Glutamate transporter EAAT2: regulation, function, and potential as a therapeutic target for neurological and psychiatric disease.

Glutamate is the predominant excitatory neurotransmitter in the central nervous system. Excitatory amino acid transporter 2 (EAAT2) is primarily responsible for clearance of extracellular glutamate to prevent neuronal excitotoxicity and hyperexcitability. EAAT2 plays a critical role in regulation of synaptic activity and plasticity. In addition, EAAT2 has been implicated in the pathogenesis of many central nervous system disorders. In this review, we summarize current understanding of EAAT2, including structure, pharmacology, physiology, and functions, as well as disease relevancy, such as in stroke, Parkinson's disease, epilepsy, amyotrophic lateral sclerosis, Alzheimer's disease, major depressive disorder, and addiction. A large number of studies have demonstrated that up-regulation of EAAT2 protein provides significant beneficial effects in many disease models suggesting EAAT2 activation is a promising therapeutic approach. Several EAAT2 activators have been identified. Further understanding of EAAT2 regulatory mechanisms could improve development of drug-like compounds that spatiotemporally regulate EAAT2.

Impact of a pyridazine derivative on tripartite synapse ultrastructure in hippocampus: a three-dimensional analysis.

Introduction: We previously discovered a pyridazine derivative compound series that can improve cognitive functions in mouse models of Alzheimer's disease. One of the advanced compounds from this series, LDN/OSU-0215111-M3, was selected as the preclinical development candidate. This compound activates local protein translation at the perisynaptic astrocytic process (PAP) and enhances synaptic plasticity sequentially. While biochemical evidence supports the hypothesis that the compound enhances the structural plasticity of the tripartite synapse, its direct structural impact has not been investigated. Methods: Volume electron microscopy was used to study the hippocampal tripartite synapse three-dimensional structure in 3-month-old wild-type FVB/NJ mice after LDN/OSU-0215111-M3 treatment. Results: LDN/OSU-0215111-M3 increased the size of tertiary apical dendrites, the volume of mushroom spines, the proportion of mushroom spines containing spine apparatus, and alterations in the spine distribution across the surface area of tertiary dendrites. Compound also increased the number of the PAP interacting with the mushroom spines as well as the size of the PAP in contact with the spines. Furthermore, proteomic analysis of the isolated synaptic terminals indicated an increase in dendritic and synaptic proteins as well as suggested a possible involvement of the phospholipase D signaling pathway. To further validate that LDN/OSU-0215111-M3 altered synaptic function, electrophysiological studies showed increased long-term potentiation following compound treatment. Discussion: This study provides direct evidence that pyridazine derivatives enhance the structural and functional plasticity of the tripartite synapse.

Increased glial glutamate transporter EAAT2 expression reduces epileptogenic processes following pilocarpine-induced status epilepticus.

Several lines of evidence indicate that glutamate plays a crucial role in the initiation of seizures and their propagation; abnormal glutamate release causes synchronous firing of large populations of neurons, leading to seizures. In the present study, we investigated whether enhanced glutamate uptake by increased glial glutamate transporter EAAT2, the major glutamate transporter, could prevent seizure activity and reduce epileptogenic processes. EAAT2 transgenic mice, which have a 1.5-2 fold increase in EAAT2 protein levels as compared to their non-transgenic counterparts, were tested in a pilocarpine-induced status epilepticus (SE) model. Several striking phenomena were observed in EAAT2 transgenic mice compared with their non-transgenic littermates. First, the post-SE mortality rate and chronic seizure frequency were significantly decreased. Second, neuronal degeneration in hippocampal subfields after SE were significantly reduced. Third, the SE-induced neurogenesis and mossy fiber sprouting were significantly decreased. The severity of cell loss in epileptic mice was positively correlated with that of mossy fiber sprouting and chronic seizure frequency. Our results suggest that increased EAAT2 expression can protect mice against SE-induced death, neuropathological changes, and chronic seizure development. This study suggests that enhancing EAAT2 protein expression is a potential therapeutic approach.

Structure-activity relationship study of pyridazine derivatives as glutamate transporter EAAT2 activators.

