The pathophysiology underlying spinal cord injury is complex. Mechanistic understanding of the adaptive responses to injury is critical for targeted therapy aimed at reestablishing lost connections between proximal and distal neurons. After injury, cell-type specific gene transcription programs govern distinct cellular behaviors, and chromatin regulators play a central role in shaping the chromatin landscape to adjust transcriptional profiles in a context-dependent manner. In this review, we summarize recent progress on the pleiotropic roles of chromatin regulators in mediating the diverse adaptive behaviors of neurons and glial cells after spinal cord injury, and wherever possible, discuss the underlying mechanisms and genomic targets. We specifically draw attention to the perspective that takes into consideration the impact of epigenetic modulation on axon growth potential, together with its effect on wound-healing properties of glial cells. Epigenetic modulation of chromatin state represents an emerging therapeutic direction to promote neural repair and axon regeneration after spinal cord injury.
epigenetics; chromatin; spinal cord injury; axon regeneration; neural repair
Intraneuronal activation of B-RAF kinase is sufficient to drive the growth of peripheral axon projections and enables robust regenerative axon growth in the injured optic nerve.
Activation of intrinsic growth programs that promote developmental axon growth may also facilitate axon regeneration in injured adult neurons. Here, we demonstrate that conditional activation of B-RAF kinase alone in mouse embryonic neurons is sufficient to drive the growth of long-range peripheral sensory axon projections in vivo in the absence of upstream neurotrophin signaling. We further show that activated B-RAF signaling enables robust regenerative growth of sensory axons into the spinal cord after a dorsal root crush as well as substantial axon regrowth in the crush-lesioned optic nerve. Finally, the combination of B-RAF gain-of-function and PTEN loss-of-function promotes optic nerve axon extension beyond what would be predicted for a simple additive effect. We conclude that cell-intrinsic RAF signaling is a crucial pathway promoting developmental and regenerative axon growth in the peripheral and central nervous systems.
Axon regeneration is hindered by a decline of intrinsic axon growth capability in mature neurons. Reversing this decline is associated with the induction of a large repertoire of regeneration-associated genes (RAGs), but the underlying regulatory mechanisms of the transcriptional changes are largely unknown. Here, we establish a correlation between diminished axon growth potential and histone 4 (H4) hypoacetylation. When neurons are triggered into a growth state, as in the conditioning lesion paradigm, H4 acetylation is restored, and RAG transcription is initiated. We have identified a set of target genes of Smad1, a proregenerative transcription factor, in conditioned DRG neurons. We also show that, during the epigenetic reprogramming process, histone-modifying enzymes work together with Smad1 to facilitate transcriptional regulation of RAGs. Importantly, targeted pharmacological modulation of the activity of histone-modifying enzymes, such as histone deacetylases, leads to induction of multiple RAGs and promotion of sensory axon regeneration in a mouse model of spinal cord injury. Our findings suggest epigenetic modulation as a potential therapeutic strategy to enhance axon regeneration.
axon regeneration; conditioning lesion; DRG neurons; epigenetic regulation; Smad1; spinal cord injury
Inflammatory monocytes -- but not the non-inflammatory subset -- depend on the chemokine receptor CCR2 for distribution to injured tissue and stimulate disease progression. Precise therapeutic targeting of this inflammatory monocyte subset could spare innate immunity's essential functions for maintenance of homeostasis and thus limit unwanted effects. Here we developed siRNA nanoparticles targeting CCR2 expression in inflammatory monocytes. We identified an optimized lipid nanoparticle and silencing siRNA sequence that when administered systemically, had rapid blood clearance, accumulated in spleen and bone marrow and showed high cellular localization of fluorescently tagged siRNA inside monocytes. Efficient degradation of CCR2 mRNA in monocytes prevented their accumulation in sites of inflammation. Specifically, the treatment attenuated their number in atherosclerotic plaques, reduced infarct size following coronary artery occlusion, prolonged normoglycemia in diabetic mice after pancreatic islet transplantation and resulted in reduced tumor volumes and lower numbers of tumor-associated macrophages. Taken together, siRNA nanoparticle-mediated CCR2 gene silencing in leukocytes selectively modulates functions of innate immune cell subtypes and may allow for the development of specific anti-inflammatory therapy.
