Low Molecular Weight Variant Research Articles

Proton-driven sodium secretion in a saline water animal

Aquatic animals residing in saline habitats either allow extracellular sodium concentration to conform to environmental values or regulate sodium to lower levels. The latter strategy requires an energy-driven process to move sodium against a large concentration gradient to eliminate excess sodium that diffuses into the animal. Previous studies of invertebrate and vertebrate species indicate a sodium pump, Na+/K+ ATPase, powers sodium secretion. We provide the first functional evidence of a saline-water animal, Aedes taeniorhynchus mosquito larva, utilizing a proton pump to power this process. Vacuolar-type H+ ATPase (VHA) protein is highly expressed on the apical membrane of the posterior rectal cells, and in situ sodium flux across this epithelium increases significantly in larvae held in higher salinity and is sensitive to Bafilomycin A1, an inhibitor of VHA. We also report the first evidence of splice variants of the sodium/proton exchanger, NHE3, with both high and low molecular weight variants highly expressed on the apical membrane of the posterior rectal cells. Evidence of NHE3 function was indicated with in situ sodium transport significantly inhibited by a NHE3 antagonist, S3226. We propose that the outward proton pumping by VHA establishes a favourable electromotive gradient to drive sodium secretion via NHE3 thus producing a hyperosmotic, sodium-rich urine. This H+- driven Na+ secretion process is the primary mechanism of ion regulation in salt-tolerant culicine mosquito species and was first investigated over 80 years ago.

Alzheimer’s Disease as a potential diabetes type III: Pilot Study of comorbidities related to AD

AbstractBackgroundAlzheimer’s disease (AD) is the most prevalent cause of dementia in the elderly population. It is a multifactorial disease in which metabolic dysregulation and glucose metabolism impairment plays a major role in the etiopathogenesis. In this research, we evaluate the comorbidities in a clinical trial for the novel biomarker Alz‐tau®, based on platelet tau, in cognitive healthy controls, mild‐cognitive impairment (MCI) and AD patients, in particular TII diabetes. Alterations in this biomarker appear to indicate high risk of developing AD, as demonstrated in previous clinical trials.MethodsFrom a cohort of 111 subjects, neuropsychology tests were performed to assign the respective group (control, MCI or AD). In addition, a health questionnaire to evaluate comorbidities. For the Alz‐tau® biomarker, platelets were obtained from plasma by centrifugation, lysed to obtain platelet protein extracts (containing tau) and further processed for a western blot analysis. The immuno‐ detection was performed with our novel monoclonal antibody tau51, which detects two variants of tau protein: a high molecular weight variant (HMW) and a low molecular weight variant (LMW). The results obtained were subjected to a densitometric analysis with ImageJ and the algorithm HWM/LMW was obtained.ResultFrom 111 patients, 17 presented diabetes type 2 as a comorbidity (15,3%). Out of them, 11 were cognitive healthy subjects (64.7%), 4 EA patients (23.5%) and 2 MCI (11.8%). Out of the control patients, 6 have the Alz‐tau® biomarker altered (54.5%), which was similar to what was observed in the EA patients (2 patients, 50%) and the MCI (1 patient, 50%). Of the 17 patients with diabetes, 53% presented an altered Alz‐tau® biomarker (cut‐off 1.2 algorithm), which is considered a risk factor for AD. It should be considered that the area under the curve (AUC) of this novel biomarker Alz‐tau® is 0.69, appropriate for a novel technique.ConclusionHere, we discovered that this early detection biomarker is correlated with the high‐risk factor of having AD in the future, since non‐symptomatic subjects exhibited an altered biomarker. Moreover, more than 50% of the TII diabetic positive cognitive healthy controls, exhibit an altered biomarker, sustaining our hypothesis of the correlation between glucose‐metabolism impairment, AD and oligomeric tau.

