Faculty Bibliography

Intracellular protein degradation decreases with age, altering the important balance between protein synthesis and breakdown. Slowly, protein accumulation events increase causing axonopathy, synaptic deterioration, and subsequent cell death. As toxic species accumulate, autophagy-lysosomal protein degradation pathways are activated. Responses include autophagic vacuoles that degrade damaged cellular components and long-lived proteins, as well as enhanced levels of lysosomal hydrolases. Although such changes correlate with neuronal atrophy in age-related neurodegenerative disorders and in related models of protein accumulation, the autophagic/lysosomal responses appear to be compensatory reactions. Recent studies indicate that protein oligomerization/ aggregation induces autophagy and activates lysosomal protein degradation in an attempt to clear toxic accumulations. Such compensatory responses may delay cell death and account for the gradual nature of protein deposition pathology that can extend over months/years in model systems and years/decades in the human diseases. Correspondingly, enhancement of compensatory pathways shifts the balance from pathogenesis to protection. Positive modulation of protein degradation processes represents a strategy to promote clearance of toxic accumulations and to slow the synaptopathogenesis and associated cognitive decline in aging-related dementias.

Dysfunction of cholinergic basal forebrain (CBF) neurons of the nucleus basalis (NB) is a cardinal feature of Alzheimer's disease (AD) and correlates with cognitive decline. Survival of CBF neurons depends upon binding of nerve growth factor (NGF) with high-affinity (trkA) and low-affinity (p75(NTR)) neurotrophin receptors produced within CBF neurons. Since trkA and p75(NTR) protein levels are reduced within CBF neurons of people with mild cognitive impairment (MCI) and mild AD, trkA and/or p75(NTR) gene expression deficits may drive NB degeneration. Using single cell expression profiling methods coupled with custom-designed cDNA arrays and validation with real-time quantitative PCR (qPCR) and in situ hybridization, individual cholinergic NB neurons displayed a significant down regulation of trkA, trkB, and trkC expression during the progression of AD. An intermediate reduction was observed in MCI, with the greatest decrement in mild to moderate AD as compared to controls. Importantly, trk down regulation is associated with cognitive decline measured by the Global Cognitive Score (GCS) and the Mini-Mental State Examination (MMSE). In contrast, there is a lack of regulation of p75(NTR) expression. Thus, trk defects may be a molecular marker for the transition from no cognitive impairment (NCI) to MCI, and from MCI to frank AD

Alpha-calcium/calmodulin-dependent kinase II (alphaCaMKII) is central to synaptic plasticity but it remains unclear whether this kinase contributes to neuronal excitability changes, which are a cellular correlate of learning. Using knock-in mice with a targeted T286A mutation that prevents the autophosphorylation of alphaCaMKII (alphaCaMKII(T286A)), we studied the role of alphaCaMKII signaling in regulating hippocampal neuronal excitability during hippocampus-dependent spatial learning in the Morris water maze. Wild-type control mice showed increased excitability of CA1 pyramidal neurons, as assessed by a reduction in the postburst afterhyperpolarization (AHP), after spatial training in the water maze. Importantly, wild-type mice did not show AHP changes when they were exposed to the water maze without the escape platform and swam the same amount of time as the trained mice (swim controls), thus manifesting learning-specific increases in hippocampal CA1 excitability associated with spatial training. Meanwhile, alphaCaMKII(T286A) mice showed impairments in spatial learning but exhibited reduced levels of AHP that were similar to wild-type controls after water-maze training. Notably, both trained and swim-control groups of alphaCaMKII(T286A) mutants showed similar increased excitability, indicating that swimming by itself is enough to induce changes in excitability in the absence of normal alphaCaMKII function. This result demonstrates dissociation of alphaCaMKII-independent changes in intrinsic neuron excitability from learning and synaptic plasticity mechanisms, suggesting that increases in excitability per se are not perfectly correlated with learning. Our findings suggest that alphaCaMKII signaling may function to suppress learning-unrelated changes during training, thereby allowing hippocampal CA1 neurons to increase their excitability appropriately for encoding spatial memories

Molecular mechanisms underlying tauopathy remain undetermined. In the current study, single cell gene expression profiling was coupled with custom-designed cDNA array analysis to evaluate tau expression and other cytoskeletal elements within individual neuronal populations in patients with no cognitive impairment (NCI), mild cognitive impairment (MCI), and Alzheimer's disease (AD). Results revealed a shift in the ratio of three-repeat tau (3Rtau) to four-repeat tau (4Rtau) mRNAs within individual human cholinergic basal forebrain (CBF) neurons within nucleus basalis (NB) and CA1 hippocampal neurons during the progression of AD, but not during normal aging. A shift in 3Rtau to 4Rtau may precipitate a cascade of events in the selective vulnerability of neurons, ultimately leading to frank neurofibrillary tangle (NFT) formation in tauopathies including AD.

