Défaut d’exocytose des granules lytiques : Plusieurs causes, un même effet

Une réponse immune exagérée, incontrôlée et le plus souvent fatale, connue sous le nom de syndrome hémophagocytaire (SH), est associée à un défaut de la fonction cytotoxique des lymphocytes T et natural killer (NK). Les anomalies moléculaires responsables, qui sont multiples, mettent en cause dans la plupart des cas un effecteur indispensable au fonctionnement de la machinerie lytique des lymphocytes. L’étude des lymphocytes cytotoxiques déficients en l’un ou l’autre de ces effecteurs apporte des éléments nouveaux quant à l’agencement des étapes clés de la sécrétion du contenu des granules lytiques au contact de la cellule cible. Des mécanismes moléculaires proches semblent contrôler la sécrétion vésiculaire au niveau des synapses immunologique et neurologique. D’autres effecteurs de la cytotoxicité ou du contrôle de l’homéostasie lymphocytaire à l’origine de SH doivent encore être caractérisés. Quant aux mécanismes précis de l’intervention de cette voie cytotoxique dans le maintien de l’homéostasie lymphocytaire (terminaison d’une réponse immune), ils demeurent à élucider.An in vivo disturbance of lymphocyte homeostasis occurs during the course of the hemophagocytic syndrome (HS). HS is a severe and often fatal syndrome resulting from potent and uncontrolled activation and proliferation of T-lymphocytes, mainly polyclonal CD8 lymphocytes, leading to excessive macrophage activation, high level of proinflammatory cytokine production and multiple deleterious effects. The onset of HS characterizes several inherited disorders in humans. In most of these conditions, the molecular defect impairs the granule-dependent cytotoxic activity of lymphocytes, thus highlighting the determinant role of this function in driving back the immune system to a state of equilibrium following infection. Several lines of evidence suggest that an increase in the expansion phase rather than a decrease in the contraction phase of the CD8+ T cells population characterizes the HS. Failure to kill antigen presenting cells through a transaction mechanism of cytotoxic cells should favor a sustained response, although the mechanism may be more complex than simple decrease of antigen load. Defect in the granule dependent cytotoxic function of lymphocytes result from perforin mutation in familial hemophagocytic lymphohistiocytosis type 2, from Munc13-4 (UNC13D) mutation in familial hemophagocytic lymphohistiocytosis type 3, from Rab27a mutation in Griscelli syndrome type 2, and from CHS/LYST mutation in Chediak-Higashi syndrome. The characterization of the molecular causes leading to these conditions identified Rab27a and Munc13-4 as two critical effectors of the exocytic machinery, required for the terminal transport/docking or priming of the cytotoxic granules, respectively. Different members of the Rab and Munc13 family of proteins are also used in neurotransmitter release at the neurological synapse, highlighting the similarity of the mechanisms regulating both secretory pathways. Future investigations regarding HS will continue to elucidate this exocytic pathway machinery and improve our understanding of how it finely regulates the immune response, an area that is likely to be useful for therapeutic intervention

Ttc7a regulates hematopoietic stem cell functions while controlling the stress-induced response

The molecular machinery that regulates the balance between self-renewal and differentiation properties of hematopoietic stem cells (HSC) has yet to be characterized in detail. Here we found that the tetratricopeptide repeat domain 7 A (Ttc7a) protein, a putative scaffold protein expressed by HSC, acts as an intrinsic regulator of the proliferative response and the self-renewal potential of murine HSC in vivo. Loss of Ttc7a consistently enhanced the competitive repopulating ability of HSC and their intrinsic capacity to replenish the hematopoietic system after serial cell transplantations, relative to wildtype cells. Ttc7a-deficient HSC exhibit a different transcriptomic profile for a set of genes controlling the cellular response to stress, which was associated with increased proliferation in response to chemically induced stress in vitro and myeloablative stress in vivo. Our results therefore revealed a previously unrecognized role of Ttc7a as a critical regulator of HSC stemness. This role is related, at least in part, to regulation of the endoplasmic reticulum stress response.</p

Premature birth, respiratory distress, intracerebral hemorrhage, and silvery-gray hair: differential diagnosis of the 3 types of Griscelli syndrome

A preterm neonate, born to consanguineous parents, presented with respiratory distress, intracerebral hemorrhage, and a silvery-gray sheen of the hair and eyelashes. Griscelli syndrome (GS) type 3 was diagnosed after the detection of a novel homozygous mutation of the melanophilin gene. Thus, only the hypopigmentation, but not the patient's other clinical features, were attributable to this form of GS. Differential diagnosis of the various forms of GS must be performed as early as possible as GS2 is associated with a life threatening but curable immune disorder

Defective NKT cell development in mice and humans lacking the adapter SAP, the X-linked lymphoproliferative syndrome gene product