Novel AD biomarker based on platelets tau: Expanding to plasma detection

AbstractBackgroundAlzheimer’s disease (AD) is the most prevalent cause of dementia in the elderly. One of the major drawbacks of this disease is the lack of molecular biomarkers that could provide of an early detection, as the ones currently available are invasive (CSF biomarkers), not cost‐efficient (neuroimaging, PET scan) and do not provide an early detection. In this work we propose the novel biomarker Alz‐tau®, based on platelet tau, as a non‐invasive and cost‐efficient way for early detection of pathological events that may lead Alzheimer’s disease.MethodsPlasma samples were obtained by centrifugation. Platelets were obtained from blood samples of AD patients and controls, lysed to obtain protein extracts and further processed for a western blot analysis. The immuno‐ detection was performed with a novel monoclonal antibody tau51, which detects two variants of tau protein: a high molecular weight variant (HMW) and a low molecular weight variant (LMW). The results obtained were subjected to a densitometric analysis with ImageJ and the algorithm HWM/LMW was obtained.ResultsWe demonstrated that in platelets the algorithm HWM/LMW was statistically higher in AD patients when compared to controls. Similar results were obtained in plasma. Likewise, the platelet biomarker has a proven sensitivity of 71.43% and a specificity of 69.23%, while a sensibility of 80% and specificity of 66.5% was found in plasma samples.ConclusionsHere, we demonstrate the efficiency and functionality of of a novel, non‐invasive, and cost‐efficient biomarker for early detection of AD, based on 4 clinical trials. The most relevant feature of this biomarker is that it is able to detect alterations before the patient presents any symptoms, thus it would be highly beneficial for prevention of AD.

On the essentiality of the angiotensin converting enzyme 2 receptor for SARS-CoV-2 infection and the potential of soluble angiotensin converting enzyme 2 proteins as universal approach for variants causing COVID-19.

On the essentiality of the angiotensin converting enzyme 2 receptor for SARS-CoV-2 infection and the potential of soluble angiotensin converting enzyme 2 proteins as universal approach for variants causing COVID-19.

Role of harvest depth filtration in controlling product-related impurities for a bispecific antibody.

BackgroundBispecific antibodies (BsAb) belong to a novel antibody category with advantages over traditional mono-specific therapeutic antibodies. However, product variants are also commonly seen during the production of BsAb, which poses significant challenges to downstream processing. In this study, the adsorptive characteristics of a BsAb product and its variants were investigated for a set of depth filters during primary recovery of the cell culture fluid.MethodsThe retention of the BsAb product and its variants on a set of Millistak+® D0HC and X0HC depth filters were first investigated, followed by studying the mechanism of their adsorption on the depth filters. The chemical and structural properties of depth filters along with the molecular properties of the product and its variants were studied subsequently.ResultsThe X0HC filter was found to be able to retain a significant amount of low molecular weight (LMW) variants along with a low amount of main product retained. Different levels of retention, observed for these variants, were correlated to their different hydrophobic and charge characteristics in relation with the adsorptive properties of the depth filters used. Electrostatic, hydrophobic, and hydrogen bonding interactions were found to be the key forces to keep product variants retained on the depth filter where the higher hydrophobicity of the LMW variants may cause them to be preferentially retained.ConclusionHarvest depth filters potentially can be utilized for retaining the BsAb variants, which depends on relative molecular properties of the product and its variants and adsorptive properties of the depth filters used.

Recent biopharmaceutical applications of capillary electrophoresis methods on recombinant DNA technology-based products.

Biopharmaceuticals (recombinant technology-based products, vaccines, whole blood and blood components, gene therapy, cells, tissues, etc.,) are described as biological medical products produced from various living sources such as human, microbial, animal, and so on by manufacturing, extraction, or semi-synthesis. They are complex molecules having high molecular weights. For their safety and efficacy, their structural, clinical, physicochemical, and chemical features must be carefully controlled, and they must be well characterized by analytical techniques before the approval of the final product. Capillary electrophoresis (CE) having versatile modes can provide valuable safety and efficacy information, such as amino acid sequence, size variants (low and high molecular weight variants), charged variants (acidic and basic impurities), aggregates, N-linked glycosylation, and O-linked glycosylation. There are numerous applications of CE in the literature. In this review, the most significant and recent studies on the analysis of recombinant DNA technology-based products using different CE modes in the last ten years have been overviewed. It was seen that the researches mostly focus on the analysis of mAbs and IgG. In addition, in recent years, researchers have started to prefer CE combined mass spectrometry (MS) techniques to provide a more detailed characterization for protein and peptide fragments.

