Biophysical and network mechanisms of high frequency extracellular potentials in the rat hippocampus

A fundamental question in neuroscience is how distributed networks of neurons communicate and coordinate dynamically and specifically. Several models propose that oscillating local networks can transiently couple to each other through phase-locked firing. Coherent local field potentials (LFP) between synaptically connected regions is often presented as evidence for such coupling. The physiological correlates of LFP signals depend on many anatomical and physiological factors, however, and how the underlying neural processes collectively generate features of different spatiotemporal scales is poorly understood. High frequency oscillations in the hippocampus, including gamma rhythms (30-100 Hz) that are organized by the theta oscillations (5-10 Hz) during active exploration and REM sleep, as well as sharp wave-ripples (SWRs, 140-200 Hz) during immobility or slow wave sleep, have each been associated with various aspects of learning and memory. Deciphering their physiology and functional consequences is crucial to understanding the operation of the hippocampal network.

We investigated the origins and coordination of high frequency LFPs in the hippocampo-entorhinal network using both biophysical models and analyses of large-scale recordings in behaving and sleeping rats. We found that the synchronization of pyramidal cell spikes substantially shapes, or even dominates, the electrical signature of SWRs in area CA1 of the hippocampus. The precise mechanisms coordinating this synchrony are still unresolved, but they appear to also affect CA1 activity during theta oscillations. The input to CA1, which often arrives in the form of gamma-frequency waves of activity from area CA3 and layer 3 of entorhinal cortex (EC3), did not strongly influence the timing of CA1 pyramidal cells. Rather, our data are more consistent with local network interactions governing pyramidal cells' spike timing during the integration of their inputs. Furthermore, the relative timing of input from EC3 and CA3 during the theta cycle matched that found in previous work to engage mechanisms for synapse modification and active dendritic processes. Our work demonstrates how local networks interact with upstream inputs to generate a coordinated hippocampal output during behavior and sleep, in the form of theta-gamma coupling and SWRs.

Mechanistic aspects of the Ziegler-Natta polymerization

The isotope effect on propagation rate was determined for four homogeneous ethylene polymerization systems. The catalytic system Cp_2Ti(Et)Cl + EtA1Cl_2 has a k^H_p/k^D_p = 1.035 ± 0.03. This result strongly supports an insertion mechanism which does not involve a hydrogen migration during the rate determining step of propagation (Cossee mechanism). Three metal-alkyl free systems were also studied. The catalyst I_2 (PMe_3)_3Ta(neopentylidene)(H) has a k^H_p/k^D_p = 1.709. It is interpreted as a primary isotope effect involving a non-linear a-hydrogen migration during the rate determining step of propagation (Green mechanism). The lanthanide complexes Cp*_2LuMe•Et_2O and Cp*_2YbMe•Et_2O have a k^H_p/k^D_p = 1.46 and 1.25, respectively. They are interpreted as primary isotope effects due to a partial hydrogen migration during the rate determining step of propagation.

The presence of a precoordination or other intermediate species during the polymerization of ethylene by the mentioned metal-alkyl free catalysts was sought by low temperature NMR spectroscopy. However, no evidence for such species was found. If they exist, their concentrations are very small or their lifetimes are shorter than the NMR time scale.

Two titanocene (alkenyl)chlorides (hexenyl 1 and heptenyl 2 were prepared from titanocene dichloride and a THF solution of the corresponding alkenylmagnesium chloride. They do not cyclize in solution when alone, but cyclization to their respective titanocene(methyl(cycloalkyl) chlorides occurs readily in the presence of a Lewis acid. It is demonstrated that such cyclization occurs with the alkenyl ligand within the coordination sphere of the titanium atom. Cyclization of 1 with EtAlCl_2 at 0°C occurs in less than 95 msec (ethylene insertion time), as shown by the presence of 97% cyclopentyl-capped oligomers when polymerizing ethylene with this system. Some alkyl exchange occurs (3%). Cyclization of 2 is slower under the same reaction conditions and is not complete in 95 msec as shown by the presence of both cyclohexyl-capped oligomers (35%) and odd number α-olefin oligomers (50%). Alkyl exchange is more extensive as evidenced by the even number n-alkanes (15%).

Cyclization of 2-d_1 (titanocene(hept-6-en-1-yl-1-d_1)chloride) with EtA1Cl_2 demonstrated that for this system there is no α-hydrogen participation during said process. The cyclization is believed to occur by a Cossee-type mechanism. There was no evidence for precoordination of the alkenyl double bond during the cyclization process.

MicroRNA-132 is a physiological regulator of hematopoietic stem cell function and B-cell development

MicroRNAs are a class of small non-coding RNAs that negatively regulate gene expression. Several microRNAs have been implicated in altering hematopoietic cell fate decisions. Importantly, deregulation of many microRNAs can lead to deleterious consequences in the hematopoietic system, including the onset of cancer, autoimmunity, or a failure to respond effectively to infection. As such, microRNAs fine-tune the balance between normal hematopoietic output and pathologic consequences. In this work, we explore the role of two microRNAs, miR-132 and miR-125b, in regulating hematopoietic stem cell (HSC) function and B cell development. In particular, we uncover the role of miR-132 in maintaining the appropriate balance between self-renewal, differentiation, and survival in aging HSCs by buffering the expression of a critical transcription factor, FOXO3. By maintain this balance, miR-132 may play a critical role in preventing aging-associated hematopoietic conditions such as autoimmune disease and cancer. We also find that miR-132 plays a critical role in B cell development by targeting a key transcription factor, Sox4, that is responsible for the differentiation of pro-B cells into pre-B cells. We find that miR-132 regulates B cell apoptosis, and by delivering miR-132 to mice that are predisposed to developing B cell cancers, we can inhibit the formation of these cancers and improve the survival of these mice. In addition to miR-132, we uncovered the role of another critical microRNA, miR-125b, that potentiates hematopoietic stem cell function. We found that enforced expression of miR-125b causes an aggressive myeloid leukemia by downregulation of its target Lin28a. Importantly, miR-125b also plays a critical role in inhibiting the formation of pro-B cells. Thus, we have discovered two microRNAs with important roles in regulating normal hematopoiesis, and whose dregulation can lead to deleterious consequences such as cancer in the aging hematopoietic system. Both miR-132 and miR-125b may therefore be targeted for therapeutics to inhibit age-related immune diseases associated with the loss of HSC function and cancer progression.