of 1.35 with an area under the curve of 0.906 and sensitivity and specificity of 95.8% and 75%, respectively. Our findings encourage the use of plasma MMP-9/TIMP-1 ratio as a biomarker in the diagnosis of AD.

The Role of Extracellular Matrix in Human Neurodegenerative Diseases

The dense neuropil of the central nervous system leaves only limited space for extracellular substances free. The advent of immunohistochemistry, soon followed by advanced diagnostic tools, enabled us to explore the biochemical heterogeneity and compartmentalization of the brain extracellular matrix in exploratory and clinical research alike. The composition of the extracellular matrix is critical to shape neuronal function; changes in its assembly trigger or reflect brain/spinal cord malfunction. In this study, we focus on extracellular matrix changes in neurodegenerative disorders. We summarize its phenotypic appearance and biochemical characteristics, as well as the major enzymes which regulate and remodel matrix establishment in disease. The specifically built basement membrane of the central nervous system, perineuronal nets and perisynaptic axonal coats can protect neurons from toxic agents, and biochemical analysis revealed how the individual glycosaminoglycan and proteoglycan components interact with these molecules. Depending on the site, type and progress of the disease, select matrix components can either proactively trigger the formation of disease-specific harmful products, or reactively accumulate, likely to reduce tissue breakdown and neuronal loss. We review the diagnostic use and the increasing importance of medical screening of extracellular matrix components, especially enzymes, which informs us about disease status and, better yet, allows us to forecast illness.

Tau Protein Modulates Perineuronal Extracellular Matrix Expression in the TauP301L-acan Mouse Model

Tau mutations promote the formation of tau oligomers and filaments, which are neuropathological signs of several tau-associated dementias. Types of neurons in the CNS are spared of tau pathology and are surrounded by a specialized form of extracellular matrix; called perineuronal nets (PNs). Aggrecan, the major PN proteoglycans, is suggested to mediate PNs neuroprotective function by forming an external shield preventing the internalization of misfolded tau. We recently demonstrated a correlation between aggrecan amount and the expression and phosphorylation of tau in a TauP310L-acan mouse model, generated by crossbreeding heterozygous aggrecan mice with a significant reduction of aggrecan and homozygous TauP301L mice. Neurodegenerative processes have been associated with changes of PN structure and protein signature. In this study, we hypothesized that the structure and protein expression of PNs in this TauP310L-acan mouse is regulated by tau. Immunohistochemical and biochemical analyses demonstrate that protein levels of PN components differ between TauP301L^HET-acan^WT and TauP301L^HET-acan^HET mice, accompanied by changes in the expression of protein phosphatase 2 A. In addition, tau can modulate PN components such as brevican. Co-immunoprecipitation experiments revealed a physical connection between PN components and tau. These data demonstrate a complex, mutual interrelation of tau and the proteoglycans of the PN.

Role of Lipocalin-2 in Amyloid-Beta Oligomer-Induced Mouse Model of Alzheimer's Disease

Lipocalin-2 (LCN2) is an inflammatory protein with diverse functions in the brain. Although many studies have investigated the mechanism of LCN2 in brain injuries, the effect of LCN2 on amyloid-toxicity-related memory deficits in a mouse model of Alzheimer’s disease (AD) has been less studied. We investigated the role of LCN2 in human AD patients using a mouse model of AD. We created an AD mouse model by injecting amyloid-beta oligomer (AβO) into the hippocampus. In this model, animals exhibited impaired learning and memory. We found LCN2 upregulation in the human brain frontal lobe, as well as a positive correlation between white matter ischemic changes and serum LCN2. We also found increased astrocytic LCN2, microglia activation, iron accumulation, and blood–brain barrier disruption in AβO-treated hippocampi. These findings suggest that LCN2 is involved in a variety of amyloid toxicity mechanisms, especially neuroinflammation and oxidative stress.

MMP9 modulation improves specific neurobehavioral deficits in a mouse model of Alzheimer's disease

BACKGROUND

Matrix metallopeptidase 9 (MMP9) has been implicated in a variety of neurological disorders, including Alzheimer's disease (AD), where MMP9 levels are elevated in the brain and cerebrovasculature. Previously our group demonstrated apolipoprotein E4 (apoE4) was less efficient in regulating MMP9 activity in the brain than other apoE isoforms, and that MMP9 inhibition facilitated beta-amyloid (Aβ) elimination across the blood-brain barrier (BBB) METHODS: In the current studies, we evaluated the impact of MMP9 modulation on Aβ disposition and neurobehavior in AD using two approaches, (1) pharmacological inhibition of MMP9 with SB-3CT in apoE4 x AD (E4FAD) mice, and (2) gene deletion of MMP9 in AD mice (MMP9KO/5xFAD) RESULTS: Treatment with the MMP9 inhibitor SB-3CT in E4FAD mice led to reduced anxiety compared to placebo using the elevated plus maze. Deletion of the MMP9 gene in 5xFAD mice also reduced anxiety using the open field test, in addition to improving sociability and social recognition memory, particularly in male mice, as assessed through the three-chamber task, indicating certain behavioral alterations in AD may be mediated by MMP9. However, neither pharmacological inhibition of MMP9 or gene deletion of MMP9 affected spatial learning or memory in the AD animals, as determined through the radial arm water maze. Moreover, the effect of MMP9 modulation on AD neurobehavior was not due to changes in Aβ disposition, as both brain and plasma Aβ levels were unchanged in the SB-3CT-treated E4FAD animals and MMP9KO/AD mice compared to their respective controls.

CONCLUSIONS

In total, while MMP9 inhibition did improve specific neurobehavioral deficits associated with AD, such as anxiety and social recognition memory, modulation of MMP9 did not alter spatial learning and memory or Aβ tissue levels in AD animals. While targeting MMP9 may represent a therapeutic strategy to mitigate aspects of neurobehavioral decline in AD, further work is necessary to understand the nature of the relationship between MMP9 activity and neurological dysfunction.

GPR18 drives FAAH inhibition-induced neuroprotection against HIV-1 Tat-induced neurodegeneration

Human immunodeficiency virus type 1 (HIV-1) is known to provoke microglial immune responses which likely play a paramount role in the development of chronic neuroinflammatory conditions and neuronal damage related to HIV-1 associated neurocognitive disorders (HAND). In particular, HIV-1 Tat protein is a proinflammatory neurotoxin which predisposes neurons to synaptodendritic injury. Drugs targeting the degradative enzymes of endogenous cannabinoids have shown promise in reducing inflammation with minimal side effects in rodent models. Considering that markers of neuroinflammation can predict the extent of neuronal injury in HAND patients, we evaluated the neurotoxic effect of HIV-1 Tat-exposed microglia following blockade of fatty acid amid hydrolyze (FAAH), a catabolic enzyme responsible for degradation of endocannabinoids, e.g. anandamide (AEA). In the present study, cultured murine microglia were incubated with Tat and/or a FAAH inhibitor (PF3845). After 24 h, cells were imaged for morphological analysis and microglial conditioned media (MCM) was collected. Frontal cortex neuron cultures (DIV 7–11) were then exposed to MCM, and neurotoxicity was assessed via live cell calcium imaging and staining of actin positive dendritic structures. Results demonstrate a strong attenuation of microglial responses to Tat by PF3845 pretreatment, which is indicated by 1) microglial changes in morphology to a less proinflammatory phenotype using fractal analysis, 2) a decrease in release of neurotoxic cytokines/chemokines (MCP-1/CCL2) and matrix metalloproteinases (MMPs; MMP-9) using ELISA/multiplex assays, and 3) enhanced production of endocannabinoids (AEA) using LC/MS/MS. Additionally, PF3845’s effects on Tat-induced microglial-mediated neurotoxicity, decreased dysregulation of neuronal intracellular calcium and prevented the loss of actin-positive staining and punctate structure in frontal cortex neuron cultures. Interestingly, these observed neuroprotective effects appeared to be independent of cannabinoid receptor activity (CB₁R & CB₂R). We found that a purported GPR18 antagonist, CID-85469571, blocked the neuroprotective effects of PF3845 in all experiments. Collectively, these experiments increase understanding of the role of FAAH inhibition and Tat in mediating microglial neurotoxicity in the HAND condition.