Biosimilar or Not: Physicochemical and Biological Characterization of MabThera and Its Two Biosimilar Candidates.

The development of therapeutic biosimilar antibodies has become an important driving force of the modern biopharmaceutical industry. In this study, physiochemical characteristics (amino acid sequence, intact/subunit molecular weight, isoelectric point, post-translation modification, and disulfide linkage pattern), purity (charge variants, high and low molecular weight variants), antigen binding activity, Fc receptor binding affinity and Fc-effector function (CDC and ADCC) were analyzed by using an extensive set of state-of-the-art and orthogonal analytical technologies to provide a comprehensive characterization of the innovative product rituximab and two biosimilar candidates. The similarity study showed that biosimilar candidate 1 (BC1) and the reference product (RP) MabThera had an identical protein amino acid sequences and highly similar primary structures along with similar purity, heterogeneity profiles, antigen binding activity, Fc receptor binding affinity, and Fc-effector functions. Biosimilar candidate 2 (BC2), which had an amino acid replacement at a constant region, a different N-glycosylation profiling, and purity, was not analytically similar to RP. Although BC2 showed improvement such as an increased level of afucose, another IgG1 allotype, and similar biological activities, it was not recommended to be applied as a biosimilar compound in drug registration because the biosimilar manufacturer must first show that its primary structure was identical to that of RP. Our physicochemical characterizations and bioassay comparability study provided a deepened understanding of the structure-function relationship of quality attributes.

A Role for Estrogen Receptor alpha36 in Cancer Progression.

Estrogen receptor α (ERα) functions as a ligand dependent transcription factor that directly binds specific estrogen responsive elements, thus regulating the transcription of estrogen sensitive genes. ERα has also been shown to be associated with the plasma membrane (membrane associated ERα, mERα), concentrated in lipid rafts, plasma membrane microdomains with a distinct lipid composition, where it transduces membrane-initiated estrogen-dependent activation of the mitogen-activated protein (MAP) kinase signaling pathway. Two isoforms of ERα have been described: the “traditional” ERα66 (66 kDa) and a lower molecular weight variant: the ERα46 (46 kDa). More recently, a novel ERα variant with a molecular mass of 36 kDa (ERα36) has been discovered. Notably, ERα36 has been found expressed in different human tumor cells, including both ER- positive and ER- negative breast cancer cells. Estrogen signaling at the cell membrane via ERα36 appears as capable of activating multiple pathways of importance for cancer aggressiveness and metastatic potential. The presence of serum autoantibodies reacting with mERα (anti-ERα Abs) in a large percentage of patients with breast cancer has recently been reported by our group. These anti-ERα Abs seem to act as estrogen agonists rapidly triggering MAP kinase pathway activation thus inducing tumor cell proliferation and overcoming cell resistance to anti-estrogen drug tamoxifen. In this review, we describe the involvement of ERα36 in different tumors. We also report the potential pathogenetic activity of anti-ERα Abs and their implication in drug resistance.

Acute Kidney Injury in COVID-19: Emerging Evidence of a Distinct Pathophysiology.

Acute Kidney Injury in COVID-19: Emerging Evidence of a Distinct Pathophysiology.

Sound Science before Quick Judgement Regarding RAS Blockade in COVID-19.

There has been much speculation in journals as well as social and traditional media about a link between popularly used classes of drugs that inhibit the renin-angiotensin system (RAS) and novel coronavirus (severe acute respiratory syndrome coronavirus 2 [SARS-CoV-2]) infection or coronavirus

