Introduction
Gaucher disease (GD) was first described in 1882 by a young medical student. Molecular delineation commenced in 1934 with identification of the nature of stored lipid, followed 31 years later by recognition of metabolic defect in the form of inherited deficiency of glucocerebrosidase (GCase), and 16 year later by characterization of the
GBA1 gene and description of the first disease mutations. Many of these milestones are elegantly described in a monograph edited at the occasion of centennial anniversary of the description of the disorder (Desnick et al.
1982). Knowledge of cellular pathology of this most frequent lysosomal enzymopathy is generally viewed as storage restricted to macrophages transformed into cytologically unique Gaucher cells (GC) characterized by active secretion of numerous molecules, some of which are in use as biomarkers in clinical practice (Aerts and Hollak
1997; Boven et al.
2004; Cox
2005; Deegan et al.
2005). Lysosomal storage in other cell types has never been reported. Therefore, the pathological focus has been on the pattern of GC infiltration in different organs. Interestingly, neurodegeneration, a feature of GD type II, is not associated with lysosomal storage and is believed to involve direct toxic effects of glucosylsphingosine on neuronal cells. The current understanding of the pathology of GC is described comprehensively in excellent monographs (Beutler and Grabowski
2001; Lee
1982; Lee
2007). However, there are significant gaps in our understanding of the pathophysiologic basis of GD and cell types involved. Notably, there is a discrepancy between generalized enzymopathy that occurs in GD and restriction of lipid storage structures to one cell type, which led one of us to propose a hypothesis that might bridge the persisting gap between the firmly established molecular definition and the loosely defined cell pathology (Elleder
2006). In this report, we present for the first time prominent alteration of the capillary endothelial cells (ECs) disclosed by routine ultrastructural examination of skin biopsies of patients with type II and type III GD. To the best of our knowledge, we are not aware of any study of ECs in GD, despite the voluminous literature (see above) and recent report (Chen and Wang
2008). Reports on skin pathology are limited to clinical observations in GD I (Goldblatt and Beighton
1984). Reports on all types of early-onset GD focused on epidermal abnormalities (Holleran et al.
1994a,
b,
2006; Reissner et al.
1998; Sharma et al.
2000; Sidransky et al.
1992) or general histology (Eblan et al.
2005; Mignot et al.
2003; Sherer et al.
1993; Spear et al.
2007; Stone et al.
1999; Van Gysel et al.
2002). Also, reports on mouse models of GD did not mention EC status. Our findings of dermal capillaries point to a novel aspect of pathology of GD: remarkably intensive alterations of blood capillary endothelium suggesting EC overstimulation. This prompts consideration of the presence of a novel aspect of the pathophysiology of GD, i.e., that of endothelial dysfunction and its implications in several mechanisms of GD, such as a vascular osteonecrosis and pulmonary vascular disease.
Discussion
The spectrum of changes in the blood capillary endothelium described herein in GD points to classical hypertrophy represented by an enlarged nuclear compartment rich in euchromatin, increased cytoplasmic volume, and increased numbers of organelles, with the number of free ribosomes and amount of rough endoplasmic reticulum pointing to upregulation of proteosynthesis. These changes were accompanied by a prominent modulation of cell membranes, which suggests a considerable increase in their dynamics. It was expressed mainly in the luminal apical membranes, frequently to a remarkable degree, but also at the basal EC pole. The latter often displayed a highly serrated appearance and an increased size of the endothelial basal foot projections, which frequently created a second endothelial layer (Pavelka and Roth
2005). EC hypertrophy was accompanied by a process strongly resembling both types of angiogenesis (Burri and Djonov
2002; Djonov et al.
2003; Risau
1997): (a) intussusceptive (with various degrees of transcapillary pillar formation), and (b) sprouting type (with luminal expansions of various types). Regressive changes were rare and restricted to individual ECs. However, the presence of multiple basement membranes, resembling the situation in diabetes mellitus (Vracko and Benditt
1974), suggests increased turnover either of all ECs or turnover confined to basal pole processes. It should be stressed that there were no signs of glucocerebroside storage structures.
