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Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by the degeneration of motor neurons. Mutations in the superoxide dismutase (SOD1) gene, causing protein misfolding and aggregation, were suggested as the pathogenic mechanisms involved in familial ALS cases. In the present study, we investigated the potential therapeutic effect of C4 and C5, two derivatives of the chemical chaperone 4-phenylbutyric acid (4-PBA). By combining in vivo and in vitro techniques, we show that, although C4 and C5 successfully inhibited amyloid aggregation of recombinant mutant SOD1 in a dose-dependent manner, they failed to suppress the accumulation of misfolded SOD1. Moreover, C4 or C5 daily injections to SOD1G93A mice following onset had no effect on either the accumulation of misfolded SOD1 or the neuroinflammatory response in the spinal cord and, consequently, failed to extend the survival of SOD1G93A mice or to improve their motor symptoms. Finally, pharmacokinetic (PK) studies demonstrated that high concentrations of C4 and C5 reached the brain and spinal cord but only for a short period of time. Thus, our findings suggest that use of such chemical chaperones for ALS drug development may need to be optimized for more effective results.

Macrophage migration inhibitory factor (MIF) is a conserved cytokine found as a homotrimer protein. It is found in a wide spectrum of cell types in the body including neuronal and non-neuronal cells. MIF is implicated in several biological processes; chemo-attraction, cytokine activity, and receptor binding, among other functions. More recently, a chaperone-like activity has been added to its repertoire. In this review, we focus on the implication of MIF in the central nervous system and peripheries, its role in neurological disorders, and the mechanisms by which MIF is regulated. Numerous studies have associated MIF with various disease settings. MIF plays an important role in advocating tumorigenic processes, Alzheimer's disease, and is also upregulated in autism-spectrum disorders and spinal cord injury where it contributes to the severity of the injured area. The protective effect of MIF has been reported in amyotrophic lateral sclerosis by its reduction of aggregated misfolded SOD1, subsequently reducing the severity of this disease. Interestingly, a protective as well as pathological role for MIF has been implicated in stroke and cerebral ischemia, as well as depression. Thus, the role of MIF in neurological disorders appears to be diverse with both beneficial and adversary effects. Furthermore, its modulation is rather complex and it is regulated by different proteins, either on a molecular or protein level. This complexity might be dependent on the pathophysiological context and/or cellular microenvironment. Hence, further clarification of its diverse roles in neurological pathologies is warranted to provide new mechanistic insights which may lead in the future to the development of therapeutic strategies based on MIF, to fight some of these neurological disorders.

The normal function of α-synuclein (α-syn) remains elusive. Although recent studies suggest α-syn as a physiologic attenuator of synaptic vesicle (SV) recycling, mechanisms are unclear. Here, we show that synapsin—a cytosolic protein with known roles in SV mobilization and clustering—is required for presynaptic functions of α-syn. Our data offer a critical missing link and advocate a model where α-syn and synapsin cooperate to cluster SVs and attenuate recycling.

Mutations in superoxide dismutase (SOD1) cause amyotrophic lateral sclerosis (ALS), a neurodegenerative disease characterized by the loss of upper and lower motor neurons. Transgenic mice that overexpress mutant SOD1 develop paralysis and accumulate misfolded SOD1 onto the cytoplasmic faces of intracellular organelles, including mitochondria and endoplasmic reticulum (ER). Recently, macrophage migration inhibitory factor (MIF) was shown to directly inhibit mutant SOD1 misfolding and binding to intracellular membranes. In addition, complete elimination of endogenous MIF accelerated disease onset and late disease progression, as well as shortened the lifespan of mutant SOD1 mice with higher amounts of misfolded SOD1 detected within the spinal cord. Based on these findings, we used adeno-associated viral (AAV) vectors to overexpress MIF in the spinal cord of mutant SOD1(G93A) and loxSOD1(G37R) mice. Our data show that MIF mRNA and protein levels were increased in the spinal cords of AAV2/9-MIF–injected mice. Furthermore, mutant SOD1(G93A) and loxSOD1(G37R) mice injected with AAV2/9-MIF demonstrated a significant delay in disease onset and prolonged survival compared with their AAV2/9-GFP–injected or noninjected littermates. Moreover, these mice accumulated reduced amounts of misfolded SOD1 in their spinal cords, with no observed effect on glial overactivation as a result of MIF up-regulation. Our findings indicate that MIF plays a significant role in SOD1 folding and misfolding mechanisms and strengthen the hypothesis that MIF acts as a chaperone for misfolded SOD1 in vivo and may have further implications regarding the therapeutic potential role of up-regulation of MIF in modulating the specific accumulation of misfolded SOD1.

