ccm2-like is required for cardiovascular development as a novel component of the Heg-CCM pathway

KRIT1-mediated regulation of neutrophil adhesion and motility

Loss of Krev interaction-trapped-1 (KRIT1) expression leads to the development of cerebral cavernous malformations (CCM), a disease in which abnormal blood vessel formation compromises the structure and function of the blood-brain barrier. The role of KRIT1 in regulating endothelial function is well-established. However, several studies have suggested that KRIT1 could also play a role in regulating nonendothelial cell types and, in particular, immune cells. In this study, we generated a mouse model with neutrophil-specific deletion of KRIT1 in order to investigate the effect of KRIT1 deficiency on neutrophil function. Neutrophils isolated from adult Ly6Gtm2621(cre)Arte Krit1flox/flox mice had a reduced ability to attach and spread on the extracellular matrix protein fibronectin and exhibited a subsequent increase in migration. However, adhesion to and migration on ICAM-1 was unchanged. In addition, we used a monomeric, fluorescently-labelled fragment of fibronectin to show that integrin activation is reduced in the absence of KRIT1 expression, though β1 integrin expression appears unchanged. Finally, neutrophil migration in response to lipopolysaccharide-induced inflammation in the lung was decreased, as shown by reduced cell number and myeloperoxidase activity in lavage samples from Krit1PMNKO mice. Altogether, we show that KRIT1 regulates neutrophil adhesion and migration, likely through regulation of integrin activation, which can lead to altered inflammatory responses in vivo.

HEG1 as a novel potential biomarker for the prognosis of lung adenocarcinoma

BACKGROUND

Heart development protein with EGF-like domains 1 (HEG1), generally related to angiogenesis and embryonic development, was reported to participate in the occurrence and progression of some tumors recently. However, the role of HEG1 in lung adenocarcinoma (LUAD) is unclear.

PATIENTS AND METHODS

To explore the effect of HEG1 on LUAD, GEPIA platform and UALCAN database, as well as Kaplan-Meier plotter were adopted to analyze the association of HEG1 with clinicopathological characteristics and survival outcomes for LUAD firstly. And then the HEG1 in LUAD tissues, blood and cell lines were detected by qRT-PCR, western blot, immunofluorescence, immunohistochemistry, and ELISA. Gene set enrichment analysis (GSEA) was conducted to identify pathways that might be affected by HEG1 in LUAD.

RESULTS

In this study, HEG1 in lung tissues and cell lines of LUAD were significantly downregulated compared to benign pulmonary disease tissues and alveolar epithelial cells (p < 0.05). Moreover, compared with other groups, patients with advanced tumor stage had lower HEG1 mRNA expression levels (p = 0.025), which were negatively correlated with Ki67 index in tumor tissues (r = -0.427, p = 0.033). On the other hand, the LUAD patients with lower HEG1 had shorter overall survival (OS) (HR = 0.51, 95% CI: 0.40-0.65, p < 0.001) according to Kaplan-Meier plotter. In addition, HEG1 in serum of LUAD patients was negatively associated with CEA (r = -0.636, p < 0.001). GSEA showed that HEG1 was enriched in various metabolic-related pathways, including glucose metabolism, lipid metabolism, and nucleotide metabolism signaling.

CONCLUSIONS

HEG1 was downregulated in LUAD patients and associated with poor prognosis, which indicating HEG1 may serve as a potential biomarker for diagnosis and prognosis of LUAD.

Myocardial protective effect and transcriptome profiling of Naoxintong on cardiomyopathy in zebrafish

Background

Cardiomyopathy is a kind of cardiovascular diseases, which makes it more difficult for the heart to pump blood to other parts of the body, eventually leading to heart failure. Naoxintong (NXT), as a traditional Chinese Medicine (TCM) preparation, is widely used in the treatment of cardiovascular diseases, including cardiomyopathy, while its underlying mechanism has not been fully elucidated. The purpose of this study is to investigate the therapeutic effect of NXT on cardiomyopathy and its molecular mechanism in zebrafish model.

Methods

The zebrafish cardiomyopathy model was established using terfenadine (TFD) and treated with NXT. The therapeutic effect of NXT on cardiomyopathy was evaluated by measuring the heart rate, the distance between the sinus venosus and bulbus arteriosus (SV-BA), the pericardial area, and the blood flow velocity of zebrafish. Then, the zebrafish hearts were isolated and collected; transcriptome analysis of NXT on cardiomyopathy was investigated. Moreover, the heg1 mutant of zebrafish congenital cardiomyopathy model was used to further validate the therapeutic effect of NXT on cardiomyopathy. Additionally, UPLC analysis combined with the zebrafish model investigation was performed to identify the bioactive components of NXT.

