Shobha Ratnam

Ciliary Polycystin-2 Is a Mechanosensitive Calcium Channel Involved in Nitric Oxide Signaling Cascades

Cardiovascular complications such as hypertension are a continuous concern in patients with autosomal dominant polycystic kidney disease (ADPKD). The PKD2 encoding for polycystin-2 is mutated in ≈15% of ADPKD patients. Here, we show that polycystin-2 is localized to the cilia of mouse and human vascular endothelial cells. We demonstrate that the normal expression level and localization of polycystin-2 to cilia is required for the endothelial cilia to sense fluid shear stress through a complex biochemical cascade, involving calcium, calmodulin, Akt/PKB, and protein kinase C. In response to fluid shear stress, mouse endothelial cells with knockdown or knockout of Pkd2 lose the ability to generate nitric oxide (NO). Consistent with mouse data, endothelial cells generated from ADPKD patients do not show polycystin-2 in the cilia and are unable to sense fluid flow. In the isolated artery, we further show that ciliary polycystin-2 responds specifically to shear stress and not to mechanical stretch, a pressurized biomechanical force that involves purinergic receptor activation. We propose a new role for polycystin-2 in transmitting extracellular shear stress to intracellular NO biosynthesis. Thus, aberrant expression or localization of polycystin-2 to cilia could promote high blood pressure because of inability to synthesize NO in response to an increase in shear stress (blood flow).

Endothelial cells from humans and mice with polycystic kidney disease are characterized by polyploidy and chromosome segregation defects through survivin down-regulation

Autosomal-dominant polycystic kidney disease (ADPKD) is the most common hereditary and systemic disorder associated with various cardiovascular complications. It has been implicated with dysfunction in primary cilia. We and others have shown that the immediate function of endothelial cilia is to sense extracellular signal. The long-term function of cilia is hypothesized to regulate cell cycle. Here, we show that ciliary function (polycystins) and structure (polaris) are required for proper cellular division. Cilia mutant cells undergo abnormal cell division with apparent defects in mitotic spindle formation, cellular spindle assembly checkpoint and centrosome amplification. Down-regulation of the chromosomal passenger survivin contributes to these abnormalities, which further result in cell polyploidy. Re-expression of survivin restores a competent spindle assembly checkpoint and reduces polyploidy. Aged animals show a more severe phenotype in cellular division, consistent with progression of cardiovascular complications seen in older ADPKD patients. For the first time, we show that structure and function of mechanosensory cilia are crucial in maintaining proper cellular proliferation. Furthermore, developmental aging plays a crucial role in the progression of these abnormal cellular phenotypes. We propose that abnormal function or structure of primary cilia not only causes failure to transmit extracellular signals, but also is associated with cytokinesis defects in both mice and humans with polycystic kidney disease.

Survivin-Induced Abnormal Ploidy Contributes to Cystic Kidney and Aneurysm Formation

Background— Cystic kidneys and vascular aneurysms are clinical manifestations seen in patients with polycystic kidney disease, a cilia-associated pathology (ciliopathy). Survivin overexpression is associated with cancer, but the clinical pathology associated with survivin downregulation or knockout has never been studied before. The present studies aim to examine whether and how cilia function (Pkd1 or Pkd2) and structure (Tg737) play a role in cystic kidney and aneurysm through survivin downregulation. Methods and Results— Cysts and aneurysms from polycystic kidney disease patients, Pkd mouse, and zebrafish models are characterized by chromosome instability and low survivin expression. This triggers cytokinesis defects and formation of nuclear polyploidy or aneuploidy. In vivo conditional mouse and zebrafish models confirm that survivin gene deletion in the kidneys results in a cystic phenotype. As in hypertensive Pkd1, Pkd2, and Tg737 models, aneurysm formation can also be induced in vascular-specific normotensive survivin mice. Survivin knockout also contributes to abnormal oriented cell division in both kidney and vasculature. Furthermore, survivin expression and ciliary localization are regulated by flow-induced cilia activation through protein kinase C, Akt and nuclear factor-&kgr;B. Circumventing ciliary function by re-expressing survivin can rescue polycystic kidney disease phenotypes. Conclusions— For the first time, our studies offer a unifying mechanism that explains both renal and vascular phenotypes in polycystic kidney disease. Although primary cilia dysfunction accounts for aneurysm formation and hypertension, hypertension itself does not cause aneurysm. Furthermore, aneurysm formation and cyst formation share a common cellular and molecular pathway involving cilia function or structure, survivin expression, cytokinesis, cell ploidy, symmetrical cell division, and tissue architecture orientation.

Aggressive posttransplant lymphoproliferative disease in a renal transplant patient treated with alemtuzumab.

Post transplant lymphoproliferative disease (PTLD) is a rare but potentially fatal complication after solid organ transplantation. The risk of PTLD varies with type of organ transplant, Epstein-Barr virus serostatus of the donor and recipient, age, and intensity of immunosuppression. We report a case of a 45-year-old man who developed aggressive PTLD 7 months after receiving a cadaveric renal transplant. He received 30 mg alemtuzumab intravenously intraoperatively as induction immunosuppression followed by maintenance immunosuppression with tacrolimus and mycophenolate mofetil. The patient presented with intestinal perforation and gastrointestinal bleeding. Histopathology revealed Epstein-Barr virus-positive diffuse large B-cell lymphoma with a high mitotic index involving multiple segments of small and large intestines and leading to perforation of the ileum, jejunum, and cecum. The patient had Stage IV disease and treatment consisted of immunosuppression reduction and 375 mg/m2 rituximab weekly for four doses. Unfortunately, the patient had recurrent intestinal perforation followed by fatal gastrointestinal bleeding. There have been very few case reports of PTLD after alemtuzumab induction in renal transplant and the case discussed had simultaneous multiple perforations in the small intestine and colon.

Microscopic polyangiitis triggered by recurrent methicillin-resistant Staphylococcus aureus bacteremia

Most of the purported links between microbial agents and primary small-vessel anti-neutrophilic antibody-positive (ANCA) vasculitides remain speculative. There is strong circumstantial evidence for the role of Staphylococcus aureus in the development of Wegener’s granulomatosis, but its role in other ANCA-positive vasculitis syndromes is less clear. We describe a patient who developed a non-granulomatous, necrotizing small-vessel vasculitis with a positive anti-neutrophil cytoplasmic antibody of a perinuclear type (p-ANCA), along with anti-myeloperoxidase antibodies after recurrent episodes of methicillin-resistant Staphylococcus aureus bacteremia.