Mutations in superoxide dismutase (SOD1) cause amyotrophic lateral sclerosis (ALS), a fatal neurodegenerative disease caused by the progressive loss of motor neurons in the brain and spinal cord. It has been suggested that toxicity of mutant SOD1 results from its misfolding, however, it is yet unclear why misfolded SOD1 accumulates specifically within motor neurons. We recently demonstrated that macrophage migration inhibitory factor (MIF)—a multifunctional protein with cytokine/chemokine activity and cytosolic chaperone-like properties—inhibits the accumulation of misfolded SOD1. Here, we show that MIF inhibits mutant SOD1 nuclear clearance when overexpressed in motor neuron-like NSC-34 cells. In addition, MIF alters the typical SOD1 amyloid aggregation pathway in vitro, and, instead, promotes the formation of disordered aggregates, as measured by Thioflavin T (ThT) assay and transmission electron microscopy (TEM) imaging. Moreover, we report that MIF reduces the toxicity of misfolded SOD1 by directly interacting with it, and that the chaperone function and protective effect of MIF in neuronal cultures do not require its intrinsic catalytic activities. Importantly, we report that the locked-trimeric MIFN110C mutant, which exhibits strongly impaired CD74-mediated cytokine functions, has strong chaperone activity, dissociating, for the first time, these two cellular functions. Altogether, our study implicates MIF as a potential therapeutic candidate in the treatment of ALS.

Synaptic transmission is expensive in terms of its energy demands and was recently shown to decrease the ATP concentration within presynaptic terminals transiently, an observation that we confirm. We hypothesized that, in addition to being an energy source, ATP may modulate the synapsins directly. Synapsins are abundant neuronal proteins that associate with the surface of synaptic vesicles and possess a well defined ATP-binding site of undetermined function. To examine our hypothesis, we produced a mutation (K270Q) in synapsin IIa that prevents ATP binding and reintroduced the mutant into cultured mouse hippocampal neurons devoid of all synapsins. Remarkably, staining for synaptic vesicle markers was enhanced in these neurons compared with neurons expressing wild-type synapsin IIa, suggesting overly efficient clustering of vesicles. In contrast, the mutation completely disrupted the capability of synapsin IIa to slow synaptic depression during sustained 10 Hz stimulation, indicating that it interfered with synapsin-dependent vesicle recruitment. Finally, we found that the K270Q mutation attenuated the phosphorylation of synapsin IIa on a distant PKA/CaMKI consensus site known to be essential for vesicle recruitment. We conclude that ATP binding to synapsin IIa plays a key role in modulating its function and in defining its contribution to hippocampal short-term synaptic plasticity.