Leukocytes are central regulators of inflammation and the target cells of therapies for key diseases, including autoimmune, cardiovascular, and malignant disorders. Efficient in vivo delivery of small interfering RNA (siRNA) to immune cells could thus enable novel treatment strategies with broad applicability. In this report, we develop systemic delivery methods of siRNA encapsulated in lipid nanoparticles (LNP) for durable and potent in vivo RNA interference (RNAi)-mediated silencing in myeloid cells. This work provides the first demonstration of siRNA-mediated silencing in myeloid cell types of nonhuman primates (NHPs) and establishes the feasibility of targeting multiple gene targets in rodent myeloid cells. The therapeutic potential of these formulations was demonstrated using siRNA targeting tumor necrosis factor-α (TNFα) which induced substantial attenuation of disease progression comparable to a potent antibody treatment in a mouse model of rheumatoid arthritis (RA). In summary, we demonstrate a broadly applicable and therapeutically relevant platform for silencing disease genes in immune cells.
delivery; immune cell; siRNA
Obesity is accompanied by chronic, low-grade inflammation of adipose tissue, which promotes insulin resistance and type-2 diabetes. How does fat inflammation escape the powerful armamentarium of cells and molecules normally responsible for guarding against a run-away immune response? Regulatory CD4+ T cells expressing the transcription factor Foxp3 (termed Treg cells) are a lymphocyte lineage specialized in controlling immunologic reactivity. Treg cells with a unique phenotype were highly enriched in the abdominal fat of normal mice, but were strikingly and specifically reduced at this site in insulin-resistant models of obesity. In loss-of-function and gain-of-function experiments, Treg cells regulated the inflammatory state of adipose tissue and insulin resistance. Cytokines differentially synthesized by fat-resident regulatory and conventional T cells directly impacted on the synthesis of inflammatory mediators and glucose uptake by cultured adipocytes. These findings open the door to harnessing the anti-inflammatory properties of Treg cells to inhibit elements of the metabolic syndrome.
Recent genome-wide association studies (GWASs) have identified a locus on chromosome 1p13 as strongly associated with both serum low-density lipoprotein cholesterol (LDL-C) and myocardial infarction (MI) in humans. Here we show through a series of studies in human cohorts and human-derived hepatocytes that a common noncoding polymorphism at the 1p13 locus, rs12740374, creates a C/EBP transcription factor binding site and alters the hepatic expression of the SORT1 gene. With siRNA knockdown and viral overexpression in mouse liver, we demonstrate that Sort1 alters plasma LDL-C and very low-density lipoprotein (VLDL) particle levels by modulating hepatic VLDL secretion. Thus, we provide functional evidence for a novel regulatory pathway for lipoprotein metabolism and suggest that modulation of this pathway may alter risk for MI in humans. We also demonstrate that common noncoding DNA variants identified by GWASs can directly contribute to clinical phenotypes.
Treatment with anti-CD3 is a promising therapeutic approach for autoimmune diabetes, but its mechanism of action remains unclear. Foxp3+ regulatory T (T reg) cells may be involved, but the evidence has been conflicting. We investigated this issue in mice derived from the NOD model, which were engineered so that T reg populations were perturbed, or could be manipulated by acute ablation or transfer. The data highlighted the involvement of Foxp3+ cells in anti-CD3 action. Rather than a generic influence on all T reg cells, the therapeutic effect seemed to involve an ∼50–60-fold expansion of previously constrained T reg cell populations; this expansion occurred not through conversion from Foxp3− conventional T (T conv) cells, but from a proliferative expansion. We found that T reg cells are normally constrained by TCR-specific niches in secondary lymphoid organs, and that intraclonal competition restrains their possibility for conversion and expansion in the spleen and lymph nodes, much as niche competition limits their selection in the thymus. The strong perturbations induced by anti-CD3 overcame these niche limitations, in a process dependent on receptors for interleukin-2 (IL-2) and IL-7.
There is continuing controversy about whether the cells of origin of the corticospinal tract (CST) undergo retrograde cell death following spinal cord injury (SCI). All previous attempts to assess this have utilized imaging and/or histological techniques to assess upper motoneurons in the cerebral cortex. Here we address the question in a novel way by assessing Wallerian degeneration and axon numbers in the medullary pyramid of Sprague-Dawley rats following both acute SCI, either at cervical level 5 (C5) or thoracic level 9 (T9), and chronic SCI at T9. Our findings demonstrate that only a fraction of a percent of the total axons in the medullary pyramid exhibit any sign of degeneration at any time post-SCI—no more so than in uninjured control rats. Moreover, design-based counts of myelinated axons revealed no decrease in axon number in the medullary pyramid after SCI, regardless of injury level, severity, or time post injury. Spinal cord injured rats had fewer myelinated axons in the medullary pyramid at 1-year post injury than aged matched controls suggesting that injury may affect ongoing myelination of axons during aging. We conclude that SCI does not cause death of the CST cell bodies in the cortex; therefore therapeutic strategies aimed at promoting axon regeneration of the CST in the spinal cord do not require a separate intervention to prevent retrograde degeneration of upper motoneurons in the cortex.
