Spinal muscular atrophy and a model for survival of motor neuron protein function in axonal ribonucleoprotein complexes

Delivery of Various Cargos into Cancer Cells and Tissues via Cell-Penetrating Peptides: A Review of the Last Decade

Cell-penetrating peptides (CPPs), also known as protein transduction domains, are a class of diverse amino acid sequences with the ability to cross cellular membranes. CPPs can deliver several bioactive cargos, including proteins, peptides, nucleic acids and chemotherapeutics, into cells. Ever since their discovery, synthetic and natural CPPs have been utilized in therapeutics delivery, gene editing and cell imaging in fundamental research and clinical experiments. Over the years, CPPs have gained significant attention due to their low cytotoxicity and high transduction efficacy. In the last decade, multiple investigations demonstrated the potential of CPPs as carriers for the delivery of therapeutics to treat various types of cancer. Besides their remarkable efficacy owing to fast and efficient delivery, a crucial benefit of CPP-based cancer treatments is delivering anticancer agents selectively, rather than mediating toxicities toward normal tissues. To obtain a higher therapeutic index and to improve cell and tissue selectivity, CPP-cargo constructions can also be complexed with other agents such as nanocarriers and liposomes to obtain encouraging outcomes. This review summarizes various types of CPPs conjugated to anticancer cargos. Furthermore, we present a brief history of CPP utilization as delivery systems for anticancer agents in the last decade and evaluate several reports on the applications of CPPs in basic research and preclinical studies.

Fluorescence Correlation Spectroscopy Reveals Survival Motor Neuron Oligomerization but No Active Transport in Motor Axons of a Zebrafish Model for Spinal Muscular Atrophy

Spinal Muscular Atrophy (SMA) is a progressive neurodegenerative disease affecting lower motor neurons that is caused by a deficiency in ubiquitously expressed Survival Motor Neuron (SMN) protein. Two mutually exclusive hypotheses have been discussed to explain increased motor neuron vulnerability in SMA. Reduced SMN levels have been proposed to lead to defective snRNP assembly and aberrant splicing of transcripts that are essential for motor neuron maintenance. An alternative hypothesis proposes a motor neuron-specific function for SMN in axonal transport of mRNAs and/or RNPs. To address these possibilities, we used a novel in vivo approach with fluorescence correlation spectroscopy (FCS) in transgenic zebrafish embryos to assess the subcellular dynamics of Smn in motor neuron cell bodies and axons. Using fluorescently tagged Smn we show that it exists as two freely diffusing components, a monomeric, and a complex-bound, likely oligomeric, component. This oligomer hypothesis was supported by the disappearance of the complex-bound form for a truncated Smn variant that is deficient in oligomerization and a change in its dynamics under endogenous Smn deficient conditions. Surprisingly, our FCS measurements did not provide any evidence for an active transport of Smn in axons. Instead, our in vivo observations are consistent with previous findings that SMN acts as a chaperone for the assembly of snRNP and mRNP complexes.

Decoding Neuromuscular Disorders Using Phenotypic Clusters Obtained From Co-Occurrence Networks

Neuromuscular disorders (NMDs) represent an important subset of rare diseases associated with elevated morbidity and mortality whose diagnosis can take years. Here we present a novel approach using systems biology to produce functionally-coherent phenotype clusters that provide insight into the cellular functions and phenotypic patterns underlying NMDs, using the Human Phenotype Ontology as a common framework. Gene and phenotype information was obtained for 424 NMDs in OMIM and 126 NMDs in Orphanet, and 335 and 216 phenotypes were identified as typical for NMDs, respectively. ‘Elevated serum creatine kinase’ was the most specific to NMDs, in agreement with the clinical test of elevated serum creatinine kinase that is conducted on NMD patients. The approach to obtain co-occurring NMD phenotypes was validated based on co-mention in PubMed abstracts. A total of 231 (OMIM) and 150 (Orphanet) clusters of highly connected co-occurrent NMD phenotypes were obtained. In parallel, a tripartite network based on phenotypes, diseases and genes was used to associate NMD phenotypes with functions, an approach also validated by literature co-mention, with KEGG pathways showing proportionally higher overlap than Gene Ontology and Reactome. Phenotype-function pairs were crossed with the co-occurrent NMD phenotype clusters to obtain 40 (OMIM) and 72 (Orphanet) functionally coherent phenotype clusters. As expected, many of these overlapped with known diseases and confirmed existing knowledge. Other clusters revealed interesting new findings, indicating informative phenotypes for differential diagnosis, providing deeper knowledge of NMDs, and pointing towards specific cell dysfunction caused by pleiotropic genes. This work is an example of reproducible research that i) can help better understand NMDs and support their diagnosis by providing a new tool that exploits existing information to obtain novel clusters of functionally-related phenotypes, and ii) takes us another step towards personalised medicine for NMDs.

Localization elements and zip codes in the intracellular transport and localization of messenger RNAs in Saccharomyces cerevisiae

Intracellular trafficking and localization of mRNAs provide a mechanism of regulation of expression of genes with excellent spatial control. mRNA localization followed by localized translation appears to be a mechanism of targeted protein sorting to a specific cell-compartment, which is linked to the establishment of cell polarity, cell asymmetry, embryonic axis determination, and neuronal plasticity in metazoans. However, the complexity of the mechanism and the components of mRNA localization in higher organisms prompted the use of the unicellular organism Saccharomyces cerevisiae as a simplified model organism to study this vital process. Current knowledge indicates that a variety of mRNAs are asymmetrically and selectively localized to the tip of the bud of the daughter cells, to the vicinity of endoplasmic reticulum, mitochondria, and nucleus in this organism, which are connected to diverse cellular processes. Interestingly, specific cis-acting RNA localization elements (LEs) or RNA zip codes play a crucial role in the localization and trafficking of these localized mRNAs by providing critical binding sites for the specific RNA-binding proteins (RBPs). In this review, we present a comprehensive account of mRNA localization in S. cerevisiae, various types of localization elements influencing the mRNA localization, and the RBPs, which bind to these LEs to implement a number of vital physiological processes. Finally, we emphasize the significance of this process by highlighting their connection to several neuropathological disorders and cancers. This article is categorized under: RNA Export and Localization > RNA Localization.