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disorder, with a 10% genetic linkage, of which 20% of these cases may be attributed to mutations in superoxide dismutase (SOD1). Specific mutations in SOD1 have been associated with disease duration, which can be highly variable ranging from a life expectancy of 3 to beyond 10 years. SOD1 neurotoxicity has been attributed to aberrant accumulation of misfolded SOD1, which in its soluble form binds to intracellular organelles disrupting their function or forms insoluble toxic aggregates. To understand whether these biophysical properties of the mutant protein may influence disease onset and duration, we generated 19 point mutations in the SOD1 gene, based on available clinical data of disease onset and progression from patients. By overexpressing these mutants in motor-neuron-like NSC-34 cells, we demonstrate a variability in misfolding capacity between the different mutants with a correlation between the degree of protein misfolding and mutation severity. We also show a clear variation of the different SOD1 mutants to associate with mitochondrial-enriched fractions with a correlation between mutation severity and this association. In summary, these findings reveal a correlation between the accumulation of misfolded SOD1 species and their mitochondrial association with disease duration but not with disease onset, and they have implications for the potential therapeutic role of suppressing the accumulation of misfolded SOD1.

Zinc transporter-1 (ZnT-1) is a putative zinc transporter that confers cellular resistance from zinc toxicity. In addition, ZnT-1 has important regulatory functions, including inhibition of L-type calcium channels and activation of Raf-1 kinase. Here we studied the effects of ZnT-1 on the expression and function of T-type calcium channels. In Xenopus oocytes expressing voltage-gated calcium channel (CaV) 3.1 or CaV3.2, ZnT-1 enhanced the low-threshold calcium currents ( IcaT) to 182 ± 15 and 167.95 ± 9.27% of control, respectively ( P < 0.005 for both channels). As expected, ZnT-1 also enhanced ERK phosphorylation. Coexpression of ZnT-1 and nonactive Raf-1 blocked the ZnT-1-mediated ERK phosphorylation and abolished the ZnT-1-induced augmentation of IcaT. In mammalian cells (Chinese hamster ovary), coexpression of CaV3.1 and ZnT-1 increased the IcaT to 166.37 ± 6.37% compared with cells expressing CaV3.1 alone ( P < 0.01). Interestingly, surface expression measurements using biotinylation or total internal reflection fluorescence microscopy indicated marked ZnT-1-induced enhancement of CaV3.1 surface expression. The MEK inhibitor PD-98059 abolished the ZnT-1-induced augmentation of surface expression of CaV3.1. In cultured murine cardiomyocytes (HL-1 cells), transient exposure to zinc, leading to enhanced ZnT-1 expression, also enhanced the surface expression of endogenous CaV3.1 channels. Consistently, in these cells, endothelin-1, a potent activator of Ras-ERK signaling, enhanced the surface expression of CaV3.1 channels in a PD-98059-sensitive manner. Our findings indicate that ZnT-1 enhances the activity of CaV3.1 and CaV3.2 through activation of Ras-ERK signaling. The augmentation of CaV3.1 currents by Ras-ERK activation is associated with enhanced trafficking of the channel to the plasma membrane.

The synaptic vesicle cycle encompasses the pre-synaptic events that drive neurotransmission. Influx of calcium leads to the fusion of synaptic vesicles with the plasma membrane and the release of neurotransmitter, closely followed by endocytosis. Vacated release sites are repopulated with vesicles which are then primed for release. When activity is intense, reserve vesicles may be mobilized to counteract an eventual decline in transmission. Recently, interplay between endocytosis and repopulation of the readily releasable pool of vesicles has been identified. In this study, we show that exo-endocytosis is necessary to enable detachment of synapsin from reserve pool vesicles during synaptic activity. We report that blockage of exocytosis in cultured mouse hippocampal neurons, either by tetanus toxin or by the deletion of munc13, inhibits the activity-dependent redistribution of synapsin from the pre-synaptic terminal into the axon. Likewise, perturbation of endocytosis with dynasore or by a dynamin dominant-negative mutant fully prevents synapsin redistribution. Such inhibition of synapsin redistribution occurred despite the efficient phosphorylation of synapsin at its protein kinase A/CaMKI site, indicating that disengagement of synapsin from the vesicles requires exocytosis and endocytosis in addition to phosphorylation. Our results therefore reveal hitherto unidentified feedback within the synaptic vesicle cycle involving the synapsin-managed reserve pool.

