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1. Novel types of frequency filtering in the lateral perforant path projections to dentate gyrus

   https://doi.org/10.1113/JP283012  

   Quintanilla, Julian ; Jia, Yousheng ; Lauterborn, Julie C. ; Pruess, Benedict S. ; Le, Aliza A. ; Cox, Conor D. ; Gall, Christine M. ; Lynch, Gary ; Gunn, Benjamin G. ( August 2022 , The Journal of Physiology)

   Despite its evident importance to learning theory and models, the manner in which the lateral perforant path (LPP) transforms signals from entorhinal cortex to hippocampus is not well understood. The present studies measured synaptic responses in the dentate gyrus (DG) of adult mouse hippocampal slices during different patterns of LPP stimulation. Theta (5 Hz) stimulation produced a modest within‐train facilitation that was markedly enhanced at the level of DG output. Gamma (50 Hz) activation resulted in a singular pattern with initial synaptic facilitation being followed by a progressively greater depression. DG output was absent after only two pulses. Reducing release probability with low extracellular calcium instated frequency facilitation to gamma stimulation while long‐term potentiation, which increases release by LPP terminals, enhanced within‐train depression. Relatedly, per terminal concentrations of VGLUT2, a vesicular glutamate transporter associated with high release probability, were much greater in the LPP than in CA3–CA1 connections. Attempts to circumvent the potent gamma filter using a series of short (three‐pulse) 50 Hz trains spaced by 200 ms were only partially successful: composite responses were substantially reduced after the first burst, an effect opposite to that recorded in field CA1. The interaction between bursts was surprisingly persistent (>1.0 s). Low calcium improved throughput during theta/gamma activation but buffering of postsynaptic calcium did not. In all, presynaptic specializations relating to release probability produce an unusual but potent type of frequency filtering in the LPP. Patterned burst input engages a different type of filter with substrates that are also likely to be located presynaptically.image

   The lateral perforant path (LPP)–dentate gyrus (DG) synapse operates as a low‐pass filter, where responses to a train of 50 Hz, γ frequency activation are greatly suppressed.

   Activation with brief bursts of γ frequency information engages a secondary filter that persists for prolonged periods (lasting seconds).

   Both forms of LPP frequency filtering are influenced by presynaptic, as opposed to postsynaptic, processes; this contrasts with other hippocampal synapses.

   LPP frequency filtering is modified by the unique presynaptic long‐term potentiation at this synapse.

   Computational simulations indicate that presynaptic factors associated with release probability and vesicle recycling may underlie the potent LPP–DG frequency filtering.

    

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2. K ATP channel activity and slow oscillations in pancreatic beta cells are regulated by mitochondrial ATP production

   https://doi.org/10.1113/JP284982  

   Corradi, Jeremías ; Thompson, Benjamin ; Fletcher, Patrick A. ; Bertram, Richard ; Sherman, Arthur S. ; Satin, Leslie S. ( November 2023 , The Journal of Physiology)

   Pancreatic beta cells secrete insulin in response to plasma glucose. The ATP‐sensitive potassium channel (K_ATP) links glucose metabolism to islet electrical activity in these cells by responding to increased cytosolic [ATP]/[ADP]. It was recently proposed that pyruvate kinase (PK) in close proximity to beta cell K_ATPlocally produces the ATP that inhibits K_ATPactivity. This proposal was largely based on the observation that applying phosphoenolpyruvate (PEP) and ADP to the cytoplasmic side of excised inside‐out patches inhibited K_ATP. To test the relative contributions of local vs. mitochondrial ATP production, we recorded K_ATPactivity using mouse beta cells and INS‐1 832/13 cells. In contrast to prior reports, we could not replicate inhibition of K_ATPactivity by PEP + ADP. However, when the pH of the PEP solutions was not corrected for the addition of PEP, strong channel inhibition was observed as a result of the well‐known action of protons to inhibit K_ATP. In cell‐attached recordings, perifusing either a PK activator or an inhibitor had little or no effect on K_ATPchannel closure by glucose, further suggesting that PK is not an important regulator of K_ATP. In contrast, addition of mitochondrial inhibitors robustly increased K_ATPactivity. Finally, by measuring the [ATP]/[ADP] responses to imposed calcium oscillations in mouse beta cells, we found that oxidative phosphorylation could raise [ATP]/[ADP] even when ADP was at its nadir during the burst silent phase, in agreement with our mathematical model. These results indicate that ATP produced by mitochondrial oxidative phosphorylation is the primary controller of K_ATPin pancreatic beta cells.image

   Phosphoenolpyruvate (PEP) plus adenosine diphosphate does not inhibit K_ATPactivity in excised patches. PEP solutions only inhibit K_ATPactivity if the pH is unbalanced.