Splicing Defects of the Profilin Gene Alter Actin Dynamics in an S. pombe SMN Mutant

Spinal muscular atrophy (SMA) is a devastating motor neuron disorder caused by mutations in the survival motor neuron (SMN) gene. It remains unclear how SMN deficiency leads to the loss of motor neurons. By screening Schizosaccharomyces pombe, we found that the growth defect of an SMN mutant can be alleviated by deletion of the actin-capping protein subunit gene acp1⁺. We show that SMN mutated cells have splicing defects in the profilin gene, which thus directly hinder actin cytoskeleton homeostasis including endocytosis and cytokinesis. We conclude that deletion of acp1⁺ in an SMN mutant background compensates for actin cytoskeleton alterations by restoring redistribution of actin monomers between different types of cellular actin networks. Our data reveal a direct correlation between an impaired function of SMN in snRNP assembly and defects in actin dynamics. They also point to important common features in the pathogenic mechanism of SMA and ALS.

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Splicing defects in the profilin gene in an S. pombe SMN mutant

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SMN mutant contains excessively polymerized actin

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Altered actin dynamics in the SMN mutant hinders endocytosis and cytokinesis

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Deletion of the acp1 subunit restores actin dynamics in the SMN mutant

mRNP assembly, axonal transport, and local translation in neurodegenerative diseases

The development, maturation, and maintenance of the mammalian nervous system rely on complex spatiotemporal patterns of gene expression. In neurons, this is achieved by the expression of differentially localized isoforms and specific sets of mRNA-binding proteins (mRBPs) that regulate RNA processing, mRNA trafficking, and local protein synthesis at remote sites within dendrites and axons. There is growing evidence that axons contain a specialized transcriptome and are endowed with the machinery that allows them to rapidly alter their local proteome via local translation and protein degradation. This enables axons to quickly respond to changes in their environment during development, and to facilitate axon regeneration and maintenance in adult organisms. Aside from providing autonomy to neuronal processes, local translation allows axons to send retrograde injury signals to the cell soma. In this review, we discuss evidence that disturbances in mRNP transport, granule assembly, axonal localization, and local translation contribute to pathology in various neurodegenerative diseases, including spinal muscular atrophy (SMA), amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD), and Alzheimer's disease (AD).

Axon outgrowth and neuronal differentiation defects after a-SMN and FL-SMN silencing in primary hippocampal cultures

Spinal Muscular Atrophy (SMA) is a severe autosomal recessive disease characterized by selective motor neuron degeneration, caused by disruptions of the Survival of Motor Neuron 1 (Smn1) gene. The main product of SMN1 is the full-length SMN protein (FL-SMN), that plays an established role in mRNA splicing. FL-SMN is also involved in neurite outgrowth and axonal transport. A shorter SMN isoform, axonal-SMN or a-SMN, displays a more specific axonal localization and has remarkable axonogenic properties in NSC-34. Introduction of known SMA mutations into the a-SMN transcript leads to impairment of axon growth and morphological defects similar to those observed in SMA patients and animal models. Although there is increasing evidence for the relevance of SMN axonal functions in SMA pathogenesis, the specific contributions of FL-SMN and a-SMN are not known yet. This work aimed to analyze the differential roles of FL-SMN and a-SMN in axon outgrowth and in neuronal homeostasis during differentiation of neurons into a mature phenotype. We employed primary cultures of hippocampal neurons as a well-defined model of polarization and differentiation. By analyzing subcellular localization, we showed that a-SMN is preferentially localized in the growing axonal compartment. By specifically silencing FL-SMN or a-SMN proteins, we demonstrated that both proteins play a role in axon growth, as their selective down-regulation reduces axon length without affecting dendritic arborization. a-SMN silencing, and in minor extent FL-SMN silencing, resulted in the growth of multi-neuritic neurons, impaired in the differentiation process of selecting a single axon out of multiple neurites. In these neurons, neurites often display mixed axonal and dendritic markers and abnormal distribution of the axonal initial segment protein Ankirin G, suggesting loss of neuronal polarity. Our results indicate that a-SMN and FL-SMN are needed for neuronal polarization and organization of axonal and dendritic compartments, processes that are fundamental for neuronal function and survival.

Smn-Deficiency Increases the Intrinsic Excitability of Motoneurons

During development, motoneurons experience significant changes in their size and in the number and strength of connections that they receive, which requires adaptive changes in their passive and active electrical properties. Even after reaching maturity, motoneurons continue to adjust their intrinsic excitability and synaptic activity for proper functioning of the sensorimotor circuit in accordance with physiological demands. Likewise, if some elements of the circuit become dysfunctional, the system tries to compensate for the alterations to maintain appropriate function. In Spinal Muscular Atrophy (SMA), a severe motor disease, spinal motoneurons receive less excitation from glutamatergic sensory fibers and interneurons and are electrically hyperexcitable. Currently, the origin and relationship among these alterations are not completely established. In this study, we investigated whether Survival of Motor Neuron (SMN), the ubiquitous protein defective in SMA, regulates the excitability of motoneurons before and after the establishment of the synaptic contacts. To this end, we performed patch-clamp recordings in embryonic spinal motoneurons forming complex synaptic networks in primary cultures, and in differentiated NSC-34 motoneuron-like cells in the absence of synaptic contacts. Our results show that in both conditions, Smn-deficient cells displayed lower action potential threshold, greater action potential amplitudes, and larger density of voltage-dependent sodium currents than cells with normal Smn-levels. These results indicate that Smn participates in the regulation of the cell-autonomous excitability of motoneurons at an early stage of development. This finding may contribute to a better understanding of motoneuron excitability in SMA during the development of the disease.

Dysregulation of mRNA Localization and Translation in Genetic Disease

RNA-binding proteins (RBPs) acting at various steps in the post-transcriptional regulation of gene expression play crucial roles in neuronal development and synaptic plasticity. Genetic mutations affecting several RBPs and associated factors lead to diverse neurological symptoms, as characterized by neurodevelopmental and neuropsychiatric disorders, neuromuscular and neurodegenerative diseases, and can often be multisystemic diseases. We will highlight the physiological roles of a few specific proteins in molecular mechanisms of cytoplasmic mRNA regulation, and how these processes are dysregulated in genetic disease. Recent advances in computational biology and genomewide analysis, integrated with diverse experimental approaches and model systems, have provided new insights into conserved mechanisms and the shared pathobiology of mRNA dysregulation in disease. Progress has been made to understand the pathobiology of disease mechanisms for myotonic dystrophy, spinal muscular atrophy, and fragile X syndrome, with broader implications for other RBP-associated genetic neurological diseases. This gained knowledge of underlying basic mechanisms has paved the way to the development of therapeutic strategies targeting disease mechanisms.