Results

In the TFD-induced zebrafish cardiomyopathy model, NXT treatment could significantly restore the cardiovascular malformations caused by cardiac dysfunction. Transcriptome and bioinformatics analyses of the TFD and TFD  +  NXT treated zebrafish developing hearts revealed that the differentially expressed genes were highly enriched in biological processes such as cardiac muscle contraction and heart development. As a cardiac development protein associated with cardiomyopathy, HEG1 had been identified as one of the important targets of NXT in the treatment of cardiomyopathy. The cardiovascular abnormalities of zebrafish heg1 mutant could be recovered significantly from NXT treatment, including the expanded atrial cavity and blood stagnation. qRT-PCR analysis further showed that NXT could restore cardiomyopathy phenotype in zebrafish through HEG1-CCM signaling. Among the seven components identified in NXT, paeoniflorin (PF) and salvianolic acid B (Sal B) were considered to be the main bioactive ones with myocardial protection.

Conclusion

NXT presented myocardial protective effect and could restore myocardial injury and cardiac dysfunction in zebrafish; the action mechanism was involved in HEG1-CCM signaling.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13020-021-00532-0.

Cerebral Cavernous Malformation: From Mechanism to Therapy

Cerebral cavernous malformations are acquired vascular anomalies that constitute a common cause of central nervous system hemorrhage and stroke. The past 2 decades have seen a remarkable increase in our understanding of the pathogenesis of this vascular disease. This new knowledge spans genetic causes of sporadic and familial forms of the disease, molecular signaling changes in vascular endothelial cells that underlie the disease, unexpectedly strong environmental effects on disease pathogenesis, and drivers of disease end points such as hemorrhage. These novel insights are the integrated product of human clinical studies, human genetic studies, studies in mouse and zebrafish genetic models, and basic molecular and cellular studies. This review addresses the genetic and molecular underpinnings of cerebral cavernous malformation disease, the mechanisms that lead to lesion hemorrhage, and emerging biomarkers and therapies for clinical treatment of cerebral cavernous malformation disease. It may also serve as an example for how focused basic and clinical investigation and emerging technologies can rapidly unravel a complex disease mechanism.

Cell Cycle Withdrawal Limit the Regenerative Potential of Neonatal Cardiomyocytes

PURPOSE

The neonatal mouse possesses a transient capacity for cardiac regeneration during the first few days of life. The regenerative response of neonatal mouse is primarily mediated by pre-existing cardiomyocyte (CM) proliferation, which has been identified as the primary source of myocardial regeneration. Postnatal 4-day-old (P4) mouse CMs appear to undergo a rapid transition from hyperplastic to hypertrophic growth and binucleation. By 7 days following birth this regenerative potential is lost which coincidently correspond with CM cell cycle arrest and binucleation. CCM2-like (Ccm2l) plays pivotal roles in cardiovascular development and cardiac growth, indicating a potential function in heart regeneration postnatally. The aim of this study was to determine the cardiac regeneration ability of P4 neonatal mouse using a novel and more reproducible injury model and to determine whether Ccm2l has any functional roles in heart repair following ischemic injury.

METHODS

We performed a modified left anterior descending artery (LAD) ligation procedure on P4 mice to examine cardiac regenerative responses at different time points. Additionally, we generated an endothelial-specific Ccm2l gain-of-function transgenic mouse to determine the role of Ccm2l in neonatal cardiac regeneration.

RESULTS

We found that the P4 mouse heart harbor a robust regenerative response after injury that was through the proliferation of pre-existing CMs but cardiac hypertrophy and subsequent remodeling was still evident 60 days after LAD ligation. Furthermore, we show that endothelial-specific overexpression of Ccm2l does not promote CM proliferation and heart repair after LAD ligation.

CONCLUSION

The neonatal heart at P4 harbors a robust but incomplete capacity for cardiac regeneration. Endothelial overexpression of Ccm2l has no effect on cardiac regeneration.

