Striated muscle-specific base editing enables correction of mutations causing dilated cardiomyopathy.

Dilated cardiomyopathy is the second most common cause for heart failure with no cure except a high-risk heart transplantation. Approximately 30% of patients harbor heritable mutations which are amenable to CRISPR-based gene therapy. However, challenges related to delivery of the editing complex and off-target concerns hamper the broad applicability of CRISPR agents in the heart. We employ a combination of the viral vector AAVMYO with superior targeting specificity of heart muscle tissue and CRISPR base editors to repair patient mutations in the cardiac splice factor Rbm20, which cause aggressive dilated cardiomyopathy. Using optimized conditions, we repair >70% of cardiomyocytes in two Rbm20 knock-in mouse models that we have generated to serve as an in vivo platform of our editing strategy. Treatment of juvenile mice restores the localization defect of RBM20 in 75% of cells and splicing of RBM20 targets including TTN. Three months after injection, cardiac dilation and ejection fraction reach wild-type levels. Single-nuclei RNA sequencing uncovers restoration of the transcriptional profile across all major cardiac cell types and whole-genome sequencing reveals no evidence for aberrant off-target editing. Our study highlights the potential of base editors combined with AAVMYO to achieve gene repair for treatment of hereditary cardiac diseases.

Phospholipid scramblase Xkr8 is required for developmental axon pruning via phosphatidylserine exposure.

The mature mammalian brain connectome emerges during development via the extension and pruning of neuronal connections. Glial cells have been identified as key players in the phagocytic elimination of neuronal synapses and projections. Recently, phosphatidylserine has been identified as neuronal "eat-me" signal that guides elimination of unnecessary input sources, but the associated transduction systems involved in such pruning are yet to be described. Here, we identified Xk-related protein 8 (Xkr8), a phospholipid scramblase, as a key factor for the pruning of axons in the developing mammalian brain. We found that mouse Xkr8 is highly expressed immediately after birth and required for phosphatidylserine exposure in the hippocampus. Mice lacking Xkr8 showed excess excitatory nerve terminals, increased density of cortico-cortical and cortico-spinal projections, aberrant electrophysiological profiles of hippocampal neurons, and global brain hyperconnectivity. These data identify phospholipid scrambling by Xkr8 as a central process in the labeling and discrimination of developing neuronal projections for pruning in the mammalian brain.

MiR-142-3p is a Critical Modulator of TNF-mediated Neuronal Toxicity in Multiple Sclerosis.

BACKGROUND: TNF-dependent synaptotoxicity contributes to the neuronal damage occurring in patients with Multiple Sclerosis (pwMS) and its mouse model Experimental Autoimmune Encephalomyelitis (EAE). Here, we investigated miR-142-3p, a synaptotoxic microRNA induced by inflammation in EAE and MS, as a potential downstream effector of TNF signalling. METHODS: Electrophysiological recordings, supported by molecular, biochemical and histochemical analyses, were performed to explore TNF-synaptotoxicity in the striatum of EAE and healthy mice. MiR-142 heterozygous (miR-142 HE) mice and/or LNA-anti miR-142-3p strategy were used to verify the TNF-miR-142-3p axis hypothesis. The cerebrospinal fluid (CSF) of 151 pwMS was analysed to evaluate possible correlation between TNF and miR-142-3p levels and their impact on clinical parameters (e.g. progression index (PI), age-related clinical severity (gARMSS)) and MRI measurements at diagnosis (T0). RESULTS: High levels of TNF and miR-142-3p were detected in both EAE striatum and MS-CSF. The TNF-dependent glutamatergic alterations were prevented in the inflamed striatum of EAE miR-142 HE mice. Accordingly, TNF was ineffective in healthy striatal slices incubated with LNA-anti miR- 142-3p. However, both preclinical and clinical data did not validate the TNF-miR-142-3p axis hypothesis, suggesting a permissive neuronal role of miR-142-3p on TNF-signalling. Clinical data showed a negative impact of each molecule on disease course and/or brain lesions and unveiled that their high levels exert a detrimental synergistic effect on disease activity, PI and white matter lesion volume. CONCLUSION: We propose miR-142-3p as a critical modulator of TNF-mediated neuronal toxicity and suggest a detrimental synergistic action of these molecules on MS pathology.

The long noncoding RNA Charme supervises cardiomyocyte maturation by controlling cell differentiation programs in the developing heart.