A new cis-acting motif is required for the axonal SMN-dependent Anxa2 mRNA localization

Spinal muscular atrophy (SMA) is caused by mutations and/or deletions of the survival motor neuron gene (SMN1). Besides its function in the biogenesis of spliceosomal snRNPs, SMN might possess a motor neuron specific role and could function in the transport of axonal mRNAs and in the modulation of local protein translation. Accordingly, SMN colocalizes with axonal mRNAs of differentiated NSC-34 motor neuron-like cells. We recently showed that SMN depletion gives rise to a decrease in the axonal transport of the mRNAs encoding Annexin A2 (Anxa2). In this work, we have characterized the structural features of the Anxa2 mRNA required for its axonal targeting by SMN. We found that a G-rich motif located near the 3'UTR is essential for axonal localization of the Anxa2 transcript. We also show that mutations in the motif sequence abolish targeting of Anxa2 reporter mRNAs in axon-like structures of differentiated NSC-34 cells. Finally, localization of both wild-type and mutated Anxa2 reporters is restricted to the cell body in SMN-depleted cells. Altogether, our studies show that this G-motif represents a novel and essential determinant for axonal localization of the Anxa2 mRNA mediated by the SMN complex.

Spatially and temporally regulating translation via mRNA-binding proteins in cellular and neuronal function

Coordinated regulation of mRNA localization and local translation are essential steps in cellular asymmetry and function. It is increasingly evident that mRNA-binding proteins play critical functions in controlling the fate of mRNA, including when and where translation occurs. In this review, we discuss the robust and complex roles that mRNA-binding proteins play in the regulation of local translation that impact cellular function in vertebrates. First, we discuss the role of local translation in cellular polarity and possible links to vertebrate development and patterning. Next, we discuss the expanding role for local protein synthesis in neuronal development and function, with special focus on how a number of neurological diseases have given us insight into the importance of translational regulation. Finally, we discuss the ever-increasing set of tools to study regulated translation and how these tools will be vital in pushing forward and addressing the outstanding questions in the field.

Linking amyotrophic lateral sclerosis and spinal muscular atrophy through RNA-transcriptome homeostasis: a genomics perspective

In this review, we present our most recent understanding of key biomolecular processes that underlie two motor neuron degenerative disorders, amyotrophic lateral sclerosis, and spinal muscular atrophy. We focus on the role of four multifunctional proteins involved in RNA metabolism (TDP-43, FUS, SMN, and Senataxin) that play a causal role in these diseases. Recent results have led to a novel scenario of intricate connections between these four proteins, bringing transcriptome homeostasis into the spotlight as a common theme in motor neuron degeneration. We review reported functional and physical interactions between these four proteins, highlighting their common association with nuclear bodies and small nuclear ribonucleoprotein particle biogenesis and function. We discuss how these interactions are turning out to be particularly relevant for the control of transcription and chromatin homeostasis, including the recent identification of an association between SMN and Senataxin required to ensure the resolution of DNA-RNA hybrid formation and proper termination by RNA polymerase II. These connections strongly support the existence of common pathways underlying the spinal muscular atrophy and amyotrophic lateral sclerosis phenotype. We also discuss the potential of genome-wide expression profiling, in particular RNA sequencing derived data, to contribute to unravelling the underlying mechanisms. We provide a review of publicly available datasets that have addressed both diseases using these approaches, and highlight the value of investing in cross-disease studies to promote our understanding of the pathways leading to neurodegeneration.

Deficiency of the Survival of Motor Neuron Protein Impairs mRNA Localization and Local Translation in the Growth Cone of Motor Neurons

Spinal muscular atrophy (SMA) is a neurodegenerative disease primarily affecting spinal motor neurons. It is caused by reduced levels of the survival of motor neuron (SMN) protein, which plays an essential role in the biogenesis of spliceosomal small nuclear ribonucleoproteins in all tissues. The etiology of the specific defects in the motor circuitry in SMA is still unclear, but SMN has also been implicated in mediating the axonal localization of mRNA-protein complexes, which may contribute to the axonal degeneration observed in SMA. Here, we report that SMN deficiency severely disrupts local protein synthesis within neuronal growth cones. We also identify the cytoskeleton-associated growth-associated protein 43 (GAP43) mRNA as a new target of SMN and show that motor neurons from SMA mouse models have reduced levels ofGAP43mRNA and protein in axons and growth cones. Importantly, overexpression of two mRNA-binding proteins, HuD and IMP1, restoresGAP43mRNA and protein levels in growth cones and rescues axon outgrowth defects in SMA neurons. These findings demonstrate that SMN plays an important role in the localization and local translation of mRNAs with important axonal functions and suggest that disruption of this function may contribute to the axonal defects observed in SMA.

SIGNIFICANCE STATEMENT

The motor neuron disease spinal muscular atrophy (SMA) is caused by reduced levels of the survival of motor neuron (SMN) protein, which plays a key role in assembling RNA/protein complexes that are essential for mRNA splicing. It remains unclear whether defects in this well characterized housekeeping function cause the specific degeneration of spinal motor neurons observed in SMA. Here, we describe an additional role of SMN in regulating the axonal localization and local translation of the mRNA encoding growth-associated protein 43 (GAP43). This study supports a model whereby SMN deficiency impedes transport and local translation of mRNAs important for neurite outgrowth and stabilization, thus contributing to axon degeneration, muscle denervation, and motor neuron cell death in SMA.

Large-Scale Production of Adeno-Associated Viral Vector Serotype-9 Carrying the Human Survival Motor Neuron Gene

Recombinant AAV (rAAV) vectors are a suitable vector for gene therapy studies because of desired characteristics such as low immunogenicity, transfection of non-dividing and dividing cells, and long-term expression of the transgene. In this study, the large-scale production of single stranded (ss) and self-complementary (sc) AAV9 carrying the human survival motor neuron (SMN) gene (AAV9-SMN) suitable for in vivo gene therapy studies of SMA was described. SMN cDNA has been cloned into pAAV-CB6-PI and pAAVsc-CB6-PI with and without its specific UTRs, respectively. Both plasmids bear CMV enhancer/beta-actin (CB) promoter, CMV IE enhancer, and polyadenylation signal sequences. 2.5 μg of constructed pAAV-CB6-PI-SMN and pAAVsc-CB6-PI-SMN cause to, respectively, 4.853- and 2.321-fold increases in SMN protein levels in transfected cells compared to untransfected cells. Ss and scAAV9-SMN vectors were also produced from these plasmids by transient transfection of HEK293 cells using CaCl2 solution. The silver staining and electron microscopy analysis demonstrated good quality of both isolated vectors, ssAAV9-SMN and scAAV9-SMN, with the titers of 2.00E+13 and 1.00E+13 GC/ml. The results of this study show that, the plasmid containing UTR elements causes to twice more SMN gene expression in transfected cells. The quality control results show that both produced ss and scAAV9-SMN are suitable for in vivo studies.