Excitatory amino acid transporter 2 (EAAT2) is the major glutamate transporter and functions to remove glutamate from synapses. A thiopyridazine derivative has been found to increase EAAT2 protein levels in astrocytes. A structure-activity relationship study revealed that several components of the molecule were required for activity, such as the thioether and pyridazine. Modification of the benzylthioether resulted in several derivatives (7-13, 7-15 and 7-17) that enhanced EAAT2 levels by >6-fold at concentrations < 5 µM after 24h. In addition, one of the derivatives (7-22) enhanced EAAT2 levels 3.5-3.9-fold after 24h with an EC(50) of 0.5 µM.

Oxidative damage to RNA: mechanisms, consequences, and diseases.

Overproduction of free radicals can damage cellular components resulting in progressive physiological dysfunction, which has been implicated in many human diseases. Oxidative damage to RNA received little attention until the past decade. Recent studies indicate that RNA, such as messenger RNA and ribosomal RNA, is very vulnerable to oxidative damage. RNA oxidation is not a consequence of dying cells but an early event involved in pathogenesis. Oxidative modification to RNA results in disturbance of the translational process and impairment of protein synthesis, which can cause cell deterioration or even cell death. In this review, we discuss the mechanisms of oxidative damage to RNA and the possible biological consequences of damaged RNA. Furthermore, we review recent evidence suggesting that oxidative damage to RNA may contribute to progression of many human diseases.

Emerging role of glutamate in the pathophysiology and therapeutics of Gulf War illness.

Gulf War illness (GWI) is a chronic and multi-symptomatic disorder affecting veterans who served in the Gulf War. The commonly reported symptoms in GWI veterans include mood problems, cognitive impairment, muscle and joint pain, migraine/headache, chronic fatigue, gastrointestinal complaints, skin rashes, and respiratory problems. Neuroimaging studies have revealed significant brain structure alterations in GWI veterans, including subcortical atrophy, decreased volume of the hippocampus, reduced total grey and white matter, and increased brain white matter axial diffusivity. These brain changes may contribute to or increase the severities of the GWI-related symptoms. Epidemiological studies have revealed that neurotoxic exposures and stress may be significant contributors to the development of GWI. However, the mechanism underlying how the exposure and stress could contribute to the multi-symptomatic disorder of GWI remains unclear. We and others have demonstrated that rodent models exposed to GW-related agents and stress exhibited higher extracellular glutamate levels, as well as impaired structure and function of glutamatergic synapses. Restoration of the glutamatergic synapses ameliorated the GWI-related pathological and behavioral deficits. Moreover, recent studies showed that a low-glutamate diet reduced multiple symptoms in GWI veterans, suggesting an important role of the glutamatergic system in GWI. Currently, growing evidence has indicated that abnormal glutamate neurotransmission may contribute to the GWI symptoms. This review summarizes the potential roles of glutamate dyshomeostasis and dysfunction of the glutamatergic system in linking the initial cause to the multi-symptomatic outcomes in GWI and suggests the glutamatergic system as a therapeutic target for GWI.

Effects of axon degeneration on oligodendrocyte lineage cells: dorsal rhizotomy evokes a repair response while axon degeneration rostral to spinal contusion induces both repair and apoptosis.

Wallerian degeneration in the dorsal columns (DC) after spinal cord injury (SCI) is associated with microglial activation and prolonged oligodendrocyte (OL) apoptosis that may contribute to demyelination and dysfunction after SCI. But, there is an increase in OL lineage cells after SCI that may represent a reparative response, and there is evidence for remyelination after SCI. To assess the role of axonal degeneration per se in OL apoptosis and proliferation, we cut the L2-S2 dorsal roots producing massive axonal degeneration and microglial activation in the DC, and found no evidence of OL loss or apoptosis. Rather, the numbers of OL-lineage cells positive for NG2 and APC (CC1) increased, and BrdU studies suggested new OL formation. We then tested contusion SCI (cSCI) that results in comparable degeneration in the DC rostral to the injury, microglial activation, and apoptosis of DC OLs by eight days. NG2+ cell proliferation and oligodendrogenesis was seen as after rhizotomy. The net result of this combination of proliferation and apoptosis was a reduction in DC OLs, confirming earlier studies. Using an antibody to oxidized nucleic acids, we found rapid and prolonged RNA oxidation in OLs rostral to cSCI, but no evidence of oxidative stress in DC OLs after rhizotomy. These results suggest that signals associated with axonal degeneration are sufficient to induce OL proliferation, and that secondary injury processes associated with the central SCI, including oxidative stress, rather than axonal degeneration per se, are responsible for OL apoptosis.