Adult synapsin triple-knockout mice exhibit epilepsy that manifests as generalized tonic-clonic seizures. Because in vitro recordings have shown a reduction in quantal release from inhibitory neurons, an inherent excitation-inhibition imbalance has been hypothesized as the direct culprit for epilepsy in these mice. We critically assessed this hypothesis by examining neurotransmission during the emergence of epilepsy. Using long-term video and telemetric EEG monitoring we found that synapsin triple-knockout mice exhibit an abrupt transition during early adulthood from a seizure-free presymptomatic latent state to a consistent symptomatic state of sensory-induced seizures. Electrophysiological recordings showed that during the latent period larger field responses could be elicited in slices from mutant mice. However, only after the transition to a symptomatic state in the adult mice did evoked epileptiform activity become prevalent. This state was characterized by resistance to the epileptiform-promoting effects of 4-aminopyridine, by marked hypersensitivity to blockage of GABAA receptors, and by the emergence of unresponsiveness to NMDA receptor antagonism, all of which were not observed during the latent period. Importantly, enhancement in inhibitory transmission was associated with upregulation of GAD67 expression without affecting the number of inhibitory neurons in the same brain areas where epileptiform activity was recorded. We therefore suggest that while deletion of the synapsins initially increases cortical network activity, this enhanced excitability is insufficient to elicit seizures. Rather, compensatory epileptogenic mechanisms are activated during the latent period that lead to an additional almost-balanced enhancement of both the excitatory and inhibitory components of the network, finally culminating in the emergence of epilepsy.

Contemporary research aims to understand biological processes not only by identifying participating proteins, but also by characterizing the dynamics of their interactions. Because Förster's Resonance Energy Transfer (FRET) is invaluable for the latter undertaking, its usage is steadily increasing. However, FRET measurements are notoriously error-prone, especially when its inherent efficiency is low, a not uncommon situation. Furthermore, many FRET methods are either difficult to implement, are not appropriate for observation of cellular dynamics, or report instrument-specific indices that hamper communication of results within the scientific community. We present here a novel comprehensive spectral methodology, SpRET, which substantially increases both the reliability and sensitivity of FRET microscopy, even under unfavorable conditions such as weak fluorescence or the presence of noise. While SpRET overcomes common pitfalls such as interchannel crosstalk and direct excitation of the acceptor, it also excels in removal of autofluorescence or background contaminations and in correcting chromatic aberrations, often overlooked factors that severely undermine FRET experiments. Finally, SpRET quantitatively reports absolute rather than relative FRET efficiency values, as well as the acceptor-to-donor molar ratio, which is critical for full and proper interpretation of FRET experiments. Thus, SpRET serves as an advanced, improved, and powerful tool in the cell biologist's toolbox.

Atrial fibrillation (AF), the highest occurring cardiac arrhythmia in the Western world, is associated with substantial morbidity and increased mortality. In spite of extensive research, the cause of atrial electrical remodeling, a major factor in the self-perpetuating nature of AF, is still unknown. Downregulation of L-type Ca2+ channel (LTCC) activity is the hallmark of atrial electrical remodeling. ZnT-1 is a ubiquitous membrane protein that was recently suggested to inhibit the LTCC. We have studied and shown that ZnT-1 expression inhibits LTCC function in an oocyte expression system as well as in isolated cardiomyocytes. Our data also show that rapid electrical pacing can augment ZnT-1 expression in culture as well as in the atria of rats in vivo. Finally, in a pilot study, ZnT-1 expression was found to be augmented in the atria of AF patients. These findings position ZnT-1 as a probable missing link in the mechanism underlying atrial tachycardia remodeling.