Splicing inhibition of U2AF65 leads to alternative exon skipping

U2 snRNP auxiliary factor 65 kDa (U2AF(65)) is a general splicing factor that contacts polypyrimidine (Py) tract and promotes prespliceosome assembly. In this report, we show that U2AF(65) stimulates alternative exon skipping in spinal muscular atrophy (SMA)-related survival motor neuron (SMN) pre-mRNA. A stronger 5' splice-site mutation of alternative exon abolishes the stimulatory effects of U2AF(65). U2AF(65) overexpression promotes its own binding only on the weaker, not the stronger, Py tract. We further demonstrate that U2AF(65) inhibits splicing of flanking introns of alternative exon in both three-exon and two-exon contexts. Similar U2AF(65) effects were observed in Fas (Apo-1/CD95) pre-mRNA. Strikingly, we demonstrate that U2AF(65) even inhibits general splicing of adenovirus major late (Ad ML) or β-globin pre-mRNA. Thus, we conclude that U2AF(65) possesses a splicing Inhibitory function that leads to alternative exon skipping.

Spinal muscular atrophy--recent therapeutic advances for an old challenge

In the past decade, improved understanding of spinal muscular atrophy (SMA) aetiopathogenesis has brought us to a historical turning point: we are at the verge of development of disease-modifying treatments for this hitherto incurable disease. The increasingly precise delineation of molecular targets within the survival of motor neuron (SMN) gene locus has led to the development of promising therapeutic strategies. These novel avenues in treatment for SMA include gene therapy, molecular therapy with antisense oligonucleotides, and small molecules that aim to increase expression of SMN protein. Stem cell studies of SMA have provided an in vitro model for SMA, and stem cell transplantation could be used as a complementary strategy with a potential to treat the symptomatic phases of the disease. Here, we provide an overview of established data and novel insights into SMA pathogenesis, including discussion of the crucial function of the SMN protein. Preclinical evidence and recent advances from ongoing clinical trials are thoroughly reviewed. The final remarks are dedicated to future clinical perspectives in this rapidly evolving field, with a broad discussion on the comparison between the outlined therapeutic approaches and the remaining open questions.

Presynaptic localization of Smn and hnRNP R in axon terminals of embryonic and postnatal mouse motoneurons

Spinal muscular atrophy (SMA) is caused by deficiency of the ubiquitously expressed survival motoneuron (SMN) protein. SMN is crucial component of a complex for the assembly of spliceosomal small nuclear ribonucleoprotein (snRNP) particles. Other cellular functions of SMN are less characterized so far. SMA predominantly affects lower motoneurons, but the cellular basis for this relative specificity is still unknown. In contrast to nonneuronal cells where the protein is mainly localized in perinuclear regions and the nucleus, Smn is also present in dendrites, axons and axonal growth cones of isolated motoneurons invitro. However, this distribution has not been shown invivo and it is not clear whether Smn and hnRNP R are also present in presynaptic axon terminals of motoneurons in postnatal mice. Smn also associates with components not included in the classical SMN complex like RNA-binding proteins FUS, TDP43, HuD and hnRNP R which are involved in RNA processing, subcellular localization and translation. We show here that Smn and hnRNP R are present in presynaptic compartments at neuromuscular endplates of embryonic and postnatal mice. Smn and hnRNP R are localized in close proximity to each other in axons and axon terminals both invitro and invivo. We also provide new evidence for a direct interaction of Smn and hnRNP R invitro and invivo, particularly in the cytosol of motoneurons. These data point to functions of SMN beyond snRNP assembly which could be crucial for recruitment and transport of RNA particles into axons and axon terminals, a mechanism which may contribute to SMA pathogenesis.

GEMINs: potential therapeutic targets for spinal muscular atrophy?

The motor neuron degenerative disease spinal muscular atrophy (SMA) remains one of the most frequently inherited causes of infant mortality. Afflicted patients loose the survival motor neuron 1 (SMN1) gene but retain one or more copies of SMN2, a homolog that is incorrectly spliced. Primary treatment strategies for SMA aim at boosting SMN protein levels, which are insufficient in patients. SMN is known to partner with a set of diverse proteins collectively known as GEMINs to form a macromolecular complex. The SMN-GEMINs complex is indispensible for chaperoning the assembly of small nuclear ribonucleoproteins (snRNPs), which are key for pre-mRNA splicing. Pharmaceutics that alleviate the neuromuscular phenotype by restoring the fundamental function of SMN without augmenting its levels are also crucial in the development of an effective treatment. Their use as an adjunct therapy is predicted to enhance benefit to patients. Inspired by the surprising discovery revealing a premier role for GEMINs in snRNP biogenesis together with in vivo studies documenting their requirement for the correct function of the motor system, this review speculates on whether GEMINs constitute valid targets for SMA therapeutic development.

Spinal muscular atrophy: development and implementation of potential treatments

In neurodegenerative disorders, effective treatments are urgently needed, along with methods to determine whether treatment worked. In this review, we discuss the rapid progress in the understanding of recessive proximal spinal muscular atrophy and how this is leading to exciting potential treatments of the disease. Spinal muscular atrophy is caused by loss of the survival motor neuron 1 (SMN1) gene and reduced levels of SMN protein. The critical downstream targets of SMN deficiency that result in motor neuron loss are not known. However, increasing SMN levels has a marked impact in mouse models, and these therapeutics are rapidly moving toward clinical trials. Promising preclinical therapies, the varying degree of impact on the mouse models, and potential measures of treatment effect are reviewed. One key issue discussed is the variable outcome of increasing SMN at different stages of disease progression.

