Wallerian-Like Degeneration of Central Neurons After Synchronized and Geometrically Registered Mass Axotomy in a Three-Compartmental Microfluidic Chip

Degeneration of central axons may occur following injury or due to various diseases and it involves complex molecular mechanisms that need to be elucidated. Existing in vitro axotomy models are difficult to perform, and they provide limited information on the localization of events along the axon. We present here a novel experimental model system, based on microfluidic isolation, which consists of three distinct compartments, interconnected by parallel microchannels allowing axon outgrowth. Neurons cultured in one compartment successfully elongated their axons to cross a short central compartment and invade the outermost compartment. This design provides an interesting model system for studying axonal degeneration and death mechanisms, with a previously impossible spatial and temporal control on specific molecular pathways. We provide a proof-of-concept of the system by reporting its application to a well-characterized experimental paradigm, axotomy-induced Wallerian degeneration in primary central neurons. Using this model, we applied localized central axotomy by a brief, isolated flux of detergent. We report that mouse embryonic cortical neurons exhibit rapid Wallerian-like distal degeneration but no somatic death following central axotomy. Distal axons show progressive degeneration leading to axonal beading and cytoskeletal fragmentation within a few hours after axotomy. Degeneration is asynchronous, reminiscent of in vivo Wallerian degeneration. Axonal cytoskeletal fragmentation is significantly delayed with nicotinamide adenine dinucleotide pretreatment, but it does not change when distal calpain or caspase activity is inhibited. These findings, consistent with previous experiments in vivo, confirm the power and biological relevance of this microfluidic architecture

Dynamic analysis of apoptosis in primary cortical neurons by fixed- and real-time cytofluorometry.

We describe here a cytofluorometric technology for the characterization of decision, execution, and degradation steps of neuronal apoptosis. Multiparametric flow cytometry was developed and combined to detailed fluorescence microscopy observations to establish the chronology and hierarchy of death-related events: neuron morphological changes, mitochondrial transmembrane potential (DeltaPsi(m)) collapse, caspase-3 and -9 activation, phosphatidyl-serine exposure, nuclear dismantling and final plasma membrane permeabilization. Moreover, we developed a reliable real-time flow cytometric monitoring of DeltaPsi(m) and plasma membrane integrity in response to neurotoxic insults including MPTP treatment. Taking advantage of recently developed specific fluorescent probes and a third generation pan-caspase inhibitor, this integrated approach will be pertinent to study the cell biology of neuronal apoptosis and to characterize new neuro-toxic/protective molecules

Microtubule-associated protein 2 (MAP2) is a neurosteroid receptor

The neurosteroid pregnenolone (PREG) and its chemically synthesized analog 3β-methoxypregnenolone (MePREG) bind to microtubule-associated protein 2 (MAP2) and stimulate the polymerization of microtubules. PREG, MePREG, and progesterone (PROG; the physiological immediate metabolite of PREG) significantly enhance neurite outgrowth of nerve growth factor-pretreated PC12 cells. However, PROG, although it binds to MAP2, does not increase the immunostaining of MAP2, contrary to PREG and MePREG. Nocodazole, a microtubule-disrupting agent, induces a major retraction of neurites in control cultures, but pretreatment with PREG/MePREG is protective. Decreasing MAP2 expression by RNA interference does not modify PROG action, but it prevents the stimulatory effects of PREG and MePREG on neurite extension, showing that MAP2 is their specific receptor

Differential phosphorylation of MAP1b during postnatal development of the cat brain.

Microtubule-associated protein 1b, previously also referred to as microtubule-associated protein 5 or microtubule-associated protein 1x, is a major component of the juvenile cytoskeleton, and is essential during the early differentiation of neurons. It is required for axonal growth and its function is influenced by phosphorylation. The distribution of microtubule-associated protein 1b in kitten cerebellum and cortex during postnatal development was studied with two monoclonal antibodies. Hybridoma clone AA6 detected a non-phosphorylated site, while clone 125 detected a site phosphorylated by casein-kinase II. On blots, both monoclonal antibodies stained the same two proteins of similar molecular weights, also referred to as microtubule-associated protein 5a and 5b. Antibody 125 detected a phosphorylated epitope on both microtubule-associated protein 1b forms; dephosphorylation by alkaline phosphatase abolished the immunological detection. During development of cat cortex and cerebellum, AA6 stained the perikarya and dendrites of neurons during their early differentiation, and especially labelled newly generated axons. The staining decreased during development, and axonal staining was reduced in adult tissue. In contrast to previous reports which demonstrated that antibodies against phosphorylated microtubule-associated protein 1b label exclusively axons, antibody 125 also localized microtubule-associated protein 1b in cell bodies and dendrites, even in adulthood. Some nuclear staining was observed, indicating that a phosphorylated form of microtubule-associated protein 1b may participate in nuclear function. These results demonstrate that microtubule-associated protein 1b is subject to CK2-type phosphorylation throughout neuronal maturation and suggest that phosphorylation of microtubule-associated protein 1b may participate in juvenile and mature-type microtubule functions throughout development

Inflammation Induced by Infection Potentiates Tau Pathological Features in Transgenic Mice

Comorbidities that promote the progression of Alzheimer's disease (AD) remain to be uncovered and evaluated in animal models. Because elderly individuals are vulnerable to viral and bacterial infections, these microbial agents may be considered important comorbidities that could potentiate an already existing and tenuous inflammatory condition in the brain, accelerating cognitive decline, particularly if the cellular and molecular mechanisms can be defined. Researchers have recently demonstrated that triggering inflammation in the brain exacerbates tau pathological characteristics in animal models. Herein, we explore whether inflammation induced via viral infection, compared with inflammation induced via bacterial lipopolysaccharide, modulates AD-like pathological features in the 3xTg-AD mouse model and provide evidence to support the hypothesis that infectious agents may act as a comorbidity for AD. Our study shows that infection-induced acute or chronic inflammation significantly exacerbates tau pathological characteristics, with chronic inflammation leading to impairments in spatial memory. Tau phosphorylation was increased via a glycogen synthase kinase-3β–dependent mechanism, and there was a prominent shift of tau from the detergent-soluble to the detergent-insoluble fraction. During chronic inflammation, we found that inhibiting glycogen synthase kinase-3β activity with lithium reduced tau phosphorylation and the accumulation of insoluble tau and reversed memory impairments. Taken together, infectious agents that trigger central nervous system inflammation may serve as a comorbidity for AD, leading to cognitive impairments by a mechanism that involves exacerbation of tau pathological characteristics