Stromal cell–derived factor-1 (SDF-1/CXCL12) and its receptor CXCR4 are implicated in the pathogenesis and prognosis of acute myelogenous leukemia (AML). Cellular microparticles, submicron vesicles shed from the plasma membrane of various cells, are also associated with human pathology. In the present study, we investigated the putative relationships between the SDF-1/CXCR4 axis and microparticles in AML. We detected CXCR4-expressing microparticles (CXCR4+ microparticles) in the peripheral blood and bone marrow plasma samples of normal donors and newly diagnosed adult AML patients. In samples from AML patients, levels of CXCR4+ microparticles and total SDF-1 were elevated compared with normal individuals. The majority of CXCR4+ microparticles in AML patients were CD45+, whereas in normal individuals, they were mostly CD41+. Importantly, we found a strong correlation between the levels of CXCR4+ microparticle and WBC count in the peripheral blood and bone marrow plasma obtained from the AML patients. Of interest, levels of functional, noncleaved SDF-1 were reduced in these patients compared with normal individuals and also strongly correlated with the WBC count. Furthermore, our data indicate NH2-terminal truncation of the CXCR4 molecule in the microparticles of AML patients. However, such microparticles were capable of transferring the CXCR4 molecule to AML-derived HL-60 cells, enhancing their migration to SDF-1 in vitro and increasing their homing to the bone marrow of irradiated NOD/SCID/β2mnull mice. The CXCR4 antagonist AMD3100 reduced these effects. Our findings suggest that functional CXCR4+ microparticles and SDF-1 are involved in the progression of AML. We propose that their levels are potentially valuable as an additional diagnostic AML variable. (Cancer Res 2006; 66(22): 11013-20)

The p53 tumor suppressor gene was found to play a role in the differentiation of several tissue types. We report here that p53-dependent apoptosis plays a role in the final stages of physiological differentiation of normoblasts, resulting in nuclear condensation and expulsion without cell death. Blood samples of healthy newborns, cord blood as well as bone marrow, were analysed for apoptosis by TUNEL and p53 expression by immunostaining. While some samples exhibited simultaneously several distinct patterns of apoptosis, such as perinuclear, diffused nuclear or nuclear apoptotic bodies, others presented a single defined pattern. Overexpression of p53 protein was detected in normoblasts exhibiting either perinuclear or diffused nuclear p53, corresponding to the nuclear apoptotic pattern in the same sample. Similar results were also evident with colonies cultivated for 12-14 days in culture. Differentiated erythroid colonies exhibited overexpression of p53 and positive TUNEL staining only in the normoblasts. We further examined the state of caspase 3/7 and observed a decrease of this activated enzyme during erythroid differentiation in culture. This study suggests a novel role for apoptosis in normoblast differentiation where nuclear degradation occurs with a delay in the actual cell death. A pivotal role for the p53-dependent apoptosis in the erythroid lineage development is implied. However, this apoptotic process is not fully executed because of the exhaustion in caspase 3/7 and thus cells are diverted towards final stages of differentiation.

Homing and repopulation of nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice by enriched human CD34(+) stem cells from cord blood, bone marrow, or mobilized peripheral blood are dependent on stromal cell-derived factor 1 (SDF-1)/CXCR4 interactions. Recently, human cord and fetal blood CD34(+)CD38(-)CXCR4(-) and CXCR4(+) cells, sorted with neutralizing anti-CXCR4 monoclonal antibody (mAb), were shown to have similar NOD/SCID repopulation potential. Herein we report that human cord blood CD34(+)CXCR4(+) (R4(+)) and CD34(+)CXCR4(-) (R4(-)) subsets, sorted with neutralizing anti-CXCR4 mAb, engrafted NOD/SCID mice with significantly lower levels of human cells compared with nonsorted and SDF-1-migrated CD34(+) cells. Coinjection of purified cells with 10 microg anti-CXCR4 mAb significantly reduced engraftment of all CD34(+) subsets, and 50 microg completely abrogated engraftment by R4(-) and CD34(+) cells. Importantly, R4(-) cells harbor intracellular CXCR4, which can be rapidly induced to cell surface expression within a few hours. Moreover, 48 hours of cytokine stimulation resulted in up-regulation of both cell surface and intracellular CXCR4, restoring migration capacities toward a gradient of SDF-1 and high-level NOD/SCID repopulation potential. In addition, homing of sorted R4(-) cells into the murine bone marrow and spleen was significantly slower and reduced compared to CD34(+) cells but yet CXCR4 dependent. In conclusion, R4(-) cells express intracellular CXCR4, which can be functionally expressed on the cell membrane to mediate SDF-1-dependent homing and repopulation. Our results suggest dynamic CXCR4 expression on CD34(+) stem and progenitor cells, regulating their motility and repopulation capacities.