Restoring tripartite glutamatergic synapses: A potential therapy for mood and cognitive deficits in Gulf War illness.

Gulf War illness is associated with a combination of exposure to war-related chemical agents and traumatic stress. Currently, there are no effective treatments, and the pathophysiology remains elusive. Neurological problems are among the most commonly reported symptoms. In this study, we investigated the glutamatergic system in the hippocampi of mice exposed to war-related chemical agents and stress. Mice developed Gulf War illness-like symptoms, including mood deficits, cognitive impairments, and fatigue. They exhibited the following pathological changes in hippocampi: elevated extracellular glutamate levels, impaired glutamatergic synapses, astrocyte atrophy, loss of interneurons, and decreased neurogenesis. LDN/OSU-215111 is a small-molecule that can strengthen the structure and function of both the astrocytic processes and the glutamatergic synapses that together form the tripartite synapses. We found that LDN/OSU-215111 effectively prevented the development of mood and cognitive deficits in mice when treatment was implemented immediately following the exposure. Moreover, when symptoms were already present, LDN/OSU-215111 still significantly ameliorated these deficits; impressively, benefits were sustained one month after treatment cessation, indicating disease modification. LDN/OSU-215111 effectively normalized hippocampal pathological changes. Overall, this study provides strong evidence that restoration of tripartite glutamatergic synapses by LDN/OSU-215111 is a potential therapy for Gulf War illness.

A novel noncoding RNA rescues mutant SOD1-mediated cell death.

Transgenic mice expressing mutant Cu2+/Zn2+ superoxide dismutase SOD1(G93A) develop similar clinical and pathological phenotypes to amyotrophic lateral sclerosis (ALS) patients. Here, we utilize representational difference analysis to identify the transcripts that are up-regulated in the presymptomatic stage of SOD1(G93A) mice. Unexpectedly, three predominant clones were 18S or 28S ribosomal RNA (rRNA) segments. One of these clones corresponded to a capped and polyadenylated transcript containing a large portion of 18S rRNA, named MSUR1 (mutant SOD1-up-regulated RNA 1). In vitro expression experiments show that MSUR1 is able to rescue SOD1(G93A)-mediated cell death. Expression of MSUR1 significantly reduces SOD1(G93A)-induced free radical levels and oxidative damage. Further, MSUR1 can reduce hydrogen peroxide-mediated cytotoxicity. MSUR1 does not encode a protein, suggesting its role as a functional noncoding RNA. It is widely expressed in various tissues. Searching the database of GenBank revealed that a large number of expressed sequence tag (EST) clones contain large portions of rRNA sequence, potentially indicating a heretofore overlooked class of mRNAs with functional significance.

Enhancement of tripartite synapses as a potential therapeutic strategy for Alzheimer's disease: a preclinical study in rTg4510 mice.

BACKGROUND: The lack of effective treatment options for Alzheimer's disease (AD) is of momentous societal concern. Synaptic loss is the hallmark of AD that correlates best with impaired memory and occurs early in the disease process, before the onset of clinical symptoms. We have developed a small-molecule, pyridazine-based series that enhances the structure and function of both the glial processes and the synaptic boutons that form the tripartite synapse. Previously, we have shown that these pyridazine derivatives exhibit profound efficacy in an amyloid precursor protein AD model. Here, we evaluated the efficacy of an advanced compound, LDN/OSU-0215111, in rTg4510 mice-an aggressive tauopathy model. METHODS: rTg4510 mice were treated orally with vehicle or LDN/OSU-0215111 (10 mg/kg) daily from the early symptomatic stage (2 months old) to moderate (4 months old) and severe (8 months old) disease stages. At each time point, mice were subjected to a battery of behavioral tests to assess the activity levels and cognition. Also, tissue collections were performed on a subset of mice to analyze the tripartite synaptic changes, neurodegeneration, gliosis, and tau phosphorylation as assessed by immunohistochemistry and Western blotting. At 8 months of age, a subset of rTg4510 mice treated with compound was switched to vehicle treatment and analyzed behaviorally and biochemically 30 days after treatment cessation. RESULTS: At both the moderate and severe disease stages, compound treatment normalized cognition and behavior as well as reduced synaptic loss, neurodegeneration, tau hyperphosporylation, and neuroinflammation. Importantly, after 30 days of treatment cessation, the benefits of compound treatment were sustained, indicating disease modification. We also found that compound treatment rapidly and robustly reduced tau hyperphosphorylation/deposition possibly via the inhibition of GSK3ß. CONCLUSIONS: The results show that LDN/OSU-0215111 provides benefits for multiple aspects of tauopathy-dependent pathology found in Alzheimer's disease including tripartite synapse normalization and reduction of toxic tau burden, which, in turn, likely accounted for normalized cognition and activity levels in compound-treated rTg4510 mice. This study, in combination with our previous work regarding the benefit of pyridazine derivatives against amyloid-dependent pathology, strongly supports pyridazine derivatives as a viable, clinically relevant, and disease-modifying treatment for many of the facets of Alzheimer's disease.