Long noncoding RNAs (lncRNAs) are emerging as critical regulators of heart physiology and disease, although the studies unveiling their modes of action are still limited to few examples. We recently identified pCharme, a chromatin-associated lncRNA whose functional knockout in mice results in defective myogenesis and morphological remodeling of the cardiac muscle. Here, we combined Cap-Analysis of Gene Expression (CAGE), single-cell (sc)RNA sequencing, and whole-mount in situ hybridization analyses to study pCharme cardiac expression. Since the early steps of cardiomyogenesis, we found the lncRNA being specifically restricted to cardiomyocytes, where it assists the formation of specific nuclear condensates containing MATR3, as well as important RNAs for cardiac development. In line with the functional significance of these activities, pCharme ablation in mice results in a delayed maturation of cardiomyocytes, which ultimately leads to morphological alterations of the ventricular myocardium. Since congenital anomalies in myocardium are clinically relevant in humans and predispose patients to major complications, the identification of novel genes controlling cardiac morphology becomes crucial. Our study offers unique insights into a novel lncRNA-mediated regulatory mechanism promoting cardiomyocyte maturation and bears relevance to Charme locus for future theranostic applications.

Induction of flight via midbrain projections to the cuneiform nucleus.

The dorsal periaqueductal gray is a midbrain structure implicated in the control of defensive behaviors and the processing of painful stimuli. Electrical stimulation or optogenetic activation of excitatory neurons in dorsal periaqueductal gray results in freezing or flight behavior at low and high intensity, respectively. However, the output structures that mediate these defensive behaviors remain unconfirmed. Here we carried out a targeted classification of neuron types in dorsal periaqueductal gray using multiplex in situ sequencing and then applied cell-type and projection-specific optogenetic stimulation to identify projections from dorsal periaqueductal grey to the cuneiform nucleus that promoted goal-directed flight behavior. These data confirmed that descending outputs from dorsal periaqueductal gray serve as a trigger for directed escape behavior.

Deletion of LBR N-terminal domains recapitulates Pelger-Huet anomaly phenotypes in mouse without disrupting X chromosome inactivation.

Mutations in the gene encoding Lamin B receptor (LBR), a nuclear-membrane protein with sterol reductase activity, have been linked to rare human disorders. Phenotypes range from a benign blood disorder, such as Pelger-Huet anomaly (PHA), affecting the morphology and chromatin organization of white blood cells, to embryonic lethality as for Greenberg dysplasia (GRBGD). Existing PHA mouse models do not fully recapitulate the human phenotypes, hindering efforts to understand the molecular etiology of this disorder. Here we show, using CRISPR/Cas-9 gene editing technology, that a 236bp N-terminal deletion in the mouse Lbr gene, generating a protein missing the N-terminal domains of LBR, presents a superior model of human PHA. Further, we address recent reports of a link between Lbr and defects in X chromosome inactivation (XCI) and show that our mouse mutant displays minor X chromosome inactivation defects that do not lead to any overt phenotypes in vivo. We suggest that our N-terminal deletion model provides a valuable pre-clinical tool to the research community and will aid in further understanding the etiology of PHA and the diverse functions of LBR.

Correction to: MicroRNA-34 Contributes to the Stress-related Behavior and Affects 5-HT Prefrontal/GABA Amygdalar System through Regulation of Corticotropin-releasing Factor Receptor 1.

The original version of this article unfortunately contained a mistake in Figure 3. The drawing superimposed on photomicrographs to identify the region of Dorsal raphè Nuclei was inappropriately positioned. The corrected figure is given below.

Nerve growth factor-mediated photoablation of nociceptors reduces pain behavior in mice.

Nerve growth factor (NGF) and its receptors TrkA and p75 play a key role in the development and function of peripheral nociceptive neurons. Here, we describe novel technology to selectively photoablate TrkA-positive nociceptors through delivery of a phototoxic agent coupled to an engineered NGF ligand and subsequent near-infrared illumination. We demonstrate that this approach allows for on demand and localized reversal of pain behaviors in mouse models of acute, inflammatory, neuropathic, and joint pain. To target peripheral nociceptors, we generated a SNAP-tagged NGF derivative NGF that binds to TrkA/p75 receptors but does not provoke signaling in TrkA-positive cells or elicit pain behaviors in mice. NGF was coupled to the photosensitizer IRDye700DX phthalocyanine (IR700) and injected subcutaneously. After near-infrared illumination of the injected area, behavioral responses to nociceptive mechanical and sustained thermal stimuli, but not innocuous stimuli, were substantially reduced. Similarly, in models of inflammatory, osteoarthritic, and neuropathic pain, mechanical hypersensitivity was abolished for 3 weeks after a single treatment regime. We demonstrate that this loss of pain behavior coincides with the retraction of neurons from the skin which then reinnervate the epidermis after 3 weeks corresponding with the return of mechanical hypersensitivity. Thus NGF-mediated photoablation is a minimally invasive approach to reversibly silence nociceptor input from the periphery, and control pain and hypersensitivity to mechanical stimuli.