Endogenous Mechanisms of Neuroprotection: To Boost or Not to Boost

Postmitotic cells, like neurons, must live through a lifetime. For this reason, organisms/cells have evolved with self-repair mechanisms that allow them to have a long life. The discovery workflow of neuroprotectors during the last years has focused on blocking the pathophysiological mechanisms that lead to neuronal loss in neurodegeneration. Unfortunately, only a few strategies from these studies were able to slow down or prevent neurodegeneration. There is compelling evidence demonstrating that endorsing the self-healing mechanisms that organisms/cells endogenously have, commonly referred to as cellular resilience, can arm neurons and promote their self-healing. Although enhancing these mechanisms has not yet received sufficient attention, these pathways open up new therapeutic avenues to prevent neuronal death and ameliorate neurodegeneration. Here, we highlight the main endogenous mechanisms of protection and describe their role in promoting neuron survival during neurodegeneration.

Multifaceted Role of Matrix Metalloproteinases in Neurodegenerative Diseases: Pathophysiological and Therapeutic Perspectives

Neurodegeneration is the pathological condition, in which the nervous system or neuron loses its structure, function, or both, leading to progressive degeneration or the death of neurons, and well-defined associations of tissue system, resulting in clinical manifestations. Neuroinflammation has been shown to precede neurodegeneration in several neurodegenerative diseases (NDs). No drug is yet known to delay or treat neurodegeneration. Although the etiology and potential causes of NDs remain widely indefinable, matrix metalloproteinases (MMPs) evidently have a crucial role in the progression of NDs. MMPs, a protein family of zinc (Zn²⁺)-containing endopeptidases, are pivotal agents that are involved in various biological and pathological processes in the central nervous system (CNS). The current review delineates the several emerging evidence demonstrating the effects of MMPs in the progression of NDs, wherein they regulate several processes, such as (neuro)inflammation, microglial activation, amyloid peptide degradation, blood brain barrier (BBB) disruption, dopaminergic apoptosis, and α-synuclein modulation, leading to neurotoxicity and neuron death. Published papers to date were searched via PubMed, MEDLINE, etc., while using selective keywords highlighted in our manuscript. We also aim to shed a light on pathophysiological effect of MMPs in the CNS and focus our attention on its detrimental and beneficial effects in NDs, with a special focus on Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), Alzheimer's disease (AD), multiple sclerosis (MS), and Huntington's disease (HD), and discussed various therapeutic strategies targeting MMPs, which could serve as potential modulators in NDs. Over time, several agents have been developed in order to overcome challenges and open up the possibilities for making selective modulators of MMPs to decipher the multifaceted functions of MMPs in NDs. There is still a greater need to explore them in clinics.

Apolipoprotein E isoforms differentially regulate matrix metallopeptidase 9 function in Alzheimer's disease

Apolipoprotein E (APOE) has been shown to influence amyloid-β (Aβ) clearance from the brain in an isoform-specific manner. Our prior work showed that Aβ transit across the blood-brain-barrier was reduced by apoE4, compared to other apoE isoforms, due to elevated lipoprotein receptor shedding in brain endothelia. Recently, we demonstrated that matrix metallopeptidase 9 (MMP-9) induces lipoprotein receptor proteolysis in an apoE isoform-dependent manner, which impacts Aβ elimination from the brain. The current studies interrogated the relationship between apoE and MMP-9 and found that apoE impacted proMMP-9 cellular secretion from brain endothelia (apoE2 < apoE3 = apoE4). In a cell-free assay, apoE dose-dependently reduced MMP-9 activity, with apoE4 showing a significantly weaker ability to inhibit MMP-9 function than apoE2 or apoE3. Finally, we observed elevated MMP-9 expression and activity in the cerebrovasculature of both human and animal AD brain specimens with an APOE4 genotype. Collectively, these findings suggest a role for apoE in regulating MMP-9 disposition and may describe the effect of apoE4 on Aβ pathology in the AD brain.

Influence of plasma matrix metalloproteinase levels on longitudinal changes in Alzheimer's disease (AD) biomarkers and cognitive function in patients with mild cognitive impairment due to AD registered in the Alzheimer's Disease Neuroimaging Initiative database

OBJECTIVE

The present study investigated the effects of plasma matrix metalloproteinases (MMPs) on longitudinal changes in Alzheimer's disease (AD)-related biomarkers in cerebrospinal fluid (CSF), brain atrophy, and cognitive function in patients with mild cognitive impairment due to AD (MCI-AD).

METHODS

We used data from the Alzheimer's Disease Neuroimaging Initiative database. We included 95 ApoE4-positive patients with MCI-AD who were confirmed to have low Aβ₄₂ and/or high phosphorylated-tau (p-tau) in CSF. We obtained baseline demographic data, plasma MMP levels, including MMP-1, -2, -7, -9, -10, and tissue inhibitor of MMP-1 (TIMP-1), longitudinal annual data on Aβ₄₂, total tau, and p-tau in CSF, MRI-measured hippocampal volumes, and cognitive function evaluated by the Mini-Mental State Examination (MMSE) and AD Assessment Scale-11 (ADAS-11) over 4 years. We examined the effects of baseline MMP levels on longitudinal changes in CSF AD biomarkers, hippocampal volumes, and cognitive function using a linear mixed regression analysis.

RESULTS

No significant differences were observed in baseline plasma MMP levels between MCI-AD patients and control subjects, except for MMP-10, which was significantly lower in MCI-AD than in controls. The baseline levels of MMPs did not correlate with longitudinal changes in CSF biomarkers. Declines in hippocampal volumes and cognitive function evaluated by MMSE and ADAS-11 were significantly faster in MCI-AD patients with high-MMP-9 levels at baseline than in those with middle and low MMP-9 levels at baseline.

CONCLUSION

High plasma MMP-9 levels in MCI-AD patients might enhance neurodegeneration and cognitive decline.

Metalloproteinases and Their Inhibitors: Potential for the Development of New Therapeutics

The metalloproteinase (MP) family of zinc-dependent proteases, including matrix metalloproteinases (MMPs), a disintegrin and metalloproteases (ADAMs), and a disintegrin and metalloproteinase with thrombospondin motifs (ADAMTSs) plays a crucial role in the extracellular matrix (ECM) remodeling and degradation activities. A wide range of substrates of the MP family includes ECM components, chemokines, cell receptors, and growth factors. Metalloproteinases activities are tightly regulated by proteolytic activation and inhibition via their natural inhibitors, tissue inhibitors of metalloproteinases (TIMPs), and the imbalance of the activation and inhibition is responsible in progression or inhibition of several diseases, e.g., cancer, neurological disorders, and cardiovascular diseases. We provide an overview of the structure, function, and the multifaceted role of MMPs, ADAMs, and TIMPs in several diseases via their cellular functions such as proteolysis of other cell signaling factors, degradation and remodeling of the ECM, and other essential protease-independent interactions in the ECM. The significance of MP inhibitors targeting specific MMP or ADAMs with high selectivity is also discussed. Recent advances and techniques used in developing novel MP inhibitors and MP responsive drug delivery tools are also reviewed.