Presynaptic terminals are specialized sites for information transmission where vesicles fuse with the plasma membrane and are locally recycled. Recent work has extended this classical view, with the observation that a subset of functional vesicles is dynamically shared between adjacent terminals by lateral axonal transport. Conceptually, such transport would be expected to disrupt vesicle retention around the active zone, yet terminals are characterized by a high-density vesicle cluster, suggesting that counteracting stabilizing mechanisms must operate against this tendency. The synapsins are a family of proteins that associate with synaptic vesicles and determine vesicle numbers at the terminal, but their specific function remains controversial. Here, using multiple quantitative fluorescence-based approaches and electron microscopy, we show that synapsin is instrumental for resisting vesicle dispersion and serves as a regulatory element for controlling lateral vesicle sharing between synapses. Deleting synapsin disrupts the organization of presynaptic vesicle clusters, making their boundaries hard to define. Concurrently, the fraction of vesicles amenable to transport is increased, and more vesicles are translocated to the axon. Importantly, in neurons from synapsin knock-out mice the resting and recycling pools are equally mobile. Synapsin, when present, specifically restricts the mobility of resting pool vesicles without affecting the division of vesicles between these pools. Specific expression of synapsin IIa, the sole isoform affecting synaptic depression, rescues the knock-out phenotype. Together, our results show that synapsin is pivotal for maintaining synaptic vesicle cluster integrity and that it contributes to the regulated sharing of vesicles between terminals.

The synaptic vesicle cycle encompasses the pre-synaptic events that drive neurotransmission. Influx of calcium leads to the fusion of synaptic vesicles with the plasma membrane and the release of neurotransmitter, closely followed by endocytosis. Vacated release sites are repopulated with vesicles which are then primed for release. When activity is intense, reserve vesicles may be mobilized to counteract an eventual decline in transmission. Recently, interplay between endocytosis and repopulation of the readily releasable pool of vesicles has been identified. In this study, we show that exo-endocytosis is necessary to enable detachment of synapsin from reserve pool vesicles during synaptic activity. We report that blockage of exocytosis in cultured mouse hippocampal neurons, either by tetanus toxin or by the deletion of munc13, inhibits the activity-dependent redistribution of synapsin from the pre-synaptic terminal into the axon. Likewise, perturbation of endocytosis with dynasore or by a dynamin dominant-negative mutant fully prevents synapsin redistribution. Such inhibition of synapsin redistribution occurred despite the efficient phosphorylation of synapsin at its protein kinase A/CaMKI site, indicating that disengagement of synapsin from the vesicles requires exocytosis and endocytosis in addition to phosphorylation. Our results therefore reveal hitherto unidentified feedback within the synaptic vesicle cycle involving the synapsin-managed reserve pool.