Interleukin-31-mediated photoablation of pruritogenic epidermal neurons reduces itch-associated behaviours in mice.

Itch-a major symptom of many chronic skin diseases-can exacerbate inflammation by provoking scratching and subsequent skin damage. Here, we show that activation, via near infrared illumination, of a phototoxic agent that selectively targets itch-sensing cells can reduce itch-associated behaviours in mice. We generated a SNAP-tagged interleukin-31 (IL-31) ligand derivative (IL-31K138A-SNAP) that selectively binds receptors on itch-associated cells, without evoking IL-31-receptor signalling or scratching, and conjugated it to the photosensitizer IRDye 700DX phthalocyanine. Subcutaneous injection of IL-31K138A-SNAP-IR700 in mice followed by near infrared illumination resulted in the long-term reversal of the scratching behaviour evoked by the pruritogenic IL-31, an effect that was associated with the selective retraction of itch-sensing neurons in the skin. We also show that a topical preparation of IL-31K138A-SNAP-IR700 reversed the behavioural and dermatological indicators of disease in mouse models of atopic dermatitis and of the genetic skin disease familial primary localized cutaneous amyloidosis. Targeted photoablation may enable itch control for the treatment of inflammatory skin diseases.

Control of mechanical pain hypersensitivity in mice through ligand-targeted photoablation of TrkB-positive sensory neurons.

Mechanical allodynia is a major symptom of neuropathic pain whereby innocuous touch evokes severe pain. Here we identify a population of peripheral sensory neurons expressing TrkB that are both necessary and sufficient for producing pain from light touch after nerve injury in mice. Mice in which TrkB-Cre-expressing neurons are ablated are less sensitive to the lightest touch under basal conditions, and fail to develop mechanical allodynia in a model of neuropathic pain. Moreover, selective optogenetic activation of these neurons after nerve injury evokes marked nociceptive behavior. Using a phototherapeutic approach based upon BDNF, the ligand for TrkB, we perform molecule-guided laser ablation of these neurons and achieve long-term retraction of TrkB-positive neurons from the skin and pronounced reversal of mechanical allodynia across multiple types of neuropathic pain. Thus we identify the peripheral neurons which transmit pain from light touch and uncover a novel pharmacological strategy for its treatment.

MicroRNA degradation by a conserved target RNA regulates animal behavior.

microRNAs (miRNAs) repress target transcripts through partial complementarity. By contrast, highly complementary miRNA-binding sites within viral and artificially engineered transcripts induce miRNA degradation in vitro and in cell lines. Here, we show that a genome-encoded transcript harboring a near-perfect and deeply conserved miRNA-binding site for miR-29 controls zebrafish and mouse behavior. This transcript originated in basal vertebrates as a long noncoding RNA (lncRNA) and evolved to the protein-coding gene NREP in mammals, where the miR-29-binding site is located within the 3' UTR. We show that the near-perfect miRNA site selectively triggers miR-29b destabilization through 3' trimming and restricts its spatial expression in the cerebellum. Genetic disruption of the miR-29 site within mouse Nrep results in ectopic expression of cerebellar miR-29b and impaired coordination and motor learning. Thus, we demonstrate an endogenous target-RNA-directed miRNA degradation event and its requirement for animal behavior.

MicroRNA-34 Contributes to the Stress-related Behavior and Affects 5-HT Prefrontal/GABA Amygdalar System through Regulation of Corticotropin-releasing Factor Receptor 1.