Tryptophan metabolites modify brain Aβ peptide degradation: A role in Alzheimer's disease?

Among several processes, a decrease in amyloid-beta (Aβ) peptide elimination is thought to be one of the major pathophysiological factors in Alzheimer's disease (AD). Neprilysin (NEP) is a key metalloproteinase controlling the degradation and clearance of Aβ peptides in the brain. NEP is induced by several pharmacological substances, amyloid deposits and somatostatin, but the physiological regulation of its expression remains unclear. This situation hampers the exploitation of NEP regulatory factors/mechanisms to develop effective strategies against Aβ peptide accumulation-induced brain toxicity. Based on recent data aimed at elucidating this major question, the present paper addresses and critically discusses the role of 5-hydroxyindole-acetic acid (5-HIAA) and kynurenic acid (KYNA) in the regulation of NEP activity/expression in the brain. Both 5-HIAA and KYNA are endogenous metabolites of tryptophan, an essential amino-acid obtained through diet and gut microbiome. By interacting with the aryl hydrocarbon receptor, various tryptophan metabolites modulate several metalloproteinases regulating brain Aβ peptide levels under normal and pathological conditions such as AD. In particular, interesting data reviewed here show that 5-HIAA and KYNA stimulate NEP activity/expression to prevent Aβ peptide-induced neurotoxicity. These data open promising perspectives for the development of tryptophan metabolite-based therapies against AD.

Medroxyprogesterone Acetate Impairs Amyloid Beta Degradation in a Matrix Metalloproteinase-9 Dependent Manner

Despite the extensive use of hormonal methods as either contraception or menopausal hormone therapy (HT), there is very little known about the potential effects of these compounds on the cellular processes of the brain. Medroxyprogesterone Acetate (MPA) is a progestogen used globally in the hormonal contraceptive, Depo Provera, by women in their reproductive prime and is a major compound found in HT formulations used by menopausal women. MPA promotes changes in the circulating levels of matrix metalloproteinases (MMPs), such as MMP-9, in the endometrium, yet limited literature studying the effects of MPA on neurons and astroglia cells has been conducted. Additionally, the dysregulation of MMPs has been implicated in the pathology of Alzheimer's disease (AD), where inhibiting the secretion of MMP-9 from astroglia reduces the proteolytic degradation of amyloid-beta. Thus, we hypothesize that exposure to MPA disrupts proteolytic degradation of amyloid-beta through the downregulation of MMP-9 expression and subsequent secretion. To assess the effect of progestins on MMP-9 and amyloid-beta, in vitro, C6 rat glial cells were exposed to MPA for 48 h and then the enzymatic, secretory, and amyloid-beta degrading capacity of MMP-9 was assessed from the conditioned culture medium. We found that MPA treatment inhibited transcription of MMP-9, which resulted in a subsequent decrease in the production and secretion of MMP-9 protein, in part through the glucocorticoid receptor. Additionally, we investigated the consequences of amyloid beta-degrading activity and found that MPA treatment decreased proteolytic degradation of amyloid-beta. Our results suggest MPA suppresses amyloid-beta degradation in an MMP-9-dependent manner, in vitro, and potentially compromises the clearance of amyloid-beta in vivo.

Evaluation of Matrix Metalloproteinase-2 (MMP-2) and -9 (MMP-9) and Their Tissue Inhibitors (TIMP-1 and TIMP-2) in Plasma from Patients with Neurodegenerative Dementia

Matrix metalloproteinases (MMPs) are substantial regulators of learning and memory and might be involved in neurodegeneration. It is known that MMPs are involved in pathogenesis of Alzheimer's disease (AD) and are particularly involved in the amyloid-β processing pathway. However, information on circulating levels of these proteins and their tissue inhibitors (TIMPs) in AD and other neurodegenerative dementia (ND) diseases such as dementia with Lewy bodies (DLB) and frontotemporal dementia (FTD) is not clear. Therefore, this study was directed toward finding out how plasma levels of MMP-2, MMP-9, TIMP-1, and TIMP-2 vary in AD, DLB, and FTD; and investigating the correlation of the levels of MMPs and their inhibitors with clinical parameters of the patients. MMP-2, MMP-9, TIMP-1, and TIMP-2 levels were measured by enzyme linked immunosorbent assay (ELISA). Plasma MMP-2 levels were significantly lower in all the patient groups than in the age-matched healthy controls (HCs) (p < 0.05). MMP-9 levels were significantly lower in the FTD patients than in the HCs (p < 0.05). Also, TIMP-1 levels were lower in the AD and FTD patients than in the HCs (p < 0.05). TIMP-2 levels were similar in all the groups. These findings highlight the importance of circulating MMPs in ND and suggest that MMPs and their inhibitors might play a role in impaired amyloid-β peptide metabolism which is responsible for the genesis and progression of ND. Furthermore, measurement of MMP-2 and MMP-9 and their inhibitors may be of great importance for large scale basic research and clinical studies of ND.

The role of UCH-L1, MMP-9, and GFAP as peripheral markers of different susceptibility to seizure development in a preclinical model of epilepsy

In our study, we assessed the potency of the brain-derived proteins ubiquitin carboxy-terminal hydrolase L1 (UCH-L1), matrix metalloproteinase 9 (MMP-9), glial fibrillary acidic protein (GFAP) and the immune activation indicators interleukin 1β (IL-1β) and interleukin 6 (IL-6) as peripheral biomarkers of different susceptibilities to kindling in a preclinical model. We observed increased plasma UCH-L1 levels in kindled vs. control animals. Furthermore, MMP-9 and IL-1β concentrations were the lowest in rats resistant to kindling. In summary, UCH-L1 is an indicator of neuronal loss and BBB disruption after seizure. MMP-9 and IL-1β may indicate resistance to kindling. UCH-L1, MMP-9 and IL-1β may have utility as peripheral biomarkers with translational potency in the clinic.

A New Perspective in Utilizing MMP-9 as a Therapeutic Target for Alzheimer's Disease and Type 2 Diabetes Mellitus

Matrix metalloprotease 9 (MMP-9) is a 92 kDa type IV collagenase and a member of the family of endopeptidases. MMP-9 is involved in the degradation of extracellular matrix components, tissue remodeling, cellular receptor stripping, and processing of various signaling molecules. In the CNS, the effects of MMP-9 are quite complex, since it exerts beneficial effects including neurogenesis, angiogenesis, myelogenesis, axonal growth, and inhibition of apoptosis, or destructive effects including apoptosis, blood-brain barrier disorder, and demyelination. Likewise, in the periphery, physiological events, as the involvement of MMP-9 in angiogenesis, for instance in wound healing, can be turned into pathological, such as in tumor metastasis, depending on the state of the organism. Alzheimer's disease is a neurodegenerative disorder, characterized by amyloid accumulation and deposition in the brain. Amyloidogenesis, however, also occurs in diseases of the periphery, such as type II diabetes mellitus, where an analogous type of amyloid, is deposited in the pancreas. Interestingly, both diseases exhibit similar pathology and disease progression, with insulin resistance being a major common denominator. Hence, combinatorial strategies searching new or existing molecules to apply for therapeutic use for both diseases are gaining momentum. MMP-9 is extensively studied due to its association with a variety of physiological and pathological processes. Consequently, meticulous design could render MMP-9 into a potential therapeutic target for Alzheimer's disease and type 2 diabetes mellitus; two seemingly unrelated diseases.

