Biology subjects

Almeida, A. R.

Publications and source records attributed to Almeida, A. R..

3 recordsLinked to original sources

bioRxiv2022

Early myelination involves the dynamic and repetitive ensheathment of axons which resolves through a low and consistent stabilization rate

Oligodendrocytes in the central nervous system exhibit significant variability in the number of myelin sheaths that are supported by each cell, ranging from just a few to about 50 (1-4). This begs the question, why does sheath number vary so widely across individual oligodendrocytes? Myelin production during development is dynamic and involves both sheath formation and loss (5-9). However, how these parameters are balanced to determine overall sheath number has not been thoroughly investigated. To explore this question, we combined extensive time-lapse and longitudinal imaging of oligodendrocytes in the developing zebrafish spinal cord to quantify sheath initiation and loss. Oligodendrocytes repetitively ensheathed the same axons multiple times before stable sheaths were formed. Interestingly, while there was a variable number of ensheathments produced by each oligodendrocyte, ~80-90% of these structures were always lost, which was an unexpectedly high, but consistent rate of sheath destabilization. The dynamics of this process indicated rapid membrane turn-over as ensheathments were formed and lost repetitively on each axon. To test whether membrane recycling impacts this sheath stabilization rate the endocytic pathway was disrupted by expressing a dominant-negative mutant form of Rab5. Oligodendrocytes over-expressing this mutant lost an even higher percentage of sheaths. Overall, oligodendrocytes initiate a highly variable number of axonal ensheathments, but these cells all exhibit a similar sheath stabilization rate, which is dependent on the endocytic recycling pathway.

neuroscience↗

bioRxiv2022

mTORC2 loss in oligodendrocyte progenitor cells results in regional hypomyelination in the central nervous system

In the central nervous system (CNS), oligodendrocyte progenitor cells (OPCs) differentiate into mature oligodendrocytes to generate myelin, which is essential for normal nervous system function. OPC differentiation is driven by signaling pathways such as mTOR (Mechanistic Target of Rapamycin), which functions in two distinct complexes: mTOR complex 1 (mTORC1) and mTOR complex 2 (mTORC2), containing Raptor or Rictor respectively. In the current studies, mTORC2 signaling was selectively deleted from OPCs in PDGFR-Cre X Rictorfl/fl mice. This study examined developmental myelination in male and female mice, comparing the impact of mTORC2 deletion in the corpus callosum and spinal cord. In both corpus callosum and spinal cord, Rictor loss in OPCs resulted in early reduction in myelin RNAs and some myelin proteins. However, these deficits rapidly recovered in spinal cord, where normal myelin abundance and thickness was noted at post-natal day 21 and 1.5 months. By contrast, the losses in corpus callosum resulted in severe hypomyelination, and increased unmyelinated axons. The current studies focus on uniquely altered signaling pathways following mTORC2 loss in developing oligodendrocytes. A major mTORC2 substrate is phospho-Akt-S473, which was significantly reduced throughout development in both corpus callosum and spinal cord at all ages measured, yet this had little impact in spinal cord. Loss of mTORC2 signaling resulted in decreased expression of actin regulators such as gelsolin in corpus callosum, but only minimal loss in spinal cord. The current study establishes a regionally-specific role for mTORC2 signaling in OPCs, particularly in the corpus callosum. Significance StatementmTORC1 and mTORC2 signaling have differential impact on myelination in the central nervous system. Numerous studies identify a role for mTORC1, but deletion of Rictor (mTORC2 signaling) in late-stage oligodendrocytes had little impact on myelination in the CNS. However, the current studies establish that deletion of mTORC2 signaling from oligodendrocyte progenitor cells results in reduced myelination of brain axons. These studies also establish a regional impact of mTORC2, with little change in spinal cord in these conditional Rictor deletion mice. Importantly, in both, brain and spinal cord, mTORC2 downstream signaling targets were impacted by Rictor deletion. Yet, these signaling changes had little impact on myelination in spinal cord, while they resulted in long term alterations in myelination in brain.

developmental biology↗

bioRxiv2018

The Chromatin-binding Protein Spn1 contributes to Genome Instability in Saccharomyces cerevisiae

Cells expend a large amount of energy to maintain their DNA sequence. DNA repair pathways, cell cycle checkpoint activation, proofreading polymerases, and chromatin structure are ways in which the cell minimizes changes to the genome. During replication, the DNA damage tolerance pathway allows the replication forks to bypass damage on the template strand. This avoids prolonged replication fork stalling, which can contribute to genome instability. The DNA damage tolerance pathway includes two sub-pathways: translesion synthesis and template switch. Post-translational modification of PCNA and the histone tails, cell cycle phase, and local DNA structure have all been shown to influence sub-pathway choice. Chromatin architecture contributes to maintaining genome stability by providing physical protection of the DNA and by regulating DNA processing pathways. As such, chromatin-binding factors have been implicated in maintaining genome stability. Using Saccharomyces cerevisiae, we examined the role of Spn1, a chromatin binding and transcription elongation factor, in DNA damage tolerance. Expression of a mutant allele of SPN1 results in increased resistance to the DNA damaging agent methyl methanesulfonate, lower spontaneous and damage-induced mutation rates, along with increased chronological lifespan. We attribute these effects to an increased usage of the template switch branch of the DNA damage tolerance pathway in the spn1 strain. This provides evidence for a role of wild type Spn1 in promoting genome instability, as well as having ties to overcoming replication stress and contributing to chronological aging.

genetics↗