Recent studies show that microRNA-34 (miR-34) family is critical in the regulation of stress response also suggesting that it may contribute to the individual responsiveness to stress. We have recently demonstrated that mice carrying a genetic deletion of all miR-34 isoforms (triple knockout, TKO) lack the stress-induced serotonin (5-HT) and GABA release in the medial prefrontal cortex (mpFC) and basolateral amygdala (BLA), respectively. Here, we evaluated if the absence of miR-34 was also able to modify the stress-coping strategy in the forced swimming test. We found that the blunted neurochemical response to stress was associated with lower levels of immobility (index of active coping behavior) in TKO compared to WT mice. Interestingly, among the brain regions mostly involved in the stress-related behaviors, the miR-34 displayed the strongest expression in the dorsal raphe nuclei (DRN) of wild-type (WT) mice. In the DRN, the corticotropin-releasing factor receptors (CRFR) 1 and 2, contribute to determine the stress-coping style and the CRFR1 is a target of miR-34. Thus, we hypothesized that the miR-34-dependent modulation of CRFR1 expression may be involved in the DRN regulation of stress-coping strategies. In line with this hypothesis, we found increased CRFR1 levels in the DNR of TKO compared to WT mice. Moreover, infusion of CRFR1 antagonist in the DRN of TKO mice reverted their behavioral and neurochemical phenotype. We propose that miR-34 modulate the mpFC 5-HT/BLA GABA response to stress acting on CRFR1 in the DRN and that this mechanism could contribute to determine individual stress-coping strategy.

miR-135a Regulates Synaptic Transmission and Anxiety-Like Behavior in Amygdala.

MicroRNAs are a class of non-coding RNAs with a growing relevance in the regulation of gene expression related to brain function and plasticity. They have the potential to orchestrate complex phenomena, such as the neuronal response to homeostatic challenges. We previously demonstrated the involvement of miR-135a in the regulation of early stress response. In the present study, we examine the role of miR-135a in stress-related behavior. We show that the knockdown (KD) of miR-135a in the mouse amygdala induces an increase in anxiety-like behavior. Consistently with behavioral studies, electrophysiological experiments in acute brain slices indicate an increase of amygdala spontaneous excitatory postsynaptic currents, as a result of miR-135a KD. Furthermore, we presented direct evidences, by in vitro assays and in vivo miRNA overexpression in the amygdala, that two key regulators of synaptic vesicle fusion, complexin-1 and complexin-2, are direct targets of miR-135a. In vitro analysis of miniature excitatory postsynaptic currents on miR-135a KD primary neurons indicates unpaired quantal excitatory neurotransmission. Finally, increased levels of complexin-1 and complexin-2 proteins were detected in the mouse amygdala after acute stress, accordingly to the previously observed stress-induced miR-135a downregulation. Overall, our results unravel a previously unknown miRNA-dependent mechanism in the amygdala for regulating anxiety-like behavior, providing evidences of a physiological role of miR-135a in the modulation of presynaptic mechanisms of glutamatergic neurotransmission.

miR-142-3p Is a Key Regulator of IL-1ß-Dependent Synaptopathy in Neuroinflammation.

MicroRNAs (miRNA) play an important role in post-transcriptional gene regulation of several physiological and pathological processes. In multiple sclerosis (MS), a chronic inflammatory and degenerative disease of the CNS, and in its mouse model, the experimental autoimmune encephalomyelitis (EAE), miRNA dysregulation has been mainly related to immune system dysfunction and white matter (WM) pathology. However, little is known about their role in gray matter pathology. Here, we explored miRNA involvement in the inflammation-driven alterations of synaptic structure and function, collectively known as synaptopathy, a neuropathological process contributing to excitotoxic neurodegeneration in MS/EAE. Particularly, we observed that miR-142-3p is increased in the CSF of patients with active MS and in EAE brains. We propose miR-142-3p as a molecular mediator of the IL-1ß-dependent downregulation of the glial glutamate-aspartate transporter (GLAST), which causes an enhancement of the glutamatergic transmission in the EAE cerebellum. The synaptic abnormalities mediated by IL-1ß and the clinical and neuropathological manifestations of EAE disappeared in miR-142 knock-out mice. Furthermore, we observed that in vivo miR-142-3p inhibition, either by a preventive and local treatment or by a therapeutic and systemic strategy, abolished IL-1ß- and GLAST-dependent synaptopathy in EAE wild-type mice. Consistently, miR-142-3p was responsible for the glutamatergic synaptic alterations caused by CSF of patients with MS, and CSF levels of miR-142-3p correlated with prospective MS disease progression. Our findings highlight miR-142-3p as key molecular player in IL-1ß-mediated synaptic dysfunction, possibly leading to excitotoxic damage in both EAE and MS diseases. Inhibition of miR-142-3p could be neuroprotective in MS. SIGNIFICANCE STATEMENT: Current studies suggest the role of glutamate excitotoxicity in the development and progression of multiple sclerosis (MS) and of its mouse model experimental autoimmune encephalomyelitis (EAE). The molecular mechanisms linking inflammation and synaptic alterations in MS/EAE are still unknown. Here, we identified miR-142-3p as a determinant molecular actor in inflammation-dependent synaptopathy typical of both MS and EAE. miR-142-3p was upregulated in the CSF of MS patients and in EAE cerebellum. Inhibition of miR-142-3p, locally in EAE brain and in a MS chimeric ex vivo model, recovered glutamatergic synaptic enhancement typical of EAE/MS. We proved that miR-142-3p promoted the IL-1ß-dependent glutamate dysfunction by targeting glutamate-aspartate transporter (GLAST), a crucial glial transporter involved in glutamate homeostasis. Finally, we suggest miR-142-3p as a negative prognostic factor in patients with relapsing-remitting multiple sclerosis.