Activation of ERK signaling may promote cardioprotection from ischemia–reperfusion (I/R) injury. ZnT-1, a protein that confers resistance from zinc toxicity, was found to interact with Raf-1 kinase through its C-terminal domain, leading to downstream activation of ERK. In the present study, we evaluated the effects of ZnT-1 in cultured murine cardiomyocytes (HL-1 cells) that were exposed to simulated-I/R. Cellular injury was evaluated by lactate dehydrogenase (LDH) release and by staining for pro-apoptotic caspase activation. Overexpression of ZnT-1 markedly reduced LDH release and caspase activation following I/R. Knockdown of endogenous ZnT-1 augmented the I/R-induced release of LDH and increased caspase activation following I/R. Phospho-ERK levels were significantly increased following I/R in cells overexpressing ZnT-1, while knockdown of ZnT-1 reduced phospho-ERK levels. Pretreatment of cells with the MEK inhibitor PD98059 abolished the protective effect of ZnT-1 following I/R. Accordingly, a truncated form of ZnT-1 lacking the C-terminal domain failed to induce ERK activation and did not protect the cells from I/R injury. In contrast, expression of the C-terminal domain by itself was sufficient to induce ERK activation and I/R protection. Interestingly, the C-terminal of the ZnT-1 did not have protective effect against the toxicity of zinc. In the isolated rat heart, global ischemic injury rapidly increased the endogenous levels of ZnT-1. However, following reperfusion ZnT-1 levels were found to be decreased. Our findings indicate that ZnT-1 may have important role in the ischemic myocardium through its ability to interact with Raf-1 kinase.

Atrial fibrillation (AF), the highest occurring cardiac arrhythmia in the Western world, is associated with substantial morbidity and increased mortality. In spite of extensive research, the cause of atrial electrical remodeling, a major factor in the self-perpetuating nature of AF, is still unknown. Downregulation of L-type Ca2+ channel (LTCC) activity is the hallmark of atrial electrical remodeling. ZnT-1 is a ubiquitous membrane protein that was recently suggested to inhibit the LTCC. We have studied and shown that ZnT-1 expression inhibits LTCC function in an oocyte expression system as well as in isolated cardiomyocytes. Our data also show that rapid electrical pacing can augment ZnT-1 expression in culture as well as in the atria of rats in vivo. Finally, in a pilot study, ZnT-1 expression was found to be augmented in the atria of AF patients. These findings position ZnT-1 as a probable missing link in the mechanism underlying atrial tachycardia remodeling.

SummaryDuring embryogenesis, organ development is dependent upon maintaining appropriate progenitor cell commitment. Synovial joints develop from a pool of progenitor cells that differentiate into various cell types constituting the mature joint. The involvement of the musculature in joint formation has long been recognized. However, the mechanism by which the musculature regulates joint formation has remained elusive. In this study, we demonstrate, utilizing various murine models devoid of limb musculature or its contraction, that the contracting musculature is fundamental in maintaining joint progenitors committed to their fate, a requirement for correct joint cavitation and morphogenesis. Furthermore, contraction-dependent activation of β-catenin, a key modulator of joint formation, provides a molecular mechanism for this regulation. In conclusion, our findings provide the missing link between progenitor cell fate determination and embryonic movement, two processes shown to be essential for correct organogenesis.

A major limitation to clinical stem cell–mediated gene therapy protocols is the low levels of engraftment by transduced progenitors. We report that CXCR4 overexpression on human CD34+ progenitors using a lentiviral gene transfer technique helped navigate these cells to the murine bone marrow and spleen in response to stromal-derived factor 1 (SDF-1) signaling. Cells overexpressing CXCR4 exhibited significant increases in SDF-1–mediated chemotaxis and actin polymerization compared with control cells. A major advantage of CXCR4 overexpression was demonstrated by the ability of transduced CD34+ cells to respond to lower, physiologic levels of SDF-1 when compared to control cells, leading to improved SDF-1–induced migration and proliferation/survival, and finally resulting in significantly higher levels of in vivo repopulation of nonobese diabetic/severe combined immunodeficiency (NOD/SCID) mice including primitive CD34+/CD38-/low cells. Importantly, no cellular transformation was observed following transduction with the CXCR4 vector. Unexpectedly, we documented lack of receptor internalization in response to high levels of SDF-1, which can also contribute to increased migration and proliferation by the transduced CD34+ cells. Our results suggest CXCR4 overexpression for improved definitive human stem cell motility, retention, and multilineage repopulation, which could be beneficial for in vivo navigation and expansion of hematopoietic progenitors. (Blood. 2004;103:2942-2949)