Taken together, there were signs of prominent EC hypertrophy, including a certain degree of angiogenic hyperplasia and focal regressive changes suggesting occurrence of a chronic stimulatory effect. EC changes resembled EC activation described in several experimental systems (Agha-Majzoub et al.
2005; Dye et al.
2004; Fujimoto et al.
2004; Walski and Frontczak-Baniewicz
2003) or changes described in various spontaneous conditions such as adaptive capillary growth in overloaded rat skeletal muscles, (Zhou et al.
1998) inflammation (Polosukhin
1997), acute rejection of human renal allograft (Liptak et al.
2005), and other spontaneous conditions and experimentally induced states (Martin et al.
1980; Norrby
1997; Polverini
1995), which are generally classified as EC activation.
As these known causes of EC activation were absent in the two patients, we suggest two possible explanations of the phenomenon, both capable of exerting chronic EC activation effect. First, it is challenging to assess the direct role of enzymopathy. If enzymopathy plays a role in the development of the EC abnormalities described herein, the mechanisms a priori have to involve those other than accumulation of lysosomal glucocerebroside structures, as these are absent in this cell type. It should be kept in mind that the direct role of enzymopathy in type II GD results in neurodegeneration by pathways that appear not to involve lysosomal accumulation of glucocerebroside structures. It is of interest in this connection to point out that low-density lipoprotein (LDL) in patients with GD contains markedly elevated glucosylceramide and gangliosides, suggesting that many cell types, including ECs, are chronically supplied with increased amounts of glycosphingolipids destined for turnover (Ghauharali-van der Vlugt et al.
2008; Groener et al.
2008). Recently, one of us (Elleder
2006) proposed a more generalized involvement of diverse cells types in GD other than macrophages. In this paradigm, cell types other than macrophages are envisaged to have efficient transport mechanisms for accumulating glucocerebroside out of the lysosome were it becomes available as substrate for hydrolysis by neutral GCase (GBA2) (Boot et al.
2007; van Weely et al.
1993; Yildiz et al.
2006). Such a pathway may play a role in EC activation described herein via generation of ceramide and aberrant cell signaling thereof (Elleder
2006), similarly to what was described recently in studies of the mechanism of acute hypoxic pulmonary vasoconstriction (Cogolludo et al.
2009). Glucocerebroside itself may also have signaling activity, and its lysoproduct, glucosylsphingosine, also increased in GD, might contribute to EC cytotoxicity, activating cytokine imbalance. Recently, an ex vivo study in experimentally induced GC showed increase in GlcCer in microdomains (Hein et al.
2008), suggesting the possibility of their functional alteration.
Secondly, mechanisms underlying EC activation in GD could involve the role of cytokines and growth factors produced and secreted by activated macrophages as they transform into fully mature GCs (Cox
2005; Deegan et al.
2005; Hollak et al.
1997a; Michelakakis et al.
1996). This should be explored in the future by measureing cytokines and growth factors [i.e., vascular endothelial growth factor (VEGF)] in sera of GD patients, especially those who exhibit prominent vascular phenotypes such as pulmonary hypertension, hepatopulmonary syndrome, and splenic vascular lesions. In contrast to finding noted herein, in sarcoidosis, another disease involving macrophage activation, there are predominantly regressive changes of ECs (Planes et al.
1994; Tsukada et al.
1995) combined with inflammation and necrosis (Takemura et al.
1997). A picture of reactive angioendotheliomatosis with prominent endothelial capillary proliferation rarely seen in sarcoidosis (Shyong et al.
2002) was absent in our patients.