Discovery and Characterization of Novel Anti-schistosomal Properties of the Anti-anginal Drug, Perhexiline and Its Impact on Schistosoma mansoni Male and Female Reproductive Systems.

BACKGROUND: Schistosomiasis, one of the world's greatest human neglected tropical diseases, is caused by parasitic trematodes of the genus Schistosoma. A unique feature of schistosome biology is that the induction of sexual maturation as well as the maintenance of the differentiation status of female reproductive organs and egg production, necessary for both disease transmission and pathogenesis, are strictly dependent on the male. The treatment and most control initiatives of schistosomiasis rely today on the long-term application of a single drug, praziquantel (PZQ), mostly by campaigns of mass drug administration. PZQ, while very active on adult parasites, has much lower activity against juvenile worms. Monotherapy also favors the selection of drug resistance and, therefore, new drugs are urgently needed. METHODS AND FINDINGS: Following the screening of a small compound library with an ATP-based luminescent assay on Schistosoma mansoni schistosomula, we here report the identification and characterization of novel antischistosomal properties of the anti-anginal drug perhexiline maleate (PHX). By phenotypic worm survival assays and confocal microscopy studies we show that PHX, in vitro, has a marked lethal effect on all S. mansoni parasite life stages (newly transformed schistosomula, juvenile and adult worms) of the definitive host. We further demonstrate that sub-lethal doses of PHX significantly impair egg production and lipid depletion within the vitellarium of adult female worms. Moreover, we highlighted tegumental damage in adult male worms and remarkable reproductive system alterations in both female and male adult parasites. The in vivo study in S. mansoni-patent mice showed a notable variability of worm burdens in the individual experiments, with an overall minimal schistosomicidal effect upon PHX treatment. The short PHX half-life in mice, together with its very high rodent plasma proteins binding could be the cause of the modest efficacy of PHX in the schistosomiasis murine model. CONCLUSIONS/SIGNIFICANCE: Overall, our data indicate that PHX could represent a promising starting point for novel schistosomicidal drug discovery programmes.

TMEM16F Regulates Spinal Microglial Function in Neuropathic Pain States.

Neuropathic pain is a widespread chronic pain state that results from injury to the nervous system. Spinal microglia play a causative role in the pathogenesis of neuropathic pain through secretion of growth factors and cytokines. Here, we investigated the contribution of TMEM16F, a protein that functions as a Ca(2+)-dependent ion channel and a phospholipid scramblase, to microglial activity during neuropathic pain. We demonstrate that mice with a conditional ablation of TMEM16F in microglia do not develop mechanical hypersensitivity upon nerve injury. In the absence of TMEM16F, microglia display deficits in process motility and phagocytosis. Moreover, loss of GABA immunoreactivity upon injury is spared in TMEM16F conditional knockout mice. Collectively, these data indicate that TMEM16F is an essential component of the microglial response to injury and suggest the importance of microglial phagocytosis in the pathogenesis of neuropathic pain.

A subpopulation of itch-sensing neurons marked by Ret and somatostatin expression.