Influence of Matrix Metallopeptidase 9 on Beta-Amyloid Elimination Across the Blood-Brain Barrier

Lipoprotein receptor transport across the blood-brain barrier (BBB) mediates beta-amyloid (Aβ) accumulation in the brain and may be a contributing factor in Alzheimer's disease (AD) pathogenesis. Lipoprotein receptors are susceptible to proteolytic shedding at the cell surface, which precludes the endocytic transport of ligands. A ligand that closely interacts with the lipoprotein receptors is apolipoprotein E (apoE), which exists as three isoforms (apoE2, apoE3, apoE4). Our prior work showed an inverse relationship between lipoprotein receptor shedding and Aβ transport across the BBB, which was apoE-isoform dependent. To interrogate this further, the current studies investigated an enzyme implicated in lipoprotein receptor shedding, matrix metalloproteinase 9 (MMP9). Treatment with MMP9 dose-dependently elevated lipoprotein receptor shedding in brain endothelial cells and freshly isolated mouse cerebrovessels. Furthermore, treatment with a MMP9 inhibitor (SB-3CT) mitigated Aβ-induced lipoprotein receptor shedding in brain endothelial cells and the brains of apoE4 animals. In terms of BBB transit, SB-3CT treatment increased the transport of Aβ across an in vitro model of the BBB. In vivo, administration of SB-3CT to apoE4 animals significantly enhanced Aβ clearance from the brain to the periphery following intracranial administration of Aβ. The current studies show that MMP9 impacts lipoprotein receptor shedding and Aβ transit across the BBB, in an apoE  isoform-specific manner. In total, MMP9 inhibition can facilitate Aβ clearance across the BBB, which could be an effective approach to lowering Aβ levels in the brain and mitigating the AD phenotype, particularly in subjects carrying the apoE4 allele.

Differential protective effects of connective tissue growth factor against Aβ neurotoxicity on neurons and glia

Impaired clearance of amyloid-β peptide (Aβ) leads to abnormal extracellular accumulation of this neurotoxic protein that drives neurodegeneration in sporadic Alzheimer's disease (AD). Connective tissue growth factor (CTGF/CCN2) expression is elevated in plaque-surrounding astrocytes in AD patients. However, the role of CTGF in AD pathogenesis remains unclear. Here we characterized the neuroprotective activity of CTGF. We found that CTGF facilitated Aβ uptake and subsequent degradation within primary glia and neuroblastoma cells. CTGF enhanced extracellular Aβ degradation via membrane-bound matrix metalloproteinase-14 (MMP14) in glia and extracellular MMP13 in neurons. In the brain of a Drosophila AD model, glial-expression of CTGF reduced Aβ deposits, improved locomotor function, and rescued memory deficits. Neuroprotective potential of CTGF against Aβ42-induced photoreceptor degeneration was disrupted through silencing MMPs. Therefore, CTGF may represent a node for potential AD therapeutics as it intervenes in glia-neuron communication via specific MMPs to alleviate Aβ neurotoxicity in the central nervous system.

Altered Expression of Matrix Metalloproteinases and Their Endogenous Inhibitors in a Human Isogenic Stem Cell Model of Huntington's Disease

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder characterized by a progressive movement disorder, psychiatric symptoms, and cognitive impairments. HD is caused by a CAG repeat expansion encoding a stretch of polyglutamine residues in the N-terminus of mutant huntingtin (mHTT) protein. Proteolytic processing of mHTT yields toxic fragments, which cause neurotoxicity and massive neuronal cell death predominantly in the striatum and cortex. Inhibition of mHTT cleavage reduces neuronal toxicity suggesting mHTT proteolysis contributes to HD pathogenesis. A previously conducted unbiased siRNA screen in our lab for known human proteases identified matrix metalloproteinases (MMPs) as modifiers of mHTT proteolysis and toxicity. To further study MMP activation in HD, isogenic HD, and control corrected (C116) neural stem cells (NSCs) prepared from HD patient-derived induced pluripotent stem cells were used to examine the role of MMPs and their endogenous inhibitors in this highly relevant model system. We found altered expression of MMP-2 and MMP-9 (gelatinases), MMP-3/10, and MMP-14, activity in HD-NSCs when compared to control C116-NSCs. Dysregulation in MMP activity was accompanied with concomitant changes in levels of endogenous inhibitors of MMPs, called tissue inhibitors of matrix metalloproteinases (TIMPs). Specifically, we observed decreased levels of TIMP-1 and TIMP-2 in HD-NSCs, suggesting part of the altered expression and activity of MMPs is due to lower abundance of these endogenous inhibitors. Immunofluorescence analysis revealed increased MMP/TIMP localization in the nucleus or aggregates of HD-NSCs, suggesting potential interaction with mHTT. TIMP-1 was found to associate with mHTT aggregates in discrete punctate structures in HD-NSCs. These events collectively contribute to increased neurotoxicity in HD. Previous characterization of these NSCs revealed transforming growth factor beta (TGF-β) pathway as the top dysregulated pathway in HD. TGF-β was significantly upregulated in HD-NSCs and addition of TGF-β to HD-NSCs was found to be neuroprotective. To determine if TGF-β regulated MMP and TIMP activity, C116- and HD-NSCs were exogenously treated with recombinant TGF-β. TIMP-1 levels were found to be elevated in response to TGF-β treatment, representing a potential mechanism through which elevated TGF-β levels confer neuroprotection in HD. Studying the mechanism of action of MMPs and TIMPs, and their interactions with mHTT in human isogenic patient-derived NSCs elucidates new mechanisms of HD neurotoxicity and will likely provide novel therapeutics for treatment of HD.

Multifaceted Roles of Metzincins in CNS Physiology and Pathology: From Synaptic Plasticity and Cognition to Neurodegenerative Disorders

The extracellular matrix (ECM) and membrane proteolysis play a key role in structural and functional synaptic plasticity associated with development and learning. A growing body of evidence underscores the multifaceted role of members of the metzincin superfamily, including metalloproteinases (MMPs), A Disintegrin and Metalloproteinases (ADAMs), A Disintegrin and Metalloproteinase with Thrombospondin Motifs (ADAMTSs) and astacins in physiological and pathological processes in the central nervous system (CNS). The expression and activity of metzincins are strictly controlled at different levels (e.g., through the regulation of translation, limited activation in the extracellular space, the binding of endogenous inhibitors and interactions with other proteins). Thus, unsurprising is that the dysregulation of proteolytic activity, especially the greater expression and activation of metzincins, is associated with neurodegenerative disorders that are considered synaptopathies, especially Alzheimer's disease (AD). We review current knowledge of the functions of metzincins in the development of AD, mainly the proteolytic processing of amyloid precursor protein, the degradation of amyloid β (Aβ) peptide and several pathways for Aβ clearance across brain barriers (i.e., blood-brain barrier (BBB) and blood-cerebrospinal fluid barrier (BCSFB)) that contain specific receptors that mediate the uptake of Aβ peptide. Controlling the proteolytic activity of metzincins in Aβ-induced pathological changes in AD patients' brains may be a promising therapeutic strategy.

