Stimulus responses and amyloid precursor protein processing in DAMI megakaryocytes

Platelets, when released as anuclear cells by their precursor megakaryocytes, already carry soluble proteolytic fragments of the amyloid precursor protein (APP) within their alpha-granules and intact APP in the alpha-granule membranes. In response to activation signals elicited by physiologic stimuli such as thrombin, platelets release their granules' soluble contents and translocate granule membrane-bound proteins to the plasma membrane. Because platelets carry >90% of the circulation's APP, activated platelets have been implicated as origins of the beta-amyloid peptide fragment of APP (A beta), whose deposition in the cerebrovasculature is characteristic of Alzheimer's disease. We have therefore studied the APP contents and proteolytic processing in resting DAMI human megakaryocytic cells, along with the consequences of the activation of these cells by thrombin, comparing the results in each case to those with human platelets. Resting and PMA-differentiated DAMI cell contents were examined by Western blotting, immunoprecipitation, or metabolic labeling with sulfur 35-labeled methionine during culture, while plasma membrane-bound APP was evaluated by flow cytometry. Activation was followed by changes in cytoplasmic calcium concentration ((Ca++)in) and in membrane potential. Like platelets, DAMI cells exhibited a thrombin dose-dependent delta(Ca++)in, and membrane potential change; in contrast to the surface of a platelet, the surface of an agranular resting DAMI cell expresses granule-membrane proteins (APP and CD63) that appear on platelets only after activation. DAMI cell culture with 35S-labeled methionine confirmed that megakaryocytes synthesize large amounts of APP, of slightly higher molecular weight, and degrade their APP extensively before platelets are formed.

Activated Alzheimer disease platelets retain more beta amyloid precursor protein

Upon activation, platelet alpha-granules' soluble contents are secreted and membrane-bound contents are translocated to the plasma membrane. Membrane-bound proteins include the beta-amyloid precursor protein (APP) from which the beta-amyloid (A beta) deposits found surrounding the cerebrovasculature of patients with Alzheimer's Disease (AD) may originate. We show here that activated platelets from AD patients exhibit less APP processing, retain more of the protein on their surface, and secrete less as soluble fragments than do controls. Surface labeling demonstrated that there is little APP or CD62 on the surface of resting platelets. Upon activation, control platelets exhibited more of both proteins on their surface, while advanced AD patients exhibited similar amounts of CD62 as controls, but retained significantly more surface APP. AD platelets secreted similar amounts of most soluble alpha-granule contents as controls, but less APP fragments. Together these results suggest a processing defect that may account for greater deposition of A beta-containing products in the vasculature to which activated platelets adhere.

Targeted disruption of guanosine diphosphate-dissociation inhibitor for Rho-related proteins, GDID4: normal hematopoietic differentiation but subtle defect in superoxide production by macrophages derived from in vitro embryonal stem cell differentiation

The Rho subfamily of small guanosine triphosphate (GTP)-binding proteins, through their role in cytoskeletal organization, is involved in diverse cellular functions, including cell motility and morphologic changes during differentiation. Rac also has a special role in the production of superoxide, a key component in phagocytic antimicrobial function. Guanosine diphosphate (GDP)-dissociation inhibitors (GDIs) belong to one of three classes of proteins that regulate the critical cycling of GTP-binding proteins between the inactive and active states. Two homologous GDIs for the Rho subfamily have been identified. GDID4 is preferentially expressed in hematopoietic cells, while RhoGDI is ubiquitously expressed. Whether different physiologic functions are subserved by the two GDIs is unknown. We have derived embryonal stem (ES) cells with targeted disruption of both alleles of the GDID4 gene and examined hematopoiesis and phagocytic functions of macrophages derived from in vitro ES-cell differentiation. GDID4-/- ES cells develop like wild-type cells into colonies that contain heterogeneous populations of progenitor cells and differentiated erythromyeloid cells. GDID4-/- cells express no GDID4 protein, but have normal levels of RhoGDI. GDID4-/- macrophages phagocytose yeasts and antibody-opsonized erythrocytes as effectively as wild-type macrophages. However, a slight but consistent reduction in their capacity to generate superoxide was observed, which suggests new insight into the cellular role of GDID4. The minimal phenotypic effect of a loss of function of GDID4 also indicates a significant redundancy of function between GDID4 and RhoGDI. Their functional repertoire may be better revealed by a disruption of both genes. The use of hematopoietic cells derived in vitro from genotypically altered ES cells avoids the difficulties inherent in generating knockout animals and is a useful complementary approach for evaluating the gene function.