Abstract MIP-059: MITOCHONDRIAL DYNAMICS AND DYSFUNCTION IN OVARIAN CANCER

Abstract Altered mitochondrial function remains a key feature of many tumor cells and drives pathways such gene expression, metabolic and stress responses, cell cycle progression and resistance to apoptosis. Many chemotherapeutics activate programmed cell death and it is thought that mitochondrial dysfunction may be one mechanism by which cancer cells evade killing by these compounds. Screening the metabolic profiles of ovarian cancer cell lines and patient ascites-derived tumor cells reveals that ovarian cancers fall into unique bioenergetic subgroups. For example, Ovarian Clear Cell Carcinomas (OCCC) display high oxygen consumption rate and glycolytic flux compared to the more common high grade serous adenocarcinoma (HGSA) subtype. In addition, we show that a portion of HGSAs have severe mitochondrial dysfunction, that is marked by a decrease in mitochondrial respiration, a lack of response to the uncoupler FCCP and a concomitant reliance on alternate metabolic pathways. Moreover, this is accompanied by enhanced chemoresistance to Cisplatin and Taxol. The cause of mitochondrial dysfunction has been attributed to a number of factors, including deregulation in mitochondrial fission/fusion dynamics. Moreover, fission is an integral component of apoptotic and authophagy pathways. Interestingly, the observed HGSA mitochondrial dysfunction correlates with aberrant fusion/fission dynamics and expression of a low molecular weight variant of the mitochondrial fission protein Drp1. The potential significance of Drp1 in ovarian cancer etiology is highlighted by TCGA data, where more than 15% of HGSA samples display significant increases in Drp1 mRNA levels, and associated amplification of the Drp1 gene DNM1L. Whether this represents the shorter, potentially dominant-negative Drp1 variant identified in our work is currently under active investigation. Our data suggest that compromised mitochondrial function and fission/fusion dynamics may be a hallmark of a previously unidentified subgroup of highly chemoresistant EOCs and that this is associated with aberrant expression of the fission protein Drp1. Studies are underway to identify the molecular identity and regulation of short Drp1, and the mechanistic links to alterations in fission, metabolic switching and chemoresistance. Citation Format: Dong-Hui Shin, Usawadee Dier, Patrick F. Timmins, Joshua Kesterson, Rebecca Phaeton and Nadine Hempel. MITOCHONDRIAL DYNAMICS AND DYSFUNCTION IN OVARIAN CANCER [abstract]. In: Proceedings of the 11th Biennial Ovarian Cancer Research Symposium; Sep 12-13, 2016; Seattle, WA. Philadelphia (PA): AACR; Clin Cancer Res 2017;23(11 Suppl):Abstract nr MIP-059.

Toad Heart Utilizes Exclusively Slow Skeletal Muscle Troponin T: AN EVOLUTIONARY ADAPTATION WITH POTENTIAL FUNCTIONAL BENEFITS

The three isoforms of vertebrate troponin T (TnT) are normally expressed in a muscle type-specific manner. Here we report an exception that the cardiac muscle of toad (Bufo) expresses exclusively slow skeletal muscle TnT (ssTnT) together with cardiac forms of troponin I and myosin as determined using immunoblotting, cDNA cloning, and/or LC-MS/MS. Using RT-PCR and 3'- and 5'-rapid amplification of cDNA ends on toad cardiac mRNA, we cloned full-length cDNAs encoding two alternatively spliced variants of ssTnT. Expression of the cloned cDNAs in Escherichia coli confirmed that the toad cardiac muscle expresses solely ssTnT, predominantly the low molecular weight variant with the exon 5-encoded NH(2)-terminal segment spliced out. Functional studies were performed in ex vivo working toad hearts and compared with the frog (Rana) hearts. The results showed that toad hearts had higher contractile and relaxation velocities and were able to work against a significantly higher afterload than that of frog hearts. Therefore, the unique evolutionary adaptation of utilizing exclusively ssTnT in toad cardiac muscle corresponded to a fitness value from improving systolic function of the heart. The data demonstrated a physiological importance of the functional diversity of TnT isoforms. The structure-function relationship of TnT may be explored for the development of new treatment of heart failure.