corticospinal tract; medullary pyramid; spinal cord injury; axon degeneration; retrograde cell death
We describe here an alternative procedure for assessing hindlimb locomotor function after spinal cord injury that uses the BBB scale, but tests animals in a reward-baited straight alley rather than an open field. Rats were trained to ambulate in a straight alley and habituated to the open field typically used for BBB open field testing. Three groups of rats were tested. Sprague-Dawley rats received either 200kD (n=19) or 300kD contusions (n=9) at T9 with the Infinite Horizon device. Fisher rats (n=8) received moderate contusions (12.5mm) at T8 with the NYU impactor. BBB scores were assessed at different post-injury intervals in the open field and the straight alley, and scores were compared by correlation analyses. BBB scores in the open field vs. the straight alley were highly correlated (r=0.90), validating the use of the straight alley for locomotor assessment. Rats exhibited a larger number of bouts of continuous steps in the straight alley vs. the open field (termed passes), providing more opportunities to score hindlimb use and coordination over the 4 minute testing interval. Comparisons of scores across days revealed higher day-to-day correlations in the straight alley vs. the open field (r2 values of 0.90 and 0.74 for the straight alley and open field respectively), revealing that the straight alley yielded more reliable scores.
Spinal cord injury; hindlimb motor function; recovery of function
This study tracks the fate of antigen-reactive B cells through follicular and extrafollicular responses and addresses the function of CD40 in these processes. The unique feature of this system is the use of transgenic B cells in which the heavy chain locus has been altered by site-directed insertion of a rearranged VH DJH exon such that they are able to clonally expand, isotype-switch and follow a normal course of differentiation upon immunization. These Ig transgenic B cells when adoptively transferred into non-transgenic (Tg) mice in measured amounts expanded and differentiated distinctively in response to T cell-independent (TI) or T cell-dependent (TD) antigens. The capacity of these Tg B cells to faithfully recapitulate the humoral immune response to TI and TD antigens provides the means to track clonal B cell behavior in vivo. Challenge with TI antigen in the presence of agonistic anti-CD40 mAb resulted in well-defined alterations of the TI response. In vivo triggering of Tg B cells with TI antigen and CD40 caused an increase in the levels IgG produced and a broadening of the Ig isotype profile, characteristics which partially mimic TD responses. Although some TD characteristics were induced by TI antigen and CD40 triggering, the Tg B cells failed to acquire a germinal center phenotype and failed to generate a memory response. Therefore, TD-like immunity can be only partially reconstituted with CD40 agonists and TI antigens, suggesting that there are additional signals required for germinal center formation and development of memory.
B lymphocyte; Isotype switching; Memory; Adoptive transfer; Transgenic mouse
Foxp3-expressing regulatory T (T reg) cells derive primarily from selection in the thymus. Yet conversion of mature conventional CD4+ T (T conv) cell lymphocytes can be achieved in several conditions, such as transforming growth factor β treatment, homeostatic expansion, or chronic exposure to low-dose antigen. Such conversion might provide a means to generate peripheral tolerance by “converting” potentially damaging T cells that react to self-antigens. We tested this hypothesis in mice transgenic for the BDC2.5 T cell receptor (TCR), which is representative of a diabetogenic specificity that is naturally present in NOD mice and reactive against a pancreatic self-antigen. In the thymus, before any exposure to antigen, clonotype-positive T reg and T conv cells express a second TCRα chain derived from endogenous loci. High-throughput single-cell sequencing of secondary TCRs of the Vα2 family showed their joining CDR3α regions to be very different in T reg and T conv cell thymocytes. These specific CDR3α motifs, thus, provided a “tag” with which to test the actual impact of T conv to T reg cell conversion in response to peripheral self-antigen; should the autoreactive clonotypic TCR induce T conv to T reg cell conversion upon encounter of cognate antigen in the pancreas or draining lymph node, one would expect to detect tag CDR3α motifs from T conv cells in the T reg cell populations. Sequencing large numbers of peripheral BDC+Vα2+ cells showed that little to no conversion occurs in response to this pancreatic autoantigen.
We have generated a monoclonal B-cell mouse by introducing homozygous, nonfunctional
RAG-2 alleles and a λ1 light-chain transgene into the quasi-monoclonal (QM) mouse, which
contains a “knocked-in” VHDJH rearrangement. Thus, this mouse, which we call MonoB, is
devoid of T cells and contains preformed heavy- and light-chain genes encoding immunoglobulin
with an anti-NP specificity. The MonoB mouse allows us to examine immunoglobulin
diversity in the absence of processes mediated by V(D)J recombination and T cells. Here we
report that not only is the MonoB's primary immunoglobulin repertoire monoclonal, but also
that its secondary repertoire is not further diversified by V-gene replacement or gene conversion.
Among 99 heavy-chain and 41 λ light-chain genes from peripheral B cells of the
MonoB mouse, there were no V-gene replacements. When compared to the QM mouse,
which has RAG activity, and for which V-gene replacement is the major diversifying mechanism,
these data suggest that V-gene replacement is mediated by V(D)J recombination and
not by other recombination systems.
MonoB mouse; QM mouse; RAG; transgenic; knockin; flow cytometry