Podocyte Injury and Albuminuria in Mice with Podocyte-Specific Overexpression of the Ste20-Like Kinase, SLK

Abstract

SLK expression and activity are increased during kidney development and recovery from renal ischemiareperfusion injury. In cultured cells, SLK promotes F-actin destabilization as well as apoptosis, partially via the p38 kinase pathway. To better understand the effects of SLK in vivo, a transgenic mouse model was developed where SLK was expressed in a podocytespecific manner using the mouse nephrin promoter. Offspring of four founder mice carried the SLK transgene. Among male transgenic mice, 66% developed albuminuria at approximately 3 months of age, and the albuminuric mice originated from three of four founders. Overall, the male transgenic mice demonstrated about fivefold greater urinary albumin/creatinine compared with male non-transgenic mice. Transgenic and non-transgenic female mice did not develop albuminuria, suggesting that females were less susceptible to glomerular filtration barrier damage than their male counterparts. In transgenic mice, electron microscopy revealed striking podocyte injury, including poorly formed or effaced foot processes, and edematous and vacuolated cell bodies. By immunoblotting, nephrin expression was decreased in glomeruli of the albuminuric transgenic mice. Activation-specific phosphorylation of p38 was increased in transgenic mice compared with non-transgenic animals. Glomeruli of SLK transgenic mice showed around 30% fewer podocytes, and a reduction in F-actin compared with control glomeruli. Thus, podocyte SLK overexpression in vivo results in injury and podocyte loss, consistent with the effects of SLK in cultured cells.

Activity of the Ste20-like kinase, SLK, is enhanced by homodimerization

“…SLK promotes apoptosis in various cultured cells (including kidney cells) and in podocytes in vivo (10,11,13,23,40). To determine whether dimerization of the kinase domain affected apoptosis, COS-1 cells were transiently transfected with Fv-SLK 1-373 or control vector and were treated with or without AP20187.…”

“…SLK is a member of the GCK V subfamily and is distantly related to Mst1 and Mst2, while sharing high homology to lymphocyte-oriented kinase, another member of the GCK V subfamily (14,15). SLK has been shown to induce apoptosis when expressed in cultured fibroblasts (40,41), as well as kidney tubular and glomerular epithelial cells (GECs; podocytes) in culture, and recently, podocyte-specific overexpression of SLK in transgenic mice resulted in severe podocyte injury and loss of podocytes, in keeping with apoptosis (10,11,13,23). Moreover, SLK enhanced apoptosis in kidney cells after ischemia-reperfusion injury in vitro, and apoptotic signaling occurred via the JNK and p38 pathways (23), and involved p53 (10,37).…”

Upregulation of podocyte-secreted angiopoietin-like-4 in diabetic nephropathy

“…Therefore, the discovery of a specific, reliable biomarker for podocyte injury in DN is imperative. Several studies have also focused on the production of podocyte-specific proteins in patients with DN and have attempted to detect earlier functional abnormalities in podocytes [10][11][12].…”

Regulation of the Ste20-like Kinase, SLK

“…Endogenous SLK was also shown to be phosphorylated in the developing kidney (10) and in renal ischemia-reperfusion injury models. 4 Podocyte-specific overexpression of SLK in mice resulted in albuminuria and podocyte injury, consistent with apoptosis (46). These results suggest that SLK may exacerbate cell damage in glomerular disease and acute kidney injury.…”