Messenger RNA oxidation is an early event preceding cell death and causes reduced protein expression.

We previously reported that up to 50% of messenger RNAs (mRNA) are oxidatively damaged in the affected area of Alzheimer's disease (AD) brains. The role of RNA oxidation in the cell death process is unknown. In the present study, we used cortical primary dissociated cultures to investigate the relationship between RNA oxidation and neuron degeneration induced by various insults, including hydrogen peroxide, glutamate, and amyloid beta peptide. These insults mediate the production of reactive oxygen species and thus induce oxidative stress. The results showed that RNA oxidation was an early event far preceding cell death, not merely a consequence of dying cells. RNA oxidation occurred primarily in a distinct group of neurons that died later. Identification of oxidized RNA species revealed that significant amounts of mRNAs were oxidized and that some mRNA species were more susceptible to oxidative damage, consistent with findings in the AD brain. The level of protein corresponding to the oxidized mRNA species was significantly decreased. Polyribosome analysis indicated that oxidized bases in mRNAs caused ribosome stalling on the transcripts, which led to a decrease of protein expression. These results suggest that RNA oxidation may be directly associated with neuronal deterioration, rather than harmless epiphenomena, during the process of neurodegeneration.

Pyridazine-derivatives Enhance Structural and Functional Plasticity of Tripartite Synapse Via Activation of Local Translation in Astrocytic Processes.

Excitatory amino acid transporter 2 (EAAT2) is primarily located in perisynaptic astrocytic processes (PAP) where it plays a critical role in synaptic glutamate homeostasis. Dysregulation of EAAT2 at the translational level has been implicated in a myriad of neurological diseases. We previously discovered that pyridazine analogs can activate EAAT2 translation. Here, we sought to further refine the site and mechanism of compound action. We found that in vivo, compound treatment increased EAAT2 expression only in the PAP of astrocytes where EAAT2 mRNA also was identified. Direct application of compound to isolated PAP induced de novo EAAT2 protein synthesis, indicating that compound activates translation locally in the PAP. Using a screening process, we identified a set of PAP proteins that are rapidly up-regulated following compound treatment and a subset of these PAP proteins may be locally synthesized in the PAP. Importantly, these identified proteins are associated with the structural and functional capacity of the PAP, indicating compound enhanced plasticity of the PAP. Concomitantly, we found that pyridazine analogs increase synaptic protein expression in the synapse and enhance hippocampal long-term potentiation. This was not dependent upon compound-mediated local translation in neurons. This suggests that compound enhances the structural and functional capacity of the PAP which in turn facilitates enhanced plasticity of the tripartite synapse. Overall, this provides insight into the mechanism action site of pyridazine derivatives as well as the growing appreciation of the dynamic regulation and functional aspects of the PAP at the tripartite synapse.

The identification and characterization of oxidized RNAs in Alzheimer's disease.

It has been shown that cytoplasmic RNA oxidation occurs to a great extent in the brains of Alzheimer's disease (AD) patients. The goal of this study was to isolate and identify oxidized RNA species in AD. We show that significant amounts of poly(A)+ mRNAs are oxidized in AD brains. RNA oxidation is not random but highly selective. Importantly, many identified oxidized mRNA species have been implicated in the pathogenesis of AD. Quantitative analysis revealed that some mRNA species are more susceptible to oxidative damage. We also investigated the biological consequence of oxidatively damaged mRNAs by expressing them in cell lines. Our data indicated that abnormal processing of proteins occurred to the oxidized mRNAs. This may implicate the potential contribution of RNA oxidation in the pathogenesis of AD.

Restored glial glutamate transporter EAAT2 function as a potential therapeutic approach for Alzheimer's disease.