Phosphorylation of the carboxyl tail domains of the neurofilament heavy (NF-H) and middle molecular weight (NF-M) subunits has been proposed to regulate the axonal transport of neurofilaments. To test this hypothesis, we recently constructed mice lacking the extensively phosphorylated NF-H tail domain (NF-HtailDelta) and showed that the transport rate of neurofilaments in optic axons is unaltered in the absence of this domain [M.V. Rao, M.L. Garcia, Y. Miyazaki, T. Gotow, A. Yuan, S. Mattina, C.M. Ward, N.A. Calcutt, Y. Uchiyama, R.A. Nixon, D.W. Cleveland, Gene replacement in mice reveals that the heavily phosphorylated tail of neurofilament heavy subunit does not affect axonal caliber or the transit of cargoes in slow axonal transport, J. Cell Biol. 158 (2002) 681-693]. However, Shea et al. proposed that deletion of NF-H carboxyl-terminal region accelerates the transport of a subpopulation of neurofilaments based on minor differences between tail-deleted and control mice in our axonal transport analysis. Here, we present additional evidence that neurofilament transport rate is unchanged after deleting the phosphorylated NF-H tail domain, establishing unequivocally that the NF-H tail domain alone does not regulate the rate of neurofilament transport in optic axons in vivo. Possible roles for tail domains as cross-bridges between a neurofilament and its neighbors or other cytoskeletal elements is discussed

A variant of the cysteine protease inhibitor, cystatin C, forms amyloid deposited in the cerebral vasculature of patients with hereditary cerebral hemorrhage with amyloidosis, Icelandic type (HCHWA-I), leading to cerebral hemorrhages early in life. However, cystatin C is also implicated in neuronal degenerative diseases in which it does not form the amyloid protein, such as Alzheimer disease (AD). Accumulating data suggest involvement of cystatin C in the pathogenic processes leading to amyloid deposition in cerebral vasculature and most significantly to cerebral hemorrhage in patients with cerebral amyloid angiopathy (CAA). This review focuses on cell culture and animal models used to study the role of cystatin C in these processes

Raf kinases are downstream effectors of Ras and upstream activators of the MEK-ERK cascade. Ras and MEK-ERK signaling play roles in learning and memory (L&M) and neural plasticity, but the roles of Raf kinases in L&M and plasticity are unclear. Among Raf isoforms, B-raf is preferentially expressed in the brain. To determine whether B-raf has a role in synaptic plasticity and L&M, we used the Cre-LoxP gene targeting system to derive forebrain excitatory neuron B-raf knockout mice. This conditional knockout resulted in deficits in ERK activation and hippocampal long-term potentiation (LTP) and impairments in hippocampus-dependent L&M, including spatial learning and contextual discrimination. Despite the widespread expression of B-raf, this mutation did not disrupt other forms of L&M, such as cued fear conditioning and conditioned taste aversion. Our findings demonstrate that B-raf plays a role in hippocampal ERK activation, synaptic plasticity, and L&M

Transgenic mouse models of Alzheimer's disease (AD) exhibit amyloid-beta (Abeta) accumulation and related cognitive impairments. Although deficits in hippocampus-dependent place learning have been well characterized in Alzheimer's transgenic mice, little is known about temporal memory function in these AD models. Here, we applied trace fear conditioning to two different Alzheimer's mouse models and investigated the relationship between pathogenic Abeta and temporal memory deficits. This behavioral test requires hippocampus-dependent temporal memory processing as the conditioned and unconditioned stimuli are separated by a trace interval of 30 s. We found that both amyloid precursor protein (APP) transgenic (Tg2576) and APP/presenilin (PS)1 transgenic (Tg6799) mice were impaired in memorizing this association across the time gap. Both transgenic groups performed as well as wild-type control mice in delay fear conditioning when the trace interval was removed, indicating that the trace conditioning deficits are hippocampus-specific. Importantly, Tg6799 mice engineered to lack the major Alzheimer's beta-secretase (beta-site APP-cleaving enzyme 1: BACE1) showed behavioral rescue from temporal memory deficits. Elevated levels of soluble Abeta oligomers found in Tg6799+ mouse brains returned to wild-type control levels without changes in APP/PS1 transgene expression in BACE1-/- * Tg6799+ bigenic mouse brains, suggesting Abeta oligomers as potential mediators of memory loss. Thus, trace fear conditioning is a useful assay to test the mechanisms and therapeutic interventions for Abeta-dependent deficits in temporal associative memory. Our gene-based approach suggests that lowering soluble Abeta oligomers by inhibiting BACE1 may be beneficial for alleviating cognitive disorders in AD