Molecular cloning, expression, purification and characterization of truncated forms of human plasminogen in Pichia pastoris expression system

Human plasminogen (HPG), which contains a catalytic domain together with five kringle domains, can be readily purified from blood plasma by chromatography on lysine-agarose. However, its truncated derivatives are needed for various important therapeutic applications. The proteolytic digestion of plasminogen in vitro yields several low molecular weight variants viz. the kringle-less catalytic domain, known as micro-plasminogen (microPG), or miniplasminogen (miniPG), which consists of microPG with an intact kringle-5 domain. However, this method is extremely cumbersome due to a requirement of stringent control on the limited proteolysis process, which often leads to very poor recoveries of the desired product/s, apart from the potentially serious safety-regulatory issues associated with blood-derived therapeutic products. Here, we describe the high-level secretory expression of these important plasminogen derivatives employing Pichia pastoris as the expression host with an engineered alpha-mating factor signal sequence. The purified proteins were found to be functionally comparable with human blood plasminogen-derived ‘native’ forms in terms of their N-terminal amino acid sequences and molecular mass, as well as functional properties. This study paves the way for the facile large-scale production of recombinant human plasminogen derivatives required for thrombolytic and other life-saver therapies.

Expression of a thyroglobulin (Tg) variant in mouse kidney glomerulus

Thyroglobulin (Tg) is an essential substrate for thyroid hormone biosynthesis whose production is primarily limited to the thyroid follicular cell. We have previously identified an ∼1.2 kb fragment of Tg mRNA in cultured mouse mesangial cells, and in the present study provide evidence showing that this transcript is transcribed and translated into a unique protein (kTg) in the kidney, but not the thyroid gland. Cloning of kTg from a mouse kidney cDNA library showed that transcription starts in the middle of intron 41 of the Tg gene and continues in-frame with the remaining coding sequence of thyroid-derived Tg beginning with exon 42. Translation of this mRNA is predicted to yield a protein of 367 amino acids (40 kDa) containing a unique 13 amino acid sequence serving as a signal peptide followed by a 354 amino acid segment identical to the carboxy-terminal end of thyroid Tg. Western blot analysis with an antibody directed against the C-terminus of thyroid Tg detected a 40 kDa protein expressed in the kidney. Immunohistochemistry with this antibody showed that immunoreactive Tg was localized in podocytes and the mesangial area of the renal glomerulus. A part of a homologous transcript was also detected in human kidney, and the kTg protein was recognized by sera from Hashimoto’s thyroiditis but not from controls. Together these results suggest that a unique low molecular weight variant of Tg is expressed in the kidney, where it could serve both physiological and pathological roles, including that of an autoantigen.

Interaction of Antithrombin with Sulfated, Low Molecular Weight Lignins: OPPORTUNITIES FOR POTENT, SELECTIVE MODULATION OF ANTITHROMBIN FUNCTION

Antithrombin, a major regulator of coagulation and angiogenesis, is known to interact with several natural sulfated polysaccharides. Previously, we prepared sulfated low molecular weight variants of natural lignins, called sulfated dehydrogenation polymers (DHPs) (Henry, B. L., Monien, B. H., Bock, P. E., and Desai, U. R. (2007) J. Biol. Chem. 282, 31891-31899), which have now been found to exhibit interesting antithrombin binding properties. Sulfated DHPs represent a library of diverse noncarbohydrate aromatic scaffolds that possess structures completely different from heparin and heparan sulfate. Fluorescence binding studies indicate that sulfated DHPs bind to antithrombin with micromolar affinity under physiological conditions. Salt dependence of binding affinity indicates that the antithrombin-sulfated DHP interaction involves a massive 80-87% non-ionic component to the free energy of binding. Competitive binding studies with heparin pentasaccharide, epicatechin sulfate, and full-length heparin indicate that sulfated DHPs bind to both the pentasaccharide-binding site and extended heparin-binding site of antithrombin. Affinity capillary electrophoresis resolves a limited number of peaks of antithrombin co-complexes suggesting preferential binding of selected DHP structures to the serpin. Computational genetic algorithm-based virtual screening study shows that only one sulfated DHP structure, out of the 11 present in a library of plausible sequences, bound in the heparin-binding site with a high calculated score supporting selectivity of recognition. Enzyme inhibition studies indicate that only one of the three sulfated DHPs studied is a potent inhibitor of free factor VIIa in the presence of antithrombin. Overall, the chemo-enzymatic origin and antithrombin binding properties of sulfated DHPs present novel opportunities for potent and selective modulation of the serpin function, especially for inhibiting the initiation phase of hemostasis.