Glutamatergic systems play a critical role in cognitive functions and are known to be defective in Alzheimer's disease (AD) patients. Previous literature has indicated that glial glutamate transporter EAAT2 plays an essential role in cognitive functions and that loss of EAAT2 protein is a common phenomenon observed in AD patients and animal models. In the current study, we investigated whether restored EAAT2 protein and function could benefit cognitive functions and pathology in APPSw,Ind mice, an animal model of AD. A transgenic mouse approach via crossing EAAT2 transgenic mice with APPSw,Ind. mice and a pharmacological approach using a novel EAAT2 translational activator, LDN/OSU-0212320, were conducted. Findings from both approaches demonstrated that restored EAAT2 protein function significantly improved cognitive functions, restored synaptic integrity, and reduced amyloid plaques. Importantly, the observed benefits were sustained one month after compound treatment cessation, suggesting that EAAT2 is a potential disease modifier with therapeutic potential for AD.

Molecular cloning, gene structure, expression profile and functional characterization of the mouse glutamate transporter (EAAT3) interacting protein GTRAP3-18.

Glutamate is an important amino acid implicated in energy metabolism, protein biosynthesis and neurotransmission. The Na(+)-dependent high-affinity excitatory amino acid transporter EAAT3 (EAAC1) facilitates glutamate uptake into most cells. Recently, a novel rat EAAT3-interacting protein called GTRAP3-18 has been identified by a yeast two-hybrid screening. GTRAP3-18 functions as a negative modulator of EAAT3-mediated glutamate transport. In order to further understand the function and regulation of GTRAP3-18, we cloned the mouse orthologue to GTRAP3-18 and determined its gene structure and its expression pattern. GTRAP3-18 encodes a 188-residue hydrophobic protein whose sequence is highly conserved amongst vertebrates. Mouse and human GTRAP3-18 genes contain three exons separated by two introns. The GTRAP3-18 gene is found on mouse chromosome 6D3 and on human chromosome 3p14, a susceptibility locus for cancer and epilepsy. GTRAP3-18 protein and RNA were found both in neuronal rich regions of the brain and in non-neuronal tissues such as the kidney, heart and skeletal muscle. Mouse GTRAP3-18 inhibited EAAT3-mediated glutamate transport in a dose-dependent manner. These studies show that GTRAP3-18 is a ubiquitously expressed protein that functions as a negative regulator of EAAT3 function.

Small-molecule activator of glutamate transporter EAAT2 translation provides neuroprotection.

Glial glutamate transporter EAAT2 plays a major role in glutamate clearance in synaptic clefts. Several lines of evidence indicate that strategies designed to increase EAAT2 expression have potential for preventing excitotoxicity, which contributes to neuronal injury and death in neurodegenerative diseases. We previously discovered several classes of compounds that can increase EAAT2 expression through translational activation. Here, we present efficacy studies of the compound LDN/OSU-0212320, which is a pyridazine derivative from one of our lead series. In a murine model, LDN/OSU-0212320 had good potency, adequate pharmacokinetic properties, no observed toxicity at the doses examined, and low side effect/toxicity potential. Additionally, LDN/OSU-0212320 protected cultured neurons from glutamate-mediated excitotoxic injury and death via EAAT2 activation. Importantly, LDN/OSU-0212320 markedly delayed motor function decline and extended lifespan in an animal model of amyotrophic lateral sclerosis (ALS). We also found that LDN/OSU-0212320 substantially reduced mortality, neuronal death, and spontaneous recurrent seizures in a pilocarpine-induced temporal lobe epilepsy model. Moreover, our study demonstrated that LDN/OSU-0212320 treatment results in activation of PKC and subsequent Y-box-binding protein 1 (YB-1) activation, which regulates activation of EAAT2 translation. Our data indicate that the use of small molecules to enhance EAAT2 translation may be a therapeutic strategy for the treatment of neurodegenerative diseases.

Glutamate transporter EAAT2: a new target for the treatment of neurodegenerative diseases.

Glutamate is the primary excitatory amino acid neurotransmitter in the CNS. The concentration of glutamate in the synaptic cleft is tightly controlled by interplay between glutamate release and glutamate clearance. Abnormal glutamate release and/or dysfunction of glutamate clearance can cause overstimulation of glutamate receptors and result in neuronal injury known as excitotoxicity. The glial glutamate transporter EAAT2 plays a major role in glutamate clearance. Dysfunction or reduced expression of EAAT2 has been documented in many neurodegenerative diseases. In addition, many studies in animal models of disease indicate that increased EAAT2 expression provides neuronal protection. Here, we summarize these studies and suggest that EAAT2 is a potential target for the prevention of excitotoxicity. EAAT2 can be upregulated by transcriptional or translational activation. We discuss current progress in the search for EAAT2 activators, which is a promising direction for the treatment of neurodegenerative diseases.