The attenuating effects of 1,2,3,4,6 penta-O-galloyl-β-d-glucose on inflammatory cytokines release from activated BV-2 microglial cells

Alzheimer's disease (AD) is the most common cause of neurodegeneration and dementia in the elderly. Dysregulated, chronic activation of microglia, the brain's resident macrophages, induces the release of excessive amounts of pro-inflammatory cytokines which has been implicated in the early stages of AD pathology. Therefore, suppressing the expression of these inflammatory mediators may decrease or delay the progression of AD. Many natural compounds derived from plants have shown anti-inflammatory activity. The naturally occurring 1,2,3,4,6 Penta-O-Galloyl-β-d-Glucose (PGG), is a polyphenolic compound highly enriched in Rhus chinensis Millplant. It is a potent anti-inflammatory agent that act through the inhibition of many cytokines in different experimental models. In the present study, we investigated the role of PGG as an anti-inflammatory agent in LPS/IFNγ activated BV-2 microglia cells. Mouse cytokine antibody arrays were used to assess the effect of PGG on the release of pro-inflammatory cytokines, and ELISA experiments were performed to validate the results from the arrays. The results obtained from the cytokine arrays, and ELISA assays showed that PGG decreased the expression of monocyte chemotactic protein-5 (MCP-5) 8-fold, and pro-matrix metalloproteinase 9 (Pro MMP-9) 10-fold. Both of these cytokines are upregulated during the inflammatory process and have been shown to be involved in brain injury, inflammation, and neurodegeneration. Therefore, these findings suggest that the anti-inflammatory effect of PGG on activated microglia involving the attenuation of MCP-5 and Pro MMP-9 cytokines.

NETosis in Alzheimer's Disease

Alzheimer’s disease (AD) is a neurodegenerative disorder characterized by the progressive deterioration of cognitive functions. Its neuropathological features include amyloid-β (Aβ) accumulation, the formation of neurofibrillary tangles, and the loss of neurons and synapses. Neuroinflammation is a well-established feature of AD pathogenesis, and a better understanding of its mechanisms could facilitate the development of new therapeutic approaches. Recent studies in transgenic mouse models of AD have shown that neutrophils adhere to blood vessels and migrate inside the parenchyma. Moreover, studies in human AD subjects have also shown that neutrophils adhere and spread inside brain vessels and invade the parenchyma, suggesting these cells play a role in AD pathogenesis. Indeed, neutrophil depletion and the therapeutic inhibition of neutrophil trafficking, achieved by blocking LFA-1 integrin in AD mouse models, significantly reduced memory loss and the neuropathological features of AD. We observed that neutrophils release neutrophil extracellular traps (NETs) inside blood vessels and in the parenchyma of AD mice, potentially harming the blood–brain barrier and neural cells. Furthermore, confocal microscopy confirmed the presence of NETs inside the cortical vessels and parenchyma of subjects with AD, providing more evidence that neutrophils and NETs play a role in AD-related tissue destruction. The discovery of NETs inside the AD brain suggests that these formations may exacerbate neuro-inflammatory processes, promoting vascular and parenchymal damage during AD. The inhibition of NET formation has achieved therapeutic benefits in several models of chronic inflammatory diseases, including autoimmune diseases affecting the brain. Therefore, the targeting of NETs may delay AD pathogenesis and offer a novel approach for the treatment of this increasingly prevalent disease.

Fingolimod Limits Acute Aβ Neurotoxicity and Promotes Synaptic Versus Extrasynaptic NMDA Receptor Functionality in Hippocampal Neurons

Fingolimod, also known as FTY720, is an analogue of the sphingolipid sphingosine, which has been proved to be neuroprotective in rodent models of Alzheimer’s disease (AD). Several cellular and molecular targets underlying the neuroprotective effects of FTY720 have been recently identified. However, whether the drug directly protects neurons from toxicity of amyloid-beta (Aβ) still remains poorly defined. Using a combination of biochemical assays, live imaging and electrophysiology we demonstrate that FTY720 induces a rapid increase in GLUN2A-containing neuroprotective NMDARs on the surface of dendritic spines in cultured hippocampal neurons. In addition, the drug mobilizes extrasynaptic GLUN2B-containing NMDARs, which are coupled to cell death, to the synapses. Altered ratio of synaptic/extrasynaptic NMDARs decreases calcium responsiveness of neurons to neurotoxic soluble Aβ 1–42 and renders neurons resistant to early alteration of calcium homeostasis. The fast defensive response of FTY720 occurs through a Sphingosine-1-phosphate receptor (S1P-R) -dependent mechanism, as it is lost in the presence of S1P-R1 and S1P-R3 antagonists. We propose that rapid synaptic relocation of NMDARs might have direct impact on amelioration of cognitive performance in transgenic APPswe/PS1dE9 AD mice upon sub-chronic treatment with FTY720.

The Infectious Etiology of Alzheimer's Disease

BACKGROUND

Inflammation is a part of the first line of defense of the body against invasive pathogens, and plays a crucial role in tissue regeneration and repair. A proper inflammatory response ensures the suitable resolution of inflammation and elimination of harmful stimuli, but when the inflammatory reactions are inappropriate it can lead to damage of the surrounding normal cells. The relationship between infections and Alzheimer's Disease (AD) etiology, especially lateonset AD (LOAD) has been continuously debated over the past three decades.

METHODS

This review discusses whether infections could be a causative factor that promotes the progression of AD and summarizes recent investigations associating infectious agents and chronic inflammation with AD. Preventive and therapeutic approaches to AD in the context of an infectious etiology of the disease are also discussed.

RESULTS

Emerging evidence supports the hypothesis of the role of neurotropic viruses from the Herpesviridae family, especially Human herpesvirus 1 (HHV-1), Cytomegalovirus (CMV), and Human herpesvirus 2 (HHV-2), in AD neuropathology. Recent investigations also indicate the association between Hepatitis C virus (HCV) infection and dementia. Among bacteria special attention is focused on spirochetes family and on periodontal pathogens such as Porphyromonas gingivalis or Treponema denticola that could cause chronic periodontitis and possibly contribute to the clinical onset of AD.

CONCLUSION

Chronic viral, bacterial and fungal infections might be causative factors for the inflammatory pathway in AD.

Vascular inter-regulation of inflammation: molecular and cellular targets for CNS therapy

Angiogenesis and inflammation are clearly interconnected and interdependent processes that are dysregulated in a series of systemic and brain pathologies. Herein, key aspects regarding endothelial cell function and tissue remodelling that are particularly affected or aggravated by inflammation are presented. Most importantly, the cellular and molecular mechanisms involved in the vascular regulation of the inflammatory processes occurring in several brain disorders and how they impact on disease/injury progression are detailed, highlighting potential targets for therapy. Finally, nanomedicine-based approaches designed to overcome limitations pertaining to low systemic bioavailability, light, pH and temperature sensitivity and/or rapid degradation of these targets, and to optimize their mode of action are discussed. Ultimately, we expect this review to provide new insight and to suggest novel approaches for the treatment of blood-brain barrier dysfunction per se or as a means to treat the injured or diseased central nervous system.