Hematopoietic stem cells rarely contribute to hepatic regeneration, however, the mechanisms governing their homing to the liver, which is a crucial first step, are poorly understood. The chemokine stromal cell-derived factor-1 (SDF-1), which attracts human and murine progenitors, is expressed by liver bile duct epithelium. Neutralization of the SDF-1 receptor CXCR4 abolished homing and engraftment of the murine liver by human CD34+ hematopoietic progenitors, while local injection of human SDF-1 increased their homing. Engrafted human cells were localized in clusters surrounding the bile ducts, in close proximity to SDF-1-expressing epithelial cells, and differentiated into albumin-producing cells. Irradiation or inflammation increased SDF-1 levels and hepatic injury induced MMP-9 activity, leading to both increased CXCR4 expression and SDF-1-mediated recruitment of hematopoietic progenitors to the liver. Unexpectedly, HGF, which is increased following liver injury, promoted protrusion formation, CXCR4 upregulation, and SDF-1-mediated directional migration by human CD34+ progenitors, and synergized with stem cell factor. Thus, stress-induced signals, such as increased expression of SDF-1, MMP-9, and HGF, recruit human CD34+ progenitors with hematopoietic and/or hepatic-like potential to the liver of NOD/SCID mice. Our results suggest the potential of hematopoietic CD34+/CXCR4+cells to respond to stress signals from nonhematopoietic injured organs as an important mechanism for tissue targeting and repair.

Stromal cell-derived factor-1 (SDF-1/CXCL12) and its receptor CXCR4 are implicated in the pathogenesis and prognosis of AML. Cellular microparticles (MPs), the submicron vesicles shed from the plasma membrane of various circulating cells, are associated with numerous human disorders. In the present study, we studied the putative relationships between CXCR4/SDF-1 axis and MPs in AML. We detected CXCR4 expressing MPs (CXCR4+MPs) in the peripheral blood and bone marrow plasma samples of normal donors (n=24) and newly diagnosed adult AML patients (n=26). The majority of CXCR4+MPs in AML patients were CD45+ whereas in normal individuals they were mostly CD41+. In samples from AML patients, the levels of CXCR4+MPs and total SDF-1 were significantly elevated as compared to normal individuals. Importantly, we found a strong correlation between the levels of CXCR4+MP and white blood cell (WBC) counts in the peripheral blood and bone marrow plasma obtained from the AML patients. Of interest, functional, non-cleaved SDF-1 levels were reduced in these patients compared to normal individuals, and also strongly correlated with the WBC counts. Furthermore, our data indicate N-terminal truncation of the CXCR4 molecule in the MPs of AML patients. This was found also in MPs obtained from the conditioned media of normal human CD34+ progenitors lentiviarlly transduced with CXCR4 vector in vitro. Appearance of MPs possessing N-terminally truncated CXCR4 in AML patients is likely to be dependent on proteolytic enzymes, such as elastase, which was elevated. However, MPs isolated from AML patients were capable of transferring functional CXCR4 molecule to the AML-derived HL-60 cells, enhancing their migration to SDF-1 in vitro and increasing their homing to the bone marrow of irradiated NOD/SCID mice. The CXCR4 antagonist AMD3100 reduced the increased migration and homing of MP treated HL-60 cells. Taken together, these findings suggest that functional CXCR4+MPs and SDF-1 are involved in the progression of AML. We propose that their levels are potentially valuable as an additional diagnostic AML parameter. Moreover, our findings suggest also the need for CXCR4- and SDF-1-target therapeutic approaches, clinically relevant in AML in the near future.