Taken together, the findings described herein strongly implicated EC activation and dysfunction in the pathophysiology of GD. The mechanisms responsible for EC transformation in GD can be most efficiently elucidated by exploring cultured ECs from GD patients and concurrent studies of newly developed authentic mouse models of the disease. Moreover, there is now the prospect of developing double-knock-out mouse models of
GBA1 and
GBA2 genes, which is especially pertinent to the concepts outlined herein. Such studies may challenge the macrophage-centric view of GD and help explain several features of the disease that currently defy explanation (Elleder
2006) i. e., tendency to pulmonary hypertension (Elstein et al.
2005) or permanent hypercoagulability and its consequences in GD (Adar et al.
2008; Elstein et al.
2000; Hollak et al.
1997b), as well as why patients with GD suffer from avascular osteonecrosis, when in fact there is evidence of increased vascularity (P. Mistry, unpublished observation). It is worth mentioning that recently advanced proteomics technology available to sensitively quantify plasma proteins revealed in GD patients marked abnormalities in coagulation and complement pathways that were partly corrected by enzyme replacement therapy (Vissers et al.
2007).
Numerous extensions at the apical pole membrane suggest greatly increased tendency for endothelial microparticle formation (Lynch and Ludlam
2007). In fact, their increased production of EC-derived microparticles has been demonstrated (JM Aerts, unpublished observations), especially in splenectomized GD patients. In patients with splenomegaly, increased clearance of microparticles by the enlarged spleen most likely explains the relatively low levels of circulating EC-derived microparticles.
In the series of Fabry hemizygotes, the blood capillaries manifested typical storage processes in ECs as well as in pericytes. The ECs engorged with lysosomal storage material in this setting displayed cytoplasmic enlargement proportional to the degree of lysosomal storage. In Fabry disease, alterations of apical and basal cell membranes are not found in contrast to those present in our GD patients. The apical membrane projections were present but only exceptionally prominent. The subendothelial basement membranes were frequently multiplied. In angiokeratoma of Fabry disease, in contrast, ECs were thin, despite the presence of storage lysosomes (Elleder
2003). It would be instructive to compare endothelial function and morphology in GD with Fabry disease. In the mouse model of Fabry disease, signs of EC dysfunction have been described (Eitzman et al.
2003; Park et al.
2008). Moreover, in Fabry disease patients, enzyme replacement therapy led to a decrease in circulating endothelial microparticles (Gelderman et al.
2007). A recent survey of a cohort of 36 Fabry disease patients, however, revealed only minimal abnormalities in endothelial microparticles, and of markers for platelet and endothelial activation, coagulation activation and fibrinolysis. The earlier reported abnormalities in severely affected Fabry disease patients seem to be better explained by renal insufficiency than Fabry disease itself (Vedder et al.
2009 ).
In GD patients, EC changes dominated over those seen in pericytes, which were changed to a lesser degree. However, there were definite signs of their activation, and it is important to keep in mind that their activation may be connected with calcifications (Armulik et al.
2005), repeatedly seen, e.g., in GD II (Mignot et al.
2003; Sherer et al.
1993; Spear et al.
2007).
Our results are descriptive but make a compelling case for further study of ECs in GD. The unique morphological pattern in individual capillaries taken together with remarkable tendency to EC loose anchoring as well as their protrusion into the lumen argues strongly for a pathophysiologic role of resident ECs and their possible increased exchange within the pool of circulating EC precursors (Hristov et al.
2003; Xu
2007).
Acknowledgment
Dr. Věra Malinová, Charles University, 1st Faculty of Medicine and General Teaching Hospital, Department of Pediatrics and Adolescent Medicine, Prague, Czech Republic, provided clinical data of patient 1. Dr. F. Stehling, Department of Neuropediatrics, Universitätsklinikum, Essen, Germany, is thanked for providing the skin biopsy and clinical data of patient 2; and Dr. E. Mengel, Center for Lysosomal Diseases, University of Mainz, Germany, for providing neuro-ophthalmologic data of that patient.
The study was supported by a research project of the Ministry of Education Youth and Sports Czech Republic (Grant No. MSM 0021620806).