Identification of translational activators of glial glutamate transporter EAAT2 through cell-based high-throughput screening: an approach to prevent excitotoxicity.

Excitotoxicity has been implicated as the mechanism of neuronal damage resulting from acute insults such as stroke, epilepsy, and trauma, as well as during the progression of adult-onset neurodegenerative disorders such as Alzheimer's disease and amyotrophic lateral sclerosis (ALS). Excitotoxicity is defined as excessive exposure to the neurotransmitter glutamate or overstimulation of its membrane receptors, leading to neuronal injury or death. One potential approach to protect against excitotoxic neuronal damage is enhanced glutamate reuptake. The glial glutamate transporter EAAT2 is the quantitatively dominant glutamate transporter and plays a major role in clearance of glutamate. Expression of EAAT2 protein is highly regulated at the translational level. In an effort to identify compounds that can induce translation of EAAT2 transcripts, a cell-based enzyme-linked immunosorbent assay was developed using a primary astrocyte line stably transfected with a vector designed to identify modulators of EAAT2 translation. This assay was optimized for high-throughput screening, and a library of approximately 140,000 compounds was tested. In the initial screen, 293 compounds were identified as hits. These 293 hits were retested at 3 concentrations, and a total of 61 compounds showed a dose-dependent increase in EAAT2 protein levels. Selected compounds were tested in full 12-point dose-response experiments in the screening assay to assess potency as well as confirmed by Western blot, immunohistochemistry, and glutamate uptake assays to evaluate the localization and function of the elevated EAAT2 protein. These hits provide excellent starting points for developing therapeutic agents to prevent excitotoxicity.

Increased glial glutamate transporter EAAT2 expression reduces visceral nociceptive response in mice.

Visceral hypersensitivity is the leading complaint of functional bowel disorders. Central sensitization mediated by glutamate receptor activation is implicated in pathophysiology of visceral pain. The glial glutamate transporter EAAT2 is the principal mediator of glutamate clearance to terminate glutamate-mediated responses. Transgenic mice overexpressing human EAAT2 (EAAT2 mice), which exhibited a twofold enhanced glutamate uptake, showed 39% less writhing response to intraperitoneal acetic acid than nontransgenic littermates. Moreover, EAAT2 transgenic mice showed a 53-64% reduction in visceromotor response (VMR) to colorectal distension (CRD) in assessments of the response to graded increase in pressures. Corroborating the involvement of enhanced glutamate uptake, wild-type mice treated for 1 wk with ceftriaxone, an EAAT2 expression activator, showed a 49-70% reduction in VMR to CRD. Moreover, systemic pretreatment with the selective EAAT2 transporter blocker dihydrokainate reversed the ceftriaxone-blunted nociceptive response to CRD. However, the enhanced VMR to CRD produced by intracolonic ethanol was not significantly attenuated by 1-wk ceftriaxone pretreatment. The data suggest that enhanced glutamate uptake provides protective effects against colonic distension-induced nociception and represents an exciting new mechanistic approach leading to better therapeutic options to visceral pain disorders.

The presence of rRNA sequences in polyadenylated RNA and its potential functions.

Accumulating evidence has shown that various lengths of ribosomal RNA (rRNA) sequences are widely present in polyadenylated RNA. This review article will discuss these polyadenylated rRNA containing transcripts (PART). PART are highly abundant and widely expressed in various tissues. It appears that there may be two types of PART. One type, type I, contains the rRNA segments (from approximately 10 nucleotides up to several hundred nucleotides) located within the transcripts. It has been demonstrated that short rRNA sequences within type I PART may function as cis-regulatory elements that regulate translational efficiency. The other type, type II, contains large portions or almost entire sequences of rRNA with a cap at the 5' end and poly(A) at 3' end. Recent work has shown that some type II PART have functional significance for some neurodegenerative disease processes and may play an important role in the pathogenesis of diseases. Further investigation in this area is critical to understanding the basic biology of PART and the potential role of PART in diseases.