Construction of biosensor systems for determining the pathophysiological potential of carrageenan variants

In vitro systems for monitoring safety of nutritional additives are desirable for high-throughput screenings and as a substitute for animal models. Carrageenan (CGN) is a sulfated polysaccharide widely used as a thickener and texturizer in human nutrition and is intensely discussed regarding its pathophysiological potential. Low molecular weight (lm) variants of CGN are considered to exert more profound pathophysiological effects in vivo than high molecular weight (hm) variants. We used a systematic approach to construct reporter systems allowing distinction between CGN-variants with different pathophysiological potential. Reporter systems utilizing segments of the CGN-responsive DMBT1 promoter did not display substantial activity in SW620 cells of intestinal epithelial origin. Genome-wide profiling revealed stronger qualitative and quantitative changes in global gene activities for hm-CGN than for lm-CGN (824 versus 91 genes; -6.64 to 22.33-fold for hm-CGN versus the range of -2.65 to 2.96-fold for lm-CGN). Reporter systems with segments of the IL-8 promoter showed a specific activation in response to hm-sulfated polysaccharides with lower pathophysiological potential in vivo and provided a better classification of CGN-variants than cytotoxicity assays in vitro. IL-8 reporter systems can be used for discerning between the effects of sulfated polysaccharides in vivo. Our data further provide initial insights into the molecular mechanisms that may play a role in the different effects of CGN-variants.

A high proportion of the V delta 1+ synovial fluid gamma delta T cells in juvenile rheumatoid arthritis patients express the very early activation marker CD69, but carry the high molecular weight isoform of the leucocyte common antigen (CD45RA).

We have previously shown that gamma delta T cells in the synovial compartment of patients with juvenile rheumatoid arthritis (JRA) express activation antigens (CD69 and HLA-DR) and that they are predominantly of the V delta 1 subset. In this study we have analysed the expression of activation antigens (CD69 and HLA-DR) and different isoforms of the leucocyte common antigen (CD45RO and CD45RA) on the V delta 1 and the V delta 2 subsets of gamma delta T cells in paired samples of synovial fluid and peripheral blood of nine patients with JRA, and in the peripheral blood of five children with idiopathic scoliosis. In the synovial fluid of children with JRA, there were significantly more V delta 1+CD69+ and V delta 2+CD69+ cells compared with the peripheral blood of the same patients. In contrast, however, in the synovial fluid the V delta 1 and the V delta 2 subsets differed with respect to the expression of the two isoforms of the leucocyte common antigen. The majority of the V delta 1+ cells expressed the high molecular weight isoform (CD45RA+) while most of the V delta 2+ cells carried the low molecular weight variant (CD45RO+) of this molecule.

Sulforaphane Switches Cyclin E Isoform Expression and Blocks Leukemic Cell Cycling.

Sulforaphane (SFN), an isothiocyanate derived from cruciferous vegetables, has been shown to inhibit the growth of prostate cancer cells in vitro and in vivo. We were interested in exploring potential antileukemic effects of SFN. The viability of multiple myeloid leukemic cell lines was decreased by 25uM SFN. Pharmacokinetic studies reported in rats suggest that this serum concentration can be achieved through oral dosing. Lower SFN concentrations (1–5 uM) inhibited leukemic cell growth without affecting cell viability. Synchronized HL-60 cells exposed to 25uM SFN were blocked at the G1/S phase transition. Kinetic analysis of cell cycle proteins demonstrated that the G1/S block arose from downmodulation of cyclins D3 and E rather than upregulation of cdk-inhibitors. Interestingly, we found that HL-60 cells expressed a low molecular weight (LMW, 36 kD) variant of cyclin E rather than (50 kD) full-length cyclin E. Treatment with SFN for as little as 2 hours caused a decrease in expression of the LMW cyclin E and induced the expression of a higher molecular weight (~50 kD) cyclin E isoform. Because LMW cyclin E has been associated with increased cdk2 activity and p27 resistance compared to full-length cyclin E, we postulate that SFN-mediated cyclin E isoform-switching contributed to growth inhibition of these leukemic cells. The signaling pathway through which SFN altered cyclin E expression appeared to be distinct from MEK/ERK and JNK pathways that have been implicated in the apoptotic effects of SFN. Given that cyclin E overexpression and, particularly, LMW cyclin E expression are correlated with poor prognosis in multiple cancers, the mechanism through which SFN decreases LMW cyclin E expression in these leukemic cells could have therapeutic significance.