SAP is an adaptor protein expressed in T cells and natural killer cells. It plays a critical role in immunity, as it is mutated in humans with X-linked lymphoproliferative syndrome (XLP), a fatal immunodeficiency characterized by an abnormal response to Epstein-Barr virus (EBV) infection. SAP interacts with the SLAM family receptors and promotes transduction signal events by these receptors through its capacity to recruit and activate the Src kinase FynT. Because it has been previously established that FynT is selectively required for the development of NKT cells, we examined NKT cells in SAP-deficient mice and in humans with XLP. In the absence of SAP, the development of NKT cells is severely impaired both in mice and in humans. These results imply that SAP is a potent regulator of NKT cell development. They also identify for the first time a defect in NKT cells associated with a human primary immunodeficiency, revealing a potential role of NKT cells in the immune response to EBV

Rab27a: A key to melanosome transport in human melanocytes

Normal pigmentation depends on the uniform distribution of melanin-containing vesicles, the melanosomes, in the epidermis. Griscelli syndrome (GS) is a rare autosomal recessive disease, characterized by an immune deficiency and a partial albinism that has been ascribed to an abnormal melanosome distribution. GS maps to 15q21 and was first associated with mutations in the myosin-V gene. However, it was demonstrated recently that GS can also be caused by a mutation in the Rab27a gene. These observations prompted us to investigate the role of Rab27a in melanosome transport. Using immunofluorescence and immunoelectron microscopy studies, we show that in normal melanocytes Rab27a colocalizes with melanosomes. In melanocytes isolated from a patient with GS, we show an abnormal melanosome distribution and a lack of Rab27a expression. Finally, reexpression of Rab27a in GS melanocytes restored melanosome transport to dendrite tips, leading to a phenotypic reversion of the diseased cells. These results identify Rab27a as a key component of vesicle transport machinery in melanocytes

Tetratricopeptide repeat domain 7A is a nuclear factor that modulates transcription and chromatin structure

A loss-of-function mutation in tetratricopeptide repeat domain 7A (TTC7A) is a recently identified cause of human intestinal and immune disorders. However, clues to related underlying molecular dysfunctions remain elusive. It is now shown based on the study of TTC7A-deficient and wild-type cells that TTC7A is an essential nuclear protein. It binds to chromatin, preferentially at actively transcribed regions. Its depletion results in broad range of epigenomic changes at proximal and distal transcriptional regulatory elements and in altered control of the transcriptional program. Loss of WT_TTC7A induces general decrease in chromatin compaction, unbalanced cellular distribution of histones, higher nucleosome accessibility to nuclease digestion along with genome instability, and reduced cell viability. Our observations characterize for the first time unreported functions for TTC7A in the nucleus that exert a critical role in chromatin organization and gene regulation to safeguard healthy immune and intestinal status.</p

Rab27A and its effector MyRIP link secretory granules to F-actin and control their motion towards release sites

The GTPase Rab27A interacts with myosin-VIIa and myosin-Va via MyRIP or melanophilin and mediates melanosome binding to actin. Here we show that Rab27A and MyRIP are associated with secretory granules (SGs) in adrenal chromaffin cells and PC12 cells. Overexpression of Rab27A, GTPase-deficient Rab27A-Q78L, or MyRIP reduced secretory responses of PC12 cells. Amperometric recordings of single adrenal chromaffin cells revealed that Rab27A-Q78L and MyRIP reduced the sustained component of release. Moreover, these effects on secretion were partly suppressed by the actin-depolymerizing drug latrunculin but strengthened by jasplakinolide, which stabilizes the actin cortex. Finally, MyRIP and Rab27A-Q78L restricted the motion of SGs in the subplasmalemmal region of PC12 cells, as measured by evanescent-wave fluorescence microscopy. In contrast, the Rab27A-binding domain of MyRIP and a MyRIP construct that interacts with myosin-Va but not with actin increased the mobility of SGs. We propose that Rab27A and MyRIP link SGs to F-actin and control their motion toward release sites through the actin cortex

Complementation of a pathogenic IFNGR2 misfolding mutation with modifiers of N-glycosylation

Germline mutations may cause human disease by various mechanisms. Missense and other in-frame mutations may be deleterious because the mutant proteins are not correctly targeted, do not function correctly, or both. We studied a child with mycobacterial disease caused by homozygosity for a novel in-frame microinsertion in IFNGR2. In cells transfected with the mutant allele, most of the interferon γ receptor 2 (IFN-γR2) protein was retained within the cell, and that expressed on the cell surface had an abnormally high molecular weight (MW). The misfolding mutation was not gain-of-glycosylation, as it created no new N-glycosylation site. The mutant IFNGR2 allele was null, as the patient's cells did not respond to IFN-γ. Based on the well-established relationship between protein N-glycosylation and protein quality control processes, we tested 29 compounds affecting maturation by N-glycosylation in the secretory pathway. Remarkably, up to 13 of these compounds reduced the MW of surface-expressed mutant IFN-γR2 molecules and restored cellular responsiveness to IFN-γ. Modifiers of N-glycosylation may therefore complement human cells carrying in-frame and misfolding, but not necessarily gain-of-glycosylation, mutations in genes encoding proteins subject to trafficking via the secretory pathway. Some of these compounds are available for clinical use, paving the way for clinical trials of chemical complementation for various human genetic traits