Non-amyloidogenic effects of α2 adrenergic agonists: implications for brimonidine-mediated neuroprotection

The amyloid beta (Aβ) pathway is strongly implicated in neurodegenerative conditions such as Alzheimer's disease and more recently, glaucoma. Here, we identify the α2 adrenergic receptor agonists (α2ARA) used to lower intraocular pressure can prevent retinal ganglion cell (RGC) death via the non-amyloidogenic Aβ-pathway. Neuroprotective effects were confirmed in vivo and in vitro in different glaucoma-related models using α2ARAs brimonidine (BMD), clonidine (Clo) and dexmedetomidine. α2ARA treatment significantly reduced RGC apoptosis in experimental-glaucoma models by 97.7% and 92.8% (BMD, P<0.01) and 98% and 92.3% (Clo, P<0.01)) at 3 and 8 weeks, respectively. A reduction was seen in an experimental Aβ-induced neurotoxicity model (67% BMD and 88.6% Clo, both P<0.01, respectively), and in vitro, where α2ARAs significantly (P<0.05) prevented cell death, under both hypoxic (CoCl₂) and stress (UV) conditions. In experimental-glaucoma, BMD induced ninefold and 25-fold and 36-fold and fourfold reductions in Aβ and amyloid precursor protein (APP) levels at 3 and 8 weeks, respectively, in the RGC layer, with similar results with Clo, and in vitro with all three α2ARAs. BMD significantly increased soluble APPα (sAPPα) levels at 3 and 8 weeks (2.1 and 1.6-fold) in vivo and in vitro with the CoCl₂ and UV-light insults. Furthermore, treatment of UV-insulted cells with an sAPPα antibody significantly reduced cell viability compared with BMD-treated control (52%), co-treatment (33%) and untreated control (27%). Finally, we show that α2ARAs modulate levels of laminin and MMP-9 in RGCs, potentially linked to changes in Aβ through APP processing. Together, these results provide new evidence that α2ARAs are neuroprotective through their effects on the Aβ pathway and sAPPα, which to our knowledge, is the first description. Studies have identified the need for α-secretase activators and sAPPα-mimetics in neurodegeneration; α2ARAs, already clinically available, present a promising therapy, with applications not only to reducing RGC death in glaucoma but also other neurodegenerative processes involving Aβ.

The role of MMP genes in recurrent depressive disorders and cognitive functions

OBJECTIVE

Among the 28 metalloproteinases described so far, 23 can be found in the human organism, but only few are expressed in the human brain. The main objective of this study was to analyse the relationship between MMP-2, MMP-9 and TIMP-2 gene expression and cognitive performance.

METHODS

The study comprised 234 subjects: patients suffering from recurrent depressive disorder (rDD, n=139) and healthy subjects (HS, n=95). The cognitive function assessment was carried out with the help of the following tests: Trail Making Test, The Stroop Test, Verbal Fluency Test and Auditory Verbal Learning Test. Gene expression on the mRNA and protein level was evaluated for MMP-2, MMP-9 and TIMP-2 in both groups using RNA extraction, reverse transcription and enzyme-linked immunosorbent assay.

RESULTS

Both mRNA and protein expression levels of all the genes were significantly lower in rDD subjects as compared with HS. Having analysed the entire experimental group (N=234), significant interrelations were found between the expression of the analysed genes and the results of the tests used to measure cognitive functions. Increased expression on both the mRNA and the protein level was associated in each case with better performance of all the tests conducted. After carrying out a separate analysis on the people from the rDD group and the HS group, similar dependencies were still observed.

CONCLUSIONS

The results of our study show decreased expression of MMP-2, MMP-9 and TIMP-2 genes on both mRNA and protein levels in depression. Elevated expression of MMP-2, MMP-9, TIMP-2 positively affects cognitive efficiency: working memory, executive functions, attention functions, direct and delayed auditory-verbal memory, the effectiveness of learning processes and verbal fluency. The study highlights the important role of peripheral matrix metalloproteinases genes in depression and cognitive functions.

Tong Luo Jiu Nao, a Chinese Medicine Formula, Reduces Inflammatory Stress in a Mouse Model of Alzheimer's Disease

AIM

The aim of this study is to extend the molecular mechanism of Tong Luo Jiu Nao (TLJN) for Alzheimer's disease (AD), which is a modern Chinese formula that has been used to treat AD.

METHODS

The senescence-accelerated mouse prone 8 strain (SAMP8) is one of the most appropriate models to study the mechanism that underlies AD. The levels of plasma amyloid β (Aβ) and the Aβ deposits were measured using enzyme-linked immunosorbent assay and immunohistochemistry. Immunoblotting was used to observe the effect of TLJN on inflammatory mediator expression in an senescence-accelerated mouse model of AD.

RESULTS

Our data showed that the TLJN-treated groups exhibited a reduction in plasma Aβ levels and reduced Aβ expression. Moreover, TLJN effectively attenuated Aβ-induced activation of extracellular signal-regulated kinase and c-Jun N-terminal kinases and blocked changes in inflammatory mediator expression.

CONCLUSION

These data suggest that TLJN might have protective effects and could potentially act to attenuate inflammatory stress in the pathogenesis of AD.

Matrix Metalloproteinases in Non-Neoplastic Disorders

The matrix metalloproteinases (MMPs) are zinc-dependent endopeptidases belonging to the metzincin superfamily. There are at least 23 members of MMPs ever reported in human, and they and their substrates are widely expressed in many tissues. Recent growing evidence has established that MMP not only can degrade a variety of components of extracellular matrix, but also can cleave and activate various non-matrix proteins, including cytokines, chemokines and growth factors, contributing to both physiological and pathological processes. In normal conditions, MMP expression and activity are tightly regulated via interactions between their activators and inhibitors. Imbalance among these factors, however, results in dysregulated MMP activity, which causes tissue destruction and functional alteration or local inflammation, leading to the development of diverse diseases, such as cardiovascular disease, arthritis, neurodegenerative disease, as well as cancer. This article focuses on the accumulated evidence supporting a wide range of roles of MMPs in various non-neoplastic diseases and provides an outlook on the therapeutic potential of inhibiting MMP action.

Transient ECM protease activity promotes synaptic plasticity

Activity-dependent proteolysis at a synapse has been recognized as a pivotal factor in controlling dynamic changes in dendritic spine shape and function; however, excessive proteolytic activity is detrimental to the cells. The exact mechanism of control of these seemingly contradictory outcomes of protease activity remains unknown. Here, we reveal that dendritic spine maturation is strictly controlled by the proteolytic activity, and its inhibition by the endogenous inhibitor (Tissue inhibitor of matrix metalloproteinases-1 – TIMP-1). Excessive proteolytic activity impairs long-term potentiation of the synaptic efficacy (LTP), and this impairment could be rescued by inhibition of protease activity. Moreover LTP is altered persistently when the ability of TIMP-1 to inhibit protease activity is abrogated, further demonstrating the role of such inhibition in the promotion of synaptic plasticity under well-defined conditions. We also show that dendritic spine maturation involves an intermediate formation of elongated spines, followed by their conversion into mushroom shape. The formation of mushroom-shaped spines is accompanied by increase in AMPA/NMDA ratio of glutamate receptors. Altogether, our results identify inhibition of protease activity as a critical regulatory mechanism for dendritic spines maturation.