   Modulating pyruvate kinase has minimal effects on K_ATPactivity.

   Mitochondrial inhibition, in contrast, robustly potentiates K_ATPactivity in cell‐attached patches.

   Although the ADP level falls during the silent phase of calcium oscillations, mitochondria can still produce enough ATP via oxidative phosphorylation to close K_ATP.

   Mitochondrial oxidative phosphorylation is therefore the main source of the ATP that inhibits the K_ATPactivity of pancreatic beta cells.

    

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3. Direct intraoperative measurement of isometric contractile properties in living human muscle

   https://doi.org/10.1113/JP284092  

   Binder‐Markey, Benjamin I. ; Persad, Lomas S. ; Shin, Alexander Y. ; Litchy, William J. ; Kaufman, Kenton R. ; Lieber, Richard L. ( April 2023 , The Journal of Physiology)

   Skeletal muscle's isometric contractile properties are one of the classic structure–function relationships in all of biology allowing for extrapolation of single fibre mechanical properties to whole muscle properties based on the muscle's optimal fibre length and physiological cross‐sectional area (PCSA). However, this relationship has only been validated in small animals and then extrapolated to human muscles, which are much larger in terms of length and PCSA. The present study aimed to measure directly thein situproperties and function of the human gracilis muscle to validate this relationship. We leveraged a unique surgical technique in which a human gracilis muscle is transferred from the thigh to the arm, restoring elbow flexion after brachial plexus injury. During this surgery, we directly measured subject specific gracilis muscle force–length relationshipin situand propertiesex vivo. Each subject's optimal fibre length was calculated from their muscle's length‐tension properties. Each subject's PCSA was calculated from their muscle volume and optimal fibre length. From these experimental data, we established a human muscle fibre‐specific tension of 171 kPa. We also determined that average gracilis optimal fibre length is 12.9 cm. Using this subject‐specific fibre length, we observed an excellent fit between experimental and theorical active length‐tension curves. However, these fibre lengths were about half of the previously reported optimal fascicle lengths of 23 cm. Thus, the long gracilis muscle appears to be composed of relatively short fibres acting in parallel that may not have been appreciated based on traditional anatomical methods.image

   Skeletal muscle's isometric contractile properties represent one of the classic structure–function relationships in all of biology and allow scaling single fibre mechanical properties to whole muscle properties based on the muscle's architecture.

   This physiological relationship has only been validated in small animals but is often extrapolated to human muscles, which are orders of magnitude larger.

   We leverage a unique surgical technique in which a human gracilis muscle is transplanted from the thigh to the arm to restore elbow flexion after brachial plexus injury, aiming to directly measure muscles propertiesin situand test directly the architectural scaling predictions.

   Using these direct measurements, we establish human muscle fibre‐specific tension of ∼170 kPa.

   Furthermore, we show that the gracilis muscle actually functions as a muscle with relatively short fibres acting in parallelvs. long fibres as previously assumed based on traditional anatomical models.