Electrophysiological Biomarkers in Spinal Muscular Atrophy: Preclinical Proof of Concept

Objective

Preclinical therapies that restore survival motor neuron (SMN) protein levels can dramatically extend survival in spinal muscular atrophy (SMA) mouse models. Biomarkers are needed to effectively translate these promising therapies to clinical trials. Our objective was to investigate electrophysiological biomarkers of compound muscle action potential (CMAP), motor unit number estimation (MUNE) and electromyography (EMG) using an SMA mouse model.

Methods

Sciatic CMAP, MUNE, and EMG were obtained in SMNΔ7 mice at ages 3–13 days and at 21 days in mice with SMN selectively reduced in motor neurons (ChAT^Cre). To investigate these measures as biomarkers of treatment response, measurements were obtained in SMNΔ7 mice treated with antisense oligonucleotide (ASO) or gene therapy.

Results

CMAP was significantly reduced in SMNΔ7 mice at days 6–13 (P < 0.01), and MUNE was reduced at days 7–13 (P < 0.01). Fibrillations were present on EMG in SMNΔ7 mice but not controls (P = 0.02). Similar findings were seen at 21 days in ChAT^Cre mice. MUNE in ASO-treated SMNΔ7 mice were similar to controls at day 12 and 30. CMAP reduction persisted in ASO-treated SMNΔ7 mice at day 12 but was corrected at day 30. Similarly, CMAP and MUNE responses were corrected with gene therapy to restore SMN.

Interpretation

These studies confirm features of preserved neuromuscular function in the early postnatal period and subsequent motor unit loss in SMNΔ7 mice. SMN restoring therapies result in preserved MUNE and gradual repair of CMAP responses. This provides preclinical evidence for the utilization of CMAP and MUNE as biomarkers in future SMA clinical trials.

Dynamics of survival of motor neuron (SMN) protein interaction with the mRNA-binding protein IMP1 facilitates its trafficking into motor neuron axons

Spinal muscular atrophy (SMA) is a lethal neurodegenerative disease specifically affecting spinal motor neurons. SMA is caused by the homozygous deletion or mutation of the survival of motor neuron 1 (SMN1) gene. The SMN protein plays an essential role in the assembly of spliceosomal ribonucleoproteins. However, it is still unclear how low levels of the ubiquitously expressed SMN protein lead to the selective degeneration of motor neurons. An additional role for SMN in the regulation of the axonal transport of mRNA-binding proteins (mRBPs) and their target mRNAs has been proposed. Indeed, several mRBPs have been shown to interact with SMN, and the axonal levels of few mRNAs, such as the β-actin mRNA, are reduced in SMA motor neurons. In this study we have identified the β-actin mRNA-binding protein IMP1/ZBP1 as a novel SMN-interacting protein. Using a combination of biochemical assays and quantitative imaging techniques in primary motor neurons, we show that IMP1 associates with SMN in individual granules that are actively transported in motor neuron axons. Furthermore, we demonstrate that IMP1 axonal localization depends on SMN levels, and that SMN deficiency in SMA motor neurons leads to a dramatic reduction of IMP1 protein levels. In contrast, no difference in IMP1 protein levels was detected in whole brain lysates from SMA mice, further suggesting neuron specific roles of SMN in IMP1 expression and localization. Taken together, our data support a role for SMN in the regulation of mRNA localization and axonal transport through its interaction with mRBPs such as IMP1.

Splicing therapy for neuromuscular disease

Duchenne muscular dystrophy (DMD) and spinal muscular atrophy (SMA) are two of the most common inherited neuromuscular diseases in humans. Both conditions are fatal and no clinically available treatments are able to significantly alter disease course in either case. However, by manipulation of pre-mRNA splicing using antisense oligonucleotides, defective transcripts from the DMD gene and from the SMN2 gene in SMA can be modified to once again produce protein and restore function. A large number of in vitro and in vivo studies have validated the applicability of this approach and an increasing number of preliminary clinical trials have either been completed or are under way. Several different oligonucleotide chemistries can be used for this purpose and various strategies are being developed to facilitate increased delivery efficiency and prolonged therapeutic effect. As these novel therapeutic compounds start to enter the clinical arena, attention must also be drawn to the question of how best to facilitate the clinical development of such personalised genetic therapies and how best to implement their provision.

Implication of the SMN complex in the biogenesis and steady state level of the signal recognition particle

Spinal muscular atrophy is a severe motor neuron disease caused by reduced levels of the ubiquitous Survival of MotoNeurons (SMN) protein. SMN is part of a complex that is essential for spliceosomal UsnRNP biogenesis. Signal recognition particle (SRP) is a ribonucleoprotein particle crucial for co-translational targeting of secretory and membrane proteins to the endoplasmic reticulum. SRP biogenesis is a nucleo-cytoplasmic multistep process in which the protein components, except SRP54, assemble with 7S RNA in the nucleolus. Then, SRP54 is incorporated after export of the pre-particle into the cytoplasm. The assembly factors necessary for SRP biogenesis remain to be identified. Here, we show that 7S RNA binds to purified SMN complexes in vitro and that SMN complexes associate with SRP in cellular extracts. We identified the RNA determinants required. Moreover, we report a specific reduction of 7S RNA levels in the spinal cord of SMN-deficient mice, and in a Schizosaccharomyces pombe strain carrying a temperature-degron allele of SMN. Additionally, microinjected antibodies directed against SMN or Gemin2 interfere with the association of SRP54 with 7S RNA in Xenopus laevis oocytes. Our data show that reduced levels of the SMN protein lead to defect in SRP steady-state level and describe the SMN complex as the first identified cellular factor required for SRP biogenesis.

Ubiquitin-specific protease 9x deubiquitinates and stabilizes the spinal muscular atrophy protein-survival motor neuron

Spinal muscular atrophy (SMA), the leading genetic disorder of infant mortality, is caused by low levels of survival motor neuron (SMN) protein. Currently it is not clear how the SMN protein levels are regulated at the post-transcriptional level. In this report, we find that Usp9x, a deubiquitinating enzyme, stably associates with the SMN complex via directly interacting with SMN. Usp9x deubiquitinates SMN that is mostly mono- and di-ubiquitinated. Knockdown of Usp9x promotes SMN degradation and reduces the protein levels of SMN and the SMN complex in cultured mammalian cells. Interestingly, Usp9x does not deubiquitinate nuclear SMNΔ7, the main protein product of the SMN2 gene, which is polyubiquitinated and rapidly degraded by the proteasome. Together, our results indicate that SMN and SMNΔ7 are differently ubiquitinated; Usp9x plays an important role in stabilizing SMN and the SMN complex, likely via antagonizing Ub-dependent SMN degradation.

