Using deep sequencing (deepCAGE), the FANTOM4 study measured the genome-wide dynamics of transcription-start-site usage in the human monocytic cell line THP-1 throughout a time course of growth arrest and differentiation. Modeling the expression dynamics in terms of predicted cis-regulatory sites, we identified the key transcription regulators, their time-dependent activities and target genes. Systematic siRNA knockdown of 52 transcription factors confirmed the roles of individual factors in the regulatory network. Our results indicate that cellular states are constrained by complex networks involving both positive and negative regulatory interactions among substantial numbers of transcription factors and that no single transcription factor is both necessary and sufficient to drive the differentiation process.

Remyelination is an important aspect of nerve regeneration after nerve injury but the underlying mechanisms are not fully understood. The neurotrophin receptor, p75NTR , in activated Schwann cells in the Wallerian degenerated nerve is up-regulated and may play a role in the remyelination of regenerating peripheral nerves. In the present study, the role of p75 NTR in remyelination of the sciatic nerve was investigated in p75 NTR mutant mice. Histological results showed that the number of myelinated axons and thickness of myelin sheath in the injured sciatic nerves were reduced in mutant mice compared with wild-type mice. The myelin sheath of axons in the intact sciatic nerve of adult mutant mice is also thinner than that of wild-type mice. Real-time RT-PCR showed that mRNA levels for myelin basic protein and P0 in the injured sciatic nerves were significantly reduced in p75 NTR mutant animals.Western blots also showed a significant reduction of P0 protein in the injured sciatic nerves of mutant animals. These results suggest that p75 NTR is important for the myelinogenesis during the regeneration of peripheral nerves after injury.

Brain-derived neurotrophic factor (BDNF) plays a pivotal role in brain development and synaptic plasticity. It is synthesized as a precursor (pro-BDNF), sorted into the secretory pathway, transported along dendrites and axons, and released in an activity-dependent manner. Mutant Huntingtin with expanded polyglutamine (polyQ) and the V66M polymorphism of BDNF reduce the dendritic distribution and axonal transport of BDNF. However, the mechanism underlying this defective transport remains unclear. Here, we report that Huntingtin-associated protein-1 (HAP1) interacts with the prodomain of BDNF and that the interaction was reduced in the presence of polyQ-expanded Huntingtin and BDNF V66M. Consistently, there was reduced coimmunoprecipitation of pro-BDNF with HAP1 in the brain homogenate of Huntington disease. Pro-BDNF distribution in the neuronal processes and its accumulation in the proximal and distal segments of crushed sciatic nerve and the activity-dependent release of pro-BDNF were abolished in HAP1 ؊/؊ mice. These results suggest that HAP1 may participate in axonal transport and activity-dependent release of pro-BDNF by interacting with the BDNF prodomain. Accordingly, the decreased interaction between HAP1 and pro-BDNF in Huntington disease may reduce the release and transport of BDNF.Neurotrophins play important roles in the proliferation, differentiation, and survival of neurons during development and in the maintenance of normal functions of the mature nervous system by activating their respective tyrosine kinase receptors TrkA, TrkB, and TrkC and the common receptor p75NTR (1-6). Neurotrophins are synthesized as precursors (proneurotrophins), which are either cleaved intracellularly by furin (2, 7, 8) and released as mature forms (9), or cleaved extracellularly by several proteases, including prohormone convertases, tissue-activated plasminogen/plasmin, 10,11). Recently, it has been shown that unprocessed, the nerve growth factor precursor and the brain-derived neurotrophic factor precursor (pro-BDNF) 3 bind both Sortilin and p75NTR with a high affinity and preferentially activate p75NTR, leading to apoptosis (12-15).Although the retrograde neurotrophic hypothesis is well recognized, accumulating evidence indicates that neurotrophins such as BDNF and neurotrophin-3 are also trafficked anterogradely within dendrites and axons, released in an activity-dependent manner, and uptaken by second-or third-order target neurons (16 -19). The anterogradely transported and released BDNF regulates neuronal survival, differentiation, dendritic morphology, and synaptic plasticity (17, 20 -22). Both Sortilin and carboxypeptidase E play important roles in post-translational Golgi sorting of BDNF to the regulated secretory pathway and activity-dependent release by interacting with the prodomain (23) and the mature domain, respectively (24). Recently, we showed that pro-BDNF, like mature BDNF, is also transported anterogradely and retrogradely within axons of sensory neurons (25). However, how pro-BDNF and mature BDNF are...

Brain-derived neurotrophic factor (BDNF) plays a critical role in the development of the central and peripheral nervous systems, and also in neuronal survival after injury. The actions of BDNF are mediated by its high-affinity receptors TrkB and p75NTR. Recent studies have shown that proneurotrophins bind p75NTR and sortilin with high affinity, and trigger apoptosis of neurons in vitro. As proneurotrophins are a dominant form of gene products in developing and adult animals, it is imperative to understand their physiological functions in animals. Here, we showed differential roles of proBDNF in injured and uninjured sensory neurons. proBDNF, p75NTR and sortilin are highly expressed in dorsal root ganglia (DRG) neurons. Recombinant proBDNF induced a dose-dependent death of PC12 cells and the death activity was completely abolished in the presence of antibodies against the prodomain of BDNF. The exogenous proBDNF enhanced the death of axotomized sensory neurons and the neutralizing antibodies to the prodomain or exogenous sortilin-extracellular domain-Fc fusion molecule reduced the death of axotomized sensory neurons. Interestingly, the treatment of neutralizing antibody in vivo increased the number of sensory neurons in the contralateral DRG. We conclude that proBDNF may induce the death of axotomized sensory neurons and suppress neuronal addition in the intact DRG in neonatal rats, and the suppression of endogenous proBDNF may protect neurons after neurotrauma.

Recent studies have shown that the precursor of brain-derived neurotrophic factor (pro-BDNF) activates p75NTR with high affinity to induce apoptosis. Here we show that pro-BDNF is transported anterogradely and retrogradely in sensory neurons of adult rats. After a crush injury of sciatic nerves, dorsal roots or dorsal column in adult Sprague-Dawley rats, the immunoreactivity for pro-BDNF accumulated at both the proximal and distal segments. The accumulation reached a maximum at 24 h after injury. Western blot analysis also revealed pro-BDNF in sciatic nerve segments proximal and distal to the ligature and in the spinal cord. Biotinylated or Alexa-488-labelled pro-BDNF injected into sciatic nerve was internalized and transported both retrogradely and anterogradely within sensory neurons. These results demonstrate that pro-BDNF is anterogradely and retrogradely transported in sensory neurons, suggesting that endogenous pro-BDNF may be released and play important functions.