    

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4. Lipin1 plays complementary roles in myofibre stability and regeneration in dystrophic muscles

   https://doi.org/10.1113/JP284085  

   Jama, Abdulrahman ; Alshudukhi, Abdullah A. ; Burke, Steve ; Dong, Lixin ; Kamau, John Karanja ; Voss, Andrew Alvin ; Ren, Hongmei ( February 2023 , The Journal of Physiology)

   Duchenne muscular dystrophy (DMD) is a severe muscle wasting disorder caused by dystrophin mutations, leading to the loss of sarcolemmal integrity, and resulting in progressive myofibre necrosis and impaired muscle function. Our previous studies suggest that lipin1 is important for skeletal muscle regeneration and myofibre integrity. Additionally, we discovered that mRNA expression levels of lipin1 were significantly reduced in skeletal muscle of DMD patients and the mdx mouse model. To understand the role of lipin1 in dystrophic muscle, we generated dystrophin/lipin1 double knockout (DKO) mice, and compared the limb muscle pathology and function of wild‐type B10, muscle‐specific lipin1 deficient (lipin1^Myf5cKO), mdx and DKO mice. We found that further knockout of lipin1 in dystrophic muscle exhibited a more severe phenotype characterized by increased necroptosis, fibrosis and exacerbated membrane damage in DKO compared to mdx mice. In barium chloride‐induced muscle injury, both lipin1^Myf5cKOand DKO showed prolonged regeneration at day 14 post‐injection, suggesting that lipin1 is critical for muscle regeneration.In situcontractile function assays showed that lipin1 deficiency in dystrophic muscle led to reduced specific force production. Using a cell culture system, we found that lipin1 deficiency led to elevated expression levels of necroptotic markers and medium creatine kinase, which could be a result of sarcolemmal damage. Most importantly, restoration of lipin1 inhibited the elevation of necroptotic markers in differentiated primary lipin1‐deficient myoblasts. Overall, our data suggests that lipin1 plays complementary roles in myofibre stability and muscle function in dystrophic muscles, and overexpression of lipin1 may serve as a potential therapeutic strategy for dystrophic muscles.image

   We identified that lipin1 mRNA expression levels are significantly reduced in skeletal muscles of Duchenne muscular dystrophy patients and mdx mice.

   We found that further depletion of lipin1 in skeletal muscles of mdx mice induces more severe dystrophic phenotypes, including enhanced myofibre sarcolemma damage, muscle necroptosis, inflammation, fibrosis and reduced specific force production.

   Lipin1 deficiency leads to elevated expression levels of necroptotic markers, whereas restoration of lipin1 inhibits their expression.

   Our results suggest that lipin1 is functionally complementary to dystrophin in muscle membrane integrity and muscle regeneration.

    

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5. Inspiratory and sigh breathing rhythms depend on distinct cellular signalling mechanisms in the preBötzinger complex

   https://doi.org/10.1113/JP285582  

   Borrus, Daniel S. ; Stettler, Marco K. ; Grover, Cameron J. ; Kalajian, Eva J. ; Gu, Jeffrey ; Conradi Smith, Gregory D. ; Del Negro, Christopher A. ( February 2024 , The Journal of Physiology)

   Breathing behaviour involves the generation of normal breaths (eupnoea) on a timescale of seconds and sigh breaths on the order of minutes. Both rhythms emerge in tandem from a single brainstem site, but whether and how a single cell population can generate two disparate rhythms remains unclear. We posit that recurrent synaptic excitation in concert with synaptic depression and cellular refractoriness gives rise to the eupnoea rhythm, whereas an intracellular calcium oscillation that is slower by orders of magnitude gives rise to the sigh rhythm. A mathematical model capturing these dynamics simultaneously generates eupnoea and sigh rhythms with disparate frequencies, which can be separately regulated by physiological parameters. We experimentally validated key model predictions regarding intracellular calcium signalling. All vertebrate brains feature a network oscillator that drives the breathing pump for regular respiration. However, in air‐breathing mammals with compliant lungs susceptible to collapse, the breathing rhythmogenic network may have refashioned ubiquitous intracellular signalling systems to produce a second slower rhythm (for sighs) that prevents atelectasis without impeding eupnoea.image

   A simplified activity‐based model of the preBötC generates inspiratory and sigh rhythms from a single neuron population.

   Inspiration is attributable to a canonical excitatory network oscillator mechanism.

   Sigh emerges from intracellular calcium signalling.

   The model predicts that perturbations of calcium uptake and release across the endoplasmic reticulum counterintuitively accelerate and decelerate sigh rhythmicity, respectively, which was experimentally validated.

   Vertebrate evolution may have adapted existing intracellular signalling mechanisms to produce slow oscillations needed to optimize pulmonary function in mammals.

    

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