Insights into the evolutionary features of human neurodegenerative diseases

Comparative analyses between human disease and non-disease genes are of great interest in understanding human disease gene evolution. However, the progression of neurodegenerative diseases (NDD) involving amyloid formation in specific brain regions is still unknown. Therefore, in this study, we mainly focused our analysis on the evolutionary features of human NDD genes with respect to non-disease genes. Here, we observed that human NDD genes are evolutionarily conserved relative to non-disease genes. To elucidate the conserved nature of NDD genes, we incorporated the evolutionary attributes like gene expression level, number of regulatory miRNAs, protein connectivity, intrinsic disorder content and relative aggregation propensity in our analysis. Our studies demonstrate that NDD genes have higher gene expression levels in favor of their lower evolutionary rates. Additionally, we observed that NDD genes have higher number of different regulatory miRNAs target sites and also have higher interaction partners than the non-disease genes. Moreover, miRNA targeted genes are known to have higher disorder content. In contrast, our analysis exclusively established that NDD genes have lower disorder content. In favor of our analysis, we found that NDD gene encoded proteins are enriched with multi interface hubs (party hubs) with lower disorder contents. Since, proteins with higher disorder content need to adapt special structure to reduce their aggregation propensity, NDD proteins found to have elevated relative aggregation propensity (RAP) in support of their lower disorder content. Finally, our categorical regression analysis confirmed the underlined relative dominance of protein connectivity, 3'UTR length, RAP, nature of hubs (singlish/multi interface) and disorder content for such evolutionary rates variation between human NDD genes and non-disease genes.

SMN-inducing compounds for the treatment of spinal muscular atrophy

Spinal muscular atrophy (SMA) is a leading genetic cause of infant mortality. A neurodegenerative disease, it is caused by loss of SMN1, although low, but essential, levels of SMN protein are produced by the nearly identical gene SMN2. While no effective treatment or therapy currently exists, a new wave of therapeutics has rapidly progressed from cell-based and preclinical animal models to the point where clinical trials have initiated for SMA-specific compounds. There are several reasons why SMA has moved relatively rapidly towards novel therapeutics, including: SMA is monogenic; the molecular understanding of SMN gene regulation has been building for nearly 20 years; and all SMA patients retain one or more copies of SMN2 that produces low levels of full-length, fully functional SMN protein. This review primarily focuses upon the biology behind the disease and examines SMN1- and SMN2-targeted therapeutics.

mRNA localization: an orchestration of assembly, traffic and synthesis

Asymmetrical mRNA localization and subsequent local translation provide efficient mechanisms for protein sorting in polarized cells. Defects in mRNA localization have been linked to developmental abnormalities and neurological diseases. Thus, it is critical to understand the machineries mediating and mechanisms underlying the asymmetrical distribution of mRNA and its regulation. The goal of this review is to summarize recent advances in the understanding of mRNA transport and localization, including the assembly and sorting of transport messenger ribonucleic protein (mRNP) granules, molecular mechanisms of active mRNP transport, cytoskeletal interactions and regulation of these events by extracellular signals.

Survival motor neuron affects plastin 3 protein levels leading to motor defects

The actin-binding protein plastin 3 (PLS3) has been identified as a modifier of the human motoneuron disease spinal muscular atrophy (SMA). SMA is caused by decreased levels of the survival motor neuron protein (SMN) and in its most severe form causes death in infants and young children. To understand the mechanism of PLS3 in SMA, we have analyzed pls3 RNA and protein in zebrafish smn mutants. We show that Pls3 protein levels are severely decreased in smn(-/-) mutants without a reduction in pls3 mRNA levels. Moreover, we show that both pls3 mRNA and protein stability are unaffected when Smn is reduced. This indicates that SMN affects PLS3 protein production. We had previously shown that, in smn mutants, the presynaptic protein SV2 is decreased at neuromuscular junctions. Transgenically driving human PLS3 in motoneurons rescues the decrease in SV2 expression. To determine whether PLS3 could also rescue function, we performed behavioral analysis on smn mutants and found that they had a significant decrease in spontaneous swimming and turning. Driving PLS3 transgenically in motoneurons rescued both of these defects. These data show that PLS3 protein levels are dependent on SMN and that PLS3 is able to rescue the neuromuscular defects and corresponding movement phenotypes caused by low levels of Smn suggesting that decreased PLS3 contributes to SMA motor phenotypes.

Conserved requirement for DEAD-box RNA helicase Gemin3 in Drosophila oogenesis

Background

DEAD-box RNA helicase Gemin3 is an essential protein since its deficiency is lethal in both vertebrates and invertebrates. In addition to playing a role in transcriptional regulation and RNA silencing, as a core member of the SMN (survival of motor neurons) complex, Gemin3 is required for the biogenesis of spliceosomal snRNPs (small nuclear ribonucleoproteins), and axonal mRNA metabolism. Studies in the mouse and C. elegans revealed that loss of Gemin3 function has a negative impact on ovarian physiology and development.

Findings

This work reports on the generation and characterisation of gemin3 mutant germline clones in Drosophila adult females. Gemin3 was found to be required for the completion of oogenesis and its loss led to egg polarity defects, oocyte mislocalisation, and abnormal chromosome morphology. Canonical Cajal bodies were absent in the majority of gemin3 mutant egg chambers and histone locus bodies displayed an atypical morphology. snRNP distribution was perturbed so that on gemin3 loss, snRNP cytoplasmic aggregates (U bodies) were only visible in wild type.

Conclusions

These findings establish a conserved requirement for Gemin3 in Drosophila oogenesis. Furthermore, in view of the similarity to the phenotypes described previously in smn mutant germ cells, the present results confirm the close functional relationship between SMN and Gemin3 on a cellular level.

Spinal muscular atrophy: the role of SMN in axonal mRNA regulation

Spinal muscular atrophy (SMA) is a neurodegenerative disease caused by homozygous mutations or deletions in the survival of motor neuron (SMN1) gene, encoding the ubiquitously expressed SMN protein. SMN associates with different proteins (Gemins 2-8, Unrip) to form a multimeric complex involved in the assembly of small nuclear ribonucleoprotein complexes (snRNPs). Since this activity is essential for the survival of all cell types, it still remains unclear why motor neurons are selectively vulnerable to low levels of SMN protein. Aside from its housekeeping role in the assembly of snRNPs, additional functions of SMN have been proposed. The well-documented localization of SMN in axonal transport granules and its interaction with numerous mRNA-binding proteins not involved in splicing regulation suggest a role in axonal RNA metabolism. This review will focus on the neuropathological and experimental evidence supporting a role for SMN in regulating the assembly, localization, or stability of axonal messenger ribonucleoprotein complexes (mRNPs). Furthermore, how defects in this non-canonical SMN function may contribute to the motor neuron pathology observed in SMA will be discussed. This article is part of a Special Issue entitled RNA-Binding Proteins.