Novel Role for Cyclin-dependent Kinase 2 in Neuregulin-induced Acetylcholine Receptor ϵ Subunit Expression in Differentiated Myotubes

Cyclin-dependent kinases (CDKs) are a family of evolutionarily conserved serine/threonine kinases. CDK2 acts as a checkpoint for the G(1)/S transition in the cell cycle. Despite a down-regulation of CDK2 activity in postmitotic cells, many cell types, including muscle cells, maintain abundant levels of CDK2 protein. This led us to hypothesize that CDK2 may have a function in postmitotic cells. We show here for the first time that CDK2 can be activated by neuregulin (NRG) in differentiated C2C12 myotubes. In addition, this activity is required for expression of the acetylcholine receptor (AChR) epsilon subunit. The switch from the fetal AChRgamma subunit to the adult-type AChRepsilon is required for synapse maturation and the neuromuscular junction. Inhibition of CDK2 activity with either the specific CDK2 inhibitory peptide Tat-LFG or by RNA interference abolished neuregulin-induced AChRepsilon expression. Neuregulin-induced activation of CDK2 also depended on the ErbB receptor, MAPK, and PI3K, all of which have previously been shown to be required for AChRepsilon expression. Neuregulin regulated CDK2 activity through coordinating phosphorylation of CDK2 on Thr-160, accumulation of CDK2 in the nucleus, and down-regulation of the CDK2 inhibitory protein p27 in the nucleus. In addition, we also observed a novel mechanism of regulation of CDK2 activity by a low molecular weight variant of cyclin E in response to NRG. These findings establish CDK2 as an intermediate molecule that integrates NRG-activated signals from both the MAPK and PI3K pathways to AChRepsilon expression and reveal an undiscovered physiological role for CDK2 in postmitotic cells.

Cellular Fate of Truncated Slow Skeletal Muscle Troponin T Produced by Glu180 Nonsense Mutation in Amish Nemaline Myopathy

A nonsense mutation at codon Glu180 in exon 11 of slow skeletal muscle troponin T (TnT) gene (TNNT1) causes an autosomal-recessive inherited nemaline myopathy. We previously reported the absence of intact or prematurely terminated slow TnT polypeptide in Amish nemaline myopathy (ANM) patient muscle. The present study further investigates the expression and fate of mutant slow TnT in muscle cells. Intact slow TnT mRNA was readily detected in patient muscle, indicating unaffected transcription and RNA splicing. Sequence of the cloned cDNAs revealed the single nucleotide mutation in two alternatively spliced isoforms of slow TnT mRNA. Mutant TNNT1 cDNA is translationally active in Escherichia coli and non-muscle eukaryotic cells, producing the expected truncated slow TnT protein. The mutant mRNA was expressed at significant levels in differentiated C2C12 myotubes, but unlike intact exogenous TnT, truncated slow TnT protein was not detected. Transfective expression in undifferentiated myoblasts produced slow TnT mRNA but not a detectable amount of truncated or intact slow TnT proteins, indicating a muscle cell-specific proteolysis of TnT when it is not integrated into myofilaments. The slow TnT-(1-179) fragment has substantially lower affinity for binding to tropomyosin, in keeping with the loss of one of two tropomyosin-binding sites. Our findings suggest that inefficient incorporation into myofilament is responsible for the instability of mutant slow TnT in ANM muscle. Rapid degradation of the truncated slow TnT protein, rather than instability of the nonsense mRNA, provides the protective mechanism against the potential dominant negative effect of the mutant TnT fragment.