Itch, the unpleasant sensation that elicits a desire to scratch, is mediated by specific subtypes of cutaneous sensory neuron. Here, we identify a subpopulation of itch-sensing neurons based on their expression of the receptor tyrosine kinase Ret. We apply flow cytometry to isolate Ret-positive neurons from dorsal root ganglia and detected a distinct population marked by low levels of Ret and absence of isolectin B4 binding. We determine the transcriptional profile of these neurons and demonstrate that they express neuropeptides such as somatostatin (Sst), the NGF receptor TrkA, and multiple transcripts associated with itch. We validate the selective expression of Sst using an Sst-Cre driver line and ablated these neurons by generating mice in which the diphtheria toxin receptor is conditionally expressed from the sensory neuron-specific Avil locus. Sst-Cre::Avil(iDTR) mice display normal nociceptive responses to thermal and mechanical stimuli. However, scratching behavior evoked by interleukin-31 (IL-31) or agonist at the 5HT1F receptor is significantly reduced. Our data provide a molecular signature for a subpopulation of neurons activated by multiple pruritogens.

Oligoasthenoteratozoospermia and infertility in mice deficient for miR-34b/c and miR-449 loci.

Male fertility requires the continuous production of high quality motile spermatozoa in abundance. Alterations in all three metrics cause oligoasthenoteratozoospermia, the leading cause of human sub/infertility. Post-mitotic spermatogenesis inclusive of several meiotic stages and spermiogenesis (terminal spermatozoa differentiation) are transcriptionally inert, indicating the potential importance for the post-transcriptional microRNA (miRNA) gene-silencing pathway therein. We found the expression of miRNA generating enzyme Dicer within spermatogenesis peaks in meiosis with critical functions in spermatogenesis. In an expression screen we identified two miRNA loci of the miR-34 family (miR-34b/c and miR-449) that are specifically and highly expressed in post-mitotic male germ cells. A reduction in several miRNAs inclusive of miR-34b/c in spermatozoa has been causally associated with reduced fertility in humans. We found that deletion of both miR34b/c and miR-449 loci resulted in oligoasthenoteratozoospermia in mice. MiR-34bc/449-deficiency impairs both meiosis and the final stages of spermatozoa maturation. Analysis of miR-34bc-/-;449-/- pachytene spermatocytes revealed a small cohort of genes deregulated that were highly enriched for miR-34 family target genes. Our results identify the miR-34 family as the first functionally important miRNAs for spermatogenesis whose deregulation is causal to oligoasthenoteratozoospermia and infertility.

Modulation of social behavior by the agouti pigmentation gene.

Agouti is a secreted neuropeptide that acts as an endogenous antagonist of melanocortin receptors. Mice and rats lacking agouti (called non-agouti) have dark fur due to a disinhibition of melanocortin signaling and pigment deposition in the hair follicle. Non-agouti animals have also been reported to exhibit altered behavior, despite no evidence for the expression of agouti outside the skin. Here we confirm that non-agouti mice show altered social behavior and uncover expression of agouti in the preputial gland, a sebaceous organ in the urinary tract that secretes molecules involved in social behavior. Non-agouti mice had enlarged preputial glands and altered levels of putative preputial pheromones and surgical removal of the gland reversed the behavioral phenotype. These findings demonstrate the existence of an autologous, out-of-skin pathway for the modulation of social behavior.

Severe protein aggregate myopathy in a knockout mouse model points to an essential role of cofilin2 in sarcomeric actin exchange and muscle maintenance.

Mutations in the human actin depolymerizing factor cofilin2 result in an autosomal dominant form of nemaline myopathy. Here, we report on the targeted ablation of murine cofilin2, which leads to a severe skeletal muscle specific phenotype within the first two weeks after birth. Apart from skeletal muscle, cofilin2 is also expressed in heart and CNS, however the pathology was restricted to skeletal muscle. The two close family members of cofilin2 - ADF and cofilin1 - were co-expressed in muscle, but unable to compensate for the loss of cofilin2. While primary myofibril assembly and muscle development were unaffected in cofilin2 mutant mice, progressive muscle degeneration was observed between postnatal days 3 and 7. Muscle pathology was characterized by sarcoplasmic protein aggregates, fiber size disproportion, mitochondrial abnormalities and internal nuclei. The observed muscle pathology differed from nemaline myopathy, but showed combined features of actin-associated myopathy and myofibrillar myopathy. In cofilin2 mutant mice, the postnatal expression pattern and turnover of sarcomeric α-actin isoforms were altered. Levels of smooth muscle α-actin were increased and remained high in developing muscles, suggesting that cofilin2 plays a crucial role during the exchange of α-actin isoforms during the early postnatal remodeling of the sarcomere.