Spinal muscular atrophy: a clinical and research update

Spinal muscular atrophy, a hereditary degenerative disorder of lower motor neurons associated with progressive muscle weakness and atrophy, is the most common genetic cause of infant mortality. It is caused by decreased levels of the "survival of motor neuron" (SMN) protein. Its inheritance pattern is autosomal recessive, resulting from mutations involving the SMN1 gene on chromosome 5q13. However, unlike many other autosomal recessive diseases, the SMN gene involves a unique structure (an inverted duplication) that presents potential therapeutic targets. Although no effective treatment for spinal muscular atrophy exists, the field of translational research in spinal muscular atrophy is active, and clinical trials are ongoing. Advances in the multidisciplinary supportive care of children with spinal muscular atrophy also offer hope for improved life expectancy and quality of life.

The contribution of mouse models to understanding the pathogenesis of spinal muscular atrophy

Spinal muscular atrophy (SMA), which is caused by inactivating mutations in the survival motor neuron 1 (SMN1) gene, is characterized by loss of lower motor neurons in the spinal cord. The gene encoding SMN is very highly conserved in evolution, allowing the disease to be modeled in a range of species. The similarities in anatomy and physiology to the human neuromuscular system, coupled with the ease of genetic manipulation, make the mouse the most suitable model for exploring the basic pathogenesis of motor neuron loss and for testing potential treatments. Therapies that increase SMN levels, either through direct viral delivery or by enhancing full-length SMN protein expression from the SMN1 paralog, SMN2, are approaching the translational stage of development. It is therefore timely to consider the role of mouse models in addressing aspects of disease pathogenesis that are most relevant to SMA therapy. Here, we review evidence suggesting that the apparent selective vulnerability of motor neurons to SMN deficiency is relative rather than absolute, signifying that therapies will need to be delivered systemically. We also consider evidence from mouse models suggesting that SMN has its predominant action on the neuromuscular system in early postnatal life, during a discrete phase of development. Data from these experiments suggest that the timing of therapy to increase SMN levels might be crucial. The extent to which SMN is required for the maintenance of motor neurons in later life and whether augmenting its levels could treat degenerative motor neuron diseases, such as amyotrophic lateral sclerosis (ALS), requires further exploration.

Invertebrate models of spinal muscular atrophy: insights into mechanisms and potential therapeutics

Invertebrate genetic models with their tractable neuromuscular systems are effective vehicles for the study of human nerve and muscle disorders. This is exemplified by insights made into spinal muscular atrophy (SMA) using the fruit fly Drosophila melanogaster and the nematode worm Caenorhabditis elegans. For speed and economy, these invertebrates offer convenient, whole-organism platforms for genetic screening as well as RNA interference (RNAi) and chemical library screens, permitting the rapid testing of hypotheses related to disease mechanisms and the exploration of new therapeutic routes and drug candidates. Here, we discuss recent developments encompassing synaptic physiology, RNA processing, and screening of compound and genome-scale RNAi libraries, showcasing the importance of invertebrate SMA models.

Temporal requirement for high SMN expression in SMA mice

Spinal muscular atrophy (SMA) is caused by loss of the survival motor neuron 1 gene (SMN1) and retention of the SMN2 gene, resulting in reduced SMN. SMA mice can be rescued with high expression of SMN in neurons, but when is this high expression required? We have developed a SMA mouse with inducible expression of SMN to address the temporal requirement for high SMN expression. Both embryonic and early postnatal induction of SMN resulted in a dramatic increase in survival with some mice living greater than 200 days. The mice had no marked motor deficits and neuromuscular junction (NMJ) function was near normal thus it appears that induction of SMN in postnatal SMA mice rescues motor function. Early postnatal SMN induction, followed by a 1-month removal of induction at 28 days of age, resulted in no morphological or electrophysiological abnormalities at the NMJ and no overt motor phenotype. Upon removal of SMN induction, five mice survived for just over 1 month and two female mice have survived past 8 months of age. We suggest that there is a postnatal period of time when high SMN levels are required. Furthermore, two copies of SMN2 provide the minimal amount of SMN necessary to maintain survival during adulthood. Finally, in the course of SMA, early induction of SMN is most efficacious.

The survival of motor neuron (SMN) protein interacts with the mRNA-binding protein HuD and regulates localization of poly(A) mRNA in primary motor neuron axons

Spinal muscular atrophy (SMA) results from reduced levels of the survival of motor neuron (SMN) protein, which has a well characterized function in spliceosomal small nuclear ribonucleoprotein assembly. Currently, it is not understood how deficiency of a housekeeping protein leads to the selective degeneration of spinal cord motor neurons. Numerous studies have shown that SMN is present in neuronal processes and has many interaction partners, including mRNA-binding proteins, suggesting a potential noncanonical role in axonal mRNA metabolism. In this study, we have established a novel technological approach using bimolecular fluorescence complementation (BiFC) and quantitative image analysis to characterize SMN-protein interactions in primary motor neurons. Consistent with biochemical studies on the SMN complex, BiFC analysis revealed that SMN dimerizes and interacts with Gemin2 in nuclear gems and axonal granules. In addition, using pull down assays, immunofluorescence, cell transfection, and BiFC, we characterized a novel interaction between SMN and the neuronal mRNA-binding protein HuD, which was dependent on the Tudor domain of SMN. A missense mutation in the SMN Tudor domain, which is known to cause SMA, impaired the interaction with HuD, but did not affect SMN axonal localization or self-association. Furthermore, time-lapse microscopy revealed SMN cotransport with HuD in live motor neurons. Importantly, SMN knockdown in primary motor neurons resulted in a specific reduction of both HuD protein and poly(A) mRNA levels in the axonal compartment. These findings reveal a noncanonical role for SMN whereby its interaction with mRNA-binding proteins may facilitate the localization of associated poly(A) mRNAs into axons.

