Impaired Reorganization of Centrosome Structure Underlies Human Infantile Dilated Cardiomyopathy

BACKGROUND

During cardiomyocyte maturation, the centrosome, which functions as a microtubule organizing center in cardiomyocytes, undergoes dramatic structural reorganization where its components reorganize from being localized at the centriole to the nuclear envelope. This developmentally programmed process, referred to as centrosome reduction, has been previously associated with cell cycle exit. However, understanding of how this process influences cardiomyocyte cell biology, and whether its disruption results in human cardiac disease, remains unknown. We studied this phenomenon in an infant with a rare case of infantile dilated cardiomyopathy (iDCM) who presented with left ventricular ejection fraction of 18% and disrupted sarcomere and mitochondria structure.

METHODS

We performed an analysis beginning with an infant who presented with a rare case of iDCM. We derived induced pluripotent stem cells from the patient to model iDCM in vitro. We performed whole exome sequencing on the patient and his parents for causal gene analysis. CRISPR/Cas9-mediated gene knockout and correction in vitro were used to confirm whole exome sequencing results. Zebrafish and Drosophila models were used for in vivo validation of the causal gene. Matrigel mattress technology and single-cell RNA sequencing were used to characterize iDCM cardiomyocytes further.

RESULTS

Whole exome sequencing and CRISPR/Cas9 gene knockout/correction identified RTTN, the gene encoding the centrosomal protein RTTN (rotatin), as the causal gene underlying the patient's condition, representing the first time a centrosome defect has been implicated in a nonsyndromic dilated cardiomyopathy. Genetic knockdowns in zebrafish and Drosophila confirmed an evolutionarily conserved requirement of RTTN for cardiac structure and function. Single-cell RNA sequencing of iDCM cardiomyocytes showed impaired maturation of iDCM cardiomyocytes, which underlie the observed cardiomyocyte structural and functional deficits. We also observed persistent localization of the centrosome at the centriole, contrasting with expected programmed perinuclear reorganization, which led to subsequent global microtubule network defects. In addition, we identified a small molecule that restored centrosome reorganization and improved the structure and contractility of iDCM cardiomyocytes.

CONCLUSIONS

This study is the first to demonstrate a case of human disease caused by a defect in centrosome reduction. We also uncovered a novel role for RTTN in perinatal cardiac development and identified a potential therapeutic strategy for centrosome-related iDCM. Future study aimed at identifying variants in centrosome components may uncover additional contributors to human cardiac disease.

Fc gamma RIIB as a potential molecular target for intravenous gamma globulin therapy

The ability of the immune system to respond appropriately to foreign antigen is dependent on a delicate balance of activating and inhibitory signals. Recently, the role of cell surface inhibitory receptors in attenuating immune responses, thereby preventing pathologic conditions including autoimmunity and atopy, has been recognized. It is postulated that the beneficial effects of intravenous gamma globulin in the treatment of immune disorders may be attributable, at least in part, to engagement of Fc gamma RIIB, a member of the recently described family of immune inhibitory receptors. Recent genetic and biochemical studies have identified the SH2 domain-containing inositol 5-phosphatase (SHIP) as a critical effector in Fc gamma RIIB inhibitory signaling. This review summarizes recent work from our laboratory and others aimed to define the mechanism(s) by which Fc gamma RIIB and its effector, SHIP, inhibit immune responses. Elucidation of these mechanisms may lead to the development of therapeutic strategies for the treatment of autoimmune and inflammatory pathologies that specifically target Fc gamma RIIB or its effector(s).

Mutational analysis reveals multiple distinct sites within Fc gamma receptor IIB that function in inhibitory signaling

The low-affinity receptor for IgG, FcgammaRIIB, functions broadly in the immune system, blocking mast cell degranulation, dampening the humoral immune response, and reducing the risk of autoimmunity. Previous studies concluded that inhibitory signal transduction by FcgammaRIIB is mediated solely by its immunoreceptor tyrosine-based inhibition motif (ITIM) that, when phosphorylated, recruits the SH2-containing inositol 5'- phosphatase SHIP and the SH2-containing tyrosine phosphatases SHP-1 and SHP-2. The mutational analysis reported here reveals that the receptor's C-terminal 16 residues are also required for detectable FcgammaRIIB association with SHIP in vivo and for FcgammaRIIB-mediated phosphatidylinositol 3-kinase hydrolysis by SHIP. Although the ITIM appears to contain all the structural information required for receptor-mediated tyrosine phosphorylation of SHIP, phosphorylation is enhanced when the C-terminal sequence is present. Additionally, FcgammaRIIB-mediated dephosphorylation of CD19 is independent of the cytoplasmic tail distal from residue 237, including the ITIM. Finally, the findings indicate that tyrosines 290, 309, and 326 are all sites of significant FcgammaRIIB1 phosphorylation following coaggregation with B cell Ag receptor. Thus, we conclude that multiple sites in FcgammaRIIB contribute uniquely to transduction of FcgammaRIIB-mediated inhibitory signals.

