Faculty Bibliography

THE CIRCUITRY OF THE DENTATE GYRUS (DG) OF THE HIPPOCAMPUS IS UNIQUE COMPARED TO OTHER HIPPOCAMPAL SUBFIELDS BECAUSE THERE ARE TWO GLUTAMATERGIC PRINCIPAL CELLS INSTEAD OF ONE: granule cells, which are the vast majority of the cells in the DG, and the so-called "mossy cells." The distinctive appearance of mossy cells, the extensive divergence of their axons, and their vulnerability to excitotoxicity relative to granule cells has led to a great deal of interest in mossy cells. Nevertheless, there is no consensus about the normal functions of mossy cells and the implications of their vulnerability. There even seems to be some ambiguity about exactly what mossy cells are. Here we review initial studies of mossy cells, characteristics that define them, and suggest a practical definition to allow investigators to distinguish mossy cells from other hilar neurons even if all morphological and physiological information is unavailable due to technical limitations of their experiments. In addition, hypotheses are discussed about the role of mossy cells in the DG network, reasons for their vulnerability and their implications for disease.

Although evidence is accumulating that diabetes mellitus is an important risk factor for sporadic Alzheimer's disease (AD), the mechanisms by which defects in insulin signaling may lead to the acceleration of AD progression remain unclear. In this study, we applied streptozotocin (STZ) to induce experimental diabetes in AD transgenic mice (5XFAD model) and investigated how insulin deficiency affects the beta-amyloidogenic processing of amyloid precursor protein (APP). Two and half months after 5XFAD mice were treated with STZ (90 mg/kg, i.p., once daily for two consecutive days), they showed significant reductions in brain insulin levels without changes in insulin receptor expression. Concentrations of cerebral amyloid-beta peptides (Abeta40 and Abeta42) were significantly increased in STZ-treated 5XFAD mice as compared with vehicle-treated 5XFAD controls. Importantly, STZ-induced insulin deficiency upregulated levels of both beta-site APP cleaving enzyme 1 (BACE1) and full-length APP in 5XFAD mouse brains, which was accompanied by dramatic elevations in the beta-cleaved C-terminal fragment (C99). Interestingly, BACE1 mRNA levels were not affected, whereas phosphorylation of the translation initiation factor eIF2alpha, a mechanism proposed to mediate the post-transcriptional upregulation of BACE1, was significantly elevated in STZ-treated 5XFAD mice. Meanwhile, levels of GGA3, an adapter protein responsible for sorting BACE1 to lysosomal degradation, are indistinguishable between STZ- and vehicle-treated 5XFAD mice. Moreover, STZ treatments did not affect levels of Abeta-degrading enzymes such as neprilysin and insulin-degrading enzyme (IDE) in 5XFAD brains. Taken together, our findings provide a mechanistic foundation for a link between diabetes and AD by demonstrating that insulin deficiency may change APP processing to favor beta-amyloidogenesis via the translational upregulation of BACE1 in combination with elevations in its substrate, APP.

Newly synthesized neurofilaments or protofilaments are incorporated into a highly stable stationary cytoskeleton network as they are transported along axons. Although the heavily phosphorylated carboxyl-terminal tail domains of the heavy and medium neurofilament (NF) subunits have been proposed to contribute to this process and particularly to stability of this structure, their function is still obscure. Here we show in NF-H/M tail deletion [NF-(H/M)(tailDelta)] mice that the deletion of both of these domains selectively lowers NF levels 3-6 fold along optic axons without altering either rates of subunit synthesis or the rate of slow axonal transport of NF. Pulse labeling studies carried out over 90 days revealed a significantly faster rate of disappearance of NF from the stationary NF network of optic axons in NF-(H/M)(tailDelta) mice. Faster NF disappearance was accompanied by elevated levels of NF-L proteolytic fragments in NF-(H/M)(tailDelta) axons. We conclude that NF-H and NF-M C-terminal domains do not normally regulate NF transport rates as previously proposed, but instead increase the proteolytic resistance of NF, thereby stabilizing the stationary neurofilament cytoskeleton along axons.

Neuronal cell culture models have been used to demonstrate the protective effects of cystatin C against a variety of insults, including the toxicity induced by oligomeric and fibrillar amyloid beta (Abeta). Here, we describe assays quantifying cystatin C protective effects against cytotoxicity induced by nutrient deprivation, oxidative stress, or cytotoxic forms of Abeta. Three methods for the evaluation of either cell death or cell survival are described: measurement of metabolic activity, cell death, and cell division. The cell culture models used are murine primary cortical neurons and murine primary cerebral smooth muscle cells. The effects of exogenously applied cystatin C are studied by comparing the viability of nonstressed control, stressed control, and cystatin C-treated stressed cells. The effect of endogenous level of cystatin C expression is studied by comparing stressed primary cells isolated from brains of cystatin C transgenic, cystatin C knockout, and wild-type mice.

Amyloid-containing tissue, whether from human patients or an animal model of a disease, is typically characterized by various biochemical and immunohistochemical techniques, many of which are described in detail in this volume. In this chapter, we describe a straightforward technique for the homogenization of tissue prior to these analyses. The technique is particularly well suited for performing a large number of different biochemical analyses on a single mouse brain hemisphere. Starting with this homogenate multiple characterizations can be done, including western blot analysis and isolation of membrane-associated proteins, both of which are described here. Additional analyses can readily be performed on the tissue homogenate, including the ELISA quantitation of Abeta in the brain of a transgenic mouse model of beta-amyloid deposition. The ELISA technique is described in detail in Chapter 34 .