MMP-9 in translation: from molecule to brain physiology, pathology, and therapy

Matrix metalloproteinase-9 (MMP-9) is a member of the metzincin family of mostly extracellularly operating proteases. Despite the fact that all of these enzymes might be target promiscuous, with largely overlapping catalogs of potential substrates, MMP-9 has recently emerged as a major and apparently unique player in brain physiology and pathology. The specificity of MMP-9 may arise from its very local and time-restricted actions, even when released in the brain from cells of various types, including neurons, glia, and leukocytes. In fact, the quantity of MMP-9 is very low in the naive brain, but it is markedly activated at the levels of enzymatic activity, protein abundance, and gene expression following various physiological stimuli and pathological insults. Neuronal MMP-9 participates in synaptic plasticity by controlling the shape of dendritic spines and function of excitatory synapses, thus playing a pivotal role in learning, memory, and cortical plasticity. When improperly unleashed, MMP-9 contributes to a large variety of brain disorders, including epilepsy, schizophrenia, autism spectrum disorder, brain injury, stroke, neurodegeneration, pain, brain tumors, etc. The foremost mechanism of action of MMP-9 in brain disorders appears to be its involvement in immune/inflammation responses that are related to the enzyme's ability to process and activate various cytokines and chemokines, as well as its contribution to blood-brain barrier disruption, facilitating the extravasation of leukocytes into brain parenchyma. However, another emerging possibility (i.e., the control of MMP-9 over synaptic plasticity) should not be neglected. The translational potential of MMP-9 has already been recognized in both the diagnosis and treatment domains. The most striking translational aspect may be the discovery of MMP-9 up-regulation in a mouse model of Fragile X syndrome, quickly followed by human studies and promising clinical trials that have sought to inhibit MMP-9. With regard to diagnosis, suggestions have been made to use MMP-9 alone or combined with tissue inhibitor of matrix metalloproteinase-1 or brain-derived neurotrophic factor as disease biomarkers. MMP-9, through cleavage of specific target proteins, plays a major role in synaptic plasticity and neuroinflammation, and by those virtues contributes to brain physiology and a host of neurological and psychiatric disorders. This article is part of the 60th Anniversary special issue.

Axonal Terminals Exposed to Amyloid-β May Not Lead to Pre-Synaptic Axonal Damage

BACKGROUND

Synaptic deficits and neuronal loss are the major pathological manifestations of Alzheimer's disease. However, the link between the early synaptic loss and subsequent neurodegeneration is not entirely clear. Cell culture studies have shown that amyloid-β (Aβ) applied to axonal terminals can cause retrograde degeneration leading to the neuronal loss, but this process has not been demonstrated in live animals.

OBJECTIVE

To test if Aβ applied to retinal ganglion cell axonal terminals can induce axonal damage in the optic nerve and optic tract in mice.

METHODS

Aβ was injected into the terminal field of the optic tract, in the left lateral geniculate nucleus of wildtype C57BL/6 mice. Following the injection, monthly diffusion tensor imaging was performed. Three months after the injection, mice underwent visual evoked potential recordings, and then sacrificed for immunohistochemical examination.

RESULTS

There were no significant changes seen with diffusion tensor imaging in the optic nerve and optic tract 3 months after the Aβ injection. The myelin and axons in these regions remained intact according to immunohistochemistry. The only significant changes observed in this study were delayed transduction and reduced amplitude of visual evoked potentials, although both Aβ and its reversed form caused similar changes.

CONCLUSION

Despite the published in vitro studies, there was no significant axonal damage in the optic nerve and optic tract after injecting Aβ onto retinal ganglion cell axonal terminals of wildtype C57BL/6 mice.

Therapeutic Targets for Neurodevelopmental Disorders Emerging from Animal Models with Perinatal Immune Activation

Increasing epidemiological evidence indicates that perinatal infection with various viral pathogens enhances the risk for several psychiatric disorders. The pathophysiological significance of astrocyte interactions with neurons and/or gut microbiomes has been reported in neurodevelopmental disorders triggered by pre- and postnatal immune insults. Recent studies with the maternal immune activation or neonatal polyriboinosinic polyribocytidylic acid models of neurodevelopmental disorders have identified various candidate molecules that could be responsible for brain dysfunction. Here, we review the functions of several candidate molecules in neurodevelopment and brain function and discuss their potential as therapeutic targets for psychiatric disorders.

Friends or Foes: Matrix Metalloproteinases and Their Multifaceted Roles in Neurodegenerative Diseases

Neurodegeneration is a chronic progressive loss of neuronal cells leading to deterioration of central nervous system (CNS) functionality. It has been shown that neuroinflammation precedes neurodegeneration in various neurodegenerative diseases. Matrix metalloproteinases (MMPs), a protein family of zinc-containing endopeptidases, are essential in (neuro)inflammation and might be involved in neurodegeneration. Although MMPs are indispensable for physiological development and functioning of the organism, they are often referred to as double-edged swords due to their ability to also inflict substantial damage in various pathological conditions. MMP activity is strictly controlled, and its dysregulation leads to a variety of pathologies. Investigation of their potential use as therapeutic targets requires a better understanding of their contributions to the development of neurodegenerative diseases. Here, we review MMPs and their roles in neurodegenerative diseases: Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), Huntington's disease (HD), and multiple sclerosis (MS). We also discuss MMP inhibition as a possible therapeutic strategy to treat neurodegenerative diseases.

Amyloid β Enhances Typical Rodent Behavior While It Impairs Contextual Memory Consolidation

Alzheimer's disease (AD) is associated with an early hippocampal dysfunction, which is likely induced by an increase in soluble amyloid beta peptide (Aβ). This hippocampal failure contributes to the initial memory deficits observed both in patients and in AD animal models and possibly to the deterioration in activities of daily living (ADL). One typical rodent behavior that has been proposed as a hippocampus-dependent assessment model of ADL in mice and rats is burrowing. Despite the fact that AD transgenic mice show some evidence of reduced burrowing, it has not been yet determined whether or not Aβ can affect this typical rodent behavior and whether this alteration correlates with the well-known Aβ-induced memory impairment. Thus, the purpose of this study was to test whether or not Aβ affects burrowing while inducing hippocampus-dependent memory impairment. Surprisingly, our results show that intrahippocampal application of Aβ increases burrowing while inducing memory impairment. We consider that this Aβ-induced increase in burrowing might be associated with a mild anxiety state, which was revealed by increased freezing behavior in the open field, and conclude that Aβ-induced hippocampal dysfunction is reflected in the impairment of ADL and memory, through mechanisms yet to be determined.

Disruption in the Blood-Brain Barrier: The Missing Link between Brain and Body Inflammation in Bipolar Disorder?

The blood-brain barrier (BBB) regulates the transport of micro- and macromolecules between the peripheral blood and the central nervous system (CNS) in order to maintain optimal levels of essential nutrients and neurotransmitters in the brain. In addition, the BBB plays a critical role protecting the CNS against neurotoxins. There has been growing evidence that BBB disruption is associated with brain inflammatory conditions such as Alzheimer's disease and multiple sclerosis. Considering the increasing role of inflammation and oxidative stress in the pathophysiology of bipolar disorder (BD), here we propose a novel model wherein transient or persistent disruption of BBB integrity is associated with decreased CNS protection and increased permeability of proinflammatory (e.g., cytokines, reactive oxygen species) substances from the peripheral blood into the brain. These events would trigger the activation of microglial cells and promote localized damage to oligodendrocytes and the myelin sheath, ultimately compromising myelination and the integrity of neural circuits. The potential implications for research in this area and directions for future studies are discussed.