Fc epsilon receptor I-associated lyn-dependent phosphorylation of Fc gamma receptor IIB during negative regulation of mast cell activation

Fc gamma RIIB are low-affinity receptors for IgG whose intracytoplasmic domain contains an immunoreceptor tyrosine-based inhibition motif (ITIM). Fc gamma RIIB inhibit cell activation triggered by receptors that signal via immunoreceptor tyrosine-based activation motifs. This inhibition requires ITIM tyrosyl phosphorylation and is correlated with the binding of SH2 domain-containing phosphatases that may mediate the inhibitory signal. In the present work, we investigated the mechanism of Fc gamma RIIB phosphorylation and its consequences in mast cells. We demonstrate that the phosphorylation of Fc gamma RIIB requires coaggregation with Fc epsilon RI and that, once phosphorylated, Fc gamma RIIB selectively recruit the inositol polyphosphate 5 phosphatase SHIP, in vivo. In vitro, however, the phosphorylated Fc gamma RIIB ITIM binds not only SHIP, but also the two protein tyrosine phosphatases, SHP-1 and SHP-2. We show that the coaggregation of Fc gamma RIIB with Fc epsilon RI does not prevent Fc epsilon RI-mediated activation of lyn and syk. Both kinases can phosphorylate Fc gamma RIIB in vitro. However, when coaggregated with Fc epsilon RI, Fc gamma RIIB was in vivo phosphorylated in syk-deficient mast cells, but not in lyn-deficient mast cells. When Fc epsilon RI are coaggregated with Fc gamma RIIB by immune complexes, Fc epsilon RI-associated lyn may thus phosphorylate Fc gamma RIIB. By this mechanism, Fc epsilon RI initiate ITIM-dependent inhibition of intracellular propagation of their own signals.

Selective in vivo recruitment of the phosphatidylinositol phosphatase SHIP by phosphorylated Fc gammaRIIB during negative regulation of IgE-dependent mouse mast cell activation

We demonstrated previously that the low-affinity IgG receptors Fc gammaRIIB, which are coexpressed with the high-affinity IgE receptors Fc epsilonRI in mouse mast cells, can inhibit IgE-induced release of inflammatory mediators and cytokines by these cells. Inhibition was found to require the coaggregation of the two receptors and to depend on the presence of a tyrosine-based inhibition motif (ITIM) in the intracytoplasmic domain of Fc gammaRIIB. We report here that the coaggregation with Fc gammaRIIB does not prevent Fc epsilonRI from triggering activation signals in BMMC and induces the tyrosine phosphorylation of Fc gammaRIIB. Phosphorylated ITIM peptides bound in vitro to three SH2 domain-containing phosphatases present in BMMC lysates: the phosphotyrosine phosphatases SHP-1 and SHP-2. and the inositolphosphate phosphatase SHIP. Using BMMC generated from the SHP-1-deficient motheaten mice, SHP-1 was found to be dispensable for inhibition of mast cell activation. When analyzed for in vivo association, SHIP coprecipitated with phosphorylated Fc gammaRIIB, whereas SHP-1 or SHP-2 did not. These observations altogether indicate that Fc epsilonRI actively participates in its own regulation and that the mechanisms by which Fc gammaRIIB inhibit cell activation might be different in mast cells and in B-cells.

The SHIP phosphatase becomes associated with Fc gammaRIIB1 and is tyrosine phosphorylated during 'negative' signaling

Immune-complex mediated co-ligation of antigen and Fc receptors on B-cells leads to abortive antigen receptor (BCR) signaling and provides a mechanism for feedback regulation of the immune response. A phosphotyrosine-containing 13 amino acid sequence (ITIM) found in the FcgammaRIIB1 cytoplasmic tail mediates this inhibition and specifically associates with the phosphotyrosine phosphatase SHP1. In vitro binding studies demonstrate that the phosphorylated ITIM binds unidentified proteins of 70 and 160 kD in addition to SHP1. Here we report the identification of p70 as SHP2 and p160 as the SH2 containing phosphatidylinositol 3,4,5-trisphosphate 5-phosphatase SHIP. SHIP is inducibly tyrosine phosphorylated following BCR-FcgammaRIIB1 co-ligation. Further, we observe SHIP association with tyrosine phosphorylated FcgammaRIIB1 in intact cells following BCR-FcgammaRIIB1 co-ligation. To a much lesser but significant degree, tyrosine phosphorylation of SHIP is also observed upon BCR ligation. These observations suggest that SHIP may play an important role in FcgammaRIIB1 dependent and independent regulation of BCR signaling.