Identification of the phosphorylation sites in the survival motor neuron protein by protein kinase A

The survival motor neuron (SMN) protein plays an essential role in the assembly of uridine-rich small nuclear ribonuclear protein complexes. Phosphorylation of SMN can regulate its function, stability, and sub-cellular localization. This study shows that protein kinase A (PKA) phosphorylates SMN both in vitro and in vivo. Bioinformatic analysis predicts 12 potential PKA phosphorylation sites in human SMN. Mass spectrometric analysis of a tryptic digest of SMN after PKA phosphorylation identified five distinct phosphorylation sites in SMN (serines 4, 5, 8, 187 and threonine 85). Mutagenesis of this subset of PKA-phosphorylated sites in SMN affects association of SMN with Gemin2 and Gemin8. This result indicates that phosphorylation of SMN by PKA may play a role in regulation of the in vivo function of SMN.

Molecular determinants of survival motor neuron (SMN) protein cleavage by the calcium-activated protease, calpain

Spinal muscular atrophy (SMA) is a leading genetic cause of childhood mortality, caused by reduced levels of survival motor neuron (SMN) protein. SMN functions as part of a large complex in the biogenesis of small nuclear ribonucleoproteins (snRNPs). It is not clear if defects in snRNP biogenesis cause SMA or if loss of some tissue-specific function causes disease. We recently demonstrated that the SMN complex localizes to the Z-discs of skeletal and cardiac muscle sarcomeres, and that SMN is a proteolytic target of calpain. Calpains are implicated in muscle and neurodegenerative disorders, although their relationship to SMA is unclear. Using mass spectrometry, we identified two adjacent calpain cleavage sites in SMN, S192 and F193. Deletion of small motifs in the region surrounding these sites inhibited cleavage. Patient-derived SMA mutations within SMN reduced calpain cleavage. SMN(D44V), reported to impair Gemin2 binding and amino-terminal SMN association, drastically inhibited cleavage, suggesting a role for these interactions in regulating calpain cleavage. Deletion of A188, a residue mutated in SMA type I (A188S), abrogated calpain cleavage, highlighting the importance of this region. Conversely, SMA mutations that interfere with self-oligomerization of SMN, Y272C and SMNΔ7, had no effect on cleavage. Removal of the recently-identified SMN degron (Δ268-294) resulted in increased calpain sensitivity, suggesting that the C-terminus of SMN is important in dictating availability of the cleavage site. Investigation into the spatial determinants of SMN cleavage revealed that endogenous calpains can cleave cytosolic, but not nuclear, SMN. Collectively, the results provide insight into a novel aspect of the post-translation regulation of SMN.

Biomarkers in rare disorders: the experience with spinal muscular atrophy

Spinal muscular atrophy (SMA) is an autosomal recessive neuromuscular disorder caused by homozygous mutations of the SMN1 gene. Based on clinical severity, three forms of SMA are recognized (type I–III). All patients have at least one (usually 2–4) copies of a highly homologous gene (SMN2) which produces insufficient levels of functional SMN protein, due to alternative splicing of exon7. Recently, evidence has been provided that SMN2 expression can be enhanced by different strategies. The availability of potential candidates to treat SMA has raised a number of issues, including the availability of data on the natural history of the disease, the reliability and sensitivity of outcome measures, the duration of the studies, and the number and clinical homogeneity of participating patients. Equally critical is the availability of reliable biomarkers. So far, different tools have been proposed as biomarkers in SMA, classifiable into two groups: instrumental (the Compound Motor Action Potential, the Motor Unit Number Estimation, and the Dual-energy X-ray absorptiometry) and molecular (SMN gene products dosage, either transcripts or protein). However, none of the biomarkers available so far can be considered the gold standard. Preclinical studies on SMA animal models and double-blind, placebo-controlled studies are crucial to evaluate the appropriateness of biomarkers, on the basis of correlations with clinical outcome.

High-efficiency transfection of cultured primary motor neurons to study protein localization, trafficking, and function

Background

Cultured spinal motor neurons are a valuable tool to study basic mechanisms of development, axon growth and pathfinding, and, importantly, to analyze the pathomechanisms underlying motor neuron diseases. However, the application of this cell culture model is limited by the lack of efficient gene transfer techniques which are available for other neurons. To address this problem, we have established magnetofection as a novel method for the simple and efficient transfection of mouse embryonic motor neurons. This technique allows for the study of the effects of gene expression and silencing on the development and survival of motor neurons.

Results

We found that magnetofection, a novel transfection technology based on the delivery of DNA-coated magnetic nanobeads, can be used to transfect primary motor neurons. Therefore, in order to use this method as a new tool for studying the localization and transport of axonal proteins, we optimized conditions and determined parameters for efficient transfection rates of >45% while minimizing toxic effects on survival and morphology. To demonstrate the potential of this method, we have used transfection with plasmids encoding fluorescent fusion-proteins to show for the first time that the spinal muscular atrophy-disease protein Smn is actively transported along axons of live primary motor neurons, supporting an axon-specific role for Smn that is different from its canonical function in mRNA splicing. We were also able to show the suitability of magnetofection for gene knockdown with shRNA-based constructs by significantly reducing Smn levels in both cell bodies and axons, opening new opportunities for the study of the function of axonal proteins in motor neurons.

Conclusions

In this study we have established an optimized magnetofection protocol as a novel transfection method for primary motor neurons that is simple, efficient and non-toxic. We anticipate that this novel approach will have a broad applicability in the study of motor neuron development, axonal trafficking, and molecular mechanisms of motor neuron diseases.

Regulation of mRNA transport and translation in axons

Movement of mRNAs into axons occurs by active transport by microtubules through the activity of molecular motor proteins. mRNAs are sequestered into granular-like particles, referred to as transport ribonucleoprotein particles (RNPs) that mediate transport into the axonal compartment. The interaction of mRNA binding proteins with targeted mRNA is a key event in regulating axonal mRNA localization and subsequent localized translation of mRNAs. Several growth-modulating stimuli have been shown to regulate axonal mRNA localization. These do so by activating specific intracellular signaling pathways that converge upon RNA binding proteins and other components of the transport RNP to regulate their activity specifically. Transport can be both positively and negatively regulated by individual stimuli with regard to individual mRNAs. Consequently, there is exquisite specificity for regulating the axon's composition of mRNAs and proteins that control expression in the axon. Finally, recent studies indicate that axotomy can also trigger changes in axonal mRNA composition by specifically shifting the populations of mRNAs that are transported into distal axons.