P2Y receptors in Alzheimer's disease

Alzheimer's disease (AD) is the most common cause of dementia, affecting more than 10% of people over the age of 65. Age is the greatest risk factor for AD, although a combination of genetic, lifestyle and environmental factors also contribute to disease development. Common features of AD are the formation of plaques composed of beta-amyloid peptides (Aβ) and neuronal death in brain regions involved in learning and memory. Although Aβ is neurotoxic, the primary mechanisms by which Aβ affects AD development remain uncertain and controversial. Mouse models overexpressing amyloid precursor protein and Aβ have revealed that Aβ has potent effects on neuroinflammation and cerebral blood flow that contribute to AD progression. Therefore, it is important to consider how endogenous signalling in the brain responds to Aβ and contributes to AD pathology. In recent years, Aβ has been shown to affect ATP release from brain and blood cells and alter the expression of G protein-coupled P2Y receptors that respond to ATP and other nucleotides. Accumulating evidence reveals a prominent role for P2Y receptors in AD pathology, including Aβ production and elimination, neuroinflammation, neuronal function and cerebral blood flow.

Differential spatio-temporal regulation of MMPs in the 5xFAD mouse model of Alzheimer's disease: evidence for a pro-amyloidogenic role of MT1-MMP

Matrix metalloproteinases (MMPs) are pleiotropic endopeptidases involved in a variety of neurodegenerative/neuroinflammatory processes through their interactions with a large number of substrates. Among those, the amyloid precursor protein (APP) and the beta amyloid peptide (Aβ) are largely associated with the development of Alzheimer’s disease (AD). However, the regulation and potential contribution of MMPs to AD remains unclear. In this study, we investigated the evolution of the expression of MMP-2, MMP-9, and membrane-type 1-MMP (MT1-MMP) in the hippocampus at different stages of the pathology (asymptomatic, prodromal-like and symptomatic) in the 5xFAD transgenic mouse AD model. In parallel we also followed the expression of functionally associated factors. Overall, the expression of MMP-2, MMP-9, and MT1-MMP was upregulated concomitantly with the tissue inhibitor of MMPs-1 (TIMP-1) and several markers of inflammatory/glial response. The three MMPs exhibited age- and cell-dependent upregulation of their expression, with MMP-2 and MMP-9 being primarily located to astrocytes, and MT1-MMP to neurons. MMP-9 and MT1-MMP were also prominently present in amyloid plaques. The levels of active MT1-MMP were highly upregulated in membrane-enriched fractions of hippocampus at 6 months of age (symptomatic phase), when the levels of APP, its metabolites APP C-terminal fragments (CTFs), and Aβ trimers were the highest. Overexpression of MT1-MMP in HEK cells carrying the human APP Swedish mutation (HEKswe) strongly increased β-secretase derived C-terminal APP fragment (C99) and Aβ levels, whereas MMP-2 overexpression nearly abolished Aβ production without affecting C99. Our data consolidate the emerging idea of a regulatory interplay between MMPs and the APP/Aβ system, and demonstrate for the first time the pro-amyloidogenic features of MT1-MMP. Further investigation will be justified to evaluate this MMP as a novel potential therapeutic target in AD.

Matrix metalloproteinases and their multiple roles in Alzheimer's disease

Alzheimer's disease (AD) is the most prevalent type of dementia. Pathological changes in the AD brain include amyloid-β (Aβ) plaques and neurofibrillary tangles (NFTs), as well as neuronal death and synaptic loss. Matrix metalloproteinases (MMPs) play an important role as inflammatory components in the pathogenesis of AD. MMP-2 might be assumed to have a protective role in AD and is the major MMP which is directly linked to Aβ in the brain. Synthesis of MMP-9 can be induced by Aβ, and the enzymes appear to exert multiple effects in AD in senile plaque homoeostasis. The proaggregatory influence on tau oligomer formation in strategic brain regions may be a potential neurotoxic side effect of MMP-9. MMP-3 levels are correlated to the duration of AD and correlate with the CSF T-tau and P-tau levels in the elderly controls. Elevated brain levels of MMP-3 might result in increased MMP-9 activity and indirectly facilitate tau aggregation. At present, the clinical utility of these proteins, particularly in plasma or serum, as potential early diagnostic biomarkers for AD remains to be established. More research is needed to understand the diverse roles of these proteases to design specific drugs and devise therapeutic strategies for AD.

In vivo diffusion tensor imaging of amyloid-β-induced white matter damage in mice

BACKGROUND

Diffusion tensor imaging (DTI) suggests the presence of white matter abnormality at the prodromal stage in human Alzheimer's disease (AD).

OBJECTIVE

To use a mouse model of AD to determine whether the white matter abnormality detected by in vivo DTI is associated with functional deficits and axon damage.

METHODS

Amyloid-β1-42 (Aβ1-42) was injected into the left lateral ventricle in mice. Two months after the injection, in vivo DTI and visual evoked potential (VEP) recordings were performed, followed by immunohistochemistry of phosphorylated neurofilament and myelin basic protein.

RESULTS

DTI of Aβ1-42-treated mice showed a significant increase of radial diffusivity in white matter including the optic nerves and tracts. The abnormality was associated with decreased amplitude and increased latency of VEP. Immunohistochemistry confirmed a significant loss of axons and myelin integrity.

CONCLUSION

White matter damage induced by Aβ1-42 in mice can be detected non-invasively by DTI.

Fingolimod increases brain-derived neurotrophic factor levels and ameliorates amyloid β-induced memory impairment

Alzheimer's disease is a progressive neurodegenerative disorder. Amyloid β, a neurotoxic protein, causes disruption of hippocampal synaptic plasticity, and induces cognitive impairment in Alzheimer's disease. We previously revealed that fingolimod, a new oral immunosuppressant used to treat multiple sclerosis, ameliorates oligomeric amyloid β-induced neuronal damage via up-regulation of neuronal brain-derived neurotrophic factor (BDNF). Here, we showed that oral administration of fingolimod ameliorated the impairment in object recognition memory and associative learning in mice injected with amyloid β. This effect was associated with restoration of normal BDNF expression levels in the cerebral cortices and hippocampi, suggesting that neuroprotection was mediated by up-regulation of neuronal BDNF levels. Therefore, fingolimod may provide therapeutic effects in patients with Alzheimer's disease.

The role of Beta-adrenergic receptor blockers in Alzheimer's disease: potential genetic and cellular signaling mechanisms

According to genetic studies, Alzheimer's disease (AD) is linked to beta-adrenergic receptor blockade through numerous factors, including human leukocyte antigen genes, the renin-angiotensin system, poly(adenosine diphosphate-ribose) polymerase 1, nerve growth factor, vascular endothelial growth factor, and the reduced form of nicotinamide adenine dinucleotide phosphate. Beta-adrenergic receptor blockade is also implicated in AD due to its effects on matrix metalloproteinases, mitogen-activated protein kinase pathways, prostaglandins, cyclooxygenase-2, and nitric oxide synthase. Beta-adrenergic receptor blockade may also have a significant role in AD, although the role is controversial. Behavioral symptoms, sex, or genetic factors, including Beta 2-adrenergic receptor variants, apolipoprotein E, and cytochrome P450 CYP2D6, may contribute to beta-adrenergic receptor blockade modulation in AD. Thus, the characterization of beta-adrenergic receptor blockade in patients with AD is needed.