The cellular prion protein promotes olfactory sensory neuron survival and axon targeting during adult neurogenesis.

The cellular prion protein (PrPC) has been associated with diverse biological processes including cell signaling, neurogenesis, and neuroprotection, but its physiological function(s) remain ambiguous. Here we determine the role of PrPC in adult neurogenesis using the olfactory system model in transgenic mice. Olfactory sensory neurons (OSNs) within the olfactory sensory epithelium (OSE) undergo neurogenesis, integration, and turnover even into adulthood. The neurogenic processes of proliferation, differentiation/maturation, and axon targeting were evaluated in wild type, PrP-overexpressing, and PrP-null transgenic mice. Our results indicate that PrPC plays a role in maintaining mature OSNs within the epithelium: overexpression of PrPC resulted in greater survival of mitotically active cells within the OSE, whereas absence of prion protein resulted in fewer cells being maintained over time. These results are supported by both quantitative PCR analysis of gene expression and protein analysis characteristic of OSN differentiation. Finally, evaluation of axon migration determined that OSN axon targeting in the olfactory bulb is PrPC dose-dependent. Together, these findings provide new mechanistic insight into the neuroprotective role for PrPC in adult OSE neurogenesis, whereby more mature neurons are stably maintained in animals expressing PrPC.

Zebrafish tbx5 paralogs demonstrate independent essential requirements in cardiac and pectoral fin development.

BACKGROUND: T-box genes constitute a large family of transcriptional regulators involved in developmental patterning. Homozygous mutation of tbx5 leads to embryonic lethal cardiac phenotypes and forelimb malformations in vertebrate models. Haploinsufficiency of tbx5 results in Holt-Oram syndrome, a human congenital disease characterized by cardiac and forelimb defects. Homozygous mutation of zebrafish tbx5a leads to lethal defects in cardiac looping morphogenesis, blocks pectoral fin initiation, and impairs outgrowth. Recently, a second zebrafish tbx5 gene was described, termed tbx5b. RESULTS: Our phylogenetic analyses confirm tbx5b as a paralog that likely arose in the teleost-specific whole genome duplication ∼270 MYA. Using morpholino depletion studies, we find that tbx5b is required in the heart for embryonic survival, and influences the timing and morphogenesis of pectoral fin development. Because tbx5a hypomorphic mutations are embryonic lethal, tbx5a and tbx5b functions in the heart must not be completely redundant. Consistent with this hypothesis, simultaneous depletion of both tbx5 paralogs did not lead to more severe phenotypes, and injection of wild-type mRNA from one tbx5 paralog was not sufficient to cross-rescue phenotypes of the paralogous gene. CONCLUSIONS: Collectively, these data indicate that, despite similar spatio-temporal expression patterns, tbx5a and tbx5b have independent functions in heart and fin development.

The cellular prion protein promotes neuronal regeneration after acute nasotoxic injury.

Adult neurogenesis, analogous to early development, is comprised of several, often concomitant, processes including proliferation, differentiation, and formation of synaptic connections. However, due to continual, asynchronous turn-over, newly-born adult olfactory sensory neurons (OSNs) must integrate into existing circuitry. Additionally, OSNs express high levels of cellular prion protein (PrPC), particularly in the axon, which implies a role in this cell type. The cellular prion has been shown to be important for proper adult OSN neurogenesis primarily by stabilizing mature olfactory neurons within this circuitry. However, the role of PrPC on each specific adult neurogenic processes remains to be investigated in detail. To tease out the subtle effects of prion protein expression level, a large population of regenerating neurons must be investigated. The thyroid drug methimazole (MTZ) causes nearly complete OSN loss in rodents and is used as a model of acute olfactory injury, providing a mechanism to induce synchronized OSN regeneration. This study investigated the effect of PrPC on adult neurogenesis after acute nasotoxic injury. Altered PrPC levels affected olfactory sensory epithelial (OSE) regeneration, cell proliferation, and differentiation. Attempts to investigate the role of PrPC level on axon regeneration did not support previous studies, and glomerular targeting did not recover to vehicle-treated levels, even by 20 weeks. Together, these studies demonstrate that the cellular prion protein is critical for regeneration of neurons, whereby increased PrPC levels promote early neurogenesis, and that lack of PrPC delays the regeneration of this tissue after acute injury.

Oligodendrocytes are a major target of the toxicity of spongiogenic murine retroviruses.

The neurovirulent retroviruses FrCasE and Moloney MLV-ts1 cause noninflammatory spongiform neurodegeneration in mice, manifested clinically by progressive spasticity and paralysis. Neurons have been thought to be the primary target of toxicity of these viruses. However the neurons themselves appear not to be infected, and the possible indirect mechanisms driving the neuronal toxicity have remained enigmatic. Here we have re-examined the cells that are damaged by these viruses, using lineage-specific markers. Surprisingly, these cells expressed the basic helix-loop-helix transcription factor Olig2, placing them in the oligodendrocyte lineage. Olig2+ cells were found to be infected, and many of these cells exhibited focal cytoplasmic vacuolation, suggesting that infection by spongiogenic retroviruses is directly toxic to these cells. As cytoplasmic vacuolation progressed, however, signs of viral protein expression appeared to wane, although residual viral RNA was detectable by in situ hybridization. Cells with the most advanced cytoplasmic effacement expressed the C/EBP-homologous protein (CHOP). This protein is up-regulated as a late event in a cellular response termed the integrated stress response. This observation may link the cellular pathology observed in the brain with cellular stress responses known to be induced by these viruses. The relevance of these observations to oligodendropathy in humans is discussed.