Molecular Genetic Features of Cerebral Cavernous Malformations (CCM) Patients: An Overall View from Genes to Endothelial Cells

Cerebral cavernous malformations (CCMs) are vascular lesions that affect predominantly microvasculature in the brain and spinal cord. CCM can occur either in sporadic or familial form, characterized by autosomal dominant inheritance and development of multiple lesions throughout the patient's life. Three genes associated with CCM are known: CCM1/KRIT1 (krev interaction trapped 1), CCM2/MGC4607 (encoding a protein named malcavernin), and CCM3/PDCD10 (programmed cell death 10). All the mutations identified in these genes cause a loss of function and compromise the protein functions needed for maintaining the vascular barrier integrity. Loss of function of CCM proteins causes molecular disorganization and dysfunction of endothelial adherens junctions. In this review, we provide an overall vision of the CCM pathology, starting with the genetic bases of the disease, describing the role of the proteins, until we reach the cellular level. Thus, we summarize the genetics of CCM, providing a description of CCM genes and mutation features, provided an updated knowledge of the CCM protein structure and function, and discuss the molecular mechanisms through which CCM proteins may act within endothelial cells, particularly in endothelial barrier maintenance/regulation and in cellular signaling.

CCM2L (Cerebral Cavernous Malformation 2 Like) Deletion Aggravates Cerebral Cavernous Malformation Through Map3k3-KLF Signaling Pathway

[Figure: see text].

Signalling through cerebral cavernous malformation protein networks

Cerebral cavernous malformations (CCMs) are neurovascular abnormalities characterized by thin, leaky blood vessels resulting in lesions that predispose to haemorrhages, stroke, epilepsy and focal neurological deficits. CCMs arise due to loss-of-function mutations in genes encoding one of three CCM complex proteins, KRIT1, CCM2 or CCM3. These widely expressed, multi-functional adaptor proteins can assemble into a CCM protein complex and (either alone or in complex) modulate signalling pathways that influence cell adhesion, cell contractility, cytoskeletal reorganization and gene expression. Recent advances, including analysis of the structures and interactions of CCM proteins, have allowed substantial progress towards understanding the molecular bases for CCM protein function and how their disruption leads to disease. Here, we review current knowledge of CCM protein signalling with a focus on three pathways which have generated the most interest—the RhoA–ROCK, MEKK3–MEK5–ERK5–KLF2/4 and cell junctional signalling pathways—but also consider ICAP1-β1 integrin and cdc42 signalling. We discuss emerging links between these pathways and the processes that drive disease pathology and highlight important open questions—key among them is the role of subcellular localization in the control of CCM protein activity.

Cerebral Cavernous Malformation Proteins in Barrier Maintenance and Regulation

Cerebral cavernous malformation (CCM) is a disease characterized by mulberry shaped clusters of dilated microvessels, primarily in the central nervous system. Such lesions can cause seizures, headaches, and stroke from brain bleeding. Loss-of-function germline and somatic mutations of a group of genes, called CCM genes, have been attributed to disease pathogenesis. In this review, we discuss the impact of CCM gene encoded proteins on cellular signaling, barrier function of endothelium and epithelium, and their contribution to CCM and potentially other diseases.

Alternatively spliced isoforms reveal a novel type of PTB domain in CCM2 protein

Cerebral cavernous malformations (CCMs) is a microvascular disorder in the central nervous system. Despite tremendous efforts, the causal genetic mutation in some CCM patients has not be identified, raising the possibility of an unknown CCM locus. The CCM2/MGC4607 gene has been identified as one of three known genes causing CCMs. In this report, we defined a total of 29 novel exons and 4 novel promoters in CCM2 genomic structure and subsequently identified a total of 50 new alternative spliced isoforms of CCM2 which eventually generated 22 novel protein isoforms. Genetic analysis of CCM2 isoforms revealed that the CCM2 isoforms can be classified into two groups based on their alternative promoters and alternative start codon exons. Our data demonstrated that CCM2 isoforms not only are specific in their subcellular compartmentation but also have distinct cellular expression patterns among various tissues and cells, indicating the pleiotropic cellular roles of CCM2 through their multiple isoforms. In fact, the complexity of the CCM2 genomic structure was reflected by the multiple layers of regulation of CCM2 expression patterns. At the transcriptional level, it is accomplished by alternative promoters, alternative splicing, and multiple transcriptional start sites and termination sites; while at the translational level, it is carried out with various cellular functions with a distinguishable CCM2 protein group pattern, specified abundance and composition of selective isoforms in a cell and tissue specific fashion. Through experimentation, we discovered a unique phosphotyrosine binding (PTB) domain, namely atypical phosphotyrosine binding (aPTB) domain. Some long CCM2 isoform proteins contain both classes of PTB domains, making them a dual PTB domain-containing protein. Both CCM1 and CCM3 can bind competitively to this aPTB domain, indicating CCM2 as the cornerstone for CCM signaling complex (CSC).

Genome-Wide Association Study of Growth and Body-Shape-Related Traits in Large Yellow Croaker (Larimichthys crocea) Using ddRAD Sequencing

Large yellow croaker (Larimichthys crocea) is an economically important marine fish species of China. Due to overfishing and marine pollution, the wild stocks of this croaker have collapsed in the past decades. Meanwhile, the cultured croaker is facing the difficulties of reduced genetic diversity and low growth rate. To explore the molecular markers related to the growth traits of croaker and providing the related SNPs for the marker-assisted selection, we used double-digest restriction-site associated DNA (ddRAD) sequencing to dissect the genetic bases of growth traits in a cultured population and identify the SNPs that associated with important growth traits by GWAS. A total of 220 individuals were genotyped by ddRAD sequencing. After quality control, 27,227 SNPs were identified in 220 samples and used for GWAS analysis. We identified 13 genome-wide significant associated SNPs of growth traits on 8 chromosomes, and the beta P of these SNPs ranged from 0.01 to 0.86. Through the definition of candidate regions and gene annotation, candidate genes related to growth were identified, including important regulators such as fgf18, fgf1, nr3c1, cyp8b1, fabp2, cyp2r1, ppara, and ccm2l. We also identified SNPs and candidate genes that significantly associated with body shape, including bmp7, col1a1, col11a2, and col18a1, which are also economically important traits for large yellow croaker aquaculture. The results provided insights into the genetic basis of growth and body shape in large yellow croaker population and would provide reliable genetic markers for molecular marker-assisted selection in the future. Meanwhile, the result established a basis for our subsequent fine mapping and related gene study.

Two Novel KRIT1 and CCM2 Mutations in Patients Affected by Cerebral Cavernous Malformations: New Information on CCM2 Penetrance

Wide comprehension of genetic features of cerebral cavernous malformations (CCM) represents the starting point to better manage patients and risk rating in relatives. The causative mutations spectrum is constantly growing. KRIT1, CCM2, and PDCD10 are the three loci to date linked to familial CCM development, although germline mutations have also been detected in patients affected by sporadic forms. In this context, the main challenge is to draw up criteria to formulate genotype-phenotype correlations. Clearly, genetic factors determining incomplete penetrance of CCM need to be identified. Here, we report two novel intronic variants probably affecting splicing. Molecular screening of CCM genes was performed on DNA purified by peripheral blood. Coding exons and intron-exon boundaries were sequenced by the Sanger method. The first was detected in a sporadic patient and involves KRIT1. The second affects CCM2 and it is harbored by a woman with familial CCM. Interestingly, molecular analysis extended to both healthy and ill relatives allowed to estimate, for the first time, a penetrance for CCM2 lower than 100%, as to date reported. Moreover, heterogeneity of clinical manifestations among those affected carrying the same genotype further confirms involvement of modifier factors in CCM development.

Recent advances in cerebral cavernous malformation research

Cerebral cavernous malformations (CCM) are manifested by microvascular lesions characterized by leaky endothelial cells with minimal intervening parenchyma predominantly in the central nervous system predisposed to hemorrhagic stroke, resulting in focal neurological defects. Till date, three proteins are implicated in this condition: CCM1 (KRIT1), CCM2 (MGC4607), and CCM3 (PDCD10). These multi-domain proteins form a protein complex via CCM2 that function as a docking site for the CCM signaling complex, which modulates many signaling pathways. Defects in the formation of this signaling complex have been shown to affect a wide range of cellular processes including cell-cell contact stability, vascular angiogenesis, oxidative damage protection and multiple biogenic events. In this review we provide an update on recent advances in structure and function of these CCM proteins, especially focusing on the signaling cascades involved in CCM pathogenesis and the resultant CCM cellular phenotypes in the past decade.

Introduction to cerebral cavernous malformation: a brief review

The disease known as cerebral cavernous malformations mostly occurs in the central nervous system, and their typical histological presentations are multiple lumen formation and vascular leakage at the brain capillary level, resulting in disruption of the blood-brain barrier. These abnormalities result in severe neurological symptoms such as seizures, focal neurological deficits and hemorrhagic strokes. CCM research has identified ‘loss of function’ mutations of three ccm genes responsible for the disease and also complex regulation of multiple signaling pathways including the WNT/β-catenin pathway, TGF-β and Notch signaling by the ccm genes. Although CCM research is a relatively new and small scientific field, as CCM research has the potential to regulate systemic blood vessel permeability and angiogenesis including that of the blood-brain barrier, this field is growing rapidly. In this review, I will provide a brief overview of CCM pathogenesis and function of ccm genes based on recent progress in CCM research. [BMB Reports 2016; 49(5): 255-262]

Cerebral Cavernous Malformations: Review of the Genetic and Protein-Protein Interactions Resulting in Disease Pathogenesis

Mutations in the genes KRIT1, CCM2, and PDCD10 are known to result in the formation of cerebral cavernous malformations (CCMs). CCMs are intracranial lesions composed of aberrantly enlarged “cavernous” endothelial channels that can result in cerebral hemorrhage, seizures, and neurologic deficits. Although these genes have been known to be associated with CCMs since the 1990s, numerous discoveries have been made that better elucidate how they and their subsequent protein products are involved in CCM pathogenesis. Since our last review of the molecular genetics of CCM pathogenesis in 2012, breakthroughs include a more thorough understanding of the protein structures of the gene products, involvement with integrin proteins, and MEKK3 signaling pathways, and the importance of CCM2–PDCD10 interactions. In this review, we highlight the advances that further our understanding of the “gene to protein to disease” relationships of CCMs.

The cerebral cavernous malformation proteins CCM2L and CCM2 prevent the activation of the MAP kinase MEKK3

Three genes, CCM1, CCM2, and CCM3, interact genetically and biochemically and are mutated in cerebral cavernous malformations (CCM). A recently described member of this CCM family of proteins, CCM2-like (CCM2L), has high homology to CCM2. Here we show that its relative expression in different tissues differs from that of CCM2 and, unlike CCM2, the expression of CCM2L in endothelial cells is regulated by density, flow, and statins. In vitro, both CCM2L and CCM2 bind MEKK3 in a complex with CCM1. Both CCM2L and CCM2 interfere with MEKK3 activation and its ability to phosphorylate MEK5, a downstream target. The in vivo relevance of this regulation was investigated in zebrafish. A knockdown of ccm2l and ccm2 in zebrafish leads to a more severe "big heart" and circulation defects compared with loss of function of ccm2 alone, and also leads to substantial body axis abnormalities. Silencing of mekk3 rescues the big heart and body axis phenotype, suggesting cross-talk between the CCM proteins and MEKK3 in vivo. In endothelial cells, CCM2 deletion leads to activation of ERK5 and a transcriptional program that are downstream of MEKK3. These findings suggest that CCM2L and CCM2 cooperate to regulate the activity of MEKK3.

CCM1 and the second life of proteins in adhesion complexes

It is well recognized that a number of proteins present within adhesion complexes perform discrete signaling functions outside these adhesion complexes, including transcriptional control. In this respect, β-catenin is a well-known example of an adhesion protein present both in cadherin complexes and in the nucleus where it regulates the TCF transcription factor. Here we discuss nuclear functions of adhesion complex proteins with a special focus on the CCM-1/KRIT-1 protein, which may turn out to be yet another adhesion complex protein with a second life.

Endothelial cell-cell adhesion during zebrafish vascular development

The vertebrate vasculature is an essential organ network with major roles in health and disease. The establishment of balanced cell-cell adhesion in the endothelium is crucial for the functionality of the vascular system. Furthermore, the correct patterning and integration of vascular endothelial cell-cell adhesion drives the morphogenesis of new vessels, and is thought to couple physical forces with signaling outcomes during development. Here, we review insights into this process that have come from studies in zebrafish. First, we describe mutants in which endothelial adhesion is perturbed, second we describe recent progress using in vivo cell biological approaches that allow the visualization of endothelial cell-cell junctions. These studies underline the profound potential of this model system to dissect in great detail the function of both known and novel regulators of endothelial cell-cell adhesion.

Structural Insights into the Molecular Recognition between Cerebral Cavernous Malformation 2 and Mitogen-Activated Protein Kinase Kinase Kinase 3

Cerebral cavernous malformation 2 (CCM2) functions as an adaptor protein implicated in various biological processes. By interacting with the mitogen-activated protein kinase MEKK3, CCM2 either mediates the activation of MEKK3 signaling in response to osmotic stress or negatively regulates MEKK3 signaling, which is important for normal cardiovascular development. However, the molecular basis governing CCM2-MEKK3 interaction is largely unknown. Here we report the crystal structure of the CCM2 C-terminal part (CCM2ct) containing both the five-helix domain (CCM2cts) and the following C-terminal tail. The end of the C-terminal tail forms an isolated helix, which interacts intramolecularly with CCM2cts. By biochemical studies we identified the N-terminal amphiphilic helix of MEKK3 (MEKK3-nhelix) as the essential structural element for CCM2ct binding. We further determined the crystal structure of CCM2cts-MEKK3-nhelix complex, in which MEKK3-nhelix binds to the same site of CCM2cts for CCM2ct intramolecular interaction. These findings build a structural framework for understanding CCM2ct-MEKK3 molecular recognition.

Cerebral cavernous malformation proteins at a glance

Loss-of-function mutations in genes encoding KRIT1 (also known as CCM1), CCM2 (also known as OSM and malcavernin) or PDCD10 (also known as CCM3) cause cerebral cavernous malformations (CCMs). These abnormalities are characterized by dilated leaky blood vessels, especially in the neurovasculature, that result in increased risk of stroke, focal neurological defects and seizures. The three CCM proteins can exist in a trimeric complex, and each of these essential multi-domain adaptor proteins also interacts with a range of signaling, cytoskeletal and adaptor proteins, presumably accounting for their roles in a range of basic cellular processes including cell adhesion, migration, polarity and apoptosis. In this Cell Science at a Glance article and the accompanying poster, we provide an overview of current models of CCM protein function focusing on how known protein-protein interactions might contribute to cellular phenotypes and highlighting gaps in our current understanding.

Signaling pathways and the cerebral cavernous malformations proteins: lessons from structural biology

Cerebral cavernous malformations (CCM) are neurovascular dysplasias that result in mulberry-shaped lesions predominantly located in brain and spinal tissues. Mutations in three genes are associated with CCM. These genes encode for the proteins KRIT1/CCM1 (krev interaction trapped 1/cerebral cavernous malformations 1), cerebral cavernous malformations 2, osmosensing scaffold for MEKK3 (CCM2/malcavernin/OSM), and cerebral cavernous malformations 3/programmed cell death 10 (CCM3/PDCD10). There have been many significant recent advances in our understanding of the structure and function of these proteins, as well as in their roles in cellular signaling. Here, we provide an update on the current knowledge of the structure of the CCM proteins and their functions within cellular signaling, particularly in cellular adhesion complexes and signaling cascades. We go on to discuss subcellular localization of the CCM proteins, the formation and regulation of the CCM complex signaling platform, and current progress towards targeted therapy for CCM disease. Recent structural studies have begun to shed new light on CCM protein function, and we focus here on how these studies have helped inform the current understanding of these roles and how they may aid future studies into both CCM-related biology and disease mechanisms.

Ion channel-kinase TRPM7 is required for maintaining cardiac automaticity

Sick sinus syndrome and atrioventricular block are common clinical problems, often necessitating permanent pacemaker placement, yet the pathophysiology of these conditions remains poorly understood. Here we show that Transient Receptor Potential Melastatin 7 (TRPM7), a divalent-permeant channel-kinase of unknown function, is highly expressed in embryonic myocardium and sinoatrial node (SAN) and is required for cardiac automaticity in these specialized tissues. TRPM7 disruption in vitro, in cultured embryonic cardiomyocytes, significantly reduces spontaneous Ca(2+) transient firing rates and is associated with robust down-regulation of Hcn4, Cav3.1, and SERCA2a mRNA. TRPM7 knockdown in zebrafish, global murine cardiac Trpm7 deletion (KO(αMHC-Cre)), and tamoxifen-inducible SAN restricted Trpm7 deletion (KO(HCN4-CreERT2)) disrupts cardiac automaticity in vivo. Telemetered and sedated KO(αMHC-Cre) and KO(HCN4-CreERT2) mice show episodes of sinus pauses and atrioventricular block. Isolated SAN from KO(αMHC-Cre) mice exhibit diminished Ca(2+) transient firing rates with a blunted diastolic increase in Ca(2+). Action potential firing rates are diminished owing to slower diastolic depolarization. Accordingly, Hcn4 mRNA and the pacemaker current, I(f), are diminished in SAN from both KO(αMHC-Cre) and KO(HCN4-CreERT2) mice. Moreover, heart rates of KO(αMHC-Cre) mice are less sensitive to the selective I(f) blocker ivabradine, and acute application of the recently identified TRPM7 blocker FTY720 has no effect on action potential firing rates of wild-type SAN cells. We conclude that TRPM7 influences diastolic membrane depolarization and automaticity in SAN